phpman > perldoc > Thread::Semaphore

NAME
    Thread::Semaphore - Thread-safe semaphores

VERSION
    This document describes Thread::Semaphore version 2.13

SYNOPSIS
        use Thread::Semaphore;
        my $s = Thread::Semaphore->new();
        $s->down();   # Also known as the semaphore P operation.
        # The guarded section is here
        $s->up();     # Also known as the semaphore V operation.

        # Decrement the semaphore only if it would immediately succeed.
        if ($s->down_nb()) {
            # The guarded section is here
            $s->up();
        }

        # Forcefully decrement the semaphore even if its count goes below 0.
        $s->down_force();

        # The default value for semaphore operations is 1
        my $s = Thread::Semaphore->new($initial_value);
        $s->down($down_value);
        $s->up($up_value);
        if ($s->down_nb($down_value)) {
            ...
            $s->up($up_value);
        }
        $s->down_force($down_value);

DESCRIPTION
    Semaphores provide a mechanism to regulate access to resources. Unlike locks, semaphores aren't
    tied to particular scalars, and so may be used to control access to anything you care to use
    them for.

    Semaphores don't limit their values to zero and one, so they can be used to control access to
    some resource that there may be more than one of (e.g., filehandles). Increment and decrement
    amounts aren't fixed at one either, so threads can reserve or return multiple resources at once.

METHODS
    ->new()
    ->new(NUMBER)
            "new" creates a new semaphore, and initializes its count to the specified number (which
            must be an integer). If no number is specified, the semaphore's count defaults to 1.

    ->down()
    ->down(NUMBER)
            The "down" method decreases the semaphore's count by the specified number (which must be
            an integer >= 1), or by one if no number is specified.

            If the semaphore's count would drop below zero, this method will block until such time
            as the semaphore's count is greater than or equal to the amount you're "down"ing the
            semaphore's count by.

            This is the semaphore "P operation" (the name derives from the Dutch word "pak", which
            means "capture" -- the semaphore operations were named by the late Dijkstra, who was
            Dutch).

    ->down_nb()
    ->down_nb(NUMBER)
            The "down_nb" method attempts to decrease the semaphore's count by the specified number
            (which must be an integer >= 1), or by one if no number is specified.

            If the semaphore's count would drop below zero, this method will return *false*, and the
            semaphore's count remains unchanged. Otherwise, the semaphore's count is decremented and
            this method returns *true*.

    ->down_force()
    ->down_force(NUMBER)
            The "down_force" method decreases the semaphore's count by the specified number (which
            must be an integer >= 1), or by one if no number is specified. This method does not
            block, and may cause the semaphore's count to drop below zero.

    ->down_timed(TIMEOUT)
    ->down_timed(TIMEOUT, NUMBER)
            The "down_timed" method attempts to decrease the semaphore's count by 1 or by the
            specified number within the specified timeout period given in seconds (which must be an
            integer >= 0).

            If the semaphore's count would drop below zero, this method will block until either the
            semaphore's count is greater than or equal to the amount you're "down"ing the
            semaphore's count by, or until the timeout is reached.

            If the timeout is reached, this method will return *false*, and the semaphore's count
            remains unchanged. Otherwise, the semaphore's count is decremented and this method
            returns *true*.

    ->up()
    ->up(NUMBER)
            The "up" method increases the semaphore's count by the number specified (which must be
            an integer >= 1), or by one if no number is specified.

            This will unblock any thread that is blocked trying to "down" the semaphore if the "up"
            raises the semaphore's count above the amount that the "down" is trying to decrement it
            by. For example, if three threads are blocked trying to "down" a semaphore by one, and
            another thread "up"s the semaphore by two, then two of the blocked threads (which two is
            indeterminate) will become unblocked.

            This is the semaphore "V operation" (the name derives from the Dutch word "vrij", which
            means "release").

NOTES
    Semaphores created by Thread::Semaphore can be used in both threaded and non-threaded
    applications. This allows you to write modules and packages that potentially make use of
    semaphores, and that will function in either environment.

SEE ALSO
    Thread::Semaphore on MetaCPAN: <https://metacpan.org/release/Thread-Semaphore>

    Code repository for CPAN distribution: <https://github.com/Dual-Life/Thread-Semaphore>

    threads, threads::shared

    Sample code in the *examples* directory of this distribution on CPAN.

MAINTAINER
    Jerry D. Hedden, <jdhedden AT cpan DOT org>

LICENSE
    This program is free software; you can redistribute it and/or modify it under the same terms as
    Perl itself.