> I do not understand what you're asking.  the cast?
> 
>  undo_option(struct hashtab *ht, struct nvlist **npp,
> +    struct nvlist ***next, const char *name, const char *type)
> 
> +               if (next != NULL && *next == (struct nvlist **) *npp)
> +                       *next = npp;

Yes.  It doesn't work if nv_next is not a first member of the struct.
&(*npp)->nv_next will be fine and makes code easier to read.

enami.