diff --git a/man2/linkat.2 b/man2/linkat.2
index ed8a9b931..f30fa57b6 100644
--- a/man2/linkat.2
+++ b/man2/linkat.2
@@ -120,6 +120,10 @@ using file descriptors received via a UNIX domain socket
 .BR SCM_RIGHTS
 in
 .BR unix (7)).
+This flag is Linux-specific; define
+.B _GNU_SOURCE
+.\" Before glibc 2.16, defining _ATFILE_SOURCE sufficed
+to obtain its definition.
 .TP
 .BR AT_SYMLINK_FOLLOW " (since Linux 2.6.18)"
 By default,