Limiting the number of instances of one application

[TheRonin]

IMHO... When I've had to limit the number of application instances in the past, I've used the port-method. With Qt it's simple to do as well. Granted, there are some problems like, what happens if another app is already using the requested port, etc. But i believe that handling those issues are simpler than creating platform specific code for locking processes or handling lock files (which in the worst case can require one or more monitoring processes to clean up after crashes etc).

Those are just my two cents, for whatever they're worth