TCP IP Sockets In C, Second Edition Practical Guide For Programmers

Morgan.Kaufmann.TCPIP.Sockets.In.C.Practical.Guide.For.Programmers.2nd.Edition.Mar.2009.ISBN.0123745403

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Cover Page
Copyright Page
- Copyright
Preface
- Preface
1 Introduction
- 1 Introduction
2 Basic TCP Sockets
- 2 Basic TCP Sockets
3 Of Names and Address Families
- 3 Of Names and Address Families
4 Using UDP Sockets
- 4 Using UDP Sockets
5 Sending and Receiving Data
- 5 Sending and Receiving Data
6 Beyond Basic Socket Programming
- 6 Beyond Basic Socket Programming
7 Under the Hood
- 7 Under the Hood
8 Socket Programming in C++
- 8 Socket Programming in C++
References
- References

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Preface to the Second Edition

When we wrote the ﬁrst edition of this book, it was not very common for college courses on

networking to include programming components. That seems diﬃcult to believe now, when

the Internet has become so important to our world, and the pedagogical beneﬁts of hands-on

programming and real-world protocol examples are so widely accepted. Although there are now

other languages that provide access to the Internet, interest in the original C-based Berkeley

Sockets remains high. The Sockets API (application programming interface) for networking

was developed at UC Berkeley in the 1980s for the BSD ﬂavor of UNIX—one of the very ﬁrst

examples of what would now be called an open-source project.

The Sockets API and the Internet both grew up in a world of many competing protocol

families—IPX, Appletalk, DECNet, OSI, and SNA in addition to Transmission Control Proto-

col/Internet Protocal (TCP/IP)—and Sockets was designed to support them all. Fewer protocol

families were in common use by the time we wrote the ﬁrst edition of this book, and the num-

ber today is even smaller. Nevertheless, as we predicted in the ﬁrst edition, the Sockets API

remains important for those who want to design and build distributed applications that use

the Internet—that is, that use TCP/IP. And the interface has proven robust enough to support

the new version of the Internet Protocol (IPv6), which is now supported on virtually all common

computing platforms.

Two main considerations motivated this second edition. First, based on our own experi-

ence and feedback from others, we found that some topics needed to be presented in more

depth and that others needed to be expanded. The second consideration is the increasing

acceptance and use of IP version 6, which is now supported by essentially all current end sys-

tem platforms. At this writing, it is not possible to use IPv6 to exchange messages with a large

fraction of hosts on the Internet, but it is possible to assign an IPv6 address to many of them.

Although it is still too early to tell whether IPv6 will take over the world, it is not too early to

start writing applications to be prepared.

xPreface

Changes from the First Edition

We have updated and considerably expanded most of the material, having added two chapters.

Major changes from the ﬁrst edition include:

IP version 6 coverage. We now include three kinds of code: IPv4-speciﬁc, IPv6-speciﬁc, and

generic. The code in the later chapters is designed to work with either protocol version

on dual-stack machines.

An additional chapter on socket programming in C++ (contributed by David B. Sturgill).

The PracticalSocket library provides wrappers for basic socket functionality. These allow

an instructor to teach socket programming to students without C programming back-

ground by giving them a library and then gradually peeling back the layers. Students

can start developing immediately after understanding addresses/ports and client/server.

Later they can be shown the details of socket programming by peeking inside the wrapper

code. Those teaching a subject that uses networking (e.g., OS) can use the library and only

selectively peel back the cover.

Enhanced coverage of data representation issues and strategies for organizing code that

sends and receives messages. In our instructional experience, we ﬁnd that students have

less and less understanding of how data is actually stored in memory,1so we have

attempted to compensate with more discussion of this important issue. At the same

time, internationalization will only increase in importance, and thus we have included

basic coverage of wide characters and encodings.

Omission of the reference section. The descriptions of most of the functions that make

up the Sockets API have been collected into the early chapters. However, with so many

online sources of reference information—including “man pages”—available, we chose to

leave out the complete listing of the API in favor of more code illustrations.

Highlighting important but subtle facts and caveats. Typographical devices call out

important concepts and information that might otherwise be missed on ﬁrst reading.

Although the scope of the book has expanded, we have not included everything that

we might have (or even that we were asked to include); examples of topics left for more

comprehensive texts (or the next edition) are raw sockets and programming with WinSock.

Intended Audience

We originally wrote this book so that we would have something to hand our students when we

wanted them to learn socket programming, so we would not have to take up valuable class time

1We speculate that this is due to the widespread use of C++ and Java, which hide such details from the

programmer, in undergraduate curricula.

Preface xi

teaching it. In the years since the ﬁrst edition, we have learned a good deal about the topics

that students need lots of help on, and those where they do not need as much handholding.

We also found that our book was appreciated at least as much by practitioners who were

looking for a gentle introduction to the subject. Therefore, this book is aimed simultaneously

at two general audiences: students in introductory courses in computer networks (graduate or

undergraduate) with a programming component, and practitioners who want to write their own

programs that communicate over the Internet. For students, it is intended as a supplement, not

as a primary text about networks. Although this second edition is signiﬁcantly bigger in size

and scope than the ﬁrst, we hope the book will still be considered a good value in that role.

For practitioners who just want to write some useful code, it should serve as a standalone

introduction—but readers in that category should be warned that this book will not make

them experts. Our philosophy of learning by doing has not changed, nor has our approach of

providing a concise tutorial suﬃcient to get one started learning on one’s own, and leaving the

comprehensive details to other authors. For both audiences, our goal is to take you far enough

so that you can start experimenting and learning on your own.

Assumed Background

We assume basic programming skills and experience with C and UNIX. You are expected to be

conversant with C concepts such as pointers and type casting, and you should have a basic

understanding of the binary representation of data. Some of our examples are factored into

ﬁles that should be compiled separately; we assume that you can deal with that.

Here is a little test: If you can puzzle out what the following code fragment does, you

should have no problem with the code in this book:

typedef struct {

int a;

short s[2];

} MSG;

MSG *mp, m = {4, 1, 0};

char *fp, *tp;

mp = (MSG *) malloc(sizeof(MSG));

for (fp = (char *)m.s, tp = (char *)mp->s; tp < (char *)(mp+1);)

*tp++ = *fp++;

If you do not understand this fragment, do not despair (there is nothing quite so convo-

luted in our code), but you might want to refer to your favorite C programming book to ﬁnd

out what is going on here.

You should also be familiar with the UNIX notions of process/address space, command-

line arguments, program termination, and regular ﬁle input and output. The material in

Chapters 4 and 6 assumes a somewhat more advanced grasp of UNIX. Some prior exposure to

networking concepts such as protocols, addresses, clients, and servers will be helpful.

xii Preface

Platform Requirements and Portability

Our presentation is UNIX-based. When we were developing this book, several people urged us

to include code for Windows as well as UNIX. It was not possible to do so for various reasons,

including the target length (and price) we set for the book.

For those who only have access to Windows platforms, please note that the examples in

the early chapters require minimal modiﬁcations to work with WinSock. (You have to change

the include ﬁles and add a setup call at the beginning of the program and a cleanup call

at the end.) Most of the other examples also require very slight additional modiﬁcations.

However, some are so dependent on the UNIX programming model that it does not make

sense to port them to WinSock. WinSock-ready versions of the other examples, as well as

detailed descriptions of the code modiﬁcations required, are available from the book’s Web

site at www.mkp.com/socket. Note also that almost all of our example code works with minimal

modiﬁcations under the Cygwin UNIX library package for Windows, which is available online.

For this second edition, we have adopted the C99 language standard. This version

of the language is supported by most compilers and oﬀers so many readability-improving

advantages—including line-delimited comments, ﬁxed-size integer types, and declarations

anywhere in a block—that we could not justify not using it.

Our code makes use of the “Basic Socket Interface Extensions for IPv6” ?. Among these

extensions is a new and diﬀerent interface to the name system. Because we rely completely

on this new interface (getaddrinfo()), our generic code may not run on some older platforms.

However, we expect that most modern systems will run our code just ﬁne.

The example programs included here have all been tested (and should compile and run

without modiﬁcation) on both *NIX and MacOS. Header (.h) ﬁle locations and dependencies are,

alas, not quite standard and may require some ﬁddling on your system. Socket option support

also varies widely across systems; we have tried to focus on those that are most universally

supported. Consult your API documentation for system speciﬁcs. (By API documentation we

mean the “man pages” for your system. To learn about this, type “man man” or use your

favorite web search tool.)

Please be aware that although we strive for a basic level of robustness, the primary goal

of our code examples is pedagogy, and the code is not production quality. We have sacriﬁced

some robustness for brevity and clarity, especially in the generic server code. (It turns out to

be nontrivial to write a server that works under all combinations of IPv4 and IPv6 protocol

conﬁgurations and also maximizes the likelihood of successful client connection under all

circumstances.)

This Book Will Not Make You an Expert!

We hope this second edition will be useful as a resource, even to those who already know quite

a bit about sockets. As with the ﬁrst edition, we learned some things in writing it. But becoming

an expert takes years of experience, as well as other, more comprehensive sources ?,?.

Preface xiii

The ﬁrst chapter is intended to give “just enough” of the big picture to get you ready to

write code. Chapter ?? shows you how to write TCP clients and servers using either IPv4 or IPv6.

Chapter ?? shows how to make your clients and servers use the network’s name service, and

also describes how to make them IP-version-independent. Chapter ?? covers User Datagram

Protocol (UDP). Chapters ?? and ?? provide background needed to write more programs, while

Chapter ?? relates some of what is going on in the Sockets implementation to the API calls;

these three are essentially independent and may be presented in any order. Finally, Chapter ??

presents a C++ class library that provides simpliﬁed access to socket functionality.

Throughout the book, certain statements are highlighted like this: This book will not

make you an expert! Our goal is to bring to your attention those subtle but important facts

and ideas that one might miss on ﬁrst reading. The marks in the margin tell you to “note well”

whatever is in bold.

Acknowledgments

Many people contributed to making this book a reality. In addition to all those who helped us

with the ﬁrst edition (Michel Barbeau, Steve Bernier, Arian Durresi, Gary Harkin, Ted Herman,

Lee Hollaar, David Hutchison, Shunge Li, Paul Linton, Ivan Marsic, Willis Marti, Kihong Park, Dan

Schmitt, Michael Scott, Robert Strader, Ben Wah, and Ellen Zegura), we especially thank David

B. Sturgill, who contributed code and text for Chapter ??, and Bobby Krupczak for his help in

reviewing the draft of this second edition. Finally, to the folks at Morgan Kaufmann/Elsevier—

Rick Adams, our editor, assistant editor Maria Alonso, and project manager Melinda Ritchie—

thank you for your patience, help, and caring about the quality of our book.

Feedback

We are very interested in weeding out errors and otherwise improving future editions/

printings, so if you ﬁnd any errors, please send an e-mail to either of us. We will maintain

an errata list on the book’s Web page.

M.J.D. jeff_donahoo@baylor.edu

K.L.C. calvert@netlab.uky.edu

chapter 1

Introduction

Today people use computers to make phone calls, watch TV, send instant messages to

their friends, play games with other people, and buy most anything you can think of—from

songs to automobiles. The ability of programs to communicate over the Internet makes all

this possible. It’s hard to say how many individual computers are now reachable over the

Internet, but we can safely say that it is growing rapidly; it won’t be long before the number is

in the billions. Moreover, new applications are being developed every day. With the push for

ever increasing bandwidth and access, the impact of the Internet will continue to grow for the

forseeable future.

How does a program communicate with another program over a network? The goal of this

book is to start you on the road to understanding the answer to that question, in the context of

the C programming language. For a long time, C was the language of choice for implementing

network communication softward. Indeed, the application programming interface (API) known

as Sockets was ﬁrst developed in C.

Before we delve into the details of sockets, however, it is worth taking a brief look at

the big picture of networks and protocols to see where our code will ﬁt in. Our goal here

is not to teach you how networks and TCP/IP work—many ﬁne texts are available for that

purpose [1, 3, 10, 15, 17]—but rather to introduce some basic concepts and terminology.

1.1 Networks, Packets, and Protocols

A computer network consists of machines interconnected by communication channels. We call

these machines hosts and routers. Hosts are computers that run applications such as your Web

2Chapter 1: Introduction

browser, your IM agent, or a ﬁle-sharing program. The application programs running on hosts

are the real “users” of the network. Routers (also called gateways) are machines whose job is

to relay, or forward, information from one communication channel to another. They may run

programs but typically do not run application programs. For our purposes, a communication

channel is a means of conveying sequences of bytes from one host to another; it may be a

wired (e.g., Ethernet), a wireless (e.g., WiFi), or other connection.

Routers are important simply because it is not practical to connect every host directly

to every other host. Instead, a few hosts connect to a router, which connects to other routers,

and so on to form the network. This arrangement lets each machine get by with a relatively

small number of communication channels; most hosts need only one. Programs that exchange

information over the network, however, do not interact directly with routers and generally

remain blissfully unaware of their existence.

By information we mean sequences of bytes that are constructed and interpreted by pro-

grams. In the context of computer networks, these byte sequences are generally called packets.

A packet contains control information that the network uses to do its job and sometimes also

includes user data. An example is information identifying the packet’s destination. Routers

use such control information to ﬁgure out how to forward each packet.

Aprotocol is an agreement about the packets exchanged by communicating programs

and what they mean. A protocol tells how packets are structured—for example, where the

destination information is located in the packet and how big it is—as well as how the infor-

mation is to be interpreted. A protocol is usually designed to solve a speciﬁc problem using

given capabilities. For example, the HyperText Transfer Protocol (HTTP) solves the problem of

transferring hypertext objects between servers, where they are stored or generated, and Web

browsers that make them visible and useful to users. Instant messaging protocols solve the

problem of enabling two or more users to exchange brief text messages.

Implementing a useful network requires solving a large number of diﬀerent problems.

To keep things manageable and modular, diﬀerent protocols are designed to solve diﬀerent

sets of problems. TCP/IP is one such collection of solutions, sometimes called a protocol suite.

It happens to be the suite of protocols used in the Internet, but it can be used in stand-alone

private networks as well. Henceforth when we talk about the network, we mean any network

that uses the TCP/IP protocol suite. The main protocols in the TCP/IP suite are the Internet

Protocol (IP), the Transmission Control Protocol (TCP), and the User Datagram Protocol (UDP).

It turns out to be useful to organize protocols into layers; TCP/IP and virtually all other

protocol suites are organized this way. Figure 1.1 shows the relationships among the proto-

cols, applications, and the Sockets API in the hosts and routers, as well as the ﬂow of data

from one application (using TCP) to another. The boxes labeled TCP and IP represent imple-

mentations of those protocols. Such implementations typically reside in the operating system

of a host. Applications access the services provided by UDP and TCP through the Sockets API,

represented as a dashed line. The arrow depicts the ﬂow of data from the application, through

the TCP and IP implementations, through the network, and back up through the IP and TCP

implementations at the other end.

1.1 Networks, Packets, and Protocols 3

Host Router Host

Socket Socket

(e.g., Ethernet)

IP Channel

TCP

Application Application

Channel

Figure 1.1: A TCP/IP network.

In TCP/IP, the bottom layer consists of the underlying communication channels—for

example, Ethernet or dial-up modem connections. Those channels are used by the network

layer, which deals with the problem of forwarding packets toward their destination (i.e., what

routers do). The single-network layer protocol in the TCP/IP suite is the Internet Protocol; it

solves the problem of making the sequence of channels and routers between any two hosts

look like a single host-to-host channel.

The Internet Protocol provides a datagram service: every packet is handled and delivered

by the network independently, like letters or parcels sent via the postal system. To make this

work, each IP packet has to contain the address of its destination, just as every package that

you mail is addressed to somebody. (We’ll say more about addresses shortly.) Although most

delivery companies guarantee delivery of a package, IP is only a best-eﬀort protocol: it attempts

to deliver each packet, but it can (and occasionally does) lose, reorder, or duplicate packets in

transit through the network.

The layer above IP is called the transport layer. It oﬀers a choice between two protocols:

TCP and UDP. Each builds on the service provided by IP, but they do so in diﬀerent ways to

provide diﬀerent kinds of transport, which are used by application protocols with diﬀerent

needs. TCP and UDP have one function in common: addressing. Recall that IP delivers packets

to hosts; clearly, a ﬁner granularity of addressing is needed to get a packet to a particular

application program, perhaps one of many using the network on the same host. Both TCP and

UDP use addresses, called port numbers, to identify applications within hosts. TCP and UDP

are called end-to-end transport protocols because they carry data all the way from one program

to another (whereas IP only carries data from one host to another).

TCP is designed to detect and recover from the losses, duplications, and other errors that

may occur in the host-to-host channel provided by IP. TCP provides a reliable byte-stream chan-

nel, so that applications do not have to deal with these problems. It is a connection-oriented

protocol: before using it to communicate, two programs must ﬁrst establish a TCP connection,

4Chapter 1: Introduction

which involves completing an exchange of handshake messages between the TCP implemen-

tations on the two communicating computers. Using TCP is also similar in many ways to ﬁle

input/output (I/O). In fact, a ﬁle that is written by one program and read by another is a rea-

sonable model of communication over a TCP connection. UDP, on the other hand, does not

attempt to recover from errors experienced by IP; it simply extends the IP best-eﬀort data-

gram service so that it works between application programs instead of between hosts. Thus,

applications that use UDP must be prepared to deal with losses, reordering, and so on.

1.2 About Addresses

When you mail a letter, you provide the address of the recipient in a form that the postal

service can understand. Before you can talk to someone on the phone, you must supply a

phone number to the telephone system. In a similar way, before a program can communicate

with another program, it must tell the network something to identify the other program. In

TCP/IP, it takes two pieces of information to identify a particular program: an Internet address,

used by IP, and a port number, the additional address interpreted by the transport protocol

(TCP or UDP).

Internet addresses are binary numbers. They come in two ﬂavors, corresponding to the

two versions of the Internet Protocol that have been standardized. The most common is ver-

sion 4 (IPv4, [12]); the other is version 6 (IPv6, [5]), which is just beginning to be deployed.

IPv4 addresses are 32 bits long; because this is only enough to identify about 4 billion distinct

destinations, they are not really big enough for today’s Internet. (That may seem like a lot,

but because of the way they are allocated, many are wasted. More than half of the total IPv4

address space has already been allocated.) For that reason, IPv6 was introduced. IPv6 addresses

are 128 bits long.

1.2.1 Writing Down IP Addresses

In representing Internet addresses for human consumption (as opposed to using them inside

programs), diﬀerent conventions are used for the two versions of IP. IPv4 addresses are con-

ventionally written as a group of four decimal numbers separated by periods (e.g., 10.1.2.3);

this is called the dotted-quad notation. The four numbers in a dotted-quad string represent the

contents of the four bytes of the Internet address—thus, each is a number between 0 and 255.

The 16 bytes of an IPv6 address, on the other hand, by convention are represented as

groups of hexadecimal digits, separated by colons (e.g., 2000:fdb8:0000:0000:0001:00ab:853c:

39a1). Each group of digits represents 2 bytes of the address; leading zeros may be omitted,

so the ﬁfth and sixth groups in the foregoing example might be rendered as just :1:ab:. Also,

one sequence of groups that contains only zeros may be omitted altogether (while leaving the

colons that would separate them from the rest of the address). So the example above could be

written as 2000:fdb8::1:00ab:853c:39a1.

1.2 About Addresses 5

Technically, each Internet address refers to the connection between a host and an

underlying communication channel—in other words, a network interface. A host may have

several interfaces; it is not uncommon, for example, for a host to have connections to both

wired (Ethernet) and wireless (WiFi) networks. Because each such network connection belongs

to a single host, an Internet address identiﬁes a host as well as its connection to the network.

However, the converse is not true, because a single host can have multiple interfaces, and each

interface can have multiple addresses. (In fact, the same interface can have both IPv4 and IPv6

addresses.)

1.2.2 Dealing with Two Versions

When the ﬁrst edition of this book was written, IPv6 was not widely supported. Today most

systems are capable of supporting IPv6 “out of the box.” To smooth the transition from IPv4

to IPv6, most systems are dual-stack, simultaneously supporting both IPv4 and IPv6. In such

systems, each network interface (channel connection) may have at least one IPv4 address and

one IPv6 address.

The existence of two versions of IP complicates life for the socket programmer. In gen-

eral, you will need to choose either IPv4 or IPv6 as the underlying protocol when you create

a socket to communicate. So how can you write an application that works with both ver-

sions? Fortunately, dual-stack systems handle interoperability by supporting both protocol

versions and allowing IPv6 sockets to communicate with either IPv4 or IPv6 applications. Of

course, IPv4 and IPv6 addresses are quite diﬀerent; however, IPv4 addresses can be mapped

into IPv6 addresses using IPv4 mapped addresses. An IPv4 mapped address is formed by pre-

ﬁxing the four bytes in the IPv4 address with ::ﬀf. For example, the IPv4 mapped address

for 132.3.23.7 is ::ﬀﬀ:132.3.23.7. To aid in human readability, the last four bytes are typi-

cally written in dotted-quad notation. We discuss protocol interoperability in greater detail in

Chapter 3.

Unfortunately, having an IPv6 Internet address is not suﬃcient to enable you to com-

municate with every other IPv6-enabled host across the Internet. To do that, you must also

arrange with your Internet Service Provider (ISP) to provide IPv6 forwarding service.

1.2.3 Port Numbers

We mentioned earlier that it takes two pieces of address to get a message to a program. The

port number in TCP or UDP is always interpreted relative to an Internet address. Returning to

our earlier analogies, a port number corresponds to a room number at a given street address,

say, that of a large building. The postal service uses the street address to get the letter to a

mailbox; whoever empties the mailbox is then responsible for getting the letter to the proper

room within the building. Or consider a company with an internal telephone system: to speak

to an individual in the company, you ﬁrst dial the company’s main phone number to connect

to the internal telephone system and then dial the extension of the particular telephone of the

individual with whom you wish to speak. In these analogies, the Internet address is the street

6Chapter 1: Introduction

address or the company’s main number, whereas the port corresponds to the room number or

telephone extension. Port numbers are the same in both IPv4 and IPv6: 16-bit unsigned binary

numbers. Thus, each one is in the range 1 to 65,535 (0 is reserved).

1.2.4 Special Addresses

In each version of IP, certain special-purpose addresses are deﬁned. One of these that is worth

knowing is the loopback address, which is always assigned to a special loopback interface,

a virtual device that simply echoes transmitted packets right back to the sender. The loop-

back interface is very useful for testing, because packets sent to that address are immediately

returned to the destination. Moreover, it is present on every host and can be used even when a

computer has no other interfaces (i.e., is not connected to the network). The loopback address

for IPv4 is 127.0.0.1;1for IPv6 it is 0:0:0:0:0:0:0:1 (or just ::1).

Another group of IPv4 addresses reserved for a special purpose includes those reserved

for “private use.” This group includes all IPv4 addresses that start with 10 or 192.168, as well

as those whose ﬁrst number is 172 and whose second number is between 16 and 31. (There

is no corresponding class for IPv6.) These addresses were originally designated for use in pri-

vate networks that are not part of the global Internet. Today they are often used in homes

and small oﬃces that are connected to the Internet through a network address translation

(NAT) [7] device. Such a device acts like a router that translates (rewrites) the addresses and

ports in packets as it forwards them. More precisely, it maps (private address, port) pairs in

packets on one of its interfaces to (public address, port) pairs on the other interface. This

enables a small group of hosts (e.g., those on a home network) to eﬀectively “share” a sin-

gle IP address. The importance of these addresses is that they cannot be reached from the

global Internet. If you are trying out the code in this book on a machine that has an address

in the private-use class (e.g., on your home network), and you are trying to communicate

with another host that does not have one of these addresses, in general you will not suc-

ceed unless the host with the private address initiates communication—and even then you

may fail.

A related class contains the link-local, or “autoconﬁguration” addresses. For IPv4, such

addresses begin with 169.254. For IPv6, any address whose ﬁrst 16-bit chunk is FE80, FE90,

FEA0, or FEB0 is a link-local address. These addresses can only be used for communication

between hosts connected to the same network; routers will not forward packets that have such

addresses as their destination.

Finally, another class consists of multicast addresses. Whereas regular IP (sometimes

called “unicast”) addresses refer to a single destination, multicast addresses potentially refer

to an arbitrary number of destinations. Multicasting is an advanced subject that we cover

brieﬂy in Chapter 6. In IPv4, multicast addresses in dotted-quad format have a ﬁrst number in

the range 224 to 239. In IPv6, multicast addresses start with FF.

1Technically, any IPv4 address beginning with 127 should loop back.

1.4 Clients and Servers 7

1.3 About Names

Most likely you are accustomed to referring to hosts by name (e.g., host.example.com).

However, the Internet protocols deal with addresses (binary numbers), not names. You should

understand that the use of names instead of addresses is a convenience feature that is inde-

pendent of the basic service provided by TCP/IP—you can write and use TCP/IP applications

without ever using a name. When you use a name to identify a communication end point, the

system does some extra work to resolve the name into an address. This extra step is often

worth it, for a couple of reasons. First, names are obviously easier for humans to remember

than dotted-quads (or, in the case of IPv6, strings of hexadecimal digits). Second, names pro-

vide a level of indirection, which insulates users from IP address changes. During the writing

of the ﬁrst edition of this book, the address of the Web server www.mkp.com changed. Because

we always refer to that Web server by name, www.mkp.com resolves to the current Internet

address instead of 208.164.121.48. The change in IP address is transparent to programs that

use the name to access the Web server.

The name-resolution service can access information from a wide variety of sources. Two

of the primary sources are the Domain Name System (DNS) and local conﬁguration databases.

The DNS [8] is a distributed database that maps domain names such as www.mkp.com to

Internet addresses and other information; the DNS protocol [9] allows hosts connected to

the Internet to retrieve information from that database using TCP or UDP. Local conﬁguration

databases are generally OS-speciﬁc mechanisms for local name-to-Internet address mappings.

1.4 Clients and Servers

In our postal and telephone analogies, each communication is initiated by one party, who

sends a letter or makes the telephone call, while the other party responds to the initiator’s

contact by sending a return letter or picking up the phone and talking. Internet communica-

tion is similar. The terms client and server refer to these roles: The client program initiates

communication, while the server program waits passively for and then responds to clients that

contact it. Together, the client and server compose the application. The terms client and server

are descriptive of the typical situation in which the server makes a particular capability—for

example, a database service—available to any client able to communicate with it.

Whether a program is acting as a client or server determines the general form of its use

of the Sockets API to establish communication with its peer. (The client is the peer of the

server and vice versa.) In addition, the client-server distinction is important because the client

needs to know the server’s address and port initially, but not vice versa. With the Sockets API,

the server can, if necessary, learn the client’s address information when it receives the initial

communication from the client. This is analogous to a telephone call—in order to be called, a

person does not need to know the telephone number of the caller. As with a telephone call,

once the connection is established, the distinction between server and client disappears.

8Chapter 1: Introduction

How does a client ﬁnd out a server’s IP address and port number? Usually, the client

knows the name of the server it wants—for example, from a Universal Resource Loca-

tor (URL) such as http://www.mkp.com—and uses the name-resolution service to learn the

corresponding Internet address.

Finding a server’s port number is a diﬀerent story. In principle, servers can use any port,

but the client must be able to learn what it is. In the Internet, there is a convention of assign-

ing well-known port numbers to certain applications. The Internet Assigned Number Authority

(IANA) oversees this assignment. For example, port number 80 has been assigned to the Hyper-

Text Transfer Protocol (HTTP). When you run an HTTP client browser, it tries to contact the

Web server on that port by default. A list of all the assigned port numbers is maintained by

the numbering authority of the Internet (see http://www.iana.org/assignments/port-numbers).

You may have heard of an alternative to client-server called peer-to-peer (P2P). In P2P,

applications both consume and provide service, unlike the traditional client-server architecture

in which servers provide service and clients consume. In fact, P2P nodes are sometimes called

“servents,” combining the words server and client. So do you need to learn a diﬀerent set

of technologies to program for P2P instead of client-server? No. In Sockets, client vs. server

merely distinguishes who makes the initial connection and who waits for connections. P2P

applications typically both initiate connections (to existing P2P nodes) and accept connections

(from other P2P nodes). After reading this book, you’ll be able to write P2P applications just

as well as client-server.

1.5 What Is a Socket?

Asocket is an abstraction through which an application may send and receive data, in much

the same way as an open-ﬁle handle allows an application to read and write data to stable

storage. A socket allows an application to plug in to the network and communicate with other

applications that are plugged in to the same network. Information written to the socket by

an application on one machine can be read by an application on a diﬀerent machine and vice

versa.

Diﬀerent types of sockets correspond to diﬀerent underlying protocol suites and diﬀer-

ent stacks of protocols within a suite. This book deals only with the TCP/IP protocol suite.

The main types of sockets in TCP/IP today are stream sockets and datagram sockets. Stream

sockets use TCP as the end-to-end protocol (with IP underneath) and thus provide a reliable

byte-stream service. A TCP/IP stream socket represents one end of a TCP connection. Data-

gram sockets use UDP (again, with IP underneath) and thus provide a best-eﬀort datagram

service that applications can use to send individual messages up to about 65,500 bytes in

length. Stream and datagram sockets are also supported by other protocol suites, but this

book deals only with TCP stream sockets and UDP datagram sockets. A TCP/IP socket is

uniquely identiﬁed by an Internet address, an end-to-end protocol (TCP or UDP), and a port

number. As you proceed, you will encounter several ways for a socket to become bound to

an address.

Exercises 9

TCP

...

TCP sockets

TCP ports 1 65535 1 2 65535 UDP ports

UDP

...

... ...

Sockets bound to ports

Descriptor references

UDP sockets

... ...

Applications

Figure 1.2: Sockets, protocols, and ports.

Figure 1.2 depicts the logical relationships among applications, socket abstractions,

protocols, and port numbers within a single host. There are several things to note about these

relationships. First, a program can have multiple sockets in use at the same time. Second, mul-

tiple programs can be using the same socket abstraction at the same time, although this is less

common. The ﬁgure shows that each socket has an associated local TCP or UDP port, which

is used to direct incoming packets to the application that is supposed to receive them. Earlier

we said that a port identiﬁes an application on a host. Actually, a port identiﬁes a socket on a

host. There is more to it than this, however, because as Figure 1.2 shows, more than one socket

can be associated with one local port. This is most common with TCP sockets; fortunately, you

need not understand the details to write client-server programs that use TCP sockets. The full

story will be revealed in Chapter 7.

Exercises

1. Report your IP addresses using the ifconfig command in *NIX or the ipconfig command

in Windows. Identify the addresses that are IPv6.

2. Report the name of the computer on which you are working by using the hostname

command.

3. Can you ﬁnd the IP address of any of your directly connected routers?

4. Use Internet search to try and discover what happened to IPv5?

5. Write the following IPv6 address using as few characters as possible:

2345:0000:0000:A432:0000:0000:0000:0023

10 Chapter 1: Introduction

6. Can you think of a real-life example of communication that does not ﬁt the client-server

model?

7. To how many diﬀerent kinds of networks is your home connected? How many support

two-way transport?

8. IP is a best-eﬀort protocol, requiring that information be broken down into datagrams,

which may be lost, duplicated, or reordered. TCP hides all of this, providing a reliable

service that takes and delivers an unbroken stream of bytes. How might you go about

providing TCP service on top of IP? Why would anybody use UDP when TCP is available?

chapter 2

Basic TCP Sockets

It’s time to learn about writing your own socket applications. We’ll start with TCP. By

now you’re probably ready to get your hands dirty with some actual code, so we begin by

going through a working example of a TCP client and server. Then we present the details of

the socket API used in basic TCP. To keep things simpler, we’ll present code initially that works

for one particular version of IP: IPv4, which at the time this is being written is still the dominant

version of the Internet Protocol, by a wide margin. At the end of this chapter we present the

(minor) modiﬁcations required to write IPv6 versions of our clients and servers. In Chapter 3

we will demonstrate the creation of protocol-independent applications.

Our example client and server implement the echo protocol. It works as follows: the client

connects to the server and sends its data. The server simply echoes whatever it receives back to

the client and disconnects. In our application, the data that the client sends is a string provided

as a command-line argument. Our client will print the data it receives from the server so we

can see what comes back. Many systems include an echo service for debugging and testing

purposes.

2.1 IPv4 TCP Client

The distinction between client and server is important because each uses the sockets interface

diﬀerently at certain steps in the communication. We ﬁrst focus on the client. Its job is to

initiate communication with a server that is passively waiting to be contacted.

12 Chapter 2: Basic TCP Sockets

The typical TCP client’s communication involves four basic steps:

1. Create a TCP socket using socket().

2. Establish a connection to the server using connect().

3. Communicate using send and recv().

4. Close the connection with close().

TCPEchoClient4.c is an implementation of a TCP echo client for IPv4.

TCPEchoClient4.c

1#include <stdio.h>

2#include <stdlib.h>

3#include <string.h>

4#include <unistd.h>

5#include <sys/types.h>

6#include <sys/socket.h>

7#include <netinet/in.h>

8#include <arpa/inet.h>

9#include "Practical.h"

11 int main(int argc, char *argv[]) {

13 if (argc<3||argc > 4) // Test for correct number of arguments

14 DieWithUserMessage("Parameter(s)",

15 "<Server Address> <Echo Word> [<Server Port>]");

17 char *servIP = argv[1]; // First arg: server IP address (dotted quad)

18 char *echoString = argv[2]; // Second arg: string to echo

20 // Third arg (optional): server port (numeric). 7 is well-known echo port

21 in_port_t servPort = (argc == 4) ? atoi(argv[3]) : 7;

23 // Create a reliable, stream socket using TCP

24 int sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

25 if (sock < 0)

26 DieWithSystemMessage("socket() failed");

28 // Construct the server address structure

29 struct sockaddr_in servAddr; // Server address

30 memset(&servAddr, 0, sizeof(servAddr)); // Zero out structure

31 servAddr.sin_family = AF_INET; // IPv4 address family

32 // Convert address

2.1 IPv4 TCP Client 13

33 int rtnVal = inet_pton(AF_INET, servIP, &servAddr.sin_addr.s_addr);

34 if (rtnVal == 0)

35 DieWithUserMessage("inet_pton() failed", "invalid address string");

36 else if (rtnVal < 0)

37 DieWithSystemMessage("inet_pton() failed");

38 servAddr.sin_port = htons(servPort); // Server port

40 // Establish the connection to the echo server

41 if (connect(sock, (struct sockaddr *) &servAddr, sizeof(servAddr)) < 0)

42 DieWithSystemMessage("connect() failed");

44 size_t echoStringLen = strlen(echoString); // Determine input length

46 // Send the string to the server

47 ssize_t numBytes = send(sock, echoString, echoStringLen, 0);

48 if (numBytes < 0)

49 DieWithSystemMessage("send() failed");

50 else if (numBytes != echoStringLen)

51 DieWithUserMessage("send()", "sent unexpected number of bytes");

53 // Receive the same string back from the server

54 unsigned int totalBytesRcvd = 0; // Count of total bytes received

55 fputs("Received: ", stdout); // Setup to print the echoed string

56 while (totalBytesRcvd < echoStringLen) {

57 char buffer[BUFSIZE]; // I/O buffer

58 /* Receive up to the buffer size (minus 1 to leave space for

59 a null terminator) bytes from the sender */

60 numBytes = recv(sock, buffer, BUFSIZE - 1, 0);

61 if (numBytes < 0)

62 DieWithSystemMessage("recv() failed");

63 else if (numBytes == 0)

64 DieWithUserMessage("recv()", "connection closed prematurely");

65 totalBytesRcvd += numBytes; // Keep tally of total bytes

66 buffer[numBytes] = '\0'; // Terminate the string!

67 fputs(buffer, stdout); // Print the echo buffer

68 }

70 fputc('\n', stdout); // Print a final linefeed

72 close(sock);

73 exit(0);

74 }

TCPEchoClient4.c

14 Chapter 2: Basic TCP Sockets

Our TCPEchoClient4.c does the following:

1. Application setup and parameter parsing: lines 1–21

Include ﬁles: lines 1–9

These header ﬁles declare the standard functions and constants of the API. Consult

your documentation (e.g., man pages) for the appropriate include ﬁles for socket func-

tions and data structures on your system. We utilize our own include ﬁle, Practical.h,

with prototypes for our own functions, which we describe below.

Typical parameter parsing and sanity checking: lines 13–21

The IPv4 address and string to echo are passed in as the ﬁrst two parameters. Option-

ally, the client takes the server port as the third parameter. If no port is provided, the

client uses the well-known echo protocol port, 7.

2. TCP socket creation: lines 23–26

We create a socket using the socket() function. The socket is for IPv4 (af_inet) using

the stream-based protocol (sock_stream) called TCP (ipproto_tcp). socket() returns an

integer-valued descriptor or “handle” for the socket if successful. If socket fails, it returns

–1, and we call our error-handling function, DieWithSystemMessage() (described later), to

print an informative hint and exit.

3. Prepare address and establish connection: lines 28–42

Prepare sockaddr_in structure to hold server address: lines 29–30

To connect a socket, we have to specify the address and port to connect to. The sock-

addr_in structure is deﬁned to be a “container” for this information. The call to memset()

ensures that any parts of the structure that we do not explicitly set contain zero.

Filling in the sockaddr_in: lines 31–38

We must set the address family (AF_INET), Internet address, and port number. The

function inet_pton() converts the string representation of the server’s Internet address

(passed as a command-line argument in dotted-quad notation) into a 32-bit binary

representation. The server’s port number was converted from a command-line string

to binary earlier; the call to htons() (“host to network short”) ensures that the binary

value is formatted as required by the API. (Reasons for this are described in Chapter 5.)

Connecting: lines 40–42

The connect() function establishes a connection between the given socket and the one

identiﬁed by the address and port in the sockaddr_in structure. Because the Sockets

API is generic, the pointer to the sockaddr_in address structure (which is speciﬁc to

IPv4 addresses) needs to be cast to the generic type (sockaddr∗), and the actual size of

the address data structure must be supplied.

4. Send echo string to server: lines 44–51

We ﬁnd the length of the argument string and save it for later use. A pointer to the

echo string is passed to the send() call; the string itself was stored somewhere (like all

command-line arguments) when the application was started. We do not really care where

2.1 IPv4 TCP Client 15

it is; we just need to know the address of the ﬁrst byte and how many bytes to send. (Note

that we do not send the end-of-string marker character (0) that is at the end of the argu-

ment string—and all strings in C). send() returns the number of bytes sent if successful

and –1 otherwise. If send() fails or sends the wrong number of bytes, we must deal with the

error. Note that sending the wrong number of bytes will not happen here. Nevertheless,

it’s a good idea to include the test because errors can occur in some contexts.

5. Receive echo server reply: lines 53–70

TCP is a byte-stream protocol. One implication of this type of protocol is that send()

boundaries are not preserved. In other words: The bytes sent by a call to send() on one

end of a connection may not all be returned by a single call to recv() on the other end.

(We discuss this issue in more detail in Chapter 7.) So we need to repeatedly receive bytes

until we have received as many as we sent. In all likelihood, this loop will only be executed

once because the data from the server will in fact be returned all at once; however, that

is not guaranteed to happen, and so we have to allow for the possibility that multiple

reads are required. This is a basic principle of writing applications that use sockets: you

must never assume anything about what the network and the program at the other

end are going to do.

Receive a block of bytes: lines 57–65

recv() blocks until data is available, returning the number of bytes copied into the

buﬀer or −1 in case of failure. A return value of zero indicates that the application at

the other end closed the TCP connection. Note that the size parameter passed to recv()

reserves space for adding a terminating null character.

Print buﬀer: lines 66–67

We print the data sent by the server as it is received. We add the terminating null

character (0) at the end of each chunk of received data so that it can be treated as

a string by fputs(). We do not check whether the bytes received are the same as the

bytes sent. The server may send something completely diﬀerent (up to the length of

the string we sent), and it will be written to the standard output.

Print newline: line 70

When we have received as many bytes as we sent, we exit the loop and print a newline.

6. Terminate connection and exit: lines 72–73

The close() function informs the remote socket that communication is ended, and then

deallocates local resources of the socket.

Our client application (and indeed all the programs in this book) makes use of two error-

handling functions:

DieWithUserMessage(const char *msg,const char *detail)

DieWithSystemMessage(const char *msg)

Both functions print a user-supplied message string (msg)tostderr, followed by a detail mes-

sage string; they then call exit() with an error return code, causing the application to terminate.

16 Chapter 2: Basic TCP Sockets

The only diﬀerence is the source of the detail message. For DieWithUserMessage(), the detail

message is user-supplied. For DieWithSystemMessage(), the detail message is supplied by the

system based on the value of the special variable errno (which describes the reason for the

most recent failure, if any, of a system call). We call DieWithSystemMessage() only if the error

situation results from a call to a system call that sets errno. (To keep our programs simple,

our examples do not contain much code devoted to recovering from errors—they simply punt

and exit. Production code generally should not give up so easily.)

Occasionally, we need to supply information to the user without exiting; we use printf()

if we need formatting capabilities, and fputs() otherwise. In particular, we try to avoid using

printf() to output ﬁxed, preformatted strings. One thing that you should never do is to pass

text received from the network as the ﬁrst argument to printf(). It creates a serious security

vulnerability. Use fputs() instead.

Note: the DieWith…() functions are declared in the header “Practical.h.” However,

the actual implementation of these functions is contained in the ﬁle DieWithMes-

sage.c, which should be compiled and linked with all example applications in this

text.

DieWithMessage.c

1#include <stdio.h>

2#include <stdlib.h>

4void DieWithUserMessage(const char *msg, const char *detail) {

5fputs(msg, stderr);

6fputs(": ", stderr);

7fputs(detail, stderr);

8fputc('\n', stderr);

9exit(1);

10 }

12 void DieWithSystemMessage(const char *msg) {

13 perror(msg);

14 exit(1);

15 }

DieWithMessage.c

If we compile TCPEchoClient4.c and DieWithMessage.c to create program TCPEchoClient4,

we can communicate with an echo server with Internet address 169.1.1.1 as follows:

%TCPEchoClient4 169.1.1.1 "Echo this!"

Received: Echo this!

2.2 IPv4 TCP Server 17

For our client to work, we need a server. Many systems include an echo server for

debugging and testing purposes; however, for security reasons, such servers are often ini-

tially disabled. If you don’t have access to an echo server, that’s okay because we’re about to

write one.

2.2 IPv4 TCP Server

We now turn our focus to constructing a TCP server. The server’s job is to set up a commu-

nication endpoint and passively wait for a connection from the client. There are four general

steps for basic TCP server communication:

1. Create a TCP socket using socket().

2. Assign a port number to the socket with bind().

3. Tell the system to allow connections to be made to that port, using listen().

4. Repeatedly do the following:

•Call accept() to get a new socket for each client connection.

•Communicate with the client via that new socket using send() and recv().

•Close the client connection using close().

Creating the socket, sending, receiving, and closing are the same as in the client. The

diﬀerences in the server’s use of sockets have to do with binding an address to the socket

and then using the socket as a way to obtain other sockets that are connected to clients. (We’ll

elaborate on this in the comments following the code.) The server’s communication with each

client is as simple as can be: it simply receives data on the client connection and sends the

same data back over to the client; it repeats this until the client closes its end of the connection,

at which point no more data will be forthcoming.

TCPEchoServer4.c

1#include <stdio.h>

2#include <stdlib.h>

3#include <string.h>

4#include <sys/types.h>

5#include <sys/socket.h>

6#include <netinet/in.h>

7#include <arpa/inet.h>

8#include "Practical.h"

10 static const int MAXPENDING = 5; // Maximum outstanding connection requests

12 int main(int argc, char *argv[]) {

18 Chapter 2: Basic TCP Sockets

14 if (argc != 2) // Test for correct number of arguments

15 DieWithUserMessage("Parameter(s)", "<Server Port>");

17 in_port_t servPort = atoi(argv[1]); // First arg: local port

19 // Create socket for incoming connections

20 int servSock; // Socket descriptor for server

21 if ((servSock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)

22 DieWithSystemMessage("socket() failed");

24 // Construct local address structure

25 struct sockaddr_in servAddr; // Local address

26 memset(&servAddr, 0, sizeof(servAddr)); // Zero out structure

27 servAddr.sin_family = AF_INET; // IPv4 address family

28 servAddr.sin_addr.s_addr = htonl(INADDR_ANY); // Any incoming interface

29 servAddr.sin_port = htons(servPort); // Local port

31 // Bind to the local address

32 if (bind(servSock, (struct sockaddr*) &servAddr, sizeof(servAddr)) < 0)

33 DieWithSystemMessage("bind() failed");

35 // Mark the socket so it will listen for incoming connections

36 if (listen(servSock, MAXPENDING) < 0)

37 DieWithSystemMessage("listen() failed");

39 for (;;) { // Run forever

40 struct sockaddr_in clntAddr; // Client address

41 // Set length of client address structure (in-out parameter)

42 socklen_t clntAddrLen = sizeof(clntAddr);

44 // Wait for a client to connect

45 int clntSock = accept(servSock, (struct sockaddr *) &clntAddr, &clntAddrLen);

46 if (clntSock < 0)

47 DieWithSystemMessage("accept() failed");

49 // clntSock is connected to a client!

51 char clntName[INET_ADDRSTRLEN]; // String to contain client address

52 if (inet_ntop(AF_INET, &clntAddr.sin_addr.s_addr, clntName,

53 sizeof(clntName)) != NULL)

54 printf("Handling client %s/%d\n", clntName, ntohs(clntAddr.sin_port));

55 else

56 puts("Unable to get client address");

2.2 IPv4 TCP Server 19

58 HandleTCPClient(clntSock);

59 }

60 // NOT REACHED

61 }

TCPEchoServer4.c

1. Program setup and parameter parsing: lines 1–17

We convert the port number from string to numeric value using atoi(); if the ﬁrst argu-

ment is not a number, atoi() will return 0, which will cause an error later when we call

bind().

2. Socket creation and setup: lines 19–37

Create a TCP socket: lines 20–22

We create a stream socket just like we did in the client.

Fill in desired endpoint address: lines 25–29

On the server, we need to associate our server socket with an address and port number

so that client connections get to the right place. Since we are writing for IPv4, we use

asockaddr_in structure for this. Because we don’t much care which address we are

on (any one assigned to the machine the server is running on will be OK), we let the

system pick it by specifying the wildcard address inaddr_any as our desired Internet

address. (This is usually the right thing to do for servers, and it saves the server from

having to ﬁnd out any actual Internet address.) Before setting both address and port

number in the sockaddr_in, we convert each to network byte order using htonl() and

htons(). (See Section 5.1.2 for details.)

Bind socket to speciﬁed address and port: lines 32–33

As noted above, the server’s socket needs to be associated with a local address and

port; the function that accomplishes this is bind(). Notice that while the client has to

supply the server’s address to connect(),the server has to specify its own address to

bind(). It is this piece of information (i.e., the server’s address and port) that they have

to agree on to communicate; neither one really needs to know the client’s address. Note

that bind() may fail for various reasons; one of the most important is that some other

socket is already bound to the speciﬁed port (see Section 7.5). Also, on some systems

special privileges are required to bind to certain ports (typically those with numbers

less than 1024).

