get_member_offset

get offset of a member of a structure by the member name
     id            - structure type ID
     member_name   - name of structure member
returns: -1 if bad structure type ID is passed
         or no such member in the structure
         otherwise returns offset of the specified member.
         NOTE: Union members are, in IDA's internals, located
               at subsequent byte offsets: member 0 -> offset 0x0,
               member 1 -> offset 0x1, etc...

long get_member_offset(long id, string member_name);