s6-linux-utils
Software
skarnet.org

The s6-devd program

s6-devd listens to the netlink interface for udev events, and launches a helper program for every event, similarly to what the hotplug interface does.

Interface

     s6-devd [ -q | -v ] [ -b kbufsz ] [ -t l:t:k ] prog...

• s6-devd binds to the netlink interface and listens for hotplug events, as the udev program does.
• For every event it receives, it spawns prog... with the event variables added to the environment, just as if prog... had been registered in /proc/sys/kernel/hotplug.
• However, unlike the kernel, s6-devd spawns the prog... helpers sequentially: it waits for an instance to finish before spawning another one. 
• s6-devd is a long-lived program; it exits 0 when it receives a SIGTERM. If a helper program is alive at that time, s6-devd waits for it to die before exiting.

Options

• -q : be more quiet.
• -v : be more verbose.
• -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 65536.
• -t l:t:k : If l, t or k is specified, they specify timeouts; by default, they are infinite. If prog... is still alive after l milliseconds, s6-devd sends it a SIGTERM. Then, if prog... is still alive after t more milliseconds, s6-devd sends it a SIGKILL. Then, if prog... is still alive after k more milliseconds, s6-devd yells and exits 99.

Notes

• s6-devd is a daemon; it should be run under a proper supervision system such as s6. (That is why it does not fork and logs to stderr.)
• The prog... helper, on the other hand, should be very short-lived, even if you are not using the -t option to s6-devd. Since helpers are spawned sequentially, slow helpers can make events queue up and fill the netlink kernel buffer.
•  If you are using busybox and want a minimal udev-style dynamic /dev handling, /sbin/mdev is a suitable prog... helper.
• The point of s6-devd is that it runs the helpers sequentially, so it solves the race condition that appears when helpers are run via the hotplug interface. When s6-devd is used, /proc/sys/kernel/hotplug should be empty.