diff --git a/pam_yubico.c b/pam_yubico.c
index 2c6c5d9..1dbad63 100644
--- a/pam_yubico.c
+++ b/pam_yubico.c
@@ -28,6 +28,8 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#define _GNU_SOURCE
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdarg.h>