73 lines
4.0 KiB
HTML
73 lines
4.0 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.Title" content="Compare and Swap" />
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<meta name="abstract" content="You can use the Machine Interface's (MI) Compare and Swap (CMPSWP) instruction to access data in a multithreaded program." />
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<meta name="description" content="You can use the Machine Interface's (MI) Compare and Swap (CMPSWP) instruction to access data in a multithreaded program." />
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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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<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>Compare and Swap</title>
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</head>
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<body id="rzahwcas-casco"><a name="rzahwcas-casco"><!-- --></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">Compare and Swap</h1>
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<div><p>You can use the Machine Interface's (MI) Compare and Swap (CMPSWP)
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instruction to access data in a multithreaded program.</p>
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<div class="section"><p>CMPSWP compares the value of a first compare operand to the value
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of a second compare operand. If they are equal, the swap operand is stored
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in the second compare operand's location. If they are unequal, the second
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compare operand is stored into the first compare operand's location. </p>
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</div>
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<div class="section"><p>When an equal comparison occurs, it is assured that no access
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by another CMPSWP instruction will occur at the second compare operand location
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between the moment that the second compare operand is fetched for comparison
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and the moment that the swap operand is stored at the second compare operand
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location.</p>
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</div>
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<div class="section"><p>When an unequal comparison occurs, no atomicity guarantees are
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made regarding the store to the first compare operand location and other CMPSWP
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instruction access. Thus only the second compare operand should be a variable
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shared for concurrent processing control.</p>
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</div>
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<div class="section"><p>The following code example for a C macro can be used to atomically
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increment or decrement an integer variable.</p>
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<pre>#ifndef __cmpswp_h
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#include <mih/cmpswp.h>
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#endif
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#define ATOMICADD ( var, val, rc ) { \
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int aatemp1 = (var); \
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int aatemp2 = aatemp1 + val; \
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while( ! _CMPSWP( &aatemp1, &var, aatemp2 ) ) \
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aatemp2 = aatemp1 + val; \
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rc = aatemp2; \
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}</pre>
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<p>In the code example, <em>var</em> is an integer
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to be incremented or decremented, <em>val</em> is an integer value to be added
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or subtracted from <em>var</em>, and <em>rc</em> is the resultant value.</p>
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</div>
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</div>
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<div>
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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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