1<html lang="en"> 2<head> 3<title>Ifdef - 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="Conditional-Syntax.html#Conditional-Syntax" title="Conditional Syntax"> 9<link rel="next" href="If.html#If" title="If"> 10<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 11<!-- 12Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 131997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 142008, 2009, 2010, 2011 15Free Software Foundation, Inc. 16 17Permission is granted to copy, distribute and/or modify this document 18under the terms of the GNU Free Documentation License, Version 1.3 or 19any later version published by the Free Software Foundation. A copy of 20the license is included in the 21section entitled ``GNU Free Documentation License''. 22 23This manual contains no Invariant Sections. The Front-Cover Texts are 24(a) (see below), and the Back-Cover Texts are (b) (see below). 25 26(a) The FSF's Front-Cover Text is: 27 28 A GNU Manual 29 30(b) The FSF's Back-Cover Text is: 31 32 You have freedom to copy and modify this GNU Manual, like GNU 33 software. Copies published by the Free Software Foundation raise 34 funds for GNU development. 35--> 36<meta http-equiv="Content-Style-Type" content="text/css"> 37<style type="text/css"><!-- 38 pre.display { font-family:inherit } 39 pre.format { font-family:inherit } 40 pre.smalldisplay { font-family:inherit; font-size:smaller } 41 pre.smallformat { font-family:inherit; font-size:smaller } 42 pre.smallexample { font-size:smaller } 43 pre.smalllisp { font-size:smaller } 44 span.sc { font-variant:small-caps } 45 span.roman { font-family:serif; font-weight:normal; } 46 span.sansserif { font-family:sans-serif; font-weight:normal; } 47--></style> 48<link rel="stylesheet" type="text/css" href="../cs.css"> 49</head> 50<body> 51<div class="node"> 52<a name="Ifdef"></a> 53<p> 54Next: <a rel="next" accesskey="n" href="If.html#If">If</a>, 55Up: <a rel="up" accesskey="u" href="Conditional-Syntax.html#Conditional-Syntax">Conditional Syntax</a> 56<hr> 57</div> 58 59<h4 class="subsection">4.2.1 Ifdef</h4> 60 61<p><a name="index-g_t_0023ifdef-85"></a><a name="index-g_t_0023endif-86"></a> 62The simplest sort of conditional is 63 64<pre class="smallexample"> #ifdef <var>MACRO</var> 65 66 <var>controlled text</var> 67 68 #endif /* <var>MACRO</var> */ 69</pre> 70 <p><a name="index-conditional-group-87"></a>This block is called a <dfn>conditional group</dfn>. <var>controlled text</var> 71will be included in the output of the preprocessor if and only if 72<var>MACRO</var> is defined. We say that the conditional <dfn>succeeds</dfn> if 73<var>MACRO</var> is defined, <dfn>fails</dfn> if it is not. 74 75 <p>The <var>controlled text</var> inside of a conditional can include 76preprocessing directives. They are executed only if the conditional 77succeeds. You can nest conditional groups inside other conditional 78groups, but they must be completely nested. In other words, 79‘<samp><span class="samp">#endif</span></samp>’ always matches the nearest ‘<samp><span class="samp">#ifdef</span></samp>’ (or 80‘<samp><span class="samp">#ifndef</span></samp>’, or ‘<samp><span class="samp">#if</span></samp>’). Also, you cannot start a conditional 81group in one file and end it in another. 82 83 <p>Even if a conditional fails, the <var>controlled text</var> inside it is 84still run through initial transformations and tokenization. Therefore, 85it must all be lexically valid C. Normally the only way this matters is 86that all comments and string literals inside a failing conditional group 87must still be properly ended. 88 89 <p>The comment following the ‘<samp><span class="samp">#endif</span></samp>’ is not required, but it is a 90good practice if there is a lot of <var>controlled text</var>, because it 91helps people match the ‘<samp><span class="samp">#endif</span></samp>’ to the corresponding ‘<samp><span class="samp">#ifdef</span></samp>’. 92Older programs sometimes put <var>MACRO</var> directly after the 93‘<samp><span class="samp">#endif</span></samp>’ without enclosing it in a comment. This is invalid code 94according to the C standard. CPP accepts it with a warning. It 95never affects which ‘<samp><span class="samp">#ifndef</span></samp>’ the ‘<samp><span class="samp">#endif</span></samp>’ matches. 96 97 <p><a name="index-g_t_0023ifndef-88"></a>Sometimes you wish to use some code if a macro is <em>not</em> defined. 98You can do this by writing ‘<samp><span class="samp">#ifndef</span></samp>’ instead of ‘<samp><span class="samp">#ifdef</span></samp>’. 99One common use of ‘<samp><span class="samp">#ifndef</span></samp>’ is to include code only the first 100time a header file is included. See <a href="Once_002dOnly-Headers.html#Once_002dOnly-Headers">Once-Only Headers</a>. 101 102 <p>Macro definitions can vary between compilations for several reasons. 103Here are some samples. 104 105 <ul> 106<li>Some macros are predefined on each kind of machine 107(see <a href="System_002dspecific-Predefined-Macros.html#System_002dspecific-Predefined-Macros">System-specific Predefined Macros</a>). This allows you to provide 108code specially tuned for a particular machine. 109 110 <li>System header files define more macros, associated with the features 111they implement. You can test these macros with conditionals to avoid 112using a system feature on a machine where it is not implemented. 113 114 <li>Macros can be defined or undefined with the <samp><span class="option">-D</span></samp> and <samp><span class="option">-U</span></samp> 115command line options when you compile the program. You can arrange to 116compile the same source file into two different programs by choosing a 117macro name to specify which program you want, writing conditionals to 118test whether or how this macro is defined, and then controlling the 119state of the macro with command line options, perhaps set in the 120Makefile. See <a href="Invocation.html#Invocation">Invocation</a>. 121 122 <li>Your program might have a special header file (often called 123<samp><span class="file">config.h</span></samp>) that is adjusted when the program is compiled. It can 124define or not define macros depending on the features of the system and 125the desired capabilities of the program. The adjustment can be 126automated by a tool such as <samp><span class="command">autoconf</span></samp>, or done by hand. 127</ul> 128 129 </body></html> 130 131