What's the meaning of the parameter, lPrivate, in the MSG structure?

Question

What's the meaning of the parameter, lPrivate, in the MSG structure?

StreamingMoon 60

(https://learn.microsoft.com/en-us/windows/win32/api/winuser/ns-winuser-msg

The link above is the document about MSG structure.)

typedef struct tagMSG {
  HWND   hwnd;
  UINT   message;
  WPARAM wParam;
  LPARAM lParam;
  DWORD  time;
  POINT  pt;
  DWORD  lPrivate;
} MSG, *PMSG, *NPMSG, *LPMSG;

According to the article, we could notice that, the seventh parameter, lPrivate, has no any explanation about it.

User's image

It's rare, because even if something is protected or meaningless now, the Microsoft's document would point that out, but nothing about lPrivate.

What we could know is only that it is a DWORD.

So, what's the actual meaning of lPrivate?

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Your answer

Answer 1

It seems that the Windows documentation doesn't accurately reflect the MSG structure as seen in the Windows SDK headers.

From a real winuser.h -

/*
 * Message structure
 */
typedef struct tagMSG {
    HWND        hwnd;
    UINT        message;
    WPARAM      wParam;
    LPARAM      lParam;
    DWORD       time;
    POINT       pt;
#ifdef _MAC
    DWORD       lPrivate;
#endif
} MSG, *PMSG, NEAR *NPMSG, FAR *LPMSG;

As you can see, the lPrivate member only exists if _MAC is defined.

The historical discussion of this macro can be found at https://stackoverflow.com/questions/2376478/whats-with-ifdef-mac-in-windows-header-files