add mrb_ary_modify(mrb,a); you have to ensure mrb_value a to be an array; ref #1554

include/mruby/array.h

@@ -35,6 +35,7 @@ struct RArray {
 #define RARRAY_PTR(a) (RARRAY(a)->ptr)
 #define MRB_ARY_SHARED      256
 
+void mrb_ary_modify(mrb_state*, mrb_value);
 void mrb_ary_decref(mrb_state*, mrb_shared_array*);
 mrb_value mrb_ary_new_capa(mrb_state*, mrb_int);
 mrb_value mrb_ary_new(mrb_state *mrb);

src/array.c

@@ -138,6 +138,16 @@ ary_modify(mrb_state *mrb, struct RArray *a)
   }
 }
 
+void
+mrb_ary_modify(mrb_state *mrb, mrb_value ary)
+{
+  struct RArray *a;
+
+  a = mrb_ary_ptr(ary);
+  mrb_write_barrier(mrb, (struct RBasic*)a);
+  ary_modify(mrb, a);
+}
+
 static void
 ary_make_shared(mrb_state *mrb, struct RArray *a)
 {