/*
** showip.c -- show IP addresses for a host given on the command line
*/
 
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>
 
int main(int argc, char *argv[])
{
    struct addrinfo hints, *res, *p;
    int status;
    char ipstr[INET6_ADDRSTRLEN];
 
    if (argc != 2) {
        fprintf(stderr,"usage: showip hostname\n");
        return 1;
    }
 
    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC; // AF_INET or AF_INET6 to force version
    hints.ai_socktype = SOCK_STREAM;
 
    if ((status = getaddrinfo(argv[1], NULL, &hints, &res)) != 0) {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(status));
        return 2;
    }
 
    printf("IP addresses for %s:\n\n", argv[1]);
 
    for(p = res;p != NULL; p = p->ai_next) {
        void *addr;
        char *ipver;
 
        // get the pointer to the address itself,
        // different fields in IPv4 and IPv6:
        if (p->ai_family == AF_INET) { // IPv4
            struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
            addr = &(ipv4->sin_addr);
            ipver = "IPv4";
        } else { // IPv6
            struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
            addr = &(ipv6->sin6_addr);
            ipver = "IPv6";
        }
 
        // convert the IP to a string and print it:
        inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);
        printf("  %s: %s\n", ipver, ipstr);
    }
 
    freeaddrinfo(res); // free the linked list
 
    return 0;
}

#ifndef INET6_ADDRSTRLEN
#define INET6_ADDRSTRLEN 46
#endif

addr = &(ipv4->sin_addr);
addr = &(ipv6->sin6_addr);

error: dereferencing pointer to incomplete type

#include <netinet/in.h>