OpenEuphoria

Here's one way to do it without using ifconfig. It creates a UDP socket, multicasts a packet to itself, and returns the from address. No error checking.

constant  
    setsockopt_ = define_c_func(-1, "setsockopt", {C_INT, C_INT, C_INT, C_POINTER, C_INT}, C_INT) 
     
    sockets:set_option(sock, SOL_SOCKET, SO_REUSEADDR, 1) 
 
    c_func(setsockopt_, {sock[1], IPPROTO_IP, IP_ADD_MEMBERSHIP, mreq, 12})