Killpg sends the signal sig to the process group pgrp. See
signal(7) for a list of signals. If pgrp is 0, killpg
sends the signal to the sending process's process group.
(POSIX says: If pgrp is less than or equal to 1, the behaviour is
For a process to have permission to send a signal it must either be privileged
(under Linux: have the CAP_KILL capability), or the real or effective
user ID of the sending process must equal the real or saved set-user-ID of the
target process. In the case of SIGCONT it suffices when the sending and
receiving processes belong to the same session.
There are various differences between the permission checking in BSD-type
systems and SYSV-type systems. See the POSIX rationale for kill(). A
difference not mentioned by POSIX concerns the return value EPERM: BSD
documents that no signal is sent and EPERM returned when the permission check
failed for at least one target process, while POSIX documents EPERM only when
the permission check failed for all target processes.