https://beej.us/guide/bgnet/html/
Beej's Guide to Network Programming
Using Internet Sockets
Brian "Beej Jorgensen" Hall
v3.1.5, Copyright (c) November 20, 2020
* 1 Intro
+ 1.1 Audience
+ 1.2 Platform and Compiler
+ 1.3 Official Homepage and Books For Sale
+ 1.4 Note for Solaris/SunOS Programmers
+ 1.5 Note for Windows Programmers
+ 1.6 Email Policy
+ 1.7 Mirroring
+ 1.8 Note for Translators
+ 1.9 Copyright, Distribution, and Legal
+ 1.10 Dedication
+ 1.11 Publishing Information
* 2 What is a socket?
+ 2.1 Two Types of Internet Sockets
+ 2.2 Low level Nonsense and Network Theory
* 3 IP Addresses, structs, and Data Munging
+ 3.1 IP Addresses, versions 4 and 6
o 3.1.1 Subnets
o 3.1.2 Port Numbers
+ 3.2 Byte Order
+ 3.3 structs
+ 3.4 IP Addresses, Part Deux
o 3.4.1 Private (Or Disconnected) Networks
* 4 Jumping from IPv4 to IPv6
* 5 System Calls or Bust
+ 5.1 getaddrinfo()--Prepare to launch!
+ 5.2 socket()--Get the File Descriptor!
+ 5.3 bind()--What port am I on?
+ 5.4 connect()--Hey, you!
+ 5.5 listen()--Will somebody please call me?
+ 5.6 accept()--"Thank you for calling port 3490."
+ 5.7 send() and recv()--Talk to me, baby!
+ 5.8 sendto() and recvfrom()--Talk to me, DGRAM-style
+ 5.9 close() and shutdown()--Get outta my face!
+ 5.10 getpeername()--Who are you?
+ 5.11 gethostname()--Who am I?
* 6 Client-Server Background
+ 6.1 A Simple Stream Server
+ 6.2 A Simple Stream Client
+ 6.3 Datagram Sockets
* 7 Slightly Advanced Techniques
+ 7.1 Blocking
+ 7.2 poll()--Synchronous I/O Multiplexing
+ 7.3 select()--Synchronous I/O Multiplexing, Old School
+ 7.4 Handling Partial send()s
+ 7.5 Serialization--How to Pack Data
+ 7.6 Son of Data Encapsulation
+ 7.7 Broadcast Packets--Hello, World!
* 8 Common Questions
* 9 Man Pages
+ 9.1 accept()
+ 9.2 bind()
+ 9.3 connect()
+ 9.4 close()
+ 9.5 getaddrinfo(), freeaddrinfo(), gai_strerror()
+ 9.6 gethostname()
+ 9.7 gethostbyname(), gethostbyaddr()
+ 9.8 getnameinfo()
+ 9.9 getpeername()
+ 9.10 errno
+ 9.11 fcntl()
+ 9.12 htons(), htonl(), ntohs(), ntohl()
+ 9.13 inet_ntoa(), inet_aton(), inet_addr
+ 9.14 inet_ntop(), inet_pton()
+ 9.15 listen()
+ 9.16 perror(), strerror()
+ 9.17 poll()
+ 9.18 recv(), recvfrom()
+ 9.19 select()
+ 9.20 setsockopt(), getsockopt()
+ 9.21 send(), sendto()
+ 9.22 shutdown()
+ 9.23 socket()
+ 9.24 struct sockaddr and pals
* 10 More References
+ 10.1 Books
+ 10.2 Web References
+ 10.3 RFCs
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:
* https://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:
* https://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.
So I still encourage you to try Linux^1, BSD^2, 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 Cygwin^3, 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.
Another thing that you should consider is the Windows Subsystem for
Linux^4. This basically allows you to install a Linux VM-ish thing on
Windows 10. That will also definitely get you situated.
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 FAQ^5 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.
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 Way^6.
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.
This source markdown document uses UTF-8 encoding.
Please note the license restrictions in the Copyright, Distribution,
and Legal 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, Distribution, and Legal
Beej's Guide to Network Programming is Copyright (c) 2019 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
https://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.
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.
Contact beej@beej.us for more information.
1.10 Dedication
Thanks to everyone who has helped in the past and future with me
getting this guide written. And 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, pandoc, 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.
1.11 Publishing Information
This book is written in Markdown using the vim editor on an Arch
Linux box loaded with GNU tools. The cover "art" and diagrams are
produced with Inkscape. The Markdown is converted to HTML and LaTex/
PDF by Python, Pandoc and XeLaTeX, using Liberation fonts. The
toolchain is composed of 100% Free and Open Source Software.
