Forking python, defunct child
Question: I have some troubles with Python child processes so I wrote a very simple
script:

    
    
    import os
    import sys
    import time
    
    pid = os.fork()
    if pid:
        #parent
        time.sleep(30)
    else:
        #child
        #os._exit(0)
        sys.exit()
    

While parent process is sleeping I launch

    
    
    ps fax | grep py[t]hon
    

And I read this output

    
    
    2577 ?        S      0:00 python /home/pi/python/GPIO/GPIODaemon.py restart
    2583 ?        Z      0:00  \_ [python] <defunct>
    

Using `sys.exit()`or`os._exit(0)` there is always a Zombie process and I'm
unable to understand why.

Working on my more complex code I was thinking that there was some resources
that child processes were keeping locked, but on this simplified code child
has no file/socket/db connection at all! Why is child process zombiefied?


Answer: To clear the child process in Unix you need to wait on the child, check one of
the os.wait(), os.waitpid(), os.wait3() or os.wait4() at
<http://docs.python.org/2/library/os.html#os.wait>

As to why this is so, this is a design decision of Unix. The child process
keeps its return value in its process state, if it was to disappear you'll
have no return value. The os.wait() also returns to you the return value and
then the child process is released and all associated resources are released.

