FIX: ReadString Gives Wrong Result Reading Long Strings

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with: Microsoft Visual C++, 32-bit Edition, version 4.0

SYMPTOMS

Reading long strings (greater than 128 characters) from a CStdioFile with the ReadString method can result in truncated or incorrect data being returned in the string.

CAUSE

The pointer math used in calculating buffer offsets was incorrect in the code for CStdioFile::ReadString(CString& rString) in FILETXT.CPP.

RESOLUTION

The solution was a rewriting of the MFC code for CStdioFile::ReadString(CString& rString). The following code is from the MFC 4.1 source:

   BOOL CStdioFile::ReadString(CString& rString)
   {
        ASSERT_VALID(this);

        rString = &afxChNil;    // empty string without deallocating
        const int nMaxSize = 128;
        LPTSTR lpsz = rString.GetBuffer(nMaxSize);
        LPTSTR lpszResult;
        int nLen;
        for (;;)
        {
             lpszResult = _fgetts(lpsz, nMaxSize+1, m_pStream);
             rString.ReleaseBuffer();

             // handle error/eof case
             if (lpszResult == NULL && !feof(m_pStream))
             {
                  clearerr(m_pStream);
                  AfxThrowFileException(CFileException::generic, _doserrno,
                       m_strFileName);
             }

             // if string is read completely or EOF
             if (lpszResult == NULL ||
                  (nLen = lstrlen(lpsz)) < nMaxSize ||
                  lpsz[nLen-1] == '\n')
             {
                  break;
             }

             nLen = rString.GetLength();
             lpsz = rString.GetBuffer(nMaxSize + nLen) + nLen;
        }

        // remove '\n' from end of string if present
        lpsz = rString.GetBuffer(0);
        nLen = rString.GetLength();
        if (nLen != 0 && lpsz[nLen-1] == '\n')
             rString.GetBufferSetLength(nLen-1);

        return lpszResult != NULL;
   }

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in the Microsoft Foundation Classes (MFC), included with: Microsoft Visual C++, 32-bit Edition, version 4.1.

MORE INFORMATION

When you invoke ReadString, CStdioFile::ReadString(CString& rString) enters an infinite loop that breaks when the string is completely read in or EOF is reached. It reads your string in chunks of 128 bytes, but the pointer that ReadString uses to position the next read into the string is being shifted by 256 bytes. This results in bytes 129 through 256 being undefined.

Since only 128 of the 140 bytes have been read before the shifting, the last 12 bytes (140 - 128 = 12) are read into the wrong location in the string. Depending upon what happens to be in positions 129-256, the string is either truncated or shows invalid characters.

Sample Code

The following code demonstrates the problem. It assumes you have a CString that has a large buffer already allocated for it, such as 256 bytes. You want to read a line that contains 140 bytes:

   void ReadStringBug()
   {
        CString strBuffer;
        CStdioFile sfMyFile( "C:\\test.txt", CFile::modeRead );

        sfMyFile.ReadString( strBuffer );
        AfxMessageBox( strBuffer );
        sfMyFile.ReadString( strBuffer );
        AfxMessageBox( strBuffer );
   }