GDB User Manual
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Debugging with gdb
The gnu Source-Level Debugger
Tenth Edition, for gdb version 8.0.50.20170503-git
(GDB)
Richard Stallman, Roland Pesch, Stan Shebs, et al.
(Send bugs and comments on gdb to http://www.gnu.org/software/gdb/bugs/.)
Debugging with gdb
TEXinfo 2011-02-14.11
Published by the Free Software Foundation
51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA
ISBN 978-0-9831592-3-0
Copyright c 1988-2017 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms
of the GNU Free Documentation License, Version 1.3 or any later version published by
the Free Software Foundation; with the Invariant Sections being “Free Software” and “Free
Software Needs Free Documentation”, with the Front-Cover Texts being “A GNU Manual,”
and with the Back-Cover Texts as in (a) below.
(a) The FSF’s Back-Cover Text is: “You are free to copy and modify this GNU Manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting
software freedom.”
i
Table of Contents
Summary of gdb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Free Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Free Software Needs Free Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Contributors to gdb. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1
A Sample gdb Session . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2
Getting In and Out of gdb . . . . . . . . . . . . . . . . . . . 11
2.1
Invoking gdb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1 Choosing Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2 Choosing Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.3 What gdb Does During Startup . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Quitting gdb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Shell Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Logging Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
gdb Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.1
3.2
3.3
4
11
12
13
16
17
17
18
Command Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Command Completion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Getting Help . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Running Programs Under gdb . . . . . . . . . . . . . . . 25
4.1 Compiling for Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Starting your Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Your Program’s Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4 Your Program’s Environment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5 Your Program’s Working Directory . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6 Your Program’s Input and Output . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7 Debugging an Already-running Process . . . . . . . . . . . . . . . . . . . . . . . .
4.8 Killing the Child Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.9 Debugging Multiple Inferiors and Programs. . . . . . . . . . . . . . . . . . . .
4.10 Debugging Programs with Multiple Threads . . . . . . . . . . . . . . . . . .
4.11 Debugging Forks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.12 Setting a Bookmark to Return to Later. . . . . . . . . . . . . . . . . . . . . . .
4.12.1 A Non-obvious Benefit of Using Checkpoints . . . . . . . . . . . . .
25
26
30
30
31
32
32
33
33
36
40
43
44
ii
5
Debugging with gdb
Stopping and Continuing . . . . . . . . . . . . . . . . . . . . . 45
5.1
Breakpoints, Watchpoints, and Catchpoints . . . . . . . . . . . . . . . . . . .
5.1.1 Setting Breakpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.2 Setting Watchpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.3 Setting Catchpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.4 Deleting Breakpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.5 Disabling Breakpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.6 Break Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.7 Breakpoint Command Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.8 Dynamic Printf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.9 How to save breakpoints to a file . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.10 Static Probe Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.11 “Cannot insert breakpoints” . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.12 “Breakpoint address adjusted...” . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Continuing and Stepping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Skipping Over Functions and Files . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4 Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5 Stopping and Starting Multi-thread Programs . . . . . . . . . . . . . . . . .
5.5.1 All-Stop Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.2 Non-Stop Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.3 Background Execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.4 Thread-Specific Breakpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.5 Interrupted System Calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.6 Observer Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
46
52
54
59
60
61
62
64
65
65
67
67
68
71
74
77
77
78
79
80
81
82
6
Running programs backward . . . . . . . . . . . . . . . . . 85
7
Recording Inferior’s Execution and Replaying
It . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
8
Examining the Stack . . . . . . . . . . . . . . . . . . . . . . . . . . 95
8.1
8.2
8.3
8.4
8.5
Stack Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Backtraces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
Selecting a Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Information About a Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
Management of Frame Filters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
iii
9
Examining Source Files . . . . . . . . . . . . . . . . . . . . . . 103
9.1
9.2
Printing Source Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Specifying a Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.2.1 Linespec Locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.2.2 Explicit Locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.2.3 Address Locations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.3 Editing Source Files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.3.1 Choosing your Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.4 Searching Source Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.5 Specifying Source Directories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.6 Source and Machine Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
Examining Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
10.1 Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2 Ambiguous Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.3 Program Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4 Artificial Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.5 Output Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.6 Examining Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.7 Automatic Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.8 Print Settings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.9 Pretty Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.9.1 Pretty-Printer Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.9.2 Pretty-Printer Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.9.3 Pretty-Printer Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.10 Value History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.11 Convenience Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.12 Convenience Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.13 Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.14 Floating Point Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.15 Vector Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.16 Operating System Auxiliary Information . . . . . . . . . . . . . . . . . . .
10.17 Memory Region Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.17.1 Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.17.1.1 Memory Access Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.17.1.2 Memory Access Size. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.17.1.3 Data Cache . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.17.2 Memory Access Checking . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.18 Copy Between Memory and a File . . . . . . . . . . . . . . . . . . . . . . . . .
10.19 How to Produce a Core File from Your Program . . . . . . . . . . .
10.20 Character Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.21 Caching Data of Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.22 Search Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.23 Value Sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
103
104
104
105
106
106
107
107
107
110
117
118
119
121
122
123
126
127
135
136
136
137
138
139
141
144
146
146
146
148
149
149
149
149
150
150
151
152
154
156
157
Debugging Optimized Code . . . . . . . . . . . . . . . . 159
11.1
11.2
Inline Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
Tail Call Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
iv
Debugging with gdb
12
C Preprocessor Macros . . . . . . . . . . . . . . . . . . . . . 163
13
Tracepoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
13.1 Commands to Set Tracepoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.1.1 Create and Delete Tracepoints . . . . . . . . . . . . . . . . . . . . . . . . .
13.1.2 Enable and Disable Tracepoints . . . . . . . . . . . . . . . . . . . . . . . .
13.1.3 Tracepoint Passcounts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.1.4 Tracepoint Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.1.5 Trace State Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.1.6 Tracepoint Action Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.1.7 Listing Tracepoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.1.8 Listing Static Tracepoint Markers . . . . . . . . . . . . . . . . . . . . . .
13.1.9 Starting and Stopping Trace Experiments . . . . . . . . . . . . . .
13.1.10 Tracepoint Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.2 Using the Collected Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.2.1 tfind n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.2.2 tdump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.2.3 save tracepoints filename . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.3 Convenience Variables for Tracepoints . . . . . . . . . . . . . . . . . . . . . . .
13.4 Using Trace Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
Debugging Programs That Use Overlays
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
14.1
14.2
14.3
14.4
15
167
168
170
170
171
171
172
174
175
176
178
179
179
181
182
182
183
How Overlays Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Overlay Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Automatic Overlay Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Overlay Sample Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
185
186
188
189
Using gdb with Different Languages . . . . . . 191
15.1 Switching Between Source Languages . . . . . . . . . . . . . . . . . . . . . . .
15.1.1 List of Filename Extensions and Languages . . . . . . . . . . . . .
15.1.2 Setting the Working Language . . . . . . . . . . . . . . . . . . . . . . . . .
15.1.3 Having gdb Infer the Source Language . . . . . . . . . . . . . . . . .
15.2 Displaying the Language . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.3 Type and Range Checking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.3.1 An Overview of Type Checking . . . . . . . . . . . . . . . . . . . . . . . .
15.3.2 An Overview of Range Checking . . . . . . . . . . . . . . . . . . . . . . .
15.4 Supported Languages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.1 C and C++ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.1.1 C and C++ Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.1.2 C and C++ Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.1.3 C++ Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.1.4 C and C++ Defaults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.1.5 C and C++ Type and Range Checks . . . . . . . . . . . . . . .
15.4.1.6 gdb and C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.1.7 gdb Features for C++ . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.1.8 Decimal Floating Point format . . . . . . . . . . . . . . . . . . . .
191
191
192
192
192
193
193
194
195
195
195
197
198
199
199
199
199
201
v
15.4.2 D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.3 Go. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.4 Objective-C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.4.1 Method Names in Commands . . . . . . . . . . . . . . . . . . . . .
15.4.4.2 The Print Command With Objective-C . . . . . . . . . . . .
15.4.5 OpenCL C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.5.1 OpenCL C Datatypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.5.2 OpenCL C Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.5.3 OpenCL C Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.6 Fortran . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.6.1 Fortran Operators and Expressions . . . . . . . . . . . . . . . .
15.4.6.2 Fortran Defaults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.6.3 Special Fortran Commands . . . . . . . . . . . . . . . . . . . . . . . .
15.4.7 Pascal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.8 Rust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.9 Modula-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.9.1 Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.9.2 Built-in Functions and Procedures . . . . . . . . . . . . . . . . .
15.4.9.3 Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.9.4 Modula-2 Types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.9.5 Modula-2 Defaults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.9.6 Deviations from Standard Modula-2 . . . . . . . . . . . . . . .
15.4.9.7 Modula-2 Type and Range Checks. . . . . . . . . . . . . . . . .
15.4.9.8 The Scope Operators :: and . . . . . . . . . . . . . . . . . . . . .
15.4.9.9 gdb and Modula-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.10 Ada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.10.2 Omissions from Ada . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.10.3 Additions to Ada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.10.4 Overloading support for Ada . . . . . . . . . . . . . . . . . . . . .
15.4.10.5 Stopping at the Very Beginning . . . . . . . . . . . . . . . . . .
15.4.10.6 Ada Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4.10.7 Extensions for Ada Tasks . . . . . . . . . . . . . . . . . . . . . . . .
15.4.10.8 Tasking Support when Debugging Core Files . . . . .
15.4.10.9 Tasking Support when using the Ravenscar Profile
........................................................
15.4.10.10 Known Peculiarities of Ada Mode . . . . . . . . . . . . . . .
15.5 Unsupported Languages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
201
201
202
202
202
203
203
203
203
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204
204
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205
205
206
207
208
210
210
210
210
211
211
211
212
213
214
215
215
215
218
219
219
220
Examining the Symbol Table . . . . . . . . . . . . . . 221
vi
Debugging with gdb
17
Altering Execution. . . . . . . . . . . . . . . . . . . . . . . . . . 229
17.1 Assignment to Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.2 Continuing at a Different Address . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.3 Giving your Program a Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.4 Returning from a Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.5 Calling Program Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.6 Patching Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.7 Compiling and injecting code in gdb . . . . . . . . . . . . . . . . . . . . . . . .
17.7.1 Compilation options for the compile command . . . . . . . . .
17.7.2 Caveats when using the compile command . . . . . . . . . . . . .
17.7.3 Compiler search for the compile command . . . . . . . . . . . . .
18
gdb Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
18.1
18.2
18.3
18.4
18.5
18.6
18.7
19
Commands to Specify Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
File Caching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Debugging Information in Separate Files . . . . . . . . . . . . . . . . . . . .
Debugging information in a special section . . . . . . . . . . . . . . . . . .
Index Files Speed Up gdb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Errors Reading Symbol Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
GDB Data Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
241
249
250
253
254
255
256
Specifying a Debugging Target . . . . . . . . . . . . 257
19.1
19.2
19.3
20
229
230
231
232
233
234
234
236
236
238
Active Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
Commands for Managing Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
Choosing Target Byte Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
Debugging Remote Programs . . . . . . . . . . . . . . 261
20.1 Connecting to a Remote Target . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20.1.1 Types of Remote Connections . . . . . . . . . . . . . . . . . . . . . . . . . .
20.1.2 Host and Target Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20.1.3 Remote Connection Commands . . . . . . . . . . . . . . . . . . . . . . . .
20.2 Sending files to a remote system . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20.3 Using the gdbserver Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20.3.1 Running gdbserver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20.3.1.1 Attaching to a Running Program . . . . . . . . . . . . . . . . . .
20.3.1.2 TCP port allocation lifecycle of gdbserver . . . . . . . .
20.3.1.3 Other Command-Line Arguments for gdbserver . . .
20.3.2 Connecting to gdbserver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20.3.3 Monitor Commands for gdbserver . . . . . . . . . . . . . . . . . . . . .
20.3.4 Tracepoints support in gdbserver . . . . . . . . . . . . . . . . . . . . . .
20.4 Remote Configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20.5 Implementing a Remote Stub . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20.5.1 What the Stub Can Do for You . . . . . . . . . . . . . . . . . . . . . . . .
20.5.2 What You Must Do for the Stub . . . . . . . . . . . . . . . . . . . . . . .
20.5.3 Putting it All Together. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
261
261
262
263
264
265
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266
266
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270
275
276
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278
vii
21
Configuration-Specific Information . . . . . . . . 281
21.1 Native . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.1.1 BSD libkvm Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.1.2 SVR4 Process Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.1.3 Features for Debugging djgpp Programs . . . . . . . . . . . . . . .
21.1.4 Features for Debugging MS Windows PE Executables . .
21.1.4.1 Support for DLLs without Debugging Symbols. . . . .
21.1.4.2 DLL Name Prefixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.1.4.3 Working with Minimal Symbols . . . . . . . . . . . . . . . . . . .
21.1.5 Commands Specific to gnu Hurd Systems . . . . . . . . . . . . . .
21.1.6 Darwin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.2 Embedded Operating Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.3 Embedded Processors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.3.1 Synopsys ARC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.3.2 ARM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.3.3 M68k . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.3.4 MicroBlaze . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.3.5 MIPS Embedded . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.3.6 PowerPC Embedded . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.3.7 Atmel AVR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.3.8 CRIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.3.9 Renesas Super-H . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.4 Architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.4.1 AArch64 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.4.2 x86 Architecture-specific Issues . . . . . . . . . . . . . . . . . . . . . . . . .
21.4.2.1 Intel Memory Protection Extensions (MPX). . . . . . .
21.4.3 Alpha. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.4.4 MIPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.4.5 HPPA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.4.6 Cell Broadband Engine SPU architecture . . . . . . . . . . . . . . .
21.4.7 PowerPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.4.8 Nios II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
281
281
281
283
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287
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291
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296
297
297
297
298
298
300
300
301
301
Controlling gdb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303
22.1 Prompt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22.2 Command Editing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22.3 Command History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22.4 Screen Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22.5 Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22.6 Configuring the Current ABI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22.7 Automatically loading associated files . . . . . . . . . . . . . . . . . . . . . . .
22.7.1 Automatically loading init file in the current directory . .
22.7.2 Automatically loading thread debugging library . . . . . . . .
22.7.3 Security restriction for auto-loading . . . . . . . . . . . . . . . . . . . .
22.7.4 Displaying files tried for auto-load . . . . . . . . . . . . . . . . . . . . . .
22.8 Optional Warnings and Messages. . . . . . . . . . . . . . . . . . . . . . . . . . . .
22.9 Optional Messages about Internal Happenings . . . . . . . . . . . . . . .
22.10 Other Miscellaneous Settings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
303
303
304
305
306
307
308
310
310
311
312
313
314
319
viii
Debugging with gdb
23
Extending gdb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321
23.1 Canned Sequences of Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.1.1 User-defined Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.1.2 User-defined Command Hooks. . . . . . . . . . . . . . . . . . . . . . . . . .
23.1.3 Command Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.1.4 Commands for Controlled Output . . . . . . . . . . . . . . . . . . . . . .
23.1.5 Controlling auto-loading native gdb scripts . . . . . . . . . . . . .
23.2 Extending gdb using Python . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.1 Python Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2 Python API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.1 Basic Python . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.2 Exception Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.3 Values From Inferior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.4 Types In Python . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.5 Pretty Printing API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.6 Selecting Pretty-Printers . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.7 Writing a Pretty-Printer . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.8 Type Printing API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.9 Filtering Frames. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.10 Decorating Frames. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.11 Writing a Frame Filter . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.12 Unwinding Frames in Python . . . . . . . . . . . . . . . . . . . .
23.2.2.13 Xmethods In Python . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.14 Xmethod API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.15 Writing an Xmethod . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.16 Inferiors In Python. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.17 Events In Python . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.18 Threads In Python . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.19 Recordings In Python . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.20 Commands In Python . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.21 Parameters In Python . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.22 Writing new convenience functions . . . . . . . . . . . . . . .
23.2.2.23 Program Spaces In Python . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.24 Objfiles In Python . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.25 Accessing inferior stack frames from Python. . . . . .
23.2.2.26 Accessing blocks from Python. . . . . . . . . . . . . . . . . . . .
23.2.2.27 Python representation of Symbols. . . . . . . . . . . . . . . .
23.2.2.28 Symbol table representation in Python. . . . . . . . . . . .
23.2.2.29 Manipulating line tables using Python . . . . . . . . . . . .
23.2.2.30 Manipulating breakpoints using Python . . . . . . . . . .
23.2.2.31 Finish Breakpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.2.32 Python representation of lazy strings. . . . . . . . . . . . .
23.2.2.33 Python representation of architectures . . . . . . . . . . . .
23.2.3 Python Auto-loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.4 Python modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.4.1 gdb.printing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.4.2 gdb.types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.2.4.3 gdb.prompt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
321
321
323
324
326
327
328
328
329
329
333
334
338
343
344
344
347
348
350
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356
358
359
361
364
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368
369
373
376
378
379
380
382
385
387
390
392
393
396
396
397
398
398
399
399
400
ix
23.3 Extending gdb using Guile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.1 Guile Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.2 Guile Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3 Guile API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.1 Basic Guile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.2 Guile Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.3 GDB Scheme Data Types . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.4 Guile Exception Handling . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.5 Values From Inferior In Guile . . . . . . . . . . . . . . . . . . . . .
23.3.3.6 Arithmetic In Guile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.7 Types In Guile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.8 Guile Pretty Printing API . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.9 Selecting Guile Pretty-Printers . . . . . . . . . . . . . . . . . . . .
23.3.3.10 Writing a Guile Pretty-Printer . . . . . . . . . . . . . . . . . . .
23.3.3.11 Commands In Guile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.12 Parameters In Guile. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.13 Program Spaces In Guile . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.14 Objfiles In Guile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.15 Accessing inferior stack frames from Guile. . . . . . . .
23.3.3.16 Accessing blocks from Guile. . . . . . . . . . . . . . . . . . . . . .
23.3.3.17 Guile representation of Symbols. . . . . . . . . . . . . . . . . .
23.3.3.18 Symbol table representation in Guile. . . . . . . . . . . . . .
23.3.3.19 Manipulating breakpoints using Guile . . . . . . . . . . . .
23.3.3.20 Guile representation of lazy strings. . . . . . . . . . . . . . .
23.3.3.21 Guile representation of architectures . . . . . . . . . . . . . .
23.3.3.22 Disassembly In Guile . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.23 I/O Ports in Guile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.24 Memory Ports in Guile . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.3.25 Iterators In Guile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.4 Guile Auto-loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.5 Guile Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.5.1 Guile Printing Module . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3.5.2 Guile Types Module. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.4 Auto-loading extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.4.1 The ‘objfile-gdb.ext’ file . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.4.2 The .debug_gdb_scripts section . . . . . . . . . . . . . . . . . . . . . .
23.4.2.1 Script File Entries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.4.2.2 Script Text Entries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.4.3 Which flavor to choose? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.5 Multiple Extension Languages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.5.1 Python comes first . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.6 Creating new spellings of existing commands . . . . . . . . . . . . . . . .
24
401
401
402
402
402
404
405
406
408
413
414
419
420
421
423
427
430
431
431
434
436
439
440
444
445
447
447
448
449
451
451
451
452
452
453
454
454
455
455
456
456
456
Command Interpreters . . . . . . . . . . . . . . . . . . . . . 459
x
Debugging with gdb
25
gdb Text User Interface . . . . . . . . . . . . . . . . . . . . 461
25.1
25.2
25.3
25.4
25.5
TUI Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TUI Key Bindings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TUI Single Key Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TUI-specific Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TUI Configuration Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
461
462
463
463
465
26
Using gdb under gnu Emacs . . . . . . . . . . . . . . . 467
27
The gdb/mi Interface . . . . . . . . . . . . . . . . . . . . . . . 469
Function and Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Notation and Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.3 gdb/mi General Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.3.1 Context management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.3.1.1 Threads and Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.3.1.2 Language . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.3.2 Asynchronous command execution and non-stop mode . .
27.3.3 Thread groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.4 gdb/mi Command Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.4.1 gdb/mi Input Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.4.2 gdb/mi Output Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.5 gdb/mi Compatibility with CLI . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.6 gdb/mi Development and Front Ends . . . . . . . . . . . . . . . . . . . . . . .
27.7 gdb/mi Output Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.7.1 gdb/mi Result Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.7.2 gdb/mi Stream Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.7.3 gdb/mi Async Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.7.4 gdb/mi Breakpoint Information . . . . . . . . . . . . . . . . . . . . . . . .
27.7.5 gdb/mi Frame Information . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.7.6 gdb/mi Thread Information . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.7.7 gdb/mi Ada Exception Information . . . . . . . . . . . . . . . . . . . .
27.8 Simple Examples of gdb/mi Interaction . . . . . . . . . . . . . . . . . . . . .
27.9 gdb/mi Command Description Format . . . . . . . . . . . . . . . . . . . . . .
27.10 gdb/mi Breakpoint Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.11 gdb/mi Catchpoint Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.11.1 Shared Library gdb/mi Catchpoints . . . . . . . . . . . . . . . . . .
27.11.2 Ada Exception gdb/mi Catchpoints . . . . . . . . . . . . . . . . . . .
27.12 gdb/mi Program Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.13 gdb/mi Thread Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.14 gdb/mi Ada Tasking Commands . . . . . . . . . . . . . . . . . . . . . . . . . .
27.15 gdb/mi Program Execution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.16 gdb/mi Stack Manipulation Commands . . . . . . . . . . . . . . . . . . . .
27.17 gdb/mi Variable Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.18 gdb/mi Data Manipulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.19 gdb/mi Tracepoint Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27.20 gdb/mi Symbol Query Commands . . . . . . . . . . . . . . . . . . . . . . . . .
27.21 gdb/mi File Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
469
469
469
470
470
471
471
472
472
473
473
475
475
476
476
476
477
481
482
483
483
483
484
485
495
495
496
497
499
501
502
508
514
525
533
539
539
xi
27.22
27.23
27.24
27.25
27.26
28
What is an Annotation? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Server Prefix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Annotation for gdb Input. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Invalidation Notices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Running the Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Displaying Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
557
558
558
559
559
559
560
JIT Compilation Interface . . . . . . . . . . . . . . . . . 561
29.1 JIT Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29.2 Registering Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29.3 Unregistering Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29.4 Custom Debug Info . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29.4.1 Using JIT Debug Info Readers . . . . . . . . . . . . . . . . . . . . . . . . .
29.4.2 Writing JIT Debug Info Readers . . . . . . . . . . . . . . . . . . . . . . .
30
542
545
546
547
549
gdb Annotations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557
28.1
28.2
28.3
28.4
28.5
28.6
28.7
29
gdb/mi Target Manipulation Commands. . . . . . . . . . . . . . . . . . .
gdb/mi File Transfer Commands . . . . . . . . . . . . . . . . . . . . . . . . . .
Ada Exceptions gdb/mi Commands . . . . . . . . . . . . . . . . . . . . . . .
gdb/mi Support Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Miscellaneous gdb/mi Commands . . . . . . . . . . . . . . . . . . . . . . . . .
561
562
562
562
563
563
In-Process Agent. . . . . . . . . . . . . . . . . . . . . . . . . . . . 565
30.1 In-Process Agent Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565
30.1.1 IPA Protocol Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 566
30.1.2 IPA Protocol Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 567
31
Reporting Bugs in gdb . . . . . . . . . . . . . . . . . . . . . 569
31.1
31.2
32
Have You Found a Bug? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 569
How to Report Bugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 569
Command Line Editing. . . . . . . . . . . . . . . . . . . . . 573
32.1 Introduction to Line Editing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32.2 Readline Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32.2.1 Readline Bare Essentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32.2.2 Readline Movement Commands . . . . . . . . . . . . . . . . . . . . . . . .
32.2.3 Readline Killing Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32.2.4 Readline Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32.2.5 Searching for Commands in the History . . . . . . . . . . . . . . . .
32.3 Readline Init File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32.3.1 Readline Init File Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32.3.2 Conditional Init Constructs . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32.3.3 Sample Init File. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32.4 Bindable Readline Commands. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32.4.1 Commands For Moving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
573
573
573
574
574
575
575
576
576
582
583
586
586
xii
Debugging with gdb
32.4.2 Commands For Manipulating The History . . . . . . . . . . . . . .
32.4.3 Commands For Changing Text . . . . . . . . . . . . . . . . . . . . . . . . .
32.4.4 Killing And Yanking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32.4.5 Specifying Numeric Arguments . . . . . . . . . . . . . . . . . . . . . . . . .
32.4.6 Letting Readline Type For You. . . . . . . . . . . . . . . . . . . . . . . . .
32.4.7 Keyboard Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32.4.8 Some Miscellaneous Commands . . . . . . . . . . . . . . . . . . . . . . . .
32.5 Readline vi Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
586
588
589
590
590
591
591
593
Using History Interactively . . . . . . . . . . . . . . . . 595
33.1 History Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33.1.1 Event Designators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33.1.2 Word Designators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33.1.3 Modifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
595
595
596
596
Appendix A
In Memoriam . . . . . . . . . . . . . . . . . . . . 599
Appendix B
Formatting Documentation . . . . . 601
Appendix C
Installing gdb . . . . . . . . . . . . . . . . . . . . 603
C.1 Requirements for Building gdb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C.2 Invoking the gdb ‘configure’ Script . . . . . . . . . . . . . . . . . . . . . . . .
C.3 Compiling gdb in Another Directory . . . . . . . . . . . . . . . . . . . . . . . .
C.4 Specifying Names for Hosts and Targets . . . . . . . . . . . . . . . . . . . . .
C.5 ‘configure’ Options. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C.6 System-wide configuration and settings . . . . . . . . . . . . . . . . . . . . . .
C.6.1 Installed System-wide Configuration Scripts . . . . . . . . . . . . .
603
604
605
606
607
608
608
Appendix D
Maintenance Commands. . . . . . . . 609
Appendix E
gdb Remote Serial Protocol . . . . 619
E.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.2 Packets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.3 Stop Reply Packets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.4 General Query Packets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.5 Architecture-Specific Protocol Details . . . . . . . . . . . . . . . . . . . . . . . .
E.5.1 ARM-specific Protocol Details . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.5.1.1 ARM Breakpoint Kinds . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.5.2 MIPS-specific Protocol Details . . . . . . . . . . . . . . . . . . . . . . . . . .
E.5.2.1 MIPS Register Packet Format . . . . . . . . . . . . . . . . . . . . . .
E.5.2.2 MIPS Breakpoint Kinds . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.6 Tracepoint Packets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.6.1 Relocate instruction reply packet. . . . . . . . . . . . . . . . . . . . . . . .
E.7 Host I/O Packets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.8 Interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.9 Notification Packets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
619
620
631
634
658
658
658
658
658
658
658
665
665
667
668
xiii
E.10 Remote Protocol Support for Non-Stop Mode . . . . . . . . . . . . . . .
E.11 Packet Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.12 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.13 File-I/O Remote Protocol Extension . . . . . . . . . . . . . . . . . . . . . . . .
E.13.1 File-I/O Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.13.2 Protocol Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.13.3 The F Request Packet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.13.4 The F Reply Packet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.13.5 The ‘Ctrl-C’ Message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.13.6 Console I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.13.7 List of Supported Calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
open . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
close . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
read . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
write . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
lseek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
rename . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unlink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
stat/fstat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
gettimeofday . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
isatty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.13.8 Protocol-specific Representation of Datatypes . . . . . . . . . .
Integral Datatypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pointer Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Memory Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
struct stat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
struct timeval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.13.9 Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Open Flags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
mode t Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Errno Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lseek Flags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.13.10 File-I/O Examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.14 Library List Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.15 Library List Format for SVR4 Targets . . . . . . . . . . . . . . . . . . . . . .
E.16 Memory Map Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.17 Thread List Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.18 Traceframe Info Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.19 Branch Trace Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.20 Branch Trace Configuration Format . . . . . . . . . . . . . . . . . . . . . . . .
670
671
671
672
672
672
673
673
674
674
674
674
676
676
676
677
677
678
678
679
679
679
680
680
680
680
681
681
682
682
682
682
683
683
683
683
684
685
686
686
687
688
xiv
Debugging with gdb
Appendix F The GDB Agent Expression
Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 689
F.1
F.2
F.3
F.4
F.5
General Bytecode Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bytecode Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Using Agent Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Varying Target Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix G
689
691
696
697
697
Target Descriptions . . . . . . . . . . . . . 701
G.1 Retrieving Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.2 Target Description Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.2.1 Inclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.2.2 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.2.3 OS ABI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.2.4 Compatible Architecture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.2.5 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.2.6 Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.2.7 Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.3 Predefined Target Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.4 Enum Target Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5 Standard Target Features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5.1 AArch64 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5.2 ARC Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5.3 ARM Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5.4 i386 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5.5 MicroBlaze Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5.6 MIPS Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5.7 M68K Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5.8 NDS32 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5.9 Nios II Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5.10 PowerPC Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5.11 S/390 and System z Features . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5.12 Sparc Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.5.13 TMS320C6x Features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
701
701
702
702
703
703
703
703
704
705
706
706
707
707
708
708
709
709
710
710
710
710
711
711
711
Appendix H Operating System Information
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713
H.1
Process list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713
Appendix I
Trace File Format . . . . . . . . . . . . . . . 715
Appendix J
.gdb_index section format. . . . . . . 717
Appendix K
Manual pages . . . . . . . . . . . . . . . . . . . . 721
xv
Appendix L
GNU GENERAL PUBLIC
LICENSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 727
Appendix M GNU Free Documentation License
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 739
Concept Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 747
Command, Variable, and Function Index . . . . . . 761
Summary of gdb
1
Summary of gdb
The purpose of a debugger such as gdb is to allow you to see what is going on “inside”
another program while it executes—or what another program was doing at the moment it
crashed.
gdb can do four main kinds of things (plus other things in support of these) to help you
catch bugs in the act:
•
•
•
•
Start your program, specifying anything that might affect its behavior.
Make your program stop on specified conditions.
Examine what has happened, when your program has stopped.
Change things in your program, so you can experiment with correcting the effects of
one bug and go on to learn about another.
You can use gdb to debug programs written in C and C++. For more information, see
Section 15.4 [Supported Languages], page 195. For more information, see Section 15.4.1 [C
and C++], page 195.
Support for D is partial. For information on D, see Section 15.4.2 [D], page 201.
Support for Modula-2 is partial.
[Modula-2], page 205.
For information on Modula-2, see Section 15.4.9
Support for OpenCL C is partial. For information on OpenCL C, see Section 15.4.5
[OpenCL C], page 203.
Debugging Pascal programs which use sets, subranges, file variables, or nested functions
does not currently work. gdb does not support entering expressions, printing values, or
similar features using Pascal syntax.
gdb can be used to debug programs written in Fortran, although it may be necessary
to refer to some variables with a trailing underscore.
gdb can be used to debug programs written in Objective-C, using either the Apple/NeXT or the GNU Objective-C runtime.
Free Software
gdb is free software, protected by the gnu General Public License (GPL). The GPL gives
you the freedom to copy or adapt a licensed program—but every person getting a copy also
gets with it the freedom to modify that copy (which means that they must get access to the
source code), and the freedom to distribute further copies. Typical software companies use
copyrights to limit your freedoms; the Free Software Foundation uses the GPL to preserve
these freedoms.
Fundamentally, the General Public License is a license which says that you have these
freedoms and that you cannot take these freedoms away from anyone else.
Free Software Needs Free Documentation
The biggest deficiency in the free software community today is not in the software—it is the
lack of good free documentation that we can include with the free software. Many of our
most important programs do not come with free reference manuals and free introductory
2
Debugging with gdb
texts. Documentation is an essential part of any software package; when an important free
software package does not come with a free manual and a free tutorial, that is a major gap.
We have many such gaps today.
Consider Perl, for instance. The tutorial manuals that people normally use are non-free.
How did this come about? Because the authors of those manuals published them with
restrictive terms—no copying, no modification, source files not available—which exclude
them from the free software world.
That wasn’t the first time this sort of thing happened, and it was far from the last.
Many times we have heard a GNU user eagerly describe a manual that he is writing, his
intended contribution to the community, only to learn that he had ruined everything by
signing a publication contract to make it non-free.
Free documentation, like free software, is a matter of freedom, not price. The problem
with the non-free manual is not that publishers charge a price for printed copies—that in
itself is fine. (The Free Software Foundation sells printed copies of manuals, too.) The
problem is the restrictions on the use of the manual. Free manuals are available in source
code form, and give you permission to copy and modify. Non-free manuals do not allow
this.
The criteria of freedom for a free manual are roughly the same as for free software.
Redistribution (including the normal kinds of commercial redistribution) must be permitted,
so that the manual can accompany every copy of the program, both on-line and on paper.
Permission for modification of the technical content is crucial too. When people modify the software, adding or changing features, if they are conscientious they will change
the manual too—so they can provide accurate and clear documentation for the modified
program. A manual that leaves you no choice but to write a new manual to document a
changed version of the program is not really available to our community.
Some kinds of limits on the way modification is handled are acceptable. For example,
requirements to preserve the original author’s copyright notice, the distribution terms, or
the list of authors, are ok. It is also no problem to require modified versions to include
notice that they were modified. Even entire sections that may not be deleted or changed
are acceptable, as long as they deal with nontechnical topics (like this one). These kinds of
restrictions are acceptable because they don’t obstruct the community’s normal use of the
manual.
However, it must be possible to modify all the technical content of the manual, and then
distribute the result in all the usual media, through all the usual channels. Otherwise, the
restrictions obstruct the use of the manual, it is not free, and we need another manual to
replace it.
Please spread the word about this issue. Our community continues to lose manuals
to proprietary publishing. If we spread the word that free software needs free reference
manuals and free tutorials, perhaps the next person who wants to contribute by writing
documentation will realize, before it is too late, that only free manuals contribute to the
free software community.
If you are writing documentation, please insist on publishing it under the GNU Free
Documentation License or another free documentation license. Remember that this decision requires your approval—you don’t have to let the publisher decide. Some commercial
publishers will use a free license if you insist, but they will not propose the option; it is up
Summary of gdb
3
to you to raise the issue and say firmly that this is what you want. If the publisher you
are dealing with refuses, please try other publishers. If you’re not sure whether a proposed
license is free, write to licensing@gnu.org.
You can encourage commercial publishers to sell more free, copylefted manuals and
tutorials by buying them, and particularly by buying copies from the publishers that paid
for their writing or for major improvements. Meanwhile, try to avoid buying non-free
documentation at all. Check the distribution terms of a manual before you buy it, and
insist that whoever seeks your business must respect your freedom. Check the history of
the book, and try to reward the publishers that have paid or pay the authors to work on it.
The Free Software Foundation maintains a list of free documentation published by other
publishers, at http://www.fsf.org/doc/other-free-books.html.
Contributors to gdb
Richard Stallman was the original author of gdb, and of many other gnu programs. Many
others have contributed to its development. This section attempts to credit major contributors. One of the virtues of free software is that everyone is free to contribute to it; with
regret, we cannot actually acknowledge everyone here. The file ‘ChangeLog’ in the gdb
distribution approximates a blow-by-blow account.
Changes much prior to version 2.0 are lost in the mists of time.
Plea: Additions to this section are particularly welcome. If you or your friends
(or enemies, to be evenhanded) have been unfairly omitted from this list, we
would like to add your names!
So that they may not regard their many labors as thankless, we particularly thank those
who shepherded gdb through major releases: Andrew Cagney (releases 6.3, 6.2, 6.1, 6.0,
5.3, 5.2, 5.1 and 5.0); Jim Blandy (release 4.18); Jason Molenda (release 4.17); Stan Shebs
(release 4.14); Fred Fish (releases 4.16, 4.15, 4.13, 4.12, 4.11, 4.10, and 4.9); Stu Grossman
and John Gilmore (releases 4.8, 4.7, 4.6, 4.5, and 4.4); John Gilmore (releases 4.3, 4.2, 4.1,
4.0, and 3.9); Jim Kingdon (releases 3.5, 3.4, and 3.3); and Randy Smith (releases 3.2, 3.1,
and 3.0).
Richard Stallman, assisted at various times by Peter TerMaat, Chris Hanson, and
Richard Mlynarik, handled releases through 2.8.
Michael Tiemann is the author of most of the gnu C++ support in gdb, with significant
additional contributions from Per Bothner and Daniel Berlin. James Clark wrote the gnu
C++ demangler. Early work on C++ was by Peter TerMaat (who also did much general
update work leading to release 3.0).
gdb uses the BFD subroutine library to examine multiple object-file formats; BFD was
a joint project of David V. Henkel-Wallace, Rich Pixley, Steve Chamberlain, and John
Gilmore.
David Johnson wrote the original COFF support; Pace Willison did the original support
for encapsulated COFF.
Brent Benson of Harris Computer Systems contributed DWARF 2 support.
Adam de Boor and Bradley Davis contributed the ISI Optimum V support. Per Bothner,
Noboyuki Hikichi, and Alessandro Forin contributed MIPS support. Jean-Daniel Fekete
contributed Sun 386i support. Chris Hanson improved the HP9000 support. Noboyuki
4
Debugging with gdb
Hikichi and Tomoyuki Hasei contributed Sony/News OS 3 support. David Johnson contributed Encore Umax support. Jyrki Kuoppala contributed Altos 3068 support. Jeff
Law contributed HP PA and SOM support. Keith Packard contributed NS32K support.
Doug Rabson contributed Acorn Risc Machine support. Bob Rusk contributed Harris
Nighthawk CX-UX support. Chris Smith contributed Convex support (and Fortran debugging). Jonathan Stone contributed Pyramid support. Michael Tiemann contributed
SPARC support. Tim Tucker contributed support for the Gould NP1 and Gould Powernode. Pace Willison contributed Intel 386 support. Jay Vosburgh contributed Symmetry
support. Marko Mlinar contributed OpenRISC 1000 support.
Andreas Schwab contributed M68K gnu/Linux support.
Rich Schaefer and Peter Schauer helped with support of SunOS shared libraries.
Jay Fenlason and Roland McGrath ensured that gdb and GAS agree about several
machine instruction sets.
Patrick Duval, Ted Goldstein, Vikram Koka and Glenn Engel helped develop remote
debugging. Intel Corporation, Wind River Systems, AMD, and ARM contributed remote
debugging modules for the i960, VxWorks, A29K UDI, and RDI targets, respectively.
Brian Fox is the author of the readline libraries providing command-line editing and
command history.
Andrew Beers of SUNY Buffalo wrote the language-switching code, the Modula-2 support, and contributed the Languages chapter of this manual.
Fred Fish wrote most of the support for Unix System Vr4. He also enhanced the
command-completion support to cover C++ overloaded symbols.
Hitachi America (now Renesas America), Ltd. sponsored the support for H8/300,
H8/500, and Super-H processors.
NEC sponsored the support for the v850, Vr4xxx, and Vr5xxx processors.
Mitsubishi (now Renesas) sponsored the support for D10V, D30V, and M32R/D processors.
Toshiba sponsored the support for the TX39 Mips processor.
Matsushita sponsored the support for the MN10200 and MN10300 processors.
Fujitsu sponsored the support for SPARClite and FR30 processors.
Kung Hsu, Jeff Law, and Rick Sladkey added support for hardware watchpoints.
Michael Snyder added support for tracepoints.
Stu Grossman wrote gdbserver.
Jim Kingdon, Peter Schauer, Ian Taylor, and Stu Grossman made nearly innumerable
bug fixes and cleanups throughout gdb.
The following people at the Hewlett-Packard Company contributed support for the PARISC 2.0 architecture, HP-UX 10.20, 10.30, and 11.0 (narrow mode), HP’s implementation
of kernel threads, HP’s aC++ compiler, and the Text User Interface (nee Terminal User
Interface): Ben Krepp, Richard Title, John Bishop, Susan Macchia, Kathy Mann, Satish
Pai, India Paul, Steve Rehrauer, and Elena Zannoni. Kim Haase provided HP-specific
information in this manual.
DJ Delorie ported gdb to MS-DOS, for the DJGPP project. Robert Hoehne made
significant contributions to the DJGPP port.
Summary of gdb
5
Cygnus Solutions has sponsored gdb maintenance and much of its development since
1991. Cygnus engineers who have worked on gdb fulltime include Mark Alexander, Jim
Blandy, Per Bothner, Kevin Buettner, Edith Epstein, Chris Faylor, Fred Fish, Martin
Hunt, Jim Ingham, John Gilmore, Stu Grossman, Kung Hsu, Jim Kingdon, John Metzler,
Fernando Nasser, Geoffrey Noer, Dawn Perchik, Rich Pixley, Zdenek Radouch, Keith Seitz,
Stan Shebs, David Taylor, and Elena Zannoni. In addition, Dave Brolley, Ian Carmichael,
Steve Chamberlain, Nick Clifton, JT Conklin, Stan Cox, DJ Delorie, Ulrich Drepper, Frank
Eigler, Doug Evans, Sean Fagan, David Henkel-Wallace, Richard Henderson, Jeff Holcomb,
Jeff Law, Jim Lemke, Tom Lord, Bob Manson, Michael Meissner, Jason Merrill, Catherine
Moore, Drew Moseley, Ken Raeburn, Gavin Romig-Koch, Rob Savoye, Jamie Smith, Mike
Stump, Ian Taylor, Angela Thomas, Michael Tiemann, Tom Tromey, Ron Unrau, Jim
Wilson, and David Zuhn have made contributions both large and small.
Andrew Cagney, Fernando Nasser, and Elena Zannoni, while working for Cygnus Solutions, implemented the original gdb/mi interface.
Jim Blandy added support for preprocessor macros, while working for Red Hat.
Andrew Cagney designed gdb’s architecture vector. Many people including Andrew
Cagney, Stephane Carrez, Randolph Chung, Nick Duffek, Richard Henderson, Mark Kettenis, Grace Sainsbury, Kei Sakamoto, Yoshinori Sato, Michael Snyder, Andreas Schwab,
Jason Thorpe, Corinna Vinschen, Ulrich Weigand, and Elena Zannoni, helped with the
migration of old architectures to this new framework.
Andrew Cagney completely re-designed and re-implemented gdb’s unwinder framework,
this consisting of a fresh new design featuring frame IDs, independent frame sniffers, and
the sentinel frame. Mark Kettenis implemented the dwarf 2 unwinder, Jeff Johnston the
libunwind unwinder, and Andrew Cagney the dummy, sentinel, tramp, and trad unwinders.
The architecture-specific changes, each involving a complete rewrite of the architecture’s
frame code, were carried out by Jim Blandy, Joel Brobecker, Kevin Buettner, Andrew
Cagney, Stephane Carrez, Randolph Chung, Orjan Friberg, Richard Henderson, Daniel
Jacobowitz, Jeff Johnston, Mark Kettenis, Theodore A. Roth, Kei Sakamoto, Yoshinori
Sato, Michael Snyder, Corinna Vinschen, and Ulrich Weigand.
Christian Zankel, Ross Morley, Bob Wilson, and Maxim Grigoriev from Tensilica, Inc.
contributed support for Xtensa processors. Others who have worked on the Xtensa port of
gdb in the past include Steve Tjiang, John Newlin, and Scott Foehner.
Michael Eager and staff of Xilinx, Inc., contributed support for the Xilinx MicroBlaze
architecture.
Initial support for the FreeBSD/mips target and native configuration was developed
by SRI International and the University of Cambridge Computer Laboratory under
DARPA/AFRL contract FA8750-10-C-0237 ("CTSRD"), as part of the DARPA CRASH
research programme.
Chapter 1: A Sample gdb Session
7
1 A Sample gdb Session
You can use this manual at your leisure to read all about gdb. However, a handful of
commands are enough to get started using the debugger. This chapter illustrates those
commands.
In this sample session, we emphasize user input like this: input, to make it easier to pick
out from the surrounding output.
One of the preliminary versions of gnu m4 (a generic macro processor) exhibits the
following bug: sometimes, when we change its quote strings from the default, the commands
used to capture one macro definition within another stop working. In the following short m4
session, we define a macro foo which expands to 0000; we then use the m4 built-in defn to
define bar as the same thing. However, when we change the open quote string to
and the close quote string to , the same procedure fails to define a new synonym
baz:
$ cd gnu/m4
$ ./m4
define(foo,0000)
foo
0000
define(bar,defn(‘foo’))
bar
0000
changequote(,)
define(baz,defn(foo))
baz
Ctrl-d
m4: End of input: 0: fatal error: EOF in string
Let us use gdb to try to see what is going on.
$ gdb m4
gdb is free software and you are welcome to distribute copies
of it under certain conditions; type "show copying" to see
the conditions.
There is absolutely no warranty for gdb; type "show warranty"
for details.
gdb 8.0.50.20170503-git, Copyright 1999 Free Software Foundation, Inc...
(gdb)
gdb reads only enough symbol data to know where to find the rest when needed; as a result,
the first prompt comes up very quickly. We now tell gdb to use a narrower display width
than usual, so that examples fit in this manual.
(gdb) set width 70
We need to see how the m4 built-in changequote works. Having looked at the source, we
know the relevant subroutine is m4_changequote, so we set a breakpoint there with the
gdb break command.
(gdb) break m4 changequote
Breakpoint 1 at 0x62f4: file builtin.c, line 879.
Using the run command, we start m4 running under gdb control; as long as control does
not reach the m4_changequote subroutine, the program runs as usual:
8
Debugging with gdb
(gdb) run
Starting program: /work/Editorial/gdb/gnu/m4/m4
define(foo,0000)
foo
0000
To trigger the breakpoint, we call changequote. gdb suspends execution of m4, displaying
information about the context where it stops.
changequote(,)
Breakpoint 1, m4_changequote (argc=3, argv=0x33c70)
at builtin.c:879
879
if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))
Now we use the command n (next) to advance execution to the next line of the current
function.
(gdb) n
882
: nil,
set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\
set_quotes looks like a promising subroutine. We can go into it by using the command s
(step) instead of next. step goes to the next line to be executed in any subroutine, so it
steps into set_quotes.
(gdb) s
set_quotes (lq=0x34c78 "", rq=0x34c88 "")
at input.c:530
530
if (lquote != def_lquote)
The display that shows the subroutine where m4 is now suspended (and its arguments) is
called a stack frame display. It shows a summary of the stack. We can use the backtrace
command (which can also be spelled bt), to see where we are in the stack as a whole: the
backtrace command displays a stack frame for each active subroutine.
(gdb) bt
#0 set_quotes (lq=0x34c78 "", rq=0x34c88 "")
at input.c:530
#1 0x6344 in m4_changequote (argc=3, argv=0x33c70)
at builtin.c:882
#2 0x8174 in expand_macro (sym=0x33320) at macro.c:242
#3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
at macro.c:71
#4 0x79dc in expand_input () at macro.c:40
#5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195
We step through a few more lines to see what happens. The first two times, we can use ‘s’;
the next two times we use n to avoid falling into the xstrdup subroutine.
(gdb) s
0x3b5c 532
if (rquote != def_rquote)
(gdb) s
0x3b80 535
lquote = (lq == nil || *lq == ’\0’) ? \
def_lquote : xstrdup(lq);
(gdb) n
536
rquote = (rq == nil || *rq == ’\0’) ? def_rquote\
: xstrdup(rq);
(gdb) n
538
len_lquote = strlen(rquote);
Chapter 1: A Sample gdb Session
9
The last line displayed looks a little odd; we can examine the variables lquote and rquote
to see if they are in fact the new left and right quotes we specified. We use the command
p (print) to see their values.
(gdb) p lquote
$1 = 0x35d40 ""
(gdb) p rquote
$2 = 0x35d50 ""
lquote and rquote are indeed the new left and right quotes. To look at some context, we
can display ten lines of source surrounding the current line with the l (list) command.
(gdb) l
533
xfree(rquote);
534
535
lquote = (lq == nil || *lq == ’\0’) ? def_lquote\
: xstrdup (lq);
536
rquote = (rq == nil || *rq == ’\0’) ? def_rquote\
: xstrdup (rq);
537
538
len_lquote = strlen(rquote);
539
len_rquote = strlen(lquote);
540
}
541
542
void
Let us step past the two lines that set len_lquote and len_rquote, and then examine the
values of those variables.
(gdb) n
539
len_rquote = strlen(lquote);
(gdb) n
540
}
(gdb) p len lquote
$3 = 9
(gdb) p len rquote
$4 = 7
That certainly looks wrong, assuming len_lquote and len_rquote are meant to be the
lengths of lquote and rquote respectively. We can set them to better values using the p
command, since it can print the value of any expression—and that expression can include
subroutine calls and assignments.
(gdb) p len lquote=strlen(lquote)
$5 = 7
(gdb) p len rquote=strlen(rquote)
$6 = 9
Is that enough to fix the problem of using the new quotes with the m4 built-in defn? We can
allow m4 to continue executing with the c (continue) command, and then try the example
that caused trouble initially:
(gdb) c
Continuing.
define(baz,defn(foo))
baz
0000
10
Debugging with gdb
Success! The new quotes now work just as well as the default ones. The problem seems to
have been just the two typos defining the wrong lengths. We allow m4 exit by giving it an
EOF as input:
Ctrl-d
Program exited normally.
The message ‘Program exited normally.’ is from gdb; it indicates m4 has finished executing. We can end our gdb session with the gdb quit command.
(gdb) quit
Chapter 2: Getting In and Out of gdb
11
2 Getting In and Out of gdb
This chapter discusses how to start gdb, and how to get out of it. The essentials are:
• type ‘gdb’ to start gdb.
• type quit or Ctrl-d to exit.
2.1 Invoking gdb
Invoke gdb by running the program gdb. Once started, gdb reads commands from the
terminal until you tell it to exit.
You can also run gdb with a variety of arguments and options, to specify more of your
debugging environment at the outset.
The command-line options described here are designed to cover a variety of situations;
in some environments, some of these options may effectively be unavailable.
The most usual way to start gdb is with one argument, specifying an executable program:
gdb program
You can also start with both an executable program and a core file specified:
gdb program core
You can, instead, specify a process ID as a second argument, if you want to debug a
running process:
gdb program 1234
would attach gdb to process 1234 (unless you also have a file named ‘1234’; gdb does check
for a core file first).
Taking advantage of the second command-line argument requires a fairly complete operating system; when you use gdb as a remote debugger attached to a bare board, there
may not be any notion of “process”, and there is often no way to get a core dump. gdb
will warn you if it is unable to attach or to read core dumps.
You can optionally have gdb pass any arguments after the executable file to the inferior
using --args. This option stops option processing.
gdb --args gcc -O2 -c foo.c
This will cause gdb to debug gcc, and to set gcc’s command-line arguments (see
Section 4.3 [Arguments], page 30) to ‘-O2 -c foo.c’.
You can run gdb without printing the front material, which describes gdb’s
non-warranty, by specifying --silent (or -q/--quiet):
gdb --silent
You can further control how gdb starts up by using command-line options. gdb itself can
remind you of the options available.
Type
gdb -help
to display all available options and briefly describe their use (‘gdb -h’ is a shorter equivalent).
All options and command line arguments you give are processed in sequential order. The
order makes a difference when the ‘-x’ option is used.
12
Debugging with gdb
2.1.1 Choosing Files
When gdb starts, it reads any arguments other than options as specifying an executable
file and core file (or process ID). This is the same as if the arguments were specified by the
‘-se’ and ‘-c’ (or ‘-p’) options respectively. (gdb reads the first argument that does not
have an associated option flag as equivalent to the ‘-se’ option followed by that argument;
and the second argument that does not have an associated option flag, if any, as equivalent
to the ‘-c’/‘-p’ option followed by that argument.) If the second argument begins with a
decimal digit, gdb will first attempt to attach to it as a process, and if that fails, attempt
to open it as a corefile. If you have a corefile whose name begins with a digit, you can
prevent gdb from treating it as a pid by prefixing it with ‘./’, e.g. ‘./12345’.
If gdb has not been configured to included core file support, such as for most embedded
targets, then it will complain about a second argument and ignore it.
Many options have both long and short forms; both are shown in the following list. gdb
also recognizes the long forms if you truncate them, so long as enough of the option is
present to be unambiguous. (If you prefer, you can flag option arguments with ‘--’ rather
than ‘-’, though we illustrate the more usual convention.)
-symbols file
-s file
Read symbol table from file file.
-exec file
-e file
Use file file as the executable file to execute when appropriate, and for examining
pure data in conjunction with a core dump.
-se file
Read symbol table from file file and use it as the executable file.
-core file
-c file
Use file file as a core dump to examine.
-pid number
-p number Connect to process ID number, as with the attach command.
-command file
-x file
Execute commands from file file. The contents of this file is evaluated exactly
as the source command would. See Section 23.1.3 [Command files], page 324.
-eval-command command
-ex command
Execute a single gdb command.
This option may be used multiple times to call multiple commands. It may also
be interleaved with ‘-command’ as required.
gdb -ex ’target sim’ -ex ’load’ \
-x setbreakpoints -ex ’run’ a.out
-init-command file
-ix file Execute commands from file file before loading the inferior (but after loading
gdbinit files). See Section 2.1.3 [Startup], page 16.
-init-eval-command command
-iex command
Execute a single gdb command before loading the inferior (but after loading
gdbinit files). See Section 2.1.3 [Startup], page 16.
Chapter 2: Getting In and Out of gdb
13
-directory directory
-d directory
Add directory to the path to search for source and script files.
-r
-readnow
Read each symbol file’s entire symbol table immediately, rather than the default,
which is to read it incrementally as it is needed. This makes startup slower,
but makes future operations faster.
2.1.2 Choosing Modes
You can run gdb in various alternative modes—for example, in batch mode or quiet mode.
-nx
-n
Do not execute commands found in any initialization file. There are three init
files, loaded in the following order:
‘system.gdbinit’
This is the system-wide init file. Its location is specified with the -with-system-gdbinit configure option (see Section C.6 [Systemwide configuration], page 608). It is loaded first when gdb starts,
before command line options have been processed.
‘~/.gdbinit’
This is the init file in your home directory. It is loaded next, after
‘system.gdbinit’, and before command options have been processed.
‘./.gdbinit’
This is the init file in the current directory. It is loaded last, after command line options other than -x and -ex have been processed. Command line options -x and -ex are processed last, after
‘./.gdbinit’ has been loaded.
For further documentation on startup processing, See Section 2.1.3 [Startup],
page 16. For documentation on how to write command files, See Section 23.1.3
[Command Files], page 324.
-nh
-quiet
-silent
-q
-batch
Do not execute commands found in ‘~/.gdbinit’, the init file in your home
directory. See Section 2.1.3 [Startup], page 16.
“Quiet”. Do not print the introductory and copyright messages. These messages are also suppressed in batch mode.
Run in batch mode. Exit with status 0 after processing all the command files
specified with ‘-x’ (and all commands from initialization files, if not inhibited
with ‘-n’). Exit with nonzero status if an error occurs in executing the gdb
commands in the command files. Batch mode also disables pagination, sets unlimited terminal width and height see Section 22.4 [Screen Size], page 305, and
acts as if set confirm off were in effect (see Section 22.8 [Messages/Warnings],
page 313).
14
Debugging with gdb
Batch mode may be useful for running gdb as a filter, for example to download
and run a program on another computer; in order to make this more useful, the
message
Program exited normally.
(which is ordinarily issued whenever a program running under gdb control
terminates) is not issued when running in batch mode.
-batch-silent
Run in batch mode exactly like ‘-batch’, but totally silently. All gdb output to
stdout is prevented (stderr is unaffected). This is much quieter than ‘-silent’
and would be useless for an interactive session.
This is particularly useful when using targets that give ‘Loading section’ messages, for example.
Note that targets that give their output via gdb, as opposed to writing directly
to stdout, will also be made silent.
-return-child-result
The return code from gdb will be the return code from the child process (the
process being debugged), with the following exceptions:
• gdb exits abnormally. E.g., due to an incorrect argument or an internal
error. In this case the exit code is the same as it would have been without
‘-return-child-result’.
• The user quits with an explicit value. E.g., ‘quit 1’.
• The child process never runs, or is not allowed to terminate, in which case
the exit code will be -1.
This option is useful in conjunction with ‘-batch’ or ‘-batch-silent’, when
gdb is being used as a remote program loader or simulator interface.
-nowindows
-nw
“No windows”. If gdb comes with a graphical user interface (GUI) built in,
then this option tells gdb to only use the command-line interface. If no GUI is
available, this option has no effect.
-windows
-w
If gdb includes a GUI, then this option requires it to be used if possible.
-cd directory
Run gdb using directory as its working directory, instead of the current directory.
-data-directory directory
-D directory
Run gdb using directory as its data directory. The data directory is where gdb
searches for its auxiliary files. See Section 18.7 [Data Files], page 256.
-fullname
-f
gnu Emacs sets this option when it runs gdb as a subprocess. It tells gdb to
output the full file name and line number in a standard, recognizable fashion
each time a stack frame is displayed (which includes each time your program
Chapter 2: Getting In and Out of gdb
15
stops). This recognizable format looks like two ‘\032’ characters, followed by
the file name, line number and character position separated by colons, and a
newline. The Emacs-to-gdb interface program uses the two ‘\032’ characters
as a signal to display the source code for the frame.
-annotate level
This option sets the annotation level inside gdb. Its effect is identical to using
‘set annotate level’ (see Chapter 28 [Annotations], page 557). The annotation level controls how much information gdb prints together with its prompt,
values of expressions, source lines, and other types of output. Level 0 is the
normal, level 1 is for use when gdb is run as a subprocess of gnu Emacs, level
3 is the maximum annotation suitable for programs that control gdb, and level
2 has been deprecated.
The annotation mechanism has largely been superseded by gdb/mi (see
Chapter 27 [GDB/MI], page 469).
--args
Change interpretation of command line so that arguments following the executable file are passed as command line arguments to the inferior. This option
stops option processing.
-baud bps
-b bps
Set the line speed (baud rate or bits per second) of any serial interface used by
gdb for remote debugging.
-l timeout
Set the timeout (in seconds) of any communication used by gdb for remote
debugging.
-tty device
-t device Run using device for your program’s standard input and output.
-tui
Activate the Text User Interface when starting. The Text User Interface manages several text windows on the terminal, showing source, assembly, registers and gdb command outputs (see Chapter 25 [gdb Text User Interface],
page 461). Do not use this option if you run gdb from Emacs (see Chapter 26
[Using gdb under gnu Emacs], page 467).
-interpreter interp
Use the interpreter interp for interface with the controlling program or device.
This option is meant to be set by programs which communicate with gdb using
it as a back end. See Chapter 24 [Command Interpreters], page 459.
‘--interpreter=mi’ (or ‘--interpreter=mi2’) causes gdb to use the gdb/mi
interface (see Chapter 27 [The gdb/mi Interface], page 469) included since gdb
version 6.0. The previous gdb/mi interface, included in gdb version 5.3 and
selected with ‘--interpreter=mi1’, is deprecated. Earlier gdb/mi interfaces
are no longer supported.
-write
Open the executable and core files for both reading and writing. This is equivalent to the ‘set write on’ command inside gdb (see Section 17.6 [Patching],
page 234).
16
Debugging with gdb
-statistics
This option causes gdb to print statistics about time and memory usage after
it completes each command and returns to the prompt.
-version
This option causes gdb to print its version number and no-warranty blurb, and
exit.
-configuration
This option causes gdb to print details about its build-time configuration parameters, and then exit. These details can be important when reporting gdb
bugs (see Chapter 31 [GDB Bugs], page 569).
2.1.3 What gdb Does During Startup
Here’s the description of what gdb does during session startup:
1. Sets up the command interpreter as specified by the command line (see Section 2.1.2
[Mode Options], page 13).
2. Reads the system-wide init file (if ‘--with-system-gdbinit’ was used when building
gdb; see Section C.6 [System-wide configuration and settings], page 608) and executes
all the commands in that file.
3. Reads the init file (if any) in your home directory1 and executes all the commands in
that file.
4. Executes commands and command files specified by the ‘-iex’ and ‘-ix’ options in
their specified order. Usually you should use the ‘-ex’ and ‘-x’ options instead, but
this way you can apply settings before gdb init files get executed and before inferior
gets loaded.
5. Processes command line options and operands.
6. Reads and executes the commands from init file (if any) in the current working directory
as long as ‘set auto-load local-gdbinit’ is set to ‘on’ (see Section 22.7.1 [Init File in
the Current Directory], page 310). This is only done if the current directory is different
from your home directory. Thus, you can have more than one init file, one generic in
your home directory, and another, specific to the program you are debugging, in the
directory where you invoke gdb.
7. If the command line specified a program to debug, or a process to attach to, or a core
file, gdb loads any auto-loaded scripts provided for the program or for its loaded shared
libraries. See Section 22.7 [Auto-loading], page 308.
If you wish to disable the auto-loading during startup, you must do something like the
following:
$ gdb -iex "set auto-load python-scripts off" myprogram
Option ‘-ex’ does not work because the auto-loading is then turned off too late.
8. Executes commands and command files specified by the ‘-ex’ and ‘-x’ options in their
specified order. See Section 23.1.3 [Command Files], page 324, for more details about
gdb command files.
9. Reads the command history recorded in the history file. See Section 22.3 [Command
History], page 304, for more details about the command history and the files where
gdb records it.
1
On DOS/Windows systems, the home directory is the one pointed to by the HOME environment variable.
Chapter 2: Getting In and Out of gdb
17
Init files use the same syntax as command files (see Section 23.1.3 [Command Files],
page 324) and are processed by gdb in the same way. The init file in your home directory
can set options (such as ‘set complaints’) that affect subsequent processing of command
line options and operands. Init files are not executed if you use the ‘-nx’ option (see
Section 2.1.2 [Choosing Modes], page 13).
To display the list of init files loaded by gdb at startup, you can use gdb --help.
The gdb init files are normally called ‘.gdbinit’. The DJGPP port of gdb uses the
name ‘gdb.ini’, due to the limitations of file names imposed by DOS filesystems. The
Windows port of gdb uses the standard name, but if it finds a ‘gdb.ini’ file in your home
directory, it warns you about that and suggests to rename the file to the standard name.
2.2 Quitting gdb
quit [expression]
q
To exit gdb, use the quit command (abbreviated q), or type an end-of-file
character (usually Ctrl-d). If you do not supply expression, gdb will terminate
normally; otherwise it will terminate using the result of expression as the error
code.
An interrupt (often Ctrl-c) does not exit from gdb, but rather terminates the action
of any gdb command that is in progress and returns to gdb command level. It is safe to
type the interrupt character at any time because gdb does not allow it to take effect until
a time when it is safe.
If you have been using gdb to control an attached process or device, you can release
it with the detach command (see Section 4.7 [Debugging an Already-running Process],
page 32).
2.3 Shell Commands
If you need to execute occasional shell commands during your debugging session, there is
no need to leave or suspend gdb; you can just use the shell command.
shell command-string
!command-string
Invoke a standard shell to execute command-string. Note that no space is
needed between ! and command-string. If it exists, the environment variable
SHELL determines which shell to run. Otherwise gdb uses the default shell
(‘/bin/sh’ on Unix systems, ‘COMMAND.COM’ on MS-DOS, etc.).
The utility make is often needed in development environments. You do not have to use
the shell command for this purpose in gdb:
make make-args
Execute the make program with the specified arguments. This is equivalent to
‘shell make make-args’.
18
Debugging with gdb
2.4 Logging Output
You may want to save the output of gdb commands to a file. There are several commands
to control gdb’s logging.
set logging on
Enable logging.
set logging off
Disable logging.
set logging file file
Change the name of the current logfile. The default logfile is ‘gdb.txt’.
set logging overwrite [on|off]
By default, gdb will append to the logfile. Set overwrite if you want set
logging on to overwrite the logfile instead.
set logging redirect [on|off]
By default, gdb output will go to both the terminal and the logfile. Set
redirect if you want output to go only to the log file.
show logging
Show the current values of the logging settings.
Chapter 3: gdb Commands
19
3 gdb Commands
You can abbreviate a gdb command to the first few letters of the command name, if that
abbreviation is unambiguous; and you can repeat certain gdb commands by typing just
RET. You can also use the TAB key to get gdb to fill out the rest of a word in a command
(or to show you the alternatives available, if there is more than one possibility).
3.1 Command Syntax
A gdb command is a single line of input. There is no limit on how long it can be. It
starts with a command name, which is followed by arguments whose meaning depends on
the command name. For example, the command step accepts an argument which is the
number of times to step, as in ‘step 5’. You can also use the step command with no
arguments. Some commands do not allow any arguments.
gdb command names may always be truncated if that abbreviation is unambiguous.
Other possible command abbreviations are listed in the documentation for individual commands. In some cases, even ambiguous abbreviations are allowed; for example, s is specially
defined as equivalent to step even though there are other commands whose names start
with s. You can test abbreviations by using them as arguments to the help command.
A blank line as input to gdb (typing just RET) means to repeat the previous command.
Certain commands (for example, run) will not repeat this way; these are commands whose
unintentional repetition might cause trouble and which you are unlikely to want to repeat.
User-defined commands can disable this feature; see Section 23.1.1 [Define], page 321.
The list and x commands, when you repeat them with RET, construct new arguments
rather than repeating exactly as typed. This permits easy scanning of source or memory.
gdb can also use RET in another way: to partition lengthy output, in a way similar to
the common utility more (see Section 22.4 [Screen Size], page 305). Since it is easy to press
one RET too many in this situation, gdb disables command repetition after any command
that generates this sort of display.
Any text from a # to the end of the line is a comment; it does nothing. This is useful
mainly in command files (see Section 23.1.3 [Command Files], page 324).
The Ctrl-o binding is useful for repeating a complex sequence of commands. This
command accepts the current line, like RET, and then fetches the next line relative to the
current line from the history for editing.
3.2 Command Completion
gdb can fill in the rest of a word in a command for you, if there is only one possibility;
it can also show you what the valid possibilities are for the next word in a command, at
any time. This works for gdb commands, gdb subcommands, and the names of symbols
in your program.
Press the TAB key whenever you want gdb to fill out the rest of a word. If there is only
one possibility, gdb fills in the word, and waits for you to finish the command (or press RET
to enter it). For example, if you type
(gdb) info bre TAB
gdb fills in the rest of the word ‘breakpoints’, since that is the only info subcommand
beginning with ‘bre’:
20
Debugging with gdb
(gdb) info breakpoints
You can either press RET at this point, to run the info breakpoints command, or backspace
and enter something else, if ‘breakpoints’ does not look like the command you expected. (If
you were sure you wanted info breakpoints in the first place, you might as well just type
RET immediately after ‘info bre’, to exploit command abbreviations rather than command
completion).
If there is more than one possibility for the next word when you press TAB, gdb sounds a
bell. You can either supply more characters and try again, or just press TAB a second time;
gdb displays all the possible completions for that word. For example, you might want to
set a breakpoint on a subroutine whose name begins with ‘make_’, but when you type b
make_TAB gdb just sounds the bell. Typing TAB again displays all the function names in
your program that begin with those characters, for example:
(gdb) b make_ TAB
gdb sounds bell; press TAB again, to see:
make_a_section_from_file
make_environ
make_abs_section
make_function_type
make_blockvector
make_pointer_type
make_cleanup
make_reference_type
make_command
make_symbol_completion_list
(gdb) b make_
After displaying the available possibilities, gdb copies your partial input (‘b make_’ in the
example) so you can finish the command.
If you just want to see the list of alternatives in the first place, you can press M-? rather
than pressing TAB twice. M-? means META ?. You can type this either by holding down a
key designated as the META shift on your keyboard (if there is one) while typing ?, or as ESC
followed by ?.
If the number of possible completions is large, gdb will print as much of the list as it
has collected, as well as a message indicating that the list may be truncated.
(gdb) b mTABTAB
main
<... the rest of the possible completions ...>
*** List may be truncated, max-completions reached. ***
(gdb) b m
This behavior can be controlled with the following commands:
set max-completions limit
set max-completions unlimited
Set the maximum number of completion candidates. gdb will stop looking for
more completions once it collects this many candidates. This is useful when
completing on things like function names as collecting all the possible candidates
can be time consuming. The default value is 200. A value of zero disables tabcompletion. Note that setting either no limit or a very large limit can make
completion slow.
show max-completions
Show the maximum number of candidates that gdb will collect and show during
completion.
Sometimes the string you need, while logically a “word”, may contain parentheses or
other characters that gdb normally excludes from its notion of a word. To permit word
Chapter 3: gdb Commands
21
completion to work in this situation, you may enclose words in ’ (single quote marks) in
gdb commands.
The most likely situation where you might need this is in typing the name of a C++
function. This is because C++ allows function overloading (multiple definitions of the same
function, distinguished by argument type). For example, when you want to set a breakpoint
you may need to distinguish whether you mean the version of name that takes an int
parameter, name(int), or the version that takes a float parameter, name(float). To use
the word-completion facilities in this situation, type a single quote ’ at the beginning of the
function name. This alerts gdb that it may need to consider more information than usual
when you press TAB or M-? to request word completion:
(gdb) b ’bubble( M-?
bubble(double,double)
(gdb) b ’bubble(
bubble(int,int)
In some cases, gdb can tell that completing a name requires using quotes. When this
happens, gdb inserts the quote for you (while completing as much as it can) if you do not
type the quote in the first place:
(gdb) b bub TAB
gdb alters your input line to the following, and rings a bell:
(gdb) b ’bubble(
In general, gdb can tell that a quote is needed (and inserts it) if you have not yet started
typing the argument list when you ask for completion on an overloaded symbol.
For more information about overloaded functions, see Section 15.4.1.3 [C++ Expressions],
page 198. You can use the command set overload-resolution off to disable overload
resolution; see Section 15.4.1.7 [gdb Features for C++], page 199.
When completing in an expression which looks up a field in a structure, gdb also tries1
to limit completions to the field names available in the type of the left-hand-side:
(gdb) p gdb_stdout.M-?
magic
to_fputs
to_data
to_isatty
to_delete
to_put
to_flush
to_read
to_rewind
to_write
to_write_async_safe
This is because the gdb_stdout is a variable of the type struct ui_file that is defined in
gdb sources as follows:
struct ui_file
{
int *magic;
ui_file_flush_ftype *to_flush;
ui_file_write_ftype *to_write;
ui_file_write_async_safe_ftype *to_write_async_safe;
ui_file_fputs_ftype *to_fputs;
ui_file_read_ftype *to_read;
ui_file_delete_ftype *to_delete;
ui_file_isatty_ftype *to_isatty;
ui_file_rewind_ftype *to_rewind;
ui_file_put_ftype *to_put;
void *to_data;
}
1
The completer can be confused by certain kinds of invalid expressions. Also, it only examines the static
type of the expression, not the dynamic type.
22
Debugging with gdb
3.3 Getting Help
You can always ask gdb itself for information on its commands, using the command help.
help
h
You can use help (abbreviated h) with no arguments to display a short list of
named classes of commands:
(gdb) help
List of classes of commands:
aliases -- Aliases of other commands
breakpoints -- Making program stop at certain points
data -- Examining data
files -- Specifying and examining files
internals -- Maintenance commands
obscure -- Obscure features
running -- Running the program
stack -- Examining the stack
status -- Status inquiries
support -- Support facilities
tracepoints -- Tracing of program execution without
stopping the program
user-defined -- User-defined commands
Type "help" followed by a class name for a list of
commands in that class.
Type "help" followed by command name for full
documentation.
Command name abbreviations are allowed if unambiguous.
(gdb)
help class
Using one of the general help classes as an argument, you can get a list of the
individual commands in that class. For example, here is the help display for
the class status:
(gdb) help status
Status inquiries.
List of commands:
info -- Generic command for showing things
about the program being debugged
show -- Generic command for showing things
about the debugger
Type "help" followed by command name for full
documentation.
Command name abbreviations are allowed if unambiguous.
(gdb)
help command
With a command name as help argument, gdb displays a short paragraph on
how to use that command.
Chapter 3: gdb Commands
23
apropos args
The apropos command searches through all of the gdb commands, and their
documentation, for the regular expression specified in args. It prints out all
matches found. For example:
apropos alias
results in:
alias -- Define a new command that is an alias of an existing command
aliases -- Aliases of other commands
d -- Delete some breakpoints or auto-display expressions
del -- Delete some breakpoints or auto-display expressions
delete -- Delete some breakpoints or auto-display expressions
complete args
The complete args command lists all the possible completions for the beginning of a command. Use args to specify the beginning of the command you
want completed. For example:
complete i
results in:
if
ignore
info
inspect
This is intended for use by gnu Emacs.
In addition to help, you can use the gdb commands info and show to inquire about
the state of your program, or the state of gdb itself. Each command supports many topics
of inquiry; this manual introduces each of them in the appropriate context. The listings
under info and under show in the Command, Variable, and Function Index point to all the
sub-commands. See [Command and Variable Index], page 761.
info
This command (abbreviated i) is for describing the state of your program. For
example, you can show the arguments passed to a function with info args,
list the registers currently in use with info registers, or list the breakpoints
you have set with info breakpoints. You can get a complete list of the info
sub-commands with help info.
set
You can assign the result of an expression to an environment variable with set.
For example, you can set the gdb prompt to a $-sign with set prompt $.
show
In contrast to info, show is for describing the state of gdb itself. You can
change most of the things you can show, by using the related command set;
for example, you can control what number system is used for displays with set
radix, or simply inquire which is currently in use with show radix.
To display all the settable parameters and their current values, you can use
show with no arguments; you may also use info set. Both commands produce
the same display.
Here are several miscellaneous show subcommands, all of which are exceptional in lacking
corresponding set commands:
24
Debugging with gdb
show version
Show what version of gdb is running. You should include this information in
gdb bug-reports. If multiple versions of gdb are in use at your site, you may
need to determine which version of gdb you are running; as gdb evolves, new
commands are introduced, and old ones may wither away. Also, many system
vendors ship variant versions of gdb, and there are variant versions of gdb in
gnu/Linux distributions as well. The version number is the same as the one
announced when you start gdb.
show copying
info copying
Display information about permission for copying gdb.
show warranty
info warranty
Display the gnu “NO WARRANTY” statement, or a warranty, if your version
of gdb comes with one.
show configuration
Display detailed information about the way gdb was configured when it was
built. This displays the optional arguments passed to the ‘configure’ script
and also configuration parameters detected automatically by configure. When
reporting a gdb bug (see Chapter 31 [GDB Bugs], page 569), it is important
to include this information in your report.
Chapter 4: Running Programs Under gdb
25
4 Running Programs Under gdb
When you run a program under gdb, you must first generate debugging information when
you compile it.
You may start gdb with its arguments, if any, in an environment of your choice. If you
are doing native debugging, you may redirect your program’s input and output, debug an
already running process, or kill a child process.
4.1 Compiling for Debugging
In order to debug a program effectively, you need to generate debugging information when
you compile it. This debugging information is stored in the object file; it describes the data
type of each variable or function and the correspondence between source line numbers and
addresses in the executable code.
To request debugging information, specify the ‘-g’ option when you run the compiler.
Programs that are to be shipped to your customers are compiled with optimizations,
using the ‘-O’ compiler option. However, some compilers are unable to handle the ‘-g’ and
‘-O’ options together. Using those compilers, you cannot generate optimized executables
containing debugging information.
gcc, the gnu C/C++ compiler, supports ‘-g’ with or without ‘-O’, making it possible
to debug optimized code. We recommend that you always use ‘-g’ whenever you compile
a program. You may think your program is correct, but there is no sense in pushing your
luck. For more information, see Chapter 11 [Optimized Code], page 159.
Older versions of the gnu C compiler permitted a variant option ‘-gg’ for debugging
information. gdb no longer supports this format; if your gnu C compiler has this option,
do not use it.
gdb knows about preprocessor macros and can show you their expansion (see Chapter 12
[Macros], page 163). Most compilers do not include information about preprocessor macros
in the debugging information if you specify the ‘-g’ flag alone. Version 3.1 and later of gcc,
the gnu C compiler, provides macro information if you are using the DWARF debugging
format, and specify the option ‘-g3’.
See Section “Options for Debugging Your Program or GCC” in Using the gnu Compiler
Collection (GCC), for more information on gcc options affecting debug information.
You will have the best debugging experience if you use the latest version of the DWARF
debugging format that your compiler supports. DWARF is currently the most expressive
and best supported debugging format in gdb.
26
Debugging with gdb
4.2 Starting your Program
run
r
Use the run command to start your program under gdb. You must first specify
the program name with an argument to gdb (see Chapter 2 [Getting In and
Out of gdb], page 11), or by using the file or exec-file command (see
Section 18.1 [Commands to Specify Files], page 241).
If you are running your program in an execution environment that supports processes,
run creates an inferior process and makes that process run your program. In some environments without processes, run jumps to the start of your program. Other targets, like
‘remote’, are always running. If you get an error message like this one:
The "remote" target does not support "run".
Try "help target" or "continue".
then use continue to run your program. You may need load first (see [load], page 259).
The execution of a program is affected by certain information it receives from its superior.
gdb provides ways to specify this information, which you must do before starting your
program. (You can change it after starting your program, but such changes only affect your
program the next time you start it.) This information may be divided into four categories:
The arguments.
Specify the arguments to give your program as the arguments of the run command. If a shell is available on your target, the shell is used to pass the arguments, so that you may use normal conventions (such as wildcard expansion or
variable substitution) in describing the arguments. In Unix systems, you can
control which shell is used with the SHELL environment variable. If you do not
define SHELL, gdb uses the default shell (‘/bin/sh’). You can disable use of
any shell with the set startup-with-shell command (see below for details).
The environment.
Your program normally inherits its environment from gdb, but you can use
the gdb commands set environment and unset environment to change parts
of the environment that affect your program. See Section 4.4 [Your Program’s
Environment], page 30.
The working directory.
Your program inherits its working directory from gdb. You can set the gdb
working directory with the cd command in gdb. See Section 4.5 [Your Program’s Working Directory], page 31.
The standard input and output.
Your program normally uses the same device for standard input and standard
output as gdb is using. You can redirect input and output in the run command
line, or you can use the tty command to set a different device for your program.
See Section 4.6 [Your Program’s Input and Output], page 32.
Warning: While input and output redirection work, you cannot use pipes to
pass the output of the program you are debugging to another program; if you
attempt this, gdb is likely to wind up debugging the wrong program.
Chapter 4: Running Programs Under gdb
27
When you issue the run command, your program begins to execute immediately. See
Chapter 5 [Stopping and Continuing], page 45, for discussion of how to arrange for your
program to stop. Once your program has stopped, you may call functions in your program,
using the print or call commands. See Chapter 10 [Examining Data], page 115.
If the modification time of your symbol file has changed since the last time gdb read its
symbols, gdb discards its symbol table, and reads it again. When it does this, gdb tries to
retain your current breakpoints.
start
The name of the main procedure can vary from language to language. With
C or C++, the main procedure name is always main, but other languages such
as Ada do not require a specific name for their main procedure. The debugger
provides a convenient way to start the execution of the program and to stop at
the beginning of the main procedure, depending on the language used.
The ‘start’ command does the equivalent of setting a temporary breakpoint
at the beginning of the main procedure and then invoking the ‘run’ command.
Some programs contain an elaboration phase where some startup code is executed before the main procedure is called. This depends on the languages used
to write your program. In C++, for instance, constructors for static and global
objects are executed before main is called. It is therefore possible that the
debugger stops before reaching the main procedure. However, the temporary
breakpoint will remain to halt execution.
Specify the arguments to give to your program as arguments to the ‘start’
command. These arguments will be given verbatim to the underlying ‘run’
command. Note that the same arguments will be reused if no argument is
provided during subsequent calls to ‘start’ or ‘run’.
It is sometimes necessary to debug the program during elaboration. In these
cases, using the start command would stop the execution of your program
too late, as the program would have already completed the elaboration phase.
Under these circumstances, insert breakpoints in your elaboration code before
running your program.
set exec-wrapper wrapper
show exec-wrapper
unset exec-wrapper
When ‘exec-wrapper’ is set, the specified wrapper is used to launch programs
for debugging. gdb starts your program with a shell command of the form exec
wrapper program. Quoting is added to program and its arguments, but not to
wrapper, so you should add quotes if appropriate for your shell. The wrapper
runs until it executes your program, and then gdb takes control.
You can use any program that eventually calls execve with its arguments as
a wrapper. Several standard Unix utilities do this, e.g. env and nohup. Any
Unix shell script ending with exec "$@" will also work.
For example, you can use env to pass an environment variable to the debugged
program, without setting the variable in your shell’s environment:
(gdb) set exec-wrapper env ’LD_PRELOAD=libtest.so’
(gdb) run
28
Debugging with gdb
This command is available when debugging locally on most targets, excluding
djgpp, Cygwin, MS Windows, and QNX Neutrino.
set startup-with-shell
set startup-with-shell on
set startup-with-shell off
show set startup-with-shell
On Unix systems, by default, if a shell is available on your target, gdb) uses it
to start your program. Arguments of the run command are passed to the shell,
which does variable substitution, expands wildcard characters and performs
redirection of I/O. In some circumstances, it may be useful to disable such use
of a shell, for example, when debugging the shell itself or diagnosing startup
failures such as:
(gdb) run
Starting program: ./a.out
During startup program terminated with signal SIGSEGV, Segmentation fault.
which indicates the shell or the wrapper specified with ‘exec-wrapper’ crashed,
not your program. Most often, this is caused by something odd in your shell’s
non-interactive mode initialization file—such as ‘.cshrc’ for C-shell, $‘.zshenv’
for the Z shell, or the file specified in the ‘BASH_ENV’ environment variable for
BASH.
set auto-connect-native-target
set auto-connect-native-target on
set auto-connect-native-target off
show auto-connect-native-target
By default, if not connected to any target yet (e.g., with target remote), the
run command starts your program as a native process under gdb, on your
local machine. If you’re sure you don’t want to debug programs on your local
machine, you can tell gdb to not connect to the native target automatically
with the set auto-connect-native-target off command.
If on, which is the default, and if gdb is not connected to a target already, the
run command automaticaly connects to the native target, if one is available.
If off, and if gdb is not connected to a target already, the run command fails
with an error:
(gdb) run
Don’t know how to run.
Try "help target".
If gdb is already connected to a target, gdb always uses it with the run command.
In any case, you can explicitly connect to the native target with the target
native command. For example,
(gdb) set auto-connect-native-target off
(gdb) run
Don’t know how to run. Try "help target".
(gdb) target native
(gdb) run
Starting program: ./a.out
[Inferior 1 (process 10421) exited normally]
Chapter 4: Running Programs Under gdb
29
In case you connected explicitly to the native target, gdb remains connected
even if all inferiors exit, ready for the next run command. Use the disconnect
command to disconnect.
Examples of other commands that likewise respect the auto-connect-nativetarget setting: attach, info proc, info os.
set disable-randomization
set disable-randomization on
This option (enabled by default in gdb) will turn off the native randomization of the virtual address space of the started program. This option is useful
for multiple debugging sessions to make the execution better reproducible and
memory addresses reusable across debugging sessions.
This feature is implemented only on certain targets, including gnu/Linux. On
gnu/Linux you can get the same behavior using
(gdb) set exec-wrapper setarch ‘uname -m‘ -R
set disable-randomization off
Leave the behavior of the started executable unchanged. Some bugs rear their
ugly heads only when the program is loaded at certain addresses. If your bug
disappears when you run the program under gdb, that might be because gdb
by default disables the address randomization on platforms, such as gnu/Linux,
which do that for stand-alone programs. Use set disable-randomization off
to try to reproduce such elusive bugs.
On targets where it is available, virtual address space randomization protects
the programs against certain kinds of security attacks. In these cases the attacker needs to know the exact location of a concrete executable code. Randomizing its location makes it impossible to inject jumps misusing a code at its
expected addresses.
Prelinking shared libraries provides a startup performance advantage but it
makes addresses in these libraries predictable for privileged processes by having
just unprivileged access at the target system. Reading the shared library binary
gives enough information for assembling the malicious code misusing it. Still
even a prelinked shared library can get loaded at a new random address just
requiring the regular relocation process during the startup. Shared libraries not
already prelinked are always loaded at a randomly chosen address.
Position independent executables (PIE) contain position independent code similar to the shared libraries and therefore such executables get loaded at a randomly chosen address upon startup. PIE executables always load even already
prelinked shared libraries at a random address. You can build such executable
using gcc -fPIE -pie.
Heap (malloc storage), stack and custom mmap areas are always placed randomly (as long as the randomization is enabled).
show disable-randomization
Show the current setting of the explicit disable of the native randomization of
the virtual address space of the started program.
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Debugging with gdb
4.3 Your Program’s Arguments
The arguments to your program can be specified by the arguments of the run command.
They are passed to a shell, which expands wildcard characters and performs redirection of
I/O, and thence to your program. Your SHELL environment variable (if it exists) specifies
what shell gdb uses. If you do not define SHELL, gdb uses the default shell (‘/bin/sh’ on
Unix).
On non-Unix systems, the program is usually invoked directly by gdb, which emulates
I/O redirection via the appropriate system calls, and the wildcard characters are expanded
by the startup code of the program, not by the shell.
run with no arguments uses the same arguments used by the previous run, or those set
by the set args command.
set args
Specify the arguments to be used the next time your program is run. If set
args has no arguments, run executes your program with no arguments. Once
you have run your program with arguments, using set args before the next
run is the only way to run it again without arguments.
show args Show the arguments to give your program when it is started.
4.4 Your Program’s Environment
The environment consists of a set of environment variables and their values. Environment
variables conventionally record such things as your user name, your home directory, your
terminal type, and your search path for programs to run. Usually you set up environment
variables with the shell and they are inherited by all the other programs you run. When
debugging, it can be useful to try running your program with a modified environment
without having to start gdb over again.
path directory
Add directory to the front of the PATH environment variable (the search path
for executables) that will be passed to your program. The value of PATH used
by gdb does not change. You may specify several directory names, separated
by whitespace or by a system-dependent separator character (‘:’ on Unix, ‘;’
on MS-DOS and MS-Windows). If directory is already in the path, it is moved
to the front, so it is searched sooner.
You can use the string ‘$cwd’ to refer to whatever is the current working directory at the time gdb searches the path. If you use ‘.’ instead, it refers to the
directory where you executed the path command. gdb replaces ‘.’ in the directory argument (with the current path) before adding directory to the search
path.
show paths
Display the list of search paths for executables (the PATH environment variable).
show environment [varname]
Print the value of environment variable varname to be given to your program
when it starts. If you do not supply varname, print the names and values of
all environment variables to be given to your program. You can abbreviate
environment as env.
Chapter 4: Running Programs Under gdb
31
set environment varname [=value]
Set environment variable varname to value. The value changes for your program (and the shell gdb uses to launch it), not for gdb itself. The value may be
any string; the values of environment variables are just strings, and any interpretation is supplied by your program itself. The value parameter is optional;
if it is eliminated, the variable is set to a null value.
For example, this command:
set env USER = foo
tells the debugged program, when subsequently run, that its user is named
‘foo’. (The spaces around ‘=’ are used for clarity here; they are not actually
required.)
Note that on Unix systems, gdb runs your program via a shell, which also
inherits the environment set with set environment. If necessary, you can avoid
that by using the ‘env’ program as a wrapper instead of using set environment.
See [set exec-wrapper], page 27, for an example doing just that.
unset environment varname
Remove variable varname from the environment to be passed to your program.
This is different from ‘set env varname =’; unset environment removes the
variable from the environment, rather than assigning it an empty value.
Warning: On Unix systems, gdb runs your program using the shell indicated by your
SHELL environment variable if it exists (or /bin/sh if not). If your SHELL variable names a
shell that runs an initialization file when started non-interactively—such as ‘.cshrc’ for Cshell, $‘.zshenv’ for the Z shell, or the file specified in the ‘BASH_ENV’ environment variable
for BASH—any variables you set in that file affect your program. You may wish to move
setting of environment variables to files that are only run when you sign on, such as ‘.login’
or ‘.profile’.
4.5 Your Program’s Working Directory
Each time you start your program with run, it inherits its working directory from the current
working directory of gdb. The gdb working directory is initially whatever it inherited from
its parent process (typically the shell), but you can specify a new working directory in gdb
with the cd command.
The gdb working directory also serves as a default for the commands that specify files
for gdb to operate on. See Section 18.1 [Commands to Specify Files], page 241.
cd [directory]
Set the gdb working directory to directory. If not given, directory uses ‘’~’’.
pwd
Print the gdb working directory.
It is generally impossible to find the current working directory of the process being
debugged (since a program can change its directory during its run). If you work on a system
where gdb is configured with the ‘/proc’ support, you can use the info proc command
(see Section 21.1.2 [SVR4 Process Information], page 281) to find out the current working
directory of the debuggee.
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Debugging with gdb
4.6 Your Program’s Input and Output
By default, the program you run under gdb does input and output to the same terminal
that gdb uses. gdb switches the terminal to its own terminal modes to interact with you,
but it records the terminal modes your program was using and switches back to them when
you continue running your program.
info terminal
Displays information recorded by gdb about the terminal modes your program
is using.
You can redirect your program’s input and/or output using shell redirection with the
run command. For example,
run > outfile
starts your program, diverting its output to the file ‘outfile’.
Another way to specify where your program should do input and output is with the
tty command. This command accepts a file name as argument, and causes this file to be
the default for future run commands. It also resets the controlling terminal for the child
process, for future run commands. For example,
tty /dev/ttyb
directs that processes started with subsequent run commands default to do input and output
on the terminal ‘/dev/ttyb’ and have that as their controlling terminal.
An explicit redirection in run overrides the tty command’s effect on the input/output
device, but not its effect on the controlling terminal.
When you use the tty command or redirect input in the run command, only the input
for your program is affected. The input for gdb still comes from your terminal. tty is an
alias for set inferior-tty.
You can use the show inferior-tty command to tell gdb to display the name of the
terminal that will be used for future runs of your program.
set inferior-tty [ tty ]
Set the tty for the program being debugged to tty. Omitting tty restores the
default behavior, which is to use the same terminal as gdb.
show inferior-tty
Show the current tty for the program being debugged.
4.7 Debugging an Already-running Process
attach process-id
This command attaches to a running process—one that was started outside
gdb. (info files shows your active targets.) The command takes as argument
a process ID. The usual way to find out the process-id of a Unix process is with
the ps utility, or with the ‘jobs -l’ shell command.
attach does not repeat if you press RET a second time after executing the
command.
To use attach, your program must be running in an environment which supports processes; for example, attach does not work for programs on bare-board targets that lack an
operating system. You must also have permission to send the process a signal.
Chapter 4: Running Programs Under gdb
33
When you use attach, the debugger finds the program running in the process first by
looking in the current working directory, then (if the program is not found) by using the
source file search path (see Section 9.5 [Specifying Source Directories], page 107). You can
also use the file command to load the program. See Section 18.1 [Commands to Specify
Files], page 241.
The first thing gdb does after arranging to debug the specified process is to stop it. You
can examine and modify an attached process with all the gdb commands that are ordinarily
available when you start processes with run. You can insert breakpoints; you can step and
continue; you can modify storage. If you would rather the process continue running, you
may use the continue command after attaching gdb to the process.
detach
When you have finished debugging the attached process, you can use the detach
command to release it from gdb control. Detaching the process continues its
execution. After the detach command, that process and gdb become completely independent once more, and you are ready to attach another process
or start one with run. detach does not repeat if you press RET again after
executing the command.
If you exit gdb while you have an attached process, you detach that process. If you use
the run command, you kill that process. By default, gdb asks for confirmation if you try
to do either of these things; you can control whether or not you need to confirm by using
the set confirm command (see Section 22.8 [Optional Warnings and Messages], page 313).
4.8 Killing the Child Process
kill
Kill the child process in which your program is running under gdb.
This command is useful if you wish to debug a core dump instead of a running process.
gdb ignores any core dump file while your program is running.
On some operating systems, a program cannot be executed outside gdb while you have
breakpoints set on it inside gdb. You can use the kill command in this situation to permit
running your program outside the debugger.
The kill command is also useful if you wish to recompile and relink your program,
since on many systems it is impossible to modify an executable file while it is running in a
process. In this case, when you next type run, gdb notices that the file has changed, and
reads the symbol table again (while trying to preserve your current breakpoint settings).
4.9 Debugging Multiple Inferiors and Programs
gdb lets you run and debug multiple programs in a single session. In addition, gdb on
some systems may let you run several programs simultaneously (otherwise you have to exit
from one before starting another). In the most general case, you can have multiple threads
of execution in each of multiple processes, launched from multiple executables.
gdb represents the state of each program execution with an object called an inferior.
An inferior typically corresponds to a process, but is more general and applies also to
targets that do not have processes. Inferiors may be created before a process runs, and may
be retained after a process exits. Inferiors have unique identifiers that are different from
process ids. Usually each inferior will also have its own distinct address space, although
34
Debugging with gdb
some embedded targets may have several inferiors running in different parts of a single
address space. Each inferior may in turn have multiple threads running in it.
To find out what inferiors exist at any moment, use info inferiors:
info inferiors
Print a list of all inferiors currently being managed by gdb.
gdb displays for each inferior (in this order):
1. the inferior number assigned by gdb
2. the target system’s inferior identifier
3. the name of the executable the inferior is running.
An asterisk ‘*’ preceding the gdb inferior number indicates the current inferior.
For example,
(gdb) info inferiors
Num Description
2
process 2307
* 1
process 3401
Executable
hello
goodbye
To switch focus between inferiors, use the inferior command:
inferior infno
Make inferior number infno the current inferior. The argument infno is the inferior number assigned by gdb, as shown in the first field of the ‘info inferiors’
display.
The debugger convenience variable ‘$_inferior’ contains the number of the current
inferior. You may find this useful in writing breakpoint conditional expressions, command
scripts, and so forth. See Section 10.11 [Convenience Variables], page 139, for general
information on convenience variables.
You can get multiple executables into a debugging session via the add-inferior and
clone-inferior commands. On some systems gdb can add inferiors to the debug session
automatically by following calls to fork and exec. To remove inferiors from the debugging
session use the remove-inferiors command.
add-inferior [ -copies n ] [ -exec executable ]
Adds n inferiors to be run using executable as the executable; n defaults to 1.
If no executable is specified, the inferiors begins empty, with no program. You
can still assign or change the program assigned to the inferior at any time by
using the file command with the executable name as its argument.
clone-inferior [ -copies n ] [ infno ]
Adds n inferiors ready to execute the same program as inferior infno; n defaults
to 1, and infno defaults to the number of the current inferior. This is a convenient command when you want to run another instance of the inferior you are
debugging.
(gdb) info inferiors
Num Description
* 1
process 29964
(gdb) clone-inferior
Added inferior 2.
1 inferiors added.
Executable
helloworld
Chapter 4: Running Programs Under gdb
(gdb) info inferiors
Num Description
2
* 1
process 29964
35
Executable
helloworld
helloworld
You can now simply switch focus to inferior 2 and run it.
remove-inferiors infno...
Removes the inferior or inferiors infno . . . . It is not possible to remove an
inferior that is running with this command. For those, use the kill or detach
command first.
To quit debugging one of the running inferiors that is not the current inferior, you
can either detach from it by using the detach inferior command (allowing it to run
independently), or kill it using the kill inferiors command:
detach inferior infno...
Detach from the inferior or inferiors identified by gdb inferior number(s)
infno . . . . Note that the inferior’s entry still stays on the list of inferiors shown
by info inferiors, but its Description will show ‘’.
kill inferiors infno...
Kill the inferior or inferiors identified by gdb inferior number(s) infno . . . .
Note that the inferior’s entry still stays on the list of inferiors shown by info
inferiors, but its Description will show ‘’.
After the successful completion of a command such as detach, detach inferiors, kill
or kill inferiors, or after a normal process exit, the inferior is still valid and listed with
info inferiors, ready to be restarted.
To be notified when inferiors are started or exit under gdb’s control use
set print inferior-events:
set print inferior-events
set print inferior-events on
set print inferior-events off
The set print inferior-events command allows you to enable or disable
printing of messages when gdb notices that new inferiors have started or that
inferiors have exited or have been detached. By default, these messages will
not be printed.
show print inferior-events
Show whether messages will be printed when gdb detects that inferiors have
started, exited or have been detached.
Many commands will work the same with multiple programs as with a single program:
e.g., print myglobal will simply display the value of myglobal in the current inferior.
Occasionaly, when debugging gdb itself, it may be useful to get more info about the
relationship of inferiors, programs, address spaces in a debug session. You can do that with
the maint info program-spaces command.
maint info program-spaces
Print a list of all program spaces currently being managed by gdb.
gdb displays for each program space (in this order):
36
Debugging with gdb
1. the program space number assigned by gdb
2. the name of the executable loaded into the program space, with e.g., the
file command.
An asterisk ‘*’ preceding the gdb program space number indicates the current
program space.
In addition, below each program space line, gdb prints extra information that
isn’t suitable to display in tabular form. For example, the list of inferiors bound
to the program space.
(gdb) maint info program-spaces
Id
Executable
* 1
hello
2
goodbye
Bound inferiors: ID 1 (process 21561)
Here we can see that no inferior is running the program hello, while process
21561 is running the program goodbye. On some targets, it is possible that
multiple inferiors are bound to the same program space. The most common
example is that of debugging both the parent and child processes of a vfork
call. For example,
(gdb) maint info program-spaces
Id
Executable
* 1
vfork-test
Bound inferiors: ID 2 (process 18050), ID 1 (process 18045)
Here, both inferior 2 and inferior 1 are running in the same program space as
a result of inferior 1 having executed a vfork call.
4.10 Debugging Programs with Multiple Threads
In some operating systems, such as GNU/Linux and Solaris, a single program may have more
than one thread of execution. The precise semantics of threads differ from one operating
system to another, but in general the threads of a single program are akin to multiple
processes—except that they share one address space (that is, they can all examine and
modify the same variables). On the other hand, each thread has its own registers and
execution stack, and perhaps private memory.
gdb provides these facilities for debugging multi-thread programs:
• automatic notification of new threads
• ‘thread thread-id’, a command to switch among threads
• ‘info threads’, a command to inquire about existing threads
• ‘thread apply [thread-id-list] [all] args’, a command to apply a command to a
list of threads
• thread-specific breakpoints
• ‘set print thread-events’, which controls printing of messages on thread start and
exit.
• ‘set libthread-db-search-path path’, which lets the user specify which libthread_
db to use if the default choice isn’t compatible with the program.
The gdb thread debugging facility allows you to observe all threads while your program
runs—but whenever gdb takes control, one thread in particular is always the focus of
Chapter 4: Running Programs Under gdb
37
debugging. This thread is called the current thread. Debugging commands show program
information from the perspective of the current thread.
Whenever gdb detects a new thread in your program, it displays the target system’s
identification for the thread with a message in the form ‘[New systag]’, where systag is a
thread identifier whose form varies depending on the particular system. For example, on
gnu/Linux, you might see
[New Thread 0x41e02940 (LWP 25582)]
when gdb notices a new thread. In contrast, on other systems, the systag is simply something like ‘process 368’, with no further qualifier.
For debugging purposes, gdb associates its own thread number —always a single
integer—with each thread of an inferior. This number is unique between all threads of an
inferior, but not unique between threads of different inferiors.
You can refer to a given thread in an inferior using the qualified inferior-num.thread-num
syntax, also known as qualified thread ID, with inferior-num being the inferior number and
thread-num being the thread number of the given inferior. For example, thread 2.3 refers
to thread number 3 of inferior 2. If you omit inferior-num (e.g., thread 3), then gdb infers
you’re referring to a thread of the current inferior.
Until you create a second inferior, gdb does not show the inferior-num part of thread
IDs, even though you can always use the full inferior-num.thread-num form to refer to
threads of inferior 1, the initial inferior.
Some commands accept a space-separated thread ID list as argument. A list element
can be:
1. A thread ID as shown in the first field of the ‘info threads’ display, with or without
an inferior qualifier. E.g., ‘2.1’ or ‘1’.
2. A range of thread numbers, again with or without an inferior qualifier, as in inf.thr1thr2 or thr1-thr2. E.g., ‘1.2-4’ or ‘2-4’.
3. All threads of an inferior, specified with a star wildcard, with or without an inferior
qualifier, as in inf.* (e.g., ‘1.*’) or *. The former refers to all threads of the given
inferior, and the latter form without an inferior qualifier refers to all threads of the
current inferior.
For example, if the current inferior is 1, and inferior 7 has one thread with ID 7.1, the
thread list ‘1 2-3 4.5 6.7-9 7.*’ includes threads 1 to 3 of inferior 1, thread 5 of inferior
4, threads 7 to 9 of inferior 6 and all threads of inferior 7. That is, in expanded qualified
form, the same as ‘1.1 1.2 1.3 4.5 6.7 6.8 6.9 7.1’.
In addition to a per-inferior number, each thread is also assigned a unique global number,
also known as global thread ID, a single integer. Unlike the thread number component of
the thread ID, no two threads have the same global ID, even when you’re debugging multiple
inferiors.
From gdb’s perspective, a process always has at least one thread. In other words,
gdb assigns a thread number to the program’s “main thread” even if the program is not
multi-threaded.
The debugger convenience variables ‘$_thread’ and ‘$_gthread’ contain, respectively,
the per-inferior thread number and the global thread number of the current thread. You
may find this useful in writing breakpoint conditional expressions, command scripts, and
38
Debugging with gdb
so forth. See Section 10.11 [Convenience Variables], page 139, for general information on
convenience variables.
If gdb detects the program is multi-threaded, it augments the usual message about
stopping at a breakpoint with the ID and name of the thread that hit the breakpoint.
Thread 2 "client" hit Breakpoint 1, send_message () at client.c:68
Likewise when the program receives a signal:
Thread 1 "main" received signal SIGINT, Interrupt.
info threads [thread-id-list]
Display information about one or more threads. With no arguments displays
information about all threads. You can specify the list of threads that you want
to display using the thread ID list syntax (see [thread ID lists], page 37).
gdb displays for each thread (in this order):
1. the per-inferior thread number assigned by gdb
2. the global thread number assigned by gdb, if the ‘-gid’ option was specified
3. the target system’s thread identifier (systag)
4. the thread’s name, if one is known. A thread can either be named by the
user (see thread name, below), or, in some cases, by the program itself.
5. the current stack frame summary for that thread
An asterisk ‘*’ to the left of the gdb thread number indicates the current thread.
For example,
(gdb) info threads
Id
Target Id
* 1
process 35 thread
2
process 35 thread
3
process 35 thread
at threadtest.c:68
Frame
13 main (argc=1, argv=0x7ffffff8)
23 0x34e5 in sigpause ()
27 0x34e5 in sigpause ()
If you’re debugging multiple inferiors, gdb displays thread IDs using the qualified
inferior-num.thread-num format. Otherwise, only thread-num is shown.
If you specify the ‘-gid’ option, gdb displays a column indicating each thread’s global
thread ID:
(gdb) info threads
Id
GId Target Id
1.1 1
process 35
1.2 3
process 35
1.3 4
process 35
* 2.1 2
process 65
thread
thread
thread
thread
13
23
27
1
Frame
main (argc=1, argv=0x7ffffff8)
0x34e5 in sigpause ()
0x34e5 in sigpause ()
main (argc=1, argv=0x7ffffff8)
On Solaris, you can display more information about user threads with a Solaris-specific
command:
maint info sol-threads
Display info on Solaris user threads.
thread thread-id
Make thread ID thread-id the current thread. The command argument threadid is the gdb thread ID, as shown in the first field of the ‘info threads’ display,
with or without an inferior qualifier (e.g., ‘2.1’ or ‘1’).
