#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <sys/time.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/types.h>

int main(int argc, char *argv[])
{
  unsigned char data;
  int port_number;
  struct sockaddr_in serv_addr, cli_addr;
  int socket_fd, cli_len, res;

  if (argc != 2)
    {
      printf("Usage: udp_server <port number> \n");
      exit(1);
    }

  port_number = atoi(argv[1]);
  if (port_number <= 1024) 
    {
      printf("Invalid port number. Port <= 1024. Aborting ...\n");
      exit(1);
    }

  if ((socket_fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    exit(1);
  bzero((char *)&serv_addr, sizeof(struct sockaddr_in));
  serv_addr.sin_family = AF_INET;
  serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
  serv_addr.sin_port = htons(port_number);
  
  if (bind(socket_fd, (struct sockaddr *)&serv_addr,
	   sizeof(struct sockaddr_in)) < 0) exit(1);
  
  while(1)
    {
      cli_len = sizeof(cli_addr);
      res = recvfrom(socket_fd, (void *)&data, 1, 0,
		   (struct sockaddr *) &cli_addr, &cli_len);
      if (res != 1)
	{
	  printf("UDP Server: Receive error. ..\n");
	  exit(-1);
	}

      if (data == 0)
	{
	  close(socket_fd);
	  exit(0);
	}

      sendto(socket_fd, (void *)&data, 1, 0, &cli_addr,  cli_len);
    }

  exit(0);
}