unexpected behaviour of finalize [ADA]

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From: stefan-lucks on 4 Apr 2008 05:33

Hi,

I have written a little program to demonstrate finalisation. The idea is
to locally declare a singelton object of a class derived from
Ada.Finalization.Limited_Controlled, and to override the Finalize method.
When the sigelton object leaves scope, Finalize than cleans up whatever
needs to be cleaned up. At a first look this appeared to work nicely. But
when done in a recursive subprogram, Finalize rather unexpectedly seems to
always read the local parameters of the innermost scope.

Here is a test case:

---begin fin_test_case.adb

with Ada.Text_IO, Ada.Finalization;

procedure Fin_Test_Case is
type Fin is new Ada.Finalization.Limited_Controlled with null record;

procedure Do_Something (I: Natural) is

type My_Fin is new Fin with null record;

overriding procedure Finalize(Self: in out My_Fin) is
S: String := Natural'Image(I);
begin
Ada.Text_IO.Put_Line(" Do_Something Finalization " & S);
end Finalize;

Singleton: My_Fin;

begin -- Do_Something
Ada.Text_IO.Put_Line(" I = " & Natural'Image(I));
if I>0 then
Do_Something(I-1);
Do_Something(I-1);
end if;
end Do_Something;

begin
Ada.Text_IO.Put_Line("Start Do_Something (2)");
Do_Something(2);
Ada.Text_IO.Put_Line("Stop Do_Something (2)");
end Fin_Test_Case;

---end fin_test_case.adb

This is what I got:

---begin output

Start Do_Something (2)
I = 2
I = 1
I = 0
Do_Something Finalization 0
I = 0
Do_Something Finalization 0
Do_Something Finalization 0
I = 1
I = 0
Do_Something Finalization 0
I = 0
Do_Something Finalization 0
Do_Something Finalization 0
Do_Something Finalization 0
Stop Do_Something (2)

---end output

Somehow, the local parameter I is always 0 when finalize is called. What I
expected was the following:

---begin expected output

Start Do_Something (2)
I = 2
I = 1
I = 0
Do_Something Finalization 0
I = 0
Do_Something Finalization 0
Do_Something Finalization 1
I = 1
I = 0
Do_Something Finalization 0
I = 0
Do_Something Finalization 0
Do_Something Finalization 1
Do_Something Finalization 2
Stop Do_Something (2)

---end expected output

Now, is my expectation wrong? Or is this a compiler bug? I am using
GNAT GPL 2007 (20070405-41).

So long

Stefan

--
------ Stefan Lucks -- Bauhaus-University Weimar -- Germany ------
Stefan dot Lucks at uni minus weimar dot de
------ I love the taste of Cryptanalysis in the morning! ------

