The mac address or the hardware address or the ethernet address of an interface is a 48 bit number that looks like this : 00:1c:c0:f8:79:ee

The mac address of an interface can be found given its name. The function to use is ioctl.

#include <stdio.h>	//printf
#include <string.h>	//strncpy
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <net/if.h>	//ifreq
#include <unistd.h>	//close

int main()
{
	int fd;
	struct ifreq ifr;
	char *iface = "eth0";
	unsigned char *mac;
	
	fd = socket(AF_INET, SOCK_DGRAM, 0);

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

	ioctl(fd, SIOCGIFHWADDR, &ifr);

	close(fd);
	
	mac = (unsigned char *)ifr.ifr_hwaddr.sa_data;
	
	//display mac address
	printf("Mac : %.2x:%.2x:%.2x:%.2x:%.2x:%.2x\n" , mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);

	return 0;
}

　　

Output

1 $ gcc interface_mac.c && ./a.out
2 Mac : 00:1c:c0:f8:79:ee