In C, one can set a socket's interface using setsockopt(socket, SOL_IP, IP_MULTICAST_IF, &mreqn, sizeof(mreqn)) where mreqn is

struct ip_mreqn { struct in_addr imr_multiaddr; /* IP multicast group address */ struct in_addr imr_address; /* IP address of local interface */ int imr_ifindex; /* interface index */ }; 

sockets2 offers the set_multicast_if_v4 function, but only accepts an address as parameter. I would like to set the interface using sockets2 based on an interface index.