Microsoft KB Archive/179671

= FIX: DFX_Text Sets CString Length Incorrectly in Unicode Build =

Article ID: 179671

Article Last Modified on 10/2/2003

-

APPLIES TO


 * Microsoft Visual C++ 5.0 Enterprise Edition

-



This article was previously published under Q179671



SYMPTOMS
In a Unicode build of an MFC DAO project, the Record Field Exchange function DFX_Text incorrectly sets the length of its CString argument to twice the actual length of the data fetched from the database. This action causes any concatenation operation with the CString to behave unexpectedly.

NOTE: This article contains the source code to corrected versions of DFX_Text and DAOStringAllocCallback.



CAUSE
At line 906 in the Daodfx.cpp file within the DFX_Text function, the length of the CString is set to the number of bytes rather than the number of characters returned from the database: value.GetBufferSetLength(  pFX->m_prs->m_pulColumnLengths[pFX->m_nField-1] - 1); Also, at line 1391 in the Daodfx.cpp file within the DAOStringAllocCallback function [used as a callback function by DFX_Text], the length of the CString is set to the number of bytes rather than the number of characters returned from the database: pstr->ReleaseBuffer(dwLen);



RESOLUTION
To resolve this problem, divide the string length by sizeof(TCHAR). In an ANSI build, a TCHAR is 1 byte in length; in a Unicode build, a TCHAR is 2 bytes in length.

This article contains the source code to corrected versions of DFX_Text and DAOStringAllocCallback, which contain the two fixes shown below:

Line 906, Daodfx.cpp, DFX_Text
Original version: pFX->m_prs->m_pulColumnLengths[pFX->m_nField-1] - 1); Fixed version:  (pFX->m_prs->m_pulColumnLengths[pFX->m_nField-1] / sizeof(TCHAR)) [ASCII 150]      sizeof(TCHAR));

Line 1391, Daodfx.cpp, DAOStringAllocCallback
Original version: lpsz = pstr->GetBuffer(dwLen); Fixed version: lpsz = pstr->GetBuffer(dwLen / sizeof(TCHAR));



STATUS
Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug has been fixed in Visual C++ version 6.0



MORE INFORMATION
Copy and paste the following two functions and function declarations into your project: ////////////////////////////////////////////////

STDAPI DaoStringAllocCallback2(DWORD dwLen, DWORD pData, void** ppv);

STDAPI DaoStringAllocCallback2(DWORD dwLen, DWORD pData, void** ppv) {     LPTSTR lpsz; CString* pstr = (CString*)pData;

#ifndef _UNICODE // If using ANSI DAO interfaces, DAO reports field length // rather than data length. In this case there will not be space // for NULL terminator if data length equals field length. Make room. dwLen++; #endif

TRY {        // Only re-allocate if necessary lpsz = pstr->GetBuffer(dwLen / sizeof(TCHAR)); pstr->ReleaseBuffer(dwLen); *ppv = (void*)(dwLen > 0 ? lpsz : NULL); }     CATCH_ALL(e) {        e->Delete; return E_OUTOFMEMORY; }     END_CATCH_ALL

return S_OK; }

void AFXAPI DFX_Text2(CDaoFieldExchange* pFX, LPCTSTR lpszName,     CString& value, int nPreAllocSize = AFX_DAO_TEXT_DEFAULT_SIZE,      DWORD dwBindOptions = AFX_DAO_ENABLE_FIELD_CACHE);

void AFXAPI DFX_Text2(CDaoFieldExchange* pFX, LPCTSTR lpszName,     CString& value, int nPreAllocSize, DWORD dwBindOptions) {     (pFX->m_nFieldType == CDaoFieldExchange::outputColumn) ? ++pFX->m_nField: ++pFX->m_nParam;

// Do nothing if op not supported for outputColumn or param type if (!pFX->IsValidOperation) return;

DWORD dwDAOType;

#ifdef _UNICODE dwDAOType = DAO_WCHAR; #else dwDAOType = DAO_CHAR; #endif

switch (pFX->m_nOperation) {     case CDaoFieldExchange::BindField: {           // Pre-allocate buffer to prevent needless re-allocations value.GetBuffer(nPreAllocSize); value.ReleaseBuffer;

LPDAOCOLUMNBINDING pcb = &pFX->m_prs->m_prgDaoColBindInfo[pFX->m_nField-1]; pcb->dwDataType = dwDAOType; pcb->dwBinding = DAOBINDING_DIRECT | DAOBINDING_CALLBACK; pcb->dwUser = (DWORD)&value; pcb->cbDataOffset = (DWORD)DaoStringAllocCallback2; pcb->cbMaxLen = INT_MAX; }        // Fall through to finish setting up column binding struct

default: pFX->Default(lpszName, (void*)&value, AFX_RFX_TEXT,           dwBindOptions); return;

case CDaoFieldExchange::Fixup: if (pFX->m_prs->m_pulColumnLengths[pFX->m_nField-1] == 0 ||           pFX->m_prs->m_pulColumnLengths[pFX->m_nField-1] == DAO_NULL) {           // If null or empty string set length zero value.GetBufferSetLength(0); }        else {           // Make sure length correct (and doesn't include NULL            //terminator) value.GetBufferSetLength(

(pFX->m_prs->m_pulColumnLengths[pFX->m_nField-1] /   sizeof(TCHAR))- sizeof(TCHAR));

}

pFX->Default(lpszName, (void*)&value,           AFX_RFX_TEXT, dwBindOptions); return;

#ifdef _DEBUG case CDaoFieldExchange::DumpField: *pFX->m_pdcDump << "\n" << lpszName << " = " << value; return; #endif //_DEBUG }  }

////////////////////////////////////////////// Below is an example of how to use the DFX_Text2 function in your project: void CDaoSet::DoFieldExchange(CDaoFieldExchange* pFX) {  #ifdef _UNICODE //AFX_FIELD_MAP DFX_Text2(pFX, _T("[CourseID]"), m_CourseID); DFX_Text2(pFX, _T("[CourseTitle]"), m_CourseTitle); #else //AFX_FIELD_MAP #endif

}

Additional query words: MFCDAO MFCDatabase kbDAO kbMFC kbVC600fix kbVC500bug kbdse kbDSupport

Keywords: kbbug kbfix KB179671

-

[mailto:TECHNET@MICROSOFT.COM Send feedback to Microsoft]

© Microsoft Corporation. All rights reserved.