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<title>s6: the s6-setlock program</title>
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<p>
<a href="index.html">s6</a><br />
<a href="http://skarnet.org/software/">Software</a><br />
<a href="http://skarnet.org/">skarnet.org</a>
</p>
<h1> The s6-setlock program </h1>
<p>
s6-setlock takes a lock on a file, then executes into another program.
</p>
<h2> Interface </h2>
<pre>
s6-setlock [ -n | -N | -t <em>timeout</em> ] [ -r | -w ] <em>file</em> <em>prog...</em>
</pre>
<ul>
<li> s6-setlock creates <em>file</em> if it does not exist and opens it for writing. </li>
<li> It locks <em>file</em>. If it cannot take the lock for any reason, it exits 1. </li>
<li> It executes into <em>prog...</em>. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-n</tt> : nonblocking lock. If s6-setlock cannot acquire the lock, it will
exit 1 immediately. </li>
<li> <tt>-N</tt> : blocking lock. s6-setlock will wait until it can acquire the lock.
This is the default. </li>
<li> <tt>-t <em>timeout</em></tt> : timed lock. If s6-setlock cannot acquire
the lock after <em>timeout</em> milliseconds, it will exit 1. </li>
<li> <tt>-r</tt> : shared lock. Other shared locks on the same file will not prevent
the lock from being acquired (but an exclusive lock will). </li>
<li> <tt>-w</tt> : exclusive lock. This is the default. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> s6-setlock leaks an open file descriptor into the <em>prog</em>
execution. This is intended: the fd holds the lock, which is released
when <em>prog</em> exits. <em>prog</em> must not touch fds it does not
know about. </li>
<li> If the timed lock option is chosen, s6-setlock does not acquire the lock
itself. Instead, it spawns a <a href="libs6/s6lockd-helper.html">s6lockd-helper</a>
process that acquires the lock while s6-setlock controls the timeout; the
s6lockd-helper process then holds the lock and lives as long as
<em>prog</em>. </li>
</ul>
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