killpg() sends the signal
sig to the process group
pgrp. See
sigaction(2) for a list of signals. If
pgrp is 0,
killpg() sends the signal to the sending process's process group.
The sending process and members of the process group must have the same effective user ID, or the sender must be the super-user. As a single special case the continue signal SIGCONT may be sent to any process that is a descendant of the current process.