@rmustacc's ARP-based retrieval of MAC addresses. Should work on any UNIX-like.

getmac_arp.c

/*
 * Test getifaddrs behavior.
 */

#include <sys/types.h>
#include <sys/socket.h>
#include <ifaddrs.h>
#include <stdio.h>
#include <net/if.h>
#include <unistd.h>
#include <stropts.h>
#include <sys/sockio.h>
#include <strings.h>
#include <net/if_arp.h>

int
main(int argc, const char *argv[])
{
        struct ifaddrs *ifa, *oif;
        struct arpreq arp;

        if (getifaddrs(&ifa) != 0) {
                perror("getifaddrs");
                return (1);
        }   

        oif = ifa;
        while (ifa != NULL) {
                int sock, i;

                printf("%s: [%d]\n", ifa->ifa_name, ifa->ifa_addr->sa_family);
                if (ifa->ifa_flags & IFF_LOOPBACK) {
                        ifa = ifa->ifa_next;
                        continue;
                }   

                sock = socket(ifa->ifa_addr->sa_family, SOCK_DGRAM, 0); 
                if (sock < 0) {
                        perror("socket");
                        freeifaddrs(oif);
                        return (1);
                }   

                bzero(&arp, sizeof (struct arpreq));
                arp.arp_pa = *ifa->ifa_addr;
                if (ioctl(sock, SIOCGARP, (void *)&arp) != 0) {
                        perror("ioctl");
                        return (1);
                }   

                for (i = 0; i < 6; i++) {
                        printf("%02x%s", (unsigned char)arp.arp_ha.sa_data[i],
                            i != 5 ? ":" : "");
                }   
                printf("\n");
                ifa = ifa->ifa_next;
        }   

        freeifaddrs(oif);

        return (0);
}