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.\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 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@gmail.com>
.\" Added note on historical rules enforced when an unprivileged process
.\" sends a signal.
.\" Modified 2004-06-16 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" Added note on CAP_KILL
.\" Modified 2004-06-24 by aeb
.\" Modified, 2004-11-30, after idea from emmanuel.colbus@ensimag.imag.fr
.\"
.TH kill 2 2022-12-04 "Linux man-pages 6.03"
.SH NAME
kill \- send signal to a process
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <signal.h>
.PP
.BI "int kill(pid_t " pid ", int " sig );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR kill ():
.nf
_POSIX_C_SOURCE
.fi
.SH DESCRIPTION
The
.BR kill ()
system call
can be used to send any signal to any process group or process.
.PP
If \fIpid\fP is positive, then signal \fIsig\fP is sent to the
process with the ID specified by \fIpid\fP.
.PP
If \fIpid\fP equals 0, then \fIsig\fP is sent to every process in the
process group of the calling process.
.PP
If \fIpid\fP equals \-1, then \fIsig\fP is sent to every process
for which the calling process has permission to send signals,
except for process 1 (\fIinit\fP), but see below.
.PP
If \fIpid\fP is less than \-1, then \fIsig\fP is sent to every process
in the process group whose ID is \fI\-pid\fP.
.PP
If \fIsig\fP 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.
.PP
For a process to have permission to send a signal,
it must either be privileged (under Linux: have the
.B 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 set-user-ID of the target process.
In the case of
.BR SIGCONT ,
it suffices when the sending and receiving
processes belong to the same session.
(Historically, the rules were different; see NOTES.)
.SH RETURN VALUE
On success (at least one signal was sent), zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
An invalid signal was specified.
.TP
.B EPERM
The calling process does not have permission to send the signal
to any of the target processes.
.TP
.B 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
.BR wait (2)ed
for.
.SH STANDARDS
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
.SH NOTES
The only signals that can be sent to process ID 1, the
.I init
process, are those for which
.I init
has explicitly installed signal handlers.
This is done to assure the
system is not brought down accidentally.
.PP
POSIX.1 requires that \fIkill(\-1,sig)\fP send \fIsig\fP
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
\fIkill(\-1,sig)\fP does not signal the calling process.
.PP
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
.BR sigwait (3),
at least one
unblocked signal must be delivered to the sending thread before the
.BR kill ()
returns.
.SS 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 the 0.* kernels things chopped and changed quite
.\" a bit - MTK, 24 Jul 02
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.
.SH 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,
.BR kill ()
failed with the error
.B EPERM
if the caller did not have permission to send the signal to \fIany\fP (rather
than \fIall\fP) 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.
.SH SEE ALSO
.BR kill (1),
.BR _exit (2),
.BR pidfd_send_signal (2),
.BR signal (2),
.BR tkill (2),
.BR exit (3),
.BR killpg (3),
.BR sigqueue (3),
.BR capabilities (7),
.BR credentials (7),
.BR signal (7)
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