diff --git a/signal_unix.go b/signal_unix.go
index 2e6eed7..016d352 100644
--- a/signal_unix.go
+++ b/signal_unix.go
@@ -2,6 +2,8 @@
 // The use of this source code is goverened by a BSD-style
 // license that can be found in the LICENSE-file.
 
+// +build darwin freebsd linux netbsd openbsd
+
 package main
 
 import (