C/C++ Linux MAC Address of all interfaces

You should stop using net-tools and the archaic ioctl interface, and start on using the modern Netlink/sysfs interfaces. You have no less than 5 possibilities:

• write your own Netlink-interfacing code
• your own NL code, in combination utilizing libmnl (-> see rtnl-link-dump in Examples
• or utilize autonomous libs like libnl3
• parse text output of ip -o link (-o is to get output meant for text parsing, unlike ifconfig)
• or use sysfs and just look at /sys/class/net/eth0/address