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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Interfaces.C questions
Date: Fri, 17 Mar 2017 23:57:58 +0200
Organization: Tidorum Ltd
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Date: 2017-03-17T23:57:58+02:00
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On 17-03-17 23:12 , hreba wrote:
> These are my first steps with Interfaces.C:
>
> ----------------------------------------------------------------------
> with Interfaces.C;
>
> package GSL is
>
>    package C renames Interfaces.C;
>
>    type size_t is new C.int;
>    type Real_Function is access function (x: C.double) return C.double;
>
>    function Bessel_J0 (x: C.double) return C.double;
>
>    function Integration_QNG
>      (f: Real_Function;
>       a, b, epsabs, epsrel: C.double;
>       result, abserr: out C.double;
>       neval: out size_t)
>      return C.int;
>
> private
>
>    pragma Import (C, Bessel_J0,        "gsl_sf_bessel_J0");
>    pragma Import (C, Integration_QNG,    "gsl_integration_qng");
>
> end GSL;
> ------------------------------------------------------------------------
>
> No problem with function Bessel_J0. But Integration_QNG gets me:
>
> gsl.ads:19:07: functions can only have "in" parameters.
>
> The C prototype of this function is:
>
> int gsl_integration_qng (const gsl function * f , double a , double b ,
> double epsabs , double epsrel , double * result , double * abserr , size
> t * neval )
>
> So my questions are:
>
> 1. How do I translate C functions which return non-void and which
> additionally use pointers for out parameters? Do I have to use pointers
> in my Ada program?

Using pointers is possible, but functions with "out" or "in out" 
parameters are allowed in the Ada 2012 standard. Check your compiler; it 
may by default use the Ada 2005 rules and may need a command-line option 
(perhaps -gnat2012) to use Ada 2012 rules.

> 2. On my platform size_t is C.int. How can I define size_t in a
> platform-independent way?

Don't define it; use Interfaces.C.size_t.

> 3. The C function above needs a pointer to a function as parameter. Is
> my translation to Ada correct?

Looks ok to me, but you didn't show the C prototype of the function (the 
gsl_function type?) so I can't compare the two.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .