diff --git a/libc/include/sys/cdefs.h b/libc/include/sys/cdefs.h
index 1ba9100..987a5e3 100644
--- a/libc/include/sys/cdefs.h
+++ b/libc/include/sys/cdefs.h
@@ -507,6 +507,7 @@
     __attribute__ ((always_inline)) \
     __attribute__ ((gnu_inline)) \
     __attribute__ ((artificial))
+#define __BIONIC_FORTIFY_UNKNOWN_SIZE ((size_t) -1)
 #endif
 
 #endif /* !_SYS_CDEFS_H_ */