It appears to me that, as things stand now, you are calling exit() on the main thread, which shuts down the entire application, but you didn’t signal the child threads, nor did you wait for them to complete.

As shown in “thread signaling,” in the Threads documentation, you need to signal each thread to complete.   Then, use join() to cause the main thread to wait-for and reap each one.   Or, also as shown there, follow the KILL with an immediate detach(), although this seems less-than-elegant to me.   I would run one loop that signals each thread (first...), then a second loop that reaps each thread.

Putting a tiny sleep() call just before the main-thread ends never hurts, either.   An unnoticeable, but intentional, delay covers over a multitude of sins.   (You know, the irritating little timing-holes that never show up in testing, but always appear when the CEO is looking at a big demo.   I don’t like finger-dancing in such situations.)