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  9. section entitled "GNU Free Documentation License".

  10. 

  11. This manual contains no Invariant Sections.  The Front-Cover Texts are

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  26. <title>System-specific Predefined Macros (The C Preprocessor)</title>

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  35. <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">

  36. <link href="Predefined-Macros.html#Predefined-Macros" rel="up" title="Predefined Macros">

  37. <link href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators" rel="next" title="C++ Named Operators">

  38. <link href="Common-Predefined-Macros.html#Common-Predefined-Macros" rel="prev" title="Common Predefined Macros">

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  71. <a name="System_002dspecific-Predefined-Macros"></a>

  72. <div class="header">

  73. <p>

  74. 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> &nbsp; [<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>

  75. </div>

  76. <hr>

  77. <a name="System_002dspecific-Predefined-Macros-1"></a>

  78. <h4 class="subsection">3.7.3 System-specific Predefined Macros</h4>

  79. 

  80. <a name="index-system_002dspecific-predefined-macros"></a>

  81. <a name="index-predefined-macros_002c-system_002dspecific"></a>

  82. <a name="index-reserved-namespace"></a>

  83. 

  84. <p>The C preprocessor normally predefines several macros that indicate what

  85. type of system and machine is in use.  They are obviously different on

  86. each target supported by GCC.  This manual, being for all systems and

  87. machines, cannot tell you what their names are, but you can use

  88. <code>cpp -dM</code> to see them all.  See <a href="Invocation.html#Invocation">Invocation</a>.  All system-specific

  89. predefined macros expand to a constant value, so you can test them with

  90. either &lsquo;<samp>#ifdef</samp>&rsquo; or &lsquo;<samp>#if</samp>&rsquo;.

  91. </p>

  92. <p>The C standard requires that all system-specific macros be part of the

  93. <em>reserved namespace</em>.  All names which begin with two underscores,

  94. or an underscore and a capital letter, are reserved for the compiler and

  95. library to use as they wish.  However, historically system-specific

  96. macros have had names with no special prefix; for instance, it is common

  97. to find <code>unix</code> defined on Unix systems.  For all such macros, GCC

  98. provides a parallel macro with two underscores added at the beginning

  99. and the end.  If <code>unix</code> is defined, <code>__unix__</code> will be defined

  100. too.  There will never be more than two underscores; the parallel of

  101. <code>_mips</code> is <code>__mips__</code>.

  102. </p>

  103. <p>When the <samp>-ansi</samp> option, or any <samp>-std</samp> option that

  104. requests strict conformance, is given to the compiler, all the

  105. system-specific predefined macros outside the reserved namespace are

  106. suppressed.  The parallel macros, inside the reserved namespace, remain

  107. defined.

  108. </p>

  109. <p>We are slowly phasing out all predefined macros which are outside the

  110. reserved namespace.  You should never use them in new programs, and we

  111. encourage you to correct older code to use the parallel macros whenever

  112. you find it.  We don&rsquo;t recommend you use the system-specific macros that

  113. are in the reserved namespace, either.  It is better in the long run to

  114. check specifically for features you need, using a tool such as

  115. <code>autoconf</code>.

  116. </p>

  117. <hr>

  118. <div class="header">

  119. <p>

  120. 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> &nbsp; [<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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