diff --git a/stdalign.h b/stdalign.h
index d138500..00105a9 100644
--- a/stdalign.h
+++ b/stdalign.h
@@ -7,14 +7,17 @@
 #define __FSTD_HDRS_STDALIGN_H_MACROS 1
 
 #ifndef __cplusplus
+#  undef alignas
 #  define alignas _Alignas
+#  undef alignof
 #  define alignof _Alignof
 
+#  undef __alignof_is_defined
 #  define __alignof_is_defined 1
+#  undef __alignas_is_defined
+#  define __alignas_is_defined 1
 #endif
 
-#define __alignas_is_defined 1
-
 #endif
 
 #endif