Name
kill — send signal to a process
Synopsis
int
kill( |
pid_t |
pid, |
| |
int |
sig); |
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 pid.
If pid equals 0,
then sig is sent to
every process in the process group of the current
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 −pid.
If sig is 0, then
no signal is sent, but error checking is still performed.
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.
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.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001
NOTES
The only signals that can be sent 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.1-2001 requires that kill(−1,sig) send
sig to all processes
that the current 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 current process.
POSIX.1-2001 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().
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 that of the
receiver, or the real user ID of the sender matched that of
the receiver. 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
receiver. The current rules, which conform to POSIX.1-2001,
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 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), sigqueue(2), tkill(2), exit(3), capabilities(7), credentials(7), signal(7)
Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
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Modified by Michael Haardt <michael@moria.de>
Modified by Thomas Koenig <ig25@rz.uni-karlsruhe.de>
Modified 1993-07-23 by Rik Faith <faith@cs.unc.edu>
Modified 1993-07-25 by Rik Faith <faith@cs.unc.edu>
Modified 1995-11-01 by Michael Haardt
<michael@cantor.informatik.rwth-aachen.de>
Modified 1996-04-14 by Andries Brouwer <aeb@cwi.nl>
[added some polishing contributed by Mike Battersby <mib@deakin.edu.au>]
Modified 1996-07-21 by Andries Brouwer <aeb@cwi.nl>
Modified 1997-01-17 by Andries Brouwer <aeb@cwi.nl>
Modified 2001-12-18 by Andries Brouwer <aeb@cwi.nl>
Modified 2002-07-24 by Michael Kerrisk <mtk-manpages@gmx.net>
Added note on historical rules enforced when an unprivileged process
sends a signal.
Modified 2004-06-16 by Michael Kerrisk <mtk-manpages@gmx.net>
Added note on CAP_KILL
Modified 2004-06-24 by aeb
Modified, 2004-11-30, after idea from emmanuel.colbus@ensimag.imag.fr
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