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Newsgroups: comp.lang.ada
Date: Mon, 7 May 2018 08:54:35 -0700 (PDT)
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Subject: Re: Recommendation of safe subset of Ada to use?
From: onox <denkpadje@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:52067
Date: 2018-05-07T08:54:35-07:00
List-Id: <comp.lang.ada>

On Saturday, May 5, 2018 at 11:23:04 PM UTC+2, joak...@kth.se wrote:
> Jere gave the following example in the other thread about how to get Ada =
across the chasm:
>=20
> 1.  Dangling references:  Keeping a reference to an object past its lifet=
ime=20
>=20
> Ada:=20
> ***********************************************=20
> with Ada.Text_IO; use Ada.Text_IO;=20
>=20
> procedure jdoodle is=20
>     type Integer_Access is access all Integer;=20
>    =20
>     function Inner(Value : aliased in out Integer) return Integer_Access =
is=20
>     begin=20
>         return Value'Access;=20
>     end Inner;=20
>    =20
>     function Outer return Integer_Access is=20
>         Value : aliased Integer :=3D 0;=20
>     begin=20
>         return Inner(Value);=20
>     end Outer;=20
>    =20
>     Ptr : Integer_Access :=3D Outer;  -- !!! Dangling reference=20
> begin=20
>     Put_Line("Hello World");=20
> end jdoodle;=20
> ***********************************************=20
> Hello World=20
>=20
> gcc -c jdoodle.adb=20
> gnatbind -x jdoodle.ali=20
> gnatlink jdoodle.ali -o jdoodle=20
>=20
> It's a 20 line application that demonstrates a dangling pointer in Ada. T=
hat's not supposed to be able to happen unless one goes outside of Ada's ty=
pe system by using Unchecked_Deallocation, Unchecked_Conversion or System.A=
ddress_To_Access_Conversion. I've tried the example with the GNAT compiler =
and it does not detect the issue. I do not believe this is a GNAT bug. Alia=
sed parameters were part of the solution to be able to safely reference ele=
ments in containers and thereby avoid unnecessary copying. By making this p=
ossible was a hole in Adas type system introduced? It means that one cannot=
 safely use all the features of Ada and be sure of memory safety instead on=
e should stick to a subset of Ada. One subset that comes to mind is SPARK. =
Another is for example sticking to Ada95 or Ada 2005. Or maybe one should j=
ust ban usage of aliased parameters but then what should one do with the st=
andard containers that one probably uses throughout one's application. I am=
 confused. Anybody that can shed light?
>=20
> /Joakim

A similar problem happens with anonymous access procedures (GNAT 7.2):

with Ada.Text_IO;

procedure A is
   type Proc_Access is access procedure (P1 : Integer);

   function Inner (Value : access procedure (P2 : Integer)) return Proc_Acc=
ess is
   begin
      return Value;
   end Inner;

   function Outer (X : Integer) return Proc_Access is
      use Ada.Text_IO;

      procedure Value (Y : Integer) is
      begin
         Put_Line ("X: " & X'Image);  --  random
         Put_Line ("Y: " & Y'Image);  --  2
      end Value;
   begin
      return Inner (Value'Access);
   end Outer;

   Ptr : Proc_Access :=3D Outer (1);
begin
   Ptr (2);
end A;