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84 lines
3.3 KiB
HTML
84 lines
3.3 KiB
HTML
4 years ago
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<title>Conditional Uses - The C Preprocessor</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="The C Preprocessor">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Conditionals.html#Conditionals" title="Conditionals">
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<link rel="next" href="Conditional-Syntax.html#Conditional-Syntax" title="Conditional Syntax">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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Copyright (C) 1987-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation. A copy of
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the license is included in the
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section entitled ``GNU Free Documentation License''.
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This manual contains no Invariant Sections. The Front-Cover Texts are
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(a) (see below), and the Back-Cover Texts are (b) (see below).
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Conditional-Uses"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Conditional-Syntax.html#Conditional-Syntax">Conditional Syntax</a>,
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Up: <a rel="up" accesskey="u" href="Conditionals.html#Conditionals">Conditionals</a>
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<hr>
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</div>
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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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<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>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>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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</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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</body></html>
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