Screen vs. socat

There is already a question: Screen fails to release pty and there is no answer (only a workaround has been provided so far).

My case is different and I would like to know the answer to the original question. So, why does socat (even recent version 1.7.3.0) ignore the screen when the last closes the pipe?

Answer

The answer is very simple at the end. We have to use open('/dev/ptmx') instead of old openpty() mechanism.