diff --git a/cmdline/portable.h b/cmdline/portable.h
index 6bb2a385..73ed0923 100644
--- a/cmdline/portable.h
+++ b/cmdline/portable.h
@@ -22,6 +22,8 @@
 #include "config.h" /* Use " to include first in the same directory of this file */
 #endif
 
+#define _GNU_SOURCE /* for fallocate and futimens */
+
 /***************************************************************************/
 /* Config */