Chapter 4: Running Programs Under gdb
39
gdb responds by displaying the system identifier of the thread you selected,
and its current stack frame summary:
(gdb) thread 2
[Switching to thread 2 (Thread 0xb7fdab70 (LWP 12747))]
#0 some_function (ignore=0x0) at example.c:8
8
printf ("hello\n");
As with the ‘[New ...]’ message, the form of the text after ‘Switching to’
depends on your system’s conventions for identifying threads.
thread apply [thread-id-list | all [-ascending]] command
The thread apply command allows you to apply the named command to one
or more threads. Specify the threads that you want affected using the thread
ID list syntax (see [thread ID lists], page 37), or specify all to apply to all
threads. To apply a command to all threads in descending order, type thread
apply all command. To apply a command to all threads in ascending order,
type thread apply all -ascending command.
thread name [name]
This command assigns a name to the current thread. If no argument is given,
any existing user-specified name is removed. The thread name appears in the
‘info threads’ display.
On some systems, such as gnu/Linux, gdb is able to determine the name of
the thread as given by the OS. On these systems, a name specified with ‘thread
name’ will override the system-give name, and removing the user-specified name
will cause gdb to once again display the system-specified name.
thread find [regexp]
Search for and display thread ids whose name or systag matches the supplied
regular expression.
As well as being the complement to the ‘thread name’ command, this command
also allows you to identify a thread by its target systag. For instance, on
gnu/Linux, the target systag is the LWP id.
(gdb) thread find 26688
Thread 4 has target id ’Thread 0x41e02940 (LWP 26688)’
(gdb) info thread 4
Id
Target Id
Frame
4
Thread 0x41e02940 (LWP 26688) 0x00000031ca6cd372 in select ()
set print thread-events
set print thread-events on
set print thread-events off
The set print thread-events command allows you to enable or disable printing of messages when gdb notices that new threads have started or that threads
have exited. By default, these messages will be printed if detection of these
events is supported by the target. Note that these messages cannot be disabled
on all targets.
show print thread-events
Show whether messages will be printed when gdb detects that threads have
started and exited.
40
Debugging with gdb
See Section 5.5 [Stopping and Starting Multi-thread Programs], page 77, for more information about how gdb behaves when you stop and start programs with multiple threads.
See Section 5.1.2 [Setting Watchpoints], page 52, for information about watchpoints in
programs with multiple threads.
set libthread-db-search-path [path]
If this variable is set, path is a colon-separated list of directories gdb will use
to search for libthread_db. If you omit path, ‘libthread-db-search-path’
will be reset to its default value ($sdir:$pdir on gnu/Linux and Solaris systems). Internally, the default value comes from the LIBTHREAD_DB_SEARCH_
PATH macro.
On gnu/Linux and Solaris systems, gdb uses a “helper” libthread_db library to obtain information about threads in the inferior process. gdb will use
‘libthread-db-search-path’ to find libthread_db. gdb also consults first if
inferior specific thread debugging library loading is enabled by ‘set auto-load
libthread-db’ (see Section 22.7.2 [libthread db.so.1 file], page 310).
A special entry ‘$sdir’ for ‘libthread-db-search-path’ refers to the default
system directories that are normally searched for loading shared libraries. The
‘$sdir’ entry is the only kind not needing to be enabled by ‘set auto-load
libthread-db’ (see Section 22.7.2 [libthread db.so.1 file], page 310).
A special entry ‘$pdir’ for ‘libthread-db-search-path’ refers to the directory
from which libpthread was loaded in the inferior process.
For any libthread_db library gdb finds in above directories, gdb attempts
to initialize it with the current inferior process. If this initialization fails
(which could happen because of a version mismatch between libthread_db
and libpthread), gdb will unload libthread_db, and continue with the next
directory. If none of libthread_db libraries initialize successfully, gdb will
issue a warning and thread debugging will be disabled.
Setting libthread-db-search-path is currently implemented only on some
platforms.
show libthread-db-search-path
Display current libthread db search path.
set debug libthread-db
show debug libthread-db
Turns on or off display of libthread_db-related events. Use 1 to enable, 0 to
disable.
4.11 Debugging Forks
On most systems, gdb has no special support for debugging programs which create additional processes using the fork function. When a program forks, gdb will continue to debug
the parent process and the child process will run unimpeded. If you have set a breakpoint
in any code which the child then executes, the child will get a SIGTRAP signal which (unless
it catches the signal) will cause it to terminate.
However, if you want to debug the child process there is a workaround which isn’t too
painful. Put a call to sleep in the code which the child process executes after the fork. It
Chapter 4: Running Programs Under gdb
41
may be useful to sleep only if a certain environment variable is set, or a certain file exists,
so that the delay need not occur when you don’t want to run gdb on the child. While the
child is sleeping, use the ps program to get its process ID. Then tell gdb (a new invocation
of gdb if you are also debugging the parent process) to attach to the child process (see
Section 4.7 [Attach], page 32). From that point on you can debug the child process just like
any other process which you attached to.
On some systems, gdb provides support for debugging programs that create additional
processes using the fork or vfork functions. On gnu/Linux platforms, this feature is
supported with kernel version 2.5.46 and later.
The fork debugging commands are supported in native mode and when connected to
gdbserver in either target remote mode or target extended-remote mode.
By default, when a program forks, gdb will continue to debug the parent process and
the child process will run unimpeded.
If you want to follow the child process instead of the parent process, use the command
set follow-fork-mode.
set follow-fork-mode mode
Set the debugger response to a program call of fork or vfork. A call to fork
or vfork creates a new process. The mode argument can be:
parent
The original process is debugged after a fork. The child process
runs unimpeded. This is the default.
child
The new process is debugged after a fork. The parent process runs
unimpeded.
show follow-fork-mode
Display the current debugger response to a fork or vfork call.
On Linux, if you want to debug both the parent and child processes, use the command
set detach-on-fork.
set detach-on-fork mode
Tells gdb whether to detach one of the processes after a fork, or retain debugger
control over them both.
on
The child process (or parent process, depending on the value of
follow-fork-mode) will be detached and allowed to run independently. This is the default.
off
Both processes will be held under the control of gdb. One process
(child or parent, depending on the value of follow-fork-mode) is
debugged as usual, while the other is held suspended.
show detach-on-fork
Show whether detach-on-fork mode is on/off.
If you choose to set ‘detach-on-fork’ mode off, then gdb will retain control of all forked
processes (including nested forks). You can list the forked processes under the control of gdb
by using the info inferiors command, and switch from one fork to another by using the
inferior command (see Section 4.9 [Debugging Multiple Inferiors and Programs], page 33).
42
Debugging with gdb
To quit debugging one of the forked processes, you can either detach from it by using
the detach inferiors command (allowing it to run independently), or kill it using the
kill inferiors command. See Section 4.9 [Debugging Multiple Inferiors and Programs],
page 33.
If you ask to debug a child process and a vfork is followed by an exec, gdb executes
the new target up to the first breakpoint in the new target. If you have a breakpoint set on
main in your original program, the breakpoint will also be set on the child process’s main.
On some systems, when a child process is spawned by vfork, you cannot debug the child
or parent until an exec call completes.
If you issue a run command to gdb after an exec call executes, the new target restarts.
To restart the parent process, use the file command with the parent executable name
as its argument. By default, after an exec call executes, gdb discards the symbols of the
previous executable image. You can change this behaviour with the set follow-exec-mode
command.
set follow-exec-mode mode
Set debugger response to a program call of exec. An exec call replaces the
program image of a process.
follow-exec-mode can be:
new
gdb creates a new inferior and rebinds the process to this new
inferior. The program the process was running before the exec call
can be restarted afterwards by restarting the original inferior.
For example:
(gdb) info inferiors
(gdb) info inferior
Id
Description
Executable
* 1
prog1
(gdb) run
process 12020 is executing new program: prog2
Program exited normally.
(gdb) info inferiors
Id
Description
Executable
1
prog1
* 2
prog2
same
gdb keeps the process bound to the same inferior. The new executable image replaces the previous executable loaded in the inferior. Restarting the inferior after the exec call, with e.g., the run
command, restarts the executable the process was running after the
exec call. This is the default mode.
For example:
(gdb) info inferiors
Id
Description
Executable
* 1
prog1
(gdb) run
process 12020 is executing new program: prog2
Program exited normally.
(gdb) info inferiors
Id
Description
Executable
* 1
prog2
Chapter 4: Running Programs Under gdb
43
follow-exec-mode is supported in native mode and target extended-remote mode.
You can use the catch command to make gdb stop whenever a fork, vfork, or exec
call is made. See Section 5.1.3 [Setting Catchpoints], page 54.
4.12 Setting a Bookmark to Return to Later
On certain operating systems1 , gdb is able to save a snapshot of a program’s state, called
a checkpoint, and come back to it later.
Returning to a checkpoint effectively undoes everything that has happened in the program since the checkpoint was saved. This includes changes in memory, registers, and even
(within some limits) system state. Effectively, it is like going back in time to the moment
when the checkpoint was saved.
Thus, if you’re stepping thru a program and you think you’re getting close to the point
where things go wrong, you can save a checkpoint. Then, if you accidentally go too far and
miss the critical statement, instead of having to restart your program from the beginning,
you can just go back to the checkpoint and start again from there.
This can be especially useful if it takes a lot of time or steps to reach the point where
you think the bug occurs.
To use the checkpoint/restart method of debugging:
checkpoint
Save a snapshot of the debugged program’s current execution state. The
checkpoint command takes no arguments, but each checkpoint is assigned
a small integer id, similar to a breakpoint id.
info checkpoints
List the checkpoints that have been saved in the current debugging session. For
each checkpoint, the following information will be listed:
Checkpoint ID
Process ID
Code Address
Source line, or label
restart checkpoint-id
Restore the program state that was saved as checkpoint number checkpoint-id.
All program variables, registers, stack frames etc. will be returned to the values
that they had when the checkpoint was saved. In essence, gdb will “wind back
the clock” to the point in time when the checkpoint was saved.
Note that breakpoints, gdb variables, command history etc. are not affected
by restoring a checkpoint. In general, a checkpoint only restores things that
reside in the program being debugged, not in the debugger.
delete checkpoint checkpoint-id
Delete the previously-saved checkpoint identified by checkpoint-id.
Returning to a previously saved checkpoint will restore the user state of the program
being debugged, plus a significant subset of the system (OS) state, including file pointers. It
1
Currently, only gnu/Linux.
44
Debugging with gdb
won’t “un-write” data from a file, but it will rewind the file pointer to the previous location,
so that the previously written data can be overwritten. For files opened in read mode, the
pointer will also be restored so that the previously read data can be read again.
Of course, characters that have been sent to a printer (or other external device) cannot
be “snatched back”, and characters received from eg. a serial device can be removed from
internal program buffers, but they cannot be “pushed back” into the serial pipeline, ready
to be received again. Similarly, the actual contents of files that have been changed cannot
be restored (at this time).
However, within those constraints, you actually can “rewind” your program to a previously saved point in time, and begin debugging it again — and you can change the course
of events so as to debug a different execution path this time.
Finally, there is one bit of internal program state that will be different when you return
to a checkpoint — the program’s process id. Each checkpoint will have a unique process id
(or pid), and each will be different from the program’s original pid. If your program has
saved a local copy of its process id, this could potentially pose a problem.
4.12.1 A Non-obvious Benefit of Using Checkpoints
On some systems such as gnu/Linux, address space randomization is performed on new
processes for security reasons. This makes it difficult or impossible to set a breakpoint, or
watchpoint, on an absolute address if you have to restart the program, since the absolute
location of a symbol will change from one execution to the next.
A checkpoint, however, is an identical copy of a process. Therefore if you create a
checkpoint at (eg.) the start of main, and simply return to that checkpoint instead of
restarting the process, you can avoid the effects of address randomization and your symbols
will all stay in the same place.
Chapter 5: Stopping and Continuing
45
5 Stopping and Continuing
The principal purposes of using a debugger are so that you can stop your program before it
terminates; or so that, if your program runs into trouble, you can investigate and find out
why.
Inside gdb, your program may stop for any of several reasons, such as a signal, a breakpoint, or reaching a new line after a gdb command such as step. You may then examine
and change variables, set new breakpoints or remove old ones, and then continue execution. Usually, the messages shown by gdb provide ample explanation of the status of your
program—but you can also explicitly request this information at any time.
info program
Display information about the status of your program: whether it is running
or not, what process it is, and why it stopped.
5.1 Breakpoints, Watchpoints, and Catchpoints
A breakpoint makes your program stop whenever a certain point in the program is reached.
For each breakpoint, you can add conditions to control in finer detail whether your program
stops. You can set breakpoints with the break command and its variants (see Section 5.1.1
[Setting Breakpoints], page 46), to specify the place where your program should stop by
line number, function name or exact address in the program.
On some systems, you can set breakpoints in shared libraries before the executable is
run.
A watchpoint is a special breakpoint that stops your program when the value of an
expression changes. The expression may be a value of a variable, or it could involve values
of one or more variables combined by operators, such as ‘a + b’. This is sometimes called
data breakpoints. You must use a different command to set watchpoints (see Section 5.1.2
[Setting Watchpoints], page 52), but aside from that, you can manage a watchpoint like any
other breakpoint: you enable, disable, and delete both breakpoints and watchpoints using
the same commands.
You can arrange to have values from your program displayed automatically whenever
gdb stops at a breakpoint. See Section 10.7 [Automatic Display], page 126.
A catchpoint is another special breakpoint that stops your program when a certain kind
of event occurs, such as the throwing of a C++ exception or the loading of a library. As with
watchpoints, you use a different command to set a catchpoint (see Section 5.1.3 [Setting
Catchpoints], page 54), but aside from that, you can manage a catchpoint like any other
breakpoint. (To stop when your program receives a signal, use the handle command; see
Section 5.4 [Signals], page 74.)
gdb assigns a number to each breakpoint, watchpoint, or catchpoint when you create
it; these numbers are successive integers starting with one. In many of the commands for
controlling various features of breakpoints you use the breakpoint number to say which
breakpoint you want to change. Each breakpoint may be enabled or disabled; if disabled,
it has no effect on your program until you enable it again.
Some gdb commands accept a space-separated list of breakpoints on which to operate.
A list element can be either a single breakpoint number, like ‘5’, or a range of such numbers,
46
Debugging with gdb
like ‘5-7’. When a breakpoint list is given to a command, all breakpoints in that list are
operated on.
5.1.1 Setting Breakpoints
Breakpoints are set with the break command (abbreviated b). The debugger convenience variable ‘$bpnum’ records the number of the breakpoint you’ve set most recently;
see Section 10.11 [Convenience Variables], page 139, for a discussion of what you can do
with convenience variables.
break location
Set a breakpoint at the given location, which can specify a function name, a line
number, or an address of an instruction. (See Section 9.2 [Specify Location],
page 104, for a list of all the possible ways to specify a location.) The breakpoint
will stop your program just before it executes any of the code in the specified
location.
When using source languages that permit overloading of symbols, such as C++,
a function name may refer to more than one possible place to break. See
Section 10.2 [Ambiguous Expressions], page 118, for a discussion of that situation.
It is also possible to insert a breakpoint that will stop the program only if a
specific thread (see Section 5.5.4 [Thread-Specific Breakpoints], page 80) or a
specific task (see Section 15.4.10.7 [Ada Tasks], page 215) hits that breakpoint.
break
When called without any arguments, break sets a breakpoint at the next instruction to be executed in the selected stack frame (see Chapter 8 [Examining
the Stack], page 95). In any selected frame but the innermost, this makes your
program stop as soon as control returns to that frame. This is similar to the
effect of a finish command in the frame inside the selected frame—except that
finish does not leave an active breakpoint. If you use break without an argument in the innermost frame, gdb stops the next time it reaches the current
location; this may be useful inside loops.
gdb normally ignores breakpoints when it resumes execution, until at least one
instruction has been executed. If it did not do this, you would be unable to proceed past a breakpoint without first disabling the breakpoint. This rule applies
whether or not the breakpoint already existed when your program stopped.
break ... if cond
Set a breakpoint with condition cond; evaluate the expression cond each time
the breakpoint is reached, and stop only if the value is nonzero—that is, if cond
evaluates as true. ‘...’ stands for one of the possible arguments described
above (or no argument) specifying where to break. See Section 5.1.6 [Break
Conditions], page 61, for more information on breakpoint conditions.
tbreak args
Set a breakpoint enabled only for one stop. The args are the same as for the
break command, and the breakpoint is set in the same way, but the breakpoint
is automatically deleted after the first time your program stops there. See
Section 5.1.5 [Disabling Breakpoints], page 60.
Chapter 5: Stopping and Continuing
47
hbreak args
Set a hardware-assisted breakpoint. The args are the same as for the break
command and the breakpoint is set in the same way, but the breakpoint requires hardware support and some target hardware may not have this support.
The main purpose of this is EPROM/ROM code debugging, so you can set
a breakpoint at an instruction without changing the instruction. This can be
used with the new trap-generation provided by SPARClite DSU and most x86based targets. These targets will generate traps when a program accesses some
data or instruction address that is assigned to the debug registers. However
the hardware breakpoint registers can take a limited number of breakpoints.
For example, on the DSU, only two data breakpoints can be set at a time, and
gdb will reject this command if more than two are used. Delete or disable
unused hardware breakpoints before setting new ones (see Section 5.1.5 [Disabling Breakpoints], page 60). See Section 5.1.6 [Break Conditions], page 61.
For remote targets, you can restrict the number of hardware breakpoints gdb
will use, see [set remote hardware-breakpoint-limit], page 271.
thbreak args
Set a hardware-assisted breakpoint enabled only for one stop. The args are the
same as for the hbreak command and the breakpoint is set in the same way.
However, like the tbreak command, the breakpoint is automatically deleted
after the first time your program stops there. Also, like the hbreak command,
the breakpoint requires hardware support and some target hardware may not
have this support. See Section 5.1.5 [Disabling Breakpoints], page 60. See also
Section 5.1.6 [Break Conditions], page 61.
rbreak regex
Set breakpoints on all functions matching the regular expression regex. This
command sets an unconditional breakpoint on all matches, printing a list of all
breakpoints it set. Once these breakpoints are set, they are treated just like the
breakpoints set with the break command. You can delete them, disable them,
or make them conditional the same way as any other breakpoint.
The syntax of the regular expression is the standard one used with tools like
‘grep’. Note that this is different from the syntax used by shells, so for instance
foo* matches all functions that include an fo followed by zero or more os. There
is an implicit .* leading and trailing the regular expression you supply, so to
match only functions that begin with foo, use ^foo.
When debugging C++ programs, rbreak is useful for setting breakpoints on
overloaded functions that are not members of any special classes.
The rbreak command can be used to set breakpoints in all the functions in a
program, like this:
(gdb) rbreak .
rbreak file:regex
If rbreak is called with a filename qualification, it limits the search for functions
matching the given regular expression to the specified file. This can be used,
for example, to set breakpoints on every function in a given file:
(gdb) rbreak file.c:.
48
Debugging with gdb
The colon separating the filename qualifier from the regex may optionally be
surrounded by spaces.
info breakpoints [list...]
info break [list...]
Print a table of all breakpoints, watchpoints, and catchpoints set and not
deleted. Optional argument n means print information only about the specified breakpoint(s) (or watchpoint(s) or catchpoint(s)). For each breakpoint,
following columns are printed:
Breakpoint Numbers
Type
Breakpoint, watchpoint, or catchpoint.
Disposition
Whether the breakpoint is marked to be disabled or deleted when
hit.
Enabled or Disabled
Enabled breakpoints are marked with ‘y’. ‘n’ marks breakpoints
that are not enabled.
Address
Where the breakpoint is in your program, as a memory address.
For a pending breakpoint whose address is not yet known, this
field will contain ‘’. Such breakpoint won’t fire until a
shared library that has the symbol or line referred by breakpoint is
loaded. See below for details. A breakpoint with several locations
will have ‘’ in this field—see below for details.
What
Where the breakpoint is in the source for your program, as a file and
line number. For a pending breakpoint, the original string passed
to the breakpoint command will be listed as it cannot be resolved
until the appropriate shared library is loaded in the future.
If a breakpoint is conditional, there are two evaluation modes: “host” and
“target”. If mode is “host”, breakpoint condition evaluation is done by gdb on
the host’s side. If it is “target”, then the condition is evaluated by the target.
The info break command shows the condition on the line following the affected
breakpoint, together with its condition evaluation mode in between parentheses.
Breakpoint commands, if any, are listed after that. A pending breakpoint is
allowed to have a condition specified for it. The condition is not parsed for
validity until a shared library is loaded that allows the pending breakpoint to
resolve to a valid location.
info break with a breakpoint number n as argument lists only that breakpoint. The convenience variable $_ and the default examining-address for the
x command are set to the address of the last breakpoint listed (see Section 10.6
[Examining Memory], page 123).
info break displays a count of the number of times the breakpoint has been
hit. This is especially useful in conjunction with the ignore command. You
can ignore a large number of breakpoint hits, look at the breakpoint info to see
how many times the breakpoint was hit, and then run again, ignoring one less
than that number. This will get you quickly to the last hit of that breakpoint.
Chapter 5: Stopping and Continuing
49
For a breakpoints with an enable count (xref) greater than 1, info break also
displays that count.
gdb allows you to set any number of breakpoints at the same place in your program.
There is nothing silly or meaningless about this. When the breakpoints are conditional,
this is even useful (see Section 5.1.6 [Break Conditions], page 61).
It is possible that a breakpoint corresponds to several locations in your program. Examples of this situation are:
• Multiple functions in the program may have the same name.
• For a C++ constructor, the gcc compiler generates several instances of the function
body, used in different cases.
• For a C++ template function, a given line in the function can correspond to any number
of instantiations.
• For an inlined function, a given source line can correspond to several places where that
function is inlined.
In all those cases, gdb will insert a breakpoint at all the relevant locations.
A breakpoint with multiple locations is displayed in the breakpoint table using several
rows—one header row, followed by one row for each breakpoint location. The header row
has ‘’ in the address column. The rows for individual locations contain the
actual addresses for locations, and show the functions to which those locations belong. The
number column for a location is of the form breakpoint-number.location-number.
For example:
Num
1
1.1
1.2
Type
Disp Enb
breakpoint
keep y
stop only if i==1
breakpoint already hit 1
y
y
Address
What
time
0x080486a2 in void foo() at t.cc:8
0x080486ca in void foo() at t.cc:8
Each location can be individually enabled or disabled by passing breakpointnumber.location-number as argument to the enable and disable commands. Note that
you cannot delete the individual locations from the list, you can only delete the entire
list of locations that belong to their parent breakpoint (with the delete num command,
where num is the number of the parent breakpoint, 1 in the above example). Disabling
or enabling the parent breakpoint (see Section 5.1.5 [Disabling], page 60) affects all of the
locations that belong to that breakpoint.
It’s quite common to have a breakpoint inside a shared library. Shared libraries can
be loaded and unloaded explicitly, and possibly repeatedly, as the program is executed.
To support this use case, gdb updates breakpoint locations whenever any shared library
is loaded or unloaded. Typically, you would set a breakpoint in a shared library at the
beginning of your debugging session, when the library is not loaded, and when the symbols
from the library are not available. When you try to set breakpoint, gdb will ask you if you
want to set a so called pending breakpoint—breakpoint whose address is not yet resolved.
After the program is run, whenever a new shared library is loaded, gdb reevaluates all the
breakpoints. When a newly loaded shared library contains the symbol or line referred to by
some pending breakpoint, that breakpoint is resolved and becomes an ordinary breakpoint.
50
Debugging with gdb
When a library is unloaded, all breakpoints that refer to its symbols or source lines become
pending again.
This logic works for breakpoints with multiple locations, too. For example, if you have
a breakpoint in a C++ template function, and a newly loaded shared library has an instantiation of that template, a new location is added to the list of locations for the breakpoint.
Except for having unresolved address, pending breakpoints do not differ from regular
breakpoints. You can set conditions or commands, enable and disable them and perform
other breakpoint operations.
gdb provides some additional commands for controlling what happens when the ‘break’
command cannot resolve breakpoint address specification to an address:
set breakpoint pending auto
This is the default behavior. When gdb cannot find the breakpoint location,
it queries you whether a pending breakpoint should be created.
set breakpoint pending on
This indicates that an unrecognized breakpoint location should automatically
result in a pending breakpoint being created.
set breakpoint pending off
This indicates that pending breakpoints are not to be created. Any unrecognized breakpoint location results in an error. This setting does not affect any
pending breakpoints previously created.
show breakpoint pending
Show the current behavior setting for creating pending breakpoints.
The settings above only affect the break command and its variants. Once breakpoint is
set, it will be automatically updated as shared libraries are loaded and unloaded.
For some targets, gdb can automatically decide if hardware or software breakpoints
should be used, depending on whether the breakpoint address is read-only or read-write.
This applies to breakpoints set with the break command as well as to internal breakpoints
set by commands like next and finish. For breakpoints set with hbreak, gdb will always
use hardware breakpoints.
You can control this automatic behaviour with the following commands:
set breakpoint auto-hw on
This is the default behavior. When gdb sets a breakpoint, it will try to use the
target memory map to decide if software or hardware breakpoint must be used.
set breakpoint auto-hw off
This indicates gdb should not automatically select breakpoint type. If the
target provides a memory map, gdb will warn when trying to set software
breakpoint at a read-only address.
gdb normally implements breakpoints by replacing the program code at the breakpoint
address with a special instruction, which, when executed, given control to the debugger.
By default, the program code is so modified only when the program is resumed. As soon as
the program stops, gdb restores the original instructions. This behaviour guards against
leaving breakpoints inserted in the target should gdb abrubptly disconnect. However, with
Chapter 5: Stopping and Continuing
51
slow remote targets, inserting and removing breakpoint can reduce the performance. This
behavior can be controlled with the following commands::
set breakpoint always-inserted off
All breakpoints, including newly added by the user, are inserted in the target
only when the target is resumed. All breakpoints are removed from the target
when it stops. This is the default mode.
set breakpoint always-inserted on
Causes all breakpoints to be inserted in the target at all times. If the user adds
a new breakpoint, or changes an existing breakpoint, the breakpoints in the
target are updated immediately. A breakpoint is removed from the target only
when breakpoint itself is deleted.
gdb handles conditional breakpoints by evaluating these conditions when a breakpoint
breaks. If the condition is true, then the process being debugged stops, otherwise the process
is resumed.
If the target supports evaluating conditions on its end, gdb may download the breakpoint, together with its conditions, to it.
This feature can be controlled via the following commands:
set breakpoint condition-evaluation host
This option commands gdb to evaluate the breakpoint conditions on the host’s
side. Unconditional breakpoints are sent to the target which in turn receives
the triggers and reports them back to GDB for condition evaluation. This is
the standard evaluation mode.
set breakpoint condition-evaluation target
This option commands gdb to download breakpoint conditions to the target at
the moment of their insertion. The target is responsible for evaluating the conditional expression and reporting breakpoint stop events back to gdb whenever
the condition is true. Due to limitations of target-side evaluation, some conditions cannot be evaluated there, e.g., conditions that depend on local data that
is only known to the host. Examples include conditional expressions involving
convenience variables, complex types that cannot be handled by the agent expression parser and expressions that are too long to be sent over to the target,
specially when the target is a remote system. In these cases, the conditions will
be evaluated by gdb.
set breakpoint condition-evaluation auto
This is the default mode. If the target supports evaluating breakpoint conditions on its end, gdb will download breakpoint conditions to the target (limitations mentioned previously apply). If the target does not support breakpoint
condition evaluation, then gdb will fallback to evaluating all these conditions
on the host’s side.
gdb itself sometimes sets breakpoints in your program for special purposes, such as
proper handling of longjmp (in C programs). These internal breakpoints are assigned
negative numbers, starting with -1; ‘info breakpoints’ does not display them. You can
see these breakpoints with the gdb maintenance command ‘maint info breakpoints’ (see
[maint info breakpoints], page 609).
52
Debugging with gdb
5.1.2 Setting Watchpoints
You can use a watchpoint to stop execution whenever the value of an expression changes,
without having to predict a particular place where this may happen. (This is sometimes
called a data breakpoint.) The expression may be as simple as the value of a single variable,
or as complex as many variables combined by operators. Examples include:
• A reference to the value of a single variable.
• An address cast to an appropriate data type. For example, ‘*(int *)0x12345678’ will
watch a 4-byte region at the specified address (assuming an int occupies 4 bytes).
• An arbitrarily complex expression, such as ‘a*b + c/d’. The expression can use any operators valid in the program’s native language (see Chapter 15 [Languages], page 191).
You can set a watchpoint on an expression even if the expression can not be evaluated yet.
For instance, you can set a watchpoint on ‘*global_ptr’ before ‘global_ptr’ is initialized.
gdb will stop when your program sets ‘global_ptr’ and the expression produces a valid
value. If the expression becomes valid in some other way than changing a variable (e.g. if
the memory pointed to by ‘*global_ptr’ becomes readable as the result of a malloc call),
gdb may not stop until the next time the expression changes.
Depending on your system, watchpoints may be implemented in software or hardware.
gdb does software watchpointing by single-stepping your program and testing the variable’s
value each time, which is hundreds of times slower than normal execution. (But this may
still be worth it, to catch errors where you have no clue what part of your program is the
culprit.)
On some systems, such as most PowerPC or x86-based targets, gdb includes support
for hardware watchpoints, which do not slow down the running of your program.
watch [-l|-location] expr [thread thread-id] [mask maskvalue]
Set a watchpoint for an expression. gdb will break when the expression expr
is written into by the program and its value changes. The simplest (and the
most popular) use of this command is to watch the value of a single variable:
(gdb) watch foo
If the command includes a [thread thread-id] argument, gdb breaks only
when the thread identified by thread-id changes the value of expr. If any other
threads change the value of expr, gdb will not break. Note that watchpoints
restricted to a single thread in this way only work with Hardware Watchpoints.
Ordinarily a watchpoint respects the scope of variables in expr (see below).
The -location argument tells gdb to instead watch the memory referred to
by expr. In this case, gdb will evaluate expr, take the address of the result, and
watch the memory at that address. The type of the result is used to determine
the size of the watched memory. If the expression’s result does not have an
address, then gdb will print an error.
The [mask maskvalue] argument allows creation of masked watchpoints, if the
current architecture supports this feature (e.g., PowerPC Embedded architecture, see Section 21.3.6 [PowerPC Embedded], page 294.) A masked watchpoint
specifies a mask in addition to an address to watch. The mask specifies that
some bits of an address (the bits which are reset in the mask) should be ignored
Chapter 5: Stopping and Continuing
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when matching the address accessed by the inferior against the watchpoint address. Thus, a masked watchpoint watches many addresses simultaneously—
those addresses whose unmasked bits are identical to the unmasked bits in the
watchpoint address. The mask argument implies -location. Examples:
(gdb) watch foo mask 0xffff00ff
(gdb) watch *0xdeadbeef mask 0xffffff00
rwatch [-l|-location] expr [thread thread-id] [mask maskvalue]
Set a watchpoint that will break when the value of expr is read by the program.
awatch [-l|-location] expr [thread thread-id] [mask maskvalue]
Set a watchpoint that will break when expr is either read from or written into
by the program.
info watchpoints [list...]
This command prints a list of watchpoints, using the same format as info
break (see Section 5.1.1 [Set Breaks], page 46).
If you watch for a change in a numerically entered address you need to dereference it, as
the address itself is just a constant number which will never change. gdb refuses to create
a watchpoint that watches a never-changing value:
(gdb) watch 0x600850
Cannot watch constant value 0x600850.
(gdb) watch *(int *) 0x600850
Watchpoint 1: *(int *) 6293584
gdb sets a hardware watchpoint if possible. Hardware watchpoints execute very quickly,
and the debugger reports a change in value at the exact instruction where the change occurs.
If gdb cannot set a hardware watchpoint, it sets a software watchpoint, which executes more
slowly and reports the change in value at the next statement, not the instruction, after the
change occurs.
You can force gdb to use only software watchpoints with the set can-use-hwwatchpoints 0 command. With this variable set to zero, gdb will never try to use
hardware watchpoints, even if the underlying system supports them.
(Note that
hardware-assisted watchpoints that were set before setting can-use-hw-watchpoints to
zero will still use the hardware mechanism of watching expression values.)
set can-use-hw-watchpoints
Set whether or not to use hardware watchpoints.
show can-use-hw-watchpoints
Show the current mode of using hardware watchpoints.
For remote targets, you can restrict the number of hardware watchpoints gdb will use,
see [set remote hardware-breakpoint-limit], page 271.
When you issue the watch command, gdb reports
Hardware watchpoint num: expr
if it was able to set a hardware watchpoint.
Currently, the awatch and rwatch commands can only set hardware watchpoints, because accesses to data that don’t change the value of the watched expression cannot be
detected without examining every instruction as it is being executed, and gdb does not do
54
Debugging with gdb
that currently. If gdb finds that it is unable to set a hardware breakpoint with the awatch
or rwatch command, it will print a message like this:
Expression cannot be implemented with read/access watchpoint.
Sometimes, gdb cannot set a hardware watchpoint because the data type of the watched
expression is wider than what a hardware watchpoint on the target machine can handle.
For example, some systems can only watch regions that are up to 4 bytes wide; on such systems you cannot set hardware watchpoints for an expression that yields a double-precision
floating-point number (which is typically 8 bytes wide). As a work-around, it might be possible to break the large region into a series of smaller ones and watch them with separate
watchpoints.
If you set too many hardware watchpoints, gdb might be unable to insert all of them
when you resume the execution of your program. Since the precise number of active watchpoints is unknown until such time as the program is about to be resumed, gdb might not be
able to warn you about this when you set the watchpoints, and the warning will be printed
only when the program is resumed:
Hardware watchpoint num: Could not insert watchpoint
If this happens, delete or disable some of the watchpoints.
Watching complex expressions that reference many variables can also exhaust the resources available for hardware-assisted watchpoints. That’s because gdb needs to watch
every variable in the expression with separately allocated resources.
If you call a function interactively using print or call, any watchpoints you have set
will be inactive until gdb reaches another kind of breakpoint or the call completes.
gdb automatically deletes watchpoints that watch local (automatic) variables, or expressions that involve such variables, when they go out of scope, that is, when the execution
leaves the block in which these variables were defined. In particular, when the program
being debugged terminates, all local variables go out of scope, and so only watchpoints
that watch global variables remain set. If you rerun the program, you will need to set all
such watchpoints again. One way of doing that would be to set a code breakpoint at the
entry to the main function and when it breaks, set all the watchpoints.
In multi-threaded programs, watchpoints will detect changes to the watched expression
from every thread.
Warning: In multi-threaded programs, software watchpoints have only limited
usefulness. If gdb creates a software watchpoint, it can only watch the value
of an expression in a single thread. If you are confident that the expression can
only change due to the current thread’s activity (and if you are also confident
that no other thread can become current), then you can use software watchpoints as usual. However, gdb may not notice when a non-current thread’s
activity changes the expression. (Hardware watchpoints, in contrast, watch an
expression in all threads.)
See [set remote hardware-watchpoint-limit], page 271.
5.1.3 Setting Catchpoints
You can use catchpoints to cause the debugger to stop for certain kinds of program events,
such as C++ exceptions or the loading of a shared library. Use the catch command to set
a catchpoint.
Chapter 5: Stopping and Continuing
55
catch event
Stop when event occurs. The event can be any of the following:
throw [regexp]
rethrow [regexp]
catch [regexp]
The throwing, re-throwing, or catching of a C++ exception.
If regexp is given, then only exceptions whose type matches the
regular expression will be caught.
The convenience variable $_exception is available at an exceptionrelated catchpoint, on some systems. This holds the exception being thrown.
There are currently some limitations to C++ exception handling in
gdb:
• The support for these commands is system-dependent.
Currently, only systems using the ‘gnu-v3’ C++ ABI (see
Section 22.6 [ABI], page 307) are supported.
• The regular expression feature and the $_exception convenience variable rely on the presence of some SDT probes in
libstdc++. If these probes are not present, then these features cannot be used. These probes were first available in the
GCC 4.8 release, but whether or not they are available in your
GCC also depends on how it was built.
• The $_exception convenience variable is only valid at the instruction at which an exception-related catchpoint is set.
• When an exception-related catchpoint is hit, gdb stops at a
location in the system library which implements runtime exception support for C++, usually libstdc++. You can use up
(see Section 8.3 [Selection], page 98) to get to your code.
• If you call a function interactively, gdb normally returns control to you when the function has finished executing. If the call
raises an exception, however, the call may bypass the mechanism that returns control to you and cause your program either
to abort or to simply continue running until it hits a breakpoint, catches a signal that gdb is listening for, or exits. This
is the case even if you set a catchpoint for the exception; catchpoints on exceptions are disabled within interactive calls. See
Section 17.5 [Calling], page 233, for information on controlling
this with set unwind-on-terminating-exception.
• You cannot raise an exception interactively.
• You cannot install an exception handler interactively.
exception
An Ada exception being raised. If an exception name is specified
at the end of the command (eg catch exception Program_Error),
the debugger will stop only when this specific exception is raised.
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Debugging with gdb
Otherwise, the debugger stops execution when any Ada exception
is raised.
When inserting an exception catchpoint on a user-defined exception
whose name is identical to one of the exceptions defined by the language, the fully qualified name must be used as the exception name.
Otherwise, gdb will assume that it should stop on the pre-defined
exception rather than the user-defined one. For instance, assuming an exception called Constraint_Error is defined in package
Pck, then the command to use to catch such exceptions is catch
exception Pck.Constraint_Error.
exception unhandled
An exception that was raised but is not handled by the program.
assert
A failed Ada assertion.
exec
A call to exec.
syscall
syscall [name | number | group:groupname | g:groupname] ...
A call to or return from a system call, a.k.a. syscall. A syscall is a
mechanism for application programs to request a service from the
operating system (OS) or one of the OS system services. gdb can
catch some or all of the syscalls issued by the debuggee, and show
the related information for each syscall. If no argument is specified,
calls to and returns from all system calls will be caught.
name can be any system call name that is valid for the underlying
OS. Just what syscalls are valid depends on the OS. On GNU and
Unix systems, you can find the full list of valid syscall names on
‘/usr/include/asm/unistd.h’.
Normally, gdb knows in advance which syscalls are valid for each
OS, so you can use the gdb command-line completion facilities (see
Section 3.2 [command completion], page 19) to list the available
choices.
You may also specify the system call numerically. A syscall’s number is the value passed to the OS’s syscall dispatcher to identify
the requested service. When you specify the syscall by its name,
gdb uses its database of syscalls to convert the name into the corresponding numeric code, but using the number directly may be
useful if gdb’s database does not have the complete list of syscalls
on your system (e.g., because gdb lags behind the OS upgrades).
You may specify a group of related syscalls to be caught at once using the group: syntax (g: is a shorter equivalent). For instance, on
some platforms gdb allows you to catch all network related syscalls,
by passing the argument group:network to catch syscall. Note
that not all syscall groups are available in every system. You can
use the command completion facilities (see Section 3.2 [command
completion], page 19) to list the syscall groups available on your
environment.
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The example below illustrates how this command works if you don’t
provide arguments to it:
(gdb) catch syscall
Catchpoint 1 (syscall)
(gdb) r
Starting program: /tmp/catch-syscall
Catchpoint 1 (call to syscall ’close’), \
0xffffe424 in __kernel_vsyscall ()
(gdb) c
Continuing.
Catchpoint 1 (returned from syscall ’close’), \
0xffffe424 in __kernel_vsyscall ()
(gdb)
Here is an example of catching a system call by name:
(gdb) catch syscall chroot
Catchpoint 1 (syscall ’chroot’ [61])
(gdb) r
Starting program: /tmp/catch-syscall
Catchpoint 1 (call to syscall ’chroot’), \
0xffffe424 in __kernel_vsyscall ()
(gdb) c
Continuing.
Catchpoint 1 (returned from syscall ’chroot’), \
0xffffe424 in __kernel_vsyscall ()
(gdb)
An example of specifying a system call numerically. In the case
below, the syscall number has a corresponding entry in the XML
file, so gdb finds its name and prints it:
(gdb) catch syscall 252
Catchpoint 1 (syscall(s) ’exit_group’)
(gdb) r
Starting program: /tmp/catch-syscall
Catchpoint 1 (call to syscall ’exit_group’), \
0xffffe424 in __kernel_vsyscall ()
(gdb) c
Continuing.
Program exited normally.
(gdb)
Here is an example of catching a syscall group:
(gdb) catch syscall group:process
Catchpoint 1 (syscalls ’exit’ [1] ’fork’ [2] ’waitpid’ [7]
’execve’ [11] ’wait4’ [114] ’clone’ [120] ’vfork’ [190]
’exit_group’ [252] ’waitid’ [284] ’unshare’ [310])
(gdb) r
Starting program: /tmp/catch-syscall
Catchpoint 1 (call to syscall fork), 0x00007ffff7df4e27 in open64 ()
from /lib64/ld-linux-x86-64.so.2
58
Debugging with gdb
(gdb) c
Continuing.
However, there can be situations when there is no corresponding
name in XML file for that syscall number. In this case, gdb prints
a warning message saying that it was not able to find the syscall
name, but the catchpoint will be set anyway. See the example
below:
(gdb) catch syscall 764
warning: The number ’764’ does not represent a known syscall.
Catchpoint 2 (syscall 764)
(gdb)
If you configure gdb using the ‘--without-expat’ option, it will
not be able to display syscall names. Also, if your architecture does
not have an XML file describing its system calls, you will not be
able to see the syscall names. It is important to notice that these
two features are used for accessing the syscall name database. In
either case, you will see a warning like this:
(gdb) catch syscall
warning: Could not open "syscalls/i386-linux.xml"
warning: Could not load the syscall XML file ’syscalls/i386-linux.xml’.
GDB will not be able to display syscall names.
Catchpoint 1 (syscall)
(gdb)
Of course, the file name will change depending on your architecture
and system.
Still using the example above, you can also try to catch a syscall
by its number. In this case, you would see something like:
(gdb) catch syscall 252
Catchpoint 1 (syscall(s) 252)
Again, in this case gdb would not be able to display syscall’s names.
fork
A call to fork.
vfork
A call to vfork.
load [regexp]
unload [regexp]
The loading or unloading of a shared library. If regexp is given,
then the catchpoint will stop only if the regular expression matches
one of the affected libraries.
signal [signal... | ‘all’]
The delivery of a signal.
With no arguments, this catchpoint will catch any signal that is not
used internally by gdb, specifically, all signals except ‘SIGTRAP’ and
‘SIGINT’.
With the argument ‘all’, all signals, including those used by gdb,
will be caught. This argument cannot be used with other signal
names.
Chapter 5: Stopping and Continuing
59
Otherwise, the arguments are a list of signal names as given to
handle (see Section 5.4 [Signals], page 74). Only signals specified
in this list will be caught.
One reason that catch signal can be more useful than handle is
that you can attach commands and conditions to the catchpoint.
When a signal is caught by a catchpoint, the signal’s stop and
print settings, as specified by handle, are ignored. However,
whether the signal is still delivered to the inferior depends on the
pass setting; this can be changed in the catchpoint’s commands.
tcatch event
Set a catchpoint that is enabled only for one stop. The catchpoint is automatically deleted after the first time the event is caught.
Use the info break command to list the current catchpoints.
5.1.4 Deleting Breakpoints
It is often necessary to eliminate a breakpoint, watchpoint, or catchpoint once it has done
its job and you no longer want your program to stop there. This is called deleting the
breakpoint. A breakpoint that has been deleted no longer exists; it is forgotten.
With the clear command you can delete breakpoints according to where they are in your
program. With the delete command you can delete individual breakpoints, watchpoints,
or catchpoints by specifying their breakpoint numbers.
It is not necessary to delete a breakpoint to proceed past it. gdb automatically ignores
breakpoints on the first instruction to be executed when you continue execution without
changing the execution address.
clear
Delete any breakpoints at the next instruction to be executed in the selected
stack frame (see Section 8.3 [Selecting a Frame], page 98). When the innermost
frame is selected, this is a good way to delete a breakpoint where your program
just stopped.
clear location
Delete any breakpoints set at the specified location. See Section 9.2 [Specify
Location], page 104, for the various forms of location; the most useful ones are
listed below:
clear function
clear filename:function
Delete any breakpoints set at entry to the named function.
clear linenum
clear filename:linenum
Delete any breakpoints set at or within the code of the specified
linenum of the specified filename.
delete [breakpoints] [list...]
Delete the breakpoints, watchpoints, or catchpoints of the breakpoint list specified as argument. If no argument is specified, delete all breakpoints (gdb
asks confirmation, unless you have set confirm off). You can abbreviate this
command as d.
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Debugging with gdb
5.1.5 Disabling Breakpoints
Rather than deleting a breakpoint, watchpoint, or catchpoint, you might prefer to disable
it. This makes the breakpoint inoperative as if it had been deleted, but remembers the
information on the breakpoint so that you can enable it again later.
You disable and enable breakpoints, watchpoints, and catchpoints with the enable and
disable commands, optionally specifying one or more breakpoint numbers as arguments.
Use info break to print a list of all breakpoints, watchpoints, and catchpoints if you do
not know which numbers to use.
Disabling and enabling a breakpoint that has multiple locations affects all of its locations.
A breakpoint, watchpoint, or catchpoint can have any of several different states of enablement:
• Enabled. The breakpoint stops your program. A breakpoint set with the break command starts out in this state.
• Disabled. The breakpoint has no effect on your program.
• Enabled once. The breakpoint stops your program, but then becomes disabled.
• Enabled for a count. The breakpoint stops your program for the next N times, then
becomes disabled.
• Enabled for deletion. The breakpoint stops your program, but immediately after it
does so it is deleted permanently. A breakpoint set with the tbreak command starts
out in this state.
You can use the following commands to enable or disable breakpoints, watchpoints, and
catchpoints:
disable [breakpoints] [list...]
Disable the specified breakpoints—or all breakpoints, if none are listed. A
disabled breakpoint has no effect but is not forgotten. All options such as
ignore-counts, conditions and commands are remembered in case the breakpoint
is enabled again later. You may abbreviate disable as dis.
enable [breakpoints] [list...]
Enable the specified breakpoints (or all defined breakpoints). They become
effective once again in stopping your program.
enable [breakpoints] once list...
Enable the specified breakpoints temporarily. gdb disables any of these breakpoints immediately after stopping your program.
enable [breakpoints] count count list...
Enable the specified breakpoints temporarily. gdb records count with each of
the specified breakpoints, and decrements a breakpoint’s count when it is hit.
When any count reaches 0, gdb disables that breakpoint. If a breakpoint has
an ignore count (see Section 5.1.6 [Break Conditions], page 61), that will be
decremented to 0 before count is affected.
enable [breakpoints] delete list...
Enable the specified breakpoints to work once, then die. gdb deletes any of
these breakpoints as soon as your program stops there. Breakpoints set by the
tbreak command start out in this state.
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Except for a breakpoint set with tbreak (see Section 5.1.1 [Setting Breakpoints],
page 46), breakpoints that you set are initially enabled; subsequently, they become
disabled or enabled only when you use one of the commands above. (The command until
can set and delete a breakpoint of its own, but it does not change the state of your other
breakpoints; see Section 5.2 [Continuing and Stepping], page 68.)
5.1.6 Break Conditions
The simplest sort of breakpoint breaks every time your program reaches a specified place.
You can also specify a condition for a breakpoint. A condition is just a Boolean expression in
your programming language (see Section 10.1 [Expressions], page 117). A breakpoint with
a condition evaluates the expression each time your program reaches it, and your program
stops only if the condition is true.
This is the converse of using assertions for program validation; in that situation, you
want to stop when the assertion is violated—that is, when the condition is false. In C, if
you want to test an assertion expressed by the condition assert, you should set the condition
‘! assert’ on the appropriate breakpoint.
Conditions are also accepted for watchpoints; you may not need them, since a watchpoint
is inspecting the value of an expression anyhow—but it might be simpler, say, to just set a
watchpoint on a variable name, and specify a condition that tests whether the new value is
an interesting one.
Break conditions can have side effects, and may even call functions in your program. This
can be useful, for example, to activate functions that log program progress, or to use your
own print functions to format special data structures. The effects are completely predictable
unless there is another enabled breakpoint at the same address. (In that case, gdb might
see the other breakpoint first and stop your program without checking the condition of
this one.) Note that breakpoint commands are usually more convenient and flexible than
break conditions for the purpose of performing side effects when a breakpoint is reached
(see Section 5.1.7 [Breakpoint Command Lists], page 62).
Breakpoint conditions can also be evaluated on the target’s side if the target supports
it. Instead of evaluating the conditions locally, gdb encodes the expression into an agent
expression (see Appendix F [Agent Expressions], page 689) suitable for execution on the
target, independently of gdb. Global variables become raw memory locations, locals become
stack accesses, and so forth.
In this case, gdb will only be notified of a breakpoint trigger when its condition evaluates
to true. This mechanism may provide faster response times depending on the performance
characteristics of the target since it does not need to keep gdb informed about every breakpoint trigger, even those with false conditions.
Break conditions can be specified when a breakpoint is set, by using ‘if’ in the arguments
to the break command. See Section 5.1.1 [Setting Breakpoints], page 46. They can also be
changed at any time with the condition command.
You can also use the if keyword with the watch command. The catch command does
not recognize the if keyword; condition is the only way to impose a further condition on
a catchpoint.
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Debugging with gdb
condition bnum expression
Specify expression as the break condition for breakpoint, watchpoint, or catchpoint number bnum. After you set a condition, breakpoint bnum stops your
program only if the value of expression is true (nonzero, in C). When you
use condition, gdb checks expression immediately for syntactic correctness,
and to determine whether symbols in it have referents in the context of your
breakpoint. If expression uses symbols not referenced in the context of the
breakpoint, gdb prints an error message:
No symbol "foo" in current context.
gdb does not actually evaluate expression at the time the condition command
(or a command that sets a breakpoint with a condition, like break if ...) is
given, however. See Section 10.1 [Expressions], page 117.
condition bnum
Remove the condition from breakpoint number bnum. It becomes an ordinary
unconditional breakpoint.
A special case of a breakpoint condition is to stop only when the breakpoint has been
reached a certain number of times. This is so useful that there is a special way to do it,
using the ignore count of the breakpoint. Every breakpoint has an ignore count, which is
an integer. Most of the time, the ignore count is zero, and therefore has no effect. But if
your program reaches a breakpoint whose ignore count is positive, then instead of stopping,
it just decrements the ignore count by one and continues. As a result, if the ignore count
value is n, the breakpoint does not stop the next n times your program reaches it.
ignore bnum count
Set the ignore count of breakpoint number bnum to count. The next count
times the breakpoint is reached, your program’s execution does not stop; other
than to decrement the ignore count, gdb takes no action.
To make the breakpoint stop the next time it is reached, specify a count of zero.
When you use continue to resume execution of your program from a breakpoint, you can specify an ignore count directly as an argument to continue,
rather than using ignore. See Section 5.2 [Continuing and Stepping], page 68.
If a breakpoint has a positive ignore count and a condition, the condition is
not checked. Once the ignore count reaches zero, gdb resumes checking the
condition.
You could achieve the effect of the ignore count with a condition such as
‘$foo-- <= 0’ using a debugger convenience variable that is decremented each
time. See Section 10.11 [Convenience Variables], page 139.
Ignore counts apply to breakpoints, watchpoints, and catchpoints.
5.1.7 Breakpoint Command Lists
You can give any breakpoint (or watchpoint or catchpoint) a series of commands to execute
when your program stops due to that breakpoint. For example, you might want to print
the values of certain expressions, or enable other breakpoints.
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commands [list...]
... command-list ...
end
Specify a list of commands for the given breakpoints. The commands themselves
appear on the following lines. Type a line containing just end to terminate the
commands.
To remove all commands from a breakpoint, type commands and follow it immediately with end; that is, give no commands.
With no argument, commands refers to the last breakpoint, watchpoint, or catchpoint set (not to the breakpoint most recently encountered). If the most recent
breakpoints were set with a single command, then the commands will apply
to all the breakpoints set by that command. This applies to breakpoints set
by rbreak, and also applies when a single break command creates multiple
breakpoints (see Section 10.2 [Ambiguous Expressions], page 118).
Pressing RET as a means of repeating the last gdb command is disabled within a
command-list.
You can use breakpoint commands to start your program up again. Simply use the
continue command, or step, or any other command that resumes execution.
Any other commands in the command list, after a command that resumes execution, are
ignored. This is because any time you resume execution (even with a simple next or step),
you may encounter another breakpoint—which could have its own command list, leading
to ambiguities about which list to execute.
If the first command you specify in a command list is silent, the usual message about
stopping at a breakpoint is not printed. This may be desirable for breakpoints that are
to print a specific message and then continue. If none of the remaining commands print
anything, you see no sign that the breakpoint was reached. silent is meaningful only at
the beginning of a breakpoint command list.
The commands echo, output, and printf allow you to print precisely controlled output,
and are often useful in silent breakpoints. See Section 23.1.4 [Commands for Controlled
Output], page 326.
For example, here is how you could use breakpoint commands to print the value of x at
entry to foo whenever x is positive.
break foo if x>0
commands
silent
printf "x is %d\n",x
cont
end
One application for breakpoint commands is to compensate for one bug so you can test
for another. Put a breakpoint just after the erroneous line of code, give it a condition
to detect the case in which something erroneous has been done, and give it commands to
assign correct values to any variables that need them. End with the continue command so
that your program does not stop, and start with the silent command so that no output
is produced. Here is an example:
break 403
commands
silent
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Debugging with gdb
set x = y + 4
cont
end
5.1.8 Dynamic Printf
The dynamic printf command dprintf combines a breakpoint with formatted printing of
your program’s data to give you the effect of inserting printf calls into your program
on-the-fly, without having to recompile it.
In its most basic form, the output goes to the GDB console. However, you can set
the variable dprintf-style for alternate handling. For instance, you can ask to format
the output by calling your program’s printf function. This has the advantage that the
characters go to the program’s output device, so they can recorded in redirects to files and
so forth.
If you are doing remote debugging with a stub or agent, you can also ask to have the
printf handled by the remote agent. In addition to ensuring that the output goes to the
remote program’s device along with any other output the program might produce, you can
also ask that the dprintf remain active even after disconnecting from the remote target.
Using the stub/agent is also more efficient, as it can do everything without needing to
communicate with gdb.
dprintf location,template,expression[,expression...]
Whenever execution reaches location, print the values of one or more expressions
under the control of the string template. To print several values, separate them
with commas.
set dprintf-style style
Set the dprintf output to be handled in one of several different styles enumerated
below. A change of style affects all existing dynamic printfs immediately. (If
you need individual control over the print commands, simply define normal
breakpoints with explicitly-supplied command lists.)
gdb
Handle the output using the gdb printf command.
call
Handle the output by calling a function in your program (normally
printf).
agent
Have the remote debugging agent (such as gdbserver) handle the
output itself. This style is only available for agents that support
running commands on the target.
set dprintf-function function
Set the function to call if the dprintf style is call. By default its value is
printf. You may set it to any expression. that gdb can evaluate to a function,
as per the call command.
set dprintf-channel channel
Set a “channel” for dprintf. If set to a non-empty value, gdb will evaluate it as
an expression and pass the result as a first argument to the dprintf-function,
in the manner of fprintf and similar functions. Otherwise, the dprintf format
string will be the first argument, in the manner of printf.
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As an example, if you wanted dprintf output to go to a logfile that is a standard
I/O stream assigned to the variable mylog, you could do the following:
(gdb) set dprintf-style call
(gdb) set dprintf-function fprintf
(gdb) set dprintf-channel mylog
(gdb) dprintf 25,"at line 25, glob=%d\n",glob
Dprintf 1 at 0x123456: file main.c, line 25.
(gdb) info break
1
dprintf
keep y
0x00123456 in main at main.c:25
call (void) fprintf (mylog,"at line 25, glob=%d\n",glob)
continue
(gdb)
Note that the info break displays the dynamic printf commands as normal
breakpoint commands; you can thus easily see the effect of the variable settings.
set disconnected-dprintf on
set disconnected-dprintf off
Choose whether dprintf commands should continue to run if gdb has disconnected from the target. This only applies if the dprintf-style is agent.
show disconnected-dprintf off
Show the current choice for disconnected dprintf.
gdb does not check the validity of function and channel, relying on you to supply values
that are meaningful for the contexts in which they are being used. For instance, the function
and channel may be the values of local variables, but if that is the case, then all enabled
dynamic prints must be at locations within the scope of those locals. If evaluation fails,
gdb will report an error.
5.1.9 How to save breakpoints to a file
To save breakpoint definitions to a file use the save breakpoints command.
save breakpoints [filename]
This command saves all current breakpoint definitions together with their commands and ignore counts, into a file ‘filename’ suitable for use in a later debugging session. This includes all types of breakpoints (breakpoints, watchpoints,
catchpoints, tracepoints). To read the saved breakpoint definitions, use the
source command (see Section 23.1.3 [Command Files], page 324). Note that
watchpoints with expressions involving local variables may fail to be recreated
because it may not be possible to access the context where the watchpoint is
valid anymore. Because the saved breakpoint definitions are simply a sequence
of gdb commands that recreate the breakpoints, you can edit the file in your
favorite editing program, and remove the breakpoint definitions you’re not interested in, or that can no longer be recreated.
5.1.10 Static Probe Points
gdb supports SDT probes in the code. SDT stands for Statically Defined Tracing, and
the probes are designed to have a tiny runtime code and data footprint, and no dynamic
relocations.
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Debugging with gdb
Currently, the following types of probes are supported on ELF-compatible systems:
• SystemTap (http://sourceware.org/systemtap/) SDT probes1 . SystemTap probes
are usable from assembly, C and C++ languages2 .
• DTrace (http://oss.oracle.com/projects/DTrace) USDT probes. DTrace probes
are usable from C and C++ languages.
Some SystemTap probes have an associated semaphore variable; for instance, this happens automatically if you defined your probe using a DTrace-style ‘.d’ file. If your probe
has a semaphore, gdb will automatically enable it when you specify a breakpoint using
the ‘-probe-stap’ notation. But, if you put a breakpoint at a probe’s location by some
other method (e.g., break file:line), then gdb will not automatically set the semaphore.
DTrace probes do not support semaphores.
You can examine the available static static probes using info probes, with optional
arguments:
info probes [type] [provider [name [objfile]]]
If given, type is either stap for listing SystemTap probes or dtrace for listing
DTrace probes. If omitted all probes are listed regardless of their types.
If given, provider is a regular expression used to match against provider names
when selecting which probes to list. If omitted, probes by all probes from all
providers are listed.
If given, name is a regular expression to match against probe names when
selecting which probes to list. If omitted, probe names are not considered when
deciding whether to display them.
If given, objfile is a regular expression used to select which object files (executable or shared libraries) to examine. If not given, all object files are considered.
info probes all
List the available static probes, from all types.
Some probe points can be enabled and/or disabled. The effect of enabling or disabling
a probe depends on the type of probe being handled. Some DTrace probes can be enabled
or disabled, but SystemTap probes cannot be disabled.
You can enable (or disable) one or more probes using the following commands, with
optional arguments:
enable probes [provider [name [objfile]]]
If given, provider is a regular expression used to match against provider names
when selecting which probes to enable. If omitted, all probes from all providers
are enabled.
If given, name is a regular expression to match against probe names when
selecting which probes to enable. If omitted, probe names are not considered
when deciding whether to enable them.
1
2
See http://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps for more information
on how to add SystemTap SDT probes in your applications.
See http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation for a good reference on
how the SDT probes are implemented.
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If given, objfile is a regular expression used to select which object files (executable or shared libraries) to examine. If not given, all object files are considered.
disable probes [provider [name [objfile]]]
See the enable probes command above for a description of the optional arguments accepted by this command.
A probe may specify up to twelve arguments. These are available at the point at which
the probe is defined—that is, when the current PC is at the probe’s location. The arguments are available using the convenience variables (see Section 10.11 [Convenience Vars],
page 139) $_probe_arg0. . . $_probe_arg11. In SystemTap probes each probe argument is
an integer of the appropriate size; types are not preserved. In DTrace probes types are
preserved provided that they are recognized as such by gdb; otherwise the value of the
probe argument will be a long integer. The convenience variable $_probe_argc holds the
number of arguments at the current probe point.
These variables are always available, but attempts to access them at any location other
than a probe point will cause gdb to give an error message.
5.1.11 “Cannot insert breakpoints”
If you request too many active hardware-assisted breakpoints and watchpoints, you will see
this error message:
Stopped; cannot insert breakpoints.
You may have requested too many hardware breakpoints and watchpoints.
This message is printed when you attempt to resume the program, since only then gdb
knows exactly how many hardware breakpoints and watchpoints it needs to insert.
When this message is printed, you need to disable or remove some of the hardwareassisted breakpoints and watchpoints, and then continue.
5.1.12 “Breakpoint address adjusted...”
Some processor architectures place constraints on the addresses at which breakpoints may
be placed. For architectures thus constrained, gdb will attempt to adjust the breakpoint’s
address to comply with the constraints dictated by the architecture.
One example of such an architecture is the Fujitsu FR-V. The FR-V is a VLIW architecture in which a number of RISC-like instructions may be bundled together for parallel
execution. The FR-V architecture constrains the location of a breakpoint instruction within
such a bundle to the instruction with the lowest address. gdb honors this constraint by
adjusting a breakpoint’s address to the first in the bundle.
It is not uncommon for optimized code to have bundles which contain instructions from
different source statements, thus it may happen that a breakpoint’s address will be adjusted
from one source statement to another. Since this adjustment may significantly alter gdb’s
breakpoint related behavior from what the user expects, a warning is printed when the
breakpoint is first set and also when the breakpoint is hit.
A warning like the one below is printed when setting a breakpoint that’s been subject
to address adjustment:
warning: Breakpoint address adjusted from 0x00010414 to 0x00010410.
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Debugging with gdb
Such warnings are printed both for user settable and gdb’s internal breakpoints. If you
see one of these warnings, you should verify that a breakpoint set at the adjusted address
will have the desired affect. If not, the breakpoint in question may be removed and other
breakpoints may be set which will have the desired behavior. E.g., it may be sufficient to
place the breakpoint at a later instruction. A conditional breakpoint may also be useful in
some cases to prevent the breakpoint from triggering too often.
gdb will also issue a warning when stopping at one of these adjusted breakpoints:
warning: Breakpoint 1 address previously adjusted from 0x00010414
to 0x00010410.
When this warning is encountered, it may be too late to take remedial action except in
cases where the breakpoint is hit earlier or more frequently than expected.
5.2 Continuing and Stepping
Continuing means resuming program execution until your program completes normally. In
contrast, stepping means executing just one more “step” of your program, where “step”
may mean either one line of source code, or one machine instruction (depending on what
particular command you use). Either when continuing or when stepping, your program may
stop even sooner, due to a breakpoint or a signal. (If it stops due to a signal, you may want
to use handle, or use ‘signal 0’ to resume execution (see Section 5.4 [Signals], page 74),
or you may step into the signal’s handler (see [stepping and signal handlers], page 75).)
continue [ignore-count]
c [ignore-count]
fg [ignore-count]
Resume program execution, at the address where your program last stopped;
any breakpoints set at that address are bypassed. The optional argument
ignore-count allows you to specify a further number of times to ignore a breakpoint at this location; its effect is like that of ignore (see Section 5.1.6 [Break
Conditions], page 61).
The argument ignore-count is meaningful only when your program stopped due
to a breakpoint. At other times, the argument to continue is ignored.
The synonyms c and fg (for foreground, as the debugged program is deemed
to be the foreground program) are provided purely for convenience, and have
exactly the same behavior as continue.
To resume execution at a different place, you can use return (see Section 17.4 [Returning
from a Function], page 232) to go back to the calling function; or jump (see Section 17.2
[Continuing at a Different Address], page 230) to go to an arbitrary location in your program.
A typical technique for using stepping is to set a breakpoint (see Section 5.1 [Breakpoints;
Watchpoints; and Catchpoints], page 45) at the beginning of the function or the section
of your program where a problem is believed to lie, run your program until it stops at
that breakpoint, and then step through the suspect area, examining the variables that are
interesting, until you see the problem happen.
step
Continue running your program until control reaches a different source line,
then stop it and return control to gdb. This command is abbreviated s.
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Warning: If you use the step command while control is within
a function that was compiled without debugging information, execution proceeds until control reaches a function that does have
debugging information. Likewise, it will not step into a function
which is compiled without debugging information. To step through
functions without debugging information, use the stepi command,
described below.
The step command only stops at the first instruction of a source line. This prevents the multiple stops that could otherwise occur in switch statements, for
loops, etc. step continues to stop if a function that has debugging information
is called within the line. In other words, step steps inside any functions called
within the line.
Also, the step command only enters a function if there is line number information for the function. Otherwise it acts like the next command. This avoids
problems when using cc -gl on MIPS machines. Previously, step entered subroutines if there was any debugging information about the routine.
step count
Continue running as in step, but do so count times. If a breakpoint is reached,
or a signal not related to stepping occurs before count steps, stepping stops
right away.
next [count]
Continue to the next source line in the current (innermost) stack frame. This
is similar to step, but function calls that appear within the line of code are
executed without stopping. Execution stops when control reaches a different
line of code at the original stack level that was executing when you gave the
next command. This command is abbreviated n.
An argument count is a repeat count, as for step.
The next command only stops at the first instruction of a source line. This
prevents multiple stops that could otherwise occur in switch statements, for
loops, etc.
set step-mode
set step-mode on
The set step-mode on command causes the step command to stop at the first
instruction of a function which contains no debug line information rather than
stepping over it.
This is useful in cases where you may be interested in inspecting the machine
instructions of a function which has no symbolic info and do not want gdb to
automatically skip over this function.
set step-mode off
Causes the step command to step over any functions which contains no debug
information. This is the default.
show step-mode
Show whether gdb will stop in or step over functions without source line debug
information.
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finish
until
u
Debugging with gdb
Continue running until just after function in the selected stack frame returns.
Print the returned value (if any). This command can be abbreviated as fin.
Contrast this with the return command (see Section 17.4 [Returning from a
Function], page 232).
Continue running until a source line past the current line, in the current stack
frame, is reached. This command is used to avoid single stepping through a loop
more than once. It is like the next command, except that when until encounters a jump, it automatically continues execution until the program counter is
greater than the address of the jump.
This means that when you reach the end of a loop after single stepping though
it, until makes your program continue execution until it exits the loop. In contrast, a next command at the end of a loop simply steps back to the beginning
of the loop, which forces you to step through the next iteration.
until always stops your program if it attempts to exit the current stack frame.
until may produce somewhat counterintuitive results if the order of machine
code does not match the order of the source lines. For example, in the following
excerpt from a debugging session, the f (frame) command shows that execution
is stopped at line 206; yet when we use until, we get to line 195:
(gdb) f
#0 main (argc=4, argv=0xf7fffae8) at m4.c:206
206
expand_input();
(gdb) until
195
for ( ; argc > 0; NEXTARG) {
This happened because, for execution efficiency, the compiler had generated
code for the loop closure test at the end, rather than the start, of the loop—
even though the test in a C for-loop is written before the body of the loop.
The until command appeared to step back to the beginning of the loop when
it advanced to this expression; however, it has not really gone to an earlier
statement—not in terms of the actual machine code.
until with no argument works by means of single instruction stepping, and
hence is slower than until with an argument.
until location
u location
Continue running your program until either the specified location is reached,
or the current stack frame returns. The location is any of the forms described
in Section 9.2 [Specify Location], page 104. This form of the command uses
temporary breakpoints, and hence is quicker than until without an argument.
The specified location is actually reached only if it is in the current frame. This
implies that until can be used to skip over recursive function invocations. For
instance in the code below, if the current location is line 96, issuing until 99
will execute the program up to line 99 in the same invocation of factorial, i.e.,
after the inner invocations have returned.
94 int factorial (int value)
95 {
96
if (value > 1) {
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98
99
100
71
value *= factorial (value - 1);
}
return (value);
}
advance location
Continue running the program up to the given location. An argument is required, which should be of one of the forms described in Section 9.2 [Specify
Location], page 104. Execution will also stop upon exit from the current stack
frame. This command is similar to until, but advance will not skip over recursive function calls, and the target location doesn’t have to be in the same
frame as the current one.
stepi
stepi arg
si
Execute one machine instruction, then stop and return to the debugger.
It is often useful to do ‘display/i $pc’ when stepping by machine instructions.
This makes gdb automatically display the next instruction to be executed, each
time your program stops. See Section 10.7 [Automatic Display], page 126.
An argument is a repeat count, as in step.
nexti
nexti arg
ni
Execute one machine instruction, but if it is a function call, proceed until the
function returns.
An argument is a repeat count, as in next.
By default, and if available, gdb makes use of target-assisted range stepping. In other
words, whenever you use a stepping command (e.g., step, next), gdb tells the target to
step the corresponding range of instruction addresses instead of issuing multiple single-steps.
This speeds up line stepping, particularly for remote targets. Ideally, there should be no
reason you would want to turn range stepping off. However, it’s possible that a bug in the
debug info, a bug in the remote stub (for remote targets), or even a bug in gdb could make
line stepping behave incorrectly when target-assisted range stepping is enabled. You can
use the following command to turn off range stepping if necessary:
set range-stepping
show range-stepping
Control whether range stepping is enabled.
If on, and the target supports it, gdb tells the target to step a range of addresses
itself, instead of issuing multiple single-steps. If off, gdb always issues singlesteps, even if range stepping is supported by the target. The default is on.
5.3 Skipping Over Functions and Files
The program you are debugging may contain some functions which are uninteresting to
debug. The skip command lets you tell gdb to skip a function, all functions in a file or a
particular function in a particular file when stepping.
For example, consider the following C function:
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Debugging with gdb
101
102
103
104
105
int func()
{
foo(boring());
bar(boring());
}
Suppose you wish to step into the functions foo and bar, but you are not interested in
stepping through boring. If you run step at line 103, you’ll enter boring(), but if you run
next, you’ll step over both foo and boring!
One solution is to step into boring and use the finish command to immediately exit
it. But this can become tedious if boring is called from many places.
A more flexible solution is to execute skip boring. This instructs gdb never to step
into boring. Now when you execute step at line 103, you’ll step over boring and directly
into foo.
Functions may be skipped by providing either a function name, linespec (see Section 9.2
[Specify Location], page 104), regular expression that matches the function’s name, file
name or a glob-style pattern that matches the file name.
On Posix systems the form of the regular expression is “Extended Regular Expressions”.
See for example ‘man 7 regex’ on gnu/Linux systems. On non-Posix systems the form of
the regular expression is whatever is provided by the regcomp function of the underlying
system. See for example ‘man 7 glob’ on gnu/Linux systems for a description of glob-style
patterns.
skip [options]
The basic form of the skip command takes zero or more options that specify
what to skip. The options argument is any useful combination of the following:
-file file
-fi file Functions in file will be skipped over when stepping.
-gfile file-glob-pattern
-gfi file-glob-pattern
Functions in files matching file-glob-pattern will be skipped over
when stepping.
(gdb) skip -gfi utils/*.c
-function linespec
-fu linespec
Functions named by linespec or the function containing the line
named by linespec will be skipped over when stepping. See
Section 9.2 [Specify Location], page 104.
-rfunction regexp
-rfu regexp
Functions whose name matches regexp will be skipped over when
stepping.
This form is useful for complex function names. For example, there
is generally no need to step into C++ std::string constructors or
destructors. Plus with C++ templates it can be hard to write out
the full name of the function, and often it doesn’t matter what the
Chapter 5: Stopping and Continuing
73
template arguments are. Specifying the function to be skipped as
a regular expression makes this easier.
(gdb) skip -rfu ^std::(allocator|basic_string)<.*>::~?\1 *\(
If you want to skip every templated C++ constructor and destructor
in the std namespace you can do:
(gdb) skip -rfu ^std::([a-zA-z0-9_]+)<.*>::~?\1 *\(
If no options are specified, the function you’re currently debugging will be
skipped.
skip function [linespec]
After running this command, the function named by linespec or the function
containing the line named by linespec will be skipped over when stepping. See
Section 9.2 [Specify Location], page 104.
If you do not specify linespec, the function you’re currently debugging will be
skipped.
(If you have a function called file that you want to skip, use skip function
file.)
skip file [filename]
After running this command, any function whose source lives in filename will
be skipped over when stepping.
(gdb) skip file boring.c
File boring.c will be skipped when stepping.
If you do not specify filename, functions whose source lives in the file you’re
currently debugging will be skipped.
Skips can be listed, deleted, disabled, and enabled, much like breakpoints. These are
the commands for managing your list of skips:
info skip [range]
Print details about the specified skip(s). If range is not specified, print a table
with details about all functions and files marked for skipping. info skip prints
the following information about each skip:
Identifier
A number identifying this skip.
Enabled or Disabled
Enabled skips are marked with ‘y’. Disabled skips are marked with
‘n’.
Glob
If the file name is a ‘glob’ pattern this is ‘y’. Otherwise it is ‘n’.
File
The name or ‘glob’ pattern of the file to be skipped. If no file is
specified this is ‘’.
RE
If the function name is a ‘regular expression’ this is ‘y’. Otherwise it is ‘n’.
Function
The name or regular expression of the function to skip. If no function is specified this is ‘’.
skip delete [range]
Delete the specified skip(s). If range is not specified, delete all skips.
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Debugging with gdb
skip enable [range]
Enable the specified skip(s). If range is not specified, enable all skips.
skip disable [range]
Disable the specified skip(s). If range is not specified, disable all skips.
5.4 Signals
A signal is an asynchronous event that can happen in a program. The operating system
defines the possible kinds of signals, and gives each kind a name and a number. For example,
in Unix SIGINT is the signal a program gets when you type an interrupt character (often
Ctrl-c); SIGSEGV is the signal a program gets from referencing a place in memory far
away from all the areas in use; SIGALRM occurs when the alarm clock timer goes off (which
happens only if your program has requested an alarm).
Some signals, including SIGALRM, are a normal part of the functioning of your program.
Others, such as SIGSEGV, indicate errors; these signals are fatal (they kill your program
immediately) if the program has not specified in advance some other way to handle the
signal. SIGINT does not indicate an error in your program, but it is normally fatal so it can
carry out the purpose of the interrupt: to kill the program.
gdb has the ability to detect any occurrence of a signal in your program. You can tell
gdb in advance what to do for each kind of signal.
Normally, gdb is set up to let the non-erroneous signals like SIGALRM be silently passed
to your program (so as not to interfere with their role in the program’s functioning) but to
stop your program immediately whenever an error signal happens. You can change these
settings with the handle command.
info signals
info handle
Print a table of all the kinds of signals and how gdb has been told to handle
each one. You can use this to see the signal numbers of all the defined types of
signals.
info signals sig
Similar, but print information only about the specified signal number.
info handle is an alias for info signals.
catch signal [signal... | ‘all’]
Set a catchpoint for the indicated signals. See Section 5.1.3 [Set Catchpoints],
page 54, for details about this command.
handle signal [keywords...]
Change the way gdb handles signal signal. The signal can be the number of a
signal or its name (with or without the ‘SIG’ at the beginning); a list of signal
numbers of the form ‘low-high’; or the word ‘all’, meaning all the known
signals. Optional arguments keywords, described below, say what change to
make.
The keywords allowed by the handle command can be abbreviated. Their full names
are:
Chapter 5: Stopping and Continuing
75
nostop
gdb should not stop your program when this signal happens. It may still print
a message telling you that the signal has come in.
stop
gdb should stop your program when this signal happens. This implies the
print keyword as well.
print
gdb should print a message when this signal happens.
noprint
gdb should not mention the occurrence of the signal at all. This implies the
nostop keyword as well.
pass
noignore
nopass
ignore
gdb should allow your program to see this signal; your program can handle the
signal, or else it may terminate if the signal is fatal and not handled. pass and
noignore are synonyms.
gdb should not allow your program to see this signal. nopass and ignore are
synonyms.
When a signal stops your program, the signal is not visible to the program until you
continue. Your program sees the signal then, if pass is in effect for the signal in question
at that time. In other words, after gdb reports a signal, you can use the handle command
with pass or nopass to control whether your program sees that signal when you continue.
The default is set to nostop, noprint, pass for non-erroneous signals such as SIGALRM,
SIGWINCH and SIGCHLD, and to stop, print, pass for the erroneous signals.
You can also use the signal command to prevent your program from seeing a signal, or
cause it to see a signal it normally would not see, or to give it any signal at any time. For
example, if your program stopped due to some sort of memory reference error, you might
store correct values into the erroneous variables and continue, hoping to see more execution;
but your program would probably terminate immediately as a result of the fatal signal once
it saw the signal. To prevent this, you can continue with ‘signal 0’. See Section 17.3
[Giving your Program a Signal], page 231.
gdb optimizes for stepping the mainline code. If a signal that has handle nostop and
handle pass set arrives while a stepping command (e.g., stepi, step, next) is in progress,
gdb lets the signal handler run and then resumes stepping the mainline code once the signal
handler returns. In other words, gdb steps over the signal handler. This prevents signals
that you’ve specified as not interesting (with handle nostop) from changing the focus of
debugging unexpectedly. Note that the signal handler itself may still hit a breakpoint, stop
for another signal that has handle stop in effect, or for any other event that normally
results in stopping the stepping command sooner. Also note that gdb still informs you that
the program received a signal if handle print is set.
If you set handle pass for a signal, and your program sets up a handler for it, then
issuing a stepping command, such as step or stepi, when your program is stopped due to
the signal will step into the signal handler (if the target supports that).
Likewise, if you use the queue-signal command to queue a signal to be delivered to
the current thread when execution of the thread resumes (see Section 17.3 [Giving your
Program a Signal], page 231), then a stepping command will step into the signal handler.
Here’s an example, using stepi to step to the first instruction of SIGUSR1’s handler:
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Debugging with gdb
(gdb) handle SIGUSR1
Signal
Stop
SIGUSR1
Yes
(gdb) c
Continuing.
Print
Yes
Pass to program Description
Yes
User defined signal 1
Program received signal SIGUSR1, User defined signal 1.
main () sigusr1.c:28
28
p = 0;
(gdb) si
sigusr1_handler () at sigusr1.c:9
9
{
The same, but using queue-signal instead of waiting for the program to receive the
signal first:
(gdb) n
28
p = 0;
(gdb) queue-signal SIGUSR1
(gdb) si
sigusr1_handler () at sigusr1.c:9
9
{
(gdb)
On some targets, gdb can inspect extra signal information associated with the intercepted signal, before it is actually delivered to the program being debugged. This information is exported by the convenience variable $_siginfo, and consists of data that is passed
by the kernel to the signal handler at the time of the receipt of a signal. The data type of
the information itself is target dependent. You can see the data type using the ptype $_
siginfo command. On Unix systems, it typically corresponds to the standard siginfo_t
type, as defined in the ‘signal.h’ system header.
Here’s an example, on a gnu/Linux system, printing the stray referenced address that
raised a segmentation fault.
(gdb) continue
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400766 in main ()
69
*(int *)p = 0;
(gdb) ptype $_siginfo
type = struct {
int si_signo;
int si_errno;
int si_code;
union {
int _pad[28];
struct {...} _kill;
struct {...} _timer;
struct {...} _rt;
struct {...} _sigchld;
struct {...} _sigfault;
struct {...} _sigpoll;
} _sifields;
}
(gdb) ptype $_siginfo._sifields._sigfault
type = struct {
void *si_addr;
}
(gdb) p $_siginfo._sifields._sigfault.si_addr
$1 = (void *) 0x7ffff7ff7000
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Depending on target support, $_siginfo may also be writable.
On some targets, a SIGSEGV can be caused by a boundary violation, i.e., accessing an address outside of the allowed range. In those cases gdb may displays additional information,
depending on how gdb has been told to handle the signal. With handle stop SIGSEGV,
gdb displays the violation kind: "Upper" or "Lower", the memory address accessed and
the bounds, while with handle nostop SIGSEGV no additional information is displayed.
The usual output of a segfault is:
Program received signal SIGSEGV, Segmentation fault
0x0000000000400d7c in upper () at i386-mpx-sigsegv.c:68
68
value = *(p + len);
While a bound violation is presented as:
Program received signal SIGSEGV, Segmentation fault
Upper bound violation while accessing address 0x7fffffffc3b3
Bounds: [lower = 0x7fffffffc390, upper = 0x7fffffffc3a3]
0x0000000000400d7c in upper () at i386-mpx-sigsegv.c:68
68
value = *(p + len);
5.5 Stopping and Starting Multi-thread Programs
gdb supports debugging programs with multiple threads (see Section 4.10 [Debugging Programs with Multiple Threads], page 36). There are two modes of controlling execution of
your program within the debugger. In the default mode, referred to as all-stop mode, when
any thread in your program stops (for example, at a breakpoint or while being stepped), all
other threads in the program are also stopped by gdb. On some targets, gdb also supports
non-stop mode, in which other threads can continue to run freely while you examine the
stopped thread in the debugger.
5.5.1 All-Stop Mode
In all-stop mode, whenever your program stops under gdb for any reason, all threads of
execution stop, not just the current thread. This allows you to examine the overall state
of the program, including switching between threads, without worrying that things may
change underfoot.
Conversely, whenever you restart the program, all threads start executing. This is true
even when single-stepping with commands like step or next.
In particular, gdb cannot single-step all threads in lockstep. Since thread scheduling
is up to your debugging target’s operating system (not controlled by gdb), other threads
may execute more than one statement while the current thread completes a single step.
Moreover, in general other threads stop in the middle of a statement, rather than at a clean
statement boundary, when the program stops.
You might even find your program stopped in another thread after continuing or even
single-stepping. This happens whenever some other thread runs into a breakpoint, a signal,
or an exception before the first thread completes whatever you requested.
Whenever gdb stops your program, due to a breakpoint or a signal, it automatically
selects the thread where that breakpoint or signal happened. gdb alerts you to the context
switch with a message such as ‘[Switching to Thread n]’ to identify the thread.
On some OSes, you can modify gdb’s default behavior by locking the OS scheduler to
allow only a single thread to run.
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Debugging with gdb
set scheduler-locking mode
Set the scheduler locking mode. It applies to normal execution, record mode,
and replay mode. If it is off, then there is no locking and any thread may
run at any time. If on, then only the current thread may run when the inferior
is resumed. The step mode optimizes for single-stepping; it prevents other
threads from preempting the current thread while you are stepping, so that the
focus of debugging does not change unexpectedly. Other threads never get a
chance to run when you step, and they are completely free to run when you
use commands like ‘continue’, ‘until’, or ‘finish’. However, unless another
thread hits a breakpoint during its timeslice, gdb does not change the current
thread away from the thread that you are debugging. The replay mode behaves
like off in record mode and like on in replay mode.
show scheduler-locking
Display the current scheduler locking mode.
By default, when you issue one of the execution commands such as continue, next
or step, gdb allows only threads of the current inferior to run. For example, if gdb is
attached to two inferiors, each with two threads, the continue command resumes only the
two threads of the current inferior. This is useful, for example, when you debug a program
that forks and you want to hold the parent stopped (so that, for instance, it doesn’t run to
exit), while you debug the child. In other situations, you may not be interested in inspecting
the current state of any of the processes gdb is attached to, and you may want to resume
them all until some breakpoint is hit. In the latter case, you can instruct gdb to allow all
threads of all the inferiors to run with the set schedule-multiple command.
set schedule-multiple
Set the mode for allowing threads of multiple processes to be resumed when an
execution command is issued. When on, all threads of all processes are allowed
to run. When off, only the threads of the current process are resumed. The
default is off. The scheduler-locking mode takes precedence when set to
on, or while you are stepping and set to step.
show schedule-multiple
Display the current mode for resuming the execution of threads of multiple
processes.
5.5.2 Non-Stop Mode
For some multi-threaded targets, gdb supports an optional mode of operation in which
you can examine stopped program threads in the debugger while other threads continue to
execute freely. This minimizes intrusion when debugging live systems, such as programs
where some threads have real-time constraints or must continue to respond to external
events. This is referred to as non-stop mode.
In non-stop mode, when a thread stops to report a debugging event, only that thread is
stopped; gdb does not stop other threads as well, in contrast to the all-stop mode behavior.
Additionally, execution commands such as continue and step apply by default only to
the current thread in non-stop mode, rather than all threads as in all-stop mode. This
allows you to control threads explicitly in ways that are not possible in all-stop mode — for
Chapter 5: Stopping and Continuing
79
example, stepping one thread while allowing others to run freely, stepping one thread while
holding all others stopped, or stepping several threads independently and simultaneously.
To enter non-stop mode, use this sequence of commands before you run or attach to
your program:
# If using the CLI, pagination breaks non-stop.
set pagination off
# Finally, turn it on!
set non-stop on
You can use these commands to manipulate the non-stop mode setting:
set non-stop on
Enable selection of non-stop mode.
set non-stop off
Disable selection of non-stop mode.
show non-stop
Show the current non-stop enablement setting.
Note these commands only reflect whether non-stop mode is enabled, not whether the
currently-executing program is being run in non-stop mode. In particular, the set nonstop preference is only consulted when gdb starts or connects to the target program, and
it is generally not possible to switch modes once debugging has started. Furthermore, since
not all targets support non-stop mode, even when you have enabled non-stop mode, gdb
may still fall back to all-stop operation by default.
In non-stop mode, all execution commands apply only to the current thread by default.
That is, continue only continues one thread. To continue all threads, issue continue -a
or c -a.
You can use gdb’s background execution commands (see Section 5.5.3 [Background
Execution], page 79) to run some threads in the background while you continue to examine
or step others from gdb. The MI execution commands (see Section 27.15 [GDB/MI Program
Execution], page 502) are always executed asynchronously in non-stop mode.
Suspending execution is done with the interrupt command when running in the background, or Ctrl-c during foreground execution. In all-stop mode, this stops the whole
process; but in non-stop mode the interrupt applies only to the current thread. To stop the
whole program, use interrupt -a.
Other execution commands do not currently support the -a option.
In non-stop mode, when a thread stops, gdb doesn’t automatically make that thread
current, as it does in all-stop mode. This is because the thread stop notifications are
asynchronous with respect to gdb’s command interpreter, and it would be confusing if gdb
unexpectedly changed to a different thread just as you entered a command to operate on
the previously current thread.
5.5.3 Background Execution
gdb’s execution commands have two variants: the normal foreground (synchronous) behavior, and a background (asynchronous) behavior. In foreground execution, gdb waits for the
program to report that some thread has stopped before prompting for another command.
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Debugging with gdb
In background execution, gdb immediately gives a command prompt so that you can issue
other commands while your program runs.
If the target doesn’t support async mode, gdb issues an error message if you attempt
to use the background execution commands.
To specify background execution, add a & to the command. For example, the background
form of the continue command is continue&, or just c&. The execution commands that
accept background execution are:
run
See Section 4.2 [Starting your Program], page 26.
attach
See Section 4.7 [Debugging an Already-running Process], page 32.
step
See Section 5.2 [Continuing and Stepping], page 68.
stepi
See Section 5.2 [Continuing and Stepping], page 68.
next
See Section 5.2 [Continuing and Stepping], page 68.
nexti
See Section 5.2 [Continuing and Stepping], page 68.
continue
See Section 5.2 [Continuing and Stepping], page 68.
finish
See Section 5.2 [Continuing and Stepping], page 68.
until
See Section 5.2 [Continuing and Stepping], page 68.
Background execution is especially useful in conjunction with non-stop mode for debugging programs with multiple threads; see Section 5.5.2 [Non-Stop Mode], page 78. However,
you can also use these commands in the normal all-stop mode with the restriction that you
cannot issue another execution command until the previous one finishes. Examples of commands that are valid in all-stop mode while the program is running include help and info
break.
You can interrupt your program while it is running in the background by using the
interrupt command.
interrupt
interrupt -a
Suspend execution of the running program. In all-stop mode, interrupt stops
the whole process, but in non-stop mode, it stops only the current thread. To
stop the whole program in non-stop mode, use interrupt -a.
5.5.4 Thread-Specific Breakpoints
When your program has multiple threads (see Section 4.10 [Debugging Programs with
Multiple Threads], page 36), you can choose whether to set breakpoints on all threads, or
on a particular thread.
break location thread thread-id
break location thread thread-id if ...
location specifies source lines; there are several ways of writing them (see
Section 9.2 [Specify Location], page 104), but the effect is always to specify
some source line.
Use the qualifier ‘thread thread-id’ with a breakpoint command to specify
that you only want gdb to stop the program when a particular thread reaches
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this breakpoint. The thread-id specifier is one of the thread identifiers assigned
by gdb, shown in the first column of the ‘info threads’ display.
If you do not specify ‘thread thread-id’ when you set a breakpoint, the breakpoint applies to all threads of your program.
You can use the thread qualifier on conditional breakpoints as well; in this
case, place ‘thread thread-id’ before or after the breakpoint condition, like
this:
(gdb) break frik.c:13 thread 28 if bartab > lim
Thread-specific breakpoints are automatically deleted when gdb detects the corresponding thread is no longer in the thread list. For example:
(gdb) c
Thread-specific breakpoint 3 deleted - thread 28 no longer in the thread list.
There are several ways for a thread to disappear, such as a regular thread exit, but also
when you detach from the process with the detach command (see Section 4.7 [Debugging an
Already-running Process], page 32), or if gdb loses the remote connection (see Chapter 20
[Remote Debugging], page 261), etc. Note that with some targets, gdb is only able to
detect a thread has exited when the user explictly asks for the thread list with the info
threads command.
5.5.5 Interrupted System Calls
There is an unfortunate side effect when using gdb to debug multi-threaded programs. If
one thread stops for a breakpoint, or for some other reason, and another thread is blocked
in a system call, then the system call may return prematurely. This is a consequence
of the interaction between multiple threads and the signals that gdb uses to implement
breakpoints and other events that stop execution.
To handle this problem, your program should check the return value of each system call
and react appropriately. This is good programming style anyways.
For example, do not write code like this:
sleep (10);
The call to sleep will return early if a different thread stops at a breakpoint or for some
other reason.
Instead, write this:
int unslept = 10;
while (unslept > 0)
unslept = sleep (unslept);
A system call is allowed to return early, so the system is still conforming to its specification. But gdb does cause your multi-threaded program to behave differently than it would
without gdb.
Also, gdb uses internal breakpoints in the thread library to monitor certain events such
as thread creation and thread destruction. When such an event happens, a system call
in another thread may return prematurely, even though your program does not appear to
stop.
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Debugging with gdb
5.5.6 Observer Mode
If you want to build on non-stop mode and observe program behavior without any chance
of disruption by gdb, you can set variables to disable all of the debugger’s attempts to
modify state, whether by writing memory, inserting breakpoints, etc. These operate at a
low level, intercepting operations from all commands.
When all of these are set to off, then gdb is said to be observer mode. As a convenience,
the variable observer can be set to disable these, plus enable non-stop mode.
Note that gdb will not prevent you from making nonsensical combinations of these
settings. For instance, if you have enabled may-insert-breakpoints but disabled maywrite-memory, then breakpoints that work by writing trap instructions into the code stream
will still not be able to be placed.
set observer on
set observer off
When set to on, this disables all the permission variables below (except for
insert-fast-tracepoints), plus enables non-stop debugging. Setting this to
off switches back to normal debugging, though remaining in non-stop mode.
show observer
Show whether observer mode is on or off.
set may-write-registers on
set may-write-registers off
This controls whether gdb will attempt to alter the values of registers, such as
with assignment expressions in print, or the jump command. It defaults to on.
show may-write-registers
Show the current permission to write registers.
set may-write-memory on
set may-write-memory off
This controls whether gdb will attempt to alter the contents of memory, such
as with assignment expressions in print. It defaults to on.
show may-write-memory
Show the current permission to write memory.
set may-insert-breakpoints on
set may-insert-breakpoints off
This controls whether gdb will attempt to insert breakpoints. This affects all
breakpoints, including internal breakpoints defined by gdb. It defaults to on.
show may-insert-breakpoints
Show the current permission to insert breakpoints.
set may-insert-tracepoints on
set may-insert-tracepoints off
This controls whether gdb will attempt to insert (regular) tracepoints at the
beginning of a tracing experiment. It affects only non-fast tracepoints, fast tracepoints being under the control of may-insert-fast-tracepoints. It defaults
to on.
Chapter 5: Stopping and Continuing
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show may-insert-tracepoints
Show the current permission to insert tracepoints.
set may-insert-fast-tracepoints on
set may-insert-fast-tracepoints off
This controls whether gdb will attempt to insert fast tracepoints at the beginning of a tracing experiment. It affects only fast tracepoints, regular (non-fast)
tracepoints being under the control of may-insert-tracepoints. It defaults
to on.
show may-insert-fast-tracepoints
Show the current permission to insert fast tracepoints.
set may-interrupt on
set may-interrupt off
This controls whether gdb will attempt to interrupt or stop program execution.
When this variable is off, the interrupt command will have no effect, nor will
Ctrl-c. It defaults to on.
show may-interrupt
Show the current permission to interrupt or stop the program.
Chapter 6: Running programs backward
85
6 Running programs backward
When you are debugging a program, it is not unusual to realize that you have gone too far,
and some event of interest has already happened. If the target environment supports it,
gdb can allow you to “rewind” the program by running it backward.
A target environment that supports reverse execution should be able to “undo” the
changes in machine state that have taken place as the program was executing normally.
Variables, registers etc. should revert to their previous values. Obviously this requires a
great deal of sophistication on the part of the target environment; not all target environments can support reverse execution.
When a program is executed in reverse, the instructions that have most recently been
executed are “un-executed”, in reverse order. The program counter runs backward, following the previous thread of execution in reverse. As each instruction is “un-executed”, the
values of memory and/or registers that were changed by that instruction are reverted to
their previous states. After executing a piece of source code in reverse, all side effects of
that code should be “undone”, and all variables should be returned to their prior values1 .
If you are debugging in a target environment that supports reverse execution, gdb
provides the following commands.
reverse-continue [ignore-count]
rc [ignore-count]
Beginning at the point where your program last stopped, start executing in
reverse. Reverse execution will stop for breakpoints and synchronous exceptions
(signals), just like normal execution. Behavior of asynchronous signals depends
on the target environment.
reverse-step [count]
Run the program backward until control reaches the start of a different source
line; then stop it, and return control to gdb.
Like the step command, reverse-step will only stop at the beginning of a
source line. It “un-executes” the previously executed source line. If the previous source line included calls to debuggable functions, reverse-step will
step (backward) into the called function, stopping at the beginning of the last
statement in the called function (typically a return statement).
Also, as with the step command, if non-debuggable functions are called,
reverse-step will run thru them backward without stopping.
reverse-stepi [count]
Reverse-execute one machine instruction. Note that the instruction to be
reverse-executed is not the one pointed to by the program counter, but the
1
Note that some side effects are easier to undo than others. For instance, memory and registers are
relatively easy, but device I/O is hard. Some targets may be able undo things like device I/O, and some
may not.
The contract between gdb and the reverse executing target requires only that the target do something
reasonable when gdb tells it to execute backwards, and then report the results back to gdb. Whatever
the target reports back to gdb, gdb will report back to the user. gdb assumes that the memory and
registers that the target reports are in a consistant state, but gdb accepts whatever it is given.
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Debugging with gdb
instruction executed prior to that one. For instance, if the last instruction was
a jump, reverse-stepi will take you back from the destination of the jump to
the jump instruction itself.
reverse-next [count]
Run backward to the beginning of the previous line executed in the current
(innermost) stack frame. If the line contains function calls, they will be “unexecuted” without stopping. Starting from the first line of a function, reversenext will take you back to the caller of that function, before the function was
called, just as the normal next command would take you from the last line of
a function back to its return to its caller2 .
reverse-nexti [count]
Like nexti, reverse-nexti executes a single instruction in reverse, except
that called functions are “un-executed” atomically. That is, if the previously
executed instruction was a return from another function, reverse-nexti will
continue to execute in reverse until the call to that function (from the current
stack frame) is reached.
reverse-finish
Just as the finish command takes you to the point where the current function
returns, reverse-finish takes you to the point where it was called. Instead
of ending up at the end of the current function invocation, you end up at the
beginning.
set exec-direction
Set the direction of target execution.
set exec-direction reverse
gdb will perform all execution commands in reverse, until the exec-direction
mode is changed to “forward”. Affected commands include step, stepi,
next, nexti, continue, and finish. The return command cannot be used
in reverse mode.
set exec-direction forward
gdb will perform all execution commands in the normal fashion. This is the
default.
2
Unless the code is too heavily optimized.
Chapter 7: Recording Inferior’s Execution and Replaying It
87
7 Recording Inferior’s Execution and Replaying It
On some platforms, gdb provides a special process record and replay target that can record
a log of the process execution, and replay it later with both forward and reverse execution
commands.
When this target is in use, if the execution log includes the record for the next instruction,
gdb will debug in replay mode. In the replay mode, the inferior does not really execute
code instructions. Instead, all the events that normally happen during code execution are
taken from the execution log. While code is not really executed in replay mode, the values
of registers (including the program counter register) and the memory of the inferior are still
changed as they normally would. Their contents are taken from the execution log.
If the record for the next instruction is not in the execution log, gdb will debug in record
mode. In this mode, the inferior executes normally, and gdb records the execution log for
future replay.
The process record and replay target supports reverse execution (see Chapter 6 [Reverse
Execution], page 85), even if the platform on which the inferior runs does not. However,
the reverse execution is limited in this case by the range of the instructions recorded in the
execution log. In other words, reverse execution on platforms that don’t support it directly
can only be done in the replay mode.
When debugging in the reverse direction, gdb will work in replay mode as long as the
execution log includes the record for the previous instruction; otherwise, it will work in
record mode, if the platform supports reverse execution, or stop if not.
For architecture environments that support process record and replay, gdb provides the
following commands:
record method
This command starts the process record and replay target. The recording
method can be specified as parameter. Without a parameter the command
uses the full recording method. The following recording methods are available:
full
Full record/replay recording using gdb’s software record and replay implementation. This method allows replaying and reverse
execution.
btrace format
Hardware-supported instruction recording. This method does not
record data. Further, the data is collected in a ring buffer so old
data will be overwritten when the buffer is full. It allows limited
reverse execution. Variables and registers are not available during
reverse execution. In remote debugging, recording continues on
disconnect. Recorded data can be inspected after reconnecting.
The recording may be stopped using record stop.
The recording format can be specified as parameter. Without a parameter the command chooses the recording format. The following
recording formats are available:
88
Debugging with gdb
bts
Use the Branch Trace Store (BTS) recording format.
In this format, the processor stores a from/to record
for each executed branch in the btrace ring buffer.
pt
Use the Intel Processor Trace recording format. In this
format, the processor stores the execution trace in a
compressed form that is afterwards decoded by gdb.
The trace can be recorded with very low overhead. The
compressed trace format also allows small trace buffers
to already contain a big number of instructions compared to BTS.
Decoding the recorded execution trace, on the other
hand, is more expensive than decoding BTS trace. This
is mostly due to the increased number of instructions to
process. You should increase the buffer-size with care.
Not all recording formats may be available on all processors.
The process record and replay target can only debug a process that is already
running. Therefore, you need first to start the process with the run or start
commands, and then start the recording with the record method command.
Displaced stepping (see Appendix D [displaced stepping], page 609) will be
automatically disabled when process record and replay target is started. That’s
because the process record and replay target doesn’t support displaced stepping.
If the inferior is in the non-stop mode (see Section 5.5.2 [Non-Stop Mode],
page 78) or in the asynchronous execution mode (see Section 5.5.3 [Background
Execution], page 79), not all recording methods are available. The full recording method does not support these two modes.
record stop
Stop the process record and replay target. When process record and replay
target stops, the entire execution log will be deleted and the inferior will either
be terminated, or will remain in its final state.
When you stop the process record and replay target in record mode (at the
end of the execution log), the inferior will be stopped at the next instruction
that would have been recorded. In other words, if you record for a while and
then stop recording, the inferior process will be left in the same state as if the
recording never happened.
On the other hand, if the process record and replay target is stopped while in
replay mode (that is, not at the end of the execution log, but at some earlier
point), the inferior process will become “live” at that earlier state, and it will
then be possible to continue the usual “live” debugging of the process from that
state.
When the inferior process exits, or gdb detaches from it, process record and
replay target will automatically stop itself.
record goto
Go to a specific location in the execution log. There are several ways to specify
the location to go to:
Chapter 7: Recording Inferior’s Execution and Replaying It
89
record goto begin
record goto start
Go to the beginning of the execution log.
record goto end
Go to the end of the execution log.
record goto n
Go to instruction number n in the execution log.
record save filename
Save the execution log to a file ‘filename’.
Default filename is
‘gdb_record.process_id’, where process id is the process ID of the inferior.
This command may not be available for all recording methods.
record restore filename
Restore the execution log from a file ‘filename’. File must have been created
with record save.
set record full insn-number-max limit
set record full insn-number-max unlimited
Set the limit of instructions to be recorded for the full recording method.
Default value is 200000.
If limit is a positive number, then gdb will start deleting instructions from the
log once the number of the record instructions becomes greater than limit. For
every new recorded instruction, gdb will delete the earliest recorded instruction to keep the number of recorded instructions at the limit. (Since deleting
recorded instructions loses information, gdb lets you control what happens
when the limit is reached, by means of the stop-at-limit option, described
below.)
If limit is unlimited or zero, gdb will never delete recorded instructions from
the execution log. The number of recorded instructions is limited only by the
available memory.
show record full insn-number-max
Show the limit of instructions to be recorded with the full recording method.
set record full stop-at-limit
Control the behavior of the full recording method when the number of recorded
instructions reaches the limit. If ON (the default), gdb will stop when the limit
is reached for the first time and ask you whether you want to stop the inferior or
continue running it and recording the execution log. If you decide to continue
recording, each new recorded instruction will cause the oldest one to be deleted.
If this option is OFF, gdb will automatically delete the oldest record to make
room for each new one, without asking.
show record full stop-at-limit
Show the current setting of stop-at-limit.
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Debugging with gdb
set record full memory-query
Control the behavior when gdb is unable to record memory changes caused by
an instruction for the full recording method. If ON, gdb will query whether
to stop the inferior in that case.
If this option is OFF (the default), gdb will automatically ignore the effect of
such instructions on memory. Later, when gdb replays this execution log, it
will mark the log of this instruction as not accessible, and it will not affect the
replay results.
show record full memory-query
Show the current setting of memory-query.
