diff --git a/src/TEPty.C b/src/TEPty.C
index 774777b7..82b02862 100644
--- a/src/TEPty.C
+++ b/src/TEPty.C
@@ -59,8 +59,8 @@
 #include <config.h>
 #endif
 
-#if defined(HAVE_GRANTPT) && defined(HAVE_PTSNAME) && defined(HAVE_UNLOCKPT) && !defined(_GNU_SOURCE)
-#define _GNU_SOURCE // make stdlib.h offer the above fcts
+#if defined(HAVE_GRANTPT) && defined(HAVE_PTSNAME) && defined(HAVE_UNLOCKPT) && !defined(_XOPEN_SOURCE)
+#define _XOPEN_SOURCE // make stdlib.h offer the above fcts
 #endif
 
 #include <stdlib.h>