Passing structures to callbacks by-value

[Ekaterina Vaartis]

Theres seems to be no way to pass structures to callbacks by value, even with libffi, although all other things support that with libffi.

[Luís Oliveira (luismbo)]

This is indeed a big oversight. Is this something you'd be interested to work on?

[Ekaterina Vaartis (vaartis)]

If you can give me some guidance on how to actually do this, i could do it. I have no experience with libffi though.

[Luís Oliveira (luismbo)]

So, the first step is to learn how to set up callbacks using libffi. I'm guessing their Closure API is the way to go. You could do this in C first and then translate that to CFFI, or start with CFFI, depending on how familiar you are with CFFI. You definitely need to be familiar with C, I think. :-)

[Ekaterina Vaartis (vaartis)]

I'll try to check it out and will write back later.

[Ekaterina Vaartis (vaartis)]

Although it'd be nice to also know how libffi integrates with CFFI, since the libffi system is pretty small and i don't see direct places that explain passing strcutures by-value.

[Luís Oliveira (luismbo)]

foreign-funcall-form/fsbv-with-libffi (
https://github.com/cffi/cffi/blob/master/libffi/funcall.lisp#L82) is the
main entry point.

[Alexey Egorov (alexey-e-egorov)]

It seems that `cffi:defcallback` doesn't currently use libffi for creation callbacks.
E.g. in case of sbcl, it calls `%defcallback`, that is defined in `cffi-sbcl.lisp` and calls SBCL's own `alien-sap`.

That means, if we want to add support for libffi callbacks, we need to define a new macro, like `cffi:defcallback-libffi` or something.

[Luís Oliveira (luismbo)]

Alexey, that shouldn't be necessary. Have a look at how DEFCFUN (and FOREIGN-FUNCALL, etc) decides to use libffi when needed without resorting to an auxiliar DEFCFUN-LIBFFI.

[Angel Peralta (acort3255)]

I have a PR up that should address this issue. Just awaiting review/feedback: https://github.com/cffi/cffi/pull/351