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<title>mdevd: the mdevd program</title>
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<p>
<a href="index.html">mdevd</a><br />
<a href="//skarnet.org/software/">Software</a><br />
<a href="//skarnet.org/">skarnet.org</a>
</p>
<h1> The <tt>mdevd</tt> program </h1>
<p>
<tt>mdevd</tt> is an uevent manager. It reads a series of
uevents on its stdin; for every uevent it reads, it performs
actions according to its configuration file. Actions can
be inserting a kernel module, creating or modifying device
entries in <tt>/dev</tt>, etc.
</p>
<p>
<tt>mdevd</tt>'s configuration file uses the exact same
format as
<a href="https://git.busybox.net/busybox/plain/docs/mdev.txt">mdev</a>.
The differences between mdevd and mdev are:
</p>
<ul>
<li> mdev needs to be registered as a hotplug manager and the
kernel spawns an instance of mdev per uevent; for every uevent,
mdev has to parse its configuration file. By contrast, there
is only one instance of mdevd, reading a series of uevents and
performing actions without forking; the configuration file is
read and parsed only once. </li>
<li> mdevd reads uevents on its stdin. It is not suitable as
a hotplug manager, and it does not connect to the netlink itself
either. It is meant to be used in conjunction with
<a href="mdevd-netlink.html">mdevd-netlink</a>, which reads
uevents from the netlink, or with
<a href="mdevd-coldplug.html">mdevd-coldplug</a>, which generates
coldplug uevents. </li>
</ul>
<h2> Interface </h2>
<pre>
mdevd [ -v <em>verbosity</em> ] [ -f <em>conffile</em> ] [ -n ] [ -s <em>slashsys</em> ] [ -d <em>slashdev</em> ] [ -F <em>fwbase</em> ]
</pre>
<ul>
<li> mdevd reads and parses its configuration file <tt>/etc/mdev.conf</tt>. </li>
<li> It then reads its stdin, waiting for uevents.
<li> It exits when it has finished scanning. </li>
</ul>
<h2> Exit codes </h2>
<ul>
<li> 0: EOF read on standard input </li>
<li> 1: received an invalid event </li>
<li> 2: syntax error in the configuration file </li>
<li> 100: wrong usage </li>
<li> 111: system call failed </li>
</ul>
<h2> Signals </h2>
<p>
mdevd reacts to the following signals:
</p>
<ul>
<li> SIGHUP: re-read the configuration file </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 or more
is seriously verbose debugging. </li>
<li> <tt>-n</tt> : dry run. mdevd will not create or delete
device nodes, and it will not spawn commands. Instead, it will print to stdout
the commands it would have spawned. </li>
<li> <tt>-f</tt> <em>conffile</em> : read the configuration
file from <em>conffile</em>. Default is <tt>/etc/mdev.conf</tt>.
<em>conffile</em> must be an absolute path. </li>
<li> <tt>-s</tt> <em>slashsys</em> : assume the sysfs
pseudo-filesystem is mounted on <em>slashsys</em>. Default is <tt>/sys</tt>.
<em>slashsys</em> must be an absolute path. </li>
<li> <tt>-d</tt> <em>slashdev</em> : assume the device nodes
are to be found in <em>slashdev</em>. Default is <tt>/dev</tt>.
<em>slashdev</em> must be an absolute path. </li>
<li> <tt>-F</tt> <em>fwbase</em> : assume the firmware files, if any,
are to be found in <em>fwbase</em>. Default is <tt>/lib/firmware</tt>.
<em>fwbase</em> must be an absolute path. </li>
</ul>
<h2> Configuration file </h2>
<p>
mdevd uses mdev's configuration file format.
A good <tt>mdev.conf</tt> example is available
<a href="https://github.com/slashbeast/mdev-like-a-boss/blob/master/mdev.conf">here.</a>
</p>
<p>
If mdevd cannot find its configuration file, it will use a simple, basic default
configuration where it can create device nodes as root or delete them, and does
nothing else.
</p>
<h2> Notes </h2>
<ul>
<li> Strictly speaking, mdevd is a short-lived program: it has
a normal exit condition, which is when it receives EOF on its stdin.
That allows it to work as a coldplug manager when paired with
<a href="mdevd-coldplug.html">mdevd-coldplug</a>. </li>
<li> However, when paired with <a href="mdevd-netlink.html">mdevd-netlink</a>,
it acts as a daemon, because mdev-netlink normally never exits until
the end of the machine lifetime and never closes its stdout, so
mdevd's stdin never receives EOF. </li>
<li> The <tt>examples/</tt> subdirectory of the mdevd package contains
examples on how to run mdevd under various init systems / supervisors. </li>
</ul>
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