151 lines
6.8 KiB
HTML
151 lines
6.8 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2016 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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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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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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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Using the GNU Compiler Collection (GCC): Variadic Macros</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC): Variadic Macros">
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<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Variadic Macros">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
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<link href="Escaped-Newlines.html#Escaped-Newlines" rel="next" title="Escaped Newlines">
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<link href="Variable-Length.html#Variable-Length" rel="prev" title="Variable Length">
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Variadic-Macros"></a>
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<div class="header">
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<p>
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Next: <a href="Escaped-Newlines.html#Escaped-Newlines" accesskey="n" rel="next">Escaped Newlines</a>, Previous: <a href="Variable-Length.html#Variable-Length" accesskey="p" rel="prev">Variable Length</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Macros-with-a-Variable-Number-of-Arguments_002e"></a>
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<h3 class="section">6.20 Macros with a Variable Number of Arguments.</h3>
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<a name="index-variable-number-of-arguments"></a>
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<a name="index-macro-with-variable-arguments"></a>
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<a name="index-rest-argument-_0028in-macro_0029"></a>
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<a name="index-variadic-macros"></a>
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<p>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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</p>
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<div class="smallexample">
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<pre class="smallexample">#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)
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</pre></div>
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<p>Here ‘<samp>…</samp>’ is a <em>variable argument</em>. 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>
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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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</p>
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<div class="smallexample">
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<pre class="smallexample">#define debug(format, args...) fprintf (stderr, format, args)
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</pre></div>
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<p>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>
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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>
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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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</p>
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<div class="smallexample">
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<pre class="smallexample">debug ("A message")
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</pre></div>
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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>
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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>##</samp>’. If instead you write
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)
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</pre></div>
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<p>and if the variable arguments are omitted or empty, the ‘<samp>##</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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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Escaped-Newlines.html#Escaped-Newlines" accesskey="n" rel="next">Escaped Newlines</a>, Previous: <a href="Variable-Length.html#Variable-Length" accesskey="p" rel="prev">Variable Length</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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