Bison Manual

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Bison
The Yacc-compatible Parser Generator
28 December 2002, Bison Version 1.875
by Charles Donnelly and Richard Stallman
This manual is for GNU Bison (version 1.875, 28 December 2002), the GNU parser generator.
Copyright c
1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, 1999, 2000, 2001, 2002 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 1.1 or any later version
published by the Free Software Foundation; with no Invariant Sections, with the
Front-Cover texts being “A GNU Manual,” and with the Back-Cover Texts as in
(a) below. A copy of the license is included in the section entitled “GNU Free
Documentation License.”
(a) The FSF’s Back-Cover Text is: “You have freedom to copy and modify this GNU
Manual, like GNU software. Copies published by the Free Software Foundation raise
funds for GNU development.”
Published by the Free Software Foundation
59 Temple Place, Suite 330
Boston, MA 02111-1307 USA
Printed copies are available from the Free Software Foundation.
ISBN 1-882114-44-2
Cover art by Etienne Suvasa.
i
Short Contents
Introduction .................................................. 1
Conditions for Using Bison ........................................ 3
GNU GENERAL PUBLIC LICENSE ................................. 5
1 The Concepts of Bison ....................................... 11
2 Examples ................................................ 19
3 Bison Grammar Files ........................................ 35
4 Parser C-Language Interface ................................... 53
5 The Bison Parser Algorithm ................................... 59
6 Error Recovery ............................................ 69
7 Handling Context Dependencies ................................. 71
8 Debugging Your Parser ....................................... 75
9 Invoking Bison ............................................. 83
10 Frequently Asked Questions .................................... 87
A Bison Symbols ............................................. 89
B Glossary ................................................. 95
C Copying This Manual ........................................ 99
Index ..................................................... 107
ii Bison 1.875
iii
Table of Contents
Introduction .............................................. 1
Conditions for Using Bison ................................. 3
GNU GENERAL PUBLIC LICENSE....................... 5
Preamble .......................................................................... 5
TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
.............................................................................. 5
Appendix: How to Apply These Terms to Your New Programs......................... 9
1 The Concepts of Bison ................................ 11
1.1 Languages and Context-Free Grammars ........................................ 11
1.2 From Formal Rules to Bison Input ............................................. 12
1.3 Semantic Values .............................................................. 13
1.4 Semantic Actions ............................................................. 13
1.5 Writing GLR Parsers.......................................................... 13
1.6 Locations .................................................................... 16
1.7 Bison Output: the Parser File ................................................. 16
1.8 Stages in Using Bison ......................................................... 17
1.9 The Overall Layout of a Bison Grammar ....................................... 17
2 Examples ............................................. 19
2.1 Reverse Polish Notation Calculator ............................................ 19
2.1.1 Declarations for rpcalc .................................................. 19
2.1.2 Grammar Rules for rpcalc ............................................... 20
2.1.2.1 Explanation of input ................................................ 20
2.1.2.2 Explanation of line ................................................. 21
2.1.2.3 Explanation of expr ................................................. 21
2.1.3 The rpcalc Lexical Analyzer ............................................. 22
2.1.4 The Controlling Function ................................................. 23
2.1.5 The Error Reporting Routine ............................................. 23
2.1.6 Running Bison to Make the Parser ........................................ 23
2.1.7 Compiling the Parser File ................................................ 24
2.2 Infix Notation Calculator: calc ............................................... 24
2.3 Simple Error Recovery ........................................................ 25
2.4 Location Tracking Calculator: ltcalc ......................................... 26
2.4.1 Declarations for ltcalc .................................................. 26
2.4.2 Grammar Rules for ltcalc ............................................... 26
2.4.3 The ltcalc Lexical Analyzer.............................................. 27
2.5 Multi-Function Calculator: mfcalc ............................................ 29
2.5.1 Declarations for mfcalc .................................................. 29
2.5.2 Grammar Rules for mfcalc ............................................... 30
2.5.3 The mfcalc Symbol Table ................................................ 30
2.6 Exercises .................................................................... 33
iv Bison 1.875
3 Bison Grammar Files .................................. 35
3.1 Outline of a Bison Grammar .................................................. 35
3.1.1 The prologue ............................................................ 35
3.1.2 The Bison Declarations Section ........................................... 35
3.1.3 The Grammar Rules Section .............................................. 36
3.1.4 The epilogue ............................................................ 36
3.2 Symbols, Terminal and Nonterminal ........................................... 36
3.3 Syntax of Grammar Rules..................................................... 38
3.4 Recursive Rules .............................................................. 38
3.5 Defining Language Semantics.................................................. 39
3.5.1 Data Types of Semantic Values ........................................... 39
3.5.2 More Than One Value Type .............................................. 39
3.5.3 Actions ................................................................. 40
3.5.4 Data Types of Values in Actions .......................................... 41
3.5.5 Actions in Mid-Rule ..................................................... 41
3.6 Tracking Locations ........................................................... 43
3.6.1 Data Type of Locations .................................................. 43
3.6.2 Actions and Locations.................................................... 43
3.6.3 Default Action for Locations .............................................. 44
3.7 Bison Declarations ........................................................... 45
3.7.1 Token Type Names ...................................................... 45
3.7.2 Operator Precedence ..................................................... 46
3.7.3 The Collection of Value Types ............................................ 46
3.7.4 Nonterminal Symbols .................................................... 47
3.7.5 Freeing Discarded Symbols ............................................... 47
3.7.6 Suppressing Conflict Warnings ............................................ 48
3.7.7 The Start-Symbol........................................................ 48
3.7.8 A Pure (Reentrant) Parser................................................ 48
3.7.9 Bison Declaration Summary .............................................. 49
3.8 Multiple Parsers in the Same Program ......................................... 51
4 Parser C-Language Interface ........................... 53
4.1 The Parser Function yyparse ................................................. 53
4.2 The Lexical Analyzer Function yylex .......................................... 53
4.2.1 Calling Convention for yylex ............................................. 53
4.2.2 Semantic Values of Tokens................................................ 54
4.2.3 Textual Positions of Tokens ............................................... 55
4.2.4 Calling Conventions for Pure Parsers ...................................... 55
4.3 The Error Reporting Function yyerror ........................................ 55
4.4 Special Features for Use in Actions ............................................ 56
5 The Bison Parser Algorithm ........................... 59
5.1 Look-Ahead Tokens .......................................................... 59
5.2 Shift/Reduce Conflicts ........................................................ 60
5.3 Operator Precedence ......................................................... 61
5.3.1 When Precedence is Needed .............................................. 61
5.3.2 Specifying Operator Precedence ........................................... 61
5.3.3 Precedence Examples .................................................... 62
5.3.4 How Precedence Works ................................................... 62
5.4 Context-Dependent Precedence ................................................ 62
5.5 Parser States................................................................. 63
5.6 Reduce/Reduce Conflicts...................................................... 63
5.7 Mysterious Reduce/Reduce Conflicts ........................................... 65
v
5.8 Generalized LR (GLR) Parsing................................................. 66
5.9 Stack Overflow, and How to Avoid It .......................................... 67
6 Error Recovery ....................................... 69
7 Handling Context Dependencies........................ 71
7.1 Semantic Info in Token Types ................................................. 71
7.2 Lexical Tie-ins ............................................................... 72
7.3 Lexical Tie-ins and Error Recovery ............................................ 72
8 Debugging Your Parser ................................ 75
8.1 Understanding Your Parser.................................................... 75
8.2 Tracing Your Parser .......................................................... 80
9 Invoking Bison ........................................ 83
9.1 Bison Options................................................................ 83
9.2 Option Cross Key ............................................................ 85
9.3 Yacc Library ................................................................. 85
10 Frequently Asked Questions .......................... 87
10.1 Parser Stack Overflow ....................................................... 87
Appendix A Bison Symbols ............................. 89
Appendix B Glossary ................................... 95
Appendix C Copying This Manual ....................... 99
C.1 GNU Free Documentation License............................................. 99
C.1.1 ADDENDUM: How to use this License for your documents ................ 105
Index ................................................... 107
vi Bison 1.875
1
Introduction
Bison is a general-purpose parser generator that converts a grammar description for an LALR(1)
context-free grammar into a C program to parse that grammar. Once you are proficient with
Bison, you may use it to develop a wide range of language parsers, from those used in simple
desk calculators to complex programming languages.
Bison is upward compatible with Yacc: all properly-written Yacc grammars ought to work
with Bison with no change. Anyone familiar with Yacc should be able to use Bison with little
trouble. You need to be fluent in C programming in order to use Bison or to understand this
manual.
We begin with tutorial chapters that explain the basic concepts of using Bison and show
three explained examples, each building on the last. If you don’t know Bison or Yacc, start by
reading these chapters. Reference chapters follow which describe specific aspects of Bison in
detail.
Bison was written primarily by Robert Corbett; Richard Stallman made it Yacc-compatible.
Wilfred Hansen of Carnegie Mellon University added multi-character string literals and other
features.
This edition corresponds to version 1.875 of Bison.
2 Bison 1.875
3
Conditions for Using Bison
As of Bison version 1.24, we have changed the distribution terms for yyparse to permit using
Bison’s output in nonfree programs when Bison is generating C code for LALR(1) parsers.
Formerly, these parsers could be used only in programs that were free software.
The other GNU programming tools, such as the GNU C compiler, have never had such a
requirement. They could always be used for nonfree software. The reason Bison was different
was not due to a special policy decision; it resulted from applying the usual General Public
License to all of the Bison source code.
The output of the Bison utility—the Bison parser file—contains a verbatim copy of a sizable
piece of Bison, which is the code for the yyparse function. (The actions from your grammar
are inserted into this function at one point, but the rest of the function is not changed.) When
we applied the GPL terms to the code for yyparse, the effect was to restrict the use of Bison
output to free software.
We didn’t change the terms because of sympathy for people who want to make software
proprietary. Software should be free. But we concluded that limiting Bison’s use to free software
was doing little to encourage people to make other software free. So we decided to make the
practical conditions for using Bison match the practical conditions for using the other GNU
tools.
This exception applies only when Bison is generating C code for a LALR(1) parser; otherwise,
the GPL terms operate as usual. You can tell whether the exception applies to your ‘.c’ output
file by inspecting it to see whether it says “As a special exception, when this file is copied by
Bison into a Bison output file, you may use that output file without restriction.”
4 Bison 1.875
5
GNU GENERAL PUBLIC LICENSE
Version 2, June 1991
Copyright c
1989, 1991 Free Software Foundation, Inc.
59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
Preamble
The licenses for most software are designed to take away your freedom to share and change it.
By contrast, the GNU General Public License is intended to guarantee your freedom to share
and change free software—to make sure the software is free for all its users. This General Public
License applies to most of the Free Software Foundation’s software and to any other program
whose authors commit to using it. (Some other Free Software Foundation software is covered
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When we speak of free software, we are referring to freedom, not price. Our General Public
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To protect your rights, we need to make restrictions that forbid anyone to deny you these
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We protect your rights with two steps: (1) copyright the software, and (2) offer you this
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The precise terms and conditions for copying, distribution and modification follow.
TERMS AND CONDITIONS FOR COPYING,
DISTRIBUTION AND MODIFICATION
0. This License applies to any program or other work which contains a notice placed by
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that is to say, a work containing the Program or a portion of it, either verbatim or with
modifications and/or translated into another language. (Hereinafter, translation is included
without limitation in the term “modification”.) Each licensee is addressed as “you”.
6 Bison 1.875
Activities other than copying, distribution and modification are not covered by this License;
they are outside its scope. The act of running the Program is not restricted, and the output
from the Program is covered only if its contents constitute a work based on the Program
(independent of having been made by running the Program). Whether that is true depends
on what the Program does.
1. You may copy and distribute verbatim copies of the Program’s source code as you receive
it, in any medium, provided that you conspicuously and appropriately publish on each copy
an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that
refer to this License and to the absence of any warranty; and give any other recipients of
the Program a copy of this License along with the Program.
You may charge a fee for the physical act of transferring a copy, and you may at your option
offer warranty protection in exchange for a fee.
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b. You must cause any work that you distribute or publish, that in whole or in part
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c. If the modified program normally reads commands interactively when run, you must
cause it, when started running for such interactive use in the most ordinary way, to
print or display an announcement including an appropriate copyright notice and a
notice that there is no warranty (or else, saying that you provide a warranty) and that
users may redistribute the program under these conditions, and telling the user how to
view a copy of this License. (Exception: if the Program itself is interactive but does
not normally print such an announcement, your work based on the Program is not
required to print an announcement.)
These requirements apply to the modified work as a whole. If identifiable sections of that
work are not derived from the Program, and can be reasonably considered independent
and separate works in themselves, then this License, and its terms, do not apply to those
sections when you distribute them as separate works. But when you distribute the same
sections as part of a whole which is a work based on the Program, the distribution of the
whole must be on the terms of this License, whose permissions for other licensees extend to
the entire whole, and thus to each and every part regardless of who wrote it.
Thus, it is not the intent of this section to claim rights or contest your rights to work
written entirely by you; rather, the intent is to exercise the right to control the distribution
of derivative or collective works based on the Program.
In addition, mere aggregation of another work not based on the Program with the Program
(or with a work based on the Program) on a volume of a storage or distribution medium
does not bring the other work under the scope of this License.
3. You may copy and distribute the Program (or a work based on it, under Section 2) in object
code or executable form under the terms of Sections 1 and 2 above provided that you also
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a. Accompany it with the complete corresponding machine-readable source code, which
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7
a complete machine-readable copy of the corresponding source code, to be distributed
under the terms of Sections 1 and 2 above on a medium customarily used for software
interchange; or,
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If any portion of this section is held invalid or unenforceable under any particular circum-
stance, the balance of the section is intended to apply and the section as a whole is intended
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It is not the purpose of this section to induce you to infringe any patents or other property
right claims or to contest validity of any such claims; this section has the sole purpose of
protecting the integrity of the free software distribution system, which is implemented by
8 Bison 1.875
public license practices. Many people have made generous contributions to the wide range
of software distributed through that system in reliance on consistent application of that
system; it is up to the author/donor to decide if he or she is willing to distribute software
through any other system and a licensee cannot impose that choice.
This section is intended to make thoroughly clear what is believed to be a consequence of
the rest of this License.
8. If the distribution and/or use of the Program is restricted in certain countries either by
patents or by copyrighted interfaces, the original copyright holder who places the Program
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such case, this License incorporates the limitation as if written in the body of this License.
9. The Free Software Foundation may publish revised and/or new versions of the General
Public License from time to time. Such new versions will be similar in spirit to the present
version, but may differ in detail to address new problems or concerns.
Each version is given a distinguishing version number. If the Program specifies a version
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following the terms and conditions either of that version or of any later version published
by the Free Software Foundation. If the Program does not specify a version number of this
License, you may choose any version ever published by the Free Software Foundation.
10. If you wish to incorporate parts of the Program into other free programs whose distribution
conditions are different, write to the author to ask for permission. For software which
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we sometimes make exceptions for this. Our decision will be guided by the two goals of
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NO WARRANTY
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BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE
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12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY
MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE
LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCI-
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ABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
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YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH
ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN
ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
END OF TERMS AND CONDITIONS
9
Appendix: How to Apply These Terms to Your New Programs
If you develop a new program, and you want it to be of the greatest possible use to the public,
the best way to achieve this is to make it free software which everyone can redistribute and
change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start
of each source file to most effectively convey the exclusion of warranty; and each file should have
at least the “copyright” line and a pointer to where the full notice is found.
one line to give the program’s name and a brief idea of what it does.
Copyright (C) yyyy name of author
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
Also add information on how to contact you by electronic and paper mail.
If the program is interactive, make it output a short notice like this when it starts in an
interactive mode:
Gnomovision version 69, Copyright (C) 19yy name of author
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type ‘show w’.
This is free software, and you are welcome to redistribute it
under certain conditions; type ‘show c’ for details.
The hypothetical commands ‘show w’ and ‘show c’ should show the appropriate parts of the
General Public License. Of course, the commands you use may be called something other than
‘show w’ and ‘show c’; they could even be mouse-clicks or menu items—whatever suits your
program.
You should also get your employer (if you work as a programmer) or your school, if any, to
sign a “copyright disclaimer” for the program, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program
‘Gnomovision’ (which makes passes at compilers) written by James Hacker.
signature of Ty Coon, 1 April 1989
Ty Coon, President of Vice
This General Public License does not permit incorporating your program into proprietary
programs. If your program is a subroutine library, you may consider it more useful to permit
linking proprietary applications with the library. If this is what you want to do, use the GNU
Library General Public License instead of this License.
10 Bison 1.875
Chapter 1: The Concepts of Bison 11
1 The Concepts of Bison
This chapter introduces many of the basic concepts without which the details of Bison will not
make sense. If you do not already know how to use Bison or Yacc, we suggest you start by
reading this chapter carefully.
1.1 Languages and Context-Free Grammars
In order for Bison to parse a language, it must be described by a context-free grammar. This
means that you specify one or more syntactic groupings and give rules for constructing them
from their parts. For example, in the C language, one kind of grouping is called an ‘expression’.
One rule for making an expression might be, “An expression can be made of a minus sign and
another expression”. Another would be, “An expression can be an integer”. As you can see,
rules are often recursive, but there must be at least one rule which leads out of the recursion.
The most common formal system for presenting such rules for humans to read is Backus-Naur
Form or “BNF”, which was developed in order to specify the language Algol 60. Any grammar
expressed in BNF is a context-free grammar. The input to Bison is essentially machine-readable
BNF.
There are various important subclasses of context-free grammar. Although it can handle
almost all context-free grammars, Bison is optimized for what are called LALR(1) grammars.
In brief, in these grammars, it must be possible to tell how to parse any portion of an input
string with just a single token of look-ahead. Strictly speaking, that is a description of an LR(1)
grammar, and LALR(1) involves additional restrictions that are hard to explain simply; but it
is rare in actual practice to find an LR(1) grammar that fails to be LALR(1). See Section 5.7
[Mysterious Reduce/Reduce Conflicts], page 65, for more information on this.
Parsers for LALR(1) grammars are deterministic, meaning roughly that the next grammar
rule to apply at any point in the input is uniquely determined by the preceding input and a
fixed, finite portion (called a look-ahead) of the remaining input. A context-free grammar can
be ambiguous, meaning that there are multiple ways to apply the grammar rules to get the
some inputs. Even unambiguous grammars can be non-deterministic, meaning that no fixed
look-ahead always suffices to determine the next grammar rule to apply. With the proper
declarations, Bison is also able to parse these more general context-free grammars, using a
technique known as GLR parsing (for Generalized LR). Bison’s GLR parsers are able to handle
any context-free grammar for which the number of possible parses of any given string is finite.
In the formal grammatical rules for a language, each kind of syntactic unit or grouping
is named by a symbol. Those which are built by grouping smaller constructs according to
grammatical rules are called nonterminal symbols; those which can’t be subdivided are called
terminal symbols or token types. We call a piece of input corresponding to a single terminal
symbol a token, and a piece corresponding to a single nonterminal symbol a grouping.
We can use the C language as an example of what symbols, terminal and nonterminal, mean.
The tokens of C are identifiers, constants (numeric and string), and the various keywords,
arithmetic operators and punctuation marks. So the terminal symbols of a grammar for C
include ‘identifier’, ‘number’, ‘string’, plus one symbol for each keyword, operator or punctuation
mark: ‘if’, ‘return’, ‘const’, ‘static’, ‘int’, ‘char’, ‘plus-sign’, ‘open-brace’, ‘close-brace’, ‘comma’
and many more. (These tokens can be subdivided into characters, but that is a matter of
lexicography, not grammar.)
Here is a simple C function subdivided into tokens:
int /* keyword ‘int’ */
square (int x) /* identifier, open-paren, identifier, identifier, close-paren */
{ /* open-brace */
return x * x; /* keyword ‘return’, identifier, asterisk, identifier, semicolon */
12 Bison 1.875
} /* close-brace */
The syntactic groupings of C include the expression, the statement, the declaration, and
the function definition. These are represented in the grammar of C by nonterminal symbols
‘expression’, ‘statement’, ‘declaration’ and ‘function definition’. The full grammar uses dozens
of additional language constructs, each with its own nonterminal symbol, in order to express the
meanings of these four. The example above is a function definition; it contains one declaration,
and one statement. In the statement, each ‘x’ is an expression and so is ‘x * x’.
Each nonterminal symbol must have grammatical rules showing how it is made out of simpler
constructs. For example, one kind of C statement is the return statement; this would be
described with a grammar rule which reads informally as follows:
A ‘statement’ can be made of a ‘return’ keyword, an ‘expression’ and a ‘semicolon’.
There would be many other rules for ‘statement’, one for each kind of statement in C.
One nonterminal symbol must be distinguished as the special one which defines a complete
utterance in the language. It is called the start symbol. In a compiler, this means a com-
plete input program. In the C language, the nonterminal symbol ‘sequence of definitions and
declarations’ plays this role.
For example, ‘1 + 2’ is a valid C expression—a valid part of a C program—but it is not
valid as an entire C program. In the context-free grammar of C, this follows from the fact that
‘expression’ is not the start symbol.
The Bison parser reads a sequence of tokens as its input, and groups the tokens using the
grammar rules. If the input is valid, the end result is that the entire token sequence reduces to
a single grouping whose symbol is the grammar’s start symbol. If we use a grammar for C, the
entire input must be a ‘sequence of definitions and declarations’. If not, the parser reports a
syntax error.
1.2 From Formal Rules to Bison Input
A formal grammar is a mathematical construct. To define the language for Bison, you must
write a file expressing the grammar in Bison syntax: a Bison grammar file. See Chapter 3 [Bison
Grammar Files], page 35.
A nonterminal symbol in the formal grammar is represented in Bison input as an identi-
fier, like an identifier in C. By convention, it should be in lower case, such as expr,stmt or
declaration.
The Bison representation for a terminal symbol is also called a token type. Token types as
well can be represented as C-like identifiers. By convention, these identifiers should be upper
case to distinguish them from nonterminals: for example, INTEGER,IDENTIFIER,IF or RETURN.
A terminal symbol that stands for a particular keyword in the language should be named after
that keyword converted to upper case. The terminal symbol error is reserved for error recovery.
See Section 3.2 [Symbols], page 36.
A terminal symbol can also be represented as a character literal, just like a C character
constant. You should do this whenever a token is just a single character (parenthesis, plus-sign,
etc.): use that same character in a literal as the terminal symbol for that token.
A third way to represent a terminal symbol is with a C string constant containing several
characters. See Section 3.2 [Symbols], page 36, for more information.
The grammar rules also have an expression in Bison syntax. For example, here is the Bison
rule for a C return statement. The semicolon in quotes is a literal character token, representing
part of the C syntax for the statement; the naked semicolon, and the colon, are Bison punctuation
used in every rule.
stmt: RETURN expr ’;’
Chapter 1: The Concepts of Bison 13
;
See Section 3.3 [Syntax of Grammar Rules], page 38.
1.3 Semantic Values
A formal grammar selects tokens only by their classifications: for example, if a rule mentions the
terminal symbol ‘integer constant’, it means that any integer constant is grammatically valid in
that position. The precise value of the constant is irrelevant to how to parse the input: if ‘x+4’
is grammatical then ‘x+1’ or ‘x+3989’ is equally grammatical.
But the precise value is very important for what the input means once it is parsed. A compiler
is useless if it fails to distinguish between 4, 1 and 3989 as constants in the program! Therefore,
each token in a Bison grammar has both a token type and a semantic value. See Section 3.5
[Defining Language Semantics], page 39, for details.
The token type is a terminal symbol defined in the grammar, such as INTEGER,IDENTIFIER
or ’,’. It tells everything you need to know to decide where the token may validly appear and
how to group it with other tokens. The grammar rules know nothing about tokens except their
types.
The semantic value has all the rest of the information about the meaning of the token, such
as the value of an integer, or the name of an identifier. (A token such as ’,’ which is just
punctuation doesn’t need to have any semantic value.)
For example, an input token might be classified as token type INTEGER and have the semantic
value 4. Another input token might have the same token type INTEGER but value 3989. When
a grammar rule says that INTEGER is allowed, either of these tokens is acceptable because each
is an INTEGER. When the parser accepts the token, it keeps track of the token’s semantic value.
Each grouping can also have a semantic value as well as its nonterminal symbol. For example,
in a calculator, an expression typically has a semantic value that is a number. In a compiler for
a programming language, an expression typically has a semantic value that is a tree structure
describing the meaning of the expression.