Set the socket to listen: lines 36–37

The listen() call tells the TCP implementation to allow incoming connections from

clients. Before the call to listen(), any incoming connection requests to the socket’s

address would be silently rejected—that is, the connect() would fail at the client.

3. Iteratively handle incoming connections: lines 39–59

Accept an incoming connection: lines 40–47

20 Chapter 2: Basic TCP Sockets

As discussed above, a TCP socket on which listen() has been called is used diﬀerently

than the one we saw in the client application. Instead of sending and receiving on the

socket, the server application calls accept(), which blocks until an incoming connec-

tion is made to the listening socket’s port number. At that point, accept() returns a

descriptor for a new socket, which is already connected to the initiating remote socket.

The second argument points to a sockaddr_in structure, and the third argument is a

pointer to the length of that structure. Upon success, the sockaddr_in contains the

Internet address and port of the client to which the returned socket is connected; the

address’s length has been written into the integer pointed to by the third argument.

Note that the socket referenced by the returned descriptor is already connected; among

other things this means it is ready for sending and receiving. (For details about what

happens in the underlying implementation, see Section 7.4.1 in Chapter 7.)

Report connected client: lines 51–56

At this point clntAddr contains the address and port number of the connecting client;

we provide a “Caller ID” function and print out the client’s information. As you might

expect, inet_ntop() is the inverse of inet_pton(), which we used in the client. It takes

the binary representation of the client’s address and converts it to a dotted-quad string.

Because the implementation deals with ports and addresses in so-called network byte

order (Section 5.1.2), we have to convert the port number before passing it to printf()

(inet_pton() takes care of this transparently for addresses).

Handle echo client: line 58

HandleTCPClient() takes care of the “application protocol.” We discuss it below. Thus,

we have factored out the “echo”-speciﬁc part of the server.

We have factored out the function that implements the “echo” part of our echo server.

Although this application protocol only takes a few lines to implement, it’s good design practice

to isolate its details from the rest of the server code. This promotes code reuse.

HandleTCPClient() receives data on the given socket and sends it back on the same socket,

iterating as long as recv() returns a positive value (indicating that something was received).

recv() blocks until something is received or the client closes the connection. When the client

closes the connection normally, recv() returns 0. You can ﬁnd HandleTCPClient() in the ﬁle

TCPServerUtility.c.

HandleTCPClient()

1void HandleTCPClient(int clntSocket) {

2char buffer[BUFSIZE]; // Buffer for echo string

4// Receive message from client

5ssize_t numBytesRcvd = recv(clntSocket, buffer, BUFSIZE, 0);

6if (numBytesRcvd < 0)

7DieWithSystemMessage("recv() failed");

2.2 IPv4 TCP Server 21

9// Send received string and receive again until end of stream

10 while (numBytesRcvd > 0) { // 0 indicates end of stream

11 // Echo message back to client

12 ssize_t numBytesSent = send(clntSocket, buffer, numBytesRcvd, 0);

13 if (numBytesSent < 0)

14 DieWithSystemMessage("send() failed");

15 else if (numBytesSent != numBytesRcvd)

16 DieWithUserMessage("send()", "sent unexpected number of bytes");

18 // See if there is more data to receive

19 numBytesRcvd = recv(clntSocket, buffer, BUFSIZE, 0);

20 if (numBytesRcvd < 0)

21 DieWithSystemMessage("recv() failed");

22 }

24 close(clntSocket); // Close client socket

25 }

HandleTCPClient()

Suppose we compile TCPEchoServer4.c,DieWithMessage.c,TCPServerUtility.c, and Address-

Utility.c into the executable program TCPEchoServer4, and run that program on a host with

Internet (IPv4) address 169.1.1.1, port 5000. Suppose also that we run our client on a host with

Internet address 169.1.1.2 and connect it to the server. The server’s output should look like

this:

%TCPEchoServer4 5000

Handling client 169.1.1.2

While the client’s output looks like this:

%TCPEchoClient4 169.1.1.1 "Echo this!" 5000

Received: Echo this!

The server binds its socket to port 5000 and waits for a connection request from the

client. The client connects, sends the message “Echo this!” to the server, and receives the

echoed response. In this command we have to supply TCPEchoClient with the port number on

the command line because it is talking to our echo server, which is on port 5000 rather than

the well-known port 7.

We have mentioned that a key principle for coding network applications using sockets is

Defensive Programming: your code must not make assumptions about anything received

over the network. What if you want to “play” with your TCP server to see how it responds to

various incorrect client behaviors? You could write a TCP client that sends bogus messages

and prints results; this, however, can be tedious and time-consuming. A quicker alternative

22 Chapter 2: Basic TCP Sockets

is to use the telnet program available on most systems. This is a command-line tool that

connects to a server, sends whatever text you type, and prints the response. Telnet takes

two parameters: the server and port. For example, to telnet to our example echo server from

above, try

%telnet 169.1.1.1 5000

Now type your string to echo and telnet will print the server response. The behavior of telnet

diﬀers between implementations, so you may need to research the speciﬁcs of its use on your

system.

Now that we’ve seen a complete client and server, let’s look at the individual functions

that make up the Sockets API in a bit more detail.

2.3 Creating and Destroying Sockets

To communicate using TCP or UDP, a program begins by asking the operating system to cre-

ate an instance of the socket abstraction. The function that accomplishes this is socket(); its

parameters specify the ﬂavor of socket needed by the program.

int socket(int domain,int type,int protocol)

The ﬁrst parameter determines the communication domain of the socket. Recall that the Sock-

ets API provides a generic interface for a large number of communication domains; however,

we are only interested in IPv4 (af_inet) and IPv6 (af_inet6). Note that you may see some pro-

grams use pf_xxx here instead of af_xxx. Typically, these values are equal, in which case they

are interchangeable, but this is (alas) not guaranteed.1

The second parameter speciﬁes the type of the socket. The type determines the semantics

of data transmission with the socket—for example, whether transmission is reliable, whether

message boundaries are preserved, and so on. The constant sock_stream speciﬁes a socket

with reliable byte-stream semantics, whereas sock_dgram speciﬁes a best-eﬀort datagram

socket.

The third parameter speciﬁes the particular end-to-end protocol to be used. For both

IPv4 and IPv6, we want TCP (identiﬁed by the constant ipproto_tcp) for a stream socket,

or UDP (identiﬁed by ipproto_udp) for a datagram socket. Supplying the constant 0 as the

third parameter causes the system to select the default end-to-end protocol for the speciﬁed

protocol family and type. Because there is currently only one choice for stream sockets in the

TCP/IP protocol family, we could specify 0 instead of giving the protocol number explicitly.

Someday, however, there might be other end-to-end protocols in the Internet protocol family

1Truth be told, this is an ugly part of the Sockets interface, and the documentation is simply not helpful.

2.4 Specifying Addresses 23

that implement the same semantics. In that case, specifying 0 might result in the use of a

diﬀerent protocol, which might or might not be desirable. The main thing is to ensure that the

communicating programs are using the same end-to-end protocol.

We said earlier that socket() returns a handle for the communication instance. On

Unix-derived systems, it is an integer: a nonnegative value for success and −1 for failure.

A nonfailure value should be treated as an opaque handle, like a ﬁle descriptor. (In reality, it

is a ﬁle descriptor, taken from the same space as the numbers returned by open().) This han-

dle, which we call a socket descriptor, is passed to other API functions to identify the socket

abstraction on which the operation is to be carried out.

When an application is ﬁnished with a socket, it calls close(), giving the descriptor for

the socket that is no longer needed.

int close(int socket)

close() tells the underlying protocol stack to initiate any actions required to shut down com-

munications and deallocate any resources associated with the socket. close() returns 0 on

success or −1 on failure. Once close() has been called, invoking other operations (e.g., send()

and recv()) on the socket results in an error.

2.4 Specifying Addresses

Applications using sockets need to be able to identify the remote endpoint(s) with which they

will communicate. We’ve already seen that a client must specify the address and port number

of the server application with which it needs to communicate. In addition, the sockets layer

sometimes needs to pass addresses to the application. For example, a feature analogous to

“Caller ID” in the telephone network lets a server know the address and port number of each

client that communicates with it.

In this section, we describe the data structures used as containers for this information

by the Sockets API.

2.4.1 Generic Addresses

The Sockets API deﬁnes a generic data type—the sockaddr structure—for specifying addresses

associated with sockets:

struct sockaddr {

sa_family_t sa_family; // Address family (e.g., AF_INET)

char sa_data[14]; // Family-specific address information

};

24 Chapter 2: Basic TCP Sockets

The ﬁrst part of this address structure deﬁnes the address family—the space to which the

address belongs. For our purposes, we will always use the system-deﬁned constants af_inet

and af_inet6, which specify the Internet address families for IPv4 and IPv6, respectively. The

second part is a blob of bits whose exact form depends on the address family. (This is a typical

way of dealing with heterogeneity in operating systems and networking.) As we discussed in

Section 1.2, socket addresses for the Internet protocol family have two parts: a 32-bit (IPv4) or

128-bit (IPv6) Internet address and a 16-bit port number.2

2.4.2 IPv4 Addresses

The particular form of the sockaddr structure that is used for TCP/IP socket addresses

depends on the IP version. For IPv4, use the sockaddr_in structure.

struct in_addr {

uint32_t s_addr; // Internet address (32 bits)

};

struct sockaddr_in {

sa_family_t sin_family; // Internet protocol (AF_INET)

in_port_t sin_port; // Address port (16 bits)

struct in_addr sin_addr; // IPv4 address (32 bits)

char sin_zero[8]; // Not used

};

As you can see, the sockaddr_in structure has ﬁelds for the port number and Internet

address in addition to the address family. It is important to understand that sockaddr_in is

just a more detailed view of the data in a sockaddr structure, tailored to sockets using IPv4.

Thus, we can ﬁll in the ﬁelds of a sockaddr_in and then cast (a pointer to) it to a (pointer to

a) sockaddr and pass it to the socket functions, which look at the sa_family ﬁeld to learn the

actual type, then cast back to the appropriate type.

2.4.3 IPv6 Addresses

For IPv6, use the sockaddr_in6 structure.

struct in_addr {

uint32_t s_addr[16]; // Internet address (128 bits)

};

struct sockaddr_in6 {

sa_family_t sin6_family; // Internet protocol (AF_INET6)

in_port_t sin6_port; // Address port (16 bits)

uint32_t sin6_flowinfo; // Flow information

2The astute reader may have noticed that the generic sockaddr structure is not big enough to hold both

a 16-byte IPv6 address and a 2-byte port number. We’ll deal with this diﬃculty shortly.

2.4 Specifying Addresses 25

struct in6_addr sin6_addr; // IPv6 address (128 bits)

uint32_t sin6_scope_id; // Scope identifier

};

The sockaddr_in6 structure has additional ﬁelds beyond those of a sockaddr_in. These

are intended for capabilities of the IPv6 protocol that are not commonly used. They will be

(mostly) ignored in this book.

As with sockaddr_in, we must cast (a pointer to) the sockaddr_in6 to (a pointer to) a

sockaddr in order to pass it to the various socket functions. Again, the implementation uses

the address family ﬁeld to determine the actual type of the argument.

2.4.4 Generic Address Storage

If you know anything about how data structures are allocated in C, you may have already

noticed that a sockaddr is not big enough to hold a sockaddr_in6. (If you don’t know anything

about it, don’t fear: much of what you need to know will be covered in Chapter 5.) In particular,

what if we want to allocate an address structure, but we don’t know the actual address type (e.g.,

IPv4 or IPv6)? The generic sockaddr won’t work because it’s too small for some address struc-

tures.3To solve this problem, the socket designers created the sockaddr_storage structure,

which is guaranteed to be as large as any supported address type.

struct sockaddr_storage {

sa_family_t

...

// Padding and fields to get correct length and alignment

...

};

As with sockaddr, we still have the leading family ﬁeld to determine the actual type of the

address; however, with sockaddr_storage we have suﬃcient space for any address type. (For

a hint about how this could be accomplished, refer to the discussion of how the C compiler

lays out structures in memory, in Section 5.1.6.)

One ﬁnal note on addresses. On some platforms, the address structures contain an addi-

tional ﬁeld that stores the length of the address structure in bytes. For sockaddr,sockaddr_in,

sockaddr_in6, and sockaddr_storage, the extra ﬁelds are called sa_len,sin_len,sin6_len, and

ss_len, respectively. Since a length ﬁeld is not available on all systems, avoid using it. Typically,

platforms that use this form of structure deﬁne a value (e.g., sin6_len) that can be tested for

at compile time to see if the length ﬁeld is present.

3You may wonder why this is so (we do). The reasons apparently have to do with backward-compatibility:

the Sockets API was ﬁrst speciﬁed a long time ago, before IPv6, when resources were scarcer and there

was no reason to have a bigger structure. Changing it now to make it bigger would apparently break

binary-compatibility with some applications.

26 Chapter 2: Basic TCP Sockets

2.4.5 Binary/String Address Conversion

For socket functions to understand addresses, they must be in “numeric” (i.e., binary) form;

however, addresses for human use are generally “printable” strings (e.g., 192.168.1.1 or 1::1).

We can convert addresses from printable string to numeric using the inet_pton() function

(pton =printable to numeric):

int inet_pton(int addressFamily,const char *src,void *dst)

The ﬁrst parameter, addressFamily, speciﬁes the address family of the address being converted.

Recall that the Sockets API provides a generic interface for a large number of communication

domains. However, we are only interested here in IPv4 (af_inet) and IPv6 (af_inet6). The src

parameter references a null-terminated character string containing the address to convert. The

dst parameter points to a block of memory in the caller’s space to hold the result; its length

must be suﬃcient to hold the result (at least 4 bytes for IPv4 and 16 bytes for IPv6). inet_pton()

returns 1 if the conversion succeeds, with the address referenced by dst in network byte order;

0 if the string pointed to by src is not formatted as a valid address; and −1 if the speciﬁed

address family is unknown.

We can go the other way, converting addresses from numeric to printable form, using

inet_ntop() (ntop =numeric to printable):

const char *inet_ntop(int addressFamily,const void *src,char *dst,socklen_t dstBytes)

The ﬁrst parameter, addressFamily, speciﬁes the type of the address being converted. The sec-

ond parameter src points to the ﬁrst byte of a block of memory containing the numeric address

to convert. The size of the block is determined by the address family. The dst parameter points

to a buﬀer (block of memory) allocated in the caller’s space, into which the resulting string will

be copied; its size is given by dstBytes. How do we know what size to make the block of mem-

ory? The system-deﬁned constants inet_addrstrlen (for IPv4) and inet6_addrstrlen (for

IPv6) indicate the longest possible resulting string (in bytes). inet_ntop() returns a pointer to

the string containing the printable address (i.e., the third argument) if the conversion succeeds

and NULL otherwise.

2.4.6 Getting a Socket’s Associated Addresses

The system associates a local and foreign address with each connected socket (TCP or UDP).

Later we’ll discuss the details of how these values are assigned. We can ﬁnd out these addresses

using getsockname() for the local address and getpeername() for the foreign address. Both

methods return a sockaddr structure containing the Internet address and port information.

2.6 Binding to an Address 27

int getpeername(int socket,struct sockaddr *remoteAddress,socklen_t *addressLength)

int getsockname(int socket,struct sockaddr *localAddress,socklen_t *addressLength)

The socket parameter is the descriptor of the socket whose address information we want.

The remoteAddress and localAddress parameters point to address structures into which the

address information will be placed by the implementation; they are always cast to sockaddr *

by the caller. If we don’t know the IP protocol version a priori, we should pass in a (pointer

to a) sockaddr_storage to receive the result. As with other socket calls using sockaddr, the

addressLength is an in-out parameter specifying the length of the buﬀer (input) and returned

address structure (output) in bytes.

2.5 Connecting a Socket

A TCP socket must be connected to another socket before any data can be sent through it. In

this sense using TCP sockets is something like using the telephone network. Before you can

talk, you have to specify the number you want, and a connection must be established; if the

connection cannot be established, you have to try again later. The connection establishment

process is the biggest diﬀerence between clients and servers: The client initiates the connection

while the server waits passively for clients to connect to it. (For additional details about the

connection process and how it relates to the API functions, see Section 7.4.) To establish a

connection with a server, we call connect() on the socket.

int connect(int socket,const struct sockaddr *foreignAddress,socklen_t addressLength)

The ﬁrst argument, socket, is the descriptor created by socket().foreignAddress is

declared to be a pointer to a sockaddr because the Sockets API is generic; for our purposes,

it will always be a pointer to either a sockaddr_in or sockaddr_in6 containing the Internet

address and port of the server. addressLength speciﬁes the length of the address structure,

typically given as sizeof(struct sockaddr_in) or sizeof(struct sockaddr_in6). When connect()

returns, the socket is connected, and communication can proceed with calls to send() and

recv().

2.6 Binding to an Address

As we have noted already, client and server “rendezvous” at the server’s address and port. For

that to work, the server must ﬁrst be associated with that address and port. This is accom-

plished using bind(). Again, note that the client supplies the server’s address to connect(), but

28 Chapter 2: Basic TCP Sockets

the server has to specify its own address to bind(). Neither client nor server application needs

to know the client’s address in order for them to communicate. (Of course, the server may

wish to know the client’s address for logging or other purposes.)

int bind(int socket,struct sockaddr *localAddress,socklen_t addressSize)

The ﬁrst parameter is the descriptor returned by an earlier call to socket(). As with

connect(), the address parameter is declared as a pointer to a sockaddr, but for TCP/IP appli-

cations, it will always point to a sockaddr_in (for IPv4) or sockaddr_in6 (for IPv6), containing

the Internet address of the local interface and the port to listen on. The addressSize parameter

is the size of the address structure. bind() returns 0 on success and −1 on failure.

It is important to realize that it is not possible for a program to bind a socket to an

arbitrary Internet address—if a speciﬁc Internet address is given (of either type), the call

will only succeed if that address is assigned to the host on which the program is running.

A server on a host with multiple Internet addresses might bind to a speciﬁc one because it

only wants to accept connections that arrive to that address. Typically, however, the server

wants to accept connections sent to any of the host’s addresses, and so sets the address part

of the sockaddr to the “wildcard” address inaddr_any for IPv4 or in6addr_any for IPv6. The

semantics of the wildcard address are that it matches any speciﬁc address. For a server, this

means that it will receive connections addressed to any of the host’s addresses (of the speciﬁed

type).

While bind() is mostly used by servers, a client can also use bind() to specify its local

address/port. For those TCP clients that don’t pick their own local address/port with bind(),

the local Internet address and port are determined during the call to connect(). Thus, a client

must call bind() before calling connect() if it is going to use it.

You can initialize a in6_addr structure to the wildcard address with in6addr_any_init;

however, this special constant may only be used as an “initializer” in a declaration. Note well

that while inaddr_any is deﬁned to be in host byte order and, consequently, must be

converted to network byte order with htonl() before being used as an argument to bind(),

in6addr_any and in6addr_any_init are already in network byte order.

Finally, if you supply the port number 0 to bind(), the system will select an unused local

port for you.

2.7 Handling Incoming Connections

After binding, the server socket has an address (or at least a port). Another step is required to

instruct the underlying protocol implementation to listen for connections from clients; this is

done by calling listen() on the socket.

2.7 Handling Incoming Connections 29

int listen(int socket,int queueLimit)

The listen() function causes internal state changes to the given socket, so that incoming

TCP connection requests will be processed and then queued for acceptance by the program.

(Section 7.4 in Chapter 7 has more details about the life cycle of a TCP connection.) The queue-

Limit parameter speciﬁes an upper bound on the number of incoming connections that can

be waiting at any time. The precise eﬀect of queueLimit is very system dependent, so consult

your local system’s technical speciﬁcations.4listen() returns 0 on success and −1 on failure.

Once a socket is conﬁgured to listen, the program can begin accepting client connections

on it. At ﬁrst it might seem that a server should now wait for a connection on the socket that it

has set up, send and receive through that socket, close it, and then repeat the process. However,

that is not the way it works. The socket that has been bound to a port and marked “listening”

is never actually used for sending and receiving. Instead, it is used as a way of getting new

sockets, one for each client connection; the server then sends and receives on the new sockets.

The server gets a socket for an incoming client connection by calling accept().

int accept(int socket,struct sockaddr *clientAddress,socklen_t *addressLength)

This function dequeues the next connection on the queue for socket. If the queue is

empty, accept() blocks until a connection request arrives. When successful, accept() ﬁlls in

the sockaddr structure pointed to by clientAddress, with the address and port of the client at

the other end of the connection. Upon invocation, the addressLength parameter should specify

the size of the structure pointed to by clientAddress (i.e., the space available); upon return it

contains the size of the actual address returned. A common beginner mistake is to fail to

initialize the integer that addressLength points to so it contains the length of the structure

that clientAddress points to. The following shows the correct way:

AQ1

struct sockaddr_storage address;

socklen_t addrLength = sizeof(address);

int newConnection = accept(sock, &address, &addrLength);

If successful, accept() returns a descriptor for a new socket that is connected to the

client. The socket passed as the ﬁrst parameter to accept() is unchanged (not connected to the

client) and continues to listen for new connection requests. On failure, accept() returns −1.

On most systems, accept() only fails when passed a bad socket descriptior. However, on some

platforms it may return an error if the new socket has experienced a network-level error after

being created and before being accepted.

4For information about using “man” pages, see the preface.

30 Chapter 2: Basic TCP Sockets

2.8 Communication

Once a socket is “connected,” you can begin sending and receiving data. As we’ve seen, a client

creates a connected socket by calling connect(), and a connected socket is returned by accept()

on a server. After connection, the distinction between client and server eﬀectively disappears,

at least as far as the Sockets API is concerned. Through a connected TCP socket, you can

communicate using send() and recv().

ssize_t send(int socket,const void *msg,size_t msgLength,int flags)

ssize_t recv(int socket,void *rcvBuffer,size_t bufferLength,int flags)

These functions have very similar arguments. The ﬁrst parameter socket is the descrip-

tor for the connected socket through which data is to be sent or received. For send(),msg

points to the sequence of bytes to be sent, and msgLength is the number of bytes to send. The

default behavior for send() is to block until all of the data is sent. (We revisit this behavior

in Section 6.3 and Chapter 7.) For recv(),rcvBuffer points to the buﬀer—that is, an area in

memory such as a character array—where received data will be placed, and bufferLength gives

the length of the buﬀer, which is the maximum number of bytes that can be received at once.

The default behavior for recv() is to block until at least some bytes can be transferred. (On

most systems, the minimum amount of data that will cause the caller of recv() to unblock is

1 byte.)

The flags parameter in both send() and recv() provides a way to change some aspects

of the default behavior of the socket call. Setting flags to 0 speciﬁes the default behavior.

send() and recv() return the number of bytes sent or received or −1 for failure. (See also

Section 6.3.)

Remember: TCP is a byte-stream protocol, so send() boundaries are not preserved. The

number of bytes read in a single call to recv on the receiver is not necessarily determined

by the number of bytes written by a single call to send(). If you call send() with 3000 bytes,

it may take several calls to recv() to get all 3000 bytes, even if you pass a 5000-byte buﬀer to

each recv() call. If you call send() with 100 bytes four times, you might receive all 400 bytes

with a single call to recv(). A common mistake when writing TCP socket applications involves

assuming that if you write all of the data with one send() you can read it all with one recv().

All these possibilities are illustrated in Chapter 7.

2.9 Using IPv6

So far, we’ve seen a client and server that work only with IPv4. What if you want to use IPv6? The

changes are relatively minor and basically involve using the IPv6 equivalents for the address

structure and constants. Let’s look at the IPv6 version of our TCP echo server.

2.9 Using IPv6 31

TCPEchoServer6.c

1#include <stdio.h>

2#include <stdlib.h>

3#include <string.h>

4#include <sys/types.h>

5#include <sys/socket.h>

6#include <netinet/in.h>

7#include <arpa/inet.h>

8#include "Practical.h"

10 static const int MAXPENDING = 5; // Maximum outstanding connection requests

12 int main(int argc, char *argv[]) {

14 if (argc != 2) // Test for correct number of arguments

15 DieWithUserMessage("Parameter(s)", "<Server Port>");

17 in_port_t servPort = atoi(argv[1]); // First arg: local port

19 // Create socket for incoming connections

20 int servSock = socket(AF_INET6, SOCK_STREAM, IPPROTO_TCP);

21 if (servSock < 0)

22 DieWithSystemMessage("socket() failed");

24 // Construct local address structure

25 struct sockaddr_in6 servAddr; // Local address

26 memset(&servAddr, 0, sizeof(servAddr)); // Zero out structure

27 servAddr.sin6_family = AF_INET6; // IPv6 address family

28 servAddr.sin6_addr = in6addr_any; // Any incoming interface

29 servAddr.sin6_port = htons(servPort); // Local port

31 // Bind to the local address

32 if (bind(servSock, (struct sockaddr *) &servAddr, sizeof(servAddr)) < 0)

33 DieWithSystemMessage("bind() failed");

35 // Mark the socket so it will listen for incoming connections

36 if (listen(servSock, MAXPENDING) < 0)

37 DieWithSystemMessage("listen() failed");

39 for (;;) { // Run forever

40 struct sockaddr_in6 clntAddr; // Client address

41 // Set length of client address structure (in-out parameter)

42 socklen_t clntAddrLen = sizeof(clntAddr);

32 Chapter 2: Basic TCP Sockets

44 // Wait for a client to connect

45 int clntSock = accept(servSock, (struct sockaddr *) &clntAddr, &clntAddrLen);

46 if (clntSock < 0)

47 DieWithSystemMessage("accept() failed");

49 // clntSock is connected to a client!

51 char clntName[INET6_ADDRSTRLEN]; // Array to contain client address string

52 if (inet_ntop(AF_INET6, &clntAddr.sin6_addr.s6_addr, clntName,

53 sizeof(clntName)) != NULL)

54 printf("Handling client %s/%d\n", clntName, ntohs(clntAddr.sin6_port));

55 else

56 puts("Unable to get client address");

58 HandleTCPClient(clntSock);

59 }

60 // NOT REACHED

61 }

TCPEchoServer6.c

1. Socket creation: lines 19–22

We construct an IPv6 socket by specifying the communication domain as af_inet6.

2. Fill in local address: lines 24–29

For the local address, we use the IPv6 (struct sockaddr_in6) address structure and con-

stants (af_inet6 and in6addr_any ). One subtle diﬀerence is that we do not have to

convert in6addr_any to network byte order as we did with inaddr_any.

3. Report connected client: lines 51–56

clntAddr, which contains the address of the connecting client, is declared as an IPv6

socket address structure. When we convert the numeric address representation to

a string, the maximum string length is now inet6_addrstrlen. Finally, our call to

inet_ntop() uses an IPv6 address.

You’ve now seen both IPv4- and IPv6-speciﬁc clients and servers. In Chapter 3 we will see

how they can be made to work with either type of address.

Exercises

1. Experiment with the book’s TCP echo server using telnet. What OS are you using? Does

the server appear to echo as you type (character-by-character) or only after you complete

a line?

Exercises 33

2. Use telnet to connect to your favorite Web server on port 80 and fetch the default page.

You can usually do this by sending the string “GET /” to the Web server. Report the server

address/name and the text from the default page.

3. For TCPEchoServer.c we explicitly provide an address to the socket using bind(). We said

that a socket must have an address for communication, yet we do not perform a bind()

in TCPEchoClient.c. How is the echo client’s socket given a local address?

4. Modify the client and server so that the server “talks” ﬁrst, sending a greeting message,

and the client waits until it has received the greeting before sending anything. What needs

to be agreed upon between client and server?

5. Servers are supposed to run for a long time without stopping. Therefore, they have to be

designed to provide good service no matter what their clients do. Examine the example

TCPEchoServer.c and list anything you can think of that a client might do to cause the

server to give poor service to other clients. Suggest improvements to ﬁx the problems

you ﬁnd.

6. Using getsockname() and getpeername(), modify TCPEchoClient4.c to print the local and

foreign address immediately after connect().

7. What happens when you call getpeername() on an unconnected TCP socket?

8. Using getsockname() and getpeername(), modify TCPEchoServer4.c to print the local and

foreign address for the server socket immediately before and after bind() and for the

client socket immediately after it’s returned by accept().

9. Modify TCPEchoClient4.c to use bind() so that the system selects both the address and

port.

10. Modify TCPEchoClient4.c so that the new version binds to a speciﬁc local address and

system-selected port. If the local address changed or you moved the program to a host

with a diﬀerent local address, what do you think would happen?

11. What happens when you attempt to bind after calling connect()?

12. Why does the socket interface use a special socket to accept connections? In other words,

what would be wrong with having a server create a socket, set it up using bind() and

listen(), wait for a connection, send and receive through that socket, and then when it is

ﬁnished, close it and repeat the process? (Hint: Think about what happens to connection

requests that arrive right after the server closes the previous connection.)

chapter 3

Of Names and Address Families

At this point, you know enough to build working TCP clients and servers. However, our

examples so far, though useful enough, nevertheless have a couple of features that could be

improved. First, the only way to specify a destination is with an IP address, such as 169.1.1.1

or FE80:1034::2A97:1001:1. This is a bit painful for most humans, who are—let’s face it—not

that good at dealing with long strings of numbers that have to be formatted just right. That’s

why most applications allow the use of names like www.mkp.com and server.example.com

to specify destinations, in addition to Internet addresses. But the Sockets API, as we’ve seen,

only takes numerical arguments, so applications need a way to convert names to the required

numerical form.

Another problem with our examples so far is that the choice of whether to use IPv4 or IPv6

is wired into the code—each progam we’ve seen deals with only one version of the IP protocol.

That was by design—to keep things simple. But wouldn’t it be better if we could hide this

choice from the rest of the code, letting the argument(s) determine whether a socket for IPv4

or IPv6 is created?

It turns out that the API provides solutions to both of these problems—and more! In this

chapter we’ll see how to (1) access the name service to convert between names and numeric

quantities; and (2) write code that chooses between IPv4 and IPv6 at runtime.

3.1 Mapping Names to Numbers

Identifying endpoints with strings of dot- or colon-separated numbers is not very user friendly,

but that’s not the only reason to prefer names over addresses. Another is that a host’s Internet

36 Chapter 3: Of Names and Address Families

address is tied to the part of the network to which it is connected. This is a source of inﬂexibil-

ity: If a host moves to another network or changes Internet service providers (ISPs), its Internet

address generally has to change. Then everybody who refers to the host by that address has to

be informed of the change, or they won’t be able to access the host! When this book was writ-

ten, the Web server for the publisher of this text, Morgan Kaufmann, had an Internet address

of 129.35.69.7. However, we invariably refer to that Web server as www.mkp.com. Obviously,

www.mkp.com is easier to remember than 129.35.69.7. In fact, this is most likely how you typ-

ically think of specifying a host on the Internet, by name. In addition, if Morgan Kaufmann’s

Web server changes its Internet address for some reason (e.g., new ISP, server moves to another

machine), simply changing the mapping of www.mkp.com from 129.35.69.7 to the new Inter-

net address allows the change to be transparent to all programs that use the name to identify

the Web server.1

To solve these problems, most implementations of the Sockets API provide access to

aname service that maps names to other information, including Internet addresses. You’ve

already seen names that map to Internet addresses (www.mkp.com). Names for services (e.g.,

echo) can also be mapped to port numbers. The process of mapping a name to a numeric

quantity (address or port number) is called resolution. There are a number of ways to resolve

names into binary quantities; your system probably provides access to several of these. Some

of them involve interaction with other systems “under the covers”; others are strictly local.

It is critical to remember that a name service is not required for TCP/IP to work. Names

simply provide a level of indirection, for the reasons discussed above. The host-naming ser-

vice can access information from a wide variety of sources. Two of the primary sources are the

Domain Name System (DNS) and local conﬁguration databases. The DNS [8] is a distributed

database that maps domain names such as www.mkp.com to Internet addresses and other

information; the DNS protocol [9] allows hosts connected to the Internet to retrieve infor-

mation from that database using TCP or UDP. Local conﬁguration databases are generally

operating-system-speciﬁc mechanisms for name-to-Internet-address mappings. Fortunately

for the programmer, the details of how the name service is implemented are hidden behind

the API, so the only thing we need to know is how to ask it to resolve a name.

3.1.1 Accessing the Name Service

The preferred interface to the name service interface is through the function getaddrinfo():2

int getaddrinfo (const char *hostStr,const char *serviceStr,

const struct addrinfo *hints,struct addrinfo **results)

1MK’s address was actually 208.164.121.48 when we wrote the ﬁrst edition. Presumably, they changed

their address to help us make this point.

2Historically, other functions were available for this purpose, and many applications still use them.

However, they have several shortcomings and are considered obsolescent as of the POSIX 2001 standard.

3.1 Mapping Names to Numbers 37

The ﬁrst two arguments to getaddrinfo() point to null-terminated character strings

representing a host name or address and a service name or port number, respectively. The

third argument describes the kind of information to be returned; we discuss it below. The last

argument is the location of a struct addrinfo pointer, where a pointer to a linked list contain-

ing results will be stored. The return value of getaddrinfo() indicates whether the resolution

was successful (0) or unsuccessful (nonzero error code).

Using getaddrinfo() entails using two other auxiliary functions:

void freeaddrinfo(struct addrinfo *addrList)

const char *gai_strerror(int errorCode)

getaddrinfo() creates a dynamically allocated linked list of results, which must be deallocated

after the caller is ﬁnished with the list. Given the pointer to the head of the result list, freead-

drinfo() frees all the storage allocated for the list. Failure to call this method can result in a

pernicious memory leak. The method should only be called when the program is ﬁnished

with the returned information; no information contained in the list of results is reliable after

this function has returned. In case getaddrinfo() returns a nonzero (error) value, passing it to

gai_strerror() yields a string that describes what went wrong.

Generally speaking, getaddrinfo() takes the name of a host/service pair as input and

returns a linked list of structures containing everything needed to create a socket to connect

to the named host/service, including: address/protocol family (v4 or v6), socket type (e.g.,

stream or datagram), protocol (TCP or UDP for the Internet protocol family), and numeric

socket address. Each entry in the linked list is placed into an addrinfo structure, declared as

follows:

struct addrinfo {

int ai_flags; // Flags to control info resolution

int ai_family; // Family: AF_INET, AF_INET6, AF_UNSPEC

int ai_socktype; // Socket type: SOCK_STREAM, SOCK_DGRAM

int ai_protocol; // Protocol: 0 (default) or IPPROTO_XXX

socklen_t ai_addrlen; // Length of socket address ai_addr

struct sockaddr *ai_addr; // Socket address for socket

char *ai_canonname; // Canonical name

struct addrinfo *ai_next; // Next addrinfo in linked list

};

The ai_sockaddr ﬁeld contains a sockaddr of the appropriate type, with (numeric) address and

port information ﬁlled in. It should be obvious which ﬁelds contain the address family, socket

type, and protocol information. (The ﬂags ﬁeld is not used in the result; we will discuss its use

shortly.) Actually, the results are returned in a pointer to a linked list of addrinfo structures;

the ai_next ﬁeld contains the pointers for this list.

Why a linked list? There are two reasons. First, for each combination of host and ser-

vice, there might be several diﬀerent combinations of address family (v4 or v6) and socket

38 Chapter 3: Of Names and Address Families

type/protocol (stream/TCP or datagramUDP) that represent possible endpoints. For example,

the host “server.example.net” might have instances of the “spam” service listening on port

1001 on both IPv4/TCP and IPv6/UDP. The getaddrinfo() function returns both of these. The

second reason is that a hostname can map to multiple IP addresses; getaddrinfo() helpfully

returns all of these.

Thus, getaddrinfo() returns all the viable combinations for a given hostname, service pair.

But wait—what if you don’t need options, and you know exactly what you want in advance?

You don’t want to have to write code that searches through the returned list for a particular

combination—say, IPv4/TCP. That’s where the third parameter of getaddrinfo() comes in! It

allows you to tell the system to ﬁlter the results for you. We’ll see how it is used in our example

program, GetAddrInfo.c.

GetAddrInfo.c

1#include <stdio.h>

2#include <stdlib.h>

3#include <string.h>

4#include <netdb.h>

5#include "Practical.h"

7int main(int argc, char *argv[]) {

9if (argc != 3) // Test for correct number of arguments

10 DieWithUserMessage("Parameter(s)", "<Address/Name> <Port/Service>");

12 char *addrString = argv[1]; // Server address/name

13 char *portString = argv[2]; // Server port/service

15 // Tell the system what kind(s) of address info we want

16 struct addrinfo addrCriteria; // Criteria for address match

17 memset(&addrCriteria, 0, sizeof(addrCriteria)); // Zero out structure

18 addrCriteria.ai_family = AF_UNSPEC; // Any address family

19 addrCriteria.ai_socktype = SOCK_STREAM; // Only stream sockets

20 addrCriteria.ai_protocol = IPPROTO_TCP; // Only TCP protocol

22 // Get address(es) associated with the specified name/service

23 struct addrinfo *addrList; // Holder for list of addresses returned

24 // Modify servAddr contents to reference linked list of addresses

25 int rtnVal = getaddrinfo(addrString, portString, &addrCriteria, &addrList);

26 if (rtnVal != 0)

27 DieWithUserMessage("getaddrinfo() failed", gai_strerror(rtnVal));

29 // Display returned addresses

3.1 Mapping Names to Numbers 39

30 for (struct addrinfo *addr = addrList; addr != NULL; addr = addr->ai_next) {

31 PrintSocketAddress(addr->ai_addr, stdout);

32 fputc('\n', stdout);

33 }

35 freeaddrinfo(addrList); // Free addrinfo allocated in getaddrinfo()

37 exit(0);

38 }

GetAddrInfo.c

1. Application setup and parameter parsing: lines 9–13

2. Construct address speciﬁcation: lines 15–20

The addrCriteria structure will indicate what kinds of results we are interested in.

Declare and initialize addrinfo structure: lines 16–17

Set address family: line 18

We set the family to af_unspec, which allows the returned address to come from any

family (including af_inet and af_inet6).

Set socket type: line 19

We want a stream/TCP endpoint, so we set this to sock_stream. The system will ﬁlter

out results that use diﬀerent protocols.

Set protocol: line 20

We want a TCP socket, so we set this to ipproto_tcp. Since TCP is the default protocol

for stream sockets, leaving this ﬁeld 0 would have the same result.

3. Fetch address information: lines 22–27

Declare pointer for head of result linked list: line 23

Call getaddrinfo(): line 25

We pass the desired hostname, port, and the constraints encoded in the addrCriteria

structure.

Check return value: lines 26–27

getaddrinfo() returns 0 if successful. Otherwise, the return value indicates the speciﬁc

error. The auxiliary function gai_strerror() returns a character string error message

explaining the given error return value. Note that these messages are diﬀerent from

the normal errno-based messages.

4. Print addresses: lines 29–33

Iterate over the linked list of addresses, printing each to the console. The function

PrintSocketAddress() takes an address to print and the stream on which to print. We

present its code, which is in AddressUtility.c, later in this chapter.

40 Chapter 3: Of Names and Address Families

5. Free address linked list: line 35

The system allocated storage for the linked list of addrinfo structures it returned. We

must call the auxiliary function freeaddrinfo() to free that memory when we are ﬁnished

with it.

The program GetAddrInfo.c takes two command-line parameters, a hostname (or address)

and a service name (or port number), and prints the address information returned by getad-

drinfo(). Suppose you want to ﬁnd an address for the service named “whois” on the host

named “localhost” (i.e., the one you are running on). Here’s how you use it:

%GetAddrInfo localhost whois

127.0.0.1-43

To ﬁnd the service “whois” on the host “pine.netlab.uky.edu”, do this:

%GetAddrInfo pine.uky.edu whois

128.163.170.219-43

The program can deal with any combination of name and numerical arguments:

%GetAddrInfo 169.1.1.100 time

169.1.1.100-37

%GetAddrInfo FE80:0000:0000:0000:0000:ABCD:0001:0002:0003 12345

fe80::abcd:1:2:3-12345

These examples all return a single answer. But as we noted above, some names have multiple

numeric addresses associated with them. For example, “google.com” is typically associated

with a number of Internet addresses. This allows a service (e.g., search engine) to be placed

on multiple hosts. Why do this? One reason is robustness. If any single host fails, the service

continues because the client can use any of the hosts providing the service. Another advantage

is scalability. If clients randomly select the numeric address (and corresponding host) to use,

we can spread the load over multiple servers. The good news is that getaddrinfo() returns

all of the addresses to which a name maps. You can experiment with this by executing the

program with the names of popular Web sites. (Note that supplying 0 for the second argument

results in only the address information being printed.)

3.1.2 Details, Details

As we noted above, getaddrinfo() is something of a “Swiss Army Knife” function. We’ll cover

some of the subtleties of its capabilities here. Beginning readers may wish to skip this section

and come back to it later.

The third argument (addrinfo structure) tells the system what kinds of endpoints the

caller is interested in. In GetAddrInfo.c, we set this parameter to indicate that any address

family was acceptable, and we wanted a stream/TCP socket. We could have instead speciﬁed

3.2 Writing Address-Generic Code 41

datagram/UDP or a particular address family (say, af_inet6), or we could have set the

ai_socktype and ai_protocol ﬁelds to zero, indicating that we wanted to receive all possibilities.

It is even possible to pass a NULL pointer for the third argument; the system is supposed to

treat this case as if an addrinfo structure had been passed with ai_family set to af_unspec

and everything else set to 0.

The ai_ﬂags ﬁeld in the third parameter provides additional control over the behavior of

getaddrinfo(). It is an integer, individual bits of which are interpreted as boolean variables by

the system. The meaning of each ﬂag is given below; ﬂags can be combined using the bitwise

OR operator “|” (see Section 5.1.8 for how to do this).

AI_PASSIVE If hostStr is NULL when this ﬂag is set, any returned addrinfos will have

their addresses set to the appropriate “any” address constant—inaddr_any (IPv4) or

in6addr_any_init (IPv6).

AI_CANONNAME Just as one name can resolve to many numeric addresses, multiple names

can resolve to the same IP address. However, one name is usually deﬁned to be the oﬃcial

(“canonical”) name. By setting this ﬂag in ai_ﬂags, we instruct getaddrinfo() to return a

pointer to the canonical name (if it exists) in the ai_canonname ﬁeld of the ﬁrst struct

addrinfo of the linked list.

AI_NUMERICHOST This ﬂag causes an error to be returned if hostStr does not point to a

string in valid numeric address format. Without this ﬂag, if the hostStr parameter points

to something that is not a valid string representation of a numeric address, an attempt

will be made to resolve it via the name system; this can waste time and bandwidth on

useless queries to the name service. If this ﬂag is set, a given valid address string is simply

converted and returned, a la inet_pton().

AI_ADDRCONFIG If set, getaddrinfo() returns addresses of a particular family only if the

system has an interface conﬁgured for that family. So an IPv4 address would be returned

only if the system has an interface with an IPv4 address, and similarly for IPv6.

AI_V4MAPPED If the ai_family ﬁeld contains af_inet6, and no matching IPv6 addresses are

found, then getaddrinfo() returns IPv4-mapped IPv6 addresses. This technique can be

used to provide limited interoperation between IPv4-only and IPv6 hosts.

3.2 Writing Address-Generic Code

A famous bard once wrote “To v6 or not to v6, that is the question.” Fortunately, the Socket

interface allows us to postpone answering that question until execution time. In our earlier

TCP client and server examples, we speciﬁed a particular IP protocol version to both the

socket creation and address string conversion functions using af_inet or af_inet6. How-

ever, getaddrinfo() allows us to write code that works with either address family, without

having to duplicate steps for each version. In this section we’ll use its capabilities to modify

our version-speciﬁc client and server code to make them generic.

42 Chapter 3: Of Names and Address Families

Before we do that, here’s a handy little method that prints a socket address (of either

ﬂavor). Given a sockaddr structure containing an IPv4 or IPv6 address, it prints the address

to the given output stream, using the proper format for its address family. Given any other

kind of address, it prints an error string. This function takes a generic struct sockaddr

pointer and prints the address to the speciﬁed stream. You can ﬁnd our implementation of

PrintSocketAddr() in AddressUtility.c with the function prototype included in Practical.h.

PrintSocketAddr()

1void PrintSocketAddress(const struct sockaddr *address, FILE *stream) {

2// Test for address and stream

3if (address == NULL || stream == NULL)

4return;

6void *numericAddress; // Pointer to binary address

7// Buffer to contain result (IPv6 sufficient to hold IPv4)

8char addrBuffer[INET6_ADDRSTRLEN];

9in_port_t port; // Port to print

10 // Set pointer to address based on address family

11 switch (address->sa_family) {

12 case AF_INET:

13 numericAddress = &((struct sockaddr_in *) address)->sin_addr;

14 port = ntohs(((struct sockaddr_in *) address)->sin_port);

15 break;

16 case AF_INET6:

17 numericAddress = &((struct sockaddr_in6 *) address)->sin6_addr;

18 port = ntohs(((struct sockaddr_in6 *) address)->sin6_port);

19 break;

20 default:

21 fputs("[unknown type]", stream); // Unhandled type

22 return;

23 }

24 // Convert binary to printable address

25 if (inet_ntop(address->sa_family, numericAddress, addrBuffer,

26 sizeof(addrBuffer)) == NULL)

27 fputs("[invalid address]", stream); // Unable to convert

28 else {

29 fprintf(stream, "%s", addrBuffer);

30 if (port != 0) // Zero not valid in any socket addr

31 fprintf(stream, "-%u", port);

32 }

33 }

PrintSocketAddr()

3.2 Writing Address-Generic Code 43

3.2.1 Generic TCP Client

Using getaddrinfo(), we can write clients and servers that are not speciﬁc to one IP version or

the other. Let’s begin by converting our TCP client to make it version-independent; we’ll drop

the version number and call it TCPEchoClient.c. The general strategy is to set up arguments

to getaddrinfo() that make it return both IPv4 and IPv6 addresses and use the ﬁrst address

that works. Since our address search functionality may be useful elsewhere, we factor out

the code responsible for creating and connecting the client socket, placing it in a separate

function, SetupTCPClientSocket(), in TCPClientUtility.c. The setup function takes a host and

service, speciﬁed in a string, and returns a connected socket (or -1 on failure). The host or

service may be speciﬁed as NULL.

