I don't know how to use Beej's programm from his Guide

Question

1I'm reading Beej's programming network guide. I read his code and did an important change to check ipv6 but it's not working right. Can't get an ip Address. How to use this on a Linux system? https://i.imgur.com/USVzBw1.png

#include <stdio.h>
#include <string.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 if (p->ai_family==AF_INET6) { // 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;
}```

Answer 1

This looks suspicious:

You check to make sure argc is 2 to validate you have at least one argument. Makes sense

if (argc != 2) {

But then later:

if ((status = getaddrinfo(argv[2], NULL, &hints, &res)) != 0) {

You reference argv[2] , which is undefined when the only valid indicies in argv are 0 and 1 .

Pretty sure you meant to use argv[1] as follows:

if ((status = getaddrinfo(argv[1], NULL, &hints, &res)) != 0) {

Also, this is a bug:

    // convert the IP to a string and print it:
    inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);
    printf("  %s: %s\n", ipver, ipstr);

Nothing wrong with those statements, but when p->ai_family is neither AF_INET nor AF_INET6 , then those statements get executed anyway. addr and ipver are undefined. Hence, undefined behavior. And yes, on Linux, there's plenty of address types that are not IP based.

You are welcome to reference my ResolveHostname function on Github .