diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
index 6788462c2244..88f43e3a5c3f 100644
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -253,6 +253,17 @@
 #define	__LONG_LONG_SUPPORTED
 #endif
 
+/* C++11 exposes a load of C99 stuff */
+#if __cplusplus >= 201103L
+#	define	__LONG_LONG_SUPPORTED
+#	ifndef __STDC_LIMIT_MACROS
+#		define __STDC_LIMIT_MACROS
+#	endif
+#	ifndef __STDC_CONSTANT_MACROS
+#		define __STDC_CONSTANT_MACROS
+#	endif
+#endif
+
 /*
  * GCC 2.95 provides `__restrict' as an extension to C90 to support the
  * C99-specific `restrict' type qualifier.  We happen to use `__restrict' as