From: Adam Beneschan on 4 Apr 2008 12:08

On Apr 4, 2:33 am, stefan-lu...(a)see.the.signature wrote:
> Hi,
>
> I have written a little program to demonstrate finalisation. The idea is
> to locally declare a singelton object of a class derived from
> Ada.Finalization.Limited_Controlled, and to override the Finalize method.
> When the sigelton object leaves scope, Finalize than cleans up whatever
> needs to be cleaned up. At a first look this appeared to work nicely. But
> when done in a recursive subprogram, Finalize rather unexpectedly seems to
> always read the local parameters of the innermost scope.
>
> Here is a test case:
>
> ---begin fin_test_case.adb
>
> with Ada.Text_IO, Ada.Finalization;
>
> procedure Fin_Test_Case is
> type Fin is new Ada.Finalization.Limited_Controlled with null record;
>
> procedure Do_Something (I: Natural) is
>
> type My_Fin is new Fin with null record;
>
> overriding procedure Finalize(Self: in out My_Fin) is
> S: String := Natural'Image(I);
> begin
> Ada.Text_IO.Put_Line(" Do_Something Finalization " & S);
> end Finalize;
>
> Singleton: My_Fin;
>
> begin -- Do_Something
> Ada.Text_IO.Put_Line(" I = " & Natural'Image(I));
> if I>0 then
> Do_Something(I-1);
> Do_Something(I-1);
> end if;
> end Do_Something;
>
> begin
> Ada.Text_IO.Put_Line("Start Do_Something (2)");
> Do_Something(2);
> Ada.Text_IO.Put_Line("Stop Do_Something (2)");
> end Fin_Test_Case;
>
> ---end fin_test_case.adb
>
> This is what I got:
>
> ---begin output
>
> Start Do_Something (2)
> I = 2
> I = 1
> I = 0
> Do_Something Finalization 0
> I = 0
> Do_Something Finalization 0
> Do_Something Finalization 0
> I = 1
> I = 0
> Do_Something Finalization 0
> I = 0
> Do_Something Finalization 0
> Do_Something Finalization 0
> Do_Something Finalization 0
> Stop Do_Something (2)
>
> ---end output
>
> Somehow, the local parameter I is always 0 when finalize is called. What I
> expected was the following:
>
> ---begin expected output
>
> Start Do_Something (2)
> I = 2
> I = 1
> I = 0
> Do_Something Finalization 0
> I = 0
> Do_Something Finalization 0
> Do_Something Finalization 1
> I = 1
> I = 0
> Do_Something Finalization 0
> I = 0
> Do_Something Finalization 0
> Do_Something Finalization 1
> Do_Something Finalization 2
> Stop Do_Something (2)
>
> ---end expected output
>
> Now, is my expectation wrong? Or is this a compiler bug? I am using
> GNAT GPL 2007 (20070405-41).

This looks like a pretty clear compiler bug, especially since by the
time Finalize is called by the outermost Do_Something, it's using, for
the value of I, the parameter from a Do_Something call that is no
longer active. It shouldn't be hard to modify this slightly, using
(say) a string or record as a parameter to Do_Something, to create a
test where utter garbage is displayed; then it would be clearer that
this is a compiler bug.

-- Adam

From: george.priv on 4 Apr 2008 13:34

On Apr 4, 12:08 pm, Adam Beneschan <a...(a)irvine.com> wrote:
> On Apr 4, 2:33 am, stefan-lu...(a)see.the.signature wrote:
>
>
>
> > Hi,
>
> > I have written a little program to demonstrate finalisation. The idea is
> > to locally declare a singelton object of a class derived from
> > Ada.Finalization.Limited_Controlled, and to override the Finalize method.
> > When the sigelton object leaves scope, Finalize than cleans up whatever
> > needs to be cleaned up. At a first look this appeared to work nicely. But
> > when done in a recursive subprogram, Finalize rather unexpectedly seems to
> > always read the local parameters of the innermost scope.
>
> > Here is a test case:
>
> > ---begin fin_test_case.adb
>
> > with Ada.Text_IO, Ada.Finalization;
>
> > procedure Fin_Test_Case is
> > type Fin is new Ada.Finalization.Limited_Controlled with null record;
>
> > procedure Do_Something (I: Natural) is
>
> > type My_Fin is new Fin with null record;
>
> > overriding procedure Finalize(Self: in out My_Fin) is
> > S: String := Natural'Image(I);
> > begin
> > Ada.Text_IO.Put_Line(" Do_Something Finalization " & S);
> > end Finalize;
>
> > Singleton: My_Fin;
>
> > begin -- Do_Something
> > Ada.Text_IO.Put_Line(" I = " & Natural'Image(I));
> > if I>0 then
> > Do_Something(I-1);
> > Do_Something(I-1);
> > end if;
> > end Do_Something;
>
> > begin
> > Ada.Text_IO.Put_Line("Start Do_Something (2)");
> > Do_Something(2);
> > Ada.Text_IO.Put_Line("Stop Do_Something (2)");
> > end Fin_Test_Case;
>
> > ---end fin_test_case.adb
>
> > This is what I got:
>
> > ---begin output
>
> > Start Do_Something (2)
> > I = 2
> > I = 1
> > I = 0
> > Do_Something Finalization 0
> > I = 0
> > Do_Something Finalization 0
> > Do_Something Finalization 0
> > I = 1
> > I = 0
> > Do_Something Finalization 0
> > I = 0
> > Do_Something Finalization 0
> > Do_Something Finalization 0
> > Do_Something Finalization 0
> > Stop Do_Something (2)
>
> > ---end output
>
> > Somehow, the local parameter I is always 0 when finalize is called. What I
> > expected was the following:
>
> > ---begin expected output
>
> > Start Do_Something (2)
> > I = 2
> > I = 1
> > I = 0
> > Do_Something Finalization 0
> > I = 0
> > Do_Something Finalization 0
> > Do_Something Finalization 1
> > I = 1
> > I = 0
> > Do_Something Finalization 0
> > I = 0
> > Do_Something Finalization 0
> > Do_Something Finalization 1
> > Do_Something Finalization 2
> > Stop Do_Something (2)
>
> > ---end expected output
>
> > Now, is my expectation wrong? Or is this a compiler bug? I am using
> > GNAT GPL 2007 (20070405-41).
>
> This looks like a pretty clear compiler bug, especially since by the
> time Finalize is called by the outermost Do_Something, it's using, for
> the value of I, the parameter from a Do_Something call that is no
> longer active. It shouldn't be hard to modify this slightly, using
> (say) a string or record as a parameter to Do_Something, to create a
> test where utter garbage is displayed; then it would be clearer that
> this is a compiler bug.
>
> -- Adam