The btrace record target does not trace data. As a convenience, when replaying, gdb reads read-only memory off the live program directly, assuming that
the addresses of the read-only areas don’t change. This for example makes it
possible to disassemble code while replaying, but not to print variables. In some
cases, being able to inspect variables might be useful. You can use the following
command for that:
set record btrace replay-memory-access
Control the behavior of the btrace recording method when accessing memory
during replay. If read-only (the default), gdb will only allow accesses to
read-only memory. If read-write, gdb will allow accesses to read-only and to
read-write memory. Beware that the accessed memory corresponds to the live
target and not necessarily to the current replay position.
show record btrace replay-memory-access
Show the current setting of replay-memory-access.
set record btrace bts buffer-size size
set record btrace bts buffer-size unlimited
Set the requested ring buffer size for branch tracing in BTS format. Default is
64KB.
If size is a positive number, then gdb will try to allocate a buffer of at least size
bytes for each new thread that uses the btrace recording method and the BTS
format. The actually obtained buffer size may differ from the requested size.
Use the info record command to see the actual buffer size for each thread that
uses the btrace recording method and the BTS format.
If limit is unlimited or zero, gdb will try to allocate a buffer of 4MB.
Bigger buffers mean longer traces. On the other hand, gdb will also need longer
to process the branch trace data before it can be used.
show record btrace bts buffer-size size
Show the current setting of the requested ring buffer size for branch tracing in
BTS format.
set record btrace
set record btrace
Set the
format.
pt buffer-size size
pt buffer-size unlimited
requested ring buffer size for branch tracing in Intel Processor Trace
Default is 16KB.
Chapter 7: Recording Inferior’s Execution and Replaying It
91
If size is a positive number, then gdb will try to allocate a buffer of at least
size bytes for each new thread that uses the btrace recording method and the
Intel Processor Trace format. The actually obtained buffer size may differ from
the requested size. Use the info record command to see the actual buffer size
for each thread.
If limit is unlimited or zero, gdb will try to allocate a buffer of 4MB.
Bigger buffers mean longer traces. On the other hand, gdb will also need longer
to process the branch trace data before it can be used.
show record btrace pt buffer-size size
Show the current setting of the requested ring buffer size for branch tracing in
Intel Processor Trace format.
info record
Show various statistics about the recording depending on the recording method:
full
For the full recording method, it shows the state of process record
and its in-memory execution log buffer, including:
• Whether in record mode or replay mode.
• Lowest recorded instruction number (counting from when the
current execution log started recording instructions).
• Highest recorded instruction number.
• Current instruction about to be replayed (if in replay mode).
• Number of instructions contained in the execution log.
• Maximum number of instructions that may be contained in the
execution log.
btrace
For
•
•
•
the btrace recording method, it shows:
Recording format.
Number of instructions that have been recorded.
Number of blocks of sequential control-flow formed by the
recorded instructions.
• Whether in record mode or replay mode.
For the bts recording format, it also shows:
• Size of the perf ring buffer.
For the pt recording format, it also shows:
• Size of the perf ring buffer.
record delete
When record target runs in replay mode (“in the past”), delete the subsequent
execution log and begin to record a new execution log starting from the current
address. This means you will abandon the previously recorded “future” and
begin recording a new “future”.
record instruction-history
Disassembles instructions from the recorded execution log. By default, ten
instructions are disassembled. This can be changed using the set record
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Debugging with gdb
instruction-history-size command. Instructions are printed in execution
order.
It can also print mixed source+disassembly if you specify the the /m or /s
modifier, and print the raw instructions in hex as well as in symbolic form by
specifying the /r modifier.
The current position marker is printed for the instruction at the current program
counter value. This instruction can appear multiple times in the trace and the
current position marker will be printed every time. To omit the current position
marker, specify the /p modifier.
To better align the printed instructions when the trace contains instructions
from more than one function, the function name may be omitted by specifying
the /f modifier.
Speculatively executed instructions are prefixed with ‘?’. This feature is not
available for all recording formats.
There are several ways to specify what part of the execution log to disassemble:
record instruction-history insn
Disassembles ten instructions starting from instruction number
insn.
record instruction-history insn, +/-n
Disassembles n instructions around instruction number insn. If n
is preceded with +, disassembles n instructions after instruction
number insn. If n is preceded with -, disassembles n instructions
before instruction number insn.
record instruction-history
Disassembles ten more instructions after the last disassembly.
record instruction-history Disassembles ten more instructions before the last disassembly.
record instruction-history begin, end
Disassembles instructions beginning with instruction number begin until instruction number end. The instruction number end is
included.
This command may not be available for all recording methods.
set record instruction-history-size size
set record instruction-history-size unlimited
Define how many instructions to disassemble in the record instructionhistory command. The default value is 10. A size of unlimited means
unlimited instructions.
show record instruction-history-size
Show how many instructions to disassemble in the record instructionhistory command.
record function-call-history
Prints the execution history at function granularity. It prints one line for each
sequence of instructions that belong to the same function giving the name of
Chapter 7: Recording Inferior’s Execution and Replaying It
93
that function, the source lines for this instruction sequence (if the /l modifier
is specified), and the instructions numbers that form the sequence (if the /i
modifier is specified). The function names are indented to reflect the call stack
depth if the /c modifier is specified. The /l, /i, and /c modifiers can be given
together.
(gdb) list 1, 10
1
void foo (void)
2
{
3
}
4
5
void bar (void)
6
{
7
...
8
foo ();
9
...
10 }
(gdb) record function-call-history /ilc
1 bar
inst 1,4
at foo.c:6,8
2
foo
inst 5,10
at foo.c:2,3
3 bar
inst 11,13
at foo.c:9,10
By default, ten lines are printed. This can be changed using the set record
function-call-history-size command. Functions are printed in execution
order. There are several ways to specify what to print:
record function-call-history func
Prints ten functions starting from function number func.
record function-call-history func, +/-n
Prints n functions around function number func. If n is preceded
with +, prints n functions after function number func. If n is preceded with -, prints n functions before function number func.
record function-call-history
Prints ten more functions after the last ten-line print.
record function-call-history Prints ten more functions before the last ten-line print.
record function-call-history begin, end
Prints functions beginning with function number begin until function number end. The function number end is included.
This command may not be available for all recording methods.
set record function-call-history-size size
set record function-call-history-size unlimited
Define how many lines to print in the record function-call-history command. The default value is 10. A size of unlimited means unlimited lines.
show record function-call-history-size
Show how many lines to print in the record function-call-history command.
Chapter 8: Examining the Stack
95
8 Examining the Stack
When your program has stopped, the first thing you need to know is where it stopped and
how it got there.
Each time your program performs a function call, information about the call is generated.
That information includes the location of the call in your program, the arguments of the
call, and the local variables of the function being called. The information is saved in a block
of data called a stack frame. The stack frames are allocated in a region of memory called
the call stack.
When your program stops, the gdb commands for examining the stack allow you to see
all of this information.
One of the stack frames is selected by gdb and many gdb commands refer implicitly
to the selected frame. In particular, whenever you ask gdb for the value of a variable in
your program, the value is found in the selected frame. There are special gdb commands to
select whichever frame you are interested in. See Section 8.3 [Selecting a Frame], page 98.
When your program stops, gdb automatically selects the currently executing frame and
describes it briefly, similar to the frame command (see Section 8.4 [Information about a
Frame], page 99).
8.1 Stack Frames
The call stack is divided up into contiguous pieces called stack frames, or frames for short;
each frame is the data associated with one call to one function. The frame contains the
arguments given to the function, the function’s local variables, and the address at which
the function is executing.
When your program is started, the stack has only one frame, that of the function main.
This is called the initial frame or the outermost frame. Each time a function is called, a
new frame is made. Each time a function returns, the frame for that function invocation
is eliminated. If a function is recursive, there can be many frames for the same function.
The frame for the function in which execution is actually occurring is called the innermost
frame. This is the most recently created of all the stack frames that still exist.
Inside your program, stack frames are identified by their addresses. A stack frame
consists of many bytes, each of which has its own address; each kind of computer has a convention for choosing one byte whose address serves as the address of the frame. Usually this
address is kept in a register called the frame pointer register (see Section 10.13 [Registers],
page 144) while execution is going on in that frame.
gdb assigns numbers to all existing stack frames, starting with zero for the innermost
frame, one for the frame that called it, and so on upward. These numbers do not really
exist in your program; they are assigned by gdb to give you a way of designating stack
frames in gdb commands.
Some compilers provide a way to compile functions so that they operate without stack
frames. (For example, the gcc option
‘-fomit-frame-pointer’
generates functions without a frame.) This is occasionally done with heavily used library functions to save the frame setup time. gdb has limited facilities for dealing with
96
Debugging with gdb
these function invocations. If the innermost function invocation has no stack frame, gdb
nevertheless regards it as though it had a separate frame, which is numbered zero as usual,
allowing correct tracing of the function call chain. However, gdb has no provision for
frameless functions elsewhere in the stack.
8.2 Backtraces
A backtrace is a summary of how your program got where it is. It shows one line per frame,
for many frames, starting with the currently executing frame (frame zero), followed by its
caller (frame one), and on up the stack.
backtrace
bt
Print a backtrace of the entire stack: one line per frame for all frames in the
stack.
You can stop the backtrace at any time by typing the system interrupt character, normally Ctrl-c.
backtrace n
bt n
Similar, but print only the innermost n frames.
backtrace -n
bt -n
Similar, but print only the outermost n frames.
backtrace full
bt full
bt full n
bt full -n
Print the values of the local variables also. As described above, n specifies the
number of frames to print.
backtrace no-filters
bt no-filters
bt no-filters n
bt no-filters -n
bt no-filters full
bt no-filters full n
bt no-filters full -n
Do not run Python frame filters on this backtrace. See Section 23.2.2.9 [Frame
Filter API], page 348, for more information. Additionally use [disable framefilter all], page 100 to turn off all frame filters. This is only relevant when gdb
has been configured with Python support.
The names where and info stack (abbreviated info s) are additional aliases for
backtrace.
In a multi-threaded program, gdb by default shows the backtrace only for the current
thread. To display the backtrace for several or all of the threads, use the command thread
apply (see Section 4.10 [Threads], page 36). For example, if you type thread apply all
backtrace, gdb will display the backtrace for all the threads; this is handy when you debug
a core dump of a multi-threaded program.
Each line in the backtrace shows the frame number and the function name. The program
counter value is also shown—unless you use set print address off. The backtrace also
Chapter 8: Examining the Stack
97
shows the source file name and line number, as well as the arguments to the function. The
program counter value is omitted if it is at the beginning of the code for that line number.
Here is an example of a backtrace. It was made with the command ‘bt 3’, so it shows
the innermost three frames.
#0
m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
at builtin.c:993
#1 0x6e38 in expand_macro (sym=0x2b600, data=...) at macro.c:242
#2 0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08)
at macro.c:71
(More stack frames follow...)
The display for frame zero does not begin with a program counter value, indicating that
your program has stopped at the beginning of the code for line 993 of builtin.c.
The value of parameter data in frame 1 has been replaced by .... By default, gdb prints
the value of a parameter only if it is a scalar (integer, pointer, enumeration, etc). See
command set print frame-arguments in Section 10.8 [Print Settings], page 127 for more
details on how to configure the way function parameter values are printed.
If your program was compiled with optimizations, some compilers will optimize away
arguments passed to functions if those arguments are never used after the call. Such optimizations generate code that passes arguments through registers, but doesn’t store those
arguments in the stack frame. gdb has no way of displaying such arguments in stack frames
other than the innermost one. Here’s what such a backtrace might look like:
#0
m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
at builtin.c:993
#1 0x6e38 in expand_macro (sym=) at macro.c:242
#2 0x6840 in expand_token (obs=0x0, t=, td=0xf7fffb08)
at macro.c:71
(More stack frames follow...)
The values of arguments that were not saved in their stack frames are shown as ‘’.
If you need to display the values of such optimized-out arguments, either deduce that
from other variables whose values depend on the one you are interested in, or recompile
without optimizations.
Most programs have a standard user entry point—a place where system libraries and
startup code transition into user code. For C this is main1 . When gdb finds the entry
function in a backtrace it will terminate the backtrace, to avoid tracing into highly systemspecific (and generally uninteresting) code.
If you need to examine the startup code, or limit the number of levels in a backtrace,
you can change this behavior:
set backtrace past-main
set backtrace past-main on
Backtraces will continue past the user entry point.
set backtrace past-main off
Backtraces will stop when they encounter the user entry point. This is the
default.
1
Note that embedded programs (the so-called “free-standing” environment) are not required to have a
main function as the entry point. They could even have multiple entry points.
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Debugging with gdb
show backtrace past-main
Display the current user entry point backtrace policy.
set backtrace past-entry
set backtrace past-entry on
Backtraces will continue past the internal entry point of an application. This
entry point is encoded by the linker when the application is built, and is likely
before the user entry point main (or equivalent) is called.
set backtrace past-entry off
Backtraces will stop when they encounter the internal entry point of an application. This is the default.
show backtrace past-entry
Display the current internal entry point backtrace policy.
set backtrace limit n
set backtrace limit 0
set backtrace limit unlimited
Limit the backtrace to n levels. A value of unlimited or zero means unlimited
levels.
show backtrace limit
Display the current limit on backtrace levels.
You can control how file names are displayed.
set filename-display
set filename-display relative
Display file names relative to the compilation directory. This is the default.
set filename-display basename
Display only basename of a filename.
set filename-display absolute
Display an absolute filename.
show filename-display
Show the current way to display filenames.
8.3 Selecting a Frame
Most commands for examining the stack and other data in your program work on whichever
stack frame is selected at the moment. Here are the commands for selecting a stack frame;
all of them finish by printing a brief description of the stack frame just selected.
frame n
fn
Select frame number n. Recall that frame zero is the innermost (currently
executing) frame, frame one is the frame that called the innermost one, and so
on. The highest-numbered frame is the one for main.
frame stack-addr [ pc-addr ]
f stack-addr [ pc-addr ]
Select the frame at address stack-addr. This is useful mainly if the chaining
of stack frames has been damaged by a bug, making it impossible for gdb to
Chapter 8: Examining the Stack
99
assign numbers properly to all frames. In addition, this can be useful when
your program has multiple stacks and switches between them. The optional
pc-addr can also be given to specify the value of PC for the stack frame.
up n
Move n frames up the stack; n defaults to 1. For positive numbers n, this
advances toward the outermost frame, to higher frame numbers, to frames that
have existed longer.
down n
Move n frames down the stack; n defaults to 1. For positive numbers n, this
advances toward the innermost frame, to lower frame numbers, to frames that
were created more recently. You may abbreviate down as do.
All of these commands end by printing two lines of output describing the frame. The
first line shows the frame number, the function name, the arguments, and the source file
and line number of execution in that frame. The second line shows the text of that source
line.
For example:
(gdb) up
#1 0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc)
at env.c:10
10
read_input_file (argv[i]);
After such a printout, the list command with no arguments prints ten lines centered on
the point of execution in the frame. You can also edit the program at the point of execution
with your favorite editing program by typing edit. See Section 9.1 [Printing Source Lines],
page 103, for details.
select-frame
The select-frame command is a variant of frame that does not display the
new frame after selecting it. This command is intended primarily for use in
gdb command scripts, where the output might be unnecessary and distracting.
up-silently n
down-silently n
These two commands are variants of up and down, respectively; they differ in
that they do their work silently, without causing display of the new frame. They
are intended primarily for use in gdb command scripts, where the output might
be unnecessary and distracting.
8.4 Information About a Frame
There are several other commands to print information about the selected stack frame.
frame
f
When used without any argument, this command does not change which frame
is selected, but prints a brief description of the currently selected stack frame.
It can be abbreviated f. With an argument, this command is used to select a
stack frame. See Section 8.3 [Selecting a Frame], page 98.
info frame
info f
This command prints a verbose description of the selected stack frame, including:
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Debugging with gdb
•
•
•
•
•
•
•
the address of the frame
the address of the next frame down (called by this frame)
the address of the next frame up (caller of this frame)
the language in which the source code corresponding to this frame is written
the address of the frame’s arguments
the address of the frame’s local variables
the program counter saved in it (the address of execution in the caller
frame)
• which registers were saved in the frame
The verbose description is useful when something has gone wrong that has made
the stack format fail to fit the usual conventions.
info frame addr
info f addr
Print a verbose description of the frame at address addr, without selecting that
frame. The selected frame remains unchanged by this command. This requires
the same kind of address (more than one for some architectures) that you specify
in the frame command. See Section 8.3 [Selecting a Frame], page 98.
info args Print the arguments of the selected frame, each on a separate line.
info locals
Print the local variables of the selected frame, each on a separate line. These
are all variables (declared either static or automatic) accessible at the point of
execution of the selected frame.
8.5 Management of Frame Filters.
Frame filters are Python based utilities to manage and decorate the output of frames. See
Section 23.2.2.9 [Frame Filter API], page 348, for further information.
Managing frame filters is performed by several commands available within gdb, detailed
here.
info frame-filter
Print a list of installed frame filters from all dictionaries, showing their name,
priority and enabled status.
disable frame-filter filter-dictionary filter-name
Disable a frame filter in the dictionary matching filter-dictionary and filtername. The filter-dictionary may be all, global, progspace, or the name of
the object file where the frame filter dictionary resides. When all is specified,
all frame filters across all dictionaries are disabled. The filter-name is the name
of the frame filter and is used when all is not the option for filter-dictionary.
A disabled frame-filter is not deleted, it may be enabled again later.
enable frame-filter filter-dictionary filter-name
Enable a frame filter in the dictionary matching filter-dictionary and filtername. The filter-dictionary may be all, global, progspace or the name of
Chapter 8: Examining the Stack
101
the object file where the frame filter dictionary resides. When all is specified,
all frame filters across all dictionaries are enabled. The filter-name is the name
of the frame filter and is used when all is not the option for filter-dictionary.
Example:
(gdb) info frame-filter
global frame-filters:
Priority Enabled Name
1000
No
PrimaryFunctionFilter
100
Yes
Reverse
progspace /build/test frame-filters:
Priority Enabled Name
100
Yes
ProgspaceFilter
objfile /build/test frame-filters:
Priority Enabled Name
999
Yes
BuildProgra Filter
(gdb) disable frame-filter /build/test BuildProgramFilter
(gdb) info frame-filter
global frame-filters:
Priority Enabled Name
1000
No
PrimaryFunctionFilter
100
Yes
Reverse
progspace /build/test frame-filters:
Priority Enabled Name
100
Yes
ProgspaceFilter
objfile /build/test frame-filters:
Priority Enabled Name
999
No
BuildProgramFilter
(gdb) enable frame-filter global PrimaryFunctionFilter
(gdb) info frame-filter
global frame-filters:
Priority Enabled Name
1000
Yes
PrimaryFunctionFilter
100
Yes
Reverse
progspace /build/test frame-filters:
Priority Enabled Name
100
Yes
ProgspaceFilter
objfile /build/test frame-filters:
Priority Enabled Name
999
No
BuildProgramFilter
set frame-filter priority filter-dictionary filter-name priority
Set the priority of a frame filter in the dictionary matching filter-dictionary,
and the frame filter name matching filter-name. The filter-dictionary may be
global, progspace or the name of the object file where the frame filter dictionary resides. The priority is an integer.
102
Debugging with gdb
show frame-filter priority filter-dictionary filter-name
Show the priority of a frame filter in the dictionary matching filter-dictionary,
and the frame filter name matching filter-name. The filter-dictionary may be
global, progspace or the name of the object file where the frame filter dictionary resides.
Example:
(gdb) info frame-filter
global frame-filters:
Priority Enabled Name
1000
Yes
PrimaryFunctionFilter
100
Yes
Reverse
progspace /build/test frame-filters:
Priority Enabled Name
100
Yes
ProgspaceFilter
objfile /build/test frame-filters:
Priority Enabled Name
999
No
BuildProgramFilter
(gdb) set frame-filter priority global Reverse 50
(gdb) info frame-filter
global frame-filters:
Priority Enabled Name
1000
Yes
PrimaryFunctionFilter
50
Yes
Reverse
progspace /build/test frame-filters:
Priority Enabled Name
100
Yes
ProgspaceFilter
objfile /build/test frame-filters:
Priority Enabled Name
999
No
BuildProgramFilter
Chapter 9: Examining Source Files
103
9 Examining Source Files
gdb can print parts of your program’s source, since the debugging information recorded in
the program tells gdb what source files were used to build it. When your program stops,
gdb spontaneously prints the line where it stopped. Likewise, when you select a stack frame
(see Section 8.3 [Selecting a Frame], page 98), gdb prints the line where execution in that
frame has stopped. You can print other portions of source files by explicit command.
If you use gdb through its gnu Emacs interface, you may prefer to use Emacs facilities
to view source; see Chapter 26 [Using gdb under gnu Emacs], page 467.
9.1 Printing Source Lines
To print lines from a source file, use the list command (abbreviated l). By default, ten
lines are printed. There are several ways to specify what part of the file you want to print;
see Section 9.2 [Specify Location], page 104, for the full list.
Here are the forms of the list command most commonly used:
list linenum
Print lines centered around line number linenum in the current source file.
list function
Print lines centered around the beginning of function function.
list
Print more lines. If the last lines printed were printed with a list command,
this prints lines following the last lines printed; however, if the last line printed
was a solitary line printed as part of displaying a stack frame (see Chapter 8
[Examining the Stack], page 95), this prints lines centered around that line.
list -
Print lines just before the lines last printed.
By default, gdb prints ten source lines with any of these forms of the list command.
You can change this using set listsize:
set listsize count
set listsize unlimited
Make the list command display count source lines (unless the list argument
explicitly specifies some other number). Setting count to unlimited or 0 means
there’s no limit.
show listsize
Display the number of lines that list prints.
Repeating a list command with RET discards the argument, so it is equivalent to typing
just list. This is more useful than listing the same lines again. An exception is made for
an argument of ‘-’; that argument is preserved in repetition so that each repetition moves
up in the source file.
In general, the list command expects you to supply zero, one or two locations. Locations specify source lines; there are several ways of writing them (see Section 9.2 [Specify
Location], page 104), but the effect is always to specify some source line.
Here is a complete description of the possible arguments for list:
104
Debugging with gdb
list location
Print lines centered around the line specified by location.
list first,last
Print lines from first to last. Both arguments are locations. When a list
command has two locations, and the source file of the second location is omitted,
this refers to the same source file as the first location.
list ,last
Print lines ending with last.
list first,
Print lines starting with first.
list +
Print lines just after the lines last printed.
list -
Print lines just before the lines last printed.
list
As described in the preceding table.
9.2 Specifying a Location
Several gdb commands accept arguments that specify a location of your program’s code.
Since gdb is a source-level debugger, a location usually specifies some line in the source
code. Locations may be specified using three different formats: linespec locations, explicit
locations, or address locations.
9.2.1 Linespec Locations
A linespec is a colon-separated list of source location parameters such as file name, function
name, etc. Here are all the different ways of specifying a linespec:
linenum
-offset
+offset
Specifies the line number linenum of the current source file.
Specifies the line offset lines before or after the current line. For the list
command, the current line is the last one printed; for the breakpoint commands,
this is the line at which execution stopped in the currently selected stack frame
(see Section 8.1 [Frames], page 95, for a description of stack frames.) When
used as the second of the two linespecs in a list command, this specifies the
line offset lines up or down from the first linespec.
filename:linenum
Specifies the line linenum in the source file filename. If filename is a
relative file name, then it will match any source file name with the same
trailing components.
For example, if filename is ‘gcc/expr.c’, then
it will match source file name of ‘/build/trunk/gcc/expr.c’, but not
‘/build/trunk/libcpp/expr.c’ or ‘/build/trunk/gcc/x-expr.c’.
function
Specifies the line that begins the body of the function function. For example,
in C, this is the line with the open brace.
function:label
Specifies the line where label appears in function.
Chapter 9: Examining Source Files
105
filename:function
Specifies the line that begins the body of the function function in the file filename. You only need the file name with a function name to avoid ambiguity
when there are identically named functions in different source files.
label
Specifies the line at which the label named label appears in the function corresponding to the currently selected stack frame. If there is no current selected
stack frame (for instance, if the inferior is not running), then gdb will not
search for a label.
-pstap|-probe-stap [objfile:[provider:]]name
The gnu/Linux tool SystemTap provides a way for applications to embed static
probes. See Section 5.1.10 [Static Probe Points], page 65, for more information
on finding and using static probes. This form of linespec specifies the location
of such a static probe.
If objfile is given, only probes coming from that shared library or executable
matching objfile as a regular expression are considered. If provider is given,
then only probes from that provider are considered. If several probes match
the spec, gdb will insert a breakpoint at each one of those probes.
9.2.2 Explicit Locations
Explicit locations allow the user to directly specify the source location’s parameters using
option-value pairs.
Explicit locations are useful when several functions, labels, or file names have the same
name (base name for files) in the program’s sources. In these cases, explicit locations point
to the source line you meant more accurately and unambiguously. Also, using explicit
locations might be faster in large programs.
For example, the linespec ‘foo:bar’ may refer to a function bar defined in the file named
‘foo’ or the label bar in a function named foo. gdb must search either the file system or
the symbol table to know.
The list of valid explicit location options is summarized in the following table:
-source filename
The value specifies the source file name. To differentiate between files with
the same base name, prepend as many directories as is necessary to uniquely
identify the desired file, e.g., ‘foo/bar/baz.c’. Otherwise gdb will use the first
file it finds with the given base name. This option requires the use of either
-function or -line.
-function function
The value specifies the name of a function. Operations on function locations
unmodified by other options (such as -label or -line) refer to the line that
begins the body of the function. In C, for example, this is the line with the
open brace.
-label label
The value specifies the name of a label. When the function name is not specified,
the label is searched in the function of the currently selected stack frame.
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Debugging with gdb
-line number
The value specifies a line offset for the location. The offset may either be
absolute (-line 3) or relative (-line +3), depending on the command. When
specified without any other options, the line offset is relative to the current line.
Explicit location options may be abbreviated by omitting any non-unique trailing characters from the option name, e.g., break -s main.c -li 3.
9.2.3 Address Locations
Address locations indicate a specific program address. They have the generalized form
*address.
For line-oriented commands, such as list and edit, this specifies a source line that
contains address. For break and other breakpoint-oriented commands, this can be used
to set breakpoints in parts of your program which do not have debugging information or
source files.
Here address may be any expression valid in the current working language (see
Chapter 15 [Languages], page 191) that specifies a code address. In addition, as a
convenience, gdb extends the semantics of expressions used in locations to cover several
situations that frequently occur during debugging. Here are the various forms of address:
expression
Any expression valid in the current working language.
funcaddr
An address of a function or procedure derived from its name. In C, C++,
Objective-C, Fortran, minimal, and assembly, this is simply the function’s name
function (and actually a special case of a valid expression). In Pascal and
Modula-2, this is &function. In Ada, this is function’Address (although the
Pascal form also works).
This form specifies the address of the function’s first instruction, before the
stack frame and arguments have been set up.
’filename’:funcaddr
Like funcaddr above, but also specifies the name of the source file explicitly.
This is useful if the name of the function does not specify the function unambiguously, e.g., if there are several functions with identical names in different
source files.
9.3 Editing Source Files
To edit the lines in a source file, use the edit command. The editing program of your
choice is invoked with the current line set to the active line in the program. Alternatively,
there are several ways to specify what part of the file you want to print if you want to see
other parts of the program:
edit location
Edit the source file specified by location. Editing starts at that location,
e.g., at the specified source line of the specified file. See Section 9.2 [Specify
Location], page 104, for all the possible forms of the location argument; here
are the forms of the edit command most commonly used:
Chapter 9: Examining Source Files
107
edit number
Edit the current source file with number as the active line number.
edit function
Edit the file containing function at the beginning of its definition.
9.3.1 Choosing your Editor
You can customize gdb to use any editor you want1 . By default, it is ‘/bin/ex’, but you
can change this by setting the environment variable EDITOR before using gdb. For example,
to configure gdb to use the vi editor, you could use these commands with the sh shell:
EDITOR=/usr/bin/vi
export EDITOR
gdb ...
or in the csh shell,
setenv EDITOR /usr/bin/vi
gdb ...
9.4 Searching Source Files
There are two commands for searching through the current source file for a regular expression.
forward-search regexp
search regexp
The command ‘forward-search regexp’ checks each line, starting with the
one following the last line listed, for a match for regexp. It lists the line that is
found. You can use the synonym ‘search regexp’ or abbreviate the command
name as fo.
reverse-search regexp
The command ‘reverse-search regexp’ checks each line, starting with the one
before the last line listed and going backward, for a match for regexp. It lists
the line that is found. You can abbreviate this command as rev.
9.5 Specifying Source Directories
Executable programs sometimes do not record the directories of the source files from which
they were compiled, just the names. Even when they do, the directories could be moved
between the compilation and your debugging session. gdb has a list of directories to search
for source files; this is called the source path. Each time gdb wants a source file, it tries all
the directories in the list, in the order they are present in the list, until it finds a file with
the desired name.
For example, suppose an executable references the file ‘/usr/src/foo-1.0/lib/foo.c’,
and our source path is ‘/mnt/cross’. The file is first looked up literally; if this fails,
‘/mnt/cross/usr/src/foo-1.0/lib/foo.c’ is tried; if this fails, ‘/mnt/cross/foo.c’ is
1
The only restriction is that your editor (say ex), recognizes the following command-line syntax:
ex +number file
The optional numeric value +number specifies the number of the line in the file where to start editing.
108
Debugging with gdb
opened; if this fails, an error message is printed. gdb does not look up the parts of the source
file name, such as ‘/mnt/cross/src/foo-1.0/lib/foo.c’. Likewise, the subdirectories of
the source path are not searched: if the source path is ‘/mnt/cross’, and the binary refers
to ‘foo.c’, gdb would not find it under ‘/mnt/cross/usr/src/foo-1.0/lib’.
Plain file names, relative file names with leading directories, file names containing dots,
etc. are all treated as described above; for instance, if the source path is ‘/mnt/cross’, and
the source file is recorded as ‘../lib/foo.c’, gdb would first try ‘../lib/foo.c’, then
‘/mnt/cross/../lib/foo.c’, and after that—‘/mnt/cross/foo.c’.
Note that the executable search path is not used to locate the source files.
Whenever you reset or rearrange the source path, gdb clears out any information it has
cached about where source files are found and where each line is in the file.
When you start gdb, its source path includes only ‘cdir’ and ‘cwd’, in that order. To
add other directories, use the directory command.
The search path is used to find both program source files and gdb script files (read using
the ‘-command’ option and ‘source’ command).
In addition to the source path, gdb provides a set of commands that manage a list of
source path substitution rules. A substitution rule specifies how to rewrite source directories
stored in the program’s debug information in case the sources were moved to a different
directory between compilation and debugging. A rule is made of two strings, the first
specifying what needs to be rewritten in the path, and the second specifying how it should
be rewritten. In [set substitute-path], page 109, we name these two parts from and to
respectively. gdb does a simple string replacement of from with to at the start of the
directory part of the source file name, and uses that result instead of the original file name
to look up the sources.
Using the previous example, suppose the ‘foo-1.0’ tree has been moved from ‘/usr/src’
to ‘/mnt/cross’, then you can tell gdb to replace ‘/usr/src’ in all source path names with
‘/mnt/cross’. The first lookup will then be ‘/mnt/cross/foo-1.0/lib/foo.c’ in place of
the original location of ‘/usr/src/foo-1.0/lib/foo.c’. To define a source path substitution rule, use the set substitute-path command (see [set substitute-path], page 109).
To avoid unexpected substitution results, a rule is applied only if the from part
of the directory name ends at a directory separator. For instance, a rule substituting
‘/usr/source’ into ‘/mnt/cross’ will be applied to ‘/usr/source/foo-1.0’ but not
to ‘/usr/sourceware/foo-2.0’. And because the substitution is applied only at the
beginning of the directory name, this rule will not be applied to ‘/root/usr/source/baz.c’
either.
In many cases, you can achieve the same result using the directory command. However,
set substitute-path can be more efficient in the case where the sources are organized in
a complex tree with multiple subdirectories. With the directory command, you need to
add each subdirectory of your project. If you moved the entire tree while preserving its
internal organization, then set substitute-path allows you to direct the debugger to all
the sources with one single command.
set substitute-path is also more than just a shortcut command. The source path
is only used if the file at the original location no longer exists. On the other hand, set
substitute-path modifies the debugger behavior to look at the rewritten location instead.
So, if for any reason a source file that is not relevant to your executable is located at the
Chapter 9: Examining Source Files
109
original location, a substitution rule is the only method available to point gdb at the new
location.
You can configure a default source path substitution rule by configuring gdb with the
‘--with-relocated-sources=dir’ option. The dir should be the name of a directory under
gdb’s configured prefix (set with ‘--prefix’ or ‘--exec-prefix’), and directory names in
debug information under dir will be adjusted automatically if the installed gdb is moved
to a new location. This is useful if gdb, libraries or executables with debug information
and corresponding source code are being moved together.
directory dirname ...
dir dirname ...
Add directory dirname to the front of the source path. Several directory names
may be given to this command, separated by ‘:’ (‘;’ on MS-DOS and MSWindows, where ‘:’ usually appears as part of absolute file names) or whitespace. You may specify a directory that is already in the source path; this
moves it forward, so gdb searches it sooner.
You can use the string ‘$cdir’ to refer to the compilation directory (if one is
recorded), and ‘$cwd’ to refer to the current working directory. ‘$cwd’ is not
the same as ‘.’—the former tracks the current working directory as it changes
during your gdb session, while the latter is immediately expanded to the current
directory at the time you add an entry to the source path.
directory
Reset the source path to its default value (‘$cdir:$cwd’ on Unix systems). This
requires confirmation.
set directories path-list
Set the source path to path-list. ‘$cdir:$cwd’ are added if missing.
show directories
Print the source path: show which directories it contains.
set substitute-path from to
Define a source path substitution rule, and add it at the end of the current list
of existing substitution rules. If a rule with the same from was already defined,
then the old rule is also deleted.
For example, if the file ‘/foo/bar/baz.c’ was moved to ‘/mnt/cross/baz.c’,
then the command
(gdb) set substitute-path /foo/bar /mnt/cross
will tell gdb to replace ‘/foo/bar’ with ‘/mnt/cross’, which will allow gdb to
find the file ‘baz.c’ even though it was moved.
In the case when more than one substitution rule have been defined, the rules
are evaluated one by one in the order where they have been defined. The first
one matching, if any, is selected to perform the substitution.
For instance, if we had entered the following commands:
(gdb) set substitute-path /usr/src/include /mnt/include
(gdb) set substitute-path /usr/src /mnt/src
110
Debugging with gdb
gdb would then rewrite ‘/usr/src/include/defs.h’ into ‘/mnt/include/defs.h’
by using the first rule. However, it would use the second rule to rewrite
‘/usr/src/lib/foo.c’ into ‘/mnt/src/lib/foo.c’.
unset substitute-path [path]
If a path is specified, search the current list of substitution rules for a rule that
would rewrite that path. Delete that rule if found. A warning is emitted by
the debugger if no rule could be found.
If no path is specified, then all substitution rules are deleted.
show substitute-path [path]
If a path is specified, then print the source path substitution rule which would
rewrite that path, if any.
If no path is specified, then print all existing source path substitution rules.
If your source path is cluttered with directories that are no longer of interest, gdb may
sometimes cause confusion by finding the wrong versions of source. You can correct the
situation as follows:
1. Use directory with no argument to reset the source path to its default value.
2. Use directory with suitable arguments to reinstall the directories you want in the
source path. You can add all the directories in one command.
9.6 Source and Machine Code
You can use the command info line to map source lines to program addresses (and vice
versa), and the command disassemble to display a range of addresses as machine instructions. You can use the command set disassemble-next-line to set whether to disassemble next source line when execution stops. When run under gnu Emacs mode, the
info line command causes the arrow to point to the line specified. Also, info line prints
addresses in symbolic form as well as hex.
info line location
Print the starting and ending addresses of the compiled code for source line location. You can specify source lines in any of the ways documented in Section 9.2
[Specify Location], page 104.
For example, we can use info line to discover the location of the object code for the
first line of function m4_changequote:
(gdb) info line m4_changequote
Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350.
We can also inquire (using *addr as the form for location) what source line covers a particular address:
(gdb) info line *0x63ff
Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404.
After info line, the default address for the x command is changed to the starting
address of the line, so that ‘x/i’ is sufficient to begin examining the machine code (see
Section 10.6 [Examining Memory], page 123). Also, this address is saved as the value of the
convenience variable $_ (see Section 10.11 [Convenience Variables], page 139).
Chapter 9: Examining Source Files
111
disassemble
disassemble /m
disassemble /s
disassemble /r
This specialized command dumps a range of memory as machine instructions.
It can also print mixed source+disassembly by specifying the /m or /s modifier
and print the raw instructions in hex as well as in symbolic form by specifying
the /r modifier. The default memory range is the function surrounding the
program counter of the selected frame. A single argument to this command
is a program counter value; gdb dumps the function surrounding this value.
When two arguments are given, they should be separated by a comma, possibly
surrounded by whitespace. The arguments specify a range of addresses to dump,
in one of two forms:
start,end
the addresses from start (inclusive) to end (exclusive)
start,+length
the addresses from start (inclusive) to start+length (exclusive).
When 2 arguments are specified, the name of the function is also printed (since
there could be several functions in the given range).
The argument(s) can be any expression yielding a numeric value, such as
‘0x32c4’, ‘&main+10’ or ‘$pc - 8’.
If the range of memory being disassembled contains current program counter,
the instruction at that location is shown with a => marker.
The following example shows the disassembly of a range of addresses of HP PA-RISC
2.0 code:
(gdb) disas 0x32c4, 0x32e4
Dump of assembler code from 0x32c4 to 0x32e4:
0x32c4 :
addil 0,dp
0x32c8 :
ldw 0x22c(sr0,r1),r26
0x32cc :
ldil 0x3000,r31
0x32d0 :
ble 0x3f8(sr4,r31)
0x32d4 :
ldo 0(r31),rp
0x32d8 :
addil -0x800,dp
0x32dc :
ldo 0x588(r1),r26
0x32e0 :
ldil 0x3000,r31
End of assembler dump.
Here is an example showing mixed source+assembly for Intel x86 with /m or /s, when
the program is stopped just after function prologue in a non-optimized function with no
inline code.
(gdb) disas /m main
Dump of assembler code
5
{
0x08048330 <+0>:
0x08048331 <+1>:
0x08048333 <+3>:
0x08048336 <+6>:
0x08048339 <+9>:
for function main:
push
mov
sub
and
sub
%ebp
%esp,%ebp
$0x8,%esp
$0xfffffff0,%esp
$0x10,%esp
112
Debugging with gdb
6
printf ("Hello.\n");
=> 0x0804833c <+12>:
movl
$0x8048440,(%esp)
0x08048343 <+19>:
call
0x8048284
7
8
return 0;
}
0x08048348 <+24>:
0x0804834d <+29>:
0x0804834e <+30>:
mov
leave
ret
$0x0,%eax
End of assembler dump.
The /m option is deprecated as its output is not useful when there is either inlined code
or re-ordered code. The /s option is the preferred choice. Here is an example for AMD
x86-64 showing the difference between /m output and /s output. This example has one
inline function defined in a header file, and the code is compiled with ‘-O2’ optimization.
Note how the /m output is missing the disassembly of several instructions that are present
in the /s output.
‘foo.h’:
int
foo (int a)
{
if (a < 0)
return a * 2;
if (a == 0)
return 1;
return a + 10;
}
‘foo.c’:
#include "foo.h"
volatile int x, y;
int
main ()
{
x = foo (y);
return 0;
}
(gdb) disas /m main
Dump of assembler code for function main:
5 {
6
x = foo (y);
0x0000000000400400 <+0>: mov
0x0000000000400417 <+23>: mov
7
return 0;
8 }
0x000000000040041d
0x000000000040041f
0x0000000000400420
0x0000000000400422
<+29>:
<+31>:
<+32>:
<+34>:
xor
retq
add
jmp
0x200c2e(%rip),%eax # 0x601034
%eax,0x200c13(%rip) # 0x601030
%eax,%eax
%eax,%eax
0x400417
End of assembler dump.
(gdb) disas /s main
Dump of assembler code for function main:
foo.c:
Chapter 9: Examining Source Files
113
5 {
6
x = foo (y);
0x0000000000400400 <+0>:
mov
0x200c2e(%rip),%eax # 0x601034
foo.h:
4
if (a < 0)
0x0000000000400406 <+6>:
0x0000000000400408 <+8>:
test
js
%eax,%eax
0x400420
6
7
8
if (a == 0)
return 1;
return a + 10;
0x000000000040040a
0x000000000040040d
0x000000000040040f
0x0000000000400414
<+10>:
<+13>:
<+15>:
<+20>:
lea
test
mov
cmovne
0xa(%rax),%edx
%eax,%eax
$0x1,%eax
%edx,%eax
foo.c:
6
x = foo (y);
0x0000000000400417 <+23>:
mov
%eax,0x200c13(%rip) # 0x601030
7
return 0;
8 }
0x000000000040041d <+29>:
0x000000000040041f <+31>:
xor
retq
%eax,%eax
add
jmp
%eax,%eax
0x400417
foo.h:
5
return a * 2;
0x0000000000400420 <+32>:
0x0000000000400422 <+34>:
End of assembler dump.
Here is another example showing raw instructions in hex for AMD x86-64,
(gdb) disas /r 0x400281,+10
Dump of assembler code from 0x400281 to 0x40028b:
0x0000000000400281: 38 36 cmp
%dh,(%rsi)
0x0000000000400283: 2d 36 34 2e 73 sub
$0x732e3436,%eax
0x0000000000400288: 6f
outsl %ds:(%rsi),(%dx)
0x0000000000400289: 2e 32 00
xor
%cs:(%rax),%al
End of assembler dump.
Addresses cannot be specified as a location (see Section 9.2 [Specify Location], page 104).
So, for example, if you want to disassemble function bar in file ‘foo.c’, you must type
‘disassemble ’foo.c’::bar’ and not ‘disassemble foo.c:bar’.
Some architectures have more than one commonly-used set of instruction mnemonics or
other syntax.
For programs that were dynamically linked and use shared libraries, instructions that
call functions or branch to locations in the shared libraries might show a seemingly bogus
location—it’s actually a location of the relocation table. On some architectures, gdb might
be able to resolve these to actual function names.
set disassembler-options option1[,option2...]
This command controls the passing of target specific information to the disassembler. For a list of valid options, please refer to the -M/--disassembleroptions section of the ‘objdump’ manual and/or the output of objdump --help
114
Debugging with gdb
(see Section “objdump” in The GNU Binary Utilities). The default value is the
empty string.
If it is necessary to specify more than one disassembler option, then multiple
options can be placed together into a comma separated list. Currently this
command is only supported on targets ARM, PowerPC and S/390.
show disassembler-options
Show the current setting of the disassembler options.
set disassembly-flavor instruction-set
Select the instruction set to use when disassembling the program via the
disassemble or x/i commands.
Currently this command is only defined for the Intel x86 family. You can set
instruction-set to either intel or att. The default is att, the AT&T flavor
used by default by Unix assemblers for x86-based targets.
show disassembly-flavor
Show the current setting of the disassembly flavor.
set disassemble-next-line
show disassemble-next-line
Control whether or not gdb will disassemble the next source line or instruction
when execution stops. If ON, gdb will display disassembly of the next source
line when execution of the program being debugged stops. This is in addition
to displaying the source line itself, which gdb always does if possible. If the
next source line cannot be displayed for some reason (e.g., if gdb cannot find
the source file, or there’s no line info in the debug info), gdb will display
disassembly of the next instruction instead of showing the next source line. If
AUTO, gdb will display disassembly of next instruction only if the source line
cannot be displayed. This setting causes gdb to display some feedback when
you step through a function with no line info or whose source file is unavailable.
The default is OFF, which means never display the disassembly of the next line
or instruction.
Chapter 10: Examining Data
115
10 Examining Data
The usual way to examine data in your program is with the print command (abbreviated p),
or its synonym inspect. It evaluates and prints the value of an expression of the language
your program is written in (see Chapter 15 [Using gdb with Different Languages], page 191).
It may also print the expression using a Python-based pretty-printer (see Section 10.9
[Pretty Printing], page 135).
print expr
print /f expr
expr is an expression (in the source language). By default the value of expr is
printed in a format appropriate to its data type; you can choose a different format by specifying ‘/f’, where f is a letter specifying the format; see Section 10.5
[Output Formats], page 122.
print
print /f
If you omit expr, gdb displays the last value again (from the value history;
see Section 10.10 [Value History], page 138). This allows you to conveniently
inspect the same value in an alternative format.
A more low-level way of examining data is with the x command. It examines data
in memory at a specified address and prints it in a specified format. See Section 10.6
[Examining Memory], page 123.
If you are interested in information about types, or about how the fields of a struct
or a class are declared, use the ptype exp command rather than print. See Chapter 16
[Examining the Symbol Table], page 221.
Another way of examining values of expressions and type information is through the
Python extension command explore (available only if the gdb build is configured with
--with-python). It offers an interactive way to start at the highest level (or, the most
abstract level) of the data type of an expression (or, the data type itself) and explore all
the way down to leaf scalar values/fields embedded in the higher level data types.
explore arg
arg is either an expression (in the source language), or a type visible in the
current context of the program being debugged.
The working of the explore command can be illustrated with an example. If a data
type struct ComplexStruct is defined in your C program as
struct SimpleStruct
{
int i;
double d;
};
struct ComplexStruct
{
struct SimpleStruct *ss_p;
int arr[10];
};
followed by variable declarations as
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Debugging with gdb
struct SimpleStruct ss = { 10, 1.11 };
struct ComplexStruct cs = { &ss, { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } };
then, the value of the variable cs can be explored using the explore command as follows.
(gdb) explore cs
The value of ‘cs’ is a struct/class of type ‘struct ComplexStruct’ with
the following fields:
ss_p =
arr =
Enter the field number of choice:
Since the fields of cs are not scalar values, you are being prompted to chose the field you
want to explore. Let’s say you choose the field ss_p by entering 0. Then, since this field
is a pointer, you will be asked if it is pointing to a single value. From the declaration of
cs above, it is indeed pointing to a single value, hence you enter y. If you enter n, then
you will be asked if it were pointing to an array of values, in which case this field will be
explored as if it were an array.
‘cs.ss_p’ is a pointer to a value of type ‘struct SimpleStruct’
Continue exploring it as a pointer to a single value [y/n]: y
The value of ‘*(cs.ss_p)’ is a struct/class of type ‘struct
SimpleStruct’ with the following fields:
i = 10 .. (Value of type ‘int’)
d = 1.1100000000000001 .. (Value of type ‘double’)
Press enter to return to parent value:
If the field arr of cs was chosen for exploration by entering 1 earlier, then since it is as
array, you will be prompted to enter the index of the element in the array that you want
to explore.
‘cs.arr’ is an array of ‘int’.
Enter the index of the element you want to explore in ‘cs.arr’: 5
‘(cs.arr)[5]’ is a scalar value of type ‘int’.
(cs.arr)[5] = 4
Press enter to return to parent value:
In general, at any stage of exploration, you can go deeper towards the leaf values by
responding to the prompts appropriately, or hit the return key to return to the enclosing
data structure (the higher level data structure).
Similar to exploring values, you can use the explore command to explore types. Instead
of specifying a value (which is typically a variable name or an expression valid in the current
context of the program being debugged), you specify a type name. If you consider the
same example as above, your can explore the type struct ComplexStruct by passing the
argument struct ComplexStruct to the explore command.
(gdb) explore struct ComplexStruct
By responding to the prompts appropriately in the subsequent interactive session, you can
explore the type struct ComplexStruct in a manner similar to how the value cs was
explored in the above example.
Chapter 10: Examining Data
117
The explore command also has two sub-commands, explore value and explore type.
The former sub-command is a way to explicitly specify that value exploration of the argument is being invoked, while the latter is a way to explicitly specify that type exploration
of the argument is being invoked.
explore value expr
This sub-command of explore explores the value of the expression expr (if expr
is an expression valid in the current context of the program being debugged).
The behavior of this command is identical to that of the behavior of the explore
command being passed the argument expr.
explore type arg
This sub-command of explore explores the type of arg (if arg is a type visible in the current context of program being debugged), or the type of the
value/expression arg (if arg is an expression valid in the current context of the
program being debugged). If arg is a type, then the behavior of this command
is identical to that of the explore command being passed the argument arg.
If arg is an expression, then the behavior of this command will be identical to
that of the explore command being passed the type of arg as the argument.
10.1 Expressions
print and many other gdb commands accept an expression and compute its value. Any
kind of constant, variable or operator defined by the programming language you are using
is valid in an expression in gdb. This includes conditional expressions, function calls, casts,
and string constants. It also includes preprocessor macros, if you compiled your program
to include this information; see Section 4.1 [Compilation], page 25.
gdb supports array constants in expressions input by the user. The syntax is {element,
element. . . }. For example, you can use the command print {1, 2, 3} to create an array
of three integers. If you pass an array to a function or assign it to a program variable, gdb
copies the array to memory that is malloced in the target program.
Because C is so widespread, most of the expressions shown in examples in this manual
are in C. See Chapter 15 [Using gdb with Different Languages], page 191, for information
on how to use expressions in other languages.
In this section, we discuss operators that you can use in gdb expressions regardless of
your programming language.
Casts are supported in all languages, not just in C, because it is so useful to cast a
number into a pointer in order to examine a structure at that address in memory.
gdb supports these operators, in addition to those common to programming languages:
@
‘@’ is a binary operator for treating parts of memory as arrays. See Section 10.4
[Artificial Arrays], page 121, for more information.
::
‘::’ allows you to specify a variable in terms of the file or function where it is
defined. See Section 10.3 [Program Variables], page 119.
{type} addr
Refers to an object of type type stored at address addr in memory. The address
addr may be any expression whose value is an integer or pointer (but parenthe-
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Debugging with gdb
ses are required around binary operators, just as in a cast). This construct is
allowed regardless of what kind of data is normally supposed to reside at addr.
10.2 Ambiguous Expressions
Expressions can sometimes contain some ambiguous elements. For instance, some programming languages (notably Ada, C++ and Objective-C) permit a single function name to be
defined several times, for application in different contexts. This is called overloading. Another example involving Ada is generics. A generic package is similar to C++ templates and
is typically instantiated several times, resulting in the same function name being defined in
different contexts.
In some cases and depending on the language, it is possible to adjust the expression to
remove the ambiguity. For instance in C++, you can specify the signature of the function
you want to break on, as in break function(types). In Ada, using the fully qualified
name of your function often makes the expression unambiguous as well.
When an ambiguity that needs to be resolved is detected, the debugger has the capability
to display a menu of numbered choices for each possibility, and then waits for the selection
with the prompt ‘>’. The first option is always ‘[0] cancel’, and typing 0 RET aborts the
current command. If the command in which the expression was used allows more than one
choice to be selected, the next option in the menu is ‘[1] all’, and typing 1 RET selects all
possible choices.
For example, the following session excerpt shows an attempt to set a breakpoint at the
overloaded symbol String::after. We choose three particular definitions of that function
name:
(gdb) b String::after
[0] cancel
[1] all
[2] file:String.cc; line number:867
[3] file:String.cc; line number:860
[4] file:String.cc; line number:875
[5] file:String.cc; line number:853
[6] file:String.cc; line number:846
[7] file:String.cc; line number:735
> 2 4 6
Breakpoint 1 at 0xb26c: file String.cc, line 867.
Breakpoint 2 at 0xb344: file String.cc, line 875.
Breakpoint 3 at 0xafcc: file String.cc, line 846.
Multiple breakpoints were set.
Use the "delete" command to delete unwanted
breakpoints.
(gdb)
set multiple-symbols mode
This option allows you to adjust the debugger behavior when an expression is
ambiguous.
By default, mode is set to all. If the command with which the expression is
used allows more than one choice, then gdb automatically selects all possible
choices. For instance, inserting a breakpoint on a function using an ambiguous
name results in a breakpoint inserted on each possible match. However, if a
unique choice must be made, then gdb uses the menu to help you disambiguate
Chapter 10: Examining Data
119
the expression. For instance, printing the address of an overloaded function will
result in the use of the menu.
When mode is set to ask, the debugger always uses the menu when an ambiguity
is detected.
Finally, when mode is set to cancel, the debugger reports an error due to the
ambiguity and the command is aborted.
show multiple-symbols
Show the current value of the multiple-symbols setting.
10.3 Program Variables
The most common kind of expression to use is the name of a variable in your program.
Variables in expressions are understood in the selected stack frame (see Section 8.3
[Selecting a Frame], page 98); they must be either:
• global (or file-static)
or
• visible according to the scope rules of the programming language from the point of
execution in that frame
This means that in the function
foo (a)
int a;
{
bar (a);
{
int b = test ();
bar (b);
}
}
you can examine and use the variable a whenever your program is executing within the
function foo, but you can only use or examine the variable b while your program is executing
inside the block where b is declared.
There is an exception: you can refer to a variable or function whose scope is a single
source file even if the current execution point is not in this file. But it is possible to have
more than one such variable or function with the same name (in different source files). If
that happens, referring to that name has unpredictable effects. If you wish, you can specify
a static variable in a particular function or file by using the colon-colon (::) notation:
file::variable
function::variable
Here file or function is the name of the context for the static variable. In the case of file
names, you can use quotes to make sure gdb parses the file name as a single word—for
example, to print a global value of x defined in ‘f2.c’:
(gdb) p ’f2.c’::x
The :: notation is normally used for referring to static variables, since you typically
disambiguate uses of local variables in functions by selecting the appropriate frame and
using the simple name of the variable. However, you may also use this notation to refer to
local variables in frames enclosing the selected frame:
120
Debugging with gdb
void
foo (int a)
{
if (a < 10)
bar (a);
else
process (a);
}
/* Stop here */
int
bar (int a)
{
foo (a + 5);
}
For example, if there is a breakpoint at the commented line, here is what you might see
when the program stops after executing the call bar(0):
(gdb) p a
$1 = 10
(gdb) p bar::a
$2 = 5
(gdb) up 2
#2 0x080483d0 in foo (a=5) at foobar.c:12
(gdb) p a
$3 = 5
(gdb) p bar::a
$4 = 0
These uses of ‘::’ are very rarely in conflict with the very similar use of the same notation
in C++. When they are in conflict, the C++ meaning takes precedence; however, this can be
overridden by quoting the file or function name with single quotes.
For example, suppose the program is stopped in a method of a class that has a field
named includefile, and there is also an include file named ‘includefile’ that defines a
variable, some_global.
(gdb) p includefile
$1 = 23
(gdb) p includefile::some_global
A syntax error in expression, near ‘’.
(gdb) p ’includefile’::some_global
$2 = 27
Warning: Occasionally, a local variable may appear to have the wrong value
at certain points in a function—just after entry to a new scope, and just before
exit.
You may see this problem when you are stepping by machine instructions. This is
because, on most machines, it takes more than one instruction to set up a stack frame
(including local variable definitions); if you are stepping by machine instructions, variables
may appear to have the wrong values until the stack frame is completely built. On exit, it
usually also takes more than one machine instruction to destroy a stack frame; after you
begin stepping through that group of instructions, local variable definitions may be gone.
This may also happen when the compiler does significant optimizations. To be sure of
always seeing accurate values, turn off all optimization when compiling.
Another possible effect of compiler optimizations is to optimize unused variables out of
existence, or assign variables to registers (as opposed to memory addresses). Depending
Chapter 10: Examining Data
121
on the support for such cases offered by the debug info format used by the compiler, gdb
might not be able to display values for such local variables. If that happens, gdb will print
a message like this:
No symbol "foo" in current context.
To solve such problems, either recompile without optimizations, or use a different debug
info format, if the compiler supports several such formats. See Section 4.1 [Compilation],
page 25, for more information on choosing compiler options. See Section 15.4.1 [C and
C++], page 195, for more information about debug info formats that are best suited to C++
programs.
If you ask to print an object whose contents are unknown to gdb, e.g., because its
data type is not completely specified by the debug information, gdb will say ‘’. See Chapter 16 [Symbols], page 221, for more about this.
If you append @entry string to a function parameter name you get its value at the time
the function got called. If the value is not available an error message is printed. Entry
values are available only with some compilers. Entry values are normally also printed at
the function parameter list according to [set print entry-values], page 131.
Breakpoint 1, d (i=30) at gdb.base/entry-value.c:29
29
i++;
(gdb) next
30
e (i);
(gdb) print i
$1 = 31
(gdb) print i@entry
$2 = 30
Strings are identified as arrays of char values without specified signedness. Arrays of
either signed char or unsigned char get printed as arrays of 1 byte sized integers. fsigned-char or -funsigned-char gcc options have no effect as gdb defines literal string
type "char" as char without a sign. For program code
char var0[] = "A";
signed char var1[] = "A";
You get during debugging
(gdb) print var0
$1 = "A"
(gdb) print var1
$2 = {65 ’A’, 0 ’\0’}
10.4 Artificial Arrays
It is often useful to print out several successive objects of the same type in memory; a
section of an array, or an array of dynamically determined size for which only a pointer
exists in the program.
You can do this by referring to a contiguous span of memory as an artificial array, using
the binary operator ‘@’. The left operand of ‘@’ should be the first element of the desired
array and be an individual object. The right operand should be the desired length of the
array. The result is an array value whose elements are all of the type of the left argument.
The first element is actually the left argument; the second element comes from bytes of
memory immediately following those that hold the first element, and so on. Here is an
example. If a program says
122
Debugging with gdb
int *array = (int *) malloc (len * sizeof (int));
you can print the contents of array with
p *array@len
The left operand of ‘@’ must reside in memory. Array values made with ‘@’ in this way
behave just like other arrays in terms of subscripting, and are coerced to pointers when
used in expressions. Artificial arrays most often appear in expressions via the value history
(see Section 10.10 [Value History], page 138), after printing one out.
Another way to create an artificial array is to use a cast. This re-interprets a value as if
it were an array. The value need not be in memory:
(gdb) p/x (short[2])0x12345678
$1 = {0x1234, 0x5678}
As a convenience, if you leave the array length out (as in ‘(type[])value’) gdb calculates the size to fill the value (as ‘sizeof(value)/sizeof(type)’:
(gdb) p/x (short[])0x12345678
$2 = {0x1234, 0x5678}
Sometimes the artificial array mechanism is not quite enough; in moderately complex
data structures, the elements of interest may not actually be adjacent—for example, if you
are interested in the values of pointers in an array. One useful work-around in this situation
is to use a convenience variable (see Section 10.11 [Convenience Variables], page 139) as
a counter in an expression that prints the first interesting value, and then repeat that
expression via RET. For instance, suppose you have an array dtab of pointers to structures,
and you are interested in the values of a field fv in each structure. Here is an example of
what you might type:
set $i = 0
p dtab[$i++]->fv
RET
RET
...
10.5 Output Formats
By default, gdb prints a value according to its data type. Sometimes this is not what you
want. For example, you might want to print a number in hex, or a pointer in decimal. Or
you might want to view data in memory at a certain address as a character string or as an
instruction. To do these things, specify an output format when you print a value.
The simplest use of output formats is to say how to print a value already computed.
This is done by starting the arguments of the print command with a slash and a format
letter. The format letters supported are:
x
Regard the bits of the value as an integer, and print the integer in hexadecimal.
d
Print as integer in signed decimal.
u
Print as integer in unsigned decimal.
o
Print as integer in octal.
t
Print as integer in binary. The letter ‘t’ stands for “two”.1
1
‘b’ cannot be used because these format letters are also used with the x command, where ‘b’ stands for
“byte”; see Section 10.6 [Examining Memory], page 123.
Chapter 10: Examining Data
a
123
Print as an address, both absolute in hexadecimal and as an offset from the
nearest preceding symbol. You can use this format used to discover where (in
what function) an unknown address is located:
(gdb) p/a 0x54320
$3 = 0x54320 <_initialize_vx+396>
The command info symbol 0x54320 yields similar results. See Chapter 16
[Symbols], page 221.
c
Regard as an integer and print it as a character constant. This prints both the
numerical value and its character representation. The character representation
is replaced with the octal escape ‘\nnn’ for characters outside the 7-bit ascii
range.
Without this format, gdb displays char, unsigned char, and signed char
data as character constants. Single-byte members of vectors are displayed as
integer data.
f
Regard the bits of the value as a floating point number and print using typical
floating point syntax.
s
Regard as a string, if possible. With this format, pointers to single-byte data are
displayed as null-terminated strings and arrays of single-byte data are displayed
as fixed-length strings. Other values are displayed in their natural types.
Without this format, gdb displays pointers to and arrays of char,
unsigned char, and signed char as strings. Single-byte members of a vector
are displayed as an integer array.
z
Like ‘x’ formatting, the value is treated as an integer and printed as hexadecimal, but leading zeros are printed to pad the value to the size of the integer
type.
r
Print using the ‘raw’ formatting. By default, gdb will use a Python-based
pretty-printer, if one is available (see Section 10.9 [Pretty Printing], page 135).
This typically results in a higher-level display of the value’s contents. The ‘r’
format bypasses any Python pretty-printer which might exist.
For example, to print the program counter in hex (see Section 10.13 [Registers],
page 144), type
p/x $pc
Note that no space is required before the slash; this is because command names in gdb
cannot contain a slash.
To reprint the last value in the value history with a different format, you can use the
print command with just a format and no expression. For example, ‘p/x’ reprints the last
value in hex.
10.6 Examining Memory
You can use the command x (for “examine”) to examine memory in any of several formats,
independently of your program’s data types.
124
Debugging with gdb
x/nfu addr
x addr
x
Use the x command to examine memory.
n, f, and u are all optional parameters that specify how much memory to display and how
to format it; addr is an expression giving the address where you want to start displaying
memory. If you use defaults for nfu, you need not type the slash ‘/’. Several commands set
convenient defaults for addr.
n, the repeat count
The repeat count is a decimal integer; the default is 1. It specifies how much
memory (counting by units u) to display. If a negative number is specified,
memory is examined backward from addr.
f, the display format
The display format is one of the formats used by print (‘x’, ‘d’, ‘u’, ‘o’, ‘t’,
‘a’, ‘c’, ‘f’, ‘s’), and in addition ‘i’ (for machine instructions). The default is
‘x’ (hexadecimal) initially. The default changes each time you use either x or
print.
u, the unit size
The unit size is any of
b
Bytes.
h
Halfwords (two bytes).
w
Words (four bytes). This is the initial default.
g
Giant words (eight bytes).
Each time you specify a unit size with x, that size becomes the default unit
the next time you use x. For the ‘i’ format, the unit size is ignored and is
normally not written. For the ‘s’ format, the unit size defaults to ‘b’, unless it
is explicitly given. Use x /hs to display 16-bit char strings and x /ws to display
32-bit strings. The next use of x /s will again display 8-bit strings. Note that
the results depend on the programming language of the current compilation
unit. If the language is C, the ‘s’ modifier will use the UTF-16 encoding while
‘w’ will use UTF-32. The encoding is set by the programming language and
cannot be altered.
addr, starting display address
addr is the address where you want gdb to begin displaying memory. The expression need not have a pointer value (though it may); it is always interpreted
as an integer address of a byte of memory. See Section 10.1 [Expressions],
page 117, for more information on expressions. The default for addr is usually just after the last address examined—but several other commands also set
the default address: info breakpoints (to the address of the last breakpoint
listed), info line (to the starting address of a line), and print (if you use it
to display a value from memory).
For example, ‘x/3uh 0x54320’ is a request to display three halfwords (h) of memory,
formatted as unsigned decimal integers (‘u’), starting at address 0x54320. ‘x/4xw $sp’
Chapter 10: Examining Data
125
prints the four words (‘w’) of memory above the stack pointer (here, ‘$sp’; see Section 10.13
[Registers], page 144) in hexadecimal (‘x’).
You can also specify a negative repeat count to examine memory backward from the given
address. For example, ‘x/-3uh 0x54320’ prints three halfwords (h) at 0x54314, 0x54328,
and 0x5431c.
Since the letters indicating unit sizes are all distinct from the letters specifying output
formats, you do not have to remember whether unit size or format comes first; either order
works. The output specifications ‘4xw’ and ‘4wx’ mean exactly the same thing. (However,
the count n must come first; ‘wx4’ does not work.)
Even though the unit size u is ignored for the formats ‘s’ and ‘i’, you might still want to
use a count n; for example, ‘3i’ specifies that you want to see three machine instructions,
including any operands. For convenience, especially when used with the display command,
the ‘i’ format also prints branch delay slot instructions, if any, beyond the count specified,
which immediately follow the last instruction that is within the count. The command
disassemble gives an alternative way of inspecting machine instructions; see Section 9.6
[Source and Machine Code], page 110.
If a negative repeat count is specified for the formats ‘s’ or ‘i’, the command displays
null-terminated strings or instructions before the given address as many as the absolute
value of the given number. For the ‘i’ format, we use line number information in the debug
info to accurately locate instruction boundaries while disassembling backward. If line info
is not available, the command stops examining memory with an error message.
All the defaults for the arguments to x are designed to make it easy to continue scanning
memory with minimal specifications each time you use x. For example, after you have
inspected three machine instructions with ‘x/3i addr’, you can inspect the next seven with
just ‘x/7’. If you use RET to repeat the x command, the repeat count n is used again; the
other arguments default as for successive uses of x.
When examining machine instructions, the instruction at current program counter is
shown with a => marker. For example:
(gdb) x/5i $pc-6
0x804837f :
0x8048381 :
0x8048382 :
=> 0x8048385 :
0x804838c :
mov
push
sub
movl
call
%esp,%ebp
%ecx
$0x4,%esp
$0x8048460,(%esp)
0x80482d4
The addresses and contents printed by the x command are not saved in the value history
because there is often too much of them and they would get in the way. Instead, gdb
makes these values available for subsequent use in expressions as values of the convenience
variables $_ and $__. After an x command, the last address examined is available for use
in expressions in the convenience variable $_. The contents of that address, as examined,
are available in the convenience variable $__.
If the x command has a repeat count, the address and contents saved are from the last
memory unit printed; this is not the same as the last address printed if several units were
printed on the last line of output.
Most targets have an addressable memory unit size of 8 bits. This means that to each
memory address are associated 8 bits of data. Some targets, however, have other addressable
memory unit sizes. Within gdb and this document, the term addressable memory unit (or
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Debugging with gdb
memory unit for short) is used when explicitly referring to a chunk of data of that size.
The word byte is used to refer to a chunk of data of 8 bits, regardless of the addressable
memory unit size of the target. For most systems, addressable memory unit is a synonym
of byte.
When you are debugging a program running on a remote target machine (see Chapter 20
[Remote Debugging], page 261), you may wish to verify the program’s image in the remote
machine’s memory against the executable file you downloaded to the target. Or, on any
target, you may want to check whether the program has corrupted its own read-only sections.
The compare-sections command is provided for such situations.
compare-sections [section-name|-r]
Compare the data of a loadable section section-name in the executable file of the
program being debugged with the same section in the target machine’s memory,
and report any mismatches. With no arguments, compares all loadable sections.
With an argument of -r, compares all loadable read-only sections.
Note: for remote targets, this command can be accelerated if the target supports computing the CRC checksum of a block of memory (see [qCRC packet],
page 635).
10.7 Automatic Display
If you find that you want to print the value of an expression frequently (to see how it
changes), you might want to add it to the automatic display list so that gdb prints its
value each time your program stops. Each expression added to the list is given a number to
identify it; to remove an expression from the list, you specify that number. The automatic
display looks like this:
2: foo = 38
3: bar[5] = (struct hack *) 0x3804
This display shows item numbers, expressions and their current values. As with displays
you request manually using x or print, you can specify the output format you prefer; in
fact, display decides whether to use print or x depending your format specification—it
uses x if you specify either the ‘i’ or ‘s’ format, or a unit size; otherwise it uses print.
display expr
Add the expression expr to the list of expressions to display each time your
program stops. See Section 10.1 [Expressions], page 117.
display does not repeat if you press RET again after using it.
display/fmt expr
For fmt specifying only a display format and not a size or count, add the
expression expr to the auto-display list but arrange to display it each time in
the specified format fmt. See Section 10.5 [Output Formats], page 122.
display/fmt addr
For fmt ‘i’ or ‘s’, or including a unit-size or a number of units, add the expression addr as a memory address to be examined each time your program stops.
Examining means in effect doing ‘x/fmt addr’. See Section 10.6 [Examining
Memory], page 123.
Chapter 10: Examining Data
127
For example, ‘display/i $pc’ can be helpful, to see the machine instruction about to
be executed each time execution stops (‘$pc’ is a common name for the program counter;
see Section 10.13 [Registers], page 144).
undisplay dnums...
delete display dnums...
Remove items from the list of expressions to display. Specify the numbers of
the displays that you want affected with the command argument dnums. It can
be a single display number, one of the numbers shown in the first field of the
‘info display’ display; or it could be a range of display numbers, as in 2-4.
undisplay does not repeat if you press RET after using it. (Otherwise you would
just get the error ‘No display number ...’.)
disable display dnums...
Disable the display of item numbers dnums. A disabled display item is not
printed automatically, but is not forgotten. It may be enabled again later.
Specify the numbers of the displays that you want affected with the command
argument dnums. It can be a single display number, one of the numbers shown
in the first field of the ‘info display’ display; or it could be a range of display
numbers, as in 2-4.
enable display dnums...
Enable display of item numbers dnums. It becomes effective once again in auto
display of its expression, until you specify otherwise. Specify the numbers of
the displays that you want affected with the command argument dnums. It can
be a single display number, one of the numbers shown in the first field of the
‘info display’ display; or it could be a range of display numbers, as in 2-4.
display
Display the current values of the expressions on the list, just as is done when
your program stops.
info display
Print the list of expressions previously set up to display automatically, each
one with its item number, but without showing the values. This includes disabled expressions, which are marked as such. It also includes expressions which
would not be displayed right now because they refer to automatic variables not
currently available.
If a display expression refers to local variables, then it does not make sense outside the
lexical context for which it was set up. Such an expression is disabled when execution enters
a context where one of its variables is not defined. For example, if you give the command
display last_char while inside a function with an argument last_char, gdb displays
this argument while your program continues to stop inside that function. When it stops
elsewhere—where there is no variable last_char—the display is disabled automatically.
The next time your program stops where last_char is meaningful, you can enable the
display expression once again.
10.8 Print Settings
gdb provides the following ways to control how arrays, structures, and symbols are printed.
These settings are useful for debugging programs in any language:
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Debugging with gdb
set print address
set print address on
gdb prints memory addresses showing the location of stack traces, structure
values, pointer values, breakpoints, and so forth, even when it also displays the
contents of those addresses. The default is on. For example, this is what a
stack frame display looks like with set print address on:
(gdb) f
#0 set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>")
at input.c:530
530
if (lquote != def_lquote)
set print address off
Do not print addresses when displaying their contents. For example, this is the
same stack frame displayed with set print address off:
(gdb) set print addr off
(gdb) f
#0 set_quotes (lq="<<", rq=">>") at input.c:530
530
if (lquote != def_lquote)
You can use ‘set print address off’ to eliminate all machine dependent displays from the gdb interface. For example, with print address off, you
should get the same text for backtraces on all machines—whether or not they
involve pointer arguments.
show print address
Show whether or not addresses are to be printed.
When gdb prints a symbolic address, it normally prints the closest earlier symbol plus
an offset. If that symbol does not uniquely identify the address (for example, it is a name
whose scope is a single source file), you may need to clarify. One way to do this is with info
line, for example ‘info line *0x4537’. Alternately, you can set gdb to print the source
file and line number when it prints a symbolic address:
set print symbol-filename on
Tell gdb to print the source file name and line number of a symbol in the
symbolic form of an address.
set print symbol-filename off
Do not print source file name and line number of a symbol. This is the default.
show print symbol-filename
Show whether or not gdb will print the source file name and line number of a
symbol in the symbolic form of an address.
Another situation where it is helpful to show symbol filenames and line numbers is when
disassembling code; gdb shows you the line number and source file that corresponds to each
instruction.
Also, you may wish to see the symbolic form only if the address being printed is reasonably close to the closest earlier symbol:
set print max-symbolic-offset max-offset
set print max-symbolic-offset unlimited
Tell gdb to only display the symbolic form of an address if the offset between
the closest earlier symbol and the address is less than max-offset. The default
Chapter 10: Examining Data
129
is unlimited, which tells gdb to always print the symbolic form of an address
if any symbol precedes it. Zero is equivalent to unlimited.
show print max-symbolic-offset
Ask how large the maximum offset is that gdb prints in a symbolic address.
If you have a pointer and you are not sure where it points, try ‘set print
symbol-filename on’. Then you can determine the name and source file location of the
variable where it points, using ‘p/a pointer’. This interprets the address in symbolic
form. For example, here gdb shows that a variable ptt points at another variable t,
defined in ‘hi2.c’:
(gdb) set print symbol-filename on
(gdb) p/a ptt
$4 = 0xe008
Warning: For pointers that point to a local variable, ‘p/a’ does not show the
symbol name and filename of the referent, even with the appropriate set print
options turned on.
You can also enable ‘/a’-like formatting all the time using ‘set print symbol on’:
set print symbol on
Tell gdb to print the symbol corresponding to an address, if one exists.
set print symbol off
Tell gdb not to print the symbol corresponding to an address. In this mode,
gdb will still print the symbol corresponding to pointers to functions. This is
the default.
show print symbol
Show whether gdb will display the symbol corresponding to an address.
Other settings control how different kinds of objects are printed:
set print array
set print array on
Pretty print arrays. This format is more convenient to read, but uses more
space. The default is off.
set print array off
Return to compressed format for arrays.
show print array
Show whether compressed or pretty format is selected for displaying arrays.
set print array-indexes
set print array-indexes on
Print the index of each element when displaying arrays. May be more convenient
to locate a given element in the array or quickly find the index of a given element
in that printed array. The default is off.
set print array-indexes off
Stop printing element indexes when displaying arrays.
show print array-indexes
Show whether the index of each element is printed when displaying arrays.
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Debugging with gdb
set print elements number-of-elements
set print elements unlimited
Set a limit on how many elements of an array gdb will print. If gdb is printing
a large array, it stops printing after it has printed the number of elements set
by the set print elements command. This limit also applies to the display of
strings. When gdb starts, this limit is set to 200. Setting number-of-elements
to unlimited or zero means that the number of elements to print is unlimited.
show print elements
Display the number of elements of a large array that gdb will print. If the
number is 0, then the printing is unlimited.
set print frame-arguments value
This command allows to control how the values of arguments are printed when
the debugger prints a frame (see Section 8.1 [Frames], page 95). The possible
values are:
all
The values of all arguments are printed.
scalars
Print the value of an argument only if it is a scalar. The value of
more complex arguments such as arrays, structures, unions, etc, is
replaced by .... This is the default. Here is an example where
only scalar arguments are shown:
#1 0x08048361 in call_me (i=3, s=..., ss=0xbf8d508c, u=..., e=green)
at frame-args.c:23
none
None of the argument values are printed. Instead, the value of each
argument is replaced by .... In this case, the example above now
becomes:
#1 0x08048361 in call_me (i=..., s=..., ss=..., u=..., e=...)
at frame-args.c:23
By default, only scalar arguments are printed. This command can be used to
configure the debugger to print the value of all arguments, regardless of their
type. However, it is often advantageous to not print the value of more complex
parameters. For instance, it reduces the amount of information printed in each
frame, making the backtrace more readable. Also, it improves performance
when displaying Ada frames, because the computation of large arguments can
sometimes be CPU-intensive, especially in large applications. Setting print
frame-arguments to scalars (the default) or none avoids this computation,
thus speeding up the display of each Ada frame.
show print frame-arguments
Show how the value of arguments should be displayed when printing a frame.
set print raw frame-arguments on
Print frame arguments in raw, non pretty-printed, form.
set print raw frame-arguments off
Print frame arguments in pretty-printed form, if there is a pretty-printer for the
value (see Section 10.9 [Pretty Printing], page 135), otherwise print the value
in raw form. This is the default.
Chapter 10: Examining Data
131
show print raw frame-arguments
Show whether to print frame arguments in raw form.
set print entry-values value
Set printing of frame argument values at function entry. In some cases gdb can
determine the value of function argument which was passed by the function
caller, even if the value was modified inside the called function and therefore is
different. With optimized code, the current value could be unavailable, but the
entry value may still be known.
The default value is default (see below for its description). Older gdb behaved
as with the setting no. Compilers not supporting this feature will behave in the
default setting the same way as with the no setting.
This functionality is currently supported only by DWARF 2 debugging format
and the compiler has to produce ‘DW_TAG_call_site’ tags. With gcc, you
need to specify ‘-O -g’ during compilation, to get this information.
The value parameter can be one of the following:
no
Print only actual parameter values, never print values from function
entry point.
#0
#0
#0
#0
#0
only
equal (val=5)
different (val=6)
lost (val=)
born (val=10)
invalid (val=)
Print only parameter values from function entry point. The actual
parameter values are never printed.
#0
#0
#0
#0
#0
equal (val@entry=5)
different (val@entry=5)
lost (val@entry=5)
born (val@entry=)
invalid (val@entry=)
preferred
Print only parameter values from function entry point. If value
from function entry point is not known while the actual value is
known, print the actual value for such parameter.
#0
#0
#0
#0
#0
equal (val@entry=5)
different (val@entry=5)
lost (val@entry=5)
born (val=10)
invalid (val@entry=)
if-needed
Print actual parameter values. If actual parameter value is not
known while value from function entry point is known, print the
entry point value for such parameter.
#0
#0
#0
#0
#0
equal (val=5)
different (val=6)
lost (val@entry=5)
born (val=10)
invalid (val=)
132
Debugging with gdb
both
Always print both the actual parameter value and its value from
function entry point, even if values of one or both are not available
due to compiler optimizations.
#0
#0
#0
#0
#0
compact
Print the actual parameter value if it is known and also its value
from function entry point if it is known. If neither is known, print
for the actual value . If not in MI mode (see
Chapter 27 [GDB/MI], page 469) and if both values are known and
identical, print the shortened param=param@entry=VALUE notation.
#0
#0
#0
#0
#0
default
equal (val=5, val@entry=5)
different (val=6, val@entry=5)
lost (val=, val@entry=5)
born (val=10, val@entry=)
invalid (val=, val@entry=)
equal (val=val@entry=5)
different (val=6, val@entry=5)
lost (val@entry=5)
born (val=10)
invalid (val=)
Always print the actual parameter value. Print also its value from
function entry point, but only if it is known. If not in MI mode (see
Chapter 27 [GDB/MI], page 469) and if both values are known and
identical, print the shortened param=param@entry=VALUE notation.
#0
#0
#0
#0
#0
equal (val=val@entry=5)
different (val=6, val@entry=5)
lost (val=, val@entry=5)
born (val=10)
invalid (val=)
For analysis messages on possible failures of frame argument values at function
entry resolution see [set debug entry-values], page 160.
show print entry-values
Show the method being used for printing of frame argument values at function
entry.
set print repeats number-of-repeats
set print repeats unlimited
Set the threshold for suppressing display of repeated array elements. When
the number of consecutive identical elements of an array exceeds the threshold,
gdb prints the string "", where n is the number of identical
repetitions, instead of displaying the identical elements themselves. Setting
the threshold to unlimited or zero will cause all elements to be individually
printed. The default threshold is 10.
show print repeats
Display the current threshold for printing repeated identical elements.
set print null-stop
Cause gdb to stop printing the characters of an array when the first null
is encountered. This is useful when large arrays actually contain only short
strings. The default is off.
Chapter 10: Examining Data
133
show print null-stop
Show whether gdb stops printing an array on the first null character.
set print pretty on
Cause gdb to print structures in an indented format with one member per line,
like this:
$1 = {
next = 0x0,
flags = {
sweet = 1,
sour = 1
},
meat = 0x54 "Pork"
}
set print pretty off
Cause gdb to print structures in a compact format, like this:
$1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \
meat = 0x54 "Pork"}
This is the default format.
show print pretty
Show which format gdb is using to print structures.
set print sevenbit-strings on
Print using only seven-bit characters; if this option is set, gdb displays any
eight-bit characters (in strings or character values) using the notation \nnn.
This setting is best if you are working in English (ascii) and you use the highorder bit of characters as a marker or “meta” bit.
set print sevenbit-strings off
Print full eight-bit characters. This allows the use of more international character sets, and is the default.
show print sevenbit-strings
Show whether or not gdb is printing only seven-bit characters.
set print union on
Tell gdb to print unions which are contained in structures and other unions.
This is the default setting.
set print union off
Tell gdb not to print unions which are contained in structures and other unions.
gdb will print "{...}" instead.
show print union
Ask gdb whether or not it will print unions which are contained in structures
and other unions.
For example, given the declarations
typedef enum {Tree, Bug} Species;
typedef enum {Big_tree, Acorn, Seedling} Tree_forms;
typedef enum {Caterpillar, Cocoon, Butterfly}
Bug_forms;
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Debugging with gdb
struct thing {
Species it;
union {
Tree_forms tree;
Bug_forms bug;
} form;
};
struct thing foo = {Tree, {Acorn}};
with set print union on in effect ‘p foo’ would print
$1 = {it = Tree, form = {tree = Acorn, bug = Cocoon}}
and with set print union off in effect it would print
$1 = {it = Tree, form = {...}}
set print union affects programs written in C-like languages and in Pascal.
These settings are of interest when debugging C++ programs:
set print demangle
set print demangle on
Print C++ names in their source form rather than in the encoded (“mangled”)
form passed to the assembler and linker for type-safe linkage. The default is
on.
show print demangle
Show whether C++ names are printed in mangled or demangled form.
set print asm-demangle
set print asm-demangle on
Print C++ names in their source form rather than their mangled form, even in
assembler code printouts such as instruction disassemblies. The default is off.
show print asm-demangle
Show whether C++ names in assembly listings are printed in mangled or demangled form.
set demangle-style style
Choose among several encoding schemes used by different compilers to represent
C++ names. The choices for style are currently:
auto
Allow gdb to choose a decoding style by inspecting your program.
This is the default.
gnu
Decode based on the gnu C++ compiler (g++) encoding algorithm.
hp
Decode based on the HP ANSI C++ (aCC) encoding algorithm.
lucid
Decode based on the Lucid C++ compiler (lcc) encoding algorithm.
arm
Decode using the algorithm in the C++ Annotated Reference Manual. Warning: this setting alone is not sufficient to allow debugging
cfront-generated executables. gdb would require further enhancement to permit that.
If you omit style, you will see a list of possible formats.
Chapter 10: Examining Data
135
show demangle-style
Display the encoding style currently in use for decoding C++ symbols.
set print object
set print object on
When displaying a pointer to an object, identify the actual (derived) type of
the object rather than the declared type, using the virtual function table. Note
that the virtual function table is required—this feature can only work for objects
that have run-time type identification; a single virtual method in the object’s
declared type is sufficient. Note that this setting is also taken into account when
working with variable objects via MI (see Chapter 27 [GDB/MI], page 469).
set print object off
Display only the declared type of objects, without reference to the virtual function table. This is the default setting.
show print object
Show whether actual, or declared, object types are displayed.
set print static-members
set print static-members on
Print static members when displaying a C++ object. The default is on.
set print static-members off
Do not print static members when displaying a C++ object.
show print static-members
Show whether C++ static members are printed or not.
set print pascal_static-members
set print pascal_static-members on
Print static members when displaying a Pascal object. The default is on.
set print pascal_static-members off
Do not print static members when displaying a Pascal object.
show print pascal_static-members
Show whether Pascal static members are printed or not.
set print vtbl
set print vtbl on
Pretty print C++ virtual function tables. The default is off. (The vtbl commands do not work on programs compiled with the HP ANSI C++ compiler
(aCC).)
set print vtbl off
Do not pretty print C++ virtual function tables.
show print vtbl
Show whether C++ virtual function tables are pretty printed, or not.
10.9 Pretty Printing
gdb provides a mechanism to allow pretty-printing of values using Python code. It greatly
simplifies the display of complex objects. This mechanism works for both MI and the CLI.
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Debugging with gdb
10.9.1 Pretty-Printer Introduction
When gdb prints a value, it first sees if there is a pretty-printer registered for the value.
If there is then gdb invokes the pretty-printer to print the value. Otherwise the value is
printed normally.
Pretty-printers are normally named. This makes them easy to manage. The ‘info
pretty-printer’ command will list all the installed pretty-printers with their names. If a
pretty-printer can handle multiple data types, then its subprinters are the printers for the
individual data types. Each such subprinter has its own name. The format of the name is
printer-name;subprinter-name.
Pretty-printers are installed by registering them with gdb. Typically they are automatically loaded and registered when the corresponding debug information is loaded, thus
making them available without having to do anything special.
There are three places where a pretty-printer can be registered.
• Pretty-printers registered globally are available when debugging all inferiors.
• Pretty-printers registered with a program space are available only when debugging that
program. See Section 23.2.2.23 [Progspaces In Python], page 379, for more details on
program spaces in Python.
• Pretty-printers registered with an objfile are loaded and unloaded with the corresponding objfile (e.g., shared library). See Section 23.2.2.24 [Objfiles In Python], page 380,
for more details on objfiles in Python.
See Section 23.2.2.6 [Selecting Pretty-Printers], page 344, for further information on how
pretty-printers are selected,
See Section 23.2.2.7 [Writing a Pretty-Printer], page 344, for implementing pretty printers for new types.
10.9.2 Pretty-Printer Example
Here is how a C++ std::string looks without a pretty-printer:
(gdb) print s
$1 = {
static npos = 4294967295,
_M_dataplus = {
> = {
<__gnu_cxx::new_allocator> = {
},
},
members of std::basic_string,
std::allocator >::_Alloc_hider:
_M_p = 0x804a014 "abcd"
}
}
With a pretty-printer for std::string only the contents are printed:
(gdb) print s
$2 = "abcd"
Chapter 10: Examining Data
137
10.9.3 Pretty-Printer Commands
info pretty-printer [object-regexp [name-regexp]]
Print the list of installed pretty-printers. This includes disabled pretty-printers,
which are marked as such.
object-regexp is a regular expression matching the objects whose pretty-printers
to list. Objects can be global, the program space’s file (see Section 23.2.2.23
[Progspaces In Python], page 379), and the object files within that program
space (see Section 23.2.2.24 [Objfiles In Python], page 380). See Section 23.2.2.6
[Selecting Pretty-Printers], page 344, for details on how gdb looks up a printer
from these three objects.
name-regexp is a regular expression matching the name of the printers to list.
disable pretty-printer [object-regexp [name-regexp]]
Disable pretty-printers matching object-regexp and name-regexp. A disabled
pretty-printer is not forgotten, it may be enabled again later.
enable pretty-printer [object-regexp [name-regexp]]
Enable pretty-printers matching object-regexp and name-regexp.
Example:
Suppose we have three pretty-printers installed: one from library1.so named foo that
prints objects of type foo, and another from library2.so named bar that prints two types
of objects, bar1 and bar2.
(gdb) info pretty-printer
library1.so:
foo
library2.so:
bar
bar1
bar2
(gdb) info pretty-printer library2
library2.so:
bar
bar1
bar2
(gdb) disable pretty-printer library1
1 printer disabled
2 of 3 printers enabled
(gdb) info pretty-printer
library1.so:
foo [disabled]
library2.so:
bar
bar1
bar2
(gdb) disable pretty-printer library2 bar:bar1
1 printer disabled
1 of 3 printers enabled
(gdb) info pretty-printer library2
library1.so:
foo [disabled]
library2.so:
bar
138
Debugging with gdb
bar1 [disabled]
bar2
(gdb) disable pretty-printer library2 bar
1 printer disabled
0 of 3 printers enabled
(gdb) info pretty-printer library2
library1.so:
foo [disabled]
library2.so:
bar [disabled]
bar1 [disabled]
bar2
Note that for bar the entire printer can be disabled, as can each individual subprinter.
10.10 Value History
Values printed by the print command are saved in the gdb value history. This allows you
to refer to them in other expressions. Values are kept until the symbol table is re-read or
discarded (for example with the file or symbol-file commands). When the symbol table
changes, the value history is discarded, since the values may contain pointers back to the
types defined in the symbol table.
The values printed are given history numbers by which you can refer to them. These
are successive integers starting with one. print shows you the history number assigned to
a value by printing ‘$num = ’ before the value; here num is the history number.
To refer to any previous value, use ‘$’ followed by the value’s history number. The way
print labels its output is designed to remind you of this. Just $ refers to the most recent
value in the history, and $$ refers to the value before that. $$n refers to the nth value from
the end; $$2 is the value just prior to $$, $$1 is equivalent to $$, and $$0 is equivalent to
$.
For example, suppose you have just printed a pointer to a structure and want to see the
contents of the structure. It suffices to type
p *$
If you have a chain of structures where the component next points to the next one, you
can print the contents of the next one with this:
p *$.next
You can print successive links in the chain by repeating this command—which you can do
by just typing RET.
Note that the history records values, not expressions. If the value of x is 4 and you type
these commands:
print x
set x=5
then the value recorded in the value history by the print command remains 4 even though
the value of x has changed.
show values
Print the last ten values in the value history, with their item numbers. This is
like ‘p $$9’ repeated ten times, except that show values does not change the
history.
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show values n
Print ten history values centered on history item number n.
show values +
Print ten history values just after the values last printed. If no more values are
available, show values + produces no display.
Pressing RET to repeat show values n has exactly the same effect as ‘show values +’.
10.11 Convenience Variables
gdb provides convenience variables that you can use within gdb to hold on to a value
and refer to it later. These variables exist entirely within gdb; they are not part of your
program, and setting a convenience variable has no direct effect on further execution of your
program. That is why you can use them freely.
Convenience variables are prefixed with ‘$’. Any name preceded by ‘$’ can be used for a
convenience variable, unless it is one of the predefined machine-specific register names (see
Section 10.13 [Registers], page 144). (Value history references, in contrast, are numbers
preceded by ‘$’. See Section 10.10 [Value History], page 138.)
You can save a value in a convenience variable with an assignment expression, just as
you would set a variable in your program. For example:
set $foo = *object_ptr
would save in $foo the value contained in the object pointed to by object_ptr.
Using a convenience variable for the first time creates it, but its value is void until you
assign a new value. You can alter the value with another assignment at any time.
Convenience variables have no fixed types. You can assign a convenience variable any
type of value, including structures and arrays, even if that variable already has a value of
a different type. The convenience variable, when used as an expression, has the type of its
current value.
show convenience
Print a list of convenience variables used so far, and their values, as well as a
list of the convenience functions. Abbreviated show conv.
init-if-undefined $variable = expression
Set a convenience variable if it has not already been set. This is useful for
user-defined commands that keep some state. It is similar, in concept, to using
local static variables with initializers in C (except that convenience variables
are global). It can also be used to allow users to override default values used in
a command script.
If the variable is already defined then the expression is not evaluated so any
side-effects do not occur.
One of the ways to use a convenience variable is as a counter to be incremented or a
pointer to be advanced. For example, to print a field from successive elements of an array
of structures:
set $i = 0
print bar[$i++]->contents
Repeat that command by typing RET.
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Some convenience variables are created automatically by gdb and given values likely to
be useful.
$_
The variable $_ is automatically set by the x command to the last address
examined (see Section 10.6 [Examining Memory], page 123). Other commands
which provide a default address for x to examine also set $_ to that address;
these commands include info line and info breakpoint. The type of $_ is
void * except when set by the x command, in which case it is a pointer to the
type of $__.
$__
The variable $__ is automatically set by the x command to the value found in
the last address examined. Its type is chosen to match the format in which the
data was printed.
$_exitcode
When the program being debugged terminates normally, gdb automatically
sets this variable to the exit code of the program, and resets $_exitsignal to
void.
$_exitsignal
When the program being debugged dies due to an uncaught signal, gdb automatically sets this variable to that signal’s number, and resets $_exitcode to
void.
To distinguish between whether the program being debugged has exited (i.e.,
$_exitcode is not void) or signalled (i.e., $_exitsignal is not void), the
convenience function $_isvoid can be used (see Section 10.12 [Convenience
Functions], page 141). For example, considering the following source code:
#include
int
main (int argc, char *argv[])
{
raise (SIGALRM);
return 0;
}
A valid way of telling whether the program being debugged has exited or signalled would be:
(gdb) define has_exited_or_signalled
Type commands for definition of ‘‘has_exited_or_signalled’’.
End with a line saying just ‘‘end’’.
>if $_isvoid ($_exitsignal)
>echo The program has exited\n
>else
>echo The program has signalled\n
>end
>end
(gdb) run
Starting program:
Program terminated with signal SIGALRM, Alarm clock.
The program no longer exists.
(gdb) has_exited_or_signalled
The program has signalled
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As can be seen, gdb correctly informs that the program being debugged has
signalled, since it calls raise and raises a SIGALRM signal. If the program being
debugged had not called raise, then gdb would report a normal exit:
(gdb) has_exited_or_signalled
The program has exited
$_exception
The variable $_exception is set to the exception object being thrown at an
exception-related catchpoint. See Section 5.1.3 [Set Catchpoints], page 54.
$_probe_argc
$_probe_arg0...$_probe_arg11
Arguments to a static probe. See Section 5.1.10 [Static Probe Points], page 65.
$_sdata
The variable $_sdata contains extra collected static tracepoint data. See
Section 13.1.6 [Tracepoint Action Lists], page 172. Note that $_sdata could
be empty, if not inspecting a trace buffer, or if extra static tracepoint data has
not been collected.
$_siginfo
The variable $_siginfo contains extra signal information (see [extra signal
information], page 76). Note that $_siginfo could be empty, if the application
has not yet received any signals. For example, it will be empty before you
execute the run command.
$_tlb
The variable $_tlb is automatically set when debugging applications running
on MS-Windows in native mode or connected to gdbserver that supports the
qGetTIBAddr request. See Section E.4 [General Query Packets], page 634. This
variable contains the address of the thread information block.
$_inferior
The number of the current inferior. See Section 4.9 [Debugging Multiple Inferiors and Programs], page 33.
$_thread
The thread number of the current thread. See [thread numbers], page 37.
$_gthread
The global number of the current thread. See [global thread numbers], page 37.
10.12 Convenience Functions
gdb also supplies some convenience functions. These have a syntax similar to convenience
variables. A convenience function can be used in an expression just like an ordinary function;
however, a convenience function is implemented internally to gdb.
These functions do not require gdb to be configured with Python support, which means
that they are always available.
$_isvoid (expr)
Return one if the expression expr is void. Otherwise it returns zero.
A void expression is an expression where the type of the result is void. For example, you can examine a convenience variable (see Section 10.11 [Convenience
Variables], page 139) to check whether it is void:
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Debugging with gdb
(gdb) print $_exitcode
$1 = void
(gdb) print $_isvoid ($_exitcode)
$2 = 1
(gdb) run
Starting program: ./a.out
[Inferior 1 (process 29572) exited normally]
(gdb) print $_exitcode
$3 = 0
(gdb) print $_isvoid ($_exitcode)
$4 = 0
In the example above, we used $_isvoid to check whether $_exitcode is void
before and after the execution of the program being debugged. Before the
execution there is no exit code to be examined, therefore $_exitcode is void.
After the execution the program being debugged returned zero, therefore $_
exitcode is zero, which means that it is not void anymore.
The void expression can also be a call of a function from the program being
debugged. For example, given the following function:
void
foo (void)
{
}
The result of calling it inside gdb is void:
(gdb) print foo ()
$1 = void
(gdb) print $_isvoid (foo ())
$2 = 1
(gdb) set $v = foo ()
(gdb) print $v
$3 = void
(gdb) print $_isvoid ($v)
$4 = 1
These functions require gdb to be configured with Python support.
$_memeq(buf1, buf2, length)
Returns one if the length bytes at the addresses given by buf1 and buf2 are
equal. Otherwise it returns zero.
$_regex(str, regex)
Returns one if the string str matches the regular expression regex. Otherwise it
returns zero. The syntax of the regular expression is that specified by Python’s
regular expression support.
$_streq(str1, str2)
Returns one if the strings str1 and str2 are equal. Otherwise it returns zero.
$_strlen(str)
Returns the length of string str.
$_caller_is(name[, number_of_frames])
Returns one if the calling function’s name is equal to name. Otherwise it returns
zero.
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If the optional argument number of frames is provided, it is the number of
frames up in the stack to look. The default is 1.
Example:
(gdb) backtrace
#0 bottom_func ()
at testsuite/gdb.python/py-caller-is.c:21
#1 0x00000000004005a0 in middle_func ()
at testsuite/gdb.python/py-caller-is.c:27
#2 0x00000000004005ab in top_func ()
at testsuite/gdb.python/py-caller-is.c:33
#3 0x00000000004005b6 in main ()
at testsuite/gdb.python/py-caller-is.c:39
(gdb) print $_caller_is ("middle_func")
$1 = 1
(gdb) print $_caller_is ("top_func", 2)
$1 = 1
$_caller_matches(regexp[, number_of_frames])
Returns one if the calling function’s name matches the regular expression regexp. Otherwise it returns zero.
If the optional argument number of frames is provided, it is the number of
frames up in the stack to look. The default is 1.
$_any_caller_is(name[, number_of_frames])
Returns one if any calling function’s name is equal to name. Otherwise it
returns zero.
If the optional argument number of frames is provided, it is the number of
frames up in the stack to look. The default is 1.
This function differs from $_caller_is in that this function checks all stack
frames from the immediate caller to the frame specified by number of frames,
whereas $_caller_is only checks the frame specified by number of frames.
$_any_caller_matches(regexp[, number_of_frames])
Returns one if any calling function’s name matches the regular expression regexp. Otherwise it returns zero.
If the optional argument number of frames is provided, it is the number of
frames up in the stack to look. The default is 1.
This function differs from $_caller_matches in that this function checks
all stack frames from the immediate caller to the frame specified by
number of frames, whereas $_caller_matches only checks the frame specified
by number of frames.
$_as_string(value)
Return the string representation of value.
This function is useful to obtain the textual label (enumerator) of an enumeration value. For example, assuming the variable node is of an enumerated type:
(gdb) printf "Visiting node of type %s\n", $_as_string(node)
Visiting node of type NODE_INTEGER
gdb provides the ability to list and get help on convenience functions.
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help function
Print a list of all convenience functions.
10.13 Registers
You can refer to machine register contents, in expressions, as variables with names starting
with ‘$’. The names of registers are different for each machine; use info registers to see
the names used on your machine.
info registers
Print the names and values of all registers except floating-point and vector
registers (in the selected stack frame).
info all-registers
Print the names and values of all registers, including floating-point and vector
registers (in the selected stack frame).
info registers regname ...
Print the relativized value of each specified register regname. As discussed in
detail below, register values are normally relative to the selected stack frame.
The regname may be any register name valid on the machine you are using,
with or without the initial ‘$’.
gdb has four “standard” register names that are available (in expressions) on most
machines—whenever they do not conflict with an architecture’s canonical mnemonics for
registers. The register names $pc and $sp are used for the program counter register and
the stack pointer. $fp is used for a register that contains a pointer to the current stack
frame, and $ps is used for a register that contains the processor status. For example, you
could print the program counter in hex with
p/x $pc
or print the instruction to be executed next with
x/i $pc
or add four to the stack pointer2 with
set $sp += 4
Whenever possible, these four standard register names are available on your machine
even though the machine has different canonical mnemonics, so long as there is no conflict.
The info registers command shows the canonical names. For example, on the SPARC,
info registers displays the processor status register as $psr but you can also refer to it
as $ps; and on x86-based machines $ps is an alias for the eflags register.
gdb always considers the contents of an ordinary register as an integer when the register
is examined in this way. Some machines have special registers which can hold nothing but
floating point; these registers are considered to have floating point values. There is no way
to refer to the contents of an ordinary register as floating point value (although you can
print it as a floating point value with ‘print/f $regname’).
2
This is a way of removing one word from the stack, on machines where stacks grow downward in memory
(most machines, nowadays). This assumes that the innermost stack frame is selected; setting $sp is not
allowed when other stack frames are selected. To pop entire frames off the stack, regardless of machine
architecture, use return; see Section 17.4 [Returning from a Function], page 232.
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Some registers have distinct “raw” and “virtual” data formats. This means that the data
format in which the register contents are saved by the operating system is not the same
one that your program normally sees. For example, the registers of the 68881 floating point
coprocessor are always saved in “extended” (raw) format, but all C programs expect to work
with “double” (virtual) format. In such cases, gdb normally works with the virtual format
only (the format that makes sense for your program), but the info registers command
prints the data in both formats.
Some machines have special registers whose contents can be interpreted in several different ways. For example, modern x86-based machines have SSE and MMX registers that can
hold several values packed together in several different formats. gdb refers to such registers
in struct notation:
(gdb) print $xmm1
$1 = {
v4_float = {0, 3.43859137e-038, 1.54142831e-044, 1.821688e-044},
v2_double = {9.92129282474342e-303, 2.7585945287983262e-313},
v16_int8 = "\000\000\000\000\3706;\001\v\000\000\000\r\000\000",
v8_int16 = {0, 0, 14072, 315, 11, 0, 13, 0},
v4_int32 = {0, 20657912, 11, 13},
v2_int64 = {88725056443645952, 55834574859},
uint128 = 0x0000000d0000000b013b36f800000000
}
To set values of such registers, you need to tell gdb which view of the register you wish to
change, as if you were assigning value to a struct member:
(gdb) set $xmm1.uint128 = 0x000000000000000000000000FFFFFFFF
Normally, register values are relative to the selected stack frame (see Section 8.3 [Selecting a Frame], page 98). This means that you get the value that the register would contain
if all stack frames farther in were exited and their saved registers restored. In order to see
the true contents of hardware registers, you must select the innermost frame (with ‘frame
0’).
Usually ABIs reserve some registers as not needed to be saved by the callee (a.k.a.:
“caller-saved”, “call-clobbered” or “volatile” registers). It may therefore not be possible for
gdb to know the value a register had before the call (in other words, in the outer frame),
if the register value has since been changed by the callee. gdb tries to deduce where
the inner frame saved (“callee-saved”) registers, from the debug info, unwind info, or the
machine code generated by your compiler. If some register is not saved, and gdb knows the
register is “caller-saved” (via its own knowledge of the ABI, or because the debug/unwind
info explicitly says the register’s value is undefined), gdb displays ‘’ as the
register’s value. With targets that gdb has no knowledge of the register saving convention,
if a register was not saved by the callee, then its value and location in the outer frame are
assumed to be the same of the inner frame. This is usually harmless, because if the register
is call-clobbered, the caller either does not care what is in the register after the call, or has
code to restore the value that it does care about. Note, however, that if you change such
a register in the outer frame, you may also be affecting the inner frame. Also, the more
“outer” the frame is you’re looking at, the more likely a call-clobbered register’s value is
to be wrong, in the sense that it doesn’t actually represent the value the register had just
before the call.
