TITLE: Casting of 0 in variadic functions


[ A variatic function is one which is declared to take a
  variable number of arguments (has an ellipsis in its
  prototype. -adc ]


PROBLEM: magsig@netcom.com (Pete Magsig)

[..the standard questions about NULL]

I use the classic definition

#define NULL 0L

and thats it. The "L" may seem overly cautious, but it guarantees
that a null pointer is passed as a long to vararg functions that
expect "real" pointers.


RESPONSE: olaf@cwi.nl (Olaf Weber)

Unfortunately, it offers no such guarantees.  Since pointer
representations don't have to be the same for all types either, you
should _always_ cast NULL if used as an "ellipsis argument".


RESPONSE: clamage@Eng.Sun.COM (Steve Clamage), 27 Oct 94

Let me expand on Olaf's point. You are not guaranteed that pointers and
longs are the same size. Simple example: On x86 in small-memory model,
using 0L for NULL will produce the wrong result when passed to a variadic
function. A plain 0 would produce the correct result. But recompile using a
large-data model and suddenly 0L gives the right result and plain 0
does not.

The hacker at this point would say, "Aha! Just #ifdef on the memory model
and I'm done." Sorry, no, you just need more #ifdef's as you move the code
to different systems. Olaf presented the best solution: Always cast a null
to the expected pointer type when passing it to a variadic function:
	somefunc(p, q, (char*)0); // or (char*)NULL