Beej’s Guide to Network Programming
Using Internet Sockets
Brian “Beej Jorgensen” Hall
beej@beej.us
Version 3.0.21
June 8, 2016
Copyright © 2015 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 © 2015 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 2
1.7. Mirroring 3
1.8. Note for Translators 3
1.9. Copyright and Distribution 3
2. What is a socket?………………………………………………………………………………………………………………….. 4
2.1. Two Types of Internet Sockets 4
2.2. Low level Nonsense and Network Theory 5
3. IP Addresses, structs, and Data Munging……………………………………………………………………………. 7
3.1. IP Addresses, versions 4 and 6 7
3.2. Byte Order 9
3.3. structs 10
3.4. IP Addresses, Part Deux 12
4. Jumping from IPv4 to IPv6…………………………………………………………………………………………………. 14
5. System Calls or Bust…………………………………………………………………………………………………………….15
5.1. getaddrinfo()—Prepare to launch! 15
5.2. socket()—Get the File Descriptor! 18
5.3. bind()—What port am I on? 18
5.4. connect()—Hey, you! 20
5.5. listen()—Will somebody please call me? 20
5.6. accept()—“Thank you for calling port 3490.” 21
5.7. send() and recv()—Talk to me, baby! 22
5.8. sendto() and recvfrom()—Talk to me, DGRAM-style 23
5.9. close() and shutdown()—Get outta my face! 23
5.10. getpeername()—Who are you? 24
5.11. gethostname()—Who am I? 24
6. Client-Server Background…………………………………………………………………………………………………….25
6.1. A Simple Stream Server 25
6.2. A Simple Stream Client 27
6.3. Datagram Sockets 29
7. Slightly Advanced Techniques……………………………………………………………………………………………… 33
7.1. Blocking 33
7.2. select()—Synchronous I/O Multiplexing 33
7.3. Handling Partial send()s 38
7.4. Serialization—How to Pack Data 39
7.5. Son of Data Encapsulation 50
7.6. Broadcast Packets—Hello, World! 51
8. Common Questions……………………………………………………………………………………………………………… 55
9. Man Pages…………………………………………………………………………………………………………………………… 60
9.1. accept() 61
iii
Contents
9.2. bind() 63
9.3. connect() 65
9.4. close() 66
9.5. getaddrinfo(), freeaddrinfo(), gai_strerror() 67
9.6. gethostname() 70
9.7. gethostbyname(), gethostbyaddr() 71
9.8. getnameinfo() 73
9.9. getpeername() 74
9.10. errno 75
9.11. fcntl() 76
9.12. htons(), htonl(), ntohs(), ntohl() 77
9.13. inet_ntoa(), inet_aton(), inet_addr 78
9.14. inet_ntop(), inet_pton() 79
9.15. listen() 81
9.16. perror(), strerror() 82
9.17. poll() 83
9.18. recv(), recvfrom() 85
9.19. select() 87
9.20. setsockopt(), getsockopt() 89
9.21. send(), sendto() 91
9.22. shutdown() 93
9.23. socket() 94
9.24. struct sockaddr and pals 95
10. More References………………………………………………………………………………………………………………… 97
10.1. Books 97
10.2. Web References 97
10.3. RFCs 98
Index 100
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 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.
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 much happier! As such, I can sit back and safely say, “Sure, feel free to use Windows!” …Ok yes, it
does make me grit my teeth to say that.
1
http://beej.us/guide/bgnet/
http://beej.us/guide/url/bgbuy
http://beej.us/guide/url/bgbuy
Beej’s Guide to Network Programming 2
So I still encourage you to try Linux1, BSD2, or some flavor of Unix, instead.
But people like what they like, and you Windows folk will be pleased to know that this information
is generally applicable to you guys, with a few minor changes, if any.
One cool thing you can do is install Cygwin3, which is a collection of Unix tools for Windows. I’ve
heard on the grapevine that doing so allows all these programs to compile unmodified.
But some of you might want to do things the Pure Windows Way. That’s very gutsy of you, and
this is what you have to do: run out and get Unix immediately! No, no—I’m kidding. I’m supposed to be
Windows-friendly(er) these days…
This is what you’ll have to do (unless you install Cygwin!): first, ignore pretty much all of the
system header files I mention in here. All you need to include is:
#include
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
{
WSADATA wsaData; // if this doesn’t work
//WSAData wsaData; // then try this instead
// MAKEWORD(1,1) for Winsock 1.1, MAKEWORD(2,0) for Winsock 2.0:
if (WSAStartup(MAKEWORD(1,1), &wsaData) != 0) {
fprintf(stderr, “WSAStartup failed.\n”);
exit(1);
}
You also have to tell your compiler to link in the Winsock library, usually called wsock32.lib
or winsock32.lib, or ws2_32.lib for Winsock 2.0. 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” (or whichever lib is your preference) 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).
There is also a socket class that you can use, CSocket. Check your compilers help pages for more
information.
To get more information about Winsock, read the Winsock FAQ4 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. If you’re not up to that, the CreateThread() is a little easier to digest…unfortunately
a discussion about multithreading is beyond the scope of this document. I can only talk about so much,
you know!
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.
1. http://www.linux.com/
2. http://www.bsd.org/
3. http://www.cygwin.com/
4. http://tangentsoft.net/wskfaq/
http://www.linux.com/
http://www.bsd.org/
http://www.cygwin.com/
http://www.cygwin.com/
http://tangentsoft.net/wskfaq/
Beej’s Guide to Network Programming 3
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. For more pointers, read ESR’s document, How To Ask Questions The
Smart Way5.
If you don’t get a response, 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 publicly or privately. If you publicly mirror
the site and want me to link to it from the main page, drop me a line at beej@beej.us.
1.8. Note for Translators
If you want to translate the guide into another language, write me at beej@beej.us and I’ll link to
your translation from the main page. Feel free to add your name and contact info to the translation.
Please note the license restrictions in the Copyright and Distribution section, below.
If you want me to host the translation, just ask. I’ll also link to it if you want to host it; either way is
fine.
1.9. Copyright and Distribution
Beej’s Guide to Network Programming is Copyright © 2015 Brian “Beej Jorgensen” Hall.
With specific exceptions for source code and translations, below, this work is licensed under the
Creative Commons Attribution- Noncommercial- No Derivative Works 3.0 License. To view a copy of
this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ or send a letter to
Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA.
One specific exception to the “No Derivative Works” portion of the license is as follows: this
guide may be freely translated into any language, provided the translation is accurate, and the guide is
reprinted in its entirety. The same license restrictions apply to the translation as to the original guide. 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, and is
completely free of any license restriction.
Educators are freely encouraged to recommend or supply copies of this guide to their students.
Contact beej@beej.us for more information.
5. http://www.catb.org/~esr/faqs/smart-questions.html
http://www.catb.org/~esr/faqs/smart-questions.html
http://www.catb.org/~esr/faqs/smart-questions.html
http://creativecommons.org/licenses/by-nc-nd/3.0/
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 programs using standard Unix file descriptors.
What?
Ok—you may have heard some Unix hacker state, “Jeez, everything in Unix is a file!” What that
person may have been talking about is the fact that when Unix programs do any sort of I/O, they do it by
reading or writing to a file descriptor. A file descriptor is simply an integer associated with an open file.
But (and here’s the catch), that file can be a network connection, a FIFO, a pipe, a terminal, a real on-
the-disk file, or just about anything else. Everything in Unix is a file! So when you want to communicate
with another program over the Internet you’re gonna do it through a file descriptor, you’d better believe it.
“Where do I get this file descriptor for network communication, Mr. Smarty-Pants?” is probably
the last question on your mind right now, but I’m going to answer it anyway: You make a call to the
socket() system routine. It returns the socket descriptor, and you communicate through it using the
specialized send() and recv() (man send, man recv) socket calls.
“But, hey!” you might be exclaiming right about now. “If it’s a file descriptor, why in the name of
Neptune can’t I just use the normal read() and write() calls to communicate through the socket?” The
short answer is, “You can!” The longer answer is, “You can, but send() and recv() offer much greater
control over your data transmission.”
What next? How about this: there are all kinds of sockets. There are DARPA Internet addresses
(Internet Sockets), path names on a local node (Unix Sockets), CCITT X.25 addresses (X.25 Sockets
that you can safely ignore), and probably many others depending on which Unix flavor you run. This
document deals only with the first: Internet Sockets.
2.1. Two Types of Internet Sockets
What’s this? There are two types of Internet sockets? Yes. Well, no. I’m lying. There are more, but I
didn’t want to scare you. I’m only going to talk about two types here. Except for this sentence, where I’m
going to tell you that “Raw Sockets” are also very powerful and you should look them up.
All right, already. What are the two types? One is “Stream Sockets”; the other is “Datagram
Sockets”, which may hereafter be referred to as “SOCK_STREAM” and “SOCK_DGRAM”, respectively.
Datagram sockets are sometimes called “connectionless sockets”. (Though they can be connect()’d if
you really want. See connect(), below.)
Stream sockets are reliable two-way connected communication streams. If you output two items into
the socket in the order “1, 2”, they will arrive in the order “1, 2” at the opposite end. They will also be
error-free. I’m so certain, in fact, they will be error-free, that I’m just going to put my fingers in my ears
and chant la la la la if anyone tries to claim otherwise.
What uses stream sockets? Well, you may have heard of the telnet application, yes? It uses stream
sockets. All the characters you type need to arrive in the same order you type them, right? Also, web
browsers use the HTTP protocol which uses stream sockets to get pages. Indeed, if you telnet to a web
site on port 80, and type “GET / HTTP/1.0” and hit RETURN twice, it’ll dump the HTML back at you!
How do stream sockets achieve this high level of data transmission quality? They use a protocol
called “The Transmission Control Protocol”, otherwise known as “TCP” (see RFC 7936 for extremely
detailed info on TCP.) TCP makes sure your data arrives sequentially and error-free. You may have heard
“TCP” before as the better half of “TCP/IP” where “IP” stands for “Internet Protocol” (see RFC 7917.)
IP deals primarily with Internet routing and is not generally responsible for data integrity.
Cool. What about Datagram sockets? Why are they called connectionless? What is the deal, here,
anyway? Why are they unreliable? Well, here are some facts: if you send a datagram, it may arrive. It
may arrive out of order. If it arrives, the data within the packet will be error-free.
6. http://tools.ietf.org/html/rfc793
7. http://tools.ietf.org/html/rfc791
4
http://tools.ietf.org/html/rfc793
http://tools.ietf.org/html/rfc791
Beej’s Guide to Network Programming 5
Datagram sockets also use IP for routing, but they don’t use TCP; they use the “User Datagram
Protocol”, or “UDP” (see RFC 7688.)
Why are they connectionless? Well, basically, it’s because you don’t have to maintain an open
connection as you do with stream sockets. You just build a packet, slap an IP header on it with
destination information, and send it out. No connection needed. They are generally used either when
a TCP stack is unavailable or when a few dropped packets here and there don’t mean the end of the
Universe. Sample applications: tftp (trivial file transfer protocol, a little brother to FTP), dhcpcd (a
DHCP client), multiplayer games, streaming audio, video conferencing, etc.
“Wait a minute! tftp and dhcpcd are used to transfer binary applications from one host to another!
Data can’t be lost if you expect the application to work when it arrives! What kind of dark magic is this?”
Well, my human friend, tftp and similar programs have their own protocol on top of UDP. For
example, the tftp protocol says that for each packet that gets sent, the recipient has to send back a packet
that says, “I got it!” (an “ACK” packet.) If the sender of the original packet gets no reply in, say, five
seconds, he’ll re-transmit the packet until he finally gets an ACK. This acknowledgment procedure is
very important when implementing reliable SOCK_DGRAM applications.
For unreliable applications like games, audio, or video, you just ignore the dropped packets, or
perhaps try to cleverly compensate for them. (Quake players will know the manifestation this effect by
the technical term: accursed lag. The word “accursed”, in this case, represents any extremely profane
utterance.)
Why would you use an unreliable underlying protocol? Two reasons: speed and speed. It’s way
faster to fire-and-forget than it is to keep track of what has arrived safely and make sure it’s in order and
all that. If you’re sending chat messages, TCP is great; if you’re sending 40 positional updates per second
of the players in the world, maybe it doesn’t matter so much if one or two get dropped, and UDP is a
good choice.
2.2. Low level Nonsense and Network Theory
Since I just mentioned layering of protocols, it’s time to talk about how networks really work, and
to show some examples of how SOCK_DGRAM packets are built. Practically, you can probably skip this
section. It’s good background, however.
Data Encapsulation.
Hey, kids, it’s time to learn about Data Encapsulation! This is very very important. It’s so important
that you might just learn about it if you take the networks course here at Chico State ;-). Basically, it
says this: a packet is born, the packet is wrapped (“encapsulated”) in a header (and rarely a footer) by
the first protocol (say, the TFTP protocol), then the whole thing (TFTP header included) is encapsulated
again by the next protocol (say, UDP), then again by the next (IP), then again by the final protocol on the
hardware (physical) layer (say, Ethernet).
When another computer receives the packet, the hardware strips the Ethernet header, the kernel
strips the IP and UDP headers, the TFTP program strips the TFTP header, and it finally has the data.
Now I can finally talk about the infamous Layered Network Model (aka “ISO/OSI”). This Network
Model describes a system of network functionality that has many advantages over other models. For
instance, you can write sockets programs that are exactly the same without caring how the data is
physically transmitted (serial, thin Ethernet, AUI, whatever) because programs on lower levels deal with
it for you. The actual network hardware and topology is transparent to the socket programmer.
Without any further ado, I’ll present the layers of the full-blown model. Remember this for network
class exams:
• Application
8. http://tools.ietf.org/html/rfc768
http://tools.ietf.org/html/rfc768
Beej’s Guide to Network Programming 6
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical
The Physical Layer is the hardware (serial, Ethernet, etc.). The Application Layer is just about as far
from the physical layer as you can imagine—it’s the place where users interact with the network.
Now, this model is so general you could probably use it as an automobile repair guide if you really
wanted to. A layered model more consistent with Unix might be:
• Application Layer (telnet, ftp, etc.)
• Host-to-Host Transport Layer (TCP, UDP)
• Internet Layer (IP and routing)
• Network Access Layer (Ethernet, wi-fi, or whatever)
At this point in time, you can probably see how these layers correspond to the encapsulation of the
original data.
See how much work there is in building a simple packet? Jeez! And you have to type in the packet
headers yourself using “cat”! Just kidding. All you have to do for stream sockets is send() the data out.
All you have to do for datagram sockets is encapsulate the packet in the method of your choosing and
sendto() it out. The kernel builds the Transport Layer and Internet Layer on for you and the hardware
does the Network Access Layer. Ah, modern technology.
So ends our brief foray into network theory. Oh yes, I forgot to tell you everything I wanted to say
about routing: nothing! That’s right, I’m not going to talk about it at all. The router strips the packet to
the IP header, consults its routing table, blah blah blah. Check out the IP RFC9 if you really really care. If
you never learn about it, well, you’ll live.
9. http://tools.ietf.org/html/rfc791
http://tools.ietf.org/html/rfc791
3. IP Addresses, structs, and Data Munging
Here’s the part of the game where we get to talk code for a change.
But first, let’s discuss more non-code! Yay! First I want to talk about IP addresses and ports for just
a tad so we have that sorted out. Then we’ll talk about how the sockets API stores and manipulates IP
addresses and other data.
3.1. IP Addresses, versions 4 and 6
In the good old days back when Ben Kenobi was still called Obi Wan Kenobi, there was a
wonderful network routing system called The Internet Protocol Version 4, also called IPv4. It had
addresses made up of four bytes (A.K.A. four “octets”), and was commonly written in “dots and
numbers” form, like so: 192.0.2.111.
You’ve probably seen it around.
In fact, as of this writing, virtually every site on the Internet uses IPv4.
Everyone, including Obi Wan, was happy. Things were great, until some naysayer by the name of
Vint Cerf warned everyone that we were about to run out of IPv4 addresses!
(Besides warning everyone of the Coming IPv4 Apocalypse Of Doom And Gloom, Vint Cerf10 is
also well-known for being The Father Of The Internet. So I really am in no position to second-guess his
judgment.)
Run out of addresses? How could this be? I mean, there are like billions of IP addresses in a 32-bit
IPv4 address. Do we really have billions of computers out there?
Yes.
Also, in the beginning, when there were only a few computers and everyone thought a billion was
an impossibly large number, some big organizations were generously allocated millions of IP addresses
for their own use. (Such as Xerox, MIT, Ford, HP, IBM, GE, AT&T, and some little company called
Apple, to name a few.)
In fact, if it weren’t for several stopgap measures, we would have run out a long time ago.
But now we’re living in an era where we’re talking about every human having an IP address, every
computer, every calculator, every phone, every parking meter, and (why not) every puppy dog, as well.
And so, IPv6 was born. Since Vint Cerf is probably immortal (even if his physical form should pass
on, heaven forbid, he is probably already existing as some kind of hyper-intelligent ELIZA11 program
out in the depths of the Internet2), no one wants to have to hear him say again “I told you so” if we don’t
have enough addresses in the next version of the Internet Protocol.
What does this suggest to you?
That we need a lot more addresses. That we need not just twice as many addresses, not a billion
times as many, not a thousand trillion times as many, but 79 MILLION BILLION TRILLION times as
many possible addresses! That’ll show ’em!
You’re saying, “Beej, is that true? I have every reason to disbelieve large numbers.” Well, the
difference between 32 bits and 128 bits might not sound like a lot; it’s only 96 more bits, right? But
remember, we’re talking powers here: 32 bits represents some 4 billion numbers (232), while 128 bits
represents about 340 trillion trillion trillion numbers (for real, 2128). That’s like a million IPv4 Internets for
every single star in the Universe.
Forget this dots-and-numbers look of IPv4, too; now we’ve got a hexadecimal
representation, with each two-byte chunk separated by a colon, like this:
2001:0db8:c9d2:aee5:73e3:934a:a5ae:9551.
That’s not all! Lots of times, you’ll have an IP address with lots of zeros in it, and you can compress
them between two colons. And you can leave off leading zeros for each byte pair. For instance, each of
these pairs of addresses are equivalent:
2001:0db8:c9d2:0012:0000:0000:0000:0051
2001:db8:c9d2:12::51
10. http://en.wikipedia.org/wiki/Vinton_Cerf
11. http://en.wikipedia.org/wiki/ELIZA
7
http://en.wikipedia.org/wiki/Vinton_Cerf
http://en.wikipedia.org/wiki/ELIZA
Beej’s Guide to Network Programming 8
2001:0db8:ab00:0000:0000:0000:0000:0000
2001:db8:ab00::
0000:0000:0000:0000:0000:0000:0000:0001
::1
The address ::1 is the loopback address. It always means “this machine I’m running on now”. In
IPv4, the loopback address is 127.0.0.1.
Finally, there’s an IPv4-compatibility mode for IPv6 addresses that you might come across. If you
want, for example, to represent the IPv4 address 192.0.2.33 as an IPv6 address, you use the following
notation: “::ffff:192.0.2.33”.
We’re talking serious fun.
In fact, it’s such serious fun, that the Creators of IPv6 have quite cavalierly lopped off trillions and
trillions of addresses for reserved use, but we have so many, frankly, who’s even counting anymore?
There are plenty left over for every man, woman, child, puppy, and parking meter on every planet in the
galaxy. And believe me, every planet in the galaxy has parking meters. You know it’s true.
3.1.1. Subnets
For organizational reasons, it’s sometimes convenient to declare that “this first part of this IP address
up through this bit is the network portion of the IP address, and the remainder is the host portion.
For instance, with IPv4, you might have 192.0.2.12, and we could say that the first three bytes are
the network and the last byte was the host. Or, put another way, we’re talking about host 12 on network
192.0.2.0 (see how we zero out the byte that was the host.)
And now for more outdated information! Ready? In the Ancient Times, there were “classes” of
subnets, where the first one, two, or three bytes of the address was the network part. If you were lucky
enough to have one byte for the network and three for the host, you could have 24 bits-worth of hosts on
your network (16 million or so). That was a “Class A” network. On the opposite end was a “Class C”,
with three bytes of network, and one byte of host (256 hosts, minus a couple that were reserved.)
So as you can see, there were just a few Class As, a huge pile of Class Cs, and some Class Bs in the
middle.
The network portion of the IP address is described by something called the netmask, which you
bitwise-AND with the IP address to get the network number out of it. The netmask usually looks
something like 255.255.255.0. (E.g. with that netmask, if your IP is 192.0.2.12, then your network
is 192.0.2.12 AND 255.255.255.0 which gives 192.0.2.0.)
Unfortunately, it turned out that this wasn’t fine-grained enough for the eventual needs of
the Internet; we were running out of Class C networks quite quickly, and we were most definitely
out of Class As, so don’t even bother to ask. To remedy this, The Powers That Be allowed for the
netmask to be an arbitrary number of bits, not just 8, 16, or 24. So you might have a netmask of, say
255.255.255.252, which is 30 bits of network, and 2 bits of host allowing for four hosts on the
network. (Note that the netmask is ALWAYS a bunch of 1-bits followed by a bunch of 0-bits.)
But it’s a bit unwieldy to use a big string of numbers like 255.192.0.0 as a netmask. First of all,
people don’t have an intuitive idea of how many bits that is, and secondly, it’s really not compact. So the
New Style came along, and it’s much nicer. You just put a slash after the IP address, and then follow that
by the number of network bits in decimal. Like this: 192.0.2.12/30.
Or, for IPv6, something like this: 2001:db8::/32 or 2001:db8:5413:4028::9db9/64.
3.1.2. Port Numbers
If you’ll kindly remember, I presented you earlier with the Layered Network Model which had the
Internet Layer (IP) split off from the Host-to-Host Transport Layer (TCP and UDP). Get up to speed on
that before the next paragraph.
Turns out that besides an IP address (used by the IP layer), there is another address that is used by
TCP (stream sockets) and, coincidentally, by UDP (datagram sockets). It is the port number. It’s a 16-bit
number that’s like the local address for the connection.
Think of the IP address as the street address of a hotel, and the port number as the room number.
That’s a decent analogy; maybe later I’ll come up with one involving the automobile industry.
Beej’s Guide to Network Programming 9
Say you want to have a computer that handles incoming mail AND web services—how do you
differentiate between the two on a computer with a single IP address?
Well, different services on the Internet have different well-known port numbers. You can see them
all in the Big IANA Port List12 or, if you’re on a Unix box, in your /etc/services file. HTTP (the
web) is port 80, telnet is port 23, SMTP is port 25, the game DOOM13 used port 666, etc. and so on. Ports
under 1024 are often considered special, and usually require special OS privileges to use.
And that’s about it!
3.2. Byte Order
By Order of the Realm! There shall be two byte orderings, hereafter to be known as Lame and
Magnificent!
I joke, but one really is better than the other. 🙂
There really is no easy way to say this, so I’ll just blurt it out: your computer might have been
storing bytes in reverse order behind your back. I know! No one wanted to have to tell you.
The thing is, everyone in the Internet world has generally agreed that if you want to represent the
two-byte hex number, say b34f, you’ll store it in two sequential bytes b3 followed by 4f. Makes sense,
and, as Wilford Brimley14 would tell you, it’s the Right Thing To Do. This number, stored with the big
end first, is called Big-Endian.
Unfortunately, a few computers scattered here and there throughout the world, namely anything with
an Intel or Intel-compatible processor, store the bytes reversed, so b34f would be stored in memory as
the sequential bytes 4f followed by b3. This storage method is called Little-Endian.
But wait, I’m not done with terminology yet! The more-sane Big-Endian is also called Network Byte
Order because that’s the order us network types like.
Your computer stores numbers in Host Byte Order. If it’s an Intel 80×86, Host Byte Order is Little-
Endian. If it’s a Motorola 68k, Host Byte Order is Big-Endian. If it’s a PowerPC, Host Byte Order is…
well, it depends!
A lot of times when you’re building packets or filling out data structures you’ll need to make sure
your two- and four-byte numbers are in Network Byte Order. But how can you do this if you don’t know
the native Host Byte Order?
Good news! You just get to assume the Host Byte Order isn’t right, and you always run the value
through a function to set it to Network Byte Order. The function will do the magic conversion if it has to,
and this way your code is portable to machines of differing endianness.
All righty. There are two types of numbers that you can convert: short (two bytes) and long (four
bytes). These functions work for the unsigned variations as well. Say you want to convert a short
from Host Byte Order to Network Byte Order. Start with “h” for “host”, follow it with “to”, then “n” for
“network”, and “s” for “short”: h-to-n-s, or htons() (read: “Host to Network Short”).
It’s almost too easy…
You can use every combination of “n”, “h”, “s”, and “l” you want, not counting the really stupid
ones. For example, there is NOT a stolh() (“Short to Long Host”) function—not at this party, anyway.
But there are:
htons() host to network short
htonl() host to network long
ntohs() network to host short
ntohl() network to host long
Basically, you’ll want to convert the numbers to Network Byte Order before they go out on the wire,
and convert them to Host Byte Order as they come in off the wire.
I don’t know of a 64-bit variant, sorry. And if you want to do floating point, check out the section on
Serialization, far below.
12. http://www.iana.org/assignments/port-numbers
13. http://en.wikipedia.org/wiki/Doom_(video_game)
14. http://en.wikipedia.org/wiki/Wilford_Brimley
http://www.iana.org/assignments/port-numbers
http://en.wikipedia.org/wiki/Doom_(video_game)
http://en.wikipedia.org/wiki/Wilford_Brimley
Beej’s Guide to Network Programming 10
Assume the numbers in this document are in Host Byte Order unless I say otherwise.
3.3. structs
Well, we’re finally here. It’s time to talk about programming. In this section, I’ll cover various data
types used by the sockets interface, since some of them are a real bear to figure out.
First the easy one: a socket descriptor. A socket descriptor is the following type:
int
Just a regular int.
Things get weird from here, so just read through and bear with me.
My First StructTM—struct addrinfo. This structure is a more recent invention, and is used to
prep the socket address structures for subsequent use. It’s also used in host name lookups, and service
name lookups. That’ll make more sense later when we get to actual usage, but just know for now that it’s
one of the first things you’ll call when making a connection.
struct addrinfo {
int ai_flags; // AI_PASSIVE, AI_CANONNAME, etc.
int ai_family; // AF_INET, AF_INET6, AF_UNSPEC
int ai_socktype; // SOCK_STREAM, SOCK_DGRAM
int ai_protocol; // use 0 for “any”
size_t ai_addrlen; // size of ai_addr in bytes
struct sockaddr *ai_addr; // struct sockaddr_in or _in6
char *ai_canonname; // full canonical hostname
struct addrinfo *ai_next; // linked list, next node
};
You’ll load this struct up a bit, and then call getaddrinfo(). It’ll return a pointer to a new linked
list of these structures filled out with all the goodies you need.
