BUG: "Out of Memory" Error When Saving to an Existing File

ID: Q163253

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:
- Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1, 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. We are researching this bug and will post new information here in the Microsoft Knowledge Base as it becomes available.

Additional query words: kbVC400bug 4.00 4.10 4.20 MfcDocView

Keywords : kbcode kbDocView kbMFC kbVC
Version : 4.00 4.10 4.20
Platform : NT WINDOWS
Issue type :