On 17-12-21, 13:02, Bartosz Golaszewski wrote:

On Fri, Dec 17, 2021 at 10:54 AM Viresh Kumar [off-list ref] wrote:

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I don't know Rust that well to be able to come up with an idea off the
top of my head but I assume inheritance in one way or another exists
in rust? Can you have an EdgeEvent interface implemented by two
specialized structs, one that's stored in the buffer (stores a pointer
to the internal event)

This is what my initial patch had, right? It isn't safe. There is no way for
libgpiod-rust to guarantee that the users don't access the EdgeEvent after the
buffer is freed and this needs to be made safer at libgpiod itself.

I'm not sure it had the same thing. In libgpiod it's simple a matter
of definition. The docs state that a pointer retrieved using
gpiod_edge_event_buffer_get_event() must not be freed while one
obtained with gpiod_edge_event_copy() does have to be disposed of. In
C++ it's safe because either you hold a const reference to the object
stored in the event buffer or you have copied it and now it will
survive the parent meaning the user can access the event even after
the buffer was deleted.

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and one that stores the copy (and frees it when
it goes out of scope)?

I don't understand what we will get out of such a scheme? Is there any benefit
of getting these values via the pointer to the internal C event ?

You avoid allocating memory for those objects. The memory exists in
the C buffer. In order to keep a similar level of performance you'd
need to replicate the behavior of the buffer in rust as well.

I was aligned with all this, I just wanted to skip providing the interface where
we use the event without copying as that has Safety issues. I will try to
provide both the options now, lets see how it looks.

-- 
viresh