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10.14 Floating Point Hardware
Depending on the configuration, gdb may be able to give you more information about the
status of the floating point hardware.
info float
Display hardware-dependent information about the floating point unit. The
exact contents and layout vary depending on the floating point chip. Currently,
‘info float’ is supported on the ARM and x86 machines.
10.15 Vector Unit
Depending on the configuration, gdb may be able to give you more information about the
status of the vector unit.
info vector
Display information about the vector unit. The exact contents and layout vary
depending on the hardware.
10.16 Operating System Auxiliary Information
gdb provides interfaces to useful OS facilities that can help you debug your program.
Some operating systems supply an auxiliary vector to programs at startup. This is akin
to the arguments and environment that you specify for a program, but contains a systemdependent variety of binary values that tell system libraries important details about the
hardware, operating system, and process. Each value’s purpose is identified by an integer tag; the meanings are well-known but system-specific. Depending on the configuration
and operating system facilities, gdb may be able to show you this information. For remote targets, this functionality may further depend on the remote stub’s support of the
‘qXfer:auxv:read’ packet, see [qXfer auxiliary vector read], page 652.
info auxv Display the auxiliary vector of the inferior, which can be either a live process
or a core dump file. gdb prints each tag value numerically, and also shows
names and text descriptions for recognized tags. Some values in the vector are
numbers, some bit masks, and some pointers to strings or other data. gdb
displays each value in the most appropriate form for a recognized tag, and in
hexadecimal for an unrecognized tag.
On some targets, gdb can access operating system-specific information and show it to
you. The types of information available will differ depending on the type of operating system
running on the target. The mechanism used to fetch the data is described in Appendix H
[Operating System Information], page 713. For remote targets, this functionality depends
on the remote stub’s support of the ‘qXfer:osdata:read’ packet, see [qXfer osdata read],
page 655.
info os infotype
Display OS information of the requested type.
On gnu/Linux, the following values of infotype are valid:
cpus
Display the list of all CPUs/cores. For each CPU/core, gdb prints
the available fields from /proc/cpuinfo. For each supported archi-
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tecture different fields are available. Two common entries are processor which gives CPU number and bogomips; a system constant
that is calculated during kernel initialization.
files
Display the list of open file descriptors on the target. For each
file descriptor, gdb prints the identifier of the process owning the
descriptor, the command of the owning process, the value of the
descriptor, and the target of the descriptor.
modules
Display the list of all loaded kernel modules on the target. For
each module, gdb prints the module name, the size of the module
in bytes, the number of times the module is used, the dependencies
of the module, the status of the module, and the address of the
loaded module in memory.
msg
Display the list of all System V message queues on the target. For
each message queue, gdb prints the message queue key, the message
queue identifier, the access permissions, the current number of bytes
on the queue, the current number of messages on the queue, the
processes that last sent and received a message on the queue, the
user and group of the owner and creator of the message queue, the
times at which a message was last sent and received on the queue,
and the time at which the message queue was last changed.
processes
Display the list of processes on the target. For each process, gdb
prints the process identifier, the name of the user, the command
corresponding to the process, and the list of processor cores that
the process is currently running on. (To understand what these
properties mean, for this and the following info types, please consult
the general gnu/Linux documentation.)
procgroups
Display the list of process groups on the target. For each process,
gdb prints the identifier of the process group that it belongs to, the
command corresponding to the process group leader, the process
identifier, and the command line of the process. The list is sorted
first by the process group identifier, then by the process identifier,
so that processes belonging to the same process group are grouped
together and the process group leader is listed first.
semaphores
Display the list of all System V semaphore sets on the target.
For each semaphore set, gdb prints the semaphore set key, the
semaphore set identifier, the access permissions, the number of
semaphores in the set, the user and group of the owner and creator
of the semaphore set, and the times at which the semaphore set
was operated upon and changed.
shm
Display the list of all System V shared-memory regions on the target. For each shared-memory region, gdb prints the region key,
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the shared-memory identifier, the access permissions, the size of
the region, the process that created the region, the process that
last attached to or detached from the region, the current number of
live attaches to the region, and the times at which the region was
last attached to, detach from, and changed.
info os
sockets
Display the list of Internet-domain sockets on the target. For each
socket, gdb prints the address and port of the local and remote
endpoints, the current state of the connection, the creator of the
socket, the IP address family of the socket, and the type of the
connection.
threads
Display the list of threads running on the target. For each thread,
gdb prints the identifier of the process that the thread belongs to,
the command of the process, the thread identifier, and the processor
core that it is currently running on. The main thread of a process
is not listed.
If infotype is omitted, then list the possible values for infotype and the kind of
OS information available for each infotype. If the target does not return a list
of possible types, this command will report an error.
10.17 Memory Region Attributes
Memory region attributes allow you to describe special handling required by regions of
your target’s memory. gdb uses attributes to determine whether to allow certain types
of memory accesses; whether to use specific width accesses; and whether to cache target
memory. By default the description of memory regions is fetched from the target (if the
current target supports this), but the user can override the fetched regions.
Defined memory regions can be individually enabled and disabled. When a memory
region is disabled, gdb uses the default attributes when accessing memory in that region.
Similarly, if no memory regions have been defined, gdb uses the default attributes when
accessing all memory.
When a memory region is defined, it is given a number to identify it; to enable, disable,
or remove a memory region, you specify that number.
mem lower upper attributes...
Define a memory region bounded by lower and upper with attributes
attributes . . . , and add it to the list of regions monitored by gdb. Note that
upper == 0 is a special case: it is treated as the target’s maximum memory
address. (0xffff on 16 bit targets, 0xffffffff on 32 bit targets, etc.)
mem auto
Discard any user changes to the memory regions and use target-supplied regions,
if available, or no regions if the target does not support.
delete mem nums...
Remove memory regions nums . . . from the list of regions monitored by gdb.
disable mem nums...
Disable monitoring of memory regions nums . . . . A disabled memory region is
not forgotten. It may be enabled again later.
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enable mem nums...
Enable monitoring of memory regions nums . . . .
info mem
Print a table of all defined memory regions, with the following columns for each
region:
Memory Region Number
Enabled or Disabled.
Enabled memory regions are marked with ‘y’. Disabled memory
regions are marked with ‘n’.
Lo Address
The address defining the inclusive lower bound of the memory region.
Hi Address
The address defining the exclusive upper bound of the memory
region.
Attributes The list of attributes set for this memory region.
10.17.1 Attributes
10.17.1.1 Memory Access Mode
The access mode attributes set whether gdb may make read or write accesses to a memory
region.
While these attributes prevent gdb from performing invalid memory accesses, they do
nothing to prevent the target system, I/O DMA, etc. from accessing memory.
ro
Memory is read only.
wo
Memory is write only.
rw
Memory is read/write. This is the default.
10.17.1.2 Memory Access Size
The access size attribute tells gdb to use specific sized accesses in the memory region. Often
memory mapped device registers require specific sized accesses. If no access size attribute
is specified, gdb may use accesses of any size.
8
Use 8 bit memory accesses.
16
Use 16 bit memory accesses.
32
Use 32 bit memory accesses.
64
Use 64 bit memory accesses.
10.17.1.3 Data Cache
The data cache attributes set whether gdb will cache target memory. While this generally
improves performance by reducing debug protocol overhead, it can lead to incorrect results
because gdb does not know about volatile variables or memory mapped device registers.
cache
Enable gdb to cache target memory.
nocache
Disable gdb from caching target memory. This is the default.
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10.17.2 Memory Access Checking
gdb can be instructed to refuse accesses to memory that is not explicitly described. This
can be useful if accessing such regions has undesired effects for a specific target, or to provide
better error checking. The following commands control this behaviour.
set mem inaccessible-by-default [on|off]
If on is specified, make gdb treat memory not explicitly described by the memory ranges as non-existent and refuse accesses to such memory. The checks are
only performed if there’s at least one memory range defined. If off is specified,
make gdb treat the memory not explicitly described by the memory ranges as
RAM. The default value is on.
show mem inaccessible-by-default
Show the current handling of accesses to unknown memory.
10.18 Copy Between Memory and a File
You can use the commands dump, append, and restore to copy data between target memory
and a file. The dump and append commands write data to a file, and the restore command
reads data from a file back into the inferior’s memory. Files may be in binary, Motorola
S-record, Intel hex, Tektronix Hex, or Verilog Hex format; however, gdb can only append
to binary files, and cannot read from Verilog Hex files.
dump [format] memory filename start_addr end_addr
dump [format] value filename expr
Dump the contents of memory from start addr to end addr, or the value of
expr, to filename in the given format.
The format parameter may be any one of:
binary
Raw binary form.
ihex
Intel hex format.
srec
Motorola S-record format.
tekhex
Tektronix Hex format.
verilog
Verilog Hex format.
gdb uses the same definitions of these formats as the gnu binary utilities, like
‘objdump’ and ‘objcopy’. If format is omitted, gdb dumps the data in raw
binary form.
append [binary] memory filename start_addr end_addr
append [binary] value filename expr
Append the contents of memory from start addr to end addr, or the value of
expr, to the file filename, in raw binary form. (gdb can only append data to
files in raw binary form.)
restore filename [binary] bias start end
Restore the contents of file filename into memory. The restore command can
automatically recognize any known bfd file format, except for raw binary. To
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restore a raw binary file you must specify the optional keyword binary after
the filename.
If bias is non-zero, its value will be added to the addresses contained in the file.
Binary files always start at address zero, so they will be restored at address
bias. Other bfd files have a built-in location; they will be restored at offset bias
from that location.
If start and/or end are non-zero, then only data between file offset start and
file offset end will be restored. These offsets are relative to the addresses in the
file, before the bias argument is applied.
10.19 How to Produce a Core File from Your Program
A core file or core dump is a file that records the memory image of a running process
and its process status (register values etc.). Its primary use is post-mortem debugging of a
program that crashed while it ran outside a debugger. A program that crashes automatically
produces a core file, unless this feature is disabled by the user. See Section 18.1 [Files],
page 241, for information on invoking gdb in the post-mortem debugging mode.
Occasionally, you may wish to produce a core file of the program you are debugging in
order to preserve a snapshot of its state. gdb has a special command for that.
generate-core-file [file]
gcore [file]
Produce a core dump of the inferior process. The optional argument file specifies
the file name where to put the core dump. If not specified, the file name defaults
to ‘core.pid’, where pid is the inferior process ID.
Note that this command is implemented only for some systems (as of this
writing, gnu/Linux, FreeBSD, Solaris, and S390).
On gnu/Linux, this command can take into account the value of the file
‘/proc/pid/coredump_filter’ when generating the core dump (see [set usecoredump-filter], page 151).
set use-coredump-filter on
set use-coredump-filter off
Enable or disable the use of the file ‘/proc/pid/coredump_filter’ when generating core dump files. This file is used by the Linux kernel to decide what
types of memory mappings will be dumped or ignored when generating a core
dump file. pid is the process ID of a currently running process.
To make use of this feature, you have to write in the ‘/proc/pid/coredump_filter’
file a value, in hexadecimal, which is a bit mask representing the memory
mapping types. If a bit is set in the bit mask, then the memory mappings of
the corresponding types will be dumped; otherwise, they will be ignored. This
configuration is inherited by child processes. For more information about the
bits that can be set in the ‘/proc/pid/coredump_filter’ file, please refer to
the manpage of core(5).
By default, this option is on. If this option is turned off, gdb does not read the
‘coredump_filter’ file and instead uses the same default value as the Linux
kernel in order to decide which pages will be dumped in the core dump file. This
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Debugging with gdb
value is currently 0x33, which means that bits 0 (anonymous private mappings),
1 (anonymous shared mappings), 4 (ELF headers) and 5 (private huge pages)
are active. This will cause these memory mappings to be dumped automatically.
10.20 Character Sets
If the program you are debugging uses a different character set to represent characters and
strings than the one gdb uses itself, gdb can automatically translate between the character
sets for you. The character set gdb uses we call the host character set; the one the inferior
program uses we call the target character set.
For example, if you are running gdb on a gnu/Linux system, which uses the ISO Latin
1 character set, but you are using gdb’s remote protocol (see Chapter 20 [Remote Debugging], page 261) to debug a program running on an IBM mainframe, which uses the ebcdic
character set, then the host character set is Latin-1, and the target character set is ebcdic.
If you give gdb the command set target-charset EBCDIC-US, then gdb translates between ebcdic and Latin 1 as you print character or string values, or use character and
string literals in expressions.
gdb has no way to automatically recognize which character set the inferior program
uses; you must tell it, using the set target-charset command, described below.
Here are the commands for controlling gdb’s character set support:
set target-charset charset
Set the current target character set to charset. To display the list of supported
target character sets, type set target-charset TABTAB.
set host-charset charset
Set the current host character set to charset.
By default, gdb uses a host character set appropriate to the system it is running on; you can override that default using the set host-charset command.
On some systems, gdb cannot automatically determine the appropriate host
character set. In this case, gdb uses ‘UTF-8’.
gdb can only use certain character sets as its host character set. If you type
set host-charset TABTAB, gdb will list the host character sets it supports.
set charset charset
Set the current host and target character sets to charset. As above, if you type
set charset TABTAB, gdb will list the names of the character sets that can be
used for both host and target.
show charset
Show the names of the current host and target character sets.
show host-charset
Show the name of the current host character set.
show target-charset
Show the name of the current target character set.
Chapter 10: Examining Data
153
set target-wide-charset charset
Set the current target’s wide character set to charset. This is the character
set used by the target’s wchar_t type. To display the list of supported wide
character sets, type set target-wide-charset TABTAB.
show target-wide-charset
Show the name of the current target’s wide character set.
Here is an example of gdb’s character set support in action. Assume that the following
source code has been placed in the file ‘charset-test.c’:
#include
char ascii_hello[]
= {72, 101, 108, 108, 111, 44, 32, 119,
111, 114, 108, 100, 33, 10, 0};
char ibm1047_hello[]
= {200, 133, 147, 147, 150, 107, 64, 166,
150, 153, 147, 132, 90, 37, 0};
main ()
{
printf ("Hello, world!\n");
}
In this program, ascii_hello and ibm1047_hello are arrays containing the string
‘Hello, world!’ followed by a newline, encoded in the ascii and ibm1047 character sets.
We compile the program, and invoke the debugger on it:
$ gcc -g charset-test.c -o charset-test
$ gdb -nw charset-test
GNU gdb 2001-12-19-cvs
Copyright 2001 Free Software Foundation, Inc.
...
(gdb)
We can use the show charset command to see what character sets gdb is currently
using to interpret and display characters and strings:
(gdb) show charset
The current host and target character set is ‘ISO-8859-1’.
(gdb)
For the sake of printing this manual, let’s use ascii as our initial character set:
(gdb) set charset ASCII
(gdb) show charset
The current host and target character set is ‘ASCII’.
(gdb)
Let’s assume that ascii is indeed the correct character set for our host system — in
other words, let’s assume that if gdb prints characters using the ascii character set, our
terminal will display them properly. Since our current target character set is also ascii, the
contents of ascii_hello print legibly:
(gdb) print ascii_hello
$1 = 0x401698 "Hello, world!\n"
(gdb) print ascii_hello[0]
$2 = 72 ’H’
(gdb)
gdb uses the target character set for character and string literals you use in expressions:
154
Debugging with gdb
(gdb) print ’+’
$3 = 43 ’+’
(gdb)
The ascii character set uses the number 43 to encode the ‘+’ character.
gdb relies on the user to tell it which character set the target program uses. If we print
ibm1047_hello while our target character set is still ascii, we get jibberish:
(gdb) print ibm1047_hello
$4 = 0x4016a8 "\310\205\223\223\226k@\246\226\231\223\204Z%"
(gdb) print ibm1047_hello[0]
$5 = 200 ’\310’
(gdb)
If we invoke the set target-charset followed by TABTAB, gdb tells us the character
sets it supports:
(gdb) set target-charset
ASCII
EBCDIC-US
IBM1047
(gdb) set target-charset
ISO-8859-1
We can select ibm1047 as our target character set, and examine the program’s strings
again. Now the ascii string is wrong, but gdb translates the contents of ibm1047_hello
from the target character set, ibm1047, to the host character set, ascii, and they display
correctly:
(gdb) set target-charset IBM1047
(gdb) show charset
The current host character set is ‘ASCII’.
The current target character set is ‘IBM1047’.
(gdb) print ascii_hello
$6 = 0x401698 "\110\145%%?\054\040\167?\162%\144\041\012"
(gdb) print ascii_hello[0]
$7 = 72 ’\110’
(gdb) print ibm1047_hello
$8 = 0x4016a8 "Hello, world!\n"
(gdb) print ibm1047_hello[0]
$9 = 200 ’H’
(gdb)
As above, gdb uses the target character set for character and string literals you use in
expressions:
(gdb) print ’+’
$10 = 78 ’+’
(gdb)
The ibm1047 character set uses the number 78 to encode the ‘+’ character.
10.21 Caching Data of Targets
gdb caches data exchanged between the debugger and a target. Each cache is associated
with the address space of the inferior. See Section 4.9 [Inferiors and Programs], page 33,
about inferior and address space. Such caching generally improves performance in remote
debugging (see Chapter 20 [Remote Debugging], page 261), because it reduces the overhead
of the remote protocol by bundling memory reads and writes into large chunks. Unfortunately, simply caching everything would lead to incorrect results, since gdb does not
necessarily know anything about volatile values, memory-mapped I/O addresses, etc. Furthermore, in non-stop mode (see Section 5.5.2 [Non-Stop Mode], page 78) memory can be
Chapter 10: Examining Data
155
changed while a gdb command is executing. Therefore, by default, gdb only caches data
known to be on the stack3 or in the code segment. Other regions of memory can be explicitly
marked as cacheable; see Section 10.17 [Memory Region Attributes], page 148.
set remotecache on
set remotecache off
This option no longer does anything; it exists for compatibility with old scripts.
show remotecache
Show the current state of the obsolete remotecache flag.
set stack-cache on
set stack-cache off
Enable or disable caching of stack accesses. When on, use caching. By default,
this option is on.
show stack-cache
Show the current state of data caching for memory accesses.
set code-cache on
set code-cache off
Enable or disable caching of code segment accesses. When on, use caching. By
default, this option is on. This improves performance of disassembly in remote
debugging.
show code-cache
Show the current state of target memory cache for code segment accesses.
info dcache [line]
Print the information about the performance of data cache of the current inferior’s address space. The information displayed includes the dcache width and
depth, and for each cache line, its number, address, and how many times it was
referenced. This command is useful for debugging the data cache operation.
If a line number is specified, the contents of that line will be printed in hex.
set dcache size size
Set maximum number of entries in dcache (dcache depth above).
set dcache line-size line-size
Set number of bytes each dcache entry caches (dcache width above). Must be
a power of 2.
show dcache size
Show maximum number of dcache entries. See Section 10.21 [Caching Target
Data], page 154.
show dcache line-size
Show default size of dcache lines.
3
In non-stop mode, it is moderately rare for a running thread to modify the stack of a stopped thread in
a way that would interfere with a backtrace, and caching of stack reads provides a significant speed up
of remote backtraces.
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Debugging with gdb
10.22 Search Memory
Memory can be searched for a particular sequence of bytes with the find command.
find [/sn] start_addr, +len, val1 [, val2, ...]
find [/sn] start_addr, end_addr, val1 [, val2, ...]
Search memory for the sequence of bytes specified by val1, val2, etc. The search
begins at address start addr and continues for either len bytes or through to
end addr inclusive.
s and n are optional parameters. They may be specified in either order, apart or together.
s, search query size
The size of each search query value.
b
bytes
h
halfwords (two bytes)
w
words (four bytes)
g
giant words (eight bytes)
All values are interpreted in the current language. This means, for example,
that if the current source language is C/C++ then searching for the string “hello”
includes the trailing ’\0’.
If the value size is not specified, it is taken from the value’s type in the current
language. This is useful when one wants to specify the search pattern as a
mixture of types. Note that this means, for example, that in the case of C-like
languages a search for an untyped 0x42 will search for ‘(int) 0x42’ which is
typically four bytes.
n, maximum number of finds
The maximum number of matches to print. The default is to print all finds.
You can use strings as search values. Quote them with double-quotes ("). The string
value is copied into the search pattern byte by byte, regardless of the endianness of the
target and the size specification.
The address of each match found is printed as well as a count of the number of matches
found.
The address of the last value found is stored in convenience variable ‘$_’. A count of the
number of matches is stored in ‘$numfound’.
For example, if stopped at the printf in this function:
void
hello ()
{
static char hello[] = "hello-hello";
static struct { char c; short s; int i; }
__attribute__ ((packed)) mixed
= { ’c’, 0x1234, 0x87654321 };
printf ("%s\n", hello);
}
you get during debugging:
Chapter 10: Examining Data
157
(gdb) find &hello[0], +sizeof(hello), "hello"
0x804956d
1 pattern found
(gdb) find &hello[0], +sizeof(hello), ’h’, ’e’, ’l’, ’l’, ’o’
0x8049567
0x804956d
2 patterns found
(gdb) find /b1 &hello[0], +sizeof(hello), ’h’, 0x65, ’l’
0x8049567
1 pattern found
(gdb) find &mixed, +sizeof(mixed), (char) ’c’, (short) 0x1234, (int) 0x87654321
0x8049560
1 pattern found
(gdb) print $numfound
$1 = 1
(gdb) print $_
$2 = (void *) 0x8049560
10.23 Value Sizes
Whenever gdb prints a value memory will be allocated within gdb to hold the contents of
the value. It is possible in some languages with dynamic typing systems, that an invalid
program may indicate a value that is incorrectly large, this in turn may cause gdb to try
and allocate an overly large ammount of memory.
set max-value-size bytes
set max-value-size unlimited
Set the maximum size of memory that gdb will allocate for the contents of a
value to bytes, trying to display a value that requires more memory than that
will result in an error.
Setting this variable does not effect values that have already been allocated
within gdb, only future allocations.
There’s a minimum size that max-value-size can be set to in order that gdb
can still operate correctly, this minimum is currently 16 bytes.
The limit applies to the results of some subexpressions as well as to complete
expressions. For example, an expression denoting a simple integer component,
such as x.y.z, may fail if the size of x.y is dynamic and exceeds bytes. On the
other hand, gdb is sometimes clever; the expression A[i], where A is an array
variable with non-constant size, will generally succeed regardless of the bounds
on A, as long as the component size is less than bytes.
The default value of max-value-size is currently 64k.
show max-value-size
Show the maximum size of memory, in bytes, that gdb will allocate for the
contents of a value.
Chapter 11: Debugging Optimized Code
159
11 Debugging Optimized Code
Almost all compilers support optimization. With optimization disabled, the compiler generates assembly code that corresponds directly to your source code, in a simplistic way.
As the compiler applies more powerful optimizations, the generated assembly code diverges
from your original source code. With help from debugging information generated by the
compiler, gdb can map from the running program back to constructs from your original
source.
gdb is more accurate with optimization disabled. If you can recompile without optimization, it is easier to follow the progress of your program during debugging. But, there
are many cases where you may need to debug an optimized version.
When you debug a program compiled with ‘-g -O’, remember that the optimizer has
rearranged your code; the debugger shows you what is really there. Do not be too surprised
when the execution path does not exactly match your source file! An extreme example: if
you define a variable, but never use it, gdb never sees that variable—because the compiler
optimizes it out of existence.
Some things do not work as well with ‘-g -O’ as with just ‘-g’, particularly on machines
with instruction scheduling. If in doubt, recompile with ‘-g’ alone, and if this fixes the
problem, please report it to us as a bug (including a test case!). See Section 10.3 [Variables],
page 119, for more information about debugging optimized code.
11.1 Inline Functions
Inlining is an optimization that inserts a copy of the function body directly at each call site,
instead of jumping to a shared routine. gdb displays inlined functions just like non-inlined
functions. They appear in backtraces. You can view their arguments and local variables,
step into them with step, skip them with next, and escape from them with finish. You
can check whether a function was inlined by using the info frame command.
For gdb to support inlined functions, the compiler must record information about inlining in the debug information — gcc using the dwarf 2 format does this, and several other compilers do also. gdb only supports inlined functions when using dwarf 2.
Versions of gcc before 4.1 do not emit two required attributes (‘DW_AT_call_file’ and
‘DW_AT_call_line’); gdb does not display inlined function calls with earlier versions of
gcc. It instead displays the arguments and local variables of inlined functions as local
variables in the caller.
The body of an inlined function is directly included at its call site; unlike a non-inlined
function, there are no instructions devoted to the call. gdb still pretends that the call site
and the start of the inlined function are different instructions. Stepping to the call site
shows the call site, and then stepping again shows the first line of the inlined function, even
though no additional instructions are executed.
This makes source-level debugging much clearer; you can see both the context of the call
and then the effect of the call. Only stepping by a single instruction using stepi or nexti
does not do this; single instruction steps always show the inlined body.
There are some ways that gdb does not pretend that inlined function calls are the same
as normal calls:
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Debugging with gdb
• Setting breakpoints at the call site of an inlined function may not work, because the
call site does not contain any code. gdb may incorrectly move the breakpoint to the
next line of the enclosing function, after the call. This limitation will be removed in
a future version of gdb; until then, set a breakpoint on an earlier line or inside the
inlined function instead.
• gdb cannot locate the return value of inlined calls after using the finish command.
This is a limitation of compiler-generated debugging information; after finish, you
can step to the next line and print a variable where your program stored the return
value.
11.2 Tail Call Frames
Function B can call function C in its very last statement. In unoptimized compilation the
call of C is immediately followed by return instruction at the end of B code. Optimizing
compiler may replace the call and return in function B into one jump to function C instead.
Such use of a jump instruction is called tail call.
During execution of function C, there will be no indication in the function call stack
frames that it was tail-called from B. If function A regularly calls function B which tail-calls
function C, then gdb will see A as the caller of C. However, in some cases gdb can determine
that C was tail-called from B, and it will then create fictitious call frame for that, with the
return address set up as if B called C normally.
This functionality is currently supported only by DWARF 2 debugging format and the
compiler has to produce ‘DW_TAG_call_site’ tags. With gcc, you need to specify ‘-O -g’
during compilation, to get this information.
info frame command (see Section 8.4 [Frame Info], page 99) will indicate the tail call
frame kind by text tail call frame such as in this sample gdb output:
(gdb) x/i $pc - 2
0x40066b : jmp 0x400640
(gdb) info frame
Stack level 1, frame at 0x7fffffffda30:
rip = 0x40066d in b (amd64-entry-value.cc:59); saved rip 0x4004c5
tail call frame, caller of frame at 0x7fffffffda30
source language c++.
Arglist at unknown address.
Locals at unknown address, Previous frame’s sp is 0x7fffffffda30
The detection of all the possible code path executions can find them ambiguous. There is
no execution history stored (possible Chapter 6 [Reverse Execution], page 85 is never used
for this purpose) and the last known caller could have reached the known callee by multiple
different jump sequences. In such case gdb still tries to show at least all the unambiguous
top tail callers and all the unambiguous bottom tail calees, if any.
set debug entry-values
When set to on, enables printing of analysis messages for both frame argument
values at function entry and tail calls. It will show all the possible valid tail
calls code paths it has considered. It will also print the intersection of them
with the final unambiguous (possibly partial or even empty) code path result.
Chapter 11: Debugging Optimized Code
161
show debug entry-values
Show the current state of analysis messages printing for both frame argument
values at function entry and tail calls.
The analysis messages for tail calls can for example show why the virtual tail call frame
for function c has not been recognized (due to the indirect reference by variable x):
static void __attribute__((noinline, noclone)) c (void);
void (*x) (void) = c;
static void __attribute__((noinline, noclone)) a (void) { x++; }
static void __attribute__((noinline, noclone)) c (void) { a (); }
int main (void) { x (); return 0; }
Breakpoint 1, DW_OP_entry_value resolving cannot find
DW_TAG_call_site 0x40039a in main
a () at t.c:3
3 static void __attribute__((noinline, noclone)) a (void) { x++; }
(gdb) bt
#0 a () at t.c:3
#1 0x000000000040039a in main () at t.c:5
Another possibility is an ambiguous virtual tail call frames resolution:
int i;
static void __attribute__((noinline,
static void __attribute__((noinline,
static void __attribute__((noinline,
static void __attribute__((noinline,
static void __attribute__((noinline,
{ if (i) c (); else e (); }
static void __attribute__((noinline,
int main (void) { a (); return 0; }
tailcall: initial: 0x4004d2(a)
tailcall: compare: 0x4004d2(a)
tailcall: reduced: 0x4004d2(a)
(gdb) bt
#0 f () at t.c:2
#1 0x00000000004004d2 in a ()
#2 0x0000000000400395 in main
noclone))
noclone))
noclone))
noclone))
noclone))
f
e
d
c
b
(void)
(void)
(void)
(void)
(void)
{
{
{
{
i++; }
f (); }
f (); }
d (); }
noclone)) a (void) { b (); }
0x4004ce(b) 0x4004b2(c) 0x4004a2(d)
0x4004cc(b) 0x400492(e)
|
at t.c:8
() at t.c:9
Frames #0 and #2 are real, #1 is a virtual tail call frame. The code can have possible
execution paths main->a->b->c->d->f or main->a->b->e->f, gdb cannot find which one
from the inferior state.
initial: state shows some random possible calling sequence gdb has found. It then
finds another possible calling sequcen - that one is prefixed by compare:. The nonambiguous intersection of these two is printed as the reduced: calling sequence. That
one could have many futher compare: and reduced: statements as long as there remain
any non-ambiguous sequence entries.
For the frame of function b in both cases there are different possible $pc values (0x4004cc
or 0x4004ce), therefore this frame is also ambigous. The only non-ambiguous frame is the
one for function a, therefore this one is displayed to the user while the ambiguous frames
are omitted.
There can be also reasons why printing of frame argument values at function entry may
fail:
int v;
162
Debugging with gdb
static void __attribute__((noinline,
static void __attribute__((noinline,
static void __attribute__((noinline,
static void __attribute__((noinline,
{ if (i) b (i - 1); else c (0); }
int main (void) { a (5); return 0; }
noclone))
noclone))
noclone))
noclone))
c
a
b
a
(int
(int
(int
(int
i) { v++; }
i);
i) { a (i); }
i)
(gdb) bt
#0 c (i=i@entry=0) at t.c:2
#1 0x0000000000400428 in a (DW_OP_entry_value resolving has found
function "a" at 0x400420 can call itself via tail calls
i=) at t.c:6
#2 0x000000000040036e in main () at t.c:7
gdb cannot find out from the inferior state if and how many times did function a call
itself (via function b) as these calls would be tail calls. Such tail calls would modify thue
i variable, therefore gdb cannot be sure the value it knows would be right - gdb prints
instead.
Chapter 12: C Preprocessor Macros
163
12 C Preprocessor Macros
Some languages, such as C and C++, provide a way to define and invoke “preprocessor
macros” which expand into strings of tokens. gdb can evaluate expressions containing
macro invocations, show the result of macro expansion, and show a macro’s definition,
including where it was defined.
You may need to compile your program specially to provide gdb with information about
preprocessor macros. Most compilers do not include macros in their debugging information,
even when you compile with the ‘-g’ flag. See Section 4.1 [Compilation], page 25.
A program may define a macro at one point, remove that definition later, and then
provide a different definition after that. Thus, at different points in the program, a macro
may have different definitions, or have no definition at all. If there is a current stack frame,
gdb uses the macros in scope at that frame’s source code line. Otherwise, gdb uses the
macros in scope at the current listing location; see Section 9.1 [List], page 103.
Whenever gdb evaluates an expression, it always expands any macro invocations present
in the expression. gdb also provides the following commands for working with macros
explicitly.
macro expand expression
macro exp expression
Show the results of expanding all preprocessor macro invocations in expression.
Since gdb simply expands macros, but does not parse the result, expression
need not be a valid expression; it can be any string of tokens.
macro expand-once expression
macro exp1 expression
(This command is not yet implemented.) Show the results of expanding those
preprocessor macro invocations that appear explicitly in expression. Macro
invocations appearing in that expansion are left unchanged. This command
allows you to see the effect of a particular macro more clearly, without being
confused by further expansions. Since gdb simply expands macros, but does
not parse the result, expression need not be a valid expression; it can be any
string of tokens.
info macro [-a|-all] [--] macro
Show the current definition or all definitions of the named macro, and describe
the source location or compiler command-line where that definition was established. The optional double dash is to signify the end of argument processing
and the beginning of macro for non C-like macros where the macro may begin
with a hyphen.
info macros location
Show all macro definitions that are in effect at the location specified by location, and describe the source location or compiler command-line where those
definitions were established.
macro define macro replacement-list
macro define macro(arglist) replacement-list
Introduce a definition for a preprocessor macro named macro, invocations of
which are replaced by the tokens given in replacement-list. The first form of
164
Debugging with gdb
this command defines an “object-like” macro, which takes no arguments; the
second form defines a “function-like” macro, which takes the arguments given
in arglist.
A definition introduced by this command is in scope in every expression evaluated in gdb, until it is removed with the macro undef command, described
below. The definition overrides all definitions for macro present in the program
being debugged, as well as any previous user-supplied definition.
macro undef macro
Remove any user-supplied definition for the macro named macro. This command only affects definitions provided with the macro define command, described above; it cannot remove definitions present in the program being debugged.
macro list
List all the macros defined using the macro define command.
Here is a transcript showing the above commands in action. First, we show our source
files:
$ cat sample.c
#include
#include "sample.h"
#define M 42
#define ADD(x) (M + x)
main ()
{
#define N 28
printf ("Hello, world!\n");
#undef N
printf ("We’re so creative.\n");
#define N 1729
printf ("Goodbye, world!\n");
}
$ cat sample.h
#define Q <
$
Now, we compile the program using the gnu C compiler, gcc. We pass the ‘-gdwarf-2’1
and ‘-g3’ flags to ensure the compiler includes information about preprocessor macros in
the debugging information.
$ gcc -gdwarf-2 -g3 sample.c -o sample
$
Now, we start gdb on our sample program:
$ gdb -nw sample
GNU gdb 2002-05-06-cvs
Copyright 2002 Free Software Foundation, Inc.
GDB is free software, ...
(gdb)
1
This is the minimum. Recent versions of gcc support ‘-gdwarf-3’ and ‘-gdwarf-4’; we recommend
always choosing the most recent version of DWARF.
Chapter 12: C Preprocessor Macros
165
We can expand macros and examine their definitions, even when the program is not
running. gdb uses the current listing position to decide which macro definitions are in
scope:
(gdb) list main
3
4
#define M 42
5
#define ADD(x) (M + x)
6
7
main ()
8
{
9
#define N 28
10
printf ("Hello, world!\n");
11
#undef N
12
printf ("We’re so creative.\n");
(gdb) info macro ADD
Defined at /home/jimb/gdb/macros/play/sample.c:5
#define ADD(x) (M + x)
(gdb) info macro Q
Defined at /home/jimb/gdb/macros/play/sample.h:1
included at /home/jimb/gdb/macros/play/sample.c:2
#define Q <
(gdb) macro expand ADD(1)
expands to: (42 + 1)
(gdb) macro expand-once ADD(1)
expands to: once (M + 1)
(gdb)
In the example above, note that macro expand-once expands only the macro invocation
explicit in the original text — the invocation of ADD — but does not expand the invocation
of the macro M, which was introduced by ADD.
Once the program is running, gdb uses the macro definitions in force at the source line
of the current stack frame:
(gdb) break main
Breakpoint 1 at 0x8048370: file sample.c, line 10.
(gdb) run
Starting program: /home/jimb/gdb/macros/play/sample
Breakpoint 1, main () at sample.c:10
10
printf ("Hello, world!\n");
(gdb)
At line 10, the definition of the macro N at line 9 is in force:
(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:9
#define N 28
(gdb) macro expand N Q M
expands to: 28 < 42
(gdb) print N Q M
$1 = 1
(gdb)
As we step over directives that remove N’s definition, and then give it a new definition,
gdb finds the definition (or lack thereof) in force at each point:
(gdb) next
Hello, world!
12
printf ("We’re so creative.\n");
(gdb) info macro N
166
Debugging with gdb
The symbol ‘N’ has no definition as a C/C++ preprocessor macro
at /home/jimb/gdb/macros/play/sample.c:12
(gdb) next
We’re so creative.
14
printf ("Goodbye, world!\n");
(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:13
#define N 1729
(gdb) macro expand N Q M
expands to: 1729 < 42
(gdb) print N Q M
$2 = 0
(gdb)
In addition to source files, macros can be defined on the compilation command line using
the ‘-Dname=value’ syntax. For macros defined in such a way, gdb displays the location of
their definition as line zero of the source file submitted to the compiler.
(gdb) info macro __STDC__
Defined at /home/jimb/gdb/macros/play/sample.c:0
-D__STDC__=1
(gdb)
Chapter 13: Tracepoints
167
13 Tracepoints
In some applications, it is not feasible for the debugger to interrupt the program’s execution
long enough for the developer to learn anything helpful about its behavior. If the program’s
correctness depends on its real-time behavior, delays introduced by a debugger might cause
the program to change its behavior drastically, or perhaps fail, even when the code itself is
correct. It is useful to be able to observe the program’s behavior without interrupting it.
Using gdb’s trace and collect commands, you can specify locations in the program,
called tracepoints, and arbitrary expressions to evaluate when those tracepoints are reached.
Later, using the tfind command, you can examine the values those expressions had when
the program hit the tracepoints. The expressions may also denote objects in memory—
structures or arrays, for example—whose values gdb should record; while visiting a particular tracepoint, you may inspect those objects as if they were in memory at that moment.
However, because gdb records these values without interacting with you, it can do so quickly
and unobtrusively, hopefully not disturbing the program’s behavior.
The tracepoint facility is currently available only for remote targets. See Chapter 19
[Targets], page 257. In addition, your remote target must know how to collect trace data.
This functionality is implemented in the remote stub; however, none of the stubs distributed
with gdb support tracepoints as of this writing. The format of the remote packets used to
implement tracepoints are described in Section E.6 [Tracepoint Packets], page 658.
It is also possible to get trace data from a file, in a manner reminiscent of corefiles;
you specify the filename, and use tfind to search through the file. See Section 13.4 [Trace
Files], page 183, for more details.
This chapter describes the tracepoint commands and features.
13.1 Commands to Set Tracepoints
Before running such a trace experiment, an arbitrary number of tracepoints can be set. A
tracepoint is actually a special type of breakpoint (see Section 5.1.1 [Set Breaks], page 46),
so you can manipulate it using standard breakpoint commands. For instance, as with
breakpoints, tracepoint numbers are successive integers starting from one, and many of the
commands associated with tracepoints take the tracepoint number as their argument, to
identify which tracepoint to work on.
For each tracepoint, you can specify, in advance, some arbitrary set of data that you
want the target to collect in the trace buffer when it hits that tracepoint. The collected data
can include registers, local variables, or global data. Later, you can use gdb commands to
examine the values these data had at the time the tracepoint was hit.
Tracepoints do not support every breakpoint feature. Ignore counts on tracepoints have
no effect, and tracepoints cannot run gdb commands when they are hit. Tracepoints may
not be thread-specific either.
Some targets may support fast tracepoints, which are inserted in a different way (such
as with a jump instead of a trap), that is faster but possibly restricted in where they may
be installed.
Regular and fast tracepoints are dynamic tracing facilities, meaning that they can be
used to insert tracepoints at (almost) any location in the target. Some targets may also support controlling static tracepoints from gdb. With static tracing, a set of instrumentation
168
Debugging with gdb
points, also known as markers, are embedded in the target program, and can be activated
or deactivated by name or address. These are usually placed at locations which facilitate
investigating what the target is actually doing. gdb’s support for static tracing includes
being able to list instrumentation points, and attach them with gdb defined high level tracepoints that expose the whole range of convenience of gdb’s tracepoints support. Namely,
support for collecting registers values and values of global or local (to the instrumentation
point) variables; tracepoint conditions and trace state variables. The act of installing a gdb
static tracepoint on an instrumentation point, or marker, is referred to as probing a static
tracepoint marker.
gdbserver supports tracepoints on some target systems. See Section 20.3 [Tracepoints
support in gdbserver], page 265.
This section describes commands to set tracepoints and associated conditions and actions.
13.1.1 Create and Delete Tracepoints
trace location
The trace command is very similar to the break command. Its argument location can be any valid location. See Section 9.2 [Specify Location], page 104.
The trace command defines a tracepoint, which is a point in the target program where the debugger will briefly stop, collect some data, and then allow
the program to continue. Setting a tracepoint or changing its actions takes
effect immediately if the remote stub supports the ‘InstallInTrace’ feature
(see [install tracepoint in tracing], page 649). If remote stub doesn’t support
the ‘InstallInTrace’ feature, all these changes don’t take effect until the next
tstart command, and once a trace experiment is running, further changes will
not have any effect until the next trace experiment starts. In addition, gdb
supports pending tracepoints—tracepoints whose address is not yet resolved.
(This is similar to pending breakpoints.) Pending tracepoints are not downloaded to the target and not installed until they are resolved. The resolution
of pending tracepoints requires gdb support—when debugging with the remote
target, and gdb disconnects from the remote stub (see [disconnected tracing],
page 176), pending tracepoints can not be resolved (and downloaded to the
remote stub) while gdb is disconnected.
Here are some examples of using the trace command:
(gdb) trace foo.c:121
(gdb) trace +2
(gdb) trace my function
// a source file and line number
// 2 lines forward
// first source line of function
(gdb) trace *my function // EXACT start address of function
(gdb) trace *0x2117c4
// an address
You can abbreviate trace as tr.
trace location if cond
Set a tracepoint with condition cond; evaluate the expression cond each time
the tracepoint is reached, and collect data only if the value is nonzero—that is,
Chapter 13: Tracepoints
169
if cond evaluates as true. See Section 13.1.4 [Tracepoint Conditions], page 171,
for more information on tracepoint conditions.
ftrace location [ if cond ]
The ftrace command sets a fast tracepoint. For targets that support them,
fast tracepoints will use a more efficient but possibly less general technique to
trigger data collection, such as a jump instruction instead of a trap, or some
sort of hardware support. It may not be possible to create a fast tracepoint at
the desired location, in which case the command will exit with an explanatory
message.
gdb handles arguments to ftrace exactly as for trace.
On 32-bit x86-architecture systems, fast tracepoints normally need to be placed
at an instruction that is 5 bytes or longer, but can be placed at 4-byte instructions if the low 64K of memory of the target program is available to install
trampolines. Some Unix-type systems, such as gnu/Linux, exclude low addresses from the program’s address space; but for instance with the Linux
kernel it is possible to let gdb use this area by doing a sysctl command to set
the mmap_min_addr kernel parameter, as in
sudo sysctl -w vm.mmap_min_addr=32768
which sets the low address to 32K, which leaves plenty of room for trampolines.
The minimum address should be set to a page boundary.
strace location [ if cond ]
The strace command sets a static tracepoint. For targets that support it,
setting a static tracepoint probes a static instrumentation point, or marker,
found at location. It may not be possible to set a static tracepoint at the
desired location, in which case the command will exit with an explanatory
message.
gdb handles arguments to strace exactly as for trace, with the addition that
the user can also specify -m marker as location. This probes the marker identified by the marker string identifier. This identifier depends on the static
tracepoint backend library your program is using. You can find all the marker
identifiers in the ‘ID’ field of the info static-tracepoint-markers command
output. See Section 13.1.8 [Listing Static Tracepoint Markers], page 175. For
example, in the following small program using the UST tracing engine:
main ()
{
trace_mark(ust, bar33, "str %s", "FOOBAZ");
}
the marker id is composed of joining the first two arguments to the trace_mark
call with a slash, which translates to:
(gdb) info static-tracepoint-markers
Cnt Enb ID
Address
What
1
n
ust/bar33 0x0000000000400ddc in main at stexample.c:22
Data: "str %s"
[etc...]
so you may probe the marker above with:
(gdb) strace -m ust/bar33
170
Debugging with gdb
Static tracepoints accept an extra collect action — collect $_sdata. This
collects arbitrary user data passed in the probe point call to the tracing library.
In the UST example above, you’ll see that the third argument to trace_mark
is a printf-like format string. The user data is then the result of running that
formating string against the following arguments. Note that info statictracepoint-markers command output lists that format string in the ‘Data:’
field.
You can inspect this data when analyzing the trace buffer, by printing the
$ sdata variable like any other variable available to gdb. See Section 13.1.6
[Tracepoint Action Lists], page 172.
The convenience variable $tpnum records the tracepoint number of the most
recently set tracepoint.
delete tracepoint [num]
Permanently delete one or more tracepoints. With no argument, the default is
to delete all tracepoints. Note that the regular delete command can remove
tracepoints also.
Examples:
(gdb) delete trace 1 2 3 // remove three tracepoints
(gdb) delete trace
// remove all tracepoints
You can abbreviate this command as del tr.
13.1.2 Enable and Disable Tracepoints
These commands are deprecated; they are equivalent to plain disable and enable.
disable tracepoint [num]
Disable tracepoint num, or all tracepoints if no argument num is given. A disabled tracepoint will have no effect during a trace experiment, but it is not forgotten. You can re-enable a disabled tracepoint using the enable tracepoint
command. If the command is issued during a trace experiment and the debug
target has support for disabling tracepoints during a trace experiment, then the
change will be effective immediately. Otherwise, it will be applied to the next
trace experiment.
enable tracepoint [num]
Enable tracepoint num, or all tracepoints. If this command is issued during a
trace experiment and the debug target supports enabling tracepoints during a
trace experiment, then the enabled tracepoints will become effective immediately. Otherwise, they will become effective the next time a trace experiment
is run.
13.1.3 Tracepoint Passcounts
passcount [n [num]]
Set the passcount of a tracepoint. The passcount is a way to automatically
stop a trace experiment. If a tracepoint’s passcount is n, then the trace experiment will be automatically stopped on the n’th time that tracepoint is hit. If
the tracepoint number num is not specified, the passcount command sets the
Chapter 13: Tracepoints
171
passcount of the most recently defined tracepoint. If no passcount is given, the
trace experiment will run until stopped explicitly by the user.
Examples:
(gdb) passcount 5 2 // Stop on the 5th execution of
// tracepoint 2
(gdb) passcount 12
(gdb)
(gdb)
(gdb)
(gdb)
(gdb)
(gdb)
trace foo
pass 3
trace bar
pass 2
trace baz
pass 1
// Stop on the 12th execution of the
// most recently defined tracepoint.
// Stop tracing when foo has been
// executed 3 times OR when bar has
// been executed 2 times
// OR when baz has been executed 1 time.
13.1.4 Tracepoint Conditions
The simplest sort of tracepoint collects data every time your program reaches a specified
place. You can also specify a condition for a tracepoint. A condition is just a Boolean
expression in your programming language (see Section 10.1 [Expressions], page 117). A
tracepoint with a condition evaluates the expression each time your program reaches it,
and data collection happens only if the condition is true.
Tracepoint conditions can be specified when a tracepoint is set, by using ‘if’ in the
arguments to the trace command. See Section 13.1.1 [Setting Tracepoints], page 168.
They can also be set or changed at any time with the condition command, just as with
breakpoints.
Unlike breakpoint conditions, gdb does not actually evaluate the conditional expression
itself. Instead, gdb encodes the expression into an agent expression (see Appendix F [Agent
Expressions], page 689) suitable for execution on the target, independently of gdb. Global
variables become raw memory locations, locals become stack accesses, and so forth.
For instance, suppose you have a function that is usually called frequently, but should
not be called after an error has occurred. You could use the following tracepoint command
to collect data about calls of that function that happen while the error code is propagating
through the program; an unconditional tracepoint could end up collecting thousands of
useless trace frames that you would have to search through.
(gdb) trace normal_operation if errcode > 0
13.1.5 Trace State Variables
A trace state variable is a special type of variable that is created and managed by target-side
code. The syntax is the same as that for GDB’s convenience variables (a string prefixed
with “$”), but they are stored on the target. They must be created explicitly, using a
tvariable command. They are always 64-bit signed integers.
Trace state variables are remembered by gdb, and downloaded to the target along with
tracepoint information when the trace experiment starts. There are no intrinsic limits on
the number of trace state variables, beyond memory limitations of the target.
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Debugging with gdb
Although trace state variables are managed by the target, you can use them in print
commands and expressions as if they were convenience variables; gdb will get the current
value from the target while the trace experiment is running. Trace state variables share the
same namespace as other “$” variables, which means that you cannot have trace state variables with names like $23 or $pc, nor can you have a trace state variable and a convenience
variable with the same name.
tvariable $name [ = expression ]
The tvariable command creates a new trace state variable named $name, and
optionally gives it an initial value of expression. The expression is evaluated
when this command is entered; the result will be converted to an integer if
possible, otherwise gdb will report an error. A subsequent tvariable command
specifying the same name does not create a variable, but instead assigns the
supplied initial value to the existing variable of that name, overwriting any
previous initial value. The default initial value is 0.
info tvariables
List all the trace state variables along with their initial values. Their current
values may also be displayed, if the trace experiment is currently running.
delete tvariable [ $name ... ]
Delete the given trace state variables, or all of them if no arguments are specified.
13.1.6 Tracepoint Action Lists
actions [num]
This command will prompt for a list of actions to be taken when the tracepoint
is hit. If the tracepoint number num is not specified, this command sets the
actions for the one that was most recently defined (so that you can define a
tracepoint and then say actions without bothering about its number). You
specify the actions themselves on the following lines, one action at a time, and
terminate the actions list with a line containing just end. So far, the only
defined actions are collect, teval, and while-stepping.
actions is actually equivalent to commands (see Section 5.1.7 [Breakpoint Command Lists], page 62), except that only the defined actions are allowed; any
other gdb command is rejected.
To remove all actions from a tracepoint, type ‘actions num’ and follow it immediately with ‘end’.
(gdb) collect data // collect some data
(gdb) while-stepping 5 // single-step 5 times, collect data
(gdb) end
// signals the end of actions.
In the following example, the action list begins with collect commands indicating the things to be collected when the tracepoint is hit. Then, in order
to single-step and collect additional data following the tracepoint, a whilestepping command is used, followed by the list of things to be collected after
each step in a sequence of single steps. The while-stepping command is ter-
Chapter 13: Tracepoints
173
minated by its own separate end command. Lastly, the action list is terminated
by an end command.
(gdb) trace foo
(gdb) actions
Enter actions for tracepoint 1, one per line:
> collect bar,baz
> collect $regs
> while-stepping 12
> collect $pc, arr[i]
> end
end
collect[/mods] expr1, expr2, ...
Collect values of the given expressions when the tracepoint is hit. This command accepts a comma-separated list of any valid expressions. In addition to
global, static, or local variables, the following special arguments are supported:
$regs
Collect all registers.
$args
Collect all function arguments.
$locals
Collect all local variables.
$_ret
Collect the return address. This is helpful if you want to see more
of a backtrace.
Note: The return address location can not always be reliably determined up front, and the wrong address / registers may end up
collected instead. On some architectures the reliability is higher
for tracepoints at function entry, while on others it’s the opposite.
When this happens, backtracing will stop because the return address is found unavailable (unless another collect rule happened to
match it).
$_probe_argc
Collects the number of arguments from the static probe at which
the tracepoint is located. See Section 5.1.10 [Static Probe Points],
page 65.
$_probe_argn
n is an integer between 0 and 11. Collects the nth argument
from the static probe at which the tracepoint is located. See
Section 5.1.10 [Static Probe Points], page 65.
$_sdata
Collect static tracepoint marker specific data. Only available for
static tracepoints. See Section 13.1.6 [Tracepoint Action Lists],
page 172. On the UST static tracepoints library backend, an instrumentation point resembles a printf function call. The tracing
library is able to collect user specified data formatted to a character
string using the format provided by the programmer that instrumented the program. Other backends have similar mechanisms.
Here’s an example of a UST marker call:
const char master_name[] = "$your_name";
trace_mark(channel1, marker1, "hello %s", master_name)
174
Debugging with gdb
In this case, collecting $_sdata collects the string ‘hello
$yourname’. When analyzing the trace buffer, you can inspect
‘$_sdata’ like any other variable available to gdb.
You can give several consecutive collect commands, each one with a single
argument, or one collect command with several arguments separated by commas; the effect is the same.
The optional mods changes the usual handling of the arguments. s requests
that pointers to chars be handled as strings, in particular collecting the contents
of the memory being pointed at, up to the first zero. The upper bound is by
default the value of the print elements variable; if s is followed by a decimal
number, that is the upper bound instead. So for instance ‘collect/s25 mystr’
collects as many as 25 characters at ‘mystr’.
The command info scope (see Chapter 16 [Symbols], page 221) is particularly
useful for figuring out what data to collect.
teval expr1, expr2, ...
Evaluate the given expressions when the tracepoint is hit. This command accepts a comma-separated list of expressions. The results are discarded, so this
is mainly useful for assigning values to trace state variables (see Section 13.1.5
[Trace State Variables], page 171) without adding those values to the trace
buffer, as would be the case if the collect action were used.
while-stepping n
Perform n single-step instruction traces after the tracepoint, collecting new data
after each step. The while-stepping command is followed by the list of what
to collect while stepping (followed by its own end command):
> while-stepping 12
> collect $regs, myglobal
> end
>
Note that $pc is not automatically collected by while-stepping; you need
to explicitly collect that register if you need it. You may abbreviate whilestepping as ws or stepping.
set default-collect expr1, expr2, ...
This variable is a list of expressions to collect at each tracepoint hit. It is
effectively an additional collect action prepended to every tracepoint action
list. The expressions are parsed individually for each tracepoint, so for instance
a variable named xyz may be interpreted as a global for one tracepoint, and a
local for another, as appropriate to the tracepoint’s location.
show default-collect
Show the list of expressions that are collected by default at each tracepoint hit.
13.1.7 Listing Tracepoints
info tracepoints [num...]
Display information about the tracepoint num. If you don’t specify a tracepoint
number, displays information about all the tracepoints defined so far. The
Chapter 13: Tracepoints
175
format is similar to that used for info breakpoints; in fact, info tracepoints
is the same command, simply restricting itself to tracepoints.
A tracepoint’s listing may include additional information specific to tracing:
• its passcount as given by the passcount n command
• the state about installed on target of each location
(gdb) info trace
Num
Type
Disp Enb Address
What
1
tracepoint
keep y
0x0804ab57 in foo() at main.cxx:7
while-stepping 20
collect globfoo, $regs
end
collect globfoo2
end
pass count 1200
2
tracepoint
keep y
collect $eip
2.1
y
0x0804859c in func4 at change-loc.h:35
installed on target
2.2
y
0xb7ffc480 in func4 at change-loc.h:35
installed on target
2.3
y
set_tracepoint
3
tracepoint
keep y
0x080485b1 in foo at change-loc.c:29
not installed on target
(gdb)
This command can be abbreviated info tp.
13.1.8 Listing Static Tracepoint Markers
info static-tracepoint-markers
Display information about all static tracepoint markers defined in the program.
For each marker, the following columns are printed:
Count
An incrementing counter, output to help readability. This is not a
stable identifier.
ID
The marker ID, as reported by the target.
Enabled or Disabled
Probed markers are tagged with ‘y’. ‘n’ identifies marks that are
not enabled.
Address
Where the marker is in your program, as a memory address.
What
Where the marker is in the source for your program, as a file and
line number. If the debug information included in the program does
not allow gdb to locate the source of the marker, this column will
be left blank.
In addition, the following information may be printed for each marker:
Data
User data passed to the tracing library by the marker call. In the
UST backend, this is the format string passed as argument to the
marker call.
176
Debugging with gdb
Static tracepoints probing the marker
The list of static tracepoints attached to the marker.
(gdb) info static-tracepoint-markers
Cnt ID
Enb Address
What
1
ust/bar2
y
0x0000000000400e1a in main at stexample.c:25
Data: number1 %d number2 %d
Probed by static tracepoints: #2
2
ust/bar33 n
0x0000000000400c87 in main at stexample.c:24
Data: str %s
(gdb)
13.1.9 Starting and Stopping Trace Experiments
tstart
This command starts the trace experiment, and begins collecting data. It has
the side effect of discarding all the data collected in the trace buffer during the
previous trace experiment. If any arguments are supplied, they are taken as a
note and stored with the trace experiment’s state. The notes may be arbitrary
text, and are especially useful with disconnected tracing in a multi-user context;
the notes can explain what the trace is doing, supply user contact information,
and so forth.
tstop
This command stops the trace experiment. If any arguments are supplied, they
are recorded with the experiment as a note. This is useful if you are stopping
a trace started by someone else, for instance if the trace is interfering with the
system’s behavior and needs to be stopped quickly.
Note: a trace experiment and data collection may stop automatically if any
tracepoint’s passcount is reached (see Section 13.1.3 [Tracepoint Passcounts],
page 170), or if the trace buffer becomes full.
tstatus
This command displays the status of the current trace data collection.
Here is an example of the commands we described so far:
(gdb) trace gdb c test
(gdb) actions
Enter actions for tracepoint #1, one per line.
> collect $regs,$locals,$args
> while-stepping 11
> collect $regs
> end
> end
(gdb) tstart
[time passes ...]
(gdb) tstop
You can choose to continue running the trace experiment even if gdb disconnects from
the target, voluntarily or involuntarily. For commands such as detach, the debugger will
ask what you want to do with the trace. But for unexpected terminations (gdb crash,
network outage), it would be unfortunate to lose hard-won trace data, so the variable
disconnected-tracing lets you decide whether the trace should continue running without
gdb.
Chapter 13: Tracepoints
177
set disconnected-tracing on
set disconnected-tracing off
Choose whether a tracing run should continue to run if gdb has disconnected
from the target. Note that detach or quit will ask you directly what to do
about a running trace no matter what this variable’s setting, so the variable is
mainly useful for handling unexpected situations, such as loss of the network.
show disconnected-tracing
Show the current choice for disconnected tracing.
When you reconnect to the target, the trace experiment may or may not still be running;
it might have filled the trace buffer in the meantime, or stopped for one of the other reasons.
If it is running, it will continue after reconnection.
Upon reconnection, the target will upload information about the tracepoints in effect.
gdb will then compare that information to the set of tracepoints currently defined, and
attempt to match them up, allowing for the possibility that the numbers may have changed
due to creation and deletion in the meantime. If one of the target’s tracepoints does not
match any in gdb, the debugger will create a new tracepoint, so that you have a number
with which to specify that tracepoint. This matching-up process is necessarily heuristic,
and it may result in useless tracepoints being created; you may simply delete them if they
are of no use.
If your target agent supports a circular trace buffer, then you can run a trace experiment
indefinitely without filling the trace buffer; when space runs out, the agent deletes alreadycollected trace frames, oldest first, until there is enough room to continue collecting. This
is especially useful if your tracepoints are being hit too often, and your trace gets terminated prematurely because the buffer is full. To ask for a circular trace buffer, simply set
‘circular-trace-buffer’ to on. You can set this at any time, including during tracing;
if the agent can do it, it will change buffer handling on the fly, otherwise it will not take
effect until the next run.
set circular-trace-buffer on
set circular-trace-buffer off
Choose whether a tracing run should use a linear or circular buffer for trace
data. A linear buffer will not lose any trace data, but may fill up prematurely,
while a circular buffer will discard old trace data, but it will have always room
for the latest tracepoint hits.
show circular-trace-buffer
Show the current choice for the trace buffer. Note that this may not match the
agent’s current buffer handling, nor is it guaranteed to match the setting that
might have been in effect during a past run, for instance if you are looking at
frames from a trace file.
set trace-buffer-size n
set trace-buffer-size unlimited
Request that the target use a trace buffer of n bytes. Not all targets will honor
the request; they may have a compiled-in size for the trace buffer, or some other
limitation. Set to a value of unlimited or -1 to let the target use whatever
size it likes. This is also the default.
178
Debugging with gdb
show trace-buffer-size
Show the current requested size for the trace buffer. Note that this will only
match the actual size if the target supports size-setting, and was able to handle
the requested size. For instance, if the target can only change buffer size between
runs, this variable will not reflect the change until the next run starts. Use
tstatus to get a report of the actual buffer size.
set trace-user text
show trace-user
set trace-notes text
Set the trace run’s notes.
show trace-notes
Show the trace run’s notes.
set trace-stop-notes text
Set the trace run’s stop notes. The handling of the note is as for tstop arguments; the set command is convenient way to fix a stop note that is mistaken
or incomplete.
show trace-stop-notes
Show the trace run’s stop notes.
13.1.10 Tracepoint Restrictions
There are a number of restrictions on the use of tracepoints. As described above, tracepoint
data gathering occurs on the target without interaction from gdb. Thus the full capabilities
of the debugger are not available during data gathering, and then at data examination time,
you will be limited by only having what was collected. The following items describe some
common problems, but it is not exhaustive, and you may run into additional difficulties not
mentioned here.
• Tracepoint expressions are intended to gather objects (lvalues). Thus the full flexibility
of GDB’s expression evaluator is not available. You cannot call functions, cast objects
to aggregate types, access convenience variables or modify values (except by assignment
to trace state variables). Some language features may implicitly call functions (for
instance Objective-C fields with accessors), and therefore cannot be collected either.
• Collection of local variables, either individually or in bulk with $locals or $args,
during while-stepping may behave erratically. The stepping action may enter a new
scope (for instance by stepping into a function), or the location of the variable may
change (for instance it is loaded into a register). The tracepoint data recorded uses the
location information for the variables that is correct for the tracepoint location. When
the tracepoint is created, it is not possible, in general, to determine where the steps
of a while-stepping sequence will advance the program—particularly if a conditional
branch is stepped.
• Collection of an incompletely-initialized or partially-destroyed object may result in
something that gdb cannot display, or displays in a misleading way.
• When gdb displays a pointer to character it automatically dereferences the pointer to
also display characters of the string being pointed to. However, collecting the pointer
during tracing does not automatically collect the string. You need to explicitly dereference the pointer and provide size information if you want to collect not only the
Chapter 13: Tracepoints
179
pointer, but the memory pointed to. For example, *ptr@50 can be used to collect the
50 element array pointed to by ptr.
• It is not possible to collect a complete stack backtrace at a tracepoint. Instead, you may
collect the registers and a few hundred bytes from the stack pointer with something
like *(unsigned char *)$esp@300 (adjust to use the name of the actual stack pointer
register on your target architecture, and the amount of stack you wish to capture).
Then the backtrace command will show a partial backtrace when using a trace frame.
The number of stack frames that can be examined depends on the sizes of the frames
in the collected stack. Note that if you ask for a block so large that it goes past the
bottom of the stack, the target agent may report an error trying to read from an invalid
address.
• If you do not collect registers at a tracepoint, gdb can infer that the value of $pc
must be the same as the address of the tracepoint and use that when you are looking
at a trace frame for that tracepoint. However, this cannot work if the tracepoint has
multiple locations (for instance if it was set in a function that was inlined), or if it has
a while-stepping loop. In those cases gdb will warn you that it can’t infer $pc, and
default it to zero.
13.2 Using the Collected Data
After the tracepoint experiment ends, you use gdb commands for examining the trace
data. The basic idea is that each tracepoint collects a trace snapshot every time it is
hit and another snapshot every time it single-steps. All these snapshots are consecutively
numbered from zero and go into a buffer, and you can examine them later. The way you
examine them is to focus on a specific trace snapshot. When the remote stub is focused on a
trace snapshot, it will respond to all gdb requests for memory and registers by reading from
the buffer which belongs to that snapshot, rather than from real memory or registers of the
program being debugged. This means that all gdb commands (print, info registers,
backtrace, etc.) will behave as if we were currently debugging the program state as it was
when the tracepoint occurred. Any requests for data that are not in the buffer will fail.
13.2.1 tfind n
The basic command for selecting a trace snapshot from the buffer is tfind n, which finds
trace snapshot number n, counting from zero. If no argument n is given, the next snapshot
is selected.
Here are the various forms of using the tfind command.
tfind start
Find the first snapshot in the buffer. This is a synonym for tfind 0 (since 0 is
the number of the first snapshot).
tfind none
Stop debugging trace snapshots, resume live debugging.
tfind end Same as ‘tfind none’.
tfind
No argument means find the next trace snapshot or find the first one if no trace
snapshot is selected.
180
Debugging with gdb
tfind -
Find the previous trace snapshot before the current one. This permits retracing
earlier steps.
tfind tracepoint num
Find the next snapshot associated with tracepoint num. Search proceeds forward from the last examined trace snapshot. If no argument num is given, it
means find the next snapshot collected for the same tracepoint as the current
snapshot.
tfind pc addr
Find the next snapshot associated with the value addr of the program counter.
Search proceeds forward from the last examined trace snapshot. If no argument
addr is given, it means find the next snapshot with the same value of PC as
the current snapshot.
tfind outside addr1, addr2
Find the next snapshot whose PC is outside the given range of addresses (exclusive).
tfind range addr1, addr2
Find the next snapshot whose PC is between addr1 and addr2 (inclusive).
tfind line [file:]n
Find the next snapshot associated with the source line n. If the optional argument file is given, refer to line n in that source file. Search proceeds forward
from the last examined trace snapshot. If no argument n is given, it means find
the next line other than the one currently being examined; thus saying tfind
line repeatedly can appear to have the same effect as stepping from line to
line in a live debugging session.
The default arguments for the tfind commands are specifically designed to make it easy
to scan through the trace buffer. For instance, tfind with no argument selects the next
trace snapshot, and tfind - with no argument selects the previous trace snapshot. So, by
giving one tfind command, and then simply hitting RET repeatedly you can examine all
the trace snapshots in order. Or, by saying tfind - and then hitting RET repeatedly you
can examine the snapshots in reverse order. The tfind line command with no argument
selects the snapshot for the next source line executed. The tfind pc command with no
argument selects the next snapshot with the same program counter (PC) as the current
frame. The tfind tracepoint command with no argument selects the next trace snapshot
collected by the same tracepoint as the current one.
In addition to letting you scan through the trace buffer manually, these commands make
it easy to construct gdb scripts that scan through the trace buffer and print out whatever
collected data you are interested in. Thus, if we want to examine the PC, FP, and SP
registers from each trace frame in the buffer, we can say this:
(gdb) tfind start
(gdb) while ($trace frame != -1)
> printf "Frame %d, PC = %08X, SP = %08X, FP = %08X\n", \
$trace_frame, $pc, $sp, $fp
> tfind
> end
Chapter 13: Tracepoints
Frame
Frame
Frame
Frame
Frame
Frame
Frame
Frame
Frame
Frame
Frame
181
0, PC = 0020DC64, SP = 0030BF3C, FP = 0030BF44
1, PC = 0020DC6C, SP = 0030BF38, FP = 0030BF44
2, PC = 0020DC70, SP = 0030BF34, FP = 0030BF44
3, PC = 0020DC74, SP = 0030BF30, FP = 0030BF44
4, PC = 0020DC78, SP = 0030BF2C, FP = 0030BF44
5, PC = 0020DC7C, SP = 0030BF28, FP = 0030BF44
6, PC = 0020DC80, SP = 0030BF24, FP = 0030BF44
7, PC = 0020DC84, SP = 0030BF20, FP = 0030BF44
8, PC = 0020DC88, SP = 0030BF1C, FP = 0030BF44
9, PC = 0020DC8E, SP = 0030BF18, FP = 0030BF44
10, PC = 00203F6C, SP = 0030BE3C, FP = 0030BF14
Or, if we want to examine the variable X at each source line in the buffer:
(gdb) tfind start
(gdb) while ($trace frame != -1)
> printf "Frame %d, X == %d\n", $trace_frame, X
> tfind line
> end
Frame 0, X = 1
Frame 7, X = 2
Frame 13, X = 255
13.2.2 tdump
This command takes no arguments. It prints all the data collected at the current trace
snapshot.
(gdb) trace 444
(gdb) actions
Enter actions for tracepoint #2, one per line:
> collect $regs, $locals, $args, gdb_long_test
> end
(gdb) tstart
(gdb) tfind line 444
#0 gdb_test (p1=0x11, p2=0x22, p3=0x33, p4=0x44, p5=0x55, p6=0x66)
at gdb_test.c:444
444
printp( "%s: arguments = 0x%X 0x%X 0x%X 0x%X 0x%X 0x%X\n", )
(gdb) tdump
Data collected
d0
d1
d2
d3
d4
d5
d6
d7
a0
a1
a2
a3
a4
a5
fp
sp
at tracepoint 2, trace frame 1:
0xc4aa0085
-995491707
0x18
24
0x80
128
0x33
51
0x71aea3d
119204413
0x22
34
0xe0
224
0x380035 3670069
0x19e24a 1696330
0x3000668
50333288
0x100
256
0x322000 3284992
0x3000698
50333336
0x1ad3cc 1758156
0x30bf3c 0x30bf3c
0x30bf34 0x30bf34
182
Debugging with gdb
ps
0x0
0
pc
0x20b2c8 0x20b2c8
fpcontrol
0x0
0
fpstatus
0x0
0
fpiaddr
0x0
0
p = 0x20e5b4 "gdb-test"
p1 = (void *) 0x11
p2 = (void *) 0x22
p3 = (void *) 0x33
p4 = (void *) 0x44
p5 = (void *) 0x55
p6 = (void *) 0x66
gdb_long_test = 17 ’\021’
(gdb)
tdump works by scanning the tracepoint’s current collection actions and printing the
value of each expression listed. So tdump can fail, if after a run, you change the tracepoint’s
actions to mention variables that were not collected during the run.
Also, for tracepoints with while-stepping loops, tdump uses the collected value of $pc
to distinguish between trace frames that were collected at the tracepoint hit, and frames
that were collected while stepping. This allows it to correctly choose whether to display
the basic list of collections, or the collections from the body of the while-stepping loop.
However, if $pc was not collected, then tdump will always attempt to dump using the basic
collection list, and may fail if a while-stepping frame does not include all the same data
that is collected at the tracepoint hit.
13.2.3 save tracepoints filename
This command saves all current tracepoint definitions together with their actions and passcounts, into a file ‘filename’ suitable for use in a later debugging session. To read the
saved tracepoint definitions, use the source command (see Section 23.1.3 [Command Files],
page 324). The save-tracepoints command is a deprecated alias for save tracepoints
13.3 Convenience Variables for Tracepoints
(int) $trace_frame
The current trace snapshot (a.k.a. frame) number, or -1 if no snapshot is selected.
(int) $tracepoint
The tracepoint for the current trace snapshot.
(int) $trace_line
The line number for the current trace snapshot.
(char []) $trace_file
The source file for the current trace snapshot.
(char []) $trace_func
The name of the function containing $tracepoint.
Note: $trace_file is not suitable for use in printf, use output instead.
Chapter 13: Tracepoints
183
Here’s a simple example of using these convenience variables for stepping through all the
trace snapshots and printing some of their data. Note that these are not the same as trace
state variables, which are managed by the target.
(gdb) tfind start
(gdb) while $trace frame != -1
> output $trace_file
> printf ", line %d (tracepoint #%d)\n", $trace_line, $tracepoint
> tfind
> end
13.4 Using Trace Files
In some situations, the target running a trace experiment may no longer be available;
perhaps it crashed, or the hardware was needed for a different activity. To handle these
cases, you can arrange to dump the trace data into a file, and later use that file as a source
of trace data, via the target tfile command.
tsave [ -r ] filename
tsave [-ctf] dirname
Save the trace data to filename. By default, this command assumes that filename refers to the host filesystem, so if necessary gdb will copy raw trace data
up from the target and then save it. If the target supports it, you can also
supply the optional argument -r (“remote”) to direct the target to save the
data directly into filename in its own filesystem, which may be more efficient
if the trace buffer is very large. (Note, however, that target tfile can only
read from files accessible to the host.) By default, this command will save trace
frame in tfile format. You can supply the optional argument -ctf to save data
in CTF format. The Common Trace Format (CTF) is proposed as a trace format that can be shared by multiple debugging and tracing tools. Please go to
http://www.efficios.com/ctf to get more information.
target tfile filename
target ctf dirname
Use the file named filename or directory named dirname as a source of trace
data. Commands that examine data work as they do with a live target, but it
is not possible to run any new trace experiments. tstatus will report the state
of the trace run at the moment the data was saved, as well as the current trace
frame you are examining. Both filename and dirname must be on a filesystem
accessible to the host.
(gdb) target ctf ctf.ctf
(gdb) tfind
Found trace frame 0, tracepoint 2
39
++a; /* set tracepoint 1 here */
(gdb) tdump
Data collected at tracepoint 2, trace frame 0:
i = 0
a = 0
b = 1 ’\001’
c = {"123", "456", "789", "123", "456", "789"}
d = {{{a = 1, b = 2}, {a = 3, b = 4}}, {{a = 5, b = 6}, {a = 7, b = 8}}}
(gdb) p b
184
Debugging with gdb
$1 = 1
Chapter 14: Debugging Programs That Use Overlays
185
14 Debugging Programs That Use Overlays
If your program is too large to fit completely in your target system’s memory, you can sometimes use overlays to work around this problem. gdb provides some support for debugging
programs that use overlays.
14.1 How Overlays Work
Suppose you have a computer whose instruction address space is only 64 kilobytes long, but
which has much more memory which can be accessed by other means: special instructions,
segment registers, or memory management hardware, for example. Suppose further that
you want to adapt a program which is larger than 64 kilobytes to run on this system.
One solution is to identify modules of your program which are relatively independent,
and need not call each other directly; call these modules overlays. Separate the overlays
from the main program, and place their machine code in the larger memory. Place your
main program in instruction memory, but leave at least enough space there to hold the
largest overlay as well.
Now, to call a function located in an overlay, you must first copy that overlay’s machine
code from the large memory into the space set aside for it in the instruction memory, and
then jump to its entry point there.
Data
Address Space
+-----------+
|
|
+-----------+
| program
|
| variables |
| and heap |
+-----------+
|
|
+-----------+
Instruction
Larger
Address Space
Address Space
+-----------+
+-----------+
|
|
|
|
+-----------+
+-----------+<-- overlay 1
|
main
|
.----| overlay 1 | load address
| program |
|
+-----------+
|
|
|
|
|
|
|
|
+-----------+<-- overlay 2
+-----------+
|
|
| load address
|
|
| .-| overlay 2 |
|
|
| | |
|
mapped --->+-----------+
| | +-----------+
address
|
|
| | |
|
| overlay | <-’ | |
|
|
area
| <---’ +-----------+<-- overlay 3
|
| <---. |
| load address
+-----------+
‘--| overlay 3 |
|
|
|
|
+-----------+
|
|
+-----------+
|
|
+-----------+
A code overlay
The diagram (see [A code overlay], page 185) shows a system with separate data and
instruction address spaces. To map an overlay, the program copies its code from the larger
address space to the instruction address space. Since the overlays shown here all use the
same mapped address, only one may be mapped at a time. For a system with a single
address space for data and instructions, the diagram would be similar, except that the
program variables and heap would share an address space with the main program and the
overlay area.
186
Debugging with gdb
An overlay loaded into instruction memory and ready for use is called a mapped overlay;
its mapped address is its address in the instruction memory. An overlay not present (or only
partially present) in instruction memory is called unmapped; its load address is its address
in the larger memory. The mapped address is also called the virtual memory address, or
VMA; the load address is also called the load memory address, or LMA.
Unfortunately, overlays are not a completely transparent way to adapt a program to
limited instruction memory. They introduce a new set of global constraints you must keep
in mind as you design your program:
• Before calling or returning to a function in an overlay, your program must make sure
that overlay is actually mapped. Otherwise, the call or return will transfer control to
the right address, but in the wrong overlay, and your program will probably crash.
• If the process of mapping an overlay is expensive on your system, you will need to
choose your overlays carefully to minimize their effect on your program’s performance.
• The executable file you load onto your system must contain each overlay’s instructions, appearing at the overlay’s load address, not its mapped address. However, each
overlay’s instructions must be relocated and its symbols defined as if the overlay were
at its mapped address. You can use GNU linker scripts to specify different load and
relocation addresses for pieces of your program; see Section “Overlay Description” in
Using ld: the GNU linker.
• The procedure for loading executable files onto your system must be able to load their
contents into the larger address space as well as the instruction and data spaces.
The overlay system described above is rather simple, and could be improved in many
ways:
• If your system has suitable bank switch registers or memory management hardware,
you could use those facilities to make an overlay’s load area contents simply appear at
their mapped address in instruction space. This would probably be faster than copying
the overlay to its mapped area in the usual way.
• If your overlays are small enough, you could set aside more than one overlay area, and
have more than one overlay mapped at a time.
• You can use overlays to manage data, as well as instructions. In general, data overlays
are even less transparent to your design than code overlays: whereas code overlays only
require care when you call or return to functions, data overlays require care every time
you access the data. Also, if you change the contents of a data overlay, you must copy
its contents back out to its load address before you can copy a different data overlay
into the same mapped area.
14.2 Overlay Commands
To use gdb’s overlay support, each overlay in your program must correspond to a separate
section of the executable file. The section’s virtual memory address and load memory
address must be the overlay’s mapped and load addresses. Identifying overlays with sections
allows gdb to determine the appropriate address of a function or variable, depending on
whether the overlay is mapped or not.
gdb’s overlay commands all start with the word overlay; you can abbreviate this as ov
or ovly. The commands are:
Chapter 14: Debugging Programs That Use Overlays
187
overlay off
Disable gdb’s overlay support. When overlay support is disabled, gdb assumes
that all functions and variables are always present at their mapped addresses.
By default, gdb’s overlay support is disabled.
overlay manual
Enable manual overlay debugging. In this mode, gdb relies on you to tell it
which overlays are mapped, and which are not, using the overlay map-overlay
and overlay unmap-overlay commands described below.
overlay map-overlay overlay
overlay map overlay
Tell gdb that overlay is now mapped; overlay must be the name of the object
file section containing the overlay. When an overlay is mapped, gdb assumes it
can find the overlay’s functions and variables at their mapped addresses. gdb
assumes that any other overlays whose mapped ranges overlap that of overlay
are now unmapped.
overlay unmap-overlay overlay
overlay unmap overlay
Tell gdb that overlay is no longer mapped; overlay must be the name of the
object file section containing the overlay. When an overlay is unmapped, gdb
assumes it can find the overlay’s functions and variables at their load addresses.
overlay auto
Enable automatic overlay debugging. In this mode, gdb consults a data structure the overlay manager maintains in the inferior to see which overlays are
mapped. For details, see Section 14.3 [Automatic Overlay Debugging], page 188.
overlay load-target
overlay load
Re-read the overlay table from the inferior. Normally, gdb re-reads the table
gdb automatically each time the inferior stops, so this command should only
be necessary if you have changed the overlay mapping yourself using gdb. This
command is only useful when using automatic overlay debugging.
overlay list-overlays
overlay list
Display a list of the overlays currently mapped, along with their mapped addresses, load addresses, and sizes.
Normally, when gdb prints a code address, it includes the name of the function the
address falls in:
(gdb) print main
$3 = {int ()} 0x11a0
When overlay debugging is enabled, gdb recognizes code in unmapped overlays, and prints
the names of unmapped functions with asterisks around them. For example, if foo is a
function in an unmapped overlay, gdb prints it this way:
(gdb) overlay list
No sections are mapped.
(gdb) print foo
188
Debugging with gdb
$5 = {int (int)} 0x100000 <*foo*>
When foo’s overlay is mapped, gdb prints the function’s name normally:
(gdb) overlay list
Section .ov.foo.text, loaded at 0x100000 - 0x100034,
mapped at 0x1016 - 0x104a
(gdb) print foo
$6 = {int (int)} 0x1016
When overlay debugging is enabled, gdb can find the correct address for functions and
variables in an overlay, whether or not the overlay is mapped. This allows most gdb commands, like break and disassemble, to work normally, even on unmapped code. However,
gdb’s breakpoint support has some limitations:
• You can set breakpoints in functions in unmapped overlays, as long as gdb can write
to the overlay at its load address.
• gdb can not set hardware or simulator-based breakpoints in unmapped overlays. However, if you set a breakpoint at the end of your overlay manager (and tell gdb which
overlays are now mapped, if you are using manual overlay management), gdb will re-set
its breakpoints properly.
14.3 Automatic Overlay Debugging
gdb can automatically track which overlays are mapped and which are not, given some
simple co-operation from the overlay manager in the inferior. If you enable automatic
overlay debugging with the overlay auto command (see Section 14.2 [Overlay Commands],
page 186), gdb looks in the inferior’s memory for certain variables describing the current
state of the overlays.
Here are the variables your overlay manager must define to support gdb’s automatic
overlay debugging:
_ovly_table:
This variable must be an array of the following structures:
struct
{
/* The overlay’s mapped address.
unsigned long vma;
*/
/* The size of the overlay, in bytes.
unsigned long size;
/* The overlay’s load address.
unsigned long lma;
*/
*/
/* Non-zero if the overlay is currently mapped;
zero otherwise. */
unsigned long mapped;
}
_novlys:
This variable must be a four-byte signed integer, holding the total number of
elements in _ovly_table.
To decide whether a particular overlay is mapped or not, gdb looks for an entry in
_ovly_table whose vma and lma members equal the VMA and LMA of the overlay’s section
Chapter 14: Debugging Programs That Use Overlays
189
in the executable file. When gdb finds a matching entry, it consults the entry’s mapped
member to determine whether the overlay is currently mapped.
In addition, your overlay manager may define a function called _ovly_debug_event. If
this function is defined, gdb will silently set a breakpoint there. If the overlay manager
then calls this function whenever it has changed the overlay table, this will enable gdb to
accurately keep track of which overlays are in program memory, and update any breakpoints
that may be set in overlays. This will allow breakpoints to work even if the overlays are
kept in ROM or other non-writable memory while they are not being executed.
14.4 Overlay Sample Program
When linking a program which uses overlays, you must place the overlays at their load
addresses, while relocating them to run at their mapped addresses. To do this, you must
write a linker script (see Section “Overlay Description” in Using ld: the GNU linker). Unfortunately, since linker scripts are specific to a particular host system, target architecture,
and target memory layout, this manual cannot provide portable sample code demonstrating
gdb’s overlay support.
However, the gdb source distribution does contain an overlaid program, with linker
scripts for a few systems, as part of its test suite. The program consists of the following
files from ‘gdb/testsuite/gdb.base’:
‘overlays.c’
The main program file.
‘ovlymgr.c’
A simple overlay manager, used by ‘overlays.c’.
‘foo.c’
‘bar.c’
‘baz.c’
‘grbx.c’
‘d10v.ld’
‘m32r.ld’
Overlay modules, loaded and used by ‘overlays.c’.
Linker scripts for linking the test program on the d10v-elf and m32r-elf
targets.
You can build the test program using the d10v-elf GCC cross-compiler like this:
$
$
$
$
$
$
$
d10v-elf-gcc
d10v-elf-gcc
d10v-elf-gcc
d10v-elf-gcc
d10v-elf-gcc
d10v-elf-gcc
d10v-elf-gcc
-g
-g
-g
-g
-g
-g
-g
-c overlays.c
-c ovlymgr.c
-c foo.c
-c bar.c
-c baz.c
-c grbx.c
overlays.o ovlymgr.o foo.o bar.o \
baz.o grbx.o -Wl,-Td10v.ld -o overlays
The build process is identical for any other architecture, except that you must substitute
the appropriate compiler and linker script for the target system for d10v-elf-gcc and
d10v.ld.
Chapter 15: Using gdb with Different Languages
191
15 Using gdb with Different Languages
Although programming languages generally have common aspects, they are rarely expressed
in the same manner. For instance, in ANSI C, dereferencing a pointer p is accomplished
by *p, but in Modula-2, it is accomplished by p^. Values can also be represented (and
displayed) differently. Hex numbers in C appear as ‘0x1ae’, while in Modula-2 they appear
as ‘1AEH’.
Language-specific information is built into gdb for some languages, allowing you to
express operations like the above in your program’s native language, and allowing gdb to
output values in a manner consistent with the syntax of your program’s native language.
The language you use to build expressions is called the working language.
15.1 Switching Between Source Languages
There are two ways to control the working language—either have gdb set it automatically,
or select it manually yourself. You can use the set language command for either purpose.
On startup, gdb defaults to setting the language automatically. The working language is
used to determine how expressions you type are interpreted, how values are printed, etc.
In addition to the working language, every source file that gdb knows about has its
own working language. For some object file formats, the compiler might indicate which
language a particular source file is in. However, most of the time gdb infers the language
from the name of the file. The language of a source file controls whether C++ names are
demangled—this way backtrace can show each frame appropriately for its own language.
There is no way to set the language of a source file from within gdb, but you can set the
language associated with a filename extension. See Section 15.2 [Displaying the Language],
page 192.
This is most commonly a problem when you use a program, such as cfront or f2c, that
generates C but is written in another language. In that case, make the program use #line
directives in its C output; that way gdb will know the correct language of the source code
of the original program, and will display that source code, not the generated C code.
15.1.1 List of Filename Extensions and Languages
If a source file name ends in one of the following extensions, then gdb infers that its language
is the one indicated.
‘.ada’
‘.ads’
‘.adb’
‘.a’
Ada source file.
‘.c’
C source file
‘.C’
‘.cc’
‘.cp’
‘.cpp’
‘.cxx’
‘.c++’
C++ source file
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‘.d’
D source file
‘.m’
Objective-C source file
‘.f’
‘.F’
Fortran source file
‘.mod’
Modula-2 source file
‘.s’
‘.S’
Assembler source file. This actually behaves almost like C, but gdb does not
skip over function prologues when stepping.
In addition, you may set the language associated with a filename extension.
Section 15.2 [Displaying the Language], page 192.
See
15.1.2 Setting the Working Language
If you allow gdb to set the language automatically, expressions are interpreted the same
way in your debugging session and your program.
If you wish, you may set the language manually. To do this, issue the command ‘set
language lang’, where lang is the name of a language, such as c or modula-2. For a list
of the supported languages, type ‘set language’.
Setting the language manually prevents gdb from updating the working language automatically. This can lead to confusion if you try to debug a program when the working
language is not the same as the source language, when an expression is acceptable to both
languages—but means different things. For instance, if the current source file were written
in C, and gdb was parsing Modula-2, a command such as:
print a = b + c
might not have the effect you intended. In C, this means to add b and c and place the
result in a. The result printed would be the value of a. In Modula-2, this means to compare
a to the result of b+c, yielding a BOOLEAN value.
15.1.3 Having gdb Infer the Source Language
To have gdb set the working language automatically, use ‘set language local’ or ‘set
language auto’. gdb then infers the working language. That is, when your program stops
in a frame (usually by encountering a breakpoint), gdb sets the working language to the
language recorded for the function in that frame. If the language for a frame is unknown
(that is, if the function or block corresponding to the frame was defined in a source file that
does not have a recognized extension), the current working language is not changed, and
gdb issues a warning.
This may not seem necessary for most programs, which are written entirely in one source
language. However, program modules and libraries written in one source language can be
used by a main program written in a different source language. Using ‘set language auto’
in this case frees you from having to set the working language manually.
15.2 Displaying the Language
The following commands help you find out which language is the working language, and
also what language source files were written in.
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show language
Display the current working language. This is the language you can use with
commands such as print to build and compute expressions that may involve
variables in your program.
info frame
Display the source language for this frame. This language becomes the working
language if you use an identifier from this frame. See Section 8.4 [Information
about a Frame], page 99, to identify the other information listed here.
info source
Display the source language of this source file. See Chapter 16 [Examining the
Symbol Table], page 221, to identify the other information listed here.
In unusual circumstances, you may have source files with extensions not in the standard
list. You can then set the extension associated with a language explicitly:
set extension-language ext language
Tell gdb that source files with extension ext are to be assumed as written in
the source language language.
info extensions
List all the filename extensions and the associated languages.
15.3 Type and Range Checking
Some languages are designed to guard you against making seemingly common errors through
a series of compile- and run-time checks. These include checking the type of arguments to
functions and operators and making sure mathematical overflows are caught at run time.
Checks such as these help to ensure a program’s correctness once it has been compiled
by eliminating type mismatches and providing active checks for range errors when your
program is running.
By default gdb checks for these errors according to the rules of the current source
language. Although gdb does not check the statements in your program, it can check
expressions entered directly into gdb for evaluation via the print command, for example.
15.3.1 An Overview of Type Checking
Some languages, such as C and C++, are strongly typed, meaning that the arguments to
operators and functions have to be of the correct type, otherwise an error occurs. These
checks prevent type mismatch errors from ever causing any run-time problems. For example,
int klass::my_method(char *b) { return
b ? 1 : 2; }
(gdb) print obj.my_method (0)
$1 = 2
but
(gdb) print obj.my_method (0x1234)
Cannot resolve method klass::my_method to any overloaded instance
The second example fails because in C++ the integer constant ‘0x1234’ is not typecompatible with the pointer parameter type.
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For the expressions you use in gdb commands, you can tell gdb to not enforce strict
type checking or to treat any mismatches as errors and abandon the expression; When type
checking is disabled, gdb successfully evaluates expressions like the second example above.
Even if type checking is off, there may be other reasons related to type that prevent gdb
from evaluating an expression. For instance, gdb does not know how to add an int and a
struct foo. These particular type errors have nothing to do with the language in use and
usually arise from expressions which make little sense to evaluate anyway.
gdb provides some additional commands for controlling type checking:
set check type on
set check type off
Set strict type checking on or off. If any type mismatches occur in evaluating an
expression while type checking is on, gdb prints a message and aborts evaluation
of the expression.
show check type
Show the current setting of type checking and whether gdb is enforcing strict
type checking rules.
15.3.2 An Overview of Range Checking
In some languages (such as Modula-2), it is an error to exceed the bounds of a type; this is
enforced with run-time checks. Such range checking is meant to ensure program correctness
by making sure computations do not overflow, or indices on an array element access do not
exceed the bounds of the array.
For expressions you use in gdb commands, you can tell gdb to treat range errors in one
of three ways: ignore them, always treat them as errors and abandon the expression, or
issue warnings but evaluate the expression anyway.
A range error can result from numerical overflow, from exceeding an array index bound,
or when you type a constant that is not a member of any type. Some languages, however,
do not treat overflows as an error. In many implementations of C, mathematical overflow
causes the result to “wrap around” to lower values—for example, if m is the largest integer
value, and s is the smallest, then
m + 1 ⇒ s
This, too, is specific to individual languages, and in some cases specific to individual
compilers or machines. See Section 15.4 [Supported Languages], page 195, for further
details on specific languages.
gdb provides some additional commands for controlling the range checker:
set check range auto
Set range checking on or off based on the current working language. See
Section 15.4 [Supported Languages], page 195, for the default settings for each
language.
set check range on
set check range off
Set range checking on or off, overriding the default setting for the current working language. A warning is issued if the setting does not match the language
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default. If a range error occurs and range checking is on, then a message is
printed and evaluation of the expression is aborted.
set check range warn
Output messages when the gdb range checker detects a range error, but attempt to evaluate the expression anyway. Evaluating the expression may still
be impossible for other reasons, such as accessing memory that the process does
not own (a typical example from many Unix systems).
show range
Show the current setting of the range checker, and whether or not it is being
set automatically by gdb.
15.4 Supported Languages
gdb supports C, C++, D, Go, Objective-C, Fortran, OpenCL C, Pascal, Rust, assembly,
Modula-2, and Ada. Some gdb features may be used in expressions regardless of the language you use: the gdb @ and :: operators, and the ‘{type}addr’ construct (see Section 10.1
[Expressions], page 117) can be used with the constructs of any supported language.
The following sections detail to what degree each source language is supported by gdb.
These sections are not meant to be language tutorials or references, but serve only as a
reference guide to what the gdb expression parser accepts, and what input and output
formats should look like for different languages. There are many good books written on
each of these languages; please look to these for a language reference or tutorial.
15.4.1 C and C++
Since C and C++ are so closely related, many features of gdb apply to both languages.
Whenever this is the case, we discuss those languages together.
The C++ debugging facilities are jointly implemented by the C++ compiler and gdb.
Therefore, to debug your C++ code effectively, you must compile your C++ programs with
a supported C++ compiler, such as gnu g++, or the HP ANSI C++ compiler (aCC).
15.4.1.1 C and C++ Operators
Operators must be defined on values of specific types. For instance, + is defined on numbers,
but not on structures. Operators are often defined on groups of types.
For the purposes of C and C++, the following definitions hold:
• Integral types include int with any of its storage-class specifiers; char; enum; and, for
C++, bool.
• Floating-point types include float, double, and long double (if supported by the
target platform).
• Pointer types include all types defined as (type *).
• Scalar types include all of the above.
The following operators are supported. They are listed here in order of increasing precedence:
,
The comma or sequencing operator. Expressions in a comma-separated list are
evaluated from left to right, with the result of the entire expression being the
last expression evaluated.
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=
Assignment. The value of an assignment expression is the value assigned. Defined on scalar types.
op=
Used in an expression of the form a op= b, and translated to a = a op b. op=
and = have the same precedence. The operator op is any one of the operators
|, ^, &, <<, >>, +, -, *, /, %.
?:
The ternary operator. a ? b : c can be thought of as: if a then b else c. The
argument a should be of an integral type.
||
Logical or. Defined on integral types.
&&
Logical and. Defined on integral types.
|
Bitwise or. Defined on integral types.
^
Bitwise exclusive-or. Defined on integral types.
&
Bitwise and. Defined on integral types.
==, !=
Equality and inequality. Defined on scalar types. The value of these expressions
is 0 for false and non-zero for true.
<, >, <=, >=
Less than, greater than, less than or equal, greater than or equal. Defined on
scalar types. The value of these expressions is 0 for false and non-zero for true.
<<, >>
left shift, and right shift. Defined on integral types.
@
The gdb “artificial array” operator (see Section 10.1 [Expressions], page 117).
+, -
Addition and subtraction. Defined on integral types, floating-point types and
pointer types.
*, /, %
Multiplication, division, and modulus. Multiplication and division are defined
on integral and floating-point types. Modulus is defined on integral types.
++, --
Increment and decrement. When appearing before a variable, the operation is
performed before the variable is used in an expression; when appearing after it,
the variable’s value is used before the operation takes place.
*
Pointer dereferencing. Defined on pointer types. Same precedence as ++.
&
Address operator. Defined on variables. Same precedence as ++.
For debugging C++, gdb implements a use of ‘&’ beyond what is allowed in the
C++ language itself: you can use ‘&(&ref)’ to examine the address where a C++
reference variable (declared with ‘&ref’) is stored.
-
Negative. Defined on integral and floating-point types. Same precedence as ++.
!
Logical negation. Defined on integral types. Same precedence as ++.
~
Bitwise complement operator. Defined on integral types. Same precedence as
++.
., ->
Structure member, and pointer-to-structure member. For convenience, gdb
regards the two as equivalent, choosing whether to dereference a pointer based
on the stored type information. Defined on struct and union data.
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.*, ->*
Dereferences of pointers to members.
[]
Array indexing. a[i] is defined as *(a+i). Same precedence as ->.
()
Function parameter list. Same precedence as ->.
::
C++ scope resolution operator. Defined on struct, union, and class types.
::
Doubled colons also represent the gdb scope operator (see Section 10.1 [Expressions], page 117). Same precedence as ::, above.
If an operator is redefined in the user code, gdb usually attempts to invoke the redefined
version instead of using the operator’s predefined meaning.
15.4.1.2 C and C++ Constants
gdb allows you to express the constants of C and C++ in the following ways:
• Integer constants are a sequence of digits. Octal constants are specified by a leading
‘0’ (i.e. zero), and hexadecimal constants by a leading ‘0x’ or ‘0X’. Constants may also
end with a letter ‘l’, specifying that the constant should be treated as a long value.
• Floating point constants are a sequence of digits, followed by a decimal point, followed
by a sequence of digits, and optionally followed by an exponent. An exponent is of
the form: ‘e[[+]|-]nnn’, where nnn is another sequence of digits. The ‘+’ is optional
for positive exponents. A floating-point constant may also end with a letter ‘f’ or ‘F’,
specifying that the constant should be treated as being of the float (as opposed to the
default double) type; or with a letter ‘l’ or ‘L’, which specifies a long double constant.
• Enumerated constants consist of enumerated identifiers, or their integral equivalents.
• Character constants are a single character surrounded by single quotes (’), or a
number—the ordinal value of the corresponding character (usually its ascii value).
Within quotes, the single character may be represented by a letter or by escape
sequences, which are of the form ‘\nnn’, where nnn is the octal representation of
the character’s ordinal value; or of the form ‘\x’, where ‘x’ is a predefined special
character—for example, ‘\n’ for newline.
Wide character constants can be written by prefixing a character constant with ‘L’,
as in C. For example, ‘L’x’’ is the wide form of ‘x’. The target wide character set is
used when computing the value of this constant (see Section 10.20 [Character Sets],
page 152).
• String constants are a sequence of character constants surrounded by double quotes (").
Any valid character constant (as described above) may appear. Double quotes within
the string must be preceded by a backslash, so for instance ‘"a\"b’c"’ is a string of
five characters.
Wide string constants can be written by prefixing a string constant with ‘L’, as in C.
The target wide character set is used when computing the value of this constant (see
Section 10.20 [Character Sets], page 152).
• Pointer constants are an integral value. You can also write pointers to constants using
the C operator ‘&’.
• Array constants are comma-separated lists surrounded by braces ‘{’ and ‘}’; for example, ‘{1,2,3}’ is a three-element array of integers, ‘{{1,2}, {3,4}, {5,6}}’ is a
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three-by-two array, and ‘{&"hi", &"there", &"fred"}’ is a three-element array of
pointers.
15.4.1.3 C++ Expressions
gdb expression handling can interpret most C++ expressions.
Warning: gdb can only debug C++ code if you use the proper compiler and
the proper debug format. Currently, gdb works best when debugging C++ code
that is compiled with the most recent version of gcc possible. The DWARF
debugging format is preferred; gcc defaults to this on most popular platforms.
Other compilers and/or debug formats are likely to work badly or not at all
when using gdb to debug C++ code. See Section 4.1 [Compilation], page 25.
1. Member function calls are allowed; you can use expressions like
count = aml->GetOriginal(x, y)
2. While a member function is active (in the selected stack frame), your expressions have
the same namespace available as the member function; that is, gdb allows implicit
references to the class instance pointer this following the same rules as C++. using
declarations in the current scope are also respected by gdb.
3. You can call overloaded functions; gdb resolves the function call to the right definition,
with some restrictions. gdb does not perform overload resolution involving user-defined
type conversions, calls to constructors, or instantiations of templates that do not exist
in the program. It also cannot handle ellipsis argument lists or default arguments.
It does perform integral conversions and promotions, floating-point promotions, arithmetic conversions, pointer conversions, conversions of class objects to base classes, and
standard conversions such as those of functions or arrays to pointers; it requires an
exact match on the number of function arguments.
Overload resolution is always performed, unless you have specified set overloadresolution off. See Section 15.4.1.7 [gdb Features for C++], page 199.
You must specify set overload-resolution off in order to use an explicit function
signature to call an overloaded function, as in
p ’foo(char,int)’(’x’, 13)
The gdb command-completion facility can simplify this; see Section 3.2 [Command
Completion], page 19.
4. gdb understands variables declared as C++ lvalue or rvalue references; you can use them
in expressions just as you do in C++ source—they are automatically dereferenced.
In the parameter list shown when gdb displays a frame, the values of reference variables
are not displayed (unlike other variables); this avoids clutter, since references are often
used for large structures. The address of a reference variable is always shown, unless
you have specified ‘set print address off’.
5. gdb supports the C++ name resolution operator ::—your expressions can use it just as
expressions in your program do. Since one scope may be defined in another, you can use
:: repeatedly if necessary, for example in an expression like ‘scope1::scope2::name’.
gdb also allows resolving name scope by reference to source files, in both C and C++
debugging (see Section 10.3 [Program Variables], page 119).
6. gdb performs argument-dependent lookup, following the C++ specification.
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15.4.1.4 C and C++ Defaults
If you allow gdb to set range checking automatically, it defaults to off whenever the working
language changes to C or C++. This happens regardless of whether you or gdb selects the
working language.
If you allow gdb to set the language automatically, it recognizes source files whose names
end with ‘.c’, ‘.C’, or ‘.cc’, etc, and when gdb enters code compiled from one of these files,
it sets the working language to C or C++. See Section 15.1.3 [Having gdb Infer the Source
Language], page 192, for further details.
15.4.1.5 C and C++ Type and Range Checks
By default, when gdb parses C or C++ expressions, strict type checking is used. However,
if you turn type checking off, gdb will allow certain non-standard conversions, such as
promoting integer constants to pointers.
Range checking, if turned on, is done on mathematical operations. Array indices are not
checked, since they are often used to index a pointer that is not itself an array.
15.4.1.6 gdb and C
The set print union and show print union commands apply to the union type. When
set to ‘on’, any union that is inside a struct or class is also printed. Otherwise, it appears
as ‘{...}’.
The @ operator aids in the debugging of dynamic arrays, formed with pointers and a
memory allocation function. See Section 10.1 [Expressions], page 117.
15.4.1.7 gdb Features for C++
Some gdb commands are particularly useful with C++, and some are designed specifically
for use with C++. Here is a summary:
breakpoint menus
When you want a breakpoint in a function whose name is overloaded, gdb
has the capability to display a menu of possible breakpoint locations to help
you specify which function definition you want. See Section 10.2 [Ambiguous
Expressions], page 118.
rbreak regex
Setting breakpoints using regular expressions is helpful for setting breakpoints
on overloaded functions that are not members of any special classes. See
Section 5.1.1 [Setting Breakpoints], page 46.
catch throw
catch rethrow
catch catch
Debug C++ exception handling using these commands. See Section 5.1.3 [Setting Catchpoints], page 54.
ptype typename
Print inheritance relationships as well as other information for type typename.
See Chapter 16 [Examining the Symbol Table], page 221.
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info vtbl expression.
The info vtbl command can be used to display the virtual method tables of
the object computed by expression. This shows one entry per virtual table;
there may be multiple virtual tables when multiple inheritance is in use.
demangle name
Demangle name. See Chapter 16 [Symbols], page 221, for a more complete
description of the demangle command.
set print demangle
show print demangle
set print asm-demangle
show print asm-demangle
Control whether C++ symbols display in their source form, both when displaying
code as C++ source and when displaying disassemblies. See Section 10.8 [Print
Settings], page 127.
set print object
show print object
Choose whether to print derived (actual) or declared types of objects. See
Section 10.8 [Print Settings], page 127.
set print vtbl
show print vtbl
Control the format for printing virtual function tables. See Section 10.8 [Print
Settings], page 127. (The vtbl commands do not work on programs compiled
with the HP ANSI C++ compiler (aCC).)
set overload-resolution on
Enable overload resolution for C++ expression evaluation. The default is on. For
overloaded functions, gdb evaluates the arguments and searches for a function
whose signature matches the argument types, using the standard C++ conversion rules (see Section 15.4.1.3 [C++ Expressions], page 198, for details). If it
cannot find a match, it emits a message.
set overload-resolution off
Disable overload resolution for C++ expression evaluation. For overloaded functions that are not class member functions, gdb chooses the first function of
the specified name that it finds in the symbol table, whether or not its arguments are of the correct type. For overloaded functions that are class member
functions, gdb searches for a function whose signature exactly matches the
argument types.
show overload-resolution
Show the current setting of overload resolution.
