Function Prototypes (Using the GNU Compiler Collection (GCC)) (original) (raw)

6.40 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, orlong. 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.