Variable argument lists.

[Chris Torek]

In article <14139 at brl-adm.ARPA> bates%falcon.dnet%fermat at bru.mayo.edu
(Cary Bates) writes:
>Does anybody know (or care) why in ANSI standard C when 
>using a variable length argument list, there is no way to 
>determine how many arguments where passed into the function?

Simple: it cannot be done on (some|many|most) architectures.

>In VAX C [by which I presume he means some version of VMS C] there
>is a macro called va_count.

I would bet that it does not work.  Try:

	int f(int x, ...) { return va_count(); }
	int main() {
		printf("%d\n", f(0, (double)0, (double)0));
		return 0;
	}

Chances are the program will print either 5 or 4.  Neither is
correct.

>Without such a feature it seems to me that the most of the 
>power of the variable argument list is wasted.

Not so.  The printf() and exec() families, for instance, can be
implemented without va_count(): the information is found some other
way, by finding `%'s in printf formats and by finding nil pointers in
exec arguments.  In particular, printf % formats convey type
information as well; without this, variadic functions that have
variable argument types (as well as or in place of variable number of
arguments) cannot be implemented even *with* va_count.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at mimsy.umd.edu	Path:	uunet!mimsy!chris