Re: [Caml-list] Wrapping var_args, or C ... in ocaml?

[Guillaume Yziquel <guillaume.yziquel@citycable.ch>]

Richard Jones a écrit :
> On Sun, Feb 14, 2010 at 04:46:20PM +0100, Guillaume Yziquel wrote:
>> Hello.
>>
>> I'm currently looking at:
>>
>> 	http://docs.python.org/c-api/arg.html
>>
>> and I would like to know how to wrap up C functions with va_list of with 
>> an ellipsis. Is this documented somewhere, or has someone already done 
>> something like this?
> 
> It really depends on the function and how it will be used.  It
> might translate to any of:
> 
> (1) A collection of functions implementing different aspects of the C
> function.  eg. The open(2) function in Unix is really a varargs
> function, and depending on whether you want to open a file for input,
> output, create, etc. you'd probably be better off with different
> functions in OCaml.  (Unix.openfile does _not_ do this ...)

Not the case.

> (2) A simple list, eg. for a C function that takes a NULL-terminated
> list of strings.

Could be.

> (3) A variant list of variants, or option labels, eg. for a C function
> that takes 'type, value'(s), such TIFFSetField in
> libtiff. (http://www.remotesensing.org/libtiff/man/TIFFSetField.3tiff.html)

No.

> (4) Something very specialized, eg. the 'printw' function in ncurses
> is like printf and so would need quite a tricky implementation in
> OCaml.  (Probably best to use ksprintf to convert to a string in OCaml
> and then pass printw ("%s", str) in C).

I do not think so.

> In libguestfs where we autogenerate bindings we avoided varargs
> altogether, because it's hard to map such a concept to all the
> different languages we support.

True.

But, I mean, from the point of view of the ABI, there's not much 
trickery in the concept. It looks that it is C that is not mapping the 
concept to its fullest potential.

I mean, it seems that varargs means on the receiving end "the number of 
arguments you'r giving me, as a function, is not limited", whereas on 
the sending end, you hard-code the number of arguments in your C code.

Is there a way to map an OCaml list to an ellipsis? Or is it a C limitation?

-- 
      Guillaume Yziquel
http://yziquel.homelinux.org/