You can force it to use IPv4 or IPv6 in the ai_family field, or leave it as AF_UNSPEC to use
whatever. This is cool because your code can be IP version-agnostic.
Note that this is a linked list: ai_next points at the next element—there could be several results
for you to choose from. I’d use the first result that worked, but you might have different business needs; I
don’t know everything, man!
You’ll see that the ai_addr field in the struct addrinfo is a pointer to a struct sockaddr.
This is where we start getting into the nitty-gritty details of what’s inside an IP address structure.
You might not usually need to write to these structures; oftentimes, a call to getaddrinfo() to
fill out your struct addrinfo for you is all you’ll need. You will, however, have to peer inside these
structs to get the values out, so I’m presenting them here.
(Also, all the code written before struct addrinfo was invented we packed all this stuff by hand,
so you’ll see a lot of IPv4 code out in the wild that does exactly that. You know, in old versions of this
guide and so on.)
Some structs are IPv4, some are IPv6, and some are both. I’ll make notes of which are what.
Anyway, the struct sockaddr holds socket address information for many types of sockets.
struct sockaddr {
unsigned short sa_family; // address family, AF_xxx
char sa_data[14]; // 14 bytes of protocol address
};
sa_family can be a variety of things, but it’ll be AF_INET (IPv4) or AF_INET6 (IPv6) for
everything we do in this document. sa_data contains a destination address and port number for the
socket. This is rather unwieldy since you don’t want to tediously pack the address in the sa_data by
hand.
To deal with struct sockaddr, programmers created a parallel structure: struct sockaddr_in
(“in” for “Internet”) to be used with IPv4.
And this is the important bit: a pointer to a struct sockaddr_in can be cast to a pointer to a
struct sockaddr and vice-versa. So even though connect() wants a struct sockaddr*, you can
still use a struct sockaddr_in and cast it at the last minute!
// (IPv4 only–see struct sockaddr_in6 for IPv6)
Beej’s Guide to Network Programming 11
struct sockaddr_in {
short int sin_family; // Address family, AF_INET
unsigned short int sin_port; // Port number
struct in_addr sin_addr; // Internet address
unsigned char sin_zero[8]; // Same size as struct sockaddr
};
This structure makes it easy to reference elements of the socket address. Note that sin_zero
(which is included to pad the structure to the length of a struct sockaddr) should be set to all zeros
with the function memset(). Also, notice that sin_family corresponds to sa_family in a struct
sockaddr and should be set to “AF_INET”. Finally, the sin_port must be in Network Byte Order (by
using htons()!)
Let’s dig deeper! You see the sin_addr field is a struct in_addr. What is that thing? Well, not
to be overly dramatic, but it’s one of the scariest unions of all time:
// (IPv4 only–see struct in6_addr for IPv6)
// Internet address (a structure for historical reasons)
struct in_addr {
uint32_t s_addr; // that’s a 32-bit int (4 bytes)
};
Whoa! Well, it used to be a union, but now those days seem to be gone. Good riddance. So if you
have declared ina to be of type struct sockaddr_in, then ina.sin_addr.s_addr references the
4-byte IP address (in Network Byte Order). Note that even if your system still uses the God-awful union
for struct in_addr, you can still reference the 4-byte IP address in exactly the same way as I did
above (this due to #defines.)
What about IPv6? Similar structs exist for it, as well:
// (IPv6 only–see struct sockaddr_in and struct in_addr for IPv4)
struct sockaddr_in6 {
u_int16_t sin6_family; // address family, AF_INET6
u_int16_t sin6_port; // port number, Network Byte Order
u_int32_t sin6_flowinfo; // IPv6 flow information
struct in6_addr sin6_addr; // IPv6 address
u_int32_t sin6_scope_id; // Scope ID
};
struct in6_addr {
unsigned char s6_addr[16]; // IPv6 address
};
Note that IPv6 has an IPv6 address and a port number, just like IPv4 has an IPv4 address and a port
number.
Also note that I’m not going to talk about the IPv6 flow information or Scope ID fields for the
moment… this is just a starter guide. 🙂
Last but not least, here is another simple structure, struct sockaddr_storage that is designed
to be large enough to hold both IPv4 and IPv6 structures. (See, for some calls, sometimes you don’t know
in advance if it’s going to fill out your struct sockaddr with an IPv4 or IPv6 address. So you pass in
this parallel structure, very similar to struct sockaddr except larger, and then cast it to the type you
need:
struct sockaddr_storage {
sa_family_t ss_family; // address family
// all this is padding, implementation specific, ignore it:
char __ss_pad1[_SS_PAD1SIZE];
int64_t __ss_align;
char __ss_pad2[_SS_PAD2SIZE];
};
Beej’s Guide to Network Programming 12
What’s important is that you can see the address family in the ss_family field—check this to see
if it’s AF_INET or AF_INET6 (for IPv4 or IPv6). Then you can cast it to a struct sockaddr_in or
struct sockaddr_in6 if you wanna.
3.4. IP Addresses, Part Deux
Fortunately for you, there are a bunch of functions that allow you to manipulate IP addresses. No
need to figure them out by hand and stuff them in a long with the << operator.
First, let's say you have a struct sockaddr_in ina, and you have an IP address
“10.12.110.57” or “2001:db8:63b3:1::3490” that you want to store into it. The function you
want to use, inet_pton(), converts an IP address in numbers-and-dots notation into either a struct
in_addr or a struct in6_addr depending on whether you specify AF_INET or AF_INET6. (“pton”
stands for “presentation to network”—you can call it “printable to network” if that's easier to remember.)
The conversion can be made as follows:
struct sockaddr_in sa; // IPv4
struct sockaddr_in6 sa6; // IPv6
inet_pton(AF_INET, "10.12.110.57", &(sa.sin_addr)); // IPv4
inet_pton(AF_INET6, "2001:db8:63b3:1::3490", &(sa6.sin6_addr)); // IPv6
(Quick note: the old way of doing things used a function called inet_addr() or another function
called inet_aton(); these are now obsolete and don't work with IPv6.)
Now, the above code snippet isn't very robust because there is no error checking. See,
inet_pton() returns -1 on error, or 0 if the address is messed up. So check to make sure the result is
greater than 0 before using!
All right, now you can convert string IP addresses to their binary representations. What about the
other way around? What if you have a struct in_addr and you want to print it in numbers-and-
dots notation? (Or a struct in6_addr that you want in, uh, “hex-and-colons” notation.) In this case,
you'll want to use the function inet_ntop() (“ntop” means “network to presentation”—you can call it
“network to printable” if that's easier to remember), like this:
// IPv4:
char ip4[INET_ADDRSTRLEN]; // space to hold the IPv4 string
struct sockaddr_in sa; // pretend this is loaded with something
inet_ntop(AF_INET, &(sa.sin_addr), ip4, INET_ADDRSTRLEN);
printf("The IPv4 address is: %s\n", ip4);
// IPv6:
char ip6[INET6_ADDRSTRLEN]; // space to hold the IPv6 string
struct sockaddr_in6 sa6; // pretend this is loaded with something
inet_ntop(AF_INET6, &(sa6.sin6_addr), ip6, INET6_ADDRSTRLEN);
printf("The address is: %s\n", ip6);
When you call it, you'll pass the address type (IPv4 or IPv6), the address, a pointer to a string
to hold the result, and the maximum length of that string. (Two macros conveniently hold the
size of the string you'll need to hold the largest IPv4 or IPv6 address: INET_ADDRSTRLEN and
INET6_ADDRSTRLEN.)
(Another quick note to mention once again the old way of doing things: the historical function to do
this conversion was called inet_ntoa(). It's also obsolete and won't work with IPv6.)
Lastly, these functions only work with numeric IP addresses—they won't do any nameserver DNS
lookup on a hostname, like “www.example.com”. You will use getaddrinfo() to do that, as you'll see
later on.
Beej's Guide to Network Programming 13
3.4.1. Private (Or Disconnected) Networks
Lots of places have a firewall that hides the network from the rest of the world for their own
protection. And often times, the firewall translates “internal” IP addresses to “external” (that everyone
else in the world knows) IP addresses using a process called Network Address Translation, or NAT.
Are you getting nervous yet? “Where's he going with all this weird stuff?”
Well, relax and buy yourself a non-alcoholic (or alcoholic) drink, because as a beginner, you don't
even have to worry about NAT, since it's done for you transparently. But I wanted to talk about the
network behind the firewall in case you started getting confused by the network numbers you were
seeing.
For instance, I have a firewall at home. I have two static IPv4 addresses allocated to me by the DSL
company, and yet I have seven computers on the network. How is this possible? Two computers can't
share the same IP address, or else the data wouldn't know which one to go to!
The answer is: they don't share the same IP addresses. They are on a private network with 24
million IP addresses allocated to it. They are all just for me. Well, all for me as far as anyone else is
concerned. Here's what's happening:
If I log into a remote computer, it tells me I'm logged in from 192.0.2.33 which is the public
IP address my ISP has provided to me. But if I ask my local computer what its IP address is, it says
10.0.0.5. Who is translating the IP address from one to the other? That's right, the firewall! It's doing
NAT!
10.x.x.x is one of a few reserved networks that are only to be used either on fully disconnected
networks, or on networks that are behind firewalls. The details of which private network numbers are
available for you to use are outlined in RFC 191815, but some common ones you'll see are 10.x.x.x and
192.168.x.x, where x is 0-255, generally. Less common is 172.y.x.x, where y goes between 16 and 31.
Networks behind a NATing firewall don't need to be on one of these reserved networks, but they
commonly are.
(Fun fact! My external IP address isn't really 192.0.2.33. The 192.0.2.x network is reserved for
make-believe “real” IP addresses to be used in documentation, just like this guide! Wowzers!)
IPv6 has private networks, too, in a sense. They'll start with fdxx: (or maybe in the future fcXX:),
as per RFC 419316. NAT and IPv6 don't generally mix, however (unless you're doing the IPv6 to IPv4
gateway thing which is beyond the scope of this document)—in theory you'll have so many addresses
at your disposal that you won't need to use NAT any longer. But if you want to allocate addresses for
yourself on a network that won't route outside, this is how to do it.
15. http://tools.ietf.org/html/rfc1918
16. http://tools.ietf.org/html/rfc4193
http://tools.ietf.org/html/rfc1918
http://tools.ietf.org/html/rfc4193
4. Jumping from IPv4 to IPv6
But I just want to know what to change in my code to get it going with IPv6! Tell me now!
Ok! Ok!
Almost everything in here is something I've gone over, above, but it's the short version for the
impatient. (Of course, there is more than this, but this is what applies to the guide.)
1. First of all, try to use getaddrinfo() to get all the struct sockaddr info, instead of
packing the structures by hand. This will keep you IP version-agnostic, and will eliminate
many of the subsequent steps.
2. Any place that you find you're hard-coding anything related to the IP version, try to wrap up in
a helper function.
3. Change AF_INET to AF_INET6.
4. Change PF_INET to PF_INET6.
5. Change INADDR_ANY assignments to in6addr_any assignments, which are slightly different:
struct sockaddr_in sa;
struct sockaddr_in6 sa6;
sa.sin_addr.s_addr = INADDR_ANY; // use my IPv4 address
sa6.sin6_addr = in6addr_any; // use my IPv6 address
Also, the value IN6ADDR_ANY_INIT can be used as an initializer when the struct
in6_addr is declared, like so:
struct in6_addr ia6 = IN6ADDR_ANY_INIT;
6. Instead of struct sockaddr_in use struct sockaddr_in6, being sure to add “6” to the
fields as appropriate (see structs, above). There is no sin6_zero field.
7. Instead of struct in_addr use struct in6_addr, being sure to add “6” to the fields as
appropriate (see structs, above).
8. Instead of inet_aton() or inet_addr(), use inet_pton().
9. Instead of inet_ntoa(), use inet_ntop().
10. Instead of gethostbyname(), use the superior getaddrinfo().
11. Instead of gethostbyaddr(), use the superior getnameinfo() (although
gethostbyaddr() can still work with IPv6).
12. INADDR_BROADCAST no longer works. Use IPv6 multicast instead.
Et voila!
14
5. System Calls or Bust
This is the section where we get into the system calls (and other library calls) that allow you to
access the network functionality of a Unix box, or any box that supports the sockets API for that matter
(BSD, Windows, Linux, Mac, what-have-you.) When you call one of these functions, the kernel takes
over and does all the work for you automagically.
The place most people get stuck around here is what order to call these things in. In that, the man
pages are no use, as you've probably discovered. Well, to help with that dreadful situation, I've tried to
lay out the system calls in the following sections in exactly (approximately) the same order that you'll
need to call them in your programs.
That, coupled with a few pieces of sample code here and there, some milk and cookies (which I fear
you will have to supply yourself), and some raw guts and courage, and you'll be beaming data around the
Internet like the Son of Jon Postel!
(Please note that for brevity, many code snippets below do not include necessary error checking.
And they very commonly assume that the result from calls to getaddrinfo() succeed and return a
valid entry in the linked list. Both of these situations are properly addressed in the stand-alone programs,
though, so use those as a model.)
5.1. getaddrinfo()—Prepare to launch!
This is a real workhorse of a function with a lot of options, but usage is actually pretty simple. It
helps set up the structs you need later on.
A tiny bit of history: it used to be that you would use a function called gethostbyname() to do
DNS lookups. Then you'd load that information by hand into a struct sockaddr_in, and use that in
your calls.
This is no longer necessary, thankfully. (Nor is it desirable, if you want to write code that works
for both IPv4 and IPv6!) In these modern times, you now have the function getaddrinfo() that does
all kinds of good stuff for you, including DNS and service name lookups, and fills out the structs you
need, besides!
Let's take a look!
#include
#include
#include
int getaddrinfo(const char *node, // e.g. “www.example.com” or IP
const char *service, // e.g. “http” or port number
const struct addrinfo *hints,
struct addrinfo **res);
You give this function three input parameters, and it gives you a pointer to a linked-list, res, of
results.
The node parameter is the host name to connect to, or an IP address.
Next is the parameter service, which can be a port number, like “80”, or the name of a particular
service (found in The IANA Port List17 or the /etc/services file on your Unix machine) like “http” or
“ftp” or “telnet” or “smtp” or whatever.
Finally, the hints parameter points to a struct addrinfo that you’ve already filled out with
relevant information.
Here’s a sample call if you’re a server who wants to listen on your host’s IP address, port 3490. Note
that this doesn’t actually do any listening or network setup; it merely sets up structures we’ll use later:
int status;
struct addrinfo hints;
struct addrinfo *servinfo; // will point to the results
memset(&hints, 0, sizeof hints); // make sure the struct is empty
hints.ai_family = AF_UNSPEC; // don’t care IPv4 or IPv6
17. http://www.iana.org/assignments/port-numbers
15
http://www.iana.org/assignments/port-numbers
Beej’s Guide to Network Programming 16
hints.ai_socktype = SOCK_STREAM; // TCP stream sockets
hints.ai_flags = AI_PASSIVE; // fill in my IP for me
if ((status = getaddrinfo(NULL, “3490”, &hints, &servinfo)) != 0) {
fprintf(stderr, “getaddrinfo error: %s\n”, gai_strerror(status));
exit(1);
}
// servinfo now points to a linked list of 1 or more struct addrinfos
// … do everything until you don’t need servinfo anymore ….
freeaddrinfo(servinfo); // free the linked-list
Notice that I set the ai_family to AF_UNSPEC, thereby saying that I don’t care if we use IPv4 or
IPv6. You can set it to AF_INET or AF_INET6 if you want one or the other specifically.
Also, you’ll see the AI_PASSIVE flag in there; this tells getaddrinfo() to assign the address of
my local host to the socket structures. This is nice because then you don’t have to hardcode it. (Or you
can put a specific address in as the first parameter to getaddrinfo() where I currently have NULL, up
there.)
Then we make the call. If there’s an error (getaddrinfo() returns non-zero), we can print it out
using the function gai_strerror(), as you see. If everything works properly, though, servinfo will
point to a linked list of struct addrinfos, each of which contains a struct sockaddr of some kind
that we can use later! Nifty!
Finally, when we’re eventually all done with the linked list that getaddrinfo() so graciously
allocated for us, we can (and should) free it all up with a call to freeaddrinfo().
Here’s a sample call if you’re a client who wants to connect to a particular server, say
“www.example.net” port 3490. Again, this doesn’t actually connect, but it sets up the structures we’ll use
later:
int status;
struct addrinfo hints;
struct addrinfo *servinfo; // will point to the results
memset(&hints, 0, sizeof hints); // make sure the struct is empty
hints.ai_family = AF_UNSPEC; // don’t care IPv4 or IPv6
hints.ai_socktype = SOCK_STREAM; // TCP stream sockets
// get ready to connect
status = getaddrinfo(“www.example.net”, “3490”, &hints, &servinfo);
// servinfo now points to a linked list of 1 or more struct addrinfos
// etc.
I keep saying that servinfo is a linked list with all kinds of address information. Let’s write a
quick demo program to show off this information. This short program18 will print the IP addresses for
whatever host you specify on the command line:
/*
** showip.c — show IP addresses for a host given on the command line
*/
#include
#include
#include
#include
#include
#include
#include
int main(int argc, char *argv[])
18. http://beej.us/guide/bgnet/examples/showip.c
http://beej.us/guide/bgnet/examples/showip.c
Beej’s Guide to Network Programming 17
{
struct addrinfo hints, *res, *p;
int status;
char ipstr[INET6_ADDRSTRLEN];
if (argc != 2) {
fprintf(stderr,”usage: showip hostname\n”);
return 1;
}
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // AF_INET or AF_INET6 to force version
hints.ai_socktype = SOCK_STREAM;
if ((status = getaddrinfo(argv[1], NULL, &hints, &res)) != 0) {
fprintf(stderr, “getaddrinfo: %s\n”, gai_strerror(status));
return 2;
}
printf(“IP addresses for %s:\n\n”, argv[1]);
for(p = res;p != NULL; p = p->ai_next) {
void *addr;
char *ipver;
// get the pointer to the address itself,
// different fields in IPv4 and IPv6:
if (p->ai_family == AF_INET) { // IPv4
struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
addr = &(ipv4->sin_addr);
ipver = “IPv4”;
} else { // IPv6
struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
addr = &(ipv6->sin6_addr);
ipver = “IPv6″;
}
// convert the IP to a string and print it:
inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);
printf(” %s: %s\n”, ipver, ipstr);
}
freeaddrinfo(res); // free the linked list
return 0;
}
As you see, the code calls getaddrinfo() on whatever you pass on the command line, that
fills out the linked list pointed to by res, and then we can iterate over the list and print stuff out or do
whatever.
(There’s a little bit of ugliness there where we have to dig into the different types of struct
sockaddrs depending on the IP version. Sorry about that! I’m not sure of a better way around it.)
Sample run! Everyone loves screenshots:
$ showip www.example.net
IP addresses for www.example.net:
IPv4: 192.0.2.88
$ showip ipv6.example.com
IP addresses for ipv6.example.com:
IPv4: 192.0.2.101
IPv6: 2001:db8:8c00:22::171
Now that we have that under control, we’ll use the results we get from getaddrinfo() to pass to
other socket functions and, at long last, get our network connection established! Keep reading!
Beej’s Guide to Network Programming 18
5.2. socket()—Get the File Descriptor!
I guess I can put it off no longer—I have to talk about the socket() system call. Here’s the
breakdown:
#include
#include
int socket(int domain, int type, int protocol);
But what are these arguments? They allow you to say what kind of socket you want (IPv4 or IPv6,
stream or datagram, and TCP or UDP).
It used to be people would hardcode these values, and you can absolutely still do that. (domain
is PF_INET or PF_INET6, type is SOCK_STREAM or SOCK_DGRAM, and protocol can be set to 0 to
choose the proper protocol for the given type. Or you can call getprotobyname() to look up the
protocol you want, “tcp” or “udp”.)
(This PF_INET thing is a close relative of the AF_INET that you can use when initializing the
sin_family field in your struct sockaddr_in. In fact, they’re so closely related that they actually
have the same value, and many programmers will call socket() and pass AF_INET as the first argument
instead of PF_INET. Now, get some milk and cookies, because it’s times for a story. Once upon a time,
a long time ago, it was thought that maybe an address family (what the “AF” in “AF_INET” stands
for) might support several protocols that were referred to by their protocol family (what the “PF” in
“PF_INET” stands for). That didn’t happen. And they all lived happily ever after, The End. So the most
correct thing to do is to use AF_INET in your struct sockaddr_in and PF_INET in your call to
socket().)
Anyway, enough of that. What you really want to do is use the values from the results of the call to
getaddrinfo(), and feed them into socket() directly like this:
int s;
struct addrinfo hints, *res;
// do the lookup
// [pretend we already filled out the “hints” struct]
getaddrinfo(“www.example.com”, “http”, &hints, &res);
// [again, you should do error-checking on getaddrinfo(), and walk
// the “res” linked list looking for valid entries instead of just
// assuming the first one is good (like many of these examples do.)
// See the section on client/server for real examples.]
s = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
socket() simply returns to you a socket descriptor that you can use in later system calls, or -1 on
error. The global variable errno is set to the error’s value (see the errno man page for more details, and
a quick note on using errno in multithreaded programs.)
Fine, fine, fine, but what good is this socket? The answer is that it’s really no good by itself, and you
need to read on and make more system calls for it to make any sense.
5.3. bind()—What port am I on?
Once you have a socket, you might have to associate that socket with a port on your local machine.
(This is commonly done if you’re going to listen() for incoming connections on a specific port—
multiplayer network games do this when they tell you to “connect to 192.168.5.10 port 3490”.) The port
number is used by the kernel to match an incoming packet to a certain process’s socket descriptor. If
you’re going to only be doing a connect() (because you’re the client, not the server), this is probably be
unnecessary. Read it anyway, just for kicks.
Here is the synopsis for the bind() system call:
#include
#include
int bind(int sockfd, struct sockaddr *my_addr, int addrlen);
Beej’s Guide to Network Programming 19
sockfd is the socket file descriptor returned by socket(). my_addr is a pointer to a struct
sockaddr that contains information about your address, namely, port and IP address. addrlen is the
length in bytes of that address.
Whew. That’s a bit to absorb in one chunk. Let’s have an example that binds the socket to the host
the program is running on, port 3490:
struct addrinfo hints, *res;
int sockfd;
// first, load up address structs with getaddrinfo():
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // use IPv4 or IPv6, whichever
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE; // fill in my IP for me
getaddrinfo(NULL, “3490”, &hints, &res);
// make a socket:
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
// bind it to the port we passed in to getaddrinfo():
bind(sockfd, res->ai_addr, res->ai_addrlen);
By using the AI_PASSIVE flag, I’m telling the program to bind to the IP of the host it’s running on.
If you want to bind to a specific local IP address, drop the AI_PASSIVE and put an IP address in for the
first argument to getaddrinfo().
bind() also returns -1 on error and sets errno to the error’s value.
Lots of old code manually packs the struct sockaddr_in before calling bind(). Obviously this
is IPv4-specific, but there’s really nothing stopping you from doing the same thing with IPv6, except that
using getaddrinfo() is going to be easier, generally. Anyway, the old code looks something like this:
// !!! THIS IS THE OLD WAY !!!
int sockfd;
struct sockaddr_in my_addr;
sockfd = socket(PF_INET, SOCK_STREAM, 0);
my_addr.sin_family = AF_INET;
my_addr.sin_port = htons(MYPORT); // short, network byte order
my_addr.sin_addr.s_addr = inet_addr(“10.12.110.57”);
memset(my_addr.sin_zero, ‘\0′, sizeof my_addr.sin_zero);
bind(sockfd, (struct sockaddr *)&my_addr, sizeof my_addr);
In the above code, you could also assign INADDR_ANY to the s_addr field if you wanted to bind to
your local IP address (like the AI_PASSIVE flag, above.) The IPv6 version of INADDR_ANY is a global
variable in6addr_any that is assigned into the sin6_addr field of your struct sockaddr_in6.
(There is also a macro IN6ADDR_ANY_INIT that you can use in a variable initializer.)
Another thing to watch out for when calling bind(): don’t go underboard with your port numbers.
All ports below 1024 are RESERVED (unless you’re the superuser)! You can have any port number
above that, right up to 65535 (provided they aren’t already being used by another program.)
Sometimes, you might notice, you try to rerun a server and bind() fails, claiming “Address already
in use.” What does that mean? Well, a little bit of a socket that was connected is still hanging around in
the kernel, and it’s hogging the port. You can either wait for it to clear (a minute or so), or add code to
your program allowing it to reuse the port, like this:
int yes=1;
//char yes=’1’; // Solaris people use this
// lose the pesky “Address already in use” error message
Beej’s Guide to Network Programming 20
if (setsockopt(listener,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof yes) == -1) {
perror(“setsockopt”);
exit(1);
}
One small extra final note about bind(): there are times when you won’t absolutely have to call it.
If you are connect()ing to a remote machine and you don’t care what your local port is (as is the case
with telnet where you only care about the remote port), you can simply call connect(), it’ll check to
see if the socket is unbound, and will bind() it to an unused local port if necessary.
5.4. connect()—Hey, you!
Let’s just pretend for a few minutes that you’re a telnet application. Your user commands you (just
like in the movie TRON) to get a socket file descriptor. You comply and call socket(). Next, the user
tells you to connect to “10.12.110.57” on port “23” (the standard telnet port.) Yow! What do you do
now?
Lucky for you, program, you’re now perusing the section on connect()—how to connect to a
remote host. So read furiously onward! No time to lose!
The connect() call is as follows:
#include
#include
int connect(int sockfd, struct sockaddr *serv_addr, int addrlen);
sockfd is our friendly neighborhood socket file descriptor, as returned by the socket() call,
serv_addr is a struct sockaddr containing the destination port and IP address, and addrlen is the
length in bytes of the server address structure.
All of this information can be gleaned from the results of the getaddrinfo() call, which rocks.
Is this starting to make more sense? I can’t hear you from here, so I’ll just have to hope that it is.
