.\"	$OpenBSD: kill.2,v 1.26 2020/02/08 01:09:57 jsg Exp $
.\"	$NetBSD: kill.2,v 1.7 1995/02/27 12:33:53 cgd Exp $
.\"
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.\"     @(#)kill.2	8.3 (Berkeley) 4/19/94
.\"
.Dd $Mdocdate: February 8 2020 $
.Dt KILL 2
.Os
.Sh NAME
.Nm kill
.Nd send signal to a process
.Sh SYNOPSIS
.In signal.h
.Ft int
.Fn kill "pid_t pid" "int sig"
.Sh DESCRIPTION
The
.Fn kill
function sends the signal given by
.Fa sig
to
.Fa pid ,
a
process or a group of processes.
.Fa sig
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 real or effective user ID of the receiving process must match
that of the sending process or the user must have appropriate privileges
(such as given by a set-user-ID program or the user is the superuser).
A single exception is the signal
.Dv SIGCONT ,
which may always be sent
to any process with the same session ID as the caller.
.Bl -tag -width Ds
.It \&If Fa pid No \&is greater than zero :
.Fa sig
is sent to the process whose ID is equal to
.Fa pid .
.It \&If Fa pid No \&is zero :
.Fa sig
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 3 .
.It \&If Fa pid No \&is -1 :
If the user has superuser privileges,
the signal is sent to all processes excluding
system processes and the process sending the signal.
If the user is not the superuser, the signal is sent to all processes
with the same uid as the user excluding the process sending the signal.
No error is returned if any process could be signaled.
.It \&If Fa pid No \&is negative but not -1 :
.Fa sig
is sent to all processes whose process group ID
is equal to the absolute value of
.Fa pid ;
this is a variant of
.Xr killpg 3 .
.El
.Pp
If the value of
.Fa pid
causes
.Fa sig
to be sent to the calling process, either
.Fa sig
or at least one pending unblocked signal will be delivered before
.Fn kill
returns unless
.Fa sig
is blocked in the calling thread,
.Fa sig
is unblocked in another thread, or another thread is waiting for
.Fa sig
in
.Fn sigwait .
.Pp
Setuid and setgid processes are dealt with slightly differently.
For the non-root user, to prevent attacks against such processes, some signal
deliveries are not permitted and return the error
.Er EPERM .
The following signals are allowed through to this class of processes:
.Dv SIGKILL ,
.Dv SIGINT ,
.Dv SIGTERM ,
.Dv SIGSTOP ,
.Dv SIGTTIN ,
.Dv SIGTTOU ,
.Dv SIGTSTP ,
.Dv SIGHUP ,
.Dv SIGUSR1 ,
.Dv SIGUSR2 .
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
.Fn kill
will fail and no signal will be sent if:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa sig
is not a valid signal number.
.It Bq Er ESRCH
No process can be found corresponding to that specified by
.Fa pid .
.It Bq Er EPERM
The sending process is not the superuser and its effective
user ID does not match the effective user ID of the receiving process.
When signaling a process group, this error is returned if none of the members
of the group could be signaled.
.El
.Sh SEE ALSO
.Xr getpgrp 2 ,
.Xr getpid 2 ,
.Xr sigaction 2 ,
.Xr killpg 3 ,
.Xr raise 3
.Sh STANDARDS
The
.Fn kill
function conforms to
.St -p1003.1-2008 .
.Sh HISTORY
The
.Fn kill
system call first appeared in
.At v2 .
The
.Fa sig
argument was introduced in
.At v4 .