1.4 Semantic Actions
In order to be useful, a program must do more than parse input; it must also produce some
output based on the input. In a Bison grammar, a grammar rule can have an action made up
of C statements. Each time the parser recognizes a match for that rule, the action is executed.
See Section 3.5.3 [Actions], page 40.
Most of the time, the purpose of an action is to compute the semantic value of the whole
construct from the semantic values of its parts. For example, suppose we have a rule which says
an expression can be the sum of two expressions. When the parser recognizes such a sum, each
of the subexpressions has a semantic value which describes how it was built up. The action for
this rule should create a similar sort of value for the newly recognized larger expression.
For example, here is a rule that says an expression can be the sum of two subexpressions:
expr: expr ’+’ expr { $$ = $1 + $3; }
;
The action says how to produce the semantic value of the sum expression from the values of the
two subexpressions.
1.5 Writing GLR Parsers
In some grammars, there will be cases where Bison’s standard LALR(1) parsing algorithm cannot
decide whether to apply a certain grammar rule at a given point. That is, it may not be able to
decide (on the basis of the input read so far) which of two possible reductions (applications of
14 Bison 1.875
a grammar rule) applies, or whether to apply a reduction or read more of the input and apply
a reduction later in the input. These are known respectively as reduce/reduce conflicts (see
Section 5.6 [Reduce/Reduce], page 63), and shift/reduce conflicts (see Section 5.2 [Shift/Reduce],
page 60).
To use a grammar that is not easily modified to be LALR(1), a more general parsing algorithm
is sometimes necessary. If you include %glr-parser among the Bison declarations in your file
(see Section 3.1 [Grammar Outline], page 35), the result will be a Generalized LR (GLR) parser.
These parsers handle Bison grammars that contain no unresolved conflicts (i.e., after applying
precedence declarations) identically to LALR(1) parsers. However, when faced with unresolved
shift/reduce and reduce/reduce conflicts, GLR parsers use the simple expedient of doing both,
effectively cloning the parser to follow both possibilities. Each of the resulting parsers can again
split, so that at any given time, there can be any number of possible parses being explored. The
parsers proceed in lockstep; that is, all of them consume (shift) a given input symbol before any
of them proceed to the next. Each of the cloned parsers eventually meets one of two possible
fates: either it runs into a parsing error, in which case it simply vanishes, or it merges with
another parser, because the two of them have reduced the input to an identical set of symbols.
During the time that there are multiple parsers, semantic actions are recorded, but not
performed. When a parser disappears, its recorded semantic actions disappear as well, and
are never performed. When a reduction makes two parsers identical, causing them to merge,
Bison records both sets of semantic actions. Whenever the last two parsers merge, reverting
to the single-parser case, Bison resolves all the outstanding actions either by precedences given
to the grammar rules involved, or by performing both actions, and then calling a designated
user-defined function on the resulting values to produce an arbitrary merged result.
Let’s consider an example, vastly simplified from a C++ grammar.
%{
#include <stdio.h>
#define YYSTYPE char const *
int yylex (void);
void yyerror (char const *);
%}
%token TYPENAME ID
%right ’=’
%left ’+’
%glr-parser
%%
prog :
| prog stmt { printf ("\n"); }
;
stmt : expr ’;’ %dprec 1
| decl %dprec 2
;
expr : ID { printf ("%s ", $$); }
| TYPENAME ’(’ expr ’)’
{ printf ("%s <cast> ", $1); }
Chapter 1: The Concepts of Bison 15
| expr ’+’ expr { printf ("+ "); }
| expr ’=’ expr { printf ("= "); }
;
decl : TYPENAME declarator ’;’
{ printf ("%s <declare> ", $1); }
| TYPENAME declarator ’=’ expr ’;’
{ printf ("%s <init-declare> ", $1); }
;
declarator : ID { printf ("\"%s\" ", $1); }
| ’(’ declarator ’)’
;
This models a problematic part of the C++ grammar—the ambiguity between certain declara-
tions and statements. For example,
T (x) = y+z;
parses as either an expr or a stmt (assuming that ‘T’ is recognized as a TYPENAME and ‘x’ as an
ID). Bison detects this as a reduce/reduce conflict between the rules expr : ID and declarator
: ID, which it cannot resolve at the time it encounters xin the example above. The two %dprec
declarations, however, give precedence to interpreting the example as a decl, which implies that
xis a declarator. The parser therefore prints
"x" y z + T <init-declare>
Consider a different input string for this parser:
T (x) + y;
Here, there is no ambiguity (this cannot be parsed as a declaration). However, at the time the
Bison parser encounters x, it does not have enough information to resolve the reduce/reduce
conflict (again, between xas an expr or a declarator). In this case, no precedence declaration
is used. Instead, the parser splits into two, one assuming that xis an expr, and the other
assuming xis a declarator. The second of these parsers then vanishes when it sees +, and the
parser prints
x T <cast> y +
Suppose that instead of resolving the ambiguity, you wanted to see all the possibilities. For
this purpose, we must merge the semantic actions of the two possible parsers, rather than
choosing one over the other. To do so, you could change the declaration of stmt as follows:
stmt : expr ’;’ %merge <stmtMerge>
| decl %merge <stmtMerge>
;
and define the stmtMerge function as:
static YYSTYPE
stmtMerge (YYSTYPE x0, YYSTYPE x1)
{
printf ("<OR> ");
return "";
}
with an accompanying forward declaration in the C declarations at the beginning of the file:
%{
#define YYSTYPE char const *
static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
16 Bison 1.875
%}
With these declarations, the resulting parser will parse the first example as both an expr and a
decl, and print
"x" y z + T <init-declare> x T <cast> y z + = <OR>
The GLR parsers require a compiler for ISO C89 or later. In addition, they use the inline
keyword, which is not C89, but is C99 and is a common extension in pre-C99 compilers. It is
up to the user of these parsers to handle portability issues. For instance, if using Autoconf and
the Autoconf macro AC_C_INLINE, a mere
%{
#include <config.h>
%}
will suffice. Otherwise, we suggest
%{
#if __STDC_VERSION__ < 199901 && ! defined __GNUC__ && ! defined inline
#define inline
#endif
%}
1.6 Locations
Many applications, like interpreters or compilers, have to produce verbose and useful error
messages. To achieve this, one must be able to keep track of the textual position, or location,
of each syntactic construct. Bison provides a mechanism for handling these locations.
Each token has a semantic value. In a similar fashion, each token has an associated location,
but the type of locations is the same for all tokens and groupings. Moreover, the output parser is
equipped with a default data structure for storing locations (see Section 3.6 [Locations], page 43,
for more details).
Like semantic values, locations can be reached in actions using a dedicated set of constructs.
In the example above, the location of the whole grouping is @$, while the locations of the
subexpressions are @1 and @3.
When a rule is matched, a default action is used to compute the semantic value of its left
hand side (see Section 3.5.3 [Actions], page 40). In the same way, another default action is used
for locations. However, the action for locations is general enough for most cases, meaning there
is usually no need to describe for each rule how @$ should be formed. When building a new
location for a given grouping, the default behavior of the output parser is to take the beginning
of the first symbol, and the end of the last symbol.
1.7 Bison Output: the Parser File
When you run Bison, you give it a Bison grammar file as input. The output is a C source file
that parses the language described by the grammar. This file is called a Bison parser. Keep in
mind that the Bison utility and the Bison parser are two distinct programs: the Bison utility is
a program whose output is the Bison parser that becomes part of your program.
The job of the Bison parser is to group tokens into groupings according to the grammar
rules—for example, to build identifiers and operators into expressions. As it does this, it runs
the actions for the grammar rules it uses.
The tokens come from a function called the lexical analyzer that you must supply in some
fashion (such as by writing it in C). The Bison parser calls the lexical analyzer each time it
wants a new token. It doesn’t know what is “inside” the tokens (though their semantic values
Chapter 1: The Concepts of Bison 17
may reflect this). Typically the lexical analyzer makes the tokens by parsing characters of text,
but Bison does not depend on this. See Section 4.2 [The Lexical Analyzer Function yylex],
page 53.
The Bison parser file is C code which defines a function named yyparse which implements
that grammar. This function does not make a complete C program: you must supply some
additional functions. One is the lexical analyzer. Another is an error-reporting function which
the parser calls to report an error. In addition, a complete C program must start with a function
called main; you have to provide this, and arrange for it to call yyparse or the parser will never
run. See Chapter 4 [Parser C-Language Interface], page 53.
Aside from the token type names and the symbols in the actions you write, all symbols defined
in the Bison parser file itself begin with ‘yy’ or ‘YY’. This includes interface functions such as the
lexical analyzer function yylex, the error reporting function yyerror and the parser function
yyparse itself. This also includes numerous identifiers used for internal purposes. Therefore,
you should avoid using C identifiers starting with ‘yy’ or ‘YY’ in the Bison grammar file except
for the ones defined in this manual.
In some cases the Bison parser file includes system headers, and in those cases your code
should respect the identifiers reserved by those headers. On some non-GNU hosts, <alloca.h>,
<stddef.h>, and <stdlib.h> are included as needed to declare memory allocators and related
types. Other system headers may be included if you define YYDEBUG to a nonzero value (see
Section 8.2 [Tracing Your Parser], page 80).
1.8 Stages in Using Bison
The actual language-design process using Bison, from grammar specification to a working com-
piler or interpreter, has these parts:
1. Formally specify the grammar in a form recognized by Bison (see Chapter 3 [Bison Grammar
Files], page 35). For each grammatical rule in the language, describe the action that is to
be taken when an instance of that rule is recognized. The action is described by a sequence
of C statements.
2. Write a lexical analyzer to process input and pass tokens to the parser. The lexical analyzer
may be written by hand in C (see Section 4.2 [The Lexical Analyzer Function yylex],
page 53). It could also be produced using Lex, but the use of Lex is not discussed in this
manual.
3. Write a controlling function that calls the Bison-produced parser.
4. Write error-reporting routines.
To turn this source code as written into a runnable program, you must follow these steps:
1. Run Bison on the grammar to produce the parser.
2. Compile the code output by Bison, as well as any other source files.
3. Link the object files to produce the finished product.
1.9 The Overall Layout of a Bison Grammar
The input file for the Bison utility is a Bison grammar file. The general form of a Bison grammar
file is as follows:
%{
Prologue
%}
Bison declarations
18 Bison 1.875
%%
Grammar rules
%%
Epilogue
The ‘%%’, ‘%{’ and ‘%}’ are punctuation that appears in every Bison grammar file to separate the
sections.
The prologue may define types and variables used in the actions. You can also use prepro-
cessor commands to define macros used there, and use #include to include header files that
do any of these things. You need to declare the lexical analyzer yylex and the error printer
yyerror here, along with any other global identifiers used by the actions in the grammar rules.
The Bison declarations declare the names of the terminal and nonterminal symbols, and may
also describe operator precedence and the data types of semantic values of various symbols.
The grammar rules define how to construct each nonterminal symbol from its parts.
The epilogue can contain any code you want to use. Often the definitions of functions declared
in the prologue go here. In a simple program, all the rest of the program can go here.
Chapter 2: Examples 19
2 Examples
Now we show and explain three sample programs written using Bison: a reverse polish notation
calculator, an algebraic (infix) notation calculator, and a multi-function calculator. All three
have been tested under BSD Unix 4.3; each produces a usable, though limited, interactive desk-
top calculator.
These examples are simple, but Bison grammars for real programming languages are written
the same way.
2.1 Reverse Polish Notation Calculator
The first example is that of a simple double-precision reverse polish notation calculator (a
calculator using postfix operators). This example provides a good starting point, since operator
precedence is not an issue. The second example will illustrate how operator precedence is
handled.
The source code for this calculator is named ‘rpcalc.y’. The ‘.y’ extension is a convention
used for Bison input files.
2.1.1 Declarations for rpcalc
Here are the C and Bison declarations for the reverse polish notation calculator. As in C,
comments are placed between ‘/*...*/’.
/* Reverse polish notation calculator. */
%{
#define YYSTYPE double
#include <math.h>
int yylex (void);
void yyerror (char const *);
%}
%token NUM
%% /* Grammar rules and actions follow. */
The declarations section (see Section 3.1.1 [The prologue], page 35) contains two preprocessor
directives and two forward declarations.
The #define directive defines the macro YYSTYPE, thus specifying the C data type for se-
mantic values of both tokens and groupings (see Section 3.5.1 [Data Types of Semantic Values],
page 39). The Bison parser will use whatever type YYSTYPE is defined as; if you don’t define it,
int is the default. Because we specify double, each token and each expression has an associated
value, which is a floating point number.
The #include directive is used to declare the exponentiation function pow.
The forward declarations for yylex and yyerror are needed because the C language requires
that functions be declared before they are used. These functions will be defined in the epilogue,
but the parser calls them so they must be declared in the prologue.
The second section, Bison declarations, provides information to Bison about the token types
(see Section 3.1.2 [The Bison Declarations Section], page 35). Each terminal symbol that is not
a single-character literal must be declared here. (Single-character literals normally don’t need
to be declared.) In this example, all the arithmetic operators are designated by single-character
literals, so the only terminal symbol that needs to be declared is NUM, the token type for numeric
constants.
20 Bison 1.875
2.1.2 Grammar Rules for rpcalc
Here are the grammar rules for the reverse polish notation calculator.
input: /* empty */
| input line
;
line: ’\n’
| exp ’\n’ { printf ("\t%.10g\n", $1); }
;
exp: NUM { $$ = $1; }
| exp exp ’+’ { $$ = $1 + $2; }
| exp exp ’-’ { $$ = $1 - $2; }
| exp exp ’*’ { $$ = $1 * $2; }
| exp exp ’/’ { $$ = $1 / $2; }
/* Exponentiation */
| exp exp ’^’ { $$ = pow ($1, $2); }
/* Unary minus */
| exp ’n’ { $$ = -$1; }
;
%%
The groupings of the rpcalc “language” defined here are the expression (given the name exp),
the line of input (line), and the complete input transcript (input). Each of these nonterminal
symbols has several alternate rules, joined by the ‘|’ punctuator which is read as “or”. The
following sections explain what these rules mean.
The semantics of the language is determined by the actions taken when a grouping is rec-
ognized. The actions are the C code that appears inside braces. See Section 3.5.3 [Actions],
page 40.
You must specify these actions in C, but Bison provides the means for passing semantic
values between the rules. In each action, the pseudo-variable $$ stands for the semantic value
for the grouping that the rule is going to construct. Assigning a value to $$ is the main job of
most actions. The semantic values of the components of the rule are referred to as $1,$2, and
so on.
2.1.2.1 Explanation of input
Consider the definition of input:
input: /* empty */
| input line
;
This definition reads as follows: “A complete input is either an empty string, or a complete
input followed by an input line”. Notice that “complete input” is defined in terms of itself. This
definition is said to be left recursive since input appears always as the leftmost symbol in the
sequence. See Section 3.4 [Recursive Rules], page 38.
The first alternative is empty because there are no symbols between the colon and the first
‘|’; this means that input can match an empty string of input (no tokens). We write the
rules this way because it is legitimate to type Ctrl-d right after you start the calculator. It’s
conventional to put an empty alternative first and write the comment ‘/* empty */’ in it.
The second alternate rule (input line) handles all nontrivial input. It means, “After reading
any number of lines, read one more line if possible.” The left recursion makes this rule into a
Chapter 2: Examples 21
loop. Since the first alternative matches empty input, the loop can be executed zero or more
times.
The parser function yyparse continues to process input until a grammatical error is seen or
the lexical analyzer says there are no more input tokens; we will arrange for the latter to happen
at end-of-input.
2.1.2.2 Explanation of line
Now consider the definition of line:
line: ’\n’
| exp ’\n’ { printf ("\t%.10g\n", $1); }
;
The first alternative is a token which is a newline character; this means that rpcalc accepts
a blank line (and ignores it, since there is no action). The second alternative is an expression
followed by a newline. This is the alternative that makes rpcalc useful. The semantic value
of the exp grouping is the value of $1 because the exp in question is the first symbol in the
alternative. The action prints this value, which is the result of the computation the user asked
for.
This action is unusual because it does not assign a value to $$. As a consequence, the
semantic value associated with the line is uninitialized (its value will be unpredictable). This
would be a bug if that value were ever used, but we don’t use it: once rpcalc has printed the
value of the user’s input line, that value is no longer needed.
2.1.2.3 Explanation of expr
The exp grouping has several rules, one for each kind of expression. The first rule handles the
simplest expressions: those that are just numbers. The second handles an addition-expression,
which looks like two expressions followed by a plus-sign. The third handles subtraction, and so
on.
exp: NUM
| exp exp ’+’ { $$ = $1 + $2; }
| exp exp ’-’ { $$ = $1 - $2; }
...
;
We have used ‘|’ to join all the rules for exp, but we could equally well have written them
separately:
exp: NUM ;
exp: exp exp ’+’ { $$ = $1 + $2; } ;
exp: exp exp ’-’ { $$ = $1 - $2; } ;
...
Most of the rules have actions that compute the value of the expression in terms of the value
of its parts. For example, in the rule for addition, $1 refers to the first component exp and
$2 refers to the second one. The third component, ’+’, has no meaningful associated semantic
value, but if it had one you could refer to it as $3. When yyparse recognizes a sum expression
using this rule, the sum of the two subexpressions’ values is produced as the value of the entire
expression. See Section 3.5.3 [Actions], page 40.
You don’t have to give an action for every rule. When a rule has no action, Bison by default
copies the value of $1 into $$. This is what happens in the first rule (the one that uses NUM).
The formatting shown here is the recommended convention, but Bison does not require it.
You can add or change white space as much as you wish. For example, this:
22 Bison 1.875
exp : NUM | exp exp ’+’ {$$ = $1 + $2; } | ...
means the same thing as this:
exp: NUM
| exp exp ’+’ { $$ = $1 + $2; }
| ...
The latter, however, is much more readable.
2.1.3 The rpcalc Lexical Analyzer
The lexical analyzer’s job is low-level parsing: converting characters or sequences of characters
into tokens. The Bison parser gets its tokens by calling the lexical analyzer. See Section 4.2
[The Lexical Analyzer Function yylex], page 53.
Only a simple lexical analyzer is needed for the RPN calculator. This lexical analyzer skips
blanks and tabs, then reads in numbers as double and returns them as NUM tokens. Any other
character that isn’t part of a number is a separate token. Note that the token-code for such a
single-character token is the character itself.
The return value of the lexical analyzer function is a numeric code which represents a token
type. The same text used in Bison rules to stand for this token type is also a C expression for the
numeric code for the type. This works in two ways. If the token type is a character literal, then
its numeric code is that of the character; you can use the same character literal in the lexical
analyzer to express the number. If the token type is an identifier, that identifier is defined by
Bison as a C macro whose definition is the appropriate number. In this example, therefore, NUM
becomes a macro for yylex to use.
The semantic value of the token (if it has one) is stored into the global variable yylval, which
is where the Bison parser will look for it. (The C data type of yylval is YYSTYPE, which was
defined at the beginning of the grammar; see Section 2.1.1 [Declarations for rpcalc], page 19.)
A token type code of zero is returned if the end-of-input is encountered. (Bison recognizes
any nonpositive value as indicating end-of-input.)
Here is the code for the lexical analyzer:
/* The lexical analyzer returns a double floating point
number on the stack and the token NUM, or the numeric code
of the character read if not a number. It skips all blanks
and tabs, and returns 0 for end-of-input. */
#include <ctype.h>
int
yylex (void)
{
int c;
/* Skip white space. */
while ((c = getchar ()) == ’ ’ || c == ’\t’)
;
/* Process numbers. */
if (c == ’.’ || isdigit (c))
{
ungetc (c, stdin);
scanf ("%lf", &yylval);
return NUM;
}
Chapter 2: Examples 23
/* Return end-of-input. */
if (c == EOF)
return 0;
/* Return a single char. */
return c;
}
2.1.4 The Controlling Function
In keeping with the spirit of this example, the controlling function is kept to the bare minimum.
The only requirement is that it call yyparse to start the process of parsing.
int
main (void)
{
return yyparse ();
}
2.1.5 The Error Reporting Routine
When yyparse detects a syntax error, it calls the error reporting function yyerror to print an
error message (usually but not always "syntax error"). It is up to the programmer to supply
yyerror (see Chapter 4 [Parser C-Language Interface], page 53), so here is the definition we will
use:
#include <stdio.h>
/* Called by yyparse on error. */
void
yyerror (char const *s)
{
printf ("%s\n", s);
}
After yyerror returns, the Bison parser may recover from the error and continue parsing if
the grammar contains a suitable error rule (see Chapter 6 [Error Recovery], page 69). Otherwise,
yyparse returns nonzero. We have not written any error rules in this example, so any invalid
input will cause the calculator program to exit. This is not clean behavior for a real calculator,
but it is adequate for the first example.
2.1.6 Running Bison to Make the Parser
Before running Bison to produce a parser, we need to decide how to arrange all the source code
in one or more source files. For such a simple example, the easiest thing is to put everything in
one file. The definitions of yylex,yyerror and main go at the end, in the epilogue of the file
(see Section 1.9 [The Overall Layout of a Bison Grammar], page 17).
For a large project, you would probably have several source files, and use make to arrange to
recompile them.
With all the source in a single file, you use the following command to convert it into a parser
file:
bison file_name.y
In this example the file was called ‘rpcalc.y’ (for “Reverse Polish calculator”). Bison produces
a file named ‘file_name.tab.c’, removing the ‘.y’ from the original file name. The file output
by Bison contains the source code for yyparse. The additional functions in the input file (yylex,
yyerror and main) are copied verbatim to the output.
24 Bison 1.875
2.1.7 Compiling the Parser File
Here is how to compile and run the parser file:
#List files in current directory.
$ls
rpcalc.tab.c rpcalc.y
#Compile the Bison parser.
#‘-lm’ tells compiler to search math library for pow.
$cc -lm -o rpcalc rpcalc.tab.c
#List files again.
$ls
rpcalc rpcalc.tab.c rpcalc.y
The file ‘rpcalc’ now contains the executable code. Here is an example session using rpcalc.
$rpcalc
4 9 +
13
3 7 +345*+-
-13
3 7 +345*+- n Note the unary minus, ‘n’
13
56/4n+
-3.166666667
3 4 ^Exponentiation
81
^DEnd-of-file indicator
$
2.2 Infix Notation Calculator: calc
We now modify rpcalc to handle infix operators instead of postfix. Infix notation involves the
concept of operator precedence and the need for parentheses nested to arbitrary depth. Here is
the Bison code for ‘calc.y’, an infix desk-top calculator.
/* Infix notation calculator. */
%{
#define YYSTYPE double
#include <math.h>
#include <stdio.h>
int yylex (void);
void yyerror (char const *);
%}
/* Bison declarations. */
%token NUM
%left ’-’ ’+’
%left ’*’ ’/’
%left NEG /* negation--unary minus */
%right ’^’ /* exponentiation */
%% /* The grammar follows. */
Chapter 2: Examples 25
input: /* empty */
| input line
;
line: ’\n’
| exp ’\n’ { printf ("\t%.10g\n", $1); }
;
exp: NUM { $$ = $1; }
| exp ’+’ exp { $$ = $1 + $3; }
| exp ’-’ exp { $$ = $1 - $3; }
| exp ’*’ exp { $$ = $1 * $3; }
| exp ’/’ exp { $$ = $1 / $3; }
| ’-’ exp %prec NEG { $$ = -$2; }
| exp ’^’ exp { $$ = pow ($1, $3); }
| ’(’ exp ’)’ { $$ = $2; }
;
%%
The functions yylex,yyerror and main can be the same as before.
There are two important new features shown in this code.
In the second section (Bison declarations), %left declares token types and says they are left-
associative operators. The declarations %left and %right (right associativity) take the place
of %token which is used to declare a token type name without associativity. (These tokens are
single-character literals, which ordinarily don’t need to be declared. We declare them here to
specify the associativity.)
Operator precedence is determined by the line ordering of the declarations; the higher the
line number of the declaration (lower on the page or screen), the higher the precedence. Hence,
exponentiation has the highest precedence, unary minus (NEG) is next, followed by ‘*’ and ‘/’,
and so on. See Section 5.3 [Operator Precedence], page 61.
The other important new feature is the %prec in the grammar section for the unary minus
operator. The %prec simply instructs Bison that the rule ‘| ’-’ exp’ has the same precedence as
NEG—in this case the next-to-highest. See Section 5.4 [Context-Dependent Precedence], page 62.
