为什么 select() 返回 1 而 recv() 返回 0？

Question

我可以清楚地看到 recvbuf 拥有我期望的所有数据，但 select() 一直返回 1。

现在它被困在else if (iBuffer == 0) {} 的边缘。

SOCKET m_ConnectSocket;
/* The socket setup is done elsewhere but just adding this for clarity
   This socket is responsible for sending from the client to the server
   and also receives anything the server sends back.

   This socket is doing the connect() & initial send()
*/

fd_set set;
struct timeval timeout;

// Set up the file descriptor set.
FD_ZERO(&set);
FD_SET(m_ConnectSocket, &set);

// Set up the struct timeval for the timeout.
timeout.tv_sec  = RECV_DELAY_SEC;
timeout.tv_usec = RECV_DELAY_USEC;

int iBuffer = 0;

do
{
    iResult = select(m_ConnectSocket, &set, NULL, NULL, &timeout);
    if (iResult > 0)
    {
        iBuffer = recv(m_ConnectSocket, recvbuf, DEFAULT_BUFLEN, 0);
        if (iBuffer > 0)
        {
            string sRecv(recvbuf);

            STrace = String::Format("Bytes Received: {0}", iBuffer);
            Trace(STrace, TRACE_INFO);

            STrace = String::Format("Data Received: [{0}]", gcnew String(sRecv.c_str()));
            Trace(STrace, TRACE_INFO);
        }
        else if (iBuffer == 0)
        {
            STrace = String::Format("iBuffer empty");
            Trace(STrace, TRACE_INFO);
        }
        else
        {
            STrace = String::Format("recv failed: {0}", WSAGetLastError());
            Trace(STrace, TRACE_ERROR);
        }
    }
    else if (iResult == 0)
    {
        STrace = String::Format("No data left in buffer");
        Trace(STrace, TRACE_INFO);

        pMessage->Data(recvbuf);
        if (iSentType != pMessage->Type())
        {
            STrace = String::Format("Message type mismatch: {0} | Expected: {1}", (int)pMessage->Type(), (int)iSentType);
            Trace(STrace, TRACE_WARNING);
        }
    }
    else if (iResult == -1)
    {
        STrace = String::Format("select() error");
        Trace(STrace, TRACE_ERROR);
    }
} while (iResult > 0);

Answer 1

到目前为止，他们所说的关于重置 FD 集的所有答案都是正确的，但他们都没有真正确定潜在的问题。

如果recv() 返回零，则表示对等方已断开连接，您必须关闭套接字。如果你不这样做，因为你不是，你将继续选择套接字为可读并继续接收零。

它确实不是意味着“缓冲区为空”。

Answer 2

由于select 将其参数作为指针传递，并且这些数据结构被select put 改变

fd_set set;
struct timeval timeout;

// Set up the file descriptor set.
FD_ZERO(&set);
FD_SET(m_ConnectSocket, &set);

// Set up the struct timeval for the timeout.
timeout.tv_sec  = RECV_DELAY_SEC;
timeout.tv_usec = RECV_DELAY_USEC;

就在select 语句之前，即在循环内。

Answer 3

您误用了select()。请阅读documentation：

参数

nfds [in]
忽略。包含 nfds 参数只是为了与 Berkeley 套接字兼容。

...

返回时，结构会更新以反映这些套接字中满足指定条件的子集。

因此，每次在循环中调用 select() 时，您必须重置 fd_set 结构。

此外，看起来当select() 超时时，您正在尝试解析接收到的任何内容，但您只解析最后一个成功的recv() 返回的最后一个缓冲区（如果有的话）。如果在数据超时之前必须多次调用recv()，则需要收集每个返回的缓冲区，然后将它们作为一个整体解析。

此外，您的错误处理通常也需要一些改进。

尝试类似的方法：

fd_set set;
struct timeval timeout;

string sBuffer;
int iBuffer;

do
{
    // Set up the file descriptor set.
    FD_ZERO(&set);
    FD_SET(m_ConnectSocket, &set);

    // Set up the struct timeval for the timeout.
    timeout.tv_sec  = RECV_DELAY_SEC;
    timeout.tv_usec = RECV_DELAY_USEC;

    iResult = select(0, &set, NULL, NULL, &timeout);
    if (iResult > 0)
    {
        iBuffer = recv(m_ConnectSocket, recvbuf, DEFAULT_BUFLEN, 0);
        if (iBuffer > 0)
        {
            string sRecv(recvbuf, iBuffer);

            STrace = String::Format("Bytes Received: {0}", iBuffer);
            Trace(STrace, TRACE_INFO);

            STrace = String::Format("Data Received: [{0}]", gcnew String(sRecv.c_str()));
            Trace(STrace, TRACE_INFO);

            sBuffer += sRecv;
        }
        else
        {
            if (iBuffer == 0)
            {
                STrace = String::Format("Connection closed");
                Trace(STrace, TRACE_INFO);
            }
            else
            {
                STrace = String::Format("recv failed: {0}", WSAGetLastError());
                Trace(STrace, TRACE_ERROR);
            }
            break;
        }
    }
    else if (iResult == 0)
    {
        STrace = String::Format("No data left in buffer");
        Trace(STrace, TRACE_INFO);

        pMessage->Data(sBuffer.c_str());
        if (iSentType != pMessage->Type())
        {
            STrace = String::Format("Message type mismatch: {0} | Expected: {1}", (int)pMessage->Type(), (int)iSentType);
            Trace(STrace, TRACE_WARNING);
        }
    }
    else
    {
        STrace = String::Format("select failed: {0}", WSAGetLastError());
        Trace(STrace, TRACE_ERROR);
    }
}
while (iResult > 0);