diff --git a/fel.c b/fel.c
index f96cc78..98e8d89 100644
--- a/fel.c
+++ b/fel.c
@@ -16,6 +16,8 @@
  */
 
 /* Needs _BSD_SOURCE for htole and letoh  */
+/* glibc 2.20+ also requires _DEFAULT_SOURCE */
+#define _DEFAULT_SOURCE
 #define _BSD_SOURCE
 #define _NETBSD_SOURCE
 
diff --git a/phoenix_info.c b/phoenix_info.c
index b76c5e4..bf84cf0 100644
--- a/phoenix_info.c
+++ b/phoenix_info.c
@@ -16,6 +16,8 @@
  */
 
 /* Needs _BSD_SOURCE for htole and letoh  */
+/* glibc 2.20+ also requires _DEFAULT_SOURCE */
+#define _DEFAULT_SOURCE
 #define _BSD_SOURCE
 
 #include <stdio.h>
diff --git a/pio.c b/pio.c
index 7ef4514..fc90190 100644
--- a/pio.c
+++ b/pio.c
@@ -18,6 +18,8 @@
  */
 
 /* needs _BSD_SOURCE for htole and letoh  */
+/* glibc 2.20+ also requires _DEFAULT_SOURCE */
+#define _DEFAULT_SOURCE
 #define _BSD_SOURCE
 
 #include <errno.h>