question about the memery map in vc++

the code is as follows, the first time MapViewOfFile can return a sucessful value,but the second time in the loop, the value is NULL,and the error code is 5, which means access is denied. I can't find which part is wrong, please help me ,thanks a lot!

HANDLE hFile = CreateFile(fileName,   
    GENERIC_READ,   
    FILE_SHARE_READ,   
    NULL,   
    OPEN_EXISTING,   
    FILE_ATTRIBUTE_NORMAL,   
    NULL);   

DWORD error = GetLastError();

if (hFile == NULL || hFile == INVALID_HANDLE_VALUE)
{
    CString msg;
    msg.Format("handle is wrong，code is %d!",error);
    AfxMessageBox(msg);
    return FALSE;
}

m_hFileMapping = CreateFileMapping(hFile,
    NULL,PAGE_READONLY,   
    0,
    0,
    NULL);   

if (m_hFileMapping == NULL)
{
    error = GetLastError();
    CloseHandle(hFile);
    return FALSE;
}

SYSTEM_INFO SysInfo;
GetSystemInfo(&SysInfo);
DWORD dwGran = SysInfo.dwAllocationGranularity;

DWORD dwFileSizeHigh = 0;
m_iFileByteCount = GetFileSize(hFile,&dwFileSizeHigh);

DWORD qwFileOffset = 0;	
DWORD dwBlockBytes = dwGran*10;	
DWORD qwFileSize = (DWORD)m_iFileByteCount;
LPCVOID lpBaseAdress = NULL;

while (qwFileSize - qwFileOffset> 0)
{
    if (qwFileSize-qwFileOffset < dwBlockBytes)
    {
        dwBlockBytes = (DWORD)(qwFileSize-qwFileOffset);
    }

    m_pBeginPointer = (BYTE*)MapViewOfFile(m_hFileMapping,
    FILE_MAP_READ,
    DWORD(dwBlockBytes>>32),
    DWORD(dwBlockBytes&0xFFFFFFFF),
    dwBlockBytes);

    DWORD error = GetLastError();

    if (m_pBeginPointer == NULL)
    {
        CloseHandle(m_hFileMapping);   
        m_hFileMapping = NULL;
        return FALSE;
    }	

    BOOL b = UnmapViewOfFile(m_pBeginPointer);
    qwFileOffset += dwBlockBytes;  
}

CloseHandle(m_hFileMapping);  

Tags: C++, Visual C++ 6.0, MFC