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| 22 | <title>Variadic Macros (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Macros.html" rel="up" title="Macros">
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| 33 | <link href="Predefined-Macros.html" rel="next" title="Predefined Macros">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Variadic-Macros"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Predefined-Macros.html" accesskey="n" rel="next">Predefined Macros</a>, Previous: <a href="Concatenation.html" accesskey="p" rel="prev">Concatenation</a>, Up: <a href="Macros.html" accesskey="u" rel="up">Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Variadic-Macros-1"></span><h4 class="subsection">26.5.6 Variadic Macros</h4>
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| 64 | <span id="index-variable-number-of-arguments"></span>
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| 65 | <span id="index-macros-with-variable-arguments"></span>
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| 66 | <span id="index-variadic-macros"></span>
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| 67 |
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| 68 | <p>A macro can be declared to accept a variable number of arguments much as
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| 69 | a function can. The syntax for defining the macro is similar to that of
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| 70 | a function. Here is an example:
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| 71 | </p>
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| 72 | <div class="example">
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| 73 | <pre class="example">#define eprintf(…) fprintf (stderr, __VA_ARGS__)
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| 74 | </pre></div>
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| 75 |
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| 76 | <p>This kind of macro is called <em>variadic</em>. When the macro is invoked,
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| 77 | all the tokens in its argument list after the last named argument (this
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| 78 | macro has none), including any commas, become the <em>variable
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| 79 | argument</em>. This sequence of tokens replaces the identifier
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| 80 | <code><span class="nolinebreak">__VA_ARGS__</span><!-- /@w --></code> in the macro body wherever it appears. Thus, we
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| 81 | have this expansion:
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| 82 | </p>
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| 83 | <div class="example">
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| 84 | <pre class="example">eprintf ("%s:%d: ", input_file, lineno)
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| 85 | → fprintf (stderr, "%s:%d: ", input_file, lineno)
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| 86 | </pre></div>
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| 87 |
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| 88 | <p>The variable argument is completely macro-expanded before it is inserted
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| 89 | into the macro expansion, just like an ordinary argument. You may use
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| 90 | the <code>#</code> and <code>##</code> operators to stringify the variable argument
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| 91 | or to paste its leading or trailing token with another token. (But see
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| 92 | below for an important special case for <code>##</code>.)
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| 93 | </p>
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| 94 | <p><strong>Warning:</strong> don’t use the identifier <code><span class="nolinebreak">__VA_ARGS__</span><!-- /@w --></code>
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| 95 | for anything other than this.
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| 96 | </p>
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| 97 | <p>If your macro is complicated, you may want a more descriptive name for
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| 98 | the variable argument than <code><span class="nolinebreak">__VA_ARGS__</span><!-- /@w --></code>. You can write an
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| 99 | argument name immediately before the ‘<samp>…</samp>’; that name is used
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| 100 | for the variable argument.<a id="DOCF9" href="#FOOT9"><sup>9</sup></a> The
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| 101 | <code>eprintf</code> macro above could be written thus:
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| 102 | </p>
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| 103 | <div class="example">
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| 104 | <pre class="example">#define eprintf(args…) fprintf (stderr, args)
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| 105 | </pre></div>
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| 106 |
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| 107 | <p>A variadic macro can have named arguments as well as variable
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| 108 | arguments, so <code>eprintf</code> can be defined like this, instead:
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| 109 | </p>
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| 110 | <div class="example">
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| 111 | <pre class="example">#define eprintf(format, …) \
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| 112 | fprintf (stderr, format, __VA_ARGS__)
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| 113 | </pre></div>
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| 114 |
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| 115 | <p>This formulation is more descriptive, but what if you want to specify
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| 116 | a format string that takes no arguments? In GNU C, you can omit the
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| 117 | comma before the variable arguments if they are empty, but that puts
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| 118 | an extra comma in the expansion:
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| 119 | </p>
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| 120 | <div class="example">
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| 121 | <pre class="example">eprintf ("success!\n")
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| 122 | → fprintf(stderr, "success!\n", );
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| 123 | </pre></div>
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| 124 |
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| 125 | <p>That’s an error in the call to <code>fprintf</code>.
