SetWindowLong

SetWindowLong is a Windows API function used to set a value associated with a window, either in the window’s extra data or, commonly, its window procedure pointer. It is part of the family that includes GetWindowLong and their pointer-sized variants. In modern code, SetWindowLongPtr is preferred for proper 64-bit compatibility.

The API has two main forms. In 32-bit applications, the signature is LONG SetWindowLong(HWND hWnd, int nIndex, LONG dwNewLong). In 64-bit applications, the portable form is LONG_PTR SetWindowLongPtr(HWND hWnd, int nIndex, LONG_PTR dwNewLongPtr). The SetWindowLong macro maps to SetWindowLongPtr on 64-bit builds, but direct use of SetWindowLong with pointers can truncate values on 64-bit systems, so SetWindowLongPtr is recommended.

The nIndex parameter selects what is being set. Common indices include GWLP_WNDPROC or GWL_WNDPROC to install a new window procedure, and GWLP_USERDATA or other GWLP/GWL values for window-associated data. When subclassing a window by replacing its WndProc, the usual pattern is to install a new procedure with GWLP_WNDPROC, store the old procedure, and forward messages to the original window procedure via CallWindowProc.

Return value: The function returns the previous value at the specified offset. If the function fails, the return value is zero and extended error information is available via GetLastError. Because a legitimate previous value can be zero, a failure must be detected with GetLastError.

Best practices: Use SetWindowLongPtr with GWLP_WNDPROC for subclassing, and prefer SetWindowLongPtr over SetWindowLong in modern, portable code. This API can affect window behavior across threads and processes, so changes should be made carefully and with proper error handling.