#include <stdio.h>

struct tmem_oid {
	unsigned long oid[3];
};

struct internal {
	unsigned int cmd;
	struct tmem_oid oid;
};
struct tmem_op {
	unsigned int x;
	union {
		struct {
			unsigned int cmd;
		} g;
		struct internal c;
	} u;
};

#define container_of(ptr, type, member) ({                      \
        typeof( ((type *)0)->member ) *__mptr = (ptr);          \
        (type *)( (char *)__mptr - offsetof(type,member) );})

#define offsetof(a,b) __builtin_offsetof(a,b)

void main(void) {

	struct tmem_op op;
	struct tmem_oid *_oid;
	struct tmem_oid *_src;
	struct internal *_i;
	struct internal *_q;
	printf("%d \n", offsetof(struct tmem_op, u.c.oid)); // 16
	printf("%d \n", offsetof(struct tmem_op, u.c.oid.oid)); // 16
	printf("%d \n", offsetof(struct internal, oid)); // 8

	_i = &op.u.c;
	printf("%d\n", (void *)_i - (void *)&op); // 8
	_oid = &op.u.c.oid;
	printf("%d\n", (void *)_oid - (void *)&op); // 16

    /* Pointer to struct tmem_oid (_oid), getting the pointer to 'struct internal' */
	_q = container_of(_oid, struct internal, oid);

	printf("%d\n", (void *)_q - (void *)&op); // 8

    /* Having the pointer to outer ('*op') we can get the pointer to the internal. */
	_src = (struct tmem_oid *)((void *)&op + offsetof(struct tmem_op, u.c.Xoid.oid));
	printf("%d\n", (void *)_src - (void *)&op); // 16
};