extract hex IP from struct addrinfo

Question

I have populated my structure struct addrinfo **res with data:

err = getaddrinfo(argv[1], NULL, &hints, &res);

if I want to print the IP address, I could use

addr.s_addr = ((struct sockaddr_in *)(res->ai_addr))->sin_addr.s_addr;
printf("ip address : %s\n", inet_ntoa(addr));

and that would print for example 10.111.1.11 . But I need the IP in hex format, ie 0A6F010B . I came up with following, which will indeed print the IP in hex format:

for (i=0; i<32; i=i+8)
    printf("%02X",(addr.s_addr>>i) % 256) ;

But it seems to me that my approach is needlessly complicated. I need two steps (two conversions). I would like to simplify my code, if possible, and print the hex IP directly from res->ai_addr->sa_data (without having to store the data in addr.s_addr first).

I know that the data is there in res->ai_addr->sa_data . I can print it with:

for (i=0; i<14; i++)
    printf("%d", res->ai_addr->sa_data[i]);
printf("\n");

and I get following output (which corresponds to 10.111.1.11 ):

001011111100000000

But the format is not as straightforward as in the previous example. For example for 172.25.0.31 I get this output:

00-842503100000000

Thus my question:

How can I convert this gibberish into hex IP adress?

Answer 1

According to the linked question and this description of sockaddr , you need to do some dynamic type mangling. With the correct types, it might look something like so:

struct addrinfo * res;
err = getaddrinfo(argv[1], NULL, &hints, &res);

// check that the operation succeeded
assert(err == 0);

// check that the dynamic type is AF_INET
assert(res->ai_addr->sa_family == AF_INET);

// downcast
struct sockaddr_in * p = (struct sockaddr_in *)(res->ai_addr);

// done
printf("Address: %08X (%s)\n", p->sin_addr.s_addr, inet_ntoa(p->sin_addr));

---

A more complete example that performs a proper dynamic type selection might look like this:

char str[128];

switch (res->ai_addr->sa_family)
{
  case AF_INET:
  {
      struct sockaddr_in * p = (struct sockaddr_in *)(res->ai_addr);
      if (inet_ntop(p->sin_family, &p->sin_addr, str, sizeof str) == NULL)
          fprintf(stderr, "Error in inet_ntop: %s\n", strerror(errno));
      else
          printf("Address: %08X (%s)\n", p->sin_addr.s_addr, str);
      break;
  }
  case AF_INET6:
  {
      struct sockaddr_in6 * p = (struct sockaddr_in6 *)(res->ai_addr);
      if (inet_ntop(p->sin6_family, &p->sin6_addr, str, sizeof str) == NULL)
          fprintf(stderr, "Error in inet_ntop: %s\n", strerror(errno));
      else
          printf("Address: %s\n", str);
      break;
  }
  default:
  {
      fprintf(stderr, "Unrecogized address family, skipping.\n");
  }
}

Answer 2

Take a look at an implementation of inet_ntoa here :

#define UC(b)   (((int)b)&0xff)

char* net_ntoa(struct in_addr in) {
    static char b[18];
    register char *p;
    p = (char *)∈
    snprintf(b, sizeof(b), "%d.%d.%d.%d", UC(p[0]), UC(p[1]), UC(p[2]), UC(p[3]));
    return (b);
}

You can change their implementation to suit your needs simply by swapping in the desired format, ie hex:

snprintf(b, sizeof(b), "%02X%02X%02X%02X", UC(p[0]), UC(p[1]), UC(p[2]), UC(p[3]));

Note that their solution is not thread-safe, because it uses a function-static buffer for the output. To make it thread-safe, change the function to take an output buffer, and require the buffer to have at least nine characters (eight for the hex output plus one for the null terminator.