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96 lines
4.9 KiB
HTML
96 lines
4.9 KiB
HTML
4 years ago
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<title>System-specific Predefined Macros - The C Preprocessor</title>
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<meta name="description" content="The C Preprocessor">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Predefined-Macros.html#Predefined-Macros" title="Predefined Macros">
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<link rel="prev" href="Common-Predefined-Macros.html#Common-Predefined-Macros" title="Common Predefined Macros">
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<link rel="next" href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators" title="C++ Named Operators">
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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="System-specific-Predefined-Macros"></a>
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<a name="System_002dspecific-Predefined-Macros"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators">C++ Named Operators</a>,
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Previous: <a rel="previous" accesskey="p" href="Common-Predefined-Macros.html#Common-Predefined-Macros">Common Predefined Macros</a>,
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Up: <a rel="up" accesskey="u" href="Predefined-Macros.html#Predefined-Macros">Predefined Macros</a>
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<hr>
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</div>
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<h4 class="subsection">3.7.3 System-specific Predefined Macros</h4>
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<p><a name="index-system_002dspecific-predefined-macros-62"></a><a name="index-predefined-macros_002c-system_002dspecific-63"></a><a name="index-reserved-namespace-64"></a>
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The C preprocessor normally predefines several macros that indicate what
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type of system and machine is in use. They are obviously different on
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each target supported by GCC. This manual, being for all systems and
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machines, cannot tell you what their names are, but you can use
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<samp><span class="command">cpp -dM</span></samp> to see them all. See <a href="Invocation.html#Invocation">Invocation</a>. All system-specific
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predefined macros expand to a constant value, so you can test them with
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either ‘<samp><span class="samp">#ifdef</span></samp>’ or ‘<samp><span class="samp">#if</span></samp>’.
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<p>The C standard requires that all system-specific macros be part of the
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<dfn>reserved namespace</dfn>. All names which begin with two underscores,
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or an underscore and a capital letter, are reserved for the compiler and
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library to use as they wish. However, historically system-specific
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macros have had names with no special prefix; for instance, it is common
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to find <code>unix</code> defined on Unix systems. For all such macros, GCC
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provides a parallel macro with two underscores added at the beginning
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and the end. If <code>unix</code> is defined, <code>__unix__</code> will be defined
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too. There will never be more than two underscores; the parallel of
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<code>_mips</code> is <code>__mips__</code>.
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<p>When the <samp><span class="option">-ansi</span></samp> option, or any <samp><span class="option">-std</span></samp> option that
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requests strict conformance, is given to the compiler, all the
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system-specific predefined macros outside the reserved namespace are
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suppressed. The parallel macros, inside the reserved namespace, remain
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defined.
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<p>We are slowly phasing out all predefined macros which are outside the
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reserved namespace. You should never use them in new programs, and we
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encourage you to correct older code to use the parallel macros whenever
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you find it. We don't recommend you use the system-specific macros that
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are in the reserved namespace, either. It is better in the long run to
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check specifically for features you need, using a tool such as
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<samp><span class="command">autoconf</span></samp>.
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</body></html>
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