在C ++中的套接字上发送和接收数据

Question

I am currently learning C++ and trying to understand sockets. I've done sockets before in high level languages in C# and Java but having problems in C++.

I've gone through the example at https://msdn.microsoft.com/en-us/library/windows/desktop/ms737593(v=vs.85).aspx .

The code I have is pretty much working except there's one thing I'm not sure of. When I've written other socket apps, I don't usually receive the response until after a new line character is received.

I thought maybe C++ add 1 character received at a time and adds to the buffer, so then when I close the socket, the buffer would contain everything that was received, but it seems to be a hex value, but the hex in an ascii converter seems to print gibberish instead of the socket data.

Below is my code

const int DEFAULT_BUF_LEN = 525;
    InitialiseLibrary initLibrary("tunnel.conf");
    if (initLibrary.initialise(0))
    {
        cout << "Failed to initialise library" << endl;
    }   

    BitsLibrary bitsLibrary;
    StaticSettings staticSettings(&bitsLibrary, "tunnel.conf");

    //Open a socket connection
    WSAData wsaData;
    int iResult;

    SOCKET listenSocket = INVALID_SOCKET;
    SOCKET clientSocket = INVALID_SOCKET;

    struct addrinfo *result = NULL;
    struct addrinfo hints;

    int iSendResult;
    char recvBuf[DEFAULT_BUF_LEN];
    int recBufLen = DEFAULT_BUF_LEN;

    iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (iResult != 0)
    {
        cout << "WSAStartup Failed with error: " << iResult << endl;
        return EXIT_FAILURE;
    }

    ZeroMemory(&hints, sizeof(hints));
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;
    hints.ai_flags = AI_PASSIVE;

    iResult = getaddrinfo(NULL, "500", &hints, &result);
    if (iResult != 0)
    {
        cout << "getaddrinfo failed with error: " << iResult << endl;
        return EXIT_FAILURE;
    }

    listenSocket = socket(result->ai_family, result->ai_socktype, result->ai_protocol);
    if (listenSocket == INVALID_SOCKET)
    {
        cout << "Socket failed with error: " << WSAGetLastError() << endl;
        freeaddrinfo(result);
        WSACleanup();
        return EXIT_FAILURE;
    }

    iResult = ::bind(listenSocket, result->ai_addr, (int)result->ai_addrlen);
    if (iResult != 0)
    {
        cout << "bind failed with error: " << iResult << endl;
        FreeAddrInfo(result);
        closesocket(listenSocket);
        WSACleanup();
        return EXIT_FAILURE;
    }

    freeaddrinfo(result);
    iResult = listen(listenSocket, SOMAXCONN);
    if (iResult == SOCKET_ERROR)
    {
        cout << "Listen failed with error: " << WSAGetLastError() << endl;
        closesocket(listenSocket);
        WSACleanup();
        return EXIT_SUCCESS;
    }
    sockaddr_in clientAddr;
    socklen_t sin_size = sizeof(struct sockaddr_in);
    clientSocket = accept(listenSocket, (struct sockaddr*)&clientAddr, &sin_size);
    send(clientSocket, "welcome", string("welcome").length(),0);
    if (clientSocket == INVALID_SOCKET)
    {
        cout << "accept failed with error: " << WSAGetLastError() << endl;
        closesocket(listenSocket);
        WSACleanup();
        return EXIT_FAILURE;
    }

    closesocket(listenSocket);

    do {
        iResult = recv(clientSocket, recvBuf, recBufLen, 0);
        if (iResult > 0)
        {
            cout << "Bytes received: " << iResult << endl;
            cout << "Received: " << recvBuf << endl;

            iSendResult = send(clientSocket, recvBuf, iResult, 0);
            if (iSendResult == SOCKET_ERROR)
            {
                cout << "send failed with error: " << WSAGetLastError() << endl;
                closesocket(clientSocket);
                WSACleanup();
                return EXIT_FAILURE;
            }
            cout << "Bytes sent " << iSendResult << endl;
        }
        else if (iResult == 0)
        {
            cout << "Connection closing" << endl;
        }
        else
        {
            cout << "Recv failed with error: " << WSAGetLastError() << endl;
            closesocket(clientSocket);
            WSACleanup();
            return EXIT_FAILURE;
        }
    } while (iResult > 0);

    iResult = shutdown(clientSocket, SD_SEND);
    if (iResult == SOCKET_ERROR)
    {
        cout << "shutdown failed with error: " << WSAGetLastError() << endl;
        closesocket(clientSocket);
        WSACleanup();
        return EXIT_FAILURE;
    }
    closesocket(clientSocket);
    WSACleanup();

    cout << "Message was: " << recvBuf << endl;

    return EXIT_SUCCESS;

If I'm not mistaken, if I was doing this in C# and connect via telnet to my socket I could send Hello but the app wouldn't do anything until I sent \\r\\n or the buffer was full.

However, in my C++ app, as soon as I enter H my app immediately responds prints out the recvBuf which just contains H plus a load of blank characters (shows as squares in cmd) which I'm assuming is the blank parts of the array buffer, and sends the reply.

Answer 1

Sockets don't care about newlines or nulls. Only higher-level streaming APIs do that. The important thing is the return value of recv() . That tells you exactly how many bytes were received. Nothing in the buffer beyond that count is valid.

Answer 2

In a streaming protocol, it is useful to establish a protocol for determining the content length.

For example, a client could send the content-length followed by data and the server would first read the content length and then read the socket in a loop and buffer the data until content-length number of bytes have been received.