[PATCH PR45098] Disallow NULL pointer in pointer arithmetic

[Jeff Law]

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On 06/17/11 08:12, Zdenek Dvorak wrote:

>>> if we do, we definitely should not -- the only point of such a construction is
>>> to bypass the pointer arithmetics restrictions,
>>
>> Ok, we don't.  Where does the C standard say that NULL + 0 is undefined?
> 
> I don't think it explicitly states that it is undefined.  But 6.5.6 #7 and #8 only
> give semantics for pointers to objects,
Right.  I think we need to review the standard closely, but IIRC
pointers are allowed to be NULL, point into an object or point to one
element beyond an object.

If that's correct P+C could only result in a NULL value if P was one
element beyond the end of an object and C happened to be the exact magic
value to wrap P to zero.  I have a hard time seeing how code could which
did that (and relied upon it) could ever be standard conforming.

Closely related, given P+C where the result is a pointer ought to tells
us that P is non-null since the only way it could be null would be if C
was the exact magic constant to make the result point to a valid object.

Jeff
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