[uxcn]

This doesn't say anything about pointer arithmetic on pointers to raw memory though. For example, using an mmap file, there isn't any object, and there aren't any bounds.

[david-given]

That's correct --- the standard doesn't say anything about raw memory. Only pointers to objects defined by C are defined to work; everything else is an implementation-specific extension (and so, covered under 'undefined behaviour').

I believe that C99 added the ability to losslessly cast from a pointer to a uintptr_t and back again, but, IIRC, the compiler didn't have to support this in C89.