The `mdevd` program

mdevd is a 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 /dev, etc.

mdevd's configuration file uses the exact same format as mdev. The differences between mdevd and mdev are:

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.
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 mdevd-netlink, which reads uevents from the netlink, or with mdevd-coldplug, which generates coldplug uevents.

Interface

     mdevd [ -v verbosity ] [ -f conffile ] [ -n ] [ -s slashsys ] [ -d slashdev ] [ -F fwbase ]

mdevd reads and parses its configuration file /etc/mdev.conf.
It then reads its stdin, waiting for uevents.
It exits when the stream of uevents ends. (EOF on stdin.)

Exit codes

0: EOF read on standard input
1: received an invalid event
2: syntax error in the configuration file
100: wrong usage
111: system call failed

Signals

mdevd reacts to the following signals:

SIGHUP: re-read the configuration file

Options

-v verbosity : be more or less verbose. Default verbosity is 1. 0 will only print fatal error messages, 3 or more is seriously verbose debugging.
-n : 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.
-f conffile : read the configuration file from conffile. Default is /etc/mdev.conf. conffile must be an absolute path.
-s slashsys : assume the sysfs pseudo-filesystem is mounted on slashsys. Default is /sys. slashsys must be an absolute path.
-d slashdev : assume the device nodes are to be found in slashdev. Default is /dev. slashdev must be an absolute path.
-F fwbase : assume the firmware files, if any, are to be found in fwbase. Default is /lib/firmware. fwbase must be an absolute path.

Configuration file

mdevd uses mdev's configuration file format. A good mdev.conf example is available here.

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.

Notes

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 mdevd-coldplug.
However, when paired with mdevd-netlink, 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.
The examples/ subdirectory of the mdevd package contains examples on how to run mdevd under various init systems / supervisors.