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110 lines
4.5 KiB
HTML
110 lines
4.5 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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Permission is granted to copy, distribute and/or modify this document
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any later version published by the Free Software Foundation. A copy of
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section entitled "GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<head>
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<title>Conditional Uses (The C Preprocessor)</title>
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Conditionals.html#Conditionals" rel="up" title="Conditionals">
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<link href="Conditional-Syntax.html#Conditional-Syntax" rel="next" title="Conditional Syntax">
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<link href="Conditionals.html#Conditionals" rel="prev" title="Conditionals">
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<a name="Conditional-Uses"></a>
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<div class="header">
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<p>
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Next: <a href="Conditional-Syntax.html#Conditional-Syntax" accesskey="n" rel="next">Conditional Syntax</a>, Up: <a href="Conditionals.html#Conditionals" accesskey="u" rel="up">Conditionals</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Conditional-Uses-1"></a>
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<h3 class="section">4.1 Conditional Uses</h3>
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<p>There are three general reasons to use a conditional.
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</p>
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<ul>
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<li> A program may need to use different code depending on the machine or
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operating system it is to run on. In some cases the code for one
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operating system may be erroneous on another operating system; for
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example, it might refer to data types or constants that do not exist on
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the other system. When this happens, it is not enough to avoid
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executing the invalid code. Its mere presence will cause the compiler
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to reject the program. With a preprocessing conditional, the offending
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code can be effectively excised from the program when it is not valid.
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</li><li> You may want to be able to compile the same source file into two
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different programs. One version might make frequent time-consuming
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consistency checks on its intermediate data, or print the values of
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those data for debugging, and the other not.
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</li><li> A conditional whose condition is always false is one way to exclude code
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from the program but keep it as a sort of comment for future reference.
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</li></ul>
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<p>Simple programs that do not need system-specific logic or complex
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debugging hooks generally will not need to use preprocessing
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conditionals.
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</p>
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</body>
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</html>
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