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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 for which the
calling process has permission to send signals, except for process 1
(init), but see below.
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
For a process to have permission to send a signal it must either be
privileged (under Linux: have the CAP_KILL capability), or the real or
effective user ID of the sending process must equal the real or saved
set-user-ID of the target process. In the case of SIGCONT it suffices
when the sending and receiving processes belong to the same session.
On success (at least one signal was sent), zero is returned. On error,
-1 is returned, and errno is set appropriately.
EINVAL An invalid signal was specified.
EPERM The process does not have permission to send the signal to any
of the target processes.
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.
The only signals that can be sent task number one, the init process,
are those for which init has explicitly installed signal handlers.
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
POSIX 1003.1-2003 requires that if a process sends a signal to itself,
and that process does not have the signal blocked, and no other thread
has it unblocked or is waiting for it in sigwait(), at least one
unblocked signal must be delivered to the sending thread before the
call of kill() returns.
In 2.6 kernels up to and including 2.6.7, there was a bug that meant
that when sending signals to a process group, kill() failed with the
error EPERM if the caller did have permission to send the signal to any
(rather than all) of the embers of the process group. Notwithstanding
this error return, the signal was still delivered to all of the
processes for which the caller had permission to signal.
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, 4.3BSD, POSIX 1003.1-2001
_exit(2), killpg(2), signal(2), sigqueue(2), tkill(2), exit(3),