diff --git a/include/dirent.h b/include/dirent.h
--- a/include/dirent.h
+++ b/include/dirent.h
@@ -116,7 +116,8 @@
 struct dirent *
 	 readdir(DIR *);
 #if __POSIX_VISIBLE >= 199506 || __XSI_VISIBLE >= 500
-int	 readdir_r(DIR *, struct dirent *, struct dirent **);
+int	 readdir_r(DIR *, struct dirent *, struct dirent **)
+	__deprecated1("Does not take variable {NAME_MAX} into account");
 #endif
 void	 rewinddir(DIR *);
 #if __POSIX_VISIBLE >= 200809 || __XSI_VISIBLE >= 700
diff --git a/lib/libc/gen/readdir.c b/lib/libc/gen/readdir.c
--- a/lib/libc/gen/readdir.c
+++ b/lib/libc/gen/readdir.c
@@ -134,3 +134,5 @@
 }
 
 __strong_reference(__readdir_r, readdir_r);
+__warn_references(readdir_r,
+    "warning: this program uses readdir_r(), which is unsafe.");