diff --git a/test/pdiff/pdiff.c b/test/pdiff/pdiff.c
index 4d50f0130..91e0cafe2 100644
--- a/test/pdiff/pdiff.c
+++ b/test/pdiff/pdiff.c
@@ -14,6 +14,8 @@
   if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 */
 
+#define _GNU_SOURCE
+
 #include "lpyramid.h"
 #include <math.h>
 #include <stdio.h>