Orphan Processes
When a parent process finishes executing before its children, the child processes are said to become orphan processes.
When this happens, the init process - the first process executed by the kernel at boot, with a PID of 1 - adopts the child. The init process then waits for the child process to finish.
In this code example, I have a script that calls fork System Call to create a child process. Then, we sleep for a second 3 times after printing its parent's PID in the child process. The parent will exit on the first iteration, thus orphaning its child.
from os import fork, getppid
from time import sleep
pid = fork()
if pid == 0:
print("Child: I'm about to become an orphan!")
for _ in range(3):
sleep(1)
print("Child: My parent is", getppid())
else:
print("Parent: I just created child", pid)
sleep(1)
> python fork_orphan.py
Parent: I just created child 19683
Child: I'm about to become an orphan!
Child: My parent is 19682
Child: My parent is 1
Child: My parent is 1
So, to prevent the child from becoming an orphan, our parent process can call the wait System Call.
wait(2) effectively waits for its children to exit and then collects information about them. The system call is available in Python as os.wait(), which returns a tuple containing the child's PID and exit status indication.
from os import fork, getppid, wait
from sys import exit
from time import sleep
pid = fork()
if pid == 0:
print("Child: Hope my parent doesn't forget me this time!")
for _ in range(3):
sleep(1)
print("Child: My parent is", getppid())
else:
wait()
print("Parent: My child has finished processing. My work here is done.")
> python fork_wait.py
Child: Hope my parent doesn't forget me this time!
Child: My parent is 20037
Child: My parent is 20037
Child: My parent is 20037
Parent: My child has finished processing. My work here is done.
The operating system uses the procedure of orphaning a process to daemonizes a process.
References
Wikipedia. Orphan process. Wikipedia, April 2021. URL: https://en.wikipedia.org/wiki/Orphan_process. ↩