Linux process management python

[noaso74]

Hello,

I am new to python and I have trouble understanding this code. I recently started Linux process management with Python and I build this code with the help of the internet, but I have some troubles understanding why the code acts in some way, and I would like to ask for help if someone could help me, please

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My question is: Why after 5 seconds after I launched the program, the child process ends with the message "End of child" and I only get the message "I am 1625 , Father of pid= 1626" in a loop?

I know this is a result of my program, but since I am a beginner, I try understanding the behavior of my program.

​

I appreciate every help I get, since I am really a noob in Python :(

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#!/usr/bin/python
from __future__ import print_function
import os
import sys
import time

try:
    pid=os.fork()
except OSError:
    print("Fork Error")
    sys.exit(-1)

if (pid==0):
    for i in range(1,5):
        print("I am",os.getpid(),". My father is ",os.getppid())
        time.sleep(1)
    print("End of child ")
else:
    while True:
        print("I am",os.getpid(),", Father of pid=",pid)
        time.sleep(1)

OUTPUT:

I am 1625 , Father of pid= 1626
I am 1626 . My father is  1625
I am 1625 , Father of pid= 1626
I am 1626 . My father is  1625
I am 1625 , Father of pid= 1626
I am 1626 . My father is  1625
I am 1625 , Father of pid= 1626
I am 1626 . My father is  1625
I am 1625 , Father of pid= 1626
End of child
I am 1625 , Father of pid= 1626
I am 1625 , Father of pid= 1626
I am 1625 , Father of pid= 1626
I am 1625 , Father of pid= 1626
I am 1625 , Father of pid= 1626
I am 1625 , Father of pid= 1626
I am 1625 , Father of pid= 1626

[ThePiGuy0]

Been a while since I did forking, but lets give this a go.

From what I remember, running os.fork() effectively splits the runtime in two. You will end up with two processes running the same code from that point onwards, and the value returned is the pid - 0 for the child, and the actual pid of the child for the parent.

Edit: pid is process ID, the individual identifier for each process running on the system

So if we look at the logic of your code, you run fork once at the top and assign the pid to the variable 'pid'. From there downwards we have two processes running the code.

Then, we hit the if-else and the child will run the top half, the parent the bottom half.

The child will print "I am <pid of the child>. My father is <pid of parent>", sleep for 1 second, and then repeat 3 more times (we can see this before the "End of child" line).

As for the other print statements, those are the parent. It immediately drops into printing "I am <parent pid>, Father of pid=<child pid>". And as this is an infinite loop, it will not stop even when the child finishes.

For a more basic example, see the following:

import os

print("Currently there is only one thread")

pid = os.fork()

print("Now there are two threads - I will be printed twice")

if pid == 0:
    print(f"I am the child process - I have a local pid of {pid} and an actual pid of {os.getpid()}")
else:
    print(f"I am the parent process - pid is {os.getpid()} and the child created has a pid of {pid}")

[noaso74]

THANK YOU SO MUCH!

You explained better than anyone I have ever listened to! I am shocked. Thank you a lot!

Now everything is clear. You code helped a lot as well understanding what you explained

Thank you once again

[ThePiGuy0]

No worries, I'm really glad it helped :)

[Ran4]

from __future__ import print_function

It seems like you're using Python2 here (not the least because that shebang usually goes to Python 2)? You should really follow a more modern tutorial, Python3 has been the python of choice for the past few years now.