Pseudoterminal master reads what it has just written

Question

I\'m working on a project that interfaces \"virtual devices\" (python processes) that use serial port connections with real devices that also use serial ports, and I\'m using ps

Answer 1

I'm pretty sure this is because echoing is on by default. To borrow from the Python termios docs, you could do:

master, slave = os.openpty()    # It's preferred to use os.openpty()
old_settings = termios.tcgetattr(master)
new_settings = termios.tcgetattr(master)   # Does this to avoid modifying a reference that also modifies old_settings
new_settings[3] = new_settings[3] & ~termios.ECHO
termios.tcsetattr(master, termios.TCSADRAIN, new_settings)

You can use the following to restore the old settings:

termios.tcsetattr(master, termios.TCSADRAIN, old_settings)

Answer 2

In case someone finds this question, and jszakmeister's answer doesn't work, here is what worked for me.

openpty seems to create pty's in canonical mode with echo turned on. This is not what one might expect. You can change the mode using the tty.setraw function, like in this example of a simple openpty echo server:

master, slave = os.openpty()
tty.setraw(master, termios.TCSANOW)
print("Connect to:", os.ttyname(slave))

while True:
    try:
        data = os.read(master, 10000)
    except OSError:
        break
    if not data:
        break
    os.write(master, data)