LOCKF(1) General Commands Manual LOCKF(1)

lockf — execute a command while holding a file lock

lockf [-knsw] [-t seconds] file command [arguments]

lockf [-s] [-t seconds] fd

The lockf utility acquires an exclusive lock on a file, creating it if necessary,

and removing the file on exit unless explicitly told not to.
While holding the lock, it executes a command with optional arguments. After the command completes, lockf releases the lock, and removes the file unless the -k option is specified. BSD-style locking is used, as described in flock(2); the mere existence of the file is not considered to constitute a lock.

lockf may also be used to operate on a file descriptor instead of a file. If no command is supplied, then fd must be a file descriptor. The version with a command may also be used with a file descriptor by supplying it as a path /dev/fd/N, where N is the desired file descriptor. The -k option is implied when a file descriptor is in use, and the -n and -w options are silently ignored. This can be used to lock inside a shell script.

If the lockf utility is being used to facilitate concurrency between a number of processes, it is recommended that the -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 -k option is not used, then no guarantees around lock ordering can be made.

The following options are supported:

Causes the lock file to be kept (not removed) after the command completes.
Causes lockf to operate silently. Failure to acquire the lock is indicated only in the exit status.
Causes lockf to fail if the specified lock file does not exist. If -n is not specified, lockf will create file if necessary.
seconds
Specifies a timeout for waiting for the lock. By default, lockf waits indefinitely to acquire the lock. If a timeout is specified with this option, lockf will wait at most the given number of seconds before giving up. A timeout of 0 may be given, in which case lockf will fail unless it can acquire the lock immediately. When a lock times out, command is executed.
Causes lockf to open 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.

In no event will lockf break a lock that is held by another process.

If lockf successfully acquires the lock, it returns the exit status produced by command. Otherwise, it returns one of the exit codes defined in sysexits(3), as follows:

The specified lock file was already locked by another process.
The lockf utility was unable to create the lock file, e.g., because of insufficient access privileges.
The -n option is specified and the specified lock file does not exist.
There was an error on the lockf command line.
A system call (e.g., fork(2)) failed unexpectedly.
The command did not exit normally, but may have been signaled or stopped.

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):

$ lockf mylock sleep 5 & lockf -t 1 mylock echo "Success"
[1] 94410
lockf: mylock: already locked

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):

$ lockf mylock sleep 1 & lockf -t 5 mylock echo "Success"
[1] 19995
Success
[1]+  Done                    lockf mylock sleep 1
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.

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

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 /tmp/my.lock, and lockf is performing the lock call exclusively via the passed in file descriptor (9). In this case -k is implied, and -w has no effect because the file has already been opened by the shell. This example assumes that ‘>’ is implemented in the shell by opening and truncating /tmp/my.lock, rather than by replacing the lock file.

(
	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

flock(2), lockf(3), sysexits(3)

A lockf utility first appeared in FreeBSD 2.2.

John Polstra <jdp@polstra.com>

November 25, 2023 Mac OS X 14