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 Hypertext Transfer Protocol (HTTP) 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!
If you don't have telnet installed and don't want to install it,
or your telnet is being picky about connecting to clients, the
guide comes with a telnet-like program called telnot^7. This
should work well for all the needs of the guide. (Note that
telnet is actually a spec'd networking protocol^8, and telnot
doesn't implement this protocol at all.)
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 793^9 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 791^10).
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.
Datagram sockets also use IP for routing, but they don't use TCP;
they use the "User Datagram Protocol", or "UDP" (see RFC 768^11).
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.
embed(dataencap.svg)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
* 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 RFC^12 if you really really care. If you never learn about
it, well, you'll live.
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 Cerf^13 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 ELIZA^14 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 (2^32), while 128 bits represents about 340 trillion
trillion trillion numbers (for real, 2^128). 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
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.
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 List^15 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 DOOM^16 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 Brimley^17 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
80x86, 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:
Function Description
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.
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 Struct(tm)--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)
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];
};
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.
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 1918^18, 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 4193^19. 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.
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!
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 List^20
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
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 program^21 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[])
{
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!
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 time 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);
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
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:
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 !!
// 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);
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.
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:
how Effect
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 1413^22 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.
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.
embed(cs.svg)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 code^23:
/*
** 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;
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);
}
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 source^24:
/*
** client.c -- a stream socket client demo
*/
#include
#include
#include
#include
#include
#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);
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.
Because datagram sockets are connectionless and just fire packets off
into the ether with callous disregard for success, we are going to
tell the client and server to use specifically IPv6. This way we
avoid the situation where the server is listening on IPv6 and the
client sends on IPv4; the data simply would not be received. (In our
connected TCP stream sockets world, we might still have the mismatch,
but the error on connect() for one address family would cause us to
retry for the other.)
Here is the source for listener.c^25:
/*
** 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;
char buf[MAXBUFLEN];
socklen_t addr_len;
char s[INET6_ADDRSTRLEN];
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_INET6; // set to AF_INET to use 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.c^26:
/*
** 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_INET6; // set to AF_INET to use IPv4
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;
}
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 poll().
7.2 poll()--Synchronous I/O Multiplexing
What you really want to be able to do is somehow monitor a bunch of
sockets at once and then handle the ones that have data ready. This
way you don't have to continously poll all those sockets to see which
are ready to read.
A word of warning: poll() is horribly slow when it comes to giant
numbers of connections. In those circumstances, you'll get better
performance out of an event library such as libevent^27 that
attempts to use the fastest possible method availabile on your
system.
So how can you avoid polling? Not slightly ironically, you can avoid
polling by using the poll() system call. In a nutshell, we're going
to ask the operating system to do all the dirty work for us, and just
let us know when some data is ready to read on which sockets. In the
meantime, our process can go to sleep, saving system resources.
The general gameplan is to keep an array of struct pollfds with
information about which socket descriptors we want to monitor, and
what kind of events we want to monitor for. The OS will block on the
poll() call until one of those events occurs (e.g. "socket ready to
read!") or until a user-specified timeout occurs.
Usefully, a listen()ing socket will return "ready to read" when a new
incoming connection is ready to be accept()ed.
That's enough banter. How do we use this?
#include
int poll(struct pollfd fds[], nfds_t nfds, int timeout);
fds is our array of information (which sockets to monitor for what),
nfds is the count of elements in the array, and timeout is a timeout
in milliseconds. It returns the number of elements in the array that
have had an event occur.
Let's have a look at that struct:
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
};
So we're going to have an array of those, and we'll see the fd field
for each element to a socket descriptor we're interested in
monitoring. And then we'll set the events field to indicate the type
of events we're interested in.
The events field is the bitwise-OR of the following:
Macro Description
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.
Once you have your array of struct pollfds in order, then you can
pass it to poll(), also passing the size of the array, as well as a
timeout value in milliseconds. (You can specify a negative timeout to
wait forever.)
After poll() returns, you can check the revents field to see if
POLLIN or POLLOUT is set, indicating that event occurred.
(There's actually more that you can do with the poll() call. See the
poll() man page, below, for more details.)
