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 <a name="Variadic-Macros"></a>
 <div class="header">
 <p>
 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> &nbsp; [<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>
 </div>
 <hr>
 <a name="Macros-with-a-Variable-Number-of-Arguments_002e"></a>
 <h3 class="section">6.20 Macros with a Variable Number of Arguments.</h3>
 <a name="index-variable-number-of-arguments"></a>
 <a name="index-macro-with-variable-arguments"></a>
 <a name="index-rest-argument-_0028in-macro_0029"></a>
 <a name="index-variadic-macros"></a>

 <p>In the ISO C standard of 1999, a macro can be declared to accept a
 variable number of arguments much as a function can.  The syntax for
 defining the macro is similar to that of a function.  Here is an
 example:
 </p>
 <div class="smallexample">
 <pre class="smallexample">#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)
 </pre></div>

 <p>Here &lsquo;<samp>&hellip;</samp>&rsquo; is a <em>variable argument</em>.  In the invocation of
 such a macro, it represents the zero or more tokens until the closing
 parenthesis that ends the invocation, including any commas.  This set of
 tokens replaces the identifier <code>__VA_ARGS__</code> in the macro body
 wherever it appears.  See the CPP manual for more information.
 </p>
 <p>GCC has long supported variadic macros, and used a different syntax that
 allowed you to give a name to the variable arguments just like any other
 argument.  Here is an example:
 </p>
 <div class="smallexample">
 <pre class="smallexample">#define debug(format, args...) fprintf (stderr, format, args)
 </pre></div>

 <p>This is in all ways equivalent to the ISO C example above, but arguably
 more readable and descriptive.
 </p>
 <p>GNU CPP has two further variadic macro extensions, and permits them to
 be used with either of the above forms of macro definition.
 </p>
 <p>In standard C, you are not allowed to leave the variable argument out
 entirely; but you are allowed to pass an empty argument.  For example,
 this invocation is invalid in ISO C, because there is no comma after
 the string:
 </p>
 <div class="smallexample">
 <pre class="smallexample">debug (&quot;A message&quot;)
 </pre></div>

 <p>GNU CPP permits you to completely omit the variable arguments in this
 way.  In the above examples, the compiler would complain, though since
 the expansion of the macro still has the extra comma after the format
 string.
 </p>
 <p>To help solve this problem, CPP behaves specially for variable arguments
 used with the token paste operator, &lsquo;<samp>##</samp>&rsquo;.  If instead you write
 </p>
 <div class="smallexample">
 <pre class="smallexample">#define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)
 </pre></div>

 <p>and if the variable arguments are omitted or empty, the &lsquo;<samp>##</samp>&rsquo;
 operator causes the preprocessor to remove the comma before it.  If you
 do provide some variable arguments in your macro invocation, GNU CPP
 does not complain about the paste operation and instead places the
 variable arguments after the comma.  Just like any other pasted macro
 argument, these arguments are not macro expanded.
 </p>
 <hr>
 <div class="header">
 <p>
 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> &nbsp; [<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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	<title>Using the GNU Compiler Collection (GCC): Variadic Macros</title>

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	<a name="Variadic-Macros"></a>
	<div class="header">
	<p>
	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>
	</div>
	<hr>
	<a name="Macros-with-a-Variable-Number-of-Arguments_002e"></a>
	<h3 class="section">6.20 Macros with a Variable Number of Arguments.</h3>
	<a name="index-variable-number-of-arguments"></a>
	<a name="index-macro-with-variable-arguments"></a>
	<a name="index-rest-argument-_0028in-macro_0029"></a>
	<a name="index-variadic-macros"></a>

	<p>In the ISO C standard of 1999, a macro can be declared to accept a
	variable number of arguments much as a function can. The syntax for
	defining the macro is similar to that of a function. Here is an
	example:
	</p>
	<div class="smallexample">
	<pre class="smallexample">#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)
	</pre></div>

	<p>Here ‘<samp>…</samp>’ is a <em>variable argument</em>. In the invocation of
	such a macro, it represents the zero or more tokens until the closing
	parenthesis that ends the invocation, including any commas. This set of
	tokens replaces the identifier <code>__VA_ARGS__</code> in the macro body
	wherever it appears. See the CPP manual for more information.
	</p>
	<p>GCC has long supported variadic macros, and used a different syntax that
	allowed you to give a name to the variable arguments just like any other
	argument. Here is an example:
	</p>
	<div class="smallexample">
	<pre class="smallexample">#define debug(format, args...) fprintf (stderr, format, args)
	</pre></div>

	<p>This is in all ways equivalent to the ISO C example above, but arguably
	more readable and descriptive.
	</p>
	<p>GNU CPP has two further variadic macro extensions, and permits them to
	be used with either of the above forms of macro definition.
	</p>
	<p>In standard C, you are not allowed to leave the variable argument out
	entirely; but you are allowed to pass an empty argument. For example,
	this invocation is invalid in ISO C, because there is no comma after
	the string:
	</p>
	<div class="smallexample">
	<pre class="smallexample">debug ("A message")
	</pre></div>

	<p>GNU CPP permits you to completely omit the variable arguments in this
	way. In the above examples, the compiler would complain, though since
	the expansion of the macro still has the extra comma after the format
	string.
	</p>
	<p>To help solve this problem, CPP behaves specially for variable arguments
	used with the token paste operator, ‘<samp>##</samp>’. If instead you write
	</p>
	<div class="smallexample">
	<pre class="smallexample">#define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)
	</pre></div>

	<p>and if the variable arguments are omitted or empty, the ‘<samp>##</samp>’
	operator causes the preprocessor to remove the comma before it. If you
	do provide some variable arguments in your macro invocation, GNU CPP
	does not complain about the paste operation and instead places the
	variable arguments after the comma. Just like any other pasted macro
	argument, these arguments are not macro expanded.
	</p>
	<hr>
	<div class="header">
	<p>
	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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