Q126820: BUG: EM_LIMITTEXT Causes GPF in Japanese and Chinese Windows

Article: Q126820
Product(s): Microsoft Windows Software Development Kit
Version(s): 3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 12-NOV-1999

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) for Windows (Japanese and Traditional Chinese), version 3.1 
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SYMPTOMS
========

Under the Japanese and Traditional Chinese versions of Windows version 3.1, if
an edit control has style ES_AUTOHSCROLL, and EM_LIMITTEXT is sent to the edit
control to limit the number of bytes the user can input to the edit control, a
general protection fault (GPF) will occur when the user is entering a
double-byte character for the last byte.

For example, if the following function is used to limit the number of bytes a
user can input to 7:

     SendMessage (hWnd, EM_LIMITEXT, 7, NULL);

The user enters ABCDEF ( all single-byte character) to the edit control, then try
to enter a double-byte 'G' to the edit control, a GPF will occur.

Also, a GPF will also occur under the Japanese version of Windows version 3.1, if
EM_LIMITTEXT is used and the edit control has style ES_UPPERCASE and the edit
control is using the MS MINCHO or MS GOTHIC font.

Hangeul (Korean) Windows version 3.1 does not GPF if EM_LIMITTEXT is used along
with the ES_UPPERCASE style and the MS MINCHO or MS GOTHIC font.

RESOLUTION
==========

Subclass the edit control, and check the WM_CHAR message coming in. If the
WM_CHAR message for the last byte is a DBCS leadbyte, discard the WM_CHAR
message along with the next WM_CHAR message. The following code demonstrates
this workaround:

  LONG FAR PASCAL SubClassFunc(HWND hWnd,
                               WORD Message,
                               WORD wParam,
                               LONG lParam)
  {
  static BOOL bInvalidchar=FALSE; // Is this the last byte?
     if ( Message == WM_CHAR )
     {
        if (bInvalidchar==TRUE) //The trailing byte of a DBCS character
        {                       //that comes in for the last byte in edit.
           bInvalidchar=FALSE;  //Reset the flag. Next char is valid.
           return 0;            //Throw away the trailing byte.
        }
        else
        {
           // Is this the last byte?  Is the incoming WM_CHAR a DBCS
           //   leadbyte?
           if ( IsDBCSLeadByte(LOBYTE(wParam)) &&
              (( TEXTLIMIT - SendMessage(hWnd, WM_GETTEXTLENGTH, 0,0)) == 1))
           {
              bInvalidchar=TRUE;   //The character is invalid.
              return 0;               // Throw away this byte.
           }
           else return CallWindowProc(lpfnOldWndProc,
                                       hWnd,
                                       Message,
                                       wParam,
                                       lParam);
        }
     }
     else return CallWindowProc(lpfnOldWndProc,
                                hWnd,
                                Message,
                                wParam,
                                lParam);
  }

STATUS
======

Microsoft has confirmed this to be a problem in the product(s) listed at the
beginning of this article.

This problem does not occur in Chinese or Japenese Windows 95 or Windows NT.

Additional query words: cwin jwin fesdk

======================================================================
Keywords          : kb16bitonly 
Technology        : kbHLangJapanese kbHLangChineseTrad kbAudDeveloper kbWin3xSearch kbSDKSearch kbZNotKeyword3 kbWinSDKSearch kbWinSDK310
Version           : :3.1

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