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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>

       threads(3), threads::shared(3)


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 v5.24.0                      2015-10-14            Thread::Semaphore(3pm)

perl 5.24 - Generated Sat Nov 26 06:38:37 CST 2016
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