Thread: Spawning multiple processes

Hi,

I would like to know how to spawn multiple processes from one process, and have them all doing something at once. I know that you can create a process with the fork() function, but aren't I in the child process when I do that?

So, if I did:
pid_t child;
child = fork();
, then if pid_t is 0, then I'm in the child process now, right? So, if I'm the child now, I could get that child process started on some task, but how do I go back up to the parent to create other processes and have them start other tasks in parallel, without waiting for this current child to die?

Thanks!

If fork() is successful, the return value is 0 in the child and > 0 (actually the PID of the child) in the parent. An example of how to do this might look something like this:

Code:

pid_t ret = fork();

if (ret == -1) {
    // no child process created - handle error
    perror("fork()");
    exit(EXIT_FAILURE);
} else if (ret == 0) {
    // this code is executed in the child process
    while (1) {
        printf("This is the child process\n");
        sleep(1);
    }
} else {
    // this code is executed in the parent process
    while (1) {
        printf("This is the parent process (our child has pid %d)\n", ret);
        sleep(1);
    }
}

In other words, if fork() is successful then both parent and child are executing concurrently (conceptually, at least...) and a program can differentiate between whether it is the child or parent by examining the return value of fork().

OK, that's very helpful. So, since parent and child are running concurrently after a successful fork(), any code executed after the fork() needs to be in a conditional statement based on the PID to prevent the code from being executed by *both* parent and child, correct?

This makes me curious. So, if I did something like this:

pid_t ret1 = fork();
pid_t ret2 = fork();
pid_t ret3 = fork();

, would there be eight processes (including the parent)? I'm thinking this would happen:

Line 1 - Parent creates a child (Child 1).
Line 2 - Parent creates a second child (Child 2), and Child 1 creates a child (Child 3).
Line 3 - Parent creates Child 4, Child 1 creates Child 5, Child 2 creates Child 6, Child 3 creates Child 7.

Exponentially growing processes, scary. Is this correct?

Thanks again.