.\" Copyright (c) 2005 Pawel Jakub Dawidek <pjd@FreeBSD.org>
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.Dd August 2, 2025
.Dt PIDFILE 3
.Os
.Sh NAME
.Nm pidfile_open ,
.Nm pidfile_write ,
.Nm pidfile_close ,
.Nm pidfile_remove ,
.Nm pidfile_fileno ,
.Nm pidfile_signal
.Nd "library for PID files handling"
.Sh LIBRARY
.Lb libutil
.Sh SYNOPSIS
.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"
.Ft int
.Fn pidfile_fileno "struct pidfh *pfh"
.Ft int
.Fn pidfile_signal "const char *path" "int sig" "pid_t *pidptr"
.Sh DESCRIPTION
The
.Nm pidfile
family of functions allows daemons to handle PID files.
It uses
.Xr flopen 3
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.
If
.Fa pidptr
argument is not
.Dv NULL
and file can not be locked, the function will use it to store a PID of an
already running daemon or
.Li -1
in case daemon did not write its PID yet.
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.
The
.Fn pidfile_open
function sets the O_CLOEXEC close-on-exec flag when opening the pidfile.
.Pp
The
.Fn pidfile_write
function writes process' PID into a previously opened file.
The file is truncated before write, so calling the
.Fn pidfile_write
function multiple times is supported.
.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.
.Pp
The
.Fn pidfile_fileno
function returns the file descriptor for the open pidfile.
.Pp
The
.Fn pidfile_signal
function looks for the pidfile specified by
.Va path ,
and if it exists and is locked, sends the signal specified by
.Va sig
to the PID it contains.
If
.Va pidptr
is not
.Dv NULL ,
the PID that was found in the pidfile is stored in the location it
points to.
Note that calling
.Fn pidfile_signal
with
.Va sig
set to zero is an effective way to verify the existence of a pidfile
and of the process that owns it.
.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.
.Pp
.Rv -std pidfile_write pidfile_close pidfile_remove
.Pp
The
.Fn pidfile_fileno
function returns the low-level file descriptor.
It returns
.Li -1
and sets
.Va errno
if a NULL
.Vt pidfh
is specified, or if the pidfile is no longer open.
.Pp
The
.Fn pidfile_signal
function returns 0 if it successfully signaled a process, and an
appropriate
.Va errno
value otherwise.
.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 ,
.Fn pidfile_close ,
and
.Fn pidfile_fileno
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: %jd.",
		    (intmax_t)otherpid);
	}
	/* If we cannot create pidfile from other reasons, only warn. */
	warn("Cannot open or create pidfile");
	/*
	 * Even though pfh is NULL we can continue, as the other pidfile_*
	 * function can handle such situation by doing nothing except setting
	 * errno to EDOOFUS.
	 */
}

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 %jd started.", (intmax_t)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.
If
.Fa pidptr
argument is not
.Dv NULL
the function will use it to store a PID of an already running daemon or
.Li -1
in case daemon did not write its PID yet.
.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 fstat 2 ,
.Xr write 2 ,
and
.Xr unlink 2
system calls and the
.Xr flopen 3
library function.
.Pp
The
.Fn pidfile_fileno
function will fail if:
.Bl -tag -width Er
.It Bq Er EDOOFUS
Improper function use.
Probably called not from the process which used
.Fn pidfile_open .
.El
.Pp
The
.Fn pidfile_signal
function will fail if:
.Bl -tag -width Er
.It Bq Er ENOENT
The pidfile does not exist, or exists but is not locked.
.It Bq Er EDOM
The pidfile contains a negative number.
.El
.Pp
The
.Fn pidfile_signal
function may also fail and return any of the
.Va errno
values specified for the
.Fn pidfile_read
function and the
.Xr kill 2
system call.
.Sh SEE ALSO
.Xr kill 2 ,
.Xr open 2 ,
.Xr daemon 3 ,
.Xr flopen 3
.Sh HISTORY
The functions
.Fn pidfile_open ,
.Fn pidfile_write ,
.Fn pidfile_close ,
and
.Fn pidfile_remove
first appeared in
.Fx 5.5 .
The
.Fn pidfile_fileno
function was added in
.Fx 9.1 .
The
.Fn pidfile_signal
function was added in
.Fx 14.0 .
.Sh AUTHORS
.An -nosplit
The
.Nm pidfile
functionality is based on ideas from
.An John-Mark Gurney Aq Mt jmg@FreeBSD.org .
.Pp
The code and manual page was written by
.An Pawel Jakub Dawidek Aq Mt pjd@FreeBSD.org .