Here's an example^28 where we'll wait 2.5 seconds for data to be
ready to read from standard input, i.e. when you hit RETURN:
#include
#include
int main(void)
{
struct pollfd pfds[1]; // More if you want to monitor more
pfds[0].fd = 0; // Standard input
pfds[0].events = POLLIN; // Tell me when ready to read
// If you needed to monitor other things, as well:
//pfds[1].fd = some_socket; // Some socket descriptor
//pfds[1].events = POLLIN; // Tell me when ready to read
printf("Hit RETURN or wait 2.5 seconds for timeout\n");
int num_events = poll(pfds, 1, 2500); // 2.5 second timeout
if (num_events == 0) {
printf("Poll timed out!\n");
} else {
int pollin_happened = pfds[0].revents & POLLIN;
if (pollin_happened) {
printf("File descriptor %d is ready to read\n", pfds[0].fd);
} else {
printf("Unexpected event occurred: %d\n", pfds[0].revents);
}
}
return 0;
}
Notice again that poll() returns the number of elements in the pfds
array for which events have occurred. It doesn't tell you which
elements in the array (you still have to scan for that), but it does
tell you how many entries have a non-zero revents field (so you can
stop scanning after you find that many).
A couple questions might come up here: how to add new file
descriptors to the set I pass to poll()? For this, simply make sure
you have enough space in the array for all you need, or realloc()
more space as needed.
What about deleting items from the set? For this, you can copy the
last element in the array over-top the one you're deleting. And then
pass in one fewer as the count to poll(). Another option is that you
can set any fd field to a negative number and poll() will ignore it.
How can we put it all together into a chat server that you can telnet
to?
What we'll do is start a listener socket, and add it to the set of
file descriptors to poll(). (It will show ready-to-read when there's
an incoming connection.)
Then we'll add new connections to our struct pollfd array. And we'll
grow it dynamically if we run out of space.
When a connection is closed, we'll remove it from the array.
And when a connection is ready-to-read, we'll read the data from it
and send that data to all the other connections so they can see what
the other users typed.
So give this poll server^29 a try. Run it in one window, then telnet
localhost 9034 from a number of other terminal windows. You should be
able to see what you type in one window in the other ones (after you
hit RETURN).
Not only that, but if you hit CTRL-] and type quit to exit telnet,
the server should detect the disconnection and remove you from the
array of file descriptors.
/*
** pollserver.c -- a cheezy multiperson chat server
*/
#include
#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);
}
return &(((struct sockaddr_in6*)sa)->sin6_addr);
}
// Return a listening socket
int get_listener_socket(void)
{
int listener; // Listening socket descriptor
int yes=1; // For setsockopt() SO_REUSEADDR, below
int rv;
struct addrinfo hints, *ai, *p;
// 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;
}
freeaddrinfo(ai); // All done with this
// If we got here, it means we didn't get bound
if (p == NULL) {
return -1;
}
// Listen
if (listen(listener, 10) == -1) {
return -1;
}
return listener;
}
// Add a new file descriptor to the set
void add_to_pfds(struct pollfd *pfds[], int newfd, int *fd_count, int *fd_size)
{
// If we don't have room, add more space in the pfds array
if (*fd_count == *fd_size) {
*fd_size *= 2; // Double it
*pfds = realloc(*pfds, sizeof(**pfds) * (*fd_size));
}
(*pfds)[*fd_count].fd = newfd;
(*pfds)[*fd_count].events = POLLIN; // Check ready-to-read
(*fd_count)++;
}
// Remove an index from the set
void del_from_pfds(struct pollfd pfds[], int i, int *fd_count)
{
// Copy the one from the end over this one
pfds[i] = pfds[*fd_count-1];
(*fd_count)--;
}
// Main
int main(void)
{
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
char remoteIP[INET6_ADDRSTRLEN];
// Start off with room for 5 connections
// (We'll realloc as necessary)
int fd_count = 0;
int fd_size = 5;
struct pollfd *pfds = malloc(sizeof *pfds * fd_size);
// Set up and get a listening socket
listener = get_listener_socket();
if (listener == -1) {
fprintf(stderr, "error getting listening socket\n");
exit(1);
}
// Add the listener to set
pfds[0].fd = listener;
pfds[0].events = POLLIN; // Report ready to read on incoming connection
fd_count = 1; // For the listener
// Main loop
for(;;) {
int poll_count = poll(pfds, fd_count, -1);
if (poll_count == -1) {
perror("poll");
exit(1);
}
// Run through the existing connections looking for data to read
for(int i = 0; i < fd_count; i++) {
// Check if someone's ready to read
if (pfds[i].revents & POLLIN) { // We got one!!
