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Date: Tue, 5 May 2020 12:19:40 -0700 (PDT)
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Subject: Re: How can one record component be local and another not?
From: Jere <jhb.chat@gmail.com>
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Xref: reader01.eternal-september.org comp.lang.ada:58605
Date: 2020-05-05T12:19:40-07:00
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On Tuesday, May 5, 2020 at 1:17:42 PM UTC-4, 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

This isn't so much a question for you directly...maybe more for
those more experienced with the RM, but I was curious about the
implications of having an "aliased" parameter inside a record with 
a C convention specified.  For some reason, I thought that the 
aliased keyword implied a certain alignment in Ada (not necessarily 
in C) and didn't know if specifying the convention undid that 
alignment or if there is a risk of the record not always being
the same layout/alignment as the expected C struct for the aliased
parameter.