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.IX Title "Thread::Semaphore 3"
.TH Thread::Semaphore 3 "2002-11-24" "perl v5.8.0" "Perl Programmers Reference Guide"
.SH "NAME"
Thread::Semaphore \- thread\-safe semaphores
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 5
\&    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.
.Ve
.PP
.Vb 4
\&    # The default semaphore value is 1.
\&    my $s = new Thread::Semaphore($initial_value);
\&    $s->up($up_value);
\&    $s->down($up_value);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
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.
.PP
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). Increment and decrement amounts aren't fixed at one
either, so threads can reserve or return multiple resources at once.
.SH "FUNCTIONS AND METHODS"
.IX Header "FUNCTIONS AND METHODS"
.IP "new" 8
.IX Item "new"
.PD 0
.IP "new \s-1NUMBER\s0" 8
.IX Item "new NUMBER"
.PD
\&\f(CW\*(C`new\*(C'\fR 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.
.IP "down" 8
.IX Item "down"
.PD 0
.IP "down \s-1NUMBER\s0" 8
.IX Item "down NUMBER"
.PD
The \f(CW\*(C`down\*(C'\fR 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 \f(CW\*(C`down\*(C'\fRing the
semaphore's count by.
.IP "up" 8
.IX Item "up"
.PD 0
.IP "up \s-1NUMBER\s0" 8
.IX Item "up NUMBER"
.PD
The \f(CW\*(C`up\*(C'\fR method increases the semaphore's count by the number specified,
or by one if no number has been specified. This will unblock any thread blocked
trying to \f(CW\*(C`down\*(C'\fR the semaphore if the \f(CW\*(C`up\*(C'\fR raises the semaphore count
above the amount that the \f(CW\*(C`down\*(C'\fRs are trying to decrement it by.