C++ Resolve a host IP address from a URL

Question

How can I resolve a host IP address, given a URL in Visual C++?

Answer 1

I'm not sure if there is a specific C++ class to do host name lookups, but you can always resort to plain C for such things. Here's my version which compiles and runs on Linux, Mac OS X, and Windows.

#include <stdio.h>

#ifdef _WIN32
#  include "winsock.h"
#else
#  include <netdb.h>
#  include <arpa/inet.h>
#endif

static void initialise(void)
{
#ifdef _WIN32
    WSADATA data;
    if (WSAStartup (MAKEWORD(1, 1), &data) != 0)
    {
        fputs ("Could not initialise Winsock.\n", stderr);
        exit (1);
    }
#endif
}

static void uninitialise (void)
{
#ifdef _WIN32
    WSACleanup ();
#endif
}

int main (int argc, char *argv[])
{
    struct hostent *he;

    if (argc == 1)
        return -1;

    initialise();

    he = gethostbyname (argv[1]);
    if (he == NULL)
    {
        switch (h_errno)
        {
            case HOST_NOT_FOUND:
                fputs ("The host was not found.\n", stderr);
                break;
            case NO_ADDRESS:
                fputs ("The name is valid but it has no address.\n", stderr);
                break;
            case NO_RECOVERY:
                fputs ("A non-recoverable name server error occurred.\n", stderr);
                break;
            case TRY_AGAIN:
                fputs ("The name server is temporarily unavailable.", stderr);
                break;
        }
    }
    else
    {
        puts (inet_ntoa (*((struct in_addr *) he->h_addr_list[0])));
    }

    uninitialise ();

    return he != NULL;
}

Once compiled, provide the host name as an argument:

$ ./a.out stackoverflow.com
69.59.196.211