GetWindowLong

GetWindowLong is a Windows API function in the user32.dll that retrieves a value associated with a specified window. It returns a LONG value based on the window handle (HWND) and an index (nIndex), which identifies the type of data to retrieve. The data can include a pointer or handle stored with the window, such as the window procedure, the instance handle, or values stored in the window’s extra memory.

Common indices used with GetWindowLong include GWL_WNDPROC (window procedure pointer), GWL_HINSTANCE (instance handle), GWL_STYLE and GWL_EXSTYLE (window styles), GWL_ID (control identifier), and GWL_USERDATA (per-window user data). These constants define the kinds of information you can query about a window.

64-bit considerations and modern usage: On 64-bit Windows, pointers and handles are 64 bits, so GetWindowLong may not be sufficient or safe for retrieving pointer-sized data. The recommended function is GetWindowLongPtr, which takes indices defined with the GWLP_ prefix, such as GWLP_WNDPROC, GWLP_HINSTANCE, GWLP_STYLE, GWLP_EXSTYLE, GWLP_ID, and GWLP_USERDATA. For compatibility, GWL_* constants map to the same meanings, but GetWindowLongPtr/SetWindowLongPtr should be used for pointer-sized data and for modifying window attributes in 64-bit code.

Both GetWindowLong and GetWindowLongPtr have ANSI and Unicode variants (GetWindowLongA/W and their GetWindowLongPtr counterparts). In 64-bit applications, GetWindowLongPtr and SetWindowLongPtr are preferred for retrieving or setting window data. Note that a zero return value may indicate either actual data or an error; use GetLastError to distinguish.