diff --git a/lib/isc/include/isc/log.h b/lib/isc/include/isc/log.h
index 5a9a7a1bb3..e2bec5355d 100644
--- a/lib/isc/include/isc/log.h
+++ b/lib/isc/include/isc/log.h
@@ -19,6 +19,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <syslog.h> /* XXXDCL NT */
+#include <unistd.h>
 
 #include <isc/formatcheck.h>
 #include <isc/lang.h>