E.  If OFFSETS is 'NULL',
     then the type will be laid out but not otherwise modified.  This
     can be useful for accessing the type's size or layout, as mentioned
     above.

     This function returns 'FFI_OK' on success; 'FFI_BAD_ABI' if ABI is
     invalid; or 'FFI_BAD_TYPEDEF' if STRUCT_TYPE is invalid in some
     way.  Note that only 'FFI_STRUCT' types are valid here.