CODEWITHSUNDEEP
windowspython-3.xsignals
Moondra
Moondra

Reputation: 4511

Trying to implement `signal.CTRL_C_EVENT` in Python3.6

I'm reading about signals and am attempting to implement signal.CTRL_C_EVENT

From what I"m understanding, if the user presses CTRC + C while the program is running, a signal will be sent to kill a program. I can specify the program as a parameter?

My attempt to test out the usage:

import sys
import signal
import time
import os


os.kill('python.exe', signal.CTRL_C_EVENT)

while(1):
    print ("Wait...")
    time.sleep(10)

However, it seems I need a pid number and 'python.exe' doesn't work. I looked under processes and I can't seem to find a PID number. I did see a PID column under services, but there were so many services -- I couldn't find a python one.

So how do I find PID number? Also, does signal_CTRL_C_EVENT always have to be used within os.kill? Can It be used for other purposes?

Thank you.

Upvotes: 3

Views: 6214

Answers (2)

brodeur
brodeur

Reputation: 96

You can get pid via os.getpid()

os.kill(os.getpid(), signal.CTRL_C_EVENT)

Upvotes: 1

Eryk Sun
Eryk Sun

Reputation: 34260

Windows doesn't implement Unix signals, so Python fakes os.kill. Unfortunately its implementation is confusing. It should have been split up into os.kill and os.killpg, but we're stuck with an implementation that mixes the two. To send Ctrl+C or Ctrl+Break, you need to use os.kill as if it were really os.killpg.

When its signal argument is either CTRL_C_EVENT (0) or CTRL_BREAK_EVENT (1), os.kill calls WinAPI GenerateConsoleCtrlEvent. This instructs the console (i.e. the conhost.exe instance that's hosting the console window of the current process) to send the event to a given process group ID (PGID). Group ID 0 is special cased to broadcast the event to all processes attached to the console. Otherwise a process group ID is the ID of the lead process in a process group. Every process is either created as the leader of a new group or inherits the group of its parent. A new group can be created via the CreateProcess creation flag CREATE_NEW_PROCESS_GROUP.

If either calling GenerateConsoleCtrlEvent fails (e.g. the current process isn't attached to a console) or the signal argument isn't one of the above-mentioned control events, then os.kill instead attempts to open a handle for the given process ID (PID) with terminate access and call WinAPI TerminateProcess. This function is like sending a SIGKILL signal in Unix, but with a variable exit code. Note the confusion in that it operates on an individual process (i.e. kill), not a process group (i.e. killpg).

Windows doesn't provide a function to get the group ID of a process, so generally the only way to get a valid PGID is to create the process yourself. You can pass the CREATE_NEW_PROCESS_GROUP flag to subprocess.Popen via its creationflags parameter. Then you can send Ctrl+Break to the child process and all of its children that are in the same group, but only if it's a console process that's attached to the same console as your current process, i.e. it won't work if you also also use any of these flags: CREATE_NEW_CONSOLE, CREATE_NO_WINDOW, or DETACHED_PROCESS. Also, Ctrl+C is disabled in such a process, unless the child manually enables it via WinAPI SetConsoleCtrlHandler.

Only use os.kill(os.getpid(), signal.CTRL_C_EVENT) when you know for certain that your current process was started as the lead process of a group. Otherwise the behavior is undefined, and in practice it works like sending to process group ID 0.

Upvotes: 7

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