man-pages/man2/kill.2

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.\" 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
.\"
.TH KILL 2 2004-06-24 "Linux 2.6.7" "Linux Programmer's Manual"
.SH NAME
kill \- send signal to a process
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.br
.B #include <signal.h>
.sp
.BI "int kill(pid_t " pid ", int " sig );
.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 \fIpid\fP.
.PP
If \fIpid\fP equals 0, then \fIsig\fP is sent to every process in the
process group of the current 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 (init), but see below.
.PP
If \fIpid\fP is less than \-1, then \fIsig\fP is sent to every process
in the process group \fI\-pid\fP.
.PP
If \fIsig\fP 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
.B 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.
.SH "RETURN VALUE"
On success (at least one signal was sent), zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EINVAL
An invalid signal was specified.
.TP
.B EPERM
The process does not have permission to send the signal
to any of the target processes.
.TP
.B 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 \fBwait\fP()ed for.
.SH NOTES
The only signals that can be sent 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.
.LP
POSIX.1-2001 requires that \fIkill(\-1,sig)\fP send \fIsig\fP
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
\fIkill(\-1,sig)\fP does not signal the current process.
.LP
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 \fIsigwait\fP(), at least one
unblocked signal must be delivered to the sending thread before the
\fIkill\fP().
.SH 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,
.BR kill ()
failed with the error
.B EPERM
if the caller did 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 "LINUX HISTORY"
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 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.
.SH "CONFORMING TO"
SVr4, 4.3BSD, POSIX.1-2001
.SH "SEE ALSO"
.BR _exit (2),
.BR killpg (2),
.BR signal (2),
.BR sigqueue (2),
.BR tkill (2),
.BR exit (3),
.BR capabilities (7),
.BR signal (7)