CODEWITHSUNDEEP
c
Adii
Adii

Reputation: 17

Problem of understanding, Why these 2 programs take 3 and 6 seconds respectively

I start programming in C and I cannot understand why these two programs take 3 and 6 seconds respectively to run. If anyone can help me thanks :)

Program 1 :

int main(){
    int i;
    for(i=1;i<=3;i++)
    {
        if(fork()==0)
        {
            sleep(i);
            exit(0);
        }
    }
    for(i=1;i<=3;i++)
    {
        wait(NULL);
    }    
    return 0;   
}

Program 2 :

int main(){
    int i;
    for(i=1;i<=3;i++)
    {
        if(fork()==0)
        {
            sleep(i);
            exit(0);
        }
        wait(NULL);
    }
    return 0;
}

Upvotes: 0

Views: 103

Answers (3)

Rehmaan Momin
Rehmaan Momin

Reputation: 151

So, here is some things to understand first:

Fork:

  • creates a new process that copies all data from the current process (like a brain transplant).
    • The child process gets 0 as a value from the fork the parent gets the ID of the child process
  • This new process then runs concurrently with the other processes.
    • This means that it won't wait for other processes to finish unless told otherwise.

Wait(NULL):

  • Waits for a child process (of this process) to finish executing
  • If there are no remainig child processes (of this process), then it returns automatically.

So for the first problem:

The max sleep time is 3 seconds (i= 3). Since the processes don't wait on each other (except for when they are about to terminate), the whole process only takes 3 seconds (the sleeps happen concurrently).

Try understanding the second on your own, feel free to ask questions.

Upvotes: 2

Lytigas
Lytigas

Reputation: 924

3 and 6 seconds are the correct amount of time each should run.

The difference between the two is that one spawns all three processes and then waits on all of them simultaneously, while the other spawns one process and then waits before spawning the other. This is the effect of moving the wait(NULL) inside or outside the loop.

Program 1:
t    parent      child 1     child 2     child 3
0    fork()x3    sleep       sleep       sleep
1    wait                    sleep       sleep
2    wait                                sleep
3    done

Program 2
t    parent      child 1     child 2     child 3
0    fork();wait sleep       
1    fork();wait             sleep
2    wait                    sleep
3    fork();wait                         sleep
4    wait                                sleep
5    wait                                sleep

Upvotes: 2

Barmar
Barmar

Reputation: 780663

In the first program, the loop that calls fork() doesn't wait for wach child to exit before it continues and creates the next child. So all 3 children are started at about the same time.

The second loop then waits for all of them to exit. It will complete when the third child exits, which is 3 seconds after the program started.

The second program calls wait() inside the loop that creates the children. So it doesn't start child 2 until after child 1 has exited. So the total time is the sum of all the sleep times, rather than just the largest time.

It's much easier to see the difference when you indent the code properly.

Upvotes: 2

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