1<html lang="en"> 2<head> 3<title>Traditional 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="Traditional-Mode.html#Traditional-Mode" title="Traditional Mode"> 9<link rel="prev" href="Traditional-lexical-analysis.html#Traditional-lexical-analysis" title="Traditional lexical analysis"> 10<link rel="next" href="Traditional-miscellany.html#Traditional-miscellany" title="Traditional miscellany"> 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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CPP removes 65all leading and trailing horizontal whitespace from a macro's 66replacement text before storing it, but preserves the form of internal 67whitespace. 68 69 <p>One consequence is that it is legitimate for the replacement text to 70contain an unmatched quote (see <a href="Traditional-lexical-analysis.html#Traditional-lexical-analysis">Traditional lexical analysis</a>). An 71unclosed string or character constant continues into the text 72following the macro call. Similarly, the text at the end of a macro's 73expansion can run together with the text after the macro invocation to 74produce a single token. 75 76 <p>Normally comments are removed from the replacement text after the 77macro is expanded, but if the <samp><span class="option">-CC</span></samp> option is passed on the 78command line comments are preserved. (In fact, the current 79implementation removes comments even before saving the macro 80replacement text, but it careful to do it in such a way that the 81observed effect is identical even in the function-like macro case.) 82 83 <p>The ISO stringification operator ‘<samp><span class="samp">#</span></samp>’ and token paste operator 84‘<samp><span class="samp">##</span></samp>’ have no special meaning. As explained later, an effect 85similar to these operators can be obtained in a different way. Macro 86names that are embedded in quotes, either from the main file or after 87macro replacement, do not expand. 88 89 <p>CPP replaces an unquoted object-like macro name with its replacement 90text, and then rescans it for further macros to replace. Unlike 91standard macro expansion, traditional macro expansion has no provision 92to prevent recursion. If an object-like macro appears unquoted in its 93replacement text, it will be replaced again during the rescan pass, 94and so on <em>ad infinitum</em>. GCC detects when it is expanding 95recursive macros, emits an error message, and continues after the 96offending macro invocation. 97 98<pre class="smallexample"> #define PLUS + 99 #define INC(x) PLUS+x 100 INC(foo); 101 ==> ++foo; 102</pre> 103 <p>Function-like macros are similar in form but quite different in 104behavior to their ISO counterparts. Their arguments are contained 105within parentheses, are comma-separated, and can cross physical lines. 106Commas within nested parentheses are not treated as argument 107separators. Similarly, a quote in an argument cannot be left 108unclosed; a following comma or parenthesis that comes before the 109closing quote is treated like any other character. There is no 110facility for handling variadic macros. 111 112 <p>This implementation removes all comments from macro arguments, unless 113the <samp><span class="option">-C</span></samp> option is given. The form of all other horizontal 114whitespace in arguments is preserved, including leading and trailing 115whitespace. In particular 116 117<pre class="smallexample"> f( ) 118</pre> 119 <p class="noindent">is treated as an invocation of the macro ‘<samp><span class="samp">f</span></samp>’ with a single 120argument consisting of a single space. If you want to invoke a 121function-like macro that takes no arguments, you must not leave any 122whitespace between the parentheses. 123 124 <p>If a macro argument crosses a new line, the new line is replaced with 125a space when forming the argument. If the previous line contained an 126unterminated quote, the following line inherits the quoted state. 127 128 <p>Traditional preprocessors replace parameters in the replacement text 129with their arguments regardless of whether the parameters are within 130quotes or not. This provides a way to stringize arguments. For 131example 132 133<pre class="smallexample"> #define str(x) "x" 134 str(/* <span class="roman">A comment</span> */some text ) 135 ==> "some text " 136</pre> 137 <p class="noindent">Note that the comment is removed, but that the trailing space is 138preserved. Here is an example of using a comment to effect token 139pasting. 140 141<pre class="smallexample"> #define suffix(x) foo_/**/x 142 suffix(bar) 143 ==> foo_bar 144</pre> 145 </body></html> 146 147