C++大数据的读写

当一个文件1G以上的这种，使用内存文件映射会提高读写效率；

下边时段出自《windows核心编程》，读取一个大文件，然后统计里边字符出现次数的函数：

__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； 

C++大数据的读写的相关教程结束。