NAME
kill —
send signal to a process
SYNOPSIS
#include
<signal.h>
int
kill(pid_t
pid, int sig);
DESCRIPTION
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 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 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 signaled.
- If pid is negative but not -1:
- sig is sent to all processes whose process group ID is equal to the absolute value of pid; this is a variant of killpg(3).
If the value of pid causes
sig to be sent to the calling process, either
sig or at least one pending unblocked signal will be
delivered before
kill()
returns unless sig is blocked in the calling thread,
sig is unblocked in another thread, or another thread
is waiting for sig in
sigwait().
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.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
ERRORS
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 specified 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.
SEE ALSO
STANDARDS
The kill() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”).
HISTORY
The kill() system call first appeared in
Version 2 AT&T UNIX. The
sig argument was introduced in
Version 4 AT&T UNIX.