Microsoft KB Archive/154895

= FIX: CInternet::File ReadString Loses Data =

Article ID: 154895

Article Last Modified on 11/21/2006

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APPLIES TO

 Microsoft Foundation Class Library 4.2, when used with:  Microsoft Visual C++ 4.2 Enterprise Edition

 Microsoft Visual C++ 4.2 Professional Edition 

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This article was previously published under Q154895



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; }

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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.

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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

Keywords: kbbug kbfix kbvc500fix KB154895

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