diff --git a/lib/ChangeLog b/lib/ChangeLog
index 3a1d1f6..2d8e5de 100644
--- a/lib/ChangeLog
+++ b/lib/ChangeLog
@@ -1,3 +1,8 @@
+2007-07-06 Andrew J. Schorr <ajschorr@alumni.princeton.edu>
+
+	* sockopt.{c,h}: (setsockopt_multicast_ipv4) Add some comments about
+	  the arguments.
+
 2007-06-01 Andrew J. Schorr <ajschorr@alumni.princeton.edu>
 
 	* zebra.h: On linux, we seem to need to include <linux/types.h> before
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
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);