if (pfds[i].fd == listener) {
// If listener is ready to read, handle new connection
addrlen = sizeof remoteaddr;
newfd = accept(listener,
(struct sockaddr *)&remoteaddr,
&addrlen);
if (newfd == -1) {
perror("accept");
} else {
add_to_pfds(&pfds, newfd, &fd_count, &fd_size);
printf("pollserver: new connection from %s on "
"socket %d\n",
inet_ntop(remoteaddr.ss_family,
get_in_addr((struct sockaddr*)&remoteaddr),
remoteIP, INET6_ADDRSTRLEN),
newfd);
}
} else {
// If not the listener, we're just a regular client
int nbytes = recv(pfds[i].fd, buf, sizeof buf, 0);
int sender_fd = pfds[i].fd;
if (nbytes <= 0) {
// Got error or connection closed by client
if (nbytes == 0) {
// Connection closed
printf("pollserver: socket %d hung up\n", sender_fd);
} else {
perror("recv");
}
close(pfds[i].fd); // Bye!
del_from_pfds(pfds, i, &fd_count);
} else {
// We got some good data from a client
for(int j = 0; j < fd_count; j++) {
// Send to everyone!
int dest_fd = pfds[j].fd;
// Except the listener and ourselves
if (dest_fd != listener && dest_fd != sender_fd) {
if (send(dest_fd, buf, nbytes, 0) == -1) {
perror("send");
}
}
}
}
} // END handle data from client
} // END got ready-to-read from poll()
} // END looping through file descriptors
} // END for(;;)--and you thought it would never end!
return 0;
}
In the next section, we'll look at a similar, older function called
select(). Both select() and poll() offer similar functionality and
performance, and only really differ in how they're used. select()
might be slightly more portable, but is perhaps a little clunkier in
use. Choose the one you like the best, as long as it's supported on
your system.
7.3 select()--Synchronous I/O Multiplexing, Old School
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.
A word of warning: select(), though very portable, is terribly
slow when it comes to giant numbers of connections. In those
circumstances, you'll get better performance out of an event
library such as libevent^30 that attempts to use the fastest
possible method availabile on your system.
Without any further ado, I'll offer the synopsis of select():
#include
#include
#include
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:
Function Description
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 _micro_seconds, 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 m (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 snippet^31 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)
{
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 program^32 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);
}
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");
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");
}
}
}
}
}
} // 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.
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.
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!
7.4 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.
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!
7.5 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)^33. 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;
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^34:
#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)
{
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-754^35. 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 format
^36. (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<>significandbits)&((1LL< 0) { result *= 2.0; shift--; }
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 // defines uintN_t types
#include // defines PRIx macros
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
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 Programming^37 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 a Protocol Buffers
implementation in C^38 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 up^39 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())
*/
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; }
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
*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;
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) {
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;
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 program^40 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;
}
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 4506^41, 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!
7.6 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 ...
(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!
7.7 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.
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.c^42:
/*
** broadcaster.c -- a datagram "client" like talker.c, except
** this one can broadcast
*/
#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 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
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 online^43.
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 code^44.
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.
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
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
project^45 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.
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++.
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. For Linux, there's what looks
like a useful Stack Overflow thread^46, 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 numbers^47 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.
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!
9.1 accept()
Accept an incoming connection on a listening socket
9.1.0.1 Synopsis
#include
#include
int accept(int s, struct sockaddr *addr, socklen_t *addrlen);
9.1.0.2 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.
Parameter Description
s The listen()ing socket descriptor.
addr This is filled in with the address of the site that's
connecting to you.
This is filled in with the sizeof() the structure returned
in the addr parameter. You can safely ignore it if you
addrlen 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.
9.1.0.3 Return Value
accept() returns the newly connected socket descriptor, or -1 on
error, with errno set appropriately.
9.1.0.4 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);
// ready to communicate on socket descriptor new_fd!
9.1.0.5 See Also
socket(), getaddrinfo(), listen(), struct sockaddr_in
9.2 bind()
Associate a socket with an IP address and port number
9.2.0.1 Synopsis
#include
#include
int bind(int sockfd, struct sockaddr *my_addr, socklen_t addrlen);
9.2.0.2 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.
9.2.0.3 Return Value
Returns zero on success, or -1 on error (and errno will be set
accordingly).
9.2.0.4 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;
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);
9.2.0.5 See Also
getaddrinfo(), socket(), struct sockaddr_in, struct in_addr
9.3 connect()
Connect a socket to a server
9.3.0.1 Synopsis
#include
#include
int connect(int sockfd, const struct sockaddr *serv_addr,
socklen_t addrlen);
9.3.0.2 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.
9.3.0.3 Return Value
Returns zero on success, or -1 on error (and errno will be set
accordingly).
9.3.0.4 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);
9.3.0.5 See Also
socket(), bind()
9.4 close()
Close a socket descriptor
9.4.0.1 Synopsis
#include
int close(int s);
9.4.0.2 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.
