diff --git a/winsup/utils/getconf.c b/winsup/utils/getconf.c
index e6b319e21..256bddb1a 100644
--- a/winsup/utils/getconf.c
+++ b/winsup/utils/getconf.c
@@ -27,6 +27,7 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
+#define _XOPEN_SOURCE
 #include <error.h>
 #include <errno.h>
 #include <limits.h>