151 lines
6.8 KiB
HTML
151 lines
6.8 KiB
HTML
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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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<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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<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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