SetWindowsHookEx()

This API allows one to execute a DLL routine upon the trigger of specific events.  The list of events are documented in winuser.h.  Some examples are

WH_KEYBOARD = 2
WH_MOUSE = 7
WH_SHELL = 10

The API accepts 4 parameters

int hooktype - event to be hooked
HOOKPROC procPtr - exported DLL routine to call
HINSTANCE dllHandle - handle to DLL containing the hook routine
DWORD dwThreadId - specific thread or all thread (set to 0) that trigger this event

It return the pointer to the hooked routine or NULL if call fails.

To release the hooked event, use UnHookWindowsHookEx()

The calling program first will call LoadLibrary() to load the DLL.  Then it uses GetProcAddress() to get the address of specific routine to used in the hook.  Finally, it issues SetWindowsHookEx() to hook to the event.

The hock routine should call CallNextHookEx() to propagate the event to the next hook, passing along the parameters.