The `mdevd` program

mdevd 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 /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, 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.

Interface

     mdevd [ -v verbosity ] [ -D notif ] [ -b kbufsz ] [ -f conffile ] [ -n ] [ -s slashsys ] [ -d slashdev ] [ -F fwbase ]

mdevd reads and parses its configuration file /etc/mdev.conf.
It then connects to the netlink and reads from it, waiting for uevents.
It exits 0 on a SIGTERM.

Exit codes

0: SIGTERM received, clean exit
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
SIGTERM: exit as soon as possible

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.
-D notif : when ready (actually listening to the netlink), write a newline to file descriptor notif then close it. This allows mdevd to use the s6 mechanism to notify readiness. notif cannot be lesser than 3. If this option is not given, no readiness notification is sent.
-b kbufsz : try and reserve a kernel buffer of kbufsz 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.
-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

The examples/ 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.