diff --git a/pam/fingerprint-strings.h b/pam/fingerprint-strings.h
index f9fd993..7528f37 100644
--- a/pam/fingerprint-strings.h
+++ b/pam/fingerprint-strings.h
@@ -20,6 +20,11 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#ifndef _GNU_SOURCE
+#error _GNU_SOURCE must be defined
+#endif
+#include <stdio.h>
+
 struct {
 	const char *dbus_name;
 	const char *place_str_generic;
diff --git a/pam/pam_fprintd.c b/pam/pam_fprintd.c
index 209d44d..ec40da7 100644
--- a/pam/pam_fprintd.c
+++ b/pam/pam_fprintd.c
@@ -20,6 +20,7 @@
 
 #include <config.h>
 
+#define _GNU_SOURCE
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
diff --git a/utils/enroll.c b/utils/enroll.c
index 366f04a..2149741 100644
--- a/utils/enroll.c
+++ b/utils/enroll.c
@@ -17,6 +17,7 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#define _GNU_SOURCE
 #include <stdlib.h>
 #include <string.h>
 #include <locale.h>