TCPClientUtility.c

1#include <string.h>

2#include <unistd.h>

3#include <sys/types.h>

4#include <sys/socket.h>

5#include <netdb.h>

6#include "Practical.h"

8int SetupTCPClientSocket(const char *host, const char *service) {

9// Tell the system what kind(s) of address info we want

10 struct addrinfo addrCriteria; // Criteria for address match

11 memset(&addrCriteria, 0, sizeof(addrCriteria)); // Zero out structure

12 addrCriteria.ai_family = AF_UNSPEC; // v4 or v6 is OK

13 addrCriteria.ai_socktype = SOCK_STREAM; // Only streaming sockets

14 addrCriteria.ai_protocol = IPPROTO_TCP; // Only TCP protocol

16 // Get address(es)

17 struct addrinfo *servAddr; // Holder for returned list of server addrs

18 int rtnVal = getaddrinfo(host, service, &addrCriteria, &servAddr);

19 if (rtnVal != 0)

20 DieWithUserMessage("getaddrinfo() failed", gai_strerror(rtnVal));

22 int sock = -1;

23 for (struct addrinfo *addr = servAddr; addr != NULL; addr = addr->ai_next) {

24 // Create a reliable, stream socket using TCP

25 sock = socket(addr->ai_family, addr->ai_socktype, addr->ai_protocol);

26 if (sock < 0)

27 continue; // Socket creation failed; try next address

29 // Establish the connection to the echo server

30 if (connect(sock, addr->ai_addr, addr->ai_addrlen) == 0)

44 Chapter 3: Of Names and Address Families

31 break; // Socket connection succeeded; break and return socket

33 close(sock); // Socket connection failed; try next address

34 sock = -1;

35 }

37 freeaddrinfo(servAddr); // Free addrinfo allocated in getaddrinfo()

38 return sock;

39 }

TCPClientUtility.c

1. Resolve the host and service: lines 10–20

The criteria we pass to getaddrinfo() speciﬁes that we don’t care which protocol is used

(af_unspec), but the socket address is for TCP (sock_stream/ipproto_tcp).

2. Attempt to create and connect a socket from the list of addresses: lines 22–35

Create appropriate socket type: lines 25–27

getaddrinfo() returns the matching domain (af_inet or af_inet6) and socket type/

protocol. We pass this information on to socket() when creating the new socket. If the

system cannot create a socket of the speciﬁed type, we move on to the next address.

Connect to speciﬁed server: lines 30–34

We use the address obtained from getaddrinfo() to attempt to connect to the server.

If the connection succeeds, we exit the address search loop. If the connection fails, we

close the socket and try the next address.

3. Free address list: line 37

To avoid a memory leak, we need to free the address linked list created by getaddrinfo().

4. Return resulting socket descriptor: line 38

If we succeed in creating and connecting a socket, return the socket descriptor. If no

addresses succeeded, return −1.

Now we are ready to see the generic client.

TCPEchoClient.c

1#include <stdio.h>

2#include <stdlib.h>

3#include <string.h>

4#include <unistd.h>

5#include <sys/types.h>

6#include <sys/socket.h>

7#include <netdb.h>

8#include "Practical.h"

3.2 Writing Address-Generic Code 45

10 int main(int argc, char *argv[]) {

12 if (argc<3||argc > 4) // Test for correct number of arguments

13 DieWithUserMessage("Parameter(s)",

14 "<Server Address/Name> <Echo Word> [<Server Port/Service>]");

16 char *server = argv[1]; // First arg: server address/name

17 char *echoString = argv[2]; // Second arg: string to echo

18 // Third arg (optional): server port/service

19 char *service = (argc == 4) ? argv[3] : "echo";

21 // Create a connected TCP socket

22 int sock = SetupTCPClientSocket(server, service);

23 if (sock < 0)

24 DieWithUserMessage("SetupTCPClientSocket() failed", "unable to connect");

26 size_t echoStringLen = strlen(echoString); // Determine input length

28 // Send the string to the server

29 ssize_t numBytes = send(sock, echoString, echoStringLen, 0);

30 if (numBytes < 0)

31 DieWithSystemMessage("send() failed");

32 else if (numBytes != echoStringLen)

33 DieWithUserMessage("send()", "sent unexpected number of bytes");

35 // Receive the same string back from the server

36 unsigned int totalBytesRcvd = 0; // Count of total bytes received

37 fputs("Received: ", stdout); // Setup to print the echoed string

38 while (totalBytesRcvd < echoStringLen) {

39 char buffer[BUFSIZE]; // I/O buffer

40 // Receive up to the buffer size (minus 1 to leave space for

41 // a null terminator) bytes from the sender

42 numBytes = recv(sock, buffer, BUFSIZE - 1, 0);

43 if (numBytes < 0)

44 DieWithSystemMessage("recv() failed");

45 else if (numBytes == 0)

46 DieWithUserMessage("recv()", "connection closed prematurely");

47 totalBytesRcvd += numBytes; // Keep tally of total bytes

48 buffer[numBytes] = '\0'; // Terminate the string!

49 fputs(buffer, stdout); // Print the buffer

50 }

52 fputc('\n', stdout); // Print a final linefeed

46 Chapter 3: Of Names and Address Families

54 close(sock);

55 exit(0);

56 }

TCPEchoClient.c

After socket creation, the remainder of TCPEchoClient.c is identical to the version-speciﬁc

clients. There is one caveat that must be mentioned with respect to this code. In line 25

of SetupTCPClientSocket(), we pass the ai_family ﬁeld of the returned addrinfo structure

as the ﬁrst argument to socket(). Strictly speaking, this value identiﬁes an address family

(af_xxx, whereas the ﬁrst argument of socket indicates the desired protocol family of the

socket (pf_xxx). In all implementations with which we have experience, these two families are

interchangeable—in particular af_inet and pf_inet are deﬁned to have the same value, as are

pf_inet6 and af_inet6.Our generic code depends on this fact. The authors contend that

these deﬁnitions will not change, but feel that full disclosure of this assumption (which allows

more concise code) is important. Elimination of this assumption is straightforward enough to

be left as an exercise.

3.2.2 Generic TCP Server

Our protocol-independent TCP echo server uses similar adaptations to those in the client.

Recall that the typical server binds to any available local address. To accomplish this, we

(1) specify the ai_passive ﬂag and (2) specify null for the hostname. Eﬀectively, this gets

an address suitable for passing to bind(), including a wildcard for the local IP address—

inaddr_any for IPv4 or in6addr_any_init for IPv6. For systems that support both IPv4

and IPv6, IPv6 will generally be returned ﬁrst by getaddrinfo() because it oﬀers more options

for interoperability. Note, however, that the problem of which options should be selected to

maximize connectivity depends on the particulars of the environment in which the server

operates—from its name service to its Internet Service Provider. The approach we present

here is essentially the simplest possible, and is likely not adequate for production servers

that need to operate across a wide variety of platforms. See the next section for additional

information.

As in our protocol-independent client, we’ve factored the steps involved in establishing a

socket into a separate function, SetupTCPServerSocket(),inTCPServerUtility.c. This setup func-

tion iterates over the addresses returned from getaddrinfo(), stopping when it can successfully

bind and listen or when it’s out of addresses.

SetupTCPServerSocket()

1static const int MAXPENDING = 5; // Maximum outstanding connection requests

3int SetupTCPServerSocket(const char *service) {

3.2 Writing Address-Generic Code 47

4// Construct the server address structure

5struct addrinfo addrCriteria; // Criteria for address match

6memset(&addrCriteria, 0, sizeof(addrCriteria)); // Zero out structure

7addrCriteria.ai_family = AF_UNSPEC; // Any address family

8addrCriteria.ai_flags = AI_PASSIVE; // Accept on any address/port

9addrCriteria.ai_socktype = SOCK_STREAM; // Only stream sockets

10 addrCriteria.ai_protocol = IPPROTO_TCP; // Only TCP protocol

12 struct addrinfo *servAddr; // List of server addresses

13 int rtnVal = getaddrinfo(NULL, service, &addrCriteria, &servAddr);

14 if (rtnVal != 0)

15 DieWithUserMessage("getaddrinfo() failed", gai_strerror(rtnVal));

17 int servSock = -1;

18 for (struct addrinfo *addr = servAddr; addr != NULL; addr = addr->ai_next) {

19 // Create a TCP socket

20 servSock = socket(servAddr->ai_family, servAddr->ai_socktype,

21 servAddr->ai_protocol);

22 if (servSock < 0)

23 continue; // Socket creation failed; try next address

25 // Bind to the local address and set socket to list

26 if ((bind(servSock, servAddr->ai_addr, servAddr->ai_addrlen) == 0) &&

27 (listen(servSock, MAXPENDING) == 0)) {

28 // Print local address of socket

29 struct sockaddr_storage localAddr;

30 socklen_t addrSize = sizeof(localAddr);

31 if (getsockname(servSock, (struct sockaddr *) &localAddr, &addrSize) < 0)

32 DieWithSystemMessage("getsockname() failed");

33 fputs("Binding to ", stdout);

34 PrintSocketAddress((struct sockaddr *) &localAddr, stdout);

35 fputc('\n', stdout);

36 break; // Bind and list successful

37 }

39 close(servSock); // Close and try again

40 servSock = -1;

41 }

43 // Free address list allocated by getaddrinfo()

44 freeaddrinfo(servAddr);

46 return servSock;

47 }

SetupTCPServerSocket()

48 Chapter 3: Of Names and Address Families

We also factor out accepting client connections into a separate function, AcceptTCPConnection(),

in TCPServerUtility.c.

AcceptTCPConnection()

1int AcceptTCPConnection(int servSock) {

2struct sockaddr_storage clntAddr; // Client address

3// Set length of client address structure (in-out parameter)

4socklen_t clntAddrLen = sizeof(clntAddr);

6// Wait for a client to connect

7int clntSock = accept(servSock, (struct sockaddr *) &clntAddr, &clntAddrLen);

8if (clntSock < 0)

9DieWithSystemMessage("accept() failed");

11 // clntSock is connected to a client!

13 fputs("Handling client ", stdout);

14 PrintSocketAddress((struct sockaddr *) &clntAddr, stdout);

15 fputc('\n', stdout);

17 return clntSock;

18 }

AcceptTCPConnection()

Note that we use getsockname() to print the local socket address. When you execute TCPE-

choServer.c, it will print the wildcard local network address. Finally, we use our new functions

in our protocol-independent echo server.

TCPEchoServer.c

1#include <stdio.h>

2#include "Practical.h"

3#include <unistd.h>

5int main(int argc, char *argv[]) {

7if (argc != 2) // Test for correct number of arguments

8DieWithUserMessage("Parameter(s)", "<Server Port/Service>");

10 char *service = argv[1]; // First arg: local port

12 // Create socket for incoming connections

3.2 Writing Address-Generic Code 49

13 int servSock = SetupTCPServerSocket(service);

14 if (servSock < 0)

15 DieWithUserMessage("SetupTCPServerSocket() failed", service);

17 for (;;) { // Run forever

18 // New connection creates a connected client socket

19 int clntSock = AcceptTCPConnection(servSock);

21 HandleTCPClient(clntSock); // Process client

22 close(clntSock);

23 }

24 // NOT REACHED

25 }

TCPEchoServer.c

3.2.3 IPv4–IPv6 Interoperation

Our generic client and server are oblivious to whether they are using IPv4 or IPv6 sockets. An

obvious question is, “What if one is using IPv4 and the other IPv6?” The answer is that if (and

only if) the program using IPv6 is a dual-stack system—that is, supports both verson 4 and

version 6—they should be able to interoperate. The existence of the special “v4-to-v6-mapped”

address class makes this possible. This mechanism allows an IPv6 socket to be connected to

an IPv4 socket. A full discussion of the implications of this and how it works is beyond the

scope of this book, but the basic idea is that the IPv6 implementation in a dual-stack system

recognizes that communication is desired between an IPv4 address and an IPv6 socket, and

translates the IPv4 address into a “v4-to-v6-mapped” address. Thus, each socket deals with an

address in its own format.

For example, if the client is a v4 socket with address 1.2.3.4, and the server is listening

on a v6 socket in a dual-stack platform, when the connection request comes in, the server-side

implementation will automatically do the conversion and tell the server that it is connected to

a v6 socket with the v4-mapped address ::ﬀﬀ:1.2.3.4. (Note that there is a bit more to it than

this; in particular, the server side implementation will ﬁrst try to match to a socket bound to

a v4 address, and do the conversion only if it fails to ﬁnd a match; see Chapter 7 for more

details.)

If the server is listening on a v4 socket, the client is trying to connect from a v6 socket on

a dual-stack platform, and the client has not bound the socket to a particular address before

calling connect(), the client-side implementation will recognize that it is connecting to an IPv4

address and assign a v4-mapped IPv6 address to the socket at connect() time. The stack will

“magically” convert the assigned address to an IPv4 address when the connection request is

sent out. Note that, in both cases, the message that goes over the network is actually an IPv4

message.

50 Chapter 3: Of Names and Address Families

While the v4-mapped addresses provide a good measure of interoperability, the reality

is that the space of possible scenarios is very large when one considers v4-only hosts, v6-only

hosts, hosts that support IPv6 but have no conﬁgured IPv6 addresses, and hosts that support

IPv6 and use it on the local network, but have no wide-area IPv6 transport available (i.e., their

providers do not support IPv6). Although our example code—a client that tries all possibilities

returned by getaddrinfo(), and a server that sets ai_passive and binds to the ﬁrst address

returned by getaddrinf()—covers the most likely possibilities, production code needs to be

very carefully designed to maximize the likelihood that clients and servers will ﬁnd each

other under all conditions. The details of achieving this are beyond the scope of this book;

the reader should refer to RFC 4038 [11] for more details.

3.3 Getting Names from Numbers

So we can get an Internet address from a hostname, but can we perform the mapping in the

other direction (hostname from an Internet address)? The answer is “usually.” There is an

“inverse” function called getnameinfo(), which takes a sockaddr address structure (really a

struct sockaddr_in for IPv4 and struct sockaddr_in6 for IPv6) and the address length. The

function returns a corresponding node and service name in the form of a null-terminated

character string—if the mapping between the number and name is stored in the name system.

Callers of getnameinfo() must preallocate the space for the node and service names and pass in

a pointer and length for the space. The maximum length of a node or service name is given by

the constants NI_MAXHOST and NI_MAXSERV, respectively. If the caller speciﬁes a length of 0

for node and/or service, getnameinfo() does not return the corresponding value. This function

returns 0 if successful and a nonzero value if failure. The nonzero failure return code can

again be passed to gai_strerror() to get the corresponding error text string.

int getnameinfo (const struct sockaddr *address,socklen_t addressLength,

char *node,socklen_t nodeLength,char * service,

socklen_t serviceLength,int flags)

As with getaddrinfo(), several ﬂags control the behavior of getnameinfo(). They are described

below; as before, some of them can be combined using bitwise OR (“|”).

NI_NOFQDN Return only the hostname, not FQDN (Fully Qualiﬁed Domain Name), for local

hosts. (The FQDN contains all parts, for example, protocols.example.com, while the

hostname is only the ﬁrst part, for example, “protocols”.)

NI_NUMERICHOST Return the numeric form of the address instead of the name. This avoids

potentially expensive name service lookups if you just want to use this service as a

substitute for inet_ntop().

Exercises 51

NI_NUMERICSERV Return the numeric form of the service instead of the name.

NI_NAMEREQD Return an error if a name cannot be found for the given address. Without this

option, the numeric form of the address is returned.

NI_DGRAM Speciﬁes datagram service; the default behavior assumes a stream service. In some

cases, a service has diﬀerent port numbers for TCP and UDP.

What if your program needs its own host’s name? gethostname() takes a buﬀer and buﬀer

length and copies the name of the host on which the calling program is running into the given

buﬀer.

int gethostname(char *nameBuffer,size_t bufferLength)

Exercises

1. GetAddrInfo.c requires two arguments. How could you get it to resolve a service name if

you don’t know any hostname?

2. Modify GetAddrInfo.c to take an optional third argument, containing “ﬂags” that are

passed to getaddrinfo() in the ai_ﬂags ﬁeld of the addrinfo argument. For example,

passing “-n” as the third argument should result in the ai_numerichost ﬂag being set.

3. Does getnameinfo() work for IPv6 addresses as well as IPv4? What does it return when

given the address ::1?

4. Modify the generic TCPEchoClient and TCPEchoServer to eliminate the assumption men-

tioned at the end of Section 3.2.1.

chapter 4

Using UDP Sockets

The User Datagram Protocol (UDP) provides a simpler end-to-end service than TCP

provides. In fact, UDP performs only two functions: (1) It adds another layer of addressing

(ports) to that of IP; and (2) it detects data corruption that may occur in transit and discards any

corrupted datagrams. Because of this simplicity, UDP (datagram) sockets have some diﬀerent

characteristics from the TCP (stream) sockets we saw earlier.

For example, UDP sockets do not have to be connected before being used. Where TCP

is analogous to telephone communication, UDP is analogous to communicating by mail: You

do not have to “connect” before you send a package or letter, but you do have to specify the

destination address for each one. In receiving, a UDP socket is like a mailbox into which letters

or packages from many diﬀerent sources can be placed.

Another diﬀerence between UDP sockets and TCP sockets is the way they deal with mes-

sage boundaries: UDP sockets preserve them. This makes receiving an application message

simpler, in some ways, than with TCP sockets. We will discuss this further in Section 4.3. A

ﬁnal diﬀerence is that the end-to-end transport service UDP provides is best eﬀort: There is no

guarantee that a message sent via a UDP socket will arrive at its destination. This means that

a program using UDP sockets must be prepared to deal with loss and reordering of messages;

we’ll see an example of that later.

Again we introduce the UDP portion of the Sockets API through simple client and server

programs. As before, they implement a trivial echo protocol. Afterward, we describe the API

functionality in more detail in Sections 4.3 and 4.4.

54 Chapter 4: Using UDP Sockets

4.1 UDP Client

Our UDP echo client, UDPEchoClient.c, looks similar to our address-family-independent

TCPEchoClient.c in the way it sets up the server address and communicates with the server.

However, it does not call connect(); it uses sendto() and recvfrom() instead of send() and recv();

and it only needs to do a single receive because UDP sockets preserve message boundaries,

unlike TCP’s byte-stream service. Of course, a UDP client only communicates with a UDP server.

Many systems include a UDP echo server for debugging and testing purposes; the server sim-

ply echoes whatever messages it receives back to wherever they came from. After setting up,

our echo client performs the following steps: (1) it sends the echo string to the server, (2) it

receives the echo, and (3) it shuts down the program.

UDPEchoClient.c

1#include <stdio.h>

2#include <stdlib.h>

3#include <string.h>

4#include <unistd.h>

5#include <sys/socket.h>

6#include <netdb.h>

7#include "Practical.h"

9int main(int argc, char *argv[]) {

11 if (argc<3||argc > 4) // Test for correct number of arguments

12 DieWithUserMessage("Parameter(s)",

13 "<Server Address/Name> <Echo Word> [<Server Port/Service>]");

15 char *server = argv[1]; // First arg: server address/name

16 char *echoString = argv[2]; // Second arg: word to echo

18 size_t echoStringLen = strlen(echoString);

19 if (echoStringLen > MAXSTRINGLENGTH) // Check input length

20 DieWithUserMessage(echoString, "string too long");

22 // Third arg (optional): server port/service

23 char *servPort = (argc == 4) ? argv[3] : "echo";

25 // Tell the system what kind(s) of address info we want

26 struct addrinfo addrCriteria; // Criteria for address match

27 memset(&addrCriteria, 0, sizeof(addrCriteria)); // Zero out structure

28 addrCriteria.ai_family = AF_UNSPEC; // Any address family

29 // For the following fields, a zero value means "don't care"

4.1 UDP Client 55

30 addrCriteria.ai_socktype = SOCK_DGRAM; // Only datagram sockets

31 addrCriteria.ai_protocol = IPPROTO_UDP; // Only UDP protocol

33 // Get address(es)

34 struct addrinfo *servAddr; // List of server addresses

35 int rtnVal = getaddrinfo(server, servPort, &addrCriteria, &servAddr);

36 if (rtnVal != 0)

37 DieWithUserMessage("getaddrinfo() failed", gai_strerror(rtnVal));

39 // Create a datagram/UDP socket

40 int sock = socket(servAddr->ai_family, servAddr->ai_socktype,

41 servAddr->ai_protocol); // Socket descriptor for client

42 if (sock < 0)

43 DieWithSystemMessage("socket() failed");

45 // Send the string to the server

46 ssize_t numBytes = sendto(sock, echoString, echoStringLen, 0,

47 servAddr->ai_addr, servAddr->ai_addrlen);

48 if (numBytes < 0)

49 DieWithSystemMessage("sendto() failed");

50 else if (numBytes != echoStringLen)

51 DieWithUserMessage("sendto() error", "sent unexpected number of bytes");

53 // Receive a response

55 struct sockaddr_storage fromAddr; // Source address of server

56 // Set length of from address structure (in-out parameter)

57 socklen_t fromAddrLen = sizeof(fromAddr);

58 char buffer[MAXSTRINGLENGTH + 1]; // I/O buffer

59 numBytes = recvfrom(sock, buffer, MAXSTRINGLENGTH, 0,

60 (struct sockaddr *) &fromAddr, &fromAddrLen);

61 if (numBytes < 0)

62 DieWithSystemMessage("recvfrom() failed");

63 else if (numBytes != echoStringLen)

64 DieWithUserMessage("recvfrom() error", "received unexpected number of bytes");

66 // Verify reception from expected source

67 if (!SockAddrsEqual(servAddr->ai_addr, (struct sockaddr *) &fromAddr))

68 DieWithUserMessage("recvfrom()", "received a packet from unknown source");

70 freeaddrinfo(servAddr);

72 buffer[echoStringLen] = '\0'; // Null-terminate received data

73 printf("Received: %s\n", buffer); // Print the echoed string

56 Chapter 4: Using UDP Sockets

75 close(sock);

76 exit(0);

77 }

UDPEchoClient.c

1. Program setup and parameter parsing: lines 1–23

The server address/name and string to echo are passed in as the ﬁrst two parameters.

We restrict the size of our echo message; therefore, we must verify that the given string

satisﬁes this restriction. Optionally, the client takes the server port or service name as

the third parameter. If no port is provided, the client uses the well-known echo protocol

service name, “echo”.

2. Get foreign address for server: lines 25–37

For the server, we may be given an IPv4 address, IPv6 address, or name to resolve. For

the optional port, we may be given a port number or service name. We use getaddrinfo()

to determine the corresponding address information (i.e., family, address, and port num-

ber). Note that we’ll accept an address for any family (af_unspec) for UDP (sock_dgram

and ipproto_udp); specifying the latter two eﬀectively restricts the returned address

families to IPv4 and IPv6. Note also that getaddrinfo() may return multiple addresses; by

simply using the ﬁrst, we may fail to communicate with the server when communication

is possible using an address later in the list. A production client should be prepared to

try all returned addresses.

3. Socket creation and setup: lines 39–43

This is almost identical to the TCP echo client, except that we create a datagram socket

using UDP. Note that we do not need to connect() before communicating with the server.

4. Send a single echo datagram: lines 45–51

With UDP we simply tell sendto() the datagram destination. If we wanted to, we could

call sendto() multiple times, changing the destination on every call, thus communicating

with multiple servers through the same socket. The ﬁrst call to sendto() also assigns

an arbitrarily chosen local port number, not in use by any other socket, to the socket

identiﬁed by sock, because we have not previously bound the socket to a port number.

We do not know (or care) what the chosen port number is, but the server will use it to

send the echoed message back to us.

5. Get and print echo reply: lines 55–73

Receive a message: lines 55–64

We initialize fromAddrLen to contain the size of the address buﬀer (fromAddr) and

then pass its address as the last parameter. recvfrom() blocks until a UDP datagram

addressed to this socket’s port arrives. It then copies the data from the ﬁrst arriving

datagram into buﬀer and copies the Internet address and (UDP) port number of its

source from the packet’s headers into the structure fromAddr. Note that the data buﬀer

4.2 UDP Server 57

is actually one byte bigger than maxstringlength, which allows us to add a null byte

to terminate the string.

Check message source: lines 67–70

Because there is no connection, a received message can come from any source. The

output parameter fromAddr informs us of the datagram’s source, and we check it

to make sure it matches the server’s Internet address. We use our own function,

SockAddrsEqual(), to perform protocol-independent comparison of socket addresses.

Although it is very unlikely that a packet would ever arrive from any other source, we

include this check to emphasize that it is possible. There’s one other complication. For

applications with multiple or repeated requests, we must keep in mind that UDP mes-

sages may be reordered and arbitrarily delayed, so simply checking the source address

and port may not be suﬃcient. (For example, the DNS protocol over UDP uses an iden-

tiﬁer ﬁeld to link requests and responses and detect duplication.) This is our last use

of the address returned from getaddrinfo(), so we can free the associated storage.

Print received string: lines 72–73

Before printing the received data as a string, we ﬁrst ensure that it is null-terminated.

6. Wrap-up: lines 75–76

This example client is ﬁne as an introduction to the UDP socket calls; it will work correctly

most of the time. However, it would not be suitable for production use, because if a message

is lost going to or from the server, the call to recvfrom() blocks forever, and the program

does not terminate. Clients generally deal with this problem through the use of timeouts,a

subject we cover later, in Section 6.3.3.

4.2 UDP Server

Our next example program implements the UDP version of the echo server, UDPEchoServer.c.

The server is very simple: It loops forever, receiving a message and then sending the same mes-

sage back to wherever it came from. Actually, the server only receives and sends back the ﬁrst

255 characters of the message; any excess is silently discarded by the sockets implementation.

(See Section 4.3 for an explanation.)

UDPEchoServer.c

1#include <stdlib.h>

2#include <string.h>

3#include <sys/types.h>

4#include <sys/socket.h>

5#include <netdb.h>

6#include "Practical.h"

58 Chapter 4: Using UDP Sockets

8int main(int argc, char *argv[]) {

10 if (argc != 2) // Test for correct number of arguments

11 DieWithUserMessage("Parameter(s)", "<Server Port/Service>");

13 char *service = argv[1]; // First arg: local port/service

15 // Construct the server address structure

16 struct addrinfo addrCriteria; // Criteria for address

17 memset(&addrCriteria, 0, sizeof(addrCriteria)); // Zero out structure

18 addrCriteria.ai_family = AF_UNSPEC; // Any address family

19 addrCriteria.ai_flags = AI_PASSIVE; // Accept on any address/port

20 addrCriteria.ai_socktype = SOCK_DGRAM; // Only datagram socket

21 addrCriteria.ai_protocol = IPPROTO_UDP; // Only UDP socket

23 struct addrinfo *servAddr; // List of server addresses

24 int rtnVal = getaddrinfo(NULL, service, &addrCriteria, &servAddr);

25 if (rtnVal != 0)

26 DieWithUserMessage("getaddrinfo() failed", gai_strerror(rtnVal));

28 // Create socket for incoming connections

29 int sock = socket(servAddr->ai_family, servAddr->ai_socktype,

30 servAddr->ai_protocol);

31 if (sock < 0)

32 DieWithSystemMessage("socket() failed");

34 // Bind to the local address

35 if (bind(sock, servAddr->ai_addr, servAddr->ai_addrlen) < 0)

36 DieWithSystemMessage("bind() failed");

38 // Free address list allocated by getaddrinfo()

39 freeaddrinfo(servAddr);

41 for (;;) { // Run forever

42 struct sockaddr_storage clntAddr; // Client address

43 // Set Length of client address structure (in-out parameter)

44 socklen_t clntAddrLen = sizeof(clntAddr);

46 // Block until receive message from a client

47 char buffer[MAXSTRINGLENGTH]; // I/O buffer

48 // Size of received message

49 ssize_t numBytesRcvd = recvfrom(sock, buffer, MAXSTRINGLENGTH, 0,

50 (struct sockaddr *) &clntAddr, &clntAddrLen);

51 if (numBytesRcvd < 0)

4.2 UDP Server 59

52 DieWithSystemMessage("recvfrom() failed");

54 fputs("Handling client ", stdout);

55 PrintSocketAddress((struct sockaddr *) &clntAddr, stdout);

56 fputc('\n', stdout);

58 // Send received datagram back to the client

59 ssize_t numBytesSent = sendto(sock, buffer, numBytesRcvd, 0,

60 (struct sockaddr *) &clntAddr, sizeof(clntAddr));

61 if (numBytesSent < 0)

62 DieWithSystemMessage("sendto() failed)");

63 else if (numBytesSent != numBytesRcvd)

64 DieWithUserMessage("sendto()", "sent unexpected number of bytes");

65 }

66 // NOT REACHED

67 }

UDPEchoServer.c

1. Program setup and parameter parsing: lines 1–13

2. Parse/resolve address/port args: lines 15–26

The port may be speciﬁed on the command line as a port number or service name. We

use getaddrinfo() to determine the actual local port number. As with our UDP client, we’ll

accept an address for any family (af_unspec) for UDP (sock_dgram and ipproto_udp).

We want our UDP server to accept echo requests from any of its interfaces. Setting the

ai_passive ﬂag makes getaddrinfo() return the wildcard Internet address (inaddr_any

for IPv4 or in6addr_any for IPv6). getaddrinfo() may return multiple addresses; we simply

use the ﬁrst.

3. Socket creation and setup: lines 28–39

This is nearly identical to the TCP echo server, except that we create a datagram socket

using UDP. Also, we do not need to call listen() because there is no connection setup—the

socket is ready to receive messages as soon as it has an address.

4. Iteratively handle incoming echo requests: lines 41–65

Several key diﬀerences between UDP and TCP servers are demonstrated in how each

communicates with the client. In the TCP server, we blocked on a call to accept() awaiting

a connection from a client. Since UDP servers do not establish a connection, we do not

need to get a new socket for each client. Instead, we can immediately call recvfrom() with

the same socket that was bound to the desired port number.

Receive an echo request: lines 42–52

recvfrom() blocks until a datagram is received from a client. Since there is no connec-

tion, each datagram may come from a diﬀerent sender, and we learn the source at

60 Chapter 4: Using UDP Sockets

the same time we receive the datagram. recvfrom() puts the address of the source in

clntAddr. The length of this address buﬀer is speciﬁed by cliAddrLen.

Send echo reply: lines 59–64

sendto() transmits the data in buﬀer back to the address speciﬁed by clntAddr. Each

received datagram is considered a single client echo request, so we only need a single

send and receive—unlike the TCP echo server, where we needed to receive until the

client closed the connection.

4.3 Sending and Receiving with UDP Sockets

As soon as it is created, a UDP socket can be used to send/receive messages to/from any

address and to/from many diﬀerent addresses in succession. To allow the destination address

to be speciﬁed for each message, the Sockets API provides a diﬀerent sending routine that is

generally used with UDP sockets: sendto(). Similarly, the recvfrom() routine returns the source

address of each received message in addition to the message itself.

ssize_t sendto(int socket,const void *msg,size_t msgLength,int flags,

const struct sockaddr *destAddr,socklen_t addrLen)

ssize_t recvfrom(int socket,void *msg,size_t msgLength,int flags,

struct sockaddr *srcAddr,socklen_t *addrLen)

The ﬁrst four parameters to sendto() are the same as those for send(). The two additional

parameters specify the message’s destination. Again, they will invariably be a pointer to a

struct sockaddr_in and its size, respectively, or a pointer to a struct sockaddr_in6 and its

size, respectively. Similarly, recvfrom() takes the same parameters as recv() but, in addition,

has two parameters that inform the caller of the source of the received datagram. One thing

to note is that addrLen is an in-out parameter in recvfrom(): On input it speciﬁes the size of

the address buﬀer srcAddr, which will typically be a struct sockaddr_storage in IP-version-

independent code. On output, it speciﬁes the size of the address that was actually copied into

the buﬀer. Two errors often made by novices are (1) passing an integer value instead of

a pointer to an integer for addrLen and (2) forgetting to initialize the pointed-to length

variable to contain the appropriate size.

We have already pointed out a subtle but important diﬀerence between TCP and UDP,

namely, that UDP preserves message boundaries. In particular, each call to recvfrom() returns

data from at most one sendto() call. Moreover, diﬀerent calls to recvfrom() will never return

data from the same call to sendto() (unless you use the msg_peek ﬂag with recvfrom()—see

the last paragraph of this section).

When a call to send() on a TCP socket returns, all the caller knows is that the data has been

copied into a buﬀer for transmission; the data may or may not have actually been transmitted

4.4 Connecting a UDP Socket 61

yet. (This is explained in more detail in Chapter 7.) However, UDP does not buﬀer data for

possible retransmission because it does not recover from errors. This means that by the time

a call to sendto() on a UDP socket returns, the message has been passed to the underlying

channel for transmission and is (or soon will be) on its way out the door.

Between the time a message arrives from the network and the time its data is returned

via recv() or recvfrom(), the data is stored in a ﬁrst-in, ﬁrst-out (FIFO) receive buﬀer. With a

connected TCP socket, all received-but-not-yet-delivered bytes are treated as one continuous

sequence (see Section 7.1). For a UDP socket, however, the bytes from diﬀerent messages may

have come from diﬀerent senders. Therefore, the boundaries between them need to be pre-

served so that the data from each message can be returned with the proper address. The buﬀer

really contains a FIFO sequence of “chunks” of data, each with an associated source address.

A call to recvfrom() will never return more than one of these chunks. However, if recvfrom() is

called with size parameter n, and the size of the ﬁrst chunk in the receive FIFO is bigger than n,

only the ﬁrst nbytes of the chunk are returned. The remaining bytes are quietly discarded,

with no indication to the receiving program.

For this reason, a receiver should always supply a buﬀer big enough to hold the largest

message allowed by its application protocol at the time it calls recvfrom(). This technique will

guarantee that no data will be lost. The maximum amount of data that can ever be returned by

recvfrom() on a UDP socket is 65,507 bytes—the largest payload that can be carried in a UDP

datagram.

Alternatively, the receiver can use the msg_peek ﬂag with recvfrom() to “peek” at the ﬁrst

chunk waiting to be received. This ﬂag causes the received data to remain in the socket’s receive

FIFO so it can be received more than once. This strategy can be useful if memory is scarce,

application messages vary widely in size, and each message carries information about its size

in the ﬁrst few bytes. The receiver ﬁrst calls recvfrom() with msg_peek and a small buﬀer,

examines the ﬁrst few bytes of the message to determine its size, and then calls recvfrom()

again (without msg_peek) with a buﬀer big enough to hold the entire message. In the usual

case where memory is not scarce, using a buﬀer big enough for the largest possible message

is simpler.

4.4 Connecting a UDP Socket

It is possible to call connect() on a UDP socket to ﬁx the destination address of future datagrams

sent over the socket. Once connected, you may use send() instead of sendto() to transmit

datagrams because you no longer need to specify the destination address. In a similar way,

you may use recv() instead of recvfrom() because a connected UDP socket can only receive

datagrams from the associated foreign address and port, so after calling connect() you know

the source address of any incoming datagrams. In fact, after connecting, you may only send

and receive to/from the address speciﬁed to connect(). Note that connecting and then using

send() and recv() with UDP does not change how UDP behaves. Message boundaries are still

62 Chapter 4: Using UDP Sockets

preserved, datagrams can be lost, and so on. You can “disconnect” by calling connect() with

an address family of AF_UNSPEC.

Another subtle advantage to calling connect() on a UDP socket is that it enables you to

receive error indications that result from earlier actions on the socket. The canonical example

is sending a datagram to a nonexistent server or port. When this happens, the send() that

eventually leads to the error returns with no indication of error. Some time later, an error

message is delivered to your host, indicating that the sent datagram encountered a problem.

Because this datagram is a control message and not a regular UDP datagram, the system can’t

always tell where to send it if your socket is unconnected, because an unconnected socket

has no associated foreign address and port. However, if your socket is connected, the system

is able to match the information in the error datagram with your socket’s associated foreign

IP address and port. (See Section 7.5 for details about this process.) Note such a control error

message being delivered to your socket will result in an error return from a subsequent system

call (for example, the recv() that was intended to get the reply), not the oﬀending send().

Exercises

1. Modify UDPEchoClient.c to use connect(). After the ﬁnal recv(), show how to disconnect

the UDP socket. Using getsockname() and getpeername(), print the local and foreign address

before and after connect(), and after disconnect.

2. Modify UDPEchoServer.c to use connect().

3. Verify experimentally the size of the largest datagram you can send and receive using a

UDP socket. Is the answer diﬀerent for IPv4 and IPv6?

4. While UDPEchoServer.c explicitly speciﬁes its local port number using bind(),wedonot

call bind() in UDPEchoClient.c. How is the UDP echo client’s socket given a port number?

Note that the answer is diﬀerent for UDP and TCP. We can select the client’s local port

using bind(). What diﬃculties might we encounter if we do this?

5. Modify UDPEchoClient.c and UDPEchoServer.c to allow the largest echo string possible

where an echo request is restricted to a single datagram.

6. Modify UDPEchoClient.c and UDPEchoServer.c to allow arbitrarily large echo strings. You

may ignore datagram loss and reordering (for now).

7. Using getsockname() and getpeername(), modify UDPEchoClient.c to print the local and

foreign address for the socket immediately before and after sendto().

8. You can use the same UDP socket to send datagrams to many diﬀerent destinations.

Modify UDPEchoClient.c to send and receive an echo datagram to/from two diﬀerent UDP

echo servers. You can use the book’s server running on multiple hosts or twice on the

same host with diﬀerent ports.

chapter 5

Sending and Receiving Data

Typically, you use sockets because your program needs to provide information to, or

use information provided by, another program. There is no magic: any programs that exchange

information must agree on how that information will be encoded—represented as a sequence

of bits—as well as which program sends what information when, and how the information

received aﬀects the behavior of the program. This agreement regarding the form and meaning

of information exchanged over a communication channel is called a protocol; a protocol used in

implementing a particular application is an application protocol. In our echo example from the

earlier chapters, the application protocol is trivial: neither the client’s nor the server’s behavior

is aﬀected by the contents of the messages they exchange. Because in most real applications

the behavior of clients and servers depends on the information they exchange, application

protocols are usually somewhat more complicated.

The TCP/IP protocols transport bytes of user data without examining or modifying them.

This allows applications great ﬂexibility in how they encode their information for transmission.

Most application protocols are deﬁned in terms of discrete messages made up of sequences

of ﬁelds. Each ﬁeld contains a speciﬁc piece of information encoded as a sequence of bits. The

application protocol speciﬁes exactly how these sequences of bits are to be arranged by the

sender and interpreted, or parsed, by the receiver so that the latter can extract the meaning

of each ﬁeld. About the only constraint imposed by TCP/IP is that information must be sent

and received in chunks whose length in bits is a multiple of eight. So from now on we consider

messages to be sequences of bytes. Given this, it may be helpful to think of a transmitted

message as a sequence or array of numbers, each between 0 and 255. That corresponds to the

range of binary values that can be encoded in 8 bits: 00000000 for zero, 00000001 for one,

00000010 for two, and so on, up to 11111111 for 255.

64 Chapter 5: Sending and Receiving Data

When you build a program to exchange information via sockets with other programs,

typically one of two situations applies: either you are designing/writing the programs on both

sides of the socket, in which case you are free to deﬁne the application protocol yourself, or

you are implementing a protocol that someone else has already speciﬁed, perhaps a proto-

col standard. In either case, the basic principles of encoding and decoding diﬀerent types of

information as bytes “on the wire” are the same. (By the way, everything in this chapter also

applies if the “wire” is a ﬁle that is written by one program and then read by another.)

5.1 Encoding Integers

Let’s ﬁrst consider the question of how integers—that is, groups of bits that can represent

whole numbers—can be sent and received via sockets. In a sense, all types of information

are ultimately encoded as ﬁxed-size integers, so the ability to send and receive them is

fundamental.

5.1.1 Sizes of Integers

We have seen that TCP and UDP sockets transmit sequences of bytes: groups of 8 bits, which

can contain whole number values in the range 0–255. Sometimes it is necessary to send integers

whose value might be bigger than 255; such integers must be encoded using multiple bytes.

To exchange ﬁxed-size, multibyte integers, the sender and receiver have to agree in advance

on several things. The ﬁrst is the size (in bytes) of each integer to be sent.

For example, an int might be stored as a 32-bit quantity. In addition to int, the C language

deﬁnes several other integer types: short,char, and long; the idea is that these integers can

be diﬀerent sizes, and the programmer can use the one that ﬁts the application. Unlike some

languages, however, the C language does not specify the exact size of each of these primitive

types. Instead, that is left up to the implementation. Thus, the size of a short integer can vary

from platform to platform.1The C language speciﬁcation does say that a char is no bigger than

ashort, which is no bigger than an int, which is no bigger than a long, which is no bigger than

along long. However, the speciﬁcation does not require that these types actually be diﬀerent

sizes—it is technically possible for a char to be the same size as a long! On most platforms,

however, the sizes do diﬀer, and it is a safe bet that they do on yours, too.

So how do you determine the exact size of an int (or char,orlong, or …) on your plat-

form? The answer is simple: use the sizeof() operator, which returns the amount of memory

(in “bytes”) occupied by its argument (a type or variable) on the current platform. Here are

a couple of things to note about sizeof(). First, the language speciﬁes that sizeof(char) is

1By “platform” in this book we mean the combination of compiler, operating system, and hardware archi-

tecture. The gcc compiler with the Linux operating system, running on Intel’s IA-32 architecture, is an

example of a platform.

5.1 Encoding Integers 65

1—always. Thus in the C language a “byte” is the amount of space occupied by a variable

of type char, and the units of sizeof() are actually sizeof(char). But exactly how big is a

C-language “byte”? That’s the second thing: the predeﬁned constant CHAR_BIT tells how many

bits it takes to represent a value of type char—usually 8, but possibly 10 or even 32.

Although it’s always possible to write a simple program to print the values returned by

sizeof() for the various primitive integer types, and thus clear up any mystery about integer

sizes on your platform, C’s lack of speciﬁcity about the size of its primitive integer types makes

it a little tricky if you want to write portable code for sending integers of a speciﬁc size over

the Internet. Consider the problem of sending a 32-bit integer over a TCP connection. Do you

use an int,along, or what? On some machines an int is 32 bits, while on others a long may be

32 bits.

The C99 language standard speciﬁcation oﬀers a solution in the form of a set of optional

types: int8_t,int16_t,int32_t, and int64_t (along with their unsigned counterparts uint8_t,

etc) all have the size (in bits) indicated by their names. On a platform where CHAR_BIT is

eight,2these are 1, 2, 4 and 8 byte integers, respectively. Although these types may not be

implemented on every platform, each is required to be deﬁned if any native primitive type has

the corresponding size. (So if, say, the size of an int on the platform is 32 bits, the “optional”

type int32_t is required to be deﬁned.) Throughout the rest of this chapter, we make use of

these types to specify the precise size of the integers we want. We will also make use of the

C99-deﬁned type long long, which is typically larger than a long. Program TestSizes.c will print

the value of CHAR_BIT, the sizes of all the primitive integer types, and the sizes of the ﬁxed-

size types deﬁned by the C99 standard. (The program will not compile if any of the optional

types are not deﬁned on your platform.)

TestSizes.c

1#include <limits.h>

2#include <stdint.h>

3#include <stdio.h>

5int main(int argc, char *argv[]) {

6printf("CHAR_BIT is %d\n\n",CHAR_BIT); // Bits in a char (usually 8!)

8printf("sizeof(char) is %d\n", sizeof(char)); // ALWAYS 1

9printf("sizeof(short) is %d\n", sizeof(short));

10 printf("sizeof(int) is %d\n", sizeof(int));

11 printf("sizeof(long) is %d\n", sizeof(long));

12 printf("sizeof(long long) is %d\n\n", sizeof(long long));

14 printf("sizeof(int8_t) is %d\n", sizeof(int8_t));

2We are not aware of any modern general-purpose computing platform where CHAR_BIT diﬀers from

eight. Throughout the rest of this book, the value of CHAR_BIT is assumed, without comment, to be 8.

66 Chapter 5: Sending and Receiving Data

15 printf("sizeof(int16_t) is %d\n", sizeof(int16_t));

16 printf("sizeof(int32_t) is %d\n", sizeof(int32_t));

17 printf("sizeof(int64_t) is %d\n\n", sizeof(int64_t));

19 printf("sizeof(uint8_t) is %d\n", sizeof(uint8_t));

20 printf("sizeof(uint16_t) is %d\n", sizeof(uint16_t));

21 printf("sizeof(uint32_t) is %d\n", sizeof(uint32_t));

22 printf("sizeof(uint64_t) is %d\n", sizeof(uint64_t));

23 }

TestSizes.c

To make things a little more concrete, in the remainder of this section we’ll consider the

problem of encoding a sequence of integers of diﬀerent sizes—speciﬁcally, of 1, 2, 4, and 8

bytes, in that order. Thus, we need a total of 15 bytes, as shown in the following ﬁgure.

uint16_t uint32_t uint64_tuint8_t

We’ll consider several diﬀerent methods of doing this, but in all cases we’ll assume that the

C99 ﬁxed-size types are supported.

5.1.2 Byte Ordering

Once the sender and receiver have speciﬁed the sizes of the integers to be transmitted, they

need to agree on some other aspects. For integers that require more than one byte to encode,

they have to answer the question of which order to send the bytes in.

There are two obvious choices: start at the “right” end of the number, with the least

signiﬁcant bits—so-called little-endian order—or at the left end, with the most signiﬁcant bits—

big-endian order. (Note that the ordering of bits within bytes is, fortunately, handled by the

implementation in a standard way.) Consider the long long value 123456787654321L. Its 64-bit

representation (in hexadecimal) is 0x0000704885F926B1. If we transmit the bytes in big-endian

order, the sequence of (decimal) byte values will look like this:

177382491337211200

Big-endian order of transmission

If we transmit them in little-endian order, the sequence will be:

133 72 112 024938177

Little-endian order of transmission

5.1 Encoding Integers 67

The main point is that for any multibyte integer quantity, the sender and receiver need to

agree on whether big-endian or little-endian order will be used.3If the sender were to use little-

endian order to send the above integer, and the receiver were expecting big-endian, instead of

the correct value, the receiver would interpret the transmitted 8-byte sequence as the value

12765164544669515776L.

Most protocols that send multibyte quantities in the Internet today use big-endian byte

order; in fact, it is sometimes called network byte order. The byte order used by the hardware

(whether it is big- or little-endian) is called the native byte order. C language platforms typically

provide functions that allow you to convert values between native and network byte orders;

you may recall that we have already encountered htons() and htonl(). Those routines, along

with ntohl() and ntohs(), handle the conversion for typical integer sizes. The functions whose

names end in “l” (for “long”) operate on 32-bit quantities, while the ones ending in “s” (for

“short”) operate on 16-bit quantities. The “h” stands for “host,” and the “n” for “network”.

Thus htons() was used in Chapter 2 to convert 16-bit port numbers from host byte order to

network byte order, because the Sockets API routines deal only with addresses and ports in

network byte order. That fact is worth repeating, because beginning programmers are often

bitten by forgetting it: addresses and ports that cross the Sockets API are always in network

byte order. We will make full use of these order-conversion functions shortly.

