Java -> C++ TCP Socket does not send until close

Question

I am facing another TCP Socket issue. I've read through a huge bunch of questions an answers to similar issues, but my problem is somehow different.

I have a Java Client and C++ Server. Everything goes as expected until I'm using different machines (equal to other issues so far) The messages from the client seem to getting stuck in den TCP Buffer. When I finally close the socket, everything is sent to the server. But these single messages are controlmessages so I need them to be send immediatly. As far as I read this is expected behavior, but how do I send reliable control messages.

Is there a way to force the messages to be sent. (I can leave the socket open for a couple of minutes with nothing is sent.)

Is there something wrong? (see the following code)

Do I have to close the socket each time to perform a REAL flush?

Should I use UDP instead, with an additional amount of protocol work?

Javacode:

mSocketSend = new Socket();
mSocketSend.connect(new InetSocketAddress(mServerIp, mSocketPortSend), mTimeOut);
PrintWriter pw = new PrintWriter(mSocketSend.getOutputStream(), true);
pw.println(data);

C++ Code:

   opening socket...(i leave that)
   char* buffer = new char[1024];
   int rc = recv(mConnectedSocket, buf, 1024, 0);

If you want more of it. Write it. I left almost everything out. ^^ I dont think its relevant. The Communication wents great usually.. No errors at all. So its just this TCPBuffer thingi.

I know there should be some delimiter or message length stuff. But in fact: A message length, which is not sent, does not help. ^^

Thanks for your help.

EDIT #01 The whole bunch of code:

mSocket->createSocketServer(22);
    char* buffer = new char[1024];
 while(true){

        int numberBytes = mSocket->receiveChars(buffer, 1024);

        if (numberBytes > 0){
            uninterestingHandlingFunction(buffer);
        }else{
            mSocket->createSocketServer(22);
        }
    }

bool Socket::createSocketServer(u_short port)
{
    if (mConnectedSocket != INVALID_SOCKET)
    {
        closesocket(mConnectedSocket);
    }

    if (s == INVALID_SOCKET)
    {
        WSADATA wsa;

        if (WSAStartup(MAKEWORD(2,0), &wsa) != 0)
            return 0;

        s = socket(AF_INET, SOCK_STREAM, 0);
        if (s == INVALID_SOCKET)
            return 0;

        SOCKADDR_IN addr;
        memset(&addr, 0, sizeof(SOCKADDR_IN));
        addr.sin_family=AF_INET;
        addr.sin_port=htons(port);
        addr.sin_addr.s_addr=ADDR_ANY;

        if (bind(s, (SOCKADDR*)&addr, sizeof(SOCKADDR_IN)) == SOCKET_ERROR)
        {
            s = INVALID_SOCKET;
        } else if (listen(s, 10) == SOCKET_ERROR)
        {
            s = INVALID_SOCKET;
        }

        if (s == INVALID_SOCKET)
        {
            closesocket(s);
            return 0;
        }
    }
    mConnectedSocket = accept(s, NULL, NULL);

    if (mConnectedSocket == INVALID_SOCKET)
    {
        closesocket(s);
        return 0;
    }
    return 1;
}

int Socket::receiveChars(char* buf, unsigned maxSize)
{
    if (mConnectedSocket == INVALID_SOCKET)
        return -1;

    int rc = recv(mConnectedSocket, buf, maxSize, 0);
    if (rc == SOCKET_ERROR)
    {
        std::cout << "Socket: error " << WSAGetLastError() << std::endl;
    }
    return rc;
}

You wanted it....

EDIT #2 Give it one more try

There are few more things I tried out. At first: This problem does not occure on a device connected over real network everytime. -> Full Reboot Client&Server -> Problem does not occure -> Full Reboot Client&Server -> Problem occures

Sadly, I don't know what to take from this habit.

Another thing I stumbled over is the bind and listen socket (in Code SOCKET s). This socket listens for connections and if the working thread needs a new connection (on startup or if the previous closes) the socket s gives the next queued connection to mConnectedSocket for recv, other connections are backlogged while one is processed. From the Java view: a Socket is connected (Device A). next socket (Device B) tries to connect. -> Connection success (its properly controlled in code if this is happens indeed) -> followed by sending data in natural matter. (The socket is still in the backlog on c++ side)

Well, this is hard to transform to the habit I experienced. I'll try to express my thoughts. Javaside: PrintWriter is created. Feeded with data and is flushed. Because the connection is not fully established (No additional connectedSocket on C++ side). The flush doesn't work. And onClose the socket finally flushes its content.

Please tell me to shut up, if you think so. I dont really know what the "Connection is backlogged" ACTUALLY mean in implementation" ^^

I know, I should open a new thread for each connection, but I can't at the moment. So stick with this server code.

PrintWriter(OutputStream, boolean autoflush) should do the work on '\n', but yes.. I tried that, too — xeed
Oh damn. yeah copied that together. The buffer is allocated somewhere else and it is passed as buf to the receiveFunc. The second question is something i dont really know. My current expectation is that recv return when atleast one byte can be received. — xeed

Francis Upton IV Francis Upton IV · Accepted Answer · 2012-05-18T20:37:48

2

votes

You need to do a flush(), that pushes the data out.

Java -> C++ TCP Socket does not send until close

7 Answers