/**************************************************************
 * connectionless server programming 
 * 
 * Function: echo server (not iterative)
 * 
 * Reference: Richard Stevens, Unix Network Programming, 1998
 *
 * Created by: Hairong Qi
 **************************************************************/
#include <stdio.h>        /* for standard I/O */
#include <unistd.h>       /* for read() and write() */
#include <strings.h>      /* for bzero() */
#include <stdlib.h>       /* for atoi() */
#include <sys/types.h>    /* for accept() and connect() */
#include <sys/socket.h>
#include <sys/errno.h>    /* for perror() */
#include <netinet/in.h>   /* for htonl() */

#define LISTENQ 2
#define BSIZE 256
#define USAGE "./serverudp portno\n"

/**
 ** output the error message and exit
 **/
void error(char *msg)
{
  perror(msg);
  exit(1);
}


/**
 ** the main function 
 **/
int main(int argc, char *argv[])
{
  int listenfd, connfd;              /* socket file descriptor */
  int portno;
  int clilen;
  char buffer[BSIZE];
  struct sockaddr_in serv_addr, cli_addr;
  int n;

  /* check the correctness of command-line inputs */
  if (argc < 2) 
    error(USAGE);

  /* create a listening socket */
  listenfd = socket(PF_INET, SOCK_DGRAM, 0);  /** change SOCK_STREAM to SOCK_DGRAM **/
  if (listenfd < 0) 
    error("ERROR opening socket");

  /* specify local address */
  bzero((char *) &serv_addr, sizeof(serv_addr));
  serv_addr.sin_family = AF_INET;
  serv_addr.sin_addr.s_addr = htonl(INADDR_ANY); 
  portno = atoi(argv[1]);
  serv_addr.sin_port = htons(portno);

  /* bind the address to the listening socket */
  if (bind(listenfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) 
    error("ERROR on binding");

  /**************************************************************************
  **  the following would be commented for connectionless service by UDP **
  **
  listen(listenfd, LISTENQ);

  clilen = sizeof(cli_addr);
  connfd = accept(listenfd, (struct sockaddr *) &cli_addr, &clilen);
  if (connfd < 0) 
    error("ERROR on accept");
  **
  ** commented part ends here **
  ***************************************************************************/

  /* start to receive data */
  bzero(buffer, BSIZE);
 
  /***************************************************************************
  ** can't use read() to receive messages as no client socket ID is created **
  **
  n = read(connfd, buffer, BSIZE-1);
  **
  ** instead, using recvfrom(). Pay attention to its usage
  ***************************************************************************/

  clilen = sizeof(cli_addr);
  n = recvfrom(listenfd, buffer, BSIZE-1, 0, (struct sockadd *)&cli_addr, &clilen);
  if (n < 0) 
    error("ERROR reading from socket");
  printf("Here is the message: %s\n", buffer);
  fflush(stdout);

  /* start to reply to client */

  /***************************************************************************
  ** similarly, can't use write(), use sendto() instead
  **
  n = write(connfd, "I got your message", 18);
  **
  ***************************************************************************/

  n = sendto(listenfd, "I got your message", 18, 0,
	     (struct sockaddr *)&cli_addr, sizeof(cli_addr));
  if (n < 0) 
    error("ERROR writing to socket");

  return 0; 
}