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POC++ UDP. Why is recvfrom() is not blocking?
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copied!<p>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?</p> <p>Here is the client side code:</p> <pre><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); } </code></pre> <p>And here is the initialization:</p> <pre><code>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); } </code></pre>

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