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在使用带有重叠 I/O 的标志 FILE_FLAG_NO_BUFFERING 时，我观察到一个奇怪的行为。我调用了一系列 ReadFile() 函数调用，稍后使用 GetOverlappedResult() 查询它们的状态。

我所说的奇怪行为是，即使文件句柄很好并且 ReadFile() 调用返回没有任何错误错误（预期的 ERROR_IO_PENDING 除外），从 GetOverlappedResult() 调用返回的“字节读取”值对于某些人来说为零文件，每次我运行代码时 - 它是一组不同的文件。 如果我删除 FILE_FLAG_NO_BUFFERING，事情就会开始正常工作，并且没有字节读取值为零。

下面是我使用 FILE_FLAG_NO_BUFFERING 实现重叠 I/O 代码的方法。

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;
}

在 FILE_FLAG_NO_BUFFERING 不存在的情况下，totalBytesRead 值为 57 MB。存在该标志时，totalBytesRead 值远低于 57 MB，并且每次运行从 2 MB 到 15 MB 的代码时都会不断变化。

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1 回答 1

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当文件大小小于 g_bytesPerPhysicalSector 时，您对 bytesToRead 的计算将产生 0 作为结果。因此，对于小文件，您请求 0 个字节。

于 2013-07-22T14:26:51.007 回答