Q152319: FIX: ReadString Gives Wrong Result Reading Long Strings

Article: Q152319
Product(s): Microsoft C Compiler
Version(s): winnt:4.0
Operating System(s): 
Keyword(s): kbFileIO kbMFC kbVC400bug kbVC410fix kbGrpDSMFCATL kbNoUpdatekbbuglist kbfixlist
Last Modified: 06-MAY-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Editions, 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 );
     }

You must supply a file (titled c:\test.txt in the example) that contains at least
two strings with more than 128 characters in each string for the problem to be
manifested.

Additional query words: 4.00

======================================================================
Keywords          : kbFileIO kbMFC kbVC400bug kbVC410fix kbGrpDSMFCATL kbNoUpdate kbbuglist kbfixlist
Technology        : kbAudDeveloper kbMFC
Version           : winnt:4.0
Issue type        : kbbug
Solution Type     : kbfix

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