windowprocedure

Window procedure, often referred to by its shorter name WndProc, is the central callback used in the Windows API to process messages for a window. It handles events generated by the operating system or other applications, such as painting requests, user input, window state changes, and notifications from controls. The window procedure receives each message with a window handle, a message identifier, and two additional parameters whose meaning depends on the message. The procedure returns a result that also depends on the message.

The window procedure has a fixed signature and is typically declared as LRESULT CALLBACK WindowProcedure(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam). The uMsg parameter identifies the message, while wParam and lParam carry message-specific data. The procedure is registered as part of a window class (via WNDCLASS or WNDCLASSEX) so that every window created from that class uses the same procedure.

Messages are delivered to the window procedure through the system’s message loop. A typical program retrieves messages with GetMessage, translates them with TranslateMessage, and dispatches them with DispatchMessage, which invokes the window procedure with the message parameters. For messages that the procedure does not handle, DefWindowProc provides default processing.

Common messages include WM_CREATE, WM_DESTROY, WM_PAINT, WM_SIZE, WM_CLOSE, WM_COMMAND, and WM_KEYDOWN, among others. Window procedures should be efficient to avoid blocking the message loop; long-running work should be performed asynchronously or in a separate thread. Correct use of DefWindowProc and appropriate handling of messages are central to reliable window-based applications.