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| <h3 class="section">6.19 Macros with a Variable Number of Arguments.</h3> |
| |
| <p><a name="index-variable-number-of-arguments-2298"></a><a name="index-macro-with-variable-arguments-2299"></a><a name="index-rest-argument-_0028in-macro_0029-2300"></a><a name="index-variadic-macros-2301"></a> |
| 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: |
| |
| <pre class="smallexample"> #define debug(format, ...) fprintf (stderr, format, __VA_ARGS__) |
| </pre> |
| <p>Here ‘<samp><span class="samp">...</span></samp>’ is a <dfn>variable argument</dfn>. 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>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: |
| |
| <pre class="smallexample"> #define debug(format, args...) fprintf (stderr, format, args) |
| </pre> |
| <p>This is in all ways equivalent to the ISO C example above, but arguably |
| more readable and descriptive. |
| |
| <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>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: |
| |
| <pre class="smallexample"> debug ("A message") |
| </pre> |
| <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>To help solve this problem, CPP behaves specially for variable arguments |
| used with the token paste operator, ‘<samp><span class="samp">##</span></samp>’. If instead you write |
| |
| <pre class="smallexample"> #define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__) |
| </pre> |
| <p>and if the variable arguments are omitted or empty, the ‘<samp><span class="samp">##</span></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. |
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