Re: [RFC] Clarification for “undefined behaviour”?

[David Laight]

On Wed, 5 Mar 2025 17:30:28 +0300
Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:

> On Wed, Mar 05, 2025 at 02:17:32PM +0000, David Laight wrote:
...
> > And the 'fun' starts because NULL isn't required to use the all-zero
> > bit pattern.
> > Regardless of the bit-pattern, things like (void *)(1 - 1) are valid
> > NULL pointers.
> > 
> > Of course, while C allows this, I doubt NULL has ever been other than 0.
> > (It was 0 on a system I used many years ago where the O/S invalid pointer
> > was ~0.)  
> 
> Kernel style guidelines don't even allow if (p == NULL) so we would be
> screwed.  :P

Doesn't matter:
	if (!p) ...
	if (p == 0) ...
	if (p == (void *)0) ...
	if (p == NULL) ...
	if (p == (void *)(constant integer expression with value 0)) ...
and the equivalent assignments all behave the same regardless of the
bit-pattern use for NULL.
So:
	union { long l; void *p; } lpu;
	lpu.p = 0;
	return lpu.l;
Returns ABI (implementation) defined constant value.
I think the only requirement is that it can never be the address
of a valid variable.

	David