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 <a name="System_002dspecific-Predefined-Macros"></a>
 <div class="header">
 <p>
 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>
 </div>
 <hr>
 <a name="System_002dspecific-Predefined-Macros-1"></a>
 <h4 class="subsection">3.7.3 System-specific Predefined Macros</h4>

 <a name="index-system_002dspecific-predefined-macros"></a>
 <a name="index-predefined-macros_002c-system_002dspecific"></a>
 <a name="index-reserved-namespace"></a>

 <p>The C preprocessor normally predefines several macros that indicate what
 type of system and machine is in use.  They are obviously different on
 each target supported by GCC.  This manual, being for all systems and
 machines, cannot tell you what their names are, but you can use
 <code>cpp -dM</code> to see them all.  See <a href="Invocation.html#Invocation">Invocation</a>.  All system-specific
 predefined macros expand to a constant value, so you can test them with
 either &lsquo;<samp>#ifdef</samp>&rsquo; or &lsquo;<samp>#if</samp>&rsquo;.
 </p>
 <p>The C standard requires that all system-specific macros be part of the
 <em>reserved namespace</em>.  All names which begin with two underscores,
 or an underscore and a capital letter, are reserved for the compiler and
 library to use as they wish.  However, historically system-specific
 macros have had names with no special prefix; for instance, it is common
 to find <code>unix</code> defined on Unix systems.  For all such macros, GCC
 provides a parallel macro with two underscores added at the beginning
 and the end.  If <code>unix</code> is defined, <code>__unix__</code> will be defined
 too.  There will never be more than two underscores; the parallel of
 <code>_mips</code> is <code>__mips__</code>.
 </p>
 <p>When the <samp>-ansi</samp> option, or any <samp>-std</samp> option that
 requests strict conformance, is given to the compiler, all the
 system-specific predefined macros outside the reserved namespace are
 suppressed.  The parallel macros, inside the reserved namespace, remain
 defined.
 </p>
 <p>We are slowly phasing out all predefined macros which are outside the
 reserved namespace.  You should never use them in new programs, and we
 encourage you to correct older code to use the parallel macros whenever
 you find it.  We don&rsquo;t recommend you use the system-specific macros that
 are in the reserved namespace, either.  It is better in the long run to
 check specifically for features you need, using a tool such as
 <code>autoconf</code>.
 </p>
 <hr>
 <div class="header">
 <p>
 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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	<a name="System_002dspecific-Predefined-Macros"></a>
	<div class="header">
	<p>
	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>
	</div>
	<hr>
	<a name="System_002dspecific-Predefined-Macros-1"></a>
	<h4 class="subsection">3.7.3 System-specific Predefined Macros</h4>

	<a name="index-system_002dspecific-predefined-macros"></a>
	<a name="index-predefined-macros_002c-system_002dspecific"></a>
	<a name="index-reserved-namespace"></a>

	<p>The C preprocessor normally predefines several macros that indicate what
	type of system and machine is in use. They are obviously different on
	each target supported by GCC. This manual, being for all systems and
	machines, cannot tell you what their names are, but you can use
	<code>cpp -dM</code> to see them all. See <a href="Invocation.html#Invocation">Invocation</a>. All system-specific
	predefined macros expand to a constant value, so you can test them with
	either ‘<samp>#ifdef</samp>’ or ‘<samp>#if</samp>’.
	</p>
	<p>The C standard requires that all system-specific macros be part of the
	<em>reserved namespace</em>. All names which begin with two underscores,
	or an underscore and a capital letter, are reserved for the compiler and
	library to use as they wish. However, historically system-specific
	macros have had names with no special prefix; for instance, it is common
	to find <code>unix</code> defined on Unix systems. For all such macros, GCC
	provides a parallel macro with two underscores added at the beginning
	and the end. If <code>unix</code> is defined, <code>__unix__</code> will be defined
	too. There will never be more than two underscores; the parallel of
	<code>_mips</code> is <code>__mips__</code>.
	</p>
	<p>When the <samp>-ansi</samp> option, or any <samp>-std</samp> option that
	requests strict conformance, is given to the compiler, all the
	system-specific predefined macros outside the reserved namespace are
	suppressed. The parallel macros, inside the reserved namespace, remain
	defined.
	</p>
	<p>We are slowly phasing out all predefined macros which are outside the
	reserved namespace. You should never use them in new programs, and we
	encourage you to correct older code to use the parallel macros whenever
	you find it. We don’t recommend you use the system-specific macros that
	are in the reserved namespace, either. It is better in the long run to
	check specifically for features you need, using a tool such as
	<code>autoconf</code>.
	</p>
	<hr>
	<div class="header">
	<p>
	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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