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

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 v5.14.2							    2011-09-19						  Thread::Semaphore(3perl)