Opaque ptr to struct ptr

#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>

#define OFFSET_MEMBER(type, member) ((size_t)(&((type *)0)->member))
#define PTR_STRUCT_BASE_FROM_MEMBER(type, member, value) ((type *)((size_t)(value) - OFFSET_MEMBER(type, member)))
struct bah{
    int member1;
    size_t member2;
};

int main() {

    bah thing = {91, 32};
    void *opaque = &thing.member2;

    bah *pointer_to_thing_from_opaque_ptr = PTR_STRUCT_BASE_FROM_MEMBER(bah, member2, opaque);
    printf("%zu\n", pointer_to_thing_from_opaque_ptr->member2);
    return 0;
}