Linux man pages : kill (2)
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 process.
If pid equals -1, then sig is sent to every process except for process
1 (init), but see below.
If pid is less than -1, then sig is sent to every process in the pro-
cess group -pid.
If sig is 0, then no signal is sent, but error checking is still per-
On success, zero is returned. On error, -1 is returned, and errno is
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 wait()ed for.
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 privi-
leges, or the real or effective user ID of the sending process
must equal the real or saved set-user-ID of the receiving pro-
cess. In the case of SIGCONT it suffices when the sending and
receiving processes belong to the same session.
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 accidentally.
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 processes. Linux allows a process to
signal itself, but on Linux the call kill(-1,sig) does not signal the
Across different kernel versions, Linux has enforced different rules
for the permissions required for an unprivileged process to send a sig-
nal 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 effec-
tive 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)