diff --git a/sys/netinet/tcp.h b/sys/netinet/tcp.h
index 24ec77cfd220..e189600fe4b6 100644
--- a/sys/netinet/tcp.h
+++ b/sys/netinet/tcp.h
@@ -37,6 +37,8 @@
 #ifndef _NETINET_TCP_H_
 #define _NETINET_TCP_H_
 
+#include <sys/cdefs.h>
+
 #if __BSD_VISIBLE
 
 typedef	u_int32_t tcp_seq;