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
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
_______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs