In C/C++, addition or subtraction on pointer is defined only if the resulting pointer lies within the original pointed complete object. Moreover, comparison of two pointers can only be performed if the two pointed objects are subobjects of a unique complete object.
What are the reasons of such limitations?
I supposed that segmented memory model (see here §1.2.1) could be one of the reasons but since compilers can actually define a total order on all pointers as demonstrated by this answer, I am doubting this.
There are architectures where program and data spaces are separated, and it's simply impossible to subtract two arbitrary pointers. A pointer to a function or to const static data will be in a completely different address space than a normal variable.
Even if you arbitrarily supplied a ranking between different address spaces, there's a possibility that the diff_t type would need to be a larger size. And the process of comparing or subtracting two pointers would be greatly complicated. That's a bad idea in a language that is designed for speed.