LinuxQuestions.org - A problem with threads creates 300+ zombie processes

I have a problem with 4 threads (I think it doesn't matter what I'm trying to do) and for each thread I call pthread_join. My problem is it makes 300+ zombie processes and after the program finishes, I have no more zombie processes, so I suppose it kills all of them somehow. The point is the problem works fine on my computer, but I'm a student and this is one of my homeworks, and I have to show this problem to a teacher at school, and I know we have there a 100 processes limit, and it won't work there.
In my thread function I call a popen like this:

Code:

    strcpy (s, "ps -axuf 2>/dev/null | grep ");

    strcat (s, a->nume_user);

    f = popen (s, "r");

I know it's not recommended to redirect the standard output or error and to make pipe linking within a popen, but I did it before and it worked fine. Could it be this? Do you have any idea why this happens?