CareerCup

#include <stdio.h>

#define offset(a,b) (&((a *)0)->b)

struct abc {
    int a;
    int b;
    char c;
    char d;
};

int main(void) {
    printf("offset: %p\n", offset(struct abc, d));

    return 0;
}

#define offset(abc,c) ((char*)(&(abc.c)) - (char*)(&(abc)))

#define offsetof(s,m) (size_t)&(((s *)0)->m) - (size_t)&(((s *)0))

((s *)0) takes the integer zero and casts it as a pointer to s.
((s *)0)->m dereferences that pointer to point to structure member m.
&(((s *)0)->m) computes the address of m.
(size_t)&(((s *)0)->m) casts the result to an appropriate data type.
By definition, the structure itself resides at address 0. It follows that the address of the field pointed to (Step 3 above) must be the offset, in bytes, from the start of the structure. At this point, we can make several observation