Hello,

Assume the following program:

with Text_Io; use Text_Io;
procedure test2 is
procedure test_a(ok : out Boolean) is
begin
if false then
ok := true;
end if;
end test_a;
procedure test_b(ok : in out Boolean) is
begin
if false then
ok := true;
end if;
end test_b;
ok_a,ok_b : Boolean := true;
begin
test_a(ok_a);
test_b(ok_b);
Put_Line("ok_a = " & ok_a'Image);
Put_Line("ok_b = " & ok_b'Image);
end test2;

I get the following output (using GNAT Community Edition):

ok_a = FALSE
ok_b = TRUE

As far as I understand, this is correct.
However, I think I remember that (some) older versions of the GNAT compiler gave a different result. Could others try? And does my program example reveal an unnecessary gotcha?

reinert

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On 13/11/21 18:46, reinert wrote:

procedure test_a(ok : out Boolean) is
begin
if false then
ok := true;
end if;
end test_a;

I get the following output (using GNAT Community Edition):

ok_a = FALSE
ok_b = TRUE

As far as I understand, this is correct.
However, I think I remember that (some) older versions of the GNAT compiler gave a different result. Could others try? And does my program example reveal an unnecessary gotcha?

reinert

Hi reinert,

The value of an 'out' only parameter needs to be explicitly set. The initial value of the 'ok_a' variable will be discarded and then set to
the value of the 'ok' parameter within 'test_a' when the call completes.
Since 'ok' is not set in 'test_a' it's value is indeterminate (and
therefore erroneous).

I'd expect GNAT to issue a warning. Probably you need to add a flag
to GNAT to enable that warning. Perhaps try adding '-gnatwa'.


Regards.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 2021-11-13 9:46, reinert wrote:

Hello,

Assume the following program:

with Text_Io; use Text_Io;
procedure test2 is
procedure test_a(ok : out Boolean) is
begin
if false then
ok := true;
end if;
end test_a;
procedure test_b(ok : in out Boolean) is
begin
if false then
ok := true;
end if;
end test_b;
ok_a,ok_b : Boolean := true;
begin
test_a(ok_a);
test_b(ok_b);
Put_Line("ok_a = " & ok_a'Image);
Put_Line("ok_b = " & ok_b'Image);
end test2;

I get the following output (using GNAT Community Edition):

ok_a = FALSE
ok_b = TRUE

As far as I understand, this is correct.

The value of ok_b is correct.

The value of ok_a is not defined by the language, I believe. The
parameter test_a.ok is passed by copy-out (but not copy-in), and is not assigned a value in test_a, therefore the returned value comes from an uninitialized local Boolean, and is undefined.

However, I think I remember that (some) older versions of the GNAT
compiler gave a different result.

That may be so, but it would not be bug in GNAT. It is a programmer error.

And does my program example reveal an unnecessary gotcha?

The gotcha is that Ada and GNAT do not detect all uses of uninitialized variables. That is regrettable, but it would be very expensive to detect
them.

Note that if the "out" parameter has a constrained subtype, say Integer
range 1 .. 10, and is not assigned a value in the subprogram, a Constraint_Error may be raised on return if the uninitialized value
fails the subtype check in the copy-out. (This has happened to me, when
I was younger and perhaps less learned.)

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On 13.11.21 09:53, Rod Kay wrote:

On 13/11/21 18:46, reinert wrote:

    procedure test_a(ok :    out Boolean) is
    begin
       if false then
          ok := true;
       end if;
    end test_a;

However, I think I remember that (some) older versions of the GNAT compiler gave a different result. Could others try?  And does my program example reveal an unnecessary gotcha?

   I'd expect GNAT to issue a warning. Probably you need to add a flag to GNAT to enable that warning. Perhaps try adding '-gnatwa'.

GCC's -Wuninitialized does as Rod Kay says.

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