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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 a uevent manager. It connects to the netlink and reads
a series of uevents; 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, mdevd is
a daemon: it's long-lived, and there is only one instance,
reading a series of uevents and
performing actions without forking; the configuration file is
read and parsed only once. </li>
<li> Even though mdev can now be run as a daemon, via the <tt>-d</tt>
option, there are still <a href="index.html#mdev">technical advantages</a>
to running mdevd over mdev. </li>
</ul>
<h2> Interface </h2>
<pre>
mdevd [ -v <em>verbosity</em> ] [ -D <em>notif</em> ] [ -o <em>outputfd</em> ] [ -b <em>kbufsz</em> ] [ -f <em>conffile</em> ] [ -n ] [ -s <em>slashsys</em> ] [ -d <em>slashdev</em> ] [ -F <em>fwbase</em> ] [ -C ]
</pre>
<ul>
<li> mdevd reads and parses its configuration file <tt>/etc/mdev.conf</tt>. </li>
<li> It then connects to the netlink and reads from it, waiting for uevents.
<li> It exits 0 on a SIGTERM. </li>
</ul>
<h2> Exit codes </h2>
<ul>
<li> 0: SIGTERM received, clean exit </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>
<li> SIGTERM: exit as soon as possible </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>-D</tt> <em>notif</em> : when ready
(actually listening to the netlink),
write a newline to file descriptor <em>notif</em> then close it.
This allows mdevd to use the
<a href="//skarnet.org/software/s6/notifywhenup.html">s6 mechanism to notify
readiness</a>. <em>notif</em> cannot be lesser than 3.
If this option is not given, no readiness notification is sent. </li>
<li> <tt>-o</tt> <em>outputfd</em> : send a copy of the complete
uevent, with possible new fields, to file descriptor <em>outputfd</em>,
<strong>after</strong> mdevd has handled them. (This can serve as a
synchronization mechanism with an external program that needs a device to
have been properly configured by the device manager before acting on the
device.)
The uevent is sent with the kernel format: fields are separated by a null
character. The uevent is terminated by an additional null character.
<em>outputfd</em> cannot be lesser than 3 and cannot be the same as <em>notif</em>.
If for any reason, at any point, mdevd fails to write to <em>outputfd</em>,
it stops writing, until it is restarted. (This is to preserve mdevd's memory
stability guarantee.) By default, the uevents are not written anywhere. </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 500 kB, which should be
enough for most systems; if you're getting "No buffer space available" errors
from mdevd at coldplug time, try increasing this number. </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 actions it would have performed. </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>
<li> <tt>-C</tt> : after initialization (when mdevd is ready), invoke
<a href="mdevd-coldplug.html">mdevd-coldplug</a>, so that a coldplug is
automatically performed when mdevd starts. This option should only be used
in situations where a drop-in replacement for <tt>mdev -d</tt> is needed, because
it has the drawback of performing a coldplug every time mdevd is started —
including when mdevd is killed and restarted. By default, mdevd does not perform
a coldplug when it starts, and it is up to the administrator to properly organize
their boot scripts to run <a href="mdevd-coldplug.html">mdevd-coldplug</a> once,
at boot time, when mdevd is ready. The <tt>examples/s6-rc</tt> subdirectory of
the mdevd package shows a way to do this with the
<a href="//skarnet.org/softwre/s6-rc/">s6-rc</a> service manager. </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> The <tt>examples/</tt> subdirectory of the mdevd package contains
examples on how to run mdevd under various init systems / supervisors. These
examples do not show how to run mdevd as a logged service, because mdevd
should normally be run early in the system's boot up sequence, before
mounting the disks - so the presence of a mounted partition suitable for
logging is not guaranteed. In normal use, mdevd is quite terse, so it
runs smoothly without a dedicated logger, with its error messages
going to the catch-all logger. </li>
</ul>
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