diff --git a/include/stddef.h b/include/stddef.h
index 694d503..f856119 100644
--- a/include/stddef.h
+++ b/include/stddef.h
@@ -8,6 +8,10 @@ typedef __PTRDIFF_TYPE__ ptrdiff_t;
 typedef __PTRDIFF_TYPE__ intptr_t;
 typedef __SIZE_TYPE__ uintptr_t;
 
+#if __STDC_VERSION__ >= 201112L
+typedef union { long long __ll; long double __ld; } max_align_t;
+#endif
+
 #ifndef __int8_t_defined
 #define __int8_t_defined
 typedef signed char int8_t;