Memory Mapped File Problem
From: Nicolas (info_at_-REMOVE-.chsoft.ch)
Date: 03/16/04
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Date: Tue, 16 Mar 2004 10:15:28 +0100
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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