Overloaded symbol names
You can specify a particular definition of an overloaded symbol, using the same
notation that is used to declare such symbols in C++: type symbol(types)
rather than just symbol. You can also use the gdb command-line word completion facilities to list the available choices, or to finish the type list for you.
See Section 3.2 [Command Completion], page 19, for details on how to do this.
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15.4.1.8 Decimal Floating Point format
gdb can examine, set and perform computations with numbers in decimal floating point format, which in the C language correspond to the _Decimal32, _Decimal64 and _Decimal128
types as specified by the extension to support decimal floating-point arithmetic.
There are two encodings in use, depending on the architecture: BID (Binary Integer
Decimal) for x86 and x86-64, and DPD (Densely Packed Decimal) for PowerPC and S/390.
gdb will use the appropriate encoding for the configured target.
Because of a limitation in ‘libdecnumber’, the library used by gdb to manipulate decimal
floating point numbers, it is not possible to convert (using a cast, for example) integers wider
than 32-bit to decimal float.
In addition, in order to imitate gdb’s behaviour with binary floating point computations,
error checking in decimal float operations ignores underflow, overflow and divide by zero
exceptions.
In the PowerPC architecture, gdb provides a set of pseudo-registers to inspect
_Decimal128 values stored in floating point registers. See Section 21.4.7 [PowerPC],
page 301 for more details.
15.4.2 D
gdb can be used to debug programs written in D and compiled with GDC, LDC or DMD
compilers. Currently gdb supports only one D specific feature — dynamic arrays.
15.4.3 Go
gdb can be used to debug programs written in Go and compiled with ‘gccgo’ or ‘6g’
compilers.
Here is a summary of the Go-specific features and restrictions:
The current Go package
The name of the current package does not need to be specified when specifying
global variables and functions.
For example, given the program:
package main
var myglob = "Shall we?"
func main () {
// ...
}
When stopped inside main either of these work:
(gdb) p myglob
(gdb) p main.myglob
Builtin Go types
The string type is recognized by gdb and is printed as a string.
Builtin Go functions
The gdb expression parser recognizes the unsafe.Sizeof function and handles
it internally.
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Restrictions on Go expressions
All Go operators are supported except &^. The Go _ “blank identifier” is not
supported. Automatic dereferencing of pointers is not supported.
15.4.4 Objective-C
This section provides information about some commands and command options that are
useful for debugging Objective-C code. See also Chapter 16 [Symbols], page 221, and
Chapter 16 [Symbols], page 221, for a few more commands specific to Objective-C support.
15.4.4.1 Method Names in Commands
The following commands have been extended to accept Objective-C method names as line
specifications:
• clear
• break
• info line
• jump
• list
A fully qualified Objective-C method name is specified as
-[Class methodName]
where the minus sign is used to indicate an instance method and a plus sign (not shown)
is used to indicate a class method. The class name Class and method name methodName
are enclosed in brackets, similar to the way messages are specified in Objective-C source
code. For example, to set a breakpoint at the create instance method of class Fruit in
the program currently being debugged, enter:
break -[Fruit create]
To list ten program lines around the initialize class method, enter:
list +[NSText initialize]
In the current version of gdb, the plus or minus sign is required. In future versions of
gdb, the plus or minus sign will be optional, but you can use it to narrow the search. It is
also possible to specify just a method name:
break create
You must specify the complete method name, including any colons. If your program’s
source files contain more than one create method, you’ll be presented with a numbered
list of classes that implement that method. Indicate your choice by number, or type ‘0’ to
exit if none apply.
As another example, to clear a breakpoint established at the makeKeyAndOrderFront:
method of the NSWindow class, enter:
clear -[NSWindow makeKeyAndOrderFront:]
15.4.4.2 The Print Command With Objective-C
The print command has also been extended to accept methods. For example:
print -[object hash]
will tell gdb to send the hash message to object and print the result. Also, an additional
command has been added, print-object or po for short, which is meant to print the
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description of an object. However, this command may only work with certain Objective-C
libraries that have a particular hook function, _NSPrintForDebugger, defined.
15.4.5 OpenCL C
This section provides information about gdbs OpenCL C support.
15.4.5.1 OpenCL C Datatypes
gdb supports the builtin scalar and vector datatypes specified by OpenCL 1.1. In addition
the half- and double-precision floating point data types of the cl_khr_fp16 and cl_khr_
fp64 OpenCL extensions are also known to gdb.
15.4.5.2 OpenCL C Expressions
gdb supports accesses to vector components including the access as lvalue where possible.
Since OpenCL C is based on C99 most C expressions supported by gdb can be used as
well.
15.4.5.3 OpenCL C Operators
gdb supports the operators specified by OpenCL 1.1 for scalar and vector data types.
15.4.6 Fortran
gdb can be used to debug programs written in Fortran, but it currently supports only the
features of Fortran 77 language.
Some Fortran compilers (gnu Fortran 77 and Fortran 95 compilers among them) append
an underscore to the names of variables and functions. When you debug programs compiled by those compilers, you will need to refer to variables and functions with a trailing
underscore.
15.4.6.1 Fortran Operators and Expressions
Operators must be defined on values of specific types. For instance, + is defined on numbers,
but not on characters or other non- arithmetic types. Operators are often defined on groups
of types.
**
The exponentiation operator. It raises the first operand to the power of the
second one.
:
The range operator. Normally used in the form of array(low:high) to represent
a section of array.
%
The access component operator. Normally used to access elements in derived
types. Also suitable for unions. As unions aren’t part of regular Fortran, this
can only happen when accessing a register that uses a gdbarch-defined union
type.
15.4.6.2 Fortran Defaults
Fortran symbols are usually case-insensitive, so gdb by default uses case-insensitive matches
for Fortran symbols. You can change that with the ‘set case-insensitive’ command, see
Chapter 16 [Symbols], page 221, for the details.
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15.4.6.3 Special Fortran Commands
gdb has some commands to support Fortran-specific features, such as displaying common
blocks.
info common [common-name]
This command prints the values contained in the Fortran COMMON block whose
name is common-name. With no argument, the names of all COMMON blocks
visible at the current program location are printed.
15.4.7 Pascal
Debugging Pascal programs which use sets, subranges, file variables, or nested functions
does not currently work. gdb does not support entering expressions, printing values, or
similar features using Pascal syntax.
The Pascal-specific command set print pascal_static-members controls whether
static members of Pascal objects are displayed. See Section 10.8 [Print Settings], page 127.
15.4.8 Rust
gdb supports the Rust Programming Language. Type- and value-printing, and expression
parsing, are reasonably complete. However, there are a few peculiarities and holes to be
aware of.
• Linespecs (see Section 9.2 [Specify Location], page 104) are never relative to the current
crate. Instead, they act as if there were a global namespace of crates, somewhat similar
to the way extern crate behaves.
That is, if gdb is stopped at a breakpoint in a function in crate ‘A’, module ‘B’, then
break B::f will attempt to set a breakpoint in a function named ‘f’ in a crate named
‘B’.
As a consequence of this approach, linespecs also cannot refer to items using ‘self::’
or ‘super::’.
• Because gdb implements Rust name-lookup semantics in expressions, it will sometimes
prepend the current crate to a name. For example, if gdb is stopped at a breakpoint
in the crate ‘K’, then print ::x::y will try to find the symbol ‘K::x::y’.
However, since it is useful to be able to refer to other crates when debugging, gdb
provides the extern extension to circumvent this. To use the extension, just put
extern before a path expression to refer to the otherwise unavailable “global” scope.
In the above example, if you wanted to refer to the symbol ‘y’ in the crate ‘x’, you
would use print extern x::y.
• The Rust expression evaluator does not support “statement-like” expressions such as
if or match, or lambda expressions.
• Tuple expressions are not implemented.
• The Rust expression evaluator does not currently implement the Drop trait. Objects
that may be created by the evaluator will never be destroyed.
• gdb does not implement type inference for generics. In order to call generic functions or
otherwise refer to generic items, you will have to specify the type parameters manually.
• gdb currently uses the C++ demangler for Rust. In most cases this does not cause any
problems. However, in an expression context, completing a generic function name will
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give syntactically invalid results. This happens because Rust requires the ‘::’ operator
between the function name and its generic arguments. For example, gdb might provide
a completion like crate::f, where the parser would require crate::f::.
• As of this writing, the Rust compiler (version 1.8) has a few holes in the debugging
information it generates. These holes prevent certain features from being implemented
by gdb:
• Method calls cannot be made via traits.
• Trait objects cannot be created or inspected.
• Operator overloading is not implemented.
• When debugging in a monomorphized function, you cannot use the generic type
names.
• The type Self is not available.
• use statements are not available, so some names may not be available in the crate.
15.4.9 Modula-2
The extensions made to gdb to support Modula-2 only support output from the gnu
Modula-2 compiler (which is currently being developed). Other Modula-2 compilers are not
currently supported, and attempting to debug executables produced by them is most likely
to give an error as gdb reads in the executable’s symbol table.
15.4.9.1 Operators
Operators must be defined on values of specific types. For instance, + is defined on numbers,
but not on structures. Operators are often defined on groups of types. For the purposes of
Modula-2, the following definitions hold:
• Integral types consist of INTEGER, CARDINAL, and their subranges.
• Character types consist of CHAR and its subranges.
• Floating-point types consist of REAL.
• Pointer types consist of anything declared as POINTER TO type.
• Scalar types consist of all of the above.
• Set types consist of SET and BITSET types.
• Boolean types consist of BOOLEAN.
The following operators are supported, and appear in order of increasing precedence:
,
Function argument or array index separator.
:=
Assignment. The value of var := value is value.
<, >
Less than, greater than on integral, floating-point, or enumerated types.
<=, >=
Less than or equal to, greater than or equal to on integral, floating-point and
enumerated types, or set inclusion on set types. Same precedence as <.
=, <>, #
Equality and two ways of expressing inequality, valid on scalar types. Same
precedence as <. In gdb scripts, only <> is available for inequality, since #
conflicts with the script comment character.
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IN
Set membership. Defined on set types and the types of their members. Same
precedence as <.
OR
Boolean disjunction. Defined on boolean types.
AND, &
Boolean conjunction. Defined on boolean types.
@
The gdb “artificial array” operator (see Section 10.1 [Expressions], page 117).
+, -
Addition and subtraction on integral and floating-point types, or union and
difference on set types.
*
Multiplication on integral and floating-point types, or set intersection on set
types.
/
Division on floating-point types, or symmetric set difference on set types. Same
precedence as *.
DIV, MOD
Integer division and remainder. Defined on integral types. Same precedence as
*.
-
Negative. Defined on INTEGER and REAL data.
^
Pointer dereferencing. Defined on pointer types.
NOT
Boolean negation. Defined on boolean types. Same precedence as ^.
.
RECORD field selector. Defined on RECORD data. Same precedence as ^.
[]
Array indexing. Defined on ARRAY data. Same precedence as ^.
()
Procedure argument list. Defined on PROCEDURE objects. Same precedence as
^.
::, .
gdb and Modula-2 scope operators.
Warning: Set expressions and their operations are not yet supported, so gdb
treats the use of the operator IN, or the use of operators +, -, *, /, =, , <>, #,
<=, and >= on sets as an error.
15.4.9.2 Built-in Functions and Procedures
Modula-2 also makes available several built-in procedures and functions. In describing these,
the following metavariables are used:
a
represents an ARRAY variable.
c
represents a CHAR constant or variable.
i
represents a variable or constant of integral type.
m
represents an identifier that belongs to a set. Generally used in the same function with the metavariable s. The type of s should be SET OF mtype (where
mtype is the type of m).
n
represents a variable or constant of integral or floating-point type.
r
represents a variable or constant of floating-point type.
t
represents a type.
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v
represents a variable.
x
represents a variable or constant of one of many types. See the explanation of
the function for details.
All Modula-2 built-in procedures also return a result, described below.
ABS(n)
Returns the absolute value of n.
CAP(c)
If c is a lower case letter, it returns its upper case equivalent, otherwise it
returns its argument.
CHR(i)
Returns the character whose ordinal value is i.
DEC(v)
Decrements the value in the variable v by one. Returns the new value.
DEC(v,i)
Decrements the value in the variable v by i. Returns the new value.
EXCL(m,s)
Removes the element m from the set s. Returns the new set.
FLOAT(i)
Returns the floating point equivalent of the integer i.
HIGH(a)
Returns the index of the last member of a.
INC(v)
Increments the value in the variable v by one. Returns the new value.
INC(v,i)
Increments the value in the variable v by i. Returns the new value.
INCL(m,s)
Adds the element m to the set s if it is not already there. Returns the new set.
MAX(t)
Returns the maximum value of the type t.
MIN(t)
Returns the minimum value of the type t.
ODD(i)
Returns boolean TRUE if i is an odd number.
ORD(x)
Returns the ordinal value of its argument. For example, the ordinal value of a
character is its ascii value (on machines supporting the ascii character set).
The argument x must be of an ordered type, which include integral, character
and enumerated types.
SIZE(x)
Returns the size of its argument. The argument x can be a variable or a type.
TRUNC(r)
Returns the integral part of r.
TSIZE(x)
Returns the size of its argument. The argument x can be a variable or a type.
VAL(t,i)
Returns the member of the type t whose ordinal value is i.
Warning: Sets and their operations are not yet supported, so gdb treats the
use of procedures INCL and EXCL as an error.
15.4.9.3 Constants
gdb allows you to express the constants of Modula-2 in the following ways:
• Integer constants are simply a sequence of digits. When used in an expression, a constant is interpreted to be type-compatible with the rest of the expression. Hexadecimal
integers are specified by a trailing ‘H’, and octal integers by a trailing ‘B’.
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• Floating point constants appear as a sequence of digits, followed by a decimal point
and another sequence of digits. An optional exponent can then be specified, in the form
‘E[+|-]nnn’, where ‘[+|-]nnn’ is the desired exponent. All of the digits of the floating
point constant must be valid decimal (base 10) digits.
• Character constants consist of a single character enclosed by a pair of like quotes, either
single (’) or double ("). They may also be expressed by their ordinal value (their ascii
value, usually) followed by a ‘C’.
• String constants consist of a sequence of characters enclosed by a pair of like quotes,
either single (’) or double ("). Escape sequences in the style of C are also allowed.
See Section 15.4.1.2 [C and C++ Constants], page 197, for a brief explanation of escape
sequences.
• Enumerated constants consist of an enumerated identifier.
• Boolean constants consist of the identifiers TRUE and FALSE.
• Pointer constants consist of integral values only.
• Set constants are not yet supported.
15.4.9.4 Modula-2 Types
Currently gdb can print the following data types in Modula-2 syntax: array types, record
types, set types, pointer types, procedure types, enumerated types, subrange types and base
types. You can also print the contents of variables declared using these type. This section
gives a number of simple source code examples together with sample gdb sessions.
The first example contains the following section of code:
VAR
s: SET OF CHAR ;
r: [20..40] ;
and you can request gdb to interrogate the type and value of r and s.
(gdb) print s
{’A’..’C’, ’Z’}
(gdb) ptype s
SET OF CHAR
(gdb) print r
21
(gdb) ptype r
[20..40]
Likewise if your source code declares s as:
VAR
s: SET [’A’..’Z’] ;
then you may query the type of s by:
(gdb) ptype s
type = SET [’A’..’Z’]
Note that at present you cannot interactively manipulate set expressions using the debugger.
The following example shows how you might declare an array in Modula-2 and how you
can interact with gdb to print its type and contents:
VAR
s: ARRAY [-10..10] OF CHAR ;
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(gdb) ptype s
ARRAY [-10..10] OF CHAR
Note that the array handling is not yet complete and although the type is printed
correctly, expression handling still assumes that all arrays have a lower bound of zero and
not -10 as in the example above.
Here are some more type related Modula-2 examples:
TYPE
colour = (blue, red, yellow, green) ;
t = [blue..yellow] ;
VAR
s: t ;
BEGIN
s := blue ;
The gdb interaction shows how you can query the data type and value of a variable.
(gdb) print s
$1 = blue
(gdb) ptype t
type = [blue..yellow]
In this example a Modula-2 array is declared and its contents displayed. Observe that the
contents are written in the same way as their C counterparts.
VAR
s: ARRAY [1..5] OF CARDINAL ;
BEGIN
s[1] := 1 ;
(gdb) print s
$1 = {1, 0, 0, 0, 0}
(gdb) ptype s
type = ARRAY [1..5] OF CARDINAL
The Modula-2 language interface to gdb also understands pointer types as shown in this
example:
VAR
s: POINTER TO ARRAY [1..5] OF CARDINAL ;
BEGIN
NEW(s) ;
s^[1] := 1 ;
and you can request that gdb describes the type of s.
(gdb) ptype s
type = POINTER TO ARRAY [1..5] OF CARDINAL
gdb handles compound types as we can see in this example. Here we combine array
types, record types, pointer types and subrange types:
TYPE
foo = RECORD
f1: CARDINAL ;
f2: CHAR ;
f3: myarray ;
END ;
myarray = ARRAY myrange OF CARDINAL ;
myrange = [-2..2] ;
VAR
s: POINTER TO ARRAY myrange OF foo ;
and you can ask gdb to describe the type of s as shown below.
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Debugging with gdb
(gdb) ptype s
type = POINTER TO ARRAY [-2..2] OF foo = RECORD
f1 : CARDINAL;
f2 : CHAR;
f3 : ARRAY [-2..2] OF CARDINAL;
END
15.4.9.5 Modula-2 Defaults
If type and range checking are set automatically by gdb, they both default to on whenever
the working language changes to Modula-2. This happens regardless of whether you or gdb
selected the working language.
If you allow gdb to set the language automatically, then entering code compiled from a
file whose name ends with ‘.mod’ sets the working language to Modula-2. See Section 15.1.3
[Having gdb Infer the Source Language], page 192, for further details.
15.4.9.6 Deviations from Standard Modula-2
A few changes have been made to make Modula-2 programs easier to debug. This is done
primarily via loosening its type strictness:
• Unlike in standard Modula-2, pointer constants can be formed by integers. This allows
you to modify pointer variables during debugging. (In standard Modula-2, the actual
address contained in a pointer variable is hidden from you; it can only be modified
through direct assignment to another pointer variable or expression that returned a
pointer.)
• C escape sequences can be used in strings and characters to represent non-printable
characters. gdb prints out strings with these escape sequences embedded. Single nonprintable characters are printed using the ‘CHR(nnn)’ format.
• The assignment operator (:=) returns the value of its right-hand argument.
• All built-in procedures both modify and return their argument.
15.4.9.7 Modula-2 Type and Range Checks
Warning: in this release, gdb does not yet perform type or range checking.
gdb considers two Modula-2 variables type equivalent if:
• They are of types that have been declared equivalent via a TYPE t1 = t2 statement
• They have been declared on the same line. (Note: This is true of the gnu Modula-2
compiler, but it may not be true of other compilers.)
As long as type checking is enabled, any attempt to combine variables whose types are
not equivalent is an error.
Range checking is done on all mathematical operations, assignment, array index bounds,
and all built-in functions and procedures.
15.4.9.8 The Scope Operators :: and .
There are a few subtle differences between the Modula-2 scope operator (.) and the gdb
scope operator (::). The two have similar syntax:
module . id
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scope :: id
where scope is the name of a module or a procedure, module the name of a module, and id
is any declared identifier within your program, except another module.
Using the :: operator makes gdb search the scope specified by scope for the identifier
id. If it is not found in the specified scope, then gdb searches all scopes enclosing the one
specified by scope.
Using the . operator makes gdb search the current scope for the identifier specified by
id that was imported from the definition module specified by module. With this operator,
it is an error if the identifier id was not imported from definition module module, or if id is
not an identifier in module.
15.4.9.9 gdb and Modula-2
Some gdb commands have little use when debugging Modula-2 programs. Five subcommands of set print and show print apply specifically to C and C++: ‘vtbl’, ‘demangle’,
‘asm-demangle’, ‘object’, and ‘union’. The first four apply to C++, and the last to the C
union type, which has no direct analogue in Modula-2.
The @ operator (see Section 10.1 [Expressions], page 117), while available with any
language, is not useful with Modula-2. Its intent is to aid the debugging of dynamic arrays,
which cannot be created in Modula-2 as they can in C or C++. However, because an address
can be specified by an integral constant, the construct ‘{type}adrexp’ is still useful.
In gdb scripts, the Modula-2 inequality operator # is interpreted as the beginning of a
comment. Use <> instead.
15.4.10 Ada
The extensions made to gdb for Ada only support output from the gnu Ada (GNAT)
compiler. Other Ada compilers are not currently supported, and attempting to debug
executables produced by them is most likely to be difficult.
15.4.10.1 Introduction
The Ada mode of gdb supports a fairly large subset of Ada expression syntax, with some
extensions. The philosophy behind the design of this subset is
• That gdb should provide basic literals and access to operations for arithmetic, dereferencing, field selection, indexing, and subprogram calls, leaving more sophisticated
computations to subprograms written into the program (which therefore may be called
from gdb).
• That type safety and strict adherence to Ada language restrictions are not particularly
important to the gdb user.
• That brevity is important to the gdb user.
Thus, for brevity, the debugger acts as if all names declared in user-written packages
are directly visible, even if they are not visible according to Ada rules, thus making it
unnecessary to fully qualify most names with their packages, regardless of context. Where
this causes ambiguity, gdb asks the user’s intent.
The debugger will start in Ada mode if it detects an Ada main program. As for other
languages, it will enter Ada mode when stopped in a program that was translated from an
Ada source file.
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While in Ada mode, you may use ‘--’ for comments. This is useful mostly for documenting command files. The standard gdb comment (‘#’) still works at the beginning of a
line in Ada mode, but not in the middle (to allow based literals).
15.4.10.2 Omissions from Ada
Here are the notable omissions from the subset:
• Only a subset of the attributes are supported:
− ’First, ’Last, and ’Length on array objects (not on types and subtypes).
− ’Min and ’Max.
− ’Pos and ’Val.
− ’Tag.
− ’Range on array objects (not subtypes), but only as the right operand of the
membership (in) operator.
− ’Access, ’Unchecked_Access, and ’Unrestricted_Access (a GNAT extension).
− ’Address.
• The names in Characters.Latin_1 are not available and concatenation is not implemented. Thus, escape characters in strings are not currently available.
• Equality tests (‘=’ and ‘/=’) on arrays test for bitwise equality of representations. They
will generally work correctly for strings and arrays whose elements have integer or
enumeration types. They may not work correctly for arrays whose element types have
user-defined equality, for arrays of real values (in particular, IEEE-conformant floating
point, because of negative zeroes and NaNs), and for arrays whose elements contain
unused bits with indeterminate values.
• The other component-by-component array operations (and, or, xor, not, and relational
tests other than equality) are not implemented.
• There is limited support for array and record aggregates. They are permitted only on
the right sides of assignments, as in these examples:
(gdb)
(gdb)
(gdb)
(gdb)
(gdb)
(gdb)
set
set
set
set
set
set
An_Array := (1, 2, 3, 4, 5, 6)
An_Array := (1, others => 0)
An_Array := (0|4 => 1, 1..3 => 2, 5 => 6)
A_2D_Array := ((1, 2, 3), (4, 5, 6), (7, 8, 9))
A_Record := (1, "Peter", True);
A_Record := (Name => "Peter", Id => 1, Alive => True)
Changing a discriminant’s value by assigning an aggregate has an undefined effect if
that discriminant is used within the record. However, you can first modify discriminants
by directly assigning to them (which normally would not be allowed in Ada), and then
performing an aggregate assignment. For example, given a variable A_Rec declared to
have a type such as:
type Rec (Len : Small_Integer := 0) is record
Id : Integer;
Vals : IntArray (1 .. Len);
end record;
you can assign a value with a different size of Vals with two assignments:
(gdb) set A_Rec.Len := 4
(gdb) set A_Rec := (Id => 42, Vals => (1, 2, 3, 4))
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•
•
•
•
•
•
213
As this example also illustrates, gdb is very loose about the usual rules concerning
aggregates. You may leave out some of the components of an array or record aggregate (such as the Len component in the assignment to A_Rec above); they will retain
their original values upon assignment. You may freely use dynamic values as indices in
component associations. You may even use overlapping or redundant component associations, although which component values are assigned in such cases is not defined.
Calls to dispatching subprograms are not implemented.
The overloading algorithm is much more limited (i.e., less selective) than that of real
Ada. It makes only limited use of the context in which a subexpression appears to
resolve its meaning, and it is much looser in its rules for allowing type matches. As a
result, some function calls will be ambiguous, and the user will be asked to choose the
proper resolution.
The new operator is not implemented.
Entry calls are not implemented.
Aside from printing, arithmetic operations on the native VAX floating-point formats
are not supported.
It is not possible to slice a packed array.
The names True and False, when not part of a qualified name, are interpreted as if
implicitly prefixed by Standard, regardless of context. Should your program redefine
these names in a package or procedure (at best a dubious practice), you will have to
use fully qualified names to access their new definitions.
15.4.10.3 Additions to Ada
As it does for other languages, gdb makes certain generic extensions to Ada (see Section 10.1
[Expressions], page 117):
• If the expression E is a variable residing in memory (typically a local variable or array
element) and N is a positive integer, then E@N displays the values of E and the N-1
adjacent variables following it in memory as an array. In Ada, this operator is generally
not necessary, since its prime use is in displaying parts of an array, and slicing will
usually do this in Ada. However, there are occasional uses when debugging programs
in which certain debugging information has been optimized away.
• B::var means “the variable named var that appears in function or file B.” When B is
a file name, you must typically surround it in single quotes.
• The expression {type} addr means “the variable of type type that appears at address
addr.”
• A name starting with ‘$’ is a convenience variable (see Section 10.11 [Convenience
Vars], page 139) or a machine register (see Section 10.13 [Registers], page 144).
In addition, gdb provides a few other shortcuts and outright additions specific to Ada:
• The assignment statement is allowed as an expression, returning its right-hand operand
as its value. Thus, you may enter
(gdb) set x := y + 3
(gdb) print A(tmp := y + 1)
• The semicolon is allowed as an “operator,” returning as its value the value of its righthand operand. This allows, for example, complex conditional breaks:
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(gdb) break f
(gdb) condition 1 (report(i); k += 1; A(k) > 100)
• Rather than use catenation and symbolic character names to introduce special characters into strings, one may instead use a special bracket notation, which is also used to
print strings. A sequence of characters of the form ‘["XX"]’ within a string or character
literal denotes the (single) character whose numeric encoding is XX in hexadecimal.
The sequence of characters ‘["""]’ also denotes a single quotation mark in strings. For
example,
"One line.["0a"]Next line.["0a"]"
contains an ASCII newline character (Ada.Characters.Latin_1.LF) after each period.
• The subtype used as a prefix for the attributes ’Pos, ’Min, and ’Max is optional (and
is ignored in any case). For example, it is valid to write
(gdb) print ’max(x, y)
• When printing arrays, gdb uses positional notation when the array has a lower bound
of 1, and uses a modified named notation otherwise. For example, a one-dimensional
array of three integers with a lower bound of 3 might print as
(3 => 10, 17, 1)
That is, in contrast to valid Ada, only the first component has a => clause.
• You may abbreviate attributes in expressions with any unique, multi-character subsequence of their names (an exact match gets preference). For example, you may use
a’len, a’gth, or a’lh in place of a’length.
• Since Ada is case-insensitive, the debugger normally maps identifiers you type to lower
case. The GNAT compiler uses upper-case characters for some of its internal identifiers,
which are normally of no interest to users. For the rare occasions when you actually
have to look at them, enclose them in angle brackets to avoid the lower-case mapping.
For example,
(gdb) print [0]
• Printing an object of class-wide type or dereferencing an access-to-class-wide value will
display all the components of the object’s specific type (as indicated by its run-time
tag). Likewise, component selection on such a value will operate on the specific type
of the object.
15.4.10.4 Overloading support for Ada
The debugger supports limited overloading. Given a subprogram call in which the function
symbol has multiple definitions, it will use the number of actual parameters and some
information about their types to attempt to narrow the set of definitions. It also makes
very limited use of context, preferring procedures to functions in the context of the call
command, and functions to procedures elsewhere.
If, after narrowing, the set of matching definitions still contains more than one definition,
gdb will display a menu to query which one it should use, for instance:
(gdb) print f(1)
Multiple matches for f
[0] cancel
[1] foo.f (integer) return boolean at foo.adb:23
[2] foo.f (foo.new_integer) return boolean at foo.adb:28
>
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In this case, just select one menu entry either to cancel expression evaluation (type 0
and press RET) or to continue evaluation with a specific instance (type the corresponding
number and press RET).
Here are a couple of commands to customize gdb’s behavior in this case:
set ada print-signatures
Control whether parameter types and return types are displayed in overloads
selection menus. It is on by default. See Section 15.4.10.4 [Overloading support
for Ada], page 214.
show ada print-signatures
Show the current setting for displaying parameter types and return types in
overloads selection menu. See Section 15.4.10.4 [Overloading support for Ada],
page 214.
15.4.10.5 Stopping at the Very Beginning
It is sometimes necessary to debug the program during elaboration, and before reaching the
main procedure. As defined in the Ada Reference Manual, the elaboration code is invoked
from a procedure called adainit. To run your program up to the beginning of elaboration,
simply use the following two commands: tbreak adainit and run.
15.4.10.6 Ada Exceptions
A command is provided to list all Ada exceptions:
info exceptions
info exceptions regexp
The info exceptions command allows you to list all Ada exceptions defined
within the program being debugged, as well as their addresses. With a regular
expression, regexp, as argument, only those exceptions whose names match
regexp are listed.
Below is a small example, showing how the command can be used, first without argument, and next with a regular expression passed as an argument.
(gdb) info exceptions
All defined Ada exceptions:
constraint_error: 0x613da0
program_error: 0x613d20
storage_error: 0x613ce0
tasking_error: 0x613ca0
const.aint_global_e: 0x613b00
(gdb) info exceptions const.aint
All Ada exceptions matching regular expression "const.aint":
constraint_error: 0x613da0
const.aint_global_e: 0x613b00
It is also possible to ask gdb to stop your program’s execution when an exception is
raised. For more details, see Section 5.1.3 [Set Catchpoints], page 54.
15.4.10.7 Extensions for Ada Tasks
Support for Ada tasks is analogous to that for threads (see Section 4.10 [Threads], page 36).
gdb provides the following task-related commands:
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info tasks
This command shows a list of current Ada tasks, as in the following example:
(gdb) info tasks
ID
TID P-ID Pri State
Name
1
8088000
0
15 Child Activation Wait main_task
2
80a4000
1
15 Accept Statement
b
3
809a800
1
15 Child Activation Wait a
* 4
80ae800
3
15 Runnable
c
In this listing, the asterisk before the last task indicates it to be the task currently being inspected.
ID
Represents gdb’s internal task number.
TID
The Ada task ID.
P-ID
The parent’s task ID (gdb’s internal task number).
Pri
The base priority of the task.
State
Current state of the task.
Unactivated
The task has been created but has not been activated.
It cannot be executing.
Runnable
The task is not blocked for any reason known to Ada.
(It may be waiting for a mutex, though.) It is conceptually "executing" in normal mode.
Terminated
The task is terminated, in the sense of ARM 9.3 (5).
Any dependents that were waiting on terminate alternatives have been awakened and have terminated themselves.
Child Activation Wait
The task is waiting for created tasks to complete activation.
Accept Statement
The task is waiting on an accept or selective wait statement.
Waiting on entry call
The task is waiting on an entry call.
Async Select Wait
The task is waiting to start the abortable part of an
asynchronous select statement.
Delay Sleep
The task is waiting on a select statement with only a
delay alternative open.
Chapter 15: Using gdb with Different Languages
217
Child Termination Wait
The task is sleeping having completed a master within
itself, and is waiting for the tasks dependent on that
master to become terminated or waiting on a terminate
Phase.
Wait Child in Term Alt
The task is sleeping waiting for tasks on terminate alternatives to finish terminating.
Accepting RV with taskno
The task is accepting a rendez-vous with the task taskno.
Name
Name of the task in the program.
info task taskno
This command shows detailled informations on the specified task, as in the
following example:
(gdb) info tasks
ID
TID P-ID Pri State
1
8077880
0 15 Child Activation Wait
* 2
807c468
1 15 Runnable
(gdb) info task 2
Ada Task: 0x807c468
Name: task_1
Thread: 0x807f378
Parent: 1 (main_task)
Base Priority: 15
State: Runnable
task
Name
main_task
task_1
This command prints the ID of the current task.
(gdb) info tasks
ID
TID P-ID Pri State
1
8077870
0 15 Child Activation Wait
* 2
807c458
1 15 Runnable
(gdb) task
[Current task is 2]
Name
main_task
t
task taskno
This command is like the thread thread-id command (see Section 4.10
[Threads], page 36). It switches the context of debugging from the current
task to the given task.
(gdb) info tasks
ID
TID P-ID Pri State
1
8077870
0 15 Child Activation Wait
* 2
807c458
1 15 Runnable
(gdb) task 1
[Switching to task 1]
#0 0x8067726 in pthread_cond_wait ()
Name
main_task
t
218
(gdb) bt
#0 0x8067726
#1 0x8056714
#2 0x805cb63
#3 0x806153e
#4 0x804aacc
Debugging with gdb
in
in
in
in
in
pthread_cond_wait ()
system.os_interface.pthread_cond_wait ()
system.task_primitives.operations.sleep ()
system.tasking.stages.activate_tasks ()
un () at un.adb:5
break location task taskno
break location task taskno if ...
These commands are like the break ... thread ... command (see Section 5.5
[Thread Stops], page 77). The location argument specifies source lines, as
described in Section 9.2 [Specify Location], page 104.
Use the qualifier ‘task taskno’ with a breakpoint command to specify that you
only want gdb to stop the program when a particular Ada task reaches this
breakpoint. The taskno is one of the numeric task identifiers assigned by gdb,
shown in the first column of the ‘info tasks’ display.
If you do not specify ‘task taskno’ when you set a breakpoint, the breakpoint
applies to all tasks of your program.
You can use the task qualifier on conditional breakpoints as well; in this case,
place ‘task taskno’ before the breakpoint condition (before the if).
For example,
(gdb) info tasks
ID
TID P-ID Pri State
Name
1 140022020
0
15 Child Activation Wait main_task
2 140045060
1
15 Accept/Select Wait
t2
3 140044840
1
15 Runnable
t1
* 4 140056040
1
15 Runnable
t3
(gdb) b 15 task 2
Breakpoint 5 at 0x120044cb0: file test_task_debug.adb, line 15.
(gdb) cont
Continuing.
task # 1 running
task # 2 running
Breakpoint 5, test_task_debug () at test_task_debug.adb:15
15
flush;
(gdb) info tasks
ID
TID P-ID Pri State
Name
1 140022020
0
15 Child Activation Wait main_task
* 2 140045060
1
15 Runnable
t2
3 140044840
1
15 Runnable
t1
4 140056040
1
15 Delay Sleep
t3
15.4.10.8 Tasking Support when Debugging Core Files
When inspecting a core file, as opposed to debugging a live program, tasking support may
be limited or even unavailable, depending on the platform being used. For instance, on
x86-linux, the list of tasks is available, but task switching is not supported.
On certain platforms, the debugger needs to perform some memory writes in order to
provide Ada tasking support. When inspecting a core file, this means that the core file must
be opened with read-write privileges, using the command ‘"set write on"’ (see Section 17.6
Chapter 15: Using gdb with Different Languages
219
[Patching], page 234). Under these circumstances, you should make a backup copy of the
core file before inspecting it with gdb.
15.4.10.9 Tasking Support when using the Ravenscar Profile
The Ravenscar Profile is a subset of the Ada tasking features, specifically designed for
systems with safety-critical real-time requirements.
set ravenscar task-switching on
Allows task switching when debugging a program that uses the Ravenscar Profile. This is the default.
set ravenscar task-switching off
Turn off task switching when debugging a program that uses the Ravenscar
Profile. This is mostly intended to disable the code that adds support for the
Ravenscar Profile, in case a bug in either gdb or in the Ravenscar runtime is
preventing gdb from working properly. To be effective, this command should
be run before the program is started.
show ravenscar task-switching
Show whether it is possible to switch from task to task in a program using the
Ravenscar Profile.
15.4.10.10 Known Peculiarities of Ada Mode
Besides the omissions listed previously (see Section 15.4.10.2 [Omissions from Ada],
page 212), we know of several problems with and limitations of Ada mode in gdb, some
of which will be fixed with planned future releases of the debugger and the GNU Ada
compiler.
• Static constants that the compiler chooses not to materialize as objects in storage are
invisible to the debugger.
• Named parameter associations in function argument lists are ignored (the argument
lists are treated as positional).
• Many useful library packages are currently invisible to the debugger.
• Fixed-point arithmetic, conversions, input, and output is carried out using floatingpoint arithmetic, and may give results that only approximate those on the host machine.
• The GNAT compiler never generates the prefix Standard for any of the standard
symbols defined by the Ada language. gdb knows about this: it will strip the prefix
from names when you use it, and will never look for a name you have so qualified
among local symbols, nor match against symbols in other packages or subprograms. If
you have defined entities anywhere in your program other than parameters and local
variables whose simple names match names in Standard, GNAT’s lack of qualification
here can cause confusion. When this happens, you can usually resolve the confusion
by qualifying the problematic names with package Standard explicitly.
Older versions of the compiler sometimes generate erroneous debugging information,
resulting in the debugger incorrectly printing the value of affected entities. In some cases,
the debugger is able to work around an issue automatically. In other cases, the debugger is
able to work around the issue, but the work-around has to be specifically enabled.
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Debugging with gdb
set ada trust-PAD-over-XVS on
Configure GDB to strictly follow the GNAT encoding when computing the
value of Ada entities, particularly when PAD and PAD___XVS types are involved
(see ada/exp_dbug.ads in the GCC sources for a complete description of the
encoding used by the GNAT compiler). This is the default.
set ada trust-PAD-over-XVS off
This is related to the encoding using by the GNAT compiler. If gdb sometimes
prints the wrong value for certain entities, changing ada trust-PAD-over-XVS
to off activates a work-around which may fix the issue. It is always safe to set
ada trust-PAD-over-XVS to off, but this incurs a slight performance penalty,
so it is recommended to leave this setting to on unless necessary.
Internally, the debugger also relies on the compiler following a number of conventions
known as the ‘GNAT Encoding’, all documented in ‘gcc/ada/exp_dbug.ads’ in the GCC
sources. This encoding describes how the debugging information should be generated for
certain types. In particular, this convention makes use of descriptive types, which are
artificial types generated purely to help the debugger.
These encodings were defined at a time when the debugging information format used was
not powerful enough to describe some of the more complex types available in Ada. Since
DWARF allows us to express nearly all Ada features, the long-term goal is to slowly replace
these descriptive types by their pure DWARF equivalent. To facilitate that transition, a
new maintenance option is available to force the debugger to ignore those descriptive types.
It allows the user to quickly evaluate how well gdb works without them.
maintenance ada set ignore-descriptive-types [on|off]
Control whether the debugger should ignore descriptive types. The default is
not to ignore descriptives types (off).
maintenance ada show ignore-descriptive-types
Show if descriptive types are ignored by gdb.
15.5 Unsupported Languages
In addition to the other fully-supported programming languages, gdb also provides a
pseudo-language, called minimal. It does not represent a real programming language, but
provides a set of capabilities close to what the C or assembly languages provide. This should
allow most simple operations to be performed while debugging an application that uses a
language currently not supported by gdb.
If the language is set to auto, gdb will automatically select this language if the current
frame corresponds to an unsupported language.
Chapter 16: Examining the Symbol Table
221
16 Examining the Symbol Table
The commands described in this chapter allow you to inquire about the symbols (names
of variables, functions and types) defined in your program. This information is inherent in
the text of your program and does not change as your program executes. gdb finds it in
your program’s symbol table, in the file indicated when you started gdb (see Section 2.1.1
[Choosing Files], page 12), or by one of the file-management commands (see Section 18.1
[Commands to Specify Files], page 241).
Occasionally, you may need to refer to symbols that contain unusual characters, which
gdb ordinarily treats as word delimiters. The most frequent case is in referring to static
variables in other source files (see Section 10.3 [Program Variables], page 119). File names
are recorded in object files as debugging symbols, but gdb would ordinarily parse a typical
file name, like ‘foo.c’, as the three words ‘foo’ ‘.’ ‘c’. To allow gdb to recognize ‘foo.c’
as a single symbol, enclose it in single quotes; for example,
p ’foo.c’::x
looks up the value of x in the scope of the file ‘foo.c’.
set case-sensitive on
set case-sensitive off
set case-sensitive auto
Normally, when gdb looks up symbols, it matches their names with case sensitivity determined by the current source language. Occasionally, you may wish
to control that. The command set case-sensitive lets you do that by specifying on for case-sensitive matches or off for case-insensitive ones. If you specify
auto, case sensitivity is reset to the default suitable for the source language.
The default is case-sensitive matches for all languages except for Fortran, for
which the default is case-insensitive matches.
show case-sensitive
This command shows the current setting of case sensitivity for symbols lookups.
set print type methods
set print type methods on
set print type methods off
Normally, when gdb prints a class, it displays any methods declared in that
class. You can control this behavior either by passing the appropriate flag
to ptype, or using set print type methods. Specifying on will cause gdb to
display the methods; this is the default. Specifying off will cause gdb to omit
the methods.
show print type methods
This command shows the current setting of method display when printing
classes.
set print type typedefs
set print type typedefs on
set print type typedefs off
Normally, when gdb prints a class, it displays any typedefs defined in that class.
You can control this behavior either by passing the appropriate flag to ptype,
222
Debugging with gdb
or using set print type typedefs. Specifying on will cause gdb to display the
typedef definitions; this is the default. Specifying off will cause gdb to omit
the typedef definitions. Note that this controls whether the typedef definition
itself is printed, not whether typedef names are substituted when printing other
types.
show print type typedefs
This command shows the current setting of typedef display when printing
classes.
info address symbol
Describe where the data for symbol is stored. For a register variable, this says
which register it is kept in. For a non-register local variable, this prints the
stack-frame offset at which the variable is always stored.
Note the contrast with ‘print &symbol’, which does not work at all for a register
variable, and for a stack local variable prints the exact address of the current
instantiation of the variable.
info symbol addr
Print the name of a symbol which is stored at the address addr. If no symbol
is stored exactly at addr, gdb prints the nearest symbol and an offset from it:
(gdb) info symbol 0x54320
_initialize_vx + 396 in section .text
This is the opposite of the info address command. You can use it to find out
the name of a variable or a function given its address.
For dynamically linked executables, the name of executable or shared library
containing the symbol is also printed:
(gdb) info symbol 0x400225
_start + 5 in section .text of /tmp/a.out
(gdb) info symbol 0x2aaaac2811cf
__read_nocancel + 6 in section .text of /usr/lib64/libc.so.6
demangle [-l language] [--] name
Demangle name. If language is provided it is the name of the language to
demangle name in. Otherwise name is demangled in the current language.
The ‘--’ option specifies the end of options, and is useful when name begins
with a dash.
The parameter demangle-style specifies how to interpret the kind of mangling
used. See Section 10.8 [Print Settings], page 127.
whatis[/flags] [arg]
Print the data type of arg, which can be either an expression or a name of a
data type. With no argument, print the data type of $, the last value in the
value history.
If arg is an expression (see Section 10.1 [Expressions], page 117), it is not
actually evaluated, and any side-effecting operations (such as assignments or
function calls) inside it do not take place.
If arg is a variable or an expression, whatis prints its literal type as it is used
in the source code. If the type was defined using a typedef, whatis will not
Chapter 16: Examining the Symbol Table
223
print the data type underlying the typedef. If the type of the variable or the
expression is a compound data type, such as struct or class, whatis never
prints their fields or methods. It just prints the struct/class name (a.k.a.
its tag). If you want to see the members of such a compound data type, use
ptype.
If arg is a type name that was defined using typedef, whatis unrolls only one
level of that typedef. Unrolling means that whatis will show the underlying
type used in the typedef declaration of arg. However, if that underlying type
is also a typedef, whatis will not unroll it.
For C code, the type names may also have the form ‘class class-name’,
‘struct struct-tag’, ‘union union-tag’ or ‘enum enum-tag’.
flags can be used to modify how the type is displayed. Available flags are:
r
Display in “raw” form. Normally, gdb substitutes template parameters and typedefs defined in a class when printing the class’
members. The /r flag disables this.
m
Do not print methods defined in the class.
M
Print methods defined in the class. This is the default, but the flag
exists in case you change the default with set print type methods.
t
Do not print typedefs defined in the class. Note that this controls
whether the typedef definition itself is printed, not whether typedef
names are substituted when printing other types.
T
Print typedefs defined in the class. This is the default, but the
flag exists in case you change the default with set print type
typedefs.
ptype[/flags] [arg]
ptype accepts the same arguments as whatis, but prints a detailed description
of the type, instead of just the name of the type. See Section 10.1 [Expressions],
page 117.
Contrary to whatis, ptype always unrolls any typedefs in its argument declaration, whether the argument is a variable, expression, or a data type. This
means that ptype of a variable or an expression will not print literally its type
as present in the source code—use whatis for that. typedefs at the pointer
or reference targets are also unrolled. Only typedefs of fields, methods and
inner class typedefs of structs, classes and unions are not unrolled even
with ptype.
For example, for this variable declaration:
typedef double real_t;
struct complex { real_t real; double imag; };
typedef struct complex complex_t;
complex_t var;
real_t *real_pointer_var;
the two commands give this output:
224
Debugging with gdb
(gdb) whatis var
type = complex_t
(gdb) ptype var
type = struct complex {
real_t real;
double imag;
}
(gdb) whatis complex_t
type = struct complex
(gdb) whatis struct complex
type = struct complex
(gdb) ptype struct complex
type = struct complex {
real_t real;
double imag;
}
(gdb) whatis real_pointer_var
type = real_t *
(gdb) ptype real_pointer_var
type = double *
As with whatis, using ptype without an argument refers to the type of $, the
last value in the value history.
Sometimes, programs use opaque data types or incomplete specifications of
complex data structure. If the debug information included in the program
does not allow gdb to display a full declaration of the data type, it will say
‘’. For example, given these declarations:
struct foo;
struct foo *fooptr;
but no definition for struct foo itself, gdb will say:
(gdb) ptype foo
$1 =
“Incomplete type” is C terminology for data types that are not completely
specified.
info types regexp
info types
Print a brief description of all types whose names match the regular expression
regexp (or all types in your program, if you supply no argument). Each complete
typename is matched as though it were a complete line; thus, ‘i type value’
gives information on all types in your program whose names include the string
value, but ‘i type ^value$’ gives information only on types whose complete
name is value.
This command differs from ptype in two ways: first, like whatis, it does not
print a detailed description; second, it lists all source files where a type is
defined.
info type-printers
Versions of gdb that ship with Python scripting enabled may have “type printers” available. When using ptype or whatis, these printers are consulted
when the name of a type is needed. See Section 23.2.2.8 [Type Printing API],
page 347, for more information on writing type printers.
Chapter 16: Examining the Symbol Table
225
info type-printers displays all the available type printers.
enable type-printer name...
disable type-printer name...
These commands can be used to enable or disable type printers.
info scope location
List all the variables local to a particular scope. This command accepts a
location argument—a function name, a source line, or an address preceded by a
‘*’, and prints all the variables local to the scope defined by that location. (See
Section 9.2 [Specify Location], page 104, for details about supported forms of
location.) For example:
(gdb) info scope command line handler
Scope for command_line_handler:
Symbol rl is an argument at stack/frame offset 8, length 4.
Symbol linebuffer is in static storage at address 0x150a18, length 4.
Symbol linelength is in static storage at address 0x150a1c, length 4.
Symbol p is a local variable in register $esi, length 4.
Symbol p1 is a local variable in register $ebx, length 4.
Symbol nline is a local variable in register $edx, length 4.
Symbol repeat is a local variable at frame offset -8, length 4.
This command is especially useful for determining what data to collect during
a trace experiment, see Section 13.1.6 [Tracepoint Actions], page 172.
info source
Show information about the current source file—that is, the source file for the
function containing the current point of execution:
• the name of the source file, and the directory containing it,
• the directory it was compiled in,
• its length, in lines,
• which programming language it is written in,
• if the debug information provides it, the program that compiled the file
(which may include, e.g., the compiler version and command line arguments),
• whether the executable includes debugging information for that file, and if
so, what format the information is in (e.g., STABS, Dwarf 2, etc.), and
• whether the debugging information includes information about preprocessor macros.
info sources
Print the names of all source files in your program for which there is debugging
information, organized into two lists: files whose symbols have already been
read, and files whose symbols will be read when needed.
info functions
Print the names and data types of all defined functions.
info functions regexp
Print the names and data types of all defined functions whose names contain a
match for regular expression regexp. Thus, ‘info fun step’ finds all functions
226
Debugging with gdb
whose names include step; ‘info fun ^step’ finds those whose names start
with step. If a function name contains characters that conflict with the regular
expression language (e.g. ‘operator*()’), they may be quoted with a backslash.
info variables
Print the names and data types of all variables that are defined outside of
functions (i.e. excluding local variables).
info variables regexp
Print the names and data types of all variables (except for local variables) whose
names contain a match for regular expression regexp.
info classes
info classes regexp
Display all Objective-C classes in your program, or (with the regexp argument)
all those matching a particular regular expression.
info selectors
info selectors regexp
Display all Objective-C selectors in your program, or (with the regexp argument) all those matching a particular regular expression.
set opaque-type-resolution on
Tell gdb to resolve opaque types. An opaque type is a type declared as a
pointer to a struct, class, or union—for example, struct MyType *—that
is used in one source file although the full declaration of struct MyType is in
another source file. The default is on.
A change in the setting of this subcommand will not take effect until the next
time symbols for a file are loaded.
set opaque-type-resolution off
Tell gdb not to resolve opaque types. In this case, the type is printed as follows:
{}
show opaque-type-resolution
Show whether opaque types are resolved or not.
set
set
set
set
print
print
print
print
symbol-loading
symbol-loading full
symbol-loading brief
symbol-loading off
The set print symbol-loading command allows you to control the printing of
messages when gdb loads symbol information. By default a message is printed
for the executable and one for each shared library, and normally this is what you
want. However, when debugging apps with large numbers of shared libraries
these messages can be annoying. When set to brief a message is printed for
each executable, and when gdb loads a collection of shared libraries at once it
will only print one message regardless of the number of shared libraries. When
set to off no messages are printed.
show print symbol-loading
Show whether messages will be printed when a gdb command entered from the
keyboard causes symbol information to be loaded.
Chapter 16: Examining the Symbol Table
maint
maint
maint
maint
maint
227
print symbols [-pc address] [filename]
print symbols [-objfile objfile] [-source source] [--] [filename]
print psymbols [-objfile objfile] [-pc address] [--] [filename]
print psymbols [-objfile objfile] [-source source] [--] [filename]
print msymbols [-objfile objfile] [--] [filename]
Write a dump of debugging symbol data into the file filename or the terminal if
filename is unspecified. If -objfile objfile is specified, only dump symbols
for that objfile. If -pc address is specified, only dump symbols for the file with
code at that address. Note that address may be a symbol like main. If -source
source is specified, only dump symbols for that source file.
These commands are used to debug the gdb symbol-reading code. These commands do not modify internal gdb state, therefore ‘maint print symbols’ will
only print symbols for already expanded symbol tables. You can use the command info sources to find out which files these are. If you use ‘maint print
psymbols’ instead, the dump shows information about symbols that gdb only
knows partially—that is, symbols defined in files that gdb has skimmed, but
not yet read completely. Finally, ‘maint print msymbols’ just dumps “minimal
symbols”, e.g., “ELF symbols”.
See Section 18.1 [Commands to Specify Files], page 241, for a discussion of how
gdb reads symbols (in the description of symbol-file).
maint info symtabs [ regexp ]
maint info psymtabs [ regexp ]
List the struct symtab or struct partial_symtab structures whose names
match regexp. If regexp is not given, list them all. The output includes expressions which you can copy into a gdb debugging this one to examine a particular
structure in more detail. For example:
(gdb) maint info psymtabs dwarf2read
{ objfile /home/gnu/build/gdb/gdb
((struct objfile *) 0x82e69d0)
{ psymtab /home/gnu/src/gdb/dwarf2read.c
((struct partial_symtab *) 0x8474b10)
readin no
fullname (null)
text addresses 0x814d3c8 -- 0x8158074
globals (* (struct partial_symbol **) 0x8507a08 @ 9)
statics (* (struct partial_symbol **) 0x40e95b78 @ 2882)
dependencies (none)
}
}
(gdb) maint info symtabs
(gdb)
We see that there is one partial symbol table whose filename contains the string
‘dwarf2read’, belonging to the ‘gdb’ executable; and we see that gdb has not
read in any symtabs yet at all. If we set a breakpoint on a function, that will
cause gdb to read the symtab for the compilation unit containing that function:
(gdb) break dwarf2_psymtab_to_symtab
Breakpoint 1 at 0x814e5da: file /home/gnu/src/gdb/dwarf2read.c,
line 1574.
(gdb) maint info symtabs
228
Debugging with gdb
{ objfile /home/gnu/build/gdb/gdb
((struct objfile *) 0x82e69d0)
{ symtab /home/gnu/src/gdb/dwarf2read.c
((struct symtab *) 0x86c1f38)
dirname (null)
fullname (null)
blockvector ((struct blockvector *) 0x86c1bd0) (primary)
linetable ((struct linetable *) 0x8370fa0)
debugformat DWARF 2
}
}
(gdb)
maint info line-table [ regexp ]
List the struct linetable from all struct symtab instances whose name
matches regexp. If regexp is not given, list the struct linetable from all
struct symtab.
maint set symbol-cache-size size
Set the size of the symbol cache to size. The default size is intended to be
good enough for debugging most applications. This option exists to allow for
experimenting with different sizes.
maint show symbol-cache-size
Show the size of the symbol cache.
maint print symbol-cache
Print the contents of the symbol cache. This is useful when debugging symbol
cache issues.
maint print symbol-cache-statistics
Print symbol cache usage statistics. This helps determine how well the cache
is being utilized.
maint flush-symbol-cache
Flush the contents of the symbol cache, all entries are removed. This command
is useful when debugging the symbol cache. It is also useful when collecting
performance data.
Chapter 17: Altering Execution
229
17 Altering Execution
Once you think you have found an error in your program, you might want to find out for
certain whether correcting the apparent error would lead to correct results in the rest of the
run. You can find the answer by experiment, using the gdb features for altering execution
of the program.
For example, you can store new values into variables or memory locations, give your program a signal, restart it at a different address, or even return prematurely from a function.
17.1 Assignment to Variables
To alter the value of a variable, evaluate an assignment expression. See Section 10.1 [Expressions], page 117. For example,
print x=4
stores the value 4 into the variable x, and then prints the value of the assignment expression
(which is 4). See Chapter 15 [Using gdb with Different Languages], page 191, for more
information on operators in supported languages.
If you are not interested in seeing the value of the assignment, use the set command
instead of the print command. set is really the same as print except that the expression’s
value is not printed and is not put in the value history (see Section 10.10 [Value History],
page 138). The expression is evaluated only for its effects.
If the beginning of the argument string of the set command appears identical to a
set subcommand, use the set variable command instead of just set. This command is
identical to set except for its lack of subcommands. For example, if your program has a
variable width, you get an error if you try to set a new value with just ‘set width=13’,
because gdb has the command set width:
(gdb) whatis width
type = double
(gdb) p width
$4 = 13
(gdb) set width=47
Invalid syntax in expression.
The invalid expression, of course, is ‘=47’. In order to actually set the program’s variable
width, use
(gdb) set var width=47
Because the set command has many subcommands that can conflict with the names of
program variables, it is a good idea to use the set variable command instead of just set.
For example, if your program has a variable g, you run into problems if you try to set a
new value with just ‘set g=4’, because gdb has the command set gnutarget, abbreviated
set g:
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(gdb) whatis g
type = double
(gdb) p g
$1 = 1
(gdb) set g=4
(gdb) p g
$2 = 1
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/smith/cc_progs/a.out
"/home/smith/cc_progs/a.out": can’t open to read symbols:
Invalid bfd target.
(gdb) show g
The current BFD target is "=4".
The program variable g did not change, and you silently set the gnutarget to an invalid
value. In order to set the variable g, use
(gdb) set var g=4
gdb allows more implicit conversions in assignments than C; you can freely store an
integer value into a pointer variable or vice versa, and you can convert any structure to any
other structure that is the same length or shorter.
To store values into arbitrary places in memory, use the ‘{...}’ construct to generate a
value of specified type at a specified address (see Section 10.1 [Expressions], page 117). For
example, {int}0x83040 refers to memory location 0x83040 as an integer (which implies a
certain size and representation in memory), and
set {int}0x83040 = 4
stores the value 4 into that memory location.
17.2 Continuing at a Different Address
Ordinarily, when you continue your program, you do so at the place where it stopped, with
the continue command. You can instead continue at an address of your own choosing,
with the following commands:
jump location
j location
Resume execution at location. Execution stops again immediately if there is a
breakpoint there. See Section 9.2 [Specify Location], page 104, for a description
of the different forms of location. It is common practice to use the tbreak
command in conjunction with jump. See Section 5.1.1 [Setting Breakpoints],
page 46.
The jump command does not change the current stack frame, or the stack
pointer, or the contents of any memory location or any register other than the
program counter. If location is in a different function from the one currently
executing, the results may be bizarre if the two functions expect different patterns of arguments or of local variables. For this reason, the jump command
requests confirmation if the specified line is not in the function currently executing. However, even bizarre results are predictable if you are well acquainted
with the machine-language code of your program.
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On many systems, you can get much the same effect as the jump command by storing
a new value into the register $pc. The difference is that this does not start your program
running; it only changes the address of where it will run when you continue. For example,
set $pc = 0x485
makes the next continue command or stepping command execute at address 0x485, rather
than at the address where your program stopped. See Section 5.2 [Continuing and Stepping],
page 68.
The most common occasion to use the jump command is to back up—perhaps with more
breakpoints set—over a portion of a program that has already executed, in order to examine
its execution in more detail.
17.3 Giving your Program a Signal
signal signal
Resume execution where your program is stopped, but immediately give it
the signal signal. The signal can be the name or the number of a signal.
For example, on many systems signal 2 and signal SIGINT are both ways
of sending an interrupt signal.
Alternatively, if signal is zero, continue execution without giving a signal. This
is useful when your program stopped on account of a signal and would ordinarily
see the signal when resumed with the continue command; ‘signal 0’ causes it
to resume without a signal.
Note: When resuming a multi-threaded program, signal is delivered to the
currently selected thread, not the thread that last reported a stop. This includes
the situation where a thread was stopped due to a signal. So if you want to
continue execution suppressing the signal that stopped a thread, you should
select that same thread before issuing the ‘signal 0’ command. If you issue
the ‘signal 0’ command with another thread as the selected one, gdb detects
that and asks for confirmation.
Invoking the signal command is not the same as invoking the kill utility from
the shell. Sending a signal with kill causes gdb to decide what to do with
the signal depending on the signal handling tables (see Section 5.4 [Signals],
page 74). The signal command passes the signal directly to your program.
signal does not repeat when you press RET a second time after executing the
command.
queue-signal signal
Queue signal to be delivered immediately to the current thread when execution
of the thread resumes. The signal can be the name or the number of a signal.
For example, on many systems signal 2 and signal SIGINT are both ways of
sending an interrupt signal. The handling of the signal must be set to pass
the signal to the program, otherwise gdb will report an error. You can control
the handling of signals from gdb with the handle command (see Section 5.4
[Signals], page 74).
Alternatively, if signal is zero, any currently queued signal for the current thread
is discarded and when execution resumes no signal will be delivered. This is
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useful when your program stopped on account of a signal and would ordinarily
see the signal when resumed with the continue command.
This command differs from the signal command in that the signal is just
queued, execution is not resumed. And queue-signal cannot be used to pass
a signal whose handling state has been set to nopass (see Section 5.4 [Signals],
page 74).
See [stepping into signal handlers], page 75, for information on how stepping commands
behave when the thread has a signal queued.
17.4 Returning from a Function
return
return expression
You can cancel execution of a function call with the return command. If you
give an expression argument, its value is used as the function’s return value.
When you use return, gdb discards the selected stack frame (and all frames within it).
You can think of this as making the discarded frame return prematurely. If you wish to
specify a value to be returned, give that value as the argument to return.
This pops the selected stack frame (see Section 8.3 [Selecting a Frame], page 98), and
any other frames inside of it, leaving its caller as the innermost remaining frame. That
frame becomes selected. The specified value is stored in the registers used for returning
values of functions.
The return command does not resume execution; it leaves the program stopped in the
state that would exist if the function had just returned. In contrast, the finish command
(see Section 5.2 [Continuing and Stepping], page 68) resumes execution until the selected
stack frame returns naturally.
gdb needs to know how the expression argument should be set for the inferior. The
concrete registers assignment depends on the OS ABI and the type being returned by the
selected stack frame. For example it is common for OS ABI to return floating point values
in FPU registers while integer values in CPU registers. Still some ABIs return even floating
point values in CPU registers. Larger integer widths (such as long long int) also have
specific placement rules. gdb already knows the OS ABI from its current target so it needs
to find out also the type being returned to make the assignment into the right register(s).
Normally, the selected stack frame has debug info. gdb will always use the debug info
instead of the implicit type of expression when the debug info is available. For example,
if you type return -1, and the function in the current stack frame is declared to return a
long long int, gdb transparently converts the implicit int value of -1 into a long long
int:
Breakpoint 1, func () at gdb.base/return-nodebug.c:29
29
return 31;
(gdb) return -1
Make func return now? (y or n) y
#0 0x004004f6 in main () at gdb.base/return-nodebug.c:43
43
printf ("result=%lld\n", func ());
(gdb)
However, if the selected stack frame does not have a debug info, e.g., if the function was
compiled without debug info, gdb has to find out the type to return from user. Specifying
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a different type by mistake may set the value in different inferior registers than the caller
code expects. For example, typing return -1 with its implicit type int would set only
a part of a long long int result for a debug info less function (on 32-bit architectures).
Therefore the user is required to specify the return type by an appropriate cast explicitly:
Breakpoint 2, 0x0040050b in func ()
(gdb) return -1
Return value type not available for selected stack frame.
Please use an explicit cast of the value to return.
(gdb) return (long long int) -1
Make selected stack frame return now? (y or n) y
#0 0x00400526 in main ()
(gdb)
17.5 Calling Program Functions
print expr
Evaluate the expression expr and display the resulting value. The expression
may include calls to functions in the program being debugged.
call expr Evaluate the expression expr without displaying void returned values.
You can use this variant of the print command if you want to execute a function
from your program that does not return anything (a.k.a. a void function), but
without cluttering the output with void returned values that gdb will otherwise
print. If the result is not void, it is printed and saved in the value history.
It is possible for the function you call via the print or call command to generate a
signal (e.g., if there’s a bug in the function, or if you passed it incorrect arguments). What
happens in that case is controlled by the set unwindonsignal command.
Similarly, with a C++ program it is possible for the function you call via the print or
call command to generate an exception that is not handled due to the constraints of the
dummy frame. In this case, any exception that is raised in the frame, but has an out-of-frame
exception handler will not be found. GDB builds a dummy-frame for the inferior function
call, and the unwinder cannot seek for exception handlers outside of this dummy-frame.
What happens in that case is controlled by the set unwind-on-terminating-exception
command.
set unwindonsignal
Set unwinding of the stack if a signal is received while in a function that gdb
called in the program being debugged. If set to on, gdb unwinds the stack it
created for the call and restores the context to what it was before the call. If
set to off (the default), gdb stops in the frame where the signal was received.
show unwindonsignal
Show the current setting of stack unwinding in the functions called by gdb.
set unwind-on-terminating-exception
Set unwinding of the stack if a C++ exception is raised, but left unhandled while
in a function that gdb called in the program being debugged. If set to on (the
default), gdb unwinds the stack it created for the call and restores the context
to what it was before the call. If set to off, gdb the exception is delivered to
the default C++ exception handler and the inferior terminated.
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show unwind-on-terminating-exception
Show the current setting of stack unwinding in the functions called by gdb.
Sometimes, a function you wish to call is actually a weak alias for another function. In
such case, gdb might not pick up the type information, including the types of the function
arguments, which causes gdb to call the inferior function incorrectly. As a result, the called
function will function erroneously and may even crash. A solution to that is to use the
name of the aliased function instead.
17.6 Patching Programs
By default, gdb opens the file containing your program’s executable code (or the corefile)
read-only. This prevents accidental alterations to machine code; but it also prevents you
from intentionally patching your program’s binary.
If you’d like to be able to patch the binary, you can specify that explicitly with the set
write command. For example, you might want to turn on internal debugging flags, or even
to make emergency repairs.
set write on
set write off
If you specify ‘set write on’, gdb opens executable and core files for both
reading and writing; if you specify set write off (the default), gdb opens
them read-only.
If you have already loaded a file, you must load it again (using the exec-file
or core-file command) after changing set write, for your new setting to take
effect.
show write
Display whether executable files and core files are opened for writing as well as
reading.
17.7 Compiling and injecting code in gdb
gdb supports on-demand compilation and code injection into programs running under gdb.
GCC 5.0 or higher built with ‘libcc1.so’ must be installed for this functionality to be
enabled. This functionality is implemented with the following commands.
compile code source-code
compile code -raw -- source-code
Compile source-code with the compiler language found as the current language
in gdb (see Chapter 15 [Languages], page 191). If compilation and injection
is not supported with the current language specified in gdb, or the compiler
does not support this feature, an error message will be printed. If source-code
compiles and links successfully, gdb will load the object-code emitted, and
execute it within the context of the currently selected inferior. It is important
to note that the compiled code is executed immediately. After execution, the
compiled code is removed from gdb and any new types or variables you have
defined will be deleted.
The command allows you to specify source-code in two ways. The simplest
method is to provide a single line of code to the command. E.g.:
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compile code printf ("hello world\n");
If you specify options on the command line as well as source code, they may
conflict. The ‘--’ delimiter can be used to separate options from actual source
code. E.g.:
compile code -r -- printf ("hello world\n");
Alternatively you can enter source code as multiple lines of text. To enter
this mode, invoke the ‘compile code’ command without any text following the
command. This will start the multiple-line editor and allow you to type as
many lines of source code as required. When you have completed typing, enter
‘end’ on its own line to exit the editor.
compile code
>printf ("hello\n");
>printf ("world\n");
>end
Specifying ‘-raw’, prohibits gdb from wrapping the provided source-code in
a callable scope. In this case, you must specify the entry point of the code
by defining a function named _gdb_expr_. The ‘-raw’ code cannot access
variables of the inferior. Using ‘-raw’ option may be needed for example when
source-code requires ‘#include’ lines which may conflict with inferior symbols
otherwise.
compile file filename
compile file -raw filename
Like compile code, but take the source code from filename.
compile file /home/user/example.c
compile print expr
compile print /f expr
Compile and execute expr with the compiler language found as the current
language in gdb (see Chapter 15 [Languages], page 191). By default the value
of expr is printed in a format appropriate to its data type; you can choose a
different format by specifying ‘/f’, where f is a letter specifying the format; see
Section 10.5 [Output Formats], page 122.
compile print
compile print /f
Alternatively you can enter the expression (source code producing it) as multiple
lines of text. To enter this mode, invoke the ‘compile print’ command without
any text following the command. This will start the multiple-line editor.
The process of compiling and injecting the code can be inspected using:
set debug compile
Turns on or off display of gdb process of compiling and injecting the code. The
default is off.
show debug compile
Displays the current state of displaying gdb process of compiling and injecting
the code.
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17.7.1 Compilation options for the compile command
gdb needs to specify the right compilation options for the code to be injected, in part to
make its ABI compatible with the inferior and in part to make the injected code compatible
with gdb’s injecting process.
The options used, in increasing precedence:
target architecture and OS options (gdbarch)
These options depend on target processor type and target operating system,
usually they specify at least 32-bit (-m32) or 64-bit (-m64) compilation option.
compilation options recorded in the target
gcc (since version 4.7) stores the options used for compilation into DW_AT_
producer part of DWARF debugging information according to the gcc option
-grecord-gcc-switches. One has to explicitly specify -g during inferior compilation otherwise gcc produces no DWARF. This feature is only relevant for
platforms where -g produces DWARF by default, otherwise one may try to
enforce DWARF by using -gdwarf-4.
compilation options set by set compile-args
You can override compilation options using the following command:
set compile-args
Set compilation options used for compiling and injecting code with the compile
commands. These options override any conflicting ones from the target architecture and/or options stored during inferior compilation.
show compile-args
Displays the current state of compilation options override. This does not
show all the options actually used during compilation, use [set debug compile],
page 235 for that.
17.7.2 Caveats when using the compile command
There are a few caveats to keep in mind when using the compile command. As the caveats
are different per language, the table below highlights specific issues on a per language basis.
C code examples and caveats
When the language in gdb is set to ‘C’, the compiler will attempt to compile
the source code with a ‘C’ compiler. The source code provided to the compile
command will have much the same access to variables and types as it normally
would if it were part of the program currently being debugged in gdb.
Below is a sample program that forms the basis of the examples that follow.
This program has been compiled and loaded into gdb, much like any other
normal debugging session.
void function1 (void)
{
int i = 42;
printf ("function 1\n");
}
void function2 (void)
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{
int j = 12;
function1 ();
}
int main(void)
{
int k = 6;
int *p;
function2 ();
return 0;
}
For the purposes of the examples in this section, the program above has been
compiled, loaded into gdb, stopped at the function main, and gdb is awaiting
input from the user.
To access variables and types for any program in gdb, the program must be
compiled and packaged with debug information. The compile command is not
an exception to this rule. Without debug information, you can still use the
compile command, but you will be very limited in what variables and types
you can access.
So with that in mind, the example above has been compiled with debug information enabled. The compile command will have access to all variables and
types (except those that may have been optimized out). Currently, as gdb has
stopped the program in the main function, the compile command would have
access to the variable k. You could invoke the compile command and type some
source code to set the value of k. You can also read it, or do anything with
that variable you would normally do in C. Be aware that changes to inferior
variables in the compile command are persistent. In the following example:
compile code k = 3;
the variable k is now 3. It will retain that value until something else in the
example program changes it, or another compile command changes it.
Normal scope and access rules apply to source code compiled and injected by
the compile command. In the example, the variables j and k are not accessible
yet, because the program is currently stopped in the main function, where these
variables are not in scope. Therefore, the following command
compile code j = 3;
will result in a compilation error message.
Once the program is continued, execution will bring these variables in scope,
and they will become accessible; then the code you specify via the compile
command will be able to access them.
You can create variables and types with the compile command as part of
your source code. Variables and types that are created as part of the compile
command are not visible to the rest of the program for the duration of its run.
This example is valid:
compile code int ff = 5; printf ("ff is %d\n", ff);
However, if you were to type the following into gdb after that command has
completed:
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compile code printf ("ff is %d\n’’, ff);
a compiler error would be raised as the variable ff no longer exists. Object
code generated and injected by the compile command is removed when its
execution ends. Caution is advised when assigning to program variables values
of variables created by the code submitted to the compile command. This
example is valid:
compile code int ff = 5; k = ff;
The value of the variable ff is assigned to k. The variable k does not require the
existence of ff to maintain the value it has been assigned. However, pointers
require particular care in assignment. If the source code compiled with the
compile command changed the address of a pointer in the example program,
perhaps to a variable created in the compile command, that pointer would
point to an invalid location when the command exits. The following example
would likely cause issues with your debugged program:
compile code int ff = 5; p = &ff;
In this example, p would point to ff when the compile command is executing
the source code provided to it. However, as variables in the (example) program
persist with their assigned values, the variable p would point to an invalid
location when the command exists. A general rule should be followed in that
you should either assign NULL to any assigned pointers, or restore a valid location
to the pointer before the command exits.
Similar caution must be exercised with any structs, unions, and typedefs defined
in compile command. Types defined in the compile command will no longer
be available in the next compile command. Therefore, if you cast a variable
to a type defined in the compile command, care must be taken to ensure that
any future need to resolve the type can be achieved.
(gdb) compile code static struct a { int a; } v = { 42 }; argv = &v;
(gdb) compile code printf ("%d\n", ((struct a *) argv)->a);
gdb command line:1:36: error: dereferencing pointer to incomplete type struct a
Compilation failed.
(gdb) compile code struct a { int a; }; printf ("%d\n", ((struct a *) argv)>a);
42
Variables that have been optimized away by the compiler are not accessible to
the code submitted to the compile command. Access to those variables will
generate a compiler error which gdb will print to the console.
17.7.3 Compiler search for the compile command
gdb needs to find gcc for the inferior being debugged which may not be obvious for remote
targets of different architecture than where gdb is running. Environment variable PATH
(PATH from shell that executed gdb, not the one set by gdb command set environment).
See Section 4.4 [Environment], page 30. PATH on gdb host is searched for gcc binary
matching the target architecture and operating system.
Specifically PATH is searched for binaries matching regular expression arch(-[^-]*)?os-gcc according to the inferior target being debugged. arch is processor name — multiarch
is supported, so for example both i386 and x86_64 targets look for pattern (x86_64|i.86)
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and both s390 and s390x targets look for pattern s390x?. os is currently supported only
for pattern linux(-gnu)?.
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18 gdb Files
gdb needs to know the file name of the program to be debugged, both in order to read its
symbol table and in order to start your program. To debug a core dump of a previous run,
you must also tell gdb the name of the core dump file.
18.1 Commands to Specify Files
You may want to specify executable and core dump file names. The usual way to do this is
at start-up time, using the arguments to gdb’s start-up commands (see Chapter 2 [Getting
In and Out of gdb], page 11).
Occasionally it is necessary to change to a different file during a gdb session. Or you
may run gdb and forget to specify a file you want to use. Or you are debugging a remote
target via gdbserver (see Section 20.3 [Using the gdbserver Program], page 265). In these
situations the gdb commands to specify new files are useful.
file filename
Use filename as the program to be debugged. It is read for its symbols and for
the contents of pure memory. It is also the program executed when you use
the run command. If you do not specify a directory and the file is not found
in the gdb working directory, gdb uses the environment variable PATH as a list
of directories to search, just as the shell does when looking for a program to
run. You can change the value of this variable, for both gdb and your program,
using the path command.
You can load unlinked object ‘.o’ files into gdb using the file command. You
will not be able to “run” an object file, but you can disassemble functions and
inspect variables. Also, if the underlying BFD functionality supports it, you
could use gdb -write to patch object files using this technique. Note that gdb
can neither interpret nor modify relocations in this case, so branches and some
initialized variables will appear to go to the wrong place. But this feature is
still handy from time to time.
file
file with no argument makes gdb discard any information it has on both
executable file and the symbol table.
exec-file [ filename ]
Specify that the program to be run (but not the symbol table) is found in filename. gdb searches the environment variable PATH if necessary to locate your
program. Omitting filename means to discard information on the executable
file.
symbol-file [ filename ]
Read symbol table information from file filename. PATH is searched when necessary. Use the file command to get both symbol table and program to run
from the same file.
symbol-file with no argument clears out gdb information on your program’s
symbol table.
The symbol-file command causes gdb to forget the contents of some breakpoints and auto-display expressions. This is because they may contain pointers
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to the internal data recording symbols and data types, which are part of the
old symbol table data being discarded inside gdb.
symbol-file does not repeat if you press RET again after executing it once.
When gdb is configured for a particular environment, it understands debugging
information in whatever format is the standard generated for that environment;
you may use either a gnu compiler, or other compilers that adhere to the local
conventions. Best results are usually obtained from gnu compilers; for example,
using gcc you can generate debugging information for optimized code.
For most kinds of object files, with the exception of old SVR3 systems using
COFF, the symbol-file command does not normally read the symbol table in
full right away. Instead, it scans the symbol table quickly to find which source
files and which symbols are present. The details are read later, one source file
at a time, as they are needed.
The purpose of this two-stage reading strategy is to make gdb start up faster.
For the most part, it is invisible except for occasional pauses while the symbol
table details for a particular source file are being read. (The set verbose
command can turn these pauses into messages if desired. See Section 22.8
[Optional Warnings and Messages], page 313.)
We have not implemented the two-stage strategy for COFF yet. When the
symbol table is stored in COFF format, symbol-file reads the symbol table
data in full right away. Note that “stabs-in-COFF” still does the two-stage
strategy, since the debug info is actually in stabs format.
symbol-file [ -readnow ] filename
file [ -readnow ] filename
You can override the gdb two-stage strategy for reading symbol tables by using the ‘-readnow’ option with any of the commands that load symbol table
information, if you want to be sure gdb has the entire symbol table available.
core-file [filename]
core
Specify the whereabouts of a core dump file to be used as the “contents of
memory”. Traditionally, core files contain only some parts of the address space
of the process that generated them; gdb can access the executable file itself for
other parts.
core-file with no argument specifies that no core file is to be used.
Note that the core file is ignored when your program is actually running under
gdb. So, if you have been running your program and you wish to debug a core
file instead, you must kill the subprocess in which the program is running. To
do this, use the kill command (see Section 4.8 [Killing the Child Process],
page 33).
add-symbol-file filename address
add-symbol-file filename address [ -readnow ]
add-symbol-file filename address -s section address ...
The add-symbol-file command reads additional symbol table information
from the file filename. You would use this command when filename has been
dynamically loaded (by some other means) into the program that is running.
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The address should give the memory address at which the file has been loaded;
gdb cannot figure this out for itself. You can additionally specify an arbitrary
number of ‘-s section address’ pairs, to give an explicit section name and
base address for that section. You can specify any address as an expression.
The symbol table of the file filename is added to the symbol table originally read
with the symbol-file command. You can use the add-symbol-file command
any number of times; the new symbol data thus read is kept in addition to the
old.
Changes can be reverted using the command remove-symbol-file.
Although filename is typically a shared library file, an executable file, or some
other object file which has been fully relocated for loading into a process, you
can also load symbolic information from relocatable ‘.o’ files, as long as:
• the file’s symbolic information refers only to linker symbols defined in that
file, not to symbols defined by other object files,
• every section the file’s symbolic information refers to has actually been
loaded into the inferior, as it appears in the file, and
• you can determine the address at which every section was loaded, and
provide these to the add-symbol-file command.
Some embedded operating systems, like Sun Chorus and VxWorks, can load
relocatable files into an already running program; such systems typically make
the requirements above easy to meet. However, it’s important to recognize that
many native systems use complex link procedures (.linkonce section factoring
and C++ constructor table assembly, for example) that make the requirements
difficult to meet. In general, one cannot assume that using add-symbol-file
to read a relocatable object file’s symbolic information will have the same effect
as linking the relocatable object file into the program in the normal way.
add-symbol-file does not repeat if you press RET after using it.
remove-symbol-file filename
remove-symbol-file -a address
Remove a symbol file added via the add-symbol-file command. The file to
remove can be identified by its filename or by an address that lies within the
boundaries of this symbol file in memory. Example:
(gdb) add-symbol-file /home/user/gdb/mylib.so 0x7ffff7ff9480
add symbol table from file "/home/user/gdb/mylib.so" at
.text_addr = 0x7ffff7ff9480
(y or n) y
Reading symbols from /home/user/gdb/mylib.so...done.
(gdb) remove-symbol-file -a 0x7ffff7ff9480
Remove symbol table from file "/home/user/gdb/mylib.so"? (y or n) y
(gdb)
remove-symbol-file does not repeat if you press RET after using it.
add-symbol-file-from-memory address
Load symbols from the given address in a dynamically loaded object file whose
image is mapped directly into the inferior’s memory. For example, the Linux
kernel maps a syscall DSO into each process’s address space; this DSO provides
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kernel-specific code for some system calls. The argument can be any expression whose evaluation yields the address of the file’s shared object file header.
For this command to work, you must have used symbol-file or exec-file
commands in advance.
section section addr
The section command changes the base address of the named section of the
exec file to addr. This can be used if the exec file does not contain section
addresses, (such as in the a.out format), or when the addresses specified in the
file itself are wrong. Each section must be changed separately. The info files
command, described below, lists all the sections and their addresses.
info files
info target
info files and info target are synonymous; both print the current target
(see Chapter 19 [Specifying a Debugging Target], page 257), including the
names of the executable and core dump files currently in use by gdb, and
the files from which symbols were loaded. The command help target lists all
possible targets rather than current ones.
maint info sections
Another command that can give you extra information about program sections
is maint info sections. In addition to the section information displayed by
info files, this command displays the flags and file offset of each section in
the executable and core dump files. In addition, maint info sections provides
the following command options (which may be arbitrarily combined):
ALLOBJ
Display sections for all loaded object files, including shared libraries.
sections
Display info only for named sections.
section-flags
Display info only for sections for which section-flags are true. The
section flags that gdb currently knows about are:
ALLOC
Section will have space allocated in the process when
loaded. Set for all sections except those containing debug information.
LOAD
Section will be loaded from the file into the child process memory. Set for pre-initialized code and data,
clear for .bss sections.
RELOC
Section needs to be relocated before loading.
READONLY
Section cannot be modified by the child process.
CODE
Section contains executable code only.
DATA
Section contains data only (no executable code).
ROM
Section will reside in ROM.
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245
CONSTRUCTOR
Section contains data for constructor/destructor lists.
HAS_CONTENTS
Section is not empty.
NEVER_LOAD
An instruction to the linker to not output the section.
COFF_SHARED_LIBRARY
A notification to the linker that the section contains
COFF shared library information.
IS_COMMON
Section contains common symbols.
set trust-readonly-sections on
Tell gdb that readonly sections in your object file really are read-only (i.e.
that their contents will not change). In that case, gdb can fetch values from
these sections out of the object file, rather than from the target program. For
some targets (notably embedded ones), this can be a significant enhancement
to debugging performance.
The default is off.
set trust-readonly-sections off
Tell gdb not to trust readonly sections. This means that the contents of the
section might change while the program is running, and must therefore be
fetched from the target when needed.
show trust-readonly-sections
Show the current setting of trusting readonly sections.
All file-specifying commands allow both absolute and relative file names as arguments.
gdb always converts the file name to an absolute file name and remembers it that way.
gdb supports gnu/Linux, MS-Windows, SunOS, Darwin/Mach-O, SVr4, IBM RS/6000
AIX, QNX Neutrino, FDPIC (FR-V), and DSBT (TIC6X) shared libraries.
On MS-Windows gdb must be linked with the Expat library to support shared libraries.
See [Expat], page 603.
gdb automatically loads symbol definitions from shared libraries when you use the run
command, or when you examine a core file. (Before you issue the run command, gdb
does not understand references to a function in a shared library, however—unless you are
debugging a core file).
There are times, however, when you may wish to not automatically load symbol definitions from shared libraries, such as when they are particularly large or there are many of
them.
To control the automatic loading of shared library symbols, use the commands:
set auto-solib-add mode
If mode is on, symbols from all shared object libraries will be loaded automatically when the inferior begins execution, you attach to an independently
246
Debugging with gdb
started inferior, or when the dynamic linker informs gdb that a new library
has been loaded. If mode is off, symbols must be loaded manually, using the
sharedlibrary command. The default value is on.
If your program uses lots of shared libraries with debug info that takes large
amounts of memory, you can decrease the gdb memory footprint by preventing it from automatically loading the symbols from shared libraries. To that
end, type set auto-solib-add off before running the inferior, then load each
library whose debug symbols you do need with sharedlibrary regexp, where
regexp is a regular expression that matches the libraries whose symbols you
want to be loaded.
show auto-solib-add
Display the current autoloading mode.
To explicitly load shared library symbols, use the sharedlibrary command:
info share regex
info sharedlibrary regex
Print the names of the shared libraries which are currently loaded that match
regex. If regex is omitted then print all shared libraries that are loaded.
info dll regex
This is an alias of info sharedlibrary.
sharedlibrary regex
share regex
Load shared object library symbols for files matching a Unix regular expression.
As with files loaded automatically, it only loads shared libraries required by your
program for a core file or after typing run. If regex is omitted all shared libraries
required by your program are loaded.
nosharedlibrary
Unload all shared object library symbols. This discards all symbols that have
been loaded from all shared libraries. Symbols from shared libraries that were
loaded by explicit user requests are not discarded.
Sometimes you may wish that gdb stops and gives you control when any of shared
library events happen. The best way to do this is to use catch load and catch unload
(see Section 5.1.3 [Set Catchpoints], page 54).
gdb also supports the the set stop-on-solib-events command for this. This command exists for historical reasons. It is less useful than setting a catchpoint, because it does
not allow for conditions or commands as a catchpoint does.
set stop-on-solib-events
This command controls whether gdb should give you control when the dynamic
linker notifies it about some shared library event. The most common event of
interest is loading or unloading of a new shared library.
show stop-on-solib-events
Show whether gdb stops and gives you control when shared library events
happen.
Chapter 18: gdb Files
247
Shared libraries are also supported in many cross or remote debugging configurations.
gdb needs to have access to the target’s libraries; this can be accomplished either by
providing copies of the libraries on the host system, or by asking gdb to automatically
retrieve the libraries from the target. If copies of the target libraries are provided, they need
to be the same as the target libraries, although the copies on the target can be stripped as
long as the copies on the host are not.
For remote debugging, you need to tell gdb where the target libraries are, so that it
can load the correct copies—otherwise, it may try to load the host’s libraries. gdb has two
variables to specify the search directories for target libraries.
set sysroot path
Use path as the system root for the program being debugged. Any absolute
shared library paths will be prefixed with path; many runtime loaders store the
absolute paths to the shared library in the target program’s memory. When
starting processes remotely, and when attaching to already-running processes
(local or remote), their executable filenames will be prefixed with path if reported to gdb as absolute by the operating system. If you use set sysroot to
find executables and shared libraries, they need to be laid out in the same way
that they are on the target, with e.g. a ‘/bin’, ‘/lib’ and ‘/usr/lib’ hierarchy
under path.
If path starts with the sequence ‘target:’ and the target system is remote
then gdb will retrieve the target binaries from the remote system. This is only
supported when using a remote target that supports the remote get command
(see Section 20.2 [Sending files to a remote system], page 264). The part of
path following the initial ‘target:’ (if present) is used as system root prefix
on the remote file system. If path starts with the sequence ‘remote:’ this is
converted to the sequence ‘target:’ by set sysroot1 . If you want to specify
a local system root using a directory that happens to be named ‘target:’ or
‘remote:’, you need to use some equivalent variant of the name like ‘./target:’.
For targets with an MS-DOS based filesystem, such as MS-Windows and SymbianOS, gdb tries prefixing a few variants of the target absolute file name with
path. But first, on Unix hosts, gdb converts all backslash directory separators
into forward slashes, because the backslash is not a directory separator on Unix:
c:\foo\bar.dll ⇒ c:/foo/bar.dll
Then, gdb attempts prefixing the target file name with path, and looks for the
resulting file name in the host file system:
c:/foo/bar.dll ⇒ /path/to/sysroot/c:/foo/bar.dll
If that does not find the binary, gdb tries removing the ‘:’ character from the
drive spec, both for convenience, and, for the case of the host file system not
supporting file names with colons:
c:/foo/bar.dll ⇒ /path/to/sysroot/c/foo/bar.dll
This makes it possible to have a system root that mirrors a target with more
than one drive. E.g., you may want to setup your local copies of the target
system shared libraries like so (note ‘c’ vs ‘z’):
1
Historically the functionality to retrieve binaries from the remote system was provided by prefixing path
with ‘remote:’
248
Debugging with gdb
‘/path/to/sysroot/c/sys/bin/foo.dll’
‘/path/to/sysroot/c/sys/bin/bar.dll’
‘/path/to/sysroot/z/sys/bin/bar.dll’
and point the system root at ‘/path/to/sysroot’, so that gdb can find the
correct copies of both ‘c:\sys\bin\foo.dll’, and ‘z:\sys\bin\bar.dll’.
If that still does not find the binary, gdb tries removing the whole drive spec
from the target file name:
c:/foo/bar.dll ⇒ /path/to/sysroot/foo/bar.dll
This last lookup makes it possible to not care about the drive name, if you
don’t want or need to.
The set solib-absolute-prefix command is an alias for set sysroot.
You can set the default system root by using the configure-time
‘--with-sysroot’ option. If the system root is inside gdb’s configured binary
prefix (set with ‘--prefix’ or ‘--exec-prefix’), then the default system root
will be updated automatically if the installed gdb is moved to a new location.
show sysroot
Display the current executable and shared library prefix.
set solib-search-path path
If this variable is set, path is a colon-separated list of directories to search for
shared libraries. ‘solib-search-path’ is used after ‘sysroot’ fails to locate the
library, or if the path to the library is relative instead of absolute. If you want
to use ‘solib-search-path’ instead of ‘sysroot’, be sure to set ‘sysroot’
to a nonexistent directory to prevent gdb from finding your host’s libraries.
‘sysroot’ is preferred; setting it to a nonexistent directory may interfere with
automatic loading of shared library symbols.
show solib-search-path
Display the current shared library search path.
set target-file-system-kind kind
Set assumed file system kind for target reported file names.
Shared library file names as reported by the target system may not make sense
as is on the system gdb is running on. For example, when remote debugging a
target that has MS-DOS based file system semantics, from a Unix host, the target may be reporting to gdb a list of loaded shared libraries with file names such
as ‘c:\Windows\kernel32.dll’. On Unix hosts, there’s no concept of drive letters, so the ‘c:\’ prefix is not normally understood as indicating an absolute file
name, and neither is the backslash normally considered a directory separator
character. In that case, the native file system would interpret this whole absolute file name as a relative file name with no directory components. This would
make it impossible to point gdb at a copy of the remote target’s shared libraries
on the host using set sysroot, and impractical with set solib-search-path.
Setting target-file-system-kind to dos-based tells gdb to interpret such
file names similarly to how the target would, and to map them to file names
valid on gdb’s native file system semantics. The value of kind can be "auto",
in addition to one of the supported file system kinds. In that case, gdb tries
Chapter 18: gdb Files
249
to determine the appropriate file system variant based on the current target’s
operating system (see Section 22.6 [Configuring the Current ABI], page 307).
The supported file system settings are:
unix
Instruct gdb to assume the target file system is of Unix kind. Only
file names starting the forward slash (‘/’) character are considered
absolute, and the directory separator character is also the forward
slash.
dos-based
Instruct gdb to assume the target file system is DOS based. File
names starting with either a forward slash, or a drive letter followed
by a colon (e.g., ‘c:’), are considered absolute, and both the slash
(‘/’) and the backslash (‘\\’) characters are considered directory
separators.
auto
Instruct gdb to use the file system kind associated with the target
operating system (see Section 22.6 [Configuring the Current ABI],
page 307). This is the default.
When processing file names provided by the user, gdb frequently needs to compare
them to the file names recorded in the program’s debug info. Normally, gdb compares
just the base names of the files as strings, which is reasonably fast even for very large
programs. (The base name of a file is the last portion of its name, after stripping all the
leading directories.) This shortcut in comparison is based upon the assumption that files
cannot have more than one base name. This is usually true, but references to files that use
symlinks or similar filesystem facilities violate that assumption. If your program records
files using such facilities, or if you provide file names to gdb using symlinks etc., you can
set basenames-may-differ to true to instruct gdb to completely canonicalize each pair
of file names it needs to compare. This will make file-name comparisons accurate, but at a
price of a significant slowdown.
set basenames-may-differ
Set whether a source file may have multiple base names.
show basenames-may-differ
Show whether a source file may have multiple base names.
18.2 File Caching
To speed up file loading, and reduce memory usage, gdb will reuse the bfd objects used to
track open files. See Section “BFD” in The Binary File Descriptor Library. The following
commands allow visibility and control of the caching behavior.
maint info bfds
This prints information about each bfd object that is known to gdb.
maint set bfd-sharing
maint show bfd-sharing
Control whether bfd objects can be shared. When sharing is enabled gdb
reuses already open bfd objects rather than reopening the same file. Turning
sharing off does not cause already shared bfd objects to be unshared, but all
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Debugging with gdb
future files that are opened will create a new bfd object. Similarly, re-enabling
sharing does not cause multiple existing bfd objects to be collapsed into a single
shared bfd object.
set debug bfd-cache level
Turns on debugging of the bfd cache, setting the level to level.
show debug bfd-cache
Show the current debugging level of the bfd cache.
18.3 Debugging Information in Separate Files
gdb allows you to put a program’s debugging information in a file separate from the executable itself, in a way that allows gdb to find and load the debugging information automatically. Since debugging information can be very large—sometimes larger than the executable
code itself—some systems distribute debugging information for their executables in separate
files, which users can install only when they need to debug a problem.
gdb supports two ways of specifying the separate debug info file:
• The executable contains a debug link that specifies the name of the separate debug
info file. The separate debug file’s name is usually ‘executable.debug’, where executable is the name of the corresponding executable file without leading directories
(e.g., ‘ls.debug’ for ‘/usr/bin/ls’). In addition, the debug link specifies a 32-bit
Cyclic Redundancy Check (CRC) checksum for the debug file, which gdb uses to validate that the executable and the debug file came from the same build.
• The executable contains a build ID, a unique bit string that is also present in the
corresponding debug info file. (This is supported only on some operating systems,
when using the ELF or PE file formats for binary files and the gnu Binutils.) For
more details about this feature, see the description of the ‘--build-id’ command-line
option in Section “Command Line Options” in The GNU Linker. The debug info file’s
name is not specified explicitly by the build ID, but can be computed from the build
ID, see below.
Depending on the way the debug info file is specified, gdb uses two different methods of
looking for the debug file:
• For the “debug link” method, gdb looks up the named file in the directory of the
executable file, then in a subdirectory of that directory named ‘.debug’, and finally
under each one of the global debug directories, in a subdirectory whose name is identical
to the leading directories of the executable’s absolute file name.
• For the “build ID” method, gdb looks in the ‘.build-id’ subdirectory of each one of
the global debug directories for a file named ‘nn/nnnnnnnn.debug’, where nn are the
first 2 hex characters of the build ID bit string, and nnnnnnnn are the rest of the bit
string. (Real build ID strings are 32 or more hex characters, not 10.)
So, for example, suppose you ask gdb to debug ‘/usr/bin/ls’, which has a debug link
that specifies the file ‘ls.debug’, and a build ID whose value in hex is abcdef1234. If the
list of the global debug directories includes ‘/usr/lib/debug’, then gdb will look for the
following debug information files, in the indicated order:
− ‘/usr/lib/debug/.build-id/ab/cdef1234.debug’
Chapter 18: gdb Files
251
− ‘/usr/bin/ls.debug’
− ‘/usr/bin/.debug/ls.debug’
− ‘/usr/lib/debug/usr/bin/ls.debug’.
Global debugging info directories default to what is set by gdb configure option
‘--with-separate-debug-dir’. During gdb run you can also set the global debugging
info directories, and view the list gdb is currently using.
set debug-file-directory directories
Set the directories which gdb searches for separate debugging information files
to directory. Multiple path components can be set concatenating them by a
path separator.
show debug-file-directory
Show the directories gdb searches for separate debugging information files.
A debug link is a special section of the executable file named .gnu_debuglink. The
section must contain:
• A filename, with any leading directory components removed, followed by a zero byte,
• zero to three bytes of padding, as needed to reach the next four-byte boundary within
the section, and
• a four-byte CRC checksum, stored in the same endianness used for the executable file
itself. The checksum is computed on the debugging information file’s full contents by
the function given below, passing zero as the crc argument.
Any executable file format can carry a debug link, as long as it can contain a section
named .gnu_debuglink with the contents described above.
The build ID is a special section in the executable file (and in other ELF binary files that
gdb may consider). This section is often named .note.gnu.build-id, but that name is
not mandatory. It contains unique identification for the built files—the ID remains the same
across multiple builds of the same build tree. The default algorithm SHA1 produces 160
bits (40 hexadecimal characters) of the content for the build ID string. The same section
with an identical value is present in the original built binary with symbols, in its stripped
variant, and in the separate debugging information file.
The debugging information file itself should be an ordinary executable, containing a full
set of linker symbols, sections, and debugging information. The sections of the debugging
information file should have the same names, addresses, and sizes as the original file, but
they need not contain any data—much like a .bss section in an ordinary executable.
The gnu binary utilities (Binutils) package includes the ‘objcopy’ utility that can produce the separated executable / debugging information file pairs using the following commands:
objcopy --only-keep-debug foo foo.debug
strip -g foo
These commands remove the debugging information from the executable file ‘foo’ and place
it in the file ‘foo.debug’. You can use the first, second or both methods to link the two
files:
• The debug link method needs the following additional command to also leave behind
a debug link in ‘foo’:
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Debugging with gdb
objcopy --add-gnu-debuglink=foo.debug foo
Ulrich Drepper’s ‘elfutils’ package, starting with version 0.53, contains a version of
the strip command such that the command strip foo -f foo.debug has the same
functionality as the two objcopy commands and the ln -s command above, together.
• Build ID gets embedded into the main executable using ld --build-id or the gcc
counterpart gcc -Wl,--build-id. Build ID support plus compatibility fixes for debug
files separation are present in gnu binary utilities (Binutils) package since version 2.18.
The CRC used in .gnu_debuglink is the CRC-32 defined in IEEE 802.3 using the
polynomial:
x32 + x26 + x23 + x22 + x16 + x12 + x11
+ x10 + x8 + x7 + x5 + x4 + x2 + x + 1
The function is computed byte at a time, taking the least significant bit of each byte
first. The initial pattern 0xffffffff is used, to ensure leading zeros affect the CRC and
the final result is inverted to ensure trailing zeros also affect the CRC.
Note: This is the same CRC polynomial as used in handling the Remote Serial Protocol
qCRC packet (see [qCRC packet], page 635). However in the case of the Remote Serial
Protocol, the CRC is computed most significant bit first, and the result is not inverted, so
trailing zeros have no effect on the CRC value.
To complete the description, we show below the code of the function which produces the
CRC used in .gnu_debuglink. Inverting the initially supplied crc argument means that an
initial call to this function passing in zero will start computing the CRC using 0xffffffff.
unsigned long
gnu_debuglink_crc32 (unsigned long crc,
unsigned char *buf, size_t len)
{
static const unsigned long crc32_table[256] =
{
0x00000000, 0x77073096, 0xee0e612c, 0x990951ba,
0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832,
0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd,
0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148,
0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f,
0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e,
0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd,
0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940,
0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac,
0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423,
0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2,
0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d,
0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589,
0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934,
0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97,
0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6,
0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49,
0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074,
0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb,
0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73,
0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa,
0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409,
0x076dc419,
0x79dcb8a4,
0xe7b82d07,
0x84be41de,
0x136c9856,
0x63066cd9,
0xd56041e4,
0xa50ab56b,
0x32d86ce3,
0x51de003a,
0xcfba9599,
0xb10be924,
0x76dc4190,
0x06b6b51f,
0x9609a88e,
0xe6635c01,
0x6c0695ed,
0x12b7e950,
0x8cd37cf3,
0xd4bb30e2,
0x4369e96a,
0x33031de5,
0xbe0b1010,
0xce61e49f,
Chapter 18: gdb Files
0x5edef90e, 0x29d9c998,
0x2eb40d81, 0xb7bd5c3b,
0x03b6e20c, 0x74b1d29a,
0x73dc1683, 0xe3630b12,
0xe40ecf0b, 0x9309ff9d,
0x8708a3d2, 0x1e01f268,
0x196c3671, 0x6e6b06e7,
0x67dd4acc, 0xf9b9df6f,
0xd6d6a3e8, 0xa1d1937e,
0xa6bc5767, 0x3fb506dd,
0x36034af6, 0x41047a60,
0x4669be79, 0xcb61b38c,
0xcc0c7795, 0xbb0b4703,
0xb2bd0b28, 0x2bb45a92,
0x2cd99e8b, 0x5bdeae1d,
0x026d930a, 0x9c0906a9,
0x95bf4a82, 0xe2b87a14,
0xe5d5be0d, 0x7cdcefb7,
0x68ddb3f8, 0x1fda836e,
0x18b74777, 0x88085ae6,
0x8f659eff, 0xf862ae69,
0xd70dd2ee, 0x4e048354,
0x4969474d, 0x3e6e77db,
0x37d83bf0, 0xa9bcae53,
0xbdbdf21c, 0xcabac28a,
0xcdd70693, 0x54de5729,
0x5d681b02, 0x2a6f2b94,
0x2d02ef8d
};
unsigned char *end;
253
0xb0d09822,
0xc0ba6cad,
0xead54739,
0x94643b84,
0x0a00ae27,
0x6906c2fe,
0xfed41b76,
0x8ebeeff9,
0x38d8c2c4,
0x48b2364b,
0xdf60efc3,
0xbc66831a,
0x220216b9,
0x5cb36a04,
0x9b64c2b0,
0xeb0e363f,
0x7bb12bae,
0x0bdbdf21,
0x81be16cd,
0xff0f6a70,
0x616bffd3,
0x3903b3c2,
0xaed16a4a,
0xdebb9ec5,
0x53b39330,
0x23d967bf,
0xb40bbe37,
0xc7d7a8b4,
0xedb88320,
0x9dd277af,
0x0d6d6a3e,
0x7d079eb1,
0xf762575d,
0x89d32be0,
0x17b7be43,
0x4fdff252,
0xd80d2bda,
0xa867df55,
0x256fd2a0,
0x5505262f,
0xc2d7ffa7,
0xec63f226,
0x72076785,
0x0cb61b38,
0x86d3d2d4,
0xf6b9265b,
0x66063bca,
0x166ccf45,
0xa7672661,
0xd9d65adc,
0x47b2cf7f,
0x24b4a3a6,
0xb3667a2e,
0xc30c8ea1,
0x59b33d17,
0x9abfb3b6,
0x04db2615,
0x7a6a5aa8,
0xf00f9344,
0x806567cb,
0x10da7a5a,
0x60b08ed5,
0xd1bb67f1,
0xaf0a1b4c,
0x316e8eef,
0x5268e236,
0xc5ba3bbe,
0xb5d0cf31,
0x756aa39c,
0x05005713,
0x92d28e9b,
0xf1d4e242,
0x6fb077e1,
0x11010b5c,
0xa00ae278,
0xd06016f7,
0x40df0b66,
0x30b5ffe9,
0xbad03605,
0xc4614ab8,
0x5a05df1b,
crc = ~crc & 0xffffffff;
for (end = buf + len; buf < end; ++buf)
crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8);
return ~crc & 0xffffffff;
}
This computation does not apply to the “build ID” method.
