c2ffi handling va_list

[Madhu]

*  Luís Oliveira <luismbo at gmail.com> <CAB-HnLQJE9Y0+kyq3-COjCk-GD3mzOOE+h2f+7upkdkCGu9+Ww at mail.gmail.com>
Wrote on Wed, 15 Jul 2020 14:50:27 +0100

> On Wed, 15 Jul 2020 at 06:16, Madhu <enometh at meer.net> wrote:
>> one of the cffi/TODO items has
>> -> Handle va_list. For now it's treated as any other argument.
>>
>> Maybe someone has thought deeply about how to do this?  Please let me
>> know if you have any suggestions.
>>
>> [It seems borodust/claw tries to handle this by converting va_list
>> args to variadic lisp arguments - but it still seems to blow up at a
>> cffi:parse-type.  Or maybe it is a defect with my Clang-7.0 stdarg
>> vs. what is included with gcc]
> The TODO item should probably be removed. DEFCFUN has syntax for
> varargs and that's implemented on top of FOREIGN-FUNCALL-VARARGS and
> FOREIGN-FUNCALL-VARARGS-POINTER. The implementation so far is simple,
> it only takes care of promoting floats to doubles, and chars/short to
> ints. The test suite tracks whether this works and it seems to work so
> far.

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.

> Not sure about what c2ffi needs to do, though.

In order to "treat va_list as any other argument" I tried adding an
opaque (cffi:defcstruct (:builtin-va-list :size 192)) -- 192 being
sizeof(va_list) with x86_64-pc-linux-gnu-gcc -- having
%json-type-to-cffi-type return that when it comes across a cffi tag
"__builtin_va_list". cffi/c2ffi can generate defcfns for functions
which use va_list in the signature.  I think the user would still be
hardpressed to find a way to use them.

Unless I'm missing something... ??