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| 126 | </p>
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| 127 | <p>To get rid of that comma, the <code>##</code> token paste operator has a
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| 128 | special meaning when placed between a comma and a variable
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| 129 | argument.<a id="DOCF10" href="#FOOT10"><sup>10</sup></a> If you write
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| 130 | </p>
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| 131 | <div class="example">
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| 132 | <pre class="example">#define eprintf(format, …) \
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| 133 | fprintf (stderr, format, ##__VA_ARGS__)
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| 134 | </pre></div>
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| 135 |
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| 136 | <p>then use the macro <code>eprintf</code> with empty variable arguments,
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| 137 | <code>##</code> deletes the preceding comma.
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| 138 | </p>
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| 139 | <div class="example">
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| 140 | <pre class="example">eprintf ("success!\n")
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| 141 | → fprintf(stderr, "success!\n");
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| 142 | </pre></div>
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| 143 |
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| 144 | <p>This does <em>not</em> happen if you pass an empty argument, nor does it
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| 145 | happen if the token preceding <code>##</code> is anything other than a
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| 146 | comma.
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| 147 | </p>
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| 148 | <p>When the only macro parameter is a variable arguments parameter, and
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| 149 | the macro call has no argument at all, it is not obvious whether that
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| 150 | means an empty argument or a missing argument. Should the comma be
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| 151 | kept, or deleted? The C standard says to keep the comma, but the
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| 152 | preexisting GNU C extension deleted the comma. Nowadays, GNU C
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| 153 | retains the comma when implementing a specific C standard, and deletes
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| 154 | it otherwise.
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| 155 | </p>
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| 156 | <p>C99 mandates that the only place the identifier <code><span class="nolinebreak">__VA_ARGS__</span><!-- /@w --></code>
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| 157 | can appear is in the replacement list of a variadic macro. It may not
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| 158 | be used as a macro name, macro parameter name, or within a different
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| 159 | type of macro. It may also be forbidden in open text; the standard is
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| 160 | ambiguous. We recommend you avoid using that name except for its
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| 161 | special purpose.
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| 162 | </p>
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| 163 | <p>Variadic macros where you specify the parameter name is a GNU C
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| 164 | feature that has been supported for a long time. Standard C, as of
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| 165 | C99, supports only the form where the parameter is called
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| 166 | <code><span class="nolinebreak">__VA_ARGS__</span><!-- /@w --></code>. For portability to previous versions of GNU C
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| 167 | you should use only named variable argument parameters. On the other
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| 168 | hand, for portability to other C99 compilers, you should use only
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| 169 | <code><span class="nolinebreak">__VA_ARGS__</span><!-- /@w --></code>.
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| 170 | </p>
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| 171 | <div class="footnote">
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| 172 | <hr>
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| 173 | <h4 class="footnotes-heading">Footnotes</h4>
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| 174 |
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| 175 | <h5><a id="FOOT9" href="#DOCF9">(9)</a></h3>
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| 176 | <p>GNU C extension.</p>
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| 177 | <h5><a id="FOOT10" href="#DOCF10">(10)</a></h3>
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| 178 | <p>GNU C extension.</p>
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| 179 | </div>
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| 180 | <hr>
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| 181 | <div class="header">
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| 182 | <p>
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| 183 | Next: <a href="Predefined-Macros.html" accesskey="n" rel="next">Predefined Macros</a>, Previous: <a href="Concatenation.html" accesskey="p" rel="prev">Concatenation</a>, Up: <a href="Macros.html" accesskey="u" rel="up">Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 184 | </div>
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| 185 |
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| 186 |
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| 187 |
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| 188 | </body>
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| 189 | </html>
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