diff --git a/man2/fcntl.2 b/man2/fcntl.2
index d096e06ef..c9f31ec34 100644
--- a/man2/fcntl.2
+++ b/man2/fcntl.2
@@ -1151,6 +1151,11 @@ are specified in POSIX.1-2001.
 
 .B F_DUPFD_CLOEXEC
 is specified in POSIX.1-2008.
+(To get this definition, define
+.B _POSIX_C_SOURCE
+with the value 200809L or greater, or
+.B _XOPEN_SOURCE
+with the value 700 or greater.)
 
 .BR F_GETOWN_EX ,
 .BR F_SETOWN_EX ,