KILL(2) Linux Programmer's Manual KILL(2)
kill - send signal to a process
int kill(pid_t pid, int sig);
The kill system call can be used to send any signal to any process group or process.
If pid is positive, then signal sig is sent to pid.
If pid equals 0, then sig is sent to every process in the process group of the current
If pid equals -1, then sig is sent to every process except for process 1 (init), but see
If pid is less than -1, then sig is sent to every process in the process group -pid.
If sig is 0, then no signal is sent, but error checking is still performed.
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
EINVAL An invalid signal was specified.
ESRCH The pid or process group does not exist. Note that an existing process might be a
zombie, a process which already committed termination, but has not yet been
EPERM The process does not have permission to send the signal to any of the receiving
processes. For a process to have permission to send a signal to process pid it
must either have root privileges, or the real or effective user ID of the sending
process must equal the real or saved set-user-ID of the receiving process. In the
case of SIGCONT it suffices when the sending and receiving processes belong to the
It is impossible to send a signal to task number one, the init process, for which it has
not installed a signal handler. This is done to assure the system is not brought down
POSIX 1003.1-2001 requires that kill(-1,sig) send sig to all processes that the current
process may send signals to, except possibly for some implementation-defined system pro-
cesses. Linux allows a process to signal itself, but on Linux the call kill(-1,sig) does
not signal the current process.
Across different kernel versions, Linux has enforced different rules for the permissions
required for an unprivileged process to send a signal to another process. In kernels 1.0
to 1.2.2, a signal could be sent if the effective user ID of the sender matched that of
the receiver, or the real user ID of the sender matched that of the receiver. From kernel
1.2.3 until 1.3.77, a signal could be sent if the effective user ID of the sender matched
either the real or effective user ID of the receiver. The current rules, which conform to
POSIX 1003.1-2001, were adopted in kernel 1.3.78.
SVr4, SVID, POSIX.1, X/OPEN, BSD 4.3, POSIX 1003.1-2001
_exit(2), exit(3), signal(2), signal(7)
Linux 2.5.0 2001-12-18 KILL(2)