diff --git a/reference/dio/functions/dio-open.xml b/reference/dio/functions/dio-open.xml
index 4d4337b297..fc5d5c37a3 100644
--- a/reference/dio/functions/dio-open.xml
+++ b/reference/dio/functions/dio-open.xml
@@ -70,6 +70,13 @@
           <constant>O_NONBLOCK</constant> - sets non blocking mode.
          </para>
         </listitem>
+        <listitem>
+         <para>
+          <constant>O_NOCTTY</constant> - prevent the OS from
+          assigning the opened file as the process's controlling
+          terminal when opening a TTY device file.
+         </para>
+        </listitem>
        </itemizedlist>
       </para>
      </listitem>