diff --git a/lib/sockopt.h b/lib/sockopt.h
index d88bfa6..65ba34f 100644
--- a/lib/sockopt.h
+++ b/lib/sockopt.h
@@ -79,9 +79,12 @@
                     ? SOPT_SIZE_CMSG_PKTINFO_IPV6())
 
 extern int setsockopt_multicast_ipv4(int sock, int optname, 
-			             struct in_addr if_addr,
+			             struct in_addr if_addr
+					 /* required: interface to join on */,
                                      unsigned int mcast_addr,
-			             unsigned int ifindex);
+			             unsigned int ifindex
+					 /* optional: if non-zero, may be used
+					 	instead of if_addr */);
 
 /* Ask for, and get, ifindex, by whatever method is supported. */
 extern int setsockopt_ifindex (int, int, int);