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127 lines
6.4 KiB
HTML
127 lines
6.4 KiB
HTML
4 years ago
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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) 1987-2018 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 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>System-specific Predefined Macros (The C Preprocessor)</title>
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<meta name="description" content="System-specific Predefined Macros (The C Preprocessor)">
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<meta name="keywords" content="System-specific Predefined Macros (The C Preprocessor)">
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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="Predefined-Macros.html#Predefined-Macros" rel="up" title="Predefined Macros">
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<link href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators" rel="next" title="C++ Named Operators">
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<link href="Common-Predefined-Macros.html#Common-Predefined-Macros" rel="prev" title="Common Predefined Macros">
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-->
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</style>
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</head>
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<body lang="en">
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<a name="System_002dspecific-Predefined-Macros"></a>
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<div class="header">
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<p>
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Next: <a href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators" accesskey="n" rel="next">C++ Named Operators</a>, Previous: <a href="Common-Predefined-Macros.html#Common-Predefined-Macros" accesskey="p" rel="prev">Common Predefined Macros</a>, Up: <a href="Predefined-Macros.html#Predefined-Macros" accesskey="u" rel="up">Predefined Macros</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="System_002dspecific-Predefined-Macros-1"></a>
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<h4 class="subsection">3.7.3 System-specific Predefined Macros</h4>
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<a name="index-system_002dspecific-predefined-macros"></a>
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<a name="index-predefined-macros_002c-system_002dspecific"></a>
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<a name="index-reserved-namespace"></a>
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<p>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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<code>cpp -dM</code> 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>#ifdef</samp>’ or ‘<samp>#if</samp>’.
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</p>
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<p>The C standard requires that all system-specific macros be part of the
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<em>reserved namespace</em>. 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>
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<p>When the <samp>-ansi</samp> option, or any <samp>-std</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>
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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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<code>autoconf</code>.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators" accesskey="n" rel="next">C++ Named Operators</a>, Previous: <a href="Common-Predefined-Macros.html#Common-Predefined-Macros" accesskey="p" rel="prev">Common Predefined Macros</a>, Up: <a href="Predefined-Macros.html#Predefined-Macros" accesskey="u" rel="up">Predefined Macros</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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</body>
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</html>
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