Here is a sample run of ‘calc.y’:
$calc
4+4.5 - (34/(8*3+-3))
6.880952381
-56 +2
-54
3^2
9
2.3 Simple Error Recovery
Up to this point, this manual has not addressed the issue of error recovery—how to continue
parsing after the parser detects a syntax error. All we have handled is error reporting with
yyerror. Recall that by default yyparse returns after calling yyerror. This means that an
erroneous input line causes the calculator program to exit. Now we show how to rectify this
deficiency.
The Bison language itself includes the reserved word error, which may be included in the
grammar rules. In the example below it has been added to one of the alternatives for line:
26 Bison 1.875
line: ’\n’
| exp ’\n’ { printf ("\t%.10g\n", $1); }
| error ’\n’ { yyerrok; }
;
This addition to the grammar allows for simple error recovery in the event of a syntax error.
If an expression that cannot be evaluated is read, the error will be recognized by the third
rule for line, and parsing will continue. (The yyerror function is still called upon to print its
message as well.) The action executes the statement yyerrok, a macro defined automatically by
Bison; its meaning is that error recovery is complete (see Chapter 6 [Error Recovery], page 69).
Note the difference between yyerrok and yyerror; neither one is a misprint.
This form of error recovery deals with syntax errors. There are other kinds of errors; for ex-
ample, division by zero, which raises an exception signal that is normally fatal. A real calculator
program must handle this signal and use longjmp to return to main and resume parsing input
lines; it would also have to discard the rest of the current line of input. We won’t discuss this
issue further because it is not specific to Bison programs.
2.4 Location Tracking Calculator: ltcalc
This example extends the infix notation calculator with location tracking. This feature will be
used to improve the error messages. For the sake of clarity, this example is a simple integer
calculator, since most of the work needed to use locations will be done in the lexical analyzer.
2.4.1 Declarations for ltcalc
The C and Bison declarations for the location tracking calculator are the same as the declarations
for the infix notation calculator.
/* Location tracking calculator. */
%{
#define YYSTYPE int
#include <math.h>
int yylex (void);
void yyerror (char const *);
%}
/* Bison declarations. */
%token NUM
%left ’-’ ’+’
%left ’*’ ’/’
%left NEG
%right ’^’
%% /* The grammar follows. */
Note there are no declarations specific to locations. Defining a data type for storing locations is
not needed: we will use the type provided by default (see Section 3.6.1 [Data Types of Locations],
page 43), which is a four member structure with the following integer fields: first_line,first_
column,last_line and last_column.
Chapter 2: Examples 27
2.4.2 Grammar Rules for ltcalc
Whether handling locations or not has no effect on the syntax of your language. Therefore,
grammar rules for this example will be very close to those of the previous example: we will only
modify them to benefit from the new information.
Here, we will use locations to report divisions by zero, and locate the wrong expressions or
subexpressions.
input : /* empty */
| input line
;
line : ’\n’
| exp ’\n’ { printf ("%d\n", $1); }
;
exp : NUM { $$ = $1; }
| exp ’+’ exp { $$ = $1 + $3; }
| exp ’-’ exp { $$ = $1 - $3; }
| exp ’*’ exp { $$ = $1 * $3; }
| exp ’/’ exp
{
if ($3)
$$ = $1 / $3;
else
{
$$ = 1;
fprintf (stderr, "%d.%d-%d.%d: division by zero",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
}
}
| ’-’ exp %preg NEG { $$ = -$2; }
| exp ’^’ exp { $$ = pow ($1, $3); }
| ’(’ exp ’)’ { $$ = $2; }
This code shows how to reach locations inside of semantic actions, by using the pseudo-
variables @nfor rule components, and the pseudo-variable @$ for groupings.
We don’t need to assign a value to @$: the output parser does it automatically. By default,
before executing the C code of each action, @$ is set to range from the beginning of @1 to the end
of @n, for a rule with ncomponents. This behavior can be redefined (see Section 3.6.3 [Default
Action for Locations], page 44), and for very specific rules, @$ can be computed by hand.
2.4.3 The ltcalc Lexical Analyzer.
Until now, we relied on Bison’s defaults to enable location tracking. The next step is to rewrite
the lexical analyzer, and make it able to feed the parser with the token locations, as it already
does for semantic values.
To this end, we must take into account every single character of the input text, to avoid the
computed locations of being fuzzy or wrong:
int
yylex (void)
{
int c;
28 Bison 1.875
/* Skip white space. */
while ((c = getchar ()) == ’ ’ || c == ’\t’)
++yylloc.last_column;
/* Step. */
yylloc.first_line = yylloc.last_line;
yylloc.first_column = yylloc.last_column;
/* Process numbers. */
if (isdigit (c))
{
yylval = c - ’0’;
++yylloc.last_column;
while (isdigit (c = getchar ()))
{
++yylloc.last_column;
yylval = yylval * 10 + c - ’0’;
}
ungetc (c, stdin);
return NUM;
}
/* Return end-of-input. */
if (c == EOF)
return 0;
/* Return a single char, and update location. */
if (c == ’\n’)
{
++yylloc.last_line;
yylloc.last_column = 0;
}
else
++yylloc.last_column;
return c;
}
Basically, the lexical analyzer performs the same processing as before: it skips blanks and
tabs, and reads numbers or single-character tokens. In addition, it updates yylloc, the global
variable (of type YYLTYPE) containing the token’s location.
Now, each time this function returns a token, the parser has its number as well as its semantic
value, and its location in the text. The last needed change is to initialize yylloc, for example
in the controlling function:
int
main (void)
{
yylloc.first_line = yylloc.last_line = 1;
yylloc.first_column = yylloc.last_column = 0;
return yyparse ();
}
Chapter 2: Examples 29
Remember that computing locations is not a matter of syntax. Every character must be
associated to a location update, whether it is in valid input, in comments, in literal strings, and
so on.
2.5 Multi-Function Calculator: mfcalc
Now that the basics of Bison have been discussed, it is time to move on to a more advanced
problem. The above calculators provided only five functions, ‘+’, ‘-’, ‘*’, ‘/’ and ‘^’. It would
be nice to have a calculator that provides other mathematical functions such as sin,cos, etc.
It is easy to add new operators to the infix calculator as long as they are only single-character
literals. The lexical analyzer yylex passes back all nonnumber characters as tokens, so new
grammar rules suffice for adding a new operator. But we want something more flexible: built-in
functions whose syntax has this form:
function_name (argument )
At the same time, we will add memory to the calculator, by allowing you to create named
variables, store values in them, and use them later. Here is a sample session with the multi-
function calculator:
$mfcalc
pi = 3.141592653589
3.1415926536
sin(pi)
0.0000000000
alpha = beta1 = 2.3
2.3000000000
alpha
2.3000000000
ln(alpha)
0.8329091229
exp(ln(beta1))
2.3000000000
$
Note that multiple assignment and nested function calls are permitted.
2.5.1 Declarations for mfcalc
Here are the C and Bison declarations for the multi-function calculator.
%{
#include <math.h> /* For math functions, cos(), sin(), etc. */
#include "calc.h" /* Contains definition of ‘symrec’. */
int yylex (void);
void yyerror (char const *);
%}
%union {
double val; /* For returning numbers. */
symrec *tptr; /* For returning symbol-table pointers. */
}
%token <val> NUM /* Simple double precision number. */
%token <tptr> VAR FNCT /* Variable and Function. */
%type <val> exp
%right ’=’
%left ’-’ ’+’
%left ’*’ ’/’
%left NEG /* negation--unary minus */
%right ’^’ /* exponentiation */
%% /* The grammar follows. */
30 Bison 1.875
The above grammar introduces only two new features of the Bison language. These features
allow semantic values to have various data types (see Section 3.5.2 [More Than One Value Type],
page 39).
The %union declaration specifies the entire list of possible types; this is instead of defining
YYSTYPE. The allowable types are now double-floats (for exp and NUM) and pointers to entries
in the symbol table. See Section 3.7.3 [The Collection of Value Types], page 46.
Since values can now have various types, it is necessary to associate a type with each grammar
symbol whose semantic value is used. These symbols are NUM,VAR,FNCT, and exp. Their decla-
rations are augmented with information about their data type (placed between angle brackets).
The Bison construct %type is used for declaring nonterminal symbols, just as %token is used
for declaring token types. We have not used %type before because nonterminal symbols are
normally declared implicitly by the rules that define them. But exp must be declared explicitly
so we can specify its value type. See Section 3.7.4 [Nonterminal Symbols], page 47.
2.5.2 Grammar Rules for mfcalc
Here are the grammar rules for the multi-function calculator. Most of them are copied directly
from calc; three rules, those which mention VAR or FNCT, are new.
input: /* empty */
| input line
;
line:
’\n’
| exp ’\n’ { printf ("\t%.10g\n", $1); }
| error ’\n’ { yyerrok; }
;
exp: NUM { $$ = $1; }
| VAR { $$ = $1->value.var; }
| VAR ’=’ exp { $$ = $3; $1->value.var = $3; }
| FNCT ’(’ exp ’)’ { $$ = (*($1->value.fnctptr))($3); }
| exp ’+’ exp { $$ = $1 + $3; }
| exp ’-’ exp { $$ = $1 - $3; }
| exp ’*’ exp { $$ = $1 * $3; }
| exp ’/’ exp { $$ = $1 / $3; }
| ’-’ exp %prec NEG { $$ = -$2; }
| exp ’^’ exp { $$ = pow ($1, $3); }
| ’(’ exp ’)’ { $$ = $2; }
;
/* End of grammar. */
%%
2.5.3 The mfcalc Symbol Table
The multi-function calculator requires a symbol table to keep track of the names and meanings
of variables and functions. This doesn’t affect the grammar rules (except for the actions) or the
Bison declarations, but it requires some additional C functions for support.
The symbol table itself consists of a linked list of records. Its definition, which is kept in the
header ‘calc.h’, is as follows. It provides for either functions or variables to be placed in the
table.
/* Function type. */
typedef double (*func_t) (double);
Chapter 2: Examples 31
/* Data type for links in the chain of symbols. */
struct symrec
{
char *name; /* name of symbol */
int type; /* type of symbol: either VAR or FNCT */
union
{
double var; /* value of a VAR */
func_t fnctptr; /* value of a FNCT */
} value;
struct symrec *next; /* link field */
};
typedef struct symrec symrec;
/* The symbol table: a chain of ‘struct symrec’. */
extern symrec *sym_table;
symrec *putsym (char const *, func_t);
symrec *getsym (char const *);
The new version of main includes a call to init_table, a function that initializes the symbol
table. Here it is, and init_table as well:
#include <stdio.h>
/* Called by yyparse on error. */
void
yyerror (char const *s)
{
printf ("%s\n", s);
}
struct init
{
char const *fname;
double (*fnct) (double);
};
struct init const arith_fncts[] =
{
"sin", sin,
"cos", cos,
"atan", atan,
"ln", log,
"exp", exp,
"sqrt", sqrt,
0, 0
};
/* The symbol table: a chain of ‘struct symrec’. */
symrec *sym_table;
/* Put arithmetic functions in table. */
void
init_table (void)
{
int i;
symrec *ptr;
for (i = 0; arith_fncts[i].fname != 0; i++)
{
ptr = putsym (arith_fncts[i].fname, FNCT);
ptr->value.fnctptr = arith_fncts[i].fnct;
}
}
32 Bison 1.875
int
main (void)
{
init_table ();
return yyparse ();
}
By simply editing the initialization list and adding the necessary include files, you can add
additional functions to the calculator.
Two important functions allow look-up and installation of symbols in the symbol table. The
function putsym is passed a name and the type (VAR or FNCT) of the object to be installed. The
object is linked to the front of the list, and a pointer to the object is returned. The function
getsym is passed the name of the symbol to look up. If found, a pointer to that symbol is
returned; otherwise zero is returned.
symrec *
putsym (char const *sym_name, int sym_type)
{
symrec *ptr;
ptr = (symrec *) malloc (sizeof (symrec));
ptr->name = (char *) malloc (strlen (sym_name) + 1);
strcpy (ptr->name,sym_name);
ptr->type = sym_type;
ptr->value.var = 0; /* Set value to 0 even if fctn. */
ptr->next = (struct symrec *)sym_table;
sym_table = ptr;
return ptr;
}
symrec *
getsym (char const *sym_name)
{
symrec *ptr;
for (ptr = sym_table; ptr != (symrec *) 0;
ptr = (symrec *)ptr->next)
if (strcmp (ptr->name,sym_name) == 0)
return ptr;
return 0;
}
The function yylex must now recognize variables, numeric values, and the single-character
arithmetic operators. Strings of alphanumeric characters with a leading non-digit are recognized
as either variables or functions depending on what the symbol table says about them.
The string is passed to getsym for look up in the symbol table. If the name appears in the
table, a pointer to its location and its type (VAR or FNCT) is returned to yyparse. If it is not
already in the table, then it is installed as a VAR using putsym. Again, a pointer and its type
(which must be VAR) is returned to yyparse.
No change is needed in the handling of numeric values and arithmetic operators in yylex.
#include <ctype.h>
int
yylex (void)
{
int c;
/* Ignore white space, get first nonwhite character. */
while ((c = getchar ()) == ’ ’ || c == ’\t’);
if (c == EOF)
return 0;
Chapter 2: Examples 33
/* Char starts a number => parse the number. */
if (c == ’.’ || isdigit (c))
{
ungetc (c, stdin);
scanf ("%lf", &yylval.val);
return NUM;
}
/* Char starts an identifier => read the name. */
if (isalpha (c))
{
symrec *s;
static char *symbuf = 0;
static int length = 0;
int i;
/* Initially make the buffer long enough
for a 40-character symbol name. */
if (length == 0)
length = 40, symbuf = (char *)malloc (length + 1);
i = 0;
do
{
/* If buffer is full, make it bigger. */
if (i == length)
{
length *= 2;
symbuf = (char *) realloc (symbuf, length + 1);
}
/* Add this character to the buffer. */
symbuf[i++] = c;
/* Get another character. */
c = getchar ();
}
while (isalnum (c));
ungetc (c, stdin);
symbuf[i] = ’\0’;
s = getsym (symbuf);
if (s == 0)
s = putsym (symbuf, VAR);
yylval.tptr = s;
return s->type;
}
/* Any other character is a token by itself. */
return c;
}
This program is both powerful and flexible. You may easily add new functions, and it is a
simple job to modify this code to install predefined variables such as pi or eas well.
2.6 Exercises
1. Add some new functions from ‘math.h’ to the initialization list.
2. Add another array that contains constants and their values. Then modify init_table to
add these constants to the symbol table. It will be easiest to give the constants type VAR.
3. Make the program report an error if the user refers to an uninitialized variable in any way
except to store a value in it.
34 Bison 1.875
Chapter 3: Bison Grammar Files 35
3 Bison Grammar Files
Bison takes as input a context-free grammar specification and produces a C-language function
that recognizes correct instances of the grammar.
The Bison grammar input file conventionally has a name ending in ‘.y’. See Chapter 9
[Invoking Bison], page 83.
3.1 Outline of a Bison Grammar
A Bison grammar file has four main sections, shown here with the appropriate delimiters:
%{
Prologue
%}
Bison declarations
%%
Grammar rules
%%
Epilogue
Comments enclosed in ‘/* ... */’ may appear in any of the sections. As a GNU extension,
‘//’ introduces a comment that continues until end of line.
3.1.1 The prologue
The Prologue section contains macro definitions and declarations of functions and variables
that are used in the actions in the grammar rules. These are copied to the beginning of the
parser file so that they precede the definition of yyparse. You can use ‘#include’ to get the
declarations from a header file. If you don’t need any C declarations, you may omit the ‘%{’ and
‘%}’ delimiters that bracket this section.
You may have more than one Prologue section, intermixed with the Bison declarations. This
allows you to have C and Bison declarations that refer to each other. For example, the %union
declaration may use types defined in a header file, and you may wish to prototype functions
that take arguments of type YYSTYPE. This can be done with two Prologue blocks, one before
and one after the %union declaration.
%{
#include <stdio.h>
#include "ptypes.h"
%}
%union {
long n;
tree t; /* tree is defined in ‘ptypes.h’. */
}
%{
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
%}
...
36 Bison 1.875
3.1.2 The Bison Declarations Section
The Bison declarations section contains declarations that define terminal and nonterminal sym-
bols, specify precedence, and so on. In some simple grammars you may not need any declarations.
See Section 3.7 [Bison Declarations], page 45.
3.1.3 The Grammar Rules Section
The grammar rules section contains one or more Bison grammar rules, and nothing else. See
Section 3.3 [Syntax of Grammar Rules], page 38.
There must always be at least one grammar rule, and the first ‘%%’ (which precedes the
grammar rules) may never be omitted even if it is the first thing in the file.
3.1.4 The epilogue
The Epilogue is copied verbatim to the end of the parser file, just as the Prologue is copied
to the beginning. This is the most convenient place to put anything that you want to have
in the parser file but which need not come before the definition of yyparse. For example, the
definitions of yylex and yyerror often go here. Because C requires functions to be declared
before being used, you often need to declare functions like yylex and yyerror in the Prologue,
even if you define them int he Epilogue. See Chapter 4 [Parser C-Language Interface], page 53.
If the last section is empty, you may omit the ‘%%’ that separates it from the grammar rules.
The Bison parser itself contains many macros and identifiers whose names start with ‘yy’
or ‘YY’, so it is a good idea to avoid using any such names (except those documented in this
manual) in the epilogue of the grammar file.
3.2 Symbols, Terminal and Nonterminal
Symbols in Bison grammars represent the grammatical classifications of the language.
Aterminal symbol (also known as a token type) represents a class of syntactically equivalent
tokens. You use the symbol in grammar rules to mean that a token in that class is allowed. The
symbol is represented in the Bison parser by a numeric code, and the yylex function returns a
token type code to indicate what kind of token has been read. You don’t need to know what
the code value is; you can use the symbol to stand for it.
Anonterminal symbol stands for a class of syntactically equivalent groupings. The symbol
name is used in writing grammar rules. By convention, it should be all lower case.
Symbol names can contain letters, digits (not at the beginning), underscores and periods.
Periods make sense only in nonterminals.
There are three ways of writing terminal symbols in the grammar:
•Anamed token type is written with an identifier, like an identifier in C. By convention, it
should be all upper case. Each such name must be defined with a Bison declaration such
as %token. See Section 3.7.1 [Token Type Names], page 45.
•Acharacter token type (or literal character token) is written in the grammar using the
same syntax used in C for character constants; for example, ’+’ is a character token type.
A character token type doesn’t need to be declared unless you need to specify its semantic
value data type (see Section 3.5.1 [Data Types of Semantic Values], page 39), associativity,
or precedence (see Section 5.3 [Operator Precedence], page 61).
By convention, a character token type is used only to represent a token that consists of
that particular character. Thus, the token type ’+’ is used to represent the character ‘+’
as a token. Nothing enforces this convention, but if you depart from it, your program will
confuse other readers.
All the usual escape sequences used in character literals in C can be used in Bison as well,
but you must not use the null character as a character literal because its numeric code,
Chapter 3: Bison Grammar Files 37
zero, signifies end-of-input (see Section 4.2.1 [Calling Convention for yylex], page 53).
Also, unlike standard C, trigraphs have no special meaning in Bison character literals, nor
is backslash-newline allowed.
•Aliteral string token is written like a C string constant; for example, "<=" is a literal
string token. A literal string token doesn’t need to be declared unless you need to specify
its semantic value data type (see Section 3.5.1 [Value Type], page 39), associativity, or
precedence (see Section 5.3 [Precedence], page 61).
You can associate the literal string token with a symbolic name as an alias, using the
%token declaration (see Section 3.7.1 [Token Declarations], page 45). If you don’t do that,
the lexical analyzer has to retrieve the token number for the literal string token from the
yytname table (see Section 4.2.1 [Calling Convention], page 53).
Warning: literal string tokens do not work in Yacc.
By convention, a literal string token is used only to represent a token that consists of that
particular string. Thus, you should use the token type "<=" to represent the string ‘<=’ as
a token. Bison does not enforce this convention, but if you depart from it, people who read
your program will be confused.
All the escape sequences used in string literals in C can be used in Bison as well. How-
ever, unlike Standard C, trigraphs have no special meaning in Bison string literals, nor is
backslash-newline allowed. A literal string token must contain two or more characters; for
a token containing just one character, use a character token (see above).
How you choose to write a terminal symbol has no effect on its grammatical meaning. That
depends only on where it appears in rules and on when the parser function returns that symbol.
The value returned by yylex is always one of the terminal symbols, except that a zero or
negative value signifies end-of-input. Whichever way you write the token type in the grammar
rules, you write it the same way in the definition of yylex. The numeric code for a character
token type is simply the positive numeric code of the character, so yylex can use the identical
value to generate the requisite code, though you may need to convert it to unsigned char to
avoid sign-extension on hosts where char is signed. Each named token type becomes a C macro
in the parser file, so yylex can use the name to stand for the code. (This is why periods don’t
make sense in terminal symbols.) See Section 4.2.1 [Calling Convention for yylex], page 53.
If yylex is defined in a separate file, you need to arrange for the token-type macro definitions
to be available there. Use the ‘-d’ option when you run Bison, so that it will write these macro
definitions into a separate header file ‘name.tab.h’ which you can include in the other source
files that need it. See Chapter 9 [Invoking Bison], page 83.
If you want to write a grammar that is portable to any Standard C host, you must use only
non-null character tokens taken from the basic execution character set of Standard C. This set
consists of the ten digits, the 52 lower- and upper-case English letters, and the characters in the
following C-language string:
"\a\b\t\n\v\f\r !\"#%&’()*+,-./:;<=>?[\\]^_{|}~"
The yylex function and Bison must use a consistent character set and encoding for character
tokens. For example, if you run Bison in an ASCII environment, but then compile and run
the resulting program in an environment that uses an incompatible character set like EBCDIC,
the resulting program may not work because the tables generated by Bison will assume ASCII
numeric values for character tokens. It is standard practice for software distributions to contain
C source files that were generated by Bison in an ASCII environment, so installers on platforms
that are incompatible with ASCII must rebuild those files before compiling them.
The symbol error is a terminal symbol reserved for error recovery (see Chapter 6 [Error
Recovery], page 69); you shouldn’t use it for any other purpose. In particular, yylex should
never return this value. The default value of the error token is 256, unless you explicitly assigned
256 to one of your tokens with a %token declaration.
38 Bison 1.875
3.3 Syntax of Grammar Rules
A Bison grammar rule has the following general form:
result :components ...
;
where result is the nonterminal symbol that this rule describes, and components are various
terminal and nonterminal symbols that are put together by this rule (see Section 3.2 [Symbols],
page 36).
For example,
exp: exp ’+’ exp
;
says that two groupings of type exp, with a ‘+’ token in between, can be combined into a larger
grouping of type exp.
White space in rules is significant only to separate symbols. You can add extra white space
as you wish.
Scattered among the components can be actions that determine the semantics of the rule.
An action looks like this:
{C statements }
Usually there is only one action and it follows the components. See Section 3.5.3 [Actions],
page 40.
Multiple rules for the same result can be written separately or can be joined with the vertical-
bar character ‘|’ as follows:
result :rule1-components ...
|rule2-components ...
...
;
They are still considered distinct rules even when joined in this way.
If components in a rule is empty, it means that result can match the empty string. For
example, here is how to define a comma-separated sequence of zero or more exp groupings:
expseq: /* empty */
| expseq1
;
expseq1: exp
| expseq1 ’,’ exp
;
It is customary to write a comment ‘/* empty */’ in each rule with no components.
3.4 Recursive Rules
A rule is called recursive when its result nonterminal appears also on its right hand side. Nearly
all Bison grammars need to use recursion, because that is the only way to define a sequence
of any number of a particular thing. Consider this recursive definition of a comma-separated
sequence of one or more expressions:
expseq1: exp
| expseq1 ’,’ exp
;
Since the recursive use of expseq1 is the leftmost symbol in the right hand side, we call this left
recursion. By contrast, here the same construct is defined using right recursion:
Chapter 3: Bison Grammar Files 39
expseq1: exp
| exp ’,’ expseq1
;
Any kind of sequence can be defined using either left recursion or right recursion, but you
should always use left recursion, because it can parse a sequence of any number of elements
with bounded stack space. Right recursion uses up space on the Bison stack in proportion to
the number of elements in the sequence, because all the elements must be shifted onto the stack
before the rule can be applied even once. See Chapter 5 [The Bison Parser Algorithm], page 59,
for further explanation of this.
Indirect or mutual recursion occurs when the result of the rule does not appear directly on
its right hand side, but does appear in rules for other nonterminals which do appear on its right
hand side.