Let’s have an example where we make a socket connection to “www.example.com”, port 3490:
struct addrinfo hints, *res;
int sockfd;
// first, load up address structs with getaddrinfo():
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
getaddrinfo(“www.example.com”, “3490”, &hints, &res);
// make a socket:
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
// connect!
connect(sockfd, res->ai_addr, res->ai_addrlen);
Again, old-school programs filled out their own struct sockaddr_ins to pass to connect().
You can do that if you want to. See the similar note in the bind() section, above.
Be sure to check the return value from connect()—it’ll return -1 on error and set the variable
errno.
Also, notice that we didn’t call bind(). Basically, we don’t care about our local port number; we
only care where we’re going (the remote port). The kernel will choose a local port for us, and the site we
connect to will automatically get this information from us. No worries.
5.5. listen()—Will somebody please call me?
Ok, time for a change of pace. What if you don’t want to connect to a remote host. Say, just for
kicks, that you want to wait for incoming connections and handle them in some way. The process is two
step: first you listen(), then you accept() (see below.)
The listen call is fairly simple, but requires a bit of explanation:
Beej’s Guide to Network Programming 21
int listen(int sockfd, int backlog);
sockfd is the usual socket file descriptor from the socket() system call. backlog is the number
of connections allowed on the incoming queue. What does that mean? Well, incoming connections are
going to wait in this queue until you accept() them (see below) and this is the limit on how many can
queue up. Most systems silently limit this number to about 20; you can probably get away with setting it
to 5 or 10.
Again, as per usual, listen() returns -1 and sets errno on error.
Well, as you can probably imagine, we need to call bind() before we call listen() so that the
server is running on a specific port. (You have to be able to tell your buddies which port to connect to!)
So if you’re going to be listening for incoming connections, the sequence of system calls you’ll make is:
getaddrinfo();
socket();
bind();
listen();
/* accept() goes here */
I’ll just leave that in the place of sample code, since it’s fairly self-explanatory. (The code in the
accept() section, below, is more complete.) The really tricky part of this whole sha-bang is the call to
accept().
5.6. accept()—“Thank you for calling port 3490.”
Get ready—the accept() call is kinda weird! What’s going to happen is this: someone far far
away will try to connect() to your machine on a port that you are listen()ing on. Their connection
will be queued up waiting to be accept()ed. You call accept() and you tell it to get the pending
connection. It’ll return to you a brand new socket file descriptor to use for this single connection! That’s
right, suddenly you have two socket file descriptors for the price of one! The original one is still listening
for more new connections, and the newly created one is finally ready to send() and recv(). We’re
there!
The call is as follows:
#include
#include
int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
sockfd is the listen()ing socket descriptor. Easy enough. addr will usually be a pointer to a
local struct sockaddr_storage. This is where the information about the incoming connection will
go (and with it you can determine which host is calling you from which port). addrlen is a local integer
variable that should be set to sizeof(struct sockaddr_storage) before its address is passed to
accept(). accept() will not put more than that many bytes into addr. If it puts fewer in, it’ll change
the value of addrlen to reflect that.
Guess what? accept() returns -1 and sets errno if an error occurs. Betcha didn’t figure that.
Like before, this is a bunch to absorb in one chunk, so here’s a sample code fragment for your
perusal:
#include
#include
#include
#include
#define MYPORT “3490” // the port users will be connecting to
#define BACKLOG 10 // how many pending connections queue will hold
int main(void)
{
struct sockaddr_storage their_addr;
socklen_t addr_size;
struct addrinfo hints, *res;
int sockfd, new_fd;
// !! don’t forget your error checking for these calls !!
Beej’s Guide to Network Programming 22
// first, load up address structs with getaddrinfo():
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // use IPv4 or IPv6, whichever
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE; // fill in my IP for me
getaddrinfo(NULL, MYPORT, &hints, &res);
// make a socket, bind it, and listen on it:
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
bind(sockfd, res->ai_addr, res->ai_addrlen);
listen(sockfd, BACKLOG);
// now accept an incoming connection:
addr_size = sizeof their_addr;
new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &addr_size);
// ready to communicate on socket descriptor new_fd!
.
.
.
Again, note that we will use the socket descriptor new_fd for all send() and recv() calls. If
you’re only getting one single connection ever, you can close() the listening sockfd in order to
prevent more incoming connections on the same port, if you so desire.
5.7. send() and recv()—Talk to me, baby!
These two functions are for communicating over stream sockets or connected datagram sockets. If
you want to use regular unconnected datagram sockets, you’ll need to see the section on sendto() and
recvfrom(), below.
The send() call:
int send(int sockfd, const void *msg, int len, int flags);
sockfd is the socket descriptor you want to send data to (whether it’s the one returned by
socket() or the one you got with accept().) msg is a pointer to the data you want to send, and len
is the length of that data in bytes. Just set flags to 0. (See the send() man page for more information
concerning flags.)
Some sample code might be:
char *msg = “Beej was here!”;
int len, bytes_sent;
.
.
.
len = strlen(msg);
bytes_sent = send(sockfd, msg, len, 0);
.
.
.
send() returns the number of bytes actually sent out—this might be less than the number you told
it to send! See, sometimes you tell it to send a whole gob of data and it just can’t handle it. It’ll fire off
as much of the data as it can, and trust you to send the rest later. Remember, if the value returned by
send() doesn’t match the value in len, it’s up to you to send the rest of the string. The good news is this:
if the packet is small (less than 1K or so) it will probably manage to send the whole thing all in one go.
Again, -1 is returned on error, and errno is set to the error number.
The recv() call is similar in many respects:
int recv(int sockfd, void *buf, int len, int flags);
Beej’s Guide to Network Programming 23
sockfd is the socket descriptor to read from, buf is the buffer to read the information into, len is
the maximum length of the buffer, and flags can again be set to 0. (See the recv() man page for flag
information.)
recv() returns the number of bytes actually read into the buffer, or -1 on error (with errno set,
accordingly.)
Wait! recv() can return 0. This can mean only one thing: the remote side has closed the connection
on you! A return value of 0 is recv()’s way of letting you know this has occurred.
There, that was easy, wasn’t it? You can now pass data back and forth on stream sockets! Whee!
You’re a Unix Network Programmer!
5.8. sendto() and recvfrom()—Talk to me, DGRAM-style
“This is all fine and dandy,” I hear you saying, “but where does this leave me with unconnected
datagram sockets?” No problemo, amigo. We have just the thing.
Since datagram sockets aren’t connected to a remote host, guess which piece of information we need
to give before we send a packet? That’s right! The destination address! Here’s the scoop:
int sendto(int sockfd, const void *msg, int len, unsigned int flags,
const struct sockaddr *to, socklen_t tolen);
As you can see, this call is basically the same as the call to send() with the addition of two other
pieces of information. to is a pointer to a struct sockaddr (which will probably be another struct
sockaddr_in or struct sockaddr_in6 or struct sockaddr_storage that you cast at the last
minute) which contains the destination IP address and port. tolen, an int deep-down, can simply be set
to sizeof *to or sizeof(struct sockaddr_storage).
To get your hands on the destination address structure, you’ll probably either get it from
getaddrinfo(), or from recvfrom(), below, or you’ll fill it out by hand.
Just like with send(), sendto() returns the number of bytes actually sent (which, again, might be
less than the number of bytes you told it to send!), or -1 on error.
Equally similar are recv() and recvfrom(). The synopsis of recvfrom() is:
int recvfrom(int sockfd, void *buf, int len, unsigned int flags,
struct sockaddr *from, int *fromlen);
Again, this is just like recv() with the addition of a couple fields. from is a pointer to a local
struct sockaddr_storage that will be filled with the IP address and port of the originating machine.
fromlen is a pointer to a local int that should be initialized to sizeof *from or sizeof(struct
sockaddr_storage). When the function returns, fromlen will contain the length of the address
actually stored in from.
recvfrom() returns the number of bytes received, or -1 on error (with errno set accordingly.)
So, here’s a question: why do we use struct sockaddr_storage as the socket type? Why not
struct sockaddr_in? Because, you see, we want to not tie ourselves down to IPv4 or IPv6. So we
use the generic struct sockaddr_storage which we know will be big enough for either.
(So… here’s another question: why isn’t struct sockaddr itself big enough for any address?
We even cast the general-purpose struct sockaddr_storage to the general-purpose struct
sockaddr! Seems extraneous and redundant, huh. The answer is, it just isn’t big enough, and I’d guess
that changing it at this point would be Problematic. So they made a new one.)
Remember, if you connect() a datagram socket, you can then simply use send() and recv()
for all your transactions. The socket itself is still a datagram socket and the packets still use UDP, but the
socket interface will automatically add the destination and source information for you.
5.9. close() and shutdown()—Get outta my face!
Whew! You’ve been send()ing and recv()ing data all day long, and you’ve had it. You’re ready
to close the connection on your socket descriptor. This is easy. You can just use the regular Unix file
descriptor close() function:
close(sockfd);
This will prevent any more reads and writes to the socket. Anyone attempting to read or write the
socket on the remote end will receive an error.
Beej’s Guide to Network Programming 24
Just in case you want a little more control over how the socket closes, you can use the shutdown()
function. It allows you to cut off communication in a certain direction, or both ways (just like close()
does.) Synopsis:
int shutdown(int sockfd, int how);
sockfd is the socket file descriptor you want to shutdown, and how is one of the following:
0 Further receives are disallowed
1 Further sends are disallowed
2 Further sends and receives are disallowed (like close())
shutdown() returns 0 on success, and -1 on error (with errno set accordingly.)
If you deign to use shutdown() on unconnected datagram sockets, it will simply make the socket
unavailable for further send() and recv() calls (remember that you can use these if you connect()
your datagram socket.)
It’s important to note that shutdown() doesn’t actually close the file descriptor—it just changes its
usability. To free a socket descriptor, you need to use close().
Nothing to it.
(Except to remember that if you’re using Windows and Winsock that you should call
closesocket() instead of close().)
5.10. getpeername()—Who are you?
This function is so easy.
It’s so easy, I almost didn’t give it its own section. But here it is anyway.
The function getpeername() will tell you who is at the other end of a connected stream socket.
The synopsis:
#include
int getpeername(int sockfd, struct sockaddr *addr, int *addrlen);
sockfd is the descriptor of the connected stream socket, addr is a pointer to a struct sockaddr
(or a struct sockaddr_in) that will hold the information about the other side of the connection,
and addrlen is a pointer to an int, that should be initialized to sizeof *addr or sizeof(struct
sockaddr).
The function returns -1 on error and sets errno accordingly.
Once you have their address, you can use inet_ntop(), getnameinfo(), or gethostbyaddr()
to print or get more information. No, you can’t get their login name. (Ok, ok. If the other computer is
running an ident daemon, this is possible. This, however, is beyond the scope of this document. Check
out RFC 141319 for more info.)
5.11. gethostname()—Who am I?
Even easier than getpeername() is the function gethostname(). It returns the name of the
computer that your program is running on. The name can then be used by gethostbyname(), below, to
determine the IP address of your local machine.
What could be more fun? I could think of a few things, but they don’t pertain to socket
programming. Anyway, here’s the breakdown:
#include
int gethostname(char *hostname, size_t size);
The arguments are simple: hostname is a pointer to an array of chars that will contain the hostname
upon the function’s return, and size is the length in bytes of the hostname array.
The function returns 0 on successful completion, and -1 on error, setting errno as usual.
19. http://tools.ietf.org/html/rfc1413
http://tools.ietf.org/html/rfc1413
6. Client-Server Background
It’s a client-server world, baby. Just about everything on the network deals with client processes
talking to server processes and vice-versa. Take telnet, for instance. When you connect to a remote host
on port 23 with telnet (the client), a program on that host (called telnetd, the server) springs to life. It
handles the incoming telnet connection, sets you up with a login prompt, etc.
Client-Server Interaction.
The exchange of information between client and server is summarized in the above diagram.
Note that the client-server pair can speak SOCK_STREAM, SOCK_DGRAM, or anything else (as long as
they’re speaking the same thing.) Some good examples of client-server pairs are telnet/telnetd, ftp/ftpd,
or Firefox/Apache. Every time you use ftp, there’s a remote program, ftpd, that serves you.
Often, there will only be one server on a machine, and that server will handle multiple clients
using fork(). The basic routine is: server will wait for a connection, accept() it, and fork() a child
process to handle it. This is what our sample server does in the next section.
6.1. A Simple Stream Server
All this server does is send the string “Hello, world!” out over a stream connection. All you
need to do to test this server is run it in one window, and telnet to it from another with:
$ telnet remotehostname 3490
where remotehostname is the name of the machine you’re running it on.
The server code20:
/*
** server.c — a stream socket server demo
*/
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#define PORT “3490” // the port users will be connecting to
#define BACKLOG 10 // how many pending connections queue will hold
void sigchld_handler(int s)
{
// waitpid() might overwrite errno, so we save and restore it:
int saved_errno = errno;
20. http://beej.us/guide/bgnet/examples/server.c
25
http://beej.us/guide/bgnet/examples/server.c
Beej’s Guide to Network Programming 26
while(waitpid(-1, NULL, WNOHANG) > 0);
errno = saved_errno;
}
// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
if (sa->sa_family == AF_INET) {
return &(((struct sockaddr_in*)sa)->sin_addr);
}
return &(((struct sockaddr_in6*)sa)->sin6_addr);
}
int main(void)
{
int sockfd, new_fd; // listen on sock_fd, new connection on new_fd
struct addrinfo hints, *servinfo, *p;
struct sockaddr_storage their_addr; // connector’s address information
socklen_t sin_size;
struct sigaction sa;
int yes=1;
char s[INET6_ADDRSTRLEN];
int rv;
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE; // use my IP
if ((rv = getaddrinfo(NULL, PORT, &hints, &servinfo)) != 0) {
fprintf(stderr, “getaddrinfo: %s\n”, gai_strerror(rv));
return 1;
}
// loop through all the results and bind to the first we can
for(p = servinfo; p != NULL; p = p->ai_next) {
if ((sockfd = socket(p->ai_family, p->ai_socktype,
p->ai_protocol)) == -1) {
perror(“server: socket”);
continue;
}
if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes,
sizeof(int)) == -1) {
perror(“setsockopt”);
exit(1);
}
if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
close(sockfd);
perror(“server: bind”);
continue;
}
break;
}
freeaddrinfo(servinfo); // all done with this structure
if (p == NULL) {
fprintf(stderr, “server: failed to bind\n”);
exit(1);
}
Beej’s Guide to Network Programming 27
if (listen(sockfd, BACKLOG) == -1) {
perror(“listen”);
exit(1);
}
sa.sa_handler = sigchld_handler; // reap all dead processes
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_RESTART;
if (sigaction(SIGCHLD, &sa, NULL) == -1) {
perror(“sigaction”);
exit(1);
}
printf(“server: waiting for connections…\n”);
while(1) { // main accept() loop
sin_size = sizeof their_addr;
new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
if (new_fd == -1) {
perror(“accept”);
continue;
}
inet_ntop(their_addr.ss_family,
get_in_addr((struct sockaddr *)&their_addr),
s, sizeof s);
printf(“server: got connection from %s\n”, s);
if (!fork()) { // this is the child process
close(sockfd); // child doesn’t need the listener
if (send(new_fd, “Hello, world!”, 13, 0) == -1)
perror(“send”);
close(new_fd);
exit(0);
}
close(new_fd); // parent doesn’t need this
}
return 0;
}
In case you’re curious, I have the code in one big main() function for (I feel) syntactic clarity. Feel
free to split it into smaller functions if it makes you feel better.
(Also, this whole sigaction() thing might be new to you—that’s ok. The code that’s there is
responsible for reaping zombie processes that appear as the fork()ed child processes exit. If you make
lots of zombies and don’t reap them, your system administrator will become agitated.)
You can get the data from this server by using the client listed in the next section.
6.2. A Simple Stream Client
This guy’s even easier than the server. All this client does is connect to the host you specify on the
command line, port 3490. It gets the string that the server sends.
The client source21:
/*
** client.c — a stream socket client demo
*/
#include
#include
#include
#include
#include
21. http://beej.us/guide/bgnet/examples/client.c
http://beej.us/guide/bgnet/examples/client.c
Beej’s Guide to Network Programming 28
#include
#include
#include
#include
#include
#define PORT “3490” // the port client will be connecting to
#define MAXDATASIZE 100 // max number of bytes we can get at once
// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
if (sa->sa_family == AF_INET) {
return &(((struct sockaddr_in*)sa)->sin_addr);
}
return &(((struct sockaddr_in6*)sa)->sin6_addr);
}
int main(int argc, char *argv[])
{
int sockfd, numbytes;
char buf[MAXDATASIZE];
struct addrinfo hints, *servinfo, *p;
int rv;
char s[INET6_ADDRSTRLEN];
if (argc != 2) {
fprintf(stderr,”usage: client hostname\n”);
exit(1);
}
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
if ((rv = getaddrinfo(argv[1], PORT, &hints, &servinfo)) != 0) {
fprintf(stderr, “getaddrinfo: %s\n”, gai_strerror(rv));
return 1;
}
// loop through all the results and connect to the first we can
for(p = servinfo; p != NULL; p = p->ai_next) {
if ((sockfd = socket(p->ai_family, p->ai_socktype,
p->ai_protocol)) == -1) {
perror(“client: socket”);
continue;
}
if (connect(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
close(sockfd);
perror(“client: connect”);
continue;
}
break;
}
if (p == NULL) {
fprintf(stderr, “client: failed to connect\n”);
return 2;
}
inet_ntop(p->ai_family, get_in_addr((struct sockaddr *)p->ai_addr),
s, sizeof s);
Beej’s Guide to Network Programming 29
printf(“client: connecting to %s\n”, s);
freeaddrinfo(servinfo); // all done with this structure
if ((numbytes = recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) {
perror(“recv”);
exit(1);
}
buf[numbytes] = ‘\0’;
printf(“client: received ‘%s’\n”,buf);
close(sockfd);
return 0;
}
Notice that if you don’t run the server before you run the client, connect() returns “Connection
refused”. Very useful.
6.3. Datagram Sockets
We’ve already covered the basics of UDP datagram sockets with our discussion of sendto() and
recvfrom(), above, so I’ll just present a couple of sample programs: talker.c and listener.c.
listener sits on a machine waiting for an incoming packet on port 4950. talker sends a packet to that
port, on the specified machine, that contains whatever the user enters on the command line.
Here is the source for listener.c22:
/*
** listener.c — a datagram sockets “server” demo
*/
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#define MYPORT “4950” // the port users will be connecting to
#define MAXBUFLEN 100
// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
if (sa->sa_family == AF_INET) {
return &(((struct sockaddr_in*)sa)->sin_addr);
}
return &(((struct sockaddr_in6*)sa)->sin6_addr);
}
int main(void)
{
int sockfd;
struct addrinfo hints, *servinfo, *p;
int rv;
int numbytes;
struct sockaddr_storage their_addr;
22. http://beej.us/guide/bgnet/examples/listener.c
http://beej.us/guide/bgnet/examples/listener.c
Beej’s Guide to Network Programming 30
char buf[MAXBUFLEN];
socklen_t addr_len;
char s[INET6_ADDRSTRLEN];
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // set to AF_INET to force IPv4
hints.ai_socktype = SOCK_DGRAM;
hints.ai_flags = AI_PASSIVE; // use my IP
if ((rv = getaddrinfo(NULL, MYPORT, &hints, &servinfo)) != 0) {
fprintf(stderr, “getaddrinfo: %s\n”, gai_strerror(rv));
return 1;
}
// loop through all the results and bind to the first we can
for(p = servinfo; p != NULL; p = p->ai_next) {
if ((sockfd = socket(p->ai_family, p->ai_socktype,
p->ai_protocol)) == -1) {
perror(“listener: socket”);
continue;
}
if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
close(sockfd);
perror(“listener: bind”);
continue;
}
break;
}
if (p == NULL) {
fprintf(stderr, “listener: failed to bind socket\n”);
return 2;
}
freeaddrinfo(servinfo);
printf(“listener: waiting to recvfrom…\n”);
addr_len = sizeof their_addr;
if ((numbytes = recvfrom(sockfd, buf, MAXBUFLEN-1 , 0,
(struct sockaddr *)&their_addr, &addr_len)) == -1) {
perror(“recvfrom”);
exit(1);
}
printf(“listener: got packet from %s\n”,
inet_ntop(their_addr.ss_family,
get_in_addr((struct sockaddr *)&their_addr),
s, sizeof s));
printf(“listener: packet is %d bytes long\n”, numbytes);
buf[numbytes] = ‘\0’;
printf(“listener: packet contains \”%s\”\n”, buf);
close(sockfd);
return 0;
}
Notice that in our call to getaddrinfo() we’re finally using SOCK_DGRAM. Also, note that there’s
no need to listen() or accept(). This is one of the perks of using unconnected datagram sockets!
Next comes the source for talker.c23:
/*
23. http://beej.us/guide/bgnet/examples/talker.c
http://beej.us/guide/bgnet/examples/talker.c
Beej’s Guide to Network Programming 31
** talker.c — a datagram “client” demo
*/
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#define SERVERPORT “4950” // the port users will be connecting to
int main(int argc, char *argv[])
{
int sockfd;
struct addrinfo hints, *servinfo, *p;
int rv;
int numbytes;
if (argc != 3) {
fprintf(stderr,”usage: talker hostname message\n”);
exit(1);
}
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_DGRAM;
if ((rv = getaddrinfo(argv[1], SERVERPORT, &hints, &servinfo)) != 0) {
fprintf(stderr, “getaddrinfo: %s\n”, gai_strerror(rv));
return 1;
}
// loop through all the results and make a socket
for(p = servinfo; p != NULL; p = p->ai_next) {
if ((sockfd = socket(p->ai_family, p->ai_socktype,
p->ai_protocol)) == -1) {
perror(“talker: socket”);
continue;
}
break;
}
if (p == NULL) {
fprintf(stderr, “talker: failed to create socket\n”);
return 2;
}
if ((numbytes = sendto(sockfd, argv[2], strlen(argv[2]), 0,
p->ai_addr, p->ai_addrlen)) == -1) {
perror(“talker: sendto”);
exit(1);
}
freeaddrinfo(servinfo);
printf(“talker: sent %d bytes to %s\n”, numbytes, argv[1]);
close(sockfd);
return 0;
}
Beej’s Guide to Network Programming 32
And that’s all there is to it! Run listener on some machine, then run talker on another. Watch them
communicate! Fun G-rated excitement for the entire nuclear family!
You don’t even have to run the server this time! You can run talker by itself, and it just happily fires
packets off into the ether where they disappear if no one is ready with a recvfrom() on the other side.
Remember: data sent using UDP datagram sockets isn’t guaranteed to arrive!
Except for one more tiny detail that I’ve mentioned many times in the past: connected datagram
sockets. I need to talk about this here, since we’re in the datagram section of the document. Let’s say that
talker calls connect() and specifies the listener’s address. From that point on, talker may only sent to
and receive from the address specified by connect(). For this reason, you don’t have to use sendto()
and recvfrom(); you can simply use send() and recv().
7. Slightly Advanced Techniques
These aren’t really advanced, but they’re getting out of the more basic levels we’ve already covered.
In fact, if you’ve gotten this far, you should consider yourself fairly accomplished in the basics of Unix
network programming! Congratulations!
So here we go into the brave new world of some of the more esoteric things you might want to learn
about sockets. Have at it!
7.1. Blocking
Blocking. You’ve heard about it—now what the heck is it? In a nutshell, “block” is techie jargon
for “sleep”. You probably noticed that when you run listener, above, it just sits there until a packet
arrives. What happened is that it called recvfrom(), there was no data, and so recvfrom() is said to
“block” (that is, sleep there) until some data arrives.
Lots of functions block. accept() blocks. All the recv() functions block. The reason they can do
this is because they’re allowed to. When you first create the socket descriptor with socket(), the kernel
sets it to blocking. If you don’t want a socket to be blocking, you have to make a call to fcntl():
#include
#include
.
.
.
sockfd = socket(PF_INET, SOCK_STREAM, 0);
fcntl(sockfd, F_SETFL, O_NONBLOCK);
.
.
.
By setting a socket to non-blocking, you can effectively “poll” the socket for information. If you try
to read from a non-blocking socket and there’s no data there, it’s not allowed to block—it will return -1
and errno will be set to EAGAIN or EWOULDBLOCK.
(Wait—it can return EAGAIN or EWOULDBLOCK? Which do you check for? The specification doesn’t
actually specify which your system will return, so for portability, check them both.)
Generally speaking, however, this type of polling is a bad idea. If you put your program in a busy-
wait looking for data on the socket, you’ll suck up CPU time like it was going out of style. A more
elegant solution for checking to see if there’s data waiting to be read comes in the following section on
select().
7.2. select()—Synchronous I/O Multiplexing
This function is somewhat strange, but it’s very useful. Take the following situation: you are a
server and you want to listen for incoming connections as well as keep reading from the connections you
already have.
No problem, you say, just an accept() and a couple of recv()s. Not so fast, buster! What if
you’re blocking on an accept() call? How are you going to recv() data at the same time? “Use non-
blocking sockets!” No way! You don’t want to be a CPU hog. What, then?
select() gives you the power to monitor several sockets at the same time. It’ll tell you which
ones are ready for reading, which are ready for writing, and which sockets have raised exceptions, if you
really want to know that.
This being said, in modern times select(), though very portable, is one of the slowest methods for
monitoring sockets. One possible alternative is libevent24, or something similar, that encapsulates all the
system-dependent stuff involved with getting socket notifications.
Without any further ado, I’ll offer the synopsis of select():
#include
#include
#include
24. http://www.monkey.org/~provos/libevent/
33
http://www.monkey.org/~provos/libevent/
Beej’s Guide to Network Programming 34
int select(int numfds, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, struct timeval *timeout);
The function monitors “sets” of file descriptors; in particular readfds, writefds, and
exceptfds. If you want to see if you can read from standard input and some socket descriptor, sockfd,
just add the file descriptors 0 and sockfd to the set readfds. The parameter numfds should be set to
the values of the highest file descriptor plus one. In this example, it should be set to sockfd+1, since it is
assuredly higher than standard input (0).
When select() returns, readfds will be modified to reflect which of the file descriptors you
selected which is ready for reading. You can test them with the macro FD_ISSET(), below.
Before progressing much further, I’ll talk about how to manipulate these sets. Each set is of the type
fd_set. The following macros operate on this type:
FD_SET(int fd, fd_set *set); Add fd to the set.
