Name
kill — send signal to a process
Synopsis
#include <sys/types.h> #include <signal.h>
int
kill( |
pid_t pid, |
int sig); |
![[Note]](../stylesheet/note.png) |
Note | ||
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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), 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.)
RETURN VALUE
On success (at least one signal was sent), zero is
returned. On error, −1 is returned, and errno is set appropriately.
ERRORS
- 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(2)ed for.
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 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.
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 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.
BUGS
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.
SEE ALSO
_exit(2), killpg(2), signal(2), tkill(2), exit(3), sigqueue(3), capabilities(7), credentials(7), signal(7)
COLOPHON
This page is part of release 4.07 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
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Copyright (c) 1992 Drew Eckhardt (drewcs.colorado.edu), March 28, 1992 %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified by Michael Haardt <michaelmoria.de> Modified by Thomas Koenig <ig25rz.uni-karlsruhe.de> Modified 1993-07-23 by Rik Faith <faithcs.unc.edu> Modified 1993-07-25 by Rik Faith <faithcs.unc.edu> Modified 1995-11-01 by Michael Haardt <michaelcantor.informatik.rwth-aachen.de> Modified 1996-04-14 by Andries Brouwer <aebcwi.nl> [added some polishing contributed by Mike Battersby <mibdeakin.edu.au>] Modified 1996-07-21 by Andries Brouwer <aebcwi.nl> Modified 1997-01-17 by Andries Brouwer <aebcwi.nl> Modified 2001-12-18 by Andries Brouwer <aebcwi.nl> Modified 2002-07-24 by Michael Kerrisk <mtk.manpagesgmail.com> Added note on historical rules enforced when an unprivileged process sends a signal. Modified 2004-06-16 by Michael Kerrisk <mtk.manpagesgmail.com> Added note on CAP_KILL Modified 2004-06-24 by aeb Modified, 2004-11-30, after idea from emmanuel.colbusensimag.imag.fr |