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

[Adrien <camaradetux@gmail.com>]

On 28/10/2009, Basile STARYNKEVITCH <basile@starynkevitch.net> wrote:
> 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)

Well, actually the only reason was that stdarg.h needs at least one
non-variable argument (as opposed to varargs.h). ;-)
Anyway, that doesn't change anything.


>> 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

I'll probably go for that. The API won't be as nice as it could but at
least, it's going to be safe and checked at compile-time.

> 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.

That could make a nicer API. Maybe for ocaml-gir 3 or 4. :-)

> 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.

On linux and x86, I could probably use an array where I would move by
sizeof(some_type). I really want portability however (I want to use
these bindings at least on windows too).

> Time to go to bed. I am saying lot of non-sense.
>
> Bye!

Thanks. :-)


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Adrien Nader