sizeof a relative pointer in a struct

Hi all.
I'm trying to get the size of a variable in a struct by his relative
postion i.e.

///
#define offsetof(s,m)   (size_t)&(((s *)0)->m)

struct ThePimp{
char rings[10];
char blings[20];

};

int sizeOfBling = sizeof( (*char)&ThePimp +
(char)offsetof(ThePimp,blings) );

///
(If I have any syntax mistakes please ignore)

You can easily predict that sizeOfBling will be 4... (sizeof(int*))
Is there any way to acomplish it?

Thanks