http://kldp.org/KoreanDoc/html/Programming_tip-KLDP/x43.html#AEN115

#include <netinet/ether.h>
#include <net/ethernet.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <stdio.h>

int main (int argc, char *argv[])
{
struct ifreq *iflist;
struct sockaddr *sa;
int fd;
char *out;

if(argc != 2)
{
printf("Usage: progname ifname (ex: progname eth0)\n");
return -1;
}

iflist = malloc (sizeof (struct ifreq));

fd = socket (PF_INET, SOCK_STREAM, 0);

strncpy (iflist->ifr_name, argv[1], strlen (argv[1]));

if (ioctl (fd, SIOCGIFHWADDR, iflist) == -1)
{
perror ("ioctl failed");
return -1;
}

sa = &(iflist->ifr_hwaddr);

out = ether_ntoa ((struct ether_addr *) sa->sa_data);

printf ("%s\n", out);

return 0;
}