Parameter mismatch legality question

[Tim Olson]

Here is an interesting question that came up recently.  What is the
"legality" of the following ("dusty deck" K&R, not ANSI) code:
	
	foo();
	
	bar()
	{
		int a;
		.
		.
		foo(a);
		.
		.
	}
	
	foo(a, b, c, d)
	int a, b, c, d;
	{
		.
		.
	}

i.e. the call to a function passes fewer parameters than are declared
in the function declaration.

Since C's parameters are "call-by-value", they can normally be
modified or destroyed by the called function.  What if a compiler with
dataflow analysis decided that the lifetimes for the parameter "b" and
a local variable were non-overlapping, and decided to use the same
space (be it memory or a register) to hold them?  In the case above,
it could end up overwriting some local variable from procedure bar()!

So the question is, is the code:

	a) Legal in all cases (at least for K&R C)

	b) Legal if there is no explicit modification of "optional"
	   variables

	c) Illegal

	d) Other?

--
	-- Tim Olson
	Advanced Micro Devices
	(tim at amd.com)