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From: Philippe Tarroux <philippe.tarroux@limsi.fr>
Newsgroups: comp.lang.ada
Subject: Finalization
Date: Tue, 21 Nov 2006 10:02:35 +0100
Organization: University Paris-Sud, France.
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Xref: g2news2.google.com comp.lang.ada:7594
Date: 2006-11-21T10:02:35+01:00
List-Id: <comp.lang.ada>

I have a problem trying to use controlled types. My purpose was to use 
finalize to deallocate a big data structure each time a reuse.

I wrote a simpler program that exhibits the problem too. here is the 
code followed by a comment on what I observed:

with Ada.Unchecked_Deallocation,
     Ada.Finalization;

package Final is
   type Vector is array (Positive range <>) of Float;
   type Vector_Ptr is access Vector;

   procedure Free is new Ada.Unchecked_Deallocation (Vector, Vector_Ptr);

   type Obj is new Ada.Finalization.Controlled with record
      X : Vector_Ptr := null;
   end record;

   overriding
   procedure Finalize (O : in out  Obj);
   procedure Process (O : in out Obj);
   function Process return Obj;
end Final;


with Ada.Text_Io;

package body Final is
   package Text_Io renames Ada.Text_Io;

   procedure Finalize (O : in out  Obj) is
   begin
      Text_Io.Put ("Finalize: "); Text_Io.New_Line;
      Free (O.X);
   end Finalize;

   procedure Process (O : in out Obj) is
   begin
      Text_Io.Put ("In process procedure"); Text_Io.New_Line;
      Finalize(O);
      O.X := new Vector (1 .. 100);
   end Process;

   function Process return Obj is
      O : Obj;
   begin
      Text_Io.Put ("In process function"); Text_Io.New_Line;
      O.Process;
      return O;
   end Process;
end Final;

with Ada.Text_Io,
     Final;

procedure main is
   O : Final.Obj;
begin
   for I in 1 .. 100 loop
      Ada.Text_Io.put(Integer'Image(I)); Ada.Text_Io.New_Line;
      O := Final.Process;
      -- O.Process;
   end loop;
   Ada.Text_Io.Put("Fin"); Ada.Text_Io.New_Line;
end Main;

and the resulting execution trace:

 1
In process function
In process procedure
Finalize:
Finalize:
Finalize:
Finalize:
 2
and so on...until:
 7
In process function
In process procedure
Finalize:
Finalize:
Finalize:
Finalize:
 8
In process function
In process procedure
Finalize:
process exited with status 128

When I use the function call the program stops with an unexpected error 
that seems to be different from one compiler to another (I tried gnat 
gcc 3.4.6 on Windows and the Debian gnat version on Linux). The message 
depends also on the type of structure to be freed (vector or 
vector'class). Even when the program raises PROGRAM_ERROR  I am unable 
to trap the exception. I tried to follow what happens under gdb and 
observed that an unexpected signal cencerning the heap is received by 
the program.

When i use the procedure call (commented out in the main program), all 
is correct but I suspect that the memory is not deallocated at each call 
in the main loop since there i s only one call to Finalize at the end of 
the program.

Does somebody has any idea of what happens? Do you think there is a 
faulty construct in my code?

Thanks for your help

Philippe Tarroux