diff --git a/write_json.cpp b/write_json.cpp
index 8bb2684..c832804 100644
--- a/write_json.cpp
+++ b/write_json.cpp
@@ -1,3 +1,8 @@
+// for vasprintf() on Linux
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>