.\" Copyright, The contributors to the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH kill 2 2025-05-06 "Linux man-pages 6.14"
.SH NAME
kill \- send signal to a process
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B #include <signal.h>
.P
.BI "int kill(pid_t " pid ", int " sig );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.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.
.P
If
.I pid
is positive,
then signal
.I sig
is sent to
the process with the ID specified by
.IR pid .
.P
If
.I pid
equals 0,
then
.I sig
is sent to
every process in
the process group of the calling process.
.P
If
.I pid
equals \-1,
then
.I sig
is sent to
every process for which
the calling process has permission to send signals,
except for process 1
.RI ( init ),
but see below.
.P
If
.I pid
is less than \-1,
then
.I sig
is sent to
every process in
the process group whose ID is
.IR \-pid .
.P
If
.I 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.
.P
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 HISTORY.)
.SH 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
.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.1-2008.
.SH HISTORY
POSIX.1-2001, SVr4, 4.3BSD.
.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 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.
.P
POSIX.1 requires that
.I kill(\-1,sig)
send
.I 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
.I kill(\-1,sig)
does not signal the calling process.
.P
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.
.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
.I any
(rather than
.IR 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.
.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)