diff --git a/graphics/twm4nx/apps/cclock.cxx b/graphics/twm4nx/apps/cclock.cxx
index acbd71645..2cb451897 100644
--- a/graphics/twm4nx/apps/cclock.cxx
+++ b/graphics/twm4nx/apps/cclock.cxx
@@ -44,6 +44,7 @@
 #include <cstring>
 #include <csched>
 #include <cassert>
+#include <cunistd>
 
 #include <semaphore.h>
 #include <debug.h>