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="Variable-Length.html#Variable-Length" title="Variable Length"> 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) 1988-2013 Free Software Foundation, Inc. 14 15Permission is granted to copy, distribute and/or modify this document 16under the terms of the GNU Free Documentation License, Version 1.3 or 17any later version published by the Free Software Foundation; with the 18Invariant Sections being ``Funding Free Software'', the Front-Cover 19Texts being (a) (see below), and with the Back-Cover Texts being (b) 20(see below). 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The syntax for 61defining the macro is similar to that of a function. Here is an 62example: 63 64<pre class="smallexample"> #define debug(format, ...) fprintf (stderr, format, __VA_ARGS__) 65</pre> 66 <p class="noindent">Here ‘<samp><span class="samp">...</span></samp>’ is a <dfn>variable argument</dfn>. In the invocation of 67such a macro, it represents the zero or more tokens until the closing 68parenthesis that ends the invocation, including any commas. This set of 69tokens replaces the identifier <code>__VA_ARGS__</code> in the macro body 70wherever it appears. See the CPP manual for more information. 71 72 <p>GCC has long supported variadic macros, and used a different syntax that 73allowed you to give a name to the variable arguments just like any other 74argument. Here is an example: 75 76<pre class="smallexample"> #define debug(format, args...) fprintf (stderr, format, args) 77</pre> 78 <p class="noindent">This is in all ways equivalent to the ISO C example above, but arguably 79more readable and descriptive. 80 81 <p>GNU CPP has two further variadic macro extensions, and permits them to 82be used with either of the above forms of macro definition. 83 84 <p>In standard C, you are not allowed to leave the variable argument out 85entirely; but you are allowed to pass an empty argument. For example, 86this invocation is invalid in ISO C, because there is no comma after 87the string: 88 89<pre class="smallexample"> debug ("A message") 90</pre> 91 <p>GNU CPP permits you to completely omit the variable arguments in this 92way. In the above examples, the compiler would complain, though since 93the expansion of the macro still has the extra comma after the format 94string. 95 96 <p>To help solve this problem, CPP behaves specially for variable arguments 97used with the token paste operator, ‘<samp><span class="samp">##</span></samp>’. If instead you write 98 99<pre class="smallexample"> #define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__) 100</pre> 101 <p class="noindent">and if the variable arguments are omitted or empty, the ‘<samp><span class="samp">##</span></samp>’ 102operator causes the preprocessor to remove the comma before it. If you 103do provide some variable arguments in your macro invocation, GNU CPP 104does not complain about the paste operation and instead places the 105variable arguments after the comma. Just like any other pasted macro 106argument, these arguments are not macro expanded. 107 108 </body></html> 109 110