MAC Address printing

Question

This is a code that gets some info about network the problem is when it prints the MAC address it prints it sometime normally and sometime with fff\'s like 00:21:84:a2:12:88

Answer 1

Here is a simple C program to find the MAC address of system:

#include<stdio.h>
#include<stdlib.h>

int main() {
    system("getmac");
    return 0;
}

Answer 2

It looks like a signed/unsigned problem.

Try to cast into unsigned char :

  printf("> Successfully received Local MAC Address : %02x:%02x:%02x:%02x:%02x:%02x\n",
  (unsigned char) ifr.ifr_hwaddr.sa_data[0],
  (unsigned char) ifr.ifr_hwaddr.sa_data[1],
  (unsigned char) ifr.ifr_hwaddr.sa_data[2],
  (unsigned char) ifr.ifr_hwaddr.sa_data[3],
  (unsigned char) ifr.ifr_hwaddr.sa_data[4],
  (unsigned char) ifr.ifr_hwaddr.sa_data[5]);

Answer 3

I prefer use an explicit length modifier in format string for values shorter than int. For example %02hhx instead of %02x for one-byte values. It allow me to not worry about those subtle conversion and promotion issues:

#include <stdio.h>

int main(void)
{
    signed char sff = '\xff';
    unsigned char uff = '\xff';

    printf("signed:   %02x %02hhx\n", sff, sff);
    printf("unsigned: %02x %02hhx\n", uff, uff);

    return 0;
}

prints

signed:   ffffffff ff
unsigned: ff ff

Answer 4

Although there is already an accepted answer here, there is a better solution in netinet/ether.h.

Given that Mac addresses are typically stored in u_int8_t types, as in the ether_addr struct:

You can simply do this:

printf("Mac Address: %s", ether_ntoa((struct ether_addr*)ar->sa));

in my Case ar is something that looks like this:

struct {
   u_int8_t sa[6];
}

You can easily copy this into another buffer with something like asprintf:

char *formatted_mac_address;
asprintf(formatted_mac_address, "Mac Address: %s", ether_ntoa((struct ether_addr*)ar->sa));

If you don't have a struct as I do, you can also just use the address of any u_int8_t in place of ar->sa.

Appropriate headers/etc should be pulled in, but that's going to look a lot neater than the accepted solution here.