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96 lines
4.1 KiB
HTML
96 lines
4.1 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Conditionals - 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="prev" href="Macros.html#Macros" title="Macros">
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<link rel="next" href="Diagnostics.html#Diagnostics" title="Diagnostics">
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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) 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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</head>
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<body>
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<div class="node">
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<a name="Conditionals"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Diagnostics.html#Diagnostics">Diagnostics</a>,
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Previous: <a rel="previous" accesskey="p" href="Macros.html#Macros">Macros</a>,
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Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
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<hr>
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</div>
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<h2 class="chapter">4 Conditionals</h2>
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<p><a name="index-conditionals-83"></a>
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A <dfn>conditional</dfn> is a directive that instructs the preprocessor to
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select whether or not to include a chunk of code in the final token
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stream passed to the compiler. Preprocessor conditionals can test
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arithmetic expressions, or whether a name is defined as a macro, or both
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simultaneously using the special <code>defined</code> operator.
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<p>A conditional in the C preprocessor resembles in some ways an <code>if</code>
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statement in C, but it is important to understand the difference between
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them. The condition in an <code>if</code> statement is tested during the
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execution of your program. Its purpose is to allow your program to
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behave differently from run to run, depending on the data it is
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operating on. The condition in a preprocessing conditional directive is
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tested when your program is compiled. Its purpose is to allow different
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code to be included in the program depending on the situation at the
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time of compilation.
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<p>However, the distinction is becoming less clear. Modern compilers often
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do test <code>if</code> statements when a program is compiled, if their
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conditions are known not to vary at run time, and eliminate code which
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can never be executed. If you can count on your compiler to do this,
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you may find that your program is more readable if you use <code>if</code>
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statements with constant conditions (perhaps determined by macros). Of
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course, you can only use this to exclude code, not type definitions or
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other preprocessing directives, and you can only do it if the code
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remains syntactically valid when it is not to be used.
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<p>GCC version 3 eliminates this kind of never-executed code even when
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not optimizing. Older versions did it only when optimizing.
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<ul class="menu">
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<li><a accesskey="1" href="Conditional-Uses.html#Conditional-Uses">Conditional Uses</a>
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<li><a accesskey="2" href="Conditional-Syntax.html#Conditional-Syntax">Conditional Syntax</a>
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<li><a accesskey="3" href="Deleted-Code.html#Deleted-Code">Deleted Code</a>
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</ul>
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</body></html>
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