How to fork only with a specific child?

Question

I am trying to build a process tree like this:

               P1
            /      \
          P3        P2
                      \
                       P4
                        \
                         ....

But i couldn't make it work. I tried this but it only forks with P1.

for(int i = 0; i < depth; i++) {
    int _pid = fork();
    if(_pid == 0) {
        printf("[son] pid %d from [parent] pid %d\n", getpid(), getppid());
        exit(0);
    }
}
wait(NULL);

Answer 1

The requirement to use only one fork call makes the solution cumbersome and ugly, but still doable like this:

Step 1: create an inner loop that will run twice if i is 0, and once for any i > 0.

Step 2: fork only in the child process after the first iteration of the outer loop.

Step 3: exit the child that was forked on the second iteration of the inner loop.

int cur_pid = 0;

printf("Root PID %d started by %d\n", getpid(), getppid());

for (int i = 0; i < depth && cur_pid == 0; i++) {
    for (int j = 0; j < (i ? 1 : 2); j++) {
        cur_pid = fork();
        if (cur_pid == 0) {
            printf("[son] pid %d from [parent] pid %d\n", getpid(), getppid());
            if (j == 1) {
                exit(0);
            } else {
                break;
            }
        } else if (cur_pid < 0) {
            printf("Error forking at PID %d\n", getpid());
        }
    }
}

wait(NULL);

Remember to wait on the child process of every iteration if you want getppid to return valid values!