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 termi-
nated 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 sig-
nal 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 target. From Linux 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 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 un-
blocked signal must be delivered to the sending thread before the
kill() returns.
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 error EPERM if the caller did not have permission to send the sig-
nal 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.
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 6.7 2023-10-31 kill(2)
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