From: george.priv on 4 Apr 2008 13:38

From: stefan-lucks on 4 Apr 2008 13:51

> This looks like a pretty clear compiler bug, especially since by the
> time Finalize is called by the outermost Do_Something, it's using, for
> the value of I, the parameter from a Do_Something call that is no
> longer active. It shouldn't be hard to modify this slightly, using
> (say) a string or record as a parameter to Do_Something, to create a
> test where utter garbage is displayed; then it would be clearer that
> this is a compiler bug.

Good idea! Yes, the following change makes the program output a bit of
garbage, as you guessed:

procedure Strong_Fin_Test_Case is

type Fin is new Ada.Finalization.Limited_Controlled with null record;

procedure Do_Something (I: Natural) is

A: String(1 .. 5); -- this is new

type My_Fin is new Fin with null record;
overriding procedure Finalize(Self: in out My_Fin) is
begin
Ada.Text_IO.Put_Line(" Do_Something Finalization " & A);
end Finalize;
Singleton: My_Fin;

begin -- Do_Something
Ada.Text_IO.Put_Line(" I = " & Natural'Image(I));
if I>0 then
Do_Something(I-1);
Do_Something(I-1);
end if;
A(1 .. 5) := ("ABCDE"); -- always the same
end Do_Something;

begin
Ada.Text_IO.Put_Line("Start Do_Something (2)");
Do_Something(2);
Ada.Text_IO.Put_Line("Stop Do_Something (2)");
end Strong_Fin_Test_Case;

The putput:

Start Do_Something (2)
I = 2
I = 1
I = 0
Do_Something Finalization ABCDE
I = 0
Do_Something Finalization ABCDE
Do_Something Finalization ABCDE
I = 1
I = 0
Do_Something Finalization ABCDE
I = 0
Do_Something Finalization ABCDE
Do_Something Finalization ABCDE
Do_Something Finalization �l
Stop Do_Something (2)

The last line before "Stop Do_Something (2)" contains some strange
characters instead of the constant "ABCDE" it should output. Furthermore,
this output is not always the same.

So long

Stefan

--
------ Stefan Lucks -- Bauhaus-University Weimar -- Germany ------
Stefan dot Lucks at uni minus weimar dot de
------ I love the taste of Cryptanalysis in the morning! ------

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