getaddrinfo.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>

int lookup_host (const char *host, int use_canonname) {
  struct addrinfo hints, *res;
  int errcode;
  char addrstr[100];
  void *ptr;

  memset (&hints, 0, sizeof (hints));
  hints.ai_family = AF_INET;
  if(use_canonname) {
    hints.ai_flags = AI_CANONNAME;
  }

  errcode = getaddrinfo (host, NULL, &hints, &res);
  printf("getaddrinfo returned %d: %s\n", errcode, gai_strerror(errcode));
  if(errcode != 0) {
      return -1;
  }

  printf ("Requested hostname: %s\n", host);
  while(res) {
      inet_ntop(res->ai_family, res->ai_addr->sa_data, addrstr, 100);

      switch(res->ai_family) {
        case AF_INET:
          ptr = &((struct sockaddr_in *) res->ai_addr)->sin_addr;
          break;
        case AF_INET6:
          ptr = &((struct sockaddr_in6 *) res->ai_addr)->sin6_addr;
          break;
        }
      inet_ntop(res->ai_family, ptr, addrstr, 100);
      printf ("IPv%d address: %s (%s)\n", res->ai_family == PF_INET6 ? 6 : 4,
              addrstr, res->ai_canonname);
      res = res->ai_next;
    }

  return 0;
}

int main (int argc, char *argv[]) {
  if (argc < 2) {
    fprintf(stderr, "usage: %s hostname\n", argv[0]);
    exit(1);
  }
  printf("looking up without no ai_flags set:\n");
  lookup_host(argv[1], 0);
  printf("\nlooking up with AI_CANONNAME flag set:\n");
  lookup_host(argv[1], 1);
}