Question

我在使用带有重叠I / O的标志FILE_FLAG_NO_BUFFERING时观察到一种奇怪的行为。我调用了一系列ReadFile（）函数调用，稍后使用GetOverlappedResult（）查询它们的状态。

我所说的奇怪的行为是，即使文件句柄很好而且返回的ReadFile（）调用没有任何错误（预期的ERROR_IO_PENDING除外），GetOverlappedResult（）调用返回的'bytes read'值是对于某些文件为零，每次运行代码时 - 它都是一组不同的文件。 如果我删除了FILE_FLAG_NO_BUFFERING，事情就会开始正常工作，没有字节读取值为零。

以下是我如何使用FILE_FLAG_NO_BUFFERING实现重叠的I / O代码。

long overlappedIO(std::vector<std::string> &filePathNameVectorRef)
{    
    long totalBytesRead = 0;
    DWORD bytesRead = 0;
    DWORD bytesToRead = 0;
    std::map<HANDLE, OVERLAPPED> handleMap;
    HANDLE handle = INVALID_HANDLE_VALUE;
    DWORD accessMode = GENERIC_READ;
    DWORD shareMode = 0;
    DWORD createDisposition = OPEN_EXISTING;
    DWORD flags = FILE_FLAG_OVERLAPPED | FILE_FLAG_NO_BUFFERING;

    DWORD fileSize;
    LARGE_INTEGER li;
    char * buffer;
    BOOL success = false;

    for(unsigned int i=0; i<filePathNameVectorRef.size(); i++)
    {
        const char* filePathName = filePathNameVectorRef[i].c_str();

        handle = CreateFile(filePathName, accessMode, shareMode, NULL, createDisposition, flags, NULL);

        if(handle == INVALID_HANDLE_VALUE){
            fprintf(stdout, "\n Error occured: %d", GetLastError());
            fprintf(stdout," getting handle: %s",filePathName);
            continue;
        }
        GetFileSizeEx(handle, &li);
        fileSize = (DWORD)li.QuadPart;

        bytesToRead = (fileSize/g_bytesPerPhysicalSector)*g_bytesPerPhysicalSector;
        buffer = static_cast<char *>(VirtualAlloc(0, bytesToRead, MEM_COMMIT, PAGE_READWRITE));

        OVERLAPPED overlapped;
        ZeroMemory(&overlapped, sizeof(overlapped));
        OVERLAPPED * lpOverlapped = &overlapped;

        success = ReadFile(handle, buffer, bytesToRead, &bytesRead, lpOverlapped);

        if(!success && GetLastError() != ERROR_IO_PENDING){ 
            fprintf(stdout, "\n Error occured: %d", GetLastError());
            fprintf(stdout, "\n reading file %s",filePathName);
            CloseHandle(handle);
            continue;
        }
        else
            handleMap[handle] = overlapped;
    }

    // Status check and bytes Read value
    for(std::map<HANDLE, OVERLAPPED>::iterator iter = handleMap.begin(); iter != handleMap.end(); iter++)
    {
        HANDLE handle = iter->first;        
        OVERLAPPED * overlappedPtr = &(iter->second);

        success = GetOverlappedResult(handle, overlappedPtr, &bytesRead, TRUE);
        if(success)
        {
                /* bytesRead value in some cases is unexpectedly zero */
                /* no file is of size zero or lesser than 512 bytes(physical volume sector size) */
            totalBytesRead += bytesRead;
            CloseHandle(handle);
        }
    }

    return totalBytesRead;
}

如果缺少FILE_FLAG_NO_BUFFERING，则totalBytesRead值为57 MB。当标志存在时，totalBytesRead值远低于57 MB，并且每次运行代码时都会不断更改，范围从2 MB到15 MB。

Answer 1

当文件大小小于g_bytesPerPhysicalSector时，您对bytesToRead的计算将产生0。因此，对于小文件，您要求0个字节。

具有重叠I / O的FILE_FLAG_NO_BUFFERING - 字节读取为零

1 个答案: