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<title>s6-linux-utils: the s6-uevent-listener program</title>
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<p>
<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-uevent-listener</tt> program </h1>
<p>
<tt>s6-uevent-listener</tt> spawns a long-lived helper program.
It then listens to the netlink interface for uevents (also called
"hotplug" or "udev" events), and passes those events to the
helper program's standard input, using a simple format.
</p>
<h2> Interface </h2>
<pre>
s6-uevent-listener [ -v <em>verbosity</em> ] [ -b kbufsz ] <em>prog...</em>
</pre>
<ul>
<li> s6-uevent-listener spawns <em>prog...</em> with a pipe writing to
<em>prog</em>' stdin. </li>
<li> s6-uevent-listener binds to the netlink interface and listens for
hotplug events, as the <em>udev</em> program does. </li>
<li> It transmits event information to <em>prog</em> via the pipe. </li>
<li> s6-uevent-listener, like <em>prog</em>, is a long-lived program.
When it receives a SIGTERM, it stops listening; it will
exit as soon as it has flushed its event queue to <em>prog</em>. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-v</tt> <em>verbosity</em> : be more or less verbose.
Default verbosity is 1. 0 will only print fatal error messages, 3 will
print warnings every time the netlink interface sends something
unexpected. </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 (which is on
the large side). </li>
</ul>
<h2> Protocol </h2>
<ul>
<li> <em>prog</em> should read a series of events on its stdin, and exit
on EOF. </li>
<li> An event is a series of null-terminated strings as they are sent by
the kernel to the netlink; s6-uevent-listener adds a final empty string
(i.e. an additional null character) to mark the end of the series. </li>
<li> The first string is a short description of the event; it normally
contains the string "@/". Other strings after the first are of the form
"VARIABLE=value", and describe the environment which a hotplug helper
for the event (registered in <tt>/proc/sys/kernel/hotplug</tt>) would be
spawned with. </li>
<li> Example (newlines added for clarity): <pre>
add@/class/input/input9/mouse2\0
ACTION=add\0
DEVPATH=/class/input/input9/mouse2\0
SUBSYSTEM=input\0
SEQNUM=106\0
PHYSDEVPATH=/devices/pci0000:00/0000:00:1d.1/usb2/22/22:1.0
PHYSDEVBUS=usb\0
PHYSDEVDRIVER=usbhid\0
MAJOR=13\0
MINOR=34\0
\0 </pre> </li>
</ul>
<h2> Notes </h2>
<ul>
<li> s6-uevent-listener is a daemon; it should be run under a proper supervision system such
as <a href="http://skarnet.org/software/s6/">s6</a>. </li>
<li> If you are running s6-uevent-listener, <em>prog...</em> should be the
only program handling uevents, which means that
<tt>/proc/sys/kernel/hotplug</tt> should be empty. </li>
<li> If you want the serialization benefit of the netlink, but still
want to spawn a program such as <a href="http://busybox.net/">busybox</a>'s
or <a href="http://landley.net/toybox/">toybox</a>'s <tt>mdev</tt>, use
<tt><a href="s6-uevent-spawner.html">s6-uevent-spawner</a> mdev</tt> as
s6-uevent-listener's <em>prog</em>. </li>
</ul>
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