send signal to a process
Standard C Library (libc, -lc)
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
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
For a process to have permission to send a signal to a process
designated by pid, the user must be the super-user, or
the real or saved user ID of the receiving process must match the real or
effective user ID of the sending process. A single exception is the signal
SIGCONT, which may always be sent to any process with the same session ID as
the sender. In addition, if the
is set to 1, the user is not a super-user, and the receiver is set-uid, then
only job control and terminal control signals may be sent (in particular,
only SIGKILL, SIGINT, SIGTERM, SIGALRM, SIGSTOP, SIGTTIN, SIGTTOU, SIGTSTP,
SIGHUP, SIGUSR1, SIGUSR2).
- If pid is greater than
- 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
- If pid is -1:
- If the user has super-user privileges, the signal is sent to all processes
excluding system processes (with
set), process with ID 1 (usually
and the process sending the signal. If the user is not the super user, the
signal is sent to all processes which the caller has permissions to,
excluding the process sending the signal. No error is returned if any
process could be signaled.
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
kill() function returns the value 0 if
successful; otherwise the value -1 is returned and the global variable
errno is set to indicate the error.
kill() system call will fail and no signal will be
- The sig argument is not a valid signal number.
- No process or process group can be found corresponding to that specified
- The sending process does not have permission to send
sig to any receiving process.
kill() system call is expected to conform to
IEEE Std 1003.1-1990 (“POSIX.1”).
A version of the
kill() function appeared in
Version 3 AT&T UNIX. The signal number was
added to the
kill() function in
Version 4 AT&T UNIX.