#include <stdio.h>

#include <string.h> /* for strncpy */

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <netinet/in.h>
#include <net/if.h>

int main(int argc, char **argv)
{
        int fd;
        struct ifreq ifr;

        fd = socket(AF_INET, SOCK_DGRAM, 0);

        ifr.ifr_addr.sa_family = AF_INET;
        strncpy(ifr.ifr_name, "eth0", IFNAMSIZ-1);

        ioctl(fd, SIOCGIFHWADDR, &ifr);

        close(fd);

        /* display result */
        printf("%.2X:%.2X:%.2X:%.2X:%.2X:%.2X\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]);

        return 0;
}

• Also see: Retrieval MAC address in Windiws