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

[Goswin von Brederlow <goswin-v-b@web.de>]

Adrien <camaradetux@gmail.com> writes:

> 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.

If you can't change the C function to use something other than varargs
and you can't limit the number of args and implement a case for each
number then the only option left is to create the varargs from an
array. That will be really ugly and architecture and compiler
dependent. So lots of #ifdef cases in your C code. Something you
really do not want to do.

> 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?

No. With >= 5 args you just need 2 functions, one for native and one
for bytecode. The docs have an example on how to do that. The problem
is calling the actual C function.

MfG
        Goswin