Why the output is 4 and not 16 in this C code?

Question

I dont understand why the output of this code is 4 and not 16, Assuming that unsigned int takes 4 bytes and long int takes 8 bytes. Any help?

#include <stdio.h>

struct test {
    unsigned int x;
    long int y : 33;
    unsigned int z;
};

int main()
{
    struct test t;
    unsigned int* ptr1 = &t.x;
    unsigned int* ptr2 = &t.z;
    printf("%d", ptr2 - ptr1);
    return 0;
}

Answer 1

I assume you expected the output to be 16, because the offset between the pointers should be 16 bytes (assuming sizes are as you mentioned in your question).

But in c when you subtract pointers, you don't get the number of bytes but the number of elements. The elements are the data the pointers point to.
Since these are unsigned int pointers, and assuming sizeof(unsigned int) is 4, the difference between thr pointers is 4 elements.

However - as @Lundin commented above this theoretical claculation is not really relevant because subtrating pointers that do not point the same array is UB (undefined behavior).

BTW - Note when you add an integral value to a pointer, a similar thing happens on the other way: the result is the address of the pointer plus the integral value times the size of the element pointed by the pointer. E.g.: if you add 1 to unsigned int* ptr1, the resulting address will be higher than ptr1 by 4.