1Thread::Semaphore(3pm) Perl Programmers Reference Guide Thread::Semaphore(3pm)
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6 Thread::Semaphore - Thread-safe semaphores
7
9 This document describes Thread::Semaphore version 2.12
10
12 use Thread::Semaphore;
13 my $s = Thread::Semaphore->new();
14 $s->down(); # Also known as the semaphore P operation.
15 # The guarded section is here
16 $s->up(); # Also known as the semaphore V operation.
17 # Decrement the semaphore only if it would immediately succeed.
19 if ($s->down_nb()) {
20 # The guarded section is here
21 $s->up();
22 }
23 # Forcefully decrement the semaphore even if its count goes below 0.
25 $s->down_force();
26 # The default value for semaphore operations is 1
28 my $s = Thread::Semaphore->new($initial_value);
29 $s->down($down_value);
30 $s->up($up_value);
31 if ($s->down_nb($down_value)) {
32 ...
33 $s->up($up_value);
34 }
35 $s->down_force($down_value);
36
38 Semaphores provide a mechanism to regulate access to resources. Unlike
39 locks, semaphores aren't tied to particular scalars, and so may be used
40 to control access to anything you care to use them for.
41 Semaphores don't limit their values to zero and one, so they can be
43 used to control access to some resource that there may be more than one
44 of (e.g., filehandles). Increment and decrement amounts aren't fixed
45 at one either, so threads can reserve or return multiple resources at
46 once.
47
49 ->new()
50 ->new(NUMBER)
51 "new" creates a new semaphore, and initializes its count to the
52 specified number (which must be an integer). If no number is
53 specified, the semaphore's count defaults to 1.
54 ->down()
56 ->down(NUMBER)
57 The "down" method decreases the semaphore's count by the
58 specified number (which must be an integer >= 1), or by one if
59 no number is specified.
60 If the semaphore's count would drop below zero, this method
62 will block until such time as the semaphore's count is greater
63 than or equal to the amount you're "down"ing the semaphore's
64 count by.
65 This is the semaphore "P operation" (the name derives from the
67 Dutch word "pak", which means "capture" -- the semaphore
68 operations were named by the late Dijkstra, who was Dutch).
69 ->down_nb()
71 ->down_nb(NUMBER)
72 The "down_nb" method attempts to decrease the semaphore's count
73 by the specified number (which must be an integer >= 1), or by
74 one if no number is specified.
75 If the semaphore's count would drop below zero, this method
77 will return false, and the semaphore's count remains unchanged.
78 Otherwise, the semaphore's count is decremented and this method
79 returns true.
80 ->down_force()
82 ->down_force(NUMBER)
83 The "down_force" method decreases the semaphore's count by the
84 specified number (which must be an integer >= 1), or by one if
85 no number is specified. This method does not block, and may
86 cause the semaphore's count to drop below zero.
87 ->up()
89 ->up(NUMBER)
90 The "up" method increases the semaphore's count by the number
91 specified (which must be an integer >= 1), or by one if no
92 number is specified.
93 This will unblock any thread that is blocked trying to "down"
95 the semaphore if the "up" raises the semaphore's count above
96 the amount that the "down" is trying to decrement it by. For
97 example, if three threads are blocked trying to "down" a
98 semaphore by one, and another thread "up"s the semaphore by
99 two, then two of the blocked threads (which two is
100 indeterminate) will become unblocked.
101 This is the semaphore "V operation" (the name derives from the
103 Dutch word "vrij", which means "release").
104
106 Semaphores created by Thread::Semaphore can be used in both threaded
107 and non-threaded applications. This allows you to write modules and
108 packages that potentially make use of semaphores, and that will
109 function in either environment.
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112 Thread::Semaphore Discussion Forum on CPAN:
113 http://www.cpanforum.com/dist/Thread-Semaphore
114 <http://www.cpanforum.com/dist/Thread-Semaphore>
115 threads, threads::shared
117
119 Jerry D. Hedden, <jdhedden AT cpan DOT org>
120
122 This program is free software; you can redistribute it and/or modify it
123 under the same terms as Perl itself.
124perl v5.16.3 2013-03-04 Thread::Semaphore(3pm)