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<title>Using the GNU Compiler Collection (GCC): Function Prototypes</title>
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<a name="Function-Prototypes"></a>
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<p>
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Next: <a href="C_002b_002b-Comments.html#C_002b_002b-Comments" accesskey="n" rel="next">C++ Comments</a>, Previous: <a href="Attribute-Syntax.html#Attribute-Syntax" accesskey="p" rel="prev">Attribute Syntax</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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<a name="Prototypes-and-Old_002dStyle-Function-Definitions"></a>
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<h3 class="section">6.37 Prototypes and Old-Style Function Definitions</h3>
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<a name="index-function-prototype-declarations"></a>
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<a name="index-old_002dstyle-function-definitions"></a>
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<a name="index-promotion-of-formal-parameters"></a>
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<p>GNU C extends ISO C to allow a function prototype to override a later
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old-style non-prototype definition. Consider the following example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">/* <span class="roman">Use prototypes unless the compiler is old-fashioned.</span> */
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#ifdef __STDC__
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#define P(x) x
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#else
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#define P(x) ()
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#endif
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/* <span class="roman">Prototype function declaration.</span> */
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int isroot P((uid_t));
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/* <span class="roman">Old-style function definition.</span> */
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int
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isroot (x) /* <span class="roman">??? lossage here ???</span> */
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uid_t x;
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{
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return x == 0;
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}
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</pre></div>
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<p>Suppose the type <code>uid_t</code> happens to be <code>short</code>. ISO C does
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not allow this example, because subword arguments in old-style
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non-prototype definitions are promoted. Therefore in this example the
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function definition’s argument is really an <code>int</code>, which does not
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match the prototype argument type of <code>short</code>.
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</p>
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<p>This restriction of ISO C makes it hard to write code that is portable
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to traditional C compilers, because the programmer does not know
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whether the <code>uid_t</code> type is <code>short</code>, <code>int</code>, or
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<code>long</code>. Therefore, in cases like these GNU C allows a prototype
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to override a later old-style definition. More precisely, in GNU C, a
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function prototype argument type overrides the argument type specified
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by a later old-style definition if the former type is the same as the
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latter type before promotion. Thus in GNU C the above example is
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equivalent to the following:
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</p>
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<div class="smallexample">
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<pre class="smallexample">int isroot (uid_t);
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int
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isroot (uid_t x)
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{
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return x == 0;
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}
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</pre></div>
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<p>GNU C++ does not support old-style function definitions, so this
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extension is irrelevant.
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</p>
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