1<html lang="en"> 2<head> 3<title>System-specific Predefined Macros - The C Preprocessor</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="The C Preprocessor"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="Predefined-Macros.html#Predefined-Macros" title="Predefined Macros"> 9<link rel="prev" href="Common-Predefined-Macros.html#Common-Predefined-Macros" title="Common Predefined Macros"> 10<link rel="next" href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators" title="C++ Named Operators"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 141997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 152008, 2009, 2010, 2011 16Free Software Foundation, Inc. 17 18Permission is granted to copy, distribute and/or modify this document 19under the terms of the GNU Free Documentation License, Version 1.3 or 20any later version published by the Free Software Foundation. A copy of 21the license is included in the 22section entitled ``GNU Free Documentation License''. 23 24This manual contains no Invariant Sections. The Front-Cover Texts are 25(a) (see below), and the Back-Cover Texts are (b) (see below). 26 27(a) The FSF's Front-Cover Text is: 28 29 A GNU Manual 30 31(b) The FSF's Back-Cover Text is: 32 33 You have freedom to copy and modify this GNU Manual, like GNU 34 software. Copies published by the Free Software Foundation raise 35 funds for GNU development. 36--> 37<meta http-equiv="Content-Style-Type" content="text/css"> 38<style type="text/css"><!-- 39 pre.display { font-family:inherit } 40 pre.format { font-family:inherit } 41 pre.smalldisplay { font-family:inherit; font-size:smaller } 42 pre.smallformat { font-family:inherit; font-size:smaller } 43 pre.smallexample { font-size:smaller } 44 pre.smalllisp { font-size:smaller } 45 span.sc { font-variant:small-caps } 46 span.roman { font-family:serif; font-weight:normal; } 47 span.sansserif { font-family:sans-serif; font-weight:normal; } 48--></style> 49<link rel="stylesheet" type="text/css" href="../cs.css"> 50</head> 51<body> 52<div class="node"> 53<a name="System-specific-Predefined-Macros"></a> 54<a name="System_002dspecific-Predefined-Macros"></a> 55<p> 56Next: <a rel="next" accesskey="n" href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators">C++ Named Operators</a>, 57Previous: <a rel="previous" accesskey="p" href="Common-Predefined-Macros.html#Common-Predefined-Macros">Common Predefined Macros</a>, 58Up: <a rel="up" accesskey="u" href="Predefined-Macros.html#Predefined-Macros">Predefined Macros</a> 59<hr> 60</div> 61 62<h4 class="subsection">3.7.3 System-specific Predefined Macros</h4> 63 64<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> 65The C preprocessor normally predefines several macros that indicate what 66type of system and machine is in use. They are obviously different on 67each target supported by GCC. This manual, being for all systems and 68machines, cannot tell you what their names are, but you can use 69<samp><span class="command">cpp -dM</span></samp> to see them all. See <a href="Invocation.html#Invocation">Invocation</a>. All system-specific 70predefined macros expand to the constant 1, so you can test them with 71either ‘<samp><span class="samp">#ifdef</span></samp>’ or ‘<samp><span class="samp">#if</span></samp>’. 72 73 <p>The C standard requires that all system-specific macros be part of the 74<dfn>reserved namespace</dfn>. All names which begin with two underscores, 75or an underscore and a capital letter, are reserved for the compiler and 76library to use as they wish. However, historically system-specific 77macros have had names with no special prefix; for instance, it is common 78to find <code>unix</code> defined on Unix systems. For all such macros, GCC 79provides a parallel macro with two underscores added at the beginning 80and the end. If <code>unix</code> is defined, <code>__unix__</code> will be defined 81too. There will never be more than two underscores; the parallel of 82<code>_mips</code> is <code>__mips__</code>. 83 84 <p>When the <samp><span class="option">-ansi</span></samp> option, or any <samp><span class="option">-std</span></samp> option that 85requests strict conformance, is given to the compiler, all the 86system-specific predefined macros outside the reserved namespace are 87suppressed. The parallel macros, inside the reserved namespace, remain 88defined. 89 90 <p>We are slowly phasing out all predefined macros which are outside the 91reserved namespace. You should never use them in new programs, and we 92encourage you to correct older code to use the parallel macros whenever 93you find it. We don't recommend you use the system-specific macros that 94are in the reserved namespace, either. It is better in the long run to 95check specifically for features you need, using a tool such as 96<samp><span class="command">autoconf</span></samp>. 97 98 </body></html> 99 100