For example:
expr: primary
| primary ’+’ primary
;
primary: constant
| ’(’ expr ’)’
;
defines two mutually-recursive nonterminals, since each refers to the other.
3.5 Defining Language Semantics
The grammar rules for a language determine only the syntax. The semantics are determined
by the semantic values associated with various tokens and groupings, and by the actions taken
when various groupings are recognized.
For example, the calculator calculates properly because the value associated with each ex-
pression is the proper number; it adds properly because the action for the grouping ‘x+y’ is
to add the numbers associated with xand y.
3.5.1 Data Types of Semantic Values
In a simple program it may be sufficient to use the same data type for the semantic values of all
language constructs. This was true in the RPN and infix calculator examples (see Section 2.1
[Reverse Polish Notation Calculator], page 19).
Bison’s default is to use type int for all semantic values. To specify some other type, define
YYSTYPE as a macro, like this:
#define YYSTYPE double
This macro definition must go in the prologue of the grammar file (see Section 3.1 [Outline of a
Bison Grammar], page 35).
3.5.2 More Than One Value Type
In most programs, you will need different data types for different kinds of tokens and groupings.
For example, a numeric constant may need type int or long, while a string constant needs type
char *, and an identifier might need a pointer to an entry in the symbol table.
To use more than one data type for semantic values in one parser, Bison requires you to do
two things:
•Specify the entire collection of possible data types, with the %union Bison declaration (see
Section 3.7.3 [The Collection of Value Types], page 46).
40 Bison 1.875
•Choose one of those types for each symbol (terminal or nonterminal) for which semantic
values are used. This is done for tokens with the %token Bison declaration (see Section 3.7.1
[Token Type Names], page 45) and for groupings with the %type Bison declaration (see
Section 3.7.4 [Nonterminal Symbols], page 47).
3.5.3 Actions
An action accompanies a syntactic rule and contains C code to be executed each time an instance
of that rule is recognized. The task of most actions is to compute a semantic value for the
grouping built by the rule from the semantic values associated with tokens or smaller groupings.
An action consists of C statements surrounded by braces, much like a compound statement
in C. An action can contain any sequence of C statements. Bison does not look for trigraphs,
though, so if your C code uses trigraphs you should ensure that they do not affect the nesting
of braces or the boundaries of comments, strings, or character literals.
An action can be placed at any position in the rule; it is executed at that position. Most rules
have just one action at the end of the rule, following all the components. Actions in the middle
of a rule are tricky and used only for special purposes (see Section 3.5.5 [Actions in Mid-Rule],
page 41).
The C code in an action can refer to the semantic values of the components matched by the
rule with the construct $n, which stands for the value of the nth component. The semantic
value for the grouping being constructed is $$. (Bison translates both of these constructs into
array element references when it copies the actions into the parser file.)
Here is a typical example:
exp: ...
| exp ’+’ exp
{$$=$1+$3;}
This rule constructs an exp from two smaller exp groupings connected by a plus-sign token. In
the action, $1 and $3 refer to the semantic values of the two component exp groupings, which
are the first and third symbols on the right hand side of the rule. The sum is stored into $$
so that it becomes the semantic value of the addition-expression just recognized by the rule. If
there were a useful semantic value associated with the ‘+’ token, it could be referred to as $2.
Note that the vertical-bar character ‘|’ is really a rule separator, and actions are attached
to a single rule. This is a difference with tools like Flex, for which ‘|’ stands for either “or”,
or “the same action as that of the next rule”. In the following example, the action is triggered
only when ‘b’ is found:
a-or-b: ’a’|’b’ { a_or_b_found = 1; };
If you don’t specify an action for a rule, Bison supplies a default: $$ = $1. Thus, the value
of the first symbol in the rule becomes the value of the whole rule. Of course, the default action
is valid only if the two data types match. There is no meaningful default action for an empty
rule; every empty rule must have an explicit action unless the rule’s value does not matter.
$nwith nzero or negative is allowed for reference to tokens and groupings on the stack before
those that match the current rule. This is a very risky practice, and to use it reliably you must
be certain of the context in which the rule is applied. Here is a case in which you can use this
reliably:
foo: expr bar ’+’ expr { ... }
| expr bar ’-’ expr { ... }
;
bar: /* empty */
{ previous_expr = $0; }
;
Chapter 3: Bison Grammar Files 41
As long as bar is used only in the fashion shown here, $0 always refers to the expr which
precedes bar in the definition of foo.
3.5.4 Data Types of Values in Actions
If you have chosen a single data type for semantic values, the $$ and $nconstructs always have
that data type.
If you have used %union to specify a variety of data types, then you must declare a choice
among these types for each terminal or nonterminal symbol that can have a semantic value.
Then each time you use $$ or $n, its data type is determined by which symbol it refers to in
the rule. In this example,
exp: ...
| exp ’+’ exp
{$$=$1+$3;}
$1 and $3 refer to instances of exp, so they all have the data type declared for the nonterminal
symbol exp. If $2 were used, it would have the data type declared for the terminal symbol ’+’,
whatever that might be.
Alternatively, you can specify the data type when you refer to the value, by inserting ‘<type >’
after the ‘$’ at the beginning of the reference. For example, if you have defined types as shown
here:
%union {
int itype;
double dtype;
}
then you can write $<itype>1 to refer to the first subunit of the rule as an integer, or $<dtype>1
to refer to it as a double.
3.5.5 Actions in Mid-Rule
Occasionally it is useful to put an action in the middle of a rule. These actions are written
just like usual end-of-rule actions, but they are executed before the parser even recognizes the
following components.
A mid-rule action may refer to the components preceding it using $n, but it may not refer
to subsequent components because it is run before they are parsed.
The mid-rule action itself counts as one of the components of the rule. This makes a difference
when there is another action later in the same rule (and usually there is another at the end):
you have to count the actions along with the symbols when working out which number nto use
in $n.
The mid-rule action can also have a semantic value. The action can set its value with an
assignment to $$, and actions later in the rule can refer to the value using $n. Since there is no
symbol to name the action, there is no way to declare a data type for the value in advance, so
you must use the ‘$<...>n’ construct to specify a data type each time you refer to this value.
There is no way to set the value of the entire rule with a mid-rule action, because assignments
to $$ do not have that effect. The only way to set the value for the entire rule is with an ordinary
action at the end of the rule.
Here is an example from a hypothetical compiler, handling a let statement that looks like
‘let (variable )statement ’ and serves to create a variable named variable temporarily for
the duration of statement. To parse this construct, we must put variable into the symbol table
while statement is parsed, then remove it afterward. Here is how it is done:
42 Bison 1.875
stmt: LET ’(’ var ’)’
{ $<context>$ = push_context ();
declare_variable ($3); }
stmt { $$ = $6;
pop_context ($<context>5); }
As soon as ‘let (variable )’ has been recognized, the first action is run. It saves a copy of the
current semantic context (the list of accessible variables) as its semantic value, using alternative
context in the data-type union. Then it calls declare_variable to add the new variable to
that list. Once the first action is finished, the embedded statement stmt can be parsed. Note
that the mid-rule action is component number 5, so the ‘stmt’ is component number 6.
After the embedded statement is parsed, its semantic value becomes the value of the entire
let-statement. Then the semantic value from the earlier action is used to restore the prior list
of variables. This removes the temporary let-variable from the list so that it won’t appear to
exist while the rest of the program is parsed.
Taking action before a rule is completely recognized often leads to conflicts since the parser
must commit to a parse in order to execute the action. For example, the following two rules,
without mid-rule actions, can coexist in a working parser because the parser can shift the open-
brace token and look at what follows before deciding whether there is a declaration or not:
compound: ’{’ declarations statements ’}’
| ’{’ statements ’}’
;
But when we add a mid-rule action as follows, the rules become nonfunctional:
compound: { prepare_for_local_variables (); }
’{’ declarations statements ’}’
| ’{’ statements ’}’
;
Now the parser is forced to decide whether to run the mid-rule action when it has read no farther
than the open-brace. In other words, it must commit to using one rule or the other, without
sufficient information to do it correctly. (The open-brace token is what is called the look-ahead
token at this time, since the parser is still deciding what to do about it. See Section 5.1 [Look-
Ahead Tokens], page 59.)
You might think that you could correct the problem by putting identical actions into the two
rules, like this:
compound: { prepare_for_local_variables (); }
’{’ declarations statements ’}’
| { prepare_for_local_variables (); }
’{’ statements ’}’
;
But this does not help, because Bison does not realize that the two actions are identical. (Bison
never tries to understand the C code in an action.)
If the grammar is such that a declaration can be distinguished from a statement by the first
token (which is true in C), then one solution which does work is to put the action after the
open-brace, like this:
compound: ’{’ { prepare_for_local_variables (); }
declarations statements ’}’
| ’{’ statements ’}’
;
Now the first token of the following declaration or statement, which would in any case tell Bison
which rule to use, can still do so.
Chapter 3: Bison Grammar Files 43
Another solution is to bury the action inside a nonterminal symbol which serves as a sub-
routine:
subroutine: /* empty */
{ prepare_for_local_variables (); }
;
compound: subroutine
’{’ declarations statements ’}’
| subroutine
’{’ statements ’}’
;
Now Bison can execute the action in the rule for subroutine without deciding which rule for
compound it will eventually use. Note that the action is now at the end of its rule. Any mid-rule
action can be converted to an end-of-rule action in this way, and this is what Bison actually
does to implement mid-rule actions.
3.6 Tracking Locations
Though grammar rules and semantic actions are enough to write a fully functional parser, it
can be useful to process some additional information, especially symbol locations.
The way locations are handled is defined by providing a data type, and actions to take when
rules are matched.
3.6.1 Data Type of Locations
Defining a data type for locations is much simpler than for semantic values, since all tokens and
groupings always use the same type.
The type of locations is specified by defining a macro called YYLTYPE. When YYLTYPE is not
defined, Bison uses a default structure type with four members:
typedef struct YYLTYPE
{
int first_line;
int first_column;
int last_line;
int last_column;
} YYLTYPE;
3.6.2 Actions and Locations
Actions are not only useful for defining language semantics, but also for describing the behavior
of the output parser with locations.
The most obvious way for building locations of syntactic groupings is very similar to the way
semantic values are computed. In a given rule, several constructs can be used to access the
locations of the elements being matched. The location of the nth component of the right hand
side is @n, while the location of the left hand side grouping is @$.
Here is a basic example using the default data type for locations:
44 Bison 1.875
exp: ...
| exp ’/’ exp
{
@$.first_column = @1.first_column;
@$.first_line = @1.first_line;
@$.last_column = @3.last_column;
@$.last_line = @3.last_line;
if ($3)
$$ = $1 / $3;
else
{
$$ = 1;
printf("Division by zero, l%d,c%d-l%d,c%d",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
}
}
As for semantic values, there is a default action for locations that is run each time a rule is
matched. It sets the beginning of @$ to the beginning of the first symbol, and the end of @$ to
the end of the last symbol.
With this default action, the location tracking can be fully automatic. The example above
simply rewrites this way:
exp: ...
| exp ’/’ exp
{
if ($3)
$$ = $1 / $3;
else
{
$$ = 1;
printf("Division by zero, l%d,c%d-l%d,c%d",
@3.first_line, @3.first_column,
@3.last_line, @3.last_column);
}
}
3.6.3 Default Action for Locations
Actually, actions are not the best place to compute locations. Since locations are much more
general than semantic values, there is room in the output parser to redefine the default action
to take for each rule. The YYLLOC_DEFAULT macro is invoked each time a rule is matched, before
the associated action is run. It is also invoked while processing a syntax error, to compute the
error’s location.
Most of the time, this macro is general enough to suppress location dedicated code from
semantic actions.
The YYLLOC_DEFAULT macro takes three parameters. The first one is the location of the
grouping (the result of the computation). When a rule is matched, the second parameter is
an array holding locations of all right hand side elements of the rule being matched, and the
third parameter is the size of the rule’s right hand side. When processing a syntax error, the
second parameter is an array holding locations of the symbols that were discarded during error
processing, and the third parameter is the number of discarded symbols.
By default, YYLLOC_DEFAULT is defined this way for simple LALR(1) parsers:
Chapter 3: Bison Grammar Files 45
#define YYLLOC_DEFAULT(Current, Rhs, N) \
Current.first_line = Rhs[1].first_line; \
Current.first_column = Rhs[1].first_column; \
Current.last_line = Rhs[N].last_line; \
Current.last_column = Rhs[N].last_column;
and like this for GLR parsers:
#define YYLLOC_DEFAULT(Current, Rhs, N) \
Current.first_line = YYRHSLOC(Rhs,1).first_line; \
Current.first_column = YYRHSLOC(Rhs,1).first_column; \
Current.last_line = YYRHSLOC(Rhs,N).last_line; \
Current.last_column = YYRHSLOC(Rhs,N).last_column;
When defining YYLLOC_DEFAULT, you should consider that:
•All arguments are free of side-effects. However, only the first one (the result) should be
modified by YYLLOC_DEFAULT.
•For consistency with semantic actions, valid indexes for the location array range from 1 to
n.
3.7 Bison Declarations
The Bison declarations section of a Bison grammar defines the symbols used in formulating the
grammar and the data types of semantic values. See Section 3.2 [Symbols], page 36.
All token type names (but not single-character literal tokens such as ’+’ and ’*’) must be
declared. Nonterminal symbols must be declared if you need to specify which data type to use
for the semantic value (see Section 3.5.2 [More Than One Value Type], page 39).
The first rule in the file also specifies the start symbol, by default. If you want some other
symbol to be the start symbol, you must declare it explicitly (see Section 1.1 [Languages and
Context-Free Grammars], page 11).
3.7.1 Token Type Names
The basic way to declare a token type name (terminal symbol) is as follows:
%token name
Bison will convert this into a #define directive in the parser, so that the function yylex (if
it is in this file) can use the name name to stand for this token type’s code.
Alternatively, you can use %left,%right, or %nonassoc instead of %token, if you wish to
specify associativity and precedence. See Section 3.7.2 [Operator Precedence], page 46.
You can explicitly specify the numeric code for a token type by appending an integer value
in the field immediately following the token name:
%token NUM 300
It is generally best, however, to let Bison choose the numeric codes for all token types. Bison
will automatically select codes that don’t conflict with each other or with normal characters.
In the event that the stack type is a union, you must augment the %token or other token
declaration to include the data type alternative delimited by angle-brackets (see Section 3.5.2
[More Than One Value Type], page 39).
For example:
%union { /* define stack type */
double val;
symrec *tptr;
}
%token <val> NUM /* define token NUM and its type */
46 Bison 1.875
You can associate a literal string token with a token type name by writing the literal string
at the end of a %token declaration which declares the name. For example:
%token arrow "=>"
For example, a grammar for the C language might specify these names with equivalent literal
string tokens:
%token <operator> OR "||"
%token <operator> LE 134 "<="
%left OR "<="
Once you equate the literal string and the token name, you can use them interchangeably in
further declarations or the grammar rules. The yylex function can use the token name or
the literal string to obtain the token type code number (see Section 4.2.1 [Calling Convention],
page 53).
3.7.2 Operator Precedence
Use the %left,%right or %nonassoc declaration to declare a token and specify its precedence
and associativity, all at once. These are called precedence declarations. See Section 5.3 [Operator
Precedence], page 61, for general information on operator precedence.
The syntax of a precedence declaration is the same as that of %token: either
%left symbols ...
or
%left <type >symbols ...
And indeed any of these declarations serves the purposes of %token. But in addition, they
specify the associativity and relative precedence for all the symbols:
•The associativity of an operator op determines how repeated uses of the operator nest:
whether ‘xopyopz’ is parsed by grouping xwith yfirst or by grouping ywith z
first. %left specifies left-associativity (grouping xwith yfirst) and %right specifies right-
associativity (grouping ywith zfirst). %nonassoc specifies no associativity, which means
that ‘xopyopz’ is considered a syntax error.
•The precedence of an operator determines how it nests with other operators. All the tokens
declared in a single precedence declaration have equal precedence and nest together accord-
ing to their associativity. When two tokens declared in different precedence declarations
associate, the one declared later has the higher precedence and is grouped first.
3.7.3 The Collection of Value Types
The %union declaration specifies the entire collection of possible data types for semantic values.
The keyword %union is followed by a pair of braces containing the same thing that goes inside
aunion in C.
For example:
%union {
double val;
symrec *tptr;
}
This says that the two alternative types are double and symrec *. They are given names val
and tptr; these names are used in the %token and %type declarations to pick one of the types
for a terminal or nonterminal symbol (see Section 3.7.4 [Nonterminal Symbols], page 47).
As an extension to POSIX, a tag is allowed after the union. For example:
Chapter 3: Bison Grammar Files 47
%union value {
double val;
symrec *tptr;
}
specifies the union tag value, so the corresponding C type is union value. If you do not
specify a tag, it defaults to YYSTYPE.
Note that, unlike making a union declaration in C, you need not write a semicolon after the
closing brace.
3.7.4 Nonterminal Symbols
When you use %union to specify multiple value types, you must declare the value type of each
nonterminal symbol for which values are used. This is done with a %type declaration, like this:
%type <type >nonterminal ...
Here nonterminal is the name of a nonterminal symbol, and type is the name given in the %union
to the alternative that you want (see Section 3.7.3 [The Collection of Value Types], page 46).
You can give any number of nonterminal symbols in the same %type declaration, if they have
the same value type. Use spaces to separate the symbol names.
You can also declare the value type of a terminal symbol. To do this, use the same <type >
construction in a declaration for the terminal symbol. All kinds of token declarations allow
<type >.
3.7.5 Freeing Discarded Symbols
Some symbols can be discarded by the parser, typically during error recovery (see Chapter 6
[Error Recovery], page 69). Basically, during error recovery, embarrassing symbols already
pushed on the stack, and embarrassing tokens coming from the rest of the file are thrown away
until the parser falls on its feet. If these symbols convey heap based information, this memory is
lost. While this behavior is tolerable for batch parsers, such as in compilers, it is unacceptable
for parsers that can possibility “never end” such as shells, or implementations of communication
protocols.
The %destructor directive allows for the definition of code that is called when a symbol is
thrown away.
[Directive]%destructor { code }symbols
Declare that the code must be invoked for each of the symbols that will be discarded by the
parser. The code should use $$ to designate the semantic value associated to the symbols.
The additional parser parameters are also avaible (see Section 4.1 [The Parser Function
yyparse], page 53).
Warning: as of Bison 1.875, this feature is still considered as experimental, as there was not
enough user feedback. In particular, the syntax might still change.
For instance:
%union
{
char *string;
}
%token <string> STRING
%type <string> string
%destructor { free ($$); } STRING string
guarantees that when a STRING or a string will be discarded, its associated memory will be
freed.
Note that in the future, Bison might also consider that right hand side members that are not
mentioned in the action can be destroyed. For instance, in:
48 Bison 1.875
comment: "/*" STRING "*/";
the parser is entitled to destroy the semantic value of the string. Of course, this will not apply
to the default action; compare:
typeless: string; // $$ = $1 does not apply; $1 is destroyed.
typefull: string; // $$ = $1 applies, $1 is not destroyed.
3.7.6 Suppressing Conflict Warnings
Bison normally warns if there are any conflicts in the grammar (see Section 5.2 [Shift/Reduce
Conflicts], page 60), but most real grammars have harmless shift/reduce conflicts which are
resolved in a predictable way and would be difficult to eliminate. It is desirable to suppress the
warning about these conflicts unless the number of conflicts changes. You can do this with the
%expect declaration.
The declaration looks like this:
%expect n
Here nis a decimal integer. The declaration says there should be no warning if there are n
shift/reduce conflicts and no reduce/reduce conflicts. An error, instead of the usual warning, is
given if there are either more or fewer conflicts, or if there are any reduce/reduce conflicts.
In general, using %expect involves these steps:
•Compile your grammar without %expect. Use the ‘-v’ option to get a verbose list of where
the conflicts occur. Bison will also print the number of conflicts.
•Check each of the conflicts to make sure that Bison’s default resolution is what you really
want. If not, rewrite the grammar and go back to the beginning.
•Add an %expect declaration, copying the number nfrom the number which Bison printed.
Now Bison will stop annoying you about the conflicts you have checked, but it will warn you
again if changes in the grammar result in additional conflicts.
3.7.7 The Start-Symbol
Bison assumes by default that the start symbol for the grammar is the first nonterminal specified
in the grammar specification section. The programmer may override this restriction with the
%start declaration as follows:
%start symbol
3.7.8 A Pure (Reentrant) Parser
Areentrant program is one which does not alter in the course of execution; in other words,
it consists entirely of pure (read-only) code. Reentrancy is important whenever asynchronous
execution is possible; for example, a non-reentrant program may not be safe to call from a signal
handler. In systems with multiple threads of control, a non-reentrant program must be called
only within interlocks.
Normally, Bison generates a parser which is not reentrant. This is suitable for most uses, and
it permits compatibility with Yacc. (The standard Yacc interfaces are inherently nonreentrant,
because they use statically allocated variables for communication with yylex, including yylval
and yylloc.)
Alternatively, you can generate a pure, reentrant parser. The Bison declaration %pure-
parser says that you want the parser to be reentrant. It looks like this:
%pure-parser
The result is that the communication variables yylval and yylloc become local variables
in yyparse, and a different calling convention is used for the lexical analyzer function yylex.
See Section 4.2.4 [Calling Conventions for Pure Parsers], page 55, for the details of this. The
Chapter 3: Bison Grammar Files 49
variable yynerrs also becomes local in yyparse (see Section 4.3 [The Error Reporting Function
yyerror], page 55). The convention for calling yyparse itself is unchanged.
Whether the parser is pure has nothing to do with the grammar rules. You can generate
either a pure parser or a nonreentrant parser from any valid grammar.
3.7.9 Bison Declaration Summary
Here is a summary of the declarations used to define a grammar:
[Directive]%union
Declare the collection of data types that semantic values may have (see Section 3.7.3 [The
Collection of Value Types], page 46).
[Directive]%token
Declare a terminal symbol (token type name) with no precedence or associativity specified
(see Section 3.7.1 [Token Type Names], page 45).
[Directive]%right
Declare a terminal symbol (token type name) that is right-associative (see Section 3.7.2
[Operator Precedence], page 46).
[Directive]%left
Declare a terminal symbol (token type name) that is left-associative (see Section 3.7.2 [Op-
erator Precedence], page 46).
[Directive]%nonassoc
Declare a terminal symbol (token type name) that is nonassociative (using it in a way that
would be associative is a syntax error)
(see Section 3.7.2 [Operator Precedence], page 46).
[Directive]%type
Declare the type of semantic values for a nonterminal symbol (see Section 3.7.4 [Nonterminal
Symbols], page 47).
[Directive]%start
Specify the grammar’s start symbol (see Section 3.7.7 [The Start-Symbol], page 48).
[Directive]%expect
Declare the expected number of shift-reduce conflicts (see Section 3.7.6 [Suppressing Conflict
Warnings], page 48).
In order to change the behavior of bison, use the following directives:
[Directive]%debug
In the parser file, define the macro YYDEBUG to 1 if it is not already defined, so that the
debugging facilities are compiled.
See Section 8.2 [Tracing Your Parser], page 80.
[Directive]%defines
Write an extra output file containing macro definitions for the token type names defined in the
grammar and the semantic value type YYSTYPE, as well as a few extern variable declarations.
If the parser output file is named ‘name.c’ then this file is named ‘name.h’.
This output file is essential if you wish to put the definition of yylex in a separate source
file, because yylex needs to be able to refer to token type codes and the variable yylval.
See Section 4.2.2 [Semantic Values of Tokens], page 54.
50 Bison 1.875
[Directive]%destructor
Specifying how the parser should reclaim the memory associated to discarded symbols. See
Section 3.7.5 [Freeing Discarded Symbols], page 47.
[Directive]%file-prefix="prefix "
Specify a prefix to use for all Bison output file names. The names are chosen as if the input
file were named ‘prefix.y’.
[Directive]%locations
Generate the code processing the locations (see Section 4.4 [Special Features for Use in
Actions], page 56). This mode is enabled as soon as the grammar uses the special ‘@n’
tokens, but if your grammar does not use it, using ‘%locations’ allows for more accurate
syntax error messages.
[Directive]%name-prefix="prefix "
Rename the external symbols used in the parser so that they start with prefix instead of ‘yy’.
The precise list of symbols renamed is yyparse,yylex,yyerror,yynerrs,yylval,yylloc,
yychar,yydebug, and possible yylloc. For example, if you use ‘%name-prefix="c_"’, the
names become c_parse,c_lex, and so on. See Section 3.8 [Multiple Parsers in the Same
Program], page 51.