9.4.0.3 Return Value
Returns zero on success, or -1 on error (and errno will be set
accordingly).
9.4.0.4 Example
s = socket(PF_INET, SOCK_DGRAM, 0);
.
.
.
// a whole lotta stuff...*BRRRONNNN!*
.
.
.
close(s); // not much to it, really.
9.4.0.5 See Also
socket(), shutdown()
9.5 getaddrinfo(), freeaddrinfo(), gai_strerror()
Get information about a host name and/or service and load up a struct
sockaddr with the result.
9.5.0.1 Synopsis
#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
};
9.5.0.2 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 List^48 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).
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).
9.5.0.3 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.
9.5.0.4 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
// 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
9.5.0.5 See Also
gethostbyname(), getnameinfo()
9.6 gethostname()
Returns the name of the system
9.6.0.1 Synopsis
#include
int gethostname(char *name, size_t len);
9.6.0.2 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.
9.6.0.3 Return Value
Returns zero on success, or -1 on error (and errno will be set
accordingly).
9.6.0.4 Example
char hostname[128];
gethostname(hostname, sizeof hostname);
printf("My hostname: %s\n", hostname);
9.6.0.5 See Also
gethostbyname()
9.7 gethostbyname(), gethostbyaddr()
Get an IP address for a hostname, or vice-versa
9.7.0.1 Synopsis
#include
#include
struct hostent *gethostbyname(const char *name); // DEPRECATED!
struct hostent *gethostbyaddr(const char *addr, int len, int type);
9.7.0.2 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.
Field Description
char *h_name The real canonical host name.
char A list of aliases that can be accessed with arrays--the
**h_aliases last element is NULL
int The result's address type, which really should be
h_addrtype AF_INET for our purposes.
int length The length of the addresses in bytes, which is 4 for IP
(version 4) addresses.
char A list of IP addresses for this host. Although this is
**h_addr_list a char**, it's really an array of struct in_addr*s in
disguise. The last array element is NULL.
A commonly defined alias for h_addr_list[0]. If you
h_addr just want any old IP address for this host (yeah, they
can have more than one) just use this field.
9.7.0.3 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.
9.7.0.4 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);
9.7.0.5 See Also
getaddrinfo(), getnameinfo(), gethostname(), errno, perror(),
strerror(), struct in_addr
9.8 getnameinfo()
Look up the host name and service name information for a given struct
sockaddr.
9.8.0.1 Synopsis
#include
#include
int getnameinfo(const struct sockaddr *sa, socklen_t salen,
char *host, size_t hostlen,
char *serv, size_t servlen, int flags);
9.8.0.2 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.
9.8.0.3 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.
9.8.0.4 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"
9.8.0.5 See Also
getaddrinfo(), gethostbyaddr()
9.9 getpeername()
Return address info about the remote side of the connection
9.9.0.1 Synopsis
#include
int getpeername(int s, struct sockaddr *addr, socklen_t *len);
9.9.0.2 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.
9.9.0.3 Return Value
Returns zero on success, or -1 on error (and errno will be set
accordingly).
9.9.0.4 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);
9.9.0.5 See Also
gethostname(), gethostbyname(), gethostbyaddr()
9.10 errno
Holds the error code for the last system call
9.10.0.1 Synopsis
#include
int errno;
9.10.0.2 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.)
9.10.0.3 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.
9.10.0.4 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);
}
9.10.0.5 See Also
perror(), strerror()
9.11 fcntl()
Control socket descriptors
9.11.0.1 Synopsis
#include
#include
int fcntl(int s, int cmd, long arg);
9.11.0.2 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.)
cmd Description
O_NONBLOCK Set the socket to be non-blocking. See the section on
blocking for more details.
Set the socket to do asynchronous I/O. When data is ready
O_ASYNC 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.
9.11.0.3 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.
9.11.0.4 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
9.11.0.5 See Also
Blocking, send()
9.12 htons(), htonl(), ntohs(), ntohl()
Convert multi-byte integer types from host byte order to network byte
order
9.12.0.1 Synopsis
#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);
9.12.0.2 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:
Function Description
htons() host to network short
htonl() host to network long
ntohs() network to host short
ntohl() network to host long
9.12.0.3 Return Value
Each function returns the converted value.
9.12.0.4 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
9.13 inet_ntoa(), inet_aton(), inet_addr
Convert IP addresses from a dots-and-number string to a struct
in_addr and back
9.13.0.1 Synopsis
#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);
9.13.0.2 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'Etat 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.
9.13.0.3 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.)
