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。程序工作正常，但如果我只启动客户端，recvfrom方法不会阻塞。但是，当我删除 sendto 方法时，recvfrom 开始阻塞。知道发生了什么事吗？

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

sendto 失败的原因有很多种。有些（例如 arp 失败）会在 sendto 期间导致错误。其他的，例如 ICMP port unreachable，可能会在您下次使用套接字时报告。

您的 recvfrom 调用实际上可能正在获取响应您的传出数据包而发送的 ICMP 数据包。

第二个 recvfrom 是否按预期阻塞？

Answer 2

套接字需要设置为 BLOCKING/NON-BLOCKING。

1. 设置阻止

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

2. 设置非阻塞

    int nMode = 1; // 1：非阻塞
    if (ioctlsocket(objSocket, FIONBIO, &nMode) == SOCKET_ERROR)
    {
       closesocket(SendingSocket);
       WSACleanup();
       返回iRet；
    }
   

Answer 3

看起来您正在以相同的方式设置服务器套接字和客户端套接字。对于服务器来说，初始化看起来不错，但对于客户端来说，您需要绑定到端口 0。

事实上，对于这两个客户端，您都可以执行 INADDR_ANY (IP 0.0.0.0)，它不会绑定到特定接口，而是允许正确端口上的任何连接。