客户端套接字可绑定但不可连接，因为已在使用

Question

I write a client, where i have to bind the client socket. This works fine. After that i try to connect the Socket and i get error 10048. (Address already in use.) I don't understand how this is possible.

I have to implement a client speaking with multiple server. Every server only accepts messages from a specific port. (every Server expects a different port). so i have to bind my client socket. The code above is to create one of these sockets. My code works some times. But very often the connect gives me the error 10048, while the binding before was fine. I know that bind can give also the error 10048 if the socket is already in use. But it doesn't. It returns 0. So i guess the port is free. Immediate after binding i call connect and get error 10048. I do not understand why? At the moment of the binding the port was obviously free.

bool TEthernetSocket::Open()
{
    WSADATA wsaData;

    if (WSAStartup((MAKEWORD(2, 0)), &wsaData) == SOCKET_ERROR)
    {
       return IsConnected();
    }

    Socket = socket(AF_INET, SOCK_STREAM, 0); // TCP

    if (Socket == INVALID_SOCKET)
    {
        return false;
    }

    //bind Socket
    struct sockaddr_in sa_loc;
    memset(&sa_loc, 0, sizeof(struct sockaddr_in));
    sa_loc.sin_family = AF_INET;
    sa_loc.sin_port = htons(ClientPort);
    sa_loc.sin_addr.s_addr = inet_addr(IPClient.substr(0, 15).c_str());

    CALL_TRACE_CB("ethernetSocket connected");

    if (!(bind(Socket, (struct sockaddr*)&sa_loc, sizeof(struct  
    sockaddr))))
    {
        CALL_TRACE_CB("Bind works");
    }
    else
    {
        AnsiString msg = AnsiString().sprintf("EN: error socket Bind: 
        %d", WSAGetLastError());
        CALL_ERROR_CB(ERROR_NO_PORT_HANDLE, msg.c_str());
        Close();
    }


    // TCP
    SOCKADDR_IN sAdd;
    sAdd.sin_family = AF_INET;
    sAdd.sin_port = htons(Port);
    sAdd.sin_addr.s_addr = inet_addr(IP.substr(0, 15).c_str());

    if (connect(Socket, (SOCKADDR*)&sAdd, sizeof(SOCKADDR_IN)) ==    
    SOCKET_ERROR)
    {
        AnsiString msg = AnsiString().sprintf("EN: error connect        
        errorcode: %d", WSAGetLastError());
    }
}

I expect that bind() returns 10048 before connect returns this error, but actual only connect() returns this error

Answer 1

I have to implement a client speaking with multiple server. The server only accepts messages from a specific port, so i have to bind my client socket.

This is an unsolvable problem. When you make an outbound TCP connection, the combination of local IP address and port is reserved exclusively for that particular outbound TCP connection. If you need to make multiple outbound TCP connections from the same port, each will have to be bound to its own local IP address. That is going to be extremely inconvenient.

There are other problems too. Say you finish one connection and then try to start a new one. The new one will have the same local IP address, local port (since the server only tolerates one), remote IP address, and remote port. How will packets from the new connection be distinguished from old, stale packets from the old one?

This is why you get the error when you try to connect . It's not until all four parameters of the connection (local and remote address and local and remote port) are known that the collision with the prior connection is detectable. That's not known until you call connect .

You need to fix the server to ignore the source port. If that absolutely cannot be done, you'll have to adopt a wait and retry mechanism to handle collisions with past connections.