1<html lang="en"> 2<head> 3<title>Variadic Macros - Using the GNU Compiler Collection (GCC)</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Using the GNU Compiler Collection (GCC)"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions"> 9<link rel="prev" href="Empty-Structures.html#Empty-Structures" title="Empty Structures"> 10<link rel="next" href="Escaped-Newlines.html#Escaped-Newlines" title="Escaped Newlines"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 141998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 152010 Free 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; with the 20Invariant Sections being ``Funding Free Software'', the Front-Cover 21Texts being (a) (see below), and with the Back-Cover Texts being (b) 22(see below). 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The syntax for 63defining the macro is similar to that of a function. Here is an 64example: 65 66<pre class="smallexample"> #define debug(format, ...) fprintf (stderr, format, __VA_ARGS__) 67</pre> 68 <p>Here ‘<samp><span class="samp">...</span></samp>’ is a <dfn>variable argument</dfn>. In the invocation of 69such a macro, it represents the zero or more tokens until the closing 70parenthesis that ends the invocation, including any commas. This set of 71tokens replaces the identifier <code>__VA_ARGS__</code> in the macro body 72wherever it appears. See the CPP manual for more information. 73 74 <p>GCC has long supported variadic macros, and used a different syntax that 75allowed you to give a name to the variable arguments just like any other 76argument. Here is an example: 77 78<pre class="smallexample"> #define debug(format, args...) fprintf (stderr, format, args) 79</pre> 80 <p>This is in all ways equivalent to the ISO C example above, but arguably 81more readable and descriptive. 82 83 <p>GNU CPP has two further variadic macro extensions, and permits them to 84be used with either of the above forms of macro definition. 85 86 <p>In standard C, you are not allowed to leave the variable argument out 87entirely; but you are allowed to pass an empty argument. For example, 88this invocation is invalid in ISO C, because there is no comma after 89the string: 90 91<pre class="smallexample"> debug ("A message") 92</pre> 93 <p>GNU CPP permits you to completely omit the variable arguments in this 94way. In the above examples, the compiler would complain, though since 95the expansion of the macro still has the extra comma after the format 96string. 97 98 <p>To help solve this problem, CPP behaves specially for variable arguments 99used with the token paste operator, ‘<samp><span class="samp">##</span></samp>’. If instead you write 100 101<pre class="smallexample"> #define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__) 102</pre> 103 <p>and if the variable arguments are omitted or empty, the ‘<samp><span class="samp">##</span></samp>’ 104operator causes the preprocessor to remove the comma before it. If you 105do provide some variable arguments in your macro invocation, GNU CPP 106does not complain about the paste operation and instead places the 107variable arguments after the comma. Just like any other pasted macro 108argument, these arguments are not macro expanded. 109 110 </body></html> 111 112