[Directive]%no-parser
Do not include any C code in the parser file; generate tables only. The parser file contains
just #define directives and static variable declarations.
This option also tells Bison to write the C code for the grammar actions into a file named
‘filename.act’, in the form of a brace-surrounded body fit for a switch statement.
[Directive]%no-lines
Don’t generate any #line preprocessor commands in the parser file. Ordinarily Bison writes
these commands in the parser file so that the C compiler and debuggers will associate errors
and object code with your source file (the grammar file). This directive causes them to
associate errors with the parser file, treating it an independent source file in its own right.
[Directive]%output="filename "
Specify the filename for the parser file.
[Directive]%pure-parser
Request a pure (reentrant) parser program (see Section 3.7.8 [A Pure (Reentrant) Parser],
page 48).
[Directive]%token-table
Generate an array of token names in the parser file. The name of the array is yytname;
yytname[i]is the name of the token whose internal Bison token code number is i. The
first three elements of yytname correspond to the predefined tokens "$end","error", and
"$undefined"; after these come the symbols defined in the grammar file.
For single-character literal tokens and literal string tokens, the name in the table includes
the single-quote or double-quote characters: for example, "’+’" is a single-character literal
and "\"<=\"" is a literal string token. All the characters of the literal string token appear
verbatim in the string found in the table; even double-quote characters are not escaped. For
example, if the token consists of three characters ‘*"*’, its string in yytname contains ‘"*"*"’.
(In C, that would be written as "\"*\"*\"").
When you specify %token-table, Bison also generates macro definitions for macros
YYNTOKENS,YYNNTS, and YYNRULES, and YYNSTATES:
Chapter 3: Bison Grammar Files 51
YYNTOKENS
The highest token number, plus one.
YYNNTS The number of nonterminal symbols.
YYNRULES The number of grammar rules,
YYNSTATES
The number of parser states (see Section 5.5 [Parser States], page 63).
[Directive]%verbose
Write an extra output file containing verbose descriptions of the parser states and what is
done for each type of look-ahead token in that state. See Section 8.1 [Understanding Your
Parser], page 75, for more information.
[Directive]%yacc
Pretend the option ‘--yacc’ was given, i.e., imitate Yacc, including its naming conventions.
See Section 9.1 [Bison Options], page 83, for more.
3.8 Multiple Parsers in the Same Program
Most programs that use Bison parse only one language and therefore contain only one Bison
parser. But what if you want to parse more than one language with the same program? Then
you need to avoid a name conflict between different definitions of yyparse,yylval, and so on.
The easy way to do this is to use the option ‘-p prefix ’ (see Chapter 9 [Invoking Bison],
page 83). This renames the interface functions and variables of the Bison parser to start with
prefix instead of ‘yy’. You can use this to give each parser distinct names that do not conflict.
The precise list of symbols renamed is yyparse,yylex,yyerror,yynerrs,yylval,yylloc,
yychar and yydebug. For example, if you use ‘-p c’, the names become cparse,clex, and so
on.
All the other variables and macros associated with Bison are not renamed. These others
are not global; there is no conflict if the same name is used in different parsers. For example,
YYSTYPE is not renamed, but defining this in different ways in different parsers causes no trouble
(see Section 3.5.1 [Data Types of Semantic Values], page 39).
The ‘-p’ option works by adding macro definitions to the beginning of the parser source file,
defining yyparse as prefix parse, and so on. This effectively substitutes one name for the other
in the entire parser file.
52 Bison 1.875
Chapter 4: Parser C-Language Interface 53
4 Parser C-Language Interface
The Bison parser is actually a C function named yyparse. Here we describe the interface
conventions of yyparse and the other functions that it needs to use.
Keep in mind that the parser uses many C identifiers starting with ‘yy’ and ‘YY’ for internal
purposes. If you use such an identifier (aside from those in this manual) in an action or in
epilogue in the grammar file, you are likely to run into trouble.
4.1 The Parser Function yyparse
You call the function yyparse to cause parsing to occur. This function reads tokens, executes
actions, and ultimately returns when it encounters end-of-input or an unrecoverable syntax
error. You can also write an action which directs yyparse to return immediately without
reading further.
[Function]int yyparse (void)
The value returned by yyparse is 0 if parsing was successful (return is due to end-of-input).
The value is 1 if parsing failed (return is due to a syntax error).
In an action, you can cause immediate return from yyparse by using these macros:
[Macro]YYACCEPT
Return immediately with value 0 (to report success).
[Macro]YYABORT
Return immediately with value 1 (to report failure).
4.2 The Lexical Analyzer Function yylex
The lexical analyzer function, yylex, recognizes tokens from the input stream and returns them
to the parser. Bison does not create this function automatically; you must write it so that
yyparse can call it. The function is sometimes referred to as a lexical scanner.
In simple programs, yylex is often defined at the end of the Bison grammar file. If yylex is
defined in a separate source file, you need to arrange for the token-type macro definitions to be
available there. To do this, use the ‘-d’ option when you run Bison, so that it will write these
macro definitions into a separate header file ‘name.tab.h’ which you can include in the other
source files that need it. See Chapter 9 [Invoking Bison], page 83.
4.2.1 Calling Convention for yylex
The value that yylex returns must be the positive numeric code for the type of token it has just
found; a zero or negative value signifies end-of-input.
When a token is referred to in the grammar rules by a name, that name in the parser file
becomes a C macro whose definition is the proper numeric code for that token type. So yylex
can use the name to indicate that type. See Section 3.2 [Symbols], page 36.
When a token is referred to in the grammar rules by a character literal, the numeric code for
that character is also the code for the token type. So yylex can simply return that character
code, possibly converted to unsigned char to avoid sign-extension. The null character must not
be used this way, because its code is zero and that signifies end-of-input.
Here is an example showing these things:
int
yylex (void)
{
54 Bison 1.875
...
if (c == EOF) /* Detect end-of-input. */
return 0;
...
if (c == ’+’ || c == ’-’)
return c; /* Assume token type for ‘+’ is ’+’. */
...
return INT; /* Return the type of the token. */
...
}
This interface has been designed so that the output from the lex utility can be used without
change as the definition of yylex.
If the grammar uses literal string tokens, there are two ways that yylex can determine the
token type codes for them:
•If the grammar defines symbolic token names as aliases for the literal string tokens, yylex
can use these symbolic names like all others. In this case, the use of the literal string tokens
in the grammar file has no effect on yylex.
•yylex can find the multicharacter token in the yytname table. The index of the token
in the table is the token type’s code. The name of a multicharacter token is recorded
in yytname with a double-quote, the token’s characters, and another double-quote. The
token’s characters are not escaped in any way; they appear verbatim in the contents of the
string in the table.
Here’s code for looking up a token in yytname, assuming that the characters of the token
are stored in token_buffer.
for (i = 0; i < YYNTOKENS; i++)
{
if (yytname[i] != 0
&& yytname[i][0] == ’"’
&& ! strncmp (yytname[i] + 1, token_buffer,
strlen (token_buffer))
&& yytname[i][strlen (token_buffer) + 1] == ’"’
&& yytname[i][strlen (token_buffer) + 2] == 0)
break;
}
The yytname table is generated only if you use the %token-table declaration. See Sec-
tion 3.7.9 [Decl Summary], page 49.
4.2.2 Semantic Values of Tokens
In an ordinary (non-reentrant) parser, the semantic value of the token must be stored into the
global variable yylval. When you are using just one data type for semantic values, yylval has
that type. Thus, if the type is int (the default), you might write this in yylex:
...
yylval = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
When you are using multiple data types, yylval’s type is a union made from the %union
declaration (see Section 3.7.3 [The Collection of Value Types], page 46). So when you store a
token’s value, you must use the proper member of the union. If the %union declaration looks
like this:
Chapter 4: Parser C-Language Interface 55
%union {
int intval;
double val;
symrec *tptr;
}
then the code in yylex might look like this:
...
yylval.intval = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
4.2.3 Textual Positions of Tokens
If you are using the ‘@n’-feature (see Section 3.6 [Tracking Locations], page 43) in actions to keep
track of the textual locations of tokens and groupings, then you must provide this information
in yylex. The function yyparse expects to find the textual location of a token just parsed in
the global variable yylloc. So yylex must store the proper data in that variable.
By default, the value of yylloc is a structure and you need only initialize the members that
are going to be used by the actions. The four members are called first_line,first_column,
last_line and last_column. Note that the use of this feature makes the parser noticeably
slower.
The data type of yylloc has the name YYLTYPE.
4.2.4 Calling Conventions for Pure Parsers
When you use the Bison declaration %pure-parser to request a pure, reentrant parser, the
global communication variables yylval and yylloc cannot be used. (See Section 3.7.8 [A Pure
(Reentrant) Parser], page 48.) In such parsers the two global variables are replaced by pointers
passed as arguments to yylex. You must declare them as shown here, and pass the information
back by storing it through those pointers.
int
yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
{
...
*lvalp = value; /* Put value onto Bison stack. */
return INT; /* Return the type of the token. */
...
}
If the grammar file does not use the ‘@’ constructs to refer to textual positions, then the type
YYLTYPE will not be defined. In this case, omit the second argument; yylex will be called with
only one argument.
4.3 The Error Reporting Function yyerror
The Bison parser detects a syntax error or parse error whenever it reads a token which cannot
satisfy any syntax rule. An action in the grammar can also explicitly proclaim an error, using
the macro YYERROR (see Section 4.4 [Special Features for Use in Actions], page 56).
The Bison parser expects to report the error by calling an error reporting function named
yyerror, which you must supply. It is called by yyparse whenever a syntax error is found, and
it receives one argument. For a syntax error, the string is normally "syntax error".
If you invoke the directive %error-verbose in the Bison declarations section (see Section 3.1.2
[The Bison Declarations Section], page 35), then Bison provides a more verbose and specific error
message string instead of just plain "syntax error".
56 Bison 1.875
The parser can detect one other kind of error: stack overflow. This happens when the
input contains constructions that are very deeply nested. It isn’t likely you will encounter
this, since the Bison parser extends its stack automatically up to a very large limit. But if
overflow happens, yyparse calls yyerror in the usual fashion, except that the argument string
is "parser stack overflow".
The following definition suffices in simple programs:
void
yyerror (char const *s)
{
fprintf (stderr, "%s\n", s);
}
After yyerror returns to yyparse, the latter will attempt error recovery if you have written
suitable error recovery grammar rules (see Chapter 6 [Error Recovery], page 69). If recovery is
impossible, yyparse will immediately return 1.
Obviously, in location tracking pure parsers, yyerror should have an access to the current
location. This is indeed the case for the GLR parsers, but not for the Yacc parser, for historical
reasons. I.e., if ‘%locations %pure-parser’ is passed then the prototypes for yyerror are:
void yyerror (char const *msg); /* Yacc parsers. */
void yyerror (YYLTYPE *locp, char const *msg); /* GLR parsers. */
The prototypes are only indications of how the code produced by Bison uses yyerror. Bison-
generated code always ignores the returned value, so yyerror can return any type, including
void. Also, yyerror can be a variadic function; that is why the message is always passed last.
Traditionally yyerror returns an int that is always ignored, but this is purely for historical
reasons, and void is preferable since it more accurately describes the return type for yyerror.
The variable yynerrs contains the number of syntax errors encountered so far. Normally
this variable is global; but if you request a pure parser (see Section 3.7.8 [A Pure (Reentrant)
Parser], page 48) then it is a local variable which only the actions can access.
4.4 Special Features for Use in Actions
Here is a table of Bison constructs, variables and macros that are useful in actions.
[Variable]$$
Acts like a variable that contains the semantic value for the grouping made by the current
rule. See Section 3.5.3 [Actions], page 40.
[Variable]$n
Acts like a variable that contains the semantic value for the nth component of the current
rule. See Section 3.5.3 [Actions], page 40.
[Variable]$<typealt >$
Like $$ but specifies alternative typealt in the union specified by the %union declaration. See
Section 3.5.4 [Data Types of Values in Actions], page 41.
[Variable]$<typealt >n
Like $nbut specifies alternative typealt in the union specified by the %union declaration.
See Section 3.5.4 [Data Types of Values in Actions], page 41.
[Macro]YYABORT;
Return immediately from yyparse, indicating failure. See Section 4.1 [The Parser Function
yyparse], page 53.
Chapter 4: Parser C-Language Interface 57
[Macro]YYACCEPT;
Return immediately from yyparse, indicating success. See Section 4.1 [The Parser Function
yyparse], page 53.
[Macro]YYBACKUP (token,value );
Unshift a token. This macro is allowed only for rules that reduce a single value, and only
when there is no look-ahead token. It is also disallowed in GLR parsers. It installs a look-
ahead token with token type token and semantic value value; then it discards the value that
was going to be reduced by this rule.
If the macro is used when it is not valid, such as when there is a look-ahead token already,
then it reports a syntax error with a message ‘cannot back up’ and performs ordinary error
recovery.
In either case, the rest of the action is not executed.
[Macro]YYEMPTY
Value stored in yychar when there is no look-ahead token.
[Macro]YYERROR;
Cause an immediate syntax error. This statement initiates error recovery just as if the
parser itself had detected an error; however, it does not call yyerror, and does not print any
message. If you want to print an error message, call yyerror explicitly before the ‘YYERROR;’
statement. See Chapter 6 [Error Recovery], page 69.
[Macro]YYRECOVERING
This macro stands for an expression that has the value 1 when the parser is recovering from
a syntax error, and 0 the rest of the time. See Chapter 6 [Error Recovery], page 69.
[Variable]yychar
Variable containing the current look-ahead token. (In a pure parser, this is actually a local
variable within yyparse.) When there is no look-ahead token, the value YYEMPTY is stored
in the variable. See Section 5.1 [Look-Ahead Tokens], page 59.
[Macro]yyclearin;
Discard the current look-ahead token. This is useful primarily in error rules. See Chapter 6
[Error Recovery], page 69.
[Macro]yyerrok;
Resume generating error messages immediately for subsequent syntax errors. This is useful
primarily in error rules. See Chapter 6 [Error Recovery], page 69.
[Value]@$
Acts like a structure variable containing information on the textual position of the grouping
made by the current rule. See Section 3.6 [Tracking Locations], page 43.
[Value]@n
Acts like a structure variable containing information on the textual position of the nth com-
ponent of the current rule. See Section 3.6 [Tracking Locations], page 43.
58 Bison 1.875
Chapter 5: The Bison Parser Algorithm 59
5 The Bison Parser Algorithm
As Bison reads tokens, it pushes them onto a stack along with their semantic values. The stack
is called the parser stack. Pushing a token is traditionally called shifting.
For example, suppose the infix calculator has read ‘1 + 5 *’, with a ‘3’ to come. The stack
will have four elements, one for each token that was shifted.
But the stack does not always have an element for each token read. When the last ntokens
and groupings shifted match the components of a grammar rule, they can be combined according
to that rule. This is called reduction. Those tokens and groupings are replaced on the stack by
a single grouping whose symbol is the result (left hand side) of that rule. Running the rule’s
action is part of the process of reduction, because this is what computes the semantic value of
the resulting grouping.
For example, if the infix calculator’s parser stack contains this:
1+5*3
and the next input token is a newline character, then the last three elements can be reduced to
15 via the rule:
expr: expr ’*’ expr;
Then the stack contains just these three elements:
1 + 15
At this point, another reduction can be made, resulting in the single value 16. Then the newline
token can be shifted.
The parser tries, by shifts and reductions, to reduce the entire input down to a single grouping
whose symbol is the grammar’s start-symbol (see Section 1.1 [Languages and Context-Free
Grammars], page 11).
This kind of parser is known in the literature as a bottom-up parser.
5.1 Look-Ahead Tokens
The Bison parser does not always reduce immediately as soon as the last ntokens and groupings
match a rule. This is because such a simple strategy is inadequate to handle most languages.
Instead, when a reduction is possible, the parser sometimes “looks ahead” at the next token in
order to decide what to do.
When a token is read, it is not immediately shifted; first it becomes the look-ahead token,
which is not on the stack. Now the parser can perform one or more reductions of tokens and
groupings on the stack, while the look-ahead token remains off to the side. When no more
reductions should take place, the look-ahead token is shifted onto the stack. This does not mean
that all possible reductions have been done; depending on the token type of the look-ahead
token, some rules may choose to delay their application.
Here is a simple case where look-ahead is needed. These three rules define expressions
which contain binary addition operators and postfix unary factorial operators (‘!’), and allow
parentheses for grouping.
expr: term ’+’ expr
| term
;
term: ’(’ expr ’)’
| term ’!’
| NUMBER
;
60 Bison 1.875
Suppose that the tokens ‘1 + 2’ have been read and shifted; what should be done? If the
following token is ‘)’, then the first three tokens must be reduced to form an expr. This is the
only valid course, because shifting the ‘)’ would produce a sequence of symbols term ’)’, and
no rule allows this.
If the following token is ‘!’, then it must be shifted immediately so that ‘2 !’ can be reduced
to make a term. If instead the parser were to reduce before shifting, ‘1 + 2’ would become an
expr. It would then be impossible to shift the ‘!’ because doing so would produce on the stack
the sequence of symbols expr ’!’. No rule allows that sequence.
The current look-ahead token is stored in the variable yychar. See Section 4.4 [Special
Features for Use in Actions], page 56.
5.2 Shift/Reduce Conflicts
Suppose we are parsing a language which has if-then and if-then-else statements, with a pair of
rules like this:
if_stmt:
IF expr THEN stmt
| IF expr THEN stmt ELSE stmt
;
Here we assume that IF,THEN and ELSE are terminal symbols for specific keyword tokens.
When the ELSE token is read and becomes the look-ahead token, the contents of the stack
(assuming the input is valid) are just right for reduction by the first rule. But it is also legitimate
to shift the ELSE, because that would lead to eventual reduction by the second rule.
This situation, where either a shift or a reduction would be valid, is called a shift/reduce
conflict. Bison is designed to resolve these conflicts by choosing to shift, unless otherwise directed
by operator precedence declarations. To see the reason for this, let’s contrast it with the other
alternative.
Since the parser prefers to shift the ELSE, the result is to attach the else-clause to the
innermost if-statement, making these two inputs equivalent:
if x then if y then win (); else lose;
if x then do; if y then win (); else lose; end;
But if the parser chose to reduce when possible rather than shift, the result would be to
attach the else-clause to the outermost if-statement, making these two inputs equivalent:
if x then if y then win (); else lose;
if x then do; if y then win (); end; else lose;
The conflict exists because the grammar as written is ambiguous: either parsing of the
simple nested if-statement is legitimate. The established convention is that these ambiguities
are resolved by attaching the else-clause to the innermost if-statement; this is what Bison ac-
complishes by choosing to shift rather than reduce. (It would ideally be cleaner to write an
unambiguous grammar, but that is very hard to do in this case.) This particular ambiguity was
first encountered in the specifications of Algol 60 and is called the “dangling else” ambiguity.
To avoid warnings from Bison about predictable, legitimate shift/reduce conflicts, use the
%expect ndeclaration. There will be no warning as long as the number of shift/reduce conflicts
is exactly n. See Section 3.7.6 [Suppressing Conflict Warnings], page 48.
The definition of if_stmt above is solely to blame for the conflict, but the conflict does
not actually appear without additional rules. Here is a complete Bison input file that actually
manifests the conflict:
Chapter 5: The Bison Parser Algorithm 61
%token IF THEN ELSE variable
%%
stmt: expr
| if_stmt
;
if_stmt:
IF expr THEN stmt
| IF expr THEN stmt ELSE stmt
;
expr: variable
;
5.3 Operator Precedence
Another situation where shift/reduce conflicts appear is in arithmetic expressions. Here shifting
is not always the preferred resolution; the Bison declarations for operator precedence allow you
to specify when to shift and when to reduce.
5.3.1 When Precedence is Needed
Consider the following ambiguous grammar fragment (ambiguous because the input ‘1-2*3’
can be parsed in two different ways):
expr: expr ’-’ expr
| expr ’*’ expr
| expr ’<’ expr
| ’(’ expr ’)’
...
;
Suppose the parser has seen the tokens ‘1’, ‘-’ and ‘2’; should it reduce them via the rule for
the subtraction operator? It depends on the next token. Of course, if the next token is ‘)’, we
must reduce; shifting is invalid because no single rule can reduce the token sequence ‘- 2 )’ or
anything starting with that. But if the next token is ‘*’ or ‘<’, we have a choice: either shifting
or reduction would allow the parse to complete, but with different results.
To decide which one Bison should do, we must consider the results. If the next operator token
op is shifted, then it must be reduced first in order to permit another opportunity to reduce
the difference. The result is (in effect) ‘1 - (2 op 3)’. On the other hand, if the subtraction
is reduced before shifting op, the result is ‘(1 - 2) op 3’. Clearly, then, the choice of shift or
reduce should depend on the relative precedence of the operators ‘-’ and op: ‘*’ should be
shifted first, but not ‘<’.
What about input such as ‘1-2-5’; should this be ‘(1 - 2) - 5’ or should it be
‘1 - (2 - 5)’? For most operators we prefer the former, which is called left association. The
latter alternative, right association, is desirable for assignment operators. The choice of left or
right association is a matter of whether the parser chooses to shift or reduce when the stack
contains ‘1 - 2’ and the look-ahead token is ‘-’: shifting makes right-associativity.
5.3.2 Specifying Operator Precedence
Bison allows you to specify these choices with the operator precedence declarations %left and
%right. Each such declaration contains a list of tokens, which are operators whose prece-
dence and associativity is being declared. The %left declaration makes all those operators
62 Bison 1.875
left-associative and the %right declaration makes them right-associative. A third alternative is
%nonassoc, which declares that it is a syntax error to find the same operator twice “in a row”.
The relative precedence of different operators is controlled by the order in which they are
declared. The first %left or %right declaration in the file declares the operators whose prece-
dence is lowest, the next such declaration declares the operators whose precedence is a little
higher, and so on.
5.3.3 Precedence Examples
In our example, we would want the following declarations:
%left ’<’
%left ’-’
%left ’*’
In a more complete example, which supports other operators as well, we would declare them
in groups of equal precedence. For example, ’+’ is declared with ’-’:
%left ’<’ ’>’ ’=’ NE LE GE
%left ’+’ ’-’
%left ’*’ ’/’
(Here NE and so on stand for the operators for “not equal” and so on. We assume that these
tokens are more than one character long and therefore are represented by names, not character
literals.)
5.3.4 How Precedence Works
The first effect of the precedence declarations is to assign precedence levels to the terminal
symbols declared. The second effect is to assign precedence levels to certain rules: each rule
gets its precedence from the last terminal symbol mentioned in the components. (You can also
specify explicitly the precedence of a rule. See Section 5.4 [Context-Dependent Precedence],
page 62.)
Finally, the resolution of conflicts works by comparing the precedence of the rule being
considered with that of the look-ahead token. If the token’s precedence is higher, the choice is
to shift. If the rule’s precedence is higher, the choice is to reduce. If they have equal precedence,
the choice is made based on the associativity of that precedence level. The verbose output file
made by ‘-v’ (see Chapter 9 [Invoking Bison], page 83) says how each conflict was resolved.
Not all rules and not all tokens have precedence. If either the rule or the look-ahead token
has no precedence, then the default is to shift.
5.4 Context-Dependent Precedence
Often the precedence of an operator depends on the context. This sounds outlandish at first,
but it is really very common. For example, a minus sign typically has a very high precedence
as a unary operator, and a somewhat lower precedence (lower than multiplication) as a binary
operator.
The Bison precedence declarations, %left,%right and %nonassoc, can only be used once for
a given token; so a token has only one precedence declared in this way. For context-dependent
precedence, you need to use an additional mechanism: the %prec modifier for rules.
The %prec modifier declares the precedence of a particular rule by specifying a terminal
symbol whose precedence should be used for that rule. It’s not necessary for that symbol to
appear otherwise in the rule. The modifier’s syntax is:
%prec terminal-symbol
and it is written after the components of the rule. Its effect is to assign the rule the precedence
of terminal-symbol, overriding the precedence that would be deduced for it in the ordinary
Chapter 5: The Bison Parser Algorithm 63
way. The altered rule precedence then affects how conflicts involving that rule are resolved (see
Section 5.3 [Operator Precedence], page 61).
Here is how %prec solves the problem of unary minus. First, declare a precedence for a
fictitious terminal symbol named UMINUS. There are no tokens of this type, but the symbol
serves to stand for its precedence:
...
