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kill — send signal to a process
Standard C Library (libc, -lc)
kill(pid_t pid, int sig);
The kill() system call sends the signal given by sig to pid, a process or
a group of processes. The sig argument 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 of pid.
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 super-user).
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:
The sig signal is sent to the process whose ID is equal to pid.
If pid is zero:
The sig signal 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(2).
If pid is -1:
If the user has super-user privileges, the signal is sent to all
processes excluding system processes (with P_SYSTEM flag set),
process with ID 1 (usually init(8)), and the process sending the
signal. If the user is not the super user, 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
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
The kill() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
The kill() system call will fail and no signal will be sent if:
[EINVAL] The sig argument is not a valid signal number.
[ESRCH] No process can be found corresponding to that
specified by pid.
[ESRCH] The process id was given as 0 but the sending process
does not have a process group.
[EPERM] The sending process is not the super-user 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 any members of the
group could not be signaled.
getpgrp(2), getpid(2), killpg(2), sigaction(2), raise(3), init(8)
The kill() system call is expected to conform to ISO/IEC 9945-1:1990
The kill() function appeared in Version 7 AT&T UNIX.