I spent some time hacking around on this from a library perspective when I had to interoperate with a bunch of Objective C on a 64-bit mac as many of the core library functions you need to FFI out to pass around pairs of Int32s as a struct small enough by the x64 ABI to get shoehorned into one register, and as I was programmatically cloning Objective C APIs via template haskell I couldn't use the usual clunky C shims.

What I was doing was just using libffi with a lot of work to cache the results of ffi_prep_cif for each signature.

It worked reasonably well for my purposes, but my need for it vanished and I abandoned the code in the middle of refactoring it for grander things.

So if nothing else, you can at least take this as a vote of confidence that your idea isn't crazy. =)

I'd also be happy to answer questions if you get stuck or need help.

-Edward


On Fri, Mar 14, 2014 at 7:50 AM, Yuras Shumovich <shumovichy@gmail.com> wrote:

Hi,

Right now ghc's FFI doesn't support c/c++ structures.

Whenever we have foreign function that accepts or returns struct by
value, we have to create wrapper that accepts or returns pointer to
struct. It is inconvenient, but actually not a big deal.

But there is no easy workaround when you want to export haskell function
to use it with c/c++ API that requires structures to be passed by value
(Usually it is a callback in c/c++ API. You can't change it's signature,
and if it doesn't provide some kind of "void* userdata", then you are
stuck.)

I'm interested in fixing that. I'm going to start with 'foreign import
"wrapper" ...' stuff.

Calling conventions for passing c/c++ structures by value are pretty
tricky and platform/compiler specific. So initially I'll use libffi for
that (it will work when USE_LIBFFI_FOR_ADJUSTORS is defined, see
rts/Adjustor.c). It will allow me to explore design space without
bothering about low level implementation details. Later it could be
implemented for native (non-libffi) adjustors.

Is anybody interested it that? I appreciate any comments/ideas.

Right now I don't have clear design. It would be nice to support plain
haskell data types that are 1) not recursive, 2) has one constructor and
3) contains only c/c++ types. But it doesn't work with c/c++ unions. Any
ideas are welcome.

An example how to use libffi with structures:
http://www.atmark-techno.com/~yashi/libffi.html#Structures

Thanks,
Yuras


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