diff --git a/sys/sys/ioccom.h b/sys/sys/ioccom.h
--- a/sys/sys/ioccom.h
+++ b/sys/sys/ioccom.h
@@ -79,6 +79,9 @@
 #define	IOCPARM_IVAL(x)	((int)(intptr_t)(void *)*(caddr_t *)(void *)(x))
 #endif
 
+#define	_IOC_INVALID	(_IOC_VOID|_IOC_INOUT)	/* Never valid cmd value,
+						   use as filler */
+
 #else
 
 #include <sys/cdefs.h>