Re: [Caml-list] Binding C libraries which use variable arguments (stdarg.h)

[Adrien <camaradetux@gmail.com>]

On 29/10/2009, Goswin von Brederlow <goswin-v-b@web.de> wrote:
> Basile STARYNKEVITCH <basile@starynkevitch.net> writes:
>
>> Adrien wrote:
>>> Hi,
>>>
>>> I am currently trying to bind a C function that takes variables
>>> arguments, like foo(int a, ...). I can't find how to make a C stub for
>>> that function.
>>
>>  I am assuming that the a is the number of actual arguments, so you call
>> foo(3, x, y, z)
>> foo(5, t, t+1, t+3, 0, 4)
>> foo(0)
>>
>>>
>>> Any other idea? Hint^WPointer? (sorry for the bad joke ;-) )
>>
>> First, you could suppose that the a has a reasonable limit, say 100.
>>
>> Then you could generate the glue code for each value of the argument
>> a. I mean generate ocaml code like
>>
>> external f0: void -> uit = "f_0"
>> external f1: int -> unit = "f_1"
>> external f2: int -> int -> unit = "f_2"
>> external f3: int -> int -> int -> unit = "f_3"
>>
>> let f a = match Array.length a with
>>  0 -> f0 ()
>> | 1 -> f1 a.[0]
>> | 2 -> f2 a.[0] a.[1]
>> | 3 -> f3 a.[0] a.[1] a.[2]
>> ....
>> | _ -> failwith "too many components for f"
>>
>> and generate C code for each of f_0 f_1 ...
>>
>> and call f with an array ...
>>
>> The specialized code generator is reasonably written in Ocaml
>>
>> There are more crazy variants, including
>>
>> try Ocaml varargs like Pierre Weis did in printf.ml. For plain mortals
>> like me this is white magic.
>>
>> Assuming a Linux system, you could lazily generate the glue code and
>> invoke dynamic linker on it. So the general case would be to call the
>> code generator.
>>
>> Time to go to bed. I am saying lot of non-sense.
>>
>> Bye!
>
> Since ocaml functions with more than 5 args use an array you only do
> that in ocaml for a few arguments and then you need to do this in
> C. So do it in C for all. The stub takes an array and then switches on
> the lentgh to call the real function.
>
> Unfortunately you can not convert an array or list into a va_list. You
> need to specifically catch each length and call foo(4, a[0], a[1],
> a[2], a[3]) for each length.

Well, my problem is more when calling the C function since I can chose
the interface of the caml functions.

There's something I've not been sure and would like to ask however:
for the sake of consitency, I've been giving both a native and
bytecode function every time no matter the number of arguments. Could
that be a problem?

 ---

Adrien Nader