operating system - OS; resources automatically clean up

Question

From this answer: When is a C++ terminate handler the Right Thing(TM)?

It would be nice to have a list of resources that 'are' and 'are not' automatically cleaned up by the OS when an application quits. In your answer it would be nice if you can specify the OS/resource and preferably a link to some documentaiton (if appropriate).

The obvious one:

Memory: Yes automatically cleaned up. Question. Are there any exceptions?

Answer 1

There are some obscure resources that Windows does not clean up when an app crashes or exits without explicitly releasing them, mostly because the OS doesn't know if they're important to leave around or not.

1. Temporary files -- as others have mentioned.
2. Globally registered WNDCLASSes ("No window classes registered by a DLL are unregistered when the DLL is unloaded. A DLL must explicitly unregister its classes when it is unloaded." MSDN) If your global window class also has a class DC, then that DC will leak as well.
3. Global ATOMs (a relatively limited resource).
4. Window message IDs created with RegisterWindowMessage. These are designed to leak, since there's no UnregisterWindowMessage.
5. Semaphores and Events aren't technically leaked, but when the owning application goes away without signalling them, then other processes can hang. This is not true for a Mutex. If the owning application goes away, other processes waiting on that Mutex are released.
6. There may be some residual weirdness on Windows XP and earlier if you don't unregister a hot key before exiting. Other applications may be unable to register the same hot key.
7. On Windows XP and earlier, it's not uncommon to have a zombie console window live on after a process crashes. (Specifically, a GUI application that also creates a console window.) It shows up on the task bar. All you can do is minimize, restore, or move the window.
8. Buggy drivers can be aggravated by apps that don't explicitly release resources when they exit. Non-paged pool leaks are fairly common.
9. Data copied to the clipboard. I guess that doesn't really count because it's owned by the OS at that point, not the application that put it there.
10. Globally installed hooks aren't unloaded when the installing process crashes before removing the hook.