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Thread::Semaphore(3pm)                Perl Programmers Reference Guide                Thread::Semaphore(3pm)



NAME
       Thread::Semaphore - Thread-safe semaphores

VERSION
       This document describes Thread::Semaphore version 2.12

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.

       ->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 Discussion Forum on CPAN: http://www.cpanforum.com/dist/Thread-Semaphore
       <http://www.cpanforum.com/dist/Thread-Semaphore>

       threads, threads::shared

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.



perl v5.16.2                                     2012-10-11                           Thread::Semaphore(3pm)