NAME

plockf - execute a command while holding a file lock

SYNOPSIS

    plockf [-kns] [-t seconds] file command [arguments]

DESCRIPTION

plockf is a perl port of the FreeBSD utility lockf(1).

The plockf 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, plockf 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 plockf 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 plockf to operate silently. Failure to acquire the lock is indicated only in the exit status.

-n

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

-t seconds

Specifies a timeout for waiting for the lock. By default, plockf waits indefinitely to acquire the lock. If a timeout is specified with this option, plockf will wait at most the given number of seconds before giving up. A timeout of 0 may be given, in which case plockf will fail unless it can acquire the lock immediately. When a lock times out, command is not executed.

Unlike the original lockf utility, plockf may handle also floating point timeouts on systems which implement Time::HiRes::alarm; on Windows systems only integer timeouts are supported.

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

IMPLEMENTATION DETAILS

On systems where open(2) handles O_EXLOCK and O_NONBLOCK (most notably on *BSD systems) the implementation follows quite closely the original C implementation of lockf(1).

On other systems (e.g. Linux) there's a possible race condition between creation and locking of the lock file. Here an extra check is done if the lock filehandle is really done on the lock file, and the lock procedure is re-done if not. Here it's especially recommended to use the -k option.

On Windows systems this extra check is incomplete, so it's even more recommended to use -k.

On Windows systems alarm() cannot interrupt blocking system calls, i.e. flock() (see "alarm" in perlport). Here the timeout handling is implemented by periodically checking if the lock can be acquired (currently the interval is one second).

EXIT STATUS

If plockf 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 (75)

The specified lock file was already locked by another process.

EX_CANTCREAT (73)

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

EX_UNAVAILABLE (69)

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

EX_USAGE (64)

There was an error on the plockf command line.

EX_OSERR (71)

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

EX_SOFTWARE (70)

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

EX_SOFTWARE is not reported on Windows systems.

SEE ALSO

flock(1), flock(2), open(2), sysexits(3), Fcntl.

AUTHORS

Author of the perl port: Slaven Rezic <srezic@cpan.org>

Author of the original FreeBSD utility: John Polstra <jdp@polstra.com>