Go4Expert - Help Needed in sockets in C

Code:

 /*

** 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;

}

I am having problems to understand this program i know the function getaddrinfo(); and the structs but having problem with the linked list and the following loop:-

Code:

     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";

        }

questions:-

1. Why do we need this loop and what is the mechanism of this p->next etc.

2.please explain me the whole loop rely seeking for your help .

and i have a problem in understanding this function
inet_ntop

and this code how it works :-

Code:

inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);

        printf("  %s: %s\n", ipver, ipstr);

please guyz i am stuck at this topic and am reading
more and more tutorials but none of them help please help me.

Rely seeking your help.