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.\" Copyright (c) 2005 Pawel Jakub Dawidek <pjd@FreeBSD.org>
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.Dd October 20, 2008
.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
.ds str-Lb-libbsd Utility functions from BSD systems (libbsd, \-lbsd)
.Lb libbsd
.Sh SYNOPSIS
.In bsd/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 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 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.
.Pp
.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: %jd.",
		    (intmax_t)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 %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.
.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.
.It Bq Er EAGAIN
Some process already holds the lock on the given pidfile, but the file
is truncated.
Most likely, the existing daemon is writing new PID into
the file.
.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 EINVAL
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 EINVAL
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.
.Sh SEE ALSO
.Xr open 2 ,
.Xr daemon 3 ,
.Xr flopen 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 .
kc3-lang/libbsd/src/pidfile.3

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src/pidfile.3
