how to listen and talk at the same time across different machines [closed]

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I now can get one machine to talk and the other one listen, but if I wanna do something like "instant messaging" at both computers such that still one talk one listen but they can switch listener/talker position. For example, I talk you listen, but if you talk, I can hear too. Now I am using std::cin to achieve string input on the talker side.

How do I achieve the half-duplex communication as I described?

Thanks.