Use weak functions in the RTEMS core?

[Gedare Bloom]

On Wed, Apr 8, 2020 at 10:53 PM Sebastian Huber
<sebastian.huber at embedded-brains.de> wrote:
>
> On 09/04/2020 03:32, Chris Johns wrote:
>
> > On 2020-04-09 03:43, Sebastian Huber wrote:
> >> On 08/04/2020 19:34, Jonathan Brandmeyer wrote:
> >>> Instead of treating weak references as a single level of
> >>> indirection, I think you have to treat them as a single overridable
> >>> interface.  In a dynamically-linked application, we might try to
> >>> perform an override using the LD_PRELOAD feature. But in a
> >>> statically-linked application we have to do it differently.  The
> >>> overriding archive must be named in full, and it must be named as an
> >>> object to be linked instead of a library to be searched.
> >>> Furthermore, an interface can have only one override that ends up in
> >>> the linked application.
> >> Yes, you can only have one strong implementation, otherwise you get
> >> multiple definition errors.
> >
> > I have only ever considered weak references as an overridable
> > interface within a user's application and not layered within RTEMS
> > itself. I am not sure about automatic indirection within an archive.
> Yes, this is one use case. You could use also a normal function in a
> separate file to get something similar.
> >
> > In the case of different heap allocators why not have confdefs.h
> > select the one we want and manage the indirection with a const table
> > of pointers?
>
> Yes it can be done with new configuration options, however, this would
> be my least favorite choice. I created an example how I would like to
> use weak functions:
>
> https://lists.rtems.org/pipermail/devel/2020-April/059196.html
>
Thanks for the code example it was helpful to me. What I understood is that:
1) An internal interface may be declared weakly aliased in RTEMS.
2) Implementations each need to go to a different file.
    * Not clear to me if there can be more than 2: 1 weak, 1 strong?
3) A strong implementation may be pulled in by invoking a different,
    non-aliased function implemented in the same file as (one of) the
    strong implementations.

Am I understanding this? I tried to map this to something in OOP but
the abstraction failed somewhere in my cortex. It is almost like an
interface class where you can instantiate objects of only one of the
implementations.

The weakly aliased interface proposal under consideration is NOT a
public API, at least so far. It is not yet clear to me how or whether
the approach can be used in conjunction with a weak public interface.

I am a little concerned that the approach is fragile still, based on
the discussion about how weak aliases may be resolved dependent upon
link order in command lines, etc.

Gedare

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