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Dd April 10, 2016
Dt PIDFILE 3
Os
Sh NAME
Nm pidfile ,
Nm pidfile_lock ,
Nm pidfile_read ,
Nm pidfile_clean
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"
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.
Sh RETURN VALUES
Fn pidfile
and
Fn pidfile_clean
returns 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.
Sh ERRORS
The
Fn pidfile
and
Fn pidfile_lock
functions will fail if:
Bl -tag -width Er
It Bq Er EEXIST
Some process already holds the lock on the given pid file, meaning that a
daemon is already running.
It Bq Er ENAMETOOLONG
Specified pidfile's name is too long.
El
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 .
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 .
Like-wise, if the program creates a pid file before
Xr fork 2 Ns ing
a child to take over, it should use the
Xr _exit 2
function instead of returning or using the
Xr exit 3
function to ensure the pid file is not cleaned.

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