unixdomain/Unix_domain_sockets.html

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<body class="article">
<div id="header">
<h1>UNIX domain sockets</h1>
<span id="author">Troy D. Hanson</span><br />
<span id="email"><code>&lt;<a href="mailto:tdh@tkhanson.net">tdh@tkhanson.net</a>&gt;</code></span><br />
</div>
<div id="content">
<div class="sect1">
<h2 id="_communication_within_a_host">Communication within a host</h2>
<div class="sectionbody">
<div class="paragraph"><p>UNIX domain sockets are a method by which processes on the same host can
communicate. Communication is bidirectional with stream sockets
and unidirectional with datagram sockets.</p></div>
<div class="literalblock">
<div class="content">
<pre><code>fd = socket(AF_UNIX, SOCK_STREAM, 0);</code></pre>
</div></div>
<div class="sect2">
<h3 id="_identity">Identity</h3>
<div class="paragraph"><p>Instead of identifying a server by an IP address and port, a UNIX domain socket
is known by a pathname. Obviously the client and server have to agree on the
pathname for them to find each other. The server binds the pathname to the socket:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>struct sockaddr_un addr;
memset(&amp;addr, 0, sizeof(addr));
addr.sun_family = AF_UNIX;
strncpy(addr.sun_path, "socket", sizeof(addr.sun_path)-1);
bind(fd, (struct sockaddr*)&amp;addr, sizeof(addr));</code></pre>
</div></div>
<div class="paragraph"><p>The resulting pathname is visible in the filesystem, like:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>% ls -l
srwxr-xr-x 1 thanson thanson     0 2011-07-02 17:11 socket</code></pre>
</div></div>
<div class="sect3">
<h4 id="_unlink_before_bind">Unlink before bind</h4>
<div class="paragraph"><p>Note that, once created, this socket file will continue to exist, even after the
server exits. If the server subsequently restarts, the file prevents re-binding:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>% ./srv
% ./srv
bind error: Address already in use</code></pre>
</div></div>
<div class="paragraph"><p>So, servers should unlink the socket pathname prior to binding it.</p></div>
</div>
</div>
<div class="sect2">
<h3 id="_file_permissions_control_who_can_connect">File permissions control who can connect</h3>
<div class="paragraph"><p>For UNIX domain sockets, file and directory permissions restrict which processes
on the host can open the file, and thus communicate with the server.  Therefore,
UNIX domain sockets provide an advantage over Internet sockets (to which anyone
can connect, unless extra authentication logic is implemented).</p></div>
</div>
<div class="sect2">
<h3 id="_comparison_with_named_pipes_for_ipc">Comparison with named pipes for IPC</h3>
<div class="paragraph"><p>IPC within a Unix host by may be accomplished by several other means including
named pipes.  What circumstances favor UNIX domain sockets versus pipes? The
choice is influenced by these factors:</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
Duplex
</dt>
<dd>
<p>
  Stream sockets provide bi-directional communication while named pipes are
  uni-directional.
</p>
</dd>
<dt class="hdlist1">
Distinct clients
</dt>
<dd>
<p>
  Clients using sockets each have an independent connection to the server.
  With named pipes, many clients may write to the pipe, but the server cannot
  distinguish the clients from each other-- the server has only one descriptor
  to read from the named pipe. Because the named pipe has only read descriptor
  and possibly-multiple writers, random interleaving can also occur if a client
  writes more than <code>PIPE_BUF</code> bytes in one operation. Since pipes have these
  limitations, UNIX domain sockets should be used if there are multiple clients
  that need to be distinguishable or which write long messages to the server.
</p>
</dd>
<dt class="hdlist1">
Method of creating and opening
</dt>
<dd>
<p>
  Sockets are created using <code>socket</code> and assigned their identity via <code>bind</code>. Named
  pipes are created using <code>mkfifo</code>. To connect to a UNIX domain socket the normal
  <code>socket</code>/<code>connect</code> calls are used, but a named pipe is written using regular file
  <code>open</code> and <code>write</code>. That makes them easier to use from a shell script for example.
</p>
</dd>
</dl></div>
</div>
<div class="sect2">
<h3 id="_linux_abstract_socket_namespace">Linux Abstract Socket Namespace</h3>
<div class="paragraph"><p>Linux has a special feature: if the pathname for a UNIX domain socket begins
with a null byte <em>\0</em>, its name is not mapped into the filesystem. Thus it
won&#8217;t collide with other names in the filesystem.  Also, when a server closes
its UNIX domain listening socket in the abstract namespace, its file is deleted;
with regular UNIX domain sockets, the file persists after the server closes it.</p></div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_resources">Resources</h2>
<div class="sectionbody">
<div class="paragraph"><p>Here are <a href="https://github.com/troydhanson/network/tree/master/unixdomain">some C programs</a> that implement a UNIX domain socket client and
server. These are placed in the public domain.</p></div>
<div class="ulist"><ul>
<li>
<p>
A <a href="https://github.com/troydhanson/network/blob/master/unixdomain/srv.c">server</a> example
</p>
</li>
<li>
<p>
A <a href="https://github.com/troydhanson/network/blob/master/unixdomain/cli.c">client</a> example
</p>
</li>
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Last updated 2015-02-16 09:32:28 EST
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