Copyright (C) 1998 John D. Polstra. All rights reserved.

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 $FreeBSD$

.Dd July 7, 1998
.Dt LOCKF 1
.Os
.Sh NAME
.Nm lockf
.Nd execute a command while holding a file lock
.Sh SYNOPSIS
.Nm
.Op Fl kns
.Op Fl t Ar seconds
.Ar file
.Ar command
.Op Ar arguments
.Sh DESCRIPTION
The
.Nm
utility acquires an exclusive lock on a
.Ar file ,
creating it if necessary,
f Em and removing the file on exit unless explicitly told not to.
.Ef
While holding the lock, it executes a
.Ar command
with optional
.Ar arguments .
After the
.Ar command
completes,
.Nm
releases the lock, and removes the
.Ar file
unless the
.Fl k
option is specified.
x Ns -style locking is used, as described in
.Xr flock 2 ;
the mere existence of the
.Ar file
is not considered to constitute a lock.

p
If the
.Nm
utility is being used to facilitate concurrency between a number
of processes, it is recommended that the
.Fl k
option be used.
This will guarantee lock ordering, as well as implement
a performance enhanced algorithm which minimizes CPU load associated
with concurrent unlink, drop and re-acquire activity.
It should be noted
that if the
.Fl k
option is not used, then no guarantees around lock ordering can be made.

p
The following options are supported:
l -tag -width ".Fl t Ar seconds" t Fl k Causes the lock file to be kept (not removed) after the command
completes.
t Fl s Causes
.Nm
to operate silently.
Failure to acquire the lock is indicated only in the exit status.
t Fl n Causes
.Nm
to fail if the specified lock
.Ar file
does not exist. If
.Fl n
is not specified,
.Nm
will create
.Ar file
if necessary.
t Fl t Ar seconds Specifies a timeout for waiting for the lock.
By default,
.Nm
waits indefinitely to acquire the lock.
If a timeout is specified with this option,
.Nm
will wait at most the given number of
.Ar seconds
before giving up.
A timeout of 0 may be given, in which case
.Nm
will fail unless it can acquire the lock immediately.
When a lock times out,
.Ar command
is
.Em not
executed.
.El

p
In no event will
.Nm
break a lock that is held by another process.
.Sh EXIT STATUS
If
.Nm
successfully acquires the lock, it returns the exit status produced by
.Ar command .
Otherwise, it returns one of the exit codes defined in
.Xr sysexits 3 ,
as follows:
l -tag -width ".Dv EX_CANTCREAT" t Dv EX_TEMPFAIL The specified lock file was already locked by another process.
t Dv EX_CANTCREAT The
.Nm
utility
was unable to create the lock file, e.g., because of insufficient access
privileges.
t Dv EX_UNAVAILABLE The
.Fl n
option is specified and the specified lock file does not exist.
t Dv EX_USAGE There was an error on the
.Nm
command line.
t Dv EX_OSERR A system call (e.g.,
.Xr fork 2 )
failed unexpectedly.
t Dv EX_SOFTWARE The
.Ar command
did not exit normally,
but may have been signaled or stopped.
.El
.Sh SEE ALSO
.Xr flock 2 ,
.Xr sysexits 3
.Sh HISTORY
A
.Nm
utility first appeared in
.Fx 2.2 .
.Sh AUTHORS
.An John Polstra Aq jdp@polstra.com