diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
index 84411e8e4b1..19b7d8fe427 100644
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -218,6 +218,7 @@
 #define	__aligned(x)	__attribute__((__aligned__(x)))
 #define	__section(x)	__attribute__((__section__(x)))
 #endif
+#define	__writeonly	__unused
 #if __GNUC_PREREQ__(4, 3) || __has_attribute(__alloc_size__)
 #define	__alloc_size(x)	__attribute__((__alloc_size__(x)))
 #define	__alloc_size2(n, x)	__attribute__((__alloc_size__(n, x)))