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From: Victor Porton <porton@narod.ru>
Newsgroups: comp.lang.ada
Subject: Re: Unchecked_Union record inside an other record - trouble
Date: Fri, 08 Aug 2014 22:52:41 +0300
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Date: 2014-08-08T22:52:41+03:00
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Adam Beneschan wrote:
> On Friday, August 8, 2014 8:36:48 AM UTC-7, Victor Porton wrote:
>> I am trying to interface my program with a C library.
> 
>> The below program does not compile:
>> main.adb:19:14: unconstrained subtype in component declaration
> 
>> If this does not work, what should I do to interface a union inside a
>> struct in a C library?
> 
>> -- main.adb
>> 
>> with Interfaces.C; use Interfaces.C;
> 
>> procedure Main is
> 
>>    type Kind is (First, Second);
>  
>>    type T(K: Kind) is
> 
> change this to
> 
>      type T(K: Kind := Kind'First) is

OK, I followed your way, Adam.

But it produces a warning with GNAT 4.9 (for the below program):

gnatmake -q -c -gnatc -u -P/home/porton/t/default.gpr main.adb --subdirs=check -cargs -g -O0 -gnato -fstack-check -gnatVa
main.adb:26:17: warning: component not present in subtype of "T" defined at line 21
main.adb:26:17: warning: "Constraint_Error" will be raised at run time

Should I just disable this warning? (BTW, how to disable this warning in
GNAT-GPS?)

It seems that there will be a "Constraint_Error" and this is really bad.

Any other idea/workarounds?

-- main.adb
with Interfaces.C; use Interfaces.C;

procedure Main is

   type Kind is (First, Second);
   
   type T(K: Kind) is
      record
         case K is
            when First =>
               X: int;
            when Second =>
               Y: char;
         end case;
      end record
      with Unchecked_Union, Convention=>C;
   
   type Container is
      record
         Z: int;
         R: T(Kind'First);
      end record;
   
   function P (C: Container) return char is
   begin
      return C.R.Y;
   end;

begin
   null;
end;


-- 
Victor Porton - http://portonvictor.org