%left ’+’ ’-’
%left ’*’
%left UMINUS
Now the precedence of UMINUS can be used in specific rules:
exp: ...
| exp ’-’ exp
...
| ’-’ exp %prec UMINUS
5.5 Parser States
The function yyparse is implemented using a finite-state machine. The values pushed on the
parser stack are not simply token type codes; they represent the entire sequence of terminal
and nonterminal symbols at or near the top of the stack. The current state collects all the
information about previous input which is relevant to deciding what to do next.
Each time a look-ahead token is read, the current parser state together with the type of look-
ahead token are looked up in a table. This table entry can say, “Shift the look-ahead token.” In
this case, it also specifies the new parser state, which is pushed onto the top of the parser stack.
Or it can say, “Reduce using rule number n.” This means that a certain number of tokens or
groupings are taken off the top of the stack, and replaced by one grouping. In other words, that
number of states are popped from the stack, and one new state is pushed.
There is one other alternative: the table can say that the look-ahead token is erroneous in the
current state. This causes error processing to begin (see Chapter 6 [Error Recovery], page 69).
5.6 Reduce/Reduce Conflicts
A reduce/reduce conflict occurs if there are two or more rules that apply to the same sequence
of input. This usually indicates a serious error in the grammar.
For example, here is an erroneous attempt to define a sequence of zero or more word groupings.
sequence: /* empty */
{ printf ("empty sequence\n"); }
| maybeword
| sequence word
{ printf ("added word %s\n", $2); }
;
maybeword: /* empty */
{ printf ("empty maybeword\n"); }
| word
{ printf ("single word %s\n", $1); }
;
The error is an ambiguity: there is more than one way to parse a single word into a sequence.
It could be reduced to a maybeword and then into a sequence via the second rule. Alternatively,
nothing-at-all could be reduced into a sequence via the first rule, and this could be combined
with the word using the third rule for sequence.
64 Bison 1.875
There is also more than one way to reduce nothing-at-all into a sequence. This can be done
directly via the first rule, or indirectly via maybeword and then the second rule.
You might think that this is a distinction without a difference, because it does not change
whether any particular input is valid or not. But it does affect which actions are run. One
parsing order runs the second rule’s action; the other runs the first rule’s action and the third
rule’s action. In this example, the output of the program changes.
Bison resolves a reduce/reduce conflict by choosing to use the rule that appears first in the
grammar, but it is very risky to rely on this. Every reduce/reduce conflict must be studied and
usually eliminated. Here is the proper way to define sequence:
sequence: /* empty */
{ printf ("empty sequence\n"); }
| sequence word
{ printf ("added word %s\n", $2); }
;
Here is another common error that yields a reduce/reduce conflict:
sequence: /* empty */
| sequence words
| sequence redirects
;
words: /* empty */
| words word
;
redirects:/* empty */
| redirects redirect
;
The intention here is to define a sequence which can contain either word or redirect groupings.
The individual definitions of sequence,words and redirects are error-free, but the three
together make a subtle ambiguity: even an empty input can be parsed in infinitely many ways!
Consider: nothing-at-all could be a words. Or it could be two words in a row, or three, or
any number. It could equally well be a redirects, or two, or any number. Or it could be a
words followed by three redirects and another words. And so on.
Here are two ways to correct these rules. First, to make it a single level of sequence:
sequence: /* empty */
| sequence word
| sequence redirect
;
Second, to prevent either a words or a redirects from being empty:
sequence: /* empty */
| sequence words
| sequence redirects
;
words: word
| words word
;
redirects:redirect
Chapter 5: The Bison Parser Algorithm 65
| redirects redirect
;
5.7 Mysterious Reduce/Reduce Conflicts
Sometimes reduce/reduce conflicts can occur that don’t look warranted. Here is an example:
%token ID
%%
def: param_spec return_spec ’,’
;
param_spec:
type
| name_list ’:’ type
;
return_spec:
type
| name ’:’ type
;
type: ID
;
name: ID
;
name_list:
name
| name ’,’ name_list
;
It would seem that this grammar can be parsed with only a single token of look-ahead: when
aparam_spec is being read, an ID is a name if a comma or colon follows, or a type if another
ID follows. In other words, this grammar is LR(1).
However, Bison, like most parser generators, cannot actually handle all LR(1) grammars. In
this grammar, two contexts, that after an ID at the beginning of a param_spec and likewise at
the beginning of a return_spec, are similar enough that Bison assumes they are the same. They
appear similar because the same set of rules would be active—the rule for reducing to a name and
that for reducing to a type. Bison is unable to determine at that stage of processing that the
rules would require different look-ahead tokens in the two contexts, so it makes a single parser
state for them both. Combining the two contexts causes a conflict later. In parser terminology,
this occurrence means that the grammar is not LALR(1).
In general, it is better to fix deficiencies than to document them. But this particular deficiency
is intrinsically hard to fix; parser generators that can handle LR(1) grammars are hard to write
and tend to produce parsers that are very large. In practice, Bison is more useful as it is now.
When the problem arises, you can often fix it by identifying the two parser states that are
being confused, and adding something to make them look distinct. In the above example, adding
one rule to return_spec as follows makes the problem go away:
66 Bison 1.875
%token BOGUS
...
%%
...
return_spec:
type
| name ’:’ type
/* This rule is never used. */
| ID BOGUS
;
This corrects the problem because it introduces the possibility of an additional active rule
in the context after the ID at the beginning of return_spec. This rule is not active in the
corresponding context in a param_spec, so the two contexts receive distinct parser states. As
long as the token BOGUS is never generated by yylex, the added rule cannot alter the way actual
input is parsed.
In this particular example, there is another way to solve the problem: rewrite the rule for
return_spec to use ID directly instead of via name. This also causes the two confusing contexts
to have different sets of active rules, because the one for return_spec activates the altered rule
for return_spec rather than the one for name.
param_spec:
type
| name_list ’:’ type
;
return_spec:
type
| ID ’:’ type
;
5.8 Generalized LR (GLR) Parsing
Bison produces deterministic parsers that choose uniquely when to reduce and which reduction
to apply based on a summary of the preceding input and on one extra token of lookahead. As a
result, normal Bison handles a proper subset of the family of context-free languages. Ambiguous
grammars, since they have strings with more than one possible sequence of reductions cannot
have deterministic parsers in this sense. The same is true of languages that require more than
one symbol of lookahead, since the parser lacks the information necessary to make a decision
at the point it must be made in a shift-reduce parser. Finally, as previously mentioned (see
Section 5.7 [Mystery Conflicts], page 65), there are languages where Bison’s particular choice of
how to summarize the input seen so far loses necessary information.
When you use the ‘%glr-parser’ declaration in your grammar file, Bison generates a parser
that uses a different algorithm, called Generalized LR (or GLR). A Bison GLR parser uses
the same basic algorithm for parsing as an ordinary Bison parser, but behaves differently in
cases where there is a shift-reduce conflict that has not been resolved by precedence rules (see
Section 5.3 [Precedence], page 61) or a reduce-reduce conflict. When a GLR parser encounters
such a situation, it effectively splits into a several parsers, one for each possible shift or reduction.
These parsers then proceed as usual, consuming tokens in lock-step. Some of the stacks may
encounter other conflicts and split further, with the result that instead of a sequence of states,
a Bison GLR parsing stack is what is in effect a tree of states.
In effect, each stack represents a guess as to what the proper parse is. Additional input
may indicate that a guess was wrong, in which case the appropriate stack silently disappears.
Otherwise, the semantics actions generated in each stack are saved, rather than being executed
Chapter 5: The Bison Parser Algorithm 67
immediately. When a stack disappears, its saved semantic actions never get executed. When a
reduction causes two stacks to become equivalent, their sets of semantic actions are both saved
with the state that results from the reduction. We say that two stacks are equivalent when they
both represent the same sequence of states, and each pair of corresponding states represents a
grammar symbol that produces the same segment of the input token stream.
Whenever the parser makes a transition from having multiple states to having one, it reverts
to the normal LALR(1) parsing algorithm, after resolving and executing the saved-up actions. At
this transition, some of the states on the stack will have semantic values that are sets (actually
multisets) of possible actions. The parser tries to pick one of the actions by first finding one
whose rule has the highest dynamic precedence, as set by the ‘%dprec’ declaration. Otherwise,
if the alternative actions are not ordered by precedence, but there the same merging function is
declared for both rules by the ‘%merge’ declaration, Bison resolves and evaluates both and then
calls the merge function on the result. Otherwise, it reports an ambiguity.
It is possible to use a data structure for the GLR parsing tree that permits the processing of
any LALR(1) grammar in linear time (in the size of the input), any unambiguous (not necessarily
LALR(1)) grammar in quadratic worst-case time, and any general (possibly ambiguous) context-
free grammar in cubic worst-case time. However, Bison currently uses a simpler data structure
that requires time proportional to the length of the input times the maximum number of stacks
required for any prefix of the input. Thus, really ambiguous or non-deterministic grammars can
require exponential time and space to process. Such badly behaving examples, however, are not
generally of practical interest. Usually, non-determinism in a grammar is local—the parser is “in
doubt” only for a few tokens at a time. Therefore, the current data structure should generally
be adequate. On LALR(1) portions of a grammar, in particular, it is only slightly slower than
with the default Bison parser.
5.9 Stack Overflow, and How to Avoid It
The Bison parser stack can overflow if too many tokens are shifted and not reduced. When this
happens, the parser function yyparse returns a nonzero value, pausing only to call yyerror to
report the overflow.
Because Bison parsers have growing stacks, hitting the upper limit usually results from using
a right recursion instead of a left recursion, See Section 3.4 [Recursive Rules], page 38.
By defining the macro YYMAXDEPTH, you can control how deep the parser stack can become
before a stack overflow occurs. Define the macro with a value that is an integer. This value is
the maximum number of tokens that can be shifted (and not reduced) before overflow. It must
be a constant expression whose value is known at compile time.
The stack space allowed is not necessarily allocated. If you specify a large value for
YYMAXDEPTH, the parser actually allocates a small stack at first, and then makes it bigger by
stages as needed. This increasing allocation happens automatically and silently. Therefore, you
do not need to make YYMAXDEPTH painfully small merely to save space for ordinary inputs that
do not need much stack.
The default value of YYMAXDEPTH, if you do not define it, is 10000.
You can control how much stack is allocated initially by defining the macro YYINITDEPTH.
This value too must be a compile-time constant integer. The default is 200.
Because of semantical differences between C and C++, the LALR(1) parsers in C produced by
Bison by compiled as C++ cannot grow. In this precise case (compiling a C parser as C++) you
are suggested to grow YYINITDEPTH. In the near future, a C++ output output will be provided
which addresses this issue.
68 Bison 1.875
Chapter 6: Error Recovery 69
6 Error Recovery
It is not usually acceptable to have a program terminate on a syntax error. For example, a
compiler should recover sufficiently to parse the rest of the input file and check it for errors; a
calculator should accept another expression.
In a simple interactive command parser where each input is one line, it may be sufficient to
allow yyparse to return 1 on error and have the caller ignore the rest of the input line when
that happens (and then call yyparse again). But this is inadequate for a compiler, because it
forgets all the syntactic context leading up to the error. A syntax error deep within a function
in the compiler input should not cause the compiler to treat the following line like the beginning
of a source file.
You can define how to recover from a syntax error by writing rules to recognize the special
token error. This is a terminal symbol that is always defined (you need not declare it) and
reserved for error handling. The Bison parser generates an error token whenever a syntax error
happens; if you have provided a rule to recognize this token in the current context, the parse
can continue.
For example:
stmnts: /* empty string */
| stmnts ’\n’
| stmnts exp ’\n’
| stmnts error ’\n’
The fourth rule in this example says that an error followed by a newline makes a valid addition
to any stmnts.
What happens if a syntax error occurs in the middle of an exp? The error recovery rule,
interpreted strictly, applies to the precise sequence of a stmnts, an error and a newline. If
an error occurs in the middle of an exp, there will probably be some additional tokens and
subexpressions on the stack after the last stmnts, and there will be tokens to read before the
next newline. So the rule is not applicable in the ordinary way.
But Bison can force the situation to fit the rule, by discarding part of the semantic context
and part of the input. First it discards states and objects from the stack until it gets back to
a state in which the error token is acceptable. (This means that the subexpressions already
parsed are discarded, back to the last complete stmnts.) At this point the error token can be
shifted. Then, if the old look-ahead token is not acceptable to be shifted next, the parser reads
tokens and discards them until it finds a token which is acceptable. In this example, Bison reads
and discards input until the next newline so that the fourth rule can apply. Note that discarded
symbols are possible sources of memory leaks, see Section 3.7.5 [Freeing Discarded Symbols],
page 47, for a means to reclaim this memory.
The choice of error rules in the grammar is a choice of strategies for error recovery. A simple
and useful strategy is simply to skip the rest of the current input line or current statement if an
error is detected:
stmnt: error ’;’ /* On error, skip until ’;’ is read. */
It is also useful to recover to the matching close-delimiter of an opening-delimiter that has
already been parsed. Otherwise the close-delimiter will probably appear to be unmatched, and
generate another, spurious error message:
primary: ’(’ expr ’)’
| ’(’ error ’)’
...
;
70 Bison 1.875
Error recovery strategies are necessarily guesses. When they guess wrong, one syntax error
often leads to another. In the above example, the error recovery rule guesses that an error is
due to bad input within one stmnt. Suppose that instead a spurious semicolon is inserted in
the middle of a valid stmnt. After the error recovery rule recovers from the first error, another
syntax error will be found straightaway, since the text following the spurious semicolon is also
an invalid stmnt.
To prevent an outpouring of error messages, the parser will output no error message for
another syntax error that happens shortly after the first; only after three consecutive input
tokens have been successfully shifted will error messages resume.
Note that rules which accept the error token may have actions, just as any other rules can.
You can make error messages resume immediately by using the macro yyerrok in an action.
If you do this in the error rule’s action, no error messages will be suppressed. This macro
requires no arguments; ‘yyerrok;’ is a valid C statement.
The previous look-ahead token is reanalyzed immediately after an error. If this is unac-
ceptable, then the macro yyclearin may be used to clear this token. Write the statement
‘yyclearin;’ in the error rule’s action.
For example, suppose that on a syntax error, an error handling routine is called that advances
the input stream to some point where parsing should once again commence. The next symbol
returned by the lexical scanner is probably correct. The previous look-ahead token ought to be
discarded with ‘yyclearin;’.
The macro YYRECOVERING stands for an expression that has the value 1 when the parser is
recovering from a syntax error, and 0 the rest of the time. A value of 1 indicates that error
messages are currently suppressed for new syntax errors.
Chapter 7: Handling Context Dependencies 71
7 Handling Context Dependencies
The Bison paradigm is to parse tokens first, then group them into larger syntactic units. In
many languages, the meaning of a token is affected by its context. Although this violates the
Bison paradigm, certain techniques (known as kludges) may enable you to write Bison parsers
for such languages.
(Actually, “kludge” means any technique that gets its job done but is neither clean nor
robust.)
7.1 Semantic Info in Token Types
The C language has a context dependency: the way an identifier is used depends on what its
current meaning is. For example, consider this:
foo (x);
This looks like a function call statement, but if foo is a typedef name, then this is actually
a declaration of x. How can a Bison parser for C decide how to parse this input?
The method used in GNU C is to have two different token types, IDENTIFIER and TYPENAME.
When yylex finds an identifier, it looks up the current declaration of the identifier in order to
decide which token type to return: TYPENAME if the identifier is declared as a typedef, IDENTIFIER
otherwise.
The grammar rules can then express the context dependency by the choice of token type
to recognize. IDENTIFIER is accepted as an expression, but TYPENAME is not. TYPENAME can
start a declaration, but IDENTIFIER cannot. In contexts where the meaning of the identifier
is not significant, such as in declarations that can shadow a typedef name, either TYPENAME or
IDENTIFIER is accepted—there is one rule for each of the two token types.
This technique is simple to use if the decision of which kinds of identifiers to allow is made
at a place close to where the identifier is parsed. But in C this is not always so: C allows a
declaration to redeclare a typedef name provided an explicit type has been specified earlier:
typedef int foo, bar, lose;
static foo (bar); /* redeclare bar as static variable */
static int foo (lose); /* redeclare foo as function */
Unfortunately, the name being declared is separated from the declaration construct itself by
a complicated syntactic structure—the “declarator”.
As a result, part of the Bison parser for C needs to be duplicated, with all the nonterminal
names changed: once for parsing a declaration in which a typedef name can be redefined, and
once for parsing a declaration in which that can’t be done. Here is a part of the duplication,
with actions omitted for brevity:
initdcl:
declarator maybeasm ’=’
init
| declarator maybeasm
;
notype_initdcl:
notype_declarator maybeasm ’=’
init
| notype_declarator maybeasm
;
Here initdcl can redeclare a typedef name, but notype_initdcl cannot. The distinction
between declarator and notype_declarator is the same sort of thing.
72 Bison 1.875
There is some similarity between this technique and a lexical tie-in (described next), in that
information which alters the lexical analysis is changed during parsing by other parts of the
program. The difference is here the information is global, and is used for other purposes in the
program. A true lexical tie-in has a special-purpose flag controlled by the syntactic context.
7.2 Lexical Tie-ins
One way to handle context-dependency is the lexical tie-in: a flag which is set by Bison actions,
whose purpose is to alter the way tokens are parsed.
For example, suppose we have a language vaguely like C, but with a special construct ‘hex
(hex-expr )’. After the keyword hex comes an expression in parentheses in which all integers
are hexadecimal. In particular, the token ‘a1b’ must be treated as an integer rather than as an
identifier if it appears in that context. Here is how you can do it:
%{
int hexflag;
int yylex (void);
void yyerror (char const *);
%}
%%
...
expr: IDENTIFIER
| constant
| HEX ’(’
{ hexflag = 1; }
expr ’)’
{ hexflag = 0;
$$ = $4; }
| expr ’+’ expr
{ $$ = make_sum ($1, $3); }
...
;
constant:
INTEGER
| STRING
;
Here we assume that yylex looks at the value of hexflag; when it is nonzero, all integers are
parsed in hexadecimal, and tokens starting with letters are parsed as integers if possible.
The declaration of hexflag shown in the prologue of the parser file is needed to make it
accessible to the actions (see Section 3.1.1 [The Prologue], page 35). You must also write the
code in yylex to obey the flag.
7.3 Lexical Tie-ins and Error Recovery
Lexical tie-ins make strict demands on any error recovery rules you have. See Chapter 6 [Error
Recovery], page 69.
The reason for this is that the purpose of an error recovery rule is to abort the parsing of
one construct and resume in some larger construct. For example, in C-like languages, a typical
error recovery rule is to skip tokens until the next semicolon, and then start a new statement,
like this:
stmt: expr ’;’
Chapter 7: Handling Context Dependencies 73
| IF ’(’ expr ’)’ stmt { ... }
...
error ’;’
{ hexflag = 0; }
;
If there is a syntax error in the middle of a ‘hex (expr )’ construct, this error rule will apply,
and then the action for the completed ‘hex (expr )’ will never run. So hexflag would remain
set for the entire rest of the input, or until the next hex keyword, causing identifiers to be
misinterpreted as integers.
To avoid this problem the error recovery rule itself clears hexflag.
There may also be an error recovery rule that works within expressions. For example, there
could be a rule which applies within parentheses and skips to the close-parenthesis:
expr: ...
| ’(’ expr ’)’
{ $$ = $2; }
| ’(’ error ’)’
...
If this rule acts within the hex construct, it is not going to abort that construct (since it
applies to an inner level of parentheses within the construct). Therefore, it should not clear the
flag: the rest of the hex construct should be parsed with the flag still in effect.
What if there is an error recovery rule which might abort out of the hex construct or might
not, depending on circumstances? There is no way you can write the action to determine whether
ahex construct is being aborted or not. So if you are using a lexical tie-in, you had better make
sure your error recovery rules are not of this kind. Each rule must be such that you can be sure
that it always will, or always won’t, have to clear the flag.
74 Bison 1.875
Chapter 8: Debugging Your Parser 75
8 Debugging Your Parser
Developing a parser can be a challenge, especially if you don’t understand the algorithm (see
Chapter 5 [The Bison Parser Algorithm], page 59). Even so, sometimes a detailed description
of the automaton can help (see Section 8.1 [Understanding Your Parser], page 75), or tracing
the execution of the parser can give some insight on why it behaves improperly (see Section 8.2
[Tracing Your Parser], page 80).
8.1 Understanding Your Parser
As documented elsewhere (see Chapter 5 [The Bison Parser Algorithm], page 59) Bison parsers
are shift/reduce automata. In some cases (much more frequent than one would hope), looking
at this automaton is required to tune or simply fix a parser. Bison provides two different
representation of it, either textually or graphically (as a VCG file).
The textual file is generated when the options ‘--report’ or ‘--verbose’ are specified, see
See Chapter 9 [Invoking Bison], page 83. Its name is made by removing ‘.tab.c’ or ‘.c’ from
the parser output file name, and adding ‘.output’ instead. Therefore, if the input file is ‘foo.y’,
then the parser file is called ‘foo.tab.c’ by default. As a consequence, the verbose output file
is called ‘foo.output’.
The following grammar file, ‘calc.y’, will be used in the sequel:
%token NUM STR
%left ’+’ ’-’
%left ’*’
%%
exp: exp ’+’ exp
| exp ’-’ exp
| exp ’*’ exp
| exp ’/’ exp
| NUM
;
useless: STR;
%%
bison reports:
calc.y: warning: 1 useless nonterminal and 1 useless rule
calc.y:11.1-7: warning: useless nonterminal: useless
calc.y:11.10-12: warning: useless rule: useless: STR
calc.y: conflicts: 7 shift/reduce
When given ‘--report=state’, in addition to ‘calc.tab.c’, it creates a file ‘calc.output’
with contents detailed below. The order of the output and the exact presentation might vary,
but the interpretation is the same.
The first section includes details on conflicts that were solved thanks to precedence and/or
associativity:
Conflict in state 8 between rule 2 and token ’+’ resolved as reduce.
Conflict in state 8 between rule 2 and token ’-’ resolved as reduce.
Conflict in state 8 between rule 2 and token ’*’ resolved as shift.
...
The next section lists states that still have conflicts.
State 8 conflicts: 1 shift/reduce
State 9 conflicts: 1 shift/reduce
State 10 conflicts: 1 shift/reduce
76 Bison 1.875
State 11 conflicts: 4 shift/reduce
The next section reports useless tokens, nonterminal and rules. Useless nonterminals and rules
are removed in order to produce a smaller parser, but useless tokens are preserved, since they
might be used by the scanner (note the difference between “useless” and “not used” below):
Useless nonterminals:
useless
Terminals which are not used:
STR
Useless rules:
#6 useless: STR;
The next section reproduces the exact grammar that Bison used:
Grammar
Number, Line, Rule
0 5 $accept -> exp $end
1 5 exp -> exp ’+’ exp
2 6 exp -> exp ’-’ exp
3 7 exp -> exp ’*’ exp
4 8 exp -> exp ’/’ exp
5 9 exp -> NUM
and reports the uses of the symbols:
Terminals, with rules where they appear
$end (0) 0
’*’ (42) 3
’+’ (43) 1
’-’ (45) 2
’/’ (47) 4
error (256)
NUM (258) 5
Nonterminals, with rules where they appear
$accept (8)
on left: 0
exp (9)
on left: 1 2 3 4 5, on right: 0 1 2 3 4
Bison then proceeds onto the automaton itself, describing each state with it set of items, also
known as pointed rules. Each item is a production rule together with a point (marked by ‘.’)
that the input cursor.
state 0
$accept -> . exp $ (rule 0)
NUM shift, and go to state 1
exp go to state 2
Chapter 8: Debugging Your Parser 77
This reads as follows: “state 0 corresponds to being at the very beginning of the parsing,
in the initial rule, right before the start symbol (here, exp). When the parser returns to this
state right after having reduced a rule that produced an exp, the control flow jumps to state
2. If there is no such transition on a nonterminal symbol, and the lookahead is a NUM, then this
token is shifted on the parse stack, and the control flow jumps to state 1. Any other lookahead
triggers a syntax error.”
