WindowProc
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In Win32 application programming, WindowProc (or window procedure), also known as WndProc is a user-defined callback function that processes messages sent to a window. This function is specified when an application registers its window class and can be named anything (not necessarily WindowProc).
The window procedure is responsible for handling all messages that are sent to a window. The function prototype of WindowProc is given by:
LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
hwnd is a handle to the window to which the message was sent and uMsg identifies the actual message by its identifier, as specified in winuser.h.
wParam and lParam are parameters whose meaning depends on the message. An application should identify the message and take the required action.