9.13.0.4 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");
9.13.0.5 See Also
inet_ntop(), inet_pton(), gethostbyname(), gethostbyaddr()
9.14 inet_ntop(), inet_pton()
Convert IP addresses to human-readable form and back.
9.14.0.1 Synopsis
#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);
9.14.0.2 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.
9.14.0.3 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.
9.14.0.4 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;
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;
}
9.14.0.5 See Also
getaddrinfo()
9.15 listen()
Tell a socket to listen for incoming connections
9.15.0.1 Synopsis
#include
int listen(int s, int backlog);
9.15.0.2 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.
9.15.0.3 Return Value
Returns zero on success, or -1 on error (and errno will be set
accordingly).
9.15.0.4 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
9.15.0.5 See Also
accept(), bind(), socket()
9.16 perror(), strerror()
Print an error as a human-readable string
9.16.0.1 Synopsis
#include
#include // for strerror()
void perror(const char *s);
char *strerror(int errnum);
9.16.0.2 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).
9.16.0.3 Return Value
strerror() returns a pointer to the error message string.
9.16.0.4 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));
}
9.16.0.5 See Also
errno
9.17 poll()
Test for events on multiple sockets simultaneously
9.17.0.1 Synopsis
#include
int poll(struct pollfd *ufds, unsigned int nfds, int timeout);
9.17.0.2 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:
Macro Description
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:
Macro Description
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?
9.17.0.3 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).
9.17.0.4 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, ...)...
// 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);
}
}
9.17.0.5 See Also
select()
9.18 recv(), recvfrom()
Receive data on a socket
9.18.0.1 Synopsis
#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);
9.18.0.2 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().
Macro Description
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
MSG_OOB 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.
If you want to call recv() "just for pretend", you can
call it with this flag. This will tell you what's waiting
MSG_PEEK 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.
Tell recv() to not return until all the data you
specified in the len parameter. It will ignore your
MSG_WAITALL 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().
9.18.0.3 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!
9.18.0.4 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);
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);
9.18.0.5 See Also
send(), sendto(), select(), poll(), Blocking
9.19 select()
Check if sockets descriptors are ready to read/write
9.19.0.1 Synopsis
#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);
9.19.0.2 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:
Macro Description
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.
9.19.0.3 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.
9.19.0.4 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(...);
//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);
}
}
9.19.0.5 See Also
poll()
9.20 setsockopt(), getsockopt()
Set various options for a socket
9.20.0.1 Synopsis
#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);
9.20.0.2 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:
optname Description
Bind this socket to a symbolic device name like eth0
SO_BINDTODEVICE instead of using bind() to bind it to an IP address.
Type the command ifconfig under Unix to see the
device names.
Allows other sockets to bind() to this port, unless
there is an active listening socket bound to the port
SO_REUSEADDR already. This enables you to get around those
"Address already in use" error messages when you try
to restart your server after a crash.
Allows UDP datagram (SOCK_DGRAM) sockets to send and
SOCK_DGRAM 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"!
9.20.0.3 Return Value
Returns zero on success, or -1 on error (and errno will be set
accordingly).
9.20.0.4 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);
// 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");
}
9.20.0.5 See Also
fcntl()
9.21 send(), sendto()
Send data out over a socket
9.21.0.1 Synopsis
#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);
9.21.0.2 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:
Macro Description
Send as "out of band" data. TCP supports this, and it's
a way to tell the receiving system that this data has a
MSG_OOB 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.
If send() would block because outbound traffic is
MSG_DONTWAIT clogged, have it return EAGAIN. This is like a "enable
non-blocking just for this send." See the section on
blocking for more details.
If you send() to a remote host which is no longer recv
MSG_NOSIGNAL ()ing, you'll typically get the signal SIGPIPE. Adding
this flag prevents that signal from being raised.
9.21.0.3 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.)
9.21.0.4 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, ...
// 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);
9.21.0.5 See Also
recv(), recvfrom()
9.22 shutdown()
Stop further sends and receives on a socket
9.22.0.1 Synopsis
#include
int shutdown(int s, int how);
9.22.0.2 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.
9.22.0.3 Return Value
Returns zero on success, or -1 on error (and errno will be set
accordingly).
9.22.0.4 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);
9.22.0.5 See Also
close()
9.23 socket()
Allocate a socket descriptor
9.23.0.1 Synopsis
#include
#include
int socket(int domain, int type, int protocol);
9.23.0.2 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.
Macro Description
domain describes what kind of socket you're interested in.
domain 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.
Also, the type parameter can be a number of things, but
you'll probably be setting it to either SOCK_STREAM for
type 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.)
