.\" Copyright (c) 2005 Pawel Jakub Dawidek <pjd@FreeBSD.org>
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.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 libpidfile
library provides functions for daemons to handle file with PID.
It uses
.Xr flock 2
to lock pidfile and detect already running daemons.
.Pp
The
.Fn pidfile_open
function opens (or creates) a file specified by
.Fa path
argument and locks it with
.Xr flock 2
syscall.
If file can not be locked, PID of already running daemon is returned in
.Fa pidptr
argument (if it is not NULL).
The function doesn't write process' PID into the file here, so it can be
used before fork()ing and exit with proper error message when needed.
If
.Fa path
argument is NULL,
.Pa /var/run/<progname>.pid
file will be used.
.Pp
The
.Fn pidfile_write
function write process' PID into previously opened file.
.Pp
The
.Fn pidfile_close
function closes pidfile.
It should be used after daemon fork()s to start a child process.
.Pp
The
.Fn pidfile_remove
function closes and removes pidfile.
.Sh RETURN VALUES
The
.Fn pidfile_open
function return a valid pointer to a pidfh structure on success or
.Dv NULL
if an error occurs.
If an error does occur,
.Va errno
will be set.
.Rv -std pidfile_write pidfile_close pidfile_remove
.Sh EXAMPLES
The following example shows in which order those functions should be used.
.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.", 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.", 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, which means,
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 ,
.Xr read 2
routines.
.Pp
The
.Fn pidfile_write
function will fail if:
.Bl -tag -width Er
.It Bq Er EDOOFUS
Inproper 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 ,
.Xr write 2
routines.
.Pp
The
.Fn pidfile_close
function may fail and set
.Va errno
for any errors specified for the
.Xr close 2 ,
.Xr fstat 2
routines.
.Pp
The
.Fn pidfile_remove
function will fail if:
.Bl -tag -width Er
.It Bq Er EDOOFUS
Inproper 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 ,
.Xr unlink 2
routines.
.Pp
.Sh SEE ALSO
.Xr flock 2 ,
.Xr open 2 ,
.Xr daemon 3
.Sh AUTHORS
.An -nosplit
The
.Xr pidfile 3
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 .