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 ] [ -l linevar ] [ -t l:t:k ] prog...

• s6-devd rewrites its command line into s6-uevent-listener | s6-uevent-spawner prog..., dispatching its options to the appropriate programs; then it execs into this new command line. It does nothing else: it's just a wrapper.
• The first executed program, s6-uevent-listener, will listen to the netlink and pass the events sequentially to s6-uevent-spawner, which will spawn a prog... helper for every event.

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.
• -l linevar : the short description of the event, given by the kernel as the first string in the netlink message, will be made available to prog under the environment variable named linevar.
• -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. The real long-lived process is named s6-uevent-listener; ir runs with the same pid as s6-devd.
• 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 buffers along the way.