lpMsg

lpMsg is a commonly used identifier in Windows API programming that denotes a pointer to a MSG structure. In the Windows API type system, LPMSG is a typedef for MSG*, i.e., a pointer to an MSG. The MSG structure holds information describing a single message in the system’s event queue, including the window handle to which the message is directed, the message identifier, and the associated parameters (wParam and lParam), as well as timing and positional data.

In a typical Windows message loop, the program retrieves messages from the thread’s message queue into a MSG object and dispatches them to the appropriate window procedure. The lpMsg/LPMSG type is used when a function requires a pointer to a MSG, such as when calling GetMessage, which accepts an LPMSG as its first parameter, or when a function wants to reference the current message without copying it.

Historically, the LP prefix stood for “Long Pointer” in 16-bit Windows, but in modern 32- and 64-bit Windows it is simply a typedef to a pointer type. Today LPMSG is effectively MSG*, and lpMsg is a common variable name for a pointer to a MSG in examples and code bases.

See also: MSG structure, GetMessage, TranslateMessage, DispatchMessage, LPMSG, Windows API.