diff --git a/glib/tests/strfuncs.c b/glib/tests/strfuncs.c
index 2ddf92693..648f37050 100644
--- a/glib/tests/strfuncs.c
+++ b/glib/tests/strfuncs.c
@@ -19,6 +19,7 @@
  * if advised of the possibility of such damage.
  */
 
+#define _XOPEN_SOURCE
 #include <ctype.h>
 #include <errno.h>
 #include <locale.h>