fork() to execute processes from bottom to up using wait()

fork() system call is used to create a process generally known as child process and the process that created it is known as parent process. Now, all the processes that are created using fork() runs concurrently. But what if we want the last process created to execute first and in this manner bottom to up execution such that parent process executes last. This can be done by using wait() system call. The parent process may then issue a wait() system call, which suspends the execution of the parent process while the child executes and when the child finishes execution, it returns exit status to operating system. Now wait() suspend the process until any of its child process finish execution. 

NOTE: This is a linux system call, so it must be executed on linux or unix variant system. 

CPP

// Program to demonstrate bottom to up execution
// of processes using fork() and wait()

#include <iostream>
#include <sys/wait.h> // for wait()
#include <unistd.h> // for fork()
int main()
{
    // creating 4 process using 2 fork calls
    // 1 parent : 2 child : 1 grand-child
    pid_t id1 = fork();
    pid_t id2 = fork();

    // parent process
    if (id1 > 0 && id2 > 0) {
        wait(NULL);
        wait(NULL);
        cout << "Parent Terminated" << endl;
    }

    // 1st child
    else if (id1 == 0 && id2 > 0) {

        // sleep the process for 2 seconds
        // to ensure 2nd child executes first
        sleep(2);
        wait(NULL);
        cout << "1st child Terminated" << endl;
    }

    // second child
    else if (id1 > 0 && id2 == 0) {
        // sleep the process for 1 second
        sleep(1);
        cout << "2nd Child Terminated" << endl;
    }

    // grand child
    else {
        cout << "Grand Child Terminated" << endl;
    }

    return 0;
}

Python3

# Program to demonstrate bottom to up execution
# of processes using fork() and wait()
import os
import time

if __name__ == "__main__":
    # creating 4 process using 2 fork calls
    # 1 parent : 2 child : 1 grand-child
    pid1 = os.fork()
    pid2 = os.fork()

    # Parent Process
    if pid1 > 0 and pid2 > 0:
        os.wait()
        os.wait()
        print("Parent terminated")

    # 1st Child
    elif pid1 == 0 and pid2 > 0:
        # sleep the process for 2 seconds
        # to ensure 2nd child executes first
        time.sleep(2)
        os.wait()
        print("1st child terminated")

    # 2nd Child
    elif pid1 > 0 and pid2 == 0:
        # sleep the process for 1 second
        time.sleep(1)
        print("2nd child terminated")

    # Grand Child
    else:
        print("Grand child terminated")

Output:

Grand Child Terminated
2nd Child Terminated
1st child Terminated
Parent Terminated

Time Complexity: O(1)
Auxiliary Space: O(1)