Sending ^C to Python subprocess objects on Windows

Question

I have a test harness (written in Python) that needs to shut down the program under test (written in C) by sending it ^C. On Unix,

proc.send_sign         


        

Answer 1

There is a solution by using a wrapper (as described in the link Vinay provided) which is started in a new console window with the Windows start command.

Code of the wrapper:

#wrapper.py
import subprocess, time, signal, sys, os

def signal_handler(signal, frame):
  time.sleep(1)
  print 'Ctrl+C received in wrapper.py'

signal.signal(signal.SIGINT, signal_handler)
print "wrapper.py started"
subprocess.Popen("python demo.py")
time.sleep(3) #Replace with your IPC code here, which waits on a fire CTRL-C request
os.kill(signal.CTRL_C_EVENT, 0)

Code of the program catching CTRL-C:

#demo.py

import signal, sys, time

def signal_handler(signal, frame):
  print 'Ctrl+C received in demo.py'
  time.sleep(1)
  sys.exit(0)

signal.signal(signal.SIGINT, signal_handler)
print 'demo.py started'
#signal.pause() # does not work under Windows
while(True):
  time.sleep(1)

Launch the wrapper like e.g.:

PythonPrompt> import subprocess
PythonPrompt> subprocess.Popen("start python wrapper.py", shell=True)

You need to add some IPC code which allows you to control the wrapper firing the os.kill(signal.CTRL_C_EVENT, 0) command. I used sockets for this purpose in my application.

Explanation:

Preinformation

• send_signal(CTRL_C_EVENT) does not work because CTRL_C_EVENT is only for os.kill. [REF1]
• os.kill(CTRL_C_EVENT) sends the signal to all processes running in the current cmd window [REF2]
• Popen(..., creationflags=CREATE_NEW_PROCESS_GROUP) does not work because CTRL_C_EVENT is ignored for process groups. [REF2] This is a bug in the python documentation [REF3]

Implemented solution

1. Let your program run in a different cmd window with the Windows shell command start.
2. Add a CTRL-C request wrapper between your control application and the application which should get the CTRL-C signal. The wrapper will run in the same cmd window as the application which should get the CTRL-C signal.
3. The wrapper will shutdown itself and the program which should get the CTRL-C signal by sending all processes in the cmd window the CTRL_C_EVENT.
4. The control program should be able to request the wrapper to send the CTRL-C signal. This might be implemnted trough IPC means, e.g. sockets.

Helpful posts were:

I had to remove the http in front of the links because I'm a new user and are not allowed to post more than two links.

• http://social.msdn.microsoft.com/Forums/en-US/windowsgeneraldevelopmentissues/thread/dc9586ab-1ee8-41aa-a775-cf4828ac1239/#6589714f-12a7-447e-b214-27372f31ca11
• Can I send a ctrl-C (SIGINT) to an application on Windows?
• Sending SIGINT to a subprocess of python
• http://bugs.python.org/issue9524
• http://ss64.com/nt/start.html
• http://objectmix.com/python/387639-sending-cntrl-c.html#post1443948

Update: IPC based CTRL-C Wrapper

Here you can find a selfwritten python module providing a CTRL-C wrapping including a socket based IPC. The syntax is quite similiar to the subprocess module.

Usage:

>>> import winctrlc
>>> p1 = winctrlc.Popen("python demo.py")
>>> p2 = winctrlc.Popen("python demo.py")
>>> p3 = winctrlc.Popen("python demo.py")
>>> p2.send_ctrl_c()
>>> p1.send_ctrl_c()
>>> p3.send_ctrl_c()

Code

import socket
import subprocess
import time
import random
import signal, os, sys


class Popen:
  _port = random.randint(10000, 50000)
  _connection = ''

  def _start_ctrl_c_wrapper(self, cmd):
    cmd_str = "start \"\" python winctrlc.py "+"\""+cmd+"\""+" "+str(self._port)
    subprocess.Popen(cmd_str, shell=True)

  def _create_connection(self):
    self._connection = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    self._connection.connect(('localhost', self._port))

  def send_ctrl_c(self):
    self._connection.send(Wrapper.TERMINATION_REQ)
    self._connection.close()

  def __init__(self, cmd):
    self._start_ctrl_c_wrapper(cmd)
    self._create_connection()


class Wrapper:
  TERMINATION_REQ = "Terminate with CTRL-C"

  def _create_connection(self, port):
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.bind(('localhost', port))
    s.listen(1)
    conn, addr = s.accept()
    return conn

  def _wait_on_ctrl_c_request(self, conn):
    while True:
      data = conn.recv(1024)
      if data == self.TERMINATION_REQ:
        ctrl_c_received = True
        break
      else:
        ctrl_c_received = False
    return ctrl_c_received

  def _cleanup_and_fire_ctrl_c(self, conn):
    conn.close()
    os.kill(signal.CTRL_C_EVENT, 0)

  def _signal_handler(self, signal, frame):
    time.sleep(1)
    sys.exit(0)

  def __init__(self, cmd, port):
    signal.signal(signal.SIGINT, self._signal_handler)
    subprocess.Popen(cmd)
    conn = self._create_connection(port)
    ctrl_c_req_received = self._wait_on_ctrl_c_request(conn)
    if ctrl_c_req_received:
      self._cleanup_and_fire_ctrl_c(conn)
    else:
      sys.exit(0)


if __name__ == "__main__":
  command_string = sys.argv[1]
  port_no = int(sys.argv[2])
  Wrapper(command_string, port_no)