5.1.3 Signedness and Sign Extension

One last detail on which the sender and receiver must agree: whether the numbers transmit-

ted will be signed or unsigned. We’ve said that bytes contain values in the range 0 to 255

(decimal). That’s true if we don’t need negative numbers, but they are necessary for many

applications. Fortunately, the same 255-bit patterns can be interpreted as integers in the range

−128 to 127. Two’s-complement representation is the usual way of representing such signed

numbers. For a k-bit number, the two’s-complement representation of the negative integer

−n,1≤n≤2k−1, is the binary value of 2k−n. The nonnegative integer p,0≤p≤2k−1−1, is

encoded simply by the k-bit binary value of p. Thus, given kbits, we can represent values in the

range −2k−1through 2k−1−1 using two’s-complement. Note that the most signiﬁcant bit (msb)

tells whether the value is positive (msb =0) or negative (msb =1). On the other hand, a k-bit

unsigned integer can encode values in the range 0 through 2k−1 directly. So for example, the

32-bit value 0xffffffff (the all-ones value) when interpreted as a signed, two’s-complement

number represents −1; when interpreted as an unsigned integer, it represents 4294967295.

The signedness of the integers being transmitted should be determined by the range of values

that need to be encoded.

Some care is required when dealing with integers of diﬀerent signedness because of sign-

extension. When a signed value is copied to any wider type, the additional bits are copied from

the sign (i.e., most signiﬁcant) bit. By way of example, suppose the variable smallInt is of type

3Other orders are possible for integers bigger than 2 bytes, but we know of no modern systems that use

them.

68 Chapter 5: Sending and Receiving Data

int8_t, that is, a signed, 8-bit integer, and widerInt is of type int16_t. Suppose also that smallInt

contains the (binary) value 01001110 (i.e., decimal 78). The assignment statement:

widerInt := smallInt;

places the binary value 0000000001001110 into widerInt. However, if smallInt has the value

11100010 (decimal −30) before the assignment statement, then afterward widerInt contains

the binary value 1111111111100010.

Now suppose the variable widerUInt is of type uint16_t, and smallInt again has the value

−30, and we do this assignment:

widerUInt := smallInt;

What do you think the value of widerUInt is afterward? The answer is again

1111111111100010, because the sign of smallInt’s value is extended as it is widened to ﬁt

in widerUInt, even though the latter variable is unsigned. If you print the resulting value of

widerUInt as a decimal number, the result will be 65506. On the other hand, if we have a vari-

able smallUInt of type uint8_t, containing the same binary value 11100010, and we copy its

value to the wider unsigned variable:

widerUInt := smallUInt;

and then print the result, we get 226, because the value of an unsigned integer type is—

reasonably enough—not sign-extended.

One ﬁnal point to remember: when expressions are evaluated, values of variables are

widened (if needed) to the “native” (int) size before any computation occurs. Thus, if you add

the values of two char variables together, the type of the result will be int, not char:

char a,b;

printf("sizeof(a+b) is %d\n", sizeof(a+b));

On the platform used in writing this book, this code prints “sizeof(a+b) is 4”. The type of the

argument to sizeof()—the expression a+b—is int. This is generally not an issue, but you need

to be aware that sign-extension also occurs during this implicit widening.

5.1.4 Encoding Integers by Hand

Having agreed on byte ordering (we’ll use big-endian) and signedness (the integers are all

unsigned), we’re ready to construct our message. We’ll ﬁrst show how to do it “by hand,”

using shifting and masking operations. The program BruteForceCoding.c features a method

EncodeIntBigEndian() that places any given primitive integer value as a sequence of the speci-

ﬁed number of bytes at a speciﬁed location in memory, using big-endian representation. The

method takes four arguments: a pointer to the starting location where the value is to be placed;

the value to be encoded (represented as a 64-bit unsigned integer, which is big enough to hold

any of the other types); the oﬀset in the array at which the value should start; and the size in

bytes of the value to be written. Of course, whatever we encode at the sender must be decodable

5.1 Encoding Integers 69

at the receiver. The DecodeIntBigEndian() method handles decoding a byte sequence of a given

length into a 64-bit integer, interpreting it as a big-endian sequence.

These methods treat all quantities as unsigned; see the exercises for other possibilities.

BruteForceCoding.c

1#include <stdint.h>

2#include <stdlib.h>

3#include <stdio.h>

4#include <limits.h>

5#include "Practical.h"

7const uint8_t val8 = 101; // One hundred and one

8const uint16_t val16 = 10001; // Ten thousand and one

9const uint32_t val32 = 100000001; // One hundred million and one

10 const uint64_t val64 = 1000000000001L; // One trillion and one

11 const int MESSAGELENGTH = sizeof(uint8_t) + sizeof(uint16_t) + sizeof(uint32_t)

12 + sizeof(uint64_t);

14 static char stringBuf[BUFSIZE];

15 char *BytesToDecString(uint8_t *byteArray, int arrayLength) {

16 char *cp = stringBuf;

17 size_t bufSpaceLeft = BUFSIZE;

18 for(inti=0;i<arrayLength && bufSpaceLeft > 0; i++) {

19 int strl = snprintf(cp, bufSpaceLeft, "%u ", byteArray[i]);

20 bufSpaceLeft -= strl;

21 cp += strl;

22 }

23 return stringBuf;

24 }

26 // Warning: Untested preconditions (e.g., 0 <= size <= 8)

27 int EncodeIntBigEndian(uint8_t dst[], uint64_t val, int offset, int size) {

28 for(inti=0;i<size; i++) {

29 dst[offset++] = (uint8_t) (val >> ((size - 1) - i) * CHAR_BIT);

30 }

31 return offset;

32 }

34 // Warning: Untested preconditions (e.g., 0 <= size <= 8)

35 uint64_t DecodeIntBigEndian(uint8_t val[], int offset, int size) {

36 uint64_t rtn = 0;

37 for(inti=0;i<size; i++) {

38 rtn = (rtn << CHAR_BIT) | val[offset + i];

39 }

40 return rtn;

70 Chapter 5: Sending and Receiving Data

41 }

43 int main(int argc, char *argv[]) {

44 uint8_t message[MESSAGELENGTH]; // Big enough to hold all four values

46 // Encode the integers in sequence in the message buffer

47 int offset = 0;

48 offset = EncodeIntBigEndian(message, val8, offset, sizeof(uint8_t));

49 offset = EncodeIntBigEndian(message, val16, offset, sizeof(uint16_t));

50 offset = EncodeIntBigEndian(message, val32, offset, sizeof(uint32_t));

51 offset = EncodeIntBigEndian(message, val64, offset, sizeof(uint64_t));

52 printf("Encoded message:\n%s\n", BytesToDecString(message, MESSAGELENGTH));

54 uint64_t value =

55 DecodeIntBigEndian(message, sizeof(uint8_t), sizeof(uint16_t));

56 printf("Decoded 2-byte integer = %u\n", (unsigned int) value);

57 value = DecodeIntBigEndian(message, sizeof(uint8_t) + sizeof(uint16_t)

58 + sizeof(uint32_t), sizeof(uint64_t));

59 printf("Decoded 8-byte integer = %llu\n", value);

61 // Show signedness

62 offset = 4;

63 int iSize = sizeof(int32_t);

64 value = DecodeIntBigEndian(message, offset, iSize);

65 printf("Decoded value (offset %d, size %d) = %lld\n", offset, iSize, value);

66 int signedVal = DecodeIntBigEndian(message, offset, iSize);

67 printf("...same as signed value %d\n", signedVal);

68 }

BruteForceCoding.c

1. Declarations and inclusions: lines 1–12

Library functions and constants: lines 1–5

Integer variables with values to be encoded: lines 7–10

Message length computation: lines 11–12

The language spec says the initializer expression evaluates to 15; we include it for

completeness.

2. BytesToDecString():lines 14–24

This auxiliary routine takes an array of bytes and its length, and returns a string

containing the value of each byte as a decimal integer in the range 0 to 255.

3. EncodeIntBigEndian():lines 27–32

We iterate over the given value size times. On each iteration, the right-hand side of the

assignment shifts the value to be encoded to the right, so the byte we are interested in is

5.1 Encoding Integers 71

in the low-order 8 bits. The resulting value is then cast to the type uint8_t, which throws

away all but the low-order 8 bits, and placed in the array at the appropriate location. The

ending value of offset is returned so that the caller does not have to recompute it when

encoding a sequence of integers (as we will).

4. DecodeIntBigEndian():lines 35–41

We construct the value in a 64-bit integer variable. Again we iterate size times, each time

shifting the accumulated value left and bitwise-ORing in the next byte’s value.

5. Demonstrate methods: lines 43–68

Declare buﬀer (array of bytes) to receive series of integers: line 44

Encode items: lines 47–51

The integers are encoded into the array in the sequence described earlier.

Print contents of encoded array: line 52

Extract and display some values from encoded message: lines 54–59

Output should show the decoded values equal to the original constants.

Signedness eﬀects: lines 62–67

At oﬀset 4, the byte value is 245 (decimal); because it has its high-order bit set, if it is

the high-order byte of a signed value, that value will be considered negative. We show

this be decoding the 4 bytes starting at oﬀset 4 and placing the result into both a signed

integer and an unsigned integer.

Note that we might consider testing several preconditions at the beginning of

EncodeIntBigEndian() and DecodeIntBigEndian(), such as 0 ≤size ≤8 and dst != NULL. Can you

name any others?

Running the program produces output showing the following (decimal) byte values:

byte short int long

101 39 17 5 245 225 1 232 212 165 16 1 000

As you can see, the brute-force method requires the programmer to do quite a bit of work:

computing and naming the oﬀset and size of each value, and invoking the encoding routine

with the appropriate arguments. Fortunately, alternative ways to build messages are often

available. We discuss these next.

5.1.5 Wrapping TCP Sockets in Streams

A way of encoding multibyte integers for transmission over a stream (TCP) socket is to use

the built-in FILE-stream facilities—the same methods you use with stdin,stdout, and so on.

To access these facilities, you need to associate one or more FILE streams with your socket

descriptor, via the fdopen() call.

72 Chapter 5: Sending and Receiving Data

FILE *fdopen(int socketdes,const char *mode)

int fclose(FILE * stream)

int fflush(FILE * stream)

The fdopen() function “wraps” the socket in a stream and returns the result (or NULL if an

error occurs); it is as if you could call fopen() on a network address. This allows buﬀered I/O

to be performed on the socket via operations like fgets(),fputs(),fread() and fwrite(); the

“mode” argument to fdopen() takes the same values as fopen().fflush() pushes buﬀered data

to underlying socket. fclose() closes the stream along with the underlying socket. fflush()

causes any buﬀered to be sent over the underlying socket.

size_t fwrite(const void * ptr,size_t size,size_t nmemb,FILE * stream)

size_t fread(void * ptr,size_t size,size_t nmemb,FILE * stream)

The fwrite() method writes the speciﬁed number of objects of the given size to the

stream. The fread() method goes in the other direction, reading the given number of objects

of the given size from the given stream and placing them sequentially in the location pointed

to by ptr. Note that the sizes are given in units of sizeof(char), while the return values of

these methods are the number of objects read/written, not the number of bytes. In particular,

fread() never reads part of an object from the stream, and similarly fwrite() never writes a

partial object. If the underlying connection terminates, these methods will return a short item

count.

By giving diﬀerent sizes to fwrite(), we can output our message to the stream sequen-

tially. Assume that the variables val8,val16, and the rest are declared and initialized as in

BruteForceCoding.c and that the integer variable sock is the descriptor of the connected TCP

socket over which we want to write our message. Finally, assume that htonll() is a function

that converts 64-bit integers from host to network byte order (see Exercise 2). Then we can

write our message to the socket one integer at a time:

sock = socket(/*...*/);

/* ... connect socket ...*/

// wrap the socket in an output stream

FILE *outstream = fdopen(sock, "w");

// send message, converting each object to network byte order before sending

if (fwrite(&val8, sizeof(val8), 1, outstream) != 1) ...

val16 = htons(val16);

if (fwrite(&val16, sizeof(val16), 1, outstream) != 1) ...

val32 = htonl(val32);

if (fwrite(&val32, sizeof(val32), 1, outstream) != 1) ...

val64 = htonll(val64);

if (fwrite(&val64, sizeof(val64), 1, outstream) != 1) ...

5.1 Encoding Integers 73

fflush(outstream); // immediately flush stream buffer to socket

... // do other work...

fclose(outstream); // flushes stream and closes socket

So much for the sending side. How does the receiver recover the transmitted values? As

you might expect, the receiving side takes the analogous steps using fread(). Suppose now the

variable csock contains the socket descriptor for a TCP connection, that the received values

are to be placed in the variables rcv8,rcv16,rcv32, and rcv64 of the expected types, and that

ntohll() is a network-to-host byte order converter for 64-bit types.

/* ... csock is connected ...*/

// wrap the socket in an input stream

FILE *instream = fdopen(csock, "r");

// receive message, converting each received object to host byte order

if (fread(&rcv8, sizeof(rcv8), 1, instream) != 1) ...

if (fread(&rcv16, sizeof(rcv16), 1, instream) != 1) ...

rcv16 = ntohs(rcv16); // convert to host order

if (fread(&rcv32, sizeof(rcv32), 1, instream) != 1) ...

rcv32 = ntohl(rcv32);

if (fread(&rcv64, sizeof(rcv64), 1, instream) != 1) ...

rcv64 = ntohll(rcv64);

...

fclose(instream); // closes the socket connection!

Among the advantages of using buﬀered FILE-streams with sockets is the ability to “put

back” a byte after reading it from the stream (via ungetc()); this can sometimes be useful when

parsing messages. We must emphasize, however, that FILE-streams can only be used with

TCP sockets.

5.1.6 Structure Overlays: Alignment and Padding

The most common approach to constructing messages containing binary data (i.e., multibyte

integers) involves overlaying a C structure on a section of memory and assigning to the ﬁelds

of the structure directly. This is possible because the C language speciﬁcation explicitly deﬁnes

how structures are laid out in memory by the compiler. It has this feature because it was

designed for implementing operating systems, where eﬃciency is a primary concern, and it is

often necessary to know precisely how data structures are represented. In combination with

the facilities for byte order conversion that we saw above, this makes it rather simple to con-

struct and parse messages made up of integers of particular sizes. (There is a signiﬁcant caveat,

however, that we will encounter shortly.)

Suppose for a moment that we are dealing with three integer components of an address:

the number on the street, which ranges from 1 to about 12000; an apartment number that

never exceeds 8000 (a nonapartment address uses an apartment number of −1); and a postal

code, which is a ﬁve-digit number between 10000 and 99999. The ﬁrst two components can be

represented using 16-bit integers; the third is too big for that, so we’ll use a 32-bit integer for

74 Chapter 5: Sending and Receiving Data

it. If we want to pass those quantities around inside a program, we could declare a structure

and pass around pointers to it:

struct addressInfo {

uint16_t streetAddress;

int16_t aptNumber;

uint32_t postalCode;

} addrInfo;

Clearly, such a structure is useful for passing data around in a program, but can we use it to

pass information between programs over the Internet? The answer is yes. The C speciﬁcation

says that this structure will be laid out as follows in memory:

streetAddress postalCodeaptNumber

To exchange information between programs, we can simply use the layout of the structure

as the message format, and take the contents directly from the structure and send them over

the network (after any needed byte-order conversions). If the variable addrInfo declared above

has been initialized to contain the values we want to send, and sock represents a connected

socket as usual, we could send the 8-byte message using the following code:

// ... put values in addrInfo ...

// convert to network byte order

addrInfo.streetAddress = htons(addrInfo.streetAddress);

addrInfo.aptNumber = htons(addrInfo.aptNumber);

addrInfo.postalCode = htonl(addrInfo.postalCode);

if (send(sock, &addrInfo, sizeof(addrInfo), 0) != sizeof(addrInfo)) ...

On the receiving end, we can again use a buﬀered input stream (see previous section) and

fread() to handle getting the right number of bytes into the structure. (Otherwise we have to

use a loop because, as we saw earlier, there is no guarantee that all the bytes of the message

will be returned in a single call to recv()). Once that’s done, all we have to do is take care of

byte ordering:

struct addressInfo addrInfo;

// ... sock is a connected socket descriptor ...

FILE *instream = fdopen(sock, "r");

if (fread(&addrInfo, sizeof(struct addressInfo), 1, instream) != 1) {

// ... handle error

}

// convert to host byte order

addrInfo.streetAddress = ntohs(addrInfo.streetAddress);

addrInfo.aptNumber = ntohs(addrInfo.aptNumber);

addrInfo.postalCode = ntohl(addrInfo.postalCode);

// use information from message...

5.1 Encoding Integers 75

Now, it would seem that we could construct our 15-byte message using a declaration like the

following:

struct integerMessage {

uint8_t oneByte;

uint16_t twoBytes;

uint32_t fourBytes;

uint64_t eightBytes;

}

Alas, this doesn’t work, because the C language rules for laying out data structures include

speciﬁc alignment requirements, including that the ﬁelds within a structure begin on certain

boundaries based on their type. The main points of the requirements can be summarized as

follows:

Data structures are maximally aligned. That is, the address of any instance of a structure

(including one in an array) will be divisible by the size of its largest native integer ﬁeld.

Fields whose type is a multibyte integer type are aligned to their size (in bytes). Thus,

an int32_t integer ﬁeld’s beginning address is always divisible by four, and a uint16_t

integer ﬁeld’s address is guaranteed to be divisible by two.

To enforce these constraints, the compiler may add padding between the ﬁelds of a structure.

Because of this, the size of the structure declared above is not 15, but 16. To see why, let’s con-

sider the constraints that apply. First, the whole structure must begin on an address divisible

by 8. The twoBytes ﬁeld must be at an even address, fourBytes must be at an address divisible

by four, and eightBytes’ address must be divisible by eight. All of these constraints will be

satisﬁed if a single byte of padding is inserted between the oneByte ﬁeld and the twoBytes

ﬁeld, like this:

pad eightBytesfourBytestwoBytesoneByte

The contents of bytes added by the compiler as padding are undeﬁned. Thus if the declaration

above is used, and the receiver is expecting the original unpadded layout, incorrect behavior

is the likely result.

The best solution is to avoid the need for padding by laying out messages so that it is

not needed. Unfortunately (and somewhat surprisingly), that’s not always possible. In the case

of our four-integer example, if we arranged the message with ﬁelds in the opposite order:

struct backwardMessage {

uint64_t eightBytes;

uint32_t fourBytes;

uint16_t twoBytes;

uint8_t oneByte;

}

76 Chapter 5: Sending and Receiving Data

no padding would be required between the ﬁelds. However, sizeof(struct backwardMessage)

would nevertheless return 16, not 15. This is because one byte of padding would be required

between instances of the structure (in an array, for example), in order to satisfy the ﬁrst

(maximal-alignment) constraint. (The invariant that must be maintained is that in an array

of structures, the address of one element plus the sizeof() the structure yields the address of

the subsequent element.) As a consequence, there is no way to specify a structure containing

any multibyte integer for which sizeof() returns an odd number.

If we can’t eliminate compiler-required padding, we can, as a last resort, include it in the

message format speciﬁcation. For our example, the message format could be deﬁned with an

explicit padding byte after the ﬁrst ﬁeld:

struct integerMessage2 {

uint8_t oneByte;

uint8_t padding; // Required for alignment

uint16_t twoBytes;

uint32_t fourBytes;

uint64_t eightBytes;

}

This structure is laid out in memory exactly like the originally declared integerMessage, except

that the contents of the padding byte can now be controlled and accessed by the programmer.

We’ll see more examples of the use of structures to encode data in Section 5.2.3.

5.1.7 Strings and Text

Old-fashioned text—strings of printable (displayable) characters—is perhaps the most com-

mon way to represent information. We’ve already seen examples of sending and receiving

strings of text in our echo client and server; you are probably also accustomed to having

programs you write generate text output.

Text is convenient because we deal with it all the time: information is represented as

strings of characters in books, newspapers, and on computer displays. Thus, once we know

how to encode text for transmission, it is straightforward to send most any other kind of data:

simply represent it as text, then encode the text. You know how to represent numbers and

boolean values as strings of text—for example "123478962","6.02e23","true","false". In this

section we deal with the question of encoding such strings as byte sequences.

To that end, we ﬁrst need to recognize that text is made up of sequences of symbols,

or characters. The C language includes the primitive type char for representing characters,

but does not include a primitive type for strings. Strings in C are traditionally represented

as arrays of char.Achar value in C is represented internally as an integer. For example, the

character ‘a’, that is, the symbol for the letter ‘a’, corresponds to the integer 97. The character

‘X’ corresponds to 88, and the symbol ‘!’ (exclamation mark) corresponds to 33.

A mapping between a set of symbols and a set of integers is called a coded character set.

You may have heard of the coded character set known as ASCII —American Standard Code for

Information Interchange. ASCII maps the letters of the English alphabet, digits, punctuation,

and some other special (nonprintable) symbols to integers between 0 and 127. It has been used

5.1 Encoding Integers 77

for data transmission since the 1960s, and is used extensively in application protocols such as

HTTP (the protocol used for the World Wide Web), even today. The C language speciﬁes a basic

character set that is a subset of ASCII. The importance of ASCII (and the C basic character set)

is that strings containing only characters from the basic set can be encoded using one byte per

character. (Note that in our echo client and server from Chapter 2 the encoding was irrelevant,

because the server did not interpret the received data at all.)

This is well and good if you speak and use a language (like English) that can be represented

using a small number of symbols. However, consider that no more than 256 distinct symbols

can be encoded using one byte per symbol, and that a large fraction of the world’s people

use languages that have more than 256 symbols, and therefore have to be encoded using more

than 8 bits per character. Clearly, the use of C presents signiﬁcant challenges for implementing

code that is “internationalizable.” Mandarin is just one prominent example of a language that

requires thousands of symbols.

The C99 extensions standard deﬁnes a type wchar_t (“wide character”) to store charac-

ters from charsets that may use more than one byte per symbol. In addition, various library

functions are deﬁned that support conversion between byte sequences and arrays of wchar_t,

in both directions. (In fact, there is a wide character string version of virtually every library

function that operates on character strings.) To convert back and forth between wide strings

and encoded char (byte) sequences suitable for transmission over the network, we would use

the wcstombs() (“wide character string to multibyte string”) and mbstowcs() functions.

#include <stdlib.h>

size_t wcstombs(char *restrict s, const wchar_t *restrict pwcs, size_t n);

size_t mbstowcs(wchar_t *restrict pwcs, const char *restrict s, size_t n);

The ﬁrst of these converts a sequence of wide characters from the array pointed to by pwcs

into a sequence of multibyte characters, and stores these multibyte characters into the array

pointed to by s, stopping if the next conversion would exceed the limit of ntotal bytes or if a

null character is stored. The second does the same thing in the other direction.

As we have seen, for a coded character set that requires larger integer values, there is more

than one way to encode those values for transmission over the network. Thus, it is necessary

that sender and receiver agree on how those integers will be encoded as byte sequences.

As an example, consider a platform that uses 16-bit integers internally to represent char-

acters, and whose character set includes ASCII as a subset—that is, the character ‘a’ maps to 97,

‘X’ to 88, and so on. Using the wide character facilities, we can declare a wide character string:

wchar_t testString[] = "Test!";

The internal representation of this string uses six 16-bit integers (including one for the ter-

minating null (0) character). However, there are several ways to encode those integers as a

sequence of 8-bit bytes:

Each character can be represented using 2 bytes, in big-endian order. In that case, the

result of calling wcstombs() on the wide string “Test!” would be the following sequence:

0, 84, 0, 101, 0, 115, 0, 116, 0, 33.

78 Chapter 5: Sending and Receiving Data

Alternatively, we could use 2 bytes in little-endian order, which would give: 84, 0, 101, 0,

115, 0, 116, 0, 33, 0.

We could use an encoding that maps symbols in the original ASCII character set to single

bytes (the same as their ASCII encoding), and uses 2 bytes for symbols beyond ASCII. If

the high-order bit of a byte is set, it indicates the next symbol is encoded with 2 bytes;

otherwise it is an ASCII symbol encoded with a single byte. In this case the result after

calling wcstombs() would be the 5-byte sequence 84, 101, 115, 116, 33.

Note that in these examples we did not include the terminating null (0) character. The terminat-

ing null is an artifact of the language, and not part of the string itself. It therefore should not

be transmitted with the string unless the protocol explicitly speciﬁes that method of marking

the end of the string.

The bad news is that C99’s wide character facilities are not designed to give the program-

mer explicit control over the encoding scheme. Indeed, they assume a single, ﬁxed charset

deﬁned according to the “locale” of the platform. Although the facilities support a variety of

charsets, they do not even provide the programmer any way to learn which charset or encoding

is in use. In fact, the C99 standard states in several situations that the eﬀect of changing the

locale’s charset at runtime is undeﬁned. What this means is that if you want to implement a

protocol using a particular charset, you’ll have to implement the encoding yourself.

5.1.8 Bit-Diddling: Encoding Booleans

Bitmaps are a very compact way to encode boolean information, which is often used in proto-

cols. The idea of a bitmap is that each of the bits of an integer type can encode one boolean

value—typically with 0 representing false and 1 representing true. To be able to manipulate

bitmaps, you need to know how to set and clear individual bits using C’s “bit-diddling” oper-

ations. A mask is an integer value that has one or more speciﬁc bits set to 1, and all others

cleared (i.e., 0). We’ll deal here mainly with 32-bit maps and masks, but everything we say

applies to types of other sizes as well.

Let’s number the bits of an integer’s binary representation from 0 to 31, where bit 0 is

the least signiﬁcant bit. In general, the uint32_t value that hasa1inbitposition i, and a zero

in all other bit positions, is just 2i. So bit 5 corresponds to the number 32, bit 12 to 4096, and

so forth. Here are some example mask declarations:

const int BIT5 = (1<<5);

const int BIT7 = 0x80;

const int BITS2AND3 = 12; // 8+4

int bitmap = 128;

To set a particular bit in an int variable, combine it with the mask for that bit using the bitwise-

OR operation (|):

bitmap |= BIT5;

// bit 5 is now one

5.2 Constructing, Framing, and Parsing Messages 79

To clear a particular bit, bitwise-AND it with the bitwise complement of the mask for that bit

(which has ones everywhere except the particular bit, which is zero). The bitwise-AND operation

inCis&, while the bitwise-complement operator is ~.

bitmap &= ~BIT7;

// bit 7 is now zero

You can set and clear multiple bits at once by OR-ing together the corresponding masks:

// clear bits 2, 3 and 5

bitmap &= ~(BITS2AND3|BIT5);

To test whether a bit is set, compare the result of the bitwise-AND of the mask and the value

with zero:

bool bit6Set = (bitmap & (1<<6)) != 0;

5.2 Constructing, Framing, and Parsing Messages

We close this chapter with an example illustrating the application of the foregoing techniques in

implementing a protocol speciﬁed by someone else. The example is a simple “voting” protocol

as shown in Figure 5.1. Here a client sends a request message to the server; the message

contains a candidate ID, which is an integer between 0 and 1000. Two types of requests are

supported. An inquiry asks the server how many votes have been cast for the given candidate.

The server sends back a response message containing the original candidate ID and the vote

total as of the time the request was received for that candidate. A voting request actually casts

a vote for the indicated candidate. The server again responds with a message containing the

candidate ID and the vote total (which now includes the vote just cast). The protocol runs over

stream sockets using TCP.

Normally, the “wire format” of the protocol messages would be precisely deﬁned as part

of the protocol. As we have seen, we might encode it in a number of ways: we could repre-

sent the information using strings of text, or as binary numbers. In order to illustrate all the

Vote Request

Candidate 5 775

Vote Response

Candidate 5 775

Vote Count 5 21527

Figure 5.1: Voting protocol.

80 Chapter 5: Sending and Receiving Data

techniques described above, we will specify several diﬀerent versions of the “wire format.”

This also helps us to make a point about protocol implementation.

When implementing any protocol, it is good practice to hide the details of the way mes-

sages are encoded from the main program logic. We’ll illustrate that here by using a generic

structure in the main programs to pass information to/from functions that process messages

sent/received over the socket. This allows us to use the same client and server code with dif-

ferent implementations of the “wire format” processing. The VoteInfo structure, deﬁned in

VoteProtocol.h, contains everything needed to construct a message: the candidate ID number

(an integer), the count of votes for that candidate (a 64-bit integer), a boolean indicating whether

the message is an “inquiry” (inquiries do not aﬀect the vote count), and another boolean indi-

cating whether the message to be sent is a response sent from client to server (true), or a

request (false). Also deﬁned in this ﬁle are constants for the largest allowable candidate ID

number and the maximum length of an encoded message on the wire. (The latter helps the

programs to size their buﬀers.)

struct VoteInfo {

uint64_t count; // invariant: !isResponse => count==0

int candidate; // invariant: 0 <= candidate <= MAX_CANDIDATE

bool isInquiry;

bool isResponse;

};

typedef struct VoteInfo VoteInfo;

enum {

MAX_CANDIDATE = 1000,

MAX_WIRE_SIZE = 500

};

Note that we have deﬁned a single VoteInfo structure for the information contained in both

request messages and response messages. This, along with a proper control structure, allows

reuse of the same message-processing code for both client and server, and both request and

response.

The message-processing code is responsible for encoding the information from a Vote-

Info structure and transmitting it over a stream socket, as well as for receiving data from a TCP

socket, parsing the incoming vote protocol message (if any), and ﬁlling in a VoteInfo structure

with the received information.

A clean design further decomposes the process into two parts. The ﬁrst is concerned with

framing, or marking the boundaries of the message, so the receiver can ﬁnd it in the stream.

The second is concerned with the actual encoding of the message, whether it is represented

using text or binary data. Notice that these two parts can be independent of each other, and in a

well-designed protocol they should be separated. In other words, we can specify the mechanism

for framing the message as a whole separately from the encoding of its diﬀerent ﬁelds. And

that is what we shall do. Our job will be somewhat easier if we can use stream-processing

functions, so we will have the client and server wrap the connected socket in a FILE stream for

input and another for output.

5.2 Constructing, Framing, and Parsing Messages 81

The interface to the framing code is deﬁned as follows in Framer.h.

int GetNextMsg(FILE *in, uint8_t *buf, size_t bufSize);

int PutMsg(uint8_t buf[], size_t msgSize, FILE *out);

The method GetNextMsg() reads data from the given stream and places it in the given buﬀer

until it runs out of room or determines that it has received a complete message. It returns

the number of bytes placed in the buﬀer (all framing information is stripped). The method

PutMsg() adds framing information to the message contained in the given buﬀer, and writes

both message and framing information to the given stream. Note that neither of these methods

needs to know anything about the message content.

The interface to the encoding and parsing code is deﬁned in VoteEncoding.h as follows:

bool Decode(uint8_t *inBuf, size_t mSize, VoteInfo *v);

size_t Encode(VoteInfo *v, uint8_t *outBuf, size_t bufSize);

The Encode() method takes a VoteInfo structure as input and converts it to a sequence of

bytes according to a particular wire format encoding; it returns the size of the resulting byte

sequence. The Decode() method takes a byte sequence of a speciﬁed size and parses it as

a message according to the protocol, ﬁlling in the VoteInfo with the information from the

message. It returns true if the message was successfully parsed, and false otherwise.

Given these interfaces to the framing and parsing code, we can now describe the voting

client and server programs that will use these methods. The client is straightforward: the can-

didate ID is given as a command-line argument, along with a ﬂag indicating that the transaction

is an inquiry (by default it is a vote request). Upon sending the request, the client waits for the

response and then closes the connection when it is received.

VoteClientTCP.c

1#include <stdio.h>

2#include <string.h>

3#include <stdlib.h>

4#include <stdint.h>

5#include <unistd.h>

6#include <errno.h>

7#include <sys/socket.h>

8#include <netinet/in.h>

9#include <arpa/inet.h>

10 #include <netdb.h>

11 #include "Practical.h"

12 #include "VoteProtocol.h"

13 #include "Framer.h"

14 #include "VoteEncoding.h"

16 int main(int argc, char *argv[]) {

17 if (argc<4||argc > 5) // Test for correct # of args

82 Chapter 5: Sending and Receiving Data

18 DieWithUserMessage("Parameter(s)", "<Server> <Port/Service> <Candidate> [I]");

20 char *server = argv[1]; // First arg: server address/name

21 char *service = argv[2]; // Second arg: string to echo

22 // Third arg: server port/service

23 int candi = atoi(argv[3]);

24 if (candi<0||candi > MAX_CANDIDATE)

25 DieWithUserMessage("Candidate # not valid", argv[3]);

27 bool inq = argc>4&&strcmp(argv[4], "I") == 0;

29 // Create a connected TCP socket

30 int sock = SetupTCPClientSocket(server, service);

31 if (sock < 0)

32 DieWithUserMessage("SetupTCPClientSocket() failed", "unable to connect");

34 FILE *str = fdopen(sock,"r+"); // Wrap for stream I/O

35 if (str == NULL)

36 DieWithSystemMessage("fdopen() failed");

38 // Set up info for a request

39 VoteInfo vi;

40 memset(&vi, 0, sizeof(vi));

42 vi.isInquiry = inq;

43 vi.candidate = candi;

45 // Encode for transmission

46 uint8_t outbuf[MAX_WIRE_SIZE];

47 size_t reqSize = Encode(&vi, outbuf, MAX_WIRE_SIZE);

49 // Print info

50 printf("Sending %d-byte %s for candidate %d...\n", reqSize,

51 (inq ? "inquiry" : "vote"), candi);

53 // Frame and send

54 if (PutMsg(outbuf, reqSize, str) < 0)

55 DieWithSystemMessage("PutMsg() failed");

57 // Receive and print response

58 uint8_t inbuf[MAX_WIRE_SIZE];

59 size_t respSize = GetNextMsg(str, inbuf, MAX_WIRE_SIZE); // Get the message

60 if (Decode(inbuf, respSize, &vi)) { // Parse it

61 printf("Received:\n");

62 if (vi.isResponse)

5.2 Constructing, Framing, and Parsing Messages 83

63 printf(" Response to ");

64 if (vi.isInquiry)

65 printf("inquiry ");

66 else

67 printf("vote ");

68 printf("for candidate %d\n", vi.candidate);

69 if (vi.isResponse)

70 printf(" count = %llu\n", vi.count);

71 }

73 // Close up

74 fclose(str);

76 exit(0);

77 }

VoteClientTCP.c

1. Access to library functions and constants: lines 1–14

2. Argument processing: lines 17–27

3. Get connected socket: lines 30–32

4. Wrap the socket in a stream: lines 34–36

We open a stream using fdopen() (mode “r+” opens for both reading and writing).

5. Prepare and send request message: lines 38–55

Prepare a VoteInfo structure with the candidate ID: lines 39–43

Encode into wire format: line 47

Print encoded message before framing: lines 50–51

Add framing and send over the output stream: lines 54–55

6. Receive, parse, and process response: lines 58–71

Call GetNextMsg():line 59

The method handles all the messy details of receiving enough data through the socket

to make up the next message; like recv(), it may block indeﬁnitely.

Pass the result to be parsed: line 60

Process the response if it is correctly formed: lines 61–70

Invalid responses are ignored.

7. Close the stream: line 74

Turning now to the server, it needs a way to keep track of the vote counts of all the

candidates. Because there are at most 1001 candidates, an array of 64-bit integers will serve

nicely. The server prepares its socket and waits for incoming connections like the other servers

we have seen. When a connection arrives, our program receives and processes messages over

84 Chapter 5: Sending and Receiving Data

that connection until the client closes it. Note that, because of the very basic interface to the

framing/parsing code, our client and server are quite simple-minded when it comes to handling

errors in received messages; the server simply ignores any message that is malformed and

immediately closes the connection.

VoteServerTCP.c

1#include <stdio.h>

2#include <string.h>

3#include <stdbool.h>

4#include <stdint.h>

5#include <unistd.h>

6#include <errno.h>

7#include <sys/socket.h>

8#include <arpa/inet.h>

9#include "Practical.h"

10 #include "VoteProtocol.h"

11 #include "VoteEncoding.h"

12 #include "Framer.h"

14 static uint64_t counts[MAX_CANDIDATE + 1];

16 int main(int argc, char *argv[]) {

17 if (argc != 2) // Test for correct number of arguments

18 DieWithUserMessage("Parameter(s)", "<Server Port/Service>");

20 int servSock = SetupTCPServerSocket(argv[1]);

21 // servSock is now ready to use to accept connections

23 for (;;) { // Run forever

25 // Wait for a client to connect

26 int clntSock = AcceptTCPConnection(servSock);

28 // Create an input stream from the socket

29 FILE *channel = fdopen(clntSock, "r+");

30 if (channel == NULL)

31 DieWithSystemMessage("fdopen() failed");

33 // Receive messages until connection closes

34 int mSize;

35 uint8_t inBuf[MAX_WIRE_SIZE];

36 VoteInfo v;

37 while ((mSize = GetNextMsg(channel, inBuf, MAX_WIRE_SIZE)) > 0) {

38 memset(&v, 0, sizeof(v)); // Clear vote information

5.2 Constructing, Framing, and Parsing Messages 85

39 printf("Received message (%d bytes)\n", mSize);

40 if (Decode(inBuf, mSize, &v)) { // Parse to get VoteInfo

41 if (!v.isResponse) { // Ignore non-requests

42 v.isResponse = true;

43 if (v.candidate >= 0 && v.candidate <= MAX_CANDIDATE) {

44 if (!v.isInquiry)

45 counts[v.candidate] += 1;

46 v.count = counts[v.candidate];

47 } // Ignore invalid candidates

48 }

49 uint8_t outBuf[MAX_WIRE_SIZE];

50 mSize = Encode(&v, outBuf, MAX_WIRE_SIZE);

51 if (PutMsg(outBuf, mSize, channel) < 0) {

52 fputs("Error framing/outputting message\n", stderr);

53 break;

54 } else {

55 printf("Processed %s for candidate %d; current count is %llu.\n",

56 (v.isInquiry ? "inquiry" : "vote"), v.candidate, v.count);

57 }

58 fflush(channel);

59 } else {

60 fputs("Parse error, closing connection.\n", stderr);

61 break;

62 }

63 }

64 puts("Client finished");

65 fclose(channel);

66 } // Each client

67 // NOT REACHED

68 }

VoteServerTCP.c

1. Access to library functions and constants: lines 1–12

2. Local declarations: line 14

Array to store vote counts.

3. Declarations and argument processing: lines 16–18

4. Set up listening socket: line 20

5. Repeatedly accept and handle clients: lines 23–66

Wait for connection: line 26

AcceptTCPConnection() prints client info.

Wrap the socket in a stream: lines 29–31

Receive and process messages until connection closes: lines 34–63

86 Chapter 5: Sending and Receiving Data

Note that the server uses the same code as the client to parse, frame, and encode

messages.

6. Close client connection: line 65

5.2.1 Framing

Application protocols typically deal with discrete messages, which are viewed as collections of

ﬁelds. Framing refers to the general problem of enabling the receiver to locate the boundaries

of a message (or part of one). Whether information is encoded as text, as multibyte binary

numbers, or as some combination of the two, the application protocol must specify how the

receiver of a message can determine when it has received all of the message.

Of course, if a complete message is sent as the payload of a UDP datagram, the problem

is trivial: every send/receive operation on a datagram socket involves a single message, so the

receiver knows exactly where that message ends. For messages sent over TCP sockets, however,

the situation can be more complicated, because TCP has no notion of message boundaries. If

the ﬁelds in a message all have ﬁxed sizes and the message is made up of a ﬁxed number of

ﬁelds, then the size of the message is known in advance and the receiver can simply read the

expected number of bytes into a buﬀer. (This technique was used in TCPEchoClient.c, where

we knew the number of bytes to expect from the server.) However, when the message can vary

in length—for example, if it contains some variable-length arbitrary text strings—we do not

know beforehand how many bytes to read.

If a receiver tries to receive more bytes from a socket than were in the message, one of

two things can happen. If no other message is in the channel, the receiver will block and will

be prevented from processing the message; if the sender is also blocked waiting for a reply,

the result will be deadlock: each side of the connection waiting for the other to send more

information. On the other hand, if another message is already in the channel, the receiver may

read some or all of it as part of the ﬁrst message, leading to other kinds of errors. Therefore

framing is an important consideration when using TCP sockets.

Note that some of the same considerations apply to ﬁnding the boundaries of the individ-

ual ﬁelds of the message: the receiver needs to know where one ends and another begins. Thus,

pretty much everything we say here about framing messages also applies to ﬁelds. However,

as was pointed out above, the cleanest code results if we deal with the problem of locating the

end of the message separately from that of parsing it into ﬁelds.

Two general techniques enable a receiver to unambiguously ﬁnd the end of the message:

Delimiter-based: The end of the message is indicated by a unique marker, a particular,

agreed-upon byte (or sequence of bytes) that the sender transmits immediately following

the data.

Explicit length: The variable-length ﬁeld or message is preceded by a length ﬁeld that tells

how many bytes it contains. The length ﬁeld is generally of a ﬁxed size; this limits the

maximum size message that can be framed.

5.2 Constructing, Framing, and Parsing Messages 87

A special case of the delimiter-based method can be used for the last message sent on a TCP

connection: the sender simply closes the sending side of the connection after sending the

message. After the receiver reads the last byte of the message, it receives an end-of-stream

indication (i.e., recv() returns 0 or fread() returns EOF) and thus can tell that it has reached

the end of the message.

The delimiter-based approach is often used with messages encoded as text: A particular

character or sequence of characters is deﬁned to mark the end of the message. The receiver

simply scans the input (as characters) looking for the delimiter sequence; it returns the charac-

ter string preceding the delimiter. The drawback is that the message itself must not contain the

delimiter; otherwise the receiver will ﬁnd the end of the message prematurely. With a delimiter-

based framing method, someone has to be responsible for ensuring that this precondition is

satisﬁed. Fortunately, so-called stuﬃng techniques allow delimiters that occur naturally in the

message to be modiﬁed so the receiver will not recognize them as such. The sending side

performs a transformation on delimiters that occur in the text; the receiver, as it scans for

the delimiter, also recognizes the transformed delimiters and restores them so that the out-

put message matches the original. The downside of such techniques is that both sender and

receiver have to scan every byte of the message.

The length-based approach is simpler but requires a known upper bound on the size of

the message. The sender ﬁrst determines the length of the message, encodes it as an integer,

and preﬁxes the result to the message. The upper bound on the message length determines

the number of bytes required to encode the length: 1 byte if messages always contain fewer

than 256 bytes, 2 bytes if they are always shorter than 65536 bytes, and so on.

The module DelimFramer.c implements delimiter-based framing using the “newline” char-

acter ("\n", byte value 10) as the delimiter. Our PutMsg() method does not do stuﬃng, but simply

fails (catastrophically) if the byte sequence to be framed already contains the delimiter. The

GetNextMsg() method scans the stream, copying each byte into the buﬀer until it reads the

delimiter or runs out of space. It returns the number of bytes placed in the buﬀer. If the mes-

sage is truncated, that is, the method returns a full buﬀer without encountering a delimiter,

the returned count is negative. If some bytes of a message are accumulated and the stream

ends without ﬁnding a delimiter, it is considered an error and a negative value is returned

(i.e., this protocol does not accept end-of-stream as a delimiter). Thus, an empty but correctly

framed message (length zero is returned) can be distinguished from the stream ending.

DelimFramer.c

1#include <stdio.h>

2#include <stdlib.h>

3#include <stdint.h>

4#include "Practical.h"

6static const char DELIMITER = '\n';

8/* Read up to bufSize bytes or until delimiter, copying into the given

9* buffer as we go.

88 Chapter 5: Sending and Receiving Data

10 * Encountering EOF after some data but before delimiter results in failure.

11 * (That is: EOF is not a valid delimiter.)

12 * Returns the number of bytes placed in buf (delimiter NOT transferred).

13 * If buffer fills without encountering delimiter, negative count is returned.

14 * If stream ends before first byte, -1 is returned.

15 * Precondition: buf has room for at least bufSize bytes.

16 */

17 int GetNextMsg(FILE *in, uint8_t *buf, size_t bufSize) {

18 int count = 0;

19 int nextChar;

20 while (count < bufSize) {

21 nextChar = getc(in);

22 if (nextChar == EOF) {

23 if (count > 0)

24 DieWithUserMessage("GetNextMsg()", "Stream ended prematurely");

25 else

26 return -1;

27 }

28 if (nextChar == DELIMITER)

29 break;

30 buf[count++] = nextChar;

31 }

32 if (nextChar != DELIMITER) { // Out of space: count==bufSize

33 return -count;

34 } else { // Found delimiter

35 return count;

36 }

37 }

39 /* Write the given message to the output stream, followed by

40 * the delimiter. Return number of bytes written, or -1 on failure.

41 */

42 int PutMsg(uint8_t buf[], size_t msgSize, FILE *out) {

43 // Check for delimiter in message

44 int i;

45 for (i = 0; i < msgSize; i++)

46 if (buf[i] == DELIMITER)

47 return -1;

48 if (fwrite(buf, 1, msgSize, out) != msgSize)

49 return -1;

50 fputc(DELIMITER, out);

51 fflush(out);

52 return msgSize;

53 }

DelimFramer.c

5.2 Constructing, Framing, and Parsing Messages 89

1. Declare constant delimiter value: line 6

2. Input method: GetNextMsg():lines 17–37

Initialize byte count: line 18

Iterate until buﬀer is full or EOF: lines 20–37

We get the next byte from the input stream, compare it to EOF, then the delimiter.

On EOF we abort if an incomplete message is in the buﬀer, else return −1. On delim-

iter we break out of the loop. If the buﬀer becomes full (count==bufSize), we return

the negation of the number of bytes read as an indication that the channel is not

empty.

Return the count of bytes transferred to buﬀer: line 35

3. Output method: PutMsg():lines 42–53

Scan the input message looking for the delimiter: lines 43–47

If we ﬁnd it, (we rather unhelpfully) kill the program.

Write message to output stream: line 48

Write delimiter byte to output stream: line 50

Flush the output stream: line 47

This ensures that the message is sent over the underlying socket.

Although our implementation makes it fairly easy to change the single character used as a

delimiter, some protocols make use of multiple-character delimiters. The HTTP protocol, which

is used in the World Wide Web, uses text-encoded messages delimited by the four-character

sequence \r\n\r\n. Extending the delimiter-based framing module to support multicharacter

delimiters and to handle stuﬃng is left as an exercise.

The module LengthFramer.c implements the framing interface using length-based fram-

ing. It works for messages up to 65535 (216 −1) bytes in length. The PutMsg() method

determines the length of the given message and writes it to the output stream as a 2-byte, big-

endian integer, followed by the complete message. On the receiving side, the fread() method

is used to read the length as an integer; after converting it to host byte order, that many bytes

are read from the channel. Note that, with this framing method, the sender does not have to

inspect the content of the message being framed; it needs only to check that the message does

not exceed the length limit.

LengthFramer.c

1#include <stdio.h>

2#include <stdlib.h>

3#include <stdint.h>

4#include <netinet/in.h>

5#include "Practical.h"

7/* Read 2-byte length and place in big-endian order.

90 Chapter 5: Sending and Receiving Data

8* Then read the indicated number of bytes.

9* If the input buffer is too small for the data, truncate to fit and

10 * return the negation of the *indicated* length. Thus a negative return

11 * other than -1 indicates that the message was truncated.

