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<html>
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<title>s6: the s6lockd internal program</title>
<meta name="Description" content="s6: the s6lockd internal program" />
<meta name="Keywords" content="s6 s6lockd lockd asynchronous timed lock daemon" />
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<a href="index.html">libs6lock</a><br />
<a href="../">s6</a><br />
<a href="http://skarnet.org/software/">Software</a><br />
<a href="http://skarnet.org/">skarnet.org</a><p />
<h1> The <tt>s6lockd</tt> program </h1>
<p>
<tt>s6lockd</tt> is the s6lock daemon. It is a program that manages
a set of lock files in a given directory, and associated timeouts.
</p>
<h2> Interface </h2>
<p>
s6lockd does not fork, does not background itself automatically,
and does not use syslog. It is not meant to be run directly by the
user: it will be spawned by the
<a href="index.html">s6lock client library</a>.
</p>
<p>
There are 2 ways to use s6lockd:
</p>
<ol>
<li> Use the <tt>s6lock_startf()</tt> library call.
A <tt>s6lockd</tt> child will then be spawned from your
calling process, and automatically reaped when you call
<tt>s6lock_end()</tt>. It requires care with applications that
trap SIGCHLD. It also requires care with lock file permissions:
a s6lockd instance might not be able
to open a lock file created by a former instance run by another
client with different permissions. </li>
<li> Use the <tt>s6lock_start()</tt> library call, together with a
<a href="http://skarnet.org/software/s6-networking/localservice.html">s6lockd service</a>.
For once, <em>this is the recommended setup</em>: s6lockd creates empty
lock files, and having all s6lockd instances run under the same user
simplifies permissions management considerably. </li>
</li>
</ol>
<p>
When run as a service, s6lockd has no "standalone" mode: it is
designed to work with a Unix
domain superserver, like
<a href="http://skarnet.org/software/s6-networking/s6-ipcserver.html">s6-ipcserver</a>.
s6lockd follows the <a href="http://cr.yp.to/proto/ucspi.txt">UCSPI</a>
interface, it can be directly executed from the superserver.
</p>
<h2> Notes </h2>
<ul>
<li> Unix does not natively provide a way to stop blocking on a lock
acquisition after a timeout. To emulate such behaviour, s6lockd actually
spawns a <a href="s6lockd-helper.html">s6lockd-helper</a> child per
requested lock. </li>
</ul>
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