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
protocol 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.
9.23.0.3 Return Value
The new socket descriptor to be used in subsequent calls, or -1 on
error (and errno will be set accordingly).
9.23.0.4 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);
9.23.0.5 See Also
accept(), bind(), getaddrinfo(), listen()
9.24 struct sockaddr and pals
Structures for handling internet addresses
9.24.0.1 Synopsis
#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];
};
9.24.0.2 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.
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.
9.24.0.3 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);
9.24.0.4 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. These redirect to affiliate links
with a popular bookseller, giving me nice kickbacks. If you're merely
feeling generous, you can paypal a donation to beej@beej.us. :-)
Unix Network Programming, volumes 1-2 by W. Richard Stevens.
Published by Addison-Wesley Professional and Prentice Hall. ISBNs for
volumes 1-2: 978-0131411555^49, 978-0130810816^50.
Internetworking with TCP/IP, volume I by Douglas E. Comer. Published
by Pearson. ISBN 978-0136085300^51.
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): 978-0201633467^52, 978-0201633542^53,
978-0201634952^54, (978-0201776317^55).
TCP/IP Network Administration by Craig Hunt. Published by O'Reilly &
Associates, Inc. ISBN 978-0596002978^56.
Advanced Programming in the UNIX Environment by W. Richard Stevens.
Published by Addison Wesley. ISBN 978-0321637734^57.
10.2 Web References
On the web:
BSD Sockets: A Quick And Dirty Primer^58 (Unix system programming
info, too!)
The Unix Socket FAQ^59
TCP/IP FAQ^60
The Winsock FAQ^61
And here are some relevant Wikipedia pages:
Berkeley Sockets^62
Internet Protocol (IP)^63
Transmission Control Protocol (TCP)^64
User Datagram Protocol (UDP)^65
Client-Server^66
Serialization^67 (packing and unpacking data)
10.3 RFCs
RFCs^68--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 1^69 --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 768^70 --The User Datagram Protocol (UDP)
RFC 791^71 --The Internet Protocol (IP)
RFC 793^72 --The Transmission Control Protocol (TCP)
RFC 854^73 --The Telnet Protocol
RFC 959^74 --File Transfer Protocol (FTP)
RFC 1350^75 --The Trivial File Transfer Protocol (TFTP)
RFC 1459^76 --Internet Relay Chat Protocol (IRC)
RFC 1918^77 --Address Allocation for Private Internets
RFC 2131^78 --Dynamic Host Configuration Protocol (DHCP)
RFC 2616^79 --Hypertext Transfer Protocol (HTTP)
RFC 2821^80 --Simple Mail Transfer Protocol (SMTP)
RFC 3330^81 --Special-Use IPv4 Addresses
RFC 3493^82 --Basic Socket Interface Extensions for IPv6
RFC 3542^83 --Advanced Sockets Application Program Interface (API) for
IPv6
RFC 3849^84 --IPv6 Address Prefix Reserved for Documentation
RFC 3920^85 --Extensible Messaging and Presence Protocol (XMPP)
RFC 3977^86 --Network News Transfer Protocol (NNTP)
RFC 4193^87 --Unique Local IPv6 Unicast Addresses
RFC 4506^88 --External Data Representation Standard (XDR)
The IETF has a nice online tool for searching and browsing RFCs^89.
---------------------------------------------------------------------
1. https://www.linux.com/-[?]
2. https://bsd.org/-[?]
3. https://cygwin.com/-[?]
4. https://docs.microsoft.com/en-us/windows/wsl/about-[?]
5. https://tangentsoft.net/wskfaq/-[?]
6. http://www.catb.org/~esr/faqs/smart-questions.html-[?]
7. https://beej.us/guide/bgnet/examples/telnot.c-[?]
8. https://tools.ietf.org/html/rfc854-[?]
9. https://tools.ietf.org/html/rfc793-[?]
10. https://tools.ietf.org/html/rfc791-[?]
11. https://tools.ietf.org/html/rfc768-[?]
12. https://tools.ietf.org/html/rfc791-[?]
13. https://en.wikipedia.org/wiki/Vint_Cerf-[?]
14. https://en.wikipedia.org/wiki/ELIZA-[?]
15. https://www.iana.org/assignments/port-numbers-[?]
16. https://en.wikipedia.org/wiki/Doom_(1993_video_game)-[?]
17. https://en.wikipedia.org/wiki/Wilford_Brimley-[?]
18. https://tools.ietf.org/html/rfc1918-[?]
19. https://tools.ietf.org/html/rfc4193-[?]
