Python subprocess communicate hangs

Question

I know that this is a frequently asked question and I've tried any solution approach which I could find here and on other sites^but couldn't solve my problem. My dilemma looks as follows (on Windows):

I have a main script (main.py) where I am creating a subprocess via Popen calling another script (sniffer.py) . Afterwards I am doing some stuff in main.py and finally want to send a character to the subprocess to finish the endless loop in sniffer.py and finally the whole subprocess.

main.py

process = Popen(["python", "sniffer.py", receiverIP, senderIP, "udp", path],stdin=PIPE)
#do some stuff
process.communicate('terminate')

sniffer.py

def check(done):
 while True:
 if sys.stdin.read() == 'terminate':
 done = True
 break
def sniff(someparams):
 done = False
 input_thread = threading.Thread(target=check, args=(done,))
 input_thread.daemon = True
 input_thread.start()
 while True:
 #do some stuff
 if done:
 break

I've also tried to combine the communicate call with a stdin.write, but it had no effect.

Note: I've noticed that, the while loop in sniffer.py doesn't continue after my communicate() call (the whole script just hangs)

• awerchniak: Could you specify how specifically it 'didn't work'? Please provide your output to aid in debugging.
• braheem38: I've edited my question

Answer 1

Nothing to do with subprocess.

You change done to True locally. You have to define it globally for the last loop to exit properly.

done = False
def check():
 global done
 while True:
 if sys.stdin.read() == 'terminate':
 done = True
 break
def sniff(someparams):
 global done
 done = False
 input_thread = threading.Thread(target=check)
 input_thread.daemon = True
 input_thread.start()
 while True:
 #do some stuff
 if done:
 break

• braheem38: It didn't help, the code still hangs on communicate(). The communicate call correctly sets done to true but the while loop in sniff() hangs after that
• Jean-François Fabre: try to add a print statement in your while True loop, and after it too to see if it's reached.