Use output_file to print to this file instead to stdout

src/main.ml

@@ -133,7 +133,14 @@ let _ =
           else (
             if !nopopt
               then (fun _ -> ())
-              else Format.printf "%a" Ast_to_c.ast_to_c)
+              else
+              (
+                  let oc = open_out !output_file in
+                  let fmt = Format.make_formatter
+                        (Stdlib.output_substring oc)
+                        (fun () -> Stdlib.flush oc) in
+                  Format.fprintf fmt "%a" Ast_to_c.ast_to_c);
+              )
         end
     end