struct sockaddr_in server;
// ...
server.sin_len = sizeof(server);
server.sin_family = AF_INET;
server.sin_addr.s_addr = INADDR_ANY;
server.sin_port = 0;
if (bind(sock, (struct sockaddr *) &server, sizeof(server)) <0) {

struct sockaddr_in6 server;
// ...
server.sin6_len = sizeof(server);
server.sin6_family = AF_INET6;
server.sin6_addr = in6addr_any;
server.sin6_port = 0;
if (bind(sock, (struct sockaddr *) &server, sizeof(server)) <0) {

int on = 1;
if (addr->sa_family == AF_INET6) {
	r = setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY, &on, sizeof(on));
	if (r)
		/* error */
}