>>> record = '12345'
>>> sub_string = 'abc'
>>> pos = 0
>>> _write_string(record, sub_string, pos)
(3, 'abc45')
>>> pos = 2
>>> _write_string(record, sub_string, pos)
(5, '12abc')
>>> pos = 7
>>> _write_string(record, sub_string, pos)
(10, '12345  abc')
