Re: Memory Mapped File Problem
From: Jase (jshelley_at_nospam.optushome.com.au)
Date: 03/16/04
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Date: Wed, 17 Mar 2004 00:02:22 +1100
You appear to have created a file mapping that is one granual long, then
tried to create a view on that mapping starting at one granual.
Jase
"Nicolas" <info@-REMOVE-.chsoft.ch> wrote in message
news:1dhd50djiepbjae9gkuh3gt1lo1aikt625@4ax.com...
> Hi
>
> I want to read a portion of a large file (50Mb+) into a memory mapped
> file for fast access. Since I don't need the full file in the memory,
> I'm trying to read only a portion into memory by using
> MapViewOfFile(...). This doesn't work somehow, I always get an error
> (GetLastError()) of 5 (ACCESS_DENIED) whenever I set the starting
> offset (dwStartOffset) for MapViewOfFile to something other than 0.
>
> Btw: Reading the whole file into memory (last two parameters of
> MapViewOfFile set to zero) works without any problems, but slows down
> the system a lot if the file to map is larger than the available
> physical memory.
>
> Here's the de-classified portion that does the file mapping. Maybe
> someone can tell me what to do to make it work....
>
> thanks in advance
> Nicolas
>
>
> --- code start ---
> const BOOL CMemMapTestDlg::ReadFile()
> {
> CString sFilename = "mytestfile.bin"; // file is about 50 MB
>
> HANDLE hFile = INVALID_HANDLE_VALUE;
> HANDLE hMapping = NULL;
> BOOL bReadOnly = TRUE;
> LPVOID lpData = NULL;
> CString sMappingName = "";
> BOOL bOpen = FALSE;
> HANDLE hMutex = NULL;
> DWORD dwLength = 0;
> LPSECURITY_ATTRIBUTES lpSecurityAttributes = NULL;
>
> SYSTEM_INFO SysInfo;
> ZeroMemory( &SysInfo, sizeof( SYSTEM_INFO ) );
> GetSystemInfo( &SysInfo );
> const unsigned long lMaxBytesToRead =
> SysInfo.dwAllocationGranularity;
>
> DWORD dwDesiredFileAccess = GENERIC_READ;
> if (!bReadOnly)
> {
> dwDesiredFileAccess |= GENERIC_WRITE;
> }
>
> DWORD dwShareMode = 0;
> //Open the real file on the file system
> hFile = CreateFile(sFilename, dwDesiredFileAccess, dwShareMode,
> NULL,
> OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
> if (hFile == INVALID_HANDLE_VALUE)
> {
> TRACE(_T("Failed in call to CreateFile, GetLastError returned
> %d\n"), GetLastError());
> //UnMap();
> return FALSE;
> }
>
> DWORD dwFileSizeHigh=0;
> DWORD dwLength_ = GetFileSize(hFile, &dwFileSizeHigh);
> if (dwLength_ == 0xFFFFFFFF)
> {
> //There was an error calling GetFileSize
> TRACE(_T("Failed in call to GetFileSize, GetLastError returned
> %d\n"), GetLastError());
> //UnMap();
> return FALSE;
> }
>
> //Fail if file is greater than 4GB in size
> if (dwFileSizeHigh)
> {
> //There was an error calling GetFileSize
> TRACE(_T("File size is greater than 4GB, Memory mapping this file
> size will not work until CMemMapFile supports Win64 !!\n"));
> // UnMap();
> return FALSE;
> }
>
> //Fail if file is 0 length in size, calling CreateFileMapping on a
> //zero length file on 95/98 can cause problems
> if (dwFileSizeHigh == 0 && dwLength_ == 0)
> {
> TRACE(_T("File size is 0, not attempting to memory map the
> file\n"));
> //UnMap();
> return FALSE;
> }
>
> //Now work out the size of the mapping we will be performing
> if (lMaxBytesToRead)
> dwLength = lMaxBytesToRead;
> else
> dwLength = dwLength_;
>
> //Create the file mapping object
> DWORD flProtect = (!bReadOnly) ? PAGE_READWRITE : PAGE_READONLY;
>
> //work out the length of the file mapping to create
> //DWORD dwLength = m_dwLength;
>
> hMapping = ::CreateFileMapping(hFile, lpSecurityAttributes,
> flProtect, 0, dwLength, sMappingName);
> if (hMapping == NULL)
> {
> TRACE(_T("Failed in call to CreateFileMapping, GetLastError
> returned %d\n"), GetLastError());
> // UnMap();
> return FALSE;
> }
>
>
> // start reading from here:
> DWORD dwStartOffset = 1 * SysInfo.dwAllocationGranularity;
>
> //Map the view
> DWORD dwDesiredAccess = (!bReadOnly) ? FILE_MAP_WRITE :
> FILE_MAP_READ;
> lpData = MapViewOfFile(hMapping, dwDesiredAccess, 0, dwStartOffset,
> dwLength);
> DWORD dwErr = GetLastError();
> //ASSERT( 0 == dwErr );
>
> // Create the mutex to sync access
> // hMutex = CreateMutex(lpSecurityAttributes, FALSE,
> CreateMutexName());
>
> // data ready??
> if( NULL != lpData )
> {
> MessageBox( "success" );
> LPVOID x = lpData;
> //.... do some processing here...
> }
>
>
>
> // clean up...
> if (lpData != NULL)
> {
> FlushViewOfFile(lpData, 0);
> UnmapViewOfFile(lpData);
> lpData = NULL;
> }
>
> //remove the file mapping
> if (hMapping != NULL)
> {
> CloseHandle(hMapping);
> hMapping = NULL;
> }
>
> //close the file system file if its open
> if (hFile != INVALID_HANDLE_VALUE)
> {
> CloseHandle(hFile);
> hFile = INVALID_HANDLE_VALUE;
> }
>
> //Close the mutex we have been using
> if (hMutex != NULL)
> {
> CloseHandle(hMutex);
> hMutex = NULL;
> }
>
> //Reset the remaining member variables
> bReadOnly = TRUE;
> sMappingName.Empty();
> dwLength = 0;
>
> return TRUE;
> }
>
>
> --- code end ---
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