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.\"     @(#)kill.2	8.3 (Berkeley) 4/19/94
.\" $FreeBSD: releng/12.2/lib/libc/sys/kill.2 355981 2019-12-21 15:33:19Z kib $
.\"
.Dd December 1, 2019
.Dt KILL 2
.Os
.Sh NAME
.Nm kill
.Nd send signal to a process
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In signal.h
.Ft int
.Fn kill "pid_t pid" "int sig"
.Sh DESCRIPTION
The
.Fn kill
system call sends the signal given by
.Fa sig
to
.Fa pid ,
a
process or a group of processes.
The
.Fa sig
argument
may be one of the signals specified in
.Xr sigaction 2
or it may be 0, in which case
error checking is performed but no
signal is actually sent.
This can be used to check the validity of
.Fa pid .
.Pp
For a process to have permission to send a signal to a process designated
by
.Fa pid ,
the user must be the super-user, or
the real or saved user ID of the receiving process must match
the real or effective user ID of the sending process.
A single exception is the signal SIGCONT, which may always be sent
to any process with the same session ID as the sender.
In addition, if the
.Va security.bsd.conservative_signals
.Xr sysctl 9
is set to 1, the user is not a super-user, and
the receiver is set-uid, then
only job control and terminal control signals may
be sent (in particular, only SIGKILL, SIGINT, SIGTERM, SIGALRM,
SIGSTOP, SIGTTIN, SIGTTOU, SIGTSTP, SIGHUP, SIGUSR1, SIGUSR2).
.Bl -tag -width Ds
.It \&If Fa pid No \&is greater than zero :
The
.Fa sig
signal
is sent to the process whose ID is equal to
.Fa pid .
.It \&If Fa pid No \&is zero :
The
.Fa sig
signal
is sent to all processes whose group ID is equal
to the process group ID of the sender, and for which the
process has permission;
this is a variant of
.Xr killpg 2 .
.It \&If Fa pid No \&is -1 :
If the user has super-user privileges,
the signal is sent to all processes excluding
system processes
(with
.Dv P_SYSTEM
flag set),
process with ID 1
(usually
.Xr init 8 ) ,
and the process sending the signal.
If the user is not the super user, the signal is sent to all processes
which the caller has permissions to, excluding the process sending the signal.
No error is returned if any process could be signaled.
.El
.Pp
If the process number is negative but not -1,
the signal is sent to all processes whose process group ID
is equal to the absolute value of the process number.
This is a variant of
.Xr killpg 2 .
.Sh RETURN VALUES
.Rv -std kill
.Sh ERRORS
The
.Fn kill
system call
will fail and no signal will be sent if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa sig
argument
is not a valid signal number.
.It Bq Er ESRCH
No process or process group can be found corresponding to that specified by
.Fa pid .
.It Bq Er EPERM
The sending process does not have permission to send
.Va sig
to any receiving process.
.El
.Sh SEE ALSO
.Xr getpgrp 2 ,
.Xr getpid 2 ,
.Xr killpg 2 ,
.Xr sigaction 2 ,
.Xr sigqueue 2 ,
.Xr raise 3 ,
.Xr init 8
.Sh STANDARDS
The
.Fn kill
system call is expected to conform to
.St -p1003.1-90 .
.Sh HISTORY
A version of the
.Fn kill
function appeared in
.At v3 .
The signal number was added to the
.Fn kill
function in
.At v4 .