// receive by UDP
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

void main(int argc, char *argv[])
{ if (argc != 2)
  { fprintf(stderr, "requires localport\n");
    exit(1); }
  int local_port = atol(argv[1]);

  int sock = socket(AF_INET, SOCK_DGRAM, 0);
  if (sock < 0)
  { perror("socket");
    exit(1); }

  struct sockaddr_in local;
  local.sin_len = sizeof(local);
  local.sin_family = AF_INET;
  local.sin_addr.s_addr = htonl(INADDR_ANY);
  local.sin_port = htons(local_port);

  int r = bind(sock, (struct sockaddr *) & local, sizeof(local));
  if (r < 0)
  { perror("bind");
    exit(1); }

  struct sockaddr_in remote;
  bzero(& remote, sizeof(remote));
  unsigned int size = sizeof(remote);

  char message[1000];
  r = recvfrom(sock, message, sizeof(message) - 1, 0,
             (struct sockaddr *) & remote, & size);
  if (r < 0)
  { perror("recvfrom");
    exit(1); }

  message[r]=0;
  printf("from %s port %d, %d characters: '%s'\n",
                  inet_ntoa(remote.sin_addr), 
                  ntohs(remote.sin_port),
                  r,
                  message);

  close(sock); }