Re: Importing C function with variable argument list

[sbelmont700@gmail.com]

Hi,

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

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 prototypes to maintain.  The second is better, but hardware and implementation specific.  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 := 0;
   
begin

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

  end loop;

  Variable_C_Function;
  
  Machine_Code.Asm (Template => "sub %%esp, %0",
                    Inputs => Integer'Asm_Input("g", ((D'last + 2) * 4)),
                    Volatile => true);
 
 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 least-worst way of dealing a truly awful kludge.

-sb