udp-echo-server.c

/* UDP echo server program -- udp-echo-server.c */

#include <stdio.h>      /* standard C i/o facilities */
#include <stdlib.h>     /* needed for atoi() */
#include <sys/types.h>  /* system data type definitions */
#if defined(_WIN32)
#include <WinSock2.h>
#pragma comment(lib, "Ws2_32.lib")
#else
#include <unistd.h>     /* defines STDIN_FILENO, system calls,etc */
#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 */
#endif // defined(_WIN32)

/* this routine echos any messages (UDP datagrams) received */

#define MAXBUF 1024*1024

void echo( int sd ) {
    int len,n;
    char *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);

    bufin = calloc(MAXBUF, sizeof(char));

    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 %d BYTES\n",n);
        /* Got something, just send it back */
        sendto(sd, bufin, n, 0, (struct sockaddr *)&remote, len);
      }
    }
    free(bufin);
}

/* server main routine */

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

  {
#if defined(_WIN32)
      WSADATA wsaData;
      if (WSAStartup(MAKEWORD(2, 2), &wsaData) != NO_ERROR) {
          printf("Error at WSAStartup()\n");
          return 1;
      }
#endif
  }

  /* 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 echo every datagram we get */
  echo(ld);
  return(0);
}