我在使用带有重叠 I/O 的标志 FILE_FLAG_NO_BUFFERING 时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，即使文件句柄良好并且 ReadFile() 调用返回时没有任何错误(预期的 ERROR_IO_PENDING 除外)，从 GetOverlappedResult() 调用返回的“字节读取”值是某些文件为零，每次我运行代码时，它都是一组不同的文件。 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。

下面是我如何使用 FILE_FLAG_NO_BUFFERING 实现重叠 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;
}

在没有 FILE_FLAG_NO_BUFFERING 的情况下，totalBytesRead 值为 57 MB。带有标志的情况下，totalBytesRead 值远低于 57 MB，并且每次运行代码时都会不断变化，范围从 2 MB 到 15 MB。

最佳答案

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。

关于c++ - 具有重叠 I/O 的 FILE_FLAG_NO_BUFFERING - 字节读为零，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/17780227/