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.