server.c

this gives details of the network programming

/* UDP server program - expects each incoming message (datagram)

   to contain a single 32 bit integer. Gets the value and prints it
   out.

*/

#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 */



#define MAXBUF 1000

/* watch for incoming UDP datagrams - for each one
   that arrives, interpret the first 4 bytes as an integer
   and printout the value
*/

void printnums( int sd ) {
    int len,n;
    char bufin[MAXBUF];
    int *theint = (int *)bufin;

    struct sockaddr_in remote;

    /* need to know how big address struct is, len must be set before the
       call to recvfrom!!! */

    len = sizeof(remote);

    while (1) {
      /* read a datagram from the socket (put result in bufin) */
      n=recvfrom(sd,bufin,MAXBUF,0,(struct sockaddr *)&remote,&len);

      /* print out the address of the sender */
      printf("Got a datagram from %s port %d\n",
	     inet_ntoa(remote.sin_addr), ntohs(remote.sin_port));

      if (n<0) {
	perror("Error receiving data");
      } else {
	printf("GOT a number: %d\n",(*theint));

      }
    }
}



/* server main routine */

int main() {
  int ld;
  struct sockaddr_in skaddr;
  int length;

  /* create a socket 
     IP protocol family (PF_INET) 
     UDP protocol (SOCK_DGRAM)
  */
  
  if ((ld = socket( PF_INET, SOCK_DGRAM, 0 )) < 0) {
    printf("Problem creating socket\n");
    exit(1);
  }
  
  /* establish our address 
     address family is AF_INET
     our IP address is INADDR_ANY (any of our IP addresses)
     the port number is assigned by the kernel 
  */

  skaddr.sin_family = AF_INET;
  skaddr.sin_addr.s_addr = htonl(INADDR_ANY);
  skaddr.sin_port = htons(0);

  if (bind(ld, (struct sockaddr *) &skaddr, sizeof(skaddr))<0) {
    printf("Problem binding\n");
    exit(0);
  }
  
  /* find out what port we were assigned and print it out */

  length = sizeof( skaddr );
  if (getsockname(ld, (struct sockaddr *) &skaddr, &length)<0) {
    printf("Error getsockname\n");
    exit(1);
  }
  
  /* port number's are network byte order, we have to convert to
     host byte order before printing !
  */
  printf("The server UDP port number is %d\n",ntohs(skaddr.sin_port));

  /* Go handle every datagram we get */
  printnums(ld);
  return(0);
}