111 lines
4.6 KiB
HTML
111 lines
4.6 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1987-2016 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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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>The C Preprocessor: Conditional Uses</title>
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<meta name="description" content="The C Preprocessor: Conditional Uses">
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<meta name="keywords" content="The C Preprocessor: Conditional Uses">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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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<style type="text/css">
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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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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