20. https://www.iana.org/assignments/port-numbers-[?]
21. https://beej.us/guide/bgnet/examples/showip.c-[?]
22. https://tools.ietf.org/html/rfc1413-[?]
23. https://beej.us/guide/bgnet/examples/server.c-[?]
24. https://beej.us/guide/bgnet/examples/client.c-[?]
25. https://beej.us/guide/bgnet/examples/listener.c-[?]
26. https://beej.us/guide/bgnet/examples/talker.c-[?]
27. https://libevent.org/-[?]
28. https://beej.us/guide/bgnet/examples/poll.c-[?]
29. https://beej.us/guide/bgnet/examples/pollserver.c-[?]
30. https://libevent.org/-[?]
31. https://beej.us/guide/bgnet/examples/select.c-[?]
32. https://beej.us/guide/bgnet/examples/selectserver.c-[?]
33. https://en.wikipedia.org/wiki/Internet_Relay_Chat-[?]
34. https://beej.us/guide/bgnet/examples/pack.c-[?]
35. https://en.wikipedia.org/wiki/IEEE_754-[?]
36. https://beej.us/guide/bgnet/examples/ieee754.c-[?]
37. https://beej.us/guide/url/tpop-[?]
38. https://github.com/protobuf-c/protobuf-c-[?]
39. https://beej.us/guide/bgnet/examples/pack2.c-[?]
40. https://beej.us/guide/bgnet/examples/pack2.c-[?]
41. https://tools.ietf.org/html/rfc4506-[?]
42. https://beej.us/guide/bgnet/examples/broadcaster.c-[?]
43. http://www.unpbook.com/src.html-[?]
44. http://www.unpbook.com/src.html-[?]
45. https://www.openssl.org/-[?]
46. https://stackoverflow.com/questions/21323023/-[?]
47. https://www.iana.org/assignments/port-numbers-[?]
48. https://www.iana.org/assignments/port-numbers-[?]
49. https://beej.us/guide/url/unixnet1-[?]
50. https://beej.us/guide/url/unixnet2-[?]
51. https://beej.us/guide/url/intertcp1-[?]
52. https://beej.us/guide/url/tcpi1-[?]
53. https://beej.us/guide/url/tcpi2-[?]
54. https://beej.us/guide/url/tcpi3-[?]
55. https://beej.us/guide/url/tcpi123-[?]
56. https://beej.us/guide/url/tcpna-[?]
57. https://beej.us/guide/url/advunix-[?]
58. https://cis.temple.edu/~giorgio/old/cis307s96/readings/docs/
sockets.html-[?]
59. https://developerweb.net/?f=70-[?]
60. http://www.faqs.org/faqs/internet/tcp-ip/tcp-ip-faq/part1/-[?]
61. https://tangentsoft.net/wskfaq/-[?]
62. https://en.wikipedia.org/wiki/Berkeley_sockets-[?]
63. https://en.wikipedia.org/wiki/Internet_Protocol-[?]
64. https://en.wikipedia.org/wiki/Transmission_Control_Protocol-[?]
65. https://en.wikipedia.org/wiki/User_Datagram_Protocol-[?]
66. https://en.wikipedia.org/wiki/Client-server-[?]
67. https://en.wikipedia.org/wiki/Serialization-[?]
68. https://www.rfc-editor.org/-[?]
69. https://tools.ietf.org/html/rfc1-[?]
70. https://tools.ietf.org/html/rfc768-[?]
71. https://tools.ietf.org/html/rfc791-[?]
72. https://tools.ietf.org/html/rfc793-[?]
73. https://tools.ietf.org/html/rfc854-[?]
74. https://tools.ietf.org/html/rfc959-[?]
75. https://tools.ietf.org/html/rfc1350-[?]
76. https://tools.ietf.org/html/rfc1459-[?]
77. https://tools.ietf.org/html/rfc1918-[?]
78. https://tools.ietf.org/html/rfc2131-[?]
79. https://tools.ietf.org/html/rfc2616-[?]
80. https://tools.ietf.org/html/rfc2821-[?]
81. https://tools.ietf.org/html/rfc3330-[?]
82. https://tools.ietf.org/html/rfc3493-[?]
83. https://tools.ietf.org/html/rfc3542-[?]
84. https://tools.ietf.org/html/rfc3849-[?]
85. https://tools.ietf.org/html/rfc3920-[?]
86. https://tools.ietf.org/html/rfc3977-[?]
87. https://tools.ietf.org/html/rfc4193-[?]
88. https://tools.ietf.org/html/rfc4506-[?]
89. https://tools.ietf.org/rfc/-[?]