kill(2) System Calls Manual kill(2)
NAME
kill - send signal to a process
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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
DESCRIPTION
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 call-
ing 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 name-
space 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 re-
ceiving processes belong to the same session. (Historically, the rules
were different; see NOTES.)
RETURN VALUE
On success (at least one signal was sent), zero is returned. On error,
-1 is returned, and errno is set to indicate the error.
ERRORS
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.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, SVr4, 4.3BSD.
Linux notes
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 Linux 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 tar-
get. From Linux 1.2.3 until 1.3.77, a signal could be sent if the ef-
fective 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 Linux 1.3.78.
NOTES
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 implemen-
tation-defined system processes. Linux allows a process to signal it-
self, 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() re-
turns.
BUGS
In Linux 2.6 up to and including Linux 2.6.7, there was a bug that meant
that when sending signals to a process group, kill() failed with the er-
ror EPERM if the caller did not have permission to send the signal to
any (rather than all) of the members of the process group. Notwith-
standing this error return, the signal was still delivered to all of the
processes for which the caller had permission to signal.
SEE ALSO
kill(1), _exit(2), pidfd_send_signal(2), signal(2), tkill(2), exit(3),
killpg(3), sigqueue(3), capabilities(7), credentials(7), signal(7)
Linux man-pages 6.9.1 2024-05-02 kill(2)
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