FD_CLR(int fd, fd_set *set); Remove fd from the set.
FD_ISSET(int fd, fd_set *set); Return true if fd is in the set.
FD_ZERO(fd_set *set); Clear all entries from the set.
Finally, what is this weirded out struct timeval? Well, sometimes you don’t want to wait
forever for someone to send you some data. Maybe every 96 seconds you want to print “Still Going…”
to the terminal even though nothing has happened. This time structure allows you to specify a timeout
period. If the time is exceeded and select() still hasn’t found any ready file descriptors, it’ll return so
you can continue processing.
The struct timeval has the follow fields:
struct timeval {
int tv_sec; // seconds
int tv_usec; // microseconds
};
Just set tv_sec to the number of seconds to wait, and set tv_usec to the number of microseconds
to wait. Yes, that’s microseconds, not milliseconds. There are 1,000 microseconds in a millisecond, and
1,000 milliseconds in a second. Thus, there are 1,000,000 microseconds in a second. Why is it “usec”?
The “u” is supposed to look like the Greek letter μ (Mu) that we use for “micro”. Also, when the function
returns, timeout might be updated to show the time still remaining. This depends on what flavor of
Unix you’re running.
Yay! We have a microsecond resolution timer! Well, don’t count on it. You’ll probably have to wait
some part of your standard Unix timeslice no matter how small you set your struct timeval.
Other things of interest: If you set the fields in your struct timeval to 0, select() will timeout
immediately, effectively polling all the file descriptors in your sets. If you set the parameter timeout to
NULL, it will never timeout, and will wait until the first file descriptor is ready. Finally, if you don’t care
about waiting for a certain set, you can just set it to NULL in the call to select().
The following code snippet25 waits 2.5 seconds for something to appear on standard input:
/*
** select.c — a select() demo
*/
#include
#include
#include
#include
#define STDIN 0 // file descriptor for standard input
int main(void)
{
25. http://beej.us/guide/bgnet/examples/select.c
http://beej.us/guide/bgnet/examples/select.c
Beej’s Guide to Network Programming 35
struct timeval tv;
fd_set readfds;
tv.tv_sec = 2;
tv.tv_usec = 500000;
FD_ZERO(&readfds);
FD_SET(STDIN, &readfds);
// don’t care about writefds and exceptfds:
select(STDIN+1, &readfds, NULL, NULL, &tv);
if (FD_ISSET(STDIN, &readfds))
printf(“A key was pressed!\n”);
else
printf(“Timed out.\n”);
return 0;
}
If you’re on a line buffered terminal, the key you hit should be RETURN or it will time out anyway.
Now, some of you might think this is a great way to wait for data on a datagram socket—and you
are right: it might be. Some Unices can use select in this manner, and some can’t. You should see what
your local man page says on the matter if you want to attempt it.
Some Unices update the time in your struct timeval to reflect the amount of time still
remaining before a timeout. But others do not. Don’t rely on that occurring if you want to be portable.
(Use gettimeofday() if you need to track time elapsed. It’s a bummer, I know, but that’s the way it is.)
What happens if a socket in the read set closes the connection? Well, in that case, select() returns
with that socket descriptor set as “ready to read”. When you actually do recv() from it, recv() will
return 0. That’s how you know the client has closed the connection.
One more note of interest about select(): if you have a socket that is listen()ing, you can
check to see if there is a new connection by putting that socket’s file descriptor in the readfds set.
And that, my friends, is a quick overview of the almighty select() function.
But, by popular demand, here is an in-depth example. Unfortunately, the difference between the dirt-
simple example, above, and this one here is significant. But have a look, then read the description that
follows it.
This program26 acts like a simple multi-user chat server. Start it running in one window, then telnet
to it (“telnet hostname 9034”) from multiple other windows. When you type something in one telnet
session, it should appear in all the others.
/*
** selectserver.c — a cheezy multiperson chat server
*/
#include
#include
#include
#include
#include
#include
#include
#include
#include
#define PORT “9034” // port we’re listening on
// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
if (sa->sa_family == AF_INET) {
return &(((struct sockaddr_in*)sa)->sin_addr);
26. http://beej.us/guide/bgnet/examples/selectserver.c
http://beej.us/guide/bgnet/examples/selectserver.c
Beej’s Guide to Network Programming 36
}
return &(((struct sockaddr_in6*)sa)->sin6_addr);
}
int main(void)
{
fd_set master; // master file descriptor list
fd_set read_fds; // temp file descriptor list for select()
int fdmax; // maximum file descriptor number
int listener; // listening socket descriptor
int newfd; // newly accept()ed socket descriptor
struct sockaddr_storage remoteaddr; // client address
socklen_t addrlen;
char buf[256]; // buffer for client data
int nbytes;
char remoteIP[INET6_ADDRSTRLEN];
int yes=1; // for setsockopt() SO_REUSEADDR, below
int i, j, rv;
struct addrinfo hints, *ai, *p;
FD_ZERO(&master); // clear the master and temp sets
FD_ZERO(&read_fds);
// get us a socket and bind it
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE;
if ((rv = getaddrinfo(NULL, PORT, &hints, &ai)) != 0) {
fprintf(stderr, “selectserver: %s\n”, gai_strerror(rv));
exit(1);
}
for(p = ai; p != NULL; p = p->ai_next) {
listener = socket(p->ai_family, p->ai_socktype, p->ai_protocol);
if (listener < 0) {
continue;
}
// lose the pesky "address already in use" error message
setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int));
if (bind(listener, p->ai_addr, p->ai_addrlen) < 0) {
close(listener);
continue;
}
break;
}
// if we got here, it means we didn't get bound
if (p == NULL) {
fprintf(stderr, "selectserver: failed to bind\n");
exit(2);
}
freeaddrinfo(ai); // all done with this
// listen
if (listen(listener, 10) == -1) {
perror("listen");
Beej's Guide to Network Programming 37
exit(3);
}
// add the listener to the master set
FD_SET(listener, &master);
// keep track of the biggest file descriptor
fdmax = listener; // so far, it's this one
// main loop
for(;;) {
read_fds = master; // copy it
if (select(fdmax+1, &read_fds, NULL, NULL, NULL) == -1) {
perror("select");
exit(4);
}
// run through the existing connections looking for data to read
for(i = 0; i <= fdmax; i++) {
if (FD_ISSET(i, &read_fds)) { // we got one!!
if (i == listener) {
// handle new connections
addrlen = sizeof remoteaddr;
newfd = accept(listener,
(struct sockaddr *)&remoteaddr,
&addrlen);
if (newfd == -1) {
perror("accept");
} else {
FD_SET(newfd, &master); // add to master set
if (newfd > fdmax) { // keep track of the max
fdmax = newfd;
}
printf(“selectserver: new connection from %s on ”
“socket %d\n”,
inet_ntop(remoteaddr.ss_family,
get_in_addr((struct sockaddr*)&remoteaddr),
remoteIP, INET6_ADDRSTRLEN),
newfd);
}
} else {
// handle data from a client
if ((nbytes = recv(i, buf, sizeof buf, 0)) <= 0) {
// got error or connection closed by client
if (nbytes == 0) {
// connection closed
printf("selectserver: socket %d hung up\n", i);
} else {
perror("recv");
}
close(i); // bye!
FD_CLR(i, &master); // remove from master set
} else {
// we got some data from a client
for(j = 0; j <= fdmax; j++) {
// send to everyone!
if (FD_ISSET(j, &master)) {
// except the listener and ourselves
if (j != listener && j != i) {
if (send(j, buf, nbytes, 0) == -1) {
perror("send");
}
}
}
}
}
Beej's Guide to Network Programming 38
} // END handle data from client
} // END got new incoming connection
} // END looping through file descriptors
} // END for(;;)--and you thought it would never end!
return 0;
}
Notice I have two file descriptor sets in the code: master and read_fds. The first, master, holds
all the socket descriptors that are currently connected, as well as the socket descriptor that is listening for
new connections.
The reason I have the master set is that select() actually changes the set you pass into it to
reflect which sockets are ready to read. Since I have to keep track of the connections from one call of
select() to the next, I must store these safely away somewhere. At the last minute, I copy the master
into the read_fds, and then call select().
But doesn't this mean that every time I get a new connection, I have to add it to the master set?
Yup! And every time a connection closes, I have to remove it from the master set? Yes, it does.
Notice I check to see when the listener socket is ready to read. When it is, it means I have a new
connection pending, and I accept() it and add it to the master set. Similarly, when a client connection
is ready to read, and recv() returns 0, I know the client has closed the connection, and I must remove it
from the master set.
If the client recv() returns non-zero, though, I know some data has been received. So I get it, and
then go through the master list and send that data to all the rest of the connected clients.
And that, my friends, is a less-than-simple overview of the almighty select() function.
In addition, here is a bonus afterthought: there is another function called poll() which behaves
much the same way select() does, but with a different system for managing the file descriptor sets.
Check it out!
7.3. Handling Partial send()s
Remember back in the section about send(), above, when I said that send() might not send all
the bytes you asked it to? That is, you want it to send 512 bytes, but it returns 412. What happened to the
remaining 100 bytes?
Well, they're still in your little buffer waiting to be sent out. Due to circumstances beyond your
control, the kernel decided not to send all the data out in one chunk, and now, my friend, it's up to you to
get the data out there.
You could write a function like this to do it, too:
#include
#include
int sendall(int s, char *buf, int *len)
{
int total = 0; // how many bytes we’ve sent
int bytesleft = *len; // how many we have left to send
int n;
while(total < *len) {
n = send(s, buf+total, bytesleft, 0);
if (n == -1) { break; }
total += n;
bytesleft -= n;
}
*len = total; // return number actually sent here
return n==-1?-1:0; // return -1 on failure, 0 on success
}
In this example, s is the socket you want to send the data to, buf is the buffer containing the data,
and len is a pointer to an int containing the number of bytes in the buffer.
Beej's Guide to Network Programming 39
The function returns -1 on error (and errno is still set from the call to send().) Also, the number
of bytes actually sent is returned in len. This will be the same number of bytes you asked it to send,
unless there was an error. sendall() will do it's best, huffing and puffing, to send the data out, but if
there's an error, it gets back to you right away.
For completeness, here's a sample call to the function:
char buf[10] = "Beej!";
int len;
len = strlen(buf);
if (sendall(s, buf, &len) == -1) {
perror("sendall");
printf("We only sent %d bytes because of the error!\n", len);
}
What happens on the receiver's end when part of a packet arrives? If the packets are variable length,
how does the receiver know when one packet ends and another begins? Yes, real-world scenarios are a
royal pain in the donkeys. You probably have to encapsulate (remember that from the data encapsulation
section way back there at the beginning?) Read on for details!
Quick note to all you Linux fans out there: sometimes, in rare circumstances, Linux's select() can
return “ready-to-read” and then not actually be ready to read! This means it will block on the read()
after the select() says it won't! Why you little—! Anyway, the workaround solution is to set the
O_NONBLOCK flag on the receiving socket so it errors with EWOULDBLOCK (which you can just safely
ignore if it occurs). See the fcntl() reference page for more info on setting a socket to non-blocking.
7.4. Serialization—How to Pack Data
It's easy enough to send text data across the network, you're finding, but what happens if you want
to send some “binary” data like ints or floats? It turns out you have a few options.
1. Convert the number into text with a function like sprintf(), then send the text. The receiver
will parse the text back into a number using a function like strtol().
2. Just send the data raw, passing a pointer to the data to send().
3. Encode the number into a portable binary form. The receiver will decode it.
Sneak preview! Tonight only!
[Curtain raises]
Beej says, “I prefer Method Three, above!”
[THE END]
(Before I begin this section in earnest, I should tell you that there are libraries out there for doing
this, and rolling your own and remaining portable and error-free is quite a challenge. So hunt around and
do your homework before deciding to implement this stuff yourself. I include the information here for
those curious about how things like this work.)
Actually all the methods, above, have their drawbacks and advantages, but, like I said, in general, I
prefer the third method. First, though, let's talk about some of the drawbacks and advantages to the other
two.
The first method, encoding the numbers as text before sending, has the advantage that you can
easily print and read the data that's coming over the wire. Sometimes a human-readable protocol is
excellent to use in a non-bandwidth-intensive situation, such as with Internet Relay Chat (IRC)27.
However, it has the disadvantage that it is slow to convert, and the results almost always take up more
space than the original number!
Method two: passing the raw data. This one is quite easy (but dangerous!): just take a pointer to the
data to send, and call send with it.
double d = 3490.15926535;
27. http://en.wikipedia.org/wiki/Internet_Relay_Chat
http://en.wikipedia.org/wiki/Internet_Relay_Chat
Beej's Guide to Network Programming 40
send(s, &d, sizeof d, 0); /* DANGER--non-portable! */
The receiver gets it like this:
double d;
recv(s, &d, sizeof d, 0); /* DANGER--non-portable! */
Fast, simple—what's not to like? Well, it turns out that not all architectures represent a double
(or int for that matter) with the same bit representation or even the same byte ordering! The code is
decidedly non-portable. (Hey—maybe you don't need portability, in which case this is nice and fast.)
When packing integer types, we've already seen how the htons()-class of functions can help keep
things portable by transforming the numbers into Network Byte Order, and how that's the Right Thing to
do. Unfortunately, there are no similar functions for float types. Is all hope lost?
Fear not! (Were you afraid there for a second? No? Not even a little bit?) There is something we can
do: we can pack (or “marshal”, or “serialize”, or one of a thousand million other names) the data into a
known binary format that the receiver can unpack on the remote side.
What do I mean by “known binary format”? Well, we've already seen the htons() example, right?
It changes (or “encodes”, if you want to think of it that way) a number from whatever the host format is
into Network Byte Order. To reverse (unencode) the number, the receiver calls ntohs().
But didn't I just get finished saying there wasn't any such function for other non-integer types? Yes.
I did. And since there's no standard way in C to do this, it's a bit of a pickle (that a gratuitous pun there
for you Python fans).
The thing to do is to pack the data into a known format and send that over the wire for decoding. For
example, to pack floats, here's something quick and dirty with plenty of room for improvement:28
#include
uint32_t htonf(float f)
{
uint32_t p;
uint32_t sign;
if (f < 0) { sign = 1; f = -f; }
else { sign = 0; }
p = ((((uint32_t)f)&0x7fff)<<16) | (sign<<31); // whole part and sign
p |= (uint32_t)(((f - (int)f) * 65536.0f))&0xffff; // fraction
return p;
}
float ntohf(uint32_t p)
{
float f = ((p>>16)&0x7fff); // whole part
f += (p&0xffff) / 65536.0f; // fraction
if (((p>>31)&0x1) == 0x1) { f = -f; } // sign bit set
return f;
}
The above code is sort of a naive implementation that stores a float in a 32-bit number. The high
bit (31) is used to store the sign of the number (“1” means negative), and the next seven bits (30-16) are
used to store the whole number portion of the float. Finally, the remaining bits (15-0) are used to store
the fractional portion of the number.
Usage is fairly straightforward:
#include
int main(void)
{
28. http://beej.us/guide/bgnet/examples/pack.c
http://beej.us/guide/bgnet/examples/pack.c
Beej’s Guide to Network Programming 41
float f = 3.1415926, f2;
uint32_t netf;
netf = htonf(f); // convert to “network” form
f2 = ntohf(netf); // convert back to test
printf(“Original: %f\n”, f); // 3.141593
printf(” Network: 0x%08X\n”, netf); // 0x0003243F
printf(“Unpacked: %f\n”, f2); // 3.141586
return 0;
}
On the plus side, it’s small, simple, and fast. On the minus side, it’s not an efficient use of space
and the range is severely restricted—try storing a number greater-than 32767 in there and it won’t be
very happy! You can also see in the above example that the last couple decimal places are not correctly
preserved.
What can we do instead? Well, The Standard for storing floating point numbers is known as
IEEE-75429. Most computers use this format internally for doing floating point math, so in those cases,
strictly speaking, conversion wouldn’t need to be done. But if you want your source code to be portable,
that’s an assumption you can’t necessarily make. (On the other hand, if you want things to be fast, you
should optimize this out on platforms that don’t need to do it! That’s what htons() and its ilk do.)
Here’s some code that encodes floats and doubles into IEEE-754 format30. (Mostly—it doesn’t
encode NaN or Infinity, but it could be modified to do that.)
#define pack754_32(f) (pack754((f), 32, 8))
#define pack754_64(f) (pack754((f), 64, 11))
#define unpack754_32(i) (unpack754((i), 32, 8))
#define unpack754_64(i) (unpack754((i), 64, 11))
uint64_t pack754(long double f, unsigned bits, unsigned expbits)
{
long double fnorm;
int shift;
long long sign, exp, significand;
unsigned significandbits = bits – expbits – 1; // -1 for sign bit
if (f == 0.0) return 0; // get this special case out of the way
// check sign and begin normalization
if (f < 0) { sign = 1; fnorm = -f; }
else { sign = 0; fnorm = f; }
// get the normalized form of f and track the exponent
shift = 0;
while(fnorm >= 2.0) { fnorm /= 2.0; shift++; }
while(fnorm < 1.0) { fnorm *= 2.0; shift--; }
fnorm = fnorm - 1.0;
// calculate the binary form (non-float) of the significand data
significand = fnorm * ((1LL<
while(shift < 0) { result /= 2.0; shift++; }
// sign it
result *= (i>>(bits-1))&1? -1.0: 1.0;
return result;
}
I put some handy macros up there at the top for packing and unpacking 32-bit (probably a float)
and 64-bit (probably a double) numbers, but the pack754() function could be called directly and told
to encode bits-worth of data (expbits of which are reserved for the normalized number’s exponent.)
Here’s sample usage:
#include
#include
#include
int main(void)
{
float f = 3.1415926, f2;
double d = 3.14159265358979323, d2;
uint32_t fi;
uint64_t di;
fi = pack754_32(f);
f2 = unpack754_32(fi);
di = pack754_64(d);
d2 = unpack754_64(di);
printf(“float before : %.7f\n”, f);
printf(“float encoded: 0x%08” PRIx32 “\n”, fi);
printf(“float after : %.7f\n\n”, f2);
printf(“double before : %.20lf\n”, d);
printf(“double encoded: 0x%016” PRIx64 “\n”, di);
printf(“double after : %.20lf\n”, d2);
return 0;
}
The above code produces this output:
float before : 3.1415925
float encoded: 0x40490FDA
float after : 3.1415925
double before : 3.14159265358979311600
double encoded: 0x400921FB54442D18
double after : 3.14159265358979311600
Beej’s Guide to Network Programming 43
Another question you might have is how do you pack structs? Unfortunately for you, the
compiler is free to put padding all over the place in a struct, and that means you can’t portably send
the whole thing over the wire in one chunk. (Aren’t you getting sick of hearing “can’t do this”, “can’t do
that”? Sorry! To quote a friend, “Whenever anything goes wrong, I always blame Microsoft.” This one
might not be Microsoft’s fault, admittedly, but my friend’s statement is completely true.)
Back to it: the best way to send the struct over the wire is to pack each field independently and
then unpack them into the struct when they arrive on the other side.
That’s a lot of work, is what you’re thinking. Yes, it is. One thing you can do is write a helper
function to help pack the data for you. It’ll be fun! Really!
In the book “The Practice of Programming31” by Kernighan and Pike, they implement printf()-
like functions called pack() and unpack() that do exactly this. I’d link to them, but apparently those
functions aren’t online with the rest of the source from the book.
(The Practice of Programming is an excellent read. Zeus saves a kitten every time I recommend it.)
At this point, I’m going to drop a pointer to the BSD-licensed Typed Parameter Language C API32
which I’ve never used, but looks completely respectable. Python and Perl programmers will want to
check out their language’s pack() and unpack() functions for accomplishing the same thing. And Java
has a big-ol’ Serializable interface that can be used in a similar way.
But if you want to write your own packing utility in C, K&P’s trick is to use variable argument lists
to make printf()-like functions to build the packets. Here’s a version I cooked up33 on my own based
on that which hopefully will be enough to give you an idea of how such a thing can work.
(This code references the pack754() functions, above. The packi*() functions operate like the
familiar htons() family, except they pack into a char array instead of another integer.)
#include
#include
#include
#include
/*
** packi16() — store a 16-bit int into a char buffer (like htons())
*/
void packi16(unsigned char *buf, unsigned int i)
{
*buf++ = i>>8; *buf++ = i;
}
/*
** packi32() — store a 32-bit int into a char buffer (like htonl())
*/
void packi32(unsigned char *buf, unsigned long int i)
{
*buf++ = i>>24; *buf++ = i>>16;
*buf++ = i>>8; *buf++ = i;
}
/*
** packi64() — store a 64-bit int into a char buffer (like htonl())
*/
void packi64(unsigned char *buf, unsigned long long int i)
{
*buf++ = i>>56; *buf++ = i>>48;
*buf++ = i>>40; *buf++ = i>>32;
*buf++ = i>>24; *buf++ = i>>16;
*buf++ = i>>8; *buf++ = i;
}
/*
** unpacki16() — unpack a 16-bit int from a char buffer (like ntohs())
31. http://cm.bell-labs.com/cm/cs/tpop/
32. http://tpl.sourceforge.net/
33. http://beej.us/guide/bgnet/examples/pack2.c
http://cm.bell-labs.com/cm/cs/tpop/
http://tpl.sourceforge.net/
http://beej.us/guide/bgnet/examples/pack2.c
Beej’s Guide to Network Programming 44
*/
int unpacki16(unsigned char *buf)
{
unsigned int i2 = ((unsigned int)buf[0]<<8) | buf[1];
int i;
// change unsigned numbers to signed
if (i2 <= 0x7fffu) { i = i2; }
else { i = -1 - (unsigned int)(0xffffu - i2); }
return i;
}
/*
** unpacku16() -- unpack a 16-bit unsigned from a char buffer (like ntohs())
*/
unsigned int unpacku16(unsigned char *buf)
{
return ((unsigned int)buf[0]<<8) | buf[1];
}
/*
** unpacki32() -- unpack a 32-bit int from a char buffer (like ntohl())
*/
long int unpacki32(unsigned char *buf)
{
unsigned long int i2 = ((unsigned long int)buf[0]<<24) |
((unsigned long int)buf[1]<<16) |
((unsigned long int)buf[2]<<8) |
buf[3];
long int i;
// change unsigned numbers to signed
if (i2 <= 0x7fffffffu) { i = i2; }
else { i = -1 - (long int)(0xffffffffu - i2); }
return i;
}
/*
** unpacku32() -- unpack a 32-bit unsigned from a char buffer (like ntohl())
*/
unsigned long int unpacku32(unsigned char *buf)
{
return ((unsigned long int)buf[0]<<24) |
((unsigned long int)buf[1]<<16) |
((unsigned long int)buf[2]<<8) |
buf[3];
}
/*
** unpacki64() -- unpack a 64-bit int from a char buffer (like ntohl())
*/
long long int unpacki64(unsigned char *buf)
{
unsigned long long int i2 = ((unsigned long long int)buf[0]<<56) |
((unsigned long long int)buf[1]<<48) |
((unsigned long long int)buf[2]<<40) |
((unsigned long long int)buf[3]<<32) |
((unsigned long long int)buf[4]<<24) |
((unsigned long long int)buf[5]<<16) |
((unsigned long long int)buf[6]<<8) |
buf[7];
long long int i;
// change unsigned numbers to signed
if (i2 <= 0x7fffffffffffffffu) { i = i2; }
Beej's Guide to Network Programming 45
else { i = -1 -(long long int)(0xffffffffffffffffu - i2); }
return i;
}
/*
** unpacku64() -- unpack a 64-bit unsigned from a char buffer (like ntohl())
*/
unsigned long long int unpacku64(unsigned char *buf)
{
return ((unsigned long long int)buf[0]<<56) |
((unsigned long long int)buf[1]<<48) |
((unsigned long long int)buf[2]<<40) |
((unsigned long long int)buf[3]<<32) |
((unsigned long long int)buf[4]<<24) |
((unsigned long long int)buf[5]<<16) |
((unsigned long long int)buf[6]<<8) |
buf[7];
}
/*
** pack() -- store data dictated by the format string in the buffer
**
** bits |signed unsigned float string
** -----+----------------------------------
** 8 | c C
** 16 | h H f
** 32 | l L d
** 64 | q Q g
** - | s
**
** (16-bit unsigned length is automatically prepended to strings)
*/
unsigned int pack(unsigned char *buf, char *format, ...)
{
va_list ap;
signed char c; // 8-bit
unsigned char C;
int h; // 16-bit
unsigned int H;
long int l; // 32-bit
unsigned long int L;
long long int q; // 64-bit
unsigned long long int Q;
float f; // floats
double d;
long double g;
unsigned long long int fhold;
char *s; // strings
unsigned int len;
unsigned int size = 0;
va_start(ap, format);
for(; *format != '\0'; format++) {
switch(*format) {
case 'c': // 8-bit
size += 1;
c = (signed char)va_arg(ap, int); // promoted
Beej's Guide to Network Programming 46
*buf++ = c;
break;
case 'C': // 8-bit unsigned
size += 1;
C = (unsigned char)va_arg(ap, unsigned int); // promoted
*buf++ = C;
break;
case 'h': // 16-bit
size += 2;
h = va_arg(ap, int);
packi16(buf, h);
buf += 2;
break;
case 'H': // 16-bit unsigned
size += 2;
H = va_arg(ap, unsigned int);
packi16(buf, H);
buf += 2;
break;
case 'l': // 32-bit
size += 4;
l = va_arg(ap, long int);
packi32(buf, l);
buf += 4;
break;
case 'L': // 32-bit unsigned
size += 4;
L = va_arg(ap, unsigned long int);
packi32(buf, L);
buf += 4;
break;
case 'q': // 64-bit
size += 8;
q = va_arg(ap, long long int);
packi64(buf, q);
buf += 8;
break;
case 'Q': // 64-bit unsigned
size += 8;
Q = va_arg(ap, unsigned long long int);
packi64(buf, Q);
buf += 8;
break;
case 'f': // float-16
size += 2;
f = (float)va_arg(ap, double); // promoted
fhold = pack754_16(f); // convert to IEEE 754
packi16(buf, fhold);
buf += 2;
break;
case 'd': // float-32
size += 4;
d = va_arg(ap, double);
fhold = pack754_32(d); // convert to IEEE 754
packi32(buf, fhold);
buf += 4;
break;
Beej's Guide to Network Programming 47
case 'g': // float-64
size += 8;
g = va_arg(ap, long double);
fhold = pack754_64(g); // convert to IEEE 754
packi64(buf, fhold);
buf += 8;
break;
case 's': // string
s = va_arg(ap, char*);
len = strlen(s);
size += len + 2;
packi16(buf, len);
buf += 2;
memcpy(buf, s, len);
buf += len;
break;
}
}
va_end(ap);
return size;
}
/*
** unpack() -- unpack data dictated by the format string into the buffer
**
** bits |signed unsigned float string
** -----+----------------------------------
** 8 | c C
** 16 | h H f
** 32 | l L d
** 64 | q Q g
** - | s
**
** (string is extracted based on its stored length, but 's' can be
** prepended with a max length)
*/
void unpack(unsigned char *buf, char *format, ...)
