c2ffi handling va_list

[Madhu]

*  Attila Lendvai <attila at lendvai.name> <CAE4vfcKMTRrYu37wy-WCRgTDrZjdB_h6nD6tRtq26civ6CibtQ at mail.gmail.com>
Wrote on Thu, 16 Jul 2020 10:06:46 +0200

>> The TODO entry was about specifically va_list.  Now I'm not sure how
>> the "new" stdargs API can be manipulated through CFFI - AFAIK it is
>> purely a C compiler thing - you cannot foreign funcall to va_start and
>> va_copy.  The only way I could use it is through lisp implementations
>> that support a form of c-inline, or through "grovelled" code which
>> would be loaded through a shared library.
>
> IIRC libffi has support for dealing with varargs.
>
> there could be a new system called cffi/c2ffi+libffi, or somesuch,
> that expands to cffi+libffi definitions? or is the cffi + libffi
> integration transparent and loaded on-demand? looks like that.

yes when you call something that passes structures by value libffi
will translate structures by value if libffi is loaded or it'll show
you an error indicating you should load the cffi-libffi system.

[BTW *foreign-structures-by-value* is intially set to a lambda
function which throws a restartable error. It offers a restart - to
load the cffi-libffi system.  when that system is loaded value of the
variable will be overwritten.

But there is a bug: the restart won't proceed with the foreign funcall
The last line withing that lambda form should be

        (funcall *foreign-structures-by-value* args)

so that instead of returning to the caller it will proceed with the
intended funcall ]


> or straight out depend on libffi by cffi/c2ffi? i don't think that's a
> good idea, though, without some hard-pressing reasons.
>
> i don't feel the urge to get into actually hacking on this, but i'll
> be happy to answer questions about the cffi/c2ffi code.

[I do have some other questions] But as luis noted varargs already
works- eg. cffi/c2ffi translates functions of the form int foo(int a,
...) to a (defcfun foo :int ((a :int) &rest)) - which you can call as
(foo 42 :char #\c)

However the varargs machinery seems to be old - it predates the fsbv
machinery and so it doesn't work when the defcfun form has struct
arguments.  I expect to hit this corner case soon and I hope I'll be
the fix will be trivial - since foreign-funcall already works.

I'd think that will be sufficient without having to introduce libffi
machinery for varargs.  Right?

[I vaguely remember seeing a comment by the author about some part of
libffi which was not yet implemented - probably on the mailing list -
or a comment or document in the code code was reorganized and
removed. was it perhaps about varargs? ]



My initial post on this thread only dealt with functions of the form

	 int foo(int a, va_list arg)

which cffi/c2ffi was skipping without generating a defcfun because it
didn't have a type for va_list.  As I noted elsewhere these would be
hard to use anyway,