wParam

WPARAM is a Windows API type used to convey information in Windows messages. It is defined as an unsigned, pointer-sized integer (typedef UINT_PTR WPARAM) and matches the platform’s pointer size: 32 bits on 32‑bit Windows and 64 bits on 64‑bit Windows. Together with LPARAM, WPARAM forms part of the standard message parameter pair delivered to a window procedure.

Usage of WPARAM depends on the specific message being processed. In a typical window procedure, WPARAM is examined in conjunction with the message code to interpret the event. Its meaning is message-specific and documented for each message. In many cases WPARAM carries identifiers, handles, or bit flags, while in other cases it may simply be a small integer.

Examples of common patterns include WM_COMMAND, where LOWORD(wParam) yields the control or menu item identifier and HIWORD(wParam) provides the notification code when the command originates from a control (for menu selections HIWORD is zero). For keyboard and mouse messages, WPARAM often contains modifier or state flags; for example, WM_KEYDOWN uses wParam for the virtual-key code, while certain mouse messages use wParam to convey key-state flags and coordinates are carried in LPARAM.

There are helper macros to extract information from the parameters, such as LOWORD, HIWORD, and specialized macros like GET_X_LPARAM and GET_Y_LPARAM to obtain coordinates from LPARAM for mouse messages. Because the exact interpretation of WPARAM varies by message, developers should consult the documentation for the specific message being handled and avoid assuming a universal meaning.

In short, WPARAM is a flexible, architecture-aware parameter in Windows messages, whose content is defined by each message and can represent identifiers, handles, flags, or small values as appropriate.