C++ UDP.为什么recvfrom()没有阻塞？

Question

我正在使用UDP编写一些简单的客户端/服务器代码.该程序工作正常,但如果我只启动客户端,recvfrom方法不会阻止.但是,当我删除sendto方法时,recvfrom开始阻塞.怎么知道发生了什么？

这是客户端代码:

    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);
    }

这是初始化:

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

有多种方法sendto可能会失败.某些(例如arp失败)将导致错误sendto.ICMP port unreachable当您下次使用套接字时,可能会报告其他内容,例如.

您的recvfrom呼叫实际上可能是获取为响应您的传出数据包而发送的ICMP数据包.

是否recvfrom按预期进行了第二次阻止？