Between Mutex, Semaphore and Horrible Hairdos

Back in the jolly days of horrible hairdos, it was possible to run several instances of a program repeatedly on a single machine: multiple windows were opened one on top of each other endlessly. Today, in most cases, we want to make sure our program runs once per machine. Mutex or Semaphores are here to offer non-recursive semantics, so your program will only run once — no hairspray needed.