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core/com.nokia.carbide.cpp.compiler.doc.user/html/c_compiler/c_volatile_var.htm
author Deepak Modgil <Deepak.Modgil@Nokia.com>
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<title>Volatile Variables</title>
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<link rel="StyleSheet" href="../../book.css" type="text/css"/>
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<h3>Volatile Variables</h3>
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<p>(ISO C, &sect;6.7.3) When you declare a volatile variable, the Carbide C compiler takes the following precautions to respect the value of the variable:</p>
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<ul>
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  <li>The compiler stores commonly used variables in processor registers to produce faster object code. However, the compiler never stores the value of a volatile variable in a processor register.</li>
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  <li>The compiler uses its common sub-expression optimization to compute the addresses of commonly used variables and the results of often-used expressions once at the beginning of a function to produce faster object code. However, every time an expression uses a volatile variable, the compiler computes both the address of the volatile variable and the results of the expression that uses it.</li>
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</ul>
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<p>Listing 1 shows an example of volatile variables.</p>
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<div class="listing">
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  <h5>Listing 1. Volatile Variables</h5>
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  <p>void main(void)<br />
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    {<br />
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    int i[100];<br />
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    volatile int a, b; /* a and b are not cached in registers. */</p>
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  <p> a = 5;<br />
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    b = 20;</p>
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  <p> i[a + b] = 15; /* compiler calculates a + b */<br />
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    i[a + b] = 30; /* compiler recalculates a + b */<br />
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  }</p>
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</div>
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<p>The compiler does not place the value of a, b, or a+b in registers. But it does recalculate a+b in both assignment statements. <br />
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</p>
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<div id="footer">Copyright &copy; 2009 Nokia Corporation and/or its subsidiary(-ies). All rights reserved. <br>License: <a href="http://www.eclipse.org/legal/epl-v10.html">http://www.eclipse.org/legal/epl-v10.html</a></div>
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FCL/sftools/dev/ide/carbidecpp