{
va_list ap;
signed char *c; // 8-bit
unsigned char *C;
int *h; // 16-bit
unsigned int *H;
long int *l; // 32-bit
unsigned long int *L;
long long int *q; // 64-bit
unsigned long long int *Q;
float *f; // floats
double *d;
long double *g;
unsigned long long int fhold;
char *s;
unsigned int len, maxstrlen=0, count;
va_start(ap, format);
for(; *format != '\0'; format++) {
switch(*format) {
Beej's Guide to Network Programming 48
case 'c': // 8-bit
c = va_arg(ap, signed char*);
if (*buf <= 0x7f) { *c = *buf;} // re-sign
else { *c = -1 - (unsigned char)(0xffu - *buf); }
buf++;
break;
case 'C': // 8-bit unsigned
C = va_arg(ap, unsigned char*);
*C = *buf++;
break;
case 'h': // 16-bit
h = va_arg(ap, int*);
*h = unpacki16(buf);
buf += 2;
break;
case 'H': // 16-bit unsigned
H = va_arg(ap, unsigned int*);
*H = unpacku16(buf);
buf += 2;
break;
case 'l': // 32-bit
l = va_arg(ap, long int*);
*l = unpacki32(buf);
buf += 4;
break;
case 'L': // 32-bit unsigned
L = va_arg(ap, unsigned long int*);
*L = unpacku32(buf);
buf += 4;
break;
case 'q': // 64-bit
q = va_arg(ap, long long int*);
*q = unpacki64(buf);
buf += 8;
break;
case 'Q': // 64-bit unsigned
Q = va_arg(ap, unsigned long long int*);
*Q = unpacku64(buf);
buf += 8;
break;
case 'f': // float
f = va_arg(ap, float*);
fhold = unpacku16(buf);
*f = unpack754_16(fhold);
buf += 2;
break;
case 'd': // float-32
d = va_arg(ap, double*);
fhold = unpacku32(buf);
*d = unpack754_32(fhold);
buf += 4;
break;
case 'g': // float-64
g = va_arg(ap, long double*);
fhold = unpacku64(buf);
*g = unpack754_64(fhold);
buf += 8;
Beej's Guide to Network Programming 49
break;
case 's': // string
s = va_arg(ap, char*);
len = unpacku16(buf);
buf += 2;
if (maxstrlen > 0 && len > maxstrlen) count = maxstrlen – 1;
else count = len;
memcpy(s, buf, count);
s[count] = ‘\0’;
buf += len;
break;
default:
if (isdigit(*format)) { // track max str len
maxstrlen = maxstrlen * 10 + (*format-‘0’);
}
}
if (!isdigit(*format)) maxstrlen = 0;
}
va_end(ap);
}
And here is a demonstration program34 of the above code that packs some data into buf and then
unpacks it into variables. Note that when calling unpack() with a string argument (format specifier
“s”), it’s wise to put a maximum length count in front of it to prevent a buffer overrun, e.g. “96s”. Be
wary when unpacking data you get over the network—a malicious user might send badly-constructed
packets in an effort to attack your system!
#include
// various bits for floating point types–
// varies for different architectures
typedef float float32_t;
typedef double float64_t;
int main(void)
{
unsigned char buf[1024];
int8_t magic;
int16_t monkeycount;
int32_t altitude;
float32_t absurdityfactor;
char *s = “Great unmitigated Zot! You’ve found the Runestaff!”;
char s2[96];
int16_t packetsize, ps2;
packetsize = pack(buf, “chhlsf”, (int8_t)’B’, (int16_t)0, (int16_t)37,
(int32_t)-5, s, (float32_t)-3490.6677);
packi16(buf+1, packetsize); // store packet size in packet for kicks
printf(“packet is %” PRId32 ” bytes\n”, packetsize);
unpack(buf, “chhl96sf”, &magic, &ps2, &monkeycount, &altitude, s2,
&absurdityfactor);
printf(“‘%c’ %” PRId32″ %” PRId16 ” %” PRId32
” \”%s\” %f\n”, magic, ps2, monkeycount,
altitude, s2, absurdityfactor);
return 0;
}
34. http://beej.us/guide/bgnet/examples/pack2.c
http://beej.us/guide/bgnet/examples/pack2.c
Beej’s Guide to Network Programming 50
Whether you roll your own code or use someone else’s, it’s a good idea to have a general set of data
packing routines for the sake of keeping bugs in check, rather than packing each bit by hand each time.
When packing the data, what’s a good format to use? Excellent question. Fortunately, RFC 450635,
the External Data Representation Standard, already defines binary formats for a bunch of different types,
like floating point types, integer types, arrays, raw data, etc. I suggest conforming to that if you’re going
to roll the data yourself. But you’re not obligated to. The Packet Police are not right outside your door. At
least, I don’t think they are.
In any case, encoding the data somehow or another before you send it is the right way of doing
things!
7.5. Son of Data Encapsulation
What does it really mean to encapsulate data, anyway? In the simplest case, it means you’ll stick a
header on there with either some identifying information or a packet length, or both.
What should your header look like? Well, it’s just some binary data that represents whatever you feel
is necessary to complete your project.
Wow. That’s vague.
Okay. For instance, let’s say you have a multi-user chat program that uses SOCK_STREAMs. When a
user types (“says”) something, two pieces of information need to be transmitted to the server: what was
said and who said it.
So far so good? “What’s the problem?” you’re asking.
The problem is that the messages can be of varying lengths. One person named “tom” might say,
“Hi”, and another person named “Benjamin” might say, “Hey guys what is up?”
So you send() all this stuff to the clients as it comes in. Your outgoing data stream looks like this:
t o m H i B e n j a m i n H e y g u y s w h a t i s u p ?
And so on. How does the client know when one message starts and another stops? You could, if you
wanted, make all messages the same length and just call the sendall() we implemented, above. But
that wastes bandwidth! We don’t want to send() 1024 bytes just so “tom” can say “Hi”.
So we encapsulate the data in a tiny header and packet structure. Both the client and server know
how to pack and unpack (sometimes referred to as “marshal” and “unmarshal”) this data. Don’t look now,
but we’re starting to define a protocol that describes how a client and server communicate!
In this case, let’s assume the user name is a fixed length of 8 characters, padded with ‘\0’. And
then let’s assume the data is variable length, up to a maximum of 128 characters. Let’s have a look a
sample packet structure that we might use in this situation:
1. len (1 byte, unsigned)—The total length of the packet, counting the 8-byte user name and chat
data.
2. name (8 bytes)—The user’s name, NUL-padded if necessary.
3. chatdata (n-bytes)—The data itself, no more than 128 bytes. The length of the packet should
be calculated as the length of this data plus 8 (the length of the name field, above).
Why did I choose the 8-byte and 128-byte limits for the fields? I pulled them out of the air,
assuming they’d be long enough. Maybe, though, 8 bytes is too restrictive for your needs, and you can
have a 30-byte name field, or whatever. The choice is up to you.
Using the above packet definition, the first packet would consist of the following information (in
hex and ASCII):
0A 74 6F 6D 00 00 00 00 00 48 69
(length) T o m (padding) H i
And the second is similar:
18 42 65 6E 6A 61 6D 69 6E 48 65 79 20 67 75 79 73 20 77 …
(length) B e n j a m i n H e y g u y s w …
35. http://tools.ietf.org/html/rfc4506
http://tools.ietf.org/html/rfc4506
Beej’s Guide to Network Programming 51
(The length is stored in Network Byte Order, of course. In this case, it’s only one byte so it doesn’t
matter, but generally speaking you’ll want all your binary integers to be stored in Network Byte Order in
your packets.)
When you’re sending this data, you should be safe and use a command similar to sendall(),
above, so you know all the data is sent, even if it takes multiple calls to send() to get it all out.
Likewise, when you’re receiving this data, you need to do a bit of extra work. To be safe, you
should assume that you might receive a partial packet (like maybe we receive “18 42 65 6E 6A” from
Benjamin, above, but that’s all we get in this call to recv()). We need to call recv() over and over
again until the packet is completely received.
But how? Well, we know the number of bytes we need to receive in total for the packet to be
complete, since that number is tacked on the front of the packet. We also know the maximum packet size
is 1+8+128, or 137 bytes (because that’s how we defined the packet.)
There are actually a couple things you can do here. Since you know every packet starts off with a
length, you can call recv() just to get the packet length. Then once you have that, you can call it again
specifying exactly the remaining length of the packet (possibly repeatedly to get all the data) until you
have the complete packet. The advantage of this method is that you only need a buffer large enough for
one packet, while the disadvantage is that you need to call recv() at least twice to get all the data.
Another option is just to call recv() and say the amount you’re willing to receive is the maximum
number of bytes in a packet. Then whatever you get, stick it onto the back of a buffer, and finally check
to see if the packet is complete. Of course, you might get some of the next packet, so you’ll need to have
room for that.
What you can do is declare an array big enough for two packets. This is your work array where you
will reconstruct packets as they arrive.
Every time you recv() data, you’ll append it into the work buffer and check to see if the packet is
complete. That is, the number of bytes in the buffer is greater than or equal to the length specified in the
header (+1, because the length in the header doesn’t include the byte for the length itself.) If the number
of bytes in the buffer is less than 1, the packet is not complete, obviously. You have to make a special
case for this, though, since the first byte is garbage and you can’t rely on it for the correct packet length.
Once the packet is complete, you can do with it what you will. Use it, and remove it from your work
buffer.
Whew! Are you juggling that in your head yet? Well, here’s the second of the one-two punch: you
might have read past the end of one packet and onto the next in a single recv() call. That is, you have a
work buffer with one complete packet, and an incomplete part of the next packet! Bloody heck. (But this
is why you made your work buffer large enough to hold two packets—in case this happened!)
Since you know the length of the first packet from the header, and you’ve been keeping track of the
number of bytes in the work buffer, you can subtract and calculate how many of the bytes in the work
buffer belong to the second (incomplete) packet. When you’ve handled the first one, you can clear it out
of the work buffer and move the partial second packet down the to front of the buffer so it’s all ready to
go for the next recv().
(Some of you readers will note that actually moving the partial second packet to the beginning of
the work buffer takes time, and the program can be coded to not require this by using a circular buffer.
Unfortunately for the rest of you, a discussion on circular buffers is beyond the scope of this article. If
you’re still curious, grab a data structures book and go from there.)
I never said it was easy. Ok, I did say it was easy. And it is; you just need practice and pretty soon
it’ll come to you naturally. By Excalibur I swear it!
7.6. Broadcast Packets—Hello, World!
So far, this guide has talked about sending data from one host to one other host. But it is possible, I
insist, that you can, with the proper authority, send data to multiple hosts at the same time!
With UDP (only UDP, not TCP) and standard IPv4, this is done through a mechanism called
broadcasting. With IPv6, broadcasting isn’t supported, and you have to resort to the often superior
technique of multicasting, which, sadly I won’t be discussing at this time. But enough of the starry-eyed
future—we’re stuck in the 32-bit present.
Beej’s Guide to Network Programming 52
But wait! You can’t just run off and start broadcasting willy-nilly; You have to set the socket option
SO_BROADCAST before you can send a broadcast packet out on the network. It’s like a one of those little
plastic covers they put over the missile launch switch! That’s just how much power you hold in your
hands!
But seriously, though, there is a danger to using broadcast packets, and that is: every system that
receives a broadcast packet must undo all the onion-skin layers of data encapsulation until it finds out
what port the data is destined to. And then it hands the data over or discards it. In either case, it’s a lot of
work for each machine that receives the broadcast packet, and since it is all of them on the local network,
that could be a lot of machines doing a lot of unnecessary work. When the game Doom first came out,
this was a complaint about its network code.
Now, there is more than one way to skin a cat… wait a minute. Is there really more than one way
to skin a cat? What kind of expression is that? Uh, and likewise, there is more than one way to send
a broadcast packet. So, to get to the meat and potatoes of the whole thing: how do you specify the
destination address for a broadcast message? There are two common ways:
1. Send the data to a specific subnet’s broadcast address. This is the subnet’s network number
with all one-bits set for the host portion of the address. For instance, at home my network
is 192.168.1.0, my netmask is 255.255.255.0, so the last byte of the address is my host
number (because the first three bytes, according to the netmask, are the network number).
So my broadcast address is 192.168.1.255. Under Unix, the ifconfig command will actually
give you all this data. (If you’re curious, the bitwise logic to get your broadcast address is
network_number OR (NOT netmask).) You can send this type of broadcast packet to remote
networks as well as your local network, but you run the risk of the packet being dropped by the
destination’s router. (If they didn’t drop it, then some random smurf could start flooding their
LAN with broadcast traffic.)
2. Send the data to the “global” broadcast address. This is 255.255.255.255, aka
INADDR_BROADCAST. Many machines will automatically bitwise AND this with your network
number to convert it to a network broadcast address, but some won’t. It varies. Routers do not
forward this type of broadcast packet off your local network, ironically enough.
So what happens if you try to send data on the broadcast address without first setting the
SO_BROADCAST socket option? Well, let’s fire up good old talker and listener and see what happens.
$ talker 192.168.1.2 foo
sent 3 bytes to 192.168.1.2
$ talker 192.168.1.255 foo
sendto: Permission denied
$ talker 255.255.255.255 foo
sendto: Permission denied
Yes, it’s not happy at all…because we didn’t set the SO_BROADCAST socket option. Do that, and now
you can sendto() anywhere you want!
In fact, that’s the only difference between a UDP application that can broadcast and one that can’t. So
let’s take the old talker application and add one section that sets the SO_BROADCAST socket option. We’ll
call this program broadcaster.c36:
/*
** broadcaster.c — a datagram “client” like talker.c, except
** this one can broadcast
*/
#include
#include
#include
#include
#include
#include
36. http://beej.us/guide/bgnet/examples/broadcaster.c
http://beej.us/guide/bgnet/examples/broadcaster.c
Beej’s Guide to Network Programming 53
#include
#include
#include
#include
#define SERVERPORT 4950 // the port users will be connecting to
int main(int argc, char *argv[])
{
int sockfd;
struct sockaddr_in their_addr; // connector’s address information
struct hostent *he;
int numbytes;
int broadcast = 1;
//char broadcast = ‘1’; // if that doesn’t work, try this
if (argc != 3) {
fprintf(stderr,”usage: broadcaster hostname message\n”);
exit(1);
}
if ((he=gethostbyname(argv[1])) == NULL) { // get the host info
perror(“gethostbyname”);
exit(1);
}
if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
perror(“socket”);
exit(1);
}
// this call is what allows broadcast packets to be sent:
if (setsockopt(sockfd, SOL_SOCKET, SO_BROADCAST, &broadcast,
sizeof broadcast) == -1) {
perror(“setsockopt (SO_BROADCAST)”);
exit(1);
}
their_addr.sin_family = AF_INET; // host byte order
their_addr.sin_port = htons(SERVERPORT); // short, network byte order
their_addr.sin_addr = *((struct in_addr *)he->h_addr);
memset(their_addr.sin_zero, ‘\0’, sizeof their_addr.sin_zero);
if ((numbytes=sendto(sockfd, argv[2], strlen(argv[2]), 0,
(struct sockaddr *)&their_addr, sizeof their_addr)) == -1) {
perror(“sendto”);
exit(1);
}
printf(“sent %d bytes to %s\n”, numbytes,
inet_ntoa(their_addr.sin_addr));
close(sockfd);
return 0;
}
What’s different between this and a “normal” UDP client/server situation? Nothing! (With the
exception of the client being allowed to send broadcast packets in this case.) As such, go ahead and run
the old UDP listener program in one window, and broadcaster in another. You should be now be able to
do all those sends that failed, above.
$ broadcaster 192.168.1.2 foo
sent 3 bytes to 192.168.1.2
$ broadcaster 192.168.1.255 foo
sent 3 bytes to 192.168.1.255
$ broadcaster 255.255.255.255 foo
Beej’s Guide to Network Programming 54
sent 3 bytes to 255.255.255.255
And you should see listener responding that it got the packets. (If listener doesn’t respond, it could
be because it’s bound to an IPv6 address. Try changing the AF_UNSPEC in listener.c to AF_INET to
force IPv4.)
Well, that’s kind of exciting. But now fire up listener on another machine next to you on the same
network so that you have two copies going, one on each machine, and run broadcaster again with your
broadcast address… Hey! Both listeners get the packet even though you only called sendto() once!
Cool!
If the listener gets data you send directly to it, but not data on the broadcast address, it could be that
you have a firewall on your local machine that is blocking the packets. (Yes, Pat and Bapper, thank you
for realizing before I did that this is why my sample code wasn’t working. I told you I’d mention you in
the guide, and here you are. So nyah.)
Again, be careful with broadcast packets. Since every machine on the LAN will be forced to deal
with the packet whether it recvfrom()s it or not, it can present quite a load to the entire computing
network. They are definitely to be used sparingly and appropriately.
8. Common Questions
Where can I get those header files?
If you don’t have them on your system already, you probably don’t need them. Check the manual for
your particular platform. If you’re building for Windows, you only need to #include
What do I do when bind() reports “Address already in use”?
You have to use setsockopt() with the SO_REUSEADDR option on the listening socket. Check out
the section on bind() and the section on select() for an example.
How do I get a list of open sockets on the system?
Use the netstat. Check the man page for full details, but you should get some good output just
typing:
$ netstat
The only trick is determining which socket is associated with which program. 🙂
How can I view the routing table?
Run the route command (in /sbin on most Linuxes) or the command netstat -r.
How can I run the client and server programs if I only have one computer? Don’t I need a network
to write network programs?
Fortunately for you, virtually all machines implement a loopback network “device” that sits in the
kernel and pretends to be a network card. (This is the interface listed as “lo” in the routing table.)
Pretend you’re logged into a machine named “goat”. Run the client in one window and the server
in another. Or start the server in the background (“server &”) and run the client in the same window. The
upshot of the loopback device is that you can either client goat or client localhost (since “localhost”
is likely defined in your /etc/hosts file) and you’ll have the client talking to the server without a
network!
In short, no changes are necessary to any of the code to make it run on a single non-networked
machine! Huzzah!
How can I tell if the remote side has closed connection?
You can tell because recv() will return 0.
How do I implement a “ping” utility? What is ICMP? Where can I find out more about raw
sockets and SOCK_RAW?
All your raw sockets questions will be answered in W. Richard Stevens’ UNIX Network
Programming books. Also, look in the ping/ subdirectory in Stevens’ UNIX Network Programming
source code, available online37.
How do I change or shorten the timeout on a call to connect()?
Instead of giving you exactly the same answer that W. Richard Stevens would give you, I’ll just refer
you to lib/connect_nonb.c in the UNIX Network Programming source code38.
The gist of it is that you make a socket descriptor with socket(), set it to non-blocking, call
connect(), and if all goes well connect() will return -1 immediately and errno will be set to
EINPROGRESS. Then you call select() with whatever timeout you want, passing the socket descriptor
in both the read and write sets. If it doesn’t timeout, it means the connect() call completed. At this
point, you’ll have to use getsockopt() with the SO_ERROR option to get the return value from the
connect() call, which should be zero if there was no error.
37. http://www.unpbook.com/src.html
38. http://www.unpbook.com/src.html
55
http://www.unpbook.com/src.html
http://www.unpbook.com/src.html
Beej’s Guide to Network Programming 56
Finally, you’ll probably want to set the socket back to be blocking again before you start transferring
data over it.
Notice that this has the added benefit of allowing your program to do something else while it’s
connecting, too. You could, for example, set the timeout to something low, like 500 ms, and update an
indicator onscreen each timeout, then call select() again. When you’ve called select() and timed-
out, say, 20 times, you’ll know it’s time to give up on the connection.
Like I said, check out Stevens’ source for a perfectly excellent example.
How do I build for Windows?
First, delete Windows and install Linux or BSD. };-). No, actually, just see the section on building
for Windows in the introduction.
How do I build for Solaris/SunOS? I keep getting linker errors when I try to compile!
The linker errors happen because Sun boxes don’t automatically compile in the socket libraries. See
the section on building for Solaris/SunOS in the introduction for an example of how to do this.
Why does select() keep falling out on a signal?
Signals tend to cause blocked system calls to return -1 with errno set to EINTR. When you set up
a signal handler with sigaction(), you can set the flag SA_RESTART, which is supposed to restart the
system call after it was interrupted.
Naturally, this doesn’t always work.
My favorite solution to this involves a goto statement. You know this irritates your professors to no
end, so go for it!
select_restart:
if ((err = select(fdmax+1, &readfds, NULL, NULL, NULL)) == -1) {
if (errno == EINTR) {
// some signal just interrupted us, so restart
goto select_restart;
}
// handle the real error here:
perror(“select”);
}
Sure, you don’t need to use goto in this case; you can use other structures to control it. But I think
the goto statement is actually cleaner.
How can I implement a timeout on a call to recv()?
Use select()! It allows you to specify a timeout parameter for socket descriptors that you’re
looking to read from. Or, you could wrap the entire functionality in a single function, like this:
#include
#include
#include
#include
int recvtimeout(int s, char *buf, int len, int timeout)
{
fd_set fds;
int n;
struct timeval tv;
// set up the file descriptor set
FD_ZERO(&fds);
FD_SET(s, &fds);
// set up the struct timeval for the timeout
tv.tv_sec = timeout;
tv.tv_usec = 0;
// wait until timeout or data received
Beej’s Guide to Network Programming 57
n = select(s+1, &fds, NULL, NULL, &tv);
if (n == 0) return -2; // timeout!
if (n == -1) return -1; // error
// data must be here, so do a normal recv()
return recv(s, buf, len, 0);
}
.
.
.
// Sample call to recvtimeout():
n = recvtimeout(s, buf, sizeof buf, 10); // 10 second timeout
if (n == -1) {
// error occurred
perror(“recvtimeout”);
}
else if (n == -2) {
// timeout occurred
} else {
// got some data in buf
}
.
.
.
Notice that recvtimeout() returns -2 in case of a timeout. Why not return 0? Well, if you recall,
a return value of 0 on a call to recv() means that the remote side closed the connection. So that return
value is already spoken for, and -1 means “error”, so I chose -2 as my timeout indicator.
How do I encrypt or compress the data before sending it through the socket?
One easy way to do encryption is to use SSL (secure sockets layer), but that’s beyond the scope of
this guide. (Check out the OpenSSL project39 for more info.)
But assuming you want to plug in or implement your own compressor or encryption system, it’s just
a matter of thinking of your data as running through a sequence of steps between both ends. Each step
changes the data in some way.
1. server reads data from file (or wherever)
2. server encrypts/compresses data (you add this part)
3. server send()s encrypted data
Now the other way around:
1. client recv()s encrypted data
2. client decrypts/decompresses data (you add this part)
3. client writes data to file (or wherever)
If you’re going to compress and encrypt, just remember to compress first. 🙂
Just as long as the client properly undoes what the server does, the data will be fine in the end no
matter how many intermediate steps you add.
So all you need to do to use my code is to find the place between where the data is read and the data
is sent (using send()) over the network, and stick some code in there that does the encryption.
What is this “PF_INET” I keep seeing? Is it related to AF_INET?
Yes, yes it is. See the section on socket() for details.
39. http://www.openssl.org/
http://www.openssl.org/
Beej’s Guide to Network Programming 58
How can I write a server that accepts shell commands from a client and executes them?
For simplicity, lets say the client connect()s, send()s, and close()s the connection (that is,
there are no subsequent system calls without the client connecting again.)
The process the client follows is this:
1. connect() to server
2. send(“/sbin/ls > /tmp/client.out”)
3. close() the connection
Meanwhile, the server is handling the data and executing it:
1. accept() the connection from the client
2. recv(str) the command string
3. close() the connection
4. system(str) to run the command
Beware! Having the server execute what the client says is like giving remote shell access and people
can do things to your account when they connect to the server. For instance, in the above example, what
if the client sends “rm -rf ~”? It deletes everything in your account, that’s what!
So you get wise, and you prevent the client from using any except for a couple utilities that you
know are safe, like the foobar utility:
if (!strncmp(str, “foobar”, 6)) {
sprintf(sysstr, “%s > /tmp/server.out”, str);
system(sysstr);
}
But you’re still unsafe, unfortunately: what if the client enters “foobar; rm -rf ~”? The safest
thing to do is to write a little routine that puts an escape (“\”) character in front of all non-alphanumeric
characters (including spaces, if appropriate) in the arguments for the command.
As you can see, security is a pretty big issue when the server starts executing things the client sends.
I’m sending a slew of data, but when I recv(), it only receives 536 bytes or 1460 bytes at a time.
But if I run it on my local machine, it receives all the data at the same time. What’s going on?
You’re hitting the MTU—the maximum size the physical medium can handle. On the local machine,
you’re using the loopback device which can handle 8K or more no problem. But on Ethernet, which can
only handle 1500 bytes with a header, you hit that limit. Over a modem, with 576 MTU (again, with
header), you hit the even lower limit.
You have to make sure all the data is being sent, first of all. (See the sendall() function
implementation for details.) Once you’re sure of that, then you need to call recv() in a loop until all
your data is read.
Read the section Son of Data Encapsulation for details on receiving complete packets of data using
multiple calls to recv().
I’m on a Windows box and I don’t have the fork() system call or any kind of struct sigaction.
What to do?
If they’re anywhere, they’ll be in POSIX libraries that may have shipped with your compiler. Since I
don’t have a Windows box, I really can’t tell you the answer, but I seem to remember that Microsoft has a
POSIX compatibility layer and that’s where fork() would be. (And maybe even sigaction.)
Search the help that came with VC++ for “fork” or “POSIX” and see if it gives you any clues.
If that doesn’t work at all, ditch the fork()/sigaction stuff and replace it with the Win32
equivalent: CreateProcess(). I don’t know how to use CreateProcess()—it takes a bazillion
arguments, but it should be covered in the docs that came with VC++.
