Here is a short example of how to get the macaddress of a network interface in linux from a c program.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

#include <net/if.h>

#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/socket.h>

int main(int argc, char **argv) {
	int sock = socket(PF_INET, SOCK_DGRAM, 0);
	struct ifreq req;
	int i = 0;

	if (argc < 2) {
		fprintf(stderr, "Usage: %s <interfacename>\n", argv[0]);
		exit(EXIT_FAILURE);
	}

	if (sock < 0) {
		perror("socket");
		exit(EXIT_FAILURE);
	}

	memset(&req, 0, sizeof(req));
	strncpy(req.ifr_name, argv[1], IF_NAMESIZE - 1);

	if (ioctl(sock, SIOCGIFHWADDR, &req) < 0) {
		perror("ioctl");
		exit(EXIT_FAILURE);
	}

	for(i=0;i<6;i++) {
		printf("%.2X", (unsigned char) req.ifr_hwaddr.sa_data[i]);
		if (i < 5)
			printf(":");
	}
	printf("\n");

	close(sock);

	exit(EXIT_SUCCESS);
	return 0;
}