[SciPy-Dev] qhull FFI

[Nil Goyette]

Hi Robert,

Thank you, this is an excellent answer. You only define what you need
because it becomes a part of SciPy code, like any other pyx file is. This
is not FFI at all.

Copying SciPy/qhull structures and types wasn't my best idea after all :D
I'll continue my work using only the qhull source then.

Nil Goyette

Le lun. 5 juill. 2021, à 10 h 19, Robert Kern <robert.kern at gmail.com> a
écrit :

> On Mon, Jul 5, 2021 at 10:12 AM Nil Goyette <nil.goyette at imeka.ca> wrote:
>
>> Hi all,
>>
>> I've been trying to use qhull in a Rust project so of course I wondered
>> how SciPy managed to do it before I tried to code my own FFI. When my
>> version didn't work, I looked deeper in SciPy and there's something I don't
>> understand. We can find several definitions In qhull_src/src/libqhull_r.h,
>> like facetT
>>
>> ctypedef struct facetT:
>>     coordT offset
>>     coordT *center
>>     coordT *normal
>>     facetT  *previous;
>>     facetT  *next;
>>     ... a total of 16 fields
>>
>> And the "corresponding" definitions in qhull.pyx
>>
>> struct facetT {
>>     coordT   offset;
>>     coordT  *normal;
>>     union { 6 fields }
>>     coordT  *center;
>>     facetT *next
>>     facetT *previoust
>>     ... A total of 37 fields
>>
>> Now I'm puzzled. There are several missing fields and they are wrongly
>> ordered! I'm not a FFI expert at all but, as a programmer, I would have
>> thought that the definitions needed to match perfectly, at least for the
>> order and the number of bytes (total and per field).
>>
>> Can someone please explain to me why only some fields match and why this
>> is not a problem? Thank you.
>>
>
> Cython is not an FFI per se. It is a language that is transpiled to C and
> is exposed to Python using Python's standard C extension module mechanism.
> When describing the `ctypedef struct`, it only needs to describe the struct
> members that you are going to refer to in the Cython code so that it can
> deal with their types properly. Because it transpiles source-to-source, it
> doesn't need to know the full details of the binary API. The C compiler is
> doing that job, and it sees the original `.h` file from QHull.
>
> --
> Robert Kern
> _______________________________________________
> SciPy-Dev mailing list
> SciPy-Dev at python.org
> https://mail.python.org/mailman/listinfo/scipy-dev
>


-- 
Nil Goyette
Développeur principal
www.imeka.ca

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