.\"	$NetBSD: pidfile.3,v 1.16 2017/10/22 16:55:32 abhinav Exp $
.\"
.\" Copyright (c) 1999, 2016 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Jason R. Thorpe and Roy Marples.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
.\" POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd April 10, 2016
.Dt PIDFILE 3
.Os
.Sh NAME
.Nm pidfile ,
.Nm pidfile_lock ,
.Nm pidfile_read ,
.Nm pidfile_clean
.Nd write a daemon pid file
.Sh LIBRARY
.Lb libutil
.Sh SYNOPSIS
.In util.h
.Ft int
.Fn pidfile "const char *path"
.Ft pid_t
.Fn pidfile_lock "const char *path"
.Ft pid_t
.Fn pidfile_read "const char *path"
.Ft int
.Fn pidfile_clean "void"
.Sh DESCRIPTION
.Fn pidfile
and
.Fn pidfile_lock
create and lock a file containing the process ID of the calling program.
The pid file can be used as a quick reference if
the process needs to be sent a signal.
The pid file is truncated and removed automatically when the program exits,
unless the program receives a fatal signal.
.Pp
If
.Ar path
is
.Dv NULL
or a plain basename (a name containing no directory components), the pid file
is created in the
.Pa /var/run
directory.
The file name has the form
.Pa /var/run/basename.pid .
The basename part is either the value of
.Ar path
if it was not
.Dv NULL ,
or the program name as returned by
.Xr getprogname 3
otherwise.
.Pp
If
.Ar path
is an absolute or relative path (i.e. it contains the
.Sq /
character),
the pid file is created in the provided location.
.Pp
If called with a new
.Ar path ,
.Fn pidfile
and
.Fn pidfile_lock
will remove the old pid file.
.Pp
The pid file is truncated, so these functions can be called multiple times and
allow a child process to take over the lock.
.Pp
.Fn pidfile_read
will read the last pid file created, or specified by
.Ar path ,
and return the process ID it contains.
.Pp
.Fn pidfile_clean
will
.Xr ftruncate 2 ,
.Xr close 2 ,
and
.Xr unlink 2
the last opening pid file if, and only if, the current process wrote it.
This function should be called if the program needs to call
.Xr _exit 2
(such as from a signal handler) and needs to clean up the pid file.
.Sh RETURN VALUES
.Fn pidfile
and
.Fn pidfile_clean
returns 0 on success and \-1 on failure.
.Pp
.Fn pidfile_lock
returns 0 on success.
Otherwise, the process ID who owns the lock is returned and if that
cannot be derived then \-1 is returned.
.Pp
.Fn pidfile_read
returns the process ID if known, otherwise \-1.
.Sh ERRORS
The
.Fn pidfile
and
.Fn pidfile_lock
functions will fail if:
.Bl -tag -width Er
.It Bq Er EEXIST
Some process already holds the lock on the given pid file, meaning that a
daemon is already running.
.It Bq Er ENAMETOOLONG
Specified pidfile's name is too long.
.El
.Sh SEE ALSO
.Xr flock 2 ,
.Xr atexit 3
.Sh HISTORY
The
.Fn pidfile
function call appeared in
.Nx 1.5 .
Support for creating pid files in any arbitrary path was added in
.Nx 6.0 .
.Pp
The
.Fn pidfile_lock ,
.Fn pidfile_read ,
and
.Fn pidfile_clean
function calls appeared in
.Nx 8 .
.Sh CAVEATS
.Fn pidfile
and
.Fn pidfile_lock
use
.Xr atexit 3
to ensure the pid file is cleaned at program exit.
However, programs that use the
.Xr _exit 2
function (for example, in signal handlers)
will not trigger this behaviour and should call
.Fn pidfile_clean .
Like-wise, if the program creates a pid file before
.Xr fork 2 Ns ing
a child to take over, it should use the
.Xr _exit 2
function instead of returning or using the
.Xr exit 3
function to ensure the pid file is not cleaned.