diff --git a/libc/include/sys/cdefs.h b/libc/include/sys/cdefs.h
index 1976d6a..1288b28 100644
--- a/libc/include/sys/cdefs.h
+++ b/libc/include/sys/cdefs.h
@@ -517,6 +517,12 @@
 
 #if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0 && defined(__OPTIMIZE__) && __OPTIMIZE__ > 0 && !defined(__clang__)
 #define __BIONIC_FORTIFY 1
+#if _FORTIFY_SOURCE == 2
+#define __bos(s) __builtin_object_size((s), 1);
+#else
+#define __bos(s) __builtin_object_size((s), 0);
+#endif
+
 #define __BIONIC_FORTIFY_INLINE \
     extern inline \
     __attribute__ ((always_inline)) \