FIX: Bad POINT Structure in MSG Returned by GetCurrentMessage

ID: Q135913

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:
- Microsoft Visual C++ 32-bit Edition, versions 2.0, 2.1

SYMPTOMS

MFC provides a function called GetCurrentMessage that returns a pointer to a MSG structure encapsulating the current message being processed. The MSG structure contains a member variable (pt), of type POINT that contains incorrect values.

CAUSE

The GetCurrentMessage function fills in the POINT structure with the following line of code:


   *((DWORD*)&pThreadState->m_lastSentMsg.pt) = ::GetMessagePos();

This code was correct in the 16-bit version of Visual C++ because a POINT structure was made up of two intergers (int) that were each 16-bits. Thus the size of a POINT was the same as a DWORD.

However, in the 32-bit version of Windows and of the Visual C++ compiler, a POINT structure is made up of two LONGs, each 32-bits, so the total size of a POINT is now 64-bits and the above assignment is not valid.

RESOLUTION

To obtain the correct value for the point in the current MSG you can use MFC's overridden CPoint constructor which takes a DWORD as an argument. For example:


   CPoint pt(::GetMessagePos());
   // Now pt has the correct values

Please note that because the GetCurrentMessage function returns a pointer to a 'const' MSG structure, you can't change the pt member of the message structure.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem was corrected in Visual C++ version 2.2.

Additional query words: 2.00 2.10 3.00 3.10

Keywords :
Version : winnt:
Platform : winnt
Issue type :