74 lines
4.7 KiB
HTML
74 lines
4.7 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html lang="en-us" xml:lang="en-us">
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<meta name="DC.Type" content="concept" />
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<meta name="DC.Title" content="Semaphores and threads" />
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<meta name="abstract" content="Semaphores (sometimes referred to as counting semaphores) can be used to control access to shared resources. A semaphore can be thought of as an intelligent counter. Every semaphore has a current count, which is greater than or equal to 0." />
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<meta name="DC.Relation" scheme="URI" content="rzahwsynco.htm" />
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<meta name="DC.Relation" scheme="URI" content="rzahwe19rx.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<meta name="DC.Rights.Owner" content="(C) Copyright IBM Corporation 1998, 2006" />
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Semaphores and threads</title>
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</head>
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<body id="rzahwsem-semco"><a name="rzahwsem-semco"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Semaphores and threads</h1>
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<div><p>Semaphores (sometimes referred to as counting semaphores) can be
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used to control access to shared resources. A semaphore can be thought of
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as an intelligent counter. Every semaphore has a current count, which is greater
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than or equal to 0. </p>
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<p>Any thread can decrement the count to lock the semaphore (this is also
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called waiting on the semaphore). Attempting to decrement the count past 0
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causes the thread that is calling to wait for another thread to unlock the
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semaphore. </p>
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<p>Any thread can increment the count to unlock the semaphore (this is also
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called posting the semaphore). Posting a semaphore might wake up a waiting
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thread if there is one present. </p>
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<p>In their simplest form (with an initial count of 1), semaphores can be
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thought of as a mutual exclusion (mutex). The important distinction between
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semaphores and mutexes is the concept of ownership. No ownership is associated
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with a semaphore. Unlike mutexes, it is possible for a thread that never waited
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for (locked) the semaphore to post (unlock) the semaphore. This can cause
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unpredictable application behavior. You should avoid this if possible.</p>
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<div class="p">Additional capabilities of some semaphore APIs provided
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by the operating system are as follows: <ul><li>More complete management capabilities, including permissions on semaphores
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that are similar to file permissions.</li>
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<li>The ability to group semaphores in sets and perform atomic operations
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on the group</li>
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<li>The ability to do multicount wait and post operations</li>
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<li>The ability to wait for a semaphore to have a count of 0</li>
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<li>The ability to undo operations that are done by another thread under certain
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conditions</li>
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</ul>
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</div>
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<div class="p"> <div class="note"><span class="notetitle">Note:</span> Java™ does not have the ability to use semaphores.</div>
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</div>
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</div>
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<div>
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<ul class="ullinks">
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<li class="ulchildlink"><strong><a href="rzahwe19rx.htm">Example: Use semaphores in Pthread programs to protect shared data</a></strong><br />
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This example shows a Pthread program starting several threads that protect access to shared data with a semaphore set.</li>
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</ul>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzahwsynco.htm" title="When you create code that is threadsafe but still benefits from sharing data or resources between threads, the most important aspect of programming becomes the ability to synchronize threads.">Synchronization techniques among threads</a></div>
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</div>
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</div>
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</body>
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</html>
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