how to get struct's start address from its member's address

Question

In C language, how to get struct's start address from its member's address?

struct type1 {
    //...
    int member1;
    //...
};
struct type1 obj1;

And ptr1 is the address of member member1 in obj1, how to define
macro #define start_add(ptr1, type1, member1) to get obj1's
start address?

Answer 1

You can do this using offsetof:

#define start_add(ptr1, type1, member1) ((type1 *)((char *)(ptr1) - offsetof(type1, member1)))