Question

I have got MAC address as under.

struct ifreq ifr;
struct ifreq *IFR;
struct ifconf ifc;
char buf[1024];
int s, i;
int ok = 0;
string macAddr = "";

s = socket(AF_INET, SOCK_DGRAM, 0);
if (s==-1) {
    return;
}

ifc.ifc_len = sizeof(buf);
ifc.ifc_buf = buf;
ioctl(s, SIOCGIFCONF, &ifc);

IFR = ifc.ifc_req;
for (i = ifc.ifc_len / sizeof(struct ifreq); --i >= 0; IFR++) {

    strcpy(ifr.ifr_name, IFR->ifr_name);
    if (ioctl(s, SIOCGIFFLAGS, &ifr) == 0) {
        if (! (ifr.ifr_flags & IFF_LOOPBACK)) {
            if (ioctl(s, SIOCGIFHWADDR, &ifr) == 0) {
                ok = 1;
                break;
            }
        }
    }
}

close(s);

int p = sizeof(ifr.ifr_hwaddr.sa_data);

cout<<"\n Size:"<<p<<"\n";

for(int i = 0; i < p; i++)
    macAddr += ifr.ifr_hwaddr.sa_data[i];

cout<<"\n MAC Address:"<<macAddr<<"\n";

I got ifr.ifr_hwaddr.sa_data data proper, but when I print out I am not getting proper value. What can be the issue for it.

Answer 1

The address in sa_data isn’t in text form, it’s the raw binary form, so you need to format it as hex yourself. You also can’t use sizeof to get the size of the address as sa_data is a generically sized array – you need to look at the sa_len member to get the length of this address. In C you want something like this:

char *separator = "";

for (int i = 0; i < ifr.ifr_hwaddr.sa_len; i++)
{
  printf("%s%02x", separator, ifr.ifr_hwaddr.sa_data[i]);
  separator = ":";
}

printf("\n");