M_SIG

The M_SIG message is sent upstream by modules or drivers to post a signal to a process. When the message reaches the front of the stream-head read queue, it evaluates the first data byte of the message as a signal number, defined in <sys/signal.h>. (The signal is not generated until it reaches the front of the stream-head read queue.) The associated signal will be sent to processes under the following conditions:

• If the signal is SIGPOLL, it is sent only to those processes that have explicitly registered to receive the signal (see I_SETSIG in streamio(7I)).

• If the signal is not SIGPOLL and the stream containing the sending module or driver is a controlling TTY, the signal is sent to the associated process group. A stream becomes the controlling TTY for its process group if, on open(2) a module or driver sends an M_SETOPTS message to the stream head with the SO_ISTTY flag set.

• If the signal is not SIGPOLL and the stream is not a controlling TTY, no signal is sent, except in case of SIOCSPGRP and TIOCSPGRP. These two ioctls set the process group field in the stream head so the stream can generate signals even if it is not a controlling TTY.