A screen saver will start, pending your choices, whenever there is no mouse or keyboard activity for the current screen saver timeout period. Some applications need to know when the screen saver has started in order to do some background processing, for example, writing or updating the data to disk.



MORE INFORMATION
When a screen saver starts, it posts a WM_SYSCOMMAND message to the foreground window with WPARAM as SC_SCREENSAVE. To detect and notify other applications of this event, use the following steps:

Install WH_GETMESSAGE Global hook.

hHook = SetWindowsHookEx(WH_GETMESSAGE, (HOOKPROC)HookProc,
(HINSTANCE) hMod, 0);



Define a user-defined registered message for example, "ScreenSaverStarted".

UINT WM_SCRNSVSTART = RegisterWindowMessage("ScreenSaverStarted");



Broadcast this message to all top-level windows in the system.

LRESULT CALLBACK HookProc(UINT code , WPARAM wParam, LPARAM lParam)
{
MSG *msg = (MSG *)lParam;
if ( msg->message == WM_SYSCOMMAND &&
msg->wParam == SC_SCREENSAVE)
{ // broadcast message to all top-level windows
// Or execute some other code here
PostMessage(HWND_BROADCAST, WM_SCRNSVSTART, 0, 0);
}
// Always call next hook in chain
return CallNextHookEx(hHook, code, wParam, lParam);
}



Uninstall the hook.

UnhookWindowsHookEx(hHook);

A screen saver will start, pending your choices, whenever there is no mouse or keyboard activity for the current screen saver timeout period. Some applications need to know when the screen saver has started in order to do some background processing, for example, writing or updating the data to disk.



MORE INFORMATION
When a screen saver starts, it posts a WM_SYSCOMMAND message to the foreground window with WPARAM as SC_SCREENSAVE. To detect and notify other applications of this event, use the following steps:

Install WH_GETMESSAGE Global hook.

hHook = SetWindowsHookEx(WH_GETMESSAGE, (HOOKPROC)HookProc,
(HINSTANCE) hMod, 0);



Define a user-defined registered message for example, "ScreenSaverStarted".

UINT WM_SCRNSVSTART = RegisterWindowMessage("ScreenSaverStarted");



Broadcast this message to all top-level windows in the system.

LRESULT CALLBACK HookProc(UINT code , WPARAM wParam, LPARAM lParam)
{
MSG *msg = (MSG *)lParam;
if ( msg->message == WM_SYSCOMMAND &&
msg->wParam == SC_SCREENSAVE)
{ // broadcast message to all top-level windows
// Or execute some other code here
PostMessage(HWND_BROADCAST, WM_SCRNSVSTART, 0, 0);
}
// Always call next hook in chain
return CallNextHookEx(hHook, code, wParam, lParam);
}



Uninstall the hook.

UnhookWindowsHookEx(hHook);