WndProc

WndProc is the window procedure in the Win32 API. It is a callback function that processes messages for a window. The system calls the window procedure with a message identifier and parameters describing the message. The function is defined with the signature LRESULT CALLBACK WndProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) and is supplied when registering a window class via WNDCLASS or WNDCLASSEX (lpfnWndProc).

Messages represent events or requests such as creation, painting, destruction, resizing, keyboard input, mouse input, or system commands (e.g., WM_PAINT, WM_SIZE, WM_DESTROY, WM_KEYDOWN, WM_LBUTTONDOWN). The WndProc should handle the messages that it cares about and pass unhandled messages to DefWindowProc to provide default processing. Applications typically run a message loop (GetMessage/TranslateMessage/DispatchMessage) in the UI thread; DispatchMessage calls the window procedure for each message addressed to that window. The return value is message-specific; if the procedure processes the message, it should return 0; otherwise DefWindowProc’s return value is used.

Window procedures can be customized for different window classes; developers may subclass an existing window by replacing its WndProc with SetWindowLongPtr and restoring it later. WndProc runs in the thread that owns the window's message queue, and interacts with other UI components through messages and window handles. Although the Windows API widely uses WndProc, newer frameworks may encapsulate it in higher‑level abstractions, while WndProc remains a core building block of traditional Win32 programming.