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.