Using the GNU Compiler Collection (GCC): Function Prototypes

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### 6.32 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.