12 * (Ambiguity is possible only if the caller passes an empty buffer.)

13 * Input stream is always left empty.

14 */

15 int GetNextMsg(FILE *in, uint8_t *buf, size_t bufSize) {

16 uint16_t mSize = 0;

17 uint16_t extra = 0;

19 if (fread(&mSize, sizeof(uint16_t), 1, in) != 1)

20 return -1;

21 mSize = ntohs(mSize);

22 if (mSize > bufSize) {

23 extra = mSize - bufSize;

24 mSize = bufSize; // Truncate

25 }

26 if (fread(buf, sizeof(uint8_t), mSize, in) != mSize) {

27 fprintf(stderr, "Framing error: expected %d, read less\n", mSize);

28 return -1;

29 }

30 if (extra > 0) { // Message was truncated

31 uint8_t waste[BUFSIZE];

32 fread(waste, sizeof(uint8_t), extra, in); // Try to flush the channel

33 return -(mSize + extra); // Negation of indicated size

34 } else

35 return mSize;

36 }

38 /* Write the given message to the output stream, followed by

39 * the delimiter. Precondition: buf[] is at least msgSize.

40 * Returns -1 on any error.

41 */

42 int PutMsg(uint8_t buf[], size_t msgSize, FILE *out) {

43 if (msgSize > UINT16_MAX)

44 return -1;

45 uint16_t payloadSize = htons(msgSize);

46 if ((fwrite(&payloadSize, sizeof(uint16_t), 1, out) != 1) || (fwrite(buf,

47 sizeof(uint8_t), msgSize, out) != msgSize))

48 return -1;

49 fflush(out);

50 return msgSize;

51 }

LengthFramer.c

5.2 Constructing, Framing, and Parsing Messages 91

1. Input method: GetNextMsg():lines 15–36

Read the preﬁx length: lines 19–20

The fread() method reads 2 bytes into the unsigned 16-bit integer mSize.

Convert to host byte order: line 21

Truncate message if necessary: lines 22–25

If the indicated size is bigger than the buﬀer provided by the caller, we truncate the

message so it will ﬁt, and remember so we can indicate via the return value that we

did so.

Read the message: line 26

Flush channel: lines 30–34

If we are returning early because the buﬀer is full, we remove the extra bytes from the

channel and return the negation of the header size.

Return the size: line 35

2. Output method: PutMsg():lines 42–51

Verify input length: lines 43–44

Because we use a 2-byte unsigned length ﬁeld, the length cannot exceed 65535 (the

value of uint16_max).

Convert length to network byte order: line 45

Output length and message: lines 46–48

Flush to ensure the message is sent: line 49

5.2.2 Text-Based Message Encoding

Now we turn to the representation of voting messages as text. Because we only have to rep-

resent numbers and a couple of indicators, we can use the basic C charset US-ASCII. The

message consists of text ﬁelds separated by one or more occurrences of the ASCII space char-

acter (decimal value 32). The message begins with a so-called magic string—a sequence of

ASCII characters that allows a recipient to quickly recognize the message as a Voting proto-

col message, as opposed to random garbage that happened to arrive over the network. The

Vote/Inquiry boolean is encoded with the character “v” for a vote or “i” for an inquiry. The

message’s status as a response is indicated by the presence of the character “R.” Then comes

the candidate ID, followed by the vote count, both encoded as strings of decimal digits. The

module VoteEncodingText.c implements this text-based encoding.

VoteEncodingText.c

1/* Routines for Text encoding of vote messages.

2* Wire Format:

3* "Voting <v|i> [R] <candidate ID> <count>"

4*/

92 Chapter 5: Sending and Receiving Data

5#include <string.h>

6#include <stdint.h>

7#include <stdbool.h>

8#include <stdlib.h>

9#include <stdio.h>

10 #include <string.h>

11 #include "Practical.h"

12 #include "VoteProtocol.h"

14 static const char *MAGIC = "Voting";

15 static const char *VOTESTR = "v";

16 static const char *INQSTR = "i";

17 static const char *RESPONSESTR = "R";

18 static const char *DELIMSTR="";

19 enum {

20 BASE = 10

21 };

23 /* Encode voting message info as a text string.

24 * WARNING: Message will be silently truncated if buffer is too small!

25 * Invariants (e.g. 0 <= candidate <= 1000) not checked.

26 */

27 size_t Encode(const VoteInfo *v, uint8_t *outBuf, const size_t bufSize) {

28 uint8_t *bufPtr = outBuf;

29 long size = (size_t) bufSize;

30 int rv = snprintf((char *) bufPtr, size, "%s %c %s %d", MAGIC,

31 (v->isInquiry ? 'i' : 'v'), (v->isResponse ? "R" : ""), v->candidate);

32 bufPtr += rv;

33 size -= rv;

34 if (v->isResponse) {

35 rv = snprintf((char *) bufPtr, size, " %llu", v->count);

36 bufPtr += rv;

37 }

38 return (size_t) (bufPtr - outBuf);

39 }

41 /* Extract message information from given buffer.

42 * Note: modifies input buffer.

43 */

44 bool Decode(uint8_t *inBuf, const size_t mSize, VoteInfo *v) {

46 char *token;

47 token = strtok((char *) inBuf, DELIMSTR);

48 // Check for magic

49 if (token == NULL || strcmp(token, MAGIC) != 0)

5.2 Constructing, Framing, and Parsing Messages 93

50 return false;

52 // Get vote/inquiry indicator

53 token = strtok(NULL, DELIMSTR);

54 if (token == NULL)

55 return false;

57 if (strcmp(token, VOTESTR) == 0)

58 v->isInquiry = false;

59 else if (strcmp(token, INQSTR) == 0)

60 v->isInquiry = true;

61 else

62 return false;

64 // Next token is either Response flag or candidate ID

65 token = strtok(NULL, DELIMSTR);

66 if (token == NULL)

67 return false; // Message too short

69 if (strcmp(token, RESPONSESTR) == 0) { // Response flag present

70 v->isResponse = true;

71 token = strtok(NULL, DELIMSTR); // Get candidate ID

72 if (token == NULL)

73 return false;

74 } else { // No response flag; token is candidate ID;

75 v->isResponse = false;

76 }

77 // Get candidate #

78 v->candidate = atoi(token);

79 if (v->isResponse) { // Response message should contain a count value

80 token = strtok(NULL, DELIMSTR);

81 if (token == NULL)

82 return false;

83 v->count = strtoll(token, NULL, BASE);

84 } else {

85 v->count = 0L;

86 }

87 return true;

88 }

VoteEncodingText.c

The Encode() method uses snprintf() to construct a string containing all the ﬁelds of the

message, separated by white space. It fails only if the caller provides an insuﬃcient amount

of space to hold the string.

94 Chapter 5: Sending and Receiving Data

The Decode() method uses the strtok() method to break the received message into tokens

(ﬁelds). The strtok() library function takes a pointer to a character array and a string containing

characters to be interpreted as delimiters. The ﬁrst time it is called, it returns the largest initial

substring consisting entirely of characters not in the delimiter string; the trailing delimiter of

that string is replaced by a null byte. On subsequent calls with a NULL ﬁrst argument, tokens

are taken left to right from the original string until there are no more tokens, at which point

NULL is returned.

Decode() ﬁrst looks for the “Magic” string; if it is not the ﬁrst thing in the message,

it simply fails and returns false.Note well: This illustrates a very important point about

implementing protocols: never assume anything about any input from the network. Your

program must always be prepared for any possible inputs, and handle them gracefully. In this

case, the Decode() method simply ignores the rest of the message and returns false if some

expected part is not present or improperly formatted.4Otherwise, Decode() gets the ﬁelds token

by token, using the library functions atoi() and strtoll() to convert tokens into integers.

5.2.3 Binary Message Encoding

Next we present a diﬀerent way to encode the Voting protocol message. In contrast with the

text-based format, the binary format uses ﬁxed-size messages. Each message begins with a

one-byte ﬁeld that contains the “magic” value 010101 in its high-order 6 bits. As with the text

format, this little bit of redundancy provides the receiver with a small degree of assurance that

it is receiving a proper voting message. The two low-order bits of the ﬁrst byte encode the two

booleans; note the use of the bitwise-or operations shown earlier, in Section 5.1.8, to set the

ﬂags. The second byte of the message always contains zeros (it is eﬀectively padding), and the

third and fourth bytes contain the candidateID. The ﬁnal 8 bytes of a response message (only)

contain the vote count.

VoteEncodingBin.c

1/* Routines for binary encoding of vote messages

2* Wire Format:

3* 111111

4*0123456789012345

5* +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

6* | Magic |Flags| ZERO |

7* +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

8* | Candidate ID |

9* +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

10 *| |

4This also illustrates a key reason why it is best to do framing before parsing: recovery from a parse error

when a message has only been partially received is much more complex because the receiver has to get

“back in sync” with the sender.

5.2 Constructing, Framing, and Parsing Messages 95

11 * | Vote Count (only in response) |

12 *| |

13 *| |

14 * +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

15 *

16 */

18 #include <string.h>

19 #include <stdbool.h>

20 #include <stdlib.h>

21 #include <stdint.h>

22 #include <netinet/in.h>

23 #include "Practical.h"

24 #include "VoteProtocol.h"

26 enum {

27 REQUEST_SIZE = 4,

28 RESPONSE_SIZE = 12,

29 COUNT_SHIFT = 32,

30 INQUIRE_FLAG = 0x0100,

31 RESPONSE_FLAG = 0x0200,

32 MAGIC = 0x5400,

33 MAGIC_MASK = 0xfc00

34 };

36 typedef struct voteMsgBin voteMsgBin;

38 struct voteMsgBin {

39 uint16_t header;

40 uint16_t candidateID;

41 uint32_t countHigh;

42 uint32_t countLow;

43 };

45 size_t Encode(VoteInfo *v, uint8_t *outBuf, size_t bufSize) {

46 if ((v->isResponse && bufSize < sizeof(voteMsgBin)) || bufSize < 2

47 * sizeof(uint16_t))

48 DieWithUserMessage("Output buffer too small", "");

49 voteMsgBin *vm = (voteMsgBin *) outBuf;

50 memset(outBuf, 0, sizeof(voteMsgBin)); // Be sure

51 vm->header = MAGIC;

52 if (v->isInquiry)

53 vm->header |= INQUIRE_FLAG;

54 if (v->isResponse)

55 vm->header |= RESPONSE_FLAG;

96 Chapter 5: Sending and Receiving Data

56 vm->header = htons(vm->header); // Byte order

57 vm->candidateID = htons(v->candidate); // Know it will fit, by invariants

58 if (v->isResponse) {

59 vm->countHigh = htonl(v->count >> COUNT_SHIFT);

60 vm->countLow = htonl((uint32_t) v->count);

61 return RESPONSE_SIZE;

62 } else {

63 return REQUEST_SIZE;

64 }

65 }

67 /* Extract message info from given buffer.

68 * Leave input unchanged.

69 */

70 bool Decode(uint8_t *inBuf, size_t mSize, VoteInfo *v) {

72 voteMsgBin *vm = (voteMsgBin *) inBuf;

74 // Attend to byte order; leave input unchanged

75 uint16_t header = ntohs(vm->header);

76 if ((mSize < REQUEST_SIZE) || ((header & MAGIC_MASK) != MAGIC))

77 return false;

78 /* message is big enough and includes correct magic number */

79 v->isResponse = ((header & RESPONSE_FLAG) != 0);

80 v->isInquiry = ((header & INQUIRE_FLAG) != 0);

81 v->candidate = ntohs(vm->candidateID);

82 if (v->isResponse && mSize >= RESPONSE_SIZE) {

83 v->count = ((uint64_t) ntohl(vm->countHigh) << COUNT_SHIFT)

84 | (uint64_t) ntohl(vm->countLow);

85 }

86 return true;

87 }

VoteEncodingBin.c

The Decode() method’s job is especially simple in this version—it simply copies the values

from the message into the VoteInfo structure, converting byte order along the way.

5.2.4 Putting It All Together

To get a working vote server, we simply compile together VoteServerTCP.c, one of the two fram-

ing modules, one of the two encoding modules, and the auxiliary modules DieWithMessage.c,

TCPClientUtility.c,TCPServerUtility.c, and AddressUtility.c. For example:

%gcc -std=gnu99-o vs VoteServerTCP.c DelimFramer.c VoteEncodingBin.c \

DieWithMessage.c TCPServerUtility.c AddressUtility.c

Exercises 97

%gcc -std=gnu99-o vc VoteClientTCP.c DelimFramer.c VoteEncodingBin.c \

DieWithMessage.c TCPClientUtility.c

All four possible combinations of framing method and encoding will work—provided the

client and server use the same combination!

5.3 Wrapping Up

We have seen how primitive types can be represented as sequences of bytes for transmission

“on the wire.” We have also considered some of the subtleties of encoding text strings, as well as

some basic methods of framing and parsing messages. We saw examples of both text-oriented

and binary-encoded protocols.

It is probably worth reiterating something we said in the Preface: this chapter will by

no means make you an expert! That takes a great deal of experience. But the code from this

chapter can be used as a starting point for further explorations.

Exercises

1. If the underlying hardware platform is little-endian, and the “network” byte order is big-

endian, is there any reason for the implementations of htonl() and ntohl() to be diﬀerent?

2. Write the function uint64_t htonll(uint64_t val), which converts a 64-bit integer from

little-endian to big-endian byte order.

3. Write the little-endian analogs of the method given in BruteForceEncoding.c, that is,

EncodeLittleEndian() and DecodeLittleEndian().

4. Write methods EncodeBigEndianSigned() and DecodeBigEndianSigned(), which return signed

values. (The input buﬀers are still unsigned types. Hint: use explicit type-casting.)

5. The EncodeIntBigEndian() method in BruteForceEncoding.c only works if several precondi-

tions such as 0 ≤size ≤8 are satisﬁed. Modify the method to test for these preconditions

and return an error indication of some sort if any are violated. What are the advantages

and disadvantages of having the program check the preconditions, versus relying on the

caller to establish them?

6. Assuming all byte values are equally likely, what is the probability that a message consist-

ing of random bits will pass the “magic test” in the binary encoding of the Voting protocol?

Suppose an ASCII-encoded text message is sent to a program expecting a binary-encoded

voteMsg. Which characters would enable the message to pass the “magic test” if they are

the ﬁrst in the message?

7. Suppose we use DelimFraming.c and VoteEncodingBin.c in building the Voting Client.

Describe circumstances in which the client fails to send a message.

98 Chapter 5: Sending and Receiving Data

8. Extend the delimiter-based framing implementation to perform “byte stuﬃng,” so that

messages that contain the delimiter can be transmitted without the caller of PutMsg()

having to worry about it. That is, the framing module transparently handles messages

that contain the delimiter. (See any decent networking text for the algorithm.)

9. Extend the delimiter-based framing implementation to handle arbitrary multiple-byte

delimiters. Be sure your implementation is eﬃcient. (Note: this problem is not trivial!

A naive approach will run very ineﬃciently in the worst case.)

10. Both GetNextMsg() implementations truncate the received message if the caller fails to

provide a big enough buﬀer. Consider the behavior on the next call to GetNextMsg() after

this happens, for both implementations. Is the behavior the same in both cases? If not,

suggest modiﬁcations so that both implementations behave the same way in all cases.

chapter 6

Beyond Basic Socket Programming

Our client and server examples demonstrate the basic model for socket programming.

The next step is to integrate these ideas into various programming models such as multi-

tasking, signalling, and broadcasting. We demonstrate these principles in the context of stan-

dard UNIX programming; however, most modern operating systems support similar features

(e.g., processes and threads).

6.1 Socket Options

The TCP/IP protocol developers spent a good deal of time thinking about the default behaviors

that would satisfy most applications. (If you doubt this, read RFCs 1122 and 1123, which

describe in excruciating detail the recommended behaviors—based on years of experience—

for implementations of the TCP/IP protocols.) For most applications, the designers did a good

job; however, it is seldom the case that “one-size-ﬁts-all” really ﬁts all. For example, each socket

has an associated receive buﬀer. How big should it be? Each implementation has a default size;

however, this value may not always be appropriate for your application (see also Section 7.1).

This particular aspect of a socket’s behavior, along with many others, is associated with a

socket option: You can change the receive buﬀer size of a socket by modifying the value of

the associated socket option. The functions getsockopt() and setsockopt() allow socket option

values to be queried and set, respectively.

100 Chapter 6: Beyond Basic Socket Programming

int getsockopt(int socket,int level,int optName,void *optVal,socklen_t *optLen)

int setsockopt(int socket,int level,int optName,const void *optVal,socklen_t optLen)

For both functions, socket must be a socket descriptor allocated by socket(). The available

socket options are divided into levels that correspond to the layers of the protocol stack; the

second parameter indicates the level of the option in question. Some options are protocol

independent and are thus handled by the socket layer itself (sol_socket), some are speciﬁc

to the transport protocol (ipproto_tcp), and some are handled by the internetwork protocol

(ipproto_ip). The option itself is speciﬁed by the integer optName, which is always speciﬁed

using a system-deﬁned constant. The parameter optVal is a pointer to a buﬀer. For getsockopt(),

the option’s current value will be placed in that buﬀer by the implementation, whereas for

setsockopt(), the socket option in the implementation will be set to the value in the buﬀer. In

both calls, optLen speciﬁes the length of the buﬀer, which must be correct for the particular

option in question. Note that in getsockopt(),optLen is an in-out parameter, initially pointing to

an integer containing the size of the buﬀer; on return the pointed-to integer contains the size

of the option value. The following code segment demonstrates how to fetch and then double

the conﬁgured size (in bytes) of the socket’s receive buﬀer:

int rcvBufferSize;

// Retrieve and print the default buffer size

int sockOptSize = sizeof(rcvBufferSize);

if (getsockopt(sock, SOL_SOCKET, SO_RCVBUF, &rcvBufferSize, &sockOptSize) < 0)

DieWithSystemMessage("getsockopt() failed");

printf("Initial Receive Buffer Size: %d\n", rcvBufferSize);

// Double the buffer size

rcvBufferSize *= 2;

if (setsockopt(sock, SOL_SOCKET, SO_RCVBUF, &rcvBufferSize, sizeof(rcvBufferSize)) < 0)

DieWithSystemMessage("setsockopt() failed");

Note that value passed to setsockopt() is not guaranteed to be the new size of the socket

buﬀer, even if the call apparently succeeds. Rather, it is best thought of as a “hint” to the

system about the value desired by the user; the system, after all, has to manage resources for

all users and may consider other factors in adjusting buﬀer size.

Table 6.1 shows some commonly used options at each level, including a description and

the data type of the buﬀer pointed to by optVal.

6.2 Signals

Signals provide a mechanism for notifying programs that certain events have occurred—for

example, the user typed the “interrupt” character, or a timer expired. Some of the events

(and therefore the notiﬁcation) may occur asynchronously, which means that the notiﬁcation

6.2 Signals 101

SOL_SOCKET

optname Type Values Description

SO_BROADCAST int 0,1 Broadcast allowed

SO_KEEPALIVE int 0,1 Keepalive messages enabled (if

implemented by protocol)

SO_LINGER linger{} time Time to delay close return waiting

for conﬁrmation (see Section 7.4)

SO_RCVBUF int bytes Bytes in the socket receive buﬀer

(see code on page 100 and

Section 7.1)

SO_RCVLOWAT int bytes Minimum number of available bytes

that will cause recv to return

SO_REUSEADDR int 0,1 Binding allowed (under certain

conditions) to an address or port

already in use (see Section 7.4)

SO_SNDLOWAT int bytes Minimum bytes to send

SO_SNDBUF int bytes Bytes in the socket send buﬀer (see

Section 7.1)

IPPROTO_TCP

optname type Values Description

TCP_MAX int seconds Seconds between keepalive

messages.

TCP_NODELAY int 0, 1 Disallow delay from Nagle’s

algorithm for data merging

IPPROTO_IP

optname type Values Description

IP_TTL int 0 - 255 Time-to-live for unicast IP packets.

IP_MULTICAST_TTL unsigned char 0 - 255 Time-to-live for multicast IP packets

(see MulticastSender.c on page 138)

IP_MULTICAST_LOOP int 0,1 Enables multicast socket to receive

packets it sent

IP_ADD_MEMBERSHIP ip_mreq{} group address Enables reception of packets

addressed to the speciﬁed multicast

group (see MulticastReceiver.c on

page 141) – Set only

IP_DROP_MEMBERSHIP ip_mreq{} group address Disables reception of packets

addressed to the speciﬁed multicast

group – Set only

Table 6.1: continued

102 Chapter 6: Beyond Basic Socket Programming

IPPROTO_IPV6

optname type Values Description

IPV6_V6ONLY int 0,1 Restrict IPv6 sockets to only IPv6

communication.

IPV6_UNICAST_HOPS int -1 - 255 Time-to-live for unicast IP packets.

IPV6_MULTICAST_HOPS int -1 - 255 Time-to-live for multicast IP packets

(see MulticastSender.c on page 138)

IPV6_MULTICAST_LOOP u_int 0,1 Enables multicast socket to receive

packets it sent

IPV6_JOIN_GROUP ipv6_mreq{} group address Enables reception of packets

addressed to the speciﬁed multicast

group (see MulticastReceiver.c on

page 141) – Set only

IPV6_LEAVE_GROUP ipv6_mreq{} group address Disables reception of packets

addressed to the speciﬁed multicast

group – Set only

Table 6.1: Socket Options

is delivered to the program regardless of which statement it is executing. When a signal is

delivered to a running program, one of four things happens:

1. The signal is ignored. The process is never aware that the signal was delivered.

2. The program is forcibly terminated by the operating system.

3. Program execution is interrupted and a signal-handling routine, speciﬁed by (and part of)

the program, is executed. This execution happens in a diﬀerent thread of control from

the main thread(s) of the program so that the program is not necessarily immediately

aware of it.

4. The signal is blocked, that is, prevented from having any eﬀect until the program takes

action to allow its delivery. Each process has a mask, indicating which signals are cur-

rently blocked in that process. (Actually, each thread in a program can have its own signal

mask.)

UNIX has dozens of diﬀerent signals, each indicating the occurrence of a diﬀerent type of

event. Each signal has a system-deﬁned default behavior, which is one of the ﬁrst two possibili-

ties listed above. For example, termination is the default behavior for sigint, which is delivered

when the interrupt character (usually Control-C) is received via the controlling terminal for that

process.

Signals are a complicated animal, and a full treatment is beyond the scope of this book.

However, some signals are frequently encountered in the context of socket programming.

6.2 Signals 103

Moreover, any program that sends on a TCP socket must explicitly deal with SIGPIPE in

order to be robust. Therefore, we present the basics of dealing with signals, focusing on these

ﬁve:

Type Triggering Event Default

sigalrm Expiration of an alarm timer termination

sigchld Child process exit ignore

sigint Interrupt char (Control-C) input termination

sigio Socket ready for I/O ignore

sigpipe Attempt to write to a closed socket termination

An application program can change the default behavior1for a particular signal using

sigaction():

int sigaction (int whichSignal,const struct sigaction *newAction,struct sigaction *oldAction)

sigaction() returns 0 on success and −1 on failure; details of its semantics, however, are a bit

more involved.

Each signal is identiﬁed by an integer constant; whichSignal speciﬁes the signal for which

the behavior is being changed. The newAction parameter points to a sigaction structure that

deﬁnes the new behavior for the given signal type; if the pointer oldAction is non-null, a

sigaction structure describing the previous behavior for the given signal is copied into it,

as shown here:

struct sigaction {

void (*sa_handler)(int); // Signal handler

sigset_t sa_mask; // Signals to be blocked during handler execution

int sa_flags; // Flags to modify default behavior

};

The ﬁeld sa_handler (of type “pointer to function of one integer parameter that returns

void”) controls which of the ﬁrst three possibilities occurs when a signal is delivered (i.e., when

it is not masked). If its value is the special constant sig_ign, the signal will be ignored. If its

value is sig_dﬂ, the default behavior for that signal will be used. If its value is the address of

a function (which is guaranteed to be diﬀerent from the two constants), that function will be

invoked with a parameter indicating the signal that was delivered. (If the same handler function

1For some signals, the default behavior cannot be changed, nor can the signal be blocked; however, this

is not true for any of the ﬁve we consider.

104 Chapter 6: Beyond Basic Socket Programming

is used for multiple signals, the parameter can be used to determine which one caused the

invocation.)

Signals can be “nested” in the following sense: While one signal is being handled, another

is delivered. As you can imagine, this can get rather complicated. Fortunately, the sigaction()

mechanism allows some signals to be temporarily blocked (in addition to those that are already

blocked by the process’s signal mask) while the speciﬁed signal is handled. The ﬁeld sa_mask

speciﬁes the signals to be blocked while handling whichSignal; it is only meaningful when

sa_handler is not sig_ign or sig_dﬂ. By default whichSignal is always blocked regardless of

whether it is reﬂected in sa_mask. (On some systems, setting the ﬂag sa_nodefer in sa_flags

allows the speciﬁed signal to be delivered while it is being handled.) The sa_flags ﬁeld controls

some further details of the way whichSignal is handled; these details are beyond the scope of

this discussion.

sa_mask is implemented as a set of boolean ﬂags, one for each type of signal. This set of

ﬂags can be manipulated with the following four functions:

int sigemptyset(sigset_t *set)

int sigfillset(sigset_t *set)

int sigaddset(sigset_t *set,int whichSignal)

int sigdelset(sigset_t *set,int whichSignal)

sigfillset() and sigemptyset() set and unset all of the ﬂags in the given set. sigaddset() and

sigdelset() set and unset individual ﬂags, speciﬁed by the signal number, in the given set. All

four functions return 0 for success and −1 for failure.

SigAction.c shows a simple sigaction() example to provide a handler for SIGINT by set-

ting up a signal handler and then entering an inﬁnite loop. When the program receives an

interrupt signal, the handler function, a pointer to which is supplied to sigaction(), executes

and exits the program.

SigAction.c

1#include <stdio.h>

2#include <signal.h>

3#include <unistd.h>

4#include <stdlib.h>

5#include "Practical.h"

7void InterruptSignalHandler(int signalType); // Interrupt signal handling function

9int main(int argc, char *argv[]) {

10 struct sigaction handler; // Signal handler specification structure

6.2 Signals 105

12 // Set InterruptSignalHandler() as handler function

13 handler.sa_handler = InterruptSignalHandler;

14 // Create mask that blocks all signals

15 if (sigfillset(&handler.sa_mask) < 0)

16 DieWithSystemMessage("sigfillset() failed");

17 handler.sa_flags = 0; // No flags

19 // Set signal handling for interrupt signal

20 if (sigaction(SIGINT, &handler, 0) < 0)

21 DieWithSystemMessage("sigaction() failed for SIGINT");

23 for (;;)

24 pause(); // Suspend program until signal received

26 exit(0);

27 }

29 void InterruptSignalHandler(int signalType) {

30 puts("Interrupt Received. Exiting program.");

31 exit(1);

32 }

SigAction.c

1. Signal handler function prototype: line 7

2. Set up signal handler: lines 10–21

Assign function to handle signal: line 13

Fill signal mask: lines 15–16

Set signal handler for SIGINT: lines 20–21

3. Loop forever until SIGINT: lines 23–24

pause() suspends the process until a signal is received.

4. Function to handle signal: lines 29–32

InterruptSignalHandler() prints a message and exits the program.

So what happens when a signal that would otherwise be delivered is blocked, say, because

another signal is being handled? Delivery is postponed until the handler completes. Such a

signal is said to be pending.It is important to realize that signals are not queued—a signal is

either pending or it is not. If the same signal is delivered more than once while it is being

handled, the handler is only executed once more after it completes the original execution.

Consider the case where three SIGINT signals arrive while the signal handler for SIGINT is

already executing. The ﬁrst of the three SIGINT signals is blocked; however, the subsequent

two signals are lost. When the SIGINT signal handler function completes, the system executes

106 Chapter 6: Beyond Basic Socket Programming

the handler only once again. We must be prepared to handle this behavior in our applications.

To see this in action, modify InterruptSignalHandler() in SigAction.c as follows:

void InterruptSignalHandler(int ignored) {

printf("Interrupt Received.\n");

sleep(3);

}

The signal handler for SIGINT sleeps for three seconds and returns, instead of exiting. Now

when you execute the program, hit the interrupt key (Control-C) several times in succession.

If you hit the interrupt key more than two times in a row, you still only see two “Interrupt

Received” messages. The ﬁrst interrupt signal invokes InterruptSignalHandler(), which sleeps

for three seconds. The second interrupt is blocked because SIGINT is already being handled.

The third and fourth interrupts are lost. Be warned that you will no longer be able to stop your

program with a keyboard interrupt. You will need to explicitly send another signal (such as

SIGTERM) to the process using the kill command.

One of the most important aspects of signals relates to the sockets interface. If a signal

is delivered while the program is blocked in a socket call (such as a recv() or connect()), and

a handler for that signal has been speciﬁed, as soon as the handler completes, the socket call

will return −1 with errno set to EINTR. Thus, your programs that catch and handle signals

need to be prepared for these erroneous returns from system calls that can block.

Later in this chapter we encounter the ﬁrst four signals mentioned above. Here we brieﬂy

describe the semantics of SIGPIPE. Consider the following scenario: A server (or client) has

a connected TCP socket, and the other end abruptly and unexpectedly closes the connection,

say, because the program crashed. How does the server ﬁnd out that the connection is broken?

The answer is that it doesn’t, until it tries to send or receive on the socket. If it tries to receive

ﬁrst, the call returns 0. If it tries to send ﬁrst, at that point, SIGPIPE is delivered. Thus, SIGPIPE

is delivered synchronously and not asynchronously. (Why not just return −1 from send()? See

exercise 5 at the end of the chapter.)

This fact is especially signiﬁcant for servers because the default behavior for SIGPIPE is

to terminate the program. Thus, servers that don’t change this behavior can be terminated

by misbehaving clients. Servers should always handle SIGPIPE so that they can detect the

client’s disappearance and reclaim any resources that were in use to service it.

6.3 Nonblocking I/O

The default behavior of a socket call is to block until the requested action is completed. For

example, the recv() function in TCPEchoClient.c (page 44) does not return until at least one

message from the echo server is received. Of course, a process with a blocked function is

suspended by the operating system.

A socket call may block for several reasons. Data reception functions (recv() and

recvfrom()) block if data is not available. A send() on a TCP socket may block if there is not

6.3 Nonblocking I/O 107

suﬃcient space to buﬀer the transmitted data (see Section 7.1). Connection-related functions

for TCP sockets block until a connection has been established. For example, accept() in

TCPEchoServer.c (page 48) blocks until a client establishes a connection with connect(). Long

round-trip times, high error rate connections, or a slow (or deceased) server may cause a call

to connect() to block for a signiﬁcant amount of time. In all of these cases, the function returns

only after the request has been satisﬁed.

What about a program that has other tasks to perform while waiting for call completion

(e.g., update busy cursor or respond to user requests)? These programs may have no time to

wait on a blocked system call. What about lost UDP datagrams? In UDPEchoClient.c (page 54),

the client sends a datagram to the server and then waits to receive a response. If either the

datagram sent from the client or the echoed datagram from the server is lost, our echo client

blocks indeﬁnitely. In this case, we need recvfrom() to unblock after some amount of time to

allow the client to handle the datagram loss. Fortunately, several mechanisms are available for

controlling unwanted blocking behaviors. We deal with three such solutions here: nonblocking

sockets, asynchronous I/O, and timeouts.

6.3.1 Nonblocking Sockets

One obvious solution to the problem of undesirable blocking is to change the behavior of the

socket so that all calls are nonblocking. For such a socket, if a requested operation can be

completed immediately, the call’s return value indicates success; otherwise it indicates failure

(usually −1). In either case the call does not block indeﬁnitely. In the case of failure, we need the

ability to distinguish between failure due to blocking and other types of failures. If the failure

occurred because the call would have blocked, the system sets errno to ewouldblock,2except

for connect(), which returns an errno of einprogress.

We can change the default blocking behavior with a call to fcntl() (“ﬁle control”).

int fcntl(int socket,int command,…)

As the name suggests, this call can be used with any kind of ﬁle: socket must be a valid ﬁle

(or socket) descriptor. The operation to be performed is given by command, which is always a

system-deﬁned constant. The behavior we want to modify is controlled by ﬂags (not the same

as socket options) associated with the descriptor, which we can get and set with the f_getﬂ

and f_setﬂ commands. When setting the socket ﬂags, we must specify the new ﬂags in a

variable-length argument list. The ﬂag that controls nonblocking behavior is o_nonblock.

When getting the socket ﬂags, the variable-length argument list is empty. We demonstrate

the use of a nonblocking socket in the next section, where we describe asynchronous I/O in

UDPEchoServer-SIGIO.c (page 109).

2Some sockets implementations return eagain. On many systems, eagain and ewouldblock are the

same error number.

108 Chapter 6: Beyond Basic Socket Programming

There are a few exceptions to this model of nonblocking sockets. For UDP sockets, there

are no send buﬀers, so send() and sendto() never return ewouldblock. For all but the connect()

socket call, the requested operation either completes before returning or none of the operation

is performed. For example, recv() either receives data from the socket or returns an error.

A nonblocking connect() is diﬀerent. For UDP, connect() simply assigns a destination address

for future data transmissions so that it never blocks. For TCP, connect() initiates the TCP

connection setup. If the connection cannot be completed without blocking, connect() returns

an error, setting errno to einprogress, indicating that the socket is still working on making

the TCP connection. Of course, subsequent data sends and receives cannot happen until the

connection is established. Determining when the connection is complete is beyond the scope

of this text,3so we recommend not setting the socket to nonblocking until after the call to

connect().

For eliminating blocking during individual send and receive operations, an alternative is

available on some platforms. The flags parameter of send(),recv(),sendto(), and recvfrom()

allows for modiﬁcation of some aspects of the behavior on a particular call. Some implemen-

tations support the msg_dontwait ﬂag, which causes nonblocking behavior in any call where

it is set in flags.

6.3.2 Asynchronous I/O

The diﬃculty with nonblocking socket calls is that there is no way of knowing when one would

succeed, except by periodically trying it until it does (a process known as “polling”). Why not

have the operating system inform the program when a socket call will be successful? That way

the program can spend its time doing other work until notiﬁed that the socket is ready for

something to happen. This is called asynchronous I/O, and it works by having the sigio signal

delivered to the process when some I/O–related event occurs on the socket.

Arranging for sigio involves three steps. First, we inform the system of the desired dis-

position of the signal using sigaction(). Then we ensure that signals related to the socket will

be delivered to this process (because multiple processes can have access to the same socket,

there might be ambiguity about which should get it) by making it the owner of the socket,

using fcntl(). Finally, we mark the socket as being primed for asynchronous I/O by setting a

ﬂag (fasync), again via fcntl().

In our next example, we adapt UDPEchoServer.c (page 57) to use asynchronous I/O with

nonblocking socket calls. The modiﬁed server is able to perform other tasks when there are no

clients needing an echo. After creating and binding the socket, instead of calling recvfrom() and

blocking until a datagram arrives, the asynchronous echo server establishes a signal handler

for SIGIO and begins doing other work. When a datagram arrives, the SIGIO signal is delivered to

the process, triggering execution of the handler function. The handler function calls recvfrom(),

3Well, mostly. Connection completion can be detected using the select() call, described in Section 6.5.

6.3 Nonblocking I/O 109

echoes back any received datagrams, and then returns, whereupon the main program continues

whatever it was doing. Our description details only the code that diﬀers from the original UDP

echo server.

UDPEchoServer–SIGIO.c

1#include <stdio.h>

2#include <stdlib.h>

3#include <string.h>

4#include <unistd.h>

5#include <fcntl.h>

6#include <sys/file.h>

7#include <signal.h>

8#include <errno.h>

9#include <sys/types.h>

10 #include <sys/socket.h>

11 #include <netdb.h>

12 #include "Practical.h"

14 void UseIdleTime(); // Execution during idle time

15 void SIGIOHandler(int signalType); // Handle SIGIO

17 int servSock; // Socket -- GLOBAL for signal handler

19 int main(int argc, char *argv[]) {

21 if (argc != 2) // Test for correct number of arguments

22 DieWithUserMessage("Parameter(s)", "<Server Port/Service>");

24 char *service = argv[1]; // First arg: local port

26 // Construct the server address structure

27 struct addrinfo addrCriteria; // Criteria for address

28 memset(&addrCriteria, 0, sizeof(addrCriteria)); // Zero out structure

29 addrCriteria.ai_family = AF_UNSPEC; // Any address family

30 addrCriteria.ai_flags = AI_PASSIVE; // Accept on any address/port

31 addrCriteria.ai_socktype = SOCK_DGRAM; // Only datagram sockets

32 addrCriteria.ai_protocol = IPPROTO_UDP; // Only UDP protocol

34 struct addrinfo *servAddr; // List of server addresses

35 int rtnVal = getaddrinfo(NULL, service, &addrCriteria, &servAddr);

36 if (rtnVal != 0)

37 DieWithUserMessage("getaddrinfo() failed", gai_strerror(rtnVal));

110 Chapter 6: Beyond Basic Socket Programming

39 // Create socket for incoming connections

40 servSock = socket(servAddr->ai_family, servAddr->ai_socktype,

41 servAddr->ai_protocol);

42 if (servSock < 0)

43 DieWithSystemMessage("socket() failed");

45 // Bind to the local address

46 if (bind(servSock, servAddr->ai_addr, servAddr->ai_addrlen) < 0)

47 DieWithSystemMessage("bind() failed");

49 // Free address list allocated by getaddrinfo()

50 freeaddrinfo(servAddr);

52 struct sigaction handler;

53 handler.sa_handler = SIGIOHandler; // Set signal handler for SIGIO

54 // Create mask that mask all signals

55 if (sigfillset(&handler.sa_mask) < 0)

56 DieWithSystemMessage("sigfillset() failed");

57 handler.sa_flags = 0; // No flags

59 if (sigaction(SIGIO, &handler, 0) < 0)

60 DieWithSystemMessage("sigaction() failed for SIGIO");

62 // We must own the socket to receive the SIGIO message

63 if (fcntl(servSock, F_SETOWN, getpid()) < 0)

64 DieWithSystemMessage("Unable to set process owner to us");

66 // Arrange for nonblocking I/O and SIGIO delivery

67 if (fcntl(servSock, F_SETFL, O_NONBLOCK | FASYNC) < 0)

68 DieWithSystemMessage(

69 "Unable to put client sock into non-blocking/async mode");

71 // Go off and do real work; echoing happens in the background

73 for (;;)

74 UseIdleTime();

75 // NOT REACHED

76 }

78 void UseIdleTime() {

79 puts(".");

80 sleep(3); // 3 seconds of activity

81 }

6.3 Nonblocking I/O 111

83 void SIGIOHandler(int signalType) {

84 ssize_t numBytesRcvd;

85 do { // As long as there is input...

86 struct sockaddr_storage clntAddr; // Address of datagram source

87 size_t clntLen = sizeof(clntAddr); // Address length in-out parameter

88 char buffer[MAXSTRINGLENGTH]; // Datagram buffer

90 numBytesRcvd = recvfrom(servSock, buffer, MAXSTRINGLENGTH, 0,

91 (struct sockaddr *) &clntAddr, &clntLen);

92 if (numBytesRcvd < 0) {

93 // Only acceptable error: recvfrom() would have blocked

94 if (errno != EWOULDBLOCK)

95 DieWithSystemMessage("recvfrom() failed");

96 } else {

97 fprintf(stdout, "Handling client ");

98 PrintSocketAddress((struct sockaddr *) &clntAddr, stdout);

99 fputc('\n', stdout);

100

101 ssize_t numBytesSent = sendto(servSock, buffer, numBytesRcvd, 0,

102 (struct sockaddr *) &clntAddr, sizeof(clntAddr));

103 if (numBytesSent < 0)

104 DieWithSystemMessage("sendto() failed");

105 else if (numBytesSent != numBytesRcvd)

106 DieWithUserMessage("sendto()", "sent unexpected number of bytes");

107 }

108 } while (numBytesRcvd >= 0);

109 // Nothing left to receive

110 }

UDPEchoServer–SIGIO.c

1. Program setup and parameter parsing: lines 1–24

2. Prototypes for signal and idle time handler: lines 14–15

UseIdleTime() simulates the other tasks of the UDP echo server. SIGIOHandler() handles

sigio signals. Note well: UseIdleTime() must be prepared for any “slow” system calls—

such as reading from a terminal device—to return −1as a result of the sigio signal

being delivered and handled (in which case it should simply verify that errno is eintr

and resume whatever it was doing).

3. Server socket descriptor: line 17

We give the socket descriptor a global scope so that it can be accessed by the sigio

handler function.

4. Set up signal handling: lines 52–69

112 Chapter 6: Beyond Basic Socket Programming

handler is the sigaction structure that describes our desired signal-handling behavior.

We ﬁll it in, giving the address of the handling routine and the set of signals we want

blocked.

Fill in the pointer to the desired handler: line 53

Specify signals to be blocked during handling: lines 55–56

Specify how to handle the SIGIO signal: lines 59–60

Arrange for SIGIO to go to this process: lines 63–64

The f_setown command identiﬁes the process to receive sigio for this socket.

Set ﬂags for nonblocking and asynchronous I/O: lines 67–69

Finally, we mark the socket (with the fasync ﬂag4) to indicate that asynchronous I/O

is in use, so sigio will be delivered on packet arrival. (Everything up to this point was

just saying how to deal with sigio.) Because we do not want SIGIOHandler() to block in

recvfrom(), we also set the o_nonblock ﬂag.

5. Run forever using idle time when available: lines 73–74

6. Perform nonechoing server tasks: lines 78–81

7. Handle asynchronous I/O: lines 83–110

This code is very similar to the loop in our earlier UDPEchoServer.c (page 57). One diﬀer-

ence is that here we loop until there are no more pending echo requests to satisfy and

then return; this technique enables the main program thread to continue what it was

doing.

Receive echo request: lines 90–99

The ﬁrst call to recvfrom() receives the datagram whose arrival prompted the sigio sig-

nal. Additional datagrams may arrive during execution of the handler, so the do/while

loop continues to call recvfrom() until no more datagrams remain to be received.

Because sock is a nonblocking socket, recvfrom() then returns −1 with errno set to

ewouldblock, terminating the loop and the handler function.

Send echo reply: lines 101–106

Just as in the original UDP echo server, sendto() repeats the message back to the client.

6.3.3 Timeouts

In the previous subsection, we relied on the system to notify our program of the occurrence

of an I/O–related event. Sometimes, however, we may actually need to know that some I/O

event has not happened for a certain time period. For example, we have already mentioned

that UDP messages can be lost; in case of such a loss, our UDP echo client (or any other client

that uses UDP, for that matter) will never receive a response to its request. Of course, the client

4The name may be diﬀerent (e.g., o_async) on some platforms.

6.3 Nonblocking I/O 113

cannot tell directly whether a loss has occurred, so it sets a limit on how long it will wait for a

response. For example, the UDP echo client might assume that if the server has not responded

to its request within two seconds, the server will never respond. The client’s reaction to this

two-second timeout might be to give up or to try again by resending the request.

The standard method of implementing timeouts is to set an alarm before calling a blocking

function.

unsigned int alarm(unsigned int secs)

alarm() starts a timer, which expires after the speciﬁed number of seconds (secs); alarm()

returns the number of seconds remaining for any previously scheduled alarm (or 0 if no alarm

was scheduled). When the timer expires, a SIGALRM signal is sent to the process, and the

handler function for SIGALRM, if any, is executed.

The code we showed earlier in UDPEchoClient.c (page 54) has a problem if either the

echo request or the response is lost: The client blocks indeﬁnitely on recvfrom(), waiting for a

datagram that will never arrive. Our next example program, UDPEchoClient-Timeout.c, modiﬁes

the original UDP echo client to retransmit the request message if a response from the echo

server is not received within a time limit of two seconds. To implement this, the new client

installs a handler for SIGALRM, and just before calling recvfrom(), it sets an alarm for two

seconds. At the end of that interval of time, the SIGALRM signal is delivered, and the han-

dler is invoked. When the handler returns, the blocked recvfrom() returns −1 with errno equal

to EINTR. The client then resends the echo request to the server. This timeout and retrans-

mission of the echo request happens up to ﬁve times before the client gives up and reports

failure. Our program description only details the code that diﬀers from the original UDP echo

client.

UDPEchoClient–Timeout.c

1#include <stdio.h>

2#include <stdlib.h>

3#include <string.h>

4#include <unistd.h>

5#include <errno.h>

6#include <signal.h>

7#include <sys/socket.h>

8#include <netinet/in.h>

9#include <netdb.h>

10 #include "Practical.h"

12 static const unsigned int TIMEOUT_SECS = 2; // Seconds between retransmits

13 static const unsigned int MAXTRIES = 5; // Tries before giving up

114 Chapter 6: Beyond Basic Socket Programming

15 unsigned int tries = 0; // Count of times sent - GLOBAL for signal-handler access

17 void CatchAlarm(int ignored); // Handler for SIGALRM

19 int main(int argc, char *argv[]) {

21 if ((argc < 3) || (argc > 4)) // Test for correct number of arguments

22 DieWithUserMessage("Parameter(s)",

23 "<Server Address/Name> <Echo Word> [<Server Port/Service>]\n");

25 char *server = argv[1]; // First arg: server address/name

26 char *echoString = argv[2]; // Second arg: word to echo

28 size_t echoStringLen = strlen(echoString);

29 if (echoStringLen > MAXSTRINGLENGTH)

30 DieWithUserMessage(echoString, "too long");

32 char *service = (argc == 4) ? argv[3] : "echo";

34 // Tell the system what kind(s) of address info we want

35 struct addrinfo addrCriteria; // Criteria for address

36 memset(&addrCriteria, 0, sizeof(addrCriteria)); // Zero out structure

37 addrCriteria.ai_family = AF_UNSPEC; // Any address family

38 addrCriteria.ai_socktype = SOCK_DGRAM; // Only datagram sockets

39 addrCriteria.ai_protocol = IPPROTO_UDP; // Only UDP protocol

41 // Get address(es)

42 struct addrinfo *servAddr; // Holder for returned list of server addrs

43 int rtnVal = getaddrinfo(server, service, &addrCriteria, &servAddr);

44 if (rtnVal != 0)

45 DieWithUserMessage("getaddrinfo() failed", gai_strerror(rtnVal));

47 // Create a reliable, stream socket using TCP

48 int sock = socket(servAddr->ai_family, servAddr->ai_socktype,

49 servAddr->ai_protocol); // Socket descriptor for client

50 if (sock < 0)

51 DieWithSystemMessage("socket() failed");

53 // Set signal handler for alarm signal

54 struct sigaction handler; // Signal handler

55 handler.sa_handler = CatchAlarm;

56 if (sigfillset(&handler.sa_mask) < 0) // Block everything in handler

57 DieWithSystemMessage("sigfillset() failed");

58 handler.sa_flags = 0;

6.3 Nonblocking I/O 115

60 if (sigaction(SIGALRM, &handler, 0) < 0)

61 DieWithSystemMessage("sigaction() failed for SIGALRM");

63 // Send the string to the server

64 ssize_t numBytes = sendto(sock, echoString, echoStringLen, 0,

65 servAddr->ai_addr, servAddr->ai_addrlen);

66 if (numBytes < 0)

67 DieWithSystemMessage("sendto() failed");

68 else if (numBytes != echoStringLen)

69 DieWithUserMessage("sendto() error", "sent unexpected number of bytes");

71 // Receive a response

73 struct sockaddr_storage fromAddr; // Source address of server

74 // Set length of from address structure (in-out parameter)

75 socklen_t fromAddrLen = sizeof(fromAddr);

76 alarm(TIMEOUT_SECS); // Set the timeout

77 char buffer[MAXSTRINGLENGTH + 1]; // I/O buffer

78 while ((numBytes = recvfrom(sock, buffer, MAXSTRINGLENGTH, 0,

79 (struct sockaddr *) &fromAddr, &fromAddrLen)) < 0) {

80 if (errno == EINTR) { // Alarm went off

81 if (tries < MAXTRIES) { // Incremented by signal handler

82 numBytes = sendto(sock, echoString, echoStringLen, 0,

83 (struct sockaddr *) servAddr->ai_addr, servAddr->ai_addrlen);

84 if (numBytes < 0)

85 DieWithSystemMessage("sendto() failed");

86 else if (numBytes != echoStringLen)

87 DieWithUserMessage("sendto() error", "sent unexpected number of bytes");

88 } else

89 DieWithUserMessage("No Response", "unable to communicate with server");

90 } else

91 DieWithSystemMessage("recvfrom() failed");

92 }

94 // recvfrom() got something -- cancel the timeout

95 alarm(0);

97 buffer[echoStringLen] = '\0'; // Null-terminate the received data

98 printf("Received: %s\n", buffer); // Print the received data

100 close(sock);

101 exit(0);

102 }

103

116 Chapter 6: Beyond Basic Socket Programming

104 // Handler for SIGALRM

105 void CatchAlarm(int ignored) {

106 tries += 1;

107 }

UDPEchoClient–Timeout.c

1. Program setup and parameter parsing: lines 1–32

2. Timeout setup: lines 12–17

tries is a global variable so that it can be accessed in the signal handler.

