Commenter bav016 asks how functions like PostQuitMessage and SetTimer(NULL) know which thread the messages should go to. Unlike some functions such as InvalidateRect which have a window handle parameter that lets you say which window you want to operate on, PostQuitMessage and SetTimer(NULL) don’t say which thread the WM_QUIT or WM_TIMER message should go to. How do they decide?

The messages go to the current thread; that is, they are delivered to the thread that called the function in the first place.

There are many functions which operate on an implicit message queue, and those cases, they operate on the message queue associated with the thread making the call. If you call GetKeyState you retrieve the calling thread’s keyboard state. If you call GetMessage you retrieve messages from the calling thread’s message queue. If you call InSendMessage, you are told about the calling thread’s message processing state. If you call GetQueueStatus you retrieve information about the calling thread’s msesage queue. You get the idea.

If you want these functions to operate on a thread different from the one that is executing, you’ll have to ask that thread to make the call for you.