DOCUMENT:Q163253  30-JUL-2001  [visualc]
TITLE   :BUG: "Out of Memory" Error When Saving to an Existing File
PRODUCT :Microsoft C Compiler
PROD/VER:winnt:4.0,4.1,4.2
OPER/SYS:
KEYWORDS:kbcode kbDocView kbFileIO kbMFC kbVC400bug kbVC410bug kbVC420bug kbGrpDSMFCATL

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

 - The Microsoft Foundation Classes (MFC), used with:
    - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1 
    - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
    - Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
-------------------------------------------------------------------------------

SYMPTOMS
========

When you save to an existing file in a Visual C++ MFC application, an "Out of
memory" error occurs. The error occurs only when you save over an existing file
on a non-NT platform, such as Windows 95, that is on a network. For example, if
you run the Scribble sample on a Windows NT machine and try to save the scribble
file to an existing file on a Windows 95 share, the error occurs.

CAUSE
=====

MFC uses a class called CMirrorFile that protects a file in cases where an error
may occur when you are writing to a file. MFC first writes out to a temporary
file and then copies over to the final file when all writing is done.

In the CMirrorFile::Open() function located in Doccore.cpp, the following code
exists:

       DWORD dwLength = 0;
       PSECURITY_DESCRIPTOR pSecurityDescriptor = NULL;
       GetFileSecurity(lpszFileName, DACL_SECURITY_INFORMATION,
            NULL, dwLength, &dwLength);
       pSecurityDescriptor = (PSECURITY_DESCRIPTOR) new BYTE[dwLength];

In the case where a file is located on a non-NT share, the GetFileSecurity call
fails. It may return a large value and this can cause the new statement to fail
with an "out of memory" error. You need to select the return value for
GetFileSecurity.

RESOLUTION
==========

There are a couple of ways to work around the problem.

 - Override the CDocument::GetFile() function to return a CFile object rather
   than a CMirrorFile object. You won't get the protection feature of
   CMirrorFile which checks to see whether you can write the whole file to the
   disk first. This may not be a problem depending upon what the application
   does. See the code the CDocument::GetFile() in \Msdev\Mfc\Src\Doccore.cpp to
   see the default implementation.

 - Create your own CMirrorFile class and check the return value of the
   GetFileSecurity() call. If the call fails, ignore the code that modifies the
   securities of the file. For example, you can create your own
   CMirrorFile-derived class like the following code demonstrates:

   **********
   #include <afxpriv.h>
   #include "c:\msdev\mfc\src\afximpl.h"

   class CMyMirrorFile: public CMirrorFile
   {
         virtual BOOL Open(LPCTSTR lpszFileName, UINT nOpenFlags,
         CFileException* pError = NULL);
   };

   BOOL CMyMirrorFile::Open(LPCTSTR lpszFileName, UINT nOpenFlags,
         CFileException* pError)
   {
       ASSERT(lpszFileName != NULL);
      m_strMirrorName.Empty();

   #ifndef _MAC
      CFileStatus status;
      if (nOpenFlags & CFile::modeCreate) //opened for writing
      {
         if (CFile::GetStatus(lpszFileName, status))
         {
            CString strRoot;
            AfxGetRoot(lpszFileName, strRoot);

            DWORD dwSecPerClus, dwBytesPerSec, dwFreeClus, dwTotalClus;
            int nBytes = 0;
            if (GetDiskFreeSpace(strRoot, &dwSecPerClus, &dwBytesPerSec,
   &dwFreeClus,
               &dwTotalClus))
            {
               nBytes = dwFreeClus*dwSecPerClus*dwBytesPerSec;
            }
            if (nBytes > 2*status.m_size) // at least 2x free space avail
            {
               // get the directory for the file
               TCHAR szPath[_MAX_PATH];
               LPTSTR lpszName;
               GetFullPathName(lpszFileName, _MAX_PATH, szPath, &lpszName);
               *lpszName = NULL;

               //let's create a temporary file name
               GetTempFileName(szPath, _T("MFC"), 0,
                  m_strMirrorName.GetBuffer(_MAX_PATH+1));
               m_strMirrorName.ReleaseBuffer();
            }
         }
      }
   #endif

      if (!m_strMirrorName.IsEmpty() &&
         CFile::Open(m_strMirrorName, nOpenFlags, pError))
      {
         m_strFileName = lpszFileName;
   #ifndef _MAC
         FILETIME ftCreate, ftAccess, ftModify;
         if (::GetFileTime((HANDLE)m_hFile, &ftCreate, &ftAccess, &ftModify))
         {
            AfxTimeToFileTime(status.m_ctime, &ftCreate);
            SetFileTime((HANDLE)m_hFile, &ftCreate, &ftAccess, &ftModify);
         }

           // If GetFileSecurity returns an error, don't do anything with
           // file securities.
         DWORD dwLength = 0;
         PSECURITY_DESCRIPTOR pSecurityDescriptor = NULL;
         if (GetFileSecurity(lpszFileName, DACL_SECURITY_INFORMATION,
            NULL, dwLength, &dwLength))
           {
               pSecurityDescriptor = (PSECURITY_DESCRIPTOR) new
   BYTE[dwLength];
             if (::GetFileSecurity(lpszFileName, DACL_SECURITY_INFORMATION,
                pSecurityDescriptor, dwLength, &dwLength))
             {
                SetFileSecurity(m_strMirrorName, DACL_SECURITY_INFORMATION,
   pSecurityDescriptor);
             }
             delete[] (BYTE*)pSecurityDescriptor;
           }
   #endif
         return TRUE;
      }
      m_strMirrorName.Empty();
      return CFile::Open(lpszFileName, nOpenFlags, pError);
   }

   *************

Then you need to override CDocument::GetFile() to return an object of your new
derived class.

Please note that the work around uses the undocumented class CMirrorFile. The
work around may have to be modified to work in a future version of MFC. Refer to
this article again in the future for any updates and modifications that may be
required for future versions.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

Additional query words:

======================================================================
Keywords          : kbcode kbDocView kbFileIO kbMFC kbVC400bug kbVC410bug kbVC420bug kbGrpDSMFCATL 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:4.0,4.1,4.2
Issue type        : kbbug

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