Task: Sleepy

Higher level - Python

Enter the labs/lab-06/tasks/sleepy directory, run make skels, open the support/src folder and go through the practice items below.

Use the tests/checker.sh script to check your solutions.

./checker.sh
sleepy_creator ...................... passed ... 30
sleepy_creator_wait ................. passed ... 30
sleepy_creator_c .................... passed ... 40
100 / 100

Head over to sleepy_creator.py.

Solve the TODO: use subprocess.Popen() to spawn 10 sleep 1000 processes.

Start the script:
```
student@os:~/.../tasks/sleepy/support$ python3 sleepy_creator.py
```
Look for the parent process:
```
student@os:~$ ps -e -H -o pid,ppid,cmd | (head -1; grep "python3 sleepy_creator.py")
```
It is a python3 process, as this is the interpreter that runs the script, but we call it the sleepy_creator.py process for simplicity. No output will be provided by the above command, as the parent process (sleepy_creator.py) dies before its child processes (the 10 sleep 1000 subprocesses) finish their execution. The parent process of the newly created child processes is an init-like process: either systemd/init or another system process that adopts orphan processes. Look for the sleep child processes using:
```
student@os:~$ ps -e -H -o pid,ppid,cmd | (head -1; grep sleep)
 PID    PPID         CMD
4164    1680     sleep 1000
4165    1680     sleep 1000
4166    1680     sleep 1000
4167    1680     sleep 1000
4168    1680     sleep 1000
4169    1680     sleep 1000
4170    1680     sleep 1000
4171    1680     sleep 1000
4172    1680     sleep 1000
4173    1680     sleep 1000
```
Notice that the child processes do not have sleepy_creator.py as a parent. What’s more, as you saw above, sleepy_creator.py doesn’t even exist anymore. The child processes have been adopted by an init-like process (in the output above, that process has PID 1680 - PPID stands for parent process ID).

Quiz
Solve the TODO: change the code in sleepy_creator_wait.py so that the sleep 1000 processes remain the children of sleepy_creator_wait.py. This means that the parent / creator process must not exit until its children have finished their execution. In other words, the parent / creator process must wait for the termination of its children. Check out Popen.wait() and add the code that makes the parent / creator process wait for its children. Before anything, terminate the sleep processes created above:
```
student@os:~$ pkill sleep
```
Start the program, again, as you did before:
```
student@os:~/.../tasks/sleepy/support$ python3 sleepy_creator.py
```
On another terminal, verify that sleepy_creator_wait.py remains the parent of the sleep processes it creates:
```
student@os:~$ ps -e -H -o pid,ppid,cmd | (head -1; grep sleep)
 PID    PPID                CMD
16107   9855         python3 sleepy_creator.py
16108   16107           sleep 1000
16109   16107           sleep 1000
16110   16107           sleep 1000
16111   16107           sleep 1000
16112   16107           sleep 1000
16113   16107           sleep 1000
16114   16107           sleep 1000
16115   16107           sleep 1000
16116   16107           sleep 1000
16117   16107           sleep 1000
```
Note that the parent process sleepy_creator_wait.py (PID 16107) is still alive, and its child processes (the 10 sleep 1000) have its ID as their PPID. You’ve successfully waited for the child processes to finish their execution.

If you’re having difficulties solving this exercise, go through this reading material.

Lower level - C

Now let’s see how to create a child process in C. There are multiple ways of doing this. For now, we’ll start with a higher-level approach.

Go to sleepy_creator.c and use system to create a sleep 1000 process.

Quiz

The man page also mentions that system calls fork() and exec() to run the command it’s given. If you want to find out more about them, head over to the Arena and create your own mini-shell.