freebsd-skq/lib/libutil/pidfile.3

.\" Copyright (c) 2005 Pawel Jakub Dawidek <pjd@FreeBSD.org>
.\" All rights reserved.
.\"
.\" 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 AUTHORS 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 AUTHORS 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.
.\"
.\" $FreeBSD$
.\"
.Dd August 22, 2005
.Dt PIDFILE 3
.Os
.Sh NAME
.Nm pidfile_open ,
.Nm pidfile_write ,
.Nm pidfile_close ,
.Nm pidfile_remove
.Nd "library for PID files handling"
.Sh LIBRARY
.Lb libutil
.Sh SYNOPSIS
.In sys/param.h
.In libutil.h
.Ft "struct pidfh *"
.Fn pidfile_open "const char *path" "mode_t mode" "pid_t *pidptr"
.Ft int
.Fn pidfile_write "struct pidfh *pfh"
.Ft int
.Fn pidfile_close "struct pidfh *pfh"
.Ft int
.Fn pidfile_remove "struct pidfh *pfh"
.Sh DESCRIPTION
The
.Nm pidfile
family of functions allows daemons to handle PID files.
It uses
.Xr flock 2
to lock a pidfile and detect already running daemons.
.Pp
The
.Fn pidfile_open
function opens (or creates) a file specified by the
.Fa path
argument and locks it with the
.Xr flock 2
system call.
If a file can not be locked, a PID of an already running daemon is returned in
the
.Fa pidptr
argument (if it is not
.Dv NULL ) .
The function does not write process' PID into the file here, so it can be
used before
.Fn fork Ns ing
and exit with a proper error message when needed.
If the
.Fa path
argument is
.Dv NULL ,
.Pa /var/run/ Ns Ao Va progname Ac Ns Pa .pid
file will be used.
.Pp
The
.Fn pidfile_write
function writes process' PID into a previously opened file.
.Pp
The
.Fn pidfile_close
function closes a pidfile.
It should be used after daemon
.Fn fork Ns s
to start a child process.
.Pp
The
.Fn pidfile_remove
function closes and removes a pidfile.
.Sh RETURN VALUES
The
.Fn pidfile_open
function returns a valid pointer to a
.Vt pidfh
structure on success, or
.Dv NULL
if an error occurs.
If an error occurs,
.Va errno
will be set.
.Rv -std pidfile_write pidfile_close pidfile_remove
.Sh EXAMPLES
The following example shows in which order these functions should be used.
Note that it is safe to pass
.Dv NULL
to
.Fn pidfile_write ,
.Fn pidfile_remove
and
.Fn pidfile_close
functions.
.Bd -literal
struct pidfh *pfh;
pid_t otherpid, childpid;

pfh = pidfile_open("/var/run/daemon.pid", 0600, &otherpid);
if (pfh == NULL) {
	if (errno == EEXIST) {
		errx(EXIT_FAILURE, "Daemon already running, pid: %d.",
		    (int)otherpid);
	}
	/* If we cannot create pidfile from other reasons, only warn. */
	warn("Cannot open or create pidfile");
}

if (daemon(0, 0) == -1) {
	warn("Cannot daemonize");
	pidfile_remove(pfh);
	exit(EXIT_FAILURE);
}

pidfile_write(pfh);

for (;;) {
	/* Do work. */
	childpid = fork();
	switch (childpid) {
	case -1:
		syslog(LOG_ERR, "Cannot fork(): %s.", strerror(errno));
		break;
	case 0:
		pidfile_close(pfh);
		/* Do child work. */
		break;
	default:
		syslog(LOG_INFO, "Child %d started.", (int)childpid);
		break;
	}
}

pidfile_remove(pfh);
exit(EXIT_SUCCESS);
.Ed
.Sh ERRORS
The
.Fn pidfile_open
function will fail if:
.Bl -tag -width Er
.It Bq Er EEXIST
Some process already holds the lock on the given pidfile, meaning that a
daemon is already running.
.It Bq Er ENAMETOOLONG
Specified pidfile's name is too long.
.It Bq Er EINVAL
Some process already holds the lock on the given pidfile, but PID read
from there is invalid.
.El
.Pp
The
.Fn pidfile_open
function may also fail and set
.Va errno
for any errors specified for the
.Xr fstat 2 ,
.Xr open 2 ,
and
.Xr read 2
calls.
.Pp
The
.Fn pidfile_write
function will fail if:
.Bl -tag -width Er
.It Bq Er EDOOFUS
Improper function use.
Probably called before
.Fn pidfile_open .
.El
.Pp
The
.Fn pidfile_write
function may also fail and set
.Va errno
for any errors specified for the
.Xr fstat 2 ,
.Xr ftruncate 2 ,
and
.Xr write 2
calls.
.Pp
The
.Fn pidfile_close
function may fail and set
.Va errno
for any errors specified for the
.Xr close 2
and
.Xr fstat 2
calls.
.Pp
The
.Fn pidfile_remove
function will fail if:
.Bl -tag -width Er
.It Bq Er EDOOFUS
Improper function use.
Probably called not from the process which made
.Fn pidfile_write .
.El
.Pp
The
.Fn pidfile_remove
function may also fail and set
.Va errno
for any errors specified for the
.Xr close 2 ,
.Xr flock 2 ,
.Xr fstat 2 ,
.Xr write 2 ,
and
.Xr unlink 2
calls.
.Sh SEE ALSO
.Xr flock 2 ,
.Xr open 2 ,
.Xr daemon 3
.Sh AUTHORS
.An -nosplit
The
.Nm pidfile
functionality is based on ideas from
.An John-Mark Gurney Aq jmg@FreeBSD.org .
.Pp
The code and manual page was written by
.An Pawel Jakub Dawidek Aq pjd@FreeBSD.org .