.\"	$NetBSD: pidfile.3,v 1.17 2026/05/30 10:09:47 roy Exp $
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.\" by Jason R. Thorpe and Roy Marples.
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.Dd May 29, 2026
.Dt PIDFILE 3
.Os
.Sh NAME
.Nm pidfile ,
.Nm pidfile_lock ,
.Nm pidfile_read ,
.Nm pidfile_clean ,
.Nm pidfile_unlock ,
.Nm pidfile_fd ,
.Nm pidfile_path ,
.Nm pidfile_unremoveable
.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"
.Ft int
.Fn pidfile_unlock "void"
.Ft int
.Fn pidfile_fd "void"
.Ft const char *
.Fn pidfile_path "void"
.Ft void
.Fn pidfile_unremoveable "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.
.Pp
.Fn pidfile_unlock
will release the resources held by the pid file, keeping it intact.
This is should be called if the process holding the pid file forks another
without the intent of taking it over.
.Pp
.Fn pidfile_fd
returns the file descriptor holding the lock for the pid file.
.Pp
.Fn pidfile_path
returns the full path of the pid file.
.Pp
.Fn pidfile_unremoveable
should be called when it's not possible for the process holding the pid file
lock to unlink the pid file.
For example, the process could be in a
.Xr chroot 2
or a sandbox environment which denies the ability to either read or unlink
the pid file.
.Sh RETURN VALUES
.Fn pidfile
and
.Fn pidfile_clean
and
.Fn pidfile_unlock
return 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.
.Pp
.Fn pidfile_fd
returns the file descriptor of the locked pid file, otherwise \-1.
.Pp
.Fn pidfile_path
returns a pointer to the path of the locked pid file, otherwise NULL.
.Sh ERRORS
The
.Fn pidfile
and
.Fn pidfile_lock
functions will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The pid file either hasn't been locked or has already been unlocked.
.It Bq Er EEXIST
Some process already holds the lock on the given pid file, meaning that a
daemon is already running.
.El
.Pp
The
.Fn pidfile
and
.Fn pidfile_lock
functions may also fail and set
.Va errno
for any errors specified by the routines
.Xr malloc 3 ,
.Xr open 2
.Xr lseek 2
.Xr ftruncate 2
and
.Xr dprintf 3 .
.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 .
.Pp
The
.Fn pidfile_fd ,
.Fn pidfile_path ,
.Fn pidfile_unlock
and
.Fn pidfile_unremoveable
function calls appeared in
.Nx 12 .
.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 .