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| <h3 class="section">3.6 Variadic Macros</h3> |
| |
| <p><a name="index-variable-number-of-arguments-56"></a><a name="index-macros-with-variable-arguments-57"></a><a name="index-variadic-macros-58"></a> |
| 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 eprintf(...) fprintf (stderr, __VA_ARGS__) |
| </pre> |
| <p>This kind of macro is called <dfn>variadic</dfn>. When the macro is invoked, |
| all the tokens in its argument list after the last named argument (this |
| macro has none), including any commas, become the <dfn>variable |
| argument</dfn>. This sequence of tokens replaces the identifier |
| <code>__VA_ARGS__<!-- /@w --></code> in the macro body wherever it appears. Thus, we |
| have this expansion: |
| |
| <pre class="smallexample"> eprintf ("%s:%d: ", input_file, lineno) |
| ==> fprintf (stderr, "%s:%d: ", input_file, lineno) |
| </pre> |
| <p>The variable argument is completely macro-expanded before it is inserted |
| into the macro expansion, just like an ordinary argument. You may use |
| the ‘<samp><span class="samp">#</span></samp>’ and ‘<samp><span class="samp">##</span></samp>’ operators to stringify the variable argument |
| or to paste its leading or trailing token with another token. (But see |
| below for an important special case for ‘<samp><span class="samp">##</span></samp>’.) |
| |
| <p>If your macro is complicated, you may want a more descriptive name for |
| the variable argument than <code>__VA_ARGS__<!-- /@w --></code>. CPP permits |
| this, as an extension. You may write an argument name immediately |
| before the ‘<samp><span class="samp">...</span></samp>’; that name is used for the variable argument. |
| The <code>eprintf</code> macro above could be written |
| |
| <pre class="smallexample"> #define eprintf(args...) fprintf (stderr, args) |
| </pre> |
| <p class="noindent">using this extension. You cannot use <code>__VA_ARGS__<!-- /@w --></code> and this |
| extension in the same macro. |
| |
| <p>You can have named arguments as well as variable arguments in a variadic |
| macro. We could define <code>eprintf</code> like this, instead: |
| |
| <pre class="smallexample"> #define eprintf(format, ...) fprintf (stderr, format, __VA_ARGS__) |
| </pre> |
| <p class="noindent">This formulation looks more descriptive, but unfortunately it is less |
| flexible: you must now supply at least one argument after the format |
| string. In standard C, you cannot omit the comma separating the named |
| argument from the variable arguments. Furthermore, if you leave the |
| variable argument empty, you will get a syntax error, because |
| there will be an extra comma after the format string. |
| |
| <pre class="smallexample"> eprintf("success!\n", ); |
| ==> fprintf(stderr, "success!\n", ); |
| </pre> |
| <p>GNU CPP has a pair of extensions which deal with this problem. First, |
| you are allowed to leave the variable argument out entirely: |
| |
| <pre class="smallexample"> eprintf ("success!\n") |
| ==> fprintf(stderr, "success!\n", ); |
| </pre> |
| <p class="noindent">Second, the ‘<samp><span class="samp">##</span></samp>’ token paste operator has a special meaning when |
| placed between a comma and a variable argument. If you write |
| |
| <pre class="smallexample"> #define eprintf(format, ...) fprintf (stderr, format, ##__VA_ARGS__) |
| </pre> |
| <p class="noindent">and the variable argument is left out when the <code>eprintf</code> macro is |
| used, then the comma before the ‘<samp><span class="samp">##</span></samp>’ will be deleted. This does |
| <em>not</em> happen if you pass an empty argument, nor does it happen if |
| the token preceding ‘<samp><span class="samp">##</span></samp>’ is anything other than a comma. |
| |
| <pre class="smallexample"> eprintf ("success!\n") |
| ==> fprintf(stderr, "success!\n"); |
| </pre> |
| <p class="noindent">The above explanation is ambiguous about the case where the only macro |
| parameter is a variable arguments parameter, as it is meaningless to |
| try to distinguish whether no argument at all is an empty argument or |
| a missing argument. In this case the C99 standard is clear that the |
| comma must remain, however the existing GCC extension used to swallow |
| the comma. So CPP retains the comma when conforming to a specific C |
| standard, and drops it otherwise. |
| |
| <p>C99 mandates that the only place the identifier <code>__VA_ARGS__<!-- /@w --></code> |
| can appear is in the replacement list of a variadic macro. It may not |
| be used as a macro name, macro argument name, or within a different type |
| of macro. It may also be forbidden in open text; the standard is |
| ambiguous. We recommend you avoid using it except for its defined |
| purpose. |
| |
| <p>Variadic macros are a new feature in C99. GNU CPP has supported them |
| for a long time, but only with a named variable argument |
| (‘<samp><span class="samp">args...</span></samp>’, not ‘<samp><span class="samp">...</span></samp>’ and <code>__VA_ARGS__<!-- /@w --></code>). If you are |
| concerned with portability to previous versions of GCC, you should use |
| only named variable arguments. On the other hand, if you are concerned |
| with portability to other conforming implementations of C99, you should |
| use only <code>__VA_ARGS__<!-- /@w --></code>. |
| |
| <p>Previous versions of CPP implemented the comma-deletion extension |
| much more generally. We have restricted it in this release to minimize |
| the differences from C99. To get the same effect with both this and |
| previous versions of GCC, the token preceding the special ‘<samp><span class="samp">##</span></samp>’ must |
| be a comma, and there must be white space between that comma and |
| whatever comes immediately before it: |
| |
| <pre class="smallexample"> #define eprintf(format, args...) fprintf (stderr, format , ##args) |
| </pre> |
| <p class="noindent">See <a href="Differences-from-previous-versions.html#Differences-from-previous-versions">Differences from previous versions</a>, for the gory details. |
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