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From: sbelmont700@gmail.com
Newsgroups: comp.lang.ada
Subject: Re: Importing C function with variable argument list
Date: Tue, 10 Apr 2012 13:25:03 -0700 (PDT)
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Date: 2012-04-10T13:25:03-07:00
List-Id: <comp.lang.ada>

Hi,

I often have occasion to suffer through using a C API with variable argumen=
t in Ada, and there really is no good choice.  I have found basically two o=
ptions:

1. Import the C function with a fixed-set of arguments, with a defaulted-to=
-zero parameter at the end.

or

2. Roll-your-own.

The first is cheap and easy, but clearly not extensible, as each version of=
 the function must be overloaded, giving you an infinite number of prototyp=
es to maintain.  The second is better, but hardware and implementation spec=
ific.  For instance, for x86, it might look something like this (where D is=
 an array of unsigned longs, or whatever the C function needs):

procedure V_A (D : Array_Type) is

   procedure Variable_C_Function;
   pragma Import (C, Variable_C_Function, "Something");
   Result : Interfaces.C.int :=3D 0;
  =20
begin

  Machine_Code.Asm (Template =3D> "push %0",
                    Inputs =3D> Interfaces.C.unsigned_long'Asm_Input("g", 0=
),
                    Volatile =3D> true);
                     =20
  for i in reverse D'range loop
 =20
    Machine_Code.Asm (Template =3D> "push %0",
                      Inputs =3D> Interfaces.C.unsigned_long'Asm_Input("g",=
 Data(i)),
                      Volatile =3D> true);

  end loop;

  Variable_C_Function;
 =20
  Machine_Code.Asm (Template =3D> "sub %%esp, %0",
                    Inputs =3D> Integer'Asm_Input("g", ((D'last + 2) * 4)),
                    Volatile =3D> true);
=20
 end V_A;

Of course, this is completly non-portable and would depend greatly on both =
the hardware, O/S, and how the C function is setup, but it's perhaps the le=
ast-worst way of dealing a truly awful kludge.

-sb