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<title>s6-linux-utils: the s6-devd program</title>
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<a href="index.html">s6-linux-utils</a><br />
<a href="//skarnet.org/software/">Software</a><br />
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<h1> The <tt>s6-devd</tt> program </h1>
<p>
<tt>s6-devd</tt> listens to the netlink interface for udev events, and
launches a helper program for every event, similarly to what the hotplug
interface does.
</p>
<h2> Interface </h2>
<pre>
s6-devd [ -q | -v ] [ -b <em>kbufsz</em> ] [ -l <em>linevar</em> ] [ -t l:t:k ] <em>prog...</em>
</pre>
<ul>
<li> s6-devd rewrites its command line into
<tt><a href="s6-uevent-listener.html">s6-uevent-listener</a> <tt>|</tt>
<a href="s6-uevent-spawner.html">s6-uevent-spawner</a> <em>prog...</em></tt>,
dispatching its options to the appropriate programs; then it execs into
this new command line. It does nothing else: it's just a wrapper. </li>
<li> The first executed program,
<a href="s6-uevent-listener.html">s6-uevent-listener</a>,
will listen to the netlink and pass the events sequentially to
<a href="s6-uevent-spawner.html">s6-uevent-spawner</a>, which will
spawn a <em>prog...</em> helper for every event. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-q</tt> : be more quiet. </li>
<li> <tt>-v</tt> : be more verbose. </li>
<li> <tt>-b</tt> <em>kbufsz</em> : try and reserve a kernel buffer of
<em>kbufsz</em> bytes for the netlink queue. Too large a buffer wastes kernel memory;
too small a buffer risks losing events. The default is 65536. </li>
<li> <tt>-l</tt> <em>linevar</em> : the short description of the
event, given by the kernel as the first string in the netlink message,
will be made available to <em>prog</em> under the environment variable
named <em>linevar</em>. </li>
<li> <tt>-t</tt> <em>l:t:k</em> : If <em>l</em>, <em>t</em> or <em>k</em> is
specified, they specify timeouts; by default, they are infinite.
If <em>prog...</em> is still alive after <em>l</em> milliseconds, s6-devd sends
it a SIGTERM. Then, if <em>prog...</em> is still alive after <em>t</em> more
milliseconds, s6-devd sends it a SIGKILL. Then, if <em>prog...</em> is still
alive after <em>k</em> more milliseconds, s6-devd yells and exits 99. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> s6-devd is a daemon; it should be run under a proper supervision system such
as <a href="//skarnet.org/software/s6/">s6</a>. The real long-lived
process is named <a href="s6-uevent-listener.html">s6-uevent-listener</a>;
ir runs with the same pid as s6-devd. </li>
<li> The <em>prog...</em> helper, on the other hand, should be very short-lived,
even if you are not using the <tt>-t</tt> option to s6-devd. Since helpers are
spawned sequentially, slow helpers can make events queue up and fill buffers
along the way. </li>
</ul>
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