Replace Parent Process with Child Process After De

Question

It is my objective to make a program with a parent and child process, and when either one them is killed, they are replaced. The part I am struggling with is the case of the parent's death. In this case, the child must step up to become the new parent, and then fork() a child of its own. When I send a SIGKILL to the parent process, my entire program seems to end abruptly, and since I cannot handle a SIGKILL, I am unsure of how to do this properly.

Is there a way to keep the child running in order to become the new parent process?

Thanks

Answer 1

Normally the child you fork shouldn't be killed when it's parent is killed, unless you do something like: How to make child process die after parent exits?

If the parent is killed, the children become a children of the init process. You probably saw on terminal that the process returns immediately after you send KILL to parent. That's because the sub-bash is waiting only on the parent's PID. But the child is actually running elsewhere.

Here is a example to show it:

#!/usr/bin/env python
# test_parent_child_kill.py
import os
import time

def child():
    print "Child process with PID= %d"%os.getpid()
    time.sleep(20)

def parent():
    print "Parent process with PID= %d"%os.getpid()
    newRef=os.fork()
    if newRef==0:
        child()
    else:
        print "Parent process and our child process has PID= %d"%newRef
        time.sleep(20)

parent()

Then within sleep period:

user@mac:/tmp|⇒  python test_parent_child_kill.py
Parent process with PID= 17430
Parent process and our child process has PID= 17431
Child process with PID= 17431

user@mac:/tmp|⇒  kill 17430

user@mac:/tmp|⇒  ps -ef | grep 17431
503 17431     1   0  9:30PM ttys000    0:00.00 /usr/local/Cellar/python/2.7.10_2/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python test_parent_child_kill.py

So the child is actually still alive.

--Edit--

1. Why when the parent is killed my program exits back to the shell?

Bash invokes the command also via folk/exec via something like this:

childPid = fork();
if (childPid == 0){
    executeCommand(cmd); //calls execvp  
} else {
    if (isBackgroundJob(cmd)){
        record in list of background jobs
    } else {
        waitpid (childPid);
    }       
}

Since from bash's point of view, the parent of your program is the child, it would return to prompt input when it returns from waitpid(childPid).

2. Is there a way to stay within the program and continue functioning as it was but with a new parent?

It might be a bit difficult if you want to "re-attach", but it's not impossible:

Attach to a processes output for viewing

https://unix.stackexchange.com/questions/58550/how-to-view-the-output-of-a-running-process-in-another-bash-session

Reference:

http://www.cs.cornell.edu/Courses/cs414/2004su/homework/shell/shell.html