Thanks to both of you for some valuable insight! I didn't realize
de-referencing on the c-side would be that simple, nor that
structs-by-value would actually be pointers anyway.
However, my problem is still not entirely solved. I am trying to interface
to the physics engines Box2D and Chipmunk, which are quite big. They both
use struct-by-value extensively. Small structs like
struct b2Vec2 {float x,y};
are passed around virtually everywhere.
When I use `chicken-bind` on my stripped-down version of Box2D.h, it
generates foreign-lambda's that return struct-by-value without complaint.
When I compile, it fails like the docs say:
Error: illegal foreign return type `b2Vec2'
Since struct-by-value is so central in both of these libraries, manually
writing wrapper-code would take too long. I am unsure of where to go from
here, but I suppose my options are:
- Modify `foreign` to support struct-by-value
- Modify `chicken-bind` to automatically write wrapper-code for
struct-by-value
- Write a script that re-parses the generated scheme-binding and inserts
the wrappers
Any thoughts on how to pursue this?
Thanks!
K.
On Wed, Feb 29, 2012 at 3:26 PM, Alaric Snell-Pym
<ala...@snell-pym.org.uk>wrote:
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> On 02/29/2012 01:29 PM, Thomas Chust wrote:
>
> > Last but not least, passing structures as pointers makes memory
> > management for them explicit and that is a good thing in this case
> > because there is simply no way the FFI could automatically figure out
> > how the optimal memory management strategy for structures passed by
> > value should look like. For example, whether such a structure can be
> > allocated as an atomic block of memory or not is impossible to answer
> > without knowing what the functions producing and consuming instances of
> > the structure actually do.
>
> I slightly disagree here. A structure passed or returned by value is
> just shoved on the stack, maybe in registers if it's a small structure.
> There's no extra memory management required; merely some extra copying
> to copy from the Chicken-side foreign-type pointer into the appropriate
> bit of stack or registers, or back again for returned values.
>
> But, indeed, it's easy to do that in C as your distim demonstration
> shows; return values are only slightly harder as you'll need to allocate
> a struct and copy the result in, then return that pointer.
>
> ABS
>
> - --
> Alaric Snell-Pym
> http://www.snell-pym.org.uk/alaric/
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