Problem running ps to find PID with multiple users - c

Question

When I run the following code and then run ps to see the processes running, I only get 4 ./a.out running processes even though there are 5 forks. Why is that? How am I able to see the other fork? Also, if multiple people are using my computer, running the same process a.out, how can I terminate only my processes, using only linux commands? Please help.
PID TTY TIME CMD
32941 ttys000 0:00.10 -bash
34098 ttys000 0:00.08 less
33651 ttys000 0:00.00 ./a.out
33652 ttys000 0:00.00 ./a.out
33654 ttys000 0:00.00 ./a.out
33655 ttys000 0:00.00 ./a.out
#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#define N 5 /* How many children to make. */
#define D 1200 /* Sleep 1200 seconds = 20 minutes */
int main(void)
{
int i;
pid_t p;
for (i = 0; i < N; i++) {
p = fork();
if (p == 0) {
close(0);
close(1);
close(2);
if (i == 2) {
setsid();
}
sleep(D);
return 0;
}
}
return 0;
}

Answer 1

All five processes are running, but you're using the ps command that only shows processes associated with your session, but the setsid() called for i=2 sets a different session, so the default ps options won't show you.
$ ps
PID TTY TIME CMD
3048 pts/0 00:00:00 bash
8288 pts/0 00:00:00 a.out
8289 pts/0 00:00:00 a.out
8291 pts/0 00:00:00 a.out
8292 pts/0 00:00:00 a.out
8301 pts/0 00:00:00 ps
$ ps -fe | grep a.out
steve 8288 1 0 13:39 pts/0 00:00:00 ./a.out
steve 8289 1 0 13:39 pts/0 00:00:00 ./a.out
steve 8290 1 0 13:39 ? 00:00:00 ./a.out <-- missing one
steve 8291 1 0 13:39 pts/0 00:00:00 ./a.out
steve 8292 1 0 13:39 pts/0 00:00:00 ./a.out
steve 8303 3048 0 13:40 pts/0 00:00:00 grep --color=auto a.out
$

Answer 2

By default, ps only displays processes with the same session id as itself and associated with a terminal. When you changed the session id of one of the processes, you disqualified it from being listed. ps x will include all of your processes.
By default, ps only lists the processes belong to you. You can be sure of this by using ps ux to include the owners of the processes in the output.
You can use the kill utility to kill these processes. Unless you are root, you can't kill other processes owned by others even if you tried.