在 unix 或 unix-like 的系统中，当一个子进程退出后，它就会变成一个僵尸进程，如果父进程没有通过 wait 系统调用来读取这个子进程的退出状态的话，这个子进程就会一直维持僵尸进程状态。
Zombie process - Wikipedia 中是这样描述的：
On Unix and Unix-like computer operating systems, a zombie process or defunct process is a process that has completed execution (via the exit system call) but still has an entry in the process table: it is a process in the "Terminated state". This occurs for child processes, where the entry is still needed to allow the parent process to read its child's exit status: once the exit status is read via the wait system call, the zombie's entry is removed from the process table and it is said to be "reaped". A child process always first becomes a zombie before being removed from the resource table. In most cases, under normal system operation zombies are immediately waited on by their parent and then reaped by the system – processes that stay zombies for a long time are generally an error and cause a resource leak.
并且僵尸进程无法通过 kill 命令来清除。
本文将探讨如何手动制造一个僵尸进程以及清除僵尸进程的办法。
手动制造一个僵尸进程
为了便于后面讲解清除僵尸进程的方法，我们使用日常开发中经常使用的 multiprocessing 模块来制造僵尸进程（准确的来说是制造一个长时间维持僵尸进程状态的子进程）：
$ 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)
测试：
$ 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)
可以看到，子进程 10091 变成了僵尸进程。
既然已经可以控制僵尸进程的产生了，那我们就可以进入下一步如何清除僵尸进程了。
清除僵尸进程有两种方法：
•第一种方法就是结束父进程。当父进程退出的时候僵尸进程随后也会被清除。
• 第二种方法就是通过 wait 调用来读取子进程退出状态。我们可以通过处理 SIGCHLD 信号，在处理程序中调用 wait 系统调用来清除僵尸进程。
处理 SIGCHLD 信号
子进程退出时系统会向父进程发送 SIGCHLD 信号，父进程可以通过注册 SIGCHLD 信号处理程序，在信号处理程序中调用 wait
系统调用来清理僵尸进程。 $ cat test_b.py
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)
效果：
$ 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
可以看到，子进程退出变成僵尸进程后，系统给父进程发送了 SIGCHLD 信号，我们在 SIGCHLD 信号的处理程序中通过 os.waitpid 调用 wait 系统调用后阻止了子进程一直处于僵尸进程状态，从而实现了清除僵尸进程的效果。
