diff --git a/lib/sockopt.c b/lib/sockopt.c
index 6ddbded..f5f1a7e 100644
--- a/lib/sockopt.c
+++ b/lib/sockopt.c
@@ -176,9 +176,10 @@
 int
 setsockopt_multicast_ipv4(int sock, 
 			int optname, 
-			struct in_addr if_addr,
+			struct in_addr if_addr /* required */,
 			unsigned int mcast_addr,
-			unsigned int ifindex)
+			unsigned int ifindex /* optional: if non-zero, may be
+						  used instead of if_addr */)
 {
 
 #ifdef HAVE_STRUCT_IP_MREQN_IMR_IFINDEX