Ardubit:
If that be the case then who do some terminals complain that the "Port is already open " when you try to connect to such a link up between two items ?
That is a different problem entirely. Within the PC it is not possible to share serial ports. And if you are connecting to the PC with a USB cable there is actually no serial data (in the Arduino sense) until the signal gets past the USB-TTL converter on the Arduino board (assuming you are using and Uno, Nano or Mega. Prior to the USB-TTL converter it is USB data.
...R