`value` argument of `gtk_level_bar_get_offset_value()` isn't an array of `gdouble`. It just a return location of `gdouble`. So you don't need to allocate an array of `gdouble`.

```c
gdouble value;
gboolean found;
found = gtk_level_bar_get_offset_value(_SELF(self), RVAL2CSTR_ACCEPT_NIL(name), &value);
if (found) {
    return DBL2RVAL(value);
} else {
    return Qnil;
}
```

We should use `RVAL2CSTR_ACCEPT_NIL()` for `name` because `name` is marked as `allow-none`.

Ref.: https://developer.gnome.org/gtk3/stable/GtkLevelBar.html#gtk-level-bar-get-offset-value (same link)