求助 VALID_FAST_IO_DISPATCH_HANDLER 理解

#define VALID_FAST_IO_DISPATCH_HANDLER(_FastIoDispatchPtr, _FieldName) \
    (((_FastIoDispatchPtr) != NULL) && \
     (((_FastIoDispatchPtr)->SizeOfFastIoDispatch) >= \
            (FIELD_OFFSET(FAST_IO_DISPATCH, _FieldName) + sizeof(void *))) && \
     ((_FastIoDispatchPtr)->_FieldName != NULL))

尤其是这句不大理解 FIELD_OFFSET(FAST_IO_DISPATCH, _FieldName) + sizeof(void *)
加sizeof(void*)干嘛？