Question

I’m sure that it’s due to a silly mistake, but I have been debuging during hours and I couldn’t solve the issue.

In my program, I have a function that returns the address of a given domain. It calls to getaddrinfo, copies the address to a new dinamically asigned variable, sets its pointer into a hostent struct and returns it. Nevertheless, the address returned wasn’t correct. After some debuging I realized that its value changes unexpectedly in some point of the code.

I made a sample program with the relevant part in this issue:

#include <iostream>
#include <string.h>
#include <string>
#include <netinet/in.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <stdlib.h>

using namespace std;
typedef unsigned long int longip_t;

int main(int argc, char** argv) {

    struct addrinfo hints, *res;
    struct hostent *final = new hostent;

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    getaddrinfo("automation.whatismyip.com", "80", &hints, &res);

    cout << inet_ntoa(((sockaddr_in*)res->ai_addr)->sin_addr) << endl;
    in_addr *addr = new in_addr;
    memcpy((void*)addr, (void*)(&((sockaddr_in*)res->ai_addr)->sin_addr), sizeof(in_addr));
    cout << inet_ntoa(*addr) << endl;

    final->h_addr_list = (char**)addr; 
    cout << inet_ntoa(*addr) << endl;
    final->h_addr = (char*)addr; // At this point the value of addr changes
    cout << inet_ntoa(*addr) << endl;
    cout << inet_ntoa(*(in_addr*)final->h_addr_list) << endl;
    exit(0);
}

The output I get:

72.233.89.195
72.233.89.195
72.233.89.195
48.116.218.9
48.116.218.9

The first three IPs are correct, but I get different values for the last two IPs every time I run the program. It looks that the value of addr changes after setting its pointer into the hostent struct.

Answer 1

From hostent Structure on MSDN

h_addr_list A NULL-terminated list of addresses for the host.
Addresses are returned in network byte order. The macro h_addr is
defined to be h_addr_list[0] for compatibility with older software.

final->h_addr = (char*)addr;

So the above line is really this

final->h_addr_list[0] = (char*)addr;

From final->h_addr_list = (char**)addr;, that line becomes this

addr[0] = (char*)addr;

Now, I believe you see the problem. 🙂

[Update]

The right way to set it is actually easy and automatic. You should call gethostbyaddr or gethostbyname, and they will return a pointer to hostent. This is well explained in MSDN. If you want to set hostent manually, which I don’t recommend, you have to create an array of pointers to addr.s_addr and set h_addr_list to the array. The array should include the last null element. I don’t see any reasons why you want to do it manually as OS does it for you.