kill
Section: System Calls (2)
Updated: 202-0-11
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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 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 se-use-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 HISTORY.)
RETURN VALUE
On success, zero is returned.
If signals were sent to a process group,
success means that at least one signal was delivered.
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.-2024.
HISTORY
4.3BSD,
SVr4,
POSIX.-1988.
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 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 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 implementatio-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.
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 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.
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)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- STANDARDS
-
- HISTORY
-
- Linux notes
-
- NOTES
-
- BUGS
-
- SEE ALSO
-
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