[vlc-devel] [PATCH 1/3] vlc_list: helpers for doubly linked lists

[Romain Vimont]

On Wed, Jun 13, 2018 at 09:55:08AM +0300, Rémi Denis-Courmont wrote:
> Err, it does not help. If I forego removal safety, I can remove it.next. I still need it.head for macro expansion safety (evaluate the head exactly once)

In think that head expansion-safety is not that important (in practice,
the head is always passed "directly").

> and it.current to avoid undefined pointer arithmetic on empty list.

In the "unsafe-against-removal" version, I think it is possible to avoid
UB (even if it's weird to return a void* to random memory). For example,
the current unsafe version in the kernel does not trigger UB IMO.

> 
> So I still need the iterator.
> 
> Le 12 juin 2018 23:06:05 GMT+03:00, Romain Vimont <rom1v at videolabs.io> a écrit :
> >On Tue, Jun 12, 2018 at 10:41:50PM +0300, Rémi Denis-Courmont wrote:
> >> Le tiistaina 12. kesäkuuta 2018, 14.35.02 EEST Romain Vimont a écrit
> >:
> >> > Anyway, my remark about the safe/unsafe version was not about
> >> > performance, but more about passing an additional argument that
> >could be
> >> > avoided in most cases (a foreach with 5 arguments is quite complex
> >to
> >> > use).
> >> 
> >> Third and fifth arguments define what is actually iterated, and seem 
> >> intrinsically necessary. The first argument defines a variable name
> >and is 
> >> necessary to avoid shadowing.
> >
> >Yes.
> >
> >> I cannot find a way to avoid the second (iterator) argument without
> >both 
> >> relying on UB and losing safety against element removal.
> >
> >I agree. But I think that it is very common to iterate over a list
> >without removing, so providing an additional macro with one parameter
> >less (and one variable declaration less "struct vlc_list_it it;" for
> >each caller) could be worth it.
> >
> >> The forth argument can be avoided with typeof, but I cannot see a
> >well-defined 
> >> and standard-compliant alternative. Specifically
> >>     (((char *)(&pos->member)) - ((char *)pos))
> >> is constant but undefined if pos is not initialized or not a valid
> >pointer. As 
> >> far as I recall, ubsan is prone to tripping on it.
> >
> >I cannot see a well-defined+standard-compliant solution either.
> >
> >However, it seems that every modern compiler supports typeof() or
> >__typeof() or __typeof__():
> >
> ><https://stackoverflow.com/a/14878370/1987178>
> >
> >Maybe we could #define some vlc_typeof() to use the right name?
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