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Prototypes and Old-Style Function Definitions

GNU C extends ANSI 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. /*
    #if __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. ANSI 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 ANSI 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 previous 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 the previous example’s extension is irrelevant.