1package Thread::Semaphore;
2
3use threads::shared;
4
5our $VERSION = '2.01';
6
7=head1 NAME
8
9Thread::Semaphore - thread-safe semaphores
10
11=head1 SYNOPSIS
12
13    use Thread::Semaphore;
14    my $s = new Thread::Semaphore;
15    $s->down;	# Also known as the semaphore P operation.
16    # The guarded section is here
17    $s->up;	# Also known as the semaphore V operation.
18
19    # The default semaphore value is 1.
20    my $s = new Thread::Semaphore($initial_value);
21    $s->down($down_value);
22    $s->up($up_value);
23
24=head1 DESCRIPTION
25
26Semaphores provide a mechanism to regulate access to resources. Semaphores,
27unlike locks, aren't tied to particular scalars, and so may be used to
28control access to anything you care to use them for.
29
30Semaphores don't limit their values to zero or one, so they can be used to
31control access to some resource that there may be more than one of. (For
32example, filehandles.) Increment and decrement amounts aren't fixed at one
33either, so threads can reserve or return multiple resources at once.
34
35=head1 FUNCTIONS AND METHODS
36
37=over 8
38
39=item new
40
41=item new NUMBER
42
43C<new> creates a new semaphore, and initializes its count to the passed
44number. If no number is passed, the semaphore's count is set to one.
45
46=item down
47
48=item down NUMBER
49
50The C<down> method decreases the semaphore's count by the specified number,
51or by one if no number has been specified. If the semaphore's count would drop
52below zero, this method will block until such time that the semaphore's
53count is equal to or larger than the amount you're C<down>ing the
54semaphore's count by.
55
56This is the semaphore "P operation" (the name derives from the Dutch
57word "pak", which means "capture" -- the semaphore operations were
58named by the late Dijkstra, who was Dutch).
59
60=item up
61
62=item up NUMBER
63
64The C<up> method increases the semaphore's count by the number specified,
65or by one if no number has been specified. This will unblock any thread blocked
66trying to C<down> the semaphore if the C<up> raises the semaphore count
67above the amount that the C<down>s are trying to decrement it by.
68
69This is the semaphore "V operation" (the name derives from the Dutch
70word "vrij", which means "release").
71
72=back
73
74=cut
75
76sub new {
77    my $class = shift;
78    my $val : shared = @_ ? shift : 1;
79    bless \$val, $class;
80}
81
82sub down {
83    my $s = shift;
84    lock($$s);
85    my $inc = @_ ? shift : 1;
86    cond_wait $$s until $$s >= $inc;
87    $$s -= $inc;
88}
89
90sub up {
91    my $s = shift;
92    lock($$s);
93    my $inc = @_ ? shift : 1;
94    ($$s += $inc) > 0 and cond_broadcast $$s;
95}
96
971;
98