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| <title>Using the GNU Compiler Collection (GCC): Function Prototypes</title> |
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| <a name="Prototypes-and-Old_002dStyle-Function-Definitions"></a> |
| <h3 class="section">6.32 Prototypes and Old-Style Function Definitions</h3> |
| <a name="index-function-prototype-declarations"></a> |
| <a name="index-old_002dstyle-function-definitions"></a> |
| <a name="index-promotion-of-formal-parameters"></a> |
| |
| <p>GNU C extends ISO C to allow a function prototype to override a later |
| old-style non-prototype definition. Consider the following example: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">/* <span class="roman">Use prototypes unless the compiler is old-fashioned.</span> */ |
| #ifdef __STDC__ |
| #define P(x) x |
| #else |
| #define P(x) () |
| #endif |
| |
| /* <span class="roman">Prototype function declaration.</span> */ |
| int isroot P((uid_t)); |
| |
| /* <span class="roman">Old-style function definition.</span> */ |
| int |
| isroot (x) /* <span class="roman">??? lossage here ???</span> */ |
| uid_t x; |
| { |
| return x == 0; |
| } |
| </pre></div> |
| |
| <p>Suppose the type <code>uid_t</code> happens to be <code>short</code>. ISO C does |
| not allow this example, because subword arguments in old-style |
| non-prototype definitions are promoted. Therefore in this example the |
| function definition’s argument is really an <code>int</code>, which does not |
| match the prototype argument type of <code>short</code>. |
| </p> |
| <p>This restriction of ISO C makes it hard to write code that is portable |
| to traditional C compilers, because the programmer does not know |
| whether the <code>uid_t</code> type is <code>short</code>, <code>int</code>, or |
| <code>long</code>. Therefore, in cases like these GNU C allows a prototype |
| to override a later old-style definition. More precisely, in GNU C, a |
| function prototype argument type overrides the argument type specified |
| by a later old-style definition if the former type is the same as the |
| latter type before promotion. Thus in GNU C the above example is |
| equivalent to the following: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">int isroot (uid_t); |
| |
| int |
| isroot (uid_t x) |
| { |
| return x == 0; |
| } |
| </pre></div> |
| |
| <p>GNU C++ does not support old-style function definitions, so this |
| extension is irrelevant. |
| </p> |
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