__int64 CountOs(void)
{
    // Get system granularity
    SYSTEM_INFO sinf;
    GetSystemInfo(&sinf);

    // open the data file
    HANDLE hFile = CreateFile(TEXT("C:\\1.TXT"), GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_FLAG_SEQUENTIAL_SCAN, NULL);

    // create the file-mapping object.
    HANDLE hFileMapping = CreateFileMapping(hFile, NULL, PAGE_READONLY, 0, 0, NULL);

    DWORD dwFileSizeHight;
    __int64 qwFileSize = GetFileSize(hFile, &dwFileSizeHight);
    qwFileSize += (((__int64)dwFileSizeHight) << 32);

    // we no longer need access to the file object‘s handle.
    CloseHandle(hFile);

    __int64 qwFileOffset = 0, qwNumOf0s = 0;

    while (qwFileSize > 0)
    {
        DWORD dwBytesInBlock = sinf.dwAllocationGranularity;
        if (qwFileSize < sinf.dwAllocationGranularity)
            dwBytesInBlock = (DWORD)qwFileSize;
        PBYTE pbFile = (PBYTE)MapViewOfFile(hFileMapping, FILE_MAP_READ, (DWORD)(qwFileOffset >> 32), (DWORD)(qwFileOffset & 0xFFFFFFFF), dwBytesInBlock);

        // count the number of 0s in this block.
        for (DWORD dwByte = 0; dwByte < dwBytesInBlock; dwByte++)
        {
            if (pbFile[dwByte] == ‘r‘)
                qwNumOf0s++;
        }

        // unmap the view; we don‘t want multiple views
        // in our address space.
        UnmapViewOfFile(pbFile);

        // skip to the next set of bytes in the file.
        qwFileOffset += dwBytesInBlock;
        qwFileSize -= dwBytesInBlock;
    }

    CloseHandle(hFileMapping);
    return qwNumOf0s;
}

如果是往里边写数据就用 memcpy把数据考入pbFile指向的内存。还有就是把读标志该成写标志；