LOCKF(1)                FreeBSD General Commands Manual               LOCKF(1)

NAME
     lockf - execute a command while holding a file lock

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

DESCRIPTION
     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.

     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:

     -k                Causes the lock file to be kept (not removed) after the
                       command completes.

     -s                Causes lockf to operate silently.  Failure to acquire
                       the lock is indicated only in the exit status.

     -n                Causes lockf to fail if the specified lock file does
                       not exist.  If -n is not specified, lockf will create
                       file if necessary.

     -t 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 not executed.

     -w                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.

EXIT STATUS
     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:

     EX_TEMPFAIL       The specified lock file was already locked by another
                       process.

     EX_CANTCREAT      The lockf utility was unable to create the lock file,
                       e.g., because of insufficient access privileges.

     EX_UNAVAILABLE    The -n option is specified and the specified lock file
                       does not exist.

     EX_USAGE          There was an error on the lockf command line.

     EX_OSERR          A system call (e.g., fork(2)) failed unexpectedly.

     EX_SOFTWARE       The command did not exit normally, but may have been
                       signaled or stopped.

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):

           $ 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

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

HISTORY
     A lockf utility first appeared in FreeBSD 2.2.

AUTHORS
     John Polstra <jdp@polstra.com>

FreeBSD 13.1-RELEASE-p6         August 26, 2020        FreeBSD 13.1-RELEASE-p6