18.4 Debugging information in a special section
Some systems ship pre-built executables and libraries that have a special ‘.gnu_debugdata’
section. This feature is called MiniDebugInfo. This section holds an LZMA-compressed
object and is used to supply extra symbols for backtraces.
The intent of this section is to provide extra minimal debugging information for use
in simple backtraces. It is not intended to be a replacement for full separate debugging
information (see Section 18.3 [Separate Debug Files], page 250). The example below shows
the intended use; however, gdb does not currently put restrictions on what sort of debugging
information might be included in the section.
gdb has support for this extension. If the section exists, then it is used provided that
no other source of debugging information can be found, and that gdb was configured with
LZMA support.
This section can be easily created using objcopy and other standard utilities:
# Extract the dynamic symbols from the main binary, there is no need
# to also have these in the normal symbol table.
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nm -D binary --format=posix --defined-only \
| awk ’{ print $1 }’ | sort > dynsyms
# Extract all the text (i.e. function) symbols from the debuginfo.
# (Note that we actually also accept "D" symbols, for the benefit
# of platforms like PowerPC64 that use function descriptors.)
nm binary --format=posix --defined-only \
| awk ’{ if ($2 == "T" || $2 == "t" || $2 == "D") print $1 }’ \
| sort > funcsyms
# Keep all the function symbols not already in the dynamic symbol
# table.
comm -13 dynsyms funcsyms > keep_symbols
# Separate full debug info into debug binary.
objcopy --only-keep-debug binary debug
# Copy the full debuginfo, keeping only a minimal set of symbols and
# removing some unnecessary sections.
objcopy -S --remove-section .gdb_index --remove-section .comment \
--keep-symbols=keep_symbols debug mini_debuginfo
# Drop the full debug info from the original binary.
strip --strip-all -R .comment binary
# Inject the compressed data into the .gnu_debugdata section of the
# original binary.
xz mini_debuginfo
objcopy --add-section .gnu_debugdata=mini_debuginfo.xz binary
18.5 Index Files Speed Up gdb
When gdb finds a symbol file, it scans the symbols in the file in order to construct an
internal symbol table. This lets most gdb operations work quickly—at the cost of a delay
early on. For large programs, this delay can be quite lengthy, so gdb provides a way to
build an index, which speeds up startup.
The index is stored as a section in the symbol file. gdb can write the index to a file,
then you can put it into the symbol file using objcopy.
To create an index file, use the save gdb-index command:
save gdb-index directory
Create an index file for each symbol file currently known by gdb. Each file is
named after its corresponding symbol file, with ‘.gdb-index’ appended, and is
written into the given directory.
Once you have created an index file you can merge it into your symbol file, here named
‘symfile’, using objcopy:
$ objcopy --add-section .gdb_index=symfile.gdb-index \
--set-section-flags .gdb_index=readonly symfile symfile
gdb will normally ignore older versions of ‘.gdb_index’ sections that have been deprecated. Usually they are deprecated because they are missing a new feature or have
performance issues. To tell gdb to use a deprecated index section anyway specify set usedeprecated-index-sections on. The default is off. This can speed up startup, but may
result in some functionality being lost. See Appendix J [Index Section Format], page 717.
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Warning: Setting use-deprecated-index-sections to on must be done before gdb
reads the file. The following will not work:
$ gdb -ex "set use-deprecated-index-sections on"
Instead you must do, for example,
$ gdb -iex "set use-deprecated-index-sections on"
There are currently some limitation on indices. They only work when for DWARF
debugging information, not stabs. And, they do not currently work for programs using
Ada.
18.6 Errors Reading Symbol Files
While reading a symbol file, gdb occasionally encounters problems, such as symbol types
it does not recognize, or known bugs in compiler output. By default, gdb does not notify
you of such problems, since they are relatively common and primarily of interest to people debugging compilers. If you are interested in seeing information about ill-constructed
symbol tables, you can either ask gdb to print only one message about each such type of
problem, no matter how many times the problem occurs; or you can ask gdb to print more
messages, to see how many times the problems occur, with the set complaints command
(see Section 22.8 [Optional Warnings and Messages], page 313).
The messages currently printed, and their meanings, include:
inner block not inside outer block in symbol
The symbol information shows where symbol scopes begin and end (such as at
the start of a function or a block of statements). This error indicates that an
inner scope block is not fully contained in its outer scope blocks.
gdb circumvents the problem by treating the inner block as if it had the same
scope as the outer block. In the error message, symbol may be shown as “(don’t
know)” if the outer block is not a function.
block at address out of order
The symbol information for symbol scope blocks should occur in order of increasing addresses. This error indicates that it does not do so.
gdb does not circumvent this problem, and has trouble locating symbols in
the source file whose symbols it is reading. (You can often determine what
source file is affected by specifying set verbose on. See Section 22.8 [Optional
Warnings and Messages], page 313.)
bad block start address patched
The symbol information for a symbol scope block has a start address smaller
than the address of the preceding source line. This is known to occur in the
SunOS 4.1.1 (and earlier) C compiler.
gdb circumvents the problem by treating the symbol scope block as starting
on the previous source line.
bad string table offset in symbol n
Symbol number n contains a pointer into the string table which is larger than
the size of the string table.
gdb circumvents the problem by considering the symbol to have the name foo,
which may cause other problems if many symbols end up with this name.
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unknown symbol type 0xnn
The symbol information contains new data types that gdb does not yet know
how to read. 0xnn is the symbol type of the uncomprehended information, in
hexadecimal.
gdb circumvents the error by ignoring this symbol information. This usually
allows you to debug your program, though certain symbols are not accessible. If
you encounter such a problem and feel like debugging it, you can debug gdb with
itself, breakpoint on complain, then go up to the function read_dbx_symtab
and examine *bufp to see the symbol.
stub type has NULL name
gdb could not find the full definition for a struct or class.
const/volatile indicator missing (ok if using g++ v1.x), got...
The symbol information for a C++ member function is missing some information
that recent versions of the compiler should have output for it.
info mismatch between compiler and debugger
gdb could not parse a type specification output by the compiler.
18.7 GDB Data Files
gdb will sometimes read an auxiliary data file. These files are kept in a directory known as
the data directory.
You can set the data directory’s name, and view the name gdb is currently using.
set data-directory directory
Set the directory which gdb searches for auxiliary data files to directory.
show data-directory
Show the directory gdb searches for auxiliary data files.
You can set the default data directory by using the configure-time ‘--with-gdb-datadir’
option. If the data directory is inside gdb’s configured binary prefix (set with ‘--prefix’
or ‘--exec-prefix’), then the default data directory will be updated automatically if the
installed gdb is moved to a new location.
The data directory may also be specified with the --data-directory command line
option. See Section 2.1.2 [Mode Options], page 13.
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19 Specifying a Debugging Target
A target is the execution environment occupied by your program.
Often, gdb runs in the same host environment as your program; in that case, the debugging target is specified as a side effect when you use the file or core commands.
When you need more flexibility—for example, running gdb on a physically separate host,
or controlling a standalone system over a serial port or a realtime system over a TCP/IP
connection—you can use the target command to specify one of the target types configured
for gdb (see Section 19.2 [Commands for Managing Targets], page 257).
It is possible to build gdb for several different target architectures. When gdb is built
like that, you can choose one of the available architectures with the set architecture
command.
set architecture arch
This command sets the current target architecture to arch. The value of arch
can be "auto", in addition to one of the supported architectures.
show architecture
Show the current target architecture.
set processor
processor
These are alias commands for, respectively, set architecture and show
architecture.
19.1 Active Targets
There are multiple classes of targets such as: processes, executable files or recording sessions.
Core files belong to the process class, making core file and process mutually exclusive.
Otherwise, gdb can work concurrently on multiple active targets, one in each class. This
allows you to (for example) start a process and inspect its activity, while still having access to
the executable file after the process finishes. Or if you start process recording (see Chapter 6
[Reverse Execution], page 85) and reverse-step there, you are presented a virtual layer of
the recording target, while the process target remains stopped at the chronologically last
point of the process execution.
Use the core-file and exec-file commands to select a new core file or executable
target (see Section 18.1 [Commands to Specify Files], page 241). To specify as a target a
process that is already running, use the attach command (see Section 4.7 [Debugging an
Already-running Process], page 32).
19.2 Commands for Managing Targets
target type parameters
Connects the gdb host environment to a target machine or process. A target
is typically a protocol for talking to debugging facilities. You use the argument
type to specify the type or protocol of the target machine.
Further parameters are interpreted by the target protocol, but typically include
things like device names or host names to connect with, process numbers, and
baud rates.
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The target command does not repeat if you press RET again after executing
the command.
help target
Displays the names of all targets available. To display targets currently selected,
use either info target or info files (see Section 18.1 [Commands to Specify
Files], page 241).
help target name
Describe a particular target, including any parameters necessary to select it.
set gnutarget args
gdb uses its own library BFD to read your files. gdb knows whether it is
reading an executable, a core, or a .o file; however, you can specify the file
format with the set gnutarget command. Unlike most target commands,
with gnutarget the target refers to a program, not a machine.
Warning: To specify a file format with set gnutarget, you must
know the actual BFD name.
See Section 18.1 [Commands to Specify Files], page 241.
show gnutarget
Use the show gnutarget command to display what file format gnutarget is set
to read. If you have not set gnutarget, gdb will determine the file format for
each file automatically, and show gnutarget displays ‘The current BFD target
is "auto"’.
Here are some common targets (available, or not, depending on the GDB configuration):
target exec program
An executable file. ‘target exec program’ is the same as ‘exec-file program’.
target core filename
A core dump file.
filename’.
‘target core filename’ is the same as ‘core-file
target remote medium
A remote system connected to gdb via a serial line or network connection. This
command tells gdb to use its own remote protocol over medium for debugging.
See Chapter 20 [Remote Debugging], page 261.
For example, if you have a board connected to ‘/dev/ttya’ on the machine
running gdb, you could say:
target remote /dev/ttya
target remote supports the load command. This is only useful if you have
some other way of getting the stub to the target system, and you can put it
somewhere in memory where it won’t get clobbered by the download.
target sim [simargs] ...
Builtin CPU simulator. gdb includes simulators for most architectures. In
general,
target sim
load
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run
works; however, you cannot assume that a specific memory map, device drivers,
or even basic I/O is available, although some simulators do provide these. For
info about any processor-specific simulator details, see the appropriate section
in Section 21.3 [Embedded Processors], page 291.
target native
Setup for local/native process debugging. Useful to make the run command
spawn native processes (likewise attach, etc.) even when set auto-connectnative-target is off (see [set auto-connect-native-target], page 28).
Different targets are available on different configurations of gdb; your configuration may
have more or fewer targets.
Many remote targets require you to download the executable’s code once you’ve successfully established a connection. You may wish to control various aspects of this process.
set hash
This command controls whether a hash mark ‘#’ is displayed while downloading
a file to the remote monitor. If on, a hash mark is displayed after each S-record
is successfully downloaded to the monitor.
show hash Show the current status of displaying the hash mark.
set debug monitor
Enable or disable display of communications messages between gdb and the
remote monitor.
show debug monitor
Show the current status of displaying communications between gdb and the
remote monitor.
load filename offset
Depending on what remote debugging facilities are configured into gdb, the
load command may be available. Where it exists, it is meant to make filename
(an executable) available for debugging on the remote system—by downloading,
or dynamic linking, for example. load also records the filename symbol table
in gdb, like the add-symbol-file command.
If your gdb does not have a load command, attempting to execute it gets the
error message “You can’t do that when your target is ...”
The file is loaded at whatever address is specified in the executable. For some
object file formats, you can specify the load address when you link the program;
for other formats, like a.out, the object file format specifies a fixed address.
It is also possible to tell gdb to load the executable file at a specific offset
described by the optional argument offset. When offset is provided, filename
must also be provided.
Depending on the remote side capabilities, gdb may be able to load programs
into flash memory.
load does not repeat if you press RET again after using it.
flash-erase
Erases all known flash memory regions on the target.
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19.3 Choosing Target Byte Order
Some types of processors, such as the MIPS, PowerPC, and Renesas SH, offer the ability
to run either big-endian or little-endian byte orders. Usually the executable or symbol will
include a bit to designate the endian-ness, and you will not need to worry about which to
use. However, you may still find it useful to adjust gdb’s idea of processor endian-ness
manually.
set endian big
Instruct gdb to assume the target is big-endian.
set endian little
Instruct gdb to assume the target is little-endian.
set endian auto
Instruct gdb to use the byte order associated with the executable.
show endian
Display gdb’s current idea of the target byte order.
Note that these commands merely adjust interpretation of symbolic data on the host,
and that they have absolutely no effect on the target system.
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20 Debugging Remote Programs
If you are trying to debug a program running on a machine that cannot run gdb in the
usual way, it is often useful to use remote debugging. For example, you might use remote
debugging on an operating system kernel, or on a small system which does not have a
general purpose operating system powerful enough to run a full-featured debugger.
Some configurations of gdb have special serial or TCP/IP interfaces to make this work
with particular debugging targets. In addition, gdb comes with a generic serial protocol
(specific to gdb, but not specific to any particular target system) which you can use if you
write the remote stubs—the code that runs on the remote system to communicate with
gdb.
Other remote targets may be available in your configuration of gdb; use help target
to list them.
20.1 Connecting to a Remote Target
This section describes how to connect to a remote target, including the types of connections
and their differences, how to set up executable and symbol files on the host and target, and
the commands used for connecting to and disconnecting from the remote target.
20.1.1 Types of Remote Connections
gdb supports two types of remote connections, target remote mode and target
extended-remote mode. Note that many remote targets support only target remote
mode. There are several major differences between the two types of connections,
enumerated here:
Result of detach or program exit
With target remote mode: When the debugged program exits or you detach
from it, gdb disconnects from the target. When using gdbserver, gdbserver
will exit.
With target extended-remote mode: When the debugged program exits or you
detach from it, gdb remains connected to the target, even though no program
is running. You can rerun the program, attach to a running program, or use
monitor commands specific to the target.
When using gdbserver in this case, it does not exit unless it was invoked
using the ‘--once’ option. If the ‘--once’ option was not used, you can ask
gdbserver to exit using the monitor exit command (see [Monitor Commands
for gdbserver], page 268).
Specifying the program to debug
For both connection types you use the file command to specify the program
on the host system. If you are using gdbserver there are some differences in
how to specify the location of the program on the target.
With target remote mode: You must either specify the program to debug on
the gdbserver command line or use the ‘--attach’ option (see [Attaching to
a Running Program], page 266).
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Debugging with gdb
With target extended-remote mode: You may specify the program to debug
on the gdbserver command line, or you can load the program or attach to it
using gdb commands after connecting to gdbserver.
You can start gdbserver without supplying an initial command to run or process ID to attach. To do this, use the ‘--multi’ command line option. Then
you can connect using target extended-remote and start the program you
want to debug (see below for details on using the run command in this scenario). Note that the conditions under which gdbserver terminates depend
on how gdb connects to it (target remote or target extended-remote). The
‘--multi’ option to gdbserver has no influence on that.
The run command
With target remote mode: The run command is not supported. Once a connection has been established, you can use all the usual gdb commands to examine
and change data. The remote program is already running, so you can use
commands like step and continue.
With target extended-remote mode: The run command is supported. The run
command uses the value set by set remote exec-file (see [set remote execfile], page 271) to select the program to run. Command line arguments are
supported, except for wildcard expansion and I/O redirection (see Section 4.3
[Arguments], page 30).
If you specify the program to debug on the command line, then the run command is not required to start execution, and you can resume using commands
like step and continue as with target remote mode.
Attaching With target remote mode: The gdb command attach is not supported. To
attach to a running program using gdbserver, you must use the ‘--attach’
option (see [Running gdbserver], page 265).
With target extended-remote mode: To attach to a running program, you may
use the attach command after the connection has been established. If you are
using gdbserver, you may also invoke gdbserver using the ‘--attach’ option
(see [Running gdbserver], page 265).
20.1.2 Host and Target Files
gdb, running on the host, needs access to symbol and debugging information for your
program running on the target. This requires access to an unstripped copy of your program,
and possibly any associated symbol files. Note that this section applies equally to both
target remote mode and target extended-remote mode.
Some remote targets (see [qXfer executable filename read], page 653, and see Section E.7
[Host I/O Packets], page 665) allow gdb to access program files over the same connection
used to communicate with gdb. With such a target, if the remote program is unstripped,
the only command you need is target remote (or target extended-remote).
If the remote program is stripped, or the target does not support remote program file
access, start up gdb using the name of the local unstripped copy of your program as the
first argument, or use the file command. Use set sysroot to specify the location (on
the host) of target libraries (unless your gdb was compiled with the correct sysroot using
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--with-sysroot). Alternatively, you may use set solib-search-path to specify how gdb
locates target libraries.
The symbol file and target libraries must exactly match the executable and libraries on
the target, with one exception: the files on the host system should not be stripped, even if
the files on the target system are. Mismatched or missing files will lead to confusing results
during debugging. On gnu/Linux targets, mismatched or missing files may also prevent
gdbserver from debugging multi-threaded programs.
20.1.3 Remote Connection Commands
gdb can communicate with the target over a serial line, or over an IP network using TCP
or UDP. In each case, gdb uses the same protocol for debugging your program; only the
medium carrying the debugging packets varies. The target remote and target extendedremote commands establish a connection to the target. Both commands accept the same
arguments, which indicate the medium to use:
target remote serial-device
target extended-remote serial-device
Use serial-device to communicate with the target. For example, to use a serial
line connected to the device named ‘/dev/ttyb’:
target remote /dev/ttyb
If you’re using a serial line, you may want to give gdb the ‘--baud’ option, or
use the set serial baud command (see Section 20.4 [Remote Configuration],
page 270) before the target command.
target
target
target
target
remote host:port
remote tcp:host:port
extended-remote host:port
extended-remote tcp:host:port
Debug using a TCP connection to port on host. The host may be either a host
name or a numeric IP address; port must be a decimal number. The host could
be the target machine itself, if it is directly connected to the net, or it might
be a terminal server which in turn has a serial line to the target.
For example, to connect to port 2828 on a terminal server named manyfarms:
target remote manyfarms:2828
If your remote target is actually running on the same machine as your debugger
session (e.g. a simulator for your target running on the same host), you can omit
the hostname. For example, to connect to port 1234 on your local machine:
target remote :1234
Note that the colon is still required here.
target remote udp:host:port
target extended-remote udp:host:port
Debug using UDP packets to port on host. For example, to connect to UDP
port 2828 on a terminal server named manyfarms:
target remote udp:manyfarms:2828
When using a UDP connection for remote debugging, you should keep in mind
that the ‘U’ stands for “Unreliable”. UDP can silently drop packets on busy or
unreliable networks, which will cause havoc with your debugging session.
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target remote | command
target extended-remote | command
Run command in the background and communicate with it using a pipe. The
command is a shell command, to be parsed and expanded by the system’s command shell, /bin/sh; it should expect remote protocol packets on its standard
input, and send replies on its standard output. You could use this to run a
stand-alone simulator that speaks the remote debugging protocol, to make net
connections using programs like ssh, or for other similar tricks.
If command closes its standard output (perhaps by exiting), gdb will try to
send it a SIGTERM signal. (If the program has already exited, this will have no
effect.)
Whenever gdb is waiting for the remote program, if you type the interrupt character
(often Ctrl-c), gdb attempts to stop the program. This may or may not succeed, depending
in part on the hardware and the serial drivers the remote system uses. If you type the
interrupt character once again, gdb displays this prompt:
Interrupted while waiting for the program.
Give up (and stop debugging it)? (y or n)
In target remote mode, if you type y, gdb abandons the remote debugging session. (If
you decide you want to try again later, you can use target remote again to connect once
more.) If you type n, gdb goes back to waiting.
In target extended-remote mode, typing n will leave gdb connected to the target.
detach
When you have finished debugging the remote program, you can use the detach
command to release it from gdb control. Detaching from the target normally resumes its execution, but the results will depend on your particular remote stub.
After the detach command in target remote mode, gdb is free to connect to
another target. In target extended-remote mode, gdb is still connected to
the target.
disconnect
The disconnect command closes the connection to the target, and the target
is generally not resumed. It will wait for gdb (this instance or another one)
to connect and continue debugging. After the disconnect command, gdb is
again free to connect to another target.
monitor cmd
This command allows you to send arbitrary commands directly to the remote
monitor. Since gdb doesn’t care about the commands it sends like this, this
command is the way to extend gdb—you can add new commands that only
the external monitor will understand and implement.
20.2 Sending files to a remote system
Some remote targets offer the ability to transfer files over the same connection used to
communicate with gdb. This is convenient for targets accessible through other means, e.g.
gnu/Linux systems running gdbserver over a network interface. For other targets, e.g.
embedded devices with only a single serial port, this may be the only way to upload or
download files.
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Not all remote targets support these commands.
remote put hostfile targetfile
Copy file hostfile from the host system (the machine running gdb) to targetfile
on the target system.
remote get targetfile hostfile
Copy file targetfile from the target system to hostfile on the host system.
remote delete targetfile
Delete targetfile from the target system.
20.3 Using the gdbserver Program
gdbserver is a control program for Unix-like systems, which allows you to connect your
program with a remote gdb via target remote or target extended-remote—but without
linking in the usual debugging stub.
gdbserver is not a complete replacement for the debugging stubs, because it requires
essentially the same operating-system facilities that gdb itself does. In fact, a system that
can run gdbserver to connect to a remote gdb could also run gdb locally! gdbserver is
sometimes useful nevertheless, because it is a much smaller program than gdb itself. It is
also easier to port than all of gdb, so you may be able to get started more quickly on a
new system by using gdbserver. Finally, if you develop code for real-time systems, you
may find that the tradeoffs involved in real-time operation make it more convenient to do
as much development work as possible on another system, for example by cross-compiling.
You can use gdbserver to make a similar choice for debugging.
gdb and gdbserver communicate via either a serial line or a TCP connection, using the
standard gdb remote serial protocol.
Warning: gdbserver does not have any built-in security. Do not run
gdbserver connected to any public network; a gdb connection to gdbserver
provides access to the target system with the same privileges as the user
running gdbserver.
20.3.1 Running gdbserver
Run gdbserver on the target system. You need a copy of the program you want to debug,
including any libraries it requires. gdbserver does not need your program’s symbol table,
so you can strip the program if necessary to save space. gdb on the host system does all
the symbol handling.
To use the server, you must tell it how to communicate with gdb; the name of your
program; and the arguments for your program. The usual syntax is:
target> gdbserver comm program [ args ... ]
comm is either a device name (to use a serial line), or a TCP hostname and portnumber,
or - or stdio to use stdin/stdout of gdbserver. For example, to debug Emacs with the
argument ‘foo.txt’ and communicate with gdb over the serial port ‘/dev/com1’:
target> gdbserver /dev/com1 emacs foo.txt
gdbserver waits passively for the host gdb to communicate with it.
To use a TCP connection instead of a serial line:
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target> gdbserver host:2345 emacs foo.txt
The only difference from the previous example is the first argument, specifying that you
are communicating with the host gdb via TCP. The ‘host:2345’ argument means that
gdbserver is to expect a TCP connection from machine ‘host’ to local TCP port 2345.
(Currently, the ‘host’ part is ignored.) You can choose any number you want for the port
number as long as it does not conflict with any TCP ports already in use on the target
system (for example, 23 is reserved for telnet).1 You must use the same port number with
the host gdb target remote command.
The stdio connection is useful when starting gdbserver with ssh:
(gdb) target remote | ssh -T hostname gdbserver - hello
The ‘-T’ option to ssh is provided because we don’t need a remote pty, and we don’t
want escape-character handling. Ssh does this by default when a command is provided, the
flag is provided to make it explicit. You could elide it if you want to.
Programs started with stdio-connected gdbserver have ‘/dev/null’ for stdin, and
stdout,stderr are sent back to gdb for display through a pipe connected to gdbserver.
Both stdout and stderr use the same pipe.
20.3.1.1 Attaching to a Running Program
On some targets, gdbserver can also attach to running programs. This is accomplished
via the --attach argument. The syntax is:
target> gdbserver --attach comm pid
pid is the process ID of a currently running process. It isn’t necessary to point gdbserver
at a binary for the running process.
In target extended-remote mode, you can also attach using the gdb attach command
(see [Attaching in Types of Remote Connections], page 262).
You can debug processes by name instead of process ID if your target has the pidof
utility:
target> gdbserver --attach comm ‘pidof program‘
In case more than one copy of program is running, or program has multiple threads,
most versions of pidof support the -s option to only return the first process ID.
20.3.1.2 TCP port allocation lifecycle of gdbserver
This section applies only when gdbserver is run to listen on a TCP port.
gdbserver normally terminates after all of its debugged processes have terminated in
target remote mode. On the other hand, for target extended-remote, gdbserver stays
running even with no processes left. gdb normally terminates the spawned debugged process on its exit, which normally also terminates gdbserver in the target remote mode.
Therefore, when the connection drops unexpectedly, and gdb cannot ask gdbserver to kill
its debugged processes, gdbserver stays running even in the target remote mode.
When gdbserver stays running, gdb can connect to it again later. Such reconnecting
is useful for features like [disconnected tracing], page 176. For completeness, at most one
gdb can be connected at a time.
1
If you choose a port number that conflicts with another service, gdbserver prints an error message and
exits.
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By default, gdbserver keeps the listening TCP port open, so that subsequent connections are possible. However, if you start gdbserver with the ‘--once’ option, it will stop
listening for any further connection attempts after connecting to the first gdb session. This
means no further connections to gdbserver will be possible after the first one. It also means
gdbserver will terminate after the first connection with remote gdb has closed, even for
unexpectedly closed connections and even in the target extended-remote mode. The
‘--once’ option allows reusing the same port number for connecting to multiple instances
of gdbserver running on the same host, since each instance closes its port after the first
connection.
20.3.1.3 Other Command-Line Arguments for gdbserver
You can use the ‘--multi’ option to start gdbserver without specifying a program to debug
or a process to attach to. Then you can attach in target extended-remote mode and run
or attach to a program. For more information, see [–multi Option in Types of Remote
Connnections], page 262.
The ‘--debug’ option tells gdbserver to display extra status information about the debugging process. The ‘--remote-debug’ option tells gdbserver to display remote protocol
debug output. These options are intended for gdbserver development and for bug reports
to the developers.
The ‘--debug-format=option1[,option2,...]’ option tells gdbserver to include additional information in each output. Possible options are:
none
Turn off all extra information in debugging output.
all
Turn on all extra information in debugging output.
timestamps
Include a timestamp in each line of debugging output.
Options are processed in order. Thus, for example, if ‘none’ appears last then no additional information is added to debugging output.
The ‘--wrapper’ option specifies a wrapper to launch programs for debugging. The
option should be followed by the name of the wrapper, then any command-line arguments
to pass to the wrapper, then -- indicating the end of the wrapper arguments.
gdbserver runs the specified wrapper program with a combined command line including
the wrapper arguments, then the name of the program to debug, then any arguments to the
program. The wrapper runs until it executes your program, and then gdb gains control.
You can use any program that eventually calls execve with its arguments as a wrapper.
Several standard Unix utilities do this, e.g. env and nohup. Any Unix shell script ending
with exec "$@" will also work.
For example, you can use env to pass an environment variable to the debugged program,
without setting the variable in gdbserver’s environment:
$ gdbserver --wrapper env LD_PRELOAD=libtest.so -- :2222 ./testprog
20.3.2 Connecting to gdbserver
The basic procedure for connecting to the remote target is:
• Run gdb on the host system.
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• Make sure you have the necessary symbol files (see [Host and target files], page 262).
Load symbols for your application using the file command before you connect. Use
set sysroot to locate target libraries (unless your gdb was compiled with the correct
sysroot using --with-sysroot).
• Connect to your target (see Section 20.1 [Connecting to a Remote Target], page 261).
For TCP connections, you must start up gdbserver prior to using the target command. Otherwise you may get an error whose text depends on the host system, but
which usually looks something like ‘Connection refused’. Don’t use the load command in gdb when using target remote mode, since the program is already on the
target.
20.3.3 Monitor Commands for gdbserver
During a gdb session using gdbserver, you can use the monitor command to send special
requests to gdbserver. Here are the available commands.
monitor help
List the available monitor commands.
monitor set debug 0
monitor set debug 1
Disable or enable general debugging messages.
monitor set remote-debug 0
monitor set remote-debug 1
Disable or enable specific debugging messages associated with the remote protocol (see Appendix E [Remote Protocol], page 619).
monitor set debug-format option1[,option2,...]
Specify additional text to add to debugging messages. Possible options are:
none
Turn off all extra information in debugging output.
all
Turn on all extra information in debugging output.
timestamps
Include a timestamp in each line of debugging output.
Options are processed in order. Thus, for example, if ‘none’ appears last then
no additional information is added to debugging output.
monitor set libthread-db-search-path [PATH]
When this command is issued, path is a colon-separated list of directories
to search for libthread_db (see Section 4.10 [set libthread-db-search-path],
page 36). If you omit path, ‘libthread-db-search-path’ will be reset to its
default value.
The special entry ‘$pdir’ for ‘libthread-db-search-path’ is not supported in
gdbserver.
monitor exit
Tell gdbserver to exit immediately. This command should be followed by
disconnect to close the debugging session. gdbserver will detach from any
attached processes and kill any processes it created. Use monitor exit to terminate gdbserver at the end of a multi-process mode debug session.
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20.3.4 Tracepoints support in gdbserver
On some targets, gdbserver supports tracepoints, fast tracepoints and static tracepoints.
For fast or static tracepoints to work, a special library called the in-process agent (IPA),
must be loaded in the inferior process. This library is built and distributed as an integral part of gdbserver. In addition, support for static tracepoints requires building the
in-process agent library with static tracepoints support. At present, the UST (LTTng
Userspace Tracer, http://lttng.org/ust) tracing engine is supported. This support is
automatically available if UST development headers are found in the standard include path
when gdbserver is built, or if gdbserver was explicitly configured using ‘--with-ust’ to
point at such headers. You can explicitly disable the support using ‘--with-ust=no’.
There are several ways to load the in-process agent in your program:
Specifying it as dependency at link time
You can link your program dynamically with the in-process agent library. On
most systems, this is accomplished by adding -linproctrace to the link command.
Using the system’s preloading mechanisms
You can force loading the in-process agent at startup time by using your system’s support for preloading shared libraries. Many Unixes support the concept
of preloading user defined libraries. In most cases, you do that by specifying
LD_PRELOAD=libinproctrace.so in the environment. See also the description
of gdbserver’s ‘--wrapper’ command line option.
Using gdb to force loading the agent at run time
On some systems, you can force the inferior to load a shared library, by calling
a dynamic loader function in the inferior that takes care of dynamically looking
up and loading a shared library. On most Unix systems, the function is dlopen.
You’ll use the call command for that. For example:
(gdb) call dlopen ("libinproctrace.so", ...)
Note that on most Unix systems, for the dlopen function to be available, the
program needs to be linked with -ldl.
On systems that have a userspace dynamic loader, like most Unix systems, when you
connect to gdbserver using target remote, you’ll find that the program is stopped at
the dynamic loader’s entry point, and no shared library has been loaded in the program’s
address space yet, including the in-process agent. In that case, before being able to use any
of the fast or static tracepoints features, you need to let the loader run and load the shared
libraries. The simplest way to do that is to run the program to the main procedure. E.g.,
if debugging a C or C++ program, start gdbserver like so:
$ gdbserver :9999 myprogram
Start GDB and connect to gdbserver like so, and run to main:
$ gdb myprogram
(gdb) target remote myhost:9999
0x00007f215893ba60 in ?? () from /lib64/ld-linux-x86-64.so.2
(gdb) b main
(gdb) continue
The in-process tracing agent library should now be loaded into the process; you can
confirm it with the info sharedlibrary command, which will list ‘libinproctrace.so’
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Debugging with gdb
as loaded in the process. You are now ready to install fast tracepoints, list static tracepoint
markers, probe static tracepoints markers, and start tracing.
20.4 Remote Configuration
This section documents the configuration options available when debugging remote programs. For the options related to the File I/O extensions of the remote protocol, see
[system], page 679.
set remoteaddresssize bits
Set the maximum size of address in a memory packet to the specified number
of bits. gdb will mask off the address bits above that number, when it passes
addresses to the remote target. The default value is the number of bits in the
target’s address.
show remoteaddresssize
Show the current value of remote address size in bits.
set serial baud n
Set the baud rate for the remote serial I/O to n baud. The value is used to set
the speed of the serial port used for debugging remote targets.
show serial baud
Show the current speed of the remote connection.
set serial parity parity
Set the parity for the remote serial I/O. Supported values of parity are: even,
none, and odd. The default is none.
show serial parity
Show the current parity of the serial port.
set remotebreak
If set to on, gdb sends a BREAK signal to the remote when you type Ctrl-c
to interrupt the program running on the remote. If set to off, gdb sends the
‘Ctrl-C’ character instead. The default is off, since most remote systems expect
to see ‘Ctrl-C’ as the interrupt signal.
show remotebreak
Show whether gdb sends BREAK or ‘Ctrl-C’ to interrupt the remote program.
set remoteflow on
set remoteflow off
Enable or disable hardware flow control (RTS/CTS) on the serial port used to
communicate to the remote target.
show remoteflow
Show the current setting of hardware flow control.
set remotelogbase base
Set the base (a.k.a. radix) of logging serial protocol communications to base.
Supported values of base are: ascii, octal, and hex. The default is ascii.
show remotelogbase
Show the current setting of the radix for logging remote serial protocol.
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set remotelogfile file
Record remote serial communications on the named file. The default is not to
record at all.
show remotelogfile.
Show the current setting of the file name on which to record the serial communications.
set remotetimeout num
Set the timeout limit to wait for the remote target to respond to num seconds.
The default is 2 seconds.
show remotetimeout
Show the current number of seconds to wait for the remote target responses.
set remote hardware-watchpoint-limit limit
set remote hardware-breakpoint-limit limit
Restrict gdb to using limit remote hardware breakpoint or watchpoints. A
limit of -1, the default, is treated as unlimited.
set remote hardware-watchpoint-length-limit limit
Restrict gdb to using limit bytes for the maximum length of a remote hardware
watchpoint. A limit of -1, the default, is treated as unlimited.
show remote hardware-watchpoint-length-limit
Show the current limit (in bytes) of the maximum length of a remote hardware
watchpoint.
set remote exec-file filename
show remote exec-file
Select the file used for run with target extended-remote. This should be set
to a filename valid on the target system. If it is not set, the target will use a
default filename (e.g. the last program run).
set remote interrupt-sequence
Allow the user to select one of ‘Ctrl-C’, a BREAK or ‘BREAK-g’ as the sequence
to the remote target in order to interrupt the execution. ‘Ctrl-C’ is a default.
Some system prefers BREAK which is high level of serial line for some certain
time. Linux kernel prefers ‘BREAK-g’, a.k.a Magic SysRq g. It is BREAK signal
followed by character g.
show interrupt-sequence
Show which of ‘Ctrl-C’, BREAK or BREAK-g is sent by gdb to interrupt the
remote program. BREAK-g is BREAK signal followed by g and also known as
Magic SysRq g.
set remote interrupt-on-connect
Specify whether interrupt-sequence is sent to remote target when gdb connects
to it. This is mostly needed when you debug Linux kernel. Linux kernel expects
BREAK followed by g which is known as Magic SysRq g in order to connect gdb.
show interrupt-on-connect
Show whether interrupt-sequence is sent to remote target when gdb connects
to it.
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Debugging with gdb
set tcp auto-retry on
Enable auto-retry for remote TCP connections. This is useful if the remote
debugging agent is launched in parallel with gdb; there is a race condition
because the agent may not become ready to accept the connection before gdb
attempts to connect. When auto-retry is enabled, if the initial attempt to
connect fails, gdb reattempts to establish the connection using the timeout
specified by set tcp connect-timeout.
set tcp auto-retry off
Do not auto-retry failed TCP connections.
show tcp auto-retry
Show the current auto-retry setting.
set tcp connect-timeout seconds
set tcp connect-timeout unlimited
Set the timeout for establishing a TCP connection to the remote target to
seconds. The timeout affects both polling to retry failed connections (enabled
by set tcp auto-retry on) and waiting for connections that are merely slow
to complete, and represents an approximate cumulative value. If seconds is
unlimited, there is no timeout and gdb will keep attempting to establish a
connection forever, unless interrupted with Ctrl-c. The default is 15 seconds.
show tcp connect-timeout
Show the current connection timeout setting.
The gdb remote protocol autodetects the packets supported by your debugging stub. If
you need to override the autodetection, you can use these commands to enable or disable
individual packets. Each packet can be set to ‘on’ (the remote target supports this packet),
‘off’ (the remote target does not support this packet), or ‘auto’ (detect remote target
support for this packet). They all default to ‘auto’. For more information about each
packet, see Appendix E [Remote Protocol], page 619.
During normal use, you should not have to use any of these commands. If you do, that
may be a bug in your remote debugging stub, or a bug in gdb. You may want to report
the problem to the gdb developers.
For each packet name, the command to enable or disable the packet is set remote
name-packet. The available settings are:
Command Name
Remote Packet
Related Features
fetch-register
p
info registers
set-register
P
set
binary-download
X
load, set
read-aux-vector
qXfer:auxv:read
info auxv
symbol-lookup
qSymbol
Detecting
threads
multiple
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attach
vAttach
attach
verbose-resume
vCont
Stepping or resuming multiple threads
run
vRun
run
software-breakpoint
Z0
break
hardware-breakpoint
Z1
hbreak
write-watchpoint
Z2
watch
read-watchpoint
Z3
rwatch
access-watchpoint
Z4
awatch
pid-to-exec-file
qXfer:exec-file:read
attach, run
target-features
qXfer:features:read
set architecture
library-info
qXfer:libraries:read
info
sharedlibrary
memory-map
qXfer:memory-map:read
info mem
read-sdata-object
qXfer:sdata:read
print $_sdata
read-spu-object
qXfer:spu:read
info spu
write-spu-object
qXfer:spu:write
info spu
read-siginfo-object
qXfer:siginfo:read
print $_siginfo
write-siginfo-object
qXfer:siginfo:write
set $_siginfo
threads
qXfer:threads:read
info threads
get-thread-localstorage-address
qGetTLSAddr
Displaying
__thread variables
get-threadinformation-blockaddress
qGetTIBAddr
Display
MSWindows
Thread
Information Block.
search-memory
qSearch:memory
find
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supported-packets
qSupported
Remote
munications
parameters
com-
catch-syscalls
QCatchSyscalls
catch syscall
pass-signals
QPassSignals
handle signal
program-signals
QProgramSignals
handle signal
hostio-close-packet
vFile:close
remote get, remote
put
hostio-open-packet
vFile:open
remote get, remote
put
hostio-pread-packet
vFile:pread
remote get, remote
put
hostio-pwrite-packet
vFile:pwrite
remote get, remote
put
hostio-unlink-packet
vFile:unlink
remote delete
hostio-readlinkpacket
hostio-fstat-packet
vFile:readlink
Host I/O
vFile:fstat
Host I/O
hostio-setfs-packet
vFile:setfs
Host I/O
noack-packet
QStartNoAckMode
Packet acknowledgment
osdata
qXfer:osdata:read
info os
query-attached
qAttached
Querying
process
state.
trace-buffer-size
QTBuffer:size
set trace-buffersize
trace-status
qTStatus
tstatus
traceframe-info
qXfer:traceframeinfo:read
Traceframe info
remote
attach
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install-in-trace
InstallInTrace
Install tracepoint in
tracing
disable-randomization
QDisableRandomization
set disablerandomization
conditionalbreakpoints-packet
Z0 and Z1
Support for
target-side
breakpoint
condition
evaluation
multiprocessextensions
multiprocess
extensions
Debug multiple processes and remote
process PID awareness
swbreak-feature
swbreak stop reason
break
hwbreak-feature
hwbreak stop reason
hbreak
fork-event-feature
fork stop reason
fork
vfork-event-feature
vfork stop reason
vfork
exec-event-feature
exec stop reason
exec
thread-events
QThreadEvents
Tracking thread lifetime.
no-resumed-stop-reply
no resumed thread left
stop reply
Tracking thread lifetime.
20.5 Implementing a Remote Stub
The stub files provided with gdb implement the target side of the communication protocol,
and the gdb side is implemented in the gdb source file ‘remote.c’. Normally, you can simply
allow these subroutines to communicate, and ignore the details. (If you’re implementing
your own stub file, you can still ignore the details: start with one of the existing stub files.
‘sparc-stub.c’ is the best organized, and therefore the easiest to read.)
To debug a program running on another machine (the debugging target machine), you
must first arrange for all the usual prerequisites for the program to run by itself. For
example, for a C program, you need:
1. A startup routine to set up the C runtime environment; these usually have a name like
‘crt0’. The startup routine may be supplied by your hardware supplier, or you may
have to write your own.
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Debugging with gdb
2. A C subroutine library to support your program’s subroutine calls, notably managing
input and output.
3. A way of getting your program to the other machine—for example, a download program. These are often supplied by the hardware manufacturer, but you may have to
write your own from hardware documentation.
The next step is to arrange for your program to use a serial port to communicate with
the machine where gdb is running (the host machine). In general terms, the scheme looks
like this:
On the host,
gdb already understands how to use this protocol; when everything else is
set up, you can simply use the ‘target remote’ command (see Chapter 19
[Specifying a Debugging Target], page 257).
On the target,
you must link with your program a few special-purpose subroutines that implement the gdb remote serial protocol. The file containing these subroutines is
called a debugging stub.
On certain remote targets, you can use an auxiliary program gdbserver instead
of linking a stub into your program. See Section 20.3 [Using the gdbserver
Program], page 265, for details.
The debugging stub is specific to the architecture of the remote machine; for example,
use ‘sparc-stub.c’ to debug programs on sparc boards.
These working remote stubs are distributed with gdb:
i386-stub.c
For Intel 386 and compatible architectures.
m68k-stub.c
For Motorola 680x0 architectures.
sh-stub.c
For Renesas SH architectures.
sparc-stub.c
For sparc architectures.
sparcl-stub.c
For Fujitsu sparclite architectures.
The ‘README’ file in the gdb distribution may list other recently added stubs.
20.5.1 What the Stub Can Do for You
The debugging stub for your architecture supplies these three subroutines:
set_debug_traps
This routine arranges for handle_exception to run when your program stops.
You must call this subroutine explicitly in your program’s startup code.
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handle_exception
This is the central workhorse, but your program never calls it explicitly—the
setup code arranges for handle_exception to run when a trap is triggered.
handle_exception takes control when your program stops during execution
(for example, on a breakpoint), and mediates communications with gdb on
the host machine. This is where the communications protocol is implemented;
handle_exception acts as the gdb representative on the target machine. It
begins by sending summary information on the state of your program, then continues to execute, retrieving and transmitting any information gdb needs, until
you execute a gdb command that makes your program resume; at that point,
handle_exception returns control to your own code on the target machine.
breakpoint
Use this auxiliary subroutine to make your program contain a breakpoint. Depending on the particular situation, this may be the only way for gdb to get
control. For instance, if your target machine has some sort of interrupt button,
you won’t need to call this; pressing the interrupt button transfers control to
handle_exception—in effect, to gdb. On some machines, simply receiving
characters on the serial port may also trigger a trap; again, in that situation,
you don’t need to call breakpoint from your own program—simply running
‘target remote’ from the host gdb session gets control.
Call breakpoint if none of these is true, or if you simply want to make certain
your program stops at a predetermined point for the start of your debugging
session.
20.5.2 What You Must Do for the Stub
The debugging stubs that come with gdb are set up for a particular chip architecture, but
they have no information about the rest of your debugging target machine.
First of all you need to tell the stub how to communicate with the serial port.
int getDebugChar()
Write this subroutine to read a single character from the serial port. It may be
identical to getchar for your target system; a different name is used to allow
you to distinguish the two if you wish.
void putDebugChar(int)
Write this subroutine to write a single character to the serial port. It may be
identical to putchar for your target system; a different name is used to allow
you to distinguish the two if you wish.
If you want gdb to be able to stop your program while it is running, you need to use
an interrupt-driven serial driver, and arrange for it to stop when it receives a ^C (‘\003’,
the control-C character). That is the character which gdb uses to tell the remote system
to stop.
Getting the debugging target to return the proper status to gdb probably requires
changes to the standard stub; one quick and dirty way is to just execute a breakpoint
instruction (the “dirty” part is that gdb reports a SIGTRAP instead of a SIGINT).
Other routines you need to supply are:
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Debugging with gdb
void exceptionHandler (int exception_number, void *exception_address)
Write this function to install exception address in the exception handling tables. You need to do this because the stub does not have any way of knowing
what the exception handling tables on your target system are like (for example, the processor’s table might be in rom, containing entries which point to a
table in ram). The exception number specifies the exception which should be
changed; its meaning is architecture-dependent (for example, different numbers
might represent divide by zero, misaligned access, etc). When this exception
occurs, control should be transferred directly to exception address, and the
processor state (stack, registers, and so on) should be just as it is when a processor exception occurs. So if you want to use a jump instruction to reach
exception address, it should be a simple jump, not a jump to subroutine.
For the 386, exception address should be installed as an interrupt gate so that
interrupts are masked while the handler runs. The gate should be at privilege
level 0 (the most privileged level). The sparc and 68k stubs are able to mask
interrupts themselves without help from exceptionHandler.
void flush_i_cache()
On sparc and sparclite only, write this subroutine to flush the instruction
cache, if any, on your target machine. If there is no instruction cache, this
subroutine may be a no-op.
On target machines that have instruction caches, gdb requires this function to
make certain that the state of your program is stable.
You must also make sure this library routine is available:
void *memset(void *, int, int)
This is the standard library function memset that sets an area of memory to a
known value. If you have one of the free versions of libc.a, memset can be found
there; otherwise, you must either obtain it from your hardware manufacturer,
or write your own.
If you do not use the GNU C compiler, you may need other standard library subroutines
as well; this varies from one stub to another, but in general the stubs are likely to use any
of the common library subroutines which gcc generates as inline code.
20.5.3 Putting it All Together
In summary, when your program is ready to debug, you must follow these steps.
1. Make sure you have defined the supporting low-level routines (see Section 20.5.2 [What
You Must Do for the Stub], page 277):
getDebugChar, putDebugChar,
flush_i_cache, memset, exceptionHandler.
2. Insert these lines in your program’s startup code, before the main procedure is called:
set_debug_traps();
breakpoint();
On some machines, when a breakpoint trap is raised, the hardware automatically makes
the PC point to the instruction after the breakpoint. If your machine doesn’t do that,
you may need to adjust handle_exception to arrange for it to return to the instruction
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after the breakpoint on this first invocation, so that your program doesn’t keep hitting
the initial breakpoint instead of making progress.
3. For the 680x0 stub only, you need to provide a variable called exceptionHook. Normally you just use:
void (*exceptionHook)() = 0;
4.
5.
6.
7.
but if before calling set_debug_traps, you set it to point to a function in your program,
that function is called when gdb continues after stopping on a trap (for example, bus
error). The function indicated by exceptionHook is called with one parameter: an int
which is the exception number.
Compile and link together: your program, the gdb debugging stub for your target
architecture, and the supporting subroutines.
Make sure you have a serial connection between your target machine and the gdb host,
and identify the serial port on the host.
Download your program to your target machine (or get it there by whatever means the
manufacturer provides), and start it.
Start gdb on the host, and connect to the target (see Section 20.1 [Connecting to a
Remote Target], page 261).
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281
21 Configuration-Specific Information
While nearly all gdb commands are available for all native and cross versions of the debugger, there are some exceptions. This chapter describes things that are only available in
certain configurations.
There are three major categories of configurations: native configurations, where the host
and target are the same, embedded operating system configurations, which are usually the
same for several different processor architectures, and bare embedded processors, which are
quite different from each other.
21.1 Native
This section describes details specific to particular native configurations.
21.1.1 BSD libkvm Interface
BSD-derived systems (FreeBSD/NetBSD/OpenBSD) have a kernel memory interface that
provides a uniform interface for accessing kernel virtual memory images, including live
systems and crash dumps. gdb uses this interface to allow you to debug live kernels and
kernel crash dumps on many native BSD configurations. This is implemented as a special
kvm debugging target. For debugging a live system, load the currently running kernel into
gdb and connect to the kvm target:
(gdb) target kvm
For debugging crash dumps, provide the file name of the crash dump as an argument:
(gdb) target kvm /var/crash/bsd.0
Once connected to the kvm target, the following commands are available:
kvm pcb
Set current context from the Process Control Block (PCB) address.
kvm proc
Set current context from proc address. This command isn’t available on modern
FreeBSD systems.
21.1.2 SVR4 Process Information
Many versions of SVR4 and compatible systems provide a facility called ‘/proc’ that can
be used to examine the image of a running process using file-system subroutines.
If gdb is configured for an operating system with this facility, the command info proc
is available to report information about the process running your program, or about any
process running on your system. This includes, as of this writing, gnu/Linux and Solaris,
for example.
This command may also work on core files that were created on a system that has the
‘/proc’ facility.
info proc
info proc process-id
Summarize available information about any running process. If a process ID
is specified by process-id, display information about that process; otherwise
display information about the program being debugged. The summary includes
the debugged process ID, the command line used to invoke it, its current working
directory, and its executable file’s absolute file name.
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On some systems, process-id can be of the form ‘[pid]/tid’ which specifies
a certain thread ID within a process. If the optional pid part is missing, it
means a thread from the process being debugged (the leading ‘/’ still needs to
be present, or else gdb will interpret the number as a process ID rather than a
thread ID).
info proc cmdline
Show the original command line of the process. This command is specific to
gnu/Linux.
info proc cwd
Show the current working directory of the process. This command is specific
to gnu/Linux.
info proc exe
Show the name of executable of the process. This command is specific to
gnu/Linux.
info proc mappings
Report the memory address space ranges accessible in the program, with information on whether the process has read, write, or execute access rights to
each range. On gnu/Linux systems, each memory range includes the object
file which is mapped to that range, instead of the memory access rights to that
range.
info proc stat
info proc status
These subcommands are specific to gnu/Linux systems. They show the processrelated information, including the user ID and group ID; how many threads are
there in the process; its virtual memory usage; the signals that are pending,
blocked, and ignored; its TTY; its consumption of system and user time; its
stack size; its ‘nice’ value; etc. For more information, see the ‘proc’ man page
(type man 5 proc from your shell prompt).
info proc all
Show all the information about the process described under all of the above
info proc subcommands.
set procfs-trace
This command enables and disables tracing of procfs API calls.
show procfs-trace
Show the current state of procfs API call tracing.
set procfs-file file
Tell gdb to write procfs API trace to the named file. gdb appends the trace
info to the previous contents of the file. The default is to display the trace on
the standard output.
show procfs-file
Show the file to which procfs API trace is written.
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proc-trace-entry
proc-trace-exit
proc-untrace-entry
proc-untrace-exit
These commands enable and disable tracing of entries into and exits from the
syscall interface.
info pidlist
For QNX Neutrino only, this command displays the list of all the processes and
all the threads within each process.
info meminfo
For QNX Neutrino only, this command displays the list of all mapinfos.
21.1.3 Features for Debugging djgpp Programs
djgpp is a port of the gnu development tools to MS-DOS and MS-Windows. djgpp
programs are 32-bit protected-mode programs that use the DPMI (DOS Protected-Mode
Interface) API to run on top of real-mode DOS systems and their emulations.
gdb supports native debugging of djgpp programs, and defines a few commands specific
to the djgpp port. This subsection describes those commands.
info dos
This is a prefix of djgpp-specific commands which print information about the
target system and important OS structures.
info dos sysinfo
This command displays assorted information about the underlying platform:
the CPU type and features, the OS version and flavor, the DPMI version, and
the available conventional and DPMI memory.
info dos gdt
info dos ldt
info dos idt
These 3 commands display entries from, respectively, Global, Local, and Interrupt Descriptor Tables (GDT, LDT, and IDT). The descriptor tables are data
structures which store a descriptor for each segment that is currently in use.
The segment’s selector is an index into a descriptor table; the table entry for
that index holds the descriptor’s base address and limit, and its attributes and
access rights.
A typical djgpp program uses 3 segments: a code segment, a data segment
(used for both data and the stack), and a DOS segment (which allows access to
DOS/BIOS data structures and absolute addresses in conventional memory).
However, the DPMI host will usually define additional segments in order to
support the DPMI environment.
These commands allow to display entries from the descriptor tables. Without
an argument, all entries from the specified table are displayed. An argument,
which should be an integer expression, means display a single entry whose index
is given by the argument. For example, here’s a convenient way to display
information about the debugged program’s data segment:
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(gdb) info dos ldt $ds
0x13f: base=0x11970000 limit=0x0009ffff 32-Bit Data (Read/Write, Exp-up)
This comes in handy when you want to see whether a pointer is outside the
data segment’s limit (i.e. garbled).
info dos pde
info dos pte
These two commands display entries from, respectively, the Page Directory and
the Page Tables. Page Directories and Page Tables are data structures which
control how virtual memory addresses are mapped into physical addresses. A
Page Table includes an entry for every page of memory that is mapped into the
program’s address space; there may be several Page Tables, each one holding
up to 4096 entries. A Page Directory has up to 4096 entries, one each for every
Page Table that is currently in use.
Without an argument, info dos pde displays the entire Page Directory, and
info dos pte displays all the entries in all of the Page Tables. An argument,
an integer expression, given to the info dos pde command means display only
that entry from the Page Directory table. An argument given to the info dos
pte command means display entries from a single Page Table, the one pointed
to by the specified entry in the Page Directory.
These commands are useful when your program uses DMA (Direct Memory
Access), which needs physical addresses to program the DMA controller.
These commands are supported only with some DPMI servers.
info dos address-pte addr
This command displays the Page Table entry for a specified linear address. The
argument addr is a linear address which should already have the appropriate
segment’s base address added to it, because this command accepts addresses
which may belong to any segment. For example, here’s how to display the Page
Table entry for the page where a variable i is stored:
(gdb) info dos address-pte __djgpp_base_address + (char *)&i
Page Table entry for address 0x11a00d30:
Base=0x02698000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0xd30
This says that i is stored at offset 0xd30 from the page whose physical base
address is 0x02698000, and shows all the attributes of that page.
Note that you must cast the addresses of variables to a char *, since otherwise
the value of __djgpp_base_address, the base address of all variables and functions in a djgpp program, will be added using the rules of C pointer arithmetics:
if i is declared an int, gdb will add 4 times the value of __djgpp_base_address
to the address of i.
Here’s another example, it displays the Page Table entry for the transfer buffer:
(gdb) info dos address-pte *((unsigned *)&_go32_info_block + 3)
Page Table entry for address 0x29110:
Base=0x00029000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0x110
(The + 3 offset is because the transfer buffer’s address is the 3rd member of the
_go32_info_block structure.) The output clearly shows that this DPMI server
maps the addresses in conventional memory 1:1, i.e. the physical (0x00029000
+ 0x110) and linear (0x29110) addresses are identical.
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This command is supported only with some DPMI servers.
In addition to native debugging, the DJGPP port supports remote debugging via a serial
data link. The following commands are specific to remote serial debugging in the DJGPP
port of gdb.
set com1base addr
This command sets the base I/O port address of the ‘COM1’ serial port.
set com1irq irq
This command sets the Interrupt Request (IRQ) line to use for the ‘COM1’ serial
port.
There are similar commands ‘set com2base’, ‘set com3irq’, etc. for setting the
port address and the IRQ lines for the other 3 COM ports.
The related commands ‘show com1base’, ‘show com1irq’ etc. display the current settings of the base address and the IRQ lines used by the COM ports.
info serial
This command prints the status of the 4 DOS serial ports. For each port,
it prints whether it’s active or not, its I/O base address and IRQ number,
whether it uses a 16550-style FIFO, its baudrate, and the counts of various
errors encountered so far.
21.1.4 Features for Debugging MS Windows PE Executables
gdb supports native debugging of MS Windows programs, including DLLs with and without
symbolic debugging information.
MS-Windows programs that call SetConsoleMode to switch off the special meaning of
the ‘Ctrl-C’ keystroke cannot be interrupted by typing C-c. For this reason, gdb on MSWindows supports C-BREAK as an alternative interrupt key sequence, which can be used to
interrupt the debuggee even if it ignores C-c.
There are various additional Cygwin-specific commands, described in this section. Working with DLLs that have no debugging symbols is described in Section 21.1.4.1 [Non-debug
DLL Symbols], page 287.
info w32
This is a prefix of MS Windows-specific commands which print information
about the target system and important OS structures.
info w32 selector
This command displays information returned by the Win32 API
GetThreadSelectorEntry function. It takes an optional argument that is
evaluated to a long value to give the information about this given selector.
Without argument, this command displays information about the six segment
registers.
info w32 thread-information-block
This command displays thread specific information stored in the Thread Information Block (readable on the X86 CPU family using $fs selector for 32-bit
programs and $gs for 64-bit programs).
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signal-event id
This command signals an event with user-provided id. Used to resume crashing
process when attached to it using MS-Windows JIT debugging (AeDebug).
To use it, create or edit the following keys in HKLM\SOFTWARE\Microsoft\Windows
NT\CurrentVersion\AeDebug and/or HKLM\SOFTWARE\Wow6432Node\Microsoft\Windows
NT\CurrentVersion\AeDebug (for x86 64 versions):
− Debugger (REG SZ) — a command to launch the debugger. Suggested
command is: fully-qualified-path-to-gdb.exe -ex "attach %ld" -ex
"signal-event %ld" -ex "continue".
The first %ld will be replaced by the process ID of the crashing process, the
second %ld will be replaced by the ID of the event that blocks the crashing
process, waiting for gdb to attach.
− Auto (REG SZ) — either 1 or 0. 1 will make the system run debugger
specified by the Debugger key automatically, 0 will cause a dialog box with
“OK” and “Cancel” buttons to appear, which allows the user to either
terminate the crashing process (OK) or debug it (Cancel).
set cygwin-exceptions mode
If mode is on, gdb will break on exceptions that happen inside the Cygwin DLL.
If mode is off, gdb will delay recognition of exceptions, and may ignore some
exceptions which seem to be caused by internal Cygwin DLL “bookkeeping”.
This option is meant primarily for debugging the Cygwin DLL itself; the default
value is off to avoid annoying gdb users with false SIGSEGV signals.
show cygwin-exceptions
Displays whether gdb will break on exceptions that happen inside the Cygwin
DLL itself.
set new-console mode
If mode is on the debuggee will be started in a new console on next start. If
mode is off, the debuggee will be started in the same console as the debugger.
show new-console
Displays whether a new console is used when the debuggee is started.
set new-group mode
This boolean value controls whether the debuggee should start a new group or
stay in the same group as the debugger. This affects the way the Windows OS
handles ‘Ctrl-C’.
show new-group
Displays current value of new-group boolean.
set debugevents
This boolean value adds debug output concerning kernel events related to the
debuggee seen by the debugger. This includes events that signal thread and
process creation and exit, DLL loading and unloading, console interrupts, and
debugging messages produced by the Windows OutputDebugString API call.
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set debugexec
This boolean value adds debug output concerning execute events (such as resume thread) seen by the debugger.
set debugexceptions
This boolean value adds debug output concerning exceptions in the debuggee
seen by the debugger.
set debugmemory
This boolean value adds debug output concerning debuggee memory reads and
writes by the debugger.
set shell This boolean values specifies whether the debuggee is called via a shell or directly (default value is on).
show shell
Displays if the debuggee will be started with a shell.
21.1.4.1 Support for DLLs without Debugging Symbols
Very often on windows, some of the DLLs that your program relies on do not include symbolic debugging information (for example, ‘kernel32.dll’). When gdb doesn’t recognize
any debugging symbols in a DLL, it relies on the minimal amount of symbolic information
contained in the DLL’s export table. This section describes working with such symbols,
known internally to gdb as “minimal symbols”.
Note that before the debugged program has started execution, no DLLs will have been
loaded. The easiest way around this problem is simply to start the program — either by
setting a breakpoint or letting the program run once to completion.
21.1.4.2 DLL Name Prefixes
In keeping with the naming conventions used by the Microsoft debugging tools, DLL
export symbols are made available with a prefix based on the DLL name, for instance
KERNEL32!CreateFileA. The plain name is also entered into the symbol table, so
CreateFileA is often sufficient. In some cases there will be name clashes within a program
(particularly if the executable itself includes full debugging symbols) necessitating the use
of the fully qualified name when referring to the contents of the DLL. Use single-quotes
around the name to avoid the exclamation mark (“!”) being interpreted as a language
operator.
Note that the internal name of the DLL may be all upper-case, even though the file
name of the DLL is lower-case, or vice-versa. Since symbols within gdb are case-sensitive
this may cause some confusion. If in doubt, try the info functions and info variables
commands or even maint print msymbols (see Chapter 16 [Symbols], page 221). Here’s an
example:
(gdb) info function CreateFileA
All functions matching regular expression "CreateFileA":
Non-debugging symbols:
0x77e885f4 CreateFileA
0x77e885f4 KERNEL32!CreateFileA
(gdb) info function !
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Debugging with gdb
All functions matching regular expression "!":
Non-debugging symbols:
0x6100114c cygwin1!__assert
0x61004034 cygwin1!_dll_crt0@0
0x61004240 cygwin1!dll_crt0(per_process *)
[etc...]
21.1.4.3 Working with Minimal Symbols
Symbols extracted from a DLL’s export table do not contain very much type information.
All that gdb can do is guess whether a symbol refers to a function or variable depending
on the linker section that contains the symbol. Also note that the actual contents of the
memory contained in a DLL are not available unless the program is running. This means
that you cannot examine the contents of a variable or disassemble a function within a DLL
without a running program.
Variables are generally treated as pointers and dereferenced automatically. For this
reason, it is often necessary to prefix a variable name with the address-of operator (“&”)
and provide explicit type information in the command. Here’s an example of the type of
problem:
(gdb) print ’cygwin1!__argv’
$1 = 268572168
(gdb) x ’cygwin1!__argv’
0x10021610:
"\230y\""
And two possible solutions:
(gdb) print ((char **)’cygwin1!__argv’)[0]
$2 = 0x22fd98 "/cygdrive/c/mydirectory/myprogram"
(gdb) x/2x &’cygwin1!__argv’
0x610c0aa8 :
0x10021608
0x00000000
(gdb) x/x 0x10021608
0x10021608:
0x0022fd98
(gdb) x/s 0x0022fd98
0x22fd98:
"/cygdrive/c/mydirectory/myprogram"
Setting a break point within a DLL is possible even before the program starts execution. However, under these circumstances, gdb can’t examine the initial instructions of the
function in order to skip the function’s frame set-up code. You can work around this by
using “*&” to set the breakpoint at a raw memory address:
(gdb) break *&’python22!PyOS_Readline’
Breakpoint 1 at 0x1e04eff0
The author of these extensions is not entirely convinced that setting a break point within
a shared DLL like ‘kernel32.dll’ is completely safe.
21.1.5 Commands Specific to gnu Hurd Systems
This subsection describes gdb commands specific to the gnu Hurd native debugging.
set signals
set sigs This command toggles the state of inferior signal interception by gdb. Mach
exceptions, such as breakpoint traps, are not affected by this command. sigs
is a shorthand alias for signals.
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show signals
show sigs Show the current state of intercepting inferior’s signals.
set signal-thread
set sigthread
This command tells gdb which thread is the libc signal thread. That thread
is run when a signal is delivered to a running process. set sigthread is the
shorthand alias of set signal-thread.
show signal-thread
show sigthread
These two commands show which thread will run when the inferior is delivered
a signal.
set stopped
This commands tells gdb that the inferior process is stopped, as with the
SIGSTOP signal. The stopped process can be continued by delivering a signal
to it.
show stopped
This command shows whether gdb thinks the debuggee is stopped.
set exceptions
Use this command to turn off trapping of exceptions in the inferior. When
exception trapping is off, neither breakpoints nor single-stepping will work. To
restore the default, set exception trapping on.
show exceptions
Show the current state of trapping exceptions in the inferior.
set task pause
This command toggles task suspension when gdb has control. Setting it to on
takes effect immediately, and the task is suspended whenever gdb gets control.
Setting it to off will take effect the next time the inferior is continued. If this
option is set to off, you can use set thread default pause on or set thread
pause on (see below) to pause individual threads.
show task pause
Show the current state of task suspension.
set task detach-suspend-count
This command sets the suspend count the task will be left with when gdb
detaches from it.
show task detach-suspend-count
Show the suspend count the task will be left with when detaching.
set task exception-port
set task excp
This command sets the task exception port to which gdb will forward exceptions. The argument should be the value of the send rights of the task. set
task excp is a shorthand alias.
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set noninvasive
This command switches gdb to a mode that is the least invasive as far as
interfering with the inferior is concerned. This is the same as using set task
pause, set exceptions, and set signals to values opposite to the defaults.
info
info
info
info
info
info
info
send-rights
receive-rights
port-rights
port-sets
dead-names
ports
psets
These commands display information about, respectively, send rights, receive
rights, port rights, port sets, and dead names of a task. There are also shorthand
aliases: info ports for info port-rights and info psets for info portsets.
set thread pause
This command toggles current thread suspension when gdb has control. Setting
it to on takes effect immediately, and the current thread is suspended whenever
gdb gets control. Setting it to off will take effect the next time the inferior is
continued. Normally, this command has no effect, since when gdb has control,
the whole task is suspended. However, if you used set task pause off (see
above), this command comes in handy to suspend only the current thread.
show thread pause
This command shows the state of current thread suspension.
set thread run
This command sets whether the current thread is allowed to run.
show thread run
Show whether the current thread is allowed to run.
set thread detach-suspend-count
This command sets the suspend count gdb will leave on a thread when detaching. This number is relative to the suspend count found by gdb when it
notices the thread; use set thread takeover-suspend-count to force it to an
absolute value.
show thread detach-suspend-count
Show the suspend count gdb will leave on the thread when detaching.
set thread exception-port
set thread excp
Set the thread exception port to which to forward exceptions. This overrides
the port set by set task exception-port (see above). set thread excp is the
shorthand alias.
set thread takeover-suspend-count
Normally, gdb’s thread suspend counts are relative to the value gdb finds
when it notices each thread. This command changes the suspend counts to be
absolute instead.
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set thread default
show thread default
Each of the above set thread commands has a set thread default counterpart (e.g., set thread default pause, set thread default exception-port,
etc.). The thread default variety of commands sets the default thread properties for all threads; you can then change the properties of individual threads
with the non-default commands.
21.1.6 Darwin
gdb provides the following commands specific to the Darwin target:
set debug darwin num
When set to a non zero value, enables debugging messages specific to the Darwin
support. Higher values produce more verbose output.
show debug darwin
Show the current state of Darwin messages.
set debug mach-o num
When set to a non zero value, enables debugging messages while gdb is reading
Darwin object files. (Mach-O is the file format used on Darwin for object
and executable files.) Higher values produce more verbose output. This is a
command to diagnose problems internal to gdb and should not be needed in
normal usage.
show debug mach-o
Show the current state of Mach-O file messages.
set mach-exceptions on
set mach-exceptions off
On Darwin, faults are first reported as a Mach exception and are then mapped
to a Posix signal. Use this command to turn on trapping of Mach exceptions
in the inferior. This might be sometimes useful to better understand the cause
of a fault. The default is off.
show mach-exceptions
Show the current state of exceptions trapping.
21.2 Embedded Operating Systems
This section describes configurations involving the debugging of embedded operating systems that are available for several different architectures.
gdb includes the ability to debug programs running on various real-time operating systems.
21.3 Embedded Processors
This section goes into details specific to particular embedded configurations.
Whenever a specific embedded processor has a simulator, gdb allows to send an arbitrary
command to the simulator.
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sim command
Send an arbitrary command string to the simulator. Consult the documentation
for the specific simulator in use for information about acceptable commands.
21.3.1 Synopsys ARC
gdb provides the following ARC-specific commands:
set debug arc
Control the level of ARC specific debug messages. Use 0 for no messages (the
default), 1 for debug messages, and 2 for even more debug messages.
show debug arc
Show the level of ARC specific debugging in operation.
maint print arc arc-instruction address
Print internal disassembler information about instruction at a given address.
21.3.2 ARM
gdb provides the following ARM-specific commands:
set arm disassembler
This commands selects from a list of disassembly styles. The "std" style is the
standard style.
show arm disassembler
Show the current disassembly style.
set arm apcs32
This command toggles ARM operation mode between 32-bit and 26-bit.
show arm apcs32
Display the current usage of the ARM 32-bit mode.
set arm fpu fputype
This command sets the ARM floating-point unit (FPU) type. The argument
fputype can be one of these:
auto
Determine the FPU type by querying the OS ABI.
softfpa
Software FPU, with mixed-endian doubles on little-endian ARM
processors.
fpa
GCC-compiled FPA co-processor.
softvfp
Software FPU with pure-endian doubles.
vfp
VFP co-processor.
show arm fpu
Show the current type of the FPU.
set arm abi
This command forces gdb to use the specified ABI.
show arm abi
Show the currently used ABI.
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set arm fallback-mode (arm|thumb|auto)
gdb uses the symbol table, when available, to determine whether instructions
are ARM or Thumb. This command controls gdb’s default behavior when the
symbol table is not available. The default is ‘auto’, which causes gdb to use
the current execution mode (from the T bit in the CPSR register).
show arm fallback-mode
Show the current fallback instruction mode.
set arm force-mode (arm|thumb|auto)
This command overrides use of the symbol table to determine whether instructions are ARM or Thumb. The default is ‘auto’, which causes gdb to use the
symbol table and then the setting of ‘set arm fallback-mode’.
show arm force-mode
Show the current forced instruction mode.
set debug arm
Toggle whether to display ARM-specific debugging messages from the ARM
target support subsystem.
show debug arm
Show whether ARM-specific debugging messages are enabled.
target sim [simargs] ...
The gdb ARM simulator accepts the following optional arguments.
--swi-support=type
Tell the simulator which SWI interfaces to support. The argument
type may be a comma separated list of the following values. The
default value is all.
none
demon
angel
redboot
all
21.3.3 M68k
The Motorola m68k configuration includes ColdFire support.
21.3.4 MicroBlaze
The MicroBlaze is a soft-core processor supported on various Xilinx FPGAs, such as Spartan or Virtex series. Boards with these processors usually have JTAG ports which connect
to a host system running the Xilinx Embedded Development Kit (EDK) or Software Development Kit (SDK). This host system is used to download the configuration bitstream
to the target FPGA. The Xilinx Microprocessor Debugger (XMD) program communicates
with the target board using the JTAG interface and presents a gdbserver interface to the
board. By default xmd uses port 1234. (While it is possible to change this default port, it
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requires the use of undocumented xmd commands. Contact Xilinx support if you need to
do this.)
Use these GDB commands to connect to the MicroBlaze target processor.
target remote :1234
Use this command to connect to the target if you are running gdb on the same
system as xmd.
target remote xmd-host:1234
Use this command to connect to the target if it is connected to xmd running on
a different system named xmd-host.
load
Use this command to download a program to the MicroBlaze target.
set debug microblaze n
Enable MicroBlaze-specific debugging messages if non-zero.
show debug microblaze n
Show MicroBlaze-specific debugging level.
21.3.5 MIPS Embedded
gdb supports these special commands for MIPS targets:
set mipsfpu double
set mipsfpu single
set mipsfpu none
set mipsfpu auto
show mipsfpu
If your target board does not support the MIPS floating point coprocessor, you
should use the command ‘set mipsfpu none’ (if you need this, you may wish to
put the command in your gdb init file). This tells gdb how to find the return
value of functions which return floating point values. It also allows gdb to avoid
saving the floating point registers when calling functions on the board. If you
are using a floating point coprocessor with only single precision floating point
support, as on the r4650 processor, use the command ‘set mipsfpu single’.
The default double precision floating point coprocessor may be selected using
‘set mipsfpu double’.
In previous versions the only choices were double precision or no floating point,
so ‘set mipsfpu on’ will select double precision and ‘set mipsfpu off’ will select no floating point.
As usual, you can inquire about the mipsfpu variable with ‘show mipsfpu’.
21.3.6 PowerPC Embedded
gdb supports using the DVC (Data Value Compare) register to implement in hardware
simple hardware watchpoint conditions of the form:
(gdb) watch ADDRESS|VARIABLE \
if ADDRESS|VARIABLE == CONSTANT EXPRESSION
The DVC register will be automatically used when gdb detects such pattern in a condition expression, and the created watchpoint uses one debug register (either the exactwatchpoints option is on and the variable is scalar, or the variable has a length of one
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byte). This feature is available in native gdb running on a Linux kernel version 2.6.34 or
newer.
When running on PowerPC embedded processors, gdb automatically uses ranged hardware watchpoints, unless the exact-watchpoints option is on, in which case watchpoints
using only one debug register are created when watching variables of scalar types.
You can create an artificial array to watch an arbitrary memory region using one of the
following commands (see Section 10.1 [Expressions], page 117):
(gdb) watch *((char *) address)@length
(gdb) watch {char[length]} address
PowerPC embedded processors support masked watchpoints. See the discussion about
the mask argument in Section 5.1.2 [Set Watchpoints], page 52.
PowerPC embedded processors support hardware accelerated ranged breakpoints. A
ranged breakpoint stops execution of the inferior whenever it executes an instruction at
any address within the range it specifies. To set a ranged breakpoint in gdb, use the
break-range command.
gdb provides the following PowerPC-specific commands:
break-range start-location, end-location
Set a breakpoint for an address range given by start-location and end-location,
which can specify a function name, a line number, an offset of lines from the
current line or from the start location, or an address of an instruction (see
Section 9.2 [Specify Location], page 104, for a list of all the possible ways to
specify a location.) The breakpoint will stop execution of the inferior whenever
it executes an instruction at any address within the specified range, (including
start-location and end-location.)
set powerpc soft-float
show powerpc soft-float
Force gdb to use (or not use) a software floating point calling convention. By
default, gdb selects the calling convention based on the selected architecture
and the provided executable file.
set powerpc vector-abi
show powerpc vector-abi
Force gdb to use the specified calling convention for vector arguments and
return values. The valid options are ‘auto’; ‘generic’, to avoid vector registers
even if they are present; ‘altivec’, to use AltiVec registers; and ‘spe’ to use
SPE registers. By default, gdb selects the calling convention based on the
selected architecture and the provided executable file.
set powerpc exact-watchpoints
show powerpc exact-watchpoints
Allow gdb to use only one debug register when watching a variable of scalar
type, thus assuming that the variable is accessed through the address of its first
byte.
21.3.7 Atmel AVR
When configured for debugging the Atmel AVR, gdb supports the following AVR-specific
commands:
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info io_registers
This command displays information about the AVR I/O registers. For each
register, gdb prints its number and value.
21.3.8 CRIS
When configured for debugging CRIS, gdb provides the following CRIS-specific commands:
set cris-version ver
Set the current CRIS version to ver, either ‘10’ or ‘32’. The CRIS version affects
register names and sizes. This command is useful in case autodetection of the
CRIS version fails.
show cris-version
Show the current CRIS version.
set cris-dwarf2-cfi
Set the usage of DWARF-2 CFI for CRIS debugging. The default is ‘on’.
Change to ‘off’ when using gcc-cris whose version is below R59.
show cris-dwarf2-cfi
Show the current state of using DWARF-2 CFI.
set cris-mode mode
Set the current CRIS mode to mode. It should only be changed when debugging
in guru mode, in which case it should be set to ‘guru’ (the default is ‘normal’).
show cris-mode
Show the current CRIS mode.
21.3.9 Renesas Super-H
For the Renesas Super-H processor, gdb provides these commands:
set sh calling-convention convention
Set the calling-convention used when calling functions from gdb. Allowed values are ‘gcc’, which is the default setting, and ‘renesas’. With the ‘gcc’ setting,
functions are called using the gcc calling convention. If the DWARF-2 information of the called function specifies that the function follows the Renesas
calling convention, the function is called using the Renesas calling convention.
If the calling convention is set to ‘renesas’, the Renesas calling convention is
always used, regardless of the DWARF-2 information. This can be used to
override the default of ‘gcc’ if debug information is missing, or the compiler
does not emit the DWARF-2 calling convention entry for a function.
show sh calling-convention
Show the current calling convention setting.
21.4 Architectures
This section describes characteristics of architectures that affect all uses of gdb with the
architecture, both native and cross.
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21.4.1 AArch64
When gdb is debugging the AArch64 architecture, it provides the following special commands:
set debug aarch64
This command determines whether AArch64 architecture-specific debugging
messages are to be displayed.
show debug aarch64
Show whether AArch64 debugging messages are displayed.
21.4.2 x86 Architecture-specific Issues
set struct-convention mode
Set the convention used by the inferior to return structs and unions from
functions to mode. Possible values of mode are "pcc", "reg", and "default"
(the default). "default" or "pcc" means that structs are returned on the
stack, while "reg" means that a struct or a union whose size is 1, 2, 4, or 8
bytes will be returned in a register.
show struct-convention
Show the current setting of the convention to return structs from functions.
21.4.2.1 Intel Memory Protection Extensions (MPX).
Memory Protection Extension (MPX) adds the bound registers ‘BND0’1 through ‘BND3’.
Bound registers store a pair of 64-bit values which are the lower bound and upper bound.
Bounds are effective addresses or memory locations. The upper bounds are architecturally
represented in 1’s complement form. A bound having lower bound = 0, and upper bound
= 0 (1’s complement of all bits set) will allow access to the entire address space.
‘BND0’ through ‘BND3’ are represented in gdb as ‘bnd0raw’ through ‘bnd3raw’. Pseudo
registers ‘bnd0’ through ‘bnd3’ display the upper bound performing the complement of one
operation on the upper bound value, i.e. when upper bound in ‘bnd0raw’ is 0 in the gdb
‘bnd0’ it will be 0xfff.... In this sense it can also be noted that the upper bounds are
inclusive.
As an example, assume that the register BND0 holds bounds for a pointer having access
allowed for the range between 0x32 and 0x71. The values present on bnd0raw and bnd
registers are presented as follows:
bnd0raw = {0x32, 0xffffffff8e}
bnd0 = {lbound = 0x32, ubound = 0x71} : size 64
This way the raw value can be accessed via bnd0raw. . . bnd3raw. Any change on
bnd0. . . bnd3 or bnd0raw. . . bnd3raw is reflect on its counterpart. When the bnd0. . . bnd3
registers are displayed via Python, the display includes the memory size, in bits, accessible
to the pointer.
Bounds can also be stored in bounds tables, which are stored in application memory.
These tables store bounds for pointers by specifying the bounds pointer’s value along with
its bounds. Evaluating and changing bounds located in bound tables is therefore interesting
while investigating bugs on MPX context. gdb provides commands for this purpose:
1
The register named with capital letters represent the architecture registers.
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show mpx bound pointer
Display bounds of the given pointer.
set mpx bound pointer, lbound, ubound
Set the bounds of a pointer in the bound table. This command takes three
parameters: pointer is the pointers whose bounds are to be changed, lbound
and ubound are new values for lower and upper bounds respectively.
When you call an inferior function on an Intel MPX enabled program, GDB sets the
inferior’s bound registers to the init (disabled) state before calling the function. As a
consequence, bounds checks for the pointer arguments passed to the function will always
pass.
This is necessary because when you call an inferior function, the program is usually in
the middle of the execution of other function. Since at that point bound registers are in
an arbitrary state, not clearing them would lead to random bound violations in the called
function.
You can still examine the influence of the bound registers on the execution of the called
function by stopping the execution of the called function at its prologue, setting bound
registers, and continuing the execution. For example:
$ break *upper
Breakpoint 2 at 0x4009de: file i386-mpx-call.c, line 47.
$ print upper (a, b, c, d, 1)
Breakpoint 2, upper (a=0x0, b=0x6e0000005b, c=0x0, d=0x0, len=48)....
$ print $bnd0
{lbound = 0x0, ubound = ffffffff} : size -1
At this last step the value of bnd0 can be changed for investigation of bound violations
caused along the execution of the call. In order to know how to set the bound registers or
bound table for the call consult the ABI.
21.4.3 Alpha
See the following section.
21.4.4 MIPS
Alpha- and MIPS-based computers use an unusual stack frame, which sometimes requires
gdb to search backward in the object code to find the beginning of a function.
To improve response time (especially for embedded applications, where gdb may be
restricted to a slow serial line for this search) you may want to limit the size of this search,
using one of these commands:
set heuristic-fence-post limit
Restrict gdb to examining at most limit bytes in its search for the beginning
of a function. A value of 0 (the default) means there is no limit. However,
except for 0, the larger the limit the more bytes heuristic-fence-post must
search and therefore the longer it takes to run. You should only need to use
this command when debugging a stripped executable.
show heuristic-fence-post
Display the current limit.
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These commands are available only when gdb is configured for debugging programs on
Alpha or MIPS processors.
Several MIPS-specific commands are available when debugging MIPS programs:
set mips abi arg
Tell gdb which MIPS ABI is used by the inferior. Possible values of arg are:
‘auto’
The default ABI associated with the current binary (this is the
default).
‘o32’
‘o64’
‘n32’
‘n64’
‘eabi32’
‘eabi64’
show mips abi
Show the MIPS ABI used by gdb to debug the inferior.
set mips compression arg
Tell gdb which MIPS compressed ISA (Instruction Set Architecture) encoding
is used by the inferior. gdb uses this for code disassembly and other internal
interpretation purposes. This setting is only referred to when no executable has
been associated with the debugging session or the executable does not provide
information about the encoding it uses. Otherwise this setting is automatically
updated from information provided by the executable.
Possible values of arg are ‘mips16’ and ‘micromips’. The default compressed
ISA encoding is ‘mips16’, as executables containing MIPS16 code frequently are
not identified as such.
This setting is “sticky”; that is, it retains its value across debugging sessions
until reset either explicitly with this command or implicitly from an executable.
The compiler and/or assembler typically add symbol table annotations to identify functions compiled for the MIPS16 or microMIPS ISAs. If these functionscope annotations are present, gdb uses them in preference to the global compressed ISA encoding setting.
show mips compression
Show the MIPS compressed ISA encoding used by gdb to debug the inferior.
set mipsfpu
show mipsfpu
See Section 21.3.5 [MIPS Embedded], page 294.
set mips mask-address arg
This command determines whether the most-significant 32 bits of 64-bit MIPS
addresses are masked off. The argument arg can be ‘on’, ‘off’, or ‘auto’. The
latter is the default setting, which lets gdb determine the correct value.
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show mips mask-address
Show whether the upper 32 bits of MIPS addresses are masked off or not.
set remote-mips64-transfers-32bit-regs
This command controls compatibility with 64-bit MIPS targets that transfer
data in 32-bit quantities. If you have an old MIPS 64 target that transfers 32
bits for some registers, like sr and fsr, and 64 bits for other registers, set this
option to ‘on’.
show remote-mips64-transfers-32bit-regs
Show the current setting of compatibility with older MIPS 64 targets.
set debug mips
This command turns on and off debugging messages for the MIPS-specific target
code in gdb.
show debug mips
Show the current setting of MIPS debugging messages.
21.4.5 HPPA
When gdb is debugging the HP PA architecture, it provides the following special commands:
set debug hppa
This command determines whether HPPA architecture-specific debugging messages are to be displayed.
show debug hppa
Show whether HPPA debugging messages are displayed.
maint print unwind address
This command displays the contents of the unwind table entry at the given
address.
21.4.6 Cell Broadband Engine SPU architecture
When gdb is debugging the Cell Broadband Engine SPU architecture, it provides the
following special commands:
info spu event
Display SPU event facility status. Shows current event mask and pending event
status.
info spu signal
Display SPU signal notification facility status. Shows pending signal-control
word and signal notification mode of both signal notification channels.
info spu mailbox
Display SPU mailbox facility status. Shows all pending entries, in order of processing, in each of the SPU Write Outbound, SPU Write Outbound Interrupt,
and SPU Read Inbound mailboxes.
info spu dma
Display MFC DMA status. Shows all pending commands in the MFC DMA
queue. For each entry, opcode, tag, class IDs, effective and local store addresses
and transfer size are shown.
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info spu proxydma
Display MFC Proxy-DMA status. Shows all pending commands in the MFC
Proxy-DMA queue. For each entry, opcode, tag, class IDs, effective and local
store addresses and transfer size are shown.
When gdb is debugging a combined PowerPC/SPU application on the Cell Broadband
Engine, it provides in addition the following special commands:
set spu stop-on-load arg
Set whether to stop for new SPE threads. When set to on, gdb will give control
to the user when a new SPE thread enters its main function. The default is
off.
show spu stop-on-load
Show whether to stop for new SPE threads.
set spu auto-flush-cache arg
Set whether to automatically flush the software-managed cache. When set to on,
gdb will automatically cause the SPE software-managed cache to be flushed
whenever SPE execution stops. This provides a consistent view of PowerPC
memory that is accessed via the cache. If an application does not use the
software-managed cache, this option has no effect.
show spu auto-flush-cache
Show whether to automatically flush the software-managed cache.
21.4.7 PowerPC
When gdb is debugging the PowerPC architecture, it provides a set of pseudo-registers to
enable inspection of 128-bit wide Decimal Floating Point numbers stored in the floating
point registers. These values must be stored in two consecutive registers, always starting
at an even register like f0 or f2.
The pseudo-registers go from $dl0 through $dl15, and are formed by joining the
even/odd register pairs f0 and f1 for $dl0, f2 and f3 for $dl1 and so on.
For POWER7 processors, gdb provides a set of pseudo-registers, the 64-bit wide Extended Floating Point Registers (‘f32’ through ‘f63’).
21.4.8 Nios II
When gdb is debugging the Nios II architecture, it provides the following special commands:
set debug nios2
This command turns on and off debugging messages for the Nios II target code
in gdb.
show debug nios2
Show the current setting of Nios II debugging messages.
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22 Controlling gdb
You can alter the way gdb interacts with you by using the set command. For commands
controlling how gdb displays data, see Section 10.8 [Print Settings], page 127. Other settings
are described here.
22.1 Prompt
gdb indicates its readiness to read a command by printing a string called the prompt.
This string is normally ‘(gdb)’. You can change the prompt string with the set prompt
command. For instance, when debugging gdb with gdb, it is useful to change the prompt
in one of the gdb sessions so that you can always tell which one you are talking to.
Note: set prompt does not add a space for you after the prompt you set. This allows
you to set a prompt which ends in a space or a prompt that does not.
set prompt newprompt
Directs gdb to use newprompt as its prompt string henceforth.
show prompt
Prints a line of the form: ‘Gdb’s prompt is: your-prompt’
Versions of gdb that ship with Python scripting enabled have prompt extensions. The
commands for interacting with these extensions are:
set extended-prompt prompt
Set an extended prompt that allows for substitutions. See Section 23.2.4.3
[gdb.prompt], page 400, for a list of escape sequences that can be used for
substitution. Any escape sequences specified as part of the prompt string are
replaced with the corresponding strings each time the prompt is displayed.
For example:
set extended-prompt Current working directory: \w (gdb)
Note that when an extended-prompt is set, it takes control of the prompt hook
hook. See [prompt hook], page 332, for further information.
show extended-prompt
Prints the extended prompt. Any escape sequences specified as part of the
prompt string with set extended-prompt, are replaced with the corresponding
strings each time the prompt is displayed.
22.2 Command Editing
gdb reads its input commands via the Readline interface. This gnu library provides consistent behavior for programs which provide a command line interface to the user. Advantages
are gnu Emacs-style or vi-style inline editing of commands, csh-like history substitution,
and a storage and recall of command history across debugging sessions.
You may control the behavior of command line editing in gdb with the command set.
set editing
set editing on
Enable command line editing (enabled by default).
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Debugging with gdb
set editing off
Disable command line editing.
show editing
Show whether command line editing is enabled.
See Chapter 32 [Command Line Editing], page 573, for more details about the Readline
interface. Users unfamiliar with gnu Emacs or vi are encouraged to read that chapter.
22.3 Command History
gdb can keep track of the commands you type during your debugging sessions, so that
you can be certain of precisely what happened. Use these commands to manage the gdb
command history facility.
gdb uses the gnu History library, a part of the Readline package, to provide the history
facility. See Chapter 33 [Using History Interactively], page 595, for the detailed description
of the History library.
To issue a command to gdb without affecting certain aspects of the state which is seen
by users, prefix it with ‘server ’ (see Section 28.2 [Server Prefix], page 558). This means
that this command will not affect the command history, nor will it affect gdb’s notion of
which command to repeat if RET is pressed on a line by itself.
The server prefix does not affect the recording of values into the value history; to print
a value without recording it into the value history, use the output command instead of the
print command.
Here is the description of gdb commands related to command history.
set history filename fname
Set the name of the gdb command history file to fname. This is the file where
gdb reads an initial command history list, and where it writes the command
history from this session when it exits. You can access this list through history
expansion or through the history command editing characters listed below.
This file defaults to the value of the environment variable GDBHISTFILE, or to
‘./.gdb_history’ (‘./_gdb_history’ on MS-DOS) if this variable is not set.
set history save
set history save on
Record command history in a file, whose name may be specified with the set
history filename command. By default, this option is disabled.
set history save off
Stop recording command history in a file.
set history size size
set history size unlimited
Set the number of commands which gdb keeps in its history list. This defaults
to the value of the environment variable GDBHISTSIZE, or to 256 if this variable
is not set. Non-numeric values of GDBHISTSIZE are ignored. If size is unlimited
or if GDBHISTSIZE is either a negative number or the empty string, then the
number of commands gdb keeps in the history list is unlimited.
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set history remove-duplicates count
set history remove-duplicates unlimited
Control the removal of duplicate history entries in the command history list.
If count is non-zero, gdb will look back at the last count history entries and
remove the first entry that is a duplicate of the current entry being added to the
command history list. If count is unlimited then this lookbehind is unbounded.
If count is 0, then removal of duplicate history entries is disabled.
Only history entries added during the current session are considered for removal.
This option is set to 0 by default.
History expansion assigns special meaning to the character !. See Section 33.1.1 [Event
Designators], page 595, for more details.
Since ! is also the logical not operator in C, history expansion is off by default. If you
decide to enable history expansion with the set history expansion on command, you may
sometimes need to follow ! (when it is used as logical not, in an expression) with a space
or a tab to prevent it from being expanded. The readline history facilities do not attempt
substitution on the strings != and !(, even when history expansion is enabled.
The commands to control history expansion are:
set history expansion on
set history expansion
Enable history expansion. History expansion is off by default.
set history expansion off
Disable history expansion.
show
show
show
show
show
history
history filename
history save
history size
history expansion
These commands display the state of the gdb history parameters.
history by itself displays all four states.
show
show commands
Display the last ten commands in the command history.
show commands n
Print ten commands centered on command number n.
show commands +
Print ten commands just after the commands last printed.
22.4 Screen Size
Certain commands to gdb may produce large amounts of information output to the screen.
To help you read all of it, gdb pauses and asks you for input at the end of each page of
output. Type RET when you want to continue the output, or q to discard the remaining
output. Also, the screen width setting determines when to wrap lines of output. Depending
on what is being printed, gdb tries to break the line at a readable place, rather than simply
letting it overflow onto the following line.
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Normally gdb knows the size of the screen from the terminal driver software. For
example, on Unix gdb uses the termcap data base together with the value of the TERM
environment variable and the stty rows and stty cols settings. If this is not correct, you
can override it with the set height and set width commands:
set height lpp
set height unlimited
show height
set width cpl
set width unlimited
show width
These set commands specify a screen height of lpp lines and a screen width of
cpl characters. The associated show commands display the current settings.
If you specify a height of either unlimited or zero lines, gdb does not pause
during output no matter how long the output is. This is useful if output is to
a file or to an editor buffer.
Likewise, you can specify ‘set width unlimited’ or ‘set width 0’ to prevent
gdb from wrapping its output.
set pagination on
set pagination off
Turn the output pagination on or off; the default is on. Turning pagination off
is the alternative to set height unlimited. Note that running gdb with the
‘--batch’ option (see Section 2.1.2 [Mode Options], page 13) also automatically
disables pagination.
show pagination
Show the current pagination mode.
22.5 Numbers
You can always enter numbers in octal, decimal, or hexadecimal in gdb by the usual
conventions: octal numbers begin with ‘0’, decimal numbers end with ‘.’, and hexadecimal
numbers begin with ‘0x’. Numbers that neither begin with ‘0’ or ‘0x’, nor end with a ‘.’
are, by default, entered in base 10; likewise, the default display for numbers—when no
particular format is specified—is base 10. You can change the default base for both input
and output with the commands described below.
set input-radix base
Set the default base for numeric input. Supported choices for base are decimal
8, 10, or 16. The base must itself be specified either unambiguously or using
the current input radix; for example, any of
set input-radix 012
set input-radix 10.
set input-radix 0xa
sets the input base to decimal. On the other hand, ‘set input-radix 10’ leaves
the input radix unchanged, no matter what it was, since ‘10’, being without any
leading or trailing signs of its base, is interpreted in the current radix. Thus,
if the current radix is 16, ‘10’ is interpreted in hex, i.e. as 16 decimal, which
doesn’t change the radix.
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set output-radix base
Set the default base for numeric display. Supported choices for base are decimal
8, 10, or 16. The base must itself be specified either unambiguously or using
the current input radix.
show input-radix
Display the current default base for numeric input.
show output-radix
Display the current default base for numeric display.
set radix [base]
show radix
These commands set and show the default base for both input and output
of numbers. set radix sets the radix of input and output to the same base;
without an argument, it resets the radix back to its default value of 10.
22.6 Configuring the Current ABI
gdb can determine the ABI (Application Binary Interface) of your application automatically. However, sometimes you need to override its conclusions. Use these commands to
manage gdb’s view of the current ABI.
One gdb configuration can debug binaries for multiple operating system targets, either
via remote debugging or native emulation. gdb will autodetect the OS ABI (Operating
System ABI) in use, but you can override its conclusion using the set osabi command. One
example where this is useful is in debugging of binaries which use an alternate C library (e.g.
uClibc for gnu/Linux) which does not have the same identifying marks that the standard
C library for your platform provides.
When gdb is debugging the AArch64 architecture, it provides a “Newlib” OS ABI. This
is useful for handling setjmp and longjmp when debugging binaries that use the newlib
C library. The “Newlib” OS ABI can be selected by set osabi Newlib.
show osabi
Show the OS ABI currently in use.
set osabi With no argument, show the list of registered available OS ABI’s.
set osabi abi
Set the current OS ABI to abi.
Generally, the way that an argument of type float is passed to a function depends on
whether the function is prototyped. For a prototyped (i.e. ANSI/ISO style) function, float
arguments are passed unchanged, according to the architecture’s convention for float. For
unprototyped (i.e. K&R style) functions, float arguments are first promoted to type double
and then passed.
Unfortunately, some forms of debug information do not reliably indicate whether a function is prototyped. If gdb calls a function that is not marked as prototyped, it consults
set coerce-float-to-double.
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Debugging with gdb
set coerce-float-to-double
set coerce-float-to-double on
Arguments of type float will be promoted to double when passed to an unprototyped function. This is the default setting.
set coerce-float-to-double off
Arguments of type float will be passed directly to unprototyped functions.
show coerce-float-to-double
Show the current setting of promoting float to double.
gdb needs to know the ABI used for your program’s C++ objects. The correct C++ ABI
depends on which C++ compiler was used to build your application. gdb only fully supports
programs with a single C++ ABI; if your program contains code using multiple C++ ABI’s
or if gdb can not identify your program’s ABI correctly, you can tell gdb which ABI to use.
Currently supported ABI’s include “gnu-v2”, for g++ versions before 3.0, “gnu-v3”, for g++
versions 3.0 and later, and “hpaCC” for the HP ANSI C++ compiler. Other C++ compilers
may use the “gnu-v2” or “gnu-v3” ABI’s as well. The default setting is “auto”.
show cp-abi
Show the C++ ABI currently in use.
set cp-abi
With no argument, show the list of supported C++ ABI’s.
set cp-abi abi
set cp-abi auto
Set the current C++ ABI to abi, or return to automatic detection.
22.7 Automatically loading associated files
gdb sometimes reads files with commands and settings automatically, without being explicitly told so by the user. We call this feature auto-loading. While auto-loading is useful
for automatically adapting gdb to the needs of your project, it can sometimes produce unexpected results or introduce security risks (e.g., if the file comes from untrusted sources).
There are various kinds of files gdb can automatically load. In addition to these files,
gdb supports auto-loading code written in various extension languages. See Section 23.4
[Auto-loading extensions], page 452.
Note that loading of these associated files (including the local ‘.gdbinit’ file) requires
accordingly configured auto-load safe-path (see Section 22.7.3 [Auto-loading safe path],
page 311).
For these reasons, gdb includes commands and options to let you control when to autoload files and which files should be auto-loaded.
set auto-load off
Globally disable loading of all auto-loaded files. You may want to use this
command with the ‘-iex’ option (see [Option -init-eval-command], page 16)
such as:
$ gdb -iex "set auto-load off" untrusted-executable corefile
Be aware that system init file (see Section C.6 [System-wide configuration],
page 608) and init files from your home directory (see [Home Directory Init
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File], page 16) still get read (as they come from generally trusted directories).
To prevent gdb from auto-loading even those init files, use the ‘-nx’ option (see
Section 2.1.2 [Mode Options], page 13), in addition to set auto-load no.
show auto-load
Show whether auto-loading of each specific ‘auto-load’ file(s) is enabled or
disabled.
(gdb) show auto-load
gdb-scripts: Auto-loading of canned sequences of commands scripts is on.
libthread-db: Auto-loading of inferior specific libthread_db is on.
local-gdbinit: Auto-loading of .gdbinit script from current directory
is on.
python-scripts: Auto-loading of Python scripts is on.
safe-path: List of directories from which it is safe to auto-load files
is $debugdir:$datadir/auto-load.
scripts-directory: List of directories from which to load auto-loaded scripts
is $debugdir:$datadir/auto-load.
info auto-load
Print whether each specific ‘auto-load’ file(s) have been auto-loaded or not.
(gdb) info auto-load
gdb-scripts:
Loaded Script
Yes
/home/user/gdb/gdb-gdb.gdb
libthread-db: No auto-loaded libthread-db.
local-gdbinit: Local .gdbinit file "/home/user/gdb/.gdbinit" has been
loaded.
python-scripts:
Loaded Script
Yes
/home/user/gdb/gdb-gdb.py
These are gdb control commands for the auto-loading:
See [set auto-load off], page 308.
See [show auto-load], page 309.
See [info auto-load], page 309.
See [set auto-load gdb-scripts], page 327.
See [show auto-load gdb-scripts], page 327.
See [info auto-load gdb-scripts], page 327.
See [set auto-load python-scripts], page 398.
See [show auto-load python-scripts], page 398.
See [info auto-load python-scripts], page 398.
See [set auto-load guile-scripts], page 451.
See [show auto-load guile-scripts], page 451.
See [info auto-load guile-scripts], page 451.
See [set auto-load scripts-directory], page 453.
See [show auto-load scripts-directory],
page 453.
See
[add-auto-load-scripts-directory],
page 453.
See [set auto-load local-gdbinit], page 310.
Disable auto-loading globally.
Show setting of all kinds of files.
Show state of all kinds of files.
Control for gdb command scripts.
Show setting of gdb command scripts.
Show state of gdb command scripts.
Control for gdb Python scripts.
Show setting of gdb Python scripts.
Show state of gdb Python scripts.
Control for gdb Guile scripts.
Show setting of gdb Guile scripts.
Show state of gdb Guile scripts.
Control for gdb auto-loaded scripts
location.
Show gdb auto-loaded scripts location.
Add directory for auto-loaded scripts location list.
Control for init file in the current directory.
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See [show auto-load local-gdbinit], page 310.
See [info auto-load local-gdbinit], page 310.
See
See
See
See
[set auto-load libthread-db], page 310.
[show auto-load libthread-db], page 310.
[info auto-load libthread-db], page 311.
[set auto-load safe-path], page 311.
See [show auto-load safe-path], page 311.
See [add-auto-load-safe-path], page 311.
Show setting of init file in the current
directory.
Show state of init file in the current
directory.
Control for thread debugging library.
Show setting of thread debugging library.
Show state of thread debugging library.
Control directories trusted for automatic
loading.
Show directories trusted for automatic
loading.
Add directory trusted for automatic
loading.
22.7.1 Automatically loading init file in the current directory
By default, gdb reads and executes the canned sequences of commands from init file (if any)
in the current working directory, see [Init File in the Current Directory during Startup],
page 16.
Note that loading of this local ‘.gdbinit’ file also requires accordingly configured autoload safe-path (see Section 22.7.3 [Auto-loading safe path], page 311).
set auto-load local-gdbinit [on|off]
Enable or disable the auto-loading of canned sequences of commands (see
Section 23.1 [Sequences], page 321) found in init file in the current directory.
show auto-load local-gdbinit
Show whether auto-loading of canned sequences of commands from init file in
the current directory is enabled or disabled.
info auto-load local-gdbinit
Print whether canned sequences of commands from init file in the current directory have been auto-loaded.
22.7.2 Automatically loading thread debugging library
This feature is currently present only on gnu/Linux native hosts.
gdb reads in some cases thread debugging library from places specific to the inferior
(see [set libthread-db-search-path], page 40).
The special ‘libthread-db-search-path’ entry ‘$sdir’ is processed without checking
this ‘set auto-load libthread-db’ switch as system libraries have to be trusted in general.
In all other cases of ‘libthread-db-search-path’ entries gdb checks first if ‘set auto-load
libthread-db’ is enabled before trying to open such thread debugging library.
Note that loading of this debugging library also requires accordingly configured autoload safe-path (see Section 22.7.3 [Auto-loading safe path], page 311).
set auto-load libthread-db [on|off]
Enable or disable the auto-loading of inferior specific thread debugging library.
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show auto-load libthread-db
Show whether auto-loading of inferior specific thread debugging library is enabled or disabled.
info auto-load libthread-db
Print the list of all loaded inferior specific thread debugging libraries and for
each such library print list of inferior pids using it.
22.7.3 Security restriction for auto-loading
As the files of inferior can come from untrusted source (such as submitted by an application
user) gdb does not always load any files automatically. gdb provides the ‘set auto-load
safe-path’ setting to list directories trusted for loading files not explicitly requested by
user. Each directory can also be a shell wildcard pattern.
If the path is not set properly you will see a warning and the file will not get loaded:
$ ./gdb -q ./gdb
Reading symbols from /home/user/gdb/gdb...done.
warning: File "/home/user/gdb/gdb-gdb.gdb" auto-loading has been
declined by your ‘auto-load safe-path’ set
to "$debugdir:$datadir/auto-load".
warning: File "/home/user/gdb/gdb-gdb.py" auto-loading has been
declined by your ‘auto-load safe-path’ set
to "$debugdir:$datadir/auto-load".
