Re: How can one record component be local and another not?

[Mark Lorenzen <mark.lorenzen@gmail.com>]

On Tuesday, May 5, 2020 at 7:17:42 PM UTC+2, hreba wrote:
> On 5/5/20 5:45 PM, Jeffrey R. Carter wrote:
> > On 5/5/20 1:04 PM, hreba wrote:
> >>
> >> (The reason for the above construction is the need to pass a pointer 
> >> to a C library function.)
> > 
> > Most likely you do not need to pass a pointer to your C function. For 
> > example, given the C function
> > 
> > void f (int* i);
> > 
> > You can do
> > 
> > procedure F (I : in out Interfaces.C.int) with Import, Convention => C, 
> > ...;
> > 
> > and the compiler will call the C function correctly.
> > 
> 
> It is more complicated, unfortunately. I got
> 
>     type gsl_odeiv2_system is record
>        c_function : access function
>             (arg1 : double;
>              arg2 : access double;
>              arg3 : access double;
>              arg4 : System.Address) return int;  -- 
> /usr/include/gsl/gsl_odeiv2.h:58
>        jacobian : access function
>             (arg1 : double;
>              arg2 : access double;
>              arg3 : access double;
>              arg4 : access double;
>              arg5 : System.Address) return int;  -- 
> /usr/include/gsl/gsl_odeiv2.h:60
>        dimension : aliased size_t;  -- /usr/include/gsl/gsl_odeiv2.h:61
>        params : System.Address;  -- /usr/include/gsl/gsl_odeiv2.h:62
>     end record;
>     pragma Convention (C_Pass_By_Copy, gsl_odeiv2_system);  -- 
> /usr/include/gsl/gsl_odeiv2.h:64
> 
> and I have to pass a variable
>     sys: access gsl_odeiv2_system
> to the initialization function of that C library, and the
> 
>    params: System.Address;
> 
> component in the record, which internally is then passed to c_function() 
> and jacobian(), posed the problem.
> 
> -- 
> Frank Hrebabetzky, Kronach	+49 / 9261 / 950 0565

You need to step back and try to get an overview of what you are trying to achieve - not blindly copy some low-level C construct. Figure out why those C functions take parameters of an access type. Are they in/out or out parameters? Then define the Ada equivalent of those function access types e.g:

type C_Function_Access is access
   function (Arg1 : double; Arg2 : in out double; Arg3 : in out double; Arg4 : in System.Address) return int with Convention => C;

Note that in Ada 2012 functions are unfortunately allowed to have in/out-mode and out-mode parameters. In previous revisions, functions were only allowed to have in-mode parameters.

As explained by Jeffrey R. Carter above, Convention => C guarantees that in/out-mode and out-mode parameters are passed by value.

Your record type then becomes something like:

type GSL_Odeiv2_System is record
   C_Function : C_Function_Access;
   ...
end record
  with Convention C_Pass_By_Copy;

You are trying to create an Ada interface to C, but you are dragging the low-level nature of C into your Ada. When you encounter access types and addresses in Ada, then step back and try to get an overview of what needs to be achieved - not how it is done in C and thus mechanically copied in the auto-generated Ada code.

Regards,
Mark L