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
 documentation and/or other materials provided with the distribution.

 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
 SUCH DAMAGE.

.Dd November 25, 2023
.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
.Nm
.Op Fl s
.Op Fl t Ar seconds
.Ar fd
.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
.Nm
may also be used to operate on a file descriptor instead of a file.
If no
.Ar command
is supplied, then
.Ar fd
must be a file descriptor.
The version with a
.Ar command
may also be used with a file descriptor by supplying it as a path

a /dev/fd/N ,
where N is the desired file descriptor.
The
.Fl k
option is implied when a file descriptor is in use, and the
.Fl n
and
.Fl w
options are silently ignored.
This can be used to lock inside a shell script.

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.
t 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

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 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):
d -literal -offset indent $ lockf mylock sleep 5 & lockf -t 1 mylock echo "Success"
[1] 94410
lockf: mylock: already locked
.Ed

p
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):
d -literal -offset indent $ lockf mylock sleep 1 & lockf -t 5 mylock echo "Success"
[1] 19995
Success
[1]+ Done lockf mylock sleep 1
.Ed
Lock a file and run a script, return immediately if the lock is not
available. Do not delete the file afterward so lock order is
guaranteed.

p
.Dl $ lockf -t 0 -k /tmp/my.lock myscript

p
Protect a section of a shell script with a lock, wait up to 5 seconds
for it to become available.
Note that the shell script has opened the lock file
.Fa /tmp/my.lock ,
and
.Nm
is performing the lock call exclusively via the passed in file descriptor (9).
In this case
.Fl k
is implied, and
.Fl w
has no effect because the file has already been opened by the shell.
This example assumes that
.Ql >
is implemented in the shell by opening and truncating

a /tmp/my.lock ,
rather than by replacing the lock file.
d -literal -offset indent (
 lockf -s -t 5 9
 if [ $? -ne 0 ]; then
 echo "Failed to obtain lock"
 exit 1
 fi

 echo Start
 # Do some stuff
 echo End
) 9>/tmp/my.lock
.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