diff --git a/opal/util/keyval/keyval_lex.l b/opal/util/keyval/keyval_lex.l
index e0cb6ec120d..4d3697cfe3f 100644
--- a/opal/util/keyval/keyval_lex.l
+++ b/opal/util/keyval/keyval_lex.l
@@ -23,6 +23,9 @@
  * $HEADER$
  */
 
+// needed for fileno()
+#define _XOPEN_SOURCE
+
 #include "opal_config.h"
 
 #include <stdio.h>