Re: [PATCH 02/10] string-list.h: add string_list_pop function.

From: Jeff King <peff@peff.net>
Date: Aug 9, 2018, 21:56 UTC
Message-ID: <20180809215637.GA12441@sigill.intra.peff.net>
In-Reply-To: <CAGZ79kZK=5gftetj3XLTbu-n3WKwRUQQDj12yxDBV0WEN8gg9Q@mail.gmail.com>

On Thu, Aug 09, 2018 at 02:52:29PM -0700, Stefan Beller wrote:

Show 16 quoted lines

> > In many cases you can just do:
> >
> >   while (list->nr) {
> >         work_on(list->items[list->nr - 1]);
> >         list_remove(list, list->nr - 1);
> >   }
> >
> > and then all of those memory ownership issues like:
> 
> [...]
> >
> > just go away. :)
> 
> The only complication here is the lack of list_remove(index),
> we do have list_remove(string), which internally searches the
> item and removes it. Hence I did not want to use it.

Heh, I almost dug into that more.

I think you could have helpers to spell the two lines above even more nicely:

  while (list->nr) {
        work_on(list_top(list));
	list_pop(list); /* note this doesn't return anything! */
  }

But yes, it's not possible with the current functions.

> Another idea I had was to keep the list immutable (except amending,
> just like a constitution ;-) and store an index of how far we got in that
> list already. That wastes memory for keeping entries around, but is safe
> for memory due to its nature.

You can also use a list.h linked-list. Then removal from the list and freeing are two separate operations (but it exercises your malloc a lot more if you're constantly pushing and popping).

Show 16 quoted lines

> > Where that falls down is if you really need work_on() to put more items
> > on the stack, but only after you've removed the current top. But then
> > writing it out may still be nicer, because it makes it clear you have to
> > do:
> >
> >   const char *cur_string = xstrdup(list->items[list->nr-1].string);
> 
> Another way would be to use
> 
>   string_list_pop(&list, &string_dst, &util_dst);
> i.e.
>   /* Returns 0 if the dst was filled */
>   int (struct string_list *, char **, void**)
> 
> as then we do not expose the internals and would not have issues
> with reallocs.

Yes, I almost suggested that, but there's the question of memory ownership of string_dst. Does it need freed or not? Is that answer dependent on the strdup_strings flag?

Show 7 quoted lines

> > if you want the data to live past the removal.
> 
> In the code proposed there are no additions (hence no reallocs)
> and the need for the data is short lived.
> 
> But I can see how the design was just fitting my purpose and
> we could come up with some better API.

Yeah, I didn't actually dig into your use case. I just want to make sure we don't add a crappy function to our API. ;)

-Peff