diff --git a/lib/libc/xdr/xdr.3 b/lib/libc/xdr/xdr.3
index 8998d06dc7f6..69f25449be67 100644
--- a/lib/libc/xdr/xdr.3
+++ b/lib/libc/xdr/xdr.3
@@ -840,3 +840,8 @@ Returns one if it succeeds, zero otherwise.
 .%O RFC1014
 .%Q "Sun Microsystems, Inc., USC\-ISI"
 .Re
+.Sh HISTORY
+The
+.Nm xdr_sizeof
+function first appeared in
+.Fx 9.0 .