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### 6.31 Prototypes and Old-Style Function Definitions

GNU C extends ISO C to allow a function prototype to override a later old-style non-prototype definition. Consider the following example:

         /* Use prototypes unless the compiler is old-fashioned.  */
         #ifdef __STDC__
         #define P(x) x
         #else
         #define P(x) ()
         #endif

         /* Prototype function declaration.  */
         int isroot P((uid_t));

         /* Old-style function definition.  */
         int
         isroot (x)   /* ??? lossage here ??? */
              uid_t x;
         {
           return x == 0;
         }

Suppose the type `uid_t` happens to be `short`. 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 `int`, which does not match the prototype argument type of `short`.

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 `uid_t` type is `short`, `int`, or `long`. 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:

         int isroot (uid_t);

         int
         isroot (uid_t x)
         {
           return x == 0;
         }

GNU C++ does not support old-style function definitions, so this extension is irrelevant.