Zombie processes

14 Feb 2022

Tags: linux

What You'll Learn

What's a zombie
How to create one
How to get rid of it

Creating a zombie

Lets start with a script to create a zombie

#!/usr/bin/env python3

import os
import sys
import time


try:
    pid = os.fork()
except Exception as e:
    print('Fork failed')
    sys.exit(1)

if pid == 0:
    # child process
    print('Hello from zombie child ' + str(os.getpid()))
    sys.exit(0)

# parent process
time.sleep(0.1)

print('Press enter to wait()')
input()
os.waitpid(pid, 0)  # The child is a zombie until we execute this line

print('Zombie child reaped')

print('Press enter to exit')
input()

Copy the code to a file zombie1.py and run it in one terminal. This will create a zombie. If pressing ENTER, the zombie will be killed.

$ python3 zombie.py
Hello from zombie child 7414
Press enter to wait()

Inspecting the zombies

Before pressing ENTER (and thus finishing the processes), we can list the zombies in another terminal

$ ps -ely | grep ^Z
S   UID   PID  PPID  C PRI  NI   RSS    SZ WCHAN  TTY          TIME CMD
Z  1000  7416  7415  0  80   0     0     0 -      pts/2    00:00:00 python3 <defunct>

A few observations:

Program is in state Z, and labeled <defunct>
Program doesn't use any memory (RSS = 0)

We can also check /proc/<pid>/smaps to validate than the process doesn't have access to any memory

$ cat /proc/7416/smaps
$

The zombie doesn't have any file descriptor (not even stdin, stdout, or stderr)

$ ls -al /proc/7416/fd/
total 0
dr-x------ 2 root   root   0 Dec 18 22:42 .
dr-xr-xr-x 9 tatref tatref 0 Dec 18 22:14 ..

How it works

Let's go into the script step by step.

First, we try to fork the current process

try:
    pid = os.fork()
except Exception as e:
    print('Fork failed')
    sys.exit(1)

If the fork() succeeded, we now have 2 processes.

When a process is created (a task in Linux parlance), an entry is inserted in the tasks list. Each entry contains a list of open files, the state of the process, the exit status...

We use the returned pid value to check if we are the parent or the child. The child just prints it's own pid, and exits

if pid == 0:
    # child
    print('Hello from zombie child ' + str(os.getpid()))
    sys.exit(0)

The parent will block until we press enter. It will then wait for the child process to terminate using waitpid()

print('Press enter to wait()')
input()
os.waitpid(pid, 0)

Here is what the lifecycle looks like:

graph LR subgraph alive running --- sleeping running --- stopped running -- Child calls exit --> zombie end zombie -- Parent calls wait --> last([deleted])

There is no transition from running to deleted, a process has to become a zombie first.

Also note that there is no dead or deleted state: when the parent calls wait(), the child just ceases to exist if it was in the zombie state.

At that moment, the kernel passes the exit status of the child to the parent, and delete the process from the tasks list.

SIGCHLD

When a child exits, the parent gets notified with SIGCHLD (17)

We can add the following piece of code at the beginning of the script to show it:

import signal

def handler(signum, frame):
    print('Got signal', signum)

signal.signal(signal.SIGCHLD, handler)

$ ./zombies.py 
Hello from child 8537
**Got signal 17**
Press enter to wait()

When this signal is received, a parent should check it's children with wait to receive their exit codes, and clean the processes from the process list.

Zombie processes

What You'll Learn

Creating a zombie

Inspecting the zombies

How it works

SIGCHLD

Conclusion

Sources & References

Comments