How do I know when a child process died?

Question

In my class I run 4 process.

from multiprocessing import Process

    procs = (
             Process(target=ClassOne, name='ClassOne'),
             Process(target=ClassTwo, name='ClassTwo'),
             Process(target=ClassThree, name='ClassThree'),
             Process(target=ClassFour, name='ClassFour'),
            )

    for p in procs:
        p.daemon = False
        p.start()

I would like to be notified when one of my children process died so i can kill the other and my self.

Answer 1

Here is a Windows solution. It uses WaitForMultipleObjects API call. For Unix os.waitpid may do the job.

import multiprocessing as mp
import ctypes


SYNCHRONIZE = 0x00100000
INFINITE = -1


def mbox(msg):
  ctypes.windll.user32.MessageBoxW(0, msg, u'spam', 0)


if __name__ == '__main__':

  # start 2 processes
  procs = [mp.Process(target=mbox, args=(msg,)) for msg in u'ab']
  for p in procs:
    p.start()

  # wait for at least one process to terminate
  handles = [
    ctypes.windll.kernel32.OpenProcess(SYNCHRONIZE, False, p.pid)
    for p in procs]
  array_type = ctypes.c_long * len(handles)
  handle_array = array_type(*handles)
  ctypes.windll.kernel32.WaitForMultipleObjects(
    len(handles), handle_array, False, INFINITE)

  # terminate the rest
  for p in procs:
    p.terminate()

  # exit
  print 'done'

Answer 2

The simplest way would be to explicitly wait until all processes are done.

while multiprocessing.active_children():
    pass

This is not event driven as you might intend, but it will get the job done in your example. Also make sure you import multiprocessing.

Answer 3

Just define a signal handler for SIGCHLD, inspect the frame returned by the just dead child to retrieve the information you need about it ... and if necessary exit() the parent too :)

Answer 4

It is possible to use os.waitpid() passing -1 as the first argument and 0 as the second one.

• The first argument means that the request pertains to any child of the current process.
• The second argument means that it behaves as wait().

The function returns a tuple with the pid of the dead child and its exit code.