为什么select（）返回1但recv（）返回0？

Question

I can see clearly that recvbuf has all the data I was expecting yet select() keeps returning 1.

Right now it's stuck in the limbo of else if (iBuffer == 0) {} .

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

All the answers so far are correct in what they say about resetting the FD sets, but none of them has actually identified the underlying problem.

If recv() returns zero it means the peer has disconnected, and you must close the socket. If you don't, as you aren't, you will continue to select the socket as readable and continue to receive zero.

It does not mean 'buffer empty'.

Answer 2

As select has it parameters passed as pointer and those data structures get altered by select put

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;

Just before the select statement ie within the loop.

Answer 3

You are misusing select() . Please read the documentation :

Parameters

nfds [in]
Ignored . The nfds parameter is included only for compatibility with Berkeley sockets.

...

Upon return, the structures are updated to reflect the subset of these sockets that meet the specified condition.

So you must reset the fd_set structure every time you call select() in your loop.

Also, it looks like when select() times out, you are trying to parse whatever was received, but you are only parsing the last buffer that was returned by the last successful recv() , if any. In case recv() has to be called multiple times before the data times out, you need to collect each returned buffer and then parse them all together as a whole.

Also, your error handling in general could use some improvement, too.

Try something more like this instead:

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);