Beej’s Guide to Network Programming 59
I’m behind a firewall—how do I let people outside the firewall know my IP address so they can
connect to my machine?
Unfortunately, the purpose of a firewall is to prevent people outside the firewall from connecting to
machines inside the firewall, so allowing them to do so is basically considered a breach of security.
This isn’t to say that all is lost. For one thing, you can still often connect() through the firewall if
it’s doing some kind of masquerading or NAT or something like that. Just design your programs so that
you’re always the one initiating the connection, and you’ll be fine.
If that’s not satisfactory, you can ask your sysadmins to poke a hole in the firewall so that people can
connect to you. The firewall can forward to you either through it’s NAT software, or through a proxy or
something like that.
Be aware that a hole in the firewall is nothing to be taken lightly. You have to make sure you don’t
give bad people access to the internal network; if you’re a beginner, it’s a lot harder to make software
secure than you might imagine.
Don’t make your sysadmin mad at me. 😉
How do I write a packet sniffer? How do I put my Ethernet interface into promiscuous mode?
For those not in the know, when a network card is in “promiscuous mode”, it will forward ALL
packets to the operating system, not just those that were addressed to this particular machine. (We’re
talking Ethernet-layer addresses here, not IP addresses–but since ethernet is lower-layer than IP, all IP
addresses are effectively forwarded as well. See the section Low Level Nonsense and Network Theory
for more info.)
This is the basis for how a packet sniffer works. It puts the interface into promiscuous mode, then
the OS gets every single packet that goes by on the wire. You’ll have a socket of some type that you can
read this data from.
Unfortunately, the answer to the question varies depending on the platform, but if you Google for,
for instance, “windows promiscuous ioctl” you’ll probably get somewhere. There’s what looks like a
decent writeup in Linux Journal40, as well.
How can I set a custom timeout value for a TCP or UDP socket?
It depends on your system. You might search the net for SO_RCVTIMEO and SO_SNDTIMEO (for use
with setsockopt()) to see if your system supports such functionality.
The Linux man page suggests using alarm() or setitimer() as a substitute.
How can I tell which ports are available to use? Is there a list of “official” port numbers?
Usually this isn’t an issue. If you’re writing, say, a web server, then it’s a good idea to use the well-
known port 80 for your software. If you’re writing just your own specialized server, then choose a port at
random (but greater than 1023) and give it a try.
If the port is already in use, you’ll get an “Address already in use” error when you try to bind().
Choose another port. (It’s a good idea to allow the user of your software to specify an alternate port either
with a config file or a command line switch.)
There is a list of official port numbers41 maintained by the Internet Assigned Numbers Authority
(IANA). Just because something (over 1023) is in that list doesn’t mean you can’t use the port. For
instance, Id Software’s DOOM uses the same port as “mdqs”, whatever that is. All that matters is that no
one else on the same machine is using that port when you want to use it.
40. http://interactive.linuxjournal.com/article/4659
41. http://www.iana.org/assignments/port-numbers
http://interactive.linuxjournal.com/article/4659
http://interactive.linuxjournal.com/article/4659
http://www.iana.org/assignments/port-numbers
9. Man Pages
In the Unix world, there are a lot of manuals. They have little sections that describe individual
functions that you have at your disposal.
Of course, manual would be too much of a thing to type. I mean, no one in the Unix world,
including myself, likes to type that much. Indeed I could go on and on at great length about how much
I prefer to be terse but instead I shall be brief and not bore you with long-winded diatribes about how
utterly amazingly brief I prefer to be in virtually all circumstances in their entirety.
[Applause]
Thank you. What I am getting at is that these pages are called “man pages” in the Unix world, and I
have included my own personal truncated variant here for your reading enjoyment. The thing is, many of
these functions are way more general purpose than I’m letting on, but I’m only going to present the parts
that are relevant for Internet Sockets Programming.
But wait! That’s not all that’s wrong with my man pages:
• They are incomplete and only show the basics from the guide.
• There are many more man pages than this in the real world.
• They are different than the ones on your system.
• The header files might be different for certain functions on your system.
• The function parameters might be different for certain functions on your system.
If you want the real information, check your local Unix man pages by typing man whatever, where
“whatever” is something that you’re incredibly interested in, such as “accept”. (I’m sure Microsoft
Visual Studio has something similar in their help section. But “man” is better because it is one byte more
concise than “help”. Unix wins again!)
So, if these are so flawed, why even include them at all in the Guide? Well, there are a few reasons,
but the best are that (a) these versions are geared specifically toward network programming and are
easier to digest than the real ones, and (b) these versions contain examples!
Oh! And speaking of the examples, I don’t tend to put in all the error checking because it really
increases the length of the code. But you should absolutely do error checking pretty much any time
you make any of the system calls unless you’re totally 100% sure it’s not going to fail, and you should
probably do it even then!
60
Beej’s Guide to Network Programming 61
9.1. accept()
Accept an incoming connection on a listening socket
Prototypes
#include
#include
int accept(int s, struct sockaddr *addr, socklen_t *addrlen);
Description
Once you’ve gone through the trouble of getting a SOCK_STREAM socket and setting it up for
incoming connections with listen(), then you call accept() to actually get yourself a new socket
descriptor to use for subsequent communication with the newly connected client.
The old socket that you are using for listening is still there, and will be used for further accept()
calls as they come in.
s The listen()ing socket descriptor.
addr This is filled in with the address of the site that’s connecting to you.
addrlen This is filled in with the sizeof() the structure returned in the addr parameter.
You can safely ignore it if you assume you’re getting a struct sockaddr_in
back, which you know you are, because that’s the type you passed in for addr.
accept() will normally block, and you can use select() to peek on the listening socket
descriptor ahead of time to see if it’s “ready to read”. If so, then there’s a new connection waiting to
be accept()ed! Yay! Alternatively, you could set the O_NONBLOCK flag on the listening socket using
fcntl(), and then it will never block, choosing instead to return -1 with errno set to EWOULDBLOCK.
The socket descriptor returned by accept() is a bona fide socket descriptor, open and connected to
the remote host. You have to close() it when you’re done with it.
Return Value
accept() returns the newly connected socket descriptor, or -1 on error, with errno set
appropriately.
Example
struct sockaddr_storage their_addr;
socklen_t addr_size;
struct addrinfo hints, *res;
int sockfd, new_fd;
// first, load up address structs with getaddrinfo():
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // use IPv4 or IPv6, whichever
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE; // fill in my IP for me
getaddrinfo(NULL, MYPORT, &hints, &res);
// make a socket, bind it, and listen on it:
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
bind(sockfd, res->ai_addr, res->ai_addrlen);
listen(sockfd, BACKLOG);
// now accept an incoming connection:
addr_size = sizeof their_addr;
new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &addr_size);
Beej’s Guide to Network Programming 62
// ready to communicate on socket descriptor new_fd!
See Also
socket(), getaddrinfo(), listen(), struct sockaddr_in
Beej’s Guide to Network Programming 63
9.2. bind()
Associate a socket with an IP address and port number
Prototypes
#include
#include
int bind(int sockfd, struct sockaddr *my_addr, socklen_t addrlen);
Description
When a remote machine wants to connect to your server program, it needs two pieces of
information: the IP address and the port number. The bind() call allows you to do just that.
First, you call getaddrinfo() to load up a struct sockaddr with the destination address and
port information. Then you call socket() to get a socket descriptor, and then you pass the socket and
address into bind(), and the IP address and port are magically (using actual magic) bound to the socket!
If you don’t know your IP address, or you know you only have one IP address on the machine, or
you don’t care which of the machine’s IP addresses is used, you can simply pass the AI_PASSIVE flag
in the hints parameter to getaddrinfo(). What this does is fill in the IP address part of the struct
sockaddr with a special value that tells bind() that it should automatically fill in this host’s IP address.
What what? What special value is loaded into the struct sockaddr’s IP address to cause it to
auto-fill the address with the current host? I’ll tell you, but keep in mind this is only if you’re filling
out the struct sockaddr by hand; if not, use the results from getaddrinfo(), as per above. In
IPv4, the sin_addr.s_addr field of the struct sockaddr_in structure is set to INADDR_ANY. In
IPv6, the sin6_addr field of the struct sockaddr_in6 structure is assigned into from the global
variable in6addr_any. Or, if you’re declaring a new struct in6_addr, you can initialize it to
IN6ADDR_ANY_INIT.
Lastly, the addrlen parameter should be set to sizeof my_addr.
Return Value
Returns zero on success, or -1 on error (and errno will be set accordingly.)
Example
// modern way of doing things with getaddrinfo()
struct addrinfo hints, *res;
int sockfd;
// first, load up address structs with getaddrinfo():
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // use IPv4 or IPv6, whichever
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE; // fill in my IP for me
getaddrinfo(NULL, “3490”, &hints, &res);
// make a socket:
// (you should actually walk the “res” linked list and error-check!)
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
// bind it to the port we passed in to getaddrinfo():
bind(sockfd, res->ai_addr, res->ai_addrlen);
// example of packing a struct by hand, IPv4
struct sockaddr_in myaddr;
int s;
Beej’s Guide to Network Programming 64
myaddr.sin_family = AF_INET;
myaddr.sin_port = htons(3490);
// you can specify an IP address:
inet_pton(AF_INET, “63.161.169.137”, &(myaddr.sin_addr));
// or you can let it automatically select one:
myaddr.sin_addr.s_addr = INADDR_ANY;
s = socket(PF_INET, SOCK_STREAM, 0);
bind(s, (struct sockaddr*)&myaddr, sizeof myaddr);
See Also
getaddrinfo(), socket(), struct sockaddr_in, struct in_addr
Beej’s Guide to Network Programming 65
9.3. connect()
Connect a socket to a server
Prototypes
#include
#include
int connect(int sockfd, const struct sockaddr *serv_addr,
socklen_t addrlen);
Description
Once you’ve built a socket descriptor with the socket() call, you can connect() that socket to
a remote server using the well-named connect() system call. All you need to do is pass it the socket
descriptor and the address of the server you’re interested in getting to know better. (Oh, and the length of
the address, which is commonly passed to functions like this.)
Usually this information comes along as the result of a call to getaddrinfo(), but you can fill out
your own struct sockaddr if you want to.
If you haven’t yet called bind() on the socket descriptor, it is automatically bound to your IP
address and a random local port. This is usually just fine with you if you’re not a server, since you
really don’t care what your local port is; you only care what the remote port is so you can put it in the
serv_addr parameter. You can call bind() if you really want your client socket to be on a specific IP
address and port, but this is pretty rare.
Once the socket is connect()ed, you’re free to send() and recv() data on it to your heart’s
content.
Special note: if you connect() a SOCK_DGRAM UDP socket to a remote host, you can use send()
and recv() as well as sendto() and recvfrom(). If you want.
Return Value
Returns zero on success, or -1 on error (and errno will be set accordingly.)
Example
// connect to www.example.com port 80 (http)
struct addrinfo hints, *res;
int sockfd;
// first, load up address structs with getaddrinfo():
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // use IPv4 or IPv6, whichever
hints.ai_socktype = SOCK_STREAM;
// we could put “80” instead on “http” on the next line:
getaddrinfo(“www.example.com”, “http”, &hints, &res);
// make a socket:
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
// connect it to the address and port we passed in to getaddrinfo():
connect(sockfd, res->ai_addr, res->ai_addrlen);
See Also
socket(), bind()
Beej’s Guide to Network Programming 66
9.4. close()
Close a socket descriptor
Prototypes
#include
int close(int s);
Description
After you’ve finished using the socket for whatever demented scheme you have concocted and you
don’t want to send() or recv() or, indeed, do anything else at all with the socket, you can close() it,
and it’ll be freed up, never to be used again.
The remote side can tell if this happens one of two ways. One: if the remote side calls recv(), it
will return 0. Two: if the remote side calls send(), it’ll receive a signal SIGPIPE and send() will return
-1 and errno will be set to EPIPE.
Windows users: the function you need to use is called closesocket(), not close(). If you try
to use close() on a socket descriptor, it’s possible Windows will get angry… And you wouldn’t like it
when it’s angry.
Return Value
Returns zero on success, or -1 on error (and errno will be set accordingly.)
Example
s = socket(PF_INET, SOCK_DGRAM, 0);
.
.
.
// a whole lotta stuff…*BRRRONNNN!*
.
.
.
close(s); // not much to it, really.
See Also
socket(), shutdown()
Beej’s Guide to Network Programming 67
9.5. getaddrinfo(), freeaddrinfo(),
gai_strerror()
Get information about a host name and/or service and load up a struct sockaddr with the result.
Prototypes
#include
#include
#include
int getaddrinfo(const char *nodename, const char *servname,
const struct addrinfo *hints, struct addrinfo **res);
void freeaddrinfo(struct addrinfo *ai);
const char *gai_strerror(int ecode);
struct addrinfo {
int ai_flags; // AI_PASSIVE, AI_CANONNAME, …
int ai_family; // AF_xxx
int ai_socktype; // SOCK_xxx
int ai_protocol; // 0 (auto) or IPPROTO_TCP, IPPROTO_UDP
socklen_t ai_addrlen; // length of ai_addr
char *ai_canonname; // canonical name for nodename
struct sockaddr *ai_addr; // binary address
struct addrinfo *ai_next; // next structure in linked list
};
Description
getaddrinfo() is an excellent function that will return information on a particular host name
(such as its IP address) and load up a struct sockaddr for you, taking care of the gritty details (like
if it’s IPv4 or IPv6.) It replaces the old functions gethostbyname() and getservbyname().The
description, below, contains a lot of information that might be a little daunting, but actual usage is pretty
simple. It might be worth it to check out the examples first.
The host name that you’re interested in goes in the nodename parameter. The address can be either a
host name, like “www.example.com”, or an IPv4 or IPv6 address (passed as a string). This parameter can
also be NULL if you’re using the AI_PASSIVE flag (see below.)
The servname parameter is basically the port number. It can be a port number (passed as a string,
like “80”), or it can be a service name, like “http” or “tftp” or “smtp” or “pop”, etc. Well-known service
names can be found in the IANA Port List42 or in your /etc/services file.
Lastly, for input parameters, we have hints. This is really where you get to define what the
getaddrinfo() function is going to do. Zero the whole structure before use with memset(). Let’s take
a look at the fields you need to set up before use.
The ai_flags can be set to a variety of things, but here are a couple important ones. (Multiple
flags can be specified by bitwise-ORing them together with the | operator.) Check your man page for the
complete list of flags.
AI_CANONNAME causes the ai_canonname of the result to the filled out with the host’s canonical
(real) name. AI_PASSIVE causes the result’s IP address to be filled out with INADDR_ANY (IPv4)or
in6addr_any (IPv6); this causes a subsequent call to bind() to auto-fill the IP address of the struct
sockaddr with the address of the current host. That’s excellent for setting up a server when you don’t
want to hardcode the address.
If you do use the AI_PASSIVE, flag, then you can pass NULL in the nodename (since bind() will
fill it in for you later.)
42. http://www.iana.org/assignments/port-numbers
http://www.iana.org/assignments/port-numbers
Beej’s Guide to Network Programming 68
Continuing on with the input paramters, you’ll likely want to set ai_family to AF_UNSPEC which
tells getaddrinfo() to look for both IPv4 and IPv6 addresses. You can also restrict yourself to one or
the other with AF_INET or AF_INET6.
Next, the socktype field should be set to SOCK_STREAM or SOCK_DGRAM, depending on which
type of socket you want.
Finally, just leave ai_protocol at 0 to automatically choose your protocol type.
Now, after you get all that stuff in there, you can finally make the call to getaddrinfo()!
Of course, this is where the fun begins. The res will now point to a linked list of struct
addrinfos, and you can go through this list to get all the addresses that match what you passed in with
the hints.
Now, it’s possible to get some addresses that don’t work for one reason or another, so what the Linux
man page does is loops through the list doing a call to socket() and connect() (or bind() if you’re
setting up a server with the AI_PASSIVE flag) until it succeeds.
Finally, when you’re done with the linked list, you need to call freeaddrinfo() to free up the
memory (or it will be leaked, and Some People will get upset.)
Return Value
Returns zero on success, or nonzero on error. If it returns nonzero, you can use the function
gai_strerror() to get a printable version of the error code in the return value.
Example
// code for a client connecting to a server
// namely a stream socket to www.example.com on port 80 (http)
// either IPv4 or IPv6
int sockfd;
struct addrinfo hints, *servinfo, *p;
int rv;
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // use AF_INET6 to force IPv6
hints.ai_socktype = SOCK_STREAM;
if ((rv = getaddrinfo(“www.example.com”, “http”, &hints, &servinfo)) != 0) {
fprintf(stderr, “getaddrinfo: %s\n”, gai_strerror(rv));
exit(1);
}
// loop through all the results and connect to the first we can
for(p = servinfo; p != NULL; p = p->ai_next) {
if ((sockfd = socket(p->ai_family, p->ai_socktype,
p->ai_protocol)) == -1) {
perror(“socket”);
continue;
}
if (connect(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
perror(“connect”);
close(sockfd);
continue;
}
break; // if we get here, we must have connected successfully
}
if (p == NULL) {
// looped off the end of the list with no connection
fprintf(stderr, “failed to connect\n”);
exit(2);
}
freeaddrinfo(servinfo); // all done with this structure
Beej’s Guide to Network Programming 69
// code for a server waiting for connections
// namely a stream socket on port 3490, on this host’s IP
// either IPv4 or IPv6.
int sockfd;
struct addrinfo hints, *servinfo, *p;
int rv;
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // use AF_INET6 to force IPv6
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE; // use my IP address
if ((rv = getaddrinfo(NULL, “3490”, &hints, &servinfo)) != 0) {
fprintf(stderr, “getaddrinfo: %s\n”, gai_strerror(rv));
exit(1);
}
// loop through all the results and bind to the first we can
for(p = servinfo; p != NULL; p = p->ai_next) {
if ((sockfd = socket(p->ai_family, p->ai_socktype,
p->ai_protocol)) == -1) {
perror(“socket”);
continue;
}
if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
close(sockfd);
perror(“bind”);
continue;
}
break; // if we get here, we must have connected successfully
}
if (p == NULL) {
// looped off the end of the list with no successful bind
fprintf(stderr, “failed to bind socket\n”);
exit(2);
}
freeaddrinfo(servinfo); // all done with this structure
See Also
gethostbyname(), getnameinfo()
Beej’s Guide to Network Programming 70
9.6. gethostname()
Returns the name of the system
Prototypes
#include
int gethostname(char *name, size_t len);
Description
Your system has a name. They all do. This is a slightly more Unixy thing than the rest of the
networky stuff we’ve been talking about, but it still has its uses.
For instance, you can get your host name, and then call gethostbyname() to find out your IP
address.
The parameter name should point to a buffer that will hold the host name, and len is the size of
that buffer in bytes. gethostname() won’t overwrite the end of the buffer (it might return an error, or it
might just stop writing), and it will NUL-terminate the string if there’s room for it in the buffer.
Return Value
Returns zero on success, or -1 on error (and errno will be set accordingly.)
Example
char hostname[128];
gethostname(hostname, sizeof hostname);
printf(“My hostname: %s\n”, hostname);
See Also
gethostbyname()
Beej’s Guide to Network Programming 71
9.7. gethostbyname(), gethostbyaddr()
Get an IP address for a hostname, or vice-versa
Prototypes
#include
#include
struct hostent *gethostbyname(const char *name); // DEPRECATED!
struct hostent *gethostbyaddr(const char *addr, int len, int type);
Description
PLEASE NOTE: these two functions are superseded by getaddrinfo() and getnameinfo()! In
particular, gethostbyname() doesn’t work well with IPv6.
These functions map back and forth between host names and IP addresses. For instance, if you have
“www.example.com”, you can use gethostbyname() to get its IP address and store it in a struct
in_addr.
Conversely, if you have a struct in_addr or a struct in6_addr, you can use
gethostbyaddr() to get the hostname back. gethostbyaddr() is IPv6 compatible, but you should
use the newer shinier getnameinfo() instead.
(If you have a string containing an IP address in dots-and-numbers format that you want to look up
the hostname of, you’d be better off using getaddrinfo() with the AI_CANONNAME flag.)
gethostbyname() takes a string like “www.yahoo.com”, and returns a struct hostent which
contains tons of information, including the IP address. (Other information is the official host name, a list
of aliases, the address type, the length of the addresses, and the list of addresses—it’s a general-purpose
structure that’s pretty easy to use for our specific purposes once you see how.)
gethostbyaddr() takes a struct in_addr or struct in6_addr and brings you up a
corresponding host name (if there is one), so it’s sort of the reverse of gethostbyname(). As for
parameters, even though addr is a char*, you actually want to pass in a pointer to a struct in_addr.
len should be sizeof(struct in_addr), and type should be AF_INET.
So what is this struct hostent that gets returned? It has a number of fields that contain
information about the host in question.
char *h_name The real canonical host name.
char **h_aliases A list of aliases that can be accessed with arrays—the last element is
NULL
int h_addrtype The result’s address type, which really should be AF_INET for our
purposes.
int length The length of the addresses in bytes, which is 4 for IP (version 4)
addresses.
char **h_addr_list A list of IP addresses for this host. Although this is a char**, it’s
really an array of struct in_addr*s in disguise. The last array
element is NULL.
h_addr A commonly defined alias for h_addr_list[0]. If you just want any
old IP address for this host (yeah, they can have more than one) just
use this field.
Return Value
Returns a pointer to a resultant struct hostent on success, or NULL on error.
Instead of the normal perror() and all that stuff you’d normally use for error reporting, these
functions have parallel results in the variable h_errno, which can be printed using the functions
herror() or hstrerror(). These work just like the classic errno, perror(), and strerror()
functions you’re used to.
Beej’s Guide to Network Programming 72
Example
// THIS IS A DEPRECATED METHOD OF GETTING HOST NAMES
// use getaddrinfo() instead!
#include
#include
#include
#include
#include
#include
#include
int main(int argc, char *argv[])
{
int i;
struct hostent *he;
struct in_addr **addr_list;
if (argc != 2) {
fprintf(stderr,”usage: ghbn hostname\n”);
return 1;
}
if ((he = gethostbyname(argv[1])) == NULL) { // get the host info
herror(“gethostbyname”);
return 2;
}
// print information about this host:
printf(“Official name is: %s\n”, he->h_name);
printf(” IP addresses: “);
addr_list = (struct in_addr **)he->h_addr_list;
for(i = 0; addr_list[i] != NULL; i++) {
printf(“%s “, inet_ntoa(*addr_list[i]));
}
printf(“\n”);
return 0;
}
// THIS HAS BEEN SUPERCEDED
// use getnameinfo() instead!
struct hostent *he;
struct in_addr ipv4addr;
struct in6_addr ipv6addr;
inet_pton(AF_INET, “192.0.2.34”, &ipv4addr);
he = gethostbyaddr(&ipv4addr, sizeof ipv4addr, AF_INET);
printf(“Host name: %s\n”, he->h_name);
inet_pton(AF_INET6, “2001:db8:63b3:1::beef”, &ipv6addr);
he = gethostbyaddr(&ipv6addr, sizeof ipv6addr, AF_INET6);
printf(“Host name: %s\n”, he->h_name);
See Also
getaddrinfo(), getnameinfo(), gethostname(), errno, perror(), strerror(), struct
in_addr
Beej’s Guide to Network Programming 73
9.8. getnameinfo()
Look up the host name and service name information for a given struct sockaddr.
Prototypes
#include
#include
int getnameinfo(const struct sockaddr *sa, socklen_t salen,
char *host, size_t hostlen,
char *serv, size_t servlen, int flags);
Description
This function is the opposite of getaddrinfo(), that is, this function takes an already
loaded struct sockaddr and does a name and service name lookup on it. It replaces the old
gethostbyaddr() and getservbyport() functions.
You have to pass in a pointer to a struct sockaddr (which in actuality is probably a struct
sockaddr_in or struct sockaddr_in6 that you’ve cast) in the sa parameter, and the length of that
struct in the salen.
The resultant host name and service name will be written to the area pointed to by the host and
serv parameters. Of course, you have to specify the max lengths of these buffers in hostlen and
servlen.
Finally, there are several flags you can pass, but here a a couple good ones. NI_NOFQDN will cause
the host to only contain the host name, not the whole domain name. NI_NAMEREQD will cause the
function to fail if the name cannot be found with a DNS lookup (if you don’t specify this flag and the
name can’t be found, getnameinfo() will put a string version of the IP address in host instead.)
As always, check your local man pages for the full scoop.
Return Value
Returns zero on success, or non-zero on error. If the return value is non-zero, it can be passed to
gai_strerror() to get a human-readable string. See getaddrinfo for more information.
Example
struct sockaddr_in6 sa; // could be IPv4 if you want
char host[1024];
char service[20];
// pretend sa is full of good information about the host and port…
getnameinfo(&sa, sizeof sa, host, sizeof host, service, sizeof service, 0);
printf(” host: %s\n”, host); // e.g. “www.example.com”
printf(“service: %s\n”, service); // e.g. “http”
See Also
getaddrinfo(), gethostbyaddr()
Beej’s Guide to Network Programming 74
9.9. getpeername()
Return address info about the remote side of the connection
Prototypes
#include
int getpeername(int s, struct sockaddr *addr, socklen_t *len);
Description
Once you have either accept()ed a remote connection, or connect()ed to a server, you now
have what is known as a peer. Your peer is simply the computer you’re connected to, identified by an IP
address and a port. So…
getpeername() simply returns a struct sockaddr_in filled with information about the
machine you’re connected to.
Why is it called a “name”? Well, there are a lot of different kinds of sockets, not just Internet
Sockets like we’re using in this guide, and so “name” was a nice generic term that covered all cases. In
our case, though, the peer’s “name” is it’s IP address and port.
Although the function returns the size of the resultant address in len, you must preload len with
the size of addr.
Return Value
Returns zero on success, or -1 on error (and errno will be set accordingly.)
