LPARAM

lParam is a parameter used in Windows messages to convey additional information to a window procedure. It is the second parameter, along with wParam, passed to the window procedure (WndProc) and its interpretation is defined by the specific message. The type of lParam is LPARAM, which is a signed, pointer-sized long: 32 bits on 32-bit builds and 64 bits on 64-bit builds.

The meaning of lParam varies by message. For some messages it carries small integer data or flags, while for others it holds a pointer to a data structure. A common example is mouse input messages such as WM_MOUSEMOVE, where the x and y coordinates are packed into the low and high words of lParam. Macros such as GET_X_LPARAM and GET_Y_LPARAM are used to extract the signed 16-bit coordinates from lParam. For other messages, lParam is a pointer to a structure, for instance lParam may point to a COPYDATASTRUCT in WM_COPYDATA or to an NMHDR structure in WM_NOTIFY, in which case you cast lParam to the appropriate pointer type.

Because lParam is architecture-dependent, code that interprets it should use the documented interpretation for each message and appropriate casting or macros. It is not universal data; its content is defined by the message’s documentation.

Security and correctness considerations include the fact that data passed in lParam may be a pointer to memory in the sending process. When messages cross process boundaries, pointers in lParam are invalid in the receiving process, so cross-process communication should avoid passing raw pointers and instead use safe IPC methods such as CopyData, shared memory, or other mechanisms.

In summary, lParam provides message-specific information to window procedures, with its type and interpretation determined by the individual Windows message being processed.