Re: Accessing elements in foreign array of structs

Sorry, the FLI doesn't have the equivalent of the points[index].x syntax in C,
mixing slot and array access, so you have to make a pointer to each struct
somehow like in Julian's code.

The only small improvement I can offer is to use

(with-coerced-pointer (current) points ...)

instead of

(let ((current (copy-pointer points))) ...)

to make the pointer object have dynamic extent.

-- 
Martin Simmons
LispWorks Ltd
http://www.lispworks.com/



>>>>> On Mon, 13 Apr 2026 08:57:00 +0200, Erik Ronström (as erik dot ronstrom at doremir dot com) said:
> 
> Thanks, that will also work!
> 
> (Although I'm still puzzled; it feels like there *has to* be some way to 
> index directly into a struct member of an array!?)
> 
> Erik
> 
> 
> Den 2026-04-12 kl. 22:53, skrev Julian Baldwin (as julian at jbaldwin 
> dot net):
> > Sorry spam (original reply didn’t include the mailing list), but does 
> > something like this work?
> >
> > (use-package "FLI")
> >
> > (define-c-struct (point (:foreign-name "tagPOINT"))
> >   (x :long)
> >   (y :long))
> >
> > (with-dynamic-foreign-objects ((points point :nelems 3))
> >   (let ((current (copy-pointer points)))
> >     (dotimes (index 3)
> >       (setf (foreign-slot-value current 'x :object-type 'point) index
> >             (foreign-slot-value current 'y :object-type 'point) index)
> >       (format t "~S: ~S ~S,~S~%"
> >               index
> >               current
> >               (foreign-slot-value current 'x)
> >               (foreign-slot-value current 'y))
> >       (incf-pointer current))))
> >
> > The compiler can optimise slot access if you pass the :object-type, 
> > but it’s not strictly necessary.
> >
> > Julian.
> >
> >
> > ------ Original Message ------
> > From "Erik Ronström (as erik dot ronstrom at doremir dot com)" 
> > <[email protected]>
> > To "LispWorks" <[email protected]>
> > Date 13/4/2026 4:53:18 am
> > Subject Accessing elements in foreign array of structs
> >
> >> Hi,
> >> I am calling a foreign function that takes an array of points as input.
> >> (fli:define-c-struct (point (:foreign-name "tagPOINT"))
> >>   (x :long)
> >>   (y :long))
> >> (let ((points (fli:allocate-foreign-object :type 'point :nelems 3)))
> >> (the-foreign-function points))
> >> I can't figure out how to access the elements of the foreign array 
> >> directly.
> >> This works:
> >> (let ((p (fli:allocate-foreign-object :type 'point)))
> >> (setf (fli:foreign-slot-value p 'x) 100
> >> (fli:foreign-slot-value p 'y) 200
> >> (fli:dereference points :index 0) p)
> >> But how do I set the elements of points without using an intermediate 
> >> object?
> >> Erik

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