How to use the pyulog.core.ULog._unpack_uint64 function in pyulog

def _read_file_header(self):
        header_data = self._file_handle.read(16)
        if len(header_data) != 16:
            raise Exception("Invalid file format (Header too short)")
        if header_data[:7] != self.HEADER_BYTES:
            raise Exception("Invalid file format (Failed to parse header)")
        self._file_version, = struct.unpack('B', header_data[7:8])
        if self._file_version > 1:
            print("Warning: unknown file version. Will attempt to read it anyway")

        # read timestamp
        self._start_timestamp, = ULog._unpack_uint64(header_data[8:])

def initialize(self, data, header, subscriptions, ulog_object):
            msg_id, = ULog._unpack_ushort(data[:2])
            if msg_id in subscriptions:
                subscription = subscriptions[msg_id]
                # accumulate data to a buffer, will be parsed later
                subscription.buffer += data[2:]
                t_off = subscription.timestamp_offset
                # TODO: the timestamp can have another size than uint64
                self.timestamp, = ULog._unpack_uint64(data[t_off+2:t_off+10])
            else:
                if not msg_id in ulog_object._filtered_message_ids:
                    # this is an error, but make it non-fatal
                    if not msg_id in ulog_object._missing_message_ids:
                        ulog_object._missing_message_ids.add(msg_id)
                        if ulog_object._debug:
                            print(ulog_object._file_handle.tell())
                        print('Warning: no subscription found for message id {:}. Continuing,'
                              ' but file is most likely corrupt'.format(msg_id))
                self.timestamp = 0