Question

For some reason my recvfrom() function for sockets is not blocking on my server code like it is supposed to. I am making a basic UDP server to create a rolling session key system.

What am I doing wrong here? It continues on after this line (before i put the (n < 1)) and was crashing. I am pretty sure recvfrom() is supposed to stop the execution of the program until it gets something from the client…

    int sockfd, portNumber;
    socklen_t clilen;
    char buffer[BUFFER_LENGTH];
    struct sockaddr_in serv_addr, from;
    int n;

    // Invalid arguments
    if (argc < 2)
        exit(0);
    else if (atoi(argv[1]) > 65535 || atoi(argv[1]) < 1)
        exit(0);

    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) 
    {
        printf("Error opening socket.\n");
        exit(0);
    }

    // Taken from reference
    bzero((char *) &serv_addr, sizeof(serv_addr));
    portNumber = atoi(argv[1]);
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = INADDR_ANY;
    serv_addr.sin_port = htons(portNumber);
    if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) 
    {
        printf("ERROR on binding.\n");
        close(sockfd);
        exit(0);
    }

    // Get initial session key request
    int fromlen = sizeof(struct sockaddr_in);
    n = recvfrom(sockfd, buffer, BUFFER_LENGTH, 0, (struct sockaddr *)&from, &fromlen);
    if (n < 0)
    {
        printf("Error in receiving.\n");
        exit(1);
    }

Thanks

Answer 1

You’re trying to use a stream socket for UDP;

sockfd = socket(AF_INET, SOCK_STREAM, 0);

What you mean to do is probably;

sockfd = socket(AF_INET, SOCK_DGRAM, 0);

Trying to do recvfrom on an unconnected stream socket will most likely return immediately with an error. Next time, you may want to check errno for a hint.