3. Establish signal handler for SIGALRM: lines 53–61

This is similar to what we did for SIGIO in UDPEchoServer-SIGIO.c.

4. Start the alarm timer: line 76

When/if the alarm timer expires, the handler CatchAlarm() will be invoked.

5. Retransmission loop: lines 78–92

We have to loop here because the SIGALRM will cause the recvfrom() to return −1. When

that happens, we decide whether or not it was a timeout and, if so, retransmit.

Attempt reception: lines 78–79

Discover the reason for recvfrom() failure: lines 80–91

If errno equals eintr,recvfrom() returned because it was interrupted by the sigalrm

while waiting for datagram arrival and not because we got a packet. In this case we

assume either the echo request or reply is lost. If we have not exceeded the maximum

number of retransmission attempts, we retransmit the request to the server; otherwise,

we report a failure. After retransmission, we reset the alarm timer to wake us again if

the timeout expires.

6. Handle echo response reception: lines 95–98

Cancel the alarm timer: line 95

Ensure that message is null-terminated: line 97

printf() will output bytes until it encounters a null byte, so we need to make sure one

is present (otherwise, our program may crash).

Print the received message: line 98

6.4 Multitasking

Our TCP echo server handles one client at a time. If additional clients connect while one is

already being serviced, their connections will be established and they will be able to send their

requests, but the server will not echo back their data until it has ﬁnished with the ﬁrst client.

This type of socket application is called an iterative server.Iterative servers work best for

6.4 Multitasking 117

applications where each client requires a small, bounded amount of work by the server.

However, if the time required to handle a client can be long, the overall connection time experi-

enced by any waiting clients may become unacceptably long. To demonstrate the problem, add

asleep() after the connect() statement in TCPEchoClient.c (page 44) and experiment with sev-

eral clients simultaneously accessing the TCP echo server. (Here, sleep() simulates an operation

that takes signiﬁcant time such as slow ﬁle or network I/O.)

Modern operating systems provide a solution to this dilemma. Using constructs like pro-

cesses or threads, we can farm out responsibility for each client to an independently executing

copy of the server. In this section, we will explore several models of such concurrent servers,

including per-client processes, per-client threads, and constrained multitasking.

6.4.1 Per-Client Processes

Processes are independently executing programs on the same host. In a per-client process

server, for each client connection request we simply create a new process to handle the

communication. Processes share the resources of the server host, each servicing its client

concurrently.

In UNIX, fork() attempts the creation of a new process, returning −1 on failure. On suc-

cess, a new process is created that is identical to the calling process, except for its process ID

and the return value it receives from fork(). The two processes thereafter execute indepen-

dently. The process invoking fork() is called the parent process, and the newly created process

is called the child. Since the processes are identical, how do the processes know whether they

are parent or child? If the return from fork() is 0, the process knows that it is the child. To the

parent, fork() returns the process ID of the new child process.

When a child process terminates, it does not automatically disappear. In UNIX parlance,

the child becomes a zombie. Zombies consume system resources until they are “harvested”

by their parent with a call to waitpid(), as demonstrated in our next example program,

TCPEchoServer-Fork.c.

We demonstrate this per-client process, multitasking approach by adapting its use for

the TCP echo server. The majority of the program is identical to the original TCPEchoServer.c

(page 48). The main diﬀerence is that the multitasking server creates a new copy of itself each

time it accepts a new connection; each copy handles one client and then terminates. No changes

are required to TCPEchoClient.c (page 44) to work with this new server.

We have decomposed this new echo server to improve readability and to allow reuse in

our later examples. Our program commentary is limited to diﬀerences between the new server

and TCPEchoServer.c (page 48).

Figure 6.1 depicts the phases involved in connection setup between the server and a

client. The server runs forever, listening for connections on a speciﬁed port, and repeatedly

(1) accepts an incoming connection from a client and then (2) creates a new process to handle

that connection. Note that only the original server process calls fork().

118 Chapter 6: Beyond Basic Socket Programming

Client

Server

Child

Client

Server

Child

ServerClient

Server

fork()

accept(servSock,...)connect(sock,...)

Phase

Server

Client

Server: close(clntSock)

Child: close(servSock)

servSock

sock

clntSock

sock

clntSock

Figure 6.1: Forking TCP echo server.

TCPEchoServer–Fork.c

1#include <stdio.h>

2#include <unistd.h>

3#include <stdlib.h>

4#include <sys/wait.h>

5#include "Practical.h"

7int main(int argc, char *argv[]) {

9if (argc != 2) // Test for correct number of arguments

10 DieWithUserMessage("Parameter(s)", "<Server Port/Service>");

12 char *service = argv[1]; // First arg: local port/service

13 int servSock = SetupTCPServerSocket(service);

14 if (servSock < 0)

15 DieWithUserMessage("SetupTCPServerSocket() failed", "unable to establish");

17 unsigned int childProcCount = 0; // Number of child processes

18 for (;;) { // Run forever

6.4 Multitasking 119

19 // New connection creates a client socket

20 int clntSock = AcceptTCPConnection(servSock);

21 // Fork child process and report any errors

22 pid_t processID = fork();

23 if (processID < 0)

24 DieWithSystemMessage("fork() failed");

25 else if (processID == 0) { // If this is the child process

26 close(servSock); // Child closes parent socket

27 HandleTCPClient(clntSock);

28 exit(0); // Child process terminates

29 }

31 printf("with child process: %d\n", processID);

32 close(clntSock); // Parent closes child socket descriptor

33 childProcCount++; // Increment number of child processes

35 while (childProcCount) { // Clean up all zombies

36 processID = waitpid((pid_t) - 1, NULL, WNOHANG); // Non-blocking wait

37 if (processID < 0) // waitpid() error?

38 DieWithSystemMessage("waitpid() failed");

39 else if (processID == 0) // No zombie to wait on

40 break;

41 else

42 childProcCount--; // Cleaned up after a child

43 }

44 }

45 // NOT REACHED

46 }

TCPEchoServer–Fork.c

1. Additional include ﬁle for waitpid():line 4

2. Create server socket: lines 13–15

SetupTCPServerSocket() allocates, binds, and marks the server socket as ready to accept

incoming connections.

3. Set up to handle multiple processes: line 17

childProcCount is a variable to count the number of processes.

4. Process dispatch loop: lines 18–44

The parent process runs forever, forking a process for each connection request.

Get the next connection: line 20

AcceptTCPConnection() blocks until a valid connection is established and returns the

socket descriptor for that connection. Connection establishment is depicted in the

transition from Phase 1 to 2 in Figure 6.1.

120 Chapter 6: Beyond Basic Socket Programming

Create a child process to handle the new connection: line 22

fork() attempts to duplicate the calling process. If the attempt fails, fork() returns −1.

If it succeeds, the child process receives a return value of 0, and the parent receives

a return value of the process ID of the child process. When fork() creates a new child

process, it copies the socket descriptors from the parent to the child; therefore, after

fork(), both the parent and child processes have socket descriptors for the listening

socket (servSock) and the newly created and connected client socket (clntSock), as

shown in Phase 3 of Figure 6.1.

Child process execution: lines 26–28

The child process is only responsible for dealing with the new client so it can close

the listening socket descriptor. However, since the parent process still has a descriptor

for the listening socket, this close does not deallocate the socket. This is depicted

in the transition from Phase 3 to 4 in Figure 6.1. It is important to note that calling

close() only terminates the speciﬁed socket if no other processes have a reference to

the socket. The child process then executes HandleTCPClient() to handle the connection.

After handling the client, the child process calls close() to deallocate the client socket.

The child process is terminated with the call to exit().

Parent execution continues: lines 31–33

Since the child is handling the new client, the parent can close the socket descrip-

tor of the new connection socket; again, this does not deallocate the socket because

the child process also contains a reference.5(See the transition from Phase 3 to 4 in

Figure 6.1.) The parent keeps a count of the number of outstanding child processes in

childProcCount.

Handle zombies: lines 35–43

After each connection request, the parent server process harvests the zombies cre-

ated by child process termination. The server repeatedly harvests zombies by calling

waitpid() until no more of them exist. The ﬁrst parameter to waitpid() (−1) is a wildcard

that instructs it to take any zombie, regardless of its process ID. The second parameter

is a placeholder where waitpid() returns the state of the zombie. Since we do not care

about the state, we specify NULL, and no state is returned. Next comes a ﬂag parameter

for customizing the behavior of waitpid().wnohang causes it to return immediately if

no zombies are found. waitpid() returns one of three value types: failure (returns −1),

found zombie (returns pid of zombie), and no zombie (returns 0). If waitpid() found a

zombie, we need to decrement childProcCount and, if more unharvested children exist

(childProcCount != 0), look for another zombie. If waitpid() returns without ﬁnding a

zombie, the parent process breaks out of the zombie harvesting loop.

5Our description of the child and parent execution assumes that the parent executes close() on the client

socket before the child. However, it is possible for the client to race ahead and execute the close() call on

clntSock before the server. In this case, the server’s close() performs the actual socket deallocation, but

this does not change the behavior of the server from the client’s perspective.

6.4 Multitasking 121

There are several other ways to deal with zombies. On some UNIX variants, the default

child termination behavior can be changed so that zombie processes are not created (e.g.,

sa_nocldwait ﬂag to sigaction()). We do not use this approach because it is not portable.

Another approach is to establish a handler function for the sigchld signal. When a child ter-

minates, a sigchld signal is delivered to the original process invoking a speciﬁed handler

function. The handler function uses waitpid() to harvest any zombies. Unfortunately, signals

may interrupt at any time, including during blocked system calls (see Section 6.2). The proper

method for restarting interrupted system calls diﬀers between UNIX variants. In some sys-

tems, restarting is the default behavior. On others, the sa_ﬂags ﬁeld of the sigaction structure

could be set to sa_restart to ensure interrupted system calls restart. On other systems, the

interrupted system calls return −1 with errno set to eintr. In this case, the program must

restart the interrupted function. We do not use any of these approaches because they are not

portable, and they complicate the program with issues that we are not addressing. We leave it

as an exercise for readers to adapt our TCP echo server to use sigchld on their systems.

For this and the rest of the server examples in this chapter, we have factored out the

code for setting up the server socket. This code can be found in TCPServerUtility.c

SetupTCPServerSocket()

1static const int MAXPENDING = 5; // Maximum outstanding connection requests

3int SetupTCPServerSocket(const char *service) {

4// Construct the server address structure

5struct addrinfo addrCriteria; // Criteria for address match

6memset(&addrCriteria, 0, sizeof(addrCriteria)); // Zero out structure

7addrCriteria.ai_family = AF_UNSPEC; // Any address family

8addrCriteria.ai_flags = AI_PASSIVE; // Accept on any address/port

9addrCriteria.ai_socktype = SOCK_STREAM; // Only stream sockets

10 addrCriteria.ai_protocol = IPPROTO_TCP; // Only TCP protocol

12 struct addrinfo *servAddr; // List of server addresses

13 int rtnVal = getaddrinfo(NULL, service, &addrCriteria, &servAddr);

14 if (rtnVal != 0)

15 DieWithUserMessage("getaddrinfo() failed", gai_strerror(rtnVal));

17 int servSock = -1;

18 for (struct addrinfo *addr = servAddr; addr != NULL; addr = addr->ai_next) {

19 // Create a TCP socket

20 servSock = socket(servAddr->ai_family, servAddr->ai_socktype,

21 servAddr->ai_protocol);

22 if (servSock < 0)

23 continue; // Socket creation failed; try next address

25 // Bind to the local address and set socket to list

122 Chapter 6: Beyond Basic Socket Programming

26 if ((bind(servSock, servAddr->ai_addr, servAddr->ai_addrlen) == 0) &&

27 (listen(servSock, MAXPENDING) == 0)) {

28 // Print local address of socket

29 struct sockaddr_storage localAddr;

30 socklen_t addrSize = sizeof(localAddr);

31 if (getsockname(servSock, (struct sockaddr *) &localAddr, &addrSize) < 0)

32 DieWithSystemMessage("getsockname() failed");

33 fputs("Binding to ", stdout);

34 PrintSocketAddress((struct sockaddr *) &localAddr, stdout);

35 fputc('\n', stdout);

36 break; // Bind and list successful

37 }

39 close(servSock); // Close and try again

40 servSock = -1;

41 }

43 // Free address list allocated by getaddrinfo()

44 freeaddrinfo(servAddr);

46 return servSock;

47 }

SetupTCPServerSocket()

The code for getting a new client connection calls accept() and prints out information on the

client’s address.

AcceptTCPConnection()

1int AcceptTCPConnection(int servSock) {

2struct sockaddr_storage clntAddr; // Client address

3// Set length of client address structure (in-out parameter)

4socklen_t clntAddrLen = sizeof(clntAddr);

6// Wait for a client to connect

7int clntSock = accept(servSock, (struct sockaddr *) &clntAddr, &clntAddrLen);

8if (clntSock < 0)

9DieWithSystemMessage("accept() failed");

11 // clntSock is connected to a client!

13 fputs("Handling client ", stdout);

14 PrintSocketAddress((struct sockaddr *) &clntAddr, stdout);

15 fputc('\n', stdout);

6.4 Multitasking 123

17 return clntSock;

18 }

AcceptTCPConnection()

After connection establishment, references to the child socket are done exclusively

through the socket descriptor (clntSock in this example). In our forking TCP echo server, the

IP address and port of the client are only known temporarily in AcceptTCPConnection(). What if

we want to know the IP address and port outside of AcceptTCPConnection()? If we didn’t want

to return that information from AcceptTCPConnection(), we could use the getpeername() and

getsockname() functions described in Section 2.4.6.

6.4.2 Per-Client Thread

Forking a new process to handle each client works, but it is expensive. Every time a pro-

cess is created, the operating system must duplicate the entire state of the parent process

including memory, stack, ﬁle/socket descriptors, and so on. Threads decrease this cost by

allowing multitasking within the same process: A newly created thread simply shares the

same address space (code and data) with the parent, negating the need to duplicate the parent

state.

The next example program, TCPEchoServer-Thread.c, demonstrates a thread-per-client

multitasking approach for the TCP echo server using POSIX threads6(“PThreads”). The majority

of the program is identical to TCPEchoServer-Fork.c (page 118). Again, no changes are required

to TCPEchoClient.c (page 44) to work with this new server. The program comments are limited

to code that diﬀers from the forking echo server.

TCPEchoServer–Thread.c

1#include <stdio.h>

2#include <stdlib.h>

3#include <string.h>

4#include <pthread.h>

5#include "Practical.h"

7void *ThreadMain(void *arg); // Main program of a thread

9// Structure of arguments to pass to client thread

10 struct ThreadArgs {

11 int clntSock; // Socket descriptor for client

6Other thread packages work in generally the same manner. We selected POSIX threads because a port of

POSIX threads exists for most operating systems.

124 Chapter 6: Beyond Basic Socket Programming

12 };

14 int main(int argc, char *argv[]) {

16 if (argc != 2) // Test for correct number of arguments

17 DieWithUserMessage("Parameter(s)", "<Server Port/Service>");

19 char *service = argv[1]; // First arg: local port/service

20 int servSock = SetupTCPServerSocket(service);

21 if (servSock < 0)

22 DieWithUserMessage("SetupTCPServerSocket() failed", "unable to establish");

23 for (;;) { // Run forever

24 int clntSock = AcceptTCPConnection(servSock);

26 // Create separate memory for client argument

27 struct ThreadArgs *threadArgs = (struct ThreadArgs *) malloc(

28 sizeof(struct ThreadArgs));

29 if (threadArgs == NULL)

30 DieWithSystemMessage("malloc() failed");

31 threadArgs->clntSock = clntSock;

33 // Create client thread

34 pthread_t threadID;

35 int returnValue = pthread_create(&threadID, NULL, ThreadMain, threadArgs);

36 if (returnValue != 0)

37 DieWithUserMessage("pthread_create() failed", strerror(returnValue));

38 printf("with thread %lu\n", (unsigned long int) threadID);

39 }

40 // NOT REACHED

41 }

43 void *ThreadMain(void *threadArgs) {

44 // Guarantees that thread resources are deallocated upon return

45 pthread_detach(pthread_self());

47 // Extract socket file descriptor from argument

48 int clntSock = ((struct ThreadArgs *) threadArgs)->clntSock;

49 free(threadArgs); // Deallocate memory for argument

51 HandleTCPClient(clntSock);

53 return (NULL);

54 }

TCPEchoServer–Thread.c

6.4 Multitasking 125

1. Additional include ﬁle for POSIX threads: line 4

2. Set up for threads: lines 7–12

ThreadMain() is the function for the POSIX thread to execute. pthread_create() allows the

caller to pass one pointer as an argument to the function to be executed in the new thread.

The ThreadArgs structure contains the “real” list of parameters. The process creating a

new thread allocates and populates the structure before calling pthread_create(). In this

program the thread function only needs a single argument (clntSock), so we could have

simply passed a pointer to an integer; however, the ThreadArgs structure provides a more

general framework for thread argument passing.

3. Population of thread argument structure: lines 26–31

We only pass the client socket descriptor to the new thread.

4. Invocation of the new thread: lines 33–38

5. Thread execution: lines 43–54

ThreadMain is the function called by pthread_create() when it creates a new thread. The

required prototype for the function to be executed by a thread is void *fcn(void *),

a function that takes a single argument of type void * and returns a void *.

Thread detach: line 45

By default, when a thread’s main function returns, state is maintained about the func-

tion return code until the parent harvests the results. This is very similar to the behavior

for processes. pthread_detach() allows the thread state to be immediately deallocated

upon completion without parent intervention. pthread_self() provides the thread ID of

the current thread as a parameter to pthread_detach(), much in the way that getpid()

provides a process its process ID.

Extracting the parameters from the ThreadArgs structure: lines 48–49

The ThreadArgs structure for this program only contains the socket descriptor of the

socket connected to the client socket by accept(). Because the ThreadArgs structure

is allocated on a per-connection basis, the new thread can deallocate threadArgs once

the parameter(s) have been extracted.

HandleTCPClient():line 51

The thread function calls the same HandleTCPClient() function that we have been using

all along.

Thread return: line 53

After creation, the parent does not need to communicate with the thread, so the thread

can return a NULL pointer.

Because the parent and thread share the same address space (and thus ﬁle/socket

descriptors), the parent thread and the per-connection thread do not close the connection

and listening sockets, respectively, before proceeding, as the parent and child processes did in

the forking example. Figure 6.2 illustrates the actions of the threaded TCP echo server. There

are a few disadvantages to using threads instead of processes:

126 Chapter 6: Beyond Basic Socket Programming

Server

(connection request)

(connection)

Client Server

Client

Client Server

Phase

connect(sock,...) accept(servSock,...)

pthread_create( )

Client

Server

Thread

sock servSock

clntSock

sock

servSocksock

servSock

clntSock

Figure 6.2: Threaded TCP echo server.

If a child process goes awry, it is easy to monitor and kill it from the command line using

its process identiﬁer. Threads may not provide this capability on some platforms, so

additional server functionality must be provided to monitor and kill individual threads.

If the operating system is oblivious to the notion of threads, it may give every process

the same size time slice. In that case a threaded Web server handling hundreds of clients

may get the same amount of CPU time as a game of solitaire.

6.4.3 Constrained Multitasking

Process and thread creation both incur overhead. In addition, the scheduling and context

switching among many processes or threads creates extra work for a system. As the number

of processes or threads increases, the operating system spends more and more time dealing

with this overhead. Eventually, the point is reached where adding another process or thread

actually decreases overall performance. That is, a client might actually experience shorter ser-

vice time if its connection request were queued until some preceding client ﬁnished, instead

of creating a new process or thread to service it.

We can avoid this problem by limiting the number of processes created by the server, an

approach we call constrained-multitasking servers. (We present a solution for processes, but it

is directly applicable to threads as well.) In this solution, the server begins as the other servers

6.4 Multitasking 127

by creating, binding, and listening to a socket. Then the server creates a set number (say, N)of

processes, each of which loops forever, accepting connections from the (same) listening socket.

This works because when multiple processes call accept() on the same listening socket at the

same time, they all block until a connection is established. Then the system picks one process,

and the socket descriptor for that new connection is returned only in that process; the others

remain blocked until the next connection is established, another lucky winner is chosen, and

so on.

Our next example program, TCPEchoServer-ForkN.c, implements this server model as a

modiﬁcation of the original TCPEchoServer.c (page 48), so we only comment on the diﬀerences.

TCPEchoServer–ForkN.c

1#include <stdlib.h>

2#include <unistd.h>

3#include <stdio.h>

4#include "Practical.h"

6void ProcessMain(int servSock); // Process main

8int main(int argc, char *argv[]) {

10 if (argc != 3) // Test for correct number of arguments

11 DieWithUserMessage("Parameter(s)", "<Server Port/Service> <Process Count>");

13 char *service = argv[1]; // First arg: local port

14 unsigned int processLimit = atoi(argv[2]); // Second arg: number of children

16 // Server socket

17 int servSock = SetupTCPServerSocket(service);

19 // Fork limit-1 child processes

20 for (int processCt = 0; processCt < processLimit - 1; processCt++) {

21 // Fork child process and report any errors

22 pid_t processID = fork();

23 if (processID < 0)

24 DieWithSystemMessage("fork() failed");

25 else if (processID == 0) // If this is the child process

26 ProcessMain(servSock);

27 }

29 // Execute last process in parent

30 ProcessMain(servSock);

31 // NOT REACHED

32 }

128 Chapter 6: Beyond Basic Socket Programming

34 void ProcessMain(int servSock) {

35 for (;;) { // Run forever

36 int clntSock = AcceptTCPConnection(servSock);

37 printf("with child process: %d\n", getpid());

38 HandleTCPClient(clntSock);

39 }

40 }

TCPEchoServer–ForkN.c

1. Prototype for “main” of forked process: line 6

Each of the Nprocesses executes the ProcessMain() function.

2. Spawning processLimit −1 processes: lines 19–27

Execute loop processLimit −1 times, each time forking a process that calls ProcessMain()

with servSock as the parameter.

3. Parent becomes last thread: line 30

4. ProcessMain():lines 34–40

ProcessMain() runs forever handling client requests. Eﬀectively, it is the same as the for(;;)

loop in TCPEchoServer.c (page 48).

Because only Nprocesses are created, we limit scheduling overhead, and because each process

lives forever handling client requests, we limit process creation overhead. Of course, if we

spawn too few processes, we can still have clients waiting unnecessarily for service.

6.5 Multiplexing

Our programs so far have dealt with I/O over a single channel; each version of our echo server

deals with only one client connection at a time. However, it is often the case that an application

needs the ability to do I/O on multiple channels simultaneously. For example, we might want to

provide echo service on several ports at once. The problem with this becomes clear as soon as

you consider what happens after the server creates and binds a socket to each port. It is ready

to accept() connections, but which socket to choose? A call to accept() (or recv())onone

socket may block, causing established connections to another socket to wait unnecessarily.

This problem can be solved using nonblocking sockets, but in that case the server ends up

continuously polling the sockets, which is wasteful. We would like to let the server block until

some socket is ready for I/O.

Fortunately, UNIX provides a way to do this. With the select() function, a program can

specify a list of descriptors to check for pending I/O; select() suspends the program until

one of the descriptors in the list becomes ready to perform I/O and returns an indication of

6.5 Multiplexing 129

which descriptors are ready. Then the program can proceed with I/O on that descriptor with

the assurance that the operation will not block.

int select(int maxDescPlus1,fd_set *readDescs,fd_set *writeDescs,fd_set *exceptionDescs,

struct timeval *timeout)

select() monitors three separate lists of descriptors. (Note that these descriptors may refer to

regular ﬁles—such as a terminal input—as well as sockets; we’ll see an example of this in our

example code later.)

readDescs: Descriptors in this list are checked for immediate input data availability; that is,

a call to recv() (or recvfrom() for a datagram socket) would not block.

writeDescs: Descriptors in this list are checked for the ability to immediately write data; that

is, a call to send() (or sendto() for a datagram socket) would not block.

exceptionDescs: Descriptors in this list are checked for pending exceptions or errors. An

example of a pending exception for a TCP socket would be if the remote end of a TCP

socket had closed while data were still in the channel; in such a case, the next read or

write operation would fail and return econnreset.

Passing NULL for any of the descriptor vectors makes select() ignore that type of I/O. For

example, passing NULL for exceptionDescs causes select() to completely ignore exceptions on

any sockets. To save space, each of these lists of descriptors is typically represented as a bit

vector. To include a descriptor in the list, we set the bit in the bit vector corresponding to the

number of its descriptor to 1. (For example, stdin is descriptor 0, so we would set the ﬁrst bit in

the vector if we want to monitor it.) Programs should not (and need not) rely on knowledge of

this implementation strategy, however, because the system provides macros for manipulating

instances of the type fd_set:

void FD_ZERO(fd_set *descriptorVector)

void FD_CLR(int descriptor,fd_set *descriptorVector)

void FD_SET(int descriptor,fd_set *descriptorVector)

int FD_ISSET(int descriptor,fd_set *descriptorVector)

FD_ZERO empties the list of descriptors. FD_CLR() and FD_SET() remove and add descriptors to

the list, respectively. Membership of a descriptor in a list is tested by FD_ISSET(), which returns

nonzero if the given descriptor is in the list, and 0 otherwise.

The maximum number of descriptors that can be contained in a list is given by the system-

deﬁned constant fd_setsize. While this number can be quite large, most applications use very

few descriptors. To make the implementation more eﬃcient, the select() function allows us

130 Chapter 6: Beyond Basic Socket Programming

to pass a hint, which indicates the largest descriptor number that needs to be considered in

any of the lists. In other words, maxDescPlus1 is the smallest descriptor number that does not

need to be considered, which is simply the maximum descriptor value plus one. For example, if

descriptors 0, 3, and 5 are set in the descriptor list, we would set maxDescPlus1 to the maximum

descriptor value (5) plus one. Notice that maxDescPlus1 applies for all three descriptor lists. If

the exception descriptor list’s largest descriptor is 7, while the read and write descriptor lists’

largest are 5 and 2, respectively, then we set maxDescPlus1 to 8.

What would you pay for the ability to listen simultaneously to so many descriptors for up

to three types of I/O? Don’t answer yet because select() does even more! The last parameter

(timeout) allows control over how long select() will wait for something to happen. The timeout

is speciﬁed with a timeval data structure:

struct timeval {

time_t tv_sec; // Seconds

time_t tv_usec; // Microseconds

};

If the time speciﬁed in the timeval structure elapses before any of the speciﬁed descriptors

becomes ready for I/O, select() returns the value 0. If timeout is NULL, select() has no timeout

bound and waits until some descriptor becomes ready. Setting both tv_sec and tv_usec to 0

causes select() to return immediately, enabling polling of I/O descriptors.

If no errors occur, select() returns the total number of descriptors prepared for I/O. To

indicate the descriptors ready for I/O, select() changes the descriptor lists so that only the

positions corresponding to ready descriptors are set. For example, if descriptors 0, 3, and 5

are set in the initial read descriptor list, the write and exception descriptor lists are NULL, and

descriptors 0 and 5 have data available for reading, select() returns 2, and only positions 0

and 5 are set in the returned read descriptor list. An error in select() is indicated by a return

value of −1.

Let’s reconsider the problem of running the echo service on multiple ports. If we create a

socket for each port, we could list these sockets in a readDescriptor list. A call to select(), given

such a list, would suspend the program until an echo request arrives for at least one of the

descriptors. We could then handle the connection setup and echo for that particular socket. Our

next example program, TCPEchoServer-Select.c, implements this model. The user can specify

an arbitrary number of ports to monitor. Notice that a connection request is considered I/O

and prepares a socket descriptor for reading by select(). To illustrate that select() works

on nonsocket descriptors as well, this server also watches for input from the standard input

stream, which it interprets as a signal to terminate itself.

TCPEchoServer–Select.c

1#include <sys/time.h>

2#include <stdlib.h>

3#include <stdio.h>

6.5 Multiplexing 131

4#include <unistd.h>

5#include <fcntl.h>

6#include <stdbool.h>

7#include "Practical.h"

9int main(int argc, char *argv[]) {

11 if (argc < 3) // Test for correct number of arguments

12 DieWithUserMessage("Parameter(s)", "<Timeout (secs.)> <Port/Service1> ...");

14 long timeout = atol(argv[1]); // First arg: Timeout

15 int noPorts = argc - 2; // Number of ports is argument count minus 2

17 // Allocate list of sockets for incoming connections

18 int servSock[noPorts];

19 // Initialize maxDescriptor for use by select()

20 int maxDescriptor = -1;

22 // Create list of ports and sockets to handle ports

23 for (int port = 0; port < noPorts; port++) {

24 // Create port socket

25 servSock[port] = SetupTCPServerSocket(argv[port + 2]);

27 // Determine if new descriptor is the largest

28 if (servSock[port] > maxDescriptor)

29 maxDescriptor = servSock[port];

30 }

32 puts("Starting server: Hit return to shutdown");

33 bool running = true; // true if server should continue running

34 fd_set sockSet; // Set of socket descriptors for select()

35 while (running) {

36 /* Zero socket descriptor vector and set for server sockets

37 This must be reset every time select() is called */

38 FD_ZERO(&sockSet);

39 // Add keyboard to descriptor vector

40 FD_SET(STDIN_FILENO, &sockSet);

41 for (int port = 0; port < noPorts; port++)

42 FD_SET(servSock[port], &sockSet);

44 // Timeout specification; must be reset every time select() is called

45 struct timeval selTimeout; // Timeout for select()

46 selTimeout.tv_sec = timeout; // Set timeout (secs.)

47 selTimeout.tv_usec = 0; // 0 microseconds

132 Chapter 6: Beyond Basic Socket Programming

49 // Suspend program until descriptor is ready or timeout

50 if (select(maxDescriptor + 1, &sockSet, NULL, NULL, &selTimeout) == 0)

51 printf("No echo requests for %ld secs...Server still alive\n", timeout);

52 else {

53 if (FD_ISSET(0, &sockSet)) { // Check keyboard

54 puts("Shutting down server");

55 getchar();

56 running = false;

57 }

59 // Process connection requests

60 for (int port = 0; port < noPorts; port++)

61 if (FD_ISSET(servSock[port], &sockSet)) {

62 printf("Request on port %d: ", port);

63 HandleTCPClient(AcceptTCPConnection(servSock[port]));

64 }

65 }

66 }

68 // Close sockets

69 for (int port = 0; port < noPorts; port++)

70 close(servSock[port]);

72 exit(0);

73 }

TCPEchoServer–Select.c

1. Set up a socket for each port: lines 23–30

We store the socket descriptors in an array, one per argument to the program.

2. Create list of ﬁle descriptors for select():line 25

3. Set timer for select():lines 44–47

4. select() execution: lines 49–65

Handle timeout: line 51

Check keyboard descriptor: lines 53–57

If the user presses return, descriptor STDIN_FILENO will be ready for reading; in that

case the server terminates itself.

Check the socket descriptors: lines 60–64

Test each descriptor, accepting and handling the valid connections.

5. Wrap-up: lines 68–72

Close all ports and exit.

6.6 Multiple Recipients 133

select() is a powerful function. It can also be used to implement a timeout version of any of

the blocking I/O functions (e.g., recv(),accept()) without using alarms.

6.6 Multiple Recipients

So far, all of our programs have dealt with communication involving two entities, usually a

server and a client. Such one-to-one communication is sometimes called unicast because only

one (“uni”) copy of the data is sent (“cast”). Sometimes we would like to send the same infor-

mation to more than one recipient. You probably know that computers in an oﬃce or home

are often connected to a local area network, and sometimes we would like to send informa-

tion that can be received by every host on the network. For example, a computer that has a

printer attached may advertise it for use by other hosts on the network by sending a message

this way; the operating systems of other machines receive these advertisements and make the

shared resources available to their users. Instead of unicasting the message to every host on

the network—which requires us not only to know the address of every host on the network,

but also to call sendto() on the message once for each host—we would like to be able to call

sendto() just once and have the network handle the duplication for us.

Or consider a typical case in which the sender has a single connection to the Internet,

such as a cable or DSL modem. Sending the same data to multiple recipients scattered through-

out the Internet with unicast requires that the same information be sent over that link many

times (Figure 6.3a). If the sender’s ﬁrst-hop connection has limited outgoing capacity (say, one

(a) (b)

Figure 6.3: (a) Unicasting the same data to multiple recipients; (b) doing the same thing with multicast.

134 Chapter 6: Beyond Basic Socket Programming

1 Mbps or so), it may not even be possible to send some kinds of information—say, a real-time

video stream at 1 Mbps—to more than one recipient without exceeding the ﬁrst-hop link capac-

ity, resulting in many lost packets and poor quality. Clearly, it would be more eﬃcient if the

information could be duplicated after it crosses the ﬁrst link, as in Figure 6.3b. This saves

bandwidth and simpliﬁes life for the sending program.

You may be surprised to learn that the sockets interface over TCP/IP provides access to

services like this—albeit with some restrictions. There are two types of network duplication

service: broadcast and multicast. With broadcast, the program calls sendto() once, and the

message is automatically delivered to all hosts on the local network. With multicast, the mes-

sage is sent once and delivered to a speciﬁc (possibly empty) group of hosts throughout the

Internet—namely, those that have indicated to the network that they should receive messages

sent to that group.

We mentioned that there are some restrictions on these services. The ﬁrst is that only

UDP sockets can use broadcast and multicast services. The second is that broadcast only

covers a local scope, typically a local area network. The third restriction is that multicast

across the entire Internet is presently not supported by most Internet service providers.

In spite of these restrictions, these services can often be useful. For example, it is often useful

to use multicast within a site such as a campus network, or broadcast to local hosts.

6.6.1 Broadcast

UDP datagrams can be sent to all nodes on an attached local network by sending them to

a special address. In IPv4 it is called the “limited broadcast address,” and it is the all-ones

address (in dotted-quad notation, 255.255.255.255). In IPv6 it is called the “all-nodes address

(link scope)” and has the value FF02::1. Routers do not forward packets addressed to either one

of these addresses, so neither one will take a datagram beyond the local network to which the

sender is connected. Each does, however, deliver the sent packet to every node on the network;

typically, this is achieved using the hardware broadcast capability of the local network. (Not all

links support broadcast; in particular, point-to-point links do not. If none of a host’s interfaces

support broadcast, any attempt to use it will result in an error.) Note also that a broadcast UDP

datagram will actually be “heard” at a host only if some program on that host is listening for

datagrams on the port to which the datagram is addressed.

What about a network-wide broadcast address to send a message to all hosts? There is

no such address. To see why, consider the impact on the network of a broadcast to every

host on the Internet. Sending a single datagram would result in an extremely large number

of packet duplications by the routers, and bandwidth would be consumed on each and every

network. The consequences of misuse (malicious or accidental) are too great, so the designers

of IP left out such an Internet-wide broadcast facility on purpose. Even with these restrictions,

broadcast on the local link can be very useful. Often it is used in state exchange for network

games where the players are all on the same local area network (e.g., Ethernet).

There is one other diﬀerence between a broadcast sender and a regular sender: before

sending to the broadcast address, the special socket option so_broadcast must be set.

6.6 Multiple Recipients 135

In eﬀect, this asks the system for “permission” to broadcast. We demonstrate the use

of UDP broadcast in BroadcastSender.c. Our sender broadcasts a given string every three

seconds to the limited broadcast address of the address family indicated by the ﬁrst

argument.

BroadcastSender.c

1#include <stdio.h>

2#include <stdlib.h>

3#include <string.h>

4#include <unistd.h>

5#include <sys/socket.h>

6#include <arpa/inet.h>

7#include "Practical.h"

9static const char *IN6ADDR_ALLNODES = "FF02::1"; // v6 addr not built in

11 int main(int argc, char *argv[]) {

13 if (argc != 4) // Test for correct number of arguments

14 DieWithUserMessage("Parameter(s)", "[4|6] <Port> <String to send>");

16 in_port_t port = htons((in_port_t) atoi(argv[2]));

18 struct sockaddr_storage destStorage;

19 memset(&destStorage, 0, sizeof(destStorage));

21 size_t addrSize = 0;

22 if (argv[1][0] == '4') {

23 struct sockaddr_in *destAddr4 = (struct sockaddr_in *) &destStorage;

24 destAddr4->sin_family = AF_INET;

25 destAddr4->sin_port = port;

26 destAddr4->sin_addr.s_addr = INADDR_BROADCAST;

27 addrSize = sizeof(struct sockaddr_in);

28 } else if (argv[1][0] == '6') {

29 struct sockaddr_in6 *destAddr6 = (struct sockaddr_in6 *) &destStorage;

30 destAddr6->sin6_family = AF_INET6;

31 destAddr6->sin6_port = port;

32 inet_pton(AF_INET6, IN6ADDR_ALLNODES, &destAddr6->sin6_addr);

33 addrSize = sizeof(struct sockaddr_in6);

34 } else {

35 DieWithUserMessage("Unknown address family", argv[1]);

36 }

38 struct sockaddr *destAddress = (struct sockaddr *) &destStorage;

136 Chapter 6: Beyond Basic Socket Programming

40 size_t msgLen = strlen(argv[3]);

41 if (msgLen > MAXSTRINGLENGTH) // Input string fits?

42 DieWithUserMessage("String too long", argv[3]);

44 // Create socket for sending/receiving datagrams

45 int sock = socket(destAddress->sa_family, SOCK_DGRAM, IPPROTO_UDP);

46 if (sock < 0)

47 DieWithSystemMessage("socket() failed");

49 // Set socket to allow broadcast

50 int broadcastPerm = 1;

51 if (setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &broadcastPerm,

52 sizeof(broadcastPerm)) < 0)

53 DieWithSystemMessage("setsockopt() failed");

55 for (;;) {// Run forever

56 // Broadcast msgString in datagram to clients every 3 seconds

57 ssize_t numBytes = sendto(sock, argv[3], msgLen, 0, destAddress, addrSize);

58 if (numBytes < 0)

59 DieWithSystemMessage("sendto() failed");

60 else if (numBytes != msgLen)

61 DieWithUserMessage("sendto()", "sent unexpected number of bytes");

63 sleep(3); // Avoid flooding the network

64 }

65 // NOT REACHED

66 }

BroadcastSender.c

1. Declare constant address: line 9

Somewhat surprisingly, the all-nodes group address is not deﬁned as a named system

constant, so we give it a name here.

2. Parameter processing: lines 13–16

3. Destination address storage: lines 18–19

We use a sockaddr_storage structure to hold the destination broadcast address, since it

may be either IPv4 or IPv6.

4. Setting up destination address: lines 21–38

We set the destination address according to the given type and remember the size for

later use. Finally, we save the address in a pointer to a generic sockaddr.

5. Socket creation: lines 44–47

6.6 Multiple Recipients 137

6. Setting permission to broadcast: lines 49–53

By default, sockets cannot broadcast. Setting the SO_BROADCAST option for the socket

enables socket broadcast.

7. Repeatedly broadcast: lines 55–64

Send the argument string every three seconds to all hosts on the network.

Note that a receiver program does not need to do anything special to receive broadcast

datagrams (except bind to the appropriate port). Writing a program to receive the broadcasts

sent out by BroadcastSender.c is left as an exercise.

6.6.2 Multicast

For the sender using multicast is very similar to using unicast. The diﬀerence is in the form

of the address. A multicast address identiﬁes a set of receivers who have “asked” the network

to deliver messages sent to that address. (This is the receiver’s responsibility; see below.)

A range of the address space is set aside for multicast in both IPv4 and IPv6. IPv4 multicast

addresses are in the range 224.0.0.0 to 239.255.255.255. IPv6 multicast addresses are those

whose ﬁrst byte contains 0xFF, that is, all ones. The IPv6 multicast address space has a fairly

complicated structure that is mostly beyond the scope of this book. (The reader is referred

to [4] for details.) For our examples we’ll use addresses beginning with FF1E; they are valid for

transient use in global applications. (The third hex digit “1” indicates a multicast address that

is not permanently assigned for any particular purpose, while the fourth digit “E” indicates

global scope.) An example would be FF1E::1234.

Our example multicast sender, shown in ﬁle MulticastSender.c, takes a multicast address

and port as an argument, and sends a given string to that address and port every three seconds.

MulticastSender.c

1#include <stdlib.h>

2#include <string.h>

3#include <unistd.h>

4#include <netdb.h>

5#include "Practical.h"

7int main(int argc, char *argv[]) {

9if (argc<4||argc > 5) // Test for number of parameters

10 DieWithUserMessage("Parameter(s)",

11 "<Multicast Address> <Port> <Send String> [<TTL>]");

13 char *multicastIPString = argv[1]; // First arg: multicast IP address

14 char *service = argv[2]; // Second arg: multicast port/service

15 char *sendString = argv[3]; // Third arg: string to multicast

138 Chapter 6: Beyond Basic Socket Programming

17 size_t sendStringLen = strlen(sendString);

18 if (sendStringLen > MAXSTRINGLENGTH) // Check input length

19 DieWithUserMessage("String too long", sendString);

21 // Fourth arg (optional): TTL for transmitting multicast packets

22 int multicastTTL = (argc == 5) ? atoi(argv[4]) : 1;

24 // Tell the system what kind(s) of address info we want

25 struct addrinfo addrCriteria; // Criteria for address match

26 memset(&addrCriteria, 0, sizeof(addrCriteria)); // Zero out structure

27 addrCriteria.ai_family = AF_UNSPEC; // v4 or v6 is OK

28 addrCriteria.ai_socktype = SOCK_DGRAM; // Only datagram sockets

29 addrCriteria.ai_protocol = IPPROTO_UDP; // Only UDP please

30 addrCriteria.ai_flags |= AI_NUMERICHOST; // Don't try to resolve address

32 struct addrinfo *multicastAddr; // Holder for returned address

33 int rtnVal= getaddrinfo(multicastIPString, service,

34 &addrCriteria, &multicastAddr);

35 if (rtnVal != 0)

36 DieWithUserMessage("getaddrinfo() failed", gai_strerror(rtnVal));

38 // Create socket for sending datagrams

39 int sock = socket(multicastAddr->ai_family, multicastAddr->ai_socktype,

40 multicastAddr->ai_protocol);

41 if (sock < 0)

42 DieWithSystemMessage("socket() failed");

44 // Set TTL of multicast packet. Unfortunately this requires

45 // address-family-specific code

46 if (multicastAddr->ai_family == AF_INET6) { // v6-specific

47 // The v6 multicast TTL socket option requires that the value be

48 // passed in as an integer

49 if (setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_HOPS,

50 &multicastTTL, sizeof(multicastTTL)) < 0)

51 DieWithSystemMessage("setsockopt(IPV6_MULTICAST_HOPS) failed");

52 } else if (multicastAddr->ai_family == AF_INET) { // v4 specific

53 // The v4 multicast TTL socket option requires that the value be

54 // passed in an unsigned char

55 u_char mcTTL = (u_char) multicastTTL;

56 if (setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, &mcTTL,

57 sizeof(mcTTL)) < 0)

58 DieWithSystemMessage("setsockopt(IP_MULTICAST_TTL) failed");

59 } else {

60 DieWithUserMessage("Unable to set TTL", "invalid address family");

6.6 Multiple Recipients 139

61 }

63 for (;;) { // Run forever

64 // Multicast the string to all who have joined the group

65 ssize_t numBytes = sendto(sock, sendString, sendStringLen, 0,

66 multicastAddr->ai_addr, multicastAddr->ai_addrlen);

67 if (numBytes < 0)

68 DieWithSystemMessage("sendto() failed");

69 else if (numBytes != sendStringLen)

70 DieWithUserMessage("sendto()", "sent unexpected number of bytes");

71 sleep(3);

72 }

73 // NOT REACHED

74 }

MulticastSender.c

Note that unlike the broadcast sender, the multicast sender does not need to set the

permission to multicast. On the other hand, the multicast sender may set the TTL (“time-to-

live”) value for the transmitted datagrams. Every packet contains a counter, which is initialized

to some default value when the packet is ﬁrst sent and decremented by each router that handles

the packet. When this counter reaches 0, the packet is discarded. The TTL mechanism (which

can be changed by setting a socket option) allows us to control the initial value of this counter

and thus limit the number of hops a multicast packet can traverse. For example, by setting

TTL = 1, the multicast packet will not go beyond the local network.

As mentioned earlier, the multicast network service duplicates and delivers the message

only to a speciﬁc set of receivers. This set of receivers, called a multicast group, is identiﬁed by

a particular multicast (or group) address. These receivers need some mechanism to notify the

network of their interest in receiving data sent to a particular multicast address. Once notiﬁed,

the network can begin forwarding the multicast messages to the receiver. This notiﬁcation of

the network, called “joining a group,” is accomplished via a multicast request (signaling) mes-

sage sent (transparently) by the underlying protocol implementation. To cause this to happen,

the receiving program needs to invoke an address-family-speciﬁc multicast socket option. For

IPv4 it is ip_add_membership; for IPv6 it is (surprisingly enough) ipv6_add_membership. This

socket option takes a structure containing the address of the multicast “group” to be joined.

