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| 22 | <title>Conditional Uses (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Conditionals.html" rel="up" title="Conditionals">
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| 33 | <link href="Conditional-Syntax.html" rel="next" title="Conditional Syntax">
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| 58 | <span id="Conditional-Uses"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Conditional-Syntax.html" accesskey="n" rel="next">Conditional Syntax</a>, Up: <a href="Conditionals.html" accesskey="u" rel="up">Conditionals</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Uses-of-Conditional-Directives"></span><h4 class="subsection">26.6.1 Uses of Conditional Directives</h4>
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| 64 |
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| 65 | <p>There are three usual reasons to use a preprocessing conditional.
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| 66 | </p>
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| 67 | <ul>
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| 68 | <li> A program may need to use different code depending on the machine or
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| 69 | operating system it is to run on. In some cases the code for one
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| 70 | operating system may be erroneous on another operating system; for
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| 71 | example, it might refer to data types or constants that do not exist on
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| 72 | the other system. When this happens, it is not enough to avoid
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| 73 | executing the invalid code. Its mere presence will cause the compiler
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| 74 | to reject the program. With a preprocessing conditional, the offending
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| 75 | code can be effectively excised from the program when it is not valid.
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| 76 |
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| 77 | </li><li> You may want to be able to compile the same source file into two
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| 78 | different programs. One version might make frequent time-consuming
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| 79 | consistency checks on its intermediate data, or print the values of
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| 80 | those data for debugging, and the other not.
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| 81 |
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| 82 | </li><li> A conditional whose condition is always false is one way to exclude code
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| 83 | from the program but keep it as a sort of comment for future reference.
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| 84 | </li></ul>
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| 85 |
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| 86 | <p>Simple programs that do not need system-specific logic or complex
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| 87 | debugging hooks generally will not need to use preprocessing
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| 88 | conditionals.
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| 89 | </p>
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| 90 |
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| 91 |
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| 92 |
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