s6-linux-utils
Software
skarnet.org

The s6-uevent-spawner program

s6-uevent-spawner listens to its standard input for netlink-style events, and launches a helper program for every event, similarly to what the hotplug interface does.

It is meant to be used together with s6-uevent-listener.

Interface

     s6-uevent-spawner [ -v verbosity ] [ -l linevar ] [ -t l:t:k ] prog...

• s6-uevent-spawner listens to its stdin for a series of strings representing kernel uevents, as formatted by s6-uevent-listener.
• For every event it reads, it spawns prog... with the event variables added to the environment, just as if prog... had been registered in /proc/sys/kernel/hotplug. However, it reads the events sequentially, and waits for a prog instance to finish before spawning another one.
• When s6-uevent-spawner receives EOF: if an instance of prog is alive, it first waits for it to die. Then it exits 0.

Options

• -v verbosity : be more or less verbose. Default verbosity is 1. 0 only prints fatal error messages. 3 is too much.
• -l linevar : make the event description (i.e. the first string in the netlink message announcing the event) available in prog under the environment variable linevar. By default, this first string is ignored - it is not needed, all the event information is already in the other variables.
• -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-uevent-spawner yells and exits 99.

Notes

• The prog... helper 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 up buffers.