Topic : Using Internet Sockets
Author : Beej
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Beej's Guide to Network Programming
Using Internet Sockets
Brian "Beej" Hall

Copyright 1995-2001 by Brian "Beej" Hall

Revision History

Revision Version 1.0.0 August, 1995 Revised by: beej
Initial version.
Revision Version 1.5.5 January 13, 1999 Revised by: beej
Latest HTML version.
Revision Version 2.0.0 March 6, 2001 Revised by: beej
Converted to DocBook XML, corrections, additions.
Revision Version 2.0.1 March 7, 2001 Revised by: beej
Minor corrections.
Revision Version 2.0.2 March 16, 2001 Revised by: beej
inet_ntoa() should have been inet_aton() in some places.
Revision Version 2.0.3 April 2, 2001 Revised by: beej
inet_aton() return values corrected, selectserver changed, typos fixed, added recvtimeout().
Revision Version 2.1.0 May 3, 2001 Revised by: beej
Fixed buffer overruns in client.c and listener.c, made server.c robustly reap zombies, added email policy.
Revision Version 2.2.0 June 24, 2001 Revised by: beej
Updates QnA, Windows, talker.c, PF_INET info, accept() demo, no more bzero()s, Solaris setsockopt().
Revision Version 2.2.1 June 25, 2001 Revised by: beej
Minor QnA update, added Amazon affiliate stuff.
Revision Version 2.3.0 July 25, 2001 Revised by: beej
Added QnA entries, polished selectserver.c, added WSACleanup(), struct in_addr size clarification.

Table of Contents
1. Intro
1.1. Audience
1.2. Platform and Compiler
1.3. Official Homepage
1.4. Note for Solaris/SunOS Programmers
1.5. Note for Windows Programmers
1.6. Email Policy
1.7. Mirroring
1.8. Note for Translators
1.9. Copyright and Distribution
2. What is a socket?
2.1. Two Types of Internet Sockets
2.2. Low level Nonsense and Network Theory
3. structs and Data Handling
3.1. Convert the Natives!
3.2. IP Addresses and How to Deal With Them
4. System Calls or Bust
4.1. socket()--Get the File Descriptor!
4.2. bind()--What port am I on?
4.3. connect()--Hey, you!
4.4. listen()--Will somebody please call me?
4.5. accept()--"Thank you for calling port 3490."
4.6. send() and recv()--Talk to me, baby!
4.7. sendto() and recvfrom()--Talk to me, DGRAM-style
4.8. close() and shutdown()--Get outta my face!
4.9. getpeername()--Who are you?
4.10. gethostname()--Who am I?
4.11. DNS--You say "", I say ""
5. Client-Server Background
5.1. A Simple Stream Server
5.2. A Simple Stream Client
5.3. Datagram Sockets
6. Slightly Advanced Techniques
6.1. Blocking
6.2. select()--Synchronous I/O Multiplexing
6.3. Handling Partial send()s
6.4. Son of Data Encapsulation
7. More References
7.1. man Pages
7.2. Books
7.3. Web References
7.4. RFCs
8. Common Questions
9. Disclaimer and Call for Help

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 structs 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.

1.1. Audience
This document has been written as a tutorial, not a 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 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
This official location of this document is at California State University, Chico, at

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
I have a particular dislike for Windows, and encourage you to try Linux, BSD, or Unix instead. That being said, you can still use this stuff under Windows.

First, ignore pretty much all of the system header files I mention in here. All you need to include is:

    #include <winsock.h>

Wait! You also have to make a call to WSAStartup() before doing anything else with the sockets library. The code to do that looks something like this:

    #include <winsock.h>

        WSADATA wsaData;   // if this doesn't work
        //WSAData wsaData; // then try this instead

        if (WSAStartup(MAKEWORD(1, 1), &wsaData) != 0) {
            fprintf(stderr, "WSAStartup failed.\n");

You also have to tell your compiler to link in the Winsock library, usually called wsock32.lib or winsock32.lib or somesuch. Under VC++, this can be done through the Project menu, under Settings.... Click the Link tab, and look for the box titled "Object/library modules". Add "wsock32.lib" to that list.

Or so I hear.

Finally, you need to call WSACleanup() when you're all through with the sockets library. See your online help for details.

Once you do that, the rest of the examples in this tutorial should generally apply, with a few exceptions. For one thing, you can't use close() to close a socket--you need to use closesocket(), instead. Also, select() only works with socket descriptors, not file descriptors (like 0 for stdin).

To get more information about Winsock, read the Winsock FAQ and go from there.

Finally, I hear that Windows has no fork() system call which is, unfortunately, used in some of my examples. Maybe you have to link in a POSIX library or something to get it to work, or you can use CreateProcess() instead. fork() takes no arguments, and CreateProcess() takes about 48 billion arguments. Welcome to the wonderful world of Win32 programming.

1.6. Email Policy

I'm generally available to help out with email questions so feel free to write in, but I can't guarantee a response. I lead a pretty busy life and there are times when I just can't answer a question you have. When that's the case, I usually just delete the message. It's nothing personal; I just won't ever have the time to give the detailed answer you require.

As a rule, the more complex the question, the less likely I am to respond. If you can narrow down your question before mailing it and be sure to include any pertinent information (like platform, compiler, error messages you're getting, and anything else you think might help me troubleshoot), you're much more likely to get a response.

If not, hack on it some more, try to find the answer, and if it's still elusive, then write me again with the information you've found and hopefully it will be enough for me to help out.

Now that I've badgered you about how to write and not write me, I'd just like to let you know that I fully appreciate all the praise the guide has received over the years. It's a real morale boost, and it gladdens me to hear that it is being used for good! :) Thank you!

1.7. Mirroring
You are more than welcome to mirror this site, whether publically or privately. If you publically mirror the site and want me to link to it from the main page, drop me a line at <>.

1.8. Note for Translators
If you want to translate the guide into another language, write me at <> and I'll link to your translation from the main page.

Feel free to add your name and email address to the translation.

Sorry, but due to space constraints, I cannot host the translations myself.

1.9. Copyright and Distribution
Beej's Guide to Network Programming is Copyright 1995-2001 Brian "Beej" Hall.

This guide may be freely reprinted in any medium provided that its content is not altered, it is presented in its entirety, and this copyright notice remains intact.

Educators are especially encouraged to recommend or supply copies of this guide to their students.

This guide may be freely translated into any language, provided the translation is accurate, and the guide is reprinted in its entirety. The translation may also include the name and contact information for the translator.

The C source code presented in this document is hereby granted to the public domain.

Contact <> for more information.

2. What is a socket?

You hear talk of "sockets" all the time, and perhaps you are wondering just what they are exactly. Well, they're this: a way to speak to other

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