C ++ UDP。 为什么recvfrom（）没有阻止？

Question

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);
    }

Answer 1

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?

Answer 2

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; } 

Answer 3

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.