Question

Is subtraction of pointers not pointing to different elements of same array valid in C?

Is something such as below guaranteed to work according to C Standards? I vaguely remember reading that this is not valid?

int * a;
int * b;
a = (int*) 100;
b = (int*) 200;
printf("%d\n", b-a);

Will this give me 25.

Answer 1

From the C spec, Appendix J.2 Undefined behaviour:

Pointers that do not point into, or just beyond, the same array object are subtracted (6.5.6).

6.5.6 Additive operators, paragraph 9 says:

When two pointers are subtracted, both shall point to elements of the same array object, or one past the last element of the array object; the result is the difference of the subscripts of the two array elements.

There you have it – your example causes undefined behaviour. That said, on most systems it will work just fine. You probably do want to change your printf format to %td to indicate that you’re printing a ptrdiff_t type.