Even though the only active rule in state 0 seems to be rule 0, the report lists NUM as a
lookahead symbol because NUM can be at the beginning of any rule deriving an exp. By default
Bison reports the so-called core or kernel of the item set, but if you want to see more detail
you can invoke bison with ‘--report=itemset’ to list all the items, include those that can be
derived:
state 0
$accept -> . exp $ (rule 0)
exp -> . exp ’+’ exp (rule 1)
exp -> . exp ’-’ exp (rule 2)
exp -> . exp ’*’ exp (rule 3)
exp -> . exp ’/’ exp (rule 4)
exp -> . NUM (rule 5)
NUM shift, and go to state 1
exp go to state 2
In the state 1...
state 1
exp -> NUM . (rule 5)
$default reduce using rule 5 (exp)
the rule 5, ‘exp: NUM;’, is completed. Whatever the lookahead (‘$default’), the parser will
reduce it. If it was coming from state 0, then, after this reduction it will return to state 0, and
will jump to state 2 (‘exp: go to state 2’).
state 2
$accept -> exp . $ (rule 0)
exp -> exp . ’+’ exp (rule 1)
exp -> exp . ’-’ exp (rule 2)
exp -> exp . ’*’ exp (rule 3)
exp -> exp . ’/’ exp (rule 4)
$ shift, and go to state 3
’+’ shift, and go to state 4
’-’ shift, and go to state 5
’*’ shift, and go to state 6
’/’ shift, and go to state 7
In state 2, the automaton can only shift a symbol. For instance, because of the item ‘exp ->
exp . ’+’ exp’, if the lookahead if ‘+’, it will be shifted on the parse stack, and the automaton
control will jump to state 4, corresponding to the item ‘exp -> exp ’+’ . exp’. Since there is
no default action, any other token than those listed above will trigger a syntax error.
The state 3 is named the final state, or the accepting state:
78 Bison 1.875
state 3
$accept -> exp $ . (rule 0)
$default accept
the initial rule is completed (the start symbol and the end of input were read), the parsing exits
successfully.
The interpretation of states 4 to 7 is straightforward, and is left to the reader.
state 4
exp -> exp ’+’ . exp (rule 1)
NUM shift, and go to state 1
exp go to state 8
state 5
exp -> exp ’-’ . exp (rule 2)
NUM shift, and go to state 1
exp go to state 9
state 6
exp -> exp ’*’ . exp (rule 3)
NUM shift, and go to state 1
exp go to state 10
state 7
exp -> exp ’/’ . exp (rule 4)
NUM shift, and go to state 1
exp go to state 11
As was announced in beginning of the report, ‘State 8 conflicts: 1 shift/reduce’:
state 8
exp -> exp . ’+’ exp (rule 1)
exp -> exp ’+’ exp . (rule 1)
exp -> exp . ’-’ exp (rule 2)
exp -> exp . ’*’ exp (rule 3)
exp -> exp . ’/’ exp (rule 4)
’*’ shift, and go to state 6
’/’ shift, and go to state 7
Chapter 8: Debugging Your Parser 79
’/’ [reduce using rule 1 (exp)]
$default reduce using rule 1 (exp)
Indeed, there are two actions associated to the lookahead ‘/’: either shifting (and going to
state 7), or reducing rule 1. The conflict means that either the grammar is ambiguous, or the
parser lacks information to make the right decision. Indeed the grammar is ambiguous, as, since
we did not specify the precedence of ‘/’, the sentence ‘NUM + NUM / NUM’ can be parsed as ‘NUM
+ (NUM / NUM)’, which corresponds to shifting ‘/’, or as ‘(NUM + NUM) / NUM’, which corresponds
to reducing rule 1.
Because in LALR(1) parsing a single decision can be made, Bison arbitrarily chose to disable
the reduction, see Section 5.2 [Shift/Reduce Conflicts], page 60. Discarded actions are reported
in between square brackets.
Note that all the previous states had a single possible action: either shifting the next token
and going to the corresponding state, or reducing a single rule. In the other cases, i.e., when
shifting and reducing is possible or when several reductions are possible, the lookahead is re-
quired to select the action. State 8 is one such state: if the lookahead is ‘*’ or ‘/’ then the
action is shifting, otherwise the action is reducing rule 1. In other words, the first two items,
corresponding to rule 1, are not eligible when the lookahead is ‘*’, since we specified that ‘*’
has higher precedence that ‘+’. More generally, some items are eligible only with some set of
possible lookaheads. When run with ‘--report=lookahead’, Bison specifies these lookaheads:
state 8
exp -> exp . ’+’ exp [$, ’+’, ’-’, ’/’] (rule 1)
exp -> exp ’+’ exp . [$, ’+’, ’-’, ’/’] (rule 1)
exp -> exp . ’-’ exp (rule 2)
exp -> exp . ’*’ exp (rule 3)
exp -> exp . ’/’ exp (rule 4)
’*’ shift, and go to state 6
’/’ shift, and go to state 7
’/’ [reduce using rule 1 (exp)]
$default reduce using rule 1 (exp)
The remaining states are similar:
state 9
exp -> exp . ’+’ exp (rule 1)
exp -> exp . ’-’ exp (rule 2)
exp -> exp ’-’ exp . (rule 2)
exp -> exp . ’*’ exp (rule 3)
exp -> exp . ’/’ exp (rule 4)
’*’ shift, and go to state 6
’/’ shift, and go to state 7
’/’ [reduce using rule 2 (exp)]
$default reduce using rule 2 (exp)
state 10
exp -> exp . ’+’ exp (rule 1)
80 Bison 1.875
exp -> exp . ’-’ exp (rule 2)
exp -> exp . ’*’ exp (rule 3)
exp -> exp ’*’ exp . (rule 3)
exp -> exp . ’/’ exp (rule 4)
’/’ shift, and go to state 7
’/’ [reduce using rule 3 (exp)]
$default reduce using rule 3 (exp)
state 11
exp -> exp . ’+’ exp (rule 1)
exp -> exp . ’-’ exp (rule 2)
exp -> exp . ’*’ exp (rule 3)
exp -> exp . ’/’ exp (rule 4)
exp -> exp ’/’ exp . (rule 4)
’+’ shift, and go to state 4
’-’ shift, and go to state 5
’*’ shift, and go to state 6
’/’ shift, and go to state 7
’+’ [reduce using rule 4 (exp)]
’-’ [reduce using rule 4 (exp)]
’*’ [reduce using rule 4 (exp)]
’/’ [reduce using rule 4 (exp)]
$default reduce using rule 4 (exp)
Observe that state 11 contains conflicts due to the lack of precedence of ‘/’ wrt ‘+’, ‘-’, and ‘*’,
but also because the associativity of ‘/’ is not specified.
8.2 Tracing Your Parser
If a Bison grammar compiles properly but doesn’t do what you want when it runs, the yydebug
parser-trace feature can help you figure out why.
There are several means to enable compilation of trace facilities:
the macro YYDEBUG
Define the macro YYDEBUG to a nonzero value when you compile the parser. This
is compliant with POSIX Yacc. You could use ‘-DYYDEBUG=1’ as a compiler option
or you could put ‘#define YYDEBUG 1’ in the prologue of the grammar file (see
Section 3.1.1 [The Prologue], page 35).
the option ‘-t’, ‘--debug’
Use the ‘-t’ option when you run Bison (see Chapter 9 [Invoking Bison], page 83).
This is POSIX compliant too.
the directive ‘%debug’
Add the %debug directive (see Section 3.7.9 [Bison Declaration Summary], page 49).
This is a Bison extension, which will prove useful when Bison will output parsers for
languages that don’t use a preprocessor. Unless POSIX and Yacc portability matter
to you, this is the preferred solution.
We suggest that you always enable the debug option so that debugging is always possible.
Chapter 8: Debugging Your Parser 81
The trace facility outputs messages with macro calls of the form YYFPRINTF (stderr, for-
mat,args )where format and args are the usual printf format and arguments. If you define
YYDEBUG to a nonzero value but do not define YYFPRINTF,<stdio.h> is automatically included
and YYPRINTF is defined to fprintf.
Once you have compiled the program with trace facilities, the way to request a trace is to
store a nonzero value in the variable yydebug. You can do this by making the C code do it (in
main, perhaps), or you can alter the value with a C debugger.
Each step taken by the parser when yydebug is nonzero produces a line or two of trace
information, written on stderr. The trace messages tell you these things:
•Each time the parser calls yylex, what kind of token was read.
•Each time a token is shifted, the depth and complete contents of the state stack (see
Section 5.5 [Parser States], page 63).
•Each time a rule is reduced, which rule it is, and the complete contents of the state stack
afterward.
To make sense of this information, it helps to refer to the listing file produced by the Bison
‘-v’ option (see Chapter 9 [Invoking Bison], page 83). This file shows the meaning of each state
in terms of positions in various rules, and also what each state will do with each possible input
token. As you read the successive trace messages, you can see that the parser is functioning
according to its specification in the listing file. Eventually you will arrive at the place where
something undesirable happens, and you will see which parts of the grammar are to blame.
The parser file is a C program and you can use C debuggers on it, but it’s not easy to interpret
what it is doing. The parser function is a finite-state machine interpreter, and aside from the
actions it executes the same code over and over. Only the values of variables show where in the
grammar it is working.
The debugging information normally gives the token type of each token read, but not its
semantic value. You can optionally define a macro named YYPRINT to provide a way to print the
value. If you define YYPRINT, it should take three arguments. The parser will pass a standard
I/O stream, the numeric code for the token type, and the token value (from yylval).
Here is an example of YYPRINT suitable for the multi-function calculator (see Section 2.5.1
[Declarations for mfcalc], page 29):
%{
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(file, type, value) print_token_value (file, type, value)
%}
... %% ... %% ...
static void
print_token_value (FILE *file, int type, YYSTYPE value)
{
if (type == VAR)
fprintf (file, "%s", value.tptr->name);
else if (type == NUM)
fprintf (file, "%d", value.val);
}
82 Bison 1.875
Chapter 9: Invoking Bison 83
9 Invoking Bison
The usual way to invoke Bison is as follows:
bison infile
Here infile is the grammar file name, which usually ends in ‘.y’. The parser file’s name is
made by replacing the ‘.y’ with ‘.tab.c’. Thus, the ‘bison foo.y’ filename yields ‘foo.tab.c’,
and the ‘bison hack/foo.y’ filename yields ‘hack/foo.tab.c’. It’s also possible, in case you are
writing C++ code instead of C in your grammar file, to name it ‘foo.ypp’ or ‘foo.y++’. Then,
the output files will take an extension like the given one as input (respectively ‘foo.tab.cpp’
and ‘foo.tab.c++’). This feature takes effect with all options that manipulate filenames like
‘-o’ or ‘-d’.
For example :
bison -d infile.yxx
will produce ‘infile.tab.cxx’ and ‘infile.tab.hxx’, and
bison -d -o output.c++ infile.y
will produce ‘output.c++’ and ‘outfile.h++’.
For compatibility with POSIX, the standard Bison distribution also contains a shell script
called yacc that invokes Bison with the ‘-y’ option.
9.1 Bison Options
Bison supports both traditional single-letter options and mnemonic long option names. Long
option names are indicated with ‘--’ instead of ‘-’. Abbreviations for option names are allowed
as long as they are unique. When a long option takes an argument, like ‘--file-prefix’,
connect the option name and the argument with ‘=’.
Here is a list of options that can be used with Bison, alphabetized by short option. It is
followed by a cross key alphabetized by long option.
Operations modes:
‘-h’
‘--help’ Print a summary of the command-line options to Bison and exit.
‘-V’
‘--version’
Print the version number of Bison and exit.
‘-y’
‘--yacc’ Equivalent to ‘-o y.tab.c’; the parser output file is called ‘y.tab.c’, and the other
outputs are called ‘y.output’ and ‘y.tab.h’. The purpose of this option is to imitate
Yacc’s output file name conventions. Thus, the following shell script can substitute
for Yacc, and the Bison distribution contains such a script for compatibility with
POSIX:
#! /bin/sh
bison -y "$¨
Tuning the parser:
‘-S file ’
‘--skeleton=file ’
Specify the skeleton to use. You probably don’t need this option unless you are
developing Bison.
84 Bison 1.875
‘-t’
‘--debug’ In the parser file, define the macro YYDEBUG to 1 if it is not already defined, so
that the debugging facilities are compiled. See Section 8.2 [Tracing Your Parser],
page 80.
‘--locations’
Pretend that %locations was specified. See Section 3.7.9 [Decl Summary], page 49.
‘-p prefix ’
‘--name-prefix=prefix ’
Pretend that %name-prefix="prefix "was specified. See Section 3.7.9 [Decl Sum-
mary], page 49.
‘-l’
‘--no-lines’
Don’t put any #line preprocessor commands in the parser file. Ordinarily Bison
puts them in the parser file so that the C compiler and debuggers will associate
errors with your source file, the grammar file. This option causes them to associate
errors with the parser file, treating it as an independent source file in its own right.
‘-n’
‘--no-parser’
Pretend that %no-parser was specified. See Section 3.7.9 [Decl Summary], page 49.
‘-k’
‘--token-table’
Pretend that %token-table was specified. See Section 3.7.9 [Decl Summary],
page 49.
Adjust the output:
‘-d’
‘--defines’
Pretend that %defines was specified, i.e., write an extra output file containing macro
definitions for the token type names defined in the grammar and the semantic value
type YYSTYPE, as well as a few extern variable declarations. See Section 3.7.9 [Decl
Summary], page 49.
‘--defines=defines-file ’
Same as above, but save in the file defines-file.
‘-b file-prefix ’
‘--file-prefix=prefix ’
Pretend that %verbose was specified, i.e, specify prefix to use for all Bison output
file names. See Section 3.7.9 [Decl Summary], page 49.
‘-r things ’
‘--report=things ’
Write an extra output file containing verbose description of the comma separated
list of things among:
state Description of the grammar, conflicts (resolved and unresolved), and
LALR automaton.
lookahead
Implies state and augments the description of the automaton with each
rule’s lookahead set.
itemset Implies state and augments the description of the automaton with the
full set of items for each state, instead of its core only.
Chapter 9: Invoking Bison 85
For instance, on the following grammar
‘-v’
‘--verbose’
Pretend that %verbose was specified, i.e, write an extra output file containing ver-
bose descriptions of the grammar and parser. See Section 3.7.9 [Decl Summary],
page 49.
‘-o filename ’
‘--output=filename ’
Specify the filename for the parser file.
The other output files’ names are constructed from filename as described under the
‘-v’ and ‘-d’ options.
‘-g’ Output a VCG definition of the LALR(1) grammar automaton computed by Bison.
If the grammar file is ‘foo.y’, the VCG output file will be ‘foo.vcg’.
‘--graph=graph-file ’
The behavior of –graph is the same than ‘-g’. The only difference is that it has an
optional argument which is the name of the output graph filename.
9.2 Option Cross Key
Here is a list of options, alphabetized by long option, to help you find the corresponding short
option.
--debug ................................. -t
--defines ................................ -d
--file-prefix .............................. -b
--graph ................................. -g
--help .................................. -h
--name-prefix .............................. -p
--no-lines ................................ -l
--no-parser ............................... -n
--output ................................. -o
--token-table .............................. -k
--verbose ................................ -v
--version ................................ -V
--yacc .................................. -y
9.3 Yacc Library
The Yacc library contains default implementations of the yyerror and main functions. These
default implementations are normally not useful, but POSIX requires them. To use the Yacc
library, link your program with the ‘-ly’ option. Note that Bison’s implementation of the Yacc
library is distributed under the terms of the GNU General Public License (see [Copying], page 5).
If you use the Yacc library’s yyerror function, you should declare yyerror as follows:
int yyerror (char const *);
Bison ignores the int value returned by this yyerror. If you use the Yacc library’s main
function, your yyparse function should have the following type signature:
int yyparse (void);
86 Bison 1.875
Chapter 10: Frequently Asked Questions 87
10 Frequently Asked Questions
Several questions about Bison come up occasionally. Here some of them are addressed.
10.1 Parser Stack Overflow
My parser returns with error with a ‘parser stack overflow’
message. What can I do?
This question is already addressed elsewhere, See Section 3.4 [Recursive Rules], page 38.
88 Bison 1.875
Appendix A: Bison Symbols 89
Appendix A Bison Symbols
[Variable]@$
In an action, the location of the left-hand side of the rule. See Section 3.6 [Locations Over-
view], page 43.
[Variable]@n
In an action, the location of the n-th symbol of the right-hand side of the rule. See Section 3.6
[Locations Overview], page 43.
[Variable]$$
In an action, the semantic value of the left-hand side of the rule. See Section 3.5.3 [Actions],
page 40.
[Variable]$n
In an action, the semantic value of the n-th symbol of the right-hand side of the rule. See
Section 3.5.3 [Actions], page 40.
[Symbol]$accept
The predefined nonterminal whose only rule is ‘$accept: start $end’, where start is the
start symbol. See Section 3.7.7 [The Start-Symbol], page 48. It cannot be used in the
grammar.
[Symbol]$end
The predefined token marking the end of the token stream. It cannot be used in the grammar.
[Symbol]$undefined
The predefined token onto which all undefined values returned by yylex are mapped. It
cannot be used in the grammar, rather, use error.
[Symbol]error
A token name reserved for error recovery. This token may be used in grammar rules so as
to allow the Bison parser to recognize an error in the grammar without halting the process.
In effect, a sentence containing an error may be recognized as valid. On a syntax error, the
token error becomes the current look-ahead token. Actions corresponding to error are then
executed, and the look-ahead token is reset to the token that originally caused the violation.
See Chapter 6 [Error Recovery], page 69.
[Macro]YYABORT
Macro to pretend that an unrecoverable syntax error has occurred, by making yyparse
return 1 immediately. The error reporting function yyerror is not called. See Section 4.1
[The Parser Function yyparse], page 53.
[Macro]YYACCEPT
Macro to pretend that a complete utterance of the language has been read, by making yyparse
return 0 immediately. See Section 4.1 [The Parser Function yyparse], page 53.
[Macro]YYBACKUP
Macro to discard a value from the parser stack and fake a look-ahead token. See Section 4.4
[Special Features for Use in Actions], page 56.
[Macro]YYDEBUG
Macro to define to equip the parser with tracing code. See Section 8.2 [Tracing Your Parser],
page 80.
90 Bison 1.875
[Macro]YYERROR
Macro to pretend that a syntax error has just been detected: call yyerror and then perform
normal error recovery if possible (see Chapter 6 [Error Recovery], page 69), or (if recovery is
impossible) make yyparse return 1. See Chapter 6 [Error Recovery], page 69.
[Macro]YYERROR_VERBOSE
An obsolete macro that you define with #define in the prologue to request verbose, specific
error message strings when yyerror is called. It doesn’t matter what definition you use for
YYERROR_VERBOSE, just whether you define it. Using %error-verbose is preferred.
[Macro]YYINITDEPTH
Macro for specifying the initial size of the parser stack. See Section 5.9 [Stack Overflow],
page 67.
[Macro]YYLEX_PARAM
An obsolete macro for specifying an extra argument (or list of extra arguments) for yyparse
to pass to yylex. he use of this macro is deprecated, and is supported only for Yacc like
parsers. See Section 4.2.4 [Calling Conventions for Pure Parsers], page 55.
[Type]YYLTYPE
Data type of yylloc; by default, a structure with four members. See Section 3.6.1 [Data
Types of Locations], page 43.
[Macro]YYMAXDEPTH
Macro for specifying the maximum size of the parser stack. See Section 5.9 [Stack Overflow],
page 67.
[Macro]YYPARSE_PARAM
An obsolete macro for specifying the name of a parameter that yyparse should accept. The
use of this macro is deprecated, and is supported only for Yacc like parsers. See Section 4.2.4
[Calling Conventions for Pure Parsers], page 55.
[Macro]YYRECOVERING
Macro whose value indicates whether the parser is recovering from a syntax error. See
Section 4.4 [Special Features for Use in Actions], page 56.
[Macro]YYSTACK_USE_ALLOCA
Macro used to control the use of alloca. If defined to ‘0’, the parser will not use alloca
but malloc when trying to grow its internal stacks. Do not define YYSTACK_USE_ALLOCA to
anything else.
[Type]YYSTYPE
Data type of semantic values; int by default. See Section 3.5.1 [Data Types of Semantic
Values], page 39.
[Variable]yychar
External integer variable that contains the integer value of the current look-ahead token. (In
a pure parser, it is a local variable within yyparse.) Error-recovery rule actions may examine
this variable. See Section 4.4 [Special Features for Use in Actions], page 56.
[Variable]yyclearin
Macro used in error-recovery rule actions. It clears the previous look-ahead token. See
Chapter 6 [Error Recovery], page 69.
Appendix A: Bison Symbols 91
[Variable]yydebug
External integer variable set to zero by default. If yydebug is given a nonzero value, the
parser will output information on input symbols and parser action. See Section 8.2 [Tracing
Your Parser], page 80.
[Macro]yyerrok
Macro to cause parser to recover immediately to its normal mode after a syntax error. See
Chapter 6 [Error Recovery], page 69.
[Function]yyerror
User-supplied function to be called by yyparse on error. See Section 4.3 [The Error Reporting
Function yyerror], page 55.
[Function]yylex
User-supplied lexical analyzer function, called with no arguments to get the next token. See
Section 4.2 [The Lexical Analyzer Function yylex], page 53.
[Variable]yylval
External variable in which yylex should place the semantic value associated with a token.
(In a pure parser, it is a local variable within yyparse, and its address is passed to yylex.)
See Section 4.2.2 [Semantic Values of Tokens], page 54.
[Variable]yylloc
External variable in which yylex should place the line and column numbers associated with
a token. (In a pure parser, it is a local variable within yyparse, and its address is passed to
yylex.) You can ignore this variable if you don’t use the ‘@’ feature in the grammar actions.
See Section 4.2.3 [Textual Positions of Tokens], page 55.
[Variable]yynerrs
Global variable which Bison increments each time there is a syntax error. (In a pure parser, it
is a local variable within yyparse.) See Section 4.3 [The Error Reporting Function yyerror],
page 55.
[Function]yyparse
The parser function produced by Bison; call this function to start parsing. See Section 4.1
[The Parser Function yyparse], page 53.
[Directive]%debug
Equip the parser for debugging. See Section 3.7.9 [Decl Summary], page 49.
[Directive]%defines
Bison declaration to create a header file meant for the scanner. See Section 3.7.9 [Decl
Summary], page 49.
[Directive]%destructor
Specifying how the parser should reclaim the memory associated to discarded symbols. See
Section 3.7.5 [Freeing Discarded Symbols], page 47.
[Directive]%dprec
Bison declaration to assign a precedence to a rule that is used at parse time to resolve
reduce/reduce conflicts. See Section 1.5 [Writing GLR Parsers], page 13.
[Directive]%error-verbose
Bison declaration to request verbose, specific error message strings when yyerror is called.
92 Bison 1.875
[Directive]%file-prefix="prefix "
Bison declaration to set the prefix of the output files. See Section 3.7.9 [Decl Summary],
page 49.
[Directive]%glr-parser
Bison declaration to produce a GLR parser. See Section 1.5 [Writing GLR Parsers], page 13.
[Directive]%left
Bison declaration to assign left associativity to token(s). See Section 3.7.2 [Operator Prece-
dence], page 46.
[Directive]%merge
Bison declaration to assign a merging function to a rule. If there is a reduce/reduce conflict
with a rule having the same merging function, the function is applied to the two semantic
values to get a single result. See Section 1.5 [Writing GLR Parsers], page 13.
[Directive]%name-prefix="prefix "
Bison declaration to rename the external symbols. See Section 3.7.9 [Decl Summary], page 49.
[Directive]%no-lines
Bison declaration to avoid generating #line directives in the parser file. See Section 3.7.9
[Decl Summary], page 49.
[Directive]%nonassoc
Bison declaration to assign non-associativity to token(s). See Section 3.7.2 [Operator Prece-
dence], page 46.
[Directive]%output="filename "
Bison declaration to set the name of the parser file. See Section 3.7.9 [Decl Summary],
page 49.
[Directive]%prec
Bison declaration to assign a precedence to a specific rule. See Section 5.4 [Context-Dependent
Precedence], page 62.
[Directive]%pure-parser
Bison declaration to request a pure (reentrant) parser. See Section 3.7.8 [A Pure (Reentrant)
Parser], page 48.
[Directive]%right
Bison declaration to assign right associativity to token(s). See Section 3.7.2 [Operator Prece-
dence], page 46.
[Directive]%start
Bison declaration to specify the start symbol. See Section 3.7.7 [The Start-Symbol], page 48.
[Directive]%token
Bison declaration to declare token(s) without specifying precedence. See Section 3.7.1 [Token
Type Names], page 45.
[Directive]%token-table
Bison declaration to include a token name table in the parser file. See Section 3.7.9 [Decl
Summary], page 49.
[Directive]%type
Bison declaration to declare nonterminals. See Section 3.7.4 [Nonterminal Symbols], page 47.
