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<!-- %MENU% A more complicated IPC mechanism, with networking support -->
<h2 class="chapter">16 Sockets</h2>

<p>This chapter describes the GNU facilities for interprocess
communication using sockets.

   <p><a name="index-socket-1630"></a><a name="index-interprocess-communication_002c-with-sockets-1631"></a>A <dfn>socket</dfn> is a generalized interprocess communication channel. 
Like a pipe, a socket is represented as a file descriptor.  Unlike pipes
sockets support communication between unrelated processes, and even
between processes running on different machines that communicate over a
network.  Sockets are the primary means of communicating with other
machines; <code>telnet</code>, <code>rlogin</code>, <code>ftp</code>, <code>talk</code> and the
other familiar network programs use sockets.

   <p>Not all operating systems support sockets.  In the GNU library, the
header file <samp><span class="file">sys/socket.h</span></samp> exists regardless of the operating
system, and the socket functions always exist, but if the system does
not really support sockets these functions always fail.

   <p><strong>Incomplete:</strong> We do not currently document the facilities for
broadcast messages or for configuring Internet interfaces.  The
reentrant functions and some newer functions that are related to IPv6
aren't documented either so far.

<ul class="menu">
<li><a accesskey="1" href="Socket-Concepts.html#Socket-Concepts">Socket Concepts</a>: 	Basic concepts you need to know about. 
<li><a accesskey="2" href="Communication-Styles.html#Communication-Styles">Communication Styles</a>: Stream communication, datagrams and other styles. 
<li><a accesskey="3" href="Socket-Addresses.html#Socket-Addresses">Socket Addresses</a>: 	How socket names (``addresses'') work. 
<li><a accesskey="4" href="Interface-Naming.html#Interface-Naming">Interface Naming</a>: 	Identifying specific network interfaces. 
<li><a accesskey="5" href="Local-Namespace.html#Local-Namespace">Local Namespace</a>: 	Details about the local namespace. 
<li><a accesskey="6" href="Internet-Namespace.html#Internet-Namespace">Internet Namespace</a>: 	Details about the Internet namespace. 
<li><a accesskey="7" href="Misc-Namespaces.html#Misc-Namespaces">Misc Namespaces</a>: 	Other namespaces not documented fully here. 
<li><a accesskey="8" href="Open_002fClose-Sockets.html#Open_002fClose-Sockets">Open/Close Sockets</a>:   Creating sockets and destroying them. 
<li><a accesskey="9" href="Connections.html#Connections">Connections</a>: 		Operations on sockets with connection state. 
<li><a href="Datagrams.html#Datagrams">Datagrams</a>: 		Operations on datagram sockets. 
<li><a href="Inetd.html#Inetd">Inetd</a>: 		Inetd is a daemon that starts servers on request. 
			   The most convenient way to write a server
			   is to make it work with Inetd. 
<li><a href="Socket-Options.html#Socket-Options">Socket Options</a>: 	Miscellaneous low-level socket options. 
<li><a href="Networks-Database.html#Networks-Database">Networks Database</a>:    Accessing the database of network names. 
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