To instruct gdb to go ahead and use the init files anyway, invoke gdb like this:
$ gdb -q -iex "set auto-load safe-path /home/user/gdb" ./gdb
The list of trusted directories is controlled by the following commands:
set auto-load safe-path [directories]
Set the list of directories (and their subdirectories) trusted for automatic loading and execution of scripts. You can also enter a specific trusted file. Each
directory can also be a shell wildcard pattern; wildcards do not match directory
separator - see FNM_PATHNAME for system function fnmatch (see Section “Wildcard Matching” in GNU C Library Reference Manual). If you omit directories,
‘auto-load safe-path’ will be reset to its default value as specified during gdb
compilation.
The list of directories uses path separator (‘:’ on GNU and Unix systems, ‘;’
on MS-Windows and MS-DOS) to separate directories, similarly to the PATH
environment variable.
show auto-load safe-path
Show the list of directories trusted for automatic loading and execution of
scripts.
add-auto-load-safe-path
Add an entry (or list of entries) to the list of directories trusted for automatic
loading and execution of scripts. Multiple entries may be delimited by the host
platform path separator in use.
This variable defaults to what --with-auto-load-dir has been configured to (see [withauto-load-dir], page 453). ‘$debugdir’ and ‘$datadir’ substitution applies the same as for
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[set auto-load scripts-directory], page 453. The default set auto-load safe-path value
can be also overriden by gdb configuration option ‘--with-auto-load-safe-path’.
Setting this variable to ‘/’ disables this security protection, corresponding gdb configuration option is ‘--without-auto-load-safe-path’. This variable is supposed to be set
to the system directories writable by the system superuser only. Users can add their source
directories in init files in their home directories (see [Home Directory Init File], page 16).
See also deprecated init file in the current directory (see [Init File in the Current Directory
during Startup], page 16).
To force gdb to load the files it declined to load in the previous example, you could use
one of the following ways:
‘~/.gdbinit’: ‘add-auto-load-safe-path ~/src/gdb’
Specify this trusted directory (or a file) as additional component of the list. You
have to specify also any existing directories displayed by by ‘show auto-load
safe-path’ (such as ‘/usr:/bin’ in this example).
gdb -iex "set auto-load safe-path /usr:/bin:~/src/gdb" ...
Specify this directory as in the previous case but just for a single gdb session.
gdb -iex "set auto-load safe-path /" ...
Disable auto-loading safety for a single gdb session. This assumes all the files
you debug during this gdb session will come from trusted sources.
./configure --without-auto-load-safe-path
During compilation of gdb you may disable any auto-loading safety. This
assumes all the files you will ever debug with this gdb come from trusted
sources.
On the other hand you can also explicitly forbid automatic files loading which also
suppresses any such warning messages:
gdb -iex "set auto-load no" ...
You can use gdb command-line option for a single gdb session.
‘~/.gdbinit’: ‘set auto-load no’
Disable auto-loading globally for the user (see [Home Directory Init File],
page 16). While it is improbable, you could also use system init file instead
(see Section C.6 [System-wide configuration], page 608).
This setting applies to the file names as entered by user. If no entry matches gdb
tries as a last resort to also resolve all the file names into their canonical form (typically
resolving symbolic links) and compare the entries again. gdb already canonicalizes most of
the filenames on its own before starting the comparison so a canonical form of directories
is recommended to be entered.
22.7.4 Displaying files tried for auto-load
For better visibility of all the file locations where you can place scripts to be auto-loaded
with inferior — or to protect yourself against accidental execution of untrusted scripts —
gdb provides a feature for printing all the files attempted to be loaded. Both existing and
non-existing files may be printed.
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For example the list of directories from which it is safe to auto-load files (see
Section 22.7.3 [Auto-loading safe path], page 311) applies also to canonicalized filenames
which may not be too obvious while setting it up.
(gdb) set debug auto-load on
(gdb) file ~/src/t/true
auto-load: Loading canned sequences of commands script "/tmp/true-gdb.gdb"
for objfile "/tmp/true".
auto-load: Updating directories of "/usr:/opt".
auto-load: Using directory "/usr".
auto-load: Using directory "/opt".
warning: File "/tmp/true-gdb.gdb" auto-loading has been declined
by your ‘auto-load safe-path’ set to "/usr:/opt".
set debug auto-load [on|off]
Set whether to print the filenames attempted to be auto-loaded.
show debug auto-load
Show whether printing of the filenames attempted to be auto-loaded is turned
on or off.
22.8 Optional Warnings and Messages
By default, gdb is silent about its inner workings. If you are running on a slow machine,
you may want to use the set verbose command. This makes gdb tell you when it does a
lengthy internal operation, so you will not think it has crashed.
Currently, the messages controlled by set verbose are those which announce that the
symbol table for a source file is being read; see symbol-file in Section 18.1 [Commands to
Specify Files], page 241.
set verbose on
Enables gdb output of certain informational messages.
set verbose off
Disables gdb output of certain informational messages.
show verbose
Displays whether set verbose is on or off.
By default, if gdb encounters bugs in the symbol table of an object file, it is silent; but if
you are debugging a compiler, you may find this information useful (see Section 18.6 [Errors
Reading Symbol Files], page 255).
set complaints limit
Permits gdb to output limit complaints about each type of unusual symbols
before becoming silent about the problem. Set limit to zero to suppress all complaints; set it to a large number to prevent complaints from being suppressed.
show complaints
Displays how many symbol complaints gdb is permitted to produce.
By default, gdb is cautious, and asks what sometimes seems to be a lot of stupid
questions to confirm certain commands. For example, if you try to run a program which is
already running:
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(gdb) run
The program being debugged has been started already.
Start it from the beginning? (y or n)
If you are willing to unflinchingly face the consequences of your own commands, you can
disable this “feature”:
set confirm off
Disables confirmation requests. Note that running gdb with the ‘--batch’
option (see Section 2.1.2 [Mode Options], page 13) also automatically disables
confirmation requests.
set confirm on
Enables confirmation requests (the default).
show confirm
Displays state of confirmation requests.
If you need to debug user-defined commands or sourced files you may find it useful to
enable command tracing. In this mode each command will be printed as it is executed, prefixed with one or more ‘+’ symbols, the quantity denoting the call depth of each command.
set trace-commands on
Enable command tracing.
set trace-commands off
Disable command tracing.
show trace-commands
Display the current state of command tracing.
22.9 Optional Messages about Internal Happenings
gdb has commands that enable optional debugging messages from various gdb subsystems;
normally these commands are of interest to gdb maintainers, or when reporting a bug. This
section documents those commands.
set exec-done-display
Turns on or off the notification of asynchronous commands’ completion. When
on, gdb will print a message when an asynchronous command finishes its execution. The default is off.
show exec-done-display
Displays the current setting of asynchronous command completion notification.
set debug aarch64
Turns on or off display of debugging messages related to ARM AArch64. The
default is off.
show debug aarch64
Displays the current state of displaying debugging messages related to ARM
AArch64.
set debug arch
Turns on or off display of gdbarch debugging info. The default is off
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show debug arch
Displays the current state of displaying gdbarch debugging info.
set debug aix-solib
Control display of debugging messages from the AIX shared library support
module. The default is off.
show debug aix-thread
Show the current state of displaying AIX shared library debugging messages.
set debug aix-thread
Display debugging messages about inner workings of the AIX thread module.
show debug aix-thread
Show the current state of AIX thread debugging info display.
set debug check-physname
Check the results of the “physname” computation. When reading DWARF
debugging information for C++, gdb attempts to compute each entity’s name.
gdb can do this computation in two different ways, depending on exactly what
information is present. When enabled, this setting causes gdb to compute the
names both ways and display any discrepancies.
show debug check-physname
Show the current state of “physname” checking.
set debug coff-pe-read
Control display of debugging messages related to reading of COFF/PE exported
symbols. The default is off.
show debug coff-pe-read
Displays the current state of displaying debugging messages related to reading
of COFF/PE exported symbols.
set debug dwarf-die
Dump DWARF DIEs after they are read in. The value is the number of nesting
levels to print. A value of zero turns off the display.
show debug dwarf-die
Show the current state of DWARF DIE debugging.
set debug dwarf-line
Turns on or off display of debugging messages related to reading DWARF line
tables. The default is 0 (off). A value of 1 provides basic information. A value
greater than 1 provides more verbose information.
show debug dwarf-line
Show the current state of DWARF line table debugging.
set debug dwarf-read
Turns on or off display of debugging messages related to reading DWARF debug
info. The default is 0 (off). A value of 1 provides basic information. A value
greater than 1 provides more verbose information.
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Debugging with gdb
show debug dwarf-read
Show the current state of DWARF reader debugging.
set debug displaced
Turns on or off display of gdb debugging info for the displaced stepping support.
The default is off.
show debug displaced
Displays the current state of displaying gdb debugging info related to displaced
stepping.
set debug event
Turns on or off display of gdb event debugging info. The default is off.
show debug event
Displays the current state of displaying gdb event debugging info.
set debug expression
Turns on or off display of debugging info about gdb expression parsing. The
default is off.
show debug expression
Displays the current state of displaying debugging info about gdb expression
parsing.
set debug fbsd-lwp
Turns on or off debugging messages from the FreeBSD LWP debug support.
show debug fbsd-lwp
Show the current state of FreeBSD LWP debugging messages.
set debug frame
Turns on or off display of gdb frame debugging info. The default is off.
show debug frame
Displays the current state of displaying gdb frame debugging info.
set debug gnu-nat
Turn on or off debugging messages from the gnu/Hurd debug support.
show debug gnu-nat
Show the current state of gnu/Hurd debugging messages.
set debug infrun
Turns on or off display of gdb debugging info for running the inferior. The
default is off. ‘infrun.c’ contains GDB’s runtime state machine used for implementing operations such as single-stepping the inferior.
show debug infrun
Displays the current state of gdb inferior debugging.
set debug jit
Turn on or off debugging messages from JIT debug support.
show debug jit
Displays the current state of gdb JIT debugging.
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set debug lin-lwp
Turn on or off debugging messages from the Linux LWP debug support.
show debug lin-lwp
Show the current state of Linux LWP debugging messages.
set debug linux-namespaces
Turn on or off debugging messages from the Linux namespaces debug support.
show debug linux-namespaces
Show the current state of Linux namespaces debugging messages.
set debug mach-o
Control display of debugging messages related to Mach-O symbols processing.
The default is off.
show debug mach-o
Displays the current state of displaying debugging messages related to reading
of COFF/PE exported symbols.
set debug notification
Turn on or off debugging messages about remote async notification. The default
is off.
show debug notification
Displays the current state of remote async notification debugging messages.
set debug observer
Turns on or off display of gdb observer debugging. This includes info such as
the notification of observable events.
show debug observer
Displays the current state of observer debugging.
set debug overload
Turns on or off display of gdb C++ overload debugging info. This includes info
such as ranking of functions, etc. The default is off.
show debug overload
Displays the current state of displaying gdb C++ overload debugging info.
set debug parser
Turns on or off the display of expression parser debugging output. Internally,
this sets the yydebug variable in the expression parser. See Section “Tracing
Your Parser” in Bison, for details. The default is off.
show debug parser
Show the current state of expression parser debugging.
set debug remote
Turns on or off display of reports on all packets sent back and forth across the
serial line to the remote machine. The info is printed on the gdb standard
output stream. The default is off.
show debug remote
Displays the state of display of remote packets.
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set debug serial
Turns on or off display of gdb serial debugging info. The default is off.
show debug serial
Displays the current state of displaying gdb serial debugging info.
set debug solib-frv
Turn on or off debugging messages for FR-V shared-library code.
show debug solib-frv
Display the current state of FR-V shared-library code debugging messages.
set debug symbol-lookup
Turns on or off display of debugging messages related to symbol lookup. The
default is 0 (off). A value of 1 provides basic information. A value greater than
1 provides more verbose information.
show debug symbol-lookup
Show the current state of symbol lookup debugging messages.
set debug symfile
Turns on or off display of debugging messages related to symbol file functions.
The default is off. See Section 18.1 [Files], page 241.
show debug symfile
Show the current state of symbol file debugging messages.
set debug symtab-create
Turns on or off display of debugging messages related to symbol table creation.
The default is 0 (off). A value of 1 provides basic information. A value greater
than 1 provides more verbose information.
show debug symtab-create
Show the current state of symbol table creation debugging.
set debug target
Turns on or off display of gdb target debugging info. This info includes what
is going on at the target level of GDB, as it happens. The default is 0. Set it
to 1 to track events, and to 2 to also track the value of large memory transfers.
show debug target
Displays the current state of displaying gdb target debugging info.
set debug timestamp
Turns on or off display of timestamps with gdb debugging info. When enabled,
seconds and microseconds are displayed before each debugging message.
show debug timestamp
Displays the current state of displaying timestamps with gdb debugging info.
set debug varobj
Turns on or off display of gdb variable object debugging info. The default is
off.
show debug varobj
Displays the current state of displaying gdb variable object debugging info.
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set debug xml
Turn on or off debugging messages for built-in XML parsers.
show debug xml
Displays the current state of XML debugging messages.
22.10 Other Miscellaneous Settings
set interactive-mode
If on, forces gdb to assume that GDB was started in a terminal. In practice,
this means that gdb should wait for the user to answer queries generated by
commands entered at the command prompt. If off, forces gdb to operate in
the opposite mode, and it uses the default answers to all queries. If auto (the
default), gdb tries to determine whether its standard input is a terminal, and
works in interactive-mode if it is, non-interactively otherwise.
In the vast majority of cases, the debugger should be able to guess correctly
which mode should be used. But this setting can be useful in certain specific
cases, such as running a MinGW gdb inside a cygwin window.
show interactive-mode
Displays whether the debugger is operating in interactive mode or not.
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23 Extending gdb
gdb provides several mechanisms for extension. gdb also provides the ability to automatically load extensions when it reads a file for debugging. This allows the user to automatically
customize gdb for the program being debugged.
To facilitate the use of extension languages, gdb is capable of evaluating the contents of
a file. When doing so, gdb can recognize which extension language is being used by looking
at the filename extension. Files with an unrecognized filename extension are always treated
as a gdb Command Files. See Section 23.1.3 [Command files], page 324.
You can control how gdb evaluates these files with the following setting:
set script-extension off
All scripts are always evaluated as gdb Command Files.
set script-extension soft
The debugger determines the scripting language based on filename extension.
If this scripting language is supported, gdb evaluates the script using that
language. Otherwise, it evaluates the file as a gdb Command File.
set script-extension strict
The debugger determines the scripting language based on filename extension,
and evaluates the script using that language. If the language is not supported,
then the evaluation fails.
show script-extension
Display the current value of the script-extension option.
23.1 Canned Sequences of Commands
Aside from breakpoint commands (see Section 5.1.7 [Breakpoint Command Lists], page 62),
gdb provides two ways to store sequences of commands for execution as a unit: user-defined
commands and command files.
23.1.1 User-defined Commands
A user-defined command is a sequence of gdb commands to which you assign a new name
as a command. This is done with the define command. User commands may accept an
unlimited number of arguments separated by whitespace. Arguments are accessed within
the user command via $arg0...$argN. A trivial example:
define adder
print $arg0 + $arg1 + $arg2
end
To execute the command use:
adder 1 2 3
This defines the command adder, which prints the sum of its three arguments. Note the
arguments are text substitutions, so they may reference variables, use complex expressions,
or even perform inferior functions calls.
In addition, $argc may be used to find out how many arguments have been passed.
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define adder
if $argc == 2
print $arg0 + $arg1
end
if $argc == 3
print $arg0 + $arg1 + $arg2
end
end
Combining with the eval command (see [eval], page 327) makes it easier to process a
variable number of arguments:
define adder
set $i = 0
set $sum = 0
while $i < $argc
eval "set $sum = $sum + $arg%d", $i
set $i = $i + 1
end
print $sum
end
define commandname
Define a command named commandname. If there is already a command by
that name, you are asked to confirm that you want to redefine it. The argument commandname may be a bare command name consisting of letters,
numbers, dashes, and underscores. It may also start with any predefined prefix command. For example, ‘define target my-target’ creates a user-defined
‘target my-target’ command.
The definition of the command is made up of other gdb command lines, which
are given following the define command. The end of these commands is marked
by a line containing end.
document commandname
Document the user-defined command commandname, so that it can be accessed by help. The command commandname must already be defined. This
command reads lines of documentation just as define reads the lines of the
command definition, ending with end. After the document command is finished, help on command commandname displays the documentation you have
written.
You may use the document command again to change the documentation of a
command. Redefining the command with define does not change the documentation.
dont-repeat
Used inside a user-defined command, this tells gdb that this command should
not be repeated when the user hits RET (see Section 3.1 [Command Syntax],
page 19).
help user-defined
List all user-defined commands and all python commands defined in class COMAND USER. The first line of the documentation or docstring is included (if
any).
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show user
show user commandname
Display the gdb commands used to define commandname (but not its documentation). If no commandname is given, display the definitions for all user-defined
commands. This does not work for user-defined python commands.
show max-user-call-depth
set max-user-call-depth
The value of max-user-call-depth controls how many recursion levels are
allowed in user-defined commands before gdb suspects an infinite recursion and
aborts the command. This does not apply to user-defined python commands.
In addition to the above commands, user-defined commands frequently use control flow
commands, described in Section 23.1.3 [Command Files], page 324.
When user-defined commands are executed, the commands of the definition are not
printed. An error in any command stops execution of the user-defined command.
If used interactively, commands that would ask for confirmation proceed without asking
when used inside a user-defined command. Many gdb commands that normally print messages to say what they are doing omit the messages when used in a user-defined command.
23.1.2 User-defined Command Hooks
You may define hooks, which are a special kind of user-defined command. Whenever you
run the command ‘foo’, if the user-defined command ‘hook-foo’ exists, it is executed (with
no arguments) before that command.
A hook may also be defined which is run after the command you executed. Whenever you
run the command ‘foo’, if the user-defined command ‘hookpost-foo’ exists, it is executed
(with no arguments) after that command. Post-execution hooks may exist simultaneously
with pre-execution hooks, for the same command.
It is valid for a hook to call the command which it hooks. If this occurs, the hook is not
re-executed, thereby avoiding infinite recursion.
In addition, a pseudo-command, ‘stop’ exists. Defining (‘hook-stop’) makes the associated commands execute every time execution stops in your program: before breakpoint
commands are run, displays are printed, or the stack frame is printed.
For example, to ignore SIGALRM signals while single-stepping, but treat them normally
during normal execution, you could define:
define hook-stop
handle SIGALRM nopass
end
define hook-run
handle SIGALRM pass
end
define hook-continue
handle SIGALRM pass
end
As a further example, to hook at the beginning and end of the echo command, and to
add extra text to the beginning and end of the message, you could define:
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define hook-echo
echo <<<--end
define hookpost-echo
echo --->>>\n
end
(gdb) echo Hello World
<<<---Hello World--->>>
(gdb)
You can define a hook for any single-word command in gdb, but not for command
aliases; you should define a hook for the basic command name, e.g. backtrace rather than
bt. You can hook a multi-word command by adding hook- or hookpost- to the last word
of the command, e.g. ‘define target hook-remote’ to add a hook to ‘target remote’.
If an error occurs during the execution of your hook, execution of gdb commands stops
and gdb issues a prompt (before the command that you actually typed had a chance to
run).
If you try to define a hook which does not match any known command, you get a warning
from the define command.
23.1.3 Command Files
A command file for gdb is a text file made of lines that are gdb commands. Comments
(lines starting with #) may also be included. An empty line in a command file does nothing;
it does not mean to repeat the last command, as it would from the terminal.
You can request the execution of a command file with the source command. Note that
the source command is also used to evaluate scripts that are not Command Files. The
exact behavior can be configured using the script-extension setting. See Chapter 23
[Extending GDB], page 321.
source [-s] [-v] filename
Execute the command file filename.
The lines in a command file are generally executed sequentially, unless the order of
execution is changed by one of the flow-control commands described below. The commands
are not printed as they are executed. An error in any command terminates execution of
the command file and control is returned to the console.
gdb first searches for filename in the current directory. If the file is not found there,
and filename does not specify a directory, then gdb also looks for the file on the source
search path (specified with the ‘directory’ command); except that ‘$cdir’ is not searched
because the compilation directory is not relevant to scripts.
If -s is specified, then gdb searches for filename on the search path even if filename specifies a directory. The search is done by appending filename to each element of the search path.
So, for example, if filename is ‘mylib/myscript’ and the search path contains ‘/home/user’
then gdb will look for the script ‘/home/user/mylib/myscript’. The search is also done
if filename is an absolute path. For example, if filename is ‘/tmp/myscript’ and the search
path contains ‘/home/user’ then gdb will look for the script ‘/home/user/tmp/myscript’.
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For DOS-like systems, if filename contains a drive specification, it is stripped before concatenation. For example, if filename is ‘d:myscript’ and the search path contains ‘c:/tmp’
then gdb will look for the script ‘c:/tmp/myscript’.
If -v, for verbose mode, is given then gdb displays each command as it is executed. The
option must be given before filename, and is interpreted as part of the filename anywhere
else.
Commands that would ask for confirmation if used interactively proceed without asking
when used in a command file. Many gdb commands that normally print messages to say
what they are doing omit the messages when called from command files.
gdb also accepts command input from standard input. In this mode, normal output
goes to standard output and error output goes to standard error. Errors in a command
file supplied on standard input do not terminate execution of the command file—execution
continues with the next command.
gdb < cmds > log 2>&1
(The syntax above will vary depending on the shell used.) This example will execute
commands from the file ‘cmds’. All output and errors would be directed to ‘log’.
Since commands stored on command files tend to be more general than commands typed
interactively, they frequently need to deal with complicated situations, such as different or
unexpected values of variables and symbols, changes in how the program being debugged
is built, etc. gdb provides a set of flow-control commands to deal with these complexities.
Using these commands, you can write complex scripts that loop over data structures, execute
commands conditionally, etc.
if
else
while
This command allows to include in your script conditionally executed commands. The if command takes a single argument, which is an expression to
evaluate. It is followed by a series of commands that are executed only if the
expression is true (its value is nonzero). There can then optionally be an else
line, followed by a series of commands that are only executed if the expression
was false. The end of the list is marked by a line containing end.
This command allows to write loops. Its syntax is similar to if: the command
takes a single argument, which is an expression to evaluate, and must be followed by the commands to execute, one per line, terminated by an end. These
commands are called the body of the loop. The commands in the body of while
are executed repeatedly as long as the expression evaluates to true.
loop_break
This command exits the while loop in whose body it is included. Execution of
the script continues after that whiles end line.
loop_continue
This command skips the execution of the rest of the body of commands in the
while loop in whose body it is included. Execution branches to the beginning
of the while loop, where it evaluates the controlling expression.
end
Terminate the block of commands that are the body of if, else, or while
flow-control commands.
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23.1.4 Commands for Controlled Output
During the execution of a command file or a user-defined command, normal gdb output
is suppressed; the only output that appears is what is explicitly printed by the commands
in the definition. This section describes three commands useful for generating exactly the
output you want.
echo text Print text. Nonprinting characters can be included in text using C escape sequences, such as ‘\n’ to print a newline. No newline is printed unless you specify
one. In addition to the standard C escape sequences, a backslash followed by a
space stands for a space. This is useful for displaying a string with spaces at the
beginning or the end, since leading and trailing spaces are otherwise trimmed
from all arguments. To print ‘ and foo = ’, use the command ‘echo \ and foo
= \ ’.
A backslash at the end of text can be used, as in C, to continue the command
onto subsequent lines. For example,
echo This is some text\n\
which is continued\n\
onto several lines.\n
produces the same output as
echo This is some text\n
echo which is continued\n
echo onto several lines.\n
output expression
Print the value of expression and nothing but that value: no newlines, no ‘$nn
= ’. The value is not entered in the value history either. See Section 10.1
[Expressions], page 117, for more information on expressions.
output/fmt expression
Print the value of expression in format fmt. You can use the same formats as
for print. See Section 10.5 [Output Formats], page 122, for more information.
printf template, expressions...
Print the values of one or more expressions under the control of the string
template. To print several values, make expressions be a comma-separated
list of individual expressions, which may be either numbers or pointers. Their
values are printed as specified by template, exactly as a C program would do
by executing the code below:
printf (template, expressions...);
As in C printf, ordinary characters in template are printed verbatim, while
conversion specification introduced by the ‘%’ character cause subsequent expressions to be evaluated, their values converted and formatted according to
type and style information encoded in the conversion specifications, and then
printed.
For example, you can print two values in hex like this:
printf "foo, bar-foo = 0x%x, 0x%x\n", foo, bar-foo
printf supports all the standard C conversion specifications, including the flags
and modifiers between the ‘%’ character and the conversion letter, with the
following exceptions:
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• The argument-ordering modifiers, such as ‘2$’, are not supported.
• The modifier ‘*’ is not supported for specifying precision or width.
• The ‘’’ flag (for separation of digits into groups according to LC_NUMERIC’)
is not supported.
• The type modifiers ‘hh’, ‘j’, ‘t’, and ‘z’ are not supported.
• The conversion letter ‘n’ (as in ‘%n’) is not supported.
• The conversion letters ‘a’ and ‘A’ are not supported.
Note that the ‘ll’ type modifier is supported only if the underlying C implementation used to build gdb supports the long long int type, and the ‘L’ type
modifier is supported only if long double type is available.
As in C, printf supports simple backslash-escape sequences, such as \n, ‘\t’,
‘\\’, ‘\"’, ‘\a’, and ‘\f’, that consist of backslash followed by a single character.
Octal and hexadecimal escape sequences are not supported.
Additionally, printf supports conversion specifications for DFP (Decimal
Floating Point) types using the following length modifiers together with a
floating point specifier. letters:
• ‘H’ for printing Decimal32 types.
• ‘D’ for printing Decimal64 types.
• ‘DD’ for printing Decimal128 types.
If the underlying C implementation used to build gdb has support for the three
length modifiers for DFP types, other modifiers such as width and precision
will also be available for gdb to use.
In case there is no such C support, no additional modifiers will be available and
the value will be printed in the standard way.
Here’s an example of printing DFP types using the above conversion letters:
printf "D32: %Hf - D64: %Df - D128: %DDf\n",1.2345df,1.2E10dd,1.2E1dl
eval template, expressions...
Convert the values of one or more expressions under the control of the string
template to a command line, and call it.
23.1.5 Controlling auto-loading native gdb scripts
When a new object file is read (for example, due to the file command, or because the
inferior has loaded a shared library), gdb will look for the command file ‘objfile-gdb.gdb’.
See Section 23.4 [Auto-loading extensions], page 452.
Auto-loading can be enabled or disabled, and the list of auto-loaded scripts can be
printed.
set auto-load gdb-scripts [on|off]
Enable or disable the auto-loading of canned sequences of commands scripts.
show auto-load gdb-scripts
Show whether auto-loading of canned sequences of commands scripts is enabled
or disabled.
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info auto-load gdb-scripts [regexp]
Print the list of all canned sequences of commands scripts that gdb auto-loaded.
If regexp is supplied only canned sequences of commands scripts with matching names
are printed.
23.2 Extending gdb using Python
You can extend gdb using the Python programming language. This feature is available
only if gdb was configured using ‘--with-python’.
Python scripts used by gdb should be installed in ‘data-directory/python’, where
data-directory is the data directory as determined at gdb startup (see Section 18.7 [Data
Files], page 256). This directory, known as the python directory, is automatically added
to the Python Search Path in order to allow the Python interpreter to locate all scripts
installed at this location.
Additionally, gdb commands and convenience functions which are written in
Python and are located in the ‘data-directory/python/gdb/command’ or ‘datadirectory/python/gdb/function’ directories are automatically imported when gdb
starts.
23.2.1 Python Commands
gdb provides two commands for accessing the Python interpreter, and one related setting:
python-interactive [command]
pi [command]
Without an argument, the python-interactive command can be used to start
an interactive Python prompt. To return to gdb, type the EOF character (e.g.,
Ctrl-D on an empty prompt).
Alternatively, a single-line Python command can be given as an argument and
evaluated. If the command is an expression, the result will be printed; otherwise, nothing will be printed. For example:
(gdb) python-interactive 2 + 3
5
python [command]
py [command]
The python command can be used to evaluate Python code.
If given an argument, the python command will evaluate the argument as a
Python command. For example:
(gdb) python print 23
23
If you do not provide an argument to python, it will act as a multi-line command, like define. In this case, the Python script is made up of subsequent
command lines, given after the python command. This command list is terminated using a line containing end. For example:
(gdb) python
Type python script
End with a line saying just "end".
>print 23
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>end
23
set python print-stack
By default, gdb will print only the message component of a Python exception
when an error occurs in a Python script. This can be controlled using set
python print-stack: if full, then full Python stack printing is enabled; if
none, then Python stack and message printing is disabled; if message, the
default, only the message component of the error is printed.
It is also possible to execute a Python script from the gdb interpreter:
source ‘script-name’
The script name must end with ‘.py’ and gdb must be configured to recognize
the script language based on filename extension using the script-extension
setting. See Chapter 23 [Extending GDB], page 321.
python execfile ("script-name")
This method is based on the execfile Python built-in function, and thus is
always available.
23.2.2 Python API
You can get quick online help for gdb’s Python API by issuing the command
python help (gdb).
Functions and methods which have two or more optional arguments allow them to be
specified using keyword syntax. This allows passing some optional arguments while skipping
others. Example: gdb.some_function (’foo’, bar = 1, baz = 2).
23.2.2.1 Basic Python
At startup, gdb overrides Python’s sys.stdout and sys.stderr to print using gdb’s
output-paging streams. A Python program which outputs to one of these streams may
have its output interrupted by the user (see Section 22.4 [Screen Size], page 305). In this
situation, a Python KeyboardInterrupt exception is thrown.
Some care must be taken when writing Python code to run in gdb. Two things worth
noting in particular:
• gdb install handlers for SIGCHLD and SIGINT. Python code must not override these,
or even change the options using sigaction. If your program changes the handling of
these signals, gdb will most likely stop working correctly. Note that it is unfortunately
common for GUI toolkits to install a SIGCHLD handler.
• gdb takes care to mark its internal file descriptors as close-on-exec. However, this
cannot be done in a thread-safe way on all platforms. Your Python programs should
be aware of this and should both create new file descriptors with the close-on-exec flag
set and arrange to close unneeded file descriptors before starting a child process.
gdb introduces a new Python module, named gdb. All methods and classes added by
gdb are placed in this module. gdb automatically imports the gdb module for use in all
scripts evaluated by the python command.
[Variable]
A string containing the python directory (see Section 23.2 [Python], page 328).
gdb.PYTHONDIR
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gdb.execute (command [, from tty [, to string]])
[Function]
Evaluate command, a string, as a gdb CLI command. If a GDB exception happens
while command runs, it is translated as described in Section 23.2.2.2 [Exception
Handling], page 333.
The from tty flag specifies whether gdb ought to consider this command as having
originated from the user invoking it interactively. It must be a boolean value. If
omitted, it defaults to False.
By default, any output produced by command is sent to gdb’s standard output (and
to the log output if logging is turned on). If the to string parameter is True, then
output will be collected by gdb.execute and returned as a string. The default is
False, in which case the return value is None. If to string is True, the gdb virtual
terminal will be temporarily set to unlimited width and height, and its pagination
will be disabled; see Section 22.4 [Screen Size], page 305.
gdb.breakpoints ()
[Function]
Return a sequence holding all of gdb’s breakpoints. See Section 23.2.2.30 [Breakpoints In Python], page 393, for more information. In gdb version 7.11 and earlier,
this function returned None if there were no breakpoints. This peculiarity was subsequently fixed, and now gdb.breakpoints returns an empty sequence in this case.
gdb.parameter (parameter)
[Function]
Return the value of a gdb parameter given by its name, a string; the parameter
name string may contain spaces if the parameter has a multi-part name. For example,
‘print object’ is a valid parameter name.
If the named parameter does not exist, this function throws a gdb.error (see
Section 23.2.2.2 [Exception Handling], page 333). Otherwise, the parameter’s value
is converted to a Python value of the appropriate type, and returned.
gdb.history (number)
[Function]
Return a value from gdb’s value history (see Section 10.10 [Value History], page 138).
The number argument indicates which history element to return. If number is negative, then gdb will take its absolute value and count backward from the last element
(i.e., the most recent element) to find the value to return. If number is zero, then
gdb will return the most recent element. If the element specified by number doesn’t
exist in the value history, a gdb.error exception will be raised.
If no exception is raised, the return value is always an instance of gdb.Value (see
Section 23.2.2.3 [Values From Inferior], page 334).
gdb.parse_and_eval (expression)
[Function]
Parse expression, which must be a string, as an expression in the current language,
evaluate it, and return the result as a gdb.Value.
This function can be useful when implementing a new command (see Section 23.2.2.20
[Commands In Python], page 373), as it provides a way to parse the command’s
argument as an expression. It is also useful simply to compute values, for example, it is
the only way to get the value of a convenience variable (see Section 10.11 [Convenience
Vars], page 139) as a gdb.Value.
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gdb.find_pc_line (pc)
[Function]
Return the gdb.Symtab_and_line object corresponding to the pc value. See
Section 23.2.2.28 [Symbol Tables In Python], page 390. If an invalid value of pc
is passed as an argument, then the symtab and line attributes of the returned
gdb.Symtab_and_line object will be None and 0 respectively.
gdb.post_event (event)
[Function]
Put event, a callable object taking no arguments, into gdb’s internal event queue.
This callable will be invoked at some later point, during gdb’s event processing.
Events posted using post_event will be run in the order in which they were posted;
however, there is no way to know when they will be processed relative to other events
inside gdb.
gdb is not thread-safe. If your Python program uses multiple threads, you must be
careful to only call gdb-specific functions in the gdb thread. post_event ensures
this. For example:
(gdb) python
>import threading
>
>class Writer():
> def __init__(self, message):
>
self.message = message;
> def __call__(self):
>
gdb.write(self.message)
>
>class MyThread1 (threading.Thread):
> def run (self):
>
gdb.post_event(Writer("Hello "))
>
>class MyThread2 (threading.Thread):
> def run (self):
>
gdb.post_event(Writer("World\n"))
>
>MyThread1().start()
>MyThread2().start()
>end
(gdb) Hello World
gdb.write (string [, stream])
[Function]
Print a string to gdb’s paginated output stream. The optional stream determines
the stream to print to. The default stream is gdb’s standard output stream. Possible
stream values are:
gdb.STDOUT
gdb’s standard output stream.
gdb.STDERR
gdb’s standard error stream.
gdb.STDLOG
gdb’s log stream (see Section 2.4 [Logging Output], page 18).
Writing to sys.stdout or sys.stderr will automatically call this function and will
automatically direct the output to the relevant stream.
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gdb.flush ()
[Function]
Flush the buffer of a gdb paginated stream so that the contents are displayed immediately. gdb will flush the contents of a stream automatically when it encounters
a newline in the buffer. The optional stream determines the stream to flush. The
default stream is gdb’s standard output stream. Possible stream values are:
gdb.STDOUT
gdb’s standard output stream.
gdb.STDERR
gdb’s standard error stream.
gdb.STDLOG
gdb’s log stream (see Section 2.4 [Logging Output], page 18).
Flushing sys.stdout or sys.stderr will automatically call this function for the
relevant stream.
gdb.target_charset ()
[Function]
Return the name of the current target character set (see Section 10.20 [Character
Sets], page 152). This differs from gdb.parameter(’target-charset’) in that ‘auto’
is never returned.
gdb.target_wide_charset ()
[Function]
Return the name of the current target wide character set (see Section 10.20 [Character
Sets], page 152). This differs from gdb.parameter(’target-wide-charset’) in that
‘auto’ is never returned.
gdb.solib_name (address)
[Function]
Return the name of the shared library holding the given address as a string, or None.
gdb.decode_line [expression]
[Function]
Return locations of the line specified by expression, or of the current line if no argument was given. This function returns a Python tuple containing two elements. The
first element contains a string holding any unparsed section of expression (or None if
the expression has been fully parsed). The second element contains either None or
another tuple that contains all the locations that match the expression represented
as gdb.Symtab_and_line objects (see Section 23.2.2.28 [Symbol Tables In Python],
page 390). If expression is provided, it is decoded the way that gdb’s inbuilt break
or edit commands do (see Section 9.2 [Specify Location], page 104).
gdb.prompt_hook (current prompt)
[Function]
If prompt hook is callable, gdb will call the method assigned to this operation before
a prompt is displayed by gdb.
The parameter current_prompt contains the current gdb prompt. This method must
return a Python string, or None. If a string is returned, the gdb prompt will be set
to that string. If None is returned, gdb will continue to use the current prompt.
Some prompts cannot be substituted in gdb. Secondary prompts such as those used
by readline for command input, and annotation related prompts are prohibited from
being changed.
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23.2.2.2 Exception Handling
When executing the python command, Python exceptions uncaught within the Python
code are translated to calls to gdb error-reporting mechanism. If the command that called
python does not handle the error, gdb will terminate it and print an error message containing the Python exception name, the associated value, and the Python call stack backtrace
at the point where the exception was raised. Example:
(gdb) python print foo
Traceback (most recent call last):
File "", line 1, in
NameError: name ’foo’ is not defined
gdb errors that happen in gdb commands invoked by Python code are converted to
Python exceptions. The type of the Python exception depends on the error.
gdb.error
This is the base class for most exceptions generated by gdb. It is derived from
RuntimeError, for compatibility with earlier versions of gdb.
If an error occurring in gdb does not fit into some more specific category, then
the generated exception will have this type.
gdb.MemoryError
This is a subclass of gdb.error which is thrown when an operation tried to
access invalid memory in the inferior.
KeyboardInterrupt
User interrupt (via C-c or by typing q at a pagination prompt) is translated to
a Python KeyboardInterrupt exception.
In all cases, your exception handler will see the gdb error message as its value and the
Python call stack backtrace at the Python statement closest to where the gdb error occured
as the traceback.
When implementing gdb commands in Python via gdb.Command, it is useful to be able
to throw an exception that doesn’t cause a traceback to be printed. For example, the user
may have invoked the command incorrectly. Use the gdb.GdbError exception to handle
this case. Example:
(gdb) python
>class HelloWorld (gdb.Command):
> """Greet the whole world."""
> def __init__ (self):
>
super (HelloWorld, self).__init__ ("hello-world", gdb.COMMAND_USER)
> def invoke (self, args, from_tty):
>
argv = gdb.string_to_argv (args)
>
if len (argv) != 0:
>
raise gdb.GdbError ("hello-world takes no arguments")
>
print "Hello, World!"
>HelloWorld ()
>end
(gdb) hello-world 42
hello-world takes no arguments
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23.2.2.3 Values From Inferior
gdb provides values it obtains from the inferior program in an object of type gdb.Value.
gdb uses this object for its internal bookkeeping of the inferior’s values, and for fetching
values when necessary.
Inferior values that are simple scalars can be used directly in Python expressions that
are valid for the value’s data type. Here’s an example for an integer or floating-point value
some_val:
bar = some_val + 2
As result of this, bar will also be a gdb.Value object whose values are of the same type as
those of some_val. Valid Python operations can also be performed on gdb.Value objects
representing a struct or class object. For such cases, the overloaded operator (if present),
is used to perform the operation. For example, if val1 and val2 are gdb.Value objects
representing instances of a class which overloads the + operator, then one can use the +
operator in their Python script as follows:
val3 = val1 + val2
The result of the operation val3 is also a gdb.Value object corresponding to the value
returned by the overloaded + operator. In general, overloaded operators are invoked for the
following operations: + (binary addition), - (binary subtraction), * (multiplication), /, %,
<<, >>, |, &, ^.
Inferior values that are structures or instances of some class can be accessed using the
Python dictionary syntax. For example, if some_val is a gdb.Value instance holding a
structure, you can access its foo element with:
bar = some_val[’foo’]
Again, bar will also be a gdb.Value object. Structure elements can also be accessed by
using gdb.Field objects as subscripts (see Section 23.2.2.4 [Types In Python], page 338,
for more information on gdb.Field objects). For example, if foo_field is a gdb.Field
object corresponding to element foo of the above structure, then bar can also be accessed
as follows:
bar = some_val[foo_field]
A gdb.Value that represents a function can be executed via inferior function call. Any
arguments provided to the call must match the function’s prototype, and must be provided
in the order specified by that prototype.
For example, some_val is a gdb.Value instance representing a function that takes two
integers as arguments. To execute this function, call it like so:
result = some_val (10,20)
Any values returned from a function call will be stored as a gdb.Value.
The following attributes are provided:
[Variable]
If this object is addressable, this read-only attribute holds a gdb.Value object representing the address. Otherwise, this attribute holds None.
Value.address
[Variable]
This read-only boolean attribute is true if the compiler optimized out this value, thus
it is not available for fetching from the inferior.
Value.is_optimized_out
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[Variable]
The type of this gdb.Value. The value of this attribute is a gdb.Type object (see
Section 23.2.2.4 [Types In Python], page 338).
Value.type
[Variable]
The dynamic type of this gdb.Value. This uses C++ run-time type information (RTTI)
to determine the dynamic type of the value. If this value is of class type, it will return
the class in which the value is embedded, if any. If this value is of pointer or reference
to a class type, it will compute the dynamic type of the referenced object, and return
a pointer or reference to that type, respectively. In all other cases, it will return the
value’s static type.
Note that this feature will only work when debugging a C++ program that includes
RTTI for the object in question. Otherwise, it will just return the static type of the
value as in ptype foo (see Chapter 16 [Symbols], page 221).
Value.dynamic_type
[Variable]
The value of this read-only boolean attribute is True if this gdb.Value has not yet
been fetched from the inferior. gdb does not fetch values until necessary, for efficiency.
For example:
Value.is_lazy
myval = gdb.parse_and_eval (’somevar’)
The value of somevar is not fetched at this time. It will be fetched when the value is
needed, or when the fetch_lazy method is invoked.
The following methods are provided:
Value.__init__ (val)
[Function]
Many Python values can be converted directly to a gdb.Value via this object initializer. Specifically:
Python boolean
A Python boolean is converted to the boolean type from the current
language.
Python integer
A Python integer is converted to the C long type for the current architecture.
Python long
A Python long is converted to the C long long type for the current
architecture.
Python float
A Python float is converted to the C double type for the current architecture.
Python string
A Python string is converted to a target string in the current target
language using the current target encoding. If a character cannot be
represented in the current target encoding, then an exception is thrown.
gdb.Value
If val is a gdb.Value, then a copy of the value is made.
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gdb.LazyString
If val is a gdb.LazyString (see Section 23.2.2.32 [Lazy Strings In
Python], page 396), then the lazy string’s value method is called, and
its result is used.
Value.cast (type)
[Function]
Return a new instance of gdb.Value that is the result of casting this instance to the
type described by type, which must be a gdb.Type object. If the cast cannot be
performed for some reason, this method throws an exception.
Value.dereference ()
[Function]
For pointer data types, this method returns a new gdb.Value object whose contents
is the object pointed to by the pointer. For example, if foo is a C pointer to an int,
declared in your C program as
int *foo;
then you can use the corresponding gdb.Value to access what foo points to like this:
bar = foo.dereference ()
The result bar will be a gdb.Value object holding the value pointed to by foo.
A similar function Value.referenced_value exists which also returns gdb.Value objects corresonding to the values pointed to by pointer values (and additionally, values
referenced by reference values). However, the behavior of Value.dereference differs
from Value.referenced_value by the fact that the behavior of Value.dereference
is identical to applying the C unary operator * on a given value. For example, consider
a reference to a pointer ptrref, declared in your C++ program as
typedef int *intptr;
...
int val = 10;
intptr ptr = &val;
intptr &ptrref = ptr;
Though ptrref is a reference value, one can apply the method Value.dereference to
the gdb.Value object corresponding to it and obtain a gdb.Value which is identical
to that corresponding to val. However, if you apply the method Value.referenced_
value, the result would be a gdb.Value object identical to that corresponding to
ptr.
py_ptrref = gdb.parse_and_eval ("ptrref")
py_val = py_ptrref.dereference ()
py_ptr = py_ptrref.referenced_value ()
The gdb.Value object py_val is identical to that corresponding to val, and py_
ptr is identical to that corresponding to ptr. In general, Value.dereference can be
applied whenever the C unary operator * can be applied to the corresponding C value.
For those cases where applying both Value.dereference and Value.referenced_
value is allowed, the results obtained need not be identical (as we have seen in the
above example). The results are however identical when applied on gdb.Value objects
corresponding to pointers (gdb.Value objects with type code TYPE_CODE_PTR) in a
C/C++ program.
Value.referenced_value ()
[Function]
For pointer or reference data types, this method returns a new gdb.Value object
corresponding to the value referenced by the pointer/reference value. For pointer data
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types, Value.dereference and Value.referenced_value produce identical results.
The difference between these methods is that Value.dereference cannot get the
values referenced by reference values. For example, consider a reference to an int,
declared in your C++ program as
int val = 10;
int &ref = val;
then applying Value.dereference to the gdb.Value object corresponding to ref will
result in an error, while applying Value.referenced_value will result in a gdb.Value
object identical to that corresponding to val.
py_ref = gdb.parse_and_eval ("ref")
er_ref = py_ref.dereference ()
py_val = py_ref.referenced_value ()
# Results in error
# Returns the referenced value
The gdb.Value object py_val is identical to that corresponding to val.
Value.reference_value ()
[Function]
Return a gdb.Value object which is a reference to the value encapsulated by this
instance.
Value.const_value ()
[Function]
Return a gdb.Value object which is a const version of the value encapsulated by this
instance.
Value.dynamic_cast (type)
[Function]
Like Value.cast, but works as if the C++ dynamic_cast operator were used. Consult
a C++ reference for details.
Value.reinterpret_cast (type)
[Function]
Like Value.cast, but works as if the C++ reinterpret_cast operator were used.
Consult a C++ reference for details.
Value.string ([encoding[, errors[, length]]])
[Function]
If this gdb.Value represents a string, then this method converts the contents to a
Python string. Otherwise, this method will throw an exception.
Values are interpreted as strings according to the rules of the current language. If
the optional length argument is given, the string will be converted to that length,
and will include any embedded zeroes that the string may contain. Otherwise, for
languages where the string is zero-terminated, the entire string will be converted.
For example, in C-like languages, a value is a string if it is a pointer to or an array of
characters or ints of type wchar_t, char16_t, or char32_t.
If the optional encoding argument is given, it must be a string naming the encoding
of the string in the gdb.Value, such as "ascii", "iso-8859-6" or "utf-8". It accepts the same encodings as the corresponding argument to Python’s string.decode
method, and the Python codec machinery will be used to convert the string. If encoding is not given, or if encoding is the empty string, then either the target-charset
(see Section 10.20 [Character Sets], page 152) will be used, or a language-specific
encoding will be used, if the current language is able to supply one.
The optional errors argument is the same as the corresponding argument to Python’s
string.decode method.
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If the optional length argument is given, the string will be fetched and converted to
the given length.
Value.lazy_string ([encoding [, length]])
[Function]
If this gdb.Value represents a string, then this method converts the contents to a
gdb.LazyString (see Section 23.2.2.32 [Lazy Strings In Python], page 396). Otherwise, this method will throw an exception.
If the optional encoding argument is given, it must be a string naming the encoding
of the gdb.LazyString. Some examples are: ‘ascii’, ‘iso-8859-6’ or ‘utf-8’. If the
encoding argument is an encoding that gdb does recognize, gdb will raise an error.
When a lazy string is printed, the gdb encoding machinery is used to convert the
string during printing. If the optional encoding argument is not provided, or is an
empty string, gdb will automatically select the encoding most suitable for the string
type. For further information on encoding in gdb please see Section 10.20 [Character
Sets], page 152.
If the optional length argument is given, the string will be fetched and encoded to
the length of characters specified. If the length argument is not provided, the string
will be fetched and encoded until a null of appropriate width is found.
Value.fetch_lazy ()
[Function]
If the gdb.Value object is currently a lazy value (gdb.Value.is_lazy is True), then
the value is fetched from the inferior. Any errors that occur in the process will produce
a Python exception.
If the gdb.Value object is not a lazy value, this method has no effect.
This method does not return a value.
23.2.2.4 Types In Python
gdb represents types from the inferior using the class gdb.Type.
The following type-related functions are available in the gdb module:
gdb.lookup_type (name [, block])
[Function]
This function looks up a type by its name, which must be a string.
If block is given, then name is looked up in that scope. Otherwise, it is searched for
globally.
Ordinarily, this function will return an instance of gdb.Type. If the named type
cannot be found, it will throw an exception.
If the type is a structure or class type, or an enum type, the fields of that type can be
accessed using the Python dictionary syntax. For example, if some_type is a gdb.Type
instance holding a structure type, you can access its foo field with:
bar = some_type[’foo’]
bar will be a gdb.Field object; see below under the description of the Type.fields
method for a description of the gdb.Field class.
An instance of Type has the following attributes:
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[Variable]
The type code for this type. The type code will be one of the TYPE_CODE_ constants
defined below.
Type.code
[Variable]
Type.name
The name of this type. If this type has no name, then None is returned.
[Variable]
The size of this type, in target char units. Usually, a target’s char type will be an
8-bit byte. However, on some unusual platforms, this type may have a different size.
Type.sizeof
[Variable]
The tag name for this type. The tag name is the name after struct, union, or enum
in C and C++; not all languages have this concept. If this type has no tag name, then
None is returned.
Type.tag
The following methods are provided:
Type.fields ()
[Function]
For structure and union types, this method returns the fields. Range types have
two fields, the minimum and maximum values. Enum types have one field per enum
constant. Function and method types have one field per parameter. The base types
of C++ classes are also represented as fields. If the type has no fields, or does not fit
into one of these categories, an empty sequence will be returned.
Each field is a gdb.Field object, with some pre-defined attributes:
bitpos
This attribute is not available for enum or static (as in C++) fields. The
value is the position, counting in bits, from the start of the containing
type.
enumval
This attribute is only available for enum fields, and its value is the enumeration member’s integer representation.
name
The name of the field, or None for anonymous fields.
artificial
This is True if the field is artificial, usually meaning that it was provided
by the compiler and not the user. This attribute is always provided, and
is False if the field is not artificial.
is_base_class
This is True if the field represents a base class of a C++ structure. This
attribute is always provided, and is False if the field is not a base class
of the type that is the argument of fields, or if that type was not a C++
class.
bitsize
If the field is packed, or is a bitfield, then this will have a non-zero value,
which is the size of the field in bits. Otherwise, this will be zero; in this
case the field’s size is given by its type.
type
The type of the field. This is usually an instance of Type, but it can be
None in some situations.
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parent_type
The type which contains this field. This is an instance of gdb.Type.
Type.array (n1 [, n2])
[Function]
Return a new gdb.Type object which represents an array of this type. If one argument
is given, it is the inclusive upper bound of the array; in this case the lower bound is
zero. If two arguments are given, the first argument is the lower bound of the array,
and the second argument is the upper bound of the array. An array’s length must
not be negative, but the bounds can be.
Type.vector (n1 [, n2])
[Function]
Return a new gdb.Type object which represents a vector of this type. If one argument
is given, it is the inclusive upper bound of the vector; in this case the lower bound is
zero. If two arguments are given, the first argument is the lower bound of the vector,
and the second argument is the upper bound of the vector. A vector’s length must
not be negative, but the bounds can be.
The difference between an array and a vector is that arrays behave like in C: when
used in expressions they decay to a pointer to the first element whereas vectors are
treated as first class values.
Type.const ()
[Function]
Return a new gdb.Type object which represents a const-qualified variant of this type.
Type.volatile ()
[Function]
Return a new gdb.Type object which represents a volatile-qualified variant of this
type.
Type.unqualified ()
[Function]
Return a new gdb.Type object which represents an unqualified variant of this type.
That is, the result is neither const nor volatile.
Type.range ()
[Function]
Return a Python Tuple object that contains two elements: the low bound of the
argument type and the high bound of that type. If the type does not have a range,
gdb will raise a gdb.error exception (see Section 23.2.2.2 [Exception Handling],
page 333).
Type.reference ()
[Function]
Return a new gdb.Type object which represents a reference to this type.
Type.pointer ()
[Function]
Return a new gdb.Type object which represents a pointer to this type.
Type.strip_typedefs ()
[Function]
Return a new gdb.Type that represents the real type, after removing all layers of
typedefs.
Type.target ()
[Function]
Return a new gdb.Type object which represents the target type of this type.
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For a pointer type, the target type is the type of the pointed-to object. For an array
type (meaning C-like arrays), the target type is the type of the elements of the array.
For a function or method type, the target type is the type of the return value. For a
complex type, the target type is the type of the elements. For a typedef, the target
type is the aliased type.
If the type does not have a target, this method will throw an exception.
Type.template_argument (n [, block])
[Function]
If this gdb.Type is an instantiation of a template, this will return a new gdb.Value or
gdb.Type which represents the value of the nth template argument (indexed starting
at 0).
If this gdb.Type is not a template type, or if the type has fewer than n template
arguments, this will throw an exception. Ordinarily, only C++ code will have template
types.
If block is given, then name is looked up in that scope. Otherwise, it is searched for
globally.
Type.optimized_out ()
[Function]
Return gdb.Value instance of this type whose value is optimized out. This allows a
frame decorator to indicate that the value of an argument or a local variable is not
known.
Each type has a code, which indicates what category this type falls into. The available
type categories are represented by constants defined in the gdb module:
gdb.TYPE_CODE_PTR
The type is a pointer.
gdb.TYPE_CODE_ARRAY
The type is an array.
gdb.TYPE_CODE_STRUCT
The type is a structure.
gdb.TYPE_CODE_UNION
The type is a union.
gdb.TYPE_CODE_ENUM
The type is an enum.
gdb.TYPE_CODE_FLAGS
A bit flags type, used for things such as status registers.
gdb.TYPE_CODE_FUNC
The type is a function.
gdb.TYPE_CODE_INT
The type is an integer type.
gdb.TYPE_CODE_FLT
A floating point type.
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gdb.TYPE_CODE_VOID
The special type void.
gdb.TYPE_CODE_SET
A Pascal set type.
gdb.TYPE_CODE_RANGE
A range type, that is, an integer type with bounds.
gdb.TYPE_CODE_STRING
A string type. Note that this is only used for certain languages with languagedefined string types; C strings are not represented this way.
gdb.TYPE_CODE_BITSTRING
A string of bits. It is deprecated.
gdb.TYPE_CODE_ERROR
An unknown or erroneous type.
gdb.TYPE_CODE_METHOD
A method type, as found in C++.
gdb.TYPE_CODE_METHODPTR
A pointer-to-member-function.
gdb.TYPE_CODE_MEMBERPTR
A pointer-to-member.
gdb.TYPE_CODE_REF
A reference type.
gdb.TYPE_CODE_RVALUE_REF
A C++11 rvalue reference type.
gdb.TYPE_CODE_CHAR
A character type.
gdb.TYPE_CODE_BOOL
A boolean type.
gdb.TYPE_CODE_COMPLEX
A complex float type.
gdb.TYPE_CODE_TYPEDEF
A typedef to some other type.
gdb.TYPE_CODE_NAMESPACE
A C++ namespace.
gdb.TYPE_CODE_DECFLOAT
A decimal floating point type.
gdb.TYPE_CODE_INTERNAL_FUNCTION
A function internal to gdb. This is the type used to represent convenience
functions.
Further support for types is provided in the gdb.types Python module (see
Section 23.2.4.2 [gdb.types], page 399).
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23.2.2.5 Pretty Printing API
An example output is provided (see Section 10.9 [Pretty Printing], page 135).
A pretty-printer is just an object that holds a value and implements a specific interface,
defined here.
pretty_printer.children (self)
[Function]
gdb will call this method on a pretty-printer to compute the children of the prettyprinter’s value.
This method must return an object conforming to the Python iterator protocol. Each
item returned by the iterator must be a tuple holding two elements. The first element
is the “name” of the child; the second element is the child’s value. The value can be
any Python object which is convertible to a gdb value.
This method is optional. If it does not exist, gdb will act as though the value has no
children.
pretty_printer.display_hint (self)
[Function]
The CLI may call this method and use its result to change the formatting of a value.
The result will also be supplied to an MI consumer as a ‘displayhint’ attribute of
the variable being printed.
This method is optional. If it does exist, this method must return a string.
Some display hints are predefined by gdb:
‘array’
Indicate that the object being printed is “array-like”. The CLI uses this to
respect parameters such as set print elements and set print array.
‘map’
Indicate that the object being printed is “map-like”, and that the children
of this value can be assumed to alternate between keys and values.
‘string’
Indicate that the object being printed is “string-like”. If the printer’s to_
string method returns a Python string of some kind, then gdb will call
its internal language-specific string-printing function to format the string.
For the CLI this means adding quotation marks, possibly escaping some
characters, respecting set print elements, and the like.
pretty_printer.to_string (self)
[Function]
gdb will call this method to display the string representation of the value passed to
the object’s constructor.
When printing from the CLI, if the to_string method exists, then gdb will prepend
its result to the values returned by children. Exactly how this formatting is done
is dependent on the display hint, and may change as more hints are added. Also,
depending on the print settings (see Section 10.8 [Print Settings], page 127), the
CLI may print just the result of to_string in a stack trace, omitting the result of
children.
If this method returns a string, it is printed verbatim.
Otherwise, if this method returns an instance of gdb.Value, then gdb prints this
value. This may result in a call to another pretty-printer.
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If instead the method returns a Python value which is convertible to a gdb.Value,
then gdb performs the conversion and prints the resulting value. Again, this may result in a call to another pretty-printer. Python scalars (integers, floats, and booleans)
and strings are convertible to gdb.Value; other types are not.
Finally, if this method returns None then no further operations are peformed in this
method and nothing is printed.
If the result is not one of these types, an exception is raised.
gdb provides a function which can be used to look up the default pretty-printer for a
gdb.Value:
gdb.default_visualizer (value)
[Function]
This function takes a gdb.Value object as an argument. If a pretty-printer for this
value exists, then it is returned. If no such printer exists, then this returns None.
23.2.2.6 Selecting Pretty-Printers
The Python list gdb.pretty_printers contains an array of functions or callable objects
that have been registered via addition as a pretty-printer. Printers in this list are called
global printers, they’re available when debugging all inferiors. Each gdb.Progspace contains a pretty_printers attribute. Each gdb.Objfile also contains a pretty_printers
attribute.
Each function on these lists is passed a single gdb.Value argument and should return
a pretty-printer object conforming to the interface definition above (see Section 23.2.2.5
[Pretty Printing API], page 343). If a function cannot create a pretty-printer for the value,
it should return None.
gdb first checks the pretty_printers attribute of each gdb.Objfile in the current program space and iteratively calls each enabled lookup routine in the list for that gdb.Objfile
until it receives a pretty-printer object. If no pretty-printer is found in the objfile lists, gdb
then searches the pretty-printer list of the current program space, calling each enabled function until an object is returned. After these lists have been exhausted, it tries the global
gdb.pretty_printers list, again calling each enabled function until an object is returned.
The order in which the objfiles are searched is not specified. For a given list, functions
are always invoked from the head of the list, and iterated over sequentially until the end of
the list, or a printer object is returned.
For various reasons a pretty-printer may not work. For example, the underlying data
structure may have changed and the pretty-printer is out of date.
The consequences of a broken pretty-printer are severe enough that gdb provides support
for enabling and disabling individual printers. For example, if print frame-arguments is
on, a backtrace can become highly illegible if any argument is printed with a broken printer.
Pretty-printers are enabled and disabled by attaching an enabled attribute to the registered function or callable object. If this attribute is present and its value is False, the
printer is disabled, otherwise the printer is enabled.
23.2.2.7 Writing a Pretty-Printer
A pretty-printer consists of two parts: a lookup function to detect if the type is supported,
and the printer itself.
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Here is an example showing how a std::string printer might be written. See
Section 23.2.2.5 [Pretty Printing API], page 343, for details on the API this class must
provide.
class StdStringPrinter(object):
"Print a std::string"
def __init__(self, val):
self.val = val
def to_string(self):
return self.val[’_M_dataplus’][’_M_p’]
def display_hint(self):
return ’string’
And here is an example showing how a lookup function for the printer example above
might be written.
def str_lookup_function(val):
lookup_tag = val.type.tag
if lookup_tag == None:
return None
regex = re.compile("^std::basic_string$")
if regex.match(lookup_tag):
return StdStringPrinter(val)
return None
The example lookup function extracts the value’s type, and attempts to match it to a
type that it can pretty-print. If it is a type the printer can pretty-print, it will return a
printer object. If not, it returns None.
We recommend that you put your core pretty-printers into a Python package. If your
pretty-printers are for use with a library, we further recommend embedding a version number
into the package name. This practice will enable gdb to load multiple versions of your
pretty-printers at the same time, because they will have different names.
You should write auto-loaded code (see Section 23.2.3 [Python Auto-loading], page 398)
such that it can be evaluated multiple times without changing its meaning. An ideal autoload file will consist solely of imports of your printer modules, followed by a call to a register
pretty-printers with the current objfile.
Taken as a whole, this approach will scale nicely to multiple inferiors, each potentially
using a different library version. Embedding a version number in the Python package name
will ensure that gdb is able to load both sets of printers simultaneously. Then, because the
search for pretty-printers is done by objfile, and because your auto-loaded code took care
to register your library’s printers with a specific objfile, gdb will find the correct printers
for the specific version of the library used by each inferior.
To continue the std::string example (see Section 23.2.2.5 [Pretty Printing API],
page 343), this code might appear in gdb.libstdcxx.v6:
def register_printers(objfile):
objfile.pretty_printers.append(str_lookup_function)
And then the corresponding contents of the auto-load file would be:
import gdb.libstdcxx.v6
gdb.libstdcxx.v6.register_printers(gdb.current_objfile())
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The previous example illustrates a basic pretty-printer. There are a few things that can
be improved on. The printer doesn’t have a name, making it hard to identify in a list of
installed printers. The lookup function has a name, but lookup functions can have arbitrary,
even identical, names.
Second, the printer only handles one type, whereas a library typically has several types.
One could install a lookup function for each desired type in the library, but one could also
have a single lookup function recognize several types. The latter is the conventional way
this is handled. If a pretty-printer can handle multiple data types, then its subprinters are
the printers for the individual data types.
The gdb.printing module provides a formal way of solving these problems (see
Section 23.2.4.1 [gdb.printing], page 399). Here is another example that handles multiple
types.
These are the types we are going to pretty-print:
struct foo { int a, b; };
struct bar { struct foo x, y; };
Here are the printers:
class fooPrinter:
"""Print a foo object."""
def __init__(self, val):
self.val = val
def to_string(self):
return ("a=<" + str(self.val["a"]) +
"> b=<" + str(self.val["b"]) + ">")
class barPrinter:
"""Print a bar object."""
def __init__(self, val):
self.val = val
def to_string(self):
return ("x=<" + str(self.val["x"]) +
"> y=<" + str(self.val["y"]) + ">")
This example doesn’t need a lookup function, that is handled by the gdb.printing
module. Instead a function is provided to build up the object that handles the lookup.
import gdb.printing
def build_pretty_printer():
pp = gdb.printing.RegexpCollectionPrettyPrinter(
"my_library")
pp.add_printer(’foo’, ’^foo$’, fooPrinter)
pp.add_printer(’bar’, ’^bar$’, barPrinter)
return pp
And here is the autoload support:
import gdb.printing
import my_library
gdb.printing.register_pretty_printer(
gdb.current_objfile(),
my_library.build_pretty_printer())
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Finally, when this printer is loaded into gdb, here is the corresponding output of ‘info
pretty-printer’:
(gdb) info pretty-printer
my_library.so:
my_library
foo
bar
23.2.2.8 Type Printing API
gdb provides a way for Python code to customize type display. This is mainly useful for
substituting canonical typedef names for types.
A type printer is just a Python object conforming to a certain protocol. A simple base
class implementing the protocol is provided; see Section 23.2.4.2 [gdb.types], page 399. A
type printer must supply at least:
[Instance Variable of type_printer]
A boolean which is True if the printer is enabled, and False otherwise. This is manipulated by the enable type-printer and disable type-printer commands.
enabled
name
[Instance Variable of type_printer]
The name of the type printer. This must be a string. This is used by the enable
type-printer and disable type-printer commands.
instantiate (self)
[Method on type_printer]
This is called by gdb at the start of type-printing. It is only called if the type printer
is enabled. This method must return a new object that supplies a recognize method,
as described below.
When displaying a type, say via the ptype command, gdb will compute a list of
type recognizers. This is done by iterating first over the per-objfile type printers (see
Section 23.2.2.24 [Objfiles In Python], page 380), followed by the per-progspace type printers (see Section 23.2.2.23 [Progspaces In Python], page 379), and finally the global type
printers.
gdb will call the instantiate method of each enabled type printer. If this method
returns None, then the result is ignored; otherwise, it is appended to the list of recognizers.
Then, when gdb is going to display a type name, it iterates over the list of recognizers.
For each one, it calls the recognition function, stopping if the function returns a non-None
value. The recognition function is defined as:
recognize (self, type)
[Method on type_recognizer]
If type is not recognized, return None. Otherwise, return a string which is to be
printed as the name of type. The type argument will be an instance of gdb.Type (see
Section 23.2.2.4 [Types In Python], page 338).
gdb uses this two-pass approach so that type printers can efficiently cache information
without holding on to it too long. For example, it can be convenient to look up type
information in a type printer and hold it for a recognizer’s lifetime; if a single pass were
done then type printers would have to make use of the event system in order to avoid
holding information that could become stale as the inferior changed.
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23.2.2.9 Filtering Frames.
Frame filters are Python objects that manipulate the visibility of a frame or frames when a
backtrace (see Section 8.2 [Backtrace], page 96) is printed by gdb.
Only commands that print a backtrace, or, in the case of gdb/mi commands (see
Chapter 27 [GDB/MI], page 469), those that return a collection of frames are affected.
The commands that work with frame filters are:
backtrace (see [The backtrace command], page 96), -stack-list-frames (see [The
-stack-list-frames command], page 511), -stack-list-variables (see [The -stack-listvariables command], page 513), -stack-list-arguments see [The -stack-list-arguments
command], page 510) and -stack-list-locals (see [The -stack-list-locals command],
page 513).
A frame filter works by taking an iterator as an argument, applying actions to the
contents of that iterator, and returning another iterator (or, possibly, the same iterator it
was provided in the case where the filter does not perform any operations). Typically, frame
filters utilize tools such as the Python’s itertools module to work with and create new
iterators from the source iterator. Regardless of how a filter chooses to apply actions, it
must not alter the underlying gdb frame or frames, or attempt to alter the call-stack within
gdb. This preserves data integrity within gdb. Frame filters are executed on a priority
basis and care should be taken that some frame filters may have been executed before, and
that some frame filters will be executed after.
An important consideration when designing frame filters, and well worth reflecting upon,
is that frame filters should avoid unwinding the call stack if possible. Some stacks can run
very deep, into the tens of thousands in some cases. To search every frame when a frame
filter executes may be too expensive at that step. The frame filter cannot know how many
frames it has to iterate over, and it may have to iterate through them all. This ends up
duplicating effort as gdb performs this iteration when it prints the frames. If the filter can
defer unwinding frames until frame decorators are executed, after the last filter has executed,
it should. See Section 23.2.2.10 [Frame Decorator API], page 350, for more information on
decorators. Also, there are examples for both frame decorators and filters in later chapters.
See Section 23.2.2.11 [Writing a Frame Filter], page 352, for more information.
The Python dictionary gdb.frame_filters contains key/object pairings that comprise a frame filter. Frame filters in this dictionary are called global frame filters, and
they are available when debugging all inferiors. These frame filters must register with
the dictionary directly. In addition to the global dictionary, there are other dictionaries that are loaded with different inferiors via auto-loading (see Section 23.2.3 [Python
Auto-loading], page 398). The two other areas where frame filter dictionaries can be
found are: gdb.Progspace which contains a frame_filters dictionary attribute, and each
gdb.Objfile object which also contains a frame_filters dictionary attribute.
When a command is executed from gdb that is compatible with frame filters, gdb
combines the global, gdb.Progspace and all gdb.Objfile dictionaries currently loaded.
All of the gdb.Objfile dictionaries are combined, as several frames, and thus several object
files, might be in use. gdb then prunes any frame filter whose enabled attribute is False.
This pruned list is then sorted according to the priority attribute in each filter.
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Once the dictionaries are combined, pruned and sorted, gdb creates an iterator which
wraps each frame in the call stack in a FrameDecorator object, and calls each filter in order.
The output from the previous filter will always be the input to the next filter, and so on.
Frame filters have a mandatory interface which each frame filter must implement, defined
here:
FrameFilter.filter (iterator)
[Function]
gdb will call this method on a frame filter when it has reached the order in the priority
list for that filter.
For example, if there are four frame filters:
Name
Priority
Filter1
Filter2
Filter3
Filter4
5
10
100
1
The order that the frame filters will be called is:
Filter3 -> Filter2 -> Filter1 -> Filter4
Note that the output from Filter3 is passed to the input of Filter2, and so on.
This filter method is passed a Python iterator. This iterator contains a sequence of
frame decorators that wrap each gdb.Frame, or a frame decorator that wraps another
frame decorator. The first filter that is executed in the sequence of frame filters will
receive an iterator entirely comprised of default FrameDecorator objects. However,
after each frame filter is executed, the previous frame filter may have wrapped some
or all of the frame decorators with their own frame decorator. As frame decorators
must also conform to a mandatory interface, these decorators can be assumed to act
in a uniform manner (see Section 23.2.2.10 [Frame Decorator API], page 350).
This method must return an object conforming to the Python iterator protocol. Each
item in the iterator must be an object conforming to the frame decorator interface.
If a frame filter does not wish to perform any operations on this iterator, it should
return that iterator untouched.
This method is not optional. If it does not exist, gdb will raise and print an error.
[Variable]
The name attribute must be Python string which contains the name of the filter displayed by gdb (see Section 8.5 [Frame Filter Management], page 100). This attribute
may contain any combination of letters or numbers. Care should be taken to ensure
that it is unique. This attribute is mandatory.
FrameFilter.name
[Variable]
The enabled attribute must be Python boolean. This attribute indicates to gdb
whether the frame filter is enabled, and should be considered when frame filters are
executed. If enabled is True, then the frame filter will be executed when any of
the backtrace commands detailed earlier in this chapter are executed. If enabled is
False, then the frame filter will not be executed. This attribute is mandatory.
FrameFilter.enabled
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[Variable]
The priority attribute must be Python integer. This attribute controls the order
of execution in relation to other frame filters. There are no imposed limits on the
range of priority other than it must be a valid integer. The higher the priority
attribute, the sooner the frame filter will be executed in relation to other frame filters.
Although priority can be negative, it is recommended practice to assume zero is
the lowest priority that a frame filter can be assigned. Frame filters that have the
same priority are executed in unsorted order in that priority slot. This attribute is
mandatory.
FrameFilter.priority
23.2.2.10 Decorating Frames.
Frame decorators are sister objects to frame filters (see Section 23.2.2.9 [Frame Filter API],
page 348). Frame decorators are applied by a frame filter and can only be used in conjunction with frame filters.
The purpose of a frame decorator is to customize the printed content of each gdb.Frame
in commands where frame filters are executed. This concept is called decorating a frame.
Frame decorators decorate a gdb.Frame with Python code contained within each API call.
This separates the actual data contained in a gdb.Frame from the decorated data produced by a frame decorator. This abstraction is necessary to maintain integrity of the data
contained in each gdb.Frame.
Frame decorators have a mandatory interface, defined below.
gdb already contains a frame decorator called FrameDecorator. This contains substantial amounts of boilerplate code to decorate the content of a gdb.Frame. It is recommended
that other frame decorators inherit and extend this object, and only to override the methods
needed.
FrameDecorator.elided (self)
[Function]
The elided method groups frames together in a hierarchical system. An example
would be an interpreter, where multiple low-level frames make up a single call in
the interpreted language. In this example, the frame filter would elide the low-level
frames and present a single high-level frame, representing the call in the interpreted
language, to the user.
The elided function must return an iterable and this iterable must contain the frames
that are being elided wrapped in a suitable frame decorator. If no frames are being
elided this function may return an empty iterable, or None. Elided frames are indented
from normal frames in a CLI backtrace, or in the case of GDB/MI, are placed in the
children field of the eliding frame.
It is the frame filter’s task to also filter out the elided frames from the source iterator.
This will avoid printing the frame twice.
FrameDecorator.function (self)
[Function]
This method returns the name of the function in the frame that is to be printed.
This method must return a Python string describing the function, or None.
If this function returns None, gdb will not print any data for this field.
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FrameDecorator.address (self)
351
[Function]
This method returns the address of the frame that is to be printed.
This method must return a Python numeric integer type of sufficient size to describe
the address of the frame, or None.
If this function returns a None, gdb will not print any data for this field.
FrameDecorator.filename (self)
[Function]
This method returns the filename and path associated with this frame.
This method must return a Python string containing the filename and the path to
the object file backing the frame, or None.
If this function returns a None, gdb will not print any data for this field.
FrameDecorator.line (self):
[Function]
This method returns the line number associated with the current position within the
function addressed by this frame.
This method must return a Python integer type, or None.
If this function returns a None, gdb will not print any data for this field.
FrameDecorator.frame_args (self)
[Function]
This method must return an iterable, or None. Returning an empty iterable, or None
means frame arguments will not be printed for this frame. This iterable must contain
objects that implement two methods, described here.
This object must implement a argument method which takes a single self parameter
and must return a gdb.Symbol (see Section 23.2.2.27 [Symbols In Python], page 387),
or a Python string. The object must also implement a value method which takes
a single self parameter and must return a gdb.Value (see Section 23.2.2.3 [Values
From Inferior], page 334), a Python value, or None. If the value method returns
None, and the argument method returns a gdb.Symbol, gdb will look-up and print
the value of the gdb.Symbol automatically.
A brief example:
class SymValueWrapper():
def __init__(self, symbol, value):
self.sym = symbol
self.val = value
def value(self):
return self.val
def symbol(self):
return self.sym
class SomeFrameDecorator()
...
...
def frame_args(self):
args = []
try:
block = self.inferior_frame.block()
except:
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return None
# Iterate over all symbols in a block. Only add
# symbols that are arguments.
for sym in block:
if not sym.is_argument:
continue
args.append(SymValueWrapper(sym,None))
# Add example synthetic argument.
args.append(SymValueWrapper(‘‘foo’’, 42))
return args
FrameDecorator.frame_locals (self)
[Function]
This method must return an iterable or None. Returning an empty iterable, or None
means frame local arguments will not be printed for this frame.
The object interface, the description of the various strategies for reading frame locals,
and the example are largely similar to those described in the frame_args function,
(see [The frame filter frame args function], page 351). Below is a modified example:
class SomeFrameDecorator()
...
...
def frame_locals(self):
vars = []
try:
block = self.inferior_frame.block()
except:
return None
# Iterate over all symbols in a block. Add all
# symbols, except arguments.
for sym in block:
if sym.is_argument:
continue
vars.append(SymValueWrapper(sym,None))
# Add an example of a synthetic local variable.
vars.append(SymValueWrapper(‘‘bar’’, 99))
return vars
FrameDecorator.inferior_frame (self):
[Function]
This method must return the underlying gdb.Frame that this frame decorator is
decorating. gdb requires the underlying frame for internal frame information to
determine how to print certain values when printing a frame.
23.2.2.11 Writing a Frame Filter
There are three basic elements that a frame filter must implement: it must correctly implement the documented interface (see Section 23.2.2.9 [Frame Filter API], page 348), it must
register itself with gdb, and finally, it must decide if it is to work on the data provided by
gdb. In all cases, whether it works on the iterator or not, each frame filter must return an
iterator. A bare-bones frame filter follows the pattern in the following example.
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import gdb
class FrameFilter():
def __init__(self):
# Frame filter attribute creation.
#
# ’name’ is the name of the filter that GDB will display.
#
# ’priority’ is the priority of the filter relative to other
# filters.
#
# ’enabled’ is a boolean that indicates whether this filter is
# enabled and should be executed.
self.name = "Foo"
self.priority = 100
self.enabled = True
# Register this frame filter with the global frame_filters
# dictionary.
gdb.frame_filters[self.name] = self
def filter(self, frame_iter):
# Just return the iterator.
return frame_iter
The frame filter in the example above implements the three requirements for all frame
filters. It implements the API, self registers, and makes a decision on the iterator (in this
case, it just returns the iterator untouched).
The first step is attribute creation and assignment, and as shown in the comments the
filter assigns the following attributes: name, priority and whether the filter should be
enabled with the enabled attribute.
The second step is registering the frame filter with the dictionary or dictionaries that
the frame filter has interest in. As shown in the comments, this filter just registers itself
with the global dictionary gdb.frame_filters. As noted earlier, gdb.frame_filters is a
dictionary that is initialized in the gdb module when gdb starts. What dictionary a filter
registers with is an important consideration. Generally, if a filter is specific to a set of code,
it should be registered either in the objfile or progspace dictionaries as they are specific
to the program currently loaded in gdb. The global dictionary is always present in gdb
and is never unloaded. Any filters registered with the global dictionary will exist until gdb
exits. To avoid filters that may conflict, it is generally better to register frame filters against
the dictionaries that more closely align with the usage of the filter currently in question.
See Section 23.2.3 [Python Auto-loading], page 398, for further information on auto-loading
Python scripts.
gdb takes a hands-off approach to frame filter registration, therefore it is the frame
filter’s responsibility to ensure registration has occurred, and that any exceptions are handled appropriately. In particular, you may wish to handle exceptions relating to Python
dictionary key uniqueness. It is mandatory that the dictionary key is the same as frame
filter’s name attribute. When a user manages frame filters (see Section 8.5 [Frame Filter Management], page 100), the names gdb will display are those contained in the name
attribute.
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The final step of this example is the implementation of the filter method. As shown
in the example comments, we define the filter method and note that the method must
take an iterator, and also must return an iterator. In this bare-bones example, the frame
filter is not very useful as it just returns the iterator untouched. However this is a valid
operation for frame filters that have the enabled attribute set, but decide not to operate
on any frames.
In the next example, the frame filter operates on all frames and utilizes a frame decorator
to perform some work on the frames. See Section 23.2.2.10 [Frame Decorator API], page 350,
for further information on the frame decorator interface.
This example works on inlined frames. It highlights frames which are inlined by tagging them with an “[inlined]” tag. By applying a frame decorator to all frames with the
Python itertools imap method, the example defers actions to the frame decorator. Frame
decorators are only processed when gdb prints the backtrace.
This introduces a new decision making topic: whether to perform decision making operations at the filtering step, or at the printing step. In this example’s approach, it does
not perform any filtering decisions at the filtering step beyond mapping a frame decorator
to each frame. This allows the actual decision making to be performed when each frame is
printed. This is an important consideration, and well worth reflecting upon when designing
a frame filter. An issue that frame filters should avoid is unwinding the stack if possible.
Some stacks can run very deep, into the tens of thousands in some cases. To search every
frame to determine if it is inlined ahead of time may be too expensive at the filtering step.
The frame filter cannot know how many frames it has to iterate over, and it would have
to iterate through them all. This ends up duplicating effort as gdb performs this iteration
when it prints the frames.
In this example decision making can be deferred to the printing step. As each frame is
printed, the frame decorator can examine each frame in turn when gdb iterates. From a
performance viewpoint, this is the most appropriate decision to make as it avoids duplicating
the effort that the printing step would undertake anyway. Also, if there are many frame
filters unwinding the stack during filtering, it can substantially delay the printing of the
backtrace which will result in large memory usage, and a poor user experience.
class InlineFilter():
def __init__(self):
self.name = "InlinedFrameFilter"
self.priority = 100
self.enabled = True
gdb.frame_filters[self.name] = self
def filter(self, frame_iter):
frame_iter = itertools.imap(InlinedFrameDecorator,
frame_iter)
return frame_iter
This frame filter is somewhat similar to the earlier example, except that the filter
method applies a frame decorator object called InlinedFrameDecorator to each element
in the iterator. The imap Python method is light-weight. It does not proactively iterate
over the iterator, but rather creates a new iterator which wraps the existing one.
Below is the frame decorator for this example.
class InlinedFrameDecorator(FrameDecorator):
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def __init__(self, fobj):
super(InlinedFrameDecorator, self).__init__(fobj)
def function(self):
frame = fobj.inferior_frame()
name = str(frame.name())
if frame.type() == gdb.INLINE_FRAME:
name = name + " [inlined]"
return name
This frame decorator only defines and overrides the function method. It lets the supplied FrameDecorator, which is shipped with gdb, perform the other work associated with
printing this frame.
The combination of these two objects create this output from a backtrace:
#0
#1
#2
0x004004e0 in bar () at inline.c:11
0x00400566 in max [inlined] (b=6, a=12) at inline.c:21
0x00400566 in main () at inline.c:31
So in the case of this example, a frame decorator is applied to all frames, regardless of
whether they may be inlined or not. As gdb iterates over the iterator produced by the
frame filters, gdb executes each frame decorator which then makes a decision on what to
print in the function callback. Using a strategy like this is a way to defer decisions on the
frame content to printing time.
Eliding Frames
It might be that the above example is not desirable for representing inlined frames, and a
hierarchical approach may be preferred. If we want to hierarchically represent frames, the
elided frame decorator interface might be preferable.
This example approaches the issue with the elided method. This example is quite long,
but very simplistic. It is out-of-scope for this section to write a complete example that
comprehensively covers all approaches of finding and printing inlined frames. However, this
example illustrates the approach an author might use.
This example comprises of three sections.
class InlineFrameFilter():
def __init__(self):
self.name = "InlinedFrameFilter"
self.priority = 100
self.enabled = True
gdb.frame_filters[self.name] = self
def filter(self, frame_iter):
return ElidingInlineIterator(frame_iter)
This frame filter is very similar to the other examples. The only difference is this frame
filter is wrapping the iterator provided to it (frame_iter) with a custom iterator called
ElidingInlineIterator. This again defers actions to when gdb prints the backtrace, as
the iterator is not traversed until printing.
The iterator for this example is as follows. It is in this section of the example where
decisions are made on the content of the backtrace.
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class ElidingInlineIterator:
def __init__(self, ii):
self.input_iterator = ii
def __iter__(self):
return self
def next(self):
frame = next(self.input_iterator)
if frame.inferior_frame().type() != gdb.INLINE_FRAME:
return frame
try:
eliding_frame = next(self.input_iterator)
except StopIteration:
return frame
return ElidingFrameDecorator(eliding_frame, [frame])
This iterator implements the Python iterator protocol. When the next function is called
(when gdb prints each frame), the iterator checks if this frame decorator, frame, is wrapping
an inlined frame. If it is not, it returns the existing frame decorator untouched. If it is
wrapping an inlined frame, it assumes that the inlined frame was contained within the
next oldest frame, eliding_frame, which it fetches. It then creates and returns a frame
decorator, ElidingFrameDecorator, which contains both the elided frame, and the eliding
frame.
class ElidingInlineDecorator(FrameDecorator):
def __init__(self, frame, elided_frames):
super(ElidingInlineDecorator, self).__init__(frame)
self.frame = frame
self.elided_frames = elided_frames
def elided(self):
return iter(self.elided_frames)
This frame decorator overrides one function and returns the inlined frame in the elided
method. As before it lets FrameDecorator do the rest of the work involved in printing this
frame. This produces the following output.
#0
#2
0x004004e0 in bar () at inline.c:11
0x00400529 in main () at inline.c:25
#1 0x00400529 in max (b=6, a=12) at inline.c:15
In that output, max which has been inlined into main is printed hierarchically. Another
approach would be to combine the function method, and the elided method to both print
a marker in the inlined frame, and also show the hierarchical relationship.
23.2.2.12 Unwinding Frames in Python
In gdb terminology “unwinding” is the process of finding the previous frame (that is,
caller’s) from the current one. An unwinder has three methods. The first one checks if it
can handle given frame (“sniff” it). For the frames it can sniff an unwinder provides two
additional methods: it can return frame’s ID, and it can fetch registers from the previous
frame. A running gdb mantains a list of the unwinders and calls each unwinder’s sniffer in
turn until it finds the one that recognizes the current frame. There is an API to register an
unwinder.
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The unwinders that come with gdb handle standard frames. However, mixed language
applications (for example, an application running Java Virtual Machine) sometimes use
frame layouts that cannot be handled by the gdb unwinders. You can write Python code
that can handle such custom frames.
You implement a frame unwinder in Python as a class with which has two attributes,
name and enabled, with obvious meanings, and a single method __call__, which examines
a given frame and returns an object (an instance of gdb.UnwindInfo class) describing it.
If an unwinder does not recognize a frame, it should return None. The code in gdb that
enables writing unwinders in Python uses this object to return frame’s ID and previous
frame registers when gdb core asks for them.
Unwinder Input
An object passed to an unwinder (a gdb.PendingFrame instance) provides a method to
read frame’s registers:
PendingFrame.read_register (reg)
[Function]
This method returns the contents of the register regn in the frame as a gdb.Value
object. reg can be either a register number or a register name; the values are platformspecific. They are usually found in the corresponding ‘platform-tdep.h’ file in the
gdb source tree.
It also provides a factory method to create a gdb.UnwindInfo instance to be returned
to gdb:
PendingFrame.create_unwind_info (frame id)
[Function]
Returns a new gdb.UnwindInfo instance identified by given frame id. The argument
is used to build gdb’s frame ID using one of functions provided by gdb. frame id’s
attributes determine which function will be used, as follows:
sp, pc, special
frame_id_build_special (frame_id.sp, frame_id.pc, frame_
id.special)
sp, pc
frame_id_build (frame_id.sp, frame_id.pc)
This is the most common case.
sp
frame_id_build_wild (frame_id.sp)
The attribute values should be gdb.Value
Unwinder Output: UnwindInfo
Use PendingFrame.create_unwind_info method described above to create a
gdb.UnwindInfo instance. Use the following method to specify caller registers that have
been saved in this frame:
gdb.UnwindInfo.add_saved_register (reg, value)
[Function]
reg identifies the register. It can be a number or a name, just as for the
PendingFrame.read_register method above. value is a register value (a gdb.Value
object).
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Unwinder Skeleton Code
gdb comes with the module containing the base Unwinder class. Derive your unwinder
class from it and structure the code as follows:
from gdb.unwinders import Unwinder
class FrameId(object):
def __init__(self, sp, pc):
self.sp = sp
self.pc = pc
class MyUnwinder(Unwinder):
def __init__(....):
supe(MyUnwinder, self).__init___()
def __call__(pending_frame):
if not :
return None
# Create UnwindInfo. Usually the frame is identified by the stack
# pointer and the program counter.
sp = pending_frame.read_register()
pc = pending_frame.read_register()
unwind_info = pending_frame.create_unwind_info(FrameId(sp, pc))
# Find the values of the registers in the caller’s frame and
# save them in the result:
unwind_info.add_saved_register(, )
....
# Return the result:
return unwind_info
Registering a Unwinder
An object file, a program space, and the gdb proper can have unwinders registered with it.
The gdb.unwinders module provides the function to register a unwinder:
gdb.unwinder.register_unwinder (locus, unwinder, replace=False)
[Function]
locus is specifies an object file or a program space to which unwinder is added.
Passing None or gdb adds unwinder to the gdb’s global unwinder list. The newly
added unwinder will be called before any other unwinder from the same locus. Two
unwinders in the same locus cannot have the same name. An attempt to add a
unwinder with already existing name raises an exception unless replace is True, in
which case the old unwinder is deleted.
Unwinder Precedence
gdb first calls the unwinders from all the object files in no particular order, then the
unwinders from the current program space, and finally the unwinders from gdb.
23.2.2.13 Xmethods In Python
Xmethods are additional methods or replacements for existing methods of a C++ class.
This feature is useful for those cases where a method defined in C++ source code could be
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inlined or optimized out by the compiler, making it unavailable to gdb. For such cases,
one can define an xmethod to serve as a replacement for the method defined in the C++
source code. gdb will then invoke the xmethod, instead of the C++ method, to evaluate
expressions. One can also use xmethods when debugging with core files. Moreover, when
debugging live programs, invoking an xmethod need not involve running the inferior (which
can potentially perturb its state). Hence, even if the C++ method is available, it is better
to use its replacement xmethod if one is defined.
The xmethods feature in Python is available via the concepts of an xmethod matcher
and an xmethod worker. To implement an xmethod, one has to implement a matcher and
a corresponding worker for it (more than one worker can be implemented, each catering to
a different overloaded instance of the method). Internally, gdb invokes the match method
of a matcher to match the class type and method name. On a match, the match method
returns a list of matching worker objects. Each worker object typically corresponds to
an overloaded instance of the xmethod. They implement a get_arg_types method which
returns a sequence of types corresponding to the arguments the xmethod requires. gdb uses
this sequence of types to perform overload resolution and picks a winning xmethod worker.
A winner is also selected from among the methods gdb finds in the C++ source code. Next,
the winning xmethod worker and the winning C++ method are compared to select an overall
winner. In case of a tie between a xmethod worker and a C++ method, the xmethod worker
is selected as the winner. That is, if a winning xmethod worker is found to be equivalent
to the winning C++ method, then the xmethod worker is treated as a replacement for the
C++ method. gdb uses the overall winner to invoke the method. If the winning xmethod
worker is the overall winner, then the corresponding xmethod is invoked via the __call__
method of the worker object.
If one wants to implement an xmethod as a replacement for an existing C++ method,
then they have to implement an equivalent xmethod which has exactly the same name and
takes arguments of exactly the same type as the C++ method. If the user wants to invoke
the C++ method even though a replacement xmethod is available for that method, then
they can disable the xmethod.
See Section 23.2.2.14 [Xmethod API], page 359, for API to implement xmethods in
Python. See Section 23.2.2.15 [Writing an Xmethod], page 361, for implementing xmethods
in Python.
23.2.2.14 Xmethod API
The gdb Python API provides classes, interfaces and functions to implement, register and
manipulate xmethods. See Section 23.2.2.13 [Xmethods In Python], page 358.
An xmethod matcher should be an instance of a class derived from XMethodMatcher
defined in the module gdb.xmethod, or an object with similar interface and attributes. An
instance of XMethodMatcher has the following attributes:
[Variable]
name
The name of the matcher.
enabled
A boolean value indicating whether the matcher is enabled or disabled.
[Variable]
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[Variable]
A list of named methods managed by the matcher. Each object in the list is an
instance of the class XMethod defined in the module gdb.xmethod, or any object with
the following attributes:
methods
name
Name of the xmethod which should be unique for each xmethod managed
by the matcher.
enabled
A boolean value indicating whether the xmethod is enabled or disabled.
The class XMethod is a convenience class with same attributes as above along with
the following constructor:
XMethod.__init__ (self, name)
[Function]
Constructs an enabled xmethod with name name.
The XMethodMatcher class has the following methods:
XMethodMatcher.__init__ (self, name)
[Function]
Constructs an enabled xmethod matcher with name name. The methods attribute is
initialized to None.
XMethodMatcher.match (self, class type, method name)
[Function]
Derived classes should override this method. It should return a xmethod worker object
(or a sequence of xmethod worker objects) matching the class type and method name.
class type is a gdb.Type object, and method name is a string value. If the matcher
manages named methods as listed in its methods attribute, then only those worker
objects whose corresponding entries in the methods list are enabled should be returned.
An xmethod worker should be an instance of a class derived from XMethodWorker defined
in the module gdb.xmethod, or support the following interface:
XMethodWorker.get_arg_types (self)
[Function]
This method returns a sequence of gdb.Type objects corresponding to the arguments
that the xmethod takes. It can return an empty sequence or None if the xmethod
does not take any arguments. If the xmethod takes a single argument, then a single
gdb.Type object corresponding to it can be returned.
XMethodWorker.get_result_type (self, *args)
[Function]
This method returns a gdb.Type object representing the type of the result of invoking
this xmethod. The args argument is the same tuple of arguments that would be passed
to the __call__ method of this worker.
XMethodWorker.__call__ (self, *args)
[Function]
This is the method which does the work of the xmethod. The args arguments is the
tuple of arguments to the xmethod. Each element in this tuple is a gdb.Value object.
The first element is always the this pointer value.
For gdb to lookup xmethods, the xmethod matchers should be registered using the
following function defined in the module gdb.xmethod:
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register_xmethod_matcher (locus, matcher, replace=False)
[Function]
The matcher is registered with locus, replacing an existing matcher with the same
name as matcher if replace is True. locus can be a gdb.Objfile object (see
Section 23.2.2.24 [Objfiles In Python], page 380), or a gdb.Progspace object (see
Section 23.2.2.23 [Progspaces In Python], page 379), or None. If it is None, then
matcher is registered globally.
23.2.2.15 Writing an Xmethod
Implementing xmethods in Python will require implementing xmethod matchers and
xmethod workers (see Section 23.2.2.13 [Xmethods In Python], page 358). Consider the
following C++ class:
class MyClass
{
public:
MyClass (int a) : a_(a) { }
int geta (void) { return a_; }
int operator+ (int b);
private:
int a_;
};
int
MyClass::operator+ (int b)
{
return a_ + b;
}
Let us define two xmethods for the class MyClass, one replacing the method geta, and
another adding an overloaded flavor of operator+ which takes a MyClass argument (the
C++ code above already has an overloaded operator+ which takes an int argument). The
xmethod matcher can be defined as follows:
class MyClass_geta(gdb.xmethod.XMethod):
def __init__(self):
gdb.xmethod.XMethod.__init__(self, ’geta’)
def get_worker(self, method_name):
if method_name == ’geta’:
return MyClassWorker_geta()
class MyClass_sum(gdb.xmethod.XMethod):
def __init__(self):
gdb.xmethod.XMethod.__init__(self, ’sum’)
def get_worker(self, method_name):
if method_name == ’operator+’:
return MyClassWorker_plus()
class MyClassMatcher(gdb.xmethod.XMethodMatcher):
def __init__(self):
gdb.xmethod.XMethodMatcher.__init__(self, ’MyClassMatcher’)
# List of methods ’managed’ by this matcher
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self.methods = [MyClass_geta(), MyClass_sum()]
def match(self, class_type, method_name):
if class_type.tag != ’MyClass’:
return None
workers = []
for method in self.methods:
if method.enabled:
worker = method.get_worker(method_name)
if worker:
workers.append(worker)
return workers
Notice that the match method of MyClassMatcher returns a worker object of type
MyClassWorker_geta for the geta method, and a worker object of type MyClassWorker_
plus for the operator+ method. This is done indirectly via helper classes derived from
gdb.xmethod.XMethod. One does not need to use the methods attribute in a matcher
as it is optional. However, if a matcher manages more than one xmethod, it is a good
practice to list the xmethods in the methods attribute of the matcher. This will then
facilitate enabling and disabling individual xmethods via the enable/disable commands.
Notice also that a worker object is returned only if the corresponding entry in the methods
attribute of the matcher is enabled.
The implementation of the worker classes returned by the matcher setup above is as
follows:
class MyClassWorker_geta(gdb.xmethod.XMethodWorker):
def get_arg_types(self):
return None
def get_result_type(self, obj):
return gdb.lookup_type(’int’)
def __call__(self, obj):
return obj[’a_’]
class MyClassWorker_plus(gdb.xmethod.XMethodWorker):
def get_arg_types(self):
return gdb.lookup_type(’MyClass’)
def get_result_type(self, obj):
return gdb.lookup_type(’int’)
def __call__(self, obj, other):
return obj[’a_’] + other[’a_’]
For gdb to actually lookup a xmethod, it has to be registered with it. The matcher
defined above is registered with gdb globally as follows:
gdb.xmethod.register_xmethod_matcher(None, MyClassMatcher())
If an object obj of type MyClass is initialized in C++ code as follows:
MyClass obj(5);
then, after loading the Python script defining the xmethod matchers and workers into GDBN,
invoking the method geta or using the operator + on obj will invoke the xmethods defined
above:
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(gdb) p obj.geta()
$1 = 5
(gdb) p obj + obj
$2 = 10
Consider another example with a C++ template class:
template
class MyTemplate
{
public:
MyTemplate () : dsize_(10), data_ (new T [10]) { }
~MyTemplate () { delete [] data_; }
int footprint (void)
{
return sizeof (T) * dsize_ + sizeof (MyTemplate);
}
private:
int dsize_;
T *data_;
};
Let us implement an xmethod for the above class which serves as a replacement for the
footprint method. The full code listing of the xmethod workers and xmethod matchers is
as follows:
class MyTemplateWorker_footprint(gdb.xmethod.XMethodWorker):
def __init__(self, class_type):
self.class_type = class_type
def get_arg_types(self):
return None
def get_result_type(self):
return gdb.lookup_type(’int’)
def __call__(self, obj):
return (self.class_type.sizeof +
obj[’dsize_’] *
self.class_type.template_argument(0).sizeof)
class MyTemplateMatcher_footprint(gdb.xmethod.XMethodMatcher):
def __init__(self):
gdb.xmethod.XMethodMatcher.__init__(self, ’MyTemplateMatcher’)
def match(self, class_type, method_name):
if (re.match(’MyTemplate<[ \t\n]*[_a-zA-Z][ _a-zA-Z0-9]*>’,
class_type.tag) and
method_name == ’footprint’):
return MyTemplateWorker_footprint(class_type)
Notice that, in this example, we have not used the methods attribute of the matcher
as the matcher manages only one xmethod. The user can enable/disable this xmethod by
enabling/disabling the matcher itself.
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23.2.2.16 Inferiors In Python
Programs which are being run under gdb are called inferiors (see Section 4.9 [Inferiors and
Programs], page 33). Python scripts can access information about and manipulate inferiors
controlled by gdb via objects of the gdb.Inferior class.
The following inferior-related functions are available in the gdb module:
gdb.inferiors ()
[Function]
Return a tuple containing all inferior objects.
gdb.selected_inferior ()
[Function]
Return an object representing the current inferior.
A gdb.Inferior object has the following attributes:
Inferior.num
[Variable]
ID of inferior, as assigned by GDB.
Inferior.pid
[Variable]
Process ID of the inferior, as assigned by the underlying operating system.
[Variable]
Boolean signaling whether the inferior was created using ‘attach’, or started by gdb
itself.
Inferior.was_attached
A gdb.Inferior object has the following methods:
Inferior.is_valid ()
[Function]
Returns True if the gdb.Inferior object is valid, False if not. A gdb.Inferior
object will become invalid if the inferior no longer exists within gdb. All other
gdb.Inferior methods will throw an exception if it is invalid at the time the method
is called.
Inferior.threads ()
[Function]
This method returns a tuple holding all the threads which are valid when it is called.
If there are no valid threads, the method will return an empty tuple.
Inferior.read_memory (address, length)
[Function]
Read length addressable memory units from the inferior, starting at address. Returns
a buffer object, which behaves much like an array or a string. It can be modified and
given to the Inferior.write_memory function. In Python 3, the return value is a
memoryview object.
Inferior.write_memory (address, buffer [, length])
[Function]
Write the contents of buffer to the inferior, starting at address. The buffer parameter
must be a Python object which supports the buffer protocol, i.e., a string, an array
or the object returned from Inferior.read_memory. If given, length determines the
number of addressable memory units from buffer to be written.
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Inferior.search_memory (address, length, pattern)
[Function]
Search a region of the inferior memory starting at address with the given length using
the search pattern supplied in pattern. The pattern parameter must be a Python
object which supports the buffer protocol, i.e., a string, an array or the object returned
from gdb.read_memory. Returns a Python Long containing the address where the
pattern was found, or None if the pattern could not be found.
23.2.2.17 Events In Python
gdb provides a general event facility so that Python code can be notified of various state
changes, particularly changes that occur in the inferior.
An event is just an object that describes some state change. The type of the object and
its attributes will vary depending on the details of the change. All the existing events are
described below.
In order to be notified of an event, you must register an event handler with an event registry. An event registry is an object in the gdb.events module which dispatches particular
events. A registry provides methods to register and unregister event handlers:
EventRegistry.connect (object)
[Function]
Add the given callable object to the registry. This object will be called when an event
corresponding to this registry occurs.
EventRegistry.disconnect (object)
[Function]
Remove the given object from the registry. Once removed, the object will no longer
receive notifications of events.
Here is an example:
def exit_handler (event):
print "event type: exit"
print "exit code: %d" % (event.exit_code)
gdb.events.exited.connect (exit_handler)
In the above example we connect our handler exit_handler to the registry
events.exited. Once connected, exit_handler gets called when the inferior exits. The
argument event in this example is of type gdb.ExitedEvent. As you can see in the
example the ExitedEvent object has an attribute which indicates the exit code of the
inferior.
The following is a listing of the event registries that are available and details of the events
they emit:
events.cont
Emits gdb.ThreadEvent.
Some events can be thread specific when gdb is running in non-stop mode.
When represented in Python, these events all extend gdb.ThreadEvent. Note,
this event is not emitted directly; instead, events which are emitted by this
or other modules might extend this event. Examples of these events are
gdb.BreakpointEvent and gdb.ContinueEvent.
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Debugging with gdb
[Variable]
In non-stop mode this attribute will be set to the specific thread which
was involved in the emitted event. Otherwise, it will be set to None.
ThreadEvent.inferior_thread
Emits gdb.ContinueEvent which extends gdb.ThreadEvent.
This event indicates that the inferior has been continued after a stop. For
inherited attribute refer to gdb.ThreadEvent above.
events.exited
Emits events.ExitedEvent which indicates that the inferior has exited.
events.ExitedEvent has two attributes:
[Variable]
An integer representing the exit code, if available, which the inferior has
returned. (The exit code could be unavailable if, for example, gdb detaches from the inferior.) If the exit code is unavailable, the attribute
does not exist.
ExitedEvent.exit_code
ExitedEvent.inferior
[Variable]
A reference to the inferior which triggered the exited event.
events.stop
Emits gdb.StopEvent which extends gdb.ThreadEvent.
Indicates that the inferior has stopped. All events emitted by this registry
extend StopEvent. As a child of gdb.ThreadEvent, gdb.StopEvent will indicate the stopped thread when gdb is running in non-stop mode. Refer to
gdb.ThreadEvent above for more details.
Emits gdb.SignalEvent which extends gdb.StopEvent.
This event indicates that the inferior or one of its threads has received as signal.
gdb.SignalEvent has the following attributes:
[Variable]
A string representing the signal received by the inferior. A list of possible
signal values can be obtained by running the command info signals in
the gdb command prompt.
SignalEvent.stop_signal
Also emits gdb.BreakpointEvent which extends gdb.StopEvent.
gdb.BreakpointEvent event indicates that one or more breakpoints have been
hit, and has the following attributes:
[Variable]
A sequence containing references to all the breakpoints (type
gdb.Breakpoint) that were hit. See Section 23.2.2.30 [Breakpoints In
Python], page 393, for details of the gdb.Breakpoint object.
