diff --git a/man2/sendmmsg.2 b/man2/sendmmsg.2
index 97c7ede2d..f9871396c 100644
--- a/man2/sendmmsg.2
+++ b/man2/sendmmsg.2
@@ -30,7 +30,7 @@
 sendmmsg \- send multiple messages on a socket
 .SH SYNOPSIS
 .nf
-.B "#define _GNU_SOURCE"
+.B "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
 .BI "#include <sys/socket.h>"
 
 .BI "int sendmmsg(int " sockfd ", struct mmsghdr *" msgvec \