Appendix A: Bison Symbols 93
[Directive]%union
Bison declaration to specify several possible data types for semantic values. See Section 3.7.3
[The Collection of Value Types], page 46.
These are the punctuation and delimiters used in Bison input:
[Delimiter]%%
Delimiter used to separate the grammar rule section from the Bison declarations section or
the epilogue. See Section 1.9 [The Overall Layout of a Bison Grammar], page 17.
[Delimiter]%{code %}
All code listed between ‘%{’ and ‘%}’ is copied directly to the output file uninterpreted. Such
code forms the prologue of the input file. See Section 3.1 [Outline of a Bison Grammar],
page 35.
[Construct]/*...*/
Comment delimiters, as in C.
[Delimiter]:
Separates a rule’s result from its components. See Section 3.3 [Syntax of Grammar Rules],
page 38.
[Delimiter];
Terminates a rule. See Section 3.3 [Syntax of Grammar Rules], page 38.
[Delimiter]|
Separates alternate rules for the same result nonterminal. See Section 3.3 [Syntax of Grammar
Rules], page 38.
94 Bison 1.875
Appendix B: Glossary 95
Appendix B Glossary
Backus-Naur Form (BNF; also called “Backus Normal Form”)
Formal method of specifying context-free grammars originally proposed by John
Backus, and slightly improved by Peter Naur in his 1960-01-02 committee document
contributing to what became the Algol 60 report. See Section 1.1 [Languages and
Context-Free Grammars], page 11.
Context-free grammars
Grammars specified as rules that can be applied regardless of context. Thus, if there
is a rule which says that an integer can be used as an expression, integers are allowed
anywhere an expression is permitted. See Section 1.1 [Languages and Context-Free
Grammars], page 11.
Dynamic allocation
Allocation of memory that occurs during execution, rather than at compile time or
on entry to a function.
Empty string
Analogous to the empty set in set theory, the empty string is a character string of
length zero.
Finite-state stack machine
A “machine” that has discrete states in which it is said to exist at each instant in
time. As input to the machine is processed, the machine moves from state to state
as specified by the logic of the machine. In the case of the parser, the input is the
language being parsed, and the states correspond to various stages in the grammar
rules. See Chapter 5 [The Bison Parser Algorithm], page 59.
Generalized LR (GLR)
A parsing algorithm that can handle all context-free grammars, including those
that are not LALR(1). It resolves situations that Bison’s usual LALR(1) algorithm
cannot by effectively splitting off multiple parsers, trying all possible parsers, and
discarding those that fail in the light of additional right context. See Section 5.8
[Generalized LR Parsing], page 66.
Grouping A language construct that is (in general) grammatically divisible; for example, ‘ex-
pression’ or ‘declaration’ in C. See Section 1.1 [Languages and Context-Free Gram-
mars], page 11.
Infix operator
An arithmetic operator that is placed between the operands on which it performs
some operation.
Input stream
A continuous flow of data between devices or programs.
Language construct
One of the typical usage schemas of the language. For example, one of the constructs
of the C language is the if statement. See Section 1.1 [Languages and Context-Free
Grammars], page 11.
Left associativity
Operators having left associativity are analyzed from left to right: ‘a+b+c’ first
computes ‘a+b’ and then combines with ‘c’. See Section 5.3 [Operator Precedence],
page 61.
96 Bison 1.875
Left recursion
A rule whose result symbol is also its first component symbol; for example, ‘expseq1
: expseq1 ’,’ exp;’. See Section 3.4 [Recursive Rules], page 38.
Left-to-right parsing
Parsing a sentence of a language by analyzing it token by token from left to right.
See Chapter 5 [The Bison Parser Algorithm], page 59.
Lexical analyzer (scanner)
A function that reads an input stream and returns tokens one by one. See Section 4.2
[The Lexical Analyzer Function yylex], page 53.
Lexical tie-in
A flag, set by actions in the grammar rules, which alters the way tokens are parsed.
See Section 7.2 [Lexical Tie-ins], page 72.
Literal string token
A token which consists of two or more fixed characters. See Section 3.2 [Symbols],
page 36.
Look-ahead token
A token already read but not yet shifted. See Section 5.1 [Look-Ahead Tokens],
page 59.
LALR(1) The class of context-free grammars that Bison (like most other parser generators)
can handle; a subset of LR(1). See Section 5.7 [Mysterious Reduce/Reduce Con-
flicts], page 65.
LR(1) The class of context-free grammars in which at most one token of look-ahead is
needed to disambiguate the parsing of any piece of input.
Nonterminal symbol
A grammar symbol standing for a grammatical construct that can be expressed
through rules in terms of smaller constructs; in other words, a construct that is not
a token. See Section 3.2 [Symbols], page 36.
Parser A function that recognizes valid sentences of a language by analyzing the syntax
structure of a set of tokens passed to it from a lexical analyzer.
Postfix operator
An arithmetic operator that is placed after the operands upon which it performs
some operation.
Reduction Replacing a string of nonterminals and/or terminals with a single nonterminal, ac-
cording to a grammar rule. See Chapter 5 [The Bison Parser Algorithm], page 59.
Reentrant A reentrant subprogram is a subprogram which can be in invoked any number of
times in parallel, without interference between the various invocations. See Sec-
tion 3.7.8 [A Pure (Reentrant) Parser], page 48.
Reverse polish notation
A language in which all operators are postfix operators.
Right recursion
A rule whose result symbol is also its last component symbol; for example, ‘expseq1:
exp ’,’ expseq1;’. See Section 3.4 [Recursive Rules], page 38.
Semantics In computer languages, the semantics are specified by the actions taken for each in-
stance of the language, i.e., the meaning of each statement. See Section 3.5 [Defining
Language Semantics], page 39.
Appendix B: Glossary 97
Shift A parser is said to shift when it makes the choice of analyzing further input from
the stream rather than reducing immediately some already-recognized rule. See
Chapter 5 [The Bison Parser Algorithm], page 59.
Single-character literal
A single character that is recognized and interpreted as is. See Section 1.2 [From
Formal Rules to Bison Input], page 12.
Start symbol
The nonterminal symbol that stands for a complete valid utterance in the language
being parsed. The start symbol is usually listed as the first nonterminal symbol in
a language specification. See Section 3.7.7 [The Start-Symbol], page 48.
Symbol table
A data structure where symbol names and associated data are stored during parsing
to allow for recognition and use of existing information in repeated uses of a symbol.
See Section 2.5 [Multi-function Calc], page 29.
Syntax error
An error encountered during parsing of an input stream due to invalid syntax. See
Chapter 6 [Error Recovery], page 69.
Token A basic, grammatically indivisible unit of a language. The symbol that describes
a token in the grammar is a terminal symbol. The input of the Bison parser is a
stream of tokens which comes from the lexical analyzer. See Section 3.2 [Symbols],
page 36.
Terminal symbol
A grammar symbol that has no rules in the grammar and therefore is grammatically
indivisible. The piece of text it represents is a token. See Section 1.1 [Languages
and Context-Free Grammars], page 11.
98 Bison 1.875
Appendix C: Copying This Manual 99
Appendix C Copying This Manual
C.1 GNU Free Documentation License
Version 1.2, November 2002
Copyright c
2000,2001,2002 Free Software Foundation, Inc.
59 Temple Place, Suite 330, Boston, MA 02111-1307, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other functional and useful
document free in the sense of freedom: to assure everyone the effective freedom to copy
and redistribute it, with or without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a way to get credit for their
work, while not being considered responsible for modifications made by others.
This License is a kind of “copyleft”, which means that derivative works of the document
must themselves be free in the same sense. It complements the GNU General Public License,
which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free
software needs free documentation: a free program should come with manuals providing the
same freedoms that the software does. But this License is not limited to software manuals;
it can be used for any textual work, regardless of subject matter or whether it is published
as a printed book. We recommend this License principally for works whose purpose is
instruction or reference.
1. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium, that contains a notice
placed by the copyright holder saying it can be distributed under the terms of this License.
Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that
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A “Modified Version” of the Document means any work containing the Document or a
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A “Secondary Section” is a named appendix or a front-matter section of the Document
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The “Invariant Sections” are certain Secondary Sections whose titles are designated, as
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If the Document does not identify any Invariant Sections then there are none.
The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or
Back-Cover Texts, in the notice that says that the Document is released under this License.
100 Bison 1.875
A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25
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A “Transparent” copy of the Document means a machine-readable copy, represented in a
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For works in formats which do not have any title page as such, “Title Page” means the
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A section “Entitled XYZ” means a named subunit of the Document whose title either
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Title” of such a section when you modify the Document means that it remains a section
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The Document may include Warranty Disclaimers next to the notice which states that
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included by reference in this License, but only as regards disclaiming warranties: any other
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meaning of this License.
2. VERBATIM COPYING
You may copy and distribute the Document in any medium, either commercially or noncom-
mercially, provided that this License, the copyright notices, and the license notice saying
this License applies to the Document are reproduced in all copies, and that you add no
other conditions whatsoever to those of this License. You may not use technical measures
to obstruct or control the reading or further copying of the copies you make or distribute.
However, you may accept compensation in exchange for copies. If you distribute a large
enough number of copies you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly
display copies.
3. COPYING IN QUANTITY
If you publish printed copies (or copies in media that commonly have printed covers) of the
Document, numbering more than 100, and the Document’s license notice requires Cover
Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both
Appendix C: Copying This Manual 101
covers must also clearly and legibly identify you as the publisher of these copies. The front
cover must present the full title with all words of the title equally prominent and visible.
You may add other material on the covers in addition. Copying with changes limited to
the covers, as long as they preserve the title of the Document and satisfy these conditions,
can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the
first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto
adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100, you
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stated location until at least one year after the last time you distribute an Opaque copy
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It is requested, but not required, that you contact the authors of the Document well before
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updated version of the Document.
4. MODIFICATIONS
You may copy and distribute a Modified Version of the Document under the conditions
of sections 2 and 3 above, provided that you release the Modified Version under precisely
this License, with the Modified Version filling the role of the Document, thus licensing
distribution and modification of the Modified Version to whoever possesses a copy of it. In
addition, you must do these things in the Modified Version:
A. Use in the Title Page (and on the covers, if any) a title distinct from that of the
Document, and from those of previous versions (which should, if there were any, be
listed in the History section of the Document). You may use the same title as a previous
version if the original publisher of that version gives permission.
B. List on the Title Page, as authors, one or more persons or entities responsible for
authorship of the modifications in the Modified Version, together with at least five of
the principal authors of the Document (all of its principal authors, if it has fewer than
five), unless they release you from this requirement.
C. State on the Title page the name of the publisher of the Modified Version, as the
publisher.
D. Preserve all the copyright notices of the Document.
E. Add an appropriate copyright notice for your modifications adjacent to the other copy-
right notices.
F. Include, immediately after the copyright notices, a license notice giving the public
permission to use the Modified Version under the terms of this License, in the form
shown in the Addendum below.
G. Preserve in that license notice the full lists of Invariant Sections and required Cover
Texts given in the Document’s license notice.
H. Include an unaltered copy of this License.
I. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating
at least the title, year, new authors, and publisher of the Modified Version as given
on the Title Page. If there is no section Entitled “History” in the Document, create
one stating the title, year, authors, and publisher of the Document as given on its
102 Bison 1.875
Title Page, then add an item describing the Modified Version as stated in the previous
sentence.
J. Preserve the network location, if any, given in the Document for public access to a
Transparent copy of the Document, and likewise the network locations given in the
Document for previous versions it was based on. These may be placed in the “History”
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gives permission.
K. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title
of the section, and preserve in the section all the substance and tone of each of the
contributor acknowledgements and/or dedications given therein.
L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their
titles. Section numbers or the equivalent are not considered part of the section titles.
M. Delete any section Entitled “Endorsements”. Such a section may not be included in
the Modified Version.
N. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title
with any Invariant Section.
O. Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or appendices that qualify as
Secondary Sections and contain no material copied from the Document, you may at your
option designate some or all of these sections as invariant. To do this, add their titles to
the list of Invariant Sections in the Modified Version’s license notice. These titles must be
distinct from any other section titles.
You may add a section Entitled “Endorsements”, provided it contains nothing but endorse-
ments of your Modified Version by various parties—for example, statements of peer review
or that the text has been approved by an organization as the authoritative definition of a
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You may add a passage of up to five words as a Front-Cover Text, and a passage of up
to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified
Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added
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old one, on explicit permission from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission to
use their names for publicity for or to assert or imply endorsement of any Modified Version.
5. COMBINING DOCUMENTS
You may combine the Document with other documents released under this License, under
the terms defined in section 4 above for modified versions, provided that you include in the
combination all of the Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its license notice, and that you
preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical
Invariant Sections may be replaced with a single copy. If there are multiple Invariant
Sections with the same name but different contents, make the title of each such section
unique by adding at the end of it, in parentheses, the name of the original author or
publisher of that section if known, or else a unique number. Make the same adjustment to
the section titles in the list of Invariant Sections in the license notice of the combined work.
Appendix C: Copying This Manual 103
In the combination, you must combine any sections Entitled “History” in the various original
documents, forming one section Entitled “History”; likewise combine any sections Entitled
“Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections
Entitled “Endorsements.”
6. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other documents released under
this License, and replace the individual copies of this License in the various documents with
a single copy that is included in the collection, provided that you follow the rules of this
License for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distribute it individually
under this License, provided you insert a copy of this License into the extracted document,
and follow this License in all other respects regarding verbatim copying of that document.
7. AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other separate and independent
documents or works, in or on a volume of a storage or distribution medium, is called an
“aggregate” if the copyright resulting from the compilation is not used to limit the legal
rights of the compilation’s users beyond what the individual works permit. When the
Document is included an aggregate, this License does not apply to the other works in the
aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document,
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Texts may be placed on covers that bracket the Document within the aggregate, or the
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appear on printed covers that bracket the whole aggregate.
8. TRANSLATION
Translation is considered a kind of modification, so you may distribute translations of the
Document under the terms of section 4. Replacing Invariant Sections with translations
requires special permission from their copyright holders, but you may include translations
of some or all Invariant Sections in addition to the original versions of these Invariant
Sections. You may include a translation of this License, and all the license notices in
the Document, and any Warrany Disclaimers, provided that you also include the original
English version of this License and the original versions of those notices and disclaimers. In
case of a disagreement between the translation and the original version of this License or a
notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”,
the requirement (section 4) to Preserve its Title (section 1) will typically require changing
the actual title.
9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as expressly
provided for under this License. Any other attempt to copy, modify, sublicense or distribute
the Document is void, and will automatically terminate your rights under this License.
However, parties who have received copies, or rights, from you under this License will not
have their licenses terminated so long as such parties remain in full compliance.
10. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the GNU Free Doc-
umentation License from time to time. Such new versions will be similar in spirit to
the present version, but may differ in detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.
104 Bison 1.875
Each version of the License is given a distinguishing version number. If the Document
specifies that a particular numbered version of this License “or any later version” applies
to it, you have the option of following the terms and conditions either of that specified
version or of any later version that has been published (not as a draft) by the Free Software
Foundation. If the Document does not specify a version number of this License, you may
choose any version ever published (not as a draft) by the Free Software Foundation.
Appendix C: Copying This Manual 105
C.1.1 ADDENDUM: How to use this License for your documents
To use this License in a document you have written, include a copy of the License in the document
and put the following copyright and license notices just after the title page:
Copyright (C) year your name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled ‘‘GNU
Free Documentation License’’.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the
“with...Texts.” line with this:
with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three,
merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing
these examples in parallel under your choice of free software license, such as the GNU General
Public License, to permit their use in free software.
106 Bison 1.875
Appendix C: Index 107
Index
$
$$...................................... 40,56,89
$<typealt >$ .................................. 56
$<typealt >n.................................. 56
$accept ....................................... 89
$end .......................................... 89
$n..................................... 40,56,89
$undefined .................................... 89
%
%% ............................................ 93
%{code %} ...................................... 93
%debug ................................. 49,80,91
%defines................................... 49,91
%destructor ............................ 47,50,91
%dprec ........................................ 91
%error-verbose ............................ 55,91
%expect.................................... 48,49
%file-prefix="prefix ".................... 50,92
%glr-parser ............................... 13,92
%left .................................. 49,61,92
%locations .................................... 50
%merge ........................................ 92
%name-prefix="prefix ".................... 50,92
%no-lines ................................. 50,92
%no-parser .................................... 50
%nonassoc .............................. 49,61,92
%output="filename "....................... 50,92
%prec ...................................... 62,92
%pure-parser ........................... 48,50,92
%right ................................. 49,61,92
%start ................................. 48,49,92
%token ................................. 45,49,92
%token-table .............................. 50,92
%type .................................. 47,49,92
%union ................................. 46,49,93
%verbose ...................................... 51
%yacc ......................................... 51
/
/*...*/ ....................................... 93
:
:............................................. 93
;
;............................................. 93
@
@$...................................... 43,57,89
@n..................................... 43,57,89
|
|.......................................... 38,93
A
action ......................................... 40
action data types............................... 41
action features summary ........................ 56
actions in mid-rule ............................. 41
actions, location ............................... 43
actions, semantic ............................... 13
additional C code section ....................... 36
algorithm of parser ............................. 59
ambiguous grammars ....................... 11,66
associativity ................................... 61
B
Backus-Naur form.............................. 11
Bison declaration summary ..................... 49
Bison declarations.............................. 45
Bison declarations (introduction) ................ 36
Bison grammar ................................ 12
Bison invocation ............................... 83
Bison parser ................... ................ 16
Bison parser algorithm ......................... 59
Bison symbols, table of ......................... 89
Bison utility ................................... 16
BNF .......................................... 11
C
C code, section for additional ................... 36
C-language interface............................ 53
calc .......................................... 24
calculator, infix notation ........................ 24
calculator, location tracking..................... 26
calculator, multi-function ....................... 29
calculator, simple .............................. 19
character token ................................ 36
compiling the parser............................ 24
conflicts.................................... 13,60
conflicts, reduce/reduce ......................... 63
conflicts, suppressing warnings of ................ 48
context-dependent precedence ................... 62
context-free grammar........................... 11
controlling function ............................ 23
core, item set .................................. 77
D
dangling else .................................. 60
data type of locations .......................... 43
data types in actions ........................... 41
data types of semantic values ................... 39
debugging ..................................... 80
declaration summary ........................... 49
declarations ................................... 35
declarations section ............................ 35
declarations, Bison ............................. 45
declarations, Bison (introduction) ............... 36
declaring literal string tokens ................... 45
declaring operator precedence ................... 46
declaring the start symbol ...................... 48
108 Bison 1.875
declaring token type names .... ................. 45
declaring value types ................... ........ 46
declaring value types, nonterminals .............. 47
default action .................................. 40
default data type............................... 39
default location type ........................... 43
default stack limit .............................. 67
default start symbol ............................ 48
defining language semantics ..................... 39
E
else, dangling ................................. 60
epilogue ....................................... 36
error ...................................... 69,89
error recovery .................................. 69
error recovery, simple ........................... 25
error reporting function......................... 55
error reporting routine.......................... 23
examples, simple ............................... 19
exercises....................................... 33
F
FDL, GNU Free Documentation License ......... 99
file format ........................... .......... 17
finite-state machine ............................ 63
formal grammar.... ............................ 12
format of grammar file.......................... 17
freeing discarded symbols ....................... 47
frequently asked questions ...................... 87
G
generalized LR (GLR) parsing ............ 11,13,66
glossary ....................................... 95
GLR parsers and inline ........................ 16
GLR parsing ............................ 11,13,66
grammar file .................................. . 17
grammar rule syntax ........................... 38
grammar rules section .......................... 36
grammar, Bison ................................ 12
grammar, context-free .......................... 11
grouping, syntactic ............................. 11
I
incline ....................................... 16
infix notation calculator ........................ 24
interface ....................................... 53
introduction .................................... 1
invoking Bison ................................. 83
item .......................................... 76
item set core ................................... 77
K
kernel, item set ................................ 77
L
LALR(1)....................................... 65
LALR(1) grammars............................. 11
language semantics, defining .................... 39
layout of Bison grammar........................ 17
left recursion .................................. 38
lexical analyzer ................................ 53
lexical analyzer, purpose ........................ 16
lexical analyzer, writing ........................ 22
lexical tie-in ................................... 72
literal string token ............................. 37
literal token ................................... 36
location .................................... 16,43
location actions ................................ 43
location tracking calculator ..................... 26
look-ahead token ............................... 59
LR(1) ......................................... 65
LR(1) grammars ............................... 11
ltcalc ........................................ 26
M
main function in simple example ................ 23
mfcalc ........................................ 29
mid-rule actions................................ 41
multi-function calculator........................ 29
multicharacter literal ........................... 37
mutual recursion ............................... 39
N
non-deterministic parsing.................... 11,66
nonterminal symbol ............................ 36
nonterminal, useless ............................ 76
O
operator precedence ............................ 61
operator precedence, declaring .................. 46
options for invoking Bison ...................... 83
overflow of parser stack ......................... 67
P
parse error.................................. ... 55
parser ......................................... 16
parser stack ................................... 59
parser stack overflow ........................... 67
parser state ................... ................. 63
pointed rule .................................. . 76
polish notation calculator ....................... 19
position, textual ............................ 16,43
precedence declarations ......................... 46
precedence of operators ......................... 61
precedence, context-dependent .................. 62
precedence, unary operator ..... ................ 62
preventing warnings about conflicts .............. 48
Prologue ...................................... 35
pure parser ................... ................. 48
Q
questions ...................................... 87
Appendix C: Index 109
R
recovery from errors ..... ....................... 69
recursive rule .................................. 38
reduce/reduce conflict .......................... 63
reduction ...................................... 59
reentrant parser ................................ 48
reverse polish notation.......................... 19
right recursion ................................. 38
rpcalc ........................................ 19
rule syntax .................................... 38
rule, pointed ................................... 76
rule, useless.................................... 76
rules section for grammar ....................... 36
running Bison (introduction) .................... 23
S
semantic actions ............................... 13
semantic value ................................. 13
semantic value type ............................ 39
shift/reduce conflicts ........................ 13,60
shifting ........................................ 59
simple examples................... ............. 19
single-character literal .......................... 36
stack overflow.................................. 67
stack, parser ................................... 59
stages in using Bison ........................... 17
start symbol ................... ................ 12
start symbol, declaring ......................... 48
state (of parser)................................ 63
string token ................................... 37
summary, action features ....................... 56
summary, Bison declaration ..................... 49
suppressing conflict warnings .................... 48
symbol ........................................ 36
symbol table example .......................... 30
symbols (abstract) ............................. 11
symbols in Bison, table of....................... 89
syntactic grouping ............................. 11
syntax error ................................... 55
syntax of grammar rules ........................ 38
T
terminal symbol........... ..................... 36
textual position............................. 16,43
token ......................................... 11
token type ..................................... 36
token type names, declaring..................... 45
token, useless .................................. 76
tracing the parser .............................. 80
U
unary operator precedence ...................... 62
useless nonterminal............................. 76
useless rule ................... ................. 76
useless token ................................... 76
using Bison .......................... .......... 17
V
value type, semantic ............................ 39
value types, declaring........................... 46
value types, nonterminals, declaring ............. 47
value, semantic ................................ 13
W
warnings, preventing ........................... 48
writing a lexical analyzer ....................... 22
Y
YYABORT.................................... 53,89
YYABORT; ...................................... 56
YYACCEPT................................... 53,89
YYACCEPT; ..................................... 57
YYBACKUP................................... 57,89
yychar ................................. 57,60,90
yyclearin ................................. 70,90
yyclearin; .................................... 57
yydebug.................................... 80,91
YYDEBUG.................................... 80,89
YYEMPTY ....................................... 57
yyerrok.................................... 70,91
yyerrok; ...................................... 57
yyerror.................................... 55,91
YYERROR.................................... 57,90
YYERROR; ...................................... 57
YYERROR_VERBOSE .............................. 90
YYINITDEPTH ............................... 67,90
yylex ...................................... 53,91
YYLEX_PARAM ................................... 90
yylloc ..................................... 55,91
YYLLOC_DEFAULT ............................... 44
YYLTYPE.................................... 55,90
yylval ..................................... 54,91
YYMAXDEPTH ................................ 67,90
yynerrs.................................... 56,91
yyparse.................................... 53,91
YYPARSE_PARAM................................. 90
YYPRINT ....................................... 81
YYRECOVERING ........................... 57,70,90
YYSTACK_USE_ALLOCA ........................... 90
YYSTYPE ....................................... 90
110 Bison 1.875

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