How to let the child process live when parent process exited?

Question

I want to use multiprocessing module to complete this.

when I do this, like:

    $ python my_process.py

I start a parent process, and then let the parent process spawn a child process,

then i want that the parent process exits itself, but the child process continues to work.

Allow me write a WRONG code to explain myself:

from multiprocessing import Process

def f(x):
    with open('out.dat', 'w') as f:
        f.write(x)

if __name__ == '__main__':
    p = Process(target=f, args=('bbb',))
    p.daemon = True    # This is key, set the daemon, then parent exits itself
    p.start()

    #p.join()    # This is WRONG code, just want to exlain what I mean.
    # the child processes will be killed, when father exit

So, how do i start a process that will not be killed when the parent process finishes?

---

20140714

Hi, you guys

My friend just told me a solution...

I just think...

Anyway, just let u see:

import os
os.system('python your_app.py&')    # SEE!? the & !!

this does work!!

Strickman

No, a daemonic process will be killed when it's parent process exit. "When a process exits, it attempts to terminate all of its daemonic child processes." from docs.python.org/2/library/… – Candler

Why would you let your child processes live without a parent? With no parent they will become zombies. – Centigrade

@Pphoenix, not really. Zombies are child processes that have exited but whose parents still haven't waited for. Orphaned processes are adopted by init and keep going on. – Gollin

@Candler Yes, you are right... I just found this in my test... The task in child do not do... – Strickman

@Centigrade As you know, every process have a father process, As @Frédéric Hamidi said, father process exited, child process' father become init process. – Strickman

Did not think of that, thank you both for clarifying! – Centigrade

@Candler do you have some solution?? – Strickman

Sounds like you want to use something like nohup or os.fork. A similar question is here: #6011735 – Depressant

@Depressant Greet thanks~ Let me have a look~ – Strickman

@Depressant I just read the link, does it mean multiprocessing or subprocess can not do this? But, linux os programming is a perfect solution. – Strickman

A trick: call os._exit to make parent process exit, in this way daemonic child processes will not be killed. – Candler

@k9x You're really not supposed to use the multiprocessing module like this. It's meant to be used as a drop-in replacement for threads, which of course cannot live beyond the life of the parent process. If you want to spawn a process that can live beyond the life of the parent, use the subprocess module. – Thermocline

@k9x See this answer for a cross-platform way of properly launching a de-coupled child process using subprocess: https://mcmap.net/q/25390/-popen-waiting-for-child-process-even-when-the-immediate-child-has-terminated – Thermocline

@Thermocline Big thanks, subprocess.Popen() did NOT block the father process, father process just exited, let child finish. U should answer this~ – Strickman

Answer 1

A trick: call os._exit to make parent process exit, in this way daemonic child processes will not be killed.

But there are some other side affects, described in the doc:

Exit the process with status n, without calling cleanup handlers, 
flushing stdio buffers, etc.

If you do not care about this, you can use it.

Candler

what if you double fork? so parent spawns child1 that spawns your daemon. then child1 calls os._exit, and parent is free to do a clean exit – Pitchford

Answer 2

Here's one way to achieve an independent child process that does not exit when __main__ exits. It uses the os._exit() tip mentioned above by @WKPlus.

Is there a way to detach matplotlib plots so that the computation can continue?

Voltmer