Q154895: FIX: CInternet::File ReadString() Loses Data

Article: Q154895
Product(s): Microsoft C Compiler
Version(s): winnt:4.2
Operating System(s): 
Keyword(s): kbInternet kbMFC kbSDKInet kbVC420bug kbVC500fix kbGrpDSInet
Last Modified: 06-MAY-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
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SYMPTOMS
========

Calling CInternetFile::ReadString() without calling SetReadBufferSize() first
can result in lost data. If the file being read is less than 4096 bytes (the
default read buffer size), a null string will be returned into the buffer
specified by the first argument of ReadString(). The ReadString() function will
also return NULL. If the file being read is greater than 4096 bytes, the first
4096 bytes will be lost and then subsequent ReadString() calls will return the
rest of the data.

You can see this problem by running the TEAR MFC sample that is included on the
Visual C++ 4.2 CD.

CAUSE
=====

There is a bug in CInternetFile::ReadString(). It calls InternetReadFile()
twice. The second call to InternetReadFile() removes the data retrieved by the
first call to InternetReadFile().

RESOLUTION
==========

To prevent ReadString from calling InternetReadFile() twice, call
SetReadBufferSize() before making the first call to ReadString. This sets a read
buffer, which means that the following code in ReadString is not executed:

     if (m_pbReadBuffer == NULL)
     {
         if (!SetReadBufferSize(4096))   // arbitrary but reasonable
             return NULL;
         if (!InternetReadFile(m_hFile, m_pbReadBuffer, m_nReadBufferSize,
                 &dwRead))
             AfxThrowInternetException(m_dwContext);
          m_nReadBufferBytes = dwRead;
     }

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug was fixed in Visual C++ version 5.0.

MORE INFORMATION
================

To fix the problem in the TEAR sample, before the ReadString() call, add a call
to SetReadBufferSize(). For example:

     TCHAR sz[1024];
     pFile->SetReadBufferSize(4096);  // Add this line
     while (pFile->ReadString(sz, 1023))
     {
         if (bStripMode)
         StripTags(sz);
         cout << sz;
     }

Additional query words: kbVC420bug kbDSupport WinInet Tear

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Keywords          : kbInternet kbMFC kbSDKInet kbVC420bug kbVC500fix kbGrpDSInet 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:4.2
Issue type        : kbbug
Solution Type     : kbfix

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