Example
// assume s is a connected socket
socklen_t len;
struct sockaddr_storage addr;
char ipstr[INET6_ADDRSTRLEN];
int port;
len = sizeof addr;
getpeername(s, (struct sockaddr*)&addr, &len);
// deal with both IPv4 and IPv6:
if (addr.ss_family == AF_INET) {
struct sockaddr_in *s = (struct sockaddr_in *)&addr;
port = ntohs(s->sin_port);
inet_ntop(AF_INET, &s->sin_addr, ipstr, sizeof ipstr);
} else { // AF_INET6
struct sockaddr_in6 *s = (struct sockaddr_in6 *)&addr;
port = ntohs(s->sin6_port);
inet_ntop(AF_INET6, &s->sin6_addr, ipstr, sizeof ipstr);
}
printf(“Peer IP address: %s\n”, ipstr);
printf(“Peer port : %d\n”, port);
See Also
gethostname(), gethostbyname(), gethostbyaddr()
Beej’s Guide to Network Programming 75
9.10. errno
Holds the error code for the last system call
Prototypes
#include
int errno;
Description
This is the variable that holds error information for a lot of system calls. If you’ll recall, things like
socket() and listen() return -1 on error, and they set the exact value of errno to let you know
specifically which error occurred.
The header file errno.h lists a bunch of constant symbolic names for errors, such as EADDRINUSE,
EPIPE, ECONNREFUSED, etc. Your local man pages will tell you what codes can be returned as an error,
and you can use these at run time to handle different errors in different ways.
Or, more commonly, you can call perror() or strerror() to get a human-readable version of
the error.
One thing to note, for you multithreading enthusiasts, is that on most systems errno is defined in
a threadsafe manner. (That is, it’s not actually a global variable, but it behaves just like a global variable
would in a single-threaded environment.)
Return Value
The value of the variable is the latest error to have transpired, which might be the code for “success”
if the last action succeeded.
Example
s = socket(PF_INET, SOCK_STREAM, 0);
if (s == -1) {
perror(“socket”); // or use strerror()
}
tryagain:
if (select(n, &readfds, NULL, NULL) == -1) {
// an error has occurred!!
// if we were only interrupted, just restart the select() call:
if (errno == EINTR) goto tryagain; // AAAA! goto!!!
// otherwise it’s a more serious error:
perror(“select”);
exit(1);
}
See Also
perror(), strerror()
Beej’s Guide to Network Programming 76
9.11. fcntl()
Control socket descriptors
Prototypes
#include
#include
int fcntl(int s, int cmd, long arg);
Description
This function is typically used to do file locking and other file-oriented stuff, but it also has a couple
socket-related functions that you might see or use from time to time.
Parameter s is the socket descriptor you wish to operate on, cmd should be set to F_SETFL, and arg
can be one of the following commands. (Like I said, there’s more to fcntl() than I’m letting on here,
but I’m trying to stay socket-oriented.)
O_NONBLOCK Set the socket to be non-blocking. See the section on blocking for more
details.
O_ASYNC Set the socket to do asynchronous I/O. When data is ready to be recv()’d
on the socket, the signal SIGIO will be raised. This is rare to see, and
beyond the scope of the guide. And I think it’s only available on certain
systems.
Return Value
Returns zero on success, or -1 on error (and errno will be set accordingly.)
Different uses of the fcntl() system call actually have different return values, but I haven’t
covered them here because they’re not socket-related. See your local fcntl() man page for more
information.
Example
int s = socket(PF_INET, SOCK_STREAM, 0);
fcntl(s, F_SETFL, O_NONBLOCK); // set to non-blocking
fcntl(s, F_SETFL, O_ASYNC); // set to asynchronous I/O
See Also
Blocking, send()
Beej’s Guide to Network Programming 77
9.12. htons(), htonl(), ntohs(), ntohl()
Convert multi-byte integer types from host byte order to network byte order
Prototypes
#include
uint32_t htonl(uint32_t hostlong);
uint16_t htons(uint16_t hostshort);
uint32_t ntohl(uint32_t netlong);
uint16_t ntohs(uint16_t netshort);
Description
Just to make you really unhappy, different computers use different byte orderings internally for their
multibyte integers (i.e. any integer that’s larger than a char.) The upshot of this is that if you send() a
two-byte short int from an Intel box to a Mac (before they became Intel boxes, too, I mean), what one
computer thinks is the number 1, the other will think is the number 256, and vice-versa.
The way to get around this problem is for everyone to put aside their differences and agree that
Motorola and IBM had it right, and Intel did it the weird way, and so we all convert our byte orderings
to “big-endian” before sending them out. Since Intel is a “little-endian” machine, it’s far more politically
correct to call our preferred byte ordering “Network Byte Order”. So these functions convert from your
native byte order to network byte order and back again.
(This means on Intel these functions swap all the bytes around, and on PowerPC they do nothing
because the bytes are already in Network Byte Order. But you should always use them in your code
anyway, since someone might want to build it on an Intel machine and still have things work properly.)
Note that the types involved are 32-bit (4 byte, probably int) and 16-bit (2 byte, very likely short)
numbers. 64-bit machines might have a htonll() for 64-bit ints, but I’ve not seen it. You’ll just have to
write your own.
Anyway, the way these functions work is that you first decide if you’re converting from host (your
machine’s) byte order or from network byte order. If “host”, the the first letter of the function you’re
going to call is “h”. Otherwise it’s “n” for “network”. The middle of the function name is always “to”
because you’re converting from one “to” another, and the penultimate letter shows what you’re converting
to. The last letter is the size of the data, “s” for short, or “l” for long. Thus:
htons() host to network short
htonl() host to network long
ntohs() network to host short
ntohl() network to host long
Return Value
Each function returns the converted value.
Example
uint32_t some_long = 10;
uint16_t some_short = 20;
uint32_t network_byte_order;
// convert and send
network_byte_order = htonl(some_long);
send(s, &network_byte_order, sizeof(uint32_t), 0);
some_short == ntohs(htons(some_short)); // this expression is true
Beej’s Guide to Network Programming 78
9.13. inet_ntoa(), inet_aton(), inet_addr
Convert IP addresses from a dots-and-number string to a struct in_addr and back
Prototypes
#include
#include
#include
// ALL THESE ARE DEPRECATED! Use inet_pton() or inet_ntop() instead!!
char *inet_ntoa(struct in_addr in);
int inet_aton(const char *cp, struct in_addr *inp);
in_addr_t inet_addr(const char *cp);
Description
These functions are deprecated because they don’t handle IPv6! Use inet_ntop() or
inet_pton() instead! They are included here because they can still be found in the wild.
All of these functions convert from a struct in_addr (part of your struct sockaddr_in,
most likely) to a string in dots-and-numbers format (e.g. “192.168.5.10”) and vice-versa. If you have
an IP address passed on the command line or something, this is the easiest way to get a struct
in_addr to connect() to, or whatever. If you need more power, try some of the DNS functions like
gethostbyname() or attempt a coup d’État in your local country.
The function inet_ntoa() converts a network address in a struct in_addr to a dots-and-
numbers format string. The “n” in “ntoa” stands for network, and the “a” stands for ASCII for historical
reasons (so it’s “Network To ASCII”—the “toa” suffix has an analogous friend in the C library called
atoi() which converts an ASCII string to an integer.)
The function inet_aton() is the opposite, converting from a dots-and-numbers string into a
in_addr_t (which is the type of the field s_addr in your struct in_addr.)
Finally, the function inet_addr() is an older function that does basically the same thing as
inet_aton(). It’s theoretically deprecated, but you’ll see it a lot and the police won’t come get you if
you use it.
Return Value
inet_aton() returns non-zero if the address is a valid one, and it returns zero if the address is
invalid.
inet_ntoa() returns the dots-and-numbers string in a static buffer that is overwritten with each
call to the function.
inet_addr() returns the address as an in_addr_t, or -1 if there’s an error. (That is the same
result as if you tried to convert the string “255.255.255.255”, which is a valid IP address. This is why
inet_aton() is better.)
Example
struct sockaddr_in antelope;
char *some_addr;
inet_aton(“10.0.0.1”, &antelope.sin_addr); // store IP in antelope
some_addr = inet_ntoa(antelope.sin_addr); // return the IP
printf(“%s\n”, some_addr); // prints “10.0.0.1”
// and this call is the same as the inet_aton() call, above:
antelope.sin_addr.s_addr = inet_addr(“10.0.0.1”);
See Also
inet_ntop(), inet_pton(), gethostbyname(), gethostbyaddr()
Beej’s Guide to Network Programming 79
9.14. inet_ntop(), inet_pton()
Convert IP addresses to human-readable form and back.
Prototypes
#include
const char *inet_ntop(int af, const void *src,
char *dst, socklen_t size);
int inet_pton(int af, const char *src, void *dst);
Description
These functions are for dealing with human-readable IP addresses and converting them to their
binary representation for use with various functions and system calls. The “n” stands for “network”, and
“p” for “presentation”. Or “text presentation”. But you can think of it as “printable”. “ntop” is “network
to printable”. See?
Sometimes you don’t want to look at a pile of binary numbers when looking at an IP address. You
want it in a nice printable form, like 192.0.2.180, or 2001:db8:8714:3a90::12. In that case,
inet_ntop() is for you.
inet_ntop() takes the address family in the af parameter (either AF_INET or AF_INET6). The
src parameter should be a pointer to either a struct in_addr or struct in6_addr containing the
address you wish to convert to a string. Finally dst and size are the pointer to the destination string and
the maximum length of that string.
What should the maximum length of the dst string be? What is the maximum length for IPv4 and
IPv6 addresses? Fortunately there are a couple of macros to help you out. The maximum lengths are:
INET_ADDRSTRLEN and INET6_ADDRSTRLEN.
Other times, you might have a string containing an IP address in readable form, and you want to
pack it into a struct sockaddr_in or a struct sockaddr_in6. In that case, the opposite funcion
inet_pton() is what you’re after.
inet_pton() also takes an address family (either AF_INET or AF_INET6) in the af parameter.
The src parameter is a pointer to a string containing the IP address in printable form. Lastly the dst
parameter points to where the result should be stored, which is probably a struct in_addr or struct
in6_addr.
These functions don’t do DNS lookups—you’ll need getaddrinfo() for that.
Return Value
inet_ntop() returns the dst parameter on success, or NULL on failure (and errno is set).
inet_pton() returns 1 on success. It returns -1 if there was an error (errno is set), or 0 if the
input isn’t a valid IP address.
Example
// IPv4 demo of inet_ntop() and inet_pton()
struct sockaddr_in sa;
char str[INET_ADDRSTRLEN];
// store this IP address in sa:
inet_pton(AF_INET, “192.0.2.33”, &(sa.sin_addr));
// now get it back and print it
inet_ntop(AF_INET, &(sa.sin_addr), str, INET_ADDRSTRLEN);
printf(“%s\n”, str); // prints “192.0.2.33”
// IPv6 demo of inet_ntop() and inet_pton()
// (basically the same except with a bunch of 6s thrown around)
struct sockaddr_in6 sa;
Beej’s Guide to Network Programming 80
char str[INET6_ADDRSTRLEN];
// store this IP address in sa:
inet_pton(AF_INET6, “2001:db8:8714:3a90::12”, &(sa.sin6_addr));
// now get it back and print it
inet_ntop(AF_INET6, &(sa.sin6_addr), str, INET6_ADDRSTRLEN);
printf(“%s\n”, str); // prints “2001:db8:8714:3a90::12”
// Helper function you can use:
//Convert a struct sockaddr address to a string, IPv4 and IPv6:
char *get_ip_str(const struct sockaddr *sa, char *s, size_t maxlen)
{
switch(sa->sa_family) {
case AF_INET:
inet_ntop(AF_INET, &(((struct sockaddr_in *)sa)->sin_addr),
s, maxlen);
break;
case AF_INET6:
inet_ntop(AF_INET6, &(((struct sockaddr_in6 *)sa)->sin6_addr),
s, maxlen);
break;
default:
strncpy(s, “Unknown AF”, maxlen);
return NULL;
}
return s;
}
See Also
getaddrinfo()
Beej’s Guide to Network Programming 81
9.15. listen()
Tell a socket to listen for incoming connections
Prototypes
#include
int listen(int s, int backlog);
Description
You can take your socket descriptor (made with the socket() system call) and tell it to listen for
incoming connections. This is what differentiates the servers from the clients, guys.
The backlog parameter can mean a couple different things depending on the system you on, but
loosely it is how many pending connections you can have before the kernel starts rejecting new ones. So
as the new connections come in, you should be quick to accept() them so that the backlog doesn’t fill.
Try setting it to 10 or so, and if your clients start getting “Connection refused” under heavy load, set it
higher.
Before calling listen(), your server should call bind() to attach itself to a specific port number.
That port number (on the server’s IP address) will be the one that clients connect to.
Return Value
Returns zero on success, or -1 on error (and errno will be set accordingly.)
Example
struct addrinfo hints, *res;
int sockfd;
// first, load up address structs with getaddrinfo():
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // use IPv4 or IPv6, whichever
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE; // fill in my IP for me
getaddrinfo(NULL, “3490”, &hints, &res);
// make a socket:
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
// bind it to the port we passed in to getaddrinfo():
bind(sockfd, res->ai_addr, res->ai_addrlen);
listen(sockfd, 10); // set s up to be a server (listening) socket
// then have an accept() loop down here somewhere
See Also
accept(), bind(), socket()
Beej’s Guide to Network Programming 82
9.16. perror(), strerror()
Print an error as a human-readable string
Prototypes
#include
#include
void perror(const char *s);
char *strerror(int errnum);
Description
Since so many functions return -1 on error and set the value of the variable errno to be some
number, it would sure be nice if you could easily print that in a form that made sense to you.
Mercifully, perror() does that. If you want more description to be printed before the error, you
can point the parameter s to it (or you can leave s as NULL and nothing additional will be printed.)
In a nutshell, this function takes errno values, like ECONNRESET, and prints them nicely, like
“Connection reset by peer.”
The function strerror() is very similar to perror(), except it returns a pointer to the error
message string for a given value (you usually pass in the variable errno.)
Return Value
strerror() returns a pointer to the error message string.
Example
int s;
s = socket(PF_INET, SOCK_STREAM, 0);
if (s == -1) { // some error has occurred
// prints “socket error: ” + the error message:
perror(“socket error”);
}
// similarly:
if (listen(s, 10) == -1) {
// this prints “an error: ” + the error message from errno:
printf(“an error: %s\n”, strerror(errno));
}
See Also
errno
Beej’s Guide to Network Programming 83
9.17. poll()
Test for events on multiple sockets simultaneously
Prototypes
#include
int poll(struct pollfd *ufds, unsigned int nfds, int timeout);
Description
This function is very similar to select() in that they both watch sets of file descriptors for events,
such as incoming data ready to recv(), socket ready to send() data to, out-of-band data ready to
recv(), errors, etc.
The basic idea is that you pass an array of nfds struct pollfds in ufds, along with a timeout in
milliseconds (1000 milliseconds in a second.) The timeout can be negative if you want to wait forever.
If no event happens on any of the socket descriptors by the timeout, poll() will return.
Each element in the array of struct pollfds represents one socket descriptor, and contains the
following fields:
struct pollfd {
int fd; // the socket descriptor
short events; // bitmap of events we’re interested in
short revents; // when poll() returns, bitmap of events that occurred
};
Before calling poll(), load fd with the socket descriptor (if you set fd to a negative number, this
struct pollfd is ignored and its revents field is set to zero) and then construct the events field by
bitwise-ORing the following macros:
POLLIN Alert me when data is ready to recv() on this socket.
POLLOUT Alert me when I can send() data to this socket without blocking.
POLLPRI Alert me when out-of-band data is ready to recv() on this socket.
Once the poll() call returns, the revents field will be constructed as a bitwise-OR of the above
fields, telling you which descriptors actually have had that event occur. Additionally, these other fields
might be present:
POLLERR An error has occurred on this socket.
POLLHUP The remote side of the connection hung up.
POLLNVAL Something was wrong with the socket descriptor fd—maybe it’s
uninitialized?
Return Value
Returns the number of elements in the ufds array that have had event occur on them; this can be
zero if the timeout occurred. Also returns -1 on error (and errno will be set accordingly.)
Example
int s1, s2;
int rv;
char buf1[256], buf2[256];
struct pollfd ufds[2];
s1 = socket(PF_INET, SOCK_STREAM, 0);
s2 = socket(PF_INET, SOCK_STREAM, 0);
// pretend we’ve connected both to a server at this point
//connect(s1, …)…
//connect(s2, …)…
Beej’s Guide to Network Programming 84
// set up the array of file descriptors.
//
// in this example, we want to know when there’s normal or out-of-band
// data ready to be recv()’d…
ufds[0].fd = s1;
ufds[0].events = POLLIN | POLLPRI; // check for normal or out-of-band
ufds[1].fd = s2;
ufds[1].events = POLLIN; // check for just normal data
// wait for events on the sockets, 3.5 second timeout
rv = poll(ufds, 2, 3500);
if (rv == -1) {
perror(“poll”); // error occurred in poll()
} else if (rv == 0) {
printf(“Timeout occurred! No data after 3.5 seconds.\n”);
} else {
// check for events on s1:
if (ufds[0].revents & POLLIN) {
recv(s1, buf1, sizeof buf1, 0); // receive normal data
}
if (ufds[0].revents & POLLPRI) {
recv(s1, buf1, sizeof buf1, MSG_OOB); // out-of-band data
}
// check for events on s2:
if (ufds[1].revents & POLLIN) {
recv(s1, buf2, sizeof buf2, 0);
}
}
See Also
select()
Beej’s Guide to Network Programming 85
9.18. recv(), recvfrom()
Receive data on a socket
Prototypes
#include
#include
ssize_t recv(int s, void *buf, size_t len, int flags);
ssize_t recvfrom(int s, void *buf, size_t len, int flags,
struct sockaddr *from, socklen_t *fromlen);
Description
Once you have a socket up and connected, you can read incoming data from the remote side using
the recv() (for TCP SOCK_STREAM sockets) and recvfrom() (for UDP SOCK_DGRAM sockets).
Both functions take the socket descriptor s, a pointer to the buffer buf, the size (in bytes) of the
buffer len, and a set of flags that control how the functions work.
Additionally, the recvfrom() takes a struct sockaddr*, from that will tell you where the
data came from, and will fill in fromlen with the size of struct sockaddr. (You must also initialize
fromlen to be the size of from or struct sockaddr.)
So what wondrous flags can you pass into this function? Here are some of them, but you should
check your local man pages for more information and what is actually supported on your system. You
bitwise-or these together, or just set flags to 0 if you want it to be a regular vanilla recv().
MSG_OOB Receive Out of Band data. This is how to get data that has been sent to
you with the MSG_OOB flag in send(). As the receiving side, you will
have had signal SIGURG raised telling you there is urgent data. In your
handler for that signal, you could call recv() with this MSG_OOB flag.
MSG_PEEK If you want to call recv() “just for pretend”, you can call it with this
flag. This will tell you what’s waiting in the buffer for when you call
recv() “for real” (i.e. without the MSG_PEEK flag. It’s like a sneak
preview into the next recv() call.
MSG_WAITALL Tell recv() to not return until all the data you specified in the len
parameter. It will ignore your wishes in extreme circumstances,
however, like if a signal interrupts the call or if some error occurs or if
the remote side closes the connection, etc. Don’t be mad with it.
When you call recv(), it will block until there is some data to read. If you want to not block, set
the socket to non-blocking or check with select() or poll() to see if there is incoming data before
calling recv() or recvfrom().
Return Value
Returns the number of bytes actually received (which might be less than you requested in the len
parameter), or -1 on error (and errno will be set accordingly.)
If the remote side has closed the connection, recv() will return 0. This is the normal method for
determining if the remote side has closed the connection. Normality is good, rebel!
Example
// stream sockets and recv()
struct addrinfo hints, *res;
int sockfd;
char buf[512];
int byte_count;
// get host info, make socket, and connect it
memset(&hints, 0, sizeof hints);
Beej’s Guide to Network Programming 86
hints.ai_family = AF_UNSPEC; // use IPv4 or IPv6, whichever
hints.ai_socktype = SOCK_STREAM;
getaddrinfo(“www.example.com”, “3490”, &hints, &res);
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
connect(sockfd, res->ai_addr, res->ai_addrlen);
// all right! now that we’re connected, we can receive some data!
byte_count = recv(sockfd, buf, sizeof buf, 0);
printf(“recv()’d %d bytes of data in buf\n”, byte_count);
// datagram sockets and recvfrom()
struct addrinfo hints, *res;
int sockfd;
int byte_count;
socklen_t fromlen;
struct sockaddr_storage addr;
char buf[512];
char ipstr[INET6_ADDRSTRLEN];
// get host info, make socket, bind it to port 4950
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // use IPv4 or IPv6, whichever
hints.ai_socktype = SOCK_DGRAM;
hints.ai_flags = AI_PASSIVE;
getaddrinfo(NULL, “4950”, &hints, &res);
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
bind(sockfd, res->ai_addr, res->ai_addrlen);
// no need to accept(), just recvfrom():
fromlen = sizeof addr;
byte_count = recvfrom(sockfd, buf, sizeof buf, 0, &addr, &fromlen);
printf(“recv()’d %d bytes of data in buf\n”, byte_count);
printf(“from IP address %s\n”,
inet_ntop(addr.ss_family,
addr.ss_family == AF_INET?
((struct sockadd_in *)&addr)->sin_addr:
((struct sockadd_in6 *)&addr)->sin6_addr,
ipstr, sizeof ipstr);
See Also
send(), sendto(), select(), poll(), Blocking
Beej’s Guide to Network Programming 87
9.19. select()
Check if sockets descriptors are ready to read/write
Prototypes
#include
int select(int n, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
struct timeval *timeout);
FD_SET(int fd, fd_set *set);
FD_CLR(int fd, fd_set *set);
FD_ISSET(int fd, fd_set *set);
FD_ZERO(fd_set *set);
Description
The select() function gives you a way to simultaneously check multiple sockets to see if they
have data waiting to be recv()d, or if you can send() data to them without blocking, or if some
exception has occurred.
You populate your sets of socket descriptors using the macros, like FD_SET(), above. Once you
have the set, you pass it into the function as one of the following parameters: readfds if you want to
know when any of the sockets in the set is ready to recv() data, writefds if any of the sockets is ready
to send() data to, and/or exceptfds if you need to know when an exception (error) occurs on any of
the sockets. Any or all of these parameters can be NULL if you’re not interested in those types of events.
After select() returns, the values in the sets will be changed to show which are ready for reading or
writing, and which have exceptions.
The first parameter, n is the highest-numbered socket descriptor (they’re just ints, remember?) plus
one.
Lastly, the struct timeval, timeout, at the end—this lets you tell select() how long to check
these sets for. It’ll return after the timeout, or when an event occurs, whichever is first. The struct
timeval has two fields: tv_sec is the number of seconds, to which is added tv_usec, the number of
microseconds (1,000,000 microseconds in a second.)
The helper macros do the following:
FD_SET(int fd, fd_set *set); Add fd to the set.
FD_CLR(int fd, fd_set *set); Remove fd from the set.
FD_ISSET(int fd, fd_set *set); Return true if fd is in the set.
FD_ZERO(fd_set *set); Clear all entries from the set.
Note for Linux users: Linux’s select() can return “ready-to-read” and then not actually be ready
to read, thus causing the subsequent read() call to block. You can work around this bug by setting
O_NONBLOCK flag on the receiving socket so it errors with EWOULDBLOCK, then ignoring this error if it
occurs. See the fcntl() reference page for more info on setting a socket to non-blocking.
Return Value
Returns the number of descriptors in the set on success, 0 if the timeout was reached, or -1 on error
(and errno will be set accordingly.) Also, the sets are modified to show which sockets are ready.
Example
int s1, s2, n;
fd_set readfds;
struct timeval tv;
char buf1[256], buf2[256];
// pretend we’ve connected both to a server at this point
//s1 = socket(…);
//s2 = socket(…);
Beej’s Guide to Network Programming 88
//connect(s1, …)…
//connect(s2, …)…
// clear the set ahead of time
FD_ZERO(&readfds);
// add our descriptors to the set
FD_SET(s1, &readfds);
FD_SET(s2, &readfds);
// since we got s2 second, it’s the “greater”, so we use that for
// the n param in select()
n = s2 + 1;
// wait until either socket has data ready to be recv()d (timeout 10.5 secs)
tv.tv_sec = 10;
tv.tv_usec = 500000;
rv = select(n, &readfds, NULL, NULL, &tv);
if (rv == -1) {
perror(“select”); // error occurred in select()
} else if (rv == 0) {
printf(“Timeout occurred! No data after 10.5 seconds.\n”);
} else {
// one or both of the descriptors have data
if (FD_ISSET(s1, &readfds)) {
recv(s1, buf1, sizeof buf1, 0);
}
if (FD_ISSET(s2, &readfds)) {
recv(s2, buf2, sizeof buf2, 0);
}
}
See Also
poll()
Beej’s Guide to Network Programming 89
9.20. setsockopt(), getsockopt()
Set various options for a socket
Prototypes
#include
#include
int getsockopt(int s, int level, int optname, void *optval,
socklen_t *optlen);
int setsockopt(int s, int level, int optname, const void *optval,
socklen_t optlen);
Description
Sockets are fairly configurable beasts. In fact, they are so configurable, I’m not even going to cover
it all here. It’s probably system-dependent anyway. But I will talk about the basics.
Obviously, these functions get and set certain options on a socket. On a Linux box, all the socket
information is in the man page for socket in section 7. (Type: “man 7 socket” to get all these goodies.)
As for parameters, s is the socket you’re talking about, level should be set to SOL_SOCKET. Then
you set the optname to the name you’re interested in. Again, see your man page for all the options, but
here are some of the most fun ones:
SO_BINDTODEVICE Bind this socket to a symbolic device name like eth0 instead of using
bind() to bind it to an IP address. Type the command ifconfig under
Unix to see the device names.
SO_REUSEADDR Allows other sockets to bind() to this port, unless there is an active
listening socket bound to the port already. This enables you to get
around those “Address already in use” error messages when you try to
restart your server after a crash.
SO_BROADCAST Allows UDP datagram (SOCK_DGRAM) sockets to send and receive
packets sent to and from the broadcast address. Does nothing
—NOTHING!!—to TCP stream sockets! Hahaha!
As for the parameter optval, it’s usually a pointer to an int indicating the value in question. For
booleans, zero is false, and non-zero is true. And that’s an absolute fact, unless it’s different on your
system. If there is no parameter to be passed, optval can be NULL.
The final parameter, optlen, should be set to the length of optval, probably sizeof(int),
but varies depending on the option. Note that in the case of getsockopt(), this is a pointer to a
socklen_t, and it specifies the maximum size object that will be stored in optval (to prevent buffer
overflows). And getsockopt() will modify the value of optlen to reflect the number of bytes actually
set.
