CODEWITHSUNDEEP
c++clientudpwinsockblocking
Python Lord
Python Lord

Reputation: 351

C++ UDP. Why is recvfrom() is not blocking?

I am writing some simple client/server code using UDP. The program works fine, but if I only start the client, the recvfrom method does not block. However, when I remove the sendto method, recvfrom starts to block. Any idea of what is going on?

Here is the client side code:

    int server_length;                      /* Length of server struct */
    char send_buffer[256] = "hi";           /* Data to send */
    time_t current_time;                    /* Time received */

    while(true)
    {

        /* Tranmsit data to get time */
        server_length = sizeof(struct sockaddr_in);
        if (sendto(m_oSocket, send_buffer, (int)strlen(send_buffer) + 1, 0, (struct sockaddr *)&m_oServer, server_length) == -1)
        {
            fprintf(stderr, "Error transmitting data.\n");
            continue;
        }

        /* Receive time */

        if (recvfrom(m_oSocket, (char *)&current_time, (int)sizeof(current_time), 0, (struct sockaddr *)&m_oServer, &server_length) < 0)
        {
            fprintf(stderr, "Error receiving data.\n");
            continue;
        }

        /* Display time */
        printf("Current time: %s\n", ctime(&current_time));

        Sleep(1000);
    }

And here is the initialization:

unsigned short m_iPortnumber;
struct sockaddr_in m_oServer;
struct sockaddr_in m_oClient;
SOCKET m_oSocket;
WSADATA w;                              /* Used to open Windows connection */
    int a1, a2, a3, a4;                     /* Server address components in xxx.xxx.xxx.xxx form */

    a1 = 192;
    a2 = 168;
    a3 = 2;
    a4 = 14;
    m_iPortnumber = 52685;

    /* Open windows connection */
    if (WSAStartup(0x0101, &w) != 0)
    {
        fprintf(stderr, "Could not open Windows connection.\n");
        exit(0);
    }

    /* Open a datagram socket */
    m_oSocket = socket(AF_INET, SOCK_DGRAM, 0);
    if (m_oSocket == INVALID_SOCKET)
    {
        fprintf(stderr, "Could not create socket.\n");
        WSACleanup();
        exit(0);
    }

    /* Clear out server struct */
    memset((void *)&m_oServer, '\0', sizeof(struct sockaddr_in));

    /* Set family and port */
    m_oServer.sin_family = AF_INET;
    m_oServer.sin_port = htons(m_iPortnumber);

    /* Set server address */
    m_oServer.sin_addr.S_un.S_un_b.s_b1 = (unsigned char)a1;
    m_oServer.sin_addr.S_un.S_un_b.s_b2 = (unsigned char)a2;
    m_oServer.sin_addr.S_un.S_un_b.s_b3 = (unsigned char)a3;
    m_oServer.sin_addr.S_un.S_un_b.s_b4 = (unsigned char)a4;

    /* Clear out client struct */
    memset((void *)&m_oClient, '\0', sizeof(struct sockaddr_in));

    /* Set family and port */
    m_oClient.sin_family = AF_INET;
    m_oClient.sin_addr.s_addr=INADDR_ANY;
    m_oClient.sin_port = htons(0);

    /* Bind local address to socket */
    if (bind(m_oSocket, (struct sockaddr *)&m_oClient, sizeof(struct sockaddr_in)) == -1)
    {
        fprintf(stderr, "Cannot bind address to socket.\n");
        closesocket(m_oSocket);
        WSACleanup();
        exit(0);
    }

Upvotes: 3

Views: 24718

Answers (3)

user1289798
user1289798

Reputation: 39

Socket required to be set BLOCKING/NON-BLOCKING.

  1. Set BLOCKING

     int nMode = 0; // 0: BLOCKING
 if (ioctlsocket (objSocket, FIONBIO, &nMode) == SOCKET_ERROR)
 {
    closesocket(SendingSocket);
    WSACleanup();
    return iRet;
 }

  2. Set NON-BLOCKING

     int nMode = 1; // 1: NON-BLOCKING
 if (ioctlsocket (objSocket, FIONBIO, &nMode) == SOCKET_ERROR)
 {
    closesocket(SendingSocket);
    WSACleanup();
    return iRet;
 }

Upvotes: 3

Ben Voigt
Ben Voigt

Reputation: 283684

There are a variety of ways that sendto can fail. Some, such as arp failure, will cause an error during sendto. Other, such as ICMP port unreachable, may be reported when you next use the socket.

Your recvfrom call could actually be fetching the ICMP packet sent in response to your outgoing packet.

Does a second recvfrom block as expected?

Upvotes: 6

Dave S
Dave S

Reputation: 21113

It looks like you're setting up the server socket and the client socket the same way. The initialization looks good for a server, but for the client, you'll want to bind to port 0.

In fact, for both of them you can do INADDR_ANY (IP 0.0.0.0), which doesn't bind to a specific interface, but instead allows any connection on the correct port.

Upvotes: 0

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