From: Mark Lorenzen <mark.lorenzen@gmail.com>
Subject: Re: How can one record component be local and another not?
Date: Tue, 5 May 2020 23:42:45 -0700 (PDT)
Date: 2020-05-05T23:42:45-07:00 [thread overview]
Message-ID: <728c5319-5f4f-4f21-9d71-cc9c12ed6016@googlegroups.com> (raw)
In-Reply-To: <hhdldkFin5dU1@mid.individual.net>
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
next prev parent reply other threads:[~2020-05-06 6:42 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-05 11:04 How can one record component be local and another not? hreba
2020-05-05 11:33 ` AdaMagica
2020-05-05 11:38 ` AdaMagica
2020-05-05 12:59 ` hreba
2020-05-05 13:19 ` J-P. Rosen
2020-05-05 13:37 ` Jere
2020-05-05 14:28 ` hreba
2020-05-05 15:18 ` AdaMagica
2020-05-05 14:32 ` hreba
2020-05-05 11:43 ` AdaMagica
2020-05-05 12:55 ` hreba
2020-05-05 11:46 ` Simon Wright
2020-05-05 13:07 ` hreba
2020-05-05 17:00 ` Dmitry A. Kazakov
2020-05-05 11:48 ` Niklas Holsti
2020-05-05 13:44 ` hreba
2020-05-05 15:45 ` Jeffrey R. Carter
2020-05-05 17:17 ` hreba
2020-05-05 19:08 ` Niklas Holsti
2020-05-06 19:31 ` hreba
2020-05-09 19:43 ` Niklas Holsti
2020-05-10 15:10 ` hreba
2020-05-05 19:19 ` Jere
2020-05-06 6:42 ` Mark Lorenzen [this message]
2020-05-06 8:26 ` Simon Wright
2020-05-06 8:33 ` Mark Lorenzen
2020-05-05 17:32 ` hreba
2020-05-05 19:04 ` Niklas Holsti
2020-05-05 20:11 ` Niklas Holsti
2020-05-06 13:13 ` hreba
2020-05-06 17:30 ` Niklas Holsti
2020-05-06 18:28 ` Jere
2020-05-06 19:09 ` Niklas Holsti
2020-05-07 9:07 ` J-P. Rosen
2020-05-07 10:15 ` Niklas Holsti
2020-05-07 13:00 ` Egil H H
2020-05-07 13:25 ` Simon Wright
2020-05-07 10:31 ` Stefan.Lucks
2020-05-07 11:58 ` J-P. Rosen
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