<html lang="en">
<head>
<title>System-specific Predefined Macros - The C Preprocessor</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="The C Preprocessor">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="Predefined-Macros.html#Predefined-Macros" title="Predefined Macros">
<link rel="prev" href="Common-Predefined-Macros.html#Common-Predefined-Macros" title="Common Predefined Macros">
<link rel="next" href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators" title="C++ Named Operators">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996,
1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007,
2008, 2009, 2010
Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation.  A copy of
the license is included in the
section entitled ``GNU Free Documentation License''.

This manual contains no Invariant Sections.  The Front-Cover Texts are
(a) (see below), and the Back-Cover Texts are (b) (see below).

(a) The FSF's Front-Cover Text is:

     A GNU Manual

(b) The FSF's Back-Cover Text is:

     You have freedom to copy and modify this GNU Manual, like GNU
     software.  Copies published by the Free Software Foundation raise
     funds for GNU development.
-->
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
  pre.display { font-family:inherit }
  pre.format  { font-family:inherit }
  pre.smalldisplay { font-family:inherit; font-size:smaller }
  pre.smallformat  { font-family:inherit; font-size:smaller }
  pre.smallexample { font-size:smaller }
  pre.smalllisp    { font-size:smaller }
  span.sc    { font-variant:small-caps }
  span.roman { font-family:serif; font-weight:normal; } 
  span.sansserif { font-family:sans-serif; font-weight:normal; } 
--></style>
<link rel="stylesheet" type="text/css" href="../cs.css">
</head>
<body>
<div class="node">
<a name="System-specific-Predefined-Macros"></a>
<a name="System_002dspecific-Predefined-Macros"></a>
<p>
Next:&nbsp;<a rel="next" accesskey="n" href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators">C++ Named Operators</a>,
Previous:&nbsp;<a rel="previous" accesskey="p" href="Common-Predefined-Macros.html#Common-Predefined-Macros">Common Predefined Macros</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="Predefined-Macros.html#Predefined-Macros">Predefined Macros</a>
<hr>
</div>

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

<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>
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
<samp><span class="command">cpp -dM</span></samp> to see them all.  See <a href="Invocation.html#Invocation">Invocation</a>.  All system-specific
predefined macros expand to the constant 1, so you can test them with
either &lsquo;<samp><span class="samp">#ifdef</span></samp>&rsquo; or &lsquo;<samp><span class="samp">#if</span></samp>&rsquo;.

   <p>The C standard requires that all system-specific macros be part of the
<dfn>reserved namespace</dfn>.  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>When the <samp><span class="option">-ansi</span></samp> option, or any <samp><span class="option">-std</span></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>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
<samp><span class="command">autoconf</span></samp>.

   </body></html>