Question

#define offsetof(type, member)  ((size_t)(&((type *)0)->member))

I am not understanding (&((type *)0)->member) what is this exactly telling me…..

here type may be a structure or something else??…

More specifically what is that 0 telling me here??

Answer 1

I break it down like this:

1. (type *)0 – casting 0 to a pointer to type “type“. i.e. imagine for a minute that there’s an object of type “type” at memory address 0.

2. ->member – look into the object for the field called member.

3. & – take the address of that.

You could also write it like this:

((size_t)((&((type *)x)->member) - x))

But we’re cheating and using 0 so that there’s no offset to take off at the end.