Alas, this structure is also diﬀerent for the two versions:

struct ip_mreq {

struct in_addr imr_multiaddr; // Group address

struct in_addr imr_interface; // local interface to join on

};

The IPv6 version diﬀers only in the type of addresses it contains:

struct ipv6_mreq {

140 Chapter 6: Beyond Basic Socket Programming

struct in6_addr ipv6mr_multiaddr; // IPv6 multicast address of group

unsigned int ipv6mr_interface; // local interface to join no

};

Our multicast receiver contains a fair amount of version-speciﬁc code to handle the

joining process.

MulticastReceiver.c

1#include <stdlib.h>

2#include <string.h>

3#include <unistd.h>

4#include <netdb.h>

5#include "Practical.h"

7int main(int argc, char *argv[]) {

9if (argc != 3)

10 DieWithUserMessage("Parameter(s)", "<Multicast Address> <Port>");

12 char *multicastAddrString = argv[1]; // First arg: multicast addr (v4 or v6!)

13 char *service = argv[2]; // Second arg: port/service

15 struct addrinfo addrCriteria; // Criteria for address match

16 memset(&addrCriteria, 0, sizeof(addrCriteria)); // Zero out structure

17 addrCriteria.ai_family = AF_UNSPEC; // v4 or v6 is OK

18 addrCriteria.ai_socktype = SOCK_DGRAM; // Only datagram sockets

19 addrCriteria.ai_protocol = IPPROTO_UDP; // Only UDP protocol

20 addrCriteria.ai_flags |= AI_NUMERICHOST; // Don't try to resolve address

22 // Get address information

23 struct addrinfo *multicastAddr; // List of server addresses

24 int rtnVal = getaddrinfo(multicastAddrString, service,

25 &addrCriteria, &multicastAddr);

26 if (rtnVal != 0)

27 DieWithUserMessage("getaddrinfo() failed", gai_strerror(rtnVal));

29 // Create socket to receive on

30 int sock = socket(multicastAddr->ai_family, multicastAddr->ai_socktype,

31 multicastAddr->ai_protocol);

32 if (sock < 0)

33 DieWithSystemMessage("socket() failed");

35 if (bind(sock, multicastAddr->ai_addr, multicastAddr->ai_addrlen) < 0)

36 DieWithSystemMessage("bind() failed");

6.6 Multiple Recipients 141

38 // Unfortunately we need some address-family-specific pieces

39 if (multicastAddr->ai_family == AF_INET6) {

40 // Now join the multicast "group" (address)

41 struct ipv6_mreq joinRequest;

42 memcpy(&joinRequest.ipv6mr_multiaddr, &((struct sockaddr_in6 *)

43 multicastAddr->ai_addr)->sin6_addr, sizeof(struct in6_addr));

44 joinRequest.ipv6mr_interface = 0; // Let system choose the i/f

45 puts("Joining IPv6 multicast group...");

46 if (setsockopt(sock, IPPROTO_IPV6, IPV6_JOIN_GROUP,

47 &joinRequest, sizeof(joinRequest)) < 0)

48 DieWithSystemMessage("setsockopt(IPV6_JOIN_GROUP) failed");

49 } else if (multicastAddr->ai_family == AF_INET) {

50 // Now join the multicast "group"

51 struct ip_mreq joinRequest;

52 joinRequest.imr_multiaddr =

53 ((struct sockaddr_in *) multicastAddr->ai_addr)->sin_addr;

54 joinRequest.imr_interface.s_addr = 0; // Let the system choose the i/f

55 printf("Joining IPv4 multicast group...\n");

56 if (setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP,

57 &joinRequest, sizeof(joinRequest)) < 0)

58 DieWithSystemMessage("setsockopt(IPV4_ADD_MEMBERSHIP) failed");

59 } else {

60 DieWithSystemMessage("Unknown address family");

61 }

62 // Free address structure(s) allocated by getaddrinfo()

63 freeaddrinfo(multicastAddr);

65 char recvString[MAXSTRINGLENGTH + 1]; // Buffer to receive into

66 // Receive a single datagram from the server

67 int recvStringLen = recvfrom(sock, recvString, MAXSTRINGLENGTH, 0, NULL, 0);

68 if (recvStringLen < 0)

69 DieWithSystemMessage("recvfrom() failed");

71 recvString[recvStringLen] = '\0'; // Terminate the received string

72 // Note: sender did not send the terminal 0

73 printf("Received: %s\n", recvString);

75 close(sock);

76 exit(0);

77 }

MulticastReceiver.c

The multicast receiver joins the group, waits to receive a message, prints it, and then

exits.

142 Chapter 6: Beyond Basic Socket Programming

6.6.3 Broadcast vs. Multicast

The decision of using broadcast or multicast in an application depends on several issues,

including the fraction of network hosts interested in receiving the data, and the knowledge

of the communicating parties. Broadcast works well if a large percentage of the network

hosts wish to receive the message; however, if few hosts need to receive the packet, broad-

cast “imposes on” all hosts in the network for the beneﬁt of a few. Multicast is preferred

because it limits the duplication of data to those that have expressed interest. The disadvan-

tages of multicast are (1) it is presently not supported globally, and (2) the sender and receiver

must agree on an IP multicast address in advance. Knowledge of an address is not required for

broadcast. In some contexts (local), this makes broadcast a better mechanism for discovery

than multicast. All hosts can receive broadcast by default, so it is simple to ask all hosts a

question like “Where’s the printer?” On the other hand, for wide-area applications, multicast

is the only choice.

Exercises

1. State precisely the conditions under which an iterative server is preferable to a multipro-

cessing server.

2. Would you ever need to implement a timeout in a client or server that uses TCP?

3. Why do we make the server socket nonblocking in UDPEchoServer-SIGIO.c? In particular,

what bad thing might happen if we did not?

4. How can you determine the minimum and maximum allowable sizes for a socket’s send

and receive buﬀers? Determine the minimums for your system.

5. This exercise considers the reasoning behind the sigpipe mechanism a little further. Recall

that sigpipe is delivered when a program tries to send on a TCP socket whose connec-

tion has gone away in the meantime. An alternative approach would be to simply have

the send() fail with econnreset. Why might the signal-based approach be better than

conveying this information by return value?

6. What do you think will happen if you use the program in MulticastReceiver.c while the

program BroadcastSender.c is running on a host connected to the same LAN?

chapter 7

Under the Hood

Some of the subtleties of network programming are diﬃcult to grasp without some

understanding of the data structures associated with each socket in the implementation and

certain details of how the underlying protocols work. This is especially true of stream (TCP)

sockets. This chapter describes some of what goes on “under the hood” when you create and

use a socket. The initial discussion and Section 7.5 apply to both datagram (UDP) and stream

(TCP) sockets; the rest applies only to TCP sockets. Please note that this description covers

only the normal sequence of events and glosses over many details. Nevertheless, we believe

that even this basic level of understanding is helpful. Readers who want the full story are

referred to the TCP speciﬁcation [13] or to one of the more comprehensive treatises on the

subject [2, 17].

Figure 7.1 is a simpliﬁed view of some of the information associated with a socket—that

is, the object created by a call to socket(). The integer returned by socket() is best thought of

as a “handle” that identiﬁes the collection of data structures for one communication endpoint

that we refer to in this chapter as the “socket structure.” As the ﬁgure indicates, more than

one descriptor can refer to the same socket structure. In fact, descriptors in diﬀerent processes

can refer to the same underlying socket structure.

By “socket structure” here we mean all data structures in the socket layer and TCP

implementation that contain state information relevant to this socket abstraction. Thus,

the socket structure contains send and receive queues and other information, including the

following:

The local and remote Internet addresses and port numbers associated with the socket.

The local Internet address (labeled “Local IP” in the ﬁgure) is one of those assigned to the

143

144 Chapter 7: Under the Hood

Application programSocket/TCP implementation

Local port

Local IP

Remote port

Remote IP

Socket structure

Closed

Descriptor(s)

SendQ

RecvQ

Figure 7.1: Data structures associated with a socket.

local host; the local port is set at bind() time. The remote address and port identify

the remote socket, if any, to which the local socket is connected. We will say more

about how and when these values are determined shortly. (Section 7.5 contains a concise

summary.)

A FIFO queue (“RecvQ ”) of received data waiting to be delivered and a FIFO queue

(“SendQ ”) for data waiting to be transmitted.

For a TCP socket, additional protocol state information relevant to the opening and closing

TCP handshakes. In Figure 7.1, the state is “Closed”; all sockets start out in the Closed

state.

Some general-purpose operating systems provide tools that enable users to obtain a

“snapshot” of these underlying data structures. On such tool is netstat, which is typically

available on both UNIX (Linux) and Windows platforms. Given appropriate options, netstat

displays exactly the information indicated in Figure 7.1: number of bytes in SendQ and RecvQ,

local and remote IP addresses and port numbers, and the connection state. Command-line

options may vary, but the output should look something like this:

7.1 Buﬀering and TCP 145

Active Internet connections (servers and established)

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 0.0.0.0:36045 0.0.0.0:* LISTEN

tcp 0 0 0.0.0.0:111 0.0.0.0:* LISTEN

tcp 0 0 0.0.0.0:53363 0.0.0.0:* LISTEN

tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN

tcp 0 0 128.133.190.219:34077 4.71.104.187:80 TIME_WAIT

tcp 0 0 128.133.190.219:43346 79.62.132.8:22 ESTABLISHED

tcp 0 0 128.133.190.219:875 128.133.190.43:2049 ESTABLISHED

tcp6 0 0 :::22 :::* LISTEN

The ﬁrst four lines and the last line depict server sockets listening for connections. (The

last line is a listening socket bound to an IPv6 address.) The ﬁfth line corresponds to a connec-

tion to a Web server (port 80) that is partially shut down (see Section 7.4.2). The next-to-last two

lines are existing TCP connections. You may want to play with netstat, if it is available on your

system, to examine the status of connections in the scenarios depicted in Figures 7.8–7.11. Be

aware, however, that because the transitions between states depicted in the ﬁgures happen so

quickly, it may be diﬃcult to catch them in the “snapshot” provided by netstat.

Knowing that these data structures exist and how they are aﬀected by the underlying

protocols is useful because they control various aspects of the behavior of the socket. For

example, because TCP provides a reliable byte-stream service, a copy of any data sent over a

TCP socket must be kept by the TCP implementation until it has been successfully received at

the other end of the connection. Completion of a call to send() on a TCP socket does not,in

general, imply that the data has actually been transmitted—only that it has been copied into

the local buﬀer. Under normal conditions, it will be transmitted soon, but the exact moment is

under the control of TCP, not the application. Moreover, the nature of the byte-stream service

means that message boundaries are not necessarily preserved in the input stream. As we saw

in Section 5.2.1, this means that most application protocols need a framing mechanism, so the

receiver can tell when it has received an entire message.

On the other hand, with a datagram (UDP) socket, packets are not buﬀered for retrans-

mission, and by the time a call to send/sendto() returns, the data has been given to the network

subsystem for transmission. If the network subsystem cannot handle the message for some

reason, the packet is silently dropped (but this is rare).

The next three sections deal with some of the subtleties of sending and receiving with

TCP’s byte-stream service. Then, Section 7.4 considers the connection establishment and ter-

mination of the TCP protocol. Finally, Section 7.5 discusses the process of matching incoming

packets to sockets and the rules about binding to port numbers.

7.1 Buﬀering and TCP

As a programmer, the most important thing to remember when using a TCP socket is this:

You cannot assume any correspondence between the sizes of writes to one end

of the connection and sizes of reads from the other end.

146 Chapter 7: Under the Hood

In particular, data passed in a single invocation of send() at the sender can be spread

across multiple invocations of recv() at the other end; a single call to recv() may return data

passed in multiple calls to send().

To see this, consider a program that does the following:

rv = connect(s,...);

...

rv = send(s, buffer0, 1000, 0);

...

rv = send(s, buffer1, 2000, 0);

...

rv = send(s, buffer2, 5000, 0);

...

close(s);

where the ellipses represent code that sets up the data in the buﬀers but contains no other

calls to send(). This TCP connection transfers 8000 bytes to the receiver. The way these 8000

bytes are grouped for delivery at the receiving end of the connection depends on the timing

between the calls to send() and recv() at the two ends of the connection—as well as the size

of the buﬀers provided to the recv() calls.

We can think of the sequence of all bytes sent (in one direction) on a TCP connection up

to a particular instant in time as being divided into three FIFO queues:

1. SendQ : Bytes buﬀered in the underlying implementation at the sender that have been

written to the output stream but not yet successfully transmitted to the receiving

host.

2. RecvQ : Bytes buﬀered in the underlying implementation at the receiver waiting to be

delivered to the receiving program—that is, read from the input stream.

3. Delivered: Bytes already read from the input stream by the receiver.

A call to send() at the sender appends bytes to SendQ. The TCP protocol is responsible

for moving bytes—in order—from SendQ to RecvQ . It is important to realize that this transfer

cannot be controlled or directly observed by the user program, and that it occurs in chunks

whose sizes are more or less independent of the size of the buﬀers passed in sends. Bytes are

moved from RecvQ to Delivered by calls to recv(); the size of the transferred chunks depends

on the amount of data in RecvQ and the size of the buﬀer given to recv().

Figure 7.2 shows one possible state of the three queues after the three sendsinthe

example above, but before any calls to recv() at the other end. The diﬀerent shading patterns

denote bytes passed in the three diﬀerent invocations of send() shown above.

The output of netstat on the sending host at the instant depicted in this ﬁgure would

contain a line like:

Active Internet connections

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 6500 10.21.44.33:43346 192.0.2.8:22 ESTABLISHED

7.1 Buﬀering and TCP 147

Receiving socket layer Receiving programSending socket layer

SendQ RecvQ Delivered

First send call (1000 bytes) Second send call (2000 bytes) Third send call (5000 bytes)

1500 bytes6500 bytes

send() TCP protocol recv()

Figure 7.2: State of the three queues after three send() calls.

SendQ RecvQ Delivered

Receiving socket layer Receiving programSending socket layer

First send call (1000 bytes) Second send call (2000 bytes)

6000 bytes 1500 bytes500 bytes

Third send call (5000 bytes)

Figure 7.3: After ﬁrst recv().

On the receiving host, netstat shows:

Active Internet connections

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 1500 0 192.0.2.8:22 10.21.44.33:43346 ESTABLISHED

Now suppose the receiver calls recv() with a byte array of size 2000. The recv() call will

move all of the 1500 bytes present in the waiting-for-delivery (RecvQ ) queue into the byte

array and return the value 1500. Note that this data includes bytes passed in both the ﬁrst and

second calls to send(). At some time later, after TCP has completed transfer of more data, the

three partitions might be in the state shown in Figure 7.3.

If the receiver now calls recv() with a buﬀer of size 4000, that many bytes will be moved

from the waiting-for-delivery (RecvQ ) queue to the already-delivered (Delivered) queue; this

includes the remaining 1500 bytes from the second send(), plus the ﬁrst 2500 bytes from the

third send(). The resulting state of the queues is shown in Figure 7.4.

The number of bytes returned by the next call to recv() depends on the size of the

buﬀer and the timing of the transfer of data over the network from the send-side socket/TCP

implementation to the receive-side implementation. The movement of data from the SendQ to

148 Chapter 7: Under the Hood

Receiving socket layer Receiving programSending socket layer

First send call (1000 bytes) Second send call (2000 bytes) Third send call (5000 bytes)

5500 bytes500 bytes 2000 bytes

RecvQ DeliveredSendQ

Figure 7.4: After another recv().

the RecvQ buﬀer has important implications for the design of application protocols. We have

already encountered the challenge of parsing messages as they are received via a socket when

in-band delimiters are used for framing (see Section 5.2). In the following sections, we consider

two more subtle ramiﬁcations.

7.2 Deadlock Danger

Application protocols have to be designed with some care to avoid deadlock—that is, a state in

which each peer is blocked waiting for the other to do something. For example, it is pretty obvi-

ous that if both client and server try to receive immediately after a connection is established,

deadlock will result. Deadlock can also occur in less immediate ways.

The buﬀers SendQ and RecvQ in the implementation have limits on their capacity.

Although the actual amount of memory they use may grow and shrink dynamically, a hard

limit is necessary to prevent all of the system’s memory from being gobbled up by a single

TCP connection under control of a misbehaving program. Because these buﬀers are ﬁnite,

they can ﬁll up. It is this fact, coupled with TCP’s ﬂow control mechanism, that leads to the

possibility of another form of deadlock.

Once RecvQ is full, the TCP ﬂow control mechanism kicks in and prevents the transfer

of any bytes from the sending host’s SendQ until space becomes available in RecvQ as a result

of the receiver calling recv(). (The purpose of the ﬂow control mechanism is to ensure that the

sender does not transmit data faster than the receiving system can handle.) A sending program

can continue to call send() until SendQ is full. However, once SendQ is full, a send() will block

until space becomes available, that is, until some bytes are transferred to the receiving socket’s

RecvQ .IfRecvQ is also full, everything stops until the receiving program calls recv and some

bytes are transferred to Delivered.

Let’s assume the sizes of SendQ and RecvQ are SQS and RQS, respectively. A call to send()

passing in a buﬀer of size nsuch that n>SQS will not return until at least n−SQS bytes have

been transferred to RecvQ at the receiving host. If nexceeds (SQS +RQS),send() cannot return

until after the receiving program has read at least n−(SQS +RQS)bytes from the input stream.

If the receiving program does not call recv(), a large send() may not complete successfully.

7.3 Performance Implications 149

Host A Host B

RecvQDelivered SendQ To be sent

RecvQ DeliveredSendQTo be sent

send(s, buffer, 1500, 0); send(s, buffer, 1500, 0);

Socket layerSocket layerProgram Program

Figure 7.5: Deadlock due to simultaneous sends to output streams at opposite ends of the connection.

In particular, if both ends of the connection call send() simultaneously, each passing a buﬀer

bigger than SQS +RQS bytes, deadlock will result: neither write will ever complete, and both

programs will remain blocked forever.

As a concrete example, consider a connection between a program on Host A and a pro-

gram on Host B. Assume SQS and RQS are 500 at both A and B. Figure 7.5 shows what happens

when both programs try to send 1500 bytes at the same time. The ﬁrst 500 bytes of data in the

program at Host A have been transferred to the other end; another 500 bytes have been copied

into SendQ at Host A. The remaining 500 bytes cannot be sent—and therefore send() will not

return—until space frees up in RecvQ at Host B. Unfortunately, the same situation holds in

the program at Host B. Therefore, neither program’s call to send() call will ever return!

The moral of the story: Design the protocol carefully to avoid sending large

quantities of data simultaneously in both directions.

7.3 Performance Implications

The TCP implementation’s need to copy user data into SendQ before sending it also has impli-

cations for performance. In particular, the sizes of the SendQ and RecvQ buﬀers aﬀect the

throughput achievable over a TCP connection. “Throughput” is the rate at which bytes of user

data from the sender are made available to the receiving program; in programs that transfer

a large amount of data, we want to maximize the number of bytes delivered per second. In

the absence of network capacity or other limitations, bigger buﬀers generally result in higher

throughput.

The reason for this has to do with the cost of transferring data into and out of the buﬀers

in the underlying implementation. If you want to transfer nbytes of data (where nis large), it

150 Chapter 7: Under the Hood

is generally much more eﬃcient to call send() once with a buﬀer of size nthan it is to call it n

times with a single byte.1However, if you call send() with a size parameter that is much larger

than SQS (the size of SendQ ), the system has to transfer the data from the user address space

in SQS-sized chunks. That is, the socket implementation ﬁlls up the SendQ buﬀer, waits for

data to be transferred out of it by the TCP protocol, reﬁlls SendQ, waits some more, and so on.

Each time the socket implementation has to wait for data to be removed from SendQ, some

time is wasted in the form of overhead (a context switch occurs). This overhead is comparable

to that incurred by a completely new call to send(). Thus the eﬀective size of a call to send()

is limited by the actual SQS. The same thing applies at the receiving end: however large the

buﬀer we pass to recv(), data will be copied out in chunks no larger than RQS, with overhead

incurred between chunks.

If you are writing a program for which throughput is an important performance metric,

you will want to change the send and receive buﬀer sizes using the SO_RCVBUF and SO_SNDBUF

socket options. Although there is always a system-imposed maximum size for each buﬀer,

it is typically signiﬁcantly larger than the default on modern systems. Remember that these

considerations apply only if your program needs to send an amount of data signiﬁcantly larger

than the buﬀer size, all at once. Note also that wrapping a TCP socket in a FILE-stream adds

another stage of buﬀering and additional overhead, and thus may negatively aﬀect throughput.

7.4 TCP Socket Life Cycle

When a new TCP socket is created, it cannot be used immediately for sending and receiving

data. First it needs to be connected to a remote endpoint. Let us therefore consider in more

detail how the underlying structure gets to and from the connected, or “Established”, state. As

we’ll see later, these details aﬀect the deﬁnition of reliability and the ability to bind a socket

to a particular port that was in use earlier.

7.4.1 Connecting

The relationship between the connect() call and the protocol events associated with connection

establishment at the client are illustrated in Figure 7.6. In this and the remaining ﬁgures of this

section, the large arrows depict external events that cause the underlying socket structures to

change state. Events that occur in the application program—that is, method calls and returns—

are shown in the upper part of the ﬁgure; events such as message arrivals are shown in the

lower part of the ﬁgure. Time proceeds left to right in these ﬁgures. The client’s Internet

address is depicted as A.B.C.D, while the server’s is W.X.Y.Z; the server’s port number is Q. (We

have depicted IPv4 addresses, but everything here applies to both IPv4 and IPv6.)

1The same thing generally applies to receiving, although calling recv() with a larger buﬀer does not guar-

antee that more data will be returned—in general, only the data present at the time of a call will be

returned.

7.4 TCP Socket Life Cycle 151

Socket/TCP implementation Application program

Local port

Local IP

Remote port

Remote IP

Closed

returns

fill in local addrs;

send conn request

to server

blocks

call connect()

Local port

Local IP

Remote port

Remote IP

Local port

Local IP

Remote port

Remote IP

A.B.C.D

W.X.Y.Z

Established

A.B.C.D

W.X.Y.Z

Connecting

handshake completes

Figure 7.6: Client-side connection establishment. (Events proceed left to right in these ﬁgures.)

When the client calls connect() with the server’s Internet address, W.X.Y.Z, and port, Q,

the underlying implementation creates a socket instance; it is initially in the Closed state. If the

client did not specify the local address/port with bind(), a local port number (P), not already

in use by another TCP socket, is chosen by the implementation. The local Internet address is

also assigned; if not explicitly speciﬁed, the address of the network interface through which

packets will be sent to the server is used. The implementation copies the local and remote

addresses and ports into the underlying socket structure, and initiates the TCP connection

establishment handshake.

The TCP opening handshake is known as a three-way handshake because it typically

involves three messages: a connection request from client to server, an acknowledgment from

server to client, and another acknowledgment from client back to server. The client TCP con-

siders the connection to be established as soon as it receives the acknowledgment from the

server. In the normal case, this happens quickly. However, the Internet is a best-eﬀort network,

and either the client’s initial message or the server’s response can get lost. For this reason, the

TCP implementation retransmits handshake messages multiple times, at increasing intervals.

If the client TCP does not receive a response from the server after some time, it times out and

gives up. In this case connect() returns −1 and sets errno to ETIMEDOUT. The implementation

tries very hard to complete the connection before giving up, and thus it can take on the order of

minutes for a connect() call to fail. After the initial handshake message is sent and before the

reply from the server is received (i.e., the middle part of Figure 7.6), the output from netstat

on the client host would look something like:

Active Internet connections

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 A.B.C.D:P W.X.Y.Z:Q SYN_SENT

152 Chapter 7: Under the Hood

Application programSocket/TCP implementation

listen()

Local port

Local IP

Remote port

Remote IP

Local port

Local IP

Remote port

Remote IP

Closed

Local port

Local IP

Remote port

Remote IP

ListeningClosed

set local address,

port

set state to

listening

(returns)

bind()

call

(returns)

call

Figure 7.7: Server-side socket setup.

where SYN_SENT is the technical name of the client’s state between the ﬁrst and second messages

of the handshake.

If the server is not accepting connections—say, if there is no program associated with

the given port at the destination—the server-side TCP will respond (immediately) with a rejec-

tion message instead of an acknowledgment, and connect() returns −1 with errno set to

ECONNREFUSED. Otherwise, after the client receives a positive reply from the server, the

netstat output would look like:

Active Internet connections

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 A.B.C.D:P W.X.Y.Z:Q ESTABLISHED

The sequence of events at the server side is rather diﬀerent; we describe it in Figures 7.7,

7.8, and 7.9. The server needs to bind to the particular TCP port known to the client. Typically,

the server speciﬁes only the port number (here, Q) in the bind() call and gives the special

wildcard address INADDR_ANY for the local IP address. In case the server host has more than

one IP address, this technique allows the socket to receive connections addressed to any of its

IP addresses. When the server calls listen(), the state of the socket is changed to “Listening”,

indicating that it is ready to accept new connections. These events are depicted in Figure 7.7.

The output from netstat on the server after this sequence would include a line like:

Active Internet connections

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 0.0.0.0:Q 0.0.0.0:0 LISTENING

7.4 TCP Socket Life Cycle 153

Local port

Local IP

Remote port

Remote IP

Listening

Local port

Local IP

Remote port

Remote IP

Listening

New connections

incoming

connection

request from

A.B.C.D/P

create new socket,

set addresses

continue handshake

Local port

Local IP

Remote port

Remote IP

W.X.Y.Z

A.B.C.D

Connecting

Local port

Local IP

Remote port

Remote IP

W.X.Y.Z

A.B.C.D

Established

completes

handshake

Socket/TCP implementation

Figure 7.8: Incoming connection request processing.

Events of Figure 6.8

Application programSocket/TCP implementation

returns descriptor for

this socket structure

call

accept()

(blocks until new

connection established)

Local port

Local IP

Remote port

Remote IP

Listening

Local port

Local IP

Remote port

Remote IP

Listening

New connections

Local port

Local IP

Remote port

Remote IP

A.B.C.D

W.X.Y.Z

Established

Local port

Local IP

Remote port

Remote IP

Listening

New connections

Remove

socket from

list of new

connections

Figure 7.9: accept().

Note that any client connection request that arrives at the server before the call to

listen() will be rejected, even if it arrives after the call to bind().

The next thing the server does is call accept(), which blocks until a connection with a

client is established. Therefore in Figure 7.8 we focus on the events that occur in the TCP

implementation when a client connection request arrives. Note that everything depicted in

this ﬁgure happens “under the covers” in the TCP implementation.

When the request for a connection arrives from the client, a new socket structure is cre-

ated for the connection. The new socket’s addresses are ﬁlled in based on the arriving packet.

154 Chapter 7: Under the Hood

The packet’s destination Internet address and port (W.X.Y.Z and Q, respectively) become

the socket’s local address and port; the packet’s source address and port (A.B.C.D and P)

become the socket’s remote Internet address and port. Note that the local port number of

the new socket is always the same as that of the listening socket. The new socket’s state is

set to Connecting (actually called SYN_RCVD in the implementation), and it is added to a list

of not-quite-connected sockets associated with the original server socket. Note well that the

original server socket does not change state. At this point the output of netstat should show

both the original, listening socket and the newly created one:

Active Internet connections

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 0.0.0.0:Q 0.0.0.0:0 LISTENING

tcp 0 0 W.X.Y.Z:Q A.B.C.D:P SYN_RCVD

In addition to creating a new underlying socket structure, the server-side TCP implemen-

tation sends an acknowledging TCP handshake message back to the client. However, the server

TCP does not consider the handshake complete until the third message of the three-way hand-

shake is received from the client. When that message eventually arrives, the new structure’s

state is set to “Established”, and it is then (and only then) moved to a list of socket struc-

tures associated with the original socket, which represent established connections ready to be

accepted. (If the third handshake message fails to arrive, eventually the “Connecting” structure

is deleted.) Output from netstat would then include:

Active Internet connections

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 0.0.0.0:Q 0.0.0.0:0 LISTENING

tcp 0 0 W.X.Y.Z:Q A.B.C.D:P ESTABLISHED

Now we can consider (in Figure 7.9) what happens when the server program calls accept().

The call unblocks as soon as there is something in the listening socket’s list of new connections.

(Note that this list may already be nonempty when accept() is called.) At that time, the new

socket structure is removed from the list, and a socket descriptor is allocated and returned as

the result of accept().

It is important to note that each structure in the server socket’s associated list represents

a fully established TCP connection with a client at the other end. Indeed, the client can send

data as soon as it receives the second message of the opening handshake—which may be long

before the server accepts the client connection!

7.4.2 Closing a TCP Connection

TCP has a graceful close mechanism that allows applications to terminate a connection with-

out having to worry about loss of data that might still be in transit. The mechanism is also

designed to allow data transfers in each direction to be terminated independently. It works

like this: the application indicates that it is ﬁnished sending data on a connected socket by

calling close() or shutdown(). At that point, the underlying TCP implementation ﬁrst transmits

7.4 TCP Socket Life Cycle 155

Application programSocket/TCP implementation

(returns immediately)

call

close()

Local port

Local IP

Remote port

Remote IP

A.B.C.D

W.X.Y.Z

Established

Local port

Local IP

Remote port

Remote IP

A.B.C.D

W.X.Y.Z

Closing

Local port

Local IP

Remote port

Remote IP

A.B.C.D

W.X.Y.Z

Half–closed

Local port

Local IP

Remote port

Remote IP

A.B.C.D

W.X.Y.Z

Time–wait

Start close HS;

deallocate

descriptor

Close HS

completes

Close HS

initiated by

remote

completes

Figure 7.10: Closing a TCP connection ﬁrst.

any data remaining in SendQ (subject to available space in RecvQ at the other end), and then

sends a closing TCP handshake message to the other end. This closing handshake message

can be thought of as an end-of-stream marker: it tells the receiving TCP that no more bytes

will be placed in RecvQ . (Note that the closing handshake message itself is not passed to the

receiving application, but that its position in the byte stream is indicated by recv() returning 0.)

The closing TCP waits for an acknowledgment of its closing handshake message, which indi-

cates that all data sent on the connection made it safely to RecvQ . Once that acknowledgment

is received, the connection is “Half closed”. The connection is not completely closed until a

symmetric handshake happens in the other direction—that is, until both ends have indicated

that they have no more data to send.

The closing event sequence in TCP can happen in two ways: either one application calls

close() (or shutdown()) and completes its closing handshake before the other does, or both close

simultaneously, so that their closing handshake messages cross in the network. Figure 7.10

shows the sequence of events in the implementation when the application invokes close()

before the other end closes. The closing handshake (HS) message is sent, the state of the

socket structure is set to “Closing”, and the call returns. After this point, further attempts to

perform any operation on the socket result in error returns. When the acknowledgment for

the close handshake is received, the state changes to “Half closed”, where it remains until the

other end’s close handshake message is received. At this point, the output of netstat on the

client would show the status of the connection as:

Active Internet connections

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 A.B.C.D:P W.X.Y.Z:Q FIN_WAIT_2

156 Chapter 7: Under the Hood

(FIN_WAIT_2 is the technical name for the “half-closed” state at the host that initiates close ﬁrst.

The state denoted by “closing” in the ﬁgure is technically called FIN_WAIT_1, but it is transient

and is diﬃcult to catch with netstat.) Note that if the remote endpoint goes away while the con-

nection is in this state, the local underlying structure will stay around indeﬁnitely. Otherwise,

when the other end’s close handshake message arrives, an acknowledgment is sent and the

state is changed to “Time-Wait”. Although the descriptor in the application program may have

long since been reclaimed (and even reused), the associated underlying socket structure con-

tinues to exist in the implementation for a minute or more; the reasons for this are discussed

at the end of this section.

The output of netstat at the right end of Figure 7.10 includes:

Active Internet connections

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 A.B.C.D:P W.X.Y.Z:Q TIME_WAIT

Figure 7.11 shows the simpler sequence of events at the endpoint that does not close

ﬁrst. When the closing handshake message arrives, an acknowledgment is sent immedi-

ately, and the connection state becomes “Close-Wait.” The output of netstat on this host

shows:

Active Internet connections

Proto Recv-Q Send-Q Local Address Foreign Address State

tcp 0 0 W.X.Y.Z:Q A.B.C.D:P CLOSE_WAIT

Application programSocket/TCP implementation

call

close()

Returns immediately

Local port

Local IP

Remote port

Remote IP

A.B.C.D

W.X.Y.Z

Established

Local port

Local IP

Remote port

Remote IP

A.B.C.D

W.X.Y.Z

Close–wait

Remote initiated

close handshake

completes

Finish close handshake

deallocate socket structure

Figure 7.11: Closing after the other end closes.

7.4 TCP Socket Life Cycle 157

At this point, it’s all over: the implementation is just waiting for the application to call

close(). When it does, the socket descriptor is deallocated and the ﬁnal close handshake is

initiated. When it completes, the underlying socket structure is deallocated.

Although most applications use close(),shutdown() actually provides more ﬂexibility. A

call to close() terminates both directions of transfer and causes the ﬁle descriptor associated

with the socket to be deallocated. Any undelivered data remaining in RecvQ is discarded, and

the ﬂow control mechanism prevents any further transfer of data from the other end’s SendQ.

All trace of the socket disappears from the calling program. Underneath, however, the asso-

ciated socket structure continues to exist until the other end initiates its closing handshake.

In contrast, shutdown() allows the sending and receiving streams to be terminated indepen-

dently. It takes an additional argument, one of SHUT_RD,SHUT_WR,orSHUT_RDWR, indicating which

stream(s) are to be shut down. A program calling shutdown() with second argument SHUT_WR

can continue to receive data on the socket; only sending is prohibited. The fact that the other

end of the connection has closed is indicated by recv() returning 0 (once RecvQ is empty, of

course) to indicate that there will be no more data available on the connection.

In view of the fact that both close() and shutdown() return without waiting for the clos-

ing handshake to complete, you may wonder how the sender can be assured that sent data

has actually made it to the receiving program (i.e., to Delivered). In fact, it is possible for an

application to call close() or shutdown() and have it complete successfully (i.e., not return −1)

while there is still data in SendQ. If either end of the connection then crashes before the data

makes it to RecvQ, data may be lost without the sending application knowing about it!

The best solution is to design the application protocol so that whichever side closes

ﬁrst, does so only after receiving application-level assurance that its data was received. For

example, when our TCPEchoClient.c program receives the same number of bytes as it sent, there

should be nothing more in transit in either direction, so it is safe for it to close the connection.

Note that there is no guarantee that the bytes received were those sent; the client is assuming

that the server implements the echo protocol. In a real application the client should certainly

not trust the server to “do the right thing.”

The other solution is to modify the semantics of close() by setting the SO_LINGER socket

option before calling it. The SO_LINGER option speciﬁes an amount of time for the TCP imple-

mentation to wait for the closing handshake to complete. The setting of SO_LINGER and the

speciﬁcation of the wait time are given to setsockopt() using the linger structure:

struct linger {

int l_onoff; // Nonzero to linger

int l_linger; // Time (secs.) to linger

};

To use the linger behavior, set l_onoff to a nonzero value and specify the time to linger in

l_linger. When SO_LINGER is set, close() blocks until the closing handshake is completed or

until the speciﬁed amount of time passes. If the handshake does not complete in time, an error

indication (ETIMEDOUT) is returned. Thus, if SO_LINGER is set and close() returns no error,

the application is assured that everything it sent reached RecvQ .

158 Chapter 7: Under the Hood

The ﬁnal subtlety of closing a TCP connection revolves around the need for the Time-Wait

state. The TCP speciﬁcation requires that when a connection terminates, at least one of the

sockets persists in the Time-Wait state for a period of time after both closing handshakes

complete. This requirement is motivated by the possibility of messages being delayed in the

network. If both ends’ underlying structures go away as soon as both closing handshakes

complete, and a new connection is immediately established between the same pair of socket

addresses, a message from the previous connection, which happened to be delayed in the

network, could arrive just after the new connection is established. Because it would contain

the same source and destination addresses, the old message could be mistaken for a mes-

sage belonging to the new connection, and its data might (incorrectly) be delivered to the

application.

Unlikely though this scenario may be, TCP employs multiple mechanisms to prevent it,

including the Time-Wait state. The Time-Wait state ensures that every TCP connection ends

with a quiet time, during which no data is sent. The quiet time is supposed to be equal to

twice the maximum amount of time a packet can remain in the network. Thus, by the time a

connection goes away completely (i.e., the socket structure leaves the Time-Wait state and is

deallocated) and clears the way for a new connection between the same pair of addresses, no

messages from the old instance can still be in the network. In practice, the length of the quiet

time is implementation dependent, because there is no real mechanism that limits how long a

packet can be delayed by the network. Values in use range from 4 minutes down to 30 seconds

or even shorter.

The most important consequence of Time-Wait is that as long as the underlying socket

structure exists, no other socket is permitted to bind to the same local port. (More on this

below.)

7.5 Demultiplexing Demystiﬁed

The fact that diﬀerent sockets on the same machine can have the same local address and port

number is implicit in the preceding discussions. For example, on a machine with only one IP

address, every new socket accept()ed via a listening socket will have the same local address

and port number as the listening socket. Clearly, the process of deciding to which socket an

incoming packet should be delivered—that is, the demultiplexing process—involves looking at

more than just the packet’s destination address and port. Otherwise there could be ambiguity

about which socket an incoming packet is intended for. The process of matching an incoming

packet to a socket is actually the same for both TCP and UDP, and can be summarized by the

following points:

The local port in the socket structure must match the destination port number in the

incoming packet.

Any address ﬁelds in the socket structure that contain the wildcard value (*) are

considered to match any value in the corresponding ﬁeld in the packet.

7.5 Demultiplexing Demystiﬁed 159

Local port

Local IP

Remote port

Remote IP

Established

192.168.3.2

30001

172.16.1.9

Local port

Local IP

Remote port

Remote IP

10.1.2.3

Listening

Local port

Local IP

Remote port

Remote IP

Listening

Local port

Local IP

Remote port

Remote IP

Established

10.5.5.8

10.1.2.3

1025

01 2

Figure 7.12: Demultiplexing with multiple matching sockets.

If more than one socket structure matches an incoming packet for all four address ﬁelds,

the one that matches using the fewest wildcards gets the packet.

For example, consider a host with two IP addresses, 10.1.2.3 and 192.168.3.2, and with

a subset of its active TCP socket structures, as shown in Figure 7.12. The structure labeled 0

is associated with a listening socket and has port 99 with a wildcard local address. Socket

structure 1 is also for a listening socket on the same port, but with the local IP address

10.1.2.3 speciﬁed (so it will only accept connection requests to that address). Structure 2 is for

a connection that was accepted via structure 0’s listening socket, and thus has the same local

port number (99), but also has its local and remote Internet addresses ﬁlled in. Other sockets

belong to other active connections. Now consider a packet with source IP address 172.16.1.10,

source port 56789, destination IP address 10.1.2.3, and destination port 99. It will be deliv-

ered to the socket associated with structure 1, because that one matches with the fewest

wildcards.

When a program attempts to bind() to a particular local port number, the existing sockets

are checked to make sure that no socket is already using that local port. The call to bind() will

fail and set EADDRINUSE if any socket matches the local port and local IP address (if any)

speciﬁed in the argument to bind(). This can cause problems in the following scenario:

1. A server’s listening socket is bound to some particular port P.

2. The server accepts a connection from a client, which enters the Established state.

3. The server terminates for some reason—say, because the programmer has created a new

version and wants to test it. When the server program exits, the underlying system auto-

matically (and virtually) calls close() on all of its existing sockets. The socket that was in

the Established state immediately transitions to the Time-Wait state.

4. The programmer starts up a new instance of the server, which attempts to bind() to

port P.

Unfortunately the new server’s call to bind() will fail with EADDRINUSE because of the old

socket in Time-Wait state.

160 Chapter 7: Under the Hood

As of this writing, there are two ways around this. One is to wait until the underlying struc-

ture leaves the Time-Wait state. The other is for the server to set the SO_REUSEADDR socket

option before calling bind(). That lets bind() succeed in spite of the existence of any sockets

representing earlier connections to the server’s port. There is no danger of ambiguity, because

the existing connections (whether still in the Established or Time-Wait state) have remote

addresses ﬁlled in, while the socket being bound does not. In general, the SO_REUSEADDR

option also enables a socket to bind to a local port to which another socket is already bound,

provided that the IP address to which it is being bound (typically the wildcard INADDR_ANY

address) is diﬀerent from that of the existing socket. The default bind() behavior is to disallow

such requests.

Exercises

1. The TCP protocol is designed so that simultaneous connection attempts will succeed.

That is, if an application using port P and Internet address W.X.Y.Z attempts to connect

to address A.B.C.D, port Q, at the same time as an application using the same address

and port tries to connect to W.X.Y.Z, port P, they will end up connected to each other.

Can this be made to happen when the programs use the sockets API?

2. The ﬁrst example of “buﬀer deadlock” in this chapter involves the programs on both

ends of a connection trying to send large messages. However, this is not necessary for

deadlock. How could the TCPEchoClient from earlier chapters be made to deadlock when

it connects to the TCPEchoServer from that chapter?

chapter 8

Socket Programming in C++*

This book is for people who want to understand sockets. It’s for people who want to

know not only how to get a couple of programs to communicate over a network but also how

and why the Sockets API works like it does. Or course, lots of developers use sockets all the

time without really understanding these details. It’s common to use sockets via a library that

oﬀers a simpliﬁed interface to socket creation, name resolution, and message transmission.

This is particularly common in object-oriented languages like C++ and Java, where it’s easy to

wrap socket functionality in a collection of related classes.

The PracticalSocket library was developed to help expose students to the basics of socket

programming without requiring a complete understanding of some of the material covered

elsewhere in this book. This library is typical of object-oriented wrappers around socket func-

tionality; it tries to oﬀer a simple interface to the most commonly used functionality. The

PracticalSocket library provides portability between Windows and UNIX platforms, and it can

serve an instructional purpose since its source code is readily available.

A reader who is more comfortable in C or who prefers to start by understanding what’s

going on underneath should save this chapter for last. It can serve as a summary and appli-

cation of many of the concepts introduced earlier. For a reader who is an experienced C++

programmer or who prefers to learn about sockets more selectively, this chapter can be read

earlier and can serve as an overview of concepts covered in much more detail in earlier chap-

ters. The examples presented here include many pointers to appropriate sections earlier in the

text, so this chapter can serve as an entry point for many other sections of the book.

∗Contributed by David Sturgill

161

162 Chapter 8: Socket Programming in C++

In this chapter, we introduce the PracticalSocket library and demonstrate its use in a

simple application. Through a series of more sophisticated applications, we expose additional

features of the library and demonstrate how PracticalSockets or a similar library might be used

in practice. Both the PracticalSocket library and the example programs used in this chapter to

demonstrate it are available from the Web site for this text.

8.1 PracticalSocket Library Overview

Figure 8.1 illustrates the classes in PracticalSockets and their inheritance relationships. All

the classes ending in “Socket” serve as wrappers around TCP or UDP sockets and provide a

simple interface for creating a socket and using it for communication. The SocketException

class provides support for error handling, and SocketAddress serves as a wrapper around an

address and port number.

We can get started using this library without understanding everything about its classes

and methods all at once. We’ll start by covering just enough to write a simple application. From

there, we can introduce new features gradually.

The TCPSocket class is the basic mechanism for communication over TCP. It is imple-

mented as a wrapper around a TCP socket and serves as an endpoint in a bidirectional

communication channel. If two applications want to communicate, they can each obtain an

instance of TCPSocket. Once these two sockets are connected, sequences of bytes sent from

one can be received at the other.

Functionality for TCPSocket is distributed across the class itself and its two base classes,

CommunicatingSocket and Socket. The Socket class is at the top of the inheritance hierarchy,

and contains only functionality common to all socket wrappers. Among other things, it has

the job of keeping up with the underlying socket descriptor, and it automatically closes its

descriptor when it is destroyed.

The CommunicatingSocket class is an abstraction for a socket that, once connected,

can exchange data with a peer socket. It provides send() and recv() methods that are wrap-

pers around the send() and recv() calls for the underlying socket descriptor. A successful

call to the send() method of a CommunicatingSocket will send the ﬁrst bufferLen bytes

Socket SocketAddress runtime_error

SocketException

iostreamTCPSocketTCPServerSocket

CommunicatingSocket

UDPSocket

Figure 8.1: PracticalSockets class diagram.

8.1 PracticalSocket Library Overview 163

pointed to by buffer to the peer CommunicatingSocket. A call to the recv() method of a

CommunicatingSocket will attempt to read up to bufferLen bytes of data from the peer and

store the result in the memory pointed to by buffer. The recv() method will block until data

is available on the socket, and it will return the number of bytes received on the socket and

written into buffer. After a socket is closed, recv() will return zero to indicate that no more

data can be received.

void CommunicatingSocket::send(const void *buffer,int bufferLen) throw(SocketException)

int CommunicatingSocket::recv(void *buffer,int bufferLen) throw(SocketException)

int CommunicatingSocket::recvFully(void *buffer,int bufferLen) throw(SocketException)

The stream of bytes transmitted between sockets may be fragmented into packets and

reconstituted by buﬀering on its way from the sender to the receiver. A group of bytes sent

in a single call to send() may not all be received in a single, corresponding call to recv().

The recvFully() method is intended to help with this. It works just like recv(), except that

it blocks until either exactly bufferLen bytes are received or the socket is closed. The return

value from recvFully() reports the number of bytes received. Ordinarily, this will be the same

as bufferLen. However, if the socket is closed before all the requested bytes are transmitted, a

value less than bufferLen may be returned.

The PracticalSocket library uses C++ exceptions to report when something goes wrong.

This is evident in the prototypes above. An instance of SocketException is thrown whenever

an error occurs in the library. This exception object inherits from runtime_error,soitcanbe

caught as an instance of SocketException by error-handling code speciﬁc to the communica-

tions portion of an application. A SocketException may be caught as a more general exception

type by more general error-handling code. The what() method of a SocketException returns a

string with a short description of the particular error that occurred.

The way to obtain an instance of TCPSocket depends on the role of the application. To

establish a pair of connected TCPSocket peers, one application must function as a server

and the other as a client. The server listens for new connections by creating an instance

of TCPServerSocket. The other creates a TCPSocket directly. The TCPServerSocket class is

derived from Socket, but not CommunicatingSocket. It is used to establish new TCP socket

connections with client applications, but it is not itself used to send and receive bytes. The

server must construct a TCPServerSocket with an application-deﬁned port number. After-

ward, a call to accept() will block until a client application attempts to connect by creating a

TCPSocket with the same port number. When this happens, accept() will return a pointer to a

new instance of TCPSocket that is connected to the TCPSocket peer on the client.

