I use Qt 4.8.6, MS Visual Studio 2008, Windows 7. I've created a GUI program. It contains main GUI thread and worker thread (I have not made QThread subclass, by the way), which makes synchronous calls to 3rd party DLL functions. These functions are rather slow. QTcpServer instance is also under worker thread. My worker class contains QTcpServer and DLL wrapper methods.
I know that quit() is preferred over terminate(), but I don't wanna wait for a minute (because of slow DLL functions) during program shutdown. When I try to terminate() worker thread, I notice warnings about stopping QTcpServer from another thread. What is a correct way of process shutdown?
Unless there is an overriding reason to do so, you should not attempt to terminate threads with user code at process-termination.
If there is no such reason, just call your OS process termination syscall, eg. ExitProcess(0). The OS can, and will will stop all process threads in any state before releasing all process resources. User code cannot do that, and should not try to terminate threads, or signal them to self-terminate, unless absolutely necessary.
Attempting to 'clean up' with user code sounds 'nice', (aparrently), but is an expensive luxury that you will pay for with extra code, extra testing and extra maintenance.
That is, if your customers don't stop buying your app because they get pissed off with it taking so long to shut down.
The OS is very good at stopping threads and cleaning up. It's had endless thousands of hours of testing during development and decades of life in the wild where problems with process termination would have become aparrent and got fixed. You will not even get close to that with your flags, events etc. as you struggle to stop threads running on another core without the benefit of an interprocessor driver.
There are surely times when you will have to resort to user code to stop threads. If you need to stop them before process termination, or you need to close some DB connection, flush some file at shutdown, deal with interprocess comms or the like issues, then you will have to resort to some of the approaches already suggested in other answers.
If not, don't try to duplicate OS functionality in the name of 'niceness'. Just ask it to terminate your process. You can get your warm, fuzzy feeling when your app shuts down immedately while other developers are still struggling to implement 'Shutdown' progress bars or trying to explain to customers why they have 15 zombie apps still running.