MirOS Manual: Thread::Semaphore(3p)

Thread::SemaphorePerl Programmers Reference Thread::Semaphore(3p)


     Thread::Semaphore - thread-safe semaphores


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

         # The default semaphore value is 1.
         my $s = new Thread::Semaphore($initial_value);


     Semaphores provide a mechanism to regulate access to
     resources. Semaphores, unlike locks, aren't tied to particu-
     lar scalars, and so may be used to control access to any-
     thing you care to use them for.

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


     new NUMBER
             "new" creates a new semaphore, and initializes its
             count to the passed number. If no number is passed,
             the semaphore's count is set to one.

     down NUMBER
             The "down" method decreases the semaphore's count by
             the specified number, or by one if no number has
             been specified. If the semaphore's count would drop
             below zero, this method will block until such time
             that the semaphore's count is equal to or larger
             than 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 "cap-
             ture" -- the semaphore operations were named by the
             late Dijkstra, who was Dutch).

     up NUMBER
             The "up" method increases the semaphore's count by

perl v5.8.8                2005-02-05                           1

Thread::SemaphorePerl Programmers Reference Thread::Semaphore(3p)

             the number specified, or by one if no number has
             been specified. This will unblock any thread blocked
             trying to "down" the semaphore if the "up" raises
             the semaphore count above the amount that the
             "down"s are trying to decrement it by.

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

perl v5.8.8                2005-02-05                           2

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