kill - send signal to a process
#include <sys/types.h>
#include <signal.h>
int kill(pid_t pid, int sig);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
kill(): _POSIX_C_SOURCE
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 the process with
the ID specified by
pid.
If
pid equals 0, then
sig is sent to every process in the process
group of the calling 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 whose ID is
-pid.
If
sig is 0, then no signal is sent, but existence and permission checks
are still performed; this can be used to check for the existence of a process
ID or process group ID that the caller is permitted to signal.
For a process to have permission to send a signal, it must either be privileged
(under Linux: have the
CAP_KILL capability in the user namespace of the
target process), 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. (Historically, the rules were different; see NOTES.)
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 calling process does not have permission to send the signal to any of
the target processes.
- ESRCH
- The target process or process group does not exist. Note that an existing
process might be a zombie, a process that has terminated execution, but
has not yet been wait(2)ed for.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
The only signals that can be sent to process ID 1, 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.1 requires that
kill(-1,sig) send
sig to all processes that
the calling 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 calling
process.
POSIX.1 requires that if a process sends a signal to itself, and the sending
thread does not have the signal blocked, and no other thread has it unblocked
or is waiting for it in
sigwait(3), at least one unblocked signal must
be delivered to the sending thread before the
kill() returns.
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 effective user ID of the target, or the real user ID
of the sender matched the real user ID of the target. 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 target. The current rules, which
conform to POSIX.1, were adopted in kernel 1.3.78.
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 not have permission to send the signal to
any (rather than
all) of the members 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.
kill(1),
_exit(2),
pidfd_send_signal(2),
signal(2),
tkill(2),
exit(3),
killpg(3),
sigqueue(3),
capabilities(7),
credentials(7),
signal(7)