The WM_TIMER message is posted to the installing thread's message queue when a timer expires. You can process the message by providing a WM_TIMER case in the window procedure. Otherwise, the default window procedure will call the TimerProc callback function specified in the call to the SetTimer function used to install the timer.
WM_TIMER
wTimerID = wParam; // timer identifier
tmprc = (TIMERPROC *) lParam; // address of timer callback
An application should return zero if it processes this message.
The WM_TIMER message is a low-priority message. The GetMessage and PeekMessage functions retrieve this message only when no other higher-priority messages are in the thread's message queue.
Windows NT: Requires version 3.1 or later.
Windows: Requires Windows 95 or later.
Windows CE: Requires version 1.0 or later.
Header: Declared in winuser.h.
Timers Overview, Timer Messages, GetMessage, PeekMessage, SetTimer, TimerProc