Troubleshooting socket programming

Question

I have a Raspberry Pi which has name resolution issues with Python's getaddrinfo. I traced the source code, perhaps wrongly, to the C function gethostbyaddr. So now I am trying to create a simple test to see what this function returns. Sockets programming, and C, is way over my head, but my attempt is:

#include <sys/socket.h>
#include <string.h>
#include <stdio.h>

static struct gai_afd {
    int a_af;
    int a_addrlen;
    int a_socklen;
    int a_off;
    const char *a_addrany;
    const char *a_loopback;
};

int main()
{
  struct hostent *hp;
  struct gai_afd *gai_afd;
  hp = gethostbyaddr("google.com", gai_afd->a_addrlen, AF_INET);
}

Compiling using gcc gives two warnings:

warning: useless storage class specifier in empty declaration [enabled by default]
In function ‘main’: warning: assignment makes pointer from integer without a cast [enabled by default]

Running a.out gives Segmentation fault.

What must I change to make the above work?

My objective is to find out why getaddrinfo is unable to resolve google.com when ping works fine on this very same machine. The trouble I am facing is here.

LPs · Accepted Answer · 2016-01-28 09:27:02Z

Pointers are not inited.

At least

int main()
{
  struct hostent *hp;
  struct gai_afd *gai_afd = malloc(sizeof(gai_afd));

  // ...

}

This is a little example to find infos:

#include <stdio.h>
#include <errno.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

int main(int argc, char *argv[])
{
    int i;
    struct hostent *he, *inner_he;
    struct in_addr **addr_list;
    unsigned long ip;
    char *addressString;

    if (argc != 2) {
        fprintf(stderr,"usage: ghbn hostname\n");
        return 1;
    }

    if ((he = gethostbyname(argv[1])) == NULL) {  // get the host info
        herror("gethostbyname");
        return 2;
    }

    // print information about this host:
    printf("Official name is: %s\n", he->h_name);
    addr_list = (struct in_addr **)he->h_addr_list;
    for(i = 0; addr_list[i] != NULL; i++)
    {
        addressString = inet_ntoa(*addr_list[i]);

        printf("    IP addresse %d: %s \n", i, addressString);

        ip = inet_addr(addressString);

        inner_he = gethostbyaddr((const char *)&ip, sizeof(ip), AF_INET);
        if (inner_he != NULL)
            printf("Host name: %s\n", inner_he->h_name);
    }
    printf("\n");

    return 0;
}

You can launch it passing www.usa.com and output will be:

Official name is: www.usa.com
    IP addresse 0: 69.10.42.209 
Host name: lawyer.com

Vasfed · Accepted Answer · 2016-01-28 08:06:19Z

0

You're not allocating any memory for your structs, and dereference an unintialised pointer

answered Jan 28, 2016 at 8:06

Vasfed

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