[Cffi-devel] Fw: Possible mem-aref bug or maybe my error on accessing member of a pointer to a struct

[Joeish W]

On Saturday, April 26, 2014 5:39 AM, Joeish W <joeish80829 at yahoo.com> wrote:
 
Thank you very much, I should have went back to my C ways of doing things, but it was so odd to see a struct in the OpenCv C++ interface I must have got a little cocky.  Again I really appreciate all your attention to these 3 matters today
>
>
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>On Saturday, April 26, 2014 4:32 AM, Willem Rein Oudshoorn <woudshoo at xs4all.nl> wrote:
> 
>Joeish W <joeish80829 at yahoo.com> writes:
>>
>>> I have this C wrapper 
>>>
>>>
>>> DMatch* cv_create_DMatch3(int _queryIdx, int _trainIdx, float _distance) {
>>>     return new DMatch(_queryIdx, _trainIdx, _distance);
>>> }
>>>
>>>
>>> it is a wrapper for these struct members
>>>
>>> http://docs.opencv.org/modules/features2d/doc/common_interfaces_of_descriptor_matchers.html?highlight=dmatch#DMatch
>>>
>>>
>>> here is my lisp wrapper for it:
>>>
>>> (defctype dmatch :pointer)
>>>
>>> (defcfun ("cv_create_DMatch3" dmatch3) (:pointer dmatch) 
>>>   "DMatch
 constructor"
>>>   (-query-idx :int)
>>>   (-train-idx :int)
>>>   (-distance :float))
>>>
>>> In C I can do this:
>>>
>>>   DMatch* a = cv_create_DMatch3(1, 2, 3.0);
>>>   cout << a->distance ;
>>>
>>> and the output is 3
>>>
>>> but in lisp when I do this:
>>>
>>> (defparameter a (dmatch3 1 2 3f0))
>>>
>>> LCV> (mem-aref a :int 0) 
>>> 1
>>> LCV> (mem-aref a :int 1) 
>>> 2
>>> LCV> (mem-aref a :float 2) 
>>> #<SINGLE-FLOAT quiet NaN> <--- this is output
>>>
>>>
>>> Any help on figuring out the reason would be much appreciated
>>
>>
>>This is not really suprising.  You are treating the resulting pointer
>>`a` as an untyped pointer.  
>>When you do `(mem-aref a :float 2)` you are basically saying:
>>
>>1. Assume a is a pointer to an array of floats
>>2. Give me the third element (element at
 index 2).
>>
>>Unfortunately `a` is not a pointer to an array of floats, but a pointer
>>to a struct DMatch.
>>
>>You should lookup `defcstruct` in the CFFI documentation:
>>
>>http://common-lisp.net/project/cffi/manual/html_node/defcstruct.html#defcstruct
>>
>>Wim Oudshoorn.
>>
>>
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>>
>>
>>
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