diff --git a/sys/sys/stddef.h b/sys/sys/stddef.h
index 8e7f206d619..15817444aef 100644
--- a/sys/sys/stddef.h
+++ b/sys/sys/stddef.h
@@ -33,7 +33,10 @@
 #include <sys/_null.h>
 #include <machine/_types.h>
 
+#ifndef _PTRDIFF_T_DECLARED
 typedef	__ptrdiff_t	ptrdiff_t;
+#define	_PTRDIFF_T_DECLARED
+#endif
 
 #define	offsetof(type, field)	__offsetof(type, field)