How to get MAC address of your machine using a C program?

Much nicer than all this socket or shell madness is simply using sysfs for this:

the file /sys/class/net/eth0/address carries your mac adress as simple string you can read with fopen()/fscanf()/fclose(). Nothing easier than that.

And if you want to support other network interfaces than eth0 (and you probably want), then simply use opendir()/readdir()/closedir() on /sys/class/net/.

You need to iterate over all the available interfaces on your machine, and use ioctl with SIOCGIFHWADDR flag to get the mac address. The mac address will be obtained as a 6-octet binary array. You also want to skip the loopback interface.

#include <sys/ioctl.h>
#include <net/if.h> 
#include <unistd.h>
#include <netinet/in.h>
#include <string.h>

int main()
{
    struct ifreq ifr;
    struct ifconf ifc;
    char buf[1024];
    int success = 0;

    int sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_IP);
    if (sock == -1) { /* handle error*/ };

    ifc.ifc_len = sizeof(buf);
    ifc.ifc_buf = buf;
    if (ioctl(sock, SIOCGIFCONF, &ifc) == -1) { /* handle error */ }

    struct ifreq* it = ifc.ifc_req;
    const struct ifreq* const end = it + (ifc.ifc_len / sizeof(struct ifreq));

    for (; it != end; ++it) {
        strcpy(ifr.ifr_name, it->ifr_name);
        if (ioctl(sock, SIOCGIFFLAGS, &ifr) == 0) {
            if (! (ifr.ifr_flags & IFF_LOOPBACK)) { // don't count loopback
                if (ioctl(sock, SIOCGIFHWADDR, &ifr) == 0) {
                    success = 1;
                    break;
                }
            }
        }
        else { /* handle error */ }
    }

    unsigned char mac_address[6];

    if (success) memcpy(mac_address, ifr.ifr_hwaddr.sa_data, 6);
}

You want to take a look at the getifaddrs(3) manual page. There is an example in C in the manpage itself that you can use. You want to get the address with the type AF_LINK.