Generating a common interface for multiple platform specific binding

[Maurizio Cimadamore]

Hi Martin,
at the time of writing, jextract does not have a solution for this.

I also believe that a general solution might not even exist - sometimes 
the bindings can vary quite wildly across different platforms, and there 
could be significant layout mismatches which would be hard to reconcile. 
For instance on Windows, C_LONG is a layout with type ValueLayout.OfInt, 
whereas on Linux x64 the type is ValueLayout.OfLong. On top of that, 
some subset of native functions might only be available on some 
platforms but not in others, etc.

Considering all this, while some automated solution might be possible 
for the use case you have in mind, most surely it would fail to scale to 
the general case - so the general recommendation is to generate 
different bindings (one per platform) and then come up with some 
abstraction layer on top (which is sort of what you are trying to do). 
Another approach that might work would be to work at a lower level - and 
generate ad-hoc downcall method handles which automagically fix up 
mismatches that can arise across platforms (e.g. if a function is 
available in two platforms with a parameter mismatch int vs. long, adapt 
the int-accepting method handle to take a long, so that a uniform 
signature can be used). Note that jextract should expose the method 
handles it generates, so perhaps this adaptation can be done 
semi-automatically based on what comes out of jextract (and based on 
what the mismatches really are in your use case).

I know that e.g. the JavaDoesUSB [1] project has faced similar issues, 
so perhaps their authors might want to share some insights here.

Cheers
Maurizio

[1] - https://github.com/manuelbl/JavaDoesUSB

On 20/12/2022 15:04, Martin Pernollet wrote:
> Hi everyone,
>
> I am back on track with OpenGL binding with Panama!
>
> I have one code design / tooling question related to JExtract : is it 
> possible to *generate a common interface that would be implemented by 
> all platform specific binding* generated by JExtract since JDK 19 or 20?
>
> Here's my use case in more detail : I've been advised to generate 
> *different binding for different OS* (and maybe version). For OpenGL, 
> this lead me to a glut_h binding for macOS, one for Windows and one 
> for Linux.
>
> To let *the user/developer face a single entry point*, I manually 
> write a GL interface 
> <https://github.com/jzy3d/panama-gl/blob/feature/fbo/src/main/java/opengl/GL.java>. 
> I then define a GL_macOS_10_15_3 
> <https://github.com/jzy3d/panama-gl/blob/feature/fbo/src/main/java/opengl/macos/GL_macOS_10_15_3.java> 
> class that wraps the binding (!). When I expand the prototype to 
> Windows, I should copy paste this to GL_Windows_10 and modify the 
> imports to reference the appropriate bindings. The goal is to write 
> code like this
>
> GL gl = Platform.selectAmong(GL_macOS_10_15_3.class, 
> GL_windows_10.class, ...)
> gl.glDoSomething()
>
> I don't think there would be another way to have java developer write 
> *applications that ignore the target hardware*. However my approach is 
> stupid : time consuming and error prone because manual. A real life 
> case may have 10 implementations and 1000 functions.
>
> *Does JExtract provide a solution to this*? Should I create this tool 
> by myself based on all generated bindings ? Would anyone *recommend 
> something smarter*?
>
> Regards,
>
> Martin
>
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