/***************************************/
/*            - UDP server -           */
/***************************************/

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h> 

#define BUFSIZE 1024
#define PORT_NO 2001
#define error(a,b) fprintf(stderr, a, b)

int main(int argc, char *argv[] ){ // arg count, arg vector 

   /************************** Declarations ***********************/
   int fd;                         // socket endpoint
   int flags;                      // receive flags
   struct sockaddr_in server;      // address of server
   struct sockaddr_in client;      // address of client
   unsigned int server_size;       // length of the sockaddr_in server
   unsigned int client_size;       // length of the sockaddr_in client
   int bytes;                      // length of buffer 
   int rcvsize;                    // received bytes
   int err;                        // error code
   char on;                        // sockopt option
   char buffer[BUFSIZE+1];         // datagram dat buffer area

   /********************** Initialization *************************/
   on    = 1;
   flags = 0;
   bytes = BUFSIZE;
   server.sin_family      = AF_INET;
   server.sin_addr.s_addr = INADDR_ANY;
   server.sin_port        = htons(PORT_NO);
   server_size = sizeof server;
   client_size = sizeof client;

   /************************ Creating socket **********************/
   fd = socket(AF_INET, SOCK_DGRAM, 0 );
   if ( fd < 0 ) {
      error("%s: Socket creation error\n",argv[0]);
      exit(1);
      }

   /************************ Setting options  *********************/
   setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, (char *)&on, sizeof on);
   setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, (char *)&on, sizeof on);

   /************************ Binding socket ***********************/
   err = bind( fd, (struct sockaddr *) &server, server_size);
   if ( err < 0 ) {
      error("%s: Cannot bind to the socket\n",argv[0]);
      exit(2);
      }

   /************************ Receive data *************************/
   rcvsize = recvfrom( fd, buffer, bytes, flags,
		  (struct sockaddr *) &client, &client_size );
   if ( rcvsize < 0 ) {
      error("%s: Cannot receive from the socket\n",argv[0]);
      exit(3);
      }

   /************************ User interface **********************/
   printf (" %i bytes have been received from client.\n Message: %s \n",
	    rcvsize-1, buffer);

   close(fd);
   exit(0);
   }