The implementation of the offsetof macro is really irrelevant.
The actual C standard defines it as in 7.17.3:
offsetof(type, member-designator)
which expands to an integer constant expression that has type size_t, the value of which is the offset in bytes, to the structure member (designated by member-designator), from the beginning of its structure (designated by type). The type and member designator shall be such that given static type t;.
Trust Adam Rosenfield's answer.
R is completely wrong, and it has many uses - especially being able to tell when code is non-portable among platforms.
(OK, it's C++, but we use it in static template compile time assertions to make sure our data structures do not change size between platforms/versions.)
