Beej - Guide to Network Programming.pdf

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Beej's Guide to Network Programming
Using Internet Sockets
Brian “Beej Jorgensen” Hall
beej@beej.us
Version 3.0.15
July 3, 2012
Copyright © 2012 Brian “Beej Jorgensen” Hall
Thanks to everyone who has helped in the past and future with me getting this guide written. Thanks to Ashley for
helping me coax the cover design into the best programmer art I could. Thank you to all the people who produce the
Free software and packages that I use to make the Guide: GNU, Linux, Slackware, vim, Python, Inkscape, Apache FOP,
Firefox, Red Hat, and many others. And finally a big thank-you to the literally thousands of you who have written in
with suggestions for improvements and words of encouragement.
I dedicate this guide to some of my biggest heroes and inpirators in the world of computers: Donald Knuth, Bruce
Schneier, W. Richard Stevens, and The Woz, my Readership, and the entire Free and Open Source Software
Community.
This book is written in XML using the vim editor on a Slackware Linux box loaded with GNU tools. The cover “art”
and diagrams are produced with Inkscape. The XML is converted into HTML and XSL-FO by custom Python scripts.
The XSL-FO output is then munged by Apache FOP to produce PDF documents, using Liberation fonts. The toolchain
is composed of 100% Free and Open Source Software.
Unless otherwise mutually agreed by the parties in writing, the author offers the work as-is and makes no representations
or warranties of any kind concerning the work, express, implied, statutory or otherwise, including, without limitation,
warranties of title, merchantibility, fitness for a particular purpose, noninfringement, or the absence of latent or other
defects, accuracy, or the presence of absence of errors, whether or not discoverable.
Except to the extent required by applicable law, in no event will the author be liable to you on any legal theory for any
special, incidental, consequential, punitive or exemplary damages arising out of the use of the work, even if the author
has been advised of the possibility of such damages.
This document is freely distributable under the terms of the Creative Commons Attribution-Noncommercial-No Derivative
Works 3.0 License. See the Copyright and Distribution section for details.
Copyright © 2012 Brian “Beej Jorgensen” Hall
Contents
1. Intro................................................................................................................................................................1
1.1. Audience
1
1.2. Platform and Compiler
1
1.3. Official Homepage and Books For Sale
1
1.4. Note for Solaris/SunOS Programmers
1
1.5. Note for Windows Programmers
1
1.6. Email Policy
3
1.7. Mirroring
3
1.8. Note for Translators
3
1.9. Copyright and Distribution
3
2. What is a socket?......................................................................................................................................... 5
2.1. Two Types of Internet Sockets
5
2.2. Low level Nonsense and Network Theory
6
3. IP Addresses,
struct
s, and Data Munging.............................................................................................. 9
3.1. IP Addresses, versions 4 and 6
9
3.2. Byte Order
11
3.3.
struct
s
12
3.4. IP Addresses, Part Deux
14
4. Jumping from IPv4 to IPv6...................................................................................................................... 17
5. System Calls or Bust.................................................................................................................................. 19
5.1.
getaddrinfo()
—Prepare to launch!
19
5.2.
socket()
—Get the File Descriptor!
22
5.3.
bind()
—What port am I on?
22
5.4.
connect()
—Hey, you!
24
5.5.
listen()
—Will somebody please call me?
25
5.6.
accept()
—“Thank you for calling port 3490.”
25
5.7.
send()
and
recv()
—Talk to me, baby!
26
5.8.
sendto()
and
recvfrom()
—Talk to me, DGRAM-style
27
5.9.
close()
and
shutdown()
—Get outta my face!
28
5.10.
getpeername()
—Who are you?
28
5.11.
gethostname()
—Who am I?
