I have sketched an application specific answer on SO.
The plugin has minimal support (a primitive) to retrieve the address of a
given symbol from a given library.
Normally, we should create an ExternalData with such address and variable
type asPointerType.
Then we can pass either the variable by reference (just pass the
ExternalData) or by value (I don't know who will dereference the
address...).
Of course, those ExternalAddress shall be cleared upon image resumption,
etc...
All this handling might deserve a specific subclass of ExternalObject, I
don't know...
(subclass of ExternalData so as to not have to modify the plugin?)

For HDF5, I'm going to assume that those hid_t never change after HDF5
library initialization, and that such initialisation happens at time of
dynamic linking/loading.

Le mer. 17 juin 2020 à 14:56, Marcel Taeumel <marcel.taeumel at hpi.de> a
écrit :

> Hi Nicolas.
>
> > So what is needed is to get the address of the variable (exactly like
> we lookup address of a function), then dereference it to get the contents...
>
> Since I am in the process of shrinking "Alien-Core" into "FFI-Callback"
> ... could this be implemented with a new interface to the FFI plugin? Maybe
> not. Unless you can provide an exported call that the plugin can intercept
> to copy the address for you? If it is not already exposed in another call.
> Then it would be easy.
>
> Best,
> Marcel
>
> Am 17.06.2020 14:11:14 schrieb Nicolas Cellier <
> nicolas.cellier.aka.nice at gmail.com>:
> Hi Marcel,
> I'm not so sure.
> It does not look like a constant, but rather a variable.
> The type of this variable is a hid_t, but the content can change at
> runtime (I believe once at HDF5 initialisation).
> So what is needed is to get the address of the variable (exactly like we
> lookup address of a function), then dereference it to get the contents...
>
> Le mer. 17 juin 2020 à 12:52, Marcel Taeumel <marcel.taeumel at hpi.de> a
> écrit :
>
>> Hi Nicolas,
>>
>> that's like to already be covered by ExternalPool. :-) I tried to write a
>> lot of documentation on how to use it in the class comment.
>>
>> Best,
>> Marcel
>>
>> Am 17.06.2020 12:50:24 schrieb Nicolas Cellier <
>> nicolas.cellier.aka.nice at gmail.com>:
>> Hi all, Marcel,
>> yet another basic requirement:
>>
>> https://stackoverflow.com/questions/62427197/how-one-refers-to-an-external-global-variable-in-squeak-ffi
>>
>> I did not even try to inquire the implementation...
>>
>>
>>
>
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