1<html lang="en"> 2<head> 3<title>Undefining and Redefining 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="Macros.html#Macros" title="Macros"> 9<link rel="prev" href="Predefined-Macros.html#Predefined-Macros" title="Predefined Macros"> 10<link rel="next" href="Directives-Within-Macro-Arguments.html#Directives-Within-Macro-Arguments" title="Directives Within Macro Arguments"> 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. 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You use the bare macro name, even if the 67macro is function-like. It is an error if anything appears on the line 68after the macro name. ‘<samp><span class="samp">#undef</span></samp>’ has no effect if the name is not a 69macro. 70 71<pre class="smallexample"> #define FOO 4 72 x = FOO; ==> x = 4; 73 #undef FOO 74 x = FOO; ==> x = FOO; 75</pre> 76 <p>Once a macro has been undefined, that identifier may be <dfn>redefined</dfn> 77as a macro by a subsequent ‘<samp><span class="samp">#define</span></samp>’ directive. The new definition 78need not have any resemblance to the old definition. 79 80 <p>However, if an identifier which is currently a macro is redefined, then 81the new definition must be <dfn>effectively the same</dfn> as the old one. 82Two macro definitions are effectively the same if: 83 <ul> 84<li>Both are the same type of macro (object- or function-like). 85<li>All the tokens of the replacement list are the same. 86<li>If there are any parameters, they are the same. 87<li>Whitespace appears in the same places in both. It need not be 88exactly the same amount of whitespace, though. Remember that comments 89count as whitespace. 90</ul> 91 92<p class="noindent">These definitions are effectively the same: 93<pre class="smallexample"> #define FOUR (2 + 2) 94 #define FOUR (2 + 2) 95 #define FOUR (2 /* <span class="roman">two</span> */ + 2) 96</pre> 97 <p class="noindent">but these are not: 98<pre class="smallexample"> #define FOUR (2 + 2) 99 #define FOUR ( 2+2 ) 100 #define FOUR (2 * 2) 101 #define FOUR(score,and,seven,years,ago) (2 + 2) 102</pre> 103 <p>If a macro is redefined with a definition that is not effectively the 104same as the old one, the preprocessor issues a warning and changes the 105macro to use the new definition. If the new definition is effectively 106the same, the redefinition is silently ignored. This allows, for 107instance, two different headers to define a common macro. The 108preprocessor will only complain if the definitions do not match. 109 110 </body></html> 111 112