29
6. Client-Server Background......................................................................................................................... 31
6.1. A Simple Stream Server
31
6.2. A Simple Stream Client
33
6.3. Datagram Sockets
35
7. Slightly Advanced Techniques...................................................................................................................39
7.1. Blocking
39
7.2.
select()
—Synchronous I/O Multiplexing
39
7.3. Handling Partial
send()
s
44
7.4. Serialization—How to Pack Data
45
7.5. Son of Data Encapsulation
53
7.6. Broadcast Packets—Hello, World!
55
iii
Contents
8. Common Questions.....................................................................................................................................59
9. Man Pages................................................................................................................................................... 65
9.1.
accept()
66
9.2.
bind()
68
9.3.
connect()
70
9.4.
close()
71
9.5.
getaddrinfo()
,
freeaddrinfo()
,
gai_strerror()
72
9.6.
gethostname()
75
9.7.
gethostbyname()
,
gethostbyaddr()
76
9.8.
getnameinfo()
79
9.9.
getpeername()
80
9.10.
errno
81
9.11.
fcntl()
82
9.12.
htons()
,
htonl()
,
ntohs()
,
ntohl()
83
9.13.
inet_ntoa()
,
inet_aton()
,
inet_addr
85
9.14.
inet_ntop()
,
inet_pton()
87
9.15.
listen()
89
9.16.
perror()
,
strerror()
90
9.17.
poll()
91
9.18.
recv()
,
recvfrom()
93
9.19.
select()
95
9.20.
setsockopt()
,
getsockopt()
97
9.21.
send()
,
sendto()
99
9.22.
shutdown()
101
9.23.
socket()
102
9.24.
struct sockaddr
and pals
103
10. More References..................................................................................................................................... 105
10.1. Books
105
10.2. Web References
105
10.3. RFCs
106
Index
109
iv
1. Intro
Hey! Socket programming got you down? Is this stuff just a little too difficult to figure out from the
man
pages? You want to do cool Internet programming, but you don't have time to wade through a gob of
struct
s trying to figure out if you have to call
bind()
before you
connect()
, etc., etc.
Well, guess what! I've already done this nasty business, and I'm dying to share the information with
everyone! You've come to the right place. This document should give the average competent C programmer
the edge s/he needs to get a grip on this networking noise.
And check it out: I've finally caught up with the future (just in the nick of time, too!) and have updated
the Guide for IPv6! Enjoy!
1.1. Audience
This document has been written as a tutorial, not a complete reference. It is probably at its best when
read by individuals who are just starting out with socket programming and are looking for a foothold. It is
certainly not the
complete and total
guide to sockets programming, by any means.
Hopefully, though, it'll be just enough for those man pages to start making sense...
:-)
1.2. Platform and Compiler
The code contained within this document was compiled on a Linux PC using Gnu's
gcc
compiler.
It should, however, build on just about any platform that uses
gcc.
Naturally, this doesn't apply if you're
programming for Windows—see the section on Windows programming, below.
1.3. Official Homepage and Books For Sale
This official location of this document is
http://beej.us/guide/bgnet/
. There you will also find
example code and translations of the guide into various languages.
To buy nicely bound print copies (some call them “books”), visit
http://beej.us/guide/url/
bgbuy
. I'll appreciate the purchase because it helps sustain my document-writing lifestyle!
1.4. Note for Solaris/SunOS Programmers
When compiling for Solaris or SunOS, you need to specify some extra command-line switches for
linking in the proper libraries. In order to do this, simply add “
-lnsl -lsocket -lresolv
” to the end of
the compile command, like so:
$ cc -o server server.c -lnsl -lsocket -lresolv
If you still get errors, you could try further adding a “
-lxnet
” to the end of that command line. I don't
know what that does, exactly, but some people seem to need it.
Another place that you might find problems is in the call to
setsockopt()
. The prototype differs from
that on my Linux box, so instead of:
int yes=1;
enter this:
char yes='1';
As I don't have a Sun box, I haven't tested any of the above information—it's just what people have told
me through email.
1.5. Note for Windows Programmers
At this point in the guide, historically, I've done a bit of bagging on Windows, simply due to the fact that
I don't like it very much. But I should really be fair and tell you that Windows has a huge install base and is
obviously a perfectly fine operating system.
They say absence makes the heart grow fonder, and in this case, I believe it to be true. (Or maybe it's
age.) But what I can say is that after a decade-plus of not using Microsoft OSes for my personal work, I'm
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