смещение данных в структуре

offsetof

libc standard.

correction - offsetof is gcc extention, no include needed:

http://gcc.gnu.org/onlinedocs/gcc/Offsetof.html#Offsetof

As can I see __builtin_offsetof is gcc extension,
offsetof need "include" stddef.h,

and my guess was right:
from stddef.h
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

any way, thanks a lot.