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;

}