Thread...maphore(3perl) Perl Programmers Reference GuideThread...maphore(3perl)
NAME
Thread::Semaphore - Thread-safe semaphores
VERSION
This document describes Thread::Semaphore version 2.13
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.
->down_timed(TIMEOUT)
->down_timed(TIMEOUT, NUMBER)
The "down_timed" method attempts to decrease the semaphore's
count by 1 or by the specified number within the specified
timeout period given in seconds (which must be an integer >= 0).
If the semaphore's count would drop below zero, this method will
block until either the semaphore's count is greater than or
equal to the amount you're "down"ing the semaphore's count by,
or until the timeout is reached.
If the timeout is reached, this method will return false, and
the semaphore's count remains unchanged. Otherwise, the
semaphore's count is decremented and this method returns true.
->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 on MetaCPAN:
<https://metacpan.org/release/Thread-Semaphore>
Code repository for CPAN distribution:
<https://github.com/Dual-Life/Thread-Semaphore>
threads, threads::shared
Sample code in the examples directory of this distribution on CPAN.
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 v5.40.1 2026-08-30 Thread...maphore(3perl)
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