See the current Perl documentation for Thread::Semaphore.
Here is our local, out-dated (pre-5.6) version:
Thread::Semaphore - thread-safe semaphores
use Thread::Semaphore; my $s = new Thread::Semaphore; $s->up; # Also known as the semaphore V -operation. # The guarded section is here $s->down; # Also known as the semaphore P -operation.
Semaphores provide a mechanism to regulate access to resources. Semaphores, unlike locks, 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 or one, so they can be used to control access to some resource that may have more than one of. (For example, filehandles) Increment and decrement amounts aren't fixed at one either, so threads can reserve or return multiple resources at once.
- new NUMBER
newcreates 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
downmethod decreases the semaphore's count by the specified number, or 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
downing the semaphore's count by.
- up NUMBER
upmethod increases the semaphore's count by the number specified, or one if no number's been specified. This will unblock any thread blocked trying to
downthe semaphore if the
upraises the semaphore count above what the
downs are trying to decrement it by.