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[Caml-list] Binding C libraries which use variable arguments (stdarg.h)
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Date: -- (:)
From: Goswin von Brederlow <goswin-v-b@w...>
Subject: Re: [Caml-list] Binding C libraries which use variable arguments (stdarg.h)
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.

MfG
        Goswin