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

[Basile STARYNKEVITCH <basile@starynkevitch.net>]

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!


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Basile STARYNKEVITCH         http://starynkevitch.net/Basile/
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