client.c

this gives details of the network programming

/* Simple UDP client - sends a 32 bit integer to the server
   and quits.

   This is just an example showing what can happen when you
   don't pay attention to network byte order!
*/ 

#include <stdio.h>      /* standard C i/o facilities */
#include <stdlib.h>     /* needed for atoi() */
#include <unistd.h>  	/* defines STDIN_FILENO, system calls,etc */
#include <sys/types.h>  /* system data type definitions */
#include <sys/socket.h> /* socket specific definitions */
#include <netinet/in.h> /* INET constants and stuff */
#include <arpa/inet.h>  /* IP address conversion stuff */
#include <netdb.h>	/* gethostbyname */


/* client program:

   The following must passed in on the command line:
      hostname of the server (argv[1])
      port number of the server (argv[2])
      the number we want to send (argv[3])
*/


int main( int argc, char **argv ) {
  int sk;
  struct sockaddr_in server;
  struct hostent *hp;
  int n_sent;
  int num;
  


  /* Make sure we have the right number of command line args */

  if (argc!=4) {
    printf("Usage: %s <server name> <port number> <number>\n",argv[0]);
    exit(0);
  }

  /* create a socket 
     IP protocol family (PF_INET) 
     UDP (SOCK_DGRAM)
  */
  
  if ((sk = socket( PF_INET, SOCK_DGRAM, 0 )) < 0)
    {
      printf("Problem creating socket\n");
      exit(1);
    }

  /* Using UDP we don't need to call bind unless we care what our
     port number is - most clients don't care */

  /* now create a sockaddr that will be used to contact the server 

     fill in an address structure that will be used to specify
     the address of the server we want to connect to

     address family is IP  (AF_INET)

     server IP address is found by calling gethostbyname with the
     name of the server (entered on the command line)

     server port number is argv[2] (entered on the command line)
  */

  server.sin_family = AF_INET;
  if ((hp = gethostbyname(argv[1]))==0) {
    printf("Invalid or unknown host\n");
    exit(1);
  }

  /* copy the IP address into the sockaddr 
     It is already in network byte order 
  */

  memcpy( &server.sin_addr.s_addr, hp->h_addr, hp->h_length);

  /* establish the server port number - we must use network byte order! */
  server.sin_port = htons(atoi(argv[2]));


  /* now get the number we want to send */
  num = atoi(argv[3]);

  //  num = htonl(num);
  /* send it to the server */
  
  n_sent = sendto(sk,&num,sizeof(num),0,
		  (struct sockaddr*) &server,sizeof(server));

  if (n_sent<0) {
    perror("Problem sending data");
    exit(1);
  }

  printf("Sendto sent %d bytes\n",n_sent);

  printf("The number sent was %d\n",num);

  return(0);
}