Skip to content

Instantly share code, notes, and snippets.

What would you like to do?
Example of getaddrinfo() program.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
lookup_host (const char *host)
struct addrinfo hints, *res;
int errcode;
char addrstr[100];
void *ptr;
memset (&hints, 0, sizeof (hints));
hints.ai_family = PF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags |= AI_CANONNAME;
errcode = getaddrinfo (host, NULL, &hints, &res);
if (errcode != 0)
perror ("getaddrinfo");
return -1;
printf ("Host: %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;
case AF_INET6:
ptr = &((struct sockaddr_in6 *) res->ai_addr)->sin6_addr;
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 (void)
char inbuf[256];
int len;
do {
bzero(inbuf, 256);
printf("Type domain name:\n");
fgets(inbuf, 256, stdin);
len = strlen(inbuf);
inbuf[len-1] = '\0';
if(strlen(inbuf) > 0)
lookup_host (inbuf);
} while(1);

This comment has been minimized.

Copy link

modelorona commented Feb 7, 2019

Hi, I am new to C and networks, so I have a question (and please correct me if I am wrong). On line 32, you use inet_ntop to get the human readable address and then under have a switch statement which determines if the res family is ipv4 or ipv6. My question is could you not remove that first inet_ntop and have the switch be the first statement in the while loop?

I am curious as to why you have two calls to inet_ntop, because when I tried locally and removed the call on line 32, it worked the same way I believe.

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.