Q241857: BUG: CComBSTR Does Not Allocate Enough Space Sometimes

Article: Q241857
Product(s): Microsoft C Compiler
Version(s): 3.0,6.0
Operating System(s): 
Keyword(s): kbString kbVC600bug kbATL300bug kbDSupport kbGrpDSMFCATL
Last Modified: 28-JUN-2001

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The information in this article applies to:

- The Microsoft Active Template Library (ATL) 3.0, included with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

The following fragment of code produces different results with ATL version 2.1
and ATL version 3.0:

  char rawStr[128] = "01234567839";
  CComBSTR bstr (4, rawStr);

In ATL 2.1, the new bstr is L"0123"; however, in ATL 3.0, the new bstr is L"012".

CAUSE
=====

This problem is due to a bug in the A2WBSTR function in the Atlconv.h file:

  inline BSTR A2WBSTR(LPCSTR lp, int nLen = -1)
  {
   USES_CONVERSION;
   BSTR str = NULL;
   int nConvertedLen= MultiByteToWideChar(_acp, 0, lp, nLen, NULL, NULL)-1;
   str = ::SysAllocStringLen(NULL, nConvertedLen);
   if (str != NULL)
   {
    MultiByteToWideChar(_acp, 0, lp, -1,
     str, nConvertedLen);
   }
   return str;
  }

If the MultiByteToWideChar function's fourth argument is -1, the string is
assumed to be NULL-terminated and the number of bytes returned will include the
NULL character. The result of this function should not subtract 1 (one) if nLen
does not equal -1; it should only do this if nLen is equal to -1.

RESOLUTION
==========

There are two ways to correct this problem:

Method 1:

Use one of the conversion macros to wrap the string passed into the constructor:

  CComBSTR bstr(5, T2OLE(buf));

  -or-

  CComBSTR bstr(5, OLESTR("01234");

This method uses the CComBSTR(int nSize, LPCOLESTR sz) constructor, which calls
::SysAllocStringLen, and then returns a BSTR with the number of characters equal
to nSize.

Method 2:

Change the incorrect code in Atlconv.h for the A2WBSTR function:

1. Change the code in Atlconv.h for the A2WBSTR function from the following:

  inline BSTR A2WBSTR(LPCSTR lp, int nLen = -1)
  {
    USES_CONVERSION;
    BSTR str = NULL;
    int nConvertedLen = MultiByteToWideChar(_acp, 0, lp, nLen, NULL, NULL)-1;
    str = ::SysAllocStringLen(NULL, nConvertedLen);
    if (str != NULL)
    {
      MultiByteToWideChar(_acp, 0, lp, -1, str, nConvertedLen);
    }
    return str;
  }

  to the following:

  inline BSTR A2WBSTR(LPCSTR lp, int nLen = -1)
  {
     USES_CONVERSION;
     BSTR str = NULL;
     int nConvertedLen = MultiByteToWideChar(_acp, 0, lp,
       nLen, NULL, NULL);
   
     // BUG FIX #1 (from Q241857): only subtract 1 from 
     // the length if the source data is nul-terminated
     if (nLen == -1)
        nConvertedLen--;
   
     str = ::SysAllocStringLen(NULL, nConvertedLen);
     if (str != NULL)
     {
       MultiByteToWideChar(_acp, 0, lp, nLen, str, nConvertedLen);
     }
     return str;
  }

2. Save Atlconv.h.

3. On the Build menu, click Rebuild All. You need to do this on a Debug or
  ReleaseMinDependency build so that the modified code above is linked into
  your code.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

Additional query words: A2WBSTR A2BSTR A2W BSTR SysAllocString

======================================================================
Keywords          : kbString kbVC600bug kbATL300bug kbDSupport kbGrpDSMFCATL 
Technology        : kbVCsearch kbAudDeveloper kbATLsearch
Version           : :3.0,6.0
Issue type        : kbbug

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