KILL(2) BSD System Calls Manual KILL(2)
kill -- send signal to a process
kill(pid_t pid, int sig);
The kill() function sends the signal given by sig to pid, a process or a
group of processes. sig may be one of the signals specified in
sigaction(2) or it may be 0, in which case error checking is performed
but no signal is actually sent. This can be used to check the validity
For a process to have permission to send a signal to a process designated
by pid, the real or effective user ID of the receiving process must match
that of the sending process or the user must have appropriate privileges
(such as given by a set-user-ID program or the user is the superuser). A
single exception is the signal SIGCONT, which may always be sent to any
process with the same session ID as the caller.
If pid is greater than zero:
sig is sent to the process whose ID is equal to pid.
If pid is zero:
sig is sent to all processes whose group ID is equal to the
process group ID of the sender, and for which the process has
permission; this is a variant of killpg(3).
If pid is -1:
If the user has superuser privileges, the signal is sent to all
processes excluding system processes and the process sending the
signal. If the user is not the superuser, the signal is sent to
all processes with the same uid as the user excluding the process
sending the signal. No error is returned if any process could be
Setuid and setgid processes are dealt with slightly differently. For the
non-root user, to prevent attacks against such processes, some signal
deliveries are not permitted and return the error EPERM. The following
signals are allowed through to this class of processes: SIGKILL, SIGINT,
SIGTERM, SIGSTOP, SIGTTIN, SIGTTOU, SIGTSTP, SIGHUP, SIGUSR1, SIGUSR2.
For compatibility with System V, if the process number is negative but
not -1, the signal is sent to all processes whose process group ID is
equal to the absolute value of the process number. This is a variant of
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
kill() will fail and no signal will be sent if:
[EINVAL] sig is not a valid signal number.
[ESRCH] No process can be found corresponding to that speci-
fied by pid.
[EPERM] The sending process is not the superuser and its
effective user ID does not match the effective user ID
of the receiving process. When signaling a process
group, this error is returned if none of the members
of the group could be signaled.
getpgrp(2), getpid(2), sigaction(2), killpg(3), raise(3)
The kill() function is expected to conform to IEEE Std 1003.1-2008
The kill() system call first appeared in Version 3 AT&T UNIX. The sig
argument was introduced in Version 4 AT&T UNIX.
IEEE Std 1003.1-2008 (``POSIX.1'') specifies that kill(0, sig) should
send signal sig to the calling process, but OpenBSD doesn't do so for
BSD May 28, 2017 BSD