$ cat test_a.py
from multiprocessing import Process, current_process
import logging
import os
import time

logging.basicConfig(
  level=logging.DEBUG,
  format='%(asctime)-15s - %(levelname)s - %(message)s'
)


def run():
  logging.info('exit child process %s', current_process().pid)
  os._exit(3)

p = Process(target=run)
p.start()
time.sleep(100)

$ python test_a.py &
[1] 10091
$ 2017-07-20 21:28:14,792 - INFO - exit child process 10106

$ ps aux |grep 10106
mozillazg       10126  0.0 0.0 2434836  740 s006 R+  0:00.00 grep 10106
mozillazg       10106  0.0 0.0    0   0 s006 Z   0:00.00 (Python)

import errno
from multiprocessing import Process, current_process
import logging
import os
import signal
import time

logging.basicConfig(
  level=logging.DEBUG,
  format='%(asctime)-15s - %(levelname)s - %(message)s'
)


def run():
  exitcode = 3
  logging.info('exit child process %s with exitcode %s',
         current_process().pid, exitcode)
  os._exit(exitcode)


def wait_child(signum, frame):
  logging.info('receive SIGCHLD')
  try:
    while True:
      # -1 表示任意子进程
      # os.WNOHANG 表示如果没有可用的需要 wait 退出状态的子进程，立即返回不阻塞
      cpid, status = os.waitpid(-1, os.WNOHANG)
      if cpid == 0:
        logging.info('no child process was immediately available')
        break
      exitcode = status >> 8
      logging.info('child process %s exit with exitcode %s', cpid, exitcode)
  except OSError as e:
    if e.errno == errno.ECHILD:
      logging.error('current process has no existing unwaited-for child processes.')
    else:
      raise
  logging.info('handle SIGCHLD end')

signal.signal(signal.SIGCHLD, wait_child)

p = Process(target=run)
p.start()

while True:
  time.sleep(100)

$ python test_b.py &
[1] 10159
$ 2017-07-20 21:28:56,085 - INFO - exit child process 10174 with exitcode 3
2017-07-20 21:28:56,088 - INFO - receive SIGCHLD
2017-07-20 21:28:56,089 - INFO - child process 10174 exit with exitcode 3
2017-07-20 21:28:56,090 - ERROR - current process has no existing unwaited-for child processes.
2017-07-20 21:28:56,090 - INFO - handle SIGCHLD end

$ ps aux |grep 10174
mozillazg       10194  0.0 0.0 2432788  556 s006 R+  0:00.00 grep 10174