How to fork n children and have them execute different tasks in C?

Question

I'm trying to create 4 children processes from a parent and have each child execute something different.

int main(void) {
int processes = 4;
int i;
for (i = 1; i <= processes; i++) {
    if (fork() == 0) {
        printf("pid: %d\n", getpid());
        exit(0);
    }
}
int status;
for (i = 1; i <= processes; i++)
    wait(&status);
}

Right now the output produces pid: 5847 pid: 5846 pid: 5845 pid: 5844

Why are the pid's in decreasing rather than increasing orders? Am I not correctly using the fork() to create the children?

Michael Michael · Accepted Answer · 2015-09-12T08:29:32

This is an optical illusion. The PIDs are in increasing order ;) Let me explain:

First, process 5844 is created
Then process 5845 is created
Then process 5846 is created
Then process 5847 is created
Then process 5847 is selected by the OS scheduler and prints "5847"
Then process 5846 is selected by the OS scheduler and prints "5846"
Then process 5845 is selected by the OS scheduler and prints "5845"
Then process 5844 is selected by the OS scheduler and prints "5844"

You just have no control over which process is selected first by the scheduler. But, if you add a sleep(1); to the end of your for loop, I'm sure the PIDs will be in increasing order (unless you hit the ceiling and they wrap around.)

At least Linux and OS X produces PIDs in increasing order, don't know about other Unix-like operating systems.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(void) {
    int processes = 4;
    int i;
    int fork_result;
    int number_of_children;
    for (i = 1; i <= processes; i++) {
        fork_result = fork();
        if (fork_result > 0) {
            printf("parent says: hello child #%d, how are you?\n", fork_result);
            number_of_children++;
        } else if (fork_result == 0) {
            printf("pid: %d\n", getpid());
            exit(0);
        } else if (fork_result < 0) {
            printf("parent says: fork() failed\n");
        }
    }
    int status;
    for (i = 1; i <= number_of_children; i++) {
        wait(&status);
    }
}

On my system (OS X 10.10.5) this prints:

parent says: hello child #2577, how are you?
parent says: hello child #2578, how are you?
pid: 2577
pid: 2578
parent says: hello child #2579, how are you?
parent says: hello child #2580, how are you?
pid: 2579
pid: 2580

What OS are you using? The "parent says:" lines should be in increasing order in any case.

How to fork n children and have them execute different tasks in C?

1 Answers