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.IX Title "Thread::Semaphore 3"
.TH Thread::Semaphore 3 2023-12-22 "perl v5.42.0" "Perl Programmers Reference Guide"
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.SH NAME
Thread::Semaphore \- Thread\-safe semaphores
.SH VERSION
.IX Header "VERSION"
This document describes Thread::Semaphore version 2.13
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 5
\&    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);
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
Semaphores provide a mechanism to regulate access to resources.  Unlike
locks, semaphores aren\*(Aqt tied to particular scalars, and so may be used to
control access to anything you care to use them for.
.PP
Semaphores don\*(Aqt 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\*(Aqt fixed at one either,
so threads can reserve or return multiple resources at once.
.SH METHODS
.IX Header "METHODS"
.IP \->\fBnew()\fR 8
.IX Item "->new()"
.PD 0
.IP \->new(NUMBER) 8
.IX Item "->new(NUMBER)"
.PD
\&\f(CW\*(C`new\*(C'\fR creates a new semaphore, and initializes its count to the specified
number (which must be an integer).  If no number is specified, the
semaphore\*(Aqs count defaults to 1.
.IP \->\fBdown()\fR 8
.IX Item "->down()"
.PD 0
.IP \->down(NUMBER) 8
.IX Item "->down(NUMBER)"
.PD
The \f(CW\*(C`down\*(C'\fR method decreases the semaphore\*(Aqs count by the specified number
(which must be an integer >= 1), or by one if no number is specified.
.Sp
If the semaphore\*(Aqs count would drop below zero, this method will block
until such time as the semaphore\*(Aqs count is greater than or equal to the
amount you\*(Aqre \f(CW\*(C`down\*(C'\fRing the semaphore\*(Aqs count by.
.Sp
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).
.IP \->\fBdown_nb()\fR 8
.IX Item "->down_nb()"
.PD 0
.IP \->down_nb(NUMBER) 8
.IX Item "->down_nb(NUMBER)"
.PD
The \f(CW\*(C`down_nb\*(C'\fR method attempts to decrease the semaphore\*(Aqs count by the
specified number (which must be an integer >= 1), or by one if no number
is specified.
.Sp
If the semaphore\*(Aqs count would drop below zero, this method will return
\&\fIfalse\fR, and the semaphore\*(Aqs count remains unchanged.  Otherwise, the
semaphore\*(Aqs count is decremented and this method returns \fItrue\fR.
.IP \->\fBdown_force()\fR 8
.IX Item "->down_force()"
.PD 0
.IP \->down_force(NUMBER) 8
.IX Item "->down_force(NUMBER)"
.PD
The \f(CW\*(C`down_force\*(C'\fR method decreases the semaphore\*(Aqs 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\*(Aqs count to drop
below zero.
.IP \->down_timed(TIMEOUT) 8
.IX Item "->down_timed(TIMEOUT)"
.PD 0
.IP "\->down_timed(TIMEOUT, NUMBER)" 8
.IX Item "->down_timed(TIMEOUT, NUMBER)"
.PD
The \f(CW\*(C`down_timed\*(C'\fR method attempts to decrease the semaphore\*(Aqs count by 1
or by the specified number within the specified timeout period given in
seconds (which must be an integer >= 0).
.Sp
If the semaphore\*(Aqs count would drop below zero, this method will block
until either the semaphore\*(Aqs count is greater than or equal to the
amount you\*(Aqre \f(CW\*(C`down\*(C'\fRing the semaphore\*(Aqs count by, or until the timeout is
reached.
.Sp
If the timeout is reached, this method will return \fIfalse\fR, and the
semaphore\*(Aqs count remains unchanged.  Otherwise, the semaphore\*(Aqs count is
decremented and this method returns \fItrue\fR.
.IP \->\fBup()\fR 8
.IX Item "->up()"
.PD 0
.IP \->up(NUMBER) 8
.IX Item "->up(NUMBER)"
.PD
The \f(CW\*(C`up\*(C'\fR method increases the semaphore\*(Aqs count by the number specified
(which must be an integer >= 1), or by one if no number is specified.
.Sp
This will unblock any thread that is blocked trying to \f(CW\*(C`down\*(C'\fR the
semaphore if the \f(CW\*(C`up\*(C'\fR raises the semaphore\*(Aqs count above the amount that
the \f(CW\*(C`down\*(C'\fR is trying to decrement it by.  For example, if three threads
are blocked trying to \f(CW\*(C`down\*(C'\fR a semaphore by one, and another thread \f(CW\*(C`up\*(C'\fRs
the semaphore by two, then two of the blocked threads (which two is
indeterminate) will become unblocked.
.Sp
This is the semaphore "V operation" (the name derives from the Dutch
word "vrij", which means "release").
.SH NOTES
.IX Header "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.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
Thread::Semaphore on MetaCPAN:
<https://metacpan.org/release/Thread\-Semaphore>
.PP
Code repository for CPAN distribution:
<https://github.com/Dual\-Life/Thread\-Semaphore>
.PP
threads, threads::shared
.PP
Sample code in the \fIexamples\fR directory of this distribution on CPAN.
.SH MAINTAINER
.IX Header "MAINTAINER"
Jerry D. Hedden, <jdhedden\ AT\ cpan\ DOT\ org>
.SH LICENSE
.IX Header "LICENSE"
This program is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.