Process groups, sessions, parents and so on

Here's a little program that can be used to help understand some of the intricate aspects:

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

int main(void) {
        pid_t p;

        printf("parent: %d\n", getpid());
        if ((p = fork()) == 0) {
                if ((p = fork()) == 0) {
                        setsid();
                        if ((p = fork()) == 0) {
                                if ((p = fork()) == 0) {
                                        pause();
                                }
                                pause();
                        }
                        exit(0);
                }
                pause();
        }
        pause();

        return 0;
}

Compile and run this, and then check the results. On Linux, like this:

stian@ubuntu04:~$ ps -C poop -o pid,ppid,pgid,sid,tty,cmd
  PID  PPID  PGID   SID TT       CMD
17017  3516 17017  3516 pts/1    ./poop
17018 17017 17017  3516 pts/1    ./poop
17019 17018 17019 17019 ?        [poop] <defunct>
17020     1 17019 17019 ?        ./poop
17021 17020 17019 17019 ?        ./poop

Now let that sink in. Questions to ask yourself:

1. Why is 17020's ppid 1?
2. Why is session and process group different to begin with, then not?
3. Why is one process defunct?
4. Why don't the last 3 have a controlling tty?
5. How come the first process got a new process group?
6. Why is UNIX so complicated?

Note: The author (Stian) doesn't know the answers to several of these.