6.50 Support for offsetof ¶

GCC implements for both C and C++ a syntactic extension to implement the offsetof macro.

primary:
        "__builtin_offsetof" "(" typename "," offsetof_member_designator ")"

offsetof_member_designator:
          identifier
        | offsetof_member_designator "." identifier
        | offsetof_member_designator "[" expr "]"

This extension is sufficient such that

#define offsetof(type, member)  __builtin_offsetof (type, member)

is a suitable definition of the offsetof macro. In C++, type may be dependent. In either case, member may consist of a single identifier, or a sequence of member accesses and array references.