class input_descr :Creates a?start_pos_in:int -> Unix.file_descr ->raw_in_channel
raw_in_channel for the passed file descriptor, which must
be open for reading.
The pos_in method returns logical positions, i.e. it counts the number
of read octets. It is not tried to determine the real file position.
The method close_in also closes the file descriptor.
start_pos_in : The position to which pos_in is initialized when
the channel is created, by default 0