This is a short example in C on linux which will open a socket and listen on a
udp port for data.

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>

#include <arpa/inet.h>
#include <sys/socket.h>

int verbose = 0;

void print_usage(FILE *fp, char *app) {
        fprintf(fp, "Usage: %s <port>\n", app);
        fprintf(fp, "\n");
        fprintf(fp, "  -h        Print this help\n");
        fprintf(fp, "\n");
        fprintf(fp, "Example: %s 5000\n", app);
        fprintf(fp, "\n");
}

int main(int argc, char **argv) {
        int c;
        int sockfd;
        struct sockaddr_in in;
        socklen_t in_size = sizeof(in);
        char buf[65535];
        int port = 0;

        while(( c = getopt(argc, argv, "hv")) != -1) {
                switch(c) {
                        case 'h':
                                print_usage(stdout, argv[0]);
                                exit(EXIT_FAILURE);
                                break;
                        case 'v':
                                verbose++;
                                break;
                        default:
                                break;
                }
        }
       if (optind < argc) {
                port = atoi(argv[optind]);
                optind++;
        } else {
                print_usage(stderr, argv[0]);
                exit(EXIT_FAILURE);
        }

        if (port == 0 || port > 65535) {
                fprintf(stderr, "%d is an invalid port\n", port);
                exit(EXIT_FAILURE);
        }

        sockfd = socket(AF_INET, SOCK_DGRAM, 0);
        if (sockfd < 0) {
                perror("socket");
                exit(EXIT_FAILURE);
        }

        in.sin_family = AF_INET;
        in.sin_port = htons(port);
        in.sin_addr.s_addr = INADDR_ANY;

        if (bind(sockfd, (struct sockaddr *) &in, sizeof(in)) < 0) {
                perror("bind");
                exit(EXIT_FAILURE);
        }

        while(1) {
                int ret = recvfrom(sockfd, buf, sizeof(buf), 0, (struct sockaddr *) &in, &in_size);

                if (ret < 0) {
                        perror("recvfrom");
                        continue;
                }
                if (verbose)
                        printf("Recived: %d Bytes From %s:%d\n", ret, inet_ntoa(in.sin_addr), ntohs(in.sin_port));

        }

        return 0;
}