Question

这不是网络问题。这是一个关于原子的问题，但是场景围绕着网络。

我有几个线程接收网络消息。每个参数都应增加一个TotalBytesReceive的原子变量，直到达到TotalBytes的值为止。我的原子行为异常，可能是由于我对某些事物的误解。

在多个线程中：

size_t BytesReceived = // number of bytes received in this thread
bool IsMessageFinished = msg->IsMsgFinished(BytesReceived);
if(IsMessageFinished) 
     {do something else}

共享的msg对象包含

class Msg
{
    std::vector<char> MessageData;          // The data received for the message
    const size_t TotalBytes;                      // The final size of the complete message
    std::atomic<size_t> TotalBytesReceived; // The bytes received so far
    std::mutex m_mutex;                     // Just for testing

public:
    Msg(size_t totalbytes) : TotalBytesReceived(0), TotalBytes(totalbytes) 
    {
        MessageData.resize(TotalBytes);
    }

    bool IsMsgFinished(size_t BytesReceived)
    {
        // std::unique_lock<std::mutex> lock(m_mutex); // uncommenting fixed the problem
        size_t newtotal = TotalBytesReceived += BytesReceived;

        std::stringstream ss;
        ss << BytesReceived << " received. New total " << newtotal << '\n';
        std::cout << ss.str(); // For diagnostics

        return newtotal == TotalBytes;
    }

    // Other functions for setting MessageData aren't important here
}

最大的问题是，当多个线程调用IsMsgFinished时，我的诊断输出显示newtotal多次具有相同的值。那不可能。（BytesReceived永远不会为0）这里有严重错误吗？

如果我在IsMsgFinished函数中使用了互斥锁和unique_lock，则一切正常，但我不想使用锁。

C ++原子fetch_add

0 个答案: