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109 lines
5.1 KiB
HTML
109 lines
5.1 KiB
HTML
4 years ago
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<html lang="en">
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<title>Variadic Macros - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Variable-Length.html#Variable-Length" title="Variable Length">
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<link rel="next" href="Escaped-Newlines.html#Escaped-Newlines" title="Escaped Newlines">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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</head>
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<body>
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<div class="node">
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<a name="Variadic-Macros"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Escaped-Newlines.html#Escaped-Newlines">Escaped Newlines</a>,
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Previous: <a rel="previous" accesskey="p" href="Variable-Length.html#Variable-Length">Variable Length</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.20 Macros with a Variable Number of Arguments.</h3>
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<p><a name="index-variable-number-of-arguments-2998"></a><a name="index-macro-with-variable-arguments-2999"></a><a name="index-rest-argument-_0028in-macro_0029-3000"></a><a name="index-variadic-macros-3001"></a>
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In the ISO C standard of 1999, a macro can be declared to accept a
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variable number of arguments much as a function can. The syntax for
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defining the macro is similar to that of a function. Here is an
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example:
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<pre class="smallexample"> #define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)
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</pre>
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<p class="noindent">Here ‘<samp><span class="samp">...</span></samp>’ is a <dfn>variable argument</dfn>. In the invocation of
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such a macro, it represents the zero or more tokens until the closing
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parenthesis that ends the invocation, including any commas. This set of
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tokens replaces the identifier <code>__VA_ARGS__</code> in the macro body
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wherever it appears. See the CPP manual for more information.
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<p>GCC has long supported variadic macros, and used a different syntax that
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allowed you to give a name to the variable arguments just like any other
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argument. Here is an example:
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<pre class="smallexample"> #define debug(format, args...) fprintf (stderr, format, args)
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</pre>
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<p class="noindent">This is in all ways equivalent to the ISO C example above, but arguably
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more readable and descriptive.
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<p>GNU CPP has two further variadic macro extensions, and permits them to
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be used with either of the above forms of macro definition.
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<p>In standard C, you are not allowed to leave the variable argument out
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entirely; but you are allowed to pass an empty argument. For example,
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this invocation is invalid in ISO C, because there is no comma after
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the string:
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<pre class="smallexample"> debug ("A message")
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</pre>
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<p>GNU CPP permits you to completely omit the variable arguments in this
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way. In the above examples, the compiler would complain, though since
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the expansion of the macro still has the extra comma after the format
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string.
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<p>To help solve this problem, CPP behaves specially for variable arguments
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used with the token paste operator, ‘<samp><span class="samp">##</span></samp>’. If instead you write
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<pre class="smallexample"> #define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)
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</pre>
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<p class="noindent">and if the variable arguments are omitted or empty, the ‘<samp><span class="samp">##</span></samp>’
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operator causes the preprocessor to remove the comma before it. If you
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do provide some variable arguments in your macro invocation, GNU CPP
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does not complain about the paste operation and instead places the
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variable arguments after the comma. Just like any other pasted macro
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argument, these arguments are not macro expanded.
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</body></html>
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