why Ghost Process appears after kill -9

Question

In my Python script, I first launch a subprocess by subprocess.Popen(). Then later on, I want to kill that subprocess by kill -9 Pid.

What I found is that after the kill is executed, the subprocess is "stopped" because the GUI window of that process disappeared immediately. But when I perform a "ps aux" right after the kill, the same process (with same pid) is still shown in the result. The difference is the command of the process is included in a pair of () like below:

root 30506 0.0 0.0 0 0 s000 Z+ 6:13PM 0:00.00 (sample process)

This breaks my process detect logical since the dead process still can be found by ps.

Anyone know why this is happening?

Thanks!

Answer 1

From the manual page of ps:

Z Defunct ("zombie") process, terminated but not reaped by its parent.

That means that the parent didn't do a waitpid() for the child that died.

Apart from waitpid(), you can avoid that by using a double fork when executing the child.

Answer 2

I think -9 signal lets the process to try to handle kill and spend some time housekeeping. You can try just kill the process without signal.

Edit: oh, its actually -15 signal, that lets process die gracefully. never mind.

Answer 3

Zombie processes are actually just an entry in the process table. They do not run, they don't consume memory; the entry just stays because the parent hasn't checked their exit code. You can either do a double fork as Gonzalo suggests, or you can filter out all ps lines with a Z in the S column.