BreakpointEvent.breakpoints
[Variable]
A reference to the first breakpoint that was hit. This function is maintained for backward compatibility and is now deprecated in favor of the
gdb.BreakpointEvent.breakpoints attribute.
BreakpointEvent.breakpoint
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events.new_objfile
Emits gdb.NewObjFileEvent which indicates that a new object file has been
loaded by gdb. gdb.NewObjFileEvent has one attribute:
[Variable]
A reference to the object file (gdb.Objfile) which has been loaded.
See Section 23.2.2.24 [Objfiles In Python], page 380, for details of the
gdb.Objfile object.
NewObjFileEvent.new_objfile
events.clear_objfiles
Emits gdb.ClearObjFilesEvent which indicates that the list of object files for
a program space has been reset. gdb.ClearObjFilesEvent has one attribute:
[Variable]
A reference to the program space (gdb.Progspace) whose objfile list has
been cleared. See Section 23.2.2.23 [Progspaces In Python], page 379.
ClearObjFilesEvent.progspace
events.inferior_call_pre
Emits gdb.InferiorCallPreEvent which indicates that a function in the inferior is about to be called.
InferiorCallPreEvent.ptid
[Variable]
The thread in which the call will be run.
InferiorCallPreEvent.address
[Variable]
The location of the function to be called.
events.inferior_call_post
Emits gdb.InferiorCallPostEvent which indicates that a function in the inferior has returned.
InferiorCallPostEvent.ptid
[Variable]
The thread in which the call was run.
InferiorCallPostEvent.address
[Variable]
The location of the function that was called.
events.memory_changed
Emits gdb.MemoryChangedEvent which indicates that the memory of the inferior has been modified by the gdb user, for instance via a command like
set *addr = value. The event has the following attributes:
MemoryChangedEvent.address
[Variable]
The start address of the changed region.
MemoryChangedEvent.length
[Variable]
Length in bytes of the changed region.
events.register_changed
Emits gdb.RegisterChangedEvent which indicates that a register in the inferior has been modified by the gdb user.
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[Variable]
A gdb.Frame object representing the frame in which the register was
modified.
RegisterChangedEvent.frame
RegisterChangedEvent.regnum
[Variable]
Denotes which register was modified.
events.breakpoint_created
This is emitted when a new breakpoint has been created. The argument that
is passed is the new gdb.Breakpoint object.
events.breakpoint_modified
This is emitted when a breakpoint has been modified in some way. The argument that is passed is the new gdb.Breakpoint object.
events.breakpoint_deleted
This is emitted when a breakpoint has been deleted. The argument that
is passed is the gdb.Breakpoint object. When this event is emitted, the
gdb.Breakpoint object will already be in its invalid state; that is, the is_
valid method will return False.
events.before_prompt
This event carries no payload. It is emitted each time gdb presents a prompt
to the user.
23.2.2.18 Threads In Python
Python scripts can access information about, and manipulate inferior threads controlled by
gdb, via objects of the gdb.InferiorThread class.
The following thread-related functions are available in the gdb module:
gdb.selected_thread ()
[Function]
This function returns the thread object for the selected thread. If there is no selected
thread, this will return None.
A gdb.InferiorThread object has the following attributes:
[Variable]
The name of the thread. If the user specified a name using thread name, then this
returns that name. Otherwise, if an OS-supplied name is available, then it is returned.
Otherwise, this returns None.
This attribute can be assigned to. The new value must be a string object, which sets
the new name, or None, which removes any user-specified thread name.
InferiorThread.name
InferiorThread.num
[Variable]
The per-inferior number of the thread, as assigned by GDB.
[Variable]
The global ID of the thread, as assigned by GDB. You can use this to make Python
breakpoints thread-specific, for example (see [The Breakpoint.thread attribute],
page 395).
InferiorThread.global_num
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[Variable]
ID of the thread, as assigned by the operating system. This attribute is a tuple containing three integers. The first is the Process ID (PID); the second is the Lightweight
Process ID (LWPID), and the third is the Thread ID (TID). Either the LWPID or
TID may be 0, which indicates that the operating system does not use that identifier.
InferiorThread.ptid
[Variable]
The inferior this thread belongs to. This attribute is represented as a gdb.Inferior
object. This attribute is not writable.
InferiorThread.inferior
A gdb.InferiorThread object has the following methods:
InferiorThread.is_valid ()
[Function]
Returns True if the gdb.InferiorThread object is valid, False if not.
A
gdb.InferiorThread object will become invalid if the thread exits, or the inferior
that the thread belongs is deleted. All other gdb.InferiorThread methods will
throw an exception if it is invalid at the time the method is called.
InferiorThread.switch ()
[Function]
This changes gdb’s currently selected thread to the one represented by this object.
InferiorThread.is_stopped ()
[Function]
Return a Boolean indicating whether the thread is stopped.
InferiorThread.is_running ()
[Function]
Return a Boolean indicating whether the thread is running.
InferiorThread.is_exited ()
[Function]
Return a Boolean indicating whether the thread is exited.
23.2.2.19 Recordings In Python
The following recordings-related functions (see Chapter 7 [Process Record and Replay],
page 87) are available in the gdb module:
gdb.start_recording ([method], [format])
[Function]
Start a recording using the given method and format. If no format is given, the
default format for the recording method is used. If no method is given, the default
method will be used. Returns a gdb.Record object on success. Throw an exception
on failure.
The following strings can be passed as method:
• "full"
• "btrace": Possible values for format: "pt", "bts" or leave out for default format.
gdb.current_recording ()
[Function]
Access a currently running recording. Return a gdb.Record object on success. Return
None if no recording is currently active.
gdb.stop_recording ()
[Function]
Stop the current recording. Throw an exception if no recording is currently active.
All record objects become invalid after this call.
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Debugging with gdb
A gdb.Record object has the following attributes:
Record.method
[Variable]
A string with the current recording method, e.g. full or btrace.
Record.format
[Variable]
A string with the current recording format, e.g. bt, pts or None.
[Variable]
A method specific instruction object representing the first instruction in this recording.
Record.begin
[Variable]
A method specific instruction object representing the current instruction, that is not
actually part of the recording.
Record.end
[Variable]
The instruction representing the current replay position. If there is no replay active,
this will be None.
Record.replay_position
Record.instruction_history
[Variable]
A list with all recorded instructions.
Record.function_call_history
[Variable]
A list with all recorded function call segments.
A gdb.Record object has the following methods:
Record.goto (instruction)
[Function]
Move the replay position to the given instruction.
The common gdb.Instruction class that recording method specific instruction objects
inherit from, has the following attributes:
Instruction.pc
[Variable]
An integer representing this instruction’s address.
[Variable]
A buffer with the raw instruction data. In Python 3, the return value is a memoryview
object.
Instruction.data
Instruction.decoded
[Variable]
A human readable string with the disassembled instruction.
Instruction.size
[Variable]
The size of the instruction in bytes.
Additionally gdb.RecordInstruction has the following attributes:
[Variable]
An integer identifying this instruction. number corresponds to the numbers seen in
record instruction-history (see Chapter 7 [Process Record and Replay], page 87).
RecordInstruction.number
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[Variable]
A gdb.Symtab_and_line object representing the associated symtab and line of this
instruction. May be None if no debug information is available.
RecordInstruction.sal
RecordInstruction.is_speculative
[Variable]
A boolean indicating whether the instruction was executed speculatively.
If an error occured during recording or decoding a recording, this error is represented by
a gdb.RecordGap object in the instruction list. It has the following attributes:
[Variable]
An integer identifying this gap. number corresponds to the numbers seen in record
instruction-history (see Chapter 7 [Process Record and Replay], page 87).
RecordGap.number
[Variable]
A numerical representation of the reason for the gap. The value is specific to the
current recording method.
RecordGap.error_code
RecordGap.error_string
[Variable]
A human readable string with the reason for the gap.
A gdb.RecordFunctionSegment object has the following attributes:
[Variable]
An integer identifying this function segment. number corresponds to the numbers
seen in record function-call-history (see Chapter 7 [Process Record and Replay],
page 87).
RecordFunctionSegment.number
[Variable]
A gdb.Symbol object representing the associated symbol. May be None if no debug
information is available.
RecordFunctionSegment.symbol
[Variable]
An integer representing the function call’s stack level. May be None if the function
call is a gap.
RecordFunctionSegment.level
[Variable]
A list of gdb.RecordInstruction or gdb.RecordGap objects associated with this
function call.
RecordFunctionSegment.instructions
[Variable]
A gdb.RecordFunctionSegment object representing the caller’s function segment. If
the call has not been recorded, this will be the function segment to which control
returns. If neither the call nor the return have been recorded, this will be None.
RecordFunctionSegment.up
[Variable]
A gdb.RecordFunctionSegment object representing the previous segment of this
function call. May be None.
RecordFunctionSegment.prev
[Variable]
A gdb.RecordFunctionSegment object representing the next segment of this function
call. May be None.
RecordFunctionSegment.next
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The following example demonstrates the usage of these objects and functions to create a
function that will rewind a record to the last time a function in a different file was executed.
This would typically be used to track the execution of user provided callback functions in
a library which typically are not visible in a back trace.
def bringback ():
rec = gdb.current_recording ()
if not rec:
return
insn = rec.instruction_history
if len (insn) == 0:
return
try:
position
except:
position
try:
filename
except:
filename
= insn.index (rec.replay_position)
= -1
= insn[position].sal.symtab.fullname ()
= None
for i in reversed (insn[:position]):
try:
current = i.sal.symtab.fullname ()
except:
current = None
if filename == current:
continue
rec.goto (i)
return
Another possible application is to write a function that counts the number of code
executions in a given line range. This line range can contain parts of functions or span
across several functions and is not limited to be contiguous.
def countrange (filename, linerange):
count = 0
def filter_only (file_name):
for call in gdb.current_recording ().function_call_history:
try:
if file_name in call.symbol.symtab.fullname ():
yield call
except:
pass
for c in filter_only (filename):
for i in c.instructions:
try:
if i.sal.line in linerange:
count += 1
break;
except:
pass
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return count
23.2.2.20 Commands In Python
You can implement new gdb CLI commands in Python. A CLI command is implemented
using an instance of the gdb.Command class, most commonly using a subclass.
Command.__init__ (name, command_class [, completer_class [,
prefix]])
[Function]
The object initializer for Command registers the new command with gdb. This initializer is normally invoked from the subclass’ own __init__ method.
name is the name of the command. If name consists of multiple words, then the initial
words are looked for as prefix commands. In this case, if one of the prefix commands
does not exist, an exception is raised.
There is no support for multi-line commands.
command class should be one of the ‘COMMAND_’ constants defined below. This argument tells gdb how to categorize the new command in the help system.
completer class is an optional argument. If given, it should be one of the ‘COMPLETE_’
constants defined below. This argument tells gdb how to perform completion for this
command. If not given, gdb will attempt to complete using the object’s complete
method (see below); if no such method is found, an error will occur when completion
is attempted.
prefix is an optional argument. If True, then the new command is a prefix command;
sub-commands of this command may be registered.
The help text for the new command is taken from the Python documentation string
for the command’s class, if there is one. If no documentation string is provided, the
default value “This command is not documented.” is used.
Command.dont_repeat ()
[Function]
By default, a gdb command is repeated when the user enters a blank line at the
command prompt. A command can suppress this behavior by invoking the dont_
repeat method. This is similar to the user command dont-repeat, see Section 23.1.1
[Define], page 321.
Command.invoke (argument, from tty)
[Function]
This method is called by gdb when this command is invoked.
argument is a string. It is the argument to the command, after leading and trailing
whitespace has been stripped.
from tty is a boolean argument. When true, this means that the command was
entered by the user at the terminal; when false it means that the command came
from elsewhere.
If this method throws an exception, it is turned into a gdb error call. Otherwise,
the return value is ignored.
To break argument up into an argv-like string use gdb.string_to_argv. This function behaves identically to gdb’s internal argument lexer buildargv. It is recommended to use this for consistency. Arguments are separated by spaces and may be
quoted. Example:
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print gdb.string_to_argv ("1 2\ \\\"3 ’4 \"5’ \"6 ’7\"")
[’1’, ’2 "3’, ’4 "5’, "6 ’7"]
Command.complete (text, word)
[Function]
This method is called by gdb when the user attempts completion on this command.
All forms of completion are handled by this method, that is, the TAB and M-? key
bindings (see Section 3.2 [Completion], page 19), and the complete command (see
Section 3.3 [Help], page 22).
The arguments text and word are both strings; text holds the complete command
line up to the cursor’s location, while word holds the last word of the command line;
this is computed using a word-breaking heuristic.
The complete method can return several values:
• If the return value is a sequence, the contents of the sequence are used as the
completions. It is up to complete to ensure that the contents actually do complete the word. A zero-length sequence is allowed, it means that there were
no completions available. Only string elements of the sequence are used; other
elements in the sequence are ignored.
• If the return value is one of the ‘COMPLETE_’ constants defined below, then the
corresponding gdb-internal completion function is invoked, and its result is used.
• All other results are treated as though there were no available completions.
When a new command is registered, it must be declared as a member of some general
class of commands. This is used to classify top-level commands in the on-line help system;
note that prefix commands are not listed under their own category but rather that of their
top-level command. The available classifications are represented by constants defined in the
gdb module:
gdb.COMMAND_NONE
The command does not belong to any particular class. A command in this
category will not be displayed in any of the help categories.
gdb.COMMAND_RUNNING
The command is related to running the inferior. For example, start, step,
and continue are in this category. Type help running at the gdb prompt to
see a list of commands in this category.
gdb.COMMAND_DATA
The command is related to data or variables. For example, call, find, and
print are in this category. Type help data at the gdb prompt to see a list of
commands in this category.
gdb.COMMAND_STACK
The command has to do with manipulation of the stack. For example,
backtrace, frame, and return are in this category. Type help stack at the
gdb prompt to see a list of commands in this category.
gdb.COMMAND_FILES
This class is used for file-related commands. For example, file, list and
section are in this category. Type help files at the gdb prompt to see a list
of commands in this category.
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gdb.COMMAND_SUPPORT
This should be used for “support facilities”, generally meaning things that are
useful to the user when interacting with gdb, but not related to the state of
the inferior. For example, help, make, and shell are in this category. Type
help support at the gdb prompt to see a list of commands in this category.
gdb.COMMAND_STATUS
The command is an ‘info’-related command, that is, related to the state of
gdb itself. For example, info, macro, and show are in this category. Type
help status at the gdb prompt to see a list of commands in this category.
gdb.COMMAND_BREAKPOINTS
The command has to do with breakpoints. For example, break, clear, and
delete are in this category. Type help breakpoints at the gdb prompt to see
a list of commands in this category.
gdb.COMMAND_TRACEPOINTS
The command has to do with tracepoints. For example, trace, actions, and
tfind are in this category. Type help tracepoints at the gdb prompt to see
a list of commands in this category.
gdb.COMMAND_USER
The command is a general purpose command for the user, and typically does
not fit in one of the other categories. Type help user-defined at the gdb
prompt to see a list of commands in this category, as well as the list of gdb
macros (see Section 23.1 [Sequences], page 321).
gdb.COMMAND_OBSCURE
The command is only used in unusual circumstances, or is not of general interest
to users. For example, checkpoint, fork, and stop are in this category. Type
help obscure at the gdb prompt to see a list of commands in this category.
gdb.COMMAND_MAINTENANCE
The command is only useful to gdb maintainers. The maintenance and
flushregs commands are in this category. Type help internals at the gdb
prompt to see a list of commands in this category.
A new command can use a predefined completion function, either by specifying it via an
argument at initialization, or by returning it from the complete method. These predefined
completion constants are all defined in the gdb module:
gdb.COMPLETE_NONE
This constant means that no completion should be done.
gdb.COMPLETE_FILENAME
This constant means that filename completion should be performed.
gdb.COMPLETE_LOCATION
This constant means that location completion should be done. See Section 9.2
[Specify Location], page 104.
gdb.COMPLETE_COMMAND
This constant means that completion should examine gdb command names.
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gdb.COMPLETE_SYMBOL
This constant means that completion should be done using symbol names as
the source.
gdb.COMPLETE_EXPRESSION
This constant means that completion should be done on expressions. Often
this means completing on symbol names, but some language parsers also have
support for completing on field names.
The following code snippet shows how a trivial CLI command can be implemented in
Python:
class HelloWorld (gdb.Command):
"""Greet the whole world."""
def __init__ (self):
super (HelloWorld, self).__init__ ("hello-world", gdb.COMMAND_USER)
def invoke (self, arg, from_tty):
print "Hello, World!"
HelloWorld ()
The last line instantiates the class, and is necessary to trigger the registration of the
command with gdb. Depending on how the Python code is read into gdb, you may need
to import the gdb module explicitly.
23.2.2.21 Parameters In Python
You can implement new gdb parameters using Python. A new parameter is implemented
as an instance of the gdb.Parameter class.
Parameters are exposed to the user via the set and show commands. See Section 3.3
[Help], page 22.
There are many parameters that already exist and can be set in gdb. Two examples are:
set follow fork and set charset. Setting these parameters influences certain behavior in
gdb. Similarly, you can define parameters that can be used to influence behavior in custom
Python scripts and commands.
Parameter.__init__ (name, command-class, parameter-class [,
enum-sequence])
[Function]
The object initializer for Parameter registers the new parameter with gdb. This
initializer is normally invoked from the subclass’ own __init__ method.
name is the name of the new parameter. If name consists of multiple words, then
the initial words are looked for as prefix parameters. An example of this can be
illustrated with the set print set of parameters. If name is print foo, then print
will be searched as the prefix parameter. In this case the parameter can subsequently
be accessed in gdb as set print foo.
If name consists of multiple words, and no prefix parameter group can be found, an
exception is raised.
command-class should be one of the ‘COMMAND_’ constants (see Section 23.2.2.20 [Commands In Python], page 373). This argument tells gdb how to categorize the new
parameter in the help system.
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parameter-class should be one of the ‘PARAM_’ constants defined below. This argument
tells gdb the type of the new parameter; this information is used for input validation
and completion.
If parameter-class is PARAM_ENUM, then enum-sequence must be a sequence of strings.
These strings represent the possible values for the parameter.
If parameter-class is not PARAM_ENUM, then the presence of a fourth argument will
cause an exception to be thrown.
The help text for the new parameter is taken from the Python documentation string
for the parameter’s class, if there is one. If there is no documentation string, a default
value is used.
[Variable]
If this attribute exists, and is a string, then its value is used as the help text for
this parameter’s set command. The value is examined when Parameter.__init__
is invoked; subsequent changes have no effect.
Parameter.set_doc
[Variable]
If this attribute exists, and is a string, then its value is used as the help text for this
parameter’s show command. The value is examined when Parameter.__init__ is
invoked; subsequent changes have no effect.
Parameter.show_doc
[Variable]
The value attribute holds the underlying value of the parameter. It can be read and
assigned to just as any other attribute. gdb does validation when assignments are
made.
Parameter.value
There are two methods that should be implemented in any Parameter class. These are:
Parameter.get_set_string (self)
[Function]
gdb will call this method when a parameter’s value has been changed via the set
API (for example, set foo off). The value attribute has already been populated
with the new value and may be used in output. This method must return a string.
Parameter.get_show_string (self, svalue)
[Function]
gdb will call this method when a parameter’s show API has been invoked (for example, show foo). The argument svalue receives the string representation of the
current value. This method must return a string.
When a new parameter is defined, its type must be specified. The available types are
represented by constants defined in the gdb module:
gdb.PARAM_BOOLEAN
The value is a plain boolean. The Python boolean values, True and False are
the only valid values.
gdb.PARAM_AUTO_BOOLEAN
The value has three possible states: true, false, and ‘auto’. In Python, true and
false are represented using boolean constants, and ‘auto’ is represented using
None.
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gdb.PARAM_UINTEGER
The value is an unsigned integer. The value of 0 should be interpreted to mean
“unlimited”.
gdb.PARAM_INTEGER
The value is a signed integer. The value of 0 should be interpreted to mean
“unlimited”.
gdb.PARAM_STRING
The value is a string. When the user modifies the string, any escape sequences,
such as ‘\t’, ‘\f’, and octal escapes, are translated into corresponding characters
and encoded into the current host charset.
gdb.PARAM_STRING_NOESCAPE
The value is a string. When the user modifies the string, escapes are passed
through untranslated.
gdb.PARAM_OPTIONAL_FILENAME
The value is a either a filename (a string), or None.
gdb.PARAM_FILENAME
The value is a filename. This is just like PARAM_STRING_NOESCAPE, but uses file
names for completion.
gdb.PARAM_ZINTEGER
The value is an integer. This is like PARAM_INTEGER, except 0 is interpreted as
itself.
gdb.PARAM_ENUM
The value is a string, which must be one of a collection string constants provided
when the parameter is created.
23.2.2.22 Writing new convenience functions
You can implement new convenience functions (see Section 10.11 [Convenience Vars],
page 139) in Python. A convenience function is an instance of a subclass of the class
gdb.Function.
Function.__init__ (name)
[Function]
The initializer for Function registers the new function with gdb. The argument
name is the name of the function, a string. The function will be visible to the user
as a convenience variable of type internal function, whose name is the same as the
given name.
The documentation for the new function is taken from the documentation string for
the new class.
Function.invoke (*args)
[Function]
When a convenience function is evaluated, its arguments are converted to instances of
gdb.Value, and then the function’s invoke method is called. Note that gdb does not
predetermine the arity of convenience functions. Instead, all available arguments are
passed to invoke, following the standard Python calling convention. In particular, a
convenience function can have default values for parameters without ill effect.
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The return value of this method is used as its value in the enclosing expression. If an
ordinary Python value is returned, it is converted to a gdb.Value following the usual
rules.
The following code snippet shows how a trivial convenience function can be implemented
in Python:
class Greet (gdb.Function):
"""Return string to greet someone.
Takes a name as argument."""
def __init__ (self):
super (Greet, self).__init__ ("greet")
def invoke (self, name):
return "Hello, %s!" % name.string ()
Greet ()
The last line instantiates the class, and is necessary to trigger the registration of the
function with gdb. Depending on how the Python code is read into gdb, you may need to
import the gdb module explicitly.
Now you can use the function in an expression:
(gdb) print $greet("Bob")
$1 = "Hello, Bob!"
23.2.2.23 Program Spaces In Python
A program space, or progspace, represents a symbolic view of an address space. It consists
of all of the objfiles of the program. See Section 23.2.2.24 [Objfiles In Python], page 380.
See Section 4.9 [Inferiors and Programs], page 33, for more details about program spaces.
The following progspace-related functions are available in the gdb module:
gdb.current_progspace ()
[Function]
This function returns the program space of the currently selected inferior. See
Section 4.9 [Inferiors and Programs], page 33.
gdb.progspaces ()
[Function]
Return a sequence of all the progspaces currently known to gdb.
Each progspace is represented by an instance of the gdb.Progspace class.
Progspace.filename
[Variable]
The file name of the progspace as a string.
Progspace.pretty_printers
The pretty_printers attribute is a list of functions.
printers. A Value is passed to each function in order;
then the search continues. Otherwise, the return value
used to format the value. See Section 23.2.2.5 [Pretty
more information.
[Variable]
It is used to look up prettyif the function returns None,
should be an object which is
Printing API], page 343, for
[Variable]
The type_printers attribute is a list of type printer objects. See Section 23.2.2.8
[Type Printing API], page 347, for more information.
Progspace.type_printers
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[Variable]
The frame_filters attribute is a dictionary of frame filter objects.
See
Section 23.2.2.9 [Frame Filter API], page 348, for more information.
Progspace.frame_filters
One may add arbitrary attributes to gdb.Progspace objects in the usual Python way.
This is useful if, for example, one needs to do some extra record keeping associated with
the program space.
In this contrived example, we want to perform some processing when an objfile with
a certain symbol is loaded, but we only want to do this once because it is expensive. To
achieve this we record the results with the program space because we can’t predict when
the desired objfile will be loaded.
(gdb) python
def clear_objfiles_handler(event):
event.progspace.expensive_computation = None
def expensive(symbol):
"""A mock routine to perform an "expensive" computation on symbol."""
print "Computing the answer to the ultimate question ..."
return 42
def new_objfile_handler(event):
objfile = event.new_objfile
progspace = objfile.progspace
if not hasattr(progspace, ’expensive_computation’) or \
progspace.expensive_computation is None:
# We use ’main’ for the symbol to keep the example simple.
# Note: There’s no current way to constrain the lookup
# to one objfile.
symbol = gdb.lookup_global_symbol(’main’)
if symbol is not None:
progspace.expensive_computation = expensive(symbol)
gdb.events.clear_objfiles.connect(clear_objfiles_handler)
gdb.events.new_objfile.connect(new_objfile_handler)
end
(gdb) file /tmp/hello
Reading symbols from /tmp/hello...done.
Computing the answer to the ultimate question ...
(gdb) python print gdb.current_progspace().expensive_computation
42
(gdb) run
Starting program: /tmp/hello
Hello.
[Inferior 1 (process 4242) exited normally]
23.2.2.24 Objfiles In Python
gdb loads symbols for an inferior from various symbol-containing files (see Section 18.1
[Files], page 241). These include the primary executable file, any shared libraries used by
the inferior, and any separate debug info files (see Section 18.3 [Separate Debug Files],
page 250). gdb calls these symbol-containing files objfiles.
The following objfile-related functions are available in the gdb module:
gdb.current_objfile ()
[Function]
When auto-loading a Python script (see Section 23.2.3 [Python Auto-loading],
page 398), gdb sets the “current objfile” to the corresponding objfile. This function
returns the current objfile. If there is no current objfile, this function returns None.
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gdb.objfiles ()
[Function]
Return a sequence of all the objfiles current known to gdb. See Section 23.2.2.24
[Objfiles In Python], page 380.
gdb.lookup_objfile (name [, by build id])
[Function]
Look up name, a file name or build ID, in the list of objfiles for the current program
space (see Section 23.2.2.23 [Progspaces In Python], page 379). If the objfile is not
found throw the Python ValueError exception.
If name is a relative file name, then it will match any source file name with the same
trailing components. For example, if name is ‘gcc/expr.c’, then it will match source
file name of ‘/build/trunk/gcc/expr.c’, but not ‘/build/trunk/libcpp/expr.c’
or ‘/build/trunk/gcc/x-expr.c’.
If by build id is provided and is True then name is the build ID of the objfile. Otherwise, name is a file name. This is supported only on some operating systems, notably
those which use the ELF format for binary files and the gnu Binutils. For more details about this feature, see the description of the ‘--build-id’ command-line option
in Section “Command Line Options” in The GNU Linker.
Each objfile is represented by an instance of the gdb.Objfile class.
Objfile.filename
[Variable]
The file name of the objfile as a string, with symbolic links resolved.
The value is None if the objfile is no longer valid. See the gdb.Objfile.is_valid
method, described below.
Objfile.username
[Variable]
The file name of the objfile as specified by the user as a string.
The value is None if the objfile is no longer valid. See the gdb.Objfile.is_valid
method, described below.
[Variable]
For separate debug info objfiles this is the corresponding gdb.Objfile object that
debug info is being provided for. Otherwise this is None. Separate debug info objfiles are added with the gdb.Objfile.add_separate_debug_file method, described
below.
Objfile.owner
[Variable]
The build ID of the objfile as a string. If the objfile does not have a build ID then
the value is None.
Objfile.build_id
This is supported only on some operating systems, notably those which use the ELF
format for binary files and the gnu Binutils. For more details about this feature, see
the description of the ‘--build-id’ command-line option in Section “Command Line
Options” in The GNU Linker.
[Variable]
The containing program space of the objfile as a gdb.Progspace object. See
Section 23.2.2.23 [Progspaces In Python], page 379.
Objfile.progspace
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Objfile.pretty_printers
The pretty_printers attribute is a list of functions.
printers. A Value is passed to each function in order;
then the search continues. Otherwise, the return value
used to format the value. See Section 23.2.2.5 [Pretty
more information.
[Variable]
It is used to look up prettyif the function returns None,
should be an object which is
Printing API], page 343, for
[Variable]
The type_printers attribute is a list of type printer objects. See Section 23.2.2.8
[Type Printing API], page 347, for more information.
Objfile.type_printers
[Variable]
The frame_filters attribute is a dictionary of frame filter objects.
See
Section 23.2.2.9 [Frame Filter API], page 348, for more information.
Objfile.frame_filters
One may add arbitrary attributes to gdb.Objfile objects in the usual Python way.
This is useful if, for example, one needs to do some extra record keeping associated with
the objfile.
In this contrived example we record the time when gdb loaded the objfile.
(gdb) python
import datetime
def new_objfile_handler(event):
# Set the time_loaded attribute of the new objfile.
event.new_objfile.time_loaded = datetime.datetime.today()
gdb.events.new_objfile.connect(new_objfile_handler)
end
(gdb) file ./hello
Reading symbols from ./hello...done.
(gdb) python print gdb.objfiles()[0].time_loaded
2014-10-09 11:41:36.770345
A gdb.Objfile object has the following methods:
Objfile.is_valid ()
[Function]
Returns True if the gdb.Objfile object is valid, False if not. A gdb.Objfile object
can become invalid if the object file it refers to is not loaded in gdb any longer. All
other gdb.Objfile methods will throw an exception if it is invalid at the time the
method is called.
Objfile.add_separate_debug_file (file)
[Function]
Add file to the list of files that gdb will search for debug information for the objfile.
This is useful when the debug info has been removed from the program and stored
in a separate file. gdb has built-in support for finding separate debug info files (see
Section 18.3 [Separate Debug Files], page 250), but if the file doesn’t live in one of
the standard places that gdb searches then this function can be used to add a debug
info file from a different place.
23.2.2.25 Accessing inferior stack frames from Python.
When the debugged program stops, gdb is able to analyze its call stack (see Section 8.1
[Stack frames], page 95). The gdb.Frame class represents a frame in the stack. A gdb.Frame
object is only valid while its corresponding frame exists in the inferior’s stack. If you try
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to use an invalid frame object, gdb will throw a gdb.error exception (see Section 23.2.2.2
[Exception Handling], page 333).
Two gdb.Frame objects can be compared for equality with the == operator, like:
(gdb) python print gdb.newest_frame() == gdb.selected_frame ()
True
The following frame-related functions are available in the gdb module:
gdb.selected_frame ()
[Function]
Return the selected frame object. (see Section 8.3 [Selecting a Frame], page 98).
gdb.newest_frame ()
[Function]
Return the newest frame object for the selected thread.
gdb.frame_stop_reason_string (reason)
[Function]
Return a string explaining the reason why gdb stopped unwinding frames, as expressed by the given reason code (an integer, see the unwind_stop_reason method
further down in this section).
[Function]
gdb internally keeps a cache of the frames that have been unwound. This function
invalidates this cache.
This function should not generally be called by ordinary Python code. It is documented for the sake of completeness.
gdb.invalidate_cached_frames
A gdb.Frame object has the following methods:
Frame.is_valid ()
[Function]
Returns true if the gdb.Frame object is valid, false if not. A frame object can become
invalid if the frame it refers to doesn’t exist anymore in the inferior. All gdb.Frame
methods will throw an exception if it is invalid at the time the method is called.
Frame.name ()
[Function]
Returns the function name of the frame, or None if it can’t be obtained.
Frame.architecture ()
[Function]
Returns the gdb.Architecture object corresponding to the frame’s architecture. See
Section 23.2.2.33 [Architectures In Python], page 397.
Frame.type ()
[Function]
Returns the type of the frame. The value can be one of:
gdb.NORMAL_FRAME
An ordinary stack frame.
gdb.DUMMY_FRAME
A fake stack frame that was created by gdb when performing an inferior
function call.
gdb.INLINE_FRAME
A frame representing an inlined function. The function was inlined into
a gdb.NORMAL_FRAME that is older than this one.
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gdb.TAILCALL_FRAME
A frame representing a tail call. See Section 11.2 [Tail Call Frames],
page 160.
gdb.SIGTRAMP_FRAME
A signal trampoline frame. This is the frame created by the OS when it
calls into a signal handler.
gdb.ARCH_FRAME
A fake stack frame representing a cross-architecture call.
gdb.SENTINEL_FRAME
This is like gdb.NORMAL_FRAME, but it is only used for the newest frame.
Frame.unwind_stop_reason ()
[Function]
Return an integer representing the reason why it’s not possible to find more frames
toward the outermost frame. Use gdb.frame_stop_reason_string to convert the
value returned by this function to a string. The value can be one of:
gdb.FRAME_UNWIND_NO_REASON
No particular reason (older frames should be available).
gdb.FRAME_UNWIND_NULL_ID
The previous frame’s analyzer returns an invalid result. This is no longer
used by gdb, and is kept only for backward compatibility.
gdb.FRAME_UNWIND_OUTERMOST
This frame is the outermost.
gdb.FRAME_UNWIND_UNAVAILABLE
Cannot unwind further, because that would require knowing the values
of registers or memory that have not been collected.
gdb.FRAME_UNWIND_INNER_ID
This frame ID looks like it ought to belong to a NEXT frame, but we
got it for a PREV frame. Normally, this is a sign of unwinder failure. It
could also indicate stack corruption.
gdb.FRAME_UNWIND_SAME_ID
This frame has the same ID as the previous one. That means that unwinding further would almost certainly give us another frame with exactly the
same ID, so break the chain. Normally, this is a sign of unwinder failure.
It could also indicate stack corruption.
gdb.FRAME_UNWIND_NO_SAVED_PC
The frame unwinder did not find any saved PC, but we needed one to
unwind further.
gdb.FRAME_UNWIND_MEMORY_ERROR
The frame unwinder caused an error while trying to access memory.
gdb.FRAME_UNWIND_FIRST_ERROR
Any stop reason greater or equal to this value indicates some kind of
error. This special value facilitates writing code that tests for errors in
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unwinding in a way that will work correctly even if the list of the other
values is modified in future gdb versions. Using it, you could write:
reason = gdb.selected_frame().unwind_stop_reason ()
reason_str = gdb.frame_stop_reason_string (reason)
if reason >= gdb.FRAME_UNWIND_FIRST_ERROR:
print "An error occured: %s" % reason_str
Frame.pc ()
[Function]
Returns the frame’s resume address.
Frame.block ()
[Function]
Return the frame’s code block. See Section 23.2.2.26 [Blocks In Python], page 385.
Frame.function ()
[Function]
Return the symbol for the function corresponding to this frame. See Section 23.2.2.27
[Symbols In Python], page 387.
Frame.older ()
[Function]
Return the frame that called this frame.
Frame.newer ()
[Function]
Return the frame called by this frame.
Frame.find_sal ()
[Function]
Return the frame’s symtab and line object. See Section 23.2.2.28 [Symbol Tables In
Python], page 390.
Frame.read_register (register)
[Function]
Return the value of register in this frame. The register argument must be a string
(e.g., ’sp’ or ’rax’). Returns a Gdb.Value object. Throws an exception if register
does not exist.
Frame.read_var (variable [, block])
[Function]
Return the value of variable in this frame. If the optional argument block is provided,
search for the variable from that block; otherwise start at the frame’s current block
(which is determined by the frame’s current program counter). The variable argument
must be a string or a gdb.Symbol object; block must be a gdb.Block object.
Frame.select ()
[Function]
Set this frame to be the selected frame. See Chapter 8 [Examining the Stack], page 95.
23.2.2.26 Accessing blocks from Python.
In gdb, symbols are stored in blocks. A block corresponds roughly to a scope in the source
code. Blocks are organized hierarchically, and are represented individually in Python as a
gdb.Block. Blocks rely on debugging information being available.
A frame has a block. Please see Section 23.2.2.25 [Frames In Python], page 382, for a
more in-depth discussion of frames.
The outermost block is known as the global block. The global block typically holds
public global variables and functions.
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The block nested just inside the global block is the static block. The static block typically
holds file-scoped variables and functions.
gdb provides a method to get a block’s superblock, but there is currently no way to
examine the sub-blocks of a block, or to iterate over all the blocks in a symbol table (see
Section 23.2.2.28 [Symbol Tables In Python], page 390).
Here is a short example that should help explain blocks:
/* This is in the global block.
int global;
*/
/* This is in the static block.
static int file_scope;
*/
/* ’function’ is in the global block, and ’argument’ is
in a block nested inside of ’function’. */
int function (int argument)
{
/* ’local’ is in a block inside ’function’. It may or may
not be in the same block as ’argument’. */
int local;
{
/* ’inner’ is in a block whose superblock is the one holding
’local’. */
int inner;
/* If this call is expanded by the compiler, you may see
a nested block here whose function is ’inline_function’
and whose superblock is the one holding ’inner’. */
inline_function ();
}
}
A gdb.Block is iterable. The iterator returns the symbols (see Section 23.2.2.27 [Symbols
In Python], page 387) local to the block. Python programs should not assume that a
specific block object will always contain a given symbol, since changes in gdb features and
infrastructure may cause symbols move across blocks in a symbol table.
The following block-related functions are available in the gdb module:
gdb.block_for_pc (pc)
[Function]
Return the innermost gdb.Block containing the given pc value. If the block cannot
be found for the pc value specified, the function will return None.
A gdb.Block object has the following methods:
Block.is_valid ()
[Function]
Returns True if the gdb.Block object is valid, False if not. A block object can
become invalid if the block it refers to doesn’t exist anymore in the inferior. All other
gdb.Block methods will throw an exception if it is invalid at the time the method is
called. The block’s validity is also checked during iteration over symbols of the block.
A gdb.Block object has the following attributes:
Block.start
The start address of the block. This attribute is not writable.
[Variable]
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Block.end
387
[Variable]
The end address of the block. This attribute is not writable.
[Variable]
The name of the block represented as a gdb.Symbol. If the block is not named, then
this attribute holds None. This attribute is not writable.
Block.function
For ordinary function blocks, the superblock is the static block. However, you should
note that it is possible for a function block to have a superblock that is not the static
block – for instance this happens for an inlined function.
[Variable]
The block containing this block. If this parent block does not exist, this attribute
holds None. This attribute is not writable.
Block.superblock
Block.global_block
[Variable]
The global block associated with this block. This attribute is not writable.
Block.static_block
[Variable]
The static block associated with this block. This attribute is not writable.
[Variable]
True if the gdb.Block object is a global block, False if not. This attribute is not
writable.
Block.is_global
[Variable]
True if the gdb.Block object is a static block, False if not. This attribute is not
writable.
Block.is_static
23.2.2.27 Python representation of Symbols.
gdb represents every variable, function and type as an entry in a symbol table. See
Chapter 16 [Examining the Symbol Table], page 221. Similarly, Python represents these
symbols in gdb with the gdb.Symbol object.
The following symbol-related functions are available in the gdb module:
gdb.lookup_symbol (name [, block [, domain]])
[Function]
This function searches for a symbol by name. The search scope can be restricted to
the parameters defined in the optional domain and block arguments.
name is the name of the symbol. It must be a string. The optional block argument
restricts the search to symbols visible in that block. The block argument must be a
gdb.Block object. If omitted, the block for the current frame is used. The optional
domain argument restricts the search to the domain type. The domain argument
must be a domain constant defined in the gdb module and described later in this
chapter.
The result is a tuple of two elements. The first element is a gdb.Symbol object or
None if the symbol is not found. If the symbol is found, the second element is True if
the symbol is a field of a method’s object (e.g., this in C++), otherwise it is False.
If the symbol is not found, the second element is False.
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Debugging with gdb
gdb.lookup_global_symbol (name [, domain])
[Function]
This function searches for a global symbol by name. The search scope can be restricted
to by the domain argument.
name is the name of the symbol. It must be a string. The optional domain argument
restricts the search to the domain type. The domain argument must be a domain
constant defined in the gdb module and described later in this chapter.
The result is a gdb.Symbol object or None if the symbol is not found.
A gdb.Symbol object has the following attributes:
[Variable]
The type of the symbol or None if no type is recorded. This attribute is represented as
a gdb.Type object. See Section 23.2.2.4 [Types In Python], page 338. This attribute
is not writable.
Symbol.type
[Variable]
The symbol table in which the symbol appears. This attribute is represented as a
gdb.Symtab object. See Section 23.2.2.28 [Symbol Tables In Python], page 390. This
attribute is not writable.
Symbol.symtab
[Variable]
The line number in the source code at which the symbol was defined. This is an
integer.
Symbol.line
Symbol.name
[Variable]
The name of the symbol as a string. This attribute is not writable.
[Variable]
The name of the symbol, as used by the linker (i.e., may be mangled). This attribute
is not writable.
Symbol.linkage_name
[Variable]
The name of the symbol in a form suitable for output. This is either name or linkage_
name, depending on whether the user asked gdb to display demangled or mangled
names.
Symbol.print_name
[Variable]
The address class of the symbol. This classifies how to find the value of a symbol.
Each address class is a constant defined in the gdb module and described later in this
chapter.
Symbol.addr_class
[Variable]
This is True if evaluating this symbol’s value requires a frame (see Section 23.2.2.25
[Frames In Python], page 382) and False otherwise. Typically, local variables will
require a frame, but other symbols will not.
Symbol.needs_frame
Symbol.is_argument
[Variable]
True if the symbol is an argument of a function.
Symbol.is_constant
True if the symbol is a constant.
[Variable]
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Symbol.is_function
389
[Variable]
True if the symbol is a function or a method.
Symbol.is_variable
[Variable]
True if the symbol is a variable.
A gdb.Symbol object has the following methods:
Symbol.is_valid ()
[Function]
Returns True if the gdb.Symbol object is valid, False if not. A gdb.Symbol object
can become invalid if the symbol it refers to does not exist in gdb any longer. All
other gdb.Symbol methods will throw an exception if it is invalid at the time the
method is called.
Symbol.value ([frame])
[Function]
Compute the value of the symbol, as a gdb.Value. For functions, this computes the
address of the function, cast to the appropriate type. If the symbol requires a frame
in order to compute its value, then frame must be given. If frame is not given, or if
frame is invalid, then this method will throw an exception.
The available domain categories in gdb.Symbol are represented as constants in the gdb
module:
gdb.SYMBOL_UNDEF_DOMAIN
This is used when a domain has not been discovered or none of the following
domains apply. This usually indicates an error either in the symbol information
or in gdb’s handling of symbols.
gdb.SYMBOL_VAR_DOMAIN
This domain contains variables, function names, typedef names and enum type
values.
gdb.SYMBOL_STRUCT_DOMAIN
This domain holds struct, union and enum type names.
gdb.SYMBOL_LABEL_DOMAIN
This domain contains names of labels (for gotos).
gdb.SYMBOL_VARIABLES_DOMAIN
This domain holds a subset of the SYMBOLS_VAR_DOMAIN; it contains everything
minus functions and types.
gdb.SYMBOL_FUNCTION_DOMAIN
This domain contains all functions.
gdb.SYMBOL_TYPES_DOMAIN
This domain contains all types.
The available address class categories in gdb.Symbol are represented as constants in the
gdb module:
gdb.SYMBOL_LOC_UNDEF
If this is returned by address class, it indicates an error either in the symbol
information or in gdb’s handling of symbols.
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gdb.SYMBOL_LOC_CONST
Value is constant int.
gdb.SYMBOL_LOC_STATIC
Value is at a fixed address.
gdb.SYMBOL_LOC_REGISTER
Value is in a register.
gdb.SYMBOL_LOC_ARG
Value is an argument. This value is at the offset stored within the symbol inside
the frame’s argument list.
gdb.SYMBOL_LOC_REF_ARG
Value address is stored in the frame’s argument list. Just like LOC_ARG except
that the value’s address is stored at the offset, not the value itself.
gdb.SYMBOL_LOC_REGPARM_ADDR
Value is a specified register. Just like LOC_REGISTER except the register holds
the address of the argument instead of the argument itself.
gdb.SYMBOL_LOC_LOCAL
Value is a local variable.
gdb.SYMBOL_LOC_TYPEDEF
Value not used. Symbols in the domain SYMBOL_STRUCT_DOMAIN all have this
class.
gdb.SYMBOL_LOC_BLOCK
Value is a block.
gdb.SYMBOL_LOC_CONST_BYTES
Value is a byte-sequence.
gdb.SYMBOL_LOC_UNRESOLVED
Value is at a fixed address, but the address of the variable has to be determined
from the minimal symbol table whenever the variable is referenced.
gdb.SYMBOL_LOC_OPTIMIZED_OUT
The value does not actually exist in the program.
gdb.SYMBOL_LOC_COMPUTED
The value’s address is a computed location.
23.2.2.28 Symbol table representation in Python.
Access to symbol table data maintained by gdb on the inferior is exposed to Python via two
objects: gdb.Symtab_and_line and gdb.Symtab. Symbol table and line data for a frame
is returned from the find_sal method in gdb.Frame object. See Section 23.2.2.25 [Frames
In Python], page 382.
For more information on gdb’s symbol table management, see Chapter 16 [Examining
the Symbol Table], page 221, for more information.
A gdb.Symtab_and_line object has the following attributes:
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[Variable]
The symbol table object (gdb.Symtab) for this frame. This attribute is not writable.
Symtab_and_line.symtab
[Variable]
Indicates the start of the address range occupied by code for the current source line.
This attribute is not writable.
Symtab_and_line.pc
[Variable]
Indicates the end of the address range occupied by code for the current source line.
This attribute is not writable.
Symtab_and_line.last
[Variable]
Indicates the current line number for this object. This attribute is not writable.
Symtab_and_line.line
A gdb.Symtab_and_line object has the following methods:
Symtab_and_line.is_valid ()
[Function]
Returns True if the gdb.Symtab_and_line object is valid, False if not. A
gdb.Symtab_and_line object can become invalid if the Symbol table and line object
it refers to does not exist in gdb any longer. All other gdb.Symtab_and_line
methods will throw an exception if it is invalid at the time the method is called.
A gdb.Symtab object has the following attributes:
Symtab.filename
[Variable]
The symbol table’s source filename. This attribute is not writable.
[Variable]
The symbol table’s backing object file. See Section 23.2.2.24 [Objfiles In Python],
page 380. This attribute is not writable.
Symtab.objfile
[Variable]
The name and possibly version number of the program that compiled the code in
the symbol table. The contents of this string is up to the compiler. If no producer
information is available then None is returned. This attribute is not writable.
Symtab.producer
A gdb.Symtab object has the following methods:
Symtab.is_valid ()
[Function]
Returns True if the gdb.Symtab object is valid, False if not. A gdb.Symtab object
can become invalid if the symbol table it refers to does not exist in gdb any longer.
All other gdb.Symtab methods will throw an exception if it is invalid at the time the
method is called.
Symtab.fullname ()
[Function]
Return the symbol table’s source absolute file name.
Symtab.global_block ()
[Function]
Return the global block of the underlying symbol table. See Section 23.2.2.26 [Blocks
In Python], page 385.
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Symtab.static_block ()
[Function]
Return the static block of the underlying symbol table. See Section 23.2.2.26 [Blocks
In Python], page 385.
Symtab.linetable ()
[Function]
Return the line table associated with the symbol table. See Section 23.2.2.29 [Line
Tables In Python], page 392.
23.2.2.29 Manipulating line tables using Python
Python code can request and inspect line table information from a symbol table that is
loaded in gdb. A line table is a mapping of source lines to their executable locations
in memory. To acquire the line table information for a particular symbol table, use the
linetable function (see Section 23.2.2.28 [Symbol Tables In Python], page 390).
A gdb.LineTable is iterable. The iterator returns LineTableEntry objects that correspond to the source line and address for each line table entry. LineTableEntry objects
have the following attributes:
[Variable]
The source line number for this line table entry. This number corresponds to the
actual line of source. This attribute is not writable.
LineTableEntry.line
[Variable]
The address that is associated with the line table entry where the executable code for
that source line resides in memory. This attribute is not writable.
LineTableEntry.pc
As there can be multiple addresses for a single source line, you may receive multiple
LineTableEntry objects with matching line attributes, but with different pc attributes.
The iterator is sorted in ascending pc order. Here is a small example illustrating iterating
over a line table.
symtab = gdb.selected_frame().find_sal().symtab
linetable = symtab.linetable()
for line in linetable:
print "Line: "+str(line.line)+" Address: "+hex(line.pc)
This will have the following output:
Line:
Line:
Line:
Line:
Line:
Line:
Line:
Line:
33
37
39
40
42
44
42
45
Address:
Address:
Address:
Address:
Address:
Address:
Address:
Address:
0x4005c8L
0x4005caL
0x4005d2L
0x4005f8L
0x4005ffL
0x400608L
0x40060cL
0x400615L
In addition to being able to iterate over a LineTable, it also has the following direct
access methods:
LineTable.line (line)
[Function]
Return a Python Tuple of LineTableEntry objects for any entries in the line table
for the given line, which specifies the source code line. If there are no entries for that
source code line, the Python None is returned.
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LineTable.has_line (line)
[Function]
Return a Python Boolean indicating whether there is an entry in the line table for
this source line. Return True if an entry is found, or False if not.
LineTable.source_lines ()
[Function]
Return a Python List of the source line numbers in the symbol table. Only lines
with executable code locations are returned. The contents of the List will just be
the source line entries represented as Python Long values.
23.2.2.30 Manipulating breakpoints using Python
Python code can manipulate breakpoints via the gdb.Breakpoint class.
Breakpoint.__init__ (spec [, type [, wp class [,internal [,temporary]]]])
[Function]
Create a new breakpoint according to spec, which is a string naming the location
of the breakpoint, or an expression that defines a watchpoint. The contents can be
any location recognized by the break command, or in the case of a watchpoint, by
the watch command. The optional type denotes the breakpoint to create from the
types defined later in this chapter. This argument can be either gdb.BP_BREAKPOINT
or gdb.BP_WATCHPOINT; it defaults to gdb.BP_BREAKPOINT. The optional internal
argument allows the breakpoint to become invisible to the user. The breakpoint
will neither be reported when created, nor will it be listed in the output from info
breakpoints (but will be listed with the maint info breakpoints command). The
optional temporary argument makes the breakpoint a temporary breakpoint. Temporary breakpoints are deleted after they have been hit. Any further access to the
Python breakpoint after it has been hit will result in a runtime error (as that breakpoint has now been automatically deleted). The optional wp class argument defines
the class of watchpoint to create, if type is gdb.BP_WATCHPOINT. If a watchpoint class
is not provided, it is assumed to be a gdb.WP_WRITE class.
The available types are represented by constants defined in the gdb module:
gdb.BP_BREAKPOINT
Normal code breakpoint.
gdb.BP_WATCHPOINT
Watchpoint breakpoint.
gdb.BP_HARDWARE_WATCHPOINT
Hardware assisted watchpoint.
gdb.BP_READ_WATCHPOINT
Hardware assisted read watchpoint.
gdb.BP_ACCESS_WATCHPOINT
Hardware assisted access watchpoint.
The available watchpoint types represented by constants are defined in the gdb module:
gdb.WP_READ
Read only watchpoint.
gdb.WP_WRITE
Write only watchpoint.
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gdb.WP_ACCESS
Read/Write watchpoint.
Breakpoint.stop (self)
[Function]
The gdb.Breakpoint class can be sub-classed and, in particular, you may choose
to implement the stop method. If this method is defined in a sub-class of
gdb.Breakpoint, it will be called when the inferior reaches any location of a
breakpoint which instantiates that sub-class. If the method returns True, the inferior
will be stopped at the location of the breakpoint, otherwise the inferior will continue.
If there are multiple breakpoints at the same location with a stop method, each one
will be called regardless of the return status of the previous. This ensures that all
stop methods have a chance to execute at that location. In this scenario if one of the
methods returns True but the others return False, the inferior will still be stopped.
You should not alter the execution state of the inferior (i.e., step, next, etc.), alter the
current frame context (i.e., change the current active frame), or alter, add or delete
any breakpoint. As a general rule, you should not alter any data within gdb or the
inferior at this time.
Example stop implementation:
class MyBreakpoint (gdb.Breakpoint):
def stop (self):
inf_val = gdb.parse_and_eval("foo")
if inf_val == 3:
return True
return False
Breakpoint.is_valid ()
[Function]
Return True if this Breakpoint object is valid, False otherwise. A Breakpoint
object can become invalid if the user deletes the breakpoint. In this case, the object
still exists, but the underlying breakpoint does not. In the cases of watchpoint scope,
the watchpoint remains valid even if execution of the inferior leaves the scope of that
watchpoint.
Breakpoint.delete ()
[Function]
Permanently deletes the gdb breakpoint.
This also invalidates the Python
Breakpoint object. Any further access to this object’s attributes or methods will
raise an error.
[Variable]
This attribute is True if the breakpoint is enabled, and False otherwise. This attribute is writable. You can use it to enable or disable the breakpoint.
Breakpoint.enabled
[Variable]
This attribute is True if the breakpoint is silent, and False otherwise. This attribute
is writable.
Note that a breakpoint can also be silent if it has commands and the first command
is silent. This is not reported by the silent attribute.
Breakpoint.silent
[Variable]
This attribute is True if the breakpoint is pending, and False otherwise. See
Section 5.1.1 [Set Breaks], page 46. This attribute is read-only.
Breakpoint.pending
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[Variable]
If the breakpoint is thread-specific, this attribute holds the thread’s global id. If the
breakpoint is not thread-specific, this attribute is None. This attribute is writable.
Breakpoint.thread
[Variable]
If the breakpoint is Ada task-specific, this attribute holds the Ada task id. If the
breakpoint is not task-specific (or the underlying language is not Ada), this attribute
is None. This attribute is writable.
Breakpoint.task
[Variable]
This attribute holds the ignore count for the breakpoint, an integer. This attribute
is writable.
Breakpoint.ignore_count
[Variable]
This attribute holds the breakpoint’s number — the identifier used by the user to
manipulate the breakpoint. This attribute is not writable.
Breakpoint.number
[Variable]
This attribute holds the breakpoint’s type — the identifier used to determine the
actual breakpoint type or use-case. This attribute is not writable.
Breakpoint.type
[Variable]
This attribute tells whether the breakpoint is visible to the user when set, or when
the ‘info breakpoints’ command is run. This attribute is not writable.
Breakpoint.visible
[Variable]
This attribute indicates whether the breakpoint was created as a temporary breakpoint. Temporary breakpoints are automatically deleted after that breakpoint has
been hit. Access to this attribute, and all other attributes and functions other than
the is_valid function, will result in an error after the breakpoint has been hit (as it
has been automatically deleted). This attribute is not writable.
Breakpoint.temporary
[Variable]
This attribute holds the hit count for the breakpoint, an integer. This attribute is
writable, but currently it can only be set to zero.
Breakpoint.hit_count
[Variable]
This attribute holds the location of the breakpoint, as specified by the user. It is a
string. If the breakpoint does not have a location (that is, it is a watchpoint) the
attribute’s value is None. This attribute is not writable.
Breakpoint.location
[Variable]
This attribute holds a breakpoint expression, as specified by the user. It is a string.
If the breakpoint does not have an expression (the breakpoint is not a watchpoint)
the attribute’s value is None. This attribute is not writable.
Breakpoint.expression
[Variable]
This attribute holds the condition of the breakpoint, as specified by the user. It is
a string. If there is no condition, this attribute’s value is None. This attribute is
writable.
Breakpoint.condition
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[Variable]
This attribute holds the commands attached to the breakpoint. If there are commands, this attribute’s value is a string holding all the commands, separated by
newlines. If there are no commands, this attribute is None. This attribute is not
writable.
Breakpoint.commands
23.2.2.31 Finish Breakpoints
A finish breakpoint is a temporary breakpoint set at the return address of a frame, based
on the finish command. gdb.FinishBreakpoint extends gdb.Breakpoint. The underlying breakpoint will be disabled and deleted when the execution will run out of the breakpoint scope (i.e. Breakpoint.stop or FinishBreakpoint.out_of_scope triggered). Finish
breakpoints are thread specific and must be create with the right thread selected.
FinishBreakpoint.__init__ ([frame] [, internal])
[Function]
Create a finish breakpoint at the return address of the gdb.Frame object frame.
If frame is not provided, this defaults to the newest frame. The optional internal
argument allows the breakpoint to become invisible to the user. See Section 23.2.2.30
[Breakpoints In Python], page 393, for further details about this argument.
FinishBreakpoint.out_of_scope (self)
[Function]
In some circumstances (e.g. longjmp, C++ exceptions, gdb return command, . . . ), a
function may not properly terminate, and thus never hit the finish breakpoint. When
gdb notices such a situation, the out_of_scope callback will be triggered.
You may want to sub-class gdb.FinishBreakpoint and override this method:
class MyFinishBreakpoint (gdb.FinishBreakpoint)
def stop (self):
print "normal finish"
return True
def out_of_scope ():
print "abnormal finish"
[Variable]
When gdb is stopped at a finish breakpoint and the frame used to build the
gdb.FinishBreakpoint object had debug symbols, this attribute will contain a
gdb.Value object corresponding to the return value of the function. The value will
be None if the function return type is void or if the return value was not computable.
This attribute is not writable.
FinishBreakpoint.return_value
23.2.2.32 Python representation of lazy strings.
A lazy string is a string whose contents is not retrieved or encoded until it is needed.
A gdb.LazyString is represented in gdb as an address that points to a region of memory, an encoding that will be used to encode that region of memory, and a length to delimit
the region of memory that represents the string. The difference between a gdb.LazyString
and a string wrapped within a gdb.Value is that a gdb.LazyString will be treated differently by gdb when printing. A gdb.LazyString is retrieved and encoded during printing,
while a gdb.Value wrapping a string is immediately retrieved and encoded on creation.
A gdb.LazyString object has the following functions:
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LazyString.value ()
[Function]
Convert the gdb.LazyString to a gdb.Value. This value will point to the string
in memory, but will lose all the delayed retrieval, encoding and handling that gdb
applies to a gdb.LazyString.
[Variable]
This attribute holds the address of the string. This attribute is not writable.
LazyString.address
[Variable]
This attribute holds the length of the string in characters. If the length is -1, then
the string will be fetched and encoded up to the first null of appropriate width. This
attribute is not writable.
LazyString.length
[Variable]
This attribute holds the encoding that will be applied to the string when the string
is printed by gdb. If the encoding is not set, or contains an empty string, then gdb
will select the most appropriate encoding when the string is printed. This attribute
is not writable.
LazyString.encoding
[Variable]
This attribute holds the type that is represented by the lazy string’s type. For a lazy
string this is a pointer or array type. To resolve this to the lazy string’s character
type, use the type’s target method. See Section 23.2.2.4 [Types In Python], page 338.
This attribute is not writable.
LazyString.type
23.2.2.33 Python representation of architectures
gdb uses architecture specific parameters and artifacts in a number of its various computations. An architecture is represented by an instance of the gdb.Architecture class.
A gdb.Architecture class has the following methods:
Architecture.name ()
[Function]
Return the name (string value) of the architecture.
Architecture.disassemble (start_pc [, end_pc [, count]])
[Function]
Return a list of disassembled instructions starting from the memory address start pc.
The optional arguments end pc and count determine the number of instructions in
the returned list. If both the optional arguments end pc and count are specified,
then a list of at most count disassembled instructions whose start address falls in the
closed memory address interval from start pc to end pc are returned. If end pc is
not specified, but count is specified, then count number of instructions starting from
the address start pc are returned. If count is not specified but end pc is specified,
then all instructions whose start address falls in the closed memory address interval
from start pc to end pc are returned. If neither end pc nor count are specified, then
a single instruction at start pc is returned. For all of these cases, each element of the
returned list is a Python dict with the following string keys:
addr
The value corresponding to this key is a Python long integer capturing
the memory address of the instruction.
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asm
The value corresponding to this key is a string value which represents the
instruction with assembly language mnemonics. The assembly language
flavor used is the same as that specified by the current CLI variable
disassembly-flavor. See Section 9.6 [Machine Code], page 110.
length
The value corresponding to this key is the length (integer value) of the
instruction in bytes.
23.2.3 Python Auto-loading
When a new object file is read (for example, due to the file command, or because the
inferior has loaded a shared library), gdb will look for Python support scripts in several
ways: ‘objfile-gdb.py’ and .debug_gdb_scripts section. See Section 23.4 [Auto-loading
extensions], page 452.
The auto-loading feature is useful for supplying application-specific debugging commands
and scripts.
Auto-loading can be enabled or disabled, and the list of auto-loaded scripts can be
printed.
set auto-load python-scripts [on|off]
Enable or disable the auto-loading of Python scripts.
show auto-load python-scripts
Show whether auto-loading of Python scripts is enabled or disabled.
info auto-load python-scripts [regexp]
Print the list of all Python scripts that gdb auto-loaded.
Also printed is the list of Python scripts that were mentioned in the .debug_
gdb_scripts section and were either not found (see Section 23.4.2 [dotdebug gdb scripts section], page 454) or were not auto-loaded due to auto-load
safe-path rejection (see Section 22.7 [Auto-loading], page 308). This is useful
because their names are not printed when gdb tries to load them and fails.
There may be many of them, and printing an error message for each one is
problematic.
If regexp is supplied only Python scripts with matching names are printed.
Example:
(gdb) info auto-load python-scripts
Loaded Script
Yes
py-section-script.py
full name: /tmp/py-section-script.py
No
my-foo-pretty-printers.py
When reading an auto-loaded file or script, gdb sets the current objfile. This is available via the gdb.current_objfile function (see Section 23.2.2.24 [Objfiles In Python],
page 380). This can be useful for registering objfile-specific pretty-printers and frame-filters.
23.2.4 Python modules
gdb comes with several modules to assist writing Python code.
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23.2.4.1 gdb.printing
This module provides a collection of utilities for working with pretty-printers.
PrettyPrinter (name, subprinters=None)
This class specifies the API that makes ‘info pretty-printer’, ‘enable
pretty-printer’ and ‘disable pretty-printer’ work.
Pretty-printers
should generally inherit from this class.
SubPrettyPrinter (name)
For printers that handle multiple types, this class specifies the corresponding
API for the subprinters.
RegexpCollectionPrettyPrinter (name)
Utility class for handling multiple printers, all recognized via regular expressions. See Section 23.2.2.7 [Writing a Pretty-Printer], page 344, for an example.
FlagEnumerationPrinter (name)
A pretty-printer which handles printing of enum values. Unlike gdb’s builtin enum printing, this printer attempts to work properly when there is some
overlap between the enumeration constants. The argument name is the name
of the printer and also the name of the enum type to look up.
register_pretty_printer (obj, printer, replace=False)
Register printer with the pretty-printer list of obj. If replace is True then any
existing copy of the printer is replaced. Otherwise a RuntimeError exception
is raised if a printer with the same name already exists.
23.2.4.2 gdb.types
This module provides a collection of utilities for working with gdb.Type objects.
get_basic_type (type)
Return type with const and volatile qualifiers stripped, and with typedefs and
C++ references converted to the underlying type.
C++ example:
typedef const int const_int;
const_int foo (3);
const_int& foo_ref (foo);
int main () { return 0; }
Then in gdb:
(gdb)
(gdb)
(gdb)
(gdb)
int
start
python import gdb.types
python foo_ref = gdb.parse_and_eval("foo_ref")
python print gdb.types.get_basic_type(foo_ref.type)
has_field (type, field)
Return True if type, assumed to be a type with fields (e.g., a structure or
union), has field field.
make_enum_dict (enum_type)
Return a Python dictionary type produced from enum type.
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deep_items (type)
Returns a Python iterator similar to the standard gdb.Type.iteritems
method, except that the iterator returned by deep_items will recursively
traverse anonymous struct or union fields. For example:
struct A
{
int a;
union {
int b0;
int b1;
};
};
Then in gdb:
(gdb) python
(gdb) python
(gdb) python
{[’a’, ’’]}
(gdb) python
{[’a’, ’b0’,
import gdb.types
struct_a = gdb.lookup_type("struct A")
print struct_a.keys ()
print [k for k,v in gdb.types.deep_items(struct_a)]
’b1’]}
get_type_recognizers ()
Return a list of the enabled type recognizers for the current context. This
is called by gdb during the type-printing process (see Section 23.2.2.8 [Type
Printing API], page 347).
apply_type_recognizers (recognizers, type_obj)
Apply the type recognizers, recognizers, to the type object type obj. If any
recognizer returns a string, return that string. Otherwise, return None. This
is called by gdb during the type-printing process (see Section 23.2.2.8 [Type
Printing API], page 347).
register_type_printer (locus, printer)
This is a convenience function to register a type printer printer. The printer
must implement the type printer protocol. The locus argument is either
a gdb.Objfile, in which case the printer is registered with that objfile; a
gdb.Progspace, in which case the printer is registered with that progspace; or
None, in which case the printer is registered globally.
TypePrinter
This is a base class that implements the type printer protocol. Type printers are
encouraged, but not required, to derive from this class. It defines a constructor:
__init__ (self, name)
[Method on TypePrinter]
Initialize the type printer with the given name. The new printer starts
in the enabled state.
23.2.4.3 gdb.prompt
This module provides a method for prompt value-substitution.
substitute_prompt (string)
Return string with escape sequences substituted by values. Some escape sequences take arguments. You can specify arguments inside “{}” immediately
following the escape sequence.
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The escape sequences you can pass to this function are:
\\
Substitute a backslash.
\e
Substitute an ESC character.
\f
Substitute the selected frame; an argument names a frame parameter.
\n
Substitute a newline.
\p
Substitute a parameter’s value; the argument names the parameter.
\r
Substitute a carriage return.
\t
Substitute the selected thread; an argument names a thread parameter.
\v
Substitute the version of GDB.
\w
Substitute the current working directory.
\[
Begin a sequence of non-printing characters. These sequences are
typically used with the ESC character, and are not counted in the
string length. Example: “\[\e[0;34m\](gdb)\[\e[0m\]” will return
a blue-colored “(gdb)” prompt where the length is five.
\]
End a sequence of non-printing characters.
For example:
substitute_prompt (‘‘frame: \f,
print arguments: \p{print frame-arguments}’’)
will return the string:
"frame: main, print arguments: scalars"
23.3 Extending gdb using Guile
You can extend gdb using the Guile implementation of the Scheme programming language.
This feature is available only if gdb was configured using ‘--with-guile’.
23.3.1 Guile Introduction
Guile is an implementation of the Scheme programming language and is the GNU project’s
official extension language.
Guile support in gdb follows the Python support in gdb reasonably closely, so concepts
there should carry over. However, some things are done differently where it makes sense.
gdb requires Guile version 2.0 or greater. Older versions are not supported.
Guile scripts used by gdb should be installed in ‘data-directory/guile’, where datadirectory is the data directory as determined at gdb startup (see Section 18.7 [Data Files],
page 256). This directory, known as the guile directory, is automatically added to the Guile
Search Path in order to allow the Guile interpreter to locate all scripts installed at this
location.
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23.3.2 Guile Commands
gdb provides two commands for accessing the Guile interpreter:
guile-repl
gr
The guile-repl command can be used to start an interactive Guile prompt or
repl. To return to gdb, type ,q or the EOF character (e.g., Ctrl-D on an empty
prompt). These commands do not take any arguments.
guile [scheme-expression]
gu [scheme-expression]
The guile command can be used to evaluate a Scheme expression.
If given an argument, gdb will pass the argument to the Guile interpreter for
evaluation.
(gdb) guile (display (+ 20 3)) (newline)
23
The result of the Scheme expression is displayed using normal Guile rules.
(gdb) guile (+ 20 3)
23
If you do not provide an argument to guile, it will act as a multi-line command,
like define. In this case, the Guile script is made up of subsequent command
lines, given after the guile command. This command list is terminated using
a line containing end. For example:
(gdb) guile
>(display 23)
>(newline)
>end
23
It is also possible to execute a Guile script from the gdb interpreter:
source ‘script-name’
The script name must end with ‘.scm’ and gdb must be configured to recognize
the script language based on filename extension using the script-extension
setting. See Chapter 23 [Extending GDB], page 321.
guile (load "script-name")
This method uses the load Guile function. It takes a string argument that is
the name of the script to load. See the Guile documentation for a description
of this function. (see Section “Loading” in GNU Guile Reference Manual).
23.3.3 Guile API
You can get quick online help for gdb’s Guile API by issuing the command help guile,
or by issuing the command ,help from an interactive Guile session. Furthermore, most
Guile procedures provided by gdb have doc strings which can be obtained with ,describe
procedure-name or ,d procedure-name from the Guile interactive prompt.
23.3.3.1 Basic Guile
At startup, gdb overrides Guile’s current-output-port and current-error-port to print
using gdb’s output-paging streams. A Guile program which outputs to one of these streams
Chapter 23: Extending gdb
403
may have its output interrupted by the user (see Section 22.4 [Screen Size], page 305). In
this situation, a Guile signal exception is thrown with value SIGINT.
Guile’s history mechanism uses the same naming as gdb’s, namely the user of dollarvariables (e.g., $1, $2, etc.). The results of evaluations in Guile and in GDB are counted
separately, $1 in Guile is not the same value as $1 in gdb.
gdb is not thread-safe. If your Guile program uses multiple threads, you must be careful
to only call gdb-specific functions in the gdb thread.
Some care must be taken when writing Guile code to run in gdb. Two things are worth
noting in particular:
• gdb installs handlers for SIGCHLD and SIGINT. Guile code must not override these, or
even change the options using sigaction. If your program changes the handling of
these signals, gdb will most likely stop working correctly. Note that it is unfortunately
common for GUI toolkits to install a SIGCHLD handler.
• gdb takes care to mark its internal file descriptors as close-on-exec. However, this
cannot be done in a thread-safe way on all platforms. Your Guile programs should be
aware of this and should both create new file descriptors with the close-on-exec flag set
and arrange to close unneeded file descriptors before starting a child process.
gdb introduces a new Guile module, named gdb. All methods and classes added by gdb
are placed in this module. gdb does not automatically import the gdb module, scripts
must do this themselves. There are various options for how to import a module, so gdb
leaves the choice of how the gdb module is imported to the user. To simplify interactive
use, it is recommended to add one of the following to your ~/.gdbinit.
guile (use-modules (gdb))
guile (use-modules ((gdb) #:renamer (symbol-prefix-proc ’gdb:)))
Which one to choose depends on your preference. The second one adds gdb: as a prefix
to all module functions and variables.
The rest of this manual assumes the gdb module has been imported without any prefix.
See the Guile documentation for use-modules for more information (see Section “Using
Guile Modules” in GNU Guile Reference Manual).
Example:
(gdb) guile (value-type (make-value 1))
ERROR: Unbound variable: value-type
Error while executing Scheme code.
(gdb) guile (use-modules (gdb))
(gdb) guile (value-type (make-value 1))
int
(gdb)
The (gdb) module provides these basic Guile functions.
execute command [#:from-tty boolean] [#:to-string boolean]
[Scheme Procedure]
Evaluate command, a string, as a gdb CLI command. If a gdb exception happens
while command runs, it is translated as described in Section 23.3.3.4 [Guile Exception
Handling], page 406.
from-tty specifies whether gdb ought to consider this command as having originated
from the user invoking it interactively. It must be a boolean value. If omitted, it
defaults to #f.
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Debugging with gdb
By default, any output produced by command is sent to gdb’s standard output (and
to the log output if logging is turned on). If the to-string parameter is #t, then output
will be collected by execute and returned as a string. The default is #f, in which
case the return value is unspecified. If to-string is #t, the gdb virtual terminal will
be temporarily set to unlimited width and height, and its pagination will be disabled;
see Section 22.4 [Screen Size], page 305.
history-ref number
[Scheme Procedure]
Return a value from gdb’s value history (see Section 10.10 [Value History], page 138).
The number argument indicates which history element to return. If number is negative, then gdb will take its absolute value and count backward from the last element
(i.e., the most recent element) to find the value to return. If number is zero, then
gdb will return the most recent element. If the element specified by number doesn’t
exist in the value history, a gdb:error exception will be raised.
If no exception is raised, the return value is always an instance of (see
Section 23.3.3.5 [Values From Inferior In Guile], page 408).
Note: gdb’s value history is independent of Guile’s. $1 in gdb’s value history contains
the result of evaluating an expression from gdb’s command line and $1 from Guile’s
history contains the result of evaluating an expression from Guile’s command line.
history-append! value
[Scheme Procedure]
Append value, an instance of , to gdb’s value history. Return its index
in the history.
Putting into history values returned by Guile extensions will allow the user convenient
access to those values via CLI history facilities.
parse-and-eval expression
[Scheme Procedure]
Parse expression as an expression in the current language, evaluate it, and return the
result as a . The expression must be a string.
This function can be useful when implementing a new command (see Section 23.3.3.11
[Commands In Guile], page 423), as it provides a way to parse the command’s arguments as an expression. It is also is useful when computing values. For example, it is
the only way to get the value of a convenience variable (see Section 10.11 [Convenience
Vars], page 139) as a .
23.3.3.2 Guile Configuration
gdb provides these Scheme functions to access various configuration parameters.
[Scheme Procedure]
Return a string containing gdb’s data directory. This directory contains gdb’s ancillary files.
data-directory
[Scheme Procedure]
Return a string containing gdb’s Guile data directory. This directory contains the
Guile modules provided by gdb.
guile-data-directory
gdb-version
Return a string containing the gdb version.
[Scheme Procedure]
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405
[Scheme Procedure]
Return a string containing the host configuration. This is the string passed to --host
when gdb was configured.
host-config
[Scheme Procedure]
Return a string containing the target configuration. This is the string passed to
--target when gdb was configured.
target-config
23.3.3.3 GDB Scheme Data Types
The values exposed by gdb to Guile are known as gdb objects. There are several kinds of
gdb object, and each is disjoint from all other types known to Guile.
gdb-object-kind object
[Scheme Procedure]
Return the kind of the gdb object, e.g., , as a symbol.
gdb defines the following object types:
See Section 23.3.3.21 [Architectures In Guile], page 445.
See Section 23.3.3.16 [Blocks In Guile], page 434.
See Section 23.3.3.16 [Blocks In Guile], page 434.
See Section 23.3.3.19 [Breakpoints In Guile], page 440.
See Section 23.3.3.11 [Commands In Guile], page 423.
See Section 23.3.3.4 [Guile Exception Handling], page 406.
See Section 23.3.3.15 [Frames In Guile], page 431.
See Section 23.3.3.25 [Iterators In Guile], page 449.
See Section 23.3.3.20 [Lazy Strings In Guile], page 444.
See Section 23.3.3.14 [Objfiles In Guile], page 431.
See Section 23.3.3.12 [Parameters In Guile], page 427.
See Section 23.3.3.8 [Guile Pretty Printing API], page 419.
See Section 23.3.3.8 [Guile Pretty Printing API], page 419.
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Debugging with gdb
See Section 23.3.3.13 [Progspaces In Guile], page 430.
See Section 23.3.3.17 [Symbols In Guile], page 436.
See Section 23.3.3.18 [Symbol Tables In Guile], page 439.
See Section 23.3.3.18 [Symbol Tables In Guile], page 439.
See Section 23.3.3.7 [Types In Guile], page 414.
See Section 23.3.3.7 [Types In Guile], page 414.
See Section 23.3.3.5 [Values From Inferior In Guile], page 408.
The following gdb objects are managed internally so that the Scheme function eq? may
be applied to them.
23.3.3.4 Guile Exception Handling
When executing the guile command, Guile exceptions uncaught within the Guile code
are translated to calls to the gdb error-reporting mechanism. If the command that called
guile does not handle the error, gdb will terminate it and report the error according to
the setting of the guile print-stack parameter.
The guile print-stack parameter has three settings:
none
Nothing is printed.
message
An error message is printed containing the Guile exception name, the associated
value, and the Guile call stack backtrace at the point where the exception was
raised. Example:
(gdb) guile (display foo)
ERROR: In procedure memoize-variable-access!:
ERROR: Unbound variable: foo
Error while executing Scheme code.
full
In addition to an error message a full backtrace is printed.
Chapter 23: Extending gdb
407
(gdb) set guile print-stack full
(gdb) guile (display foo)
Guile Backtrace:
In ice-9/boot-9.scm:
157: 10 [catch #t # ...]
In unknown file:
?: 9 [apply-smob/1 #]
In ice-9/boot-9.scm:
157: 8 [catch #t # ...]
In unknown file:
?: 7 [apply-smob/1 #]
?: 6 [call-with-input-string "(display foo)" ...]
In ice-9/boot-9.scm:
2320: 5 [save-module-excursion #]
In ice-9/eval-string.scm:
44: 4 [read-and-eval # #:lang ...]
37: 3 [lp (display foo)]
In ice-9/eval.scm:
387: 2 [eval # ()]
393: 1 [eval # ()]
In unknown file:
?: 0 [memoize-variable-access! # ...]
ERROR: In procedure memoize-variable-access!:
ERROR: Unbound variable: foo
Error while executing Scheme code.
gdb errors that happen in gdb commands invoked by Guile code are converted to Guile
exceptions. The type of the Guile exception depends on the error.
Guile procedures provided by gdb can throw the standard Guile exceptions like wrongtype-arg and out-of-range.
User interrupt (via C-c or by typing q at a pagination prompt) is translated to a Guile
signal exception with value SIGINT.
gdb Guile procedures can also throw these exceptions:
gdb:error
This exception is a catch-all for errors generated from within gdb.
gdb:invalid-object
This exception is thrown when accessing Guile objects that wrap underlying
gdb objects have become invalid. For example, a object
becomes invalid if the user deletes it from the command line. The object still
exists in Guile, but the object it represents is gone. Further operations on this
breakpoint will throw this exception.
gdb:memory-error
This exception is thrown when an operation tried to access invalid memory in
the inferior.
gdb:pp-type-error
This exception is thrown when a Guile pretty-printer passes a bad object to
gdb.
The following exception-related procedures are provided by the (gdb) module.
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Debugging with gdb
make-exception key args
[Scheme Procedure]
Return a object given by its key and args, which are the standard
Guile parameters of an exception. See the Guile documentation for more information
(see Section “Exceptions” in GNU Guile Reference Manual).
exception? object
[Scheme Procedure]
Return #t if object is a object. Otherwise return #f.
exception-key exception
[Scheme Procedure]
Return the args field of a object.
exception-args exception
[Scheme Procedure]
Return the args field of a object.
23.3.3.5 Values From Inferior In Guile
gdb provides values it obtains from the inferior program in an object of type .
gdb uses this object for its internal bookkeeping of the inferior’s values, and for fetching
values when necessary.
gdb does not memoize objects. make-value always returns a fresh object.
(gdb) guile (eq? (make-value 1) (make-value 1))
$1 = #f
(gdb) guile (equal? (make-value 1) (make-value 1))
$1 = #t
A that represents a function can be executed via inferior function call with
value-call. Any arguments provided to the call must match the function’s prototype, and
must be provided in the order specified by that prototype.
For example, some-val is a instance representing a function that takes
two integers as arguments. To execute this function, call it like so:
(define result (value-call some-val 10 20))
Any values returned from a function call are objects.
Note: Unlike Python scripting in gdb, inferior values that are simple scalars cannot be
used directly in Scheme expressions that are valid for the value’s data type. For example,
(+ (parse-and-eval "int_variable") 2) does not work. And inferior values that are
structures or instances of some class cannot be accessed using any special syntax, instead
value-field must be used.
The following value-related procedures are provided by the (gdb) module.
value? object
[Scheme Procedure]
Return #t if object is a object. Otherwise return #f.
make-value value [#:type type]
[Scheme Procedure]
Many Scheme values can be converted directly to a with this procedure.
If type is specified, the result is a value of this type, and if value can’t be represented
with this type an exception is thrown. Otherwise the type of the result is determined
from value as described below.
See Section 23.3.3.21 [Architectures In Guile], page 445, for a list of the builtin types
for an architecture.
Chapter 23: Extending gdb
409
Here’s how Scheme values are converted when type argument to make-value is not
specified:
Scheme boolean
A Scheme boolean is converted the boolean type for the current language.
Scheme integer
A Scheme integer is converted to the first of a C int, unsigned int, long,
unsigned long, long long or unsigned long long type for the current
architecture that can represent the value.
If the Scheme integer cannot be represented as a target integer an outof-range exception is thrown.
Scheme real
A Scheme real is converted to the C double type for the current architecture.
Scheme string
A Scheme string is converted to a string in the current target language using the current target encoding. Characters that cannot be represented in
the current target encoding are replaced with the corresponding escape sequence. This is Guile’s SCM_FAILED_CONVERSION_ESCAPE_SEQUENCE conversion strategy (see Section “Strings” in GNU Guile Reference Manual).
Passing type is not supported in this case, if it is provided a wrong-typearg exception is thrown.
If value is a object (see Section 23.3.3.20 [Lazy
Strings In Guile], page 444), then the lazy-string->value procedure
is called, and its result is used.
Passing type is not supported in this case, if it is provided a wrong-typearg exception is thrown.
Scheme bytevector
If value is a Scheme bytevector and type is provided, value must be the
same size, in bytes, of values of type type, and the result is essentially
created by using memcpy.
If value is a Scheme bytevector and type is not provided, the result is an
array of type uint8 of the same length.
value-optimized-out? value
[Scheme Procedure]
Return #t if the compiler optimized out value, thus it is not available for fetching
from the inferior. Otherwise return #f.
value-address value
[Scheme Procedure]
If value is addressable, returns a object representing the address. Otherwise, #f is returned.
value-type value
[Scheme Procedure]
Return the type of value as a object (see Section 23.3.3.7 [Types In
Guile], page 414).
410
Debugging with gdb
value-dynamic-type value
[Scheme Procedure]
Return the dynamic type of value. This uses C++ run-time type information (RTTI)
to determine the dynamic type of the value. If the value is of class type, it will return
the class in which the value is embedded, if any. If the value is of pointer or reference
to a class type, it will compute the dynamic type of the referenced object, and return
a pointer or reference to that type, respectively. In all other cases, it will return the
value’s static type.
Note that this feature will only work when debugging a C++ program that includes
RTTI for the object in question. Otherwise, it will just return the static type of the
value as in ptype foo. See Chapter 16 [Symbols], page 221.
value-cast value type
[Scheme Procedure]
Return a new instance of that is the result of casting value to the
type described by type, which must be a object. If the cast cannot be
performed for some reason, this method throws an exception.
value-dynamic-cast value type
[Scheme Procedure]
Like value-cast, but works as if the C++ dynamic_cast operator were used. Consult
a C++ reference for details.
value-reinterpret-cast value type
[Scheme Procedure]
Like value-cast, but works as if the C++ reinterpret_cast operator were used.
Consult a C++ reference for details.
value-dereference value
[Scheme Procedure]
For pointer data types, this method returns a new object whose contents
is the object pointed to by value. For example, if foo is a C pointer to an int, declared
in your C program as
int *foo;
then you can use the corresponding to access what foo points to like
this:
(define bar (value-dereference foo))
The result bar will be a object holding the value pointed to by foo.
A similar function value-referenced-value exists which also returns
objects corresonding to the values pointed to by pointer values (and additionally,
values referenced by reference values). However, the behavior of value-dereference
differs from value-referenced-value by the fact that the behavior of valuedereference is identical to applying the C unary operator * on a given value. For
example, consider a reference to a pointer ptrref, declared in your C++ program as
typedef int *intptr;
...
int val = 10;
intptr ptr = &val;
intptr &ptrref = ptr;
Though ptrref is a reference value, one can apply the method value-dereference
to the object corresponding to it and obtain a which is
identical to that corresponding to val. However, if you apply the method valuereferenced-value, the result would be a object identical to that corresponding to ptr.
Chapter 23: Extending gdb
411
(define scm-ptrref (parse-and-eval "ptrref"))
(define scm-val (value-dereference scm-ptrref))
(define scm-ptr (value-referenced-value scm-ptrref))
The object scm-val is identical to that corresponding to val, and scmptr is identical to that corresponding to ptr. In general, value-dereference can be
applied whenever the C unary operator * can be applied to the corresponding C value.
For those cases where applying both value-dereference and value-referencedvalue is allowed, the results obtained need not be identical (as we have seen in the
above example). The results are however identical when applied on
objects corresponding to pointers ( objects with type code TYPE_CODE_
PTR) in a C/C++ program.
value-referenced-value value
[Scheme Procedure]
For pointer or reference data types, this method returns a new object
corresponding to the value referenced by the pointer/reference value. For pointer data
types, value-dereference and value-referenced-value produce identical results.
The difference between these methods is that value-dereference cannot get the
values referenced by reference values. For example, consider a reference to an int,
declared in your C++ program as
int val = 10;
int &ref = val;
then applying value-dereference to the object corresponding to ref
will result in an error, while applying value-referenced-value will result in a
object identical to that corresponding to val.
(define scm-ref (parse-and-eval "ref"))
(define err-ref (value-dereference scm-ref))
;; error
(define scm-val (value-referenced-value scm-ref)) ;; ok
The object scm-val is identical to that corresponding to val.
value-field value field-name
[Scheme Procedure]
Return field field-name from object value.
value-subscript value index
[Scheme Procedure]
Return the value of array value at index index. The value argument must be a
subscriptable object.
value-call value arg-list
[Scheme Procedure]
Perform an inferior function call, taking value as a pointer to the function to call.
Each element of list arg-list must be a object or an object that can be
converted to a value. The result is the value returned by the function.
value->bool value
[Scheme Procedure]
Return the Scheme boolean representing value. The value must be
“integer like”. Pointers are ok.
[Scheme Procedure]
Return the Scheme integer representing value. The value must be “integer like”. Pointers are ok.
value->integer
412
Debugging with gdb
[Scheme Procedure]
Return the Scheme real number representing value. The value must be
a number.
value->real
[Scheme Procedure]
Return a Scheme bytevector with the raw contents of value. No transformation, endian or otherwise, is performed.
value->bytevector
value->string value [#:encoding encoding] [#:errors errors]
[#:length length]
[Scheme Procedure]
If value> represents a string, then this method converts the contents to a Guile string.
Otherwise, this method will throw an exception.
Values are interpreted as strings according to the rules of the current language. If
the optional length argument is given, the string will be converted to that length,
and will include any embedded zeroes that the string may contain. Otherwise, for
languages where the string is zero-terminated, the entire string will be converted.
For example, in C-like languages, a value is a string if it is a pointer to or an array of
characters or ints of type wchar_t, char16_t, or char32_t.
If the optional encoding argument is given, it must be a string naming the encoding
of the string in the , such as "ascii", "iso-8859-6" or "utf-8". It
accepts the same encodings as the corresponding argument to Guile’s scm_from_
stringn function, and the Guile codec machinery will be used to convert the string.
If encoding is not given, or if encoding is the empty string, then either the targetcharset (see Section 10.20 [Character Sets], page 152) will be used, or a languagespecific encoding will be used, if the current language is able to supply one.
The optional errors argument is one of #f, error or substitute. error and
substitute must be symbols. If errors is not specified, or if its value is #f, then
the default conversion strategy is used, which is set with the Scheme function
set-port-conversion-strategy!. If the value is ’error then an exception is
thrown if there is any conversion error. If the value is ’substitute then any
conversion error is replaced with question marks. See Section “Strings” in GNU
Guile Reference Manual.
If the optional length argument is given, the string will be fetched and converted
to the given length. The length must be a Scheme integer and not a
integer.
value->lazy-string value [#:encoding encoding] [#:length
length]
[Scheme Procedure]
If this represents a string, then this method converts value to a
integer.
value-lazy? value
[Scheme Procedure]
Return #t if value has not yet been fetched from the inferior. Otherwise return #f.
gdb does not fetch values until necessary, for efficiency. For example:
(define myval (parse-and-eval "somevar"))
The value of somevar is not fetched at this time. It will be fetched when the value is
needed, or when the fetch-lazy procedure is invoked.
make-lazy-value type address
[Scheme Procedure]
Return a that will be lazily fetched from the target. The object of
type whose value to fetch is specified by its type and its target memory
address, which is a Scheme integer.
value-fetch-lazy! value
[Scheme Procedure]
If value is a lazy value ((value-lazy? value) is #t), then the value is fetched from
the inferior. Any errors that occur in the process will produce a Guile exception.
If value is not a lazy value, this method has no effect.
The result of this function is unspecified.
value-print value
[Scheme Procedure]
Return the string representation (print form) of value.
23.3.3.6 Arithmetic In Guile
The (gdb) module provides several functions for performing arithmetic on
objects. The arithmetic is performed as if it were done by the target, and therefore has
target semantics which are not necessarily those of Scheme. For example operations work
with a fixed precision, not the arbitrary precision of Scheme.
Wherever a function takes an integer or pointer as an operand, gdb will convert appropriate Scheme values to perform the operation.
value-add a b
[Scheme Procedure]
value-sub a b
[Scheme Procedure]
value-mul a b
[Scheme Procedure]
value-div a b
[Scheme Procedure]
value-rem a b
[Scheme Procedure]
value-mod a b
[Scheme Procedure]
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Debugging with gdb
value-pow a b
[Scheme Procedure]
value-not a
[Scheme Procedure]
value-neg a
[Scheme Procedure]
value-pos a
[Scheme Procedure]
value-abs a
[Scheme Procedure]
value-lsh a b
[Scheme Procedure]
value-rsh a b
[Scheme Procedure]
value-min a b
[Scheme Procedure]
value-max a b
[Scheme Procedure]
value-lognot a
[Scheme Procedure]
value-logand a b
[Scheme Procedure]
value-logior a b
[Scheme Procedure]
value-logxor a b
[Scheme Procedure]
value=? a b
[Scheme Procedure]
value? a b
[Scheme Procedure]
value>=? a b
[Scheme Procedure]
Scheme does not provide a not-equal function, and thus Guile support in gdb does not
either.
23.3.3.7 Types In Guile
gdb represents types from the inferior in objects of type .
The following type-related procedures are provided by the (gdb) module.
type? object
[Scheme Procedure]
Return #t if object is an object of type . Otherwise return #f.
lookup-type name [#:block block]
[Scheme Procedure]
This function looks up a type by its name, which must be a string.
If block is given, it is an object of type , and name is looked up in that
scope. Otherwise, it is searched for globally.
Ordinarily, this function will return an instance of . If the named type
cannot be found, it will throw an exception.
type-code type
[Scheme Procedure]
Return the type code of type. The type code will be one of the TYPE_CODE_ constants
defined below.
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415
type-tag type
[Scheme Procedure]
Return the tag name of type. The tag name is the name after struct, union, or enum
in C and C++; not all languages have this concept. If this type has no tag name, then
#f is returned.
type-name type
[Scheme Procedure]
Return the name of type. If this type has no name, then #f is returned.
type-print-name type
[Scheme Procedure]
Return the print name of type. This returns something even for anonymous types.
For example, for an anonymous C struct "struct {...}" is returned.
type-sizeof type
[Scheme Procedure]
Return the size of this type, in target char units. Usually, a target’s char type will
be an 8-bit byte. However, on some unusual platforms, this type may have a different
size.
type-strip-typedefs type
[Scheme Procedure]
Return a new that represents the real type of type, after removing all
layers of typedefs.
type-array type n1 [n2]
[Scheme Procedure]
Return a new object which represents an array of this type. If one
argument is given, it is the inclusive upper bound of the array; in this case the lower
bound is zero. If two arguments are given, the first argument is the lower bound
of the array, and the second argument is the upper bound of the array. An array’s
length must not be negative, but the bounds can be.
type-vector type n1 [n2]
[Scheme Procedure]
Return a new object which represents a vector of this type. If one
argument is given, it is the inclusive upper bound of the vector; in this case the lower
bound is zero. If two arguments are given, the first argument is the lower bound of
the vector, and the second argument is the upper bound of the vector. A vector’s
length must not be negative, but the bounds can be.
The difference between an array and a vector is that arrays behave like in C: when
used in expressions they decay to a pointer to the first element whereas vectors are
treated as first class values.
type-pointer type
[Scheme Procedure]
Return a new object which represents a pointer to type.
type-range type
[Scheme Procedure]
Return a list of two elements: the low bound and high bound of type. If type does
not have a range, an exception is thrown.
type-reference type
[Scheme Procedure]
Return a new object which represents a reference to type.
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Debugging with gdb
type-target type
[Scheme Procedure]
Return a new object which represents the target type of type.
For a pointer type, the target type is the type of the pointed-to object. For an array
type (meaning C-like arrays), the target type is the type of the elements of the array.
For a function or method type, the target type is the type of the return value. For a
complex type, the target type is the type of the elements. For a typedef, the target
type is the aliased type.
If the type does not have a target, this method will throw an exception.
type-const type
[Scheme Procedure]
Return a new object which represents a const-qualified variant of type.
type-volatile type
[Scheme Procedure]
Return a new object which represents a volatile-qualified variant of
type.
type-unqualified type
[Scheme Procedure]
Return a new object which represents an unqualified variant of type.
That is, the result is neither const nor volatile.
type-num-fields
[Scheme Procedure]
Return the number of fields of type.
type-fields type
[Scheme Procedure]
Return the fields of type as a list. For structure and union types, fields has the
usual meaning. Range types have two fields, the minimum and maximum values.
Enum types have one field per enum constant. Function and method types have one
field per parameter. The base types of C++ classes are also represented as fields. If
the type has no fields, or does not fit into one of these categories, an empty list will
be returned. See [Fields of a type in Guile], page 418.
make-field-iterator type
[Scheme Procedure]
Return the fields of type as a object. See Section 23.3.3.25 [Iterators
In Guile], page 449.
type-field type field-name
[Scheme Procedure]
Return field named field-name in type. The result is an object of type .
See [Fields of a type in Guile], page 418. If the type does not have fields, or field-name
is not a field of type, an exception is thrown.
For example, if some-type is a instance holding a structure type, you
can access its foo field with:
(define bar (type-field some-type "foo"))
bar will be a object.
type-has-field? type name
[Scheme Procedure]
Return #t if type has field named name. Otherwise return #f.
Each type has a code, which indicates what category this type falls into. The available
type categories are represented by constants defined in the (gdb) module:
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TYPE_CODE_PTR
The type is a pointer.
TYPE_CODE_ARRAY
The type is an array.
TYPE_CODE_STRUCT
The type is a structure.
TYPE_CODE_UNION
The type is a union.
TYPE_CODE_ENUM
The type is an enum.
TYPE_CODE_FLAGS
A bit flags type, used for things such as status registers.
TYPE_CODE_FUNC
The type is a function.
TYPE_CODE_INT
The type is an integer type.
TYPE_CODE_FLT
A floating point type.
TYPE_CODE_VOID
The special type void.
TYPE_CODE_SET
A Pascal set type.
TYPE_CODE_RANGE
A range type, that is, an integer type with bounds.
TYPE_CODE_STRING
A string type. Note that this is only used for certain languages with languagedefined string types; C strings are not represented this way.
TYPE_CODE_BITSTRING
A string of bits. It is deprecated.
TYPE_CODE_ERROR
An unknown or erroneous type.
TYPE_CODE_METHOD
A method type, as found in C++.
TYPE_CODE_METHODPTR
A pointer-to-member-function.
TYPE_CODE_MEMBERPTR
A pointer-to-member.
TYPE_CODE_REF
A reference type.
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TYPE_CODE_CHAR
A character type.
TYPE_CODE_BOOL
A boolean type.
TYPE_CODE_COMPLEX
A complex float type.
TYPE_CODE_TYPEDEF
A typedef to some other type.
TYPE_CODE_NAMESPACE
A C++ namespace.
TYPE_CODE_DECFLOAT
A decimal floating point type.
TYPE_CODE_INTERNAL_FUNCTION
A function internal to gdb. This is the type used to represent convenience
functions (see Section 10.12 [Convenience Funs], page 141).
Further support for types is provided in the (gdb types) Guile module (see
Section 23.3.5.2 [Guile Types Module], page 452).
Each field is represented as an object of type .
The following field-related procedures are provided by the (gdb) module:
field? object
[Scheme Procedure]
Return #t if object is an object of type . Otherwise return #f.
field-name field
[Scheme Procedure]
Return the name of the field, or #f for anonymous fields.
field-type field
[Scheme Procedure]
Return the type of the field. This is usually an instance of , but it can
be #f in some situations.
field-enumval field
[Scheme Procedure]
Return the enum value represented by field.
field-bitpos field
[Scheme Procedure]
Return the bit position of field. This attribute is not available for
static fields (as in C++).
field-bitsize field
[Scheme Procedure]
If the field is packed, or is a bitfield, return the size of field in bits.
Otherwise, zero is returned; in which case the field’s size is given by its type.
field-artificial? field
[Scheme Procedure]
Return #t if the field is artificial, usually meaning that it was provided by the compiler
and not the user. Otherwise return #f.
field-base-class? field
[Scheme Procedure]
Return #t if the field represents a base class of a C++ structure. Otherwise return #f.
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419
23.3.3.8 Guile Pretty Printing API
An example output is provided (see Section 10.9 [Pretty Printing], page 135).
A pretty-printer is represented by an object of type . Pretty-printer
objects are created with make-pretty-printer.
The following pretty-printer-related procedures are provided by the (gdb) module:
make-pretty-printer name lookup-function
[Scheme Procedure]
Return a object named name.
lookup-function is a function of one parameter: the value to be printed. If the value
is handled by this pretty-printer, then lookup-function returns an object of type
to perform the actual pretty-printing. Otherwise lookupfunction returns #f.
pretty-printer? object
[Scheme Procedure]
Return #t if object is a object. Otherwise return #f.
pretty-printer-enabled? pretty-printer
[Scheme Procedure]
Return #t if pretty-printer is enabled. Otherwise return #f.
set-pretty-printer-enabled! pretty-printer flag
[Scheme Procedure]
Set the enabled flag of pretty-printer to flag. The value returned is unspecified.
[Scheme Procedure]
pretty-printers
Return the list of global pretty-printers.
set-pretty-printers! pretty-printers
[Scheme Procedure]
Set the list of global pretty-printers to pretty-printers. The value returned is unspecified.
make-pretty-printer-worker display-hint to-string children
[Scheme Procedure]
Return an object of type .
This function takes three parameters:
‘display-hint’
display-hint provides a hint to gdb or gdb front end via MI to change the
formatting of the value being printed. The value must be a string or #f
(meaning there is no hint). Several values for display-hint are predefined
by gdb:
‘array’
Indicate that the object being printed is “array-like”. The
CLI uses this to respect parameters such as set print
elements and set print array.
‘map’
Indicate that the object being printed is “map-like”, and that
the children of this value can be assumed to alternate between
keys and values.
‘string’
Indicate that the object being printed is “string-like”. If the
printer’s to-string function returns a Guile string of some
kind, then gdb will call its internal language-specific stringprinting function to format the string. For the CLI this means
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Debugging with gdb
adding quotation marks, possibly escaping some characters,
respecting set print elements, and the like.
‘to-string’
to-string is either a function of one parameter, the object, or #f.
When printing from the CLI, if the to-string method exists, then gdb
will prepend its result to the values returned by children. Exactly how
this formatting is done is dependent on the display hint, and may change
as more hints are added. Also, depending on the print settings (see
Section 10.8 [Print Settings], page 127), the CLI may print just the result
of to-string in a stack trace, omitting the result of children.
If this method returns a string, it is printed verbatim.
Otherwise, if this method returns an instance of , then gdb
prints this value. This may result in a call to another pretty-printer.
If instead the method returns a Guile value which is convertible to a
, then gdb performs the conversion and prints the resulting value. Again, this may result in a call to another pretty-printer.
Guile scalars (integers, floats, and booleans) and strings are convertible
to ; other types are not.
Finally, if this method returns #f then no further operations are peformed
in this method and nothing is printed.
If the result is not one of these types, an exception is raised.
to-string may also be #f in which case it is left to children to print the
value.
‘children’
children is either a function of one parameter, the object, or #f.
gdb will call this function on a pretty-printer to compute the children of
the pretty-printer’s value.
This function must return a object. Each item returned
by the iterator must be a tuple holding two elements. The first element
is the “name” of the child; the second element is the child’s value. The
value can be any Guile object which is convertible to a gdb value.
If children is #f, gdb will act as though the value has no children.
gdb provides a function which can be used to look up the default pretty-printer for a
:
default-visualizer value
[Scheme Procedure]
This function takes a object as an argument. If a pretty-printer for this
value exists, then it is returned. If no such printer exists, then this returns #f.
23.3.3.9 Selecting Guile Pretty-Printers
There are three sets of pretty-printers that gdb searches:
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421
• Per-objfile list of pretty-printers (see Section 23.3.3.14 [Objfiles In Guile], page 431).
• Per-progspace list of pretty-printers (see Section 23.3.3.13 [Progspaces In Guile],
page 430).
• The global list of pretty-printers (see Section 23.3.3.8 [Guile Pretty Printing API],
page 419). These printers are available when debugging any inferior.
Pretty-printer lookup is done by passing the value to be printed to the lookup function
of each enabled object in turn. Lookup stops when a lookup function returns a non-#f
value or when the list is exhausted. Lookup functions must return either a object or #f. Otherwise an exception is thrown.
gdb first checks the result of objfile-pretty-printers of each in the
current program space and iteratively calls each enabled lookup function in the list for that
until a non-#f object is returned. If no pretty-printer is found in the objfile
lists, gdb then searches the result of progspace-pretty-printers of the current program
space, calling each enabled function until a non-#f object is returned. After these lists have
been exhausted, it tries the global pretty-printers list, obtained with pretty-printers,
again calling each enabled function until a non-#f object is returned.
The order in which the objfiles are searched is not specified. For a given list, functions
are always invoked from the head of the list, and iterated over sequentially until the end of
the list, or a object is returned.
For various reasons a pretty-printer may not work. For example, the underlying data
structure may have changed and the pretty-printer is out of date.
The consequences of a broken pretty-printer are severe enough that gdb provides support
for enabling and disabling individual printers. For example, if print frame-arguments is
on, a backtrace can become highly illegible if any argument is printed with a broken printer.
Pretty-printers are enabled and disabled from Scheme by calling set-pretty-printerenabled!. See Section 23.3.3.8 [Guile Pretty Printing API], page 419.
23.3.3.10 Writing a Guile Pretty-Printer
A pretty-printer consists of two basic parts: a lookup function to determine if the type is
supported, and the printer itself.
Here is an example showing how a std::string printer might be written. See
Section 23.3.3.8 [Guile Pretty Printing API], page 419, for details.
(define (make-my-string-printer value)
"Print a my::string string"
(make-pretty-printer-worker
"string"
(lambda (printer)
(value-field value "_data"))
#f))
And here is an example showing how a lookup function for the printer example above
might be written.
(define (str-lookup-function pretty-printer value)
(let ((tag (type-tag (value-type value))))
(and tag
(string-prefix? "std::string<" tag)
(make-my-string-printer value))))
Then to register this printer in the global printer list:
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(append-pretty-printer!
(make-pretty-printer "my-string" str-lookup-function))
The example lookup function extracts the value’s type, and attempts to match it to a
type that it can pretty-print. If it is a type the printer can pretty-print, it will return a