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, 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.