#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;
}
