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Editing: lockf.1

.\"
.\" Copyright (C) 1998 John D. Polstra.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
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.\" THIS SOFTWARE IS PROVIDED BY JOHN D. POLSTRA AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL JOHN D. POLSTRA OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" $FreeBSD$
.\"
.Dd August 26, 2020
.Dt LOCKF 1
.Os
.Sh NAME
.Nm lockf
.Nd execute a command while holding a file lock
.Sh SYNOPSIS
.Nm
.Op Fl knsw
.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,
.Bf 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.
.Bx 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.
.Pp
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.
.Pp
The following options are supported:
.Bl -tag -width ".Fl t Ar seconds"
.It Fl k
Causes the lock file to be kept (not removed) after the command
completes.
.It Fl s
Causes
.Nm
to operate silently.
Failure to acquire the lock is indicated only in the exit status.
.It 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.
.It 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.
.It Fl w
Causes
.Nm
to open
.Ar file
for writing rather than reading.
This is necessary on filesystems (including NFSv4) where a file which
has been opened read-only cannot be exclusively locked.
.El
.Pp
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:
.Bl -tag -width ".Dv EX_CANTCREAT"
.It Dv EX_TEMPFAIL
The specified lock file was already locked by another process.
.It Dv EX_CANTCREAT
The
.Nm
utility
was unable to create the lock file, e.g., because of insufficient access
privileges.
.It Dv EX_UNAVAILABLE
The
.Fl n
option is specified and the specified lock file does not exist.
.It Dv EX_USAGE
There was an error on the
.Nm
command line.
.It Dv EX_OSERR
A system call (e.g.,
.Xr fork 2 )
failed unexpectedly.
.It Dv EX_SOFTWARE
The
.Ar command
did not exit normally,
but may have been signaled or stopped.
.El
.Sh EXAMPLES
The first job takes a lock and sleeps for 5 seconds in the background.
The second job tries to get the lock and timeouts after 1 second (PID numbers
will differ):
.Bd -literal -offset indent
$ lockf mylock sleep 5 & lockf -t 1 mylock echo "Success"
[1] 94410
lockf: mylock: already locked
.Ed
.Pp
The first job takes a lock and sleeps for 1 second in the background.
The second job waits up to 5 seconds to take the lock and echoes the message on
success (PID numbers will differ):
.Bd -literal -offset indent
$ lockf mylock sleep 1 & lockf -t 5 mylock echo "Success"
[1] 19995
Success
[1]+  Done                    lockf mylock sleep 1
.Ed
.Sh SEE ALSO
.Xr flock 2 ,
.Xr lockf 3 ,
.Xr sysexits 3
.Sh HISTORY
A
.Nm
utility first appeared in
.Fx 2.2 .
.Sh AUTHORS
.An John Polstra Aq Mt jdp@polstra.com

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