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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="http://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 kbufsz ] [ -t l:t:k ] <em>prog...</em>
</pre>
<ul>
<li> s6-devd binds to the netlink interface and listens for
hotplug events, as the <em>udev</em> program does. </li>
<li> For every event it receives, it spawns <em>prog...</em> with
the event variables added to the environment, just as if <em>prog...</em>
had been registered in <tt>/proc/sys/kernel/hotplug</tt>. </li>
<li> However, unlike the kernel, s6-devd spawns the <em>prog...</em> helpers
sequentially: it waits for an instance to finish before spawning another one. </li>
<li> s6-devd is a long-lived program; it exits 0 when it receives a
SIGTERM. If a helper program is alive at that time, s6-devd waits for it to die
before exiting. </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>-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="http://skarnet.org/software/s6/">s6</a>. (That is why it does not
fork and logs to stderr.) </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 the netlink
kernel buffer. </li>
<li> If you are using <a href="http://busybox.net/">busybox</a> and want a
minimal udev-style dynamic <tt>/dev</tt>
handling, <tt>/sbin/mdev</tt> is a suitable <em>prog...</em> helper. </li>
<li> The point of s6-devd is that it runs the helpers sequentially, so it solves
the race condition that appears when helpers are run via the hotplug interface.
When s6-devd is used, <tt>/proc/sys/kernel/hotplug</tt> should be empty. </li>
</ul>
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