callbacks and threads - technical question

when the system calls a callback function, does it create a separate thread for this?

i read, for example, that in console programming when i define my own handler(SetConsoleCtrlHandler), system creates separate thread to run this function...

is it always the case?

>> Is it always the case? <<

No. Many callbacks execute on the same thread and are called from within a function you call. For example, when you call EnumWindows(), this function will call your EnumWindowsProc as many times as needed before returning. Therefore, no new thread is needed.

Another example is the TimerProc callback. This callback is called by DefWindowProc when it receives a WM_TIMER message. Again, the order is, your thread enters DefWindowProc, DefWindowProc calls TimerProc, TimerProc runs and returns and finally DefWindowProc returns. All in the same thread.

Callbacks that execute in the same thread are more common than callbacks that execute in a new thread. Unfortunately, the documentation is not always as clear as it could be on this matter.

what about completition routins? or any function that being called upon event triggering?

do you know another examples (like in console) when new thread is created?