diff --git a/gtests/net/packetdrill/platforms.h b/gtests/net/packetdrill/platforms.h
index cd988aacb098c0ffd82b27143ed86f7d6ca67fbb..ebdeba38c2d8af40fd340c5a1468428b7026210c 100644
--- a/gtests/net/packetdrill/platforms.h
+++ b/gtests/net/packetdrill/platforms.h
@@ -136,6 +136,7 @@
 //#include <netinet/sctp.h>
 #define USE_LIBPCAP             1
 #define HAVE_TCP_INFO           1
+/* open_memstream() and fmemopen() are available in Solaris 11.4 and higher. */
 #define HAVE_FMEMOPEN           1
 #define HAVE_OPEN_MEMSTREAM     1