TCPServerSocket(in_port_t localPort) throw(SocketException)

TCPSocket *TCPServerSocket::accept() throw(SocketException)

164 Chapter 8: Socket Programming in C++

The client application creates its end of the socket connection by simply constructing an

instance of TCPSocket and providing the name or address of the server’s host and the same

port number. Once a pair of connected TCPSocket objects have been created, client and server

can communicate using the send() and recv() methods until one of the endpoints closes its

connection via the destructor or the close() method.

TCPSocket(const char *foreignAddress,in_port_t foreignPort) throw(SocketException)

void Socket::close()

8.2 Plus One Service

The few classes and methods introduced so far are enough to let us implement a simple client

and server similar to the ones in previous chapters. Here, accessing sockets through the Prac-

ticalSocket classes yields somewhat shorter source code that hides many of the details of the

underlying API. The “plus one” service is a client-server application that performs the incre-

ment operation. The client sends an unsigned integer to the server, and the server sends back

a value that is one greater.

8.2.1 Plus One Server

PlusOneServer.cpp is the server portion of the application. It accepts client connections, reads

a 32-bit unsigned integer from each client, increments it, and then sends it back.

PlusOneServer.cpp

1#include <iostream>

2#include "PracticalSocket.h"

4using namespace std;

6int main(int argc, char *argv[]) {

7try {

8// Make a socket to listen for SurveyClient connections.

9TCPServerSocket servSock(9431);

11 for (;;) { // Repeatedly accept connections

12 TCPSocket *sock = servSock.accept(); // Get next client connection

14 uint32_t val; // Read 32-bit int from client

15 if (sock->recvFully(&val, sizeof(val)) == sizeof(val)) {

8.2 Plus One Service 165

16 val = ntohl(val); // Convert to local byte order

17 val++; // Increment the value

18 val = htonl(val); // Convert to network byte order

19 sock->send(&val, sizeof(val)); // Send value back to client

20 }

22 delete sock; // Close and delete TCPSocket

23 }

24 } catch (SocketException &e) {

25 cerr << e.what() << endl; // Report errors to the console

26 }

28 return 0;

29 }

PlusOneServer.cpp

1. Application setup: lines 1–6

Access to the sockets API is hidden behind the PracticalSocket classes. The application

only needs to include the header for the library and any calls we use directly.

2. Error handling: lines 7, 24–26

Error handling is via exceptions. If an error occurs, it is caught at the end of the main

function, and the program prints an error message before terminating.

3. Create a server socket: line 9

The server creates a TCPServerSocket that listens for connections on port 9431. When

constructed like this, the TCPServerSocket automatically makes the calls to socket(),

bind() and listen() described in Sections 2.5, 2.6, and 2.7. If anything goes wrong in one

of these steps, an exception is thrown.

To keep the example simple, the server’s port number is hard-coded in the construc-

tor call. Of course, it would be more maintainable to use a named constant in a header

ﬁle as the port number.

4. Repeatedly accept client connections: lines 11–12

The server repeatedly calls the accept() method to wait for a new client connection. When

a client connects, this method returns a pointer to a new TCPSocket for communicating

with the client. This method is a wrapper around the accept() call described in Section 2.7.

If something goes wrong in accept(), the catch block reports the error and the program

terminates.

5. Read an integer from the client and convert byte order: lines 14–16

Client and server have been written to exchange ﬁxed-sized messages in the form of

unsigned 32-bit integers. The server uses a stack-allocated integer, val, to hold the

received message. Since the server is expecting a 4-byte message, it uses the recvFully()

166 Chapter 8: Socket Programming in C++

method of TCPSocket to read the message directly into the storage for val. If the client

connection is closed before the entire message arrives, recvFully() returns fewer than

the expected number of bytes and the server ignores the message.

Although client and server agree on the size of a message, if they are running on

diﬀerent hosts, they may represent integers using diﬀerent byte orders. To take care of

this possibility, we agree to only transmit values that are in big-endian byte order. The

server uses ntohl() to convert the received integer to local byte order if necessary.

Concerns over reading entire messages and adjusting byte order are really just the

issues of framing and encoding. Chapter 5 focuses speciﬁcally on these topics and oﬀers

a variety of techniques for handling framing and encoding.

6. Increment the client integer and send it back: lines 17–19

The server adds one to the client-provided value and then converts it back to network

byte order. The server sends the value back to the client by sending a copy of the 4 bytes

of memory used to represent val.

7. Close client connection: line 22

When the server destroys its instance of TCPSocket, the socket connection is closed. If

the server had neglected to destroy this object, it would have not only leaked memory but

also leaked an underlying socket descriptor with each client connection. A server with this

type of bug could very quickly reach an operating-system-enforced limit on per-process

resources.

8.2.2 Plus One Client

PlusOneClient.cpp is the client counterpart of the plus one server. It connects to the server,

sends it a copy of an integer value given on the command line, and prints out the value the

server sends back.

PlusOneClient.cpp

1#include <iostream>

2#include <cstdlib>

3#include "PracticalSocket.h"

5using namespace std;

7int main(int argc, char *argv[]) {

8if (argc != 3) { // Check number of parameters

9cerr << "Usage: PlusOneClient <server host> <starting value>" << endl;

10 return 1;

11 }

13 try {

8.2 Plus One Service 167

14 TCPSocket sock(argv[1], 9431); // Connect to the server.

16 uint32_t val = atoi(argv[2]); // Parse user-suppled value

17 val = htonl(val); // Convert to network byte order

18 sock.send(&val, sizeof(val)); // Send to server.

20 // Read the server's response, convert to local byte order and print it

21 if (sock.recvFully(&val, sizeof(val)) == sizeof(val)) {

22 val = ntohl(val);

23 cout << "Server Response: " << val << endl;

24 }

25 // Socket is closed when it goes out of scope

26 } catch(SocketException &e) {

27 cerr << e.what() << endl;

28 }

30 return 0;

31 }

PlusOneClient.cpp

1. Application setup and parameter checking: lines 1–11

The client uses the PracticalSocket classes along with support from a few other header

ﬁles. On the command line, the client expects an address or hostname for the server and

an integer value to send to the server. A usage message is printed if the wrong number

of arguments is given.

2. Error handling: lines 13, 26–28

As in the server, a socket-related exception is caught by code at the end of the program,

and an error message is printed.

3. Connect to the server: line 14

The client creates an instance of TCPSocket, passing in the user-supplied hostname and

the hard-coded port number for the server. This constructor hides a lot of detail in the

underlying sockets API. First, the given hostname is resolved to one or more addresses. As

described in Chapter 3, getaddrinfo() returns all matching addresses for a given host and

port. Some may be IPv4 addresses, and some may be IPv6. The TCPSocket constructor

creates a socket for the ﬁrst address and attempts to connect to it. If this fails, each

successive address is tried until a connection can be established. If no addresses will

connect, an exception is thrown.

4. Parse, convert, and send value to server: lines 16–18

The client parses the user-provided value from the command line, converts it to network

byte order, and sends it to the server by sending the 4 bytes starting at the value’s starting

address.

168 Chapter 8: Socket Programming in C++

5. Receive incremented value, convert, and print: lines 21–24

Using recvFully(), the client blocks until it either receives a 4-byte integer from the server

or the socket connection is closed. If all 4 bytes arrive, the received value is converted to

host byte order and printed.

6. Close the socket: line 25

Since the client’s TCPSocket is allocated on the stack, it is automatically closed and

destroyed when the socket goes out of scope.

8.2.3 Running Server and Client

The executable PlusOneServer requires no command-line arguments. Once compiled, it should

be started from the command prompt and permitted to run for as long as you want to try

it out. While the server is running, you should run a copy of PlusOneClient from a diﬀerent

command prompt, possibly on a diﬀerent host. Supply the name of the server’s host and an

integer value on the command line, and it should, with a little help from the server, produce a

value one larger than the command-line argument. For example, if the server is running on a

host named “venus,” you should be able to run the client as follows:

PlusOneClient connecting to a server on host venus

% PlusOneClient venus 345318

Server Response: 345319

Exercises

1. Instead of simply incrementing the value from a single client, modify the server so that

it will send back the sum of two values supplied by diﬀerent clients. After a ﬁrst client

connects, the server will wait for a second client. The server will read integer values from

both clients and send back the sum of these two values to both.

2. The client and server communicate using binary-encoded, ﬁxed-sized messages. Modify

these programs so that they use variable-length, text-encoded messages as described in

Section 5.2.2. The sender will write its integer value to a character array as a sequence of

ASCII-encoded digits. The receiver will read a copy of this array and parse out the integer.

8.3 Survey Service

Building on the concepts demonstrated in the plus one service and using techniques presented

in Chapters 5 and 6, we can create a distributed application that does something a little more

useful. The survey client and server implement a simple, distributed survey application. At

start-up, the server reads a list of survey questions and responses from a text ﬁle. When a

8.3 Survey Service 169

client connects, the server sends it copies of the questions and response options. The client

prints each question and sends the user’s response back to the server, where it is recorded.

The ﬁle, survey1.txt, is an example survey ﬁle the server might use. The ﬁrst line gives

the number of questions. This is followed by a description for each question. A question is

described by a one-line prompt, followed by a line giving the number of responses and a line

of text for each response. For example, the ﬁrst question on the survey below is, “What is

your favorite ﬂavor of ice cream?” The three possible responses are “Vanilla,” “Chocolate,” or

“Strawberry.” As users take the survey, the server will keep up with how many selected each

of these responses.

survey1.txt

2What is your favorite flavor of ice cream?

4Vanilla

5Chocolate

6Strawberry

7Socket programming is:

9Surprisingly easy

10 Empowering

11 Not for the faint of heart

12 Fun for the whole family

survey1.txt

8.3.1 Survey Support Functions

The client and server depend on common functionality implemented in SurveyCommon.h and

SurveyCommon.cpp. The header ﬁle provides a named constant for the server’s port number, a

Question type for representing survey questions and prototypes for provided functions.

SurveyCommon.h

1#ifndef __SURVEYCOMMON_H__

2#define __SURVEYCOMMON_H__

4#include "PracticalSocket.h"

5#include <string>

6#include <vector>

8/** Port number used by the Survey Server */

170 Chapter 8: Socket Programming in C++

9const in_port_t SURVEY_PORT = 12543;

11 /** Write an encoding of val to the socket, sock. */

12 void sendInt(CommunicatingSocket *sock, uint32_t val) throw(SocketException);

14 /** Write an encoding of str to the socket, sock. */

15 void sendString(CommunicatingSocket *sock, const std::string &str)

16 throw(SocketException);

18 /** Read from sock an integer encoded by sendInt() and return it */

19 uint32_t recvInt(CommunicatingSocket *sock) throw(std::runtime_error);

21 /** Read from sock a string encoded by sendString() and return it */

22 std::string recvString(CommunicatingSocket *sock) throw(std::runtime_error);

24 /** Representation for a survey question */

25 struct Question {

26 std::string qText; // Text of the question.

27 std::vector<std::string> rList; // List of response choices.

28 };

30 /** Read survey questions from the given stream and store them in qList. */

31 bool readSurvey(std::istream &stream, std::vector<Question> &qList);

33 #endif

SurveyCommon.h

SurveyCommon.cpp

1#include "SurveyCommon.h"

3using namespace std;

5void sendInt(CommunicatingSocket *sock, uint32_t val) throw(SocketException) {

6val = htonl(val); // Convert val to network byte order

7sock->send(&val, sizeof(val)); // Send the value through the socket

10 void sendString(CommunicatingSocket *sock, const string &str)

11 throw(SocketException) {

12 sendInt(sock, str.length()); // Send the length of string

13 sock->send(str.c_str(), str.length()); // Send string contents

14 }

8.3 Survey Service 171

16 uint32_t recvInt(CommunicatingSocket *sock) throw(runtime_error) {

17 uint32_t val; // Try to read a 32-bit int into val

18 if (sock->recvFully(&val, sizeof(val)) != sizeof(val))

19 throw runtime_error("Socket closed while reading int");

21 return ntohl(val); // Convert to host byte order, return

22 }

24 string recvString(CommunicatingSocket *sock) throw(runtime_error) {

25 uint32_t len = recvInt(sock); // Read string length

26 char *buffer = new char [len + 1]; // Temp buffer to hold string

27 if (sock->recvFully(buffer, len) != len) { // Try to read whole string

28 delete [] buffer;

29 throw runtime_error("Socket closed while reading string");

30 }

32 buffer[len] = '\0'; // Null terminate the received string

33 string result(buffer); // Convert to an std::string

34 delete [] buffer; // Free temporary buffer

35 return result;

36 }

38 bool readSurvey(istream &stream, std::vector<Question> &qList) {

39 int count = 0;

40 stream >> count; // See how many questions there are

41 stream.ignore(); // Skip past newline

42 qList = vector< Question >(count);

44 // Parse each question.

45 for (unsigned intq=0;q<qList.size(); q++ ) {

46 getline(stream, qList[q].qText); // Get the text of the question

48 count = 0;

49 stream >> count; // Read number of responses

50 stream.ignore(); // Skip past newline

52 // Initialize the response list and populate it.

53 qList[q].rList = vector< string >(count);

54 for (unsigned intr=0;r<qList[q].rList.size(); r++)

55 getline(stream, qList[q].rList[r]);

56 }

58 return stream; // Return true if stream is still good

59 }

SurveyCommon.cpp

172 Chapter 8: Socket Programming in C++

1. Integer encode and decode: lines 5–8, 16–22

In this application, client and server communicate by exchanging values of type integer

and string. Integers are handled much like they are in the plus one service. To encode a

32-bit integer, it is ﬁrst converted to network byte order and then sent out over a socket.

To receive an integer, we attempt to read 4 bytes from the socket and, if successful,

convert them to host byte order and return the result. If 4 bytes can’t be read, an exception

is thrown. Since this type of error is not produced in the PracticalSocket library itself, the

exception is reported as a runtime_error rather than a SocketException.

2. String encode and decode: lines 10–14, 24–36

Encoding and decoding of strings is more complicated because they can be of arbitrary

length. Strings are encoded by ﬁrst sending the string length and following this by the

content of the string. The decoder tries to read both of these values and, if successful,

converts the received string contents to a string object and returns it.

3. Parse survey: lines 38–59

The survey is stored in a text ﬁle. The readSurvey() function reads the survey from the

given input stream and ﬁlls in the given qList parameter with the sequence of questions.

8.3.2 Survey Server

The survey server is responsible for maintaining the list of survey questions, keeping up with

user response totals, and interacting with the client.

The plus one server was able to handle only one client connection at a time. If multiple

clients wanted to use the service, they would have to take turns. This makes sense for a simple

application, where we can expect very short exchanges with each client. However, it may not

work for the survey service. Here, users may deliberate for as long as they want over each

question. If two users want to take the survey at the same time, it isn’t reasonable to make one

wait until the other is ﬁnished.

To interact with more than one client at the same time, the survey server creates a sepa-

rate thread to handle interaction with each client. As in Section 6.4.2, the new thread manages

the session with the client. Each time the server receives a response, it tallies it in its response

count. Mutual exclusion helps the server to make sure two threads don’t modify the response

totals at the same time.

SurveyServer.cpp

1#include <iostream>

2#include <fstream>

3#include <pthread.h>

4#include "PracticalSocket.h"

5#include "SurveyCommon.h"

8.3 Survey Service 173

7using namespace std;

9static vector<Question> qList; // List of survey questions

10 static pthread_mutex_t lock; // Mutex to Protect critical sections

11 static vector<vector<int> > rCount; // Response tallies for each question

13 /** Thread main function to administer a survey over the given socket */

14 static void *conductSurvey(void *arg);

16 int main(int argc, char *argv[]) {

17 if (argc != 2) {

18 cerr << "Usage: SurveyServer <Survey File>" << endl;

19 return 1;

20 }

22 ifstream input(argv[1]); // Read survey from given file

23 if (!input || !readSurvey(input, qList) ) {

24 cerr << "Can't read survey from file: " << argv[1] << endl;

25 return 1;

26 }

28 // Initialize response tally for each question/response.

29 for (unsigned inti=0;i<qList.size(); i++)

30 rCount.push_back(vector<int>(qList[i].rList.size(), 0));

31 pthread_mutex_init(&lock, NULL); // Initialize mutex

33 try {

34 // Make a socket to listen for SurveyClient connections.

35 TCPServerSocket servSock(SURVEY_PORT);

37 for (;;) { // Repeatedly accept connections and administer the survey.

38 TCPSocket *sock = servSock.accept();

40 pthread_t newThread; // Give survey in a separate thread

41 if (pthread_create(&newThread, NULL, conductSurvey, sock) != 0) {

42 cerr << "Can't create new thread" << endl;

43 delete sock;

44 }

45 }

46 } catch (SocketException &e) {

47 cerr << e.what() << endl; // Report errors to the console.

48 }

50 return 0;

51 }

174 Chapter 8: Socket Programming in C++

53 static void *conductSurvey(void *arg) {

54 TCPSocket *sock = (TCPSocket *)arg; // Argument is really a socket

55 try {

56 sendInt(sock, qList.size()); // Tell client no of questions

58 for (unsigned intq=0;q<qList.size(); q++) {

59 // For each question, send the question text and list of responses

60 sendString(sock, qList[q].qText);

61 sendInt(sock, qList[q].rList.size());

62 for (unsigned intr=0;r<qList[q].rList.size(); r++)

63 sendString(sock, qList[q].rList[r]);

65 // Get the client's response and count it if it's in range

66 unsigned int response = recvInt(sock);

67 if (response >= 0 && response < rCount[q].size()) {

68 pthread_mutex_lock(&lock); // Lock the mutex

69 rCount[q][response]++; // Increment count for chosen item

70 pthread_mutex_unlock(&lock); // Release the lock

71 }

72 }

73 } catch (runtime_error e) {

74 cerr << e.what() << endl; // Report errors to the console.

75 }

77 delete sock; // Free the socket object (and close the connection)

78 return NULL;

79 }

SurveyServer.cpp

1. Access to library functions: lines 1–5

The server uses standard C++ I/O facilities and POSIX threads for interacting with

multiple concurrent clients.

2. Survey representation: lines 9–11

The survey is represented as a vector of Question instances. The variable, rCount, keeps

up with user response counts, with each element of rCount corresponding to a survey

question. Since each question has several possible responses, each element of rCount is

avector of totals, one for each response. Each time a user selects a response, the count

for that question and response is incremented.

If multiple clients connect to the server at the same time, it’s possible that two or

more of them will try to increment a counter at the same time. Depending on how this

increment is performed by the hardware, this could leave the count in an unknown state.

For example, an increment performed in one thread might overwrite the result of an

8.3 Survey Service 175

increment that was just completed in another thread. The chances of this happening are

remote, but this possibility should not be left to chance. The lock variable is a mutex that

is used to manage concurrent access to rCount. By using this synchronization object, it

will be easy to make sure only one thread at a time tries to modify rCount.

The server uses ﬁle-scoped variables to keep up with the survey and responses.

This makes it easy to access these data structures from any of our threads. Using the

static modiﬁer prevents these variables from polluting the global namespace, since they

are only visible to a single implementation ﬁle. However, this organization would be an

impediment to code reuse if, say, we wanted to run multiple surveys from the same

application.

3. Initialize server state: lines 17–31

The server parses the survey text from a user-supplied ﬁle. If anything goes wrong during

this process, an error message is printed and the server exits. After the Question list

has been successfully read, we know how many questions and responses there are, so

we initialize the parallel response count representation. The mutex, lock, must also be

initialized before it can be used.

4. Create server socket and repeatedly accept clients: lines 35–45

The server creates a TCPServerSocket listening on an agreed-upon port number. It repeat-

edly waits for client connections and for each connection creates a new thread to handle

interaction with the client. When creating the thread, the server gives a pointer to the

conductSurvey function as the starting point for the new thread and a pointer to the new

TCPSocket as the thread’s argument. From this point on, the new thread is responsi-

ble for the socket object and for interacting with the client. Of course, if a new thread

can’t be created, the server prints an error message and cleans up by deleting the socket

immediately.

5. Survey client handler: lines 53–79

Thread start-up: lines 53–54

The conductSurvey function serves as the main function for each new thread. The thread

will execute this function once it starts up, and it will terminate once it returns out of

the function. The parameter and return types of this function are determined by the

Pthreads API. The void pointers are intended to let us pass pointers to any type of data

we need. Here, we know we passed in a pointer to a TCPSocket instance, so the ﬁrst

thing we do is cast it to a more speciﬁc type to make it easier to use.

Depending on the number of clients connecting, there may be several copies of

conductSurvey running at the same time, each on its own thread. While all of these

threads may be running in the same function, each has its own local variables, and

each is communicating over the instance of TCPSocket that was provided at thread

start-up.

Send each question to client: lines 56–63

The server uses functions provided by SurveyCommon.cpp to simplify interaction

with the client. It sends the client the number of questions in the survey and

176 Chapter 8: Socket Programming in C++

then sends each question and its response list, waiting for a response between

questions.

Get client response and tally it: lines 66–71

The client indicates the user’s chosen response by sending its index back to the server.

Before tallying the response, the server makes sure it’s in the range of legitimate

responses. This check is very important. If the server incremented rCount using an arbi-

trary client-supplied index, it would be giving the client permission to make changes at

unknown memory locations. A malicious client could use this to try to crash the server

or, worse, take control of the server’s host. Of course, we wrote the client and the server

code. You will see that we perform a similar check on the client before we even send

a response, so what’s the point of also performing the check on the server? We’re not

concerned about the behavior of properly functioning clients. We’re more concerned

about what will happen if a malicious user creates a client that doesn’t behave as nicely

as the one we wrote.

If a legitimate response is received, the client thread locks the mutex to make sure

no other threads are trying to modify rCount at the same time. It then increments the

appropriate count and unlocks the mutex right away to let other threads make changes

to rCount as needed. This is typical of the operation of a multithreaded application. In

general, we want to lock out other threads for as short a period as possible. Consider

what would happen if threads locked the mutex once at the start of conductSurvey and

then released it when they were done. This would eliminate the need to lock and unlock

the mutex on every response, but it would completely suppress concurrency; only one

thread at a time could interact with its client.

Error handling: lines 55,73–75

An exception occurring in the client thread will terminate the thread, but the server

will continue to run and accept new connections. This makes sense as such an excep-

tion may simply result from a client terminating unexpectedly. If an exception is

caught during client interaction, execution falls through to the thread exit code. Note

that the exception is caught as the more general type, runtime_exception, since it

may be thrown either by PracticalSockets or by our own recvInt() or recvString()

functions.

Close socket and exit: lines 77–78

Since the thread in conductSurvey is responsible for its dynamically allocated TCPClient

instance, it must free the instance before it exits. Like a C++ stream, deleting the object

automatically closes the underlying socket.

8.3.3 Survey Client

The survey client is not as complicated as its server. Here, we don’t have to worry about

multithreading, concurrency, or maintaining response counts.

8.3 Survey Service 177

SurveyClient.cpp

1#include <iostream>

2#include <iomanip>

3#include "PracticalSocket.h"

4#include "SurveyCommon.h"

6using namespace std;

8int main(int argc, char *argv[]) {

9if (argc != 2) { // Make sure the user gives a host

10 cerr << "Usage: SurveyClient <Survey Server Host>" << endl;

11 return 1;

12 }

14 try {

15 // Connect to the server.

16 TCPSocket sock(argv[1], SURVEY_PORT);

18 // Find out how many questions there are.

19 int qCount = recvInt(&sock);

20 for(intq=0;q<qCount; q++) {

21 // Show each the question to the user and print the list of responses.

22 cout << "Q" << q << ": " << recvString(&sock) << endl;

23 int rCount = recvInt(&sock);

24 for(intr=0;r<rCount; r++)

25 cout << setw(2) << r << " " << recvString(&sock) << endl;

27 // Keep prompting the user until we get a legal response.

28 int response = rCount;

29 while (response<0||response >= rCount) {

30 cout << "> ";

31 cin >> response;

32 }

34 // Send the server the user's response

35 sendInt(&sock, response);

36 }

37 } catch(runtime_error &e) {

38 cerr << e.what() << endl; // Report errors to the console.

39 return 1;

40 }

42 return 0;

43 }

SurveyClient.cpp

178 Chapter 8: Socket Programming in C++

1. Access to library functions: lines 1–4

2. Connect to server: lines 9–16

The client expects the hostname of the server on the command line. If it’s not given,

an error message is printed and the client exits. The client attempts to create a TCP-

Socket object using the hostname and the server’s port number deﬁned in SurveyCommon.h.

If a connection can’t be established, the exception-handling code will report an error

and exit.

3. Receive and print survey questions: lines 19–25

Using functions provided by SurveyCommon.cpp, the client reads the number of questions

and then reads each question and its list of responses.

4. Read user responses and send them to the server: lines 28–35

For each question, the user is prompted for a response. The client checks to make sure

the response is in the proper range before sending it to the server.

8.3.4 Running Server and Client

The SurveyServer requires one command-line argument, the name of the ﬁle containing the

survey questions. For a survey ﬁle like the one above, we can run the server like:

SurveyServer oﬀering questions from survey1.txt

% SurveyServer survey1.txt

The SurveyClient requires one command-line argument, the server’s hostname or address. If

the server is running on a system named “earth,” we could run the client as:

SurveyClient connecting to a server on host earth

% SurveyClient earth

Q0: What is your favorite flavor of ice cream?

0 Vanilla

1 Chocolate

2 Strawberry

From this point, the user can respond to questions by entering the index of the desired

response. The client terminates once all questions have been answered.

8.4 Survey Service, Mark 2

Although it performs a modestly useful function as it is, the survey service is ripe for

enhancement. In this section, we add an administrative interface to report on response counts,

8.4 Survey Service, Mark 2 179

we have the server keep a list of IP addresses of clients completing the survey, and we explore

an alternative technique for encoding and decoding messages between client and server. A few

additional features of the PracticalSocket library are needed to make these enhancements to

the survey service.

8.4.1 Socket Address Support

The PracticalSocket library provides a SocketAddress class that encapsulates an IP address and

a port number. It’s actually just a wrapper around the sockaddr_storage structure introduced

in Section 2.4. Objects of this type are handy for keeping up with the endpoints of a socket

connection, and the library uses them in many diﬀerent places.

Through its constructors and static lookupAddresses() methods, SocketAddress provides

a simple interface to name resolution. An instance of SocketAddress can be constructed with

either an address or a hostname and a port number. Alternatively, the port can be described

using a service name instead of a number. Both of these constructors are wrappers around the

getaddrinfo() function described in Chapter 3. The underlying getaddrinfo() actually returns a

list of matching addresses, and these constructors simply use the ﬁrst address that’s returned.

The static lookupAddresses() methods take the same parameters as these constructors and

return a list of all matching addresses in an STL vector. This could be useful for an application

that needs to choose between IPv4 or IPv6 or when multiple network interfaces are available

on a single host.

SocketAddress(const char *host,in_port_t port) throw(SocketException)

SocketAddress(const char *host,const char *service) throw(SocketException)

static std::vectorSocketAddressSocketAddress::lookupAddresses(const char *host,

in_port_t port) throw(SocketException)

static std::vectorSocketAddresslookupAddresses(const char *host,

const char *service) throw(SocketException)

Instances of SocketAddress can also be obtained by querying the addresses at the

ends of a socket connection. The base class, Socket, provides a getLocalAddress() method

that returns a SocketAddress for the local address to which a socket is bound. In the

case of a TCPServerSocket, this will give the address on which the server is listening,

and in the case of a TCPSocket this will give the address at the local end of the connec-

tion. The CommunicatingSocket class provides a getForeignAddress() method that returns

aSocketAddress for the remote end of a connection. Once obtained, the host address

and port number of a SocketAddress can be examined via the getAddress() and getPort()

methods.

180 Chapter 8: Socket Programming in C++

SocketAddress Socket::getLocalAddress() throw(SocketException)

SocketAddress CommunicatingSocket::getForeignAddress() throw(SocketException)

std::string SocketAddress::getAddress() const throw(SocketException)

in_port_t SocketAddress::getPort() const throw(SocketException)

For the user who wants to take more control over how a socket is set up, bind() and

connect() methods are provided that take an instance of SocketAddress as a parameter.

An application can create a TCPSocket oraTCPServerSocket using the default constructor.

The socket can then be bound to a local address with the bind() method and, in the case

of TCPSocket, connected to a remote server with the connect() method. This lets the pro-

grammer manage socket creation with a level of control more similar to what’s described in

Chapter 2.

void TCPSocket::bind(const SocketAddress &localAddress) throw(SocketException)

void TCPSocket::connect(const SocketAddress &foreignAddress) throw(SocketException)

void TCPServerSocket::bind(const SocketAddress &localAddress) throw(SocketException)

8.4.2 Socket iostream Interface

The send() and recv() methods provided by CommunicatingSocket serve as a very thin veneer

over the send() and recv() functions for the underlying socket. This kind of interface is natural

for some types of messages, but it isn’t the most familiar for handling text-encoded messages,

as described in Section 5.2.2.

The getStream() method of CommunicatingSocket returns a reference to an iostream

that’s backed by the socket. For a C++ programmer, this can serve as a very convenient interface

for reading and writing text-encoded information. It’s a C++ analog to the fdopen() mecha-

nism described in Section 5.1.5. Character sequences written to the iostream will be sent over

the socket, and the sequence of characters received from the network can be read from the

iostream. With this interface, the programmer can use the familiar techniques for working

with istream and ostream to encode and decode messages.

std::iostream &CommunicatingSocket::getStream() throw(SocketException)

The iostream interface to a CommunicatingSocket provides ﬁxed-sized buﬀers for

storing part of the character sequence being sent and received. Text written to the iostream

8.4 Survey Service, Mark 2 181

will be held in memory until the buﬀer is full or until it is ﬂushed. This can oﬀer some per-

formance advantages since a message can be written to the iostream one ﬁeld at a time and

then pushed out to the network after it’s complete. However, this level of buﬀering makes it

problematic to mix I/O operations via the iostream with direct calls to the send() and recv()

methods. Consider, for example, some message Awritten to the socket via its iostream. If the

stream isn’t subsequently ﬂushed, part of message Amay remain buﬀered in the iostream.

If message Bis then sent directly via the socket’s send() method, Bwill be pushed out to the

network before the buﬀered portions of A.

8.4.3 Enhanced Survey Server

The enhanced server uses SocketAddress objects to keep up with a list of addresses for clients

completing the survey. For each client, the server records the host address by saving a copy of

the SocketAddress representing the remote end of the socket.

The server also provides an administrative socket interface for reporting the current state

of the survey. The server restricts access to the administrative interface by only permitting

connections from clients running on the same host. To do this, the server cannot use the

convenience constructor for TCPServerSocket used in previous examples. The server must

create a TCPServerSocket in the unbound state and then explicitly bind it to a SocketAddress

for the loopback interface.

The enhanced survey server makes extensive use of the iostream interface of Commu-

nicatingSocket. This makes the sending and receiving of messages look a lot like reading and

writing a ﬁle. It also permits some additional code reuse between the client and server. Instead

of using a new format to send survey questions to the client, the server encodes them in the

same format as the survey ﬁle it reads at start-up time. The client can use the same readSurvey()

function provided by SurveyCommon.cpp to read the list of survey questions all at once.

SurveyServer2.cpp

1#include <iostream>

2#include <iomanip>

3#include <fstream>

4#include <pthread.h>

5#include "PracticalSocket.h"

6#include "SurveyCommon.h"

8using namespace std;

10 static vector<Question> qList; // List of survey questions

11 static pthread_mutex_t lock; // Mutex to Protect critical sections

12 static vector<vector<int> > rCount; // Response tallies for each question

13 static vector<SocketAddress> addrList; // Address list for client history

182 Chapter 8: Socket Programming in C++

15 /** Thread main function to administer a survey over the given socket */

16 void *conductSurvey(void *arg);

18 /** Thread main function to monitor an administrative connection and

19 give reports over it. */

20 void *adminServer(void *arg);

22 int main(int argc, char *argv[]) {

23 if (argc != 2) {

24 cerr << "Usage: SurveyServer <Survey File>" << endl;

25 return 1;

26 }

28 ifstream input(argv[1]); // Read survey from given file

29 if (!input || !readSurvey(input, qList) ) {

30 cerr << "Can't read survey from file: " << argv[1] << endl;

31 return 1;

32 }

34 // Initialize response tally for each question/response.

35 for (unsigned inti=0;i<qList.size(); i++)

36 rCount.push_back(vector<int>(qList[i].rList.size(), 0));

37 pthread_mutex_init(&lock, NULL); // Initialize mutex

39 try {

40 pthread_t newThread;

42 // Make a thread to provide the administrative interface.

43 if (pthread_create(&newThread, NULL, adminServer, NULL) != 0) {

44 cerr << "Can't create administrative thread" << endl;

45 return 1;

46 }

48 // Make a socket to listen for SurveyClient connections.

49 TCPServerSocket servSock(SURVEY_PORT);

51 for (;;) { // Repeatedly accept connections and administer the survey.

52 TCPSocket *sock = servSock.accept();

53 if (pthread_create(&newThread, NULL, conductSurvey, sock) != 0) {

54 cerr << "Can't create new thread" << endl;

55 delete sock;

56 }

57 }

58 } catch (SocketException &e) {

8.4 Survey Service, Mark 2 183

59 cerr << e.what() << endl; // Report errors to the console.

60 }

62 return 0;

63 }

65 void *conductSurvey(void *arg) {

66 TCPSocket *sock = (TCPSocket *)arg; // Argument is really a socket.

67 try {

68 // Write out the survey in the same format as the input file.

69 iostream &stream = sock->getStream();

70 stream << qList.size() << "\n";

72 for (unsigned intq=0;q<qList.size(); q++) {

73 stream << qList[q].qText << "\n";

74 stream << qList[q].rList.size() << "\n";

75 for (unsigned intr=0;r<qList[q].rList.size(); r++)

76 stream << qList[q].rList[r] << "\n";

77 }

78 stream.flush();

80 // Read client responses to questions and record them

81 for (unsigned intq=0;q<qList.size(); q++) {

82 unsigned int response;

83 stream >> response;

84 if (response >= 0 && response < rCount[q].size()) {

85 pthread_mutex_lock(&lock); // Lock the mutex

86 rCount[q][response]++; // Increment count for chosen item

87 pthread_mutex_unlock(&lock); // Release the lock

88 }

89 }

91 // Log this client as completing the survey.

92 pthread_mutex_lock(&lock);

93 addrList.push_back(sock->getForeignAddress());

94 pthread_mutex_unlock(&lock);

95 } catch (runtime_error e) {

96 cerr << e.what() << endl; // Report errors to the console.

97 }

99 delete sock; // Free the socket object (and close the connection)

100 return NULL;

101 }

102

103 void *adminServer(void *arg) {

184 Chapter 8: Socket Programming in C++

104 try {

105 // Make a ServerSocket to listen for admin connections

106 TCPServerSocket adminSock;

107 adminSock.bind(SocketAddress("127.0.0.1", SURVEY_PORT + 1));

108

109 for (;;) { // Repeatedly accept administrative connections

110 TCPSocket *sock = adminSock.accept();

111 iostream &stream = sock->getStream();

112

113 try {

114 // Copy response counts and address lists

115 pthread_mutex_lock(&lock);

116 vector<vector<int> > myCount = rCount;

117 vector<SocketAddress> myList = addrList;

118 pthread_mutex_unlock(&lock);

119

120 for (unsigned intq=0;q<qList.size(); q++) {

121 // Give a report for each question.

122 stream << "Q" << q << ": " << qList[q].qText << "\n";

123 for (unsigned intr=0;r<qList[q].rList.size(); r++)

124 stream << setw(5) << myCount[q][r] <<""<<qList[q].rList[r]

125 << "\n";

126 }

127

128 // Report the list of client addresses.

129 stream << "Client Addresses:" << endl;

130 for (unsigned intc=0;c<myList.size(); c++ )

131 stream << " " << myList[c].getAddress() << "\n";

132

133 stream.flush();

134 } catch (runtime_error e) {

135 cerr << e.what() << endl;

136 }

137

138 delete sock; // Free the socket object (and close the connection)

139 }

140 } catch (SocketException e) {

141 cerr << e.what() << endl; // Report errors to the console.

142 }

143 return NULL; // Reached only on error

144 }

SurveyServer2.cpp

8.4 Survey Service, Mark 2 185

1. Access to library functions: lines 1–6

2. Survey representation: lines 10–13

This version of the server adds a vector of SocketAddress objects to keep up with the

list of clients completing the survey. Since multiple client threads may attempt to access

this list at the same time, use of the list is protected by the mutex, lock.

3. Initialize server state: lines 23–37

4. Create a thread for the administrative interface: lines 43–46

The server must accept connections over its primary TCPServerSocket and its adminis-

trative TCPServerSocket. If the same thread tried to serve both of these sockets, calling

accept() on one of them would leave it blocked and unable to accept connections from

the other. To serve both, the server creates a new thread to handle connections over its

administrative interface. The adminServer() function implements this interface.

5. Create server socket and handle each client with a new thread: lines 49–57

6. Survey client handler: lines 65–101

Encode survey and send to client: lines 69–78

A client thread ﬁrst writes a copy of the whole survey to the client, using the same

format as the original input ﬁle. It uses getStream() to obtain a copy of the iostream

for its TCPSocket and then writes out the survey as if it were writing to a ﬁle. Instead

of using endl to end each line, we use the newline character. Using endl would ﬂush

the stream after each line. As much as possible, we would like to buﬀer all or large

portions of the message and then send when the message is complete. The server

writes the entire message and then calls the stream’s flush() method to initiate the

sending of any buﬀered data.

Get client responses and tally them: lines 81–89

The client has the entire survey, so the server only needs to read its responses one

after another.

Log the client: lines 92–94

Once the client has completed the survey, the server records a copy of its address.

7. Administrative client handler: lines 103–144

Create administrative socket: lines 106–107

The administrative interface uses a port number oﬀset by one from the survey port.

Since the server is intended to only accept administrative connections from the local

host, we have to go through more explicit steps to construct the TCPServerSocket and

bind it to a local address.

Repeatedly accept administrative connections: lines 109–110

Administrative clients are served with a single thread. If two administrative clients try

to connect, the second will have to wait for the ﬁrst to be served.

186 Chapter 8: Socket Programming in C++

Snapshot reported structures: lines 115–118

Before sending a report, the server makes a copy of the client address history and the

response count lists. This frees the server from having to lock these structures repeat-

edly as it writes out the report, but, more importantly, it provides a consistent snapshot

of the state. The reported list of client addresses won’t grow while the server is printing

it out. If the server had simply locked the mutex for the entire report generation instead

of copying these lists, all client threads would have been blocked while the server tried

to send the report to the administrative client. A malicious administrative client could

fail to read from its socket and suspend the conducting of surveys indeﬁnitely.

Write administrative report: lines 120–133

We make the server responsible for formatting the administrative report. It writes a

tally for each question and response to the socket’s iostream, followed by the host

addresses for every client completing the survey.

8.4.4 Enhanced Survey Client

To communicate with the server using text-encoded messages, the client requires a few

changes. First, instead of reading survey questions individually, it obtains the socket’s iostream

and uses readSurvey() to read the whole survey at once. The client oﬀers the survey to the user

one question at a time and sends corresponding responses by writing to the stream.

SurveyClient2.cpp

1#include <iostream>

2#include <iomanip>

3#include "PracticalSocket.h"

4#include "SurveyCommon.h"

6using namespace std;

8int main(int argc, char *argv[]) {

9if (argc != 2) { // Make sure the user gives a host

10 cerr << "Usage: SurveyClient <Survey Server Host>" << endl;

11 return 1;

12 }

14 try {

15 // Connect to the server.

16 TCPSocket sock(argv[1], SURVEY_PORT);

17 iostream &stream = sock.getStream();

18 vector<Question> qList; // Read the whole survey

19 readSurvey(stream, qList);

8.4 Survey Service, Mark 2 187

21 for (unsigned intq=0;q<qList.size(); q++) {

22 // Show each the question to the user and print the list of responses.

23 cout << "Q" << q << ": " << qList[q].qText << endl;

24 for (unsigned intr=0;r<qList[q].rList.size(); r++)

25 cout << setw(2) << r << " " << qList[q].rList[r] << endl;

27 // Keep prompting the user until we get a legal response.

28 unsigned int response = qList[q].rList.size();

29 while (response<0||response >= qList[q].rList.size()) {

30 cout << "> ";

31 cin >> response;

32 }

34 stream << response << endl; // Send user response to server

35 stream.flush();

36 }

37 } catch(SocketException &e) {

38 cerr << e.what() << endl; // Report errors to the console.

39 return 1;

40 }

42 return 0;

43 }

SurveyClient2.cpp

8.4.5 Administrative Client

A very simple client is suﬃcient to access the administrative interface. Since the server writes

out response counts and address list in a human-readable format, the client can just copy

each byte it receives from the server to the console. The administrative client ﬁrst attempts to

connect to a server running on the local host. Since the client is attempting to connect using

the administrative port number, the server will automatically send it a report and then close

the socket connection.

The client creates a ﬁxed-sized buﬀer and then copies one buﬀer full after another to the

console. Even though the server writes to the socket using its iostream interface, the socket

simply conveys the sequence of bytes written, and the client is free to read the sequence using

the recv() method. In this case, recv() is probably a more convenient interface than iostream

for echoing the server’s output to the console. The client doesn’t have to worry about how the

server’s report is formatted, how many lines it contains, or how long each line is.

The server’s administrative report is just a sequence of printable characters, with no null

terminators anywhere in the report. To print out a buﬀer full of the report as if it was a string,

the client must mark the end of the buﬀer with a null terminator. Since the recv() method

188 Chapter 8: Socket Programming in C++

returns the number of bytes successfully saved in buﬀer, it’s easy to ﬁll in a null terminator at

the end. Of course, the client has to be certain to leave room for this extra character each time

a buﬀer is read. That’s why the client’s call to recv() advertises the buﬀer as one byte shorter

than its actual capacity.

AdminClient2.cpp

1#include <iostream>

2#include "PracticalSocket.h"

3#include "SurveyCommon.h"

5using namespace std;

7int main(int argc, char *argv[]) {

8try {

9// Connect to the server's administrative interface.

10 TCPSocket sock("localhost", SURVEY_PORT + 1);

12 // Read the server's report a block at a time.

13 char buffer[ 1025 ];

14 int len;

15 while ((len = sock.recv(buffer, sizeof(buffer) - 1)) != 0) {

16 buffer[len] = '\0'; // Null terminate the sequence

17 cout << buffer; // And print it like as a string

18 }

19 } catch(SocketException &e) {

20 cerr << e.what() << endl; // Report errors to the console.

21 exit(1);

22 }

24 return 0;

25 }

AdminClient2.cpp

8.4.6 Running Server and Clients

The enhanced survey server and its survey client are run with the same arguments as the

original. The administrative client doesn’t require any arguments since it automatically tries

to connect to an instance of the survey server running on the local host at a known port

number.

Exercises 189

Exercises

1. In the server’s code for the administrative interface, we made an eﬀort to get a consistent

snapshot of the response tally and the client address list before we started generating the

report. However, since the server’s conductSurvey() function records client responses as

soon as they are received, the administrative report may still include results for partially

completed surveys. Modify the server so that responses are tallied only after the client

completes the survey. In this way, both the response tallies and the list of client addresses

will reﬂect only clients that actually completed the survey.

2. As it is written, it’s possible for a client to cause a server crash. If the client closes its

socket connection while the server is sending it a message, the server will receive a SIGPIPE

signal. Consult Section 6.2 and modify the server so that it will not terminate under these

circumstances.

3. The survey server maintains an open socket and a thread on behalf of each client taking a

survey. If a client never completes the survey, the server never reclaims these resources.

Extend the server so that sockets are closed and client threads are permitted to terminate

automatically ﬁve minutes after the client begins taking the survey. When a client thread

starts up, you will also start up another thread that we’ll call the sleeper. The sleeper will

simply wait for ﬁve minutes and then check to see if the client thread is still running. If

it is, the sleeper will close the client’s socket, which will unblock the server thread and

give it a chance to exit. Doing this the right way will require the server to maintain some

additional per-client state and to perform some additional synchronization. For example,

a sleeper thread should not make a call to close() if its client thread has already ﬁnished

and deleted the socket.

References

[1] Comer, Douglas E. Internetworking with TCP/IP, volume 1, Principles, Protocols, and

Architecture (ﬁfth edition). Prentice Hall, 2005.

[2] Comer, Douglas E., and Stevens, David L. Internetworking with TCP/IP, volume 2, Design,

Implementation, and Internals (third edition). Prentice Hall, 1999.

[3] Comer, Douglas E., and Stevens, David L. Internetworking with TCP/IP, volume 3, Client-

Server Programming and Applications (BSD version, second edition). Prentice Hall, 1996.

[4] Hinden, R., and Deering, S. “IP Version 6 Addressing Architecture.” Internet Request for

Comments 4291, February 2006.

[5] Deering, S., and Hinden, R. “Internet Protocol, Version 6 (IPv6) Speciﬁcation.” Internet

Request for Comments 2460, December 1998.

[6] Gilligan, R., Thomson, S., Bound, J., McCann, J., and Stevens, W. “Basic Socket Interface

Extensions for IPv6.” Internet Request for Comments 3493, February 2003.

[7] Srisuresh, P., and Egevang, S. “Traditional IP Network Address Translator (Traditional

NAT).” Internet Request for Comments 3022, January 2001.

[8] Mockapetris, Paul. “Domain Names: Concepts and Facilities.” Internet Request for Com-

ments 1034, November 1987.

[9] Mockapetris, Paul. “Domain Names: Implementation and Speciﬁcation.” Internet Request

for Comments 1035, November 1987.

[10] Peterson, Larry L., and Davie, Bruce S. Computer Networks: A Systems Approach (fourth

edition). Morgan Kaufmann, 2007.

191

192 References

[11] M-K. Shin, et al. “Application Aspects of IPv6 Transition.” Internet Request for Comments

4038, March 2005.

[12] Postel, John. “Internet Protocol.” Internet Request for Comments 791, September 1981.

[13] Postel, John. “Transmission Control Protocol.” Internet Request for Comments 793,

September 1981.

[14] Postel, John. “User Datagram Protocol.” Internet Request for Comments 768, August 1980.

[15] Stevens, W. Richard. TCP/IP Illustrated, volume 1, The Protocols. Addison-Wesley, 1994.

[16] Stevens, W. Richard. UNIX Network Programming: Networking APIs: Sockets and XTI

(second edition). Prentice Hall, 1997.

[17] Wright, Gary R., and Stevens, W. Richard. TCP/IP Illustrated, volume 2, The Implementation.

Addison-Wesley, 1995.

TCP IP Sockets In C, Second Edition Practical Guide For Programmers

Morgan.Kaufmann.TCPIP.Sockets.In.C.Practical.Guide.For.Programmers.2nd.Edition.Mar.2009.ISBN.0123745403

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