get_first_member

get offset of the first member of a structure
     id            - structure type ID
returns: -1 if bad structure type ID is passed
         or structure has no members
         otherwise returns offset of the first member.
         NOTE: IDA allows 'holes' between members of a
               structure. It treats these 'holes'
               as unnamed arrays of bytes.
         NOTE: Union members are, in IDA's internals, located
               at subsequent byte offsets: member 0 -> offset 0x0,
               member 1 -> offset 0x1, etc...

long get_first_member(long id);