ary_make_shared(mrb_state *mrb, struct RArray *a)
{
  if (!ARY_SHARED_P(a) && !ARY_EMBED_P(a)) {
    mrb_shared_array *shared = (mrb_shared_array *)mrb_malloc(mrb, sizeof(mrb_shared_array));
    mrb_value *ptr = a->as.heap.ptr;
    mrb_int len = a->as.heap.len;

    shared->refcnt = 1;
    if (a->as.heap.aux.capa > len) {
      a->as.heap.ptr = shared->ptr = (mrb_value *)mrb_realloc(mrb, ptr, sizeof(mrb_value)*len+1);
    }
    else {
      shared->ptr = ptr;
    }
    shared->len = len;
    a->as.heap.aux.shared = shared;
    ARY_SET_SHARED_FLAG(a);
  }
}