//                   Sending 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 != 3)
  { fprintf(stderr, "requires remotehost remoteport\n");
    exit(1); }
  char * remote_host = argv[1];
  int remote_port = atol(argv[2]);

  struct hostent * he = gethostbyname(remote_host);
  if (he == NULL) 
  { herror("gethostbyname"); 
    exit(1); }
  struct in_addr * dest_addr = (struct in_addr *) he->h_addr;

  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(0);
                     // 0 means system will pick any available port >= 1024

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

  struct sockaddr_in remote;
  remote.sin_len = sizeof(remote);
  remote.sin_family = AF_INET;
  remote.sin_addr = * dest_addr;
  remote.sin_port = htons(remote_port);

  const char * message = "Hello, you.";
  r = sendto(sock, message, strlen(message), MSG_EOR,
             (struct sockaddr *) & remote, sizeof(remote));
  if (r < 0)
  { perror("sendto");
    exit(1); }
  printf("sent %d characters of '%s'\n", r, message);

  close(sock); }