Warning: on some systems (notably Sun and Windows), the option can be a char instead of an
int, and is set to, for example, a character value of ‘1’ instead of an int value of 1. Again, check your
own man pages for more info with “man setsockopt” and “man 7 socket”!
Return Value
Returns zero on success, or -1 on error (and errno will be set accordingly.)
Example
int optval;
int optlen;
char *optval2;
// set SO_REUSEADDR on a socket to true (1):
optval = 1;
setsockopt(s1, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof optval);
Beej’s Guide to Network Programming 90
// bind a socket to a device name (might not work on all systems):
optval2 = “eth1”; // 4 bytes long, so 4, below:
setsockopt(s2, SOL_SOCKET, SO_BINDTODEVICE, optval2, 4);
// see if the SO_BROADCAST flag is set:
getsockopt(s3, SOL_SOCKET, SO_BROADCAST, &optval, &optlen);
if (optval != 0) {
print(“SO_BROADCAST enabled on s3!\n”);
}
See Also
fcntl()
Beej’s Guide to Network Programming 91
9.21. send(), sendto()
Send data out over a socket
Prototypes
#include
#include
ssize_t send(int s, const void *buf, size_t len, int flags);
ssize_t sendto(int s, const void *buf, size_t len,
int flags, const struct sockaddr *to,
socklen_t tolen);
Description
These functions send data to a socket. Generally speaking, send() is used for TCP SOCK_STREAM
connected sockets, and sendto() is used for UDP SOCK_DGRAM unconnected datagram sockets. With
the unconnected sockets, you must specify the destination of a packet each time you send one, and that’s
why the last parameters of sendto() define where the packet is going.
With both send() and sendto(), the parameter s is the socket, buf is a pointer to the data you
want to send, len is the number of bytes you want to send, and flags allows you to specify more
information about how the data is to be sent. Set flags to zero if you want it to be “normal” data. Here
are some of the commonly used flags, but check your local send() man pages for more details:
MSG_OOB Send as “out of band” data. TCP supports this, and it’s a way to tell the
receiving system that this data has a higher priority than the normal
data. The receiver will receive the signal SIGURG and it can then
receive this data without first receiving all the rest of the normal data
in the queue.
MSG_DONTROUTE Don’t send this data over a router, just keep it local.
MSG_DONTWAIT If send() would block because outbound traffic is clogged, have it
return EAGAIN. This is like a “enable non-blocking just for this send.”
See the section on blocking for more details.
MSG_NOSIGNAL If you send() to a remote host which is no longer recv()ing, you’ll
typically get the signal SIGPIPE. Adding this flag prevents that signal
from being raised.
Return Value
Returns the number of bytes actually sent, or -1 on error (and errno will be set accordingly.) Note
that the number of bytes actually sent might be less than the number you asked it to send! See the section
on handling partial send()s for a helper function to get around this.
Also, if the socket has been closed by either side, the process calling send() will get the signal
SIGPIPE. (Unless send() was called with the MSG_NOSIGNAL flag.)
Example
int spatula_count = 3490;
char *secret_message = “The Cheese is in The Toaster”;
int stream_socket, dgram_socket;
struct sockaddr_in dest;
int temp;
// first with TCP stream sockets:
// assume sockets are made and connected
//stream_socket = socket(…
//connect(stream_socket, …
Beej’s Guide to Network Programming 92
// convert to network byte order
temp = htonl(spatula_count);
// send data normally:
send(stream_socket, &temp, sizeof temp, 0);
// send secret message out of band:
send(stream_socket, secret_message, strlen(secret_message)+1, MSG_OOB);
// now with UDP datagram sockets:
//getaddrinfo(…
//dest = … // assume “dest” holds the address of the destination
//dgram_socket = socket(…
// send secret message normally:
sendto(dgram_socket, secret_message, strlen(secret_message)+1, 0,
(struct sockaddr*)&dest, sizeof dest);
See Also
recv(), recvfrom()
Beej’s Guide to Network Programming 93
9.22. shutdown()
Stop further sends and receives on a socket
Prototypes
#include
int shutdown(int s, int how);
Description
That’s it! I’ve had it! No more send()s are allowed on this socket, but I still want to recv() data
on it! Or vice-versa! How can I do this?
When you close() a socket descriptor, it closes both sides of the socket for reading and
writing, and frees the socket descriptor. If you just want to close one side or the other, you can use this
shutdown() call.
As for parameters, s is obviously the socket you want to perform this action on, and what action that
is can be specified with the how parameter. How can be SHUT_RD to prevent further recv()s, SHUT_WR
to prohibit further send()s, or SHUT_RDWR to do both.
Note that shutdown() doesn’t free up the socket descriptor, so you still have to eventually
close() the socket even if it has been fully shut down.
This is a rarely used system call.
Return Value
Returns zero on success, or -1 on error (and errno will be set accordingly.)
Example
int s = socket(PF_INET, SOCK_STREAM, 0);
// …do some send()s and stuff in here…
// and now that we’re done, don’t allow any more sends()s:
shutdown(s, SHUT_WR);
See Also
close()
Beej’s Guide to Network Programming 94
9.23. socket()
Allocate a socket descriptor
Prototypes
#include
#include
int socket(int domain, int type, int protocol);
Description
Returns a new socket descriptor that you can use to do sockety things with. This is generally the
first call in the whopping process of writing a socket program, and you can use the result for subsequent
calls to listen(), bind(), accept(), or a variety of other functions.
In usual usage, you get the values for these parameters from a call to getaddrinfo(), as shown in
the example below. But you can fill them in by hand if you really want to.
domain domain describes what kind of socket you’re interested in. This can, believe
me, be a wide variety of things, but since this is a socket guide, it’s going to be
PF_INET for IPv4, and PF_INET6 for IPv6.
type Also, the type parameter can be a number of things, but you’ll probably be
setting it to either SOCK_STREAM for reliable TCP sockets (send(), recv()) or
SOCK_DGRAM for unreliable fast UDP sockets (sendto(), recvfrom().)
(Another interesting socket type is SOCK_RAW which can be used to construct
packets by hand. It’s pretty cool.)
protocol Finally, the protocol parameter tells which protocol to use with a certain
socket type. Like I’ve already said, for instance, SOCK_STREAM uses TCP.
Fortunately for you, when using SOCK_STREAM or SOCK_DGRAM, you can just set
the protocol to 0, and it’ll use the proper protocol automatically. Otherwise, you
can use getprotobyname() to look up the proper protocol number.
Return Value
The new socket descriptor to be used in subsequent calls, or -1 on error (and errno will be set
accordingly.)
Example
struct addrinfo hints, *res;
int sockfd;
// first, load up address structs with getaddrinfo():
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // AF_INET, AF_INET6, or AF_UNSPEC
hints.ai_socktype = SOCK_STREAM; // SOCK_STREAM or SOCK_DGRAM
getaddrinfo(“www.example.com”, “3490”, &hints, &res);
// make a socket using the information gleaned from getaddrinfo():
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
See Also
accept(), bind(), getaddrinfo(), listen()
Beej’s Guide to Network Programming 95
9.24. struct sockaddr and pals
Structures for handling internet addresses
Prototypes
#include
// All pointers to socket address structures are often cast to pointers
// to this type before use in various functions and system calls:
struct sockaddr {
unsigned short sa_family; // address family, AF_xxx
char sa_data[14]; // 14 bytes of protocol address
};
// IPv4 AF_INET sockets:
struct sockaddr_in {
short sin_family; // e.g. AF_INET, AF_INET6
unsigned short sin_port; // e.g. htons(3490)
struct in_addr sin_addr; // see struct in_addr, below
char sin_zero[8]; // zero this if you want to
};
struct in_addr {
unsigned long s_addr; // load with inet_pton()
};
// IPv6 AF_INET6 sockets:
struct sockaddr_in6 {
u_int16_t sin6_family; // address family, AF_INET6
u_int16_t sin6_port; // port number, Network Byte Order
u_int32_t sin6_flowinfo; // IPv6 flow information
struct in6_addr sin6_addr; // IPv6 address
u_int32_t sin6_scope_id; // Scope ID
};
struct in6_addr {
unsigned char s6_addr[16]; // load with inet_pton()
};
// General socket address holding structure, big enough to hold either
// struct sockaddr_in or struct sockaddr_in6 data:
struct sockaddr_storage {
sa_family_t ss_family; // address family
// all this is padding, implementation specific, ignore it:
char __ss_pad1[_SS_PAD1SIZE];
int64_t __ss_align;
char __ss_pad2[_SS_PAD2SIZE];
};
Description
These are the basic structures for all syscalls and functions that deal with internet addresses. Often
you’ll use getaddrinfo() to fill these structures out, and then will read them when you have to.
In memory, the struct sockaddr_in and struct sockaddr_in6 share the same beginning
structure as struct sockaddr, and you can freely cast the pointer of one type to the other without any
harm, except the possible end of the universe.
Beej’s Guide to Network Programming 96
Just kidding on that end-of-the-universe thing…if the universe does end when you cast a struct
sockaddr_in* to a struct sockaddr*, I promise you it’s pure coincidence and you shouldn’t even
worry about it.
So, with that in mind, remember that whenever a function says it takes a struct sockaddr* you
can cast your struct sockaddr_in*, struct sockaddr_in6*, or struct sockadd_storage* to
that type with ease and safety.
struct sockaddr_in is the structure used with IPv4 addresses (e.g. “192.0.2.10”). It holds an
address family (AF_INET), a port in sin_port, and an IPv4 address in sin_addr.
There’s also this sin_zero field in struct sockaddr_in which some people claim must be set
to zero. Other people don’t claim anything about it (the Linux documentation doesn’t even mention it at
all), and setting it to zero doesn’t seem to be actually necessary. So, if you feel like it, set it to zero using
memset().
Now, that struct in_addr is a weird beast on different systems. Sometimes it’s a crazy union
with all kinds of #defines and other nonsense. But what you should do is only use the s_addr field in
this structure, because many systems only implement that one.
struct sockadd_in6 and struct in6_addr are very similar, except they’re used for IPv6.
struct sockaddr_storage is a struct you can pass to accept() or recvfrom() when you’re
trying to write IP version-agnostic code and you don’t know if the new address is going to be IPv4 or
IPv6. The struct sockaddr_storage structure is large enough to hold both types, unlike the original
small struct sockaddr.
Example
// IPv4:
struct sockaddr_in ip4addr;
int s;
ip4addr.sin_family = AF_INET;
ip4addr.sin_port = htons(3490);
inet_pton(AF_INET, “10.0.0.1”, &ip4addr.sin_addr);
s = socket(PF_INET, SOCK_STREAM, 0);
bind(s, (struct sockaddr*)&ip4addr, sizeof ip4addr);
// IPv6:
struct sockaddr_in6 ip6addr;
int s;
ip6addr.sin6_family = AF_INET6;
ip6addr.sin6_port = htons(4950);
inet_pton(AF_INET6, “2001:db8:8714:3a90::12”, &ip6addr.sin6_addr);
s = socket(PF_INET6, SOCK_STREAM, 0);
bind(s, (struct sockaddr*)&ip6addr, sizeof ip6addr);
See Also
accept(), bind(), connect(), inet_aton(), inet_ntoa()
10. More References
You’ve come this far, and now you’re screaming for more! Where else can you go to learn more
about all this stuff?
10.1. Books
For old-school actual hold-it-in-your-hand pulp paper books, try some of the following excellent
books. I used to be an affiliate with a very popular internet bookseller, but their new customer tracking
system is incompatible with a print document. As such, I get no more kickbacks. If you feel compassion
for my plight, paypal a donation to beej@beej.us. 🙂
Unix Network Programming, volumes 1-2 by W. Richard Stevens. Published by Prentice Hall.
ISBNs for volumes 1-2: 013141155143, 013081081944.
Internetworking with TCP/IP, volumes I-III by Douglas E. Comer and David L. Stevens.
Published by Prentice Hall. ISBNs for volumes I, II, and III: 013187671645, 013031996146,
013032071447.
TCP/IP Illustrated, volumes 1-3 by W. Richard Stevens and Gary R. Wright. Published by
Addison Wesley. ISBNs for volumes 1, 2, and 3 (and a 3-volume set): 020163346948,
020163354X49, 020163495350, (020177631651).
TCP/IP Network Administration by Craig Hunt. Published by O’Reilly & Associates, Inc.
ISBN 059600297152.
Advanced Programming in the UNIX Environment by W. Richard Stevens. Published by
Addison Wesley. ISBN 020143307953.
10.2. Web References
On the web:
BSD Sockets: A Quick And Dirty Primer54 (Unix system programming info, too!)
The Unix Socket FAQ55
Intro to TCP/IP56
TCP/IP FAQ57
The Winsock FAQ58
And here are some relevant Wikipedia pages:
Berkeley Sockets59
Internet Protocol (IP)60
43. http://beej.us/guide/url/unixnet1
44. http://beej.us/guide/url/unixnet2
45. http://beej.us/guide/url/intertcp1
46. http://beej.us/guide/url/intertcp2
47. http://beej.us/guide/url/intertcp3
48. http://beej.us/guide/url/tcpi1
49. http://beej.us/guide/url/tcpi2
50. http://beej.us/guide/url/tcpi3
51. http://beej.us/guide/url/tcpi123
52. http://beej.us/guide/url/tcpna
53. http://beej.us/guide/url/advunix
54. http://www.cis.temple.edu/~giorgio/old/cis307s96/readings/docs/sockets.html
55. http://www.developerweb.net/forum/forumdisplay.php?f=70
56. http://pclt.cis.yale.edu/pclt/COMM/TCPIP.HTM
57. http://www.faqs.org/faqs/internet/tcp-ip/tcp-ip-faq/part1/
58. http://tangentsoft.net/wskfaq/
59. http://en.wikipedia.org/wiki/Berkeley_sockets
60. http://en.wikipedia.org/wiki/Internet_Protocol
97
http://beej.us/guide/url/unixnet1
http://beej.us/guide/url/unixnet2
http://beej.us/guide/url/intertcp1
http://beej.us/guide/url/intertcp2
http://beej.us/guide/url/intertcp3
http://beej.us/guide/url/tcpi1
http://beej.us/guide/url/tcpi2
http://beej.us/guide/url/tcpi3
http://beej.us/guide/url/tcpi123
http://beej.us/guide/url/tcpna
http://beej.us/guide/url/advunix
http://www.cis.temple.edu/~giorgio/old/cis307s96/readings/docs/sockets.html
http://www.developerweb.net/forum/forumdisplay.php?f=70
http://pclt.cis.yale.edu/pclt/COMM/TCPIP.HTM
http://www.faqs.org/faqs/internet/tcp-ip/tcp-ip-faq/part1/
http://tangentsoft.net/wskfaq/
http://en.wikipedia.org/wiki/Berkeley_sockets
http://en.wikipedia.org/wiki/Internet_Protocol
Beej’s Guide to Network Programming 98
Transmission Control Protocol (TCP)61
User Datagram Protocol (UDP)62
Client-Server63
Serialization64 (packing and unpacking data)
10.3. RFCs
RFCs65—the real dirt! These are documents that describe assigned numbers, programming APIs, and
protocols that are used on the Internet. I’ve included links to a few of them here for your enjoyment, so
grab a bucket of popcorn and put on your thinking cap:
RFC 166—The First RFC; this gives you an idea of what the “Internet” was like just as it was
coming to life, and an insight into how it was being designed from the ground up. (This
RFC is completely obsolete, obviously!)
RFC 76867—The User Datagram Protocol (UDP)
RFC 79168—The Internet Protocol (IP)
RFC 79369—The Transmission Control Protocol (TCP)
RFC 85470—The Telnet Protocol
RFC 95971—File Transfer Protocol (FTP)
RFC 135072—The Trivial File Transfer Protocol (TFTP)
RFC 145973—Internet Relay Chat Protocol (IRC)
RFC 191874—Address Allocation for Private Internets
RFC 213175—Dynamic Host Configuration Protocol (DHCP)
RFC 261676—Hypertext Transfer Protocol (HTTP)
RFC 282177—Simple Mail Transfer Protocol (SMTP)
RFC 333078—Special-Use IPv4 Addresses
RFC 349379—Basic Socket Interface Extensions for IPv6
RFC 354280—Advanced Sockets Application Program Interface (API) for IPv6
RFC 384981—IPv6 Address Prefix Reserved for Documentation
RFC 392082—Extensible Messaging and Presence Protocol (XMPP)
61. http://en.wikipedia.org/wiki/Transmission_Control_Protocol
62. http://en.wikipedia.org/wiki/User_Datagram_Protocol
63. http://en.wikipedia.org/wiki/Client-server
64. http://en.wikipedia.org/wiki/Serialization
65. http://www.rfc-editor.org/
66. http://tools.ietf.org/html/rfc1
67. http://tools.ietf.org/html/rfc768
68. http://tools.ietf.org/html/rfc791
69. http://tools.ietf.org/html/rfc793
70. http://tools.ietf.org/html/rfc854
71. http://tools.ietf.org/html/rfc959
72. http://tools.ietf.org/html/rfc1350
73. http://tools.ietf.org/html/rfc1459
74. http://tools.ietf.org/html/rfc1918
75. http://tools.ietf.org/html/rfc2131
76. http://tools.ietf.org/html/rfc2616
77. http://tools.ietf.org/html/rfc2821
78. http://tools.ietf.org/html/rfc3330
79. http://tools.ietf.org/html/rfc3493
80. http://tools.ietf.org/html/rfc3542
81. http://tools.ietf.org/html/rfc3849
82. http://tools.ietf.org/html/rfc3920
http://en.wikipedia.org/wiki/Transmission_Control_Protocol
http://en.wikipedia.org/wiki/User_Datagram_Protocol
http://en.wikipedia.org/wiki/Client-server
http://en.wikipedia.org/wiki/Serialization
http://tools.ietf.org/html/rfc1
http://tools.ietf.org/html/rfc768
http://tools.ietf.org/html/rfc791
http://tools.ietf.org/html/rfc793
http://tools.ietf.org/html/rfc854
http://tools.ietf.org/html/rfc959
http://tools.ietf.org/html/rfc1350
http://tools.ietf.org/html/rfc1459
http://tools.ietf.org/html/rfc1918
http://tools.ietf.org/html/rfc2131
http://tools.ietf.org/html/rfc2616
http://tools.ietf.org/html/rfc2821
http://tools.ietf.org/html/rfc3330
http://tools.ietf.org/html/rfc3493
http://tools.ietf.org/html/rfc3542
http://tools.ietf.org/html/rfc3849
http://tools.ietf.org/html/rfc3920
Beej’s Guide to Network Programming 99
RFC 397783—Network News Transfer Protocol (NNTP)
RFC 419384—Unique Local IPv6 Unicast Addresses
RFC 450685—External Data Representation Standard (XDR)
The IETF has a nice online tool for searching and browsing RFCs86.
83. http://tools.ietf.org/html/rfc3977
84. http://tools.ietf.org/html/rfc4193
85. http://tools.ietf.org/html/rfc4506
86. http://tools.ietf.org/rfc/
http://tools.ietf.org/html/rfc3977
http://tools.ietf.org/html/rfc4193
http://tools.ietf.org/html/rfc4506
http://tools.ietf.org/rfc/
Index
10.x.x.x 13
192.168.x.x 13
255.255.255.255 52, 78
accept() 20, 21, 61
Address already in use 19, 55
AF_INET 10, 18, 57
AF_INET6 10
asynchronous I/O 76
Bapper 54
bind() 18, 55, 63
implicit 20, 20
blah blah blah 6
blocking 33
books 97
broadcast 51
byte ordering 9, 11, 40, 77
client
datagram 30
stream 27
client/server 25
close() 23, 66
closesocket() 2, 24, 66
compilers
gcc 1
compression 57
connect() 4, 18, 20, 20, 65
on datagram sockets 23, 32, 65
Connection refused 29
CreateProcess() 2, 58
CreateThread() 2
CSocket 2
Cygwin 2
data encapsulation 5, 39
DHCP 98
disconnected network see private network.
DNS
domain name service see DNS.
donkeys 39
EAGAIN 33, 33, 91
email to Beej 2
encryption 57
EPIPE 66
errno 75, 82
Ethernet 5
EWOULDBLOCK 33, 33, 61
Excalibur 51
external data representation standard see XDR.
F_SETFL 76
fcntl() 33, 61, 76
FD_CLR() 34, 87
FD_ISSET() 34, 87
FD_SET() 34, 87
FD_ZERO() 34, 87
file descriptor 4
firewall 13, 54, 59
poking holes in 59
footer 5
fork() 2, 25, 58
FTP 98
getaddrinfo() 10, 14, 15
gethostbyaddr() 24, 71
gethostbyname() 24, 70, 71
gethostname() 24, 70
getnameinfo() 14, 24
getpeername() 24, 74
getprotobyname() 94
getsockopt() 89
gettimeofday() 35
goat 55
goto 56
header 5
header files 55
herror() 71
hstrerror() 71
htonl() 9, 77, 77
htons() 9, 11, 40, 77, 77
HTTP 98
HTTP protocol 4
ICMP 55
IEEE-754 41
INADDR_ANY
INADDR_BROADCAST 52
inet_addr() 12, 78
inet_aton() 12, 78
inet_ntoa() 12, 78
inet_ntoa() 12, 24
inet_pton() 12
Internet Control Message Protocol see ICMP.
Internet protocol see IP.
Internet Relay Chat see IRC.
ioctl() 59
IP 4, 5, 7, 12, 19, 23, 24, 98
IP address 63, 70, 71, 74
IPv4 7
IPv6 7, 11, 13, 14
IRC 39, 98
ISO/OSI 5
layered network model see ISO/OSI.
Linux 2
listen() 18, 20, 81
backlog 21
with select() 35
lo see loopback device.
100
Beej’s Guide to Network Programming 101
localhost 55
loopback device 55
man pages 60
Maximum Transmission Unit see MTU.
mirroring 3
MSG_DONTROUTE 91
MSG_DONTWAIT 91
MSG_NOSIGNAL 91
MSG_OOB 85, 91
MSG_PEEK 85
MSG_WAITALL 85
MTU 58
NAT 13
netstat 55, 55
network address translation see NAT.
NNTP 99
non-blocking sockets 33, 39, 61, 76, 87, 91
ntohl() 9, 77, 77
ntohs() 9, 77, 77
O_ASYNC see asynchronous I/O.
O_NONBLOCK see non-blocking sockets.
OpenSSL 57
out-of-band data 85, 91
packet sniffer 59
Pat 54
perror() 75, 82
PF_INET 57, 94
ping 55
poll() 38, 83
port 23, 63, 74
ports 18, 19
private network 13
promiscuous mode 59
raw sockets 4, 55
read() 4
recv() 4, 4, 22, 85
timeout 56
recvfrom() 23, 85
recvtimeout() 57
references 97
web-based 97
RFCs 98
route 55
SA_RESTART 56
Secure Sockets Layer see SSL.
security 58
select() 2, 33, 33, 55, 56, 87
with listen() 35
send() 4, 4, 6, 22, 91
sendall() 38, 50
sendto() 6, 91
serialization 39
server
datagram 29
stream 25
setsockopt() 19, 52, 55, 59, 89
shutdown() 24, 93
sigaction() 27, 56
SIGIO 76
SIGPIPE 66, 91
SIGURG 85, 91
SMTP 98
SO_BINDTODEVICE 89
SO_BROADCAST 52, 89
SO_RCVTIMEO 59
SO_REUSEADDR 19, 55, 89
SO_SNDTIMEO 59
SOCK_DGRAM see socket;datagram.
SOCK_RAW 94
SOCK_STREAM see socket;stream.
socket 4
datagram 4, 4, 5, 23, 85, 89, 91, 94
raw 4
stream 4, 4, 61, 85, 91, 94
types 4, 4
socket descriptor 4, 10
socket() 4, 18, 94
SOL_SOCKET 89
Solaris 1, 89
SSL 57
strerror() 75, 82
struct addrinfo 10
struct hostent 71
struct in_addr 95
struct pollfd 83
struct sockaddr 10, 23, 85, 95
struct sockaddr_in 10, 61, 95
struct timeval 34, 87
SunOS 1, 89
TCP 4, 94, 98
gcc 4, 98
TFTP 5, 98
timeout, setting 59
translations 3
transmission control protocol see TCP.
TRON 20
UDP 5, 5, 51, 94, 98
user datagram protocol see UDP.
Vint Cerf 7
Windows 1, 24, 55, 66, 89
Winsock 2, 24
Winsock FAQ 2
write() 4
WSACleanup() 2
WSAStartup() 2
XDR 50, 99
XMPP 98
zombie process 27
Contents
Intro
Audience
Platform and Compiler
Official Homepage and Books For Sale
Note for Solaris/SunOS Programmers
Note for Windows Programmers
Email Policy
Mirroring
Note for Translators
Copyright and Distribution
What is a socket?
Two Types of Internet Sockets
Low level Nonsense and Network Theory
IP Addresses, structs, and Data Munging
IP Addresses, versions 4 and 6
Subnets
Port Numbers
Byte Order
structs
IP Addresses, Part Deux
Private (Or Disconnected) Networks
Jumping from IPv4 to IPv6
System Calls or Bust
getaddrinfo()—Prepare to launch!
socket()—Get the File Descriptor!
bind()—What port am I on?
connect()—Hey, you!
listen()—Will somebody please call me?
accept()—”Thank you for calling port 3490.”
send() and recv()—Talk to me, baby!
sendto() and recvfrom()—Talk to me, DGRAM-style
close() and shutdown()—Get outta my face!
getpeername()—Who are you?
gethostname()—Who am I?
Client-Server Background
A Simple Stream Server
A Simple Stream Client
Datagram Sockets
Slightly Advanced Techniques
Blocking
select()—Synchronous I/O Multiplexing
Handling Partial send()s
Serialization—How to Pack Data
Son of Data Encapsulation
Broadcast Packets—Hello, World!
Common Questions
Man Pages
accept()
bind()
connect()
close()
getaddrinfo(), freeaddrinfo(), gai_strerror()
gethostname()
gethostbyname(), gethostbyaddr()
getnameinfo()
getpeername()
errno
fcntl()
htons(), htonl(), ntohs(), ntohl()
inet_ntoa(), inet_aton(), inet_addr
inet_ntop(), inet_pton()
listen()
perror(), strerror()
poll()
recv(), recvfrom()
select()
setsockopt(), getsockopt()
send(), sendto()
shutdown()
socket()
struct sockaddr and pals
More References
Books
Web References
RFCs
Index