Re: [PATCH PR45098] Disallow NULL pointer in pointer arithmetic

[Richard Guenther <richard.guenther@gmail.com>]

On Mon, Jun 20, 2011 at 2:25 PM, Zdenek Dvorak <rakdver@kam.mff.cuni.cz> wrote:
>> > I don't think we should move this kind of undefinedness from C to
>> > the GIMPLE semantics.  What do other languages allow that
>> > we have to support (what did K&R C specify?).
>>
>> I don't think there is a formal specification of K&R C, just the (somewhat
>> informal) book.  On topic of pointer arithmetics, the case of addition
>> is not completely clear.  It does say that you can only subtract pointers
>> to members of the same array, though.
>>
>> On topic of addition of integer to a pointer, it says that "The construction
>> p + n means the address of the n-th object beyond the one p currently points to. This is true
>> regardless of the kind of object p points to; n is scaled according to the size of the objects p
>> points to, which is determined by the declaration of p."
>
> Anyway, I don't think that this should be a matter of lawyer scrutiny of the specifications;
> rather, we should consider whether there is a situation where a user could reasonably expect
> NULL + 0 to be valid.  In the example by Richard,
>
> int __attribute__((noinline)) foo (void *p, int i)
> {
>  return p + i != NULL;
> }
>
> I think it would be hard to argue that this construction is natural.

Nor does it feel natural that 'p' is different from 'p + 0'.

Richard.

> Zdenek
>