C++ Socket select() before accept() 多次看到相同的连接

Question

我有一个监听函数，它检查是否有新连接在服务器套接字上等待，如果我先启动客户端然后启动服务器，我会多次看到“新连接”，如果我启动服务器并然后客户端我看到它一次。

我是否遗漏了一些 Posix 标准的知识或只是缺乏知识(最有可能)？

轮询新连接的函数是:

bool IPV4Socket::HasNewConnection( TimeoutValue *timeout )
{
    SocketSet read_fd_set;
    bool hasConnection = false;

    SocketSet_ZERO( &read_fd_set );
    SocketSet_SET( m_socket, &read_fd_set );

    if ( m_socketAdaptor->select( m_socket + 1
                                , &read_fd_set
                                , NULL
                                , NULL
                                , timeout ) < 0 )
    {
        throw( std::string( "::select() failed, unable to continue." ) );
    }

    for ( unsigned int i = 0; i <= m_socket; i++ )
    {
        if ( m_socketAdaptor->SocketSet_ISSET( i, &read_fd_set ) &&
             ( i == m_socket ) )
        {
            hasConnection = true;
        }
    }

    return hasConnection;
}

它被称为使用:

while( 1 )
{
    if ( socket->HasNewConnection( &newConnTimeout ) )
    {
        std::cout << "[INFO] A new connection is waiting...." << std::endl;

        acceptedSocket = socket->Accept( &newConnTimeout );

        if ( acceptedSocket )
        {
            std::cout << "[INFO] New connection received...." << std::endl;
        }
    }
}

连接功能是:

while( connStatus == false )
{
    socket = socketlayer->CreateSocket( socketlayer::SockType_stream );

    socket->SetNonBlocking( true );
    socket->SetSocketOption( socketlayer::SocketOption_KeepAlive, true );
    socket->SetSocketOption( socketlayer::SocketOption_ReuseAddr, true );

    connStatus = socket->Connect( "127.0.0.1", 18000, &tv );

    if ( connStatus == false ) socket->Close();
}

由于这是一段开源代码，可以在 Sourceforge 中找到

连接是通过:

bool IPV4Socket::Connect( std::string hostname
                        , unsigned short remotePort
                        , TimeoutValue *timeout )
{
    AddrInfo getResults;
    AddrInfo getaddrinfoHints;
    int connReturn = 0;
    SockAddr_In *addrData;
    //bool connectSuccess = false;
    std::string service = std::to_string( remotePort );

    getaddrinfoHints.ai_family = AddressFamily_inet;
    getaddrinfoHints.ai_socktype = SockType_stream;

    if ( m_socketAdaptor->getaddrinfo( hostname
                                     , service
                                     , &getaddrinfoHints
                                     , &getResults ) != 0 )
    {
        return false;
    }

    addrData = (SockAddr_In *)&( *getResults.ai_addr.begin() );

    connReturn = m_socketAdaptor->connect( m_socket
                                         , (const Sockaddr *)addrData
                                         , (int)getResults.ai_addrlen );

    if ( connReturn == SocketError)
    {
        int m_lastErrorCode = m_socketAdaptor->GetLastError();

        //  Connection error : FATAL 
        if ( ( m_lastErrorCode != SockErr_EWOULDBLOCK) &&
             ( m_lastErrorCode != SockErr_EALREADY ) && 
             ( m_lastErrorCode != SockErr_EINPROGRESS ) )
        {
            return false;
        }
    }

    SocketSet writeFDS;
    //SocketSet exceptFDS;
    int selectReturn = 0;

    //  Clear all the socket FDS structures
    SocketSet_ZERO( &writeFDS );
    //SocketSet_ZERO( &exceptFDS );

    //  Put the socket into the FDS structures
    SocketSet_SET( m_socket, &writeFDS );
    //SocketSet_SET( m_socket, &exceptFDS );

    selectReturn = m_socketAdaptor->select( m_socket + 1
                                            , NULL
                                            , &writeFDS
                                            , NULL
                                            , timeout );

    //  Check for Socket Error or timeout
    if ( ( selectReturn == SocketError ) || ( selectReturn == 0 ) )
    {
        return false;
    }

    return true;
}

Answer 1

套接字设置为非阻塞，并调用connect通常会返回一个错误，因为连接状态还不能确定:

With a nonblocking socket, the connection attempt cannot be completed immediately. In this case, connect will return SOCKET_ERROR, and WSAGetLastError will return WSAEWOULDBLOCK. (...)

• Use the select function to determine the completion of the connection request by checking to see if the socket is writeable.

鉴于您的客户端代码和上述文档，它应该永远无法成功连接。但是，由于您使用环回设备进行连接，因此在您的服务器已经在运行的情况下，由于计时的原因，连接可能(并且确实)会立即被接受。

因此，要么如上所述在 connect 调用之后在客户端上使用 select，要么只使用阻塞套接字。