diff --git a/sys/sys/stddef.h b/sys/sys/stddef.h
--- a/sys/sys/stddef.h
+++ b/sys/sys/stddef.h
@@ -46,6 +46,8 @@
 #define	_PTRDIFF_T_DECLARED
 #endif
 
+#ifndef offsetof
 #define	offsetof(type, field)	__offsetof(type, field)
+#endif
 
 #endif /* !_SYS_STDDEF_H_ */