Re: Storage management

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Fri, 07 Nov 2008 14:12:30 +0200, Niklas Holsti wrote:

> Dmitry A. Kazakov wrote:
>> 
>> But exactly this case is not guaranteed to work, or rather
>> is guaranteed not to work...
> 
> Aborting blocking I/O is "guaranteed not to work"? Can you explain 
> why? Is this something that has been discussed before on c.l.a.?

I don't know. But I discussed that with AdaCore people. Since the following
does not work with GNAT Pro 6.2 (Windows wavefront):

with Ada.Text_IO;  use Ada.Text_IO;
procedure Test_ATC is
begin
   Put_Line ("Type");
   select
      delay 2.0;
      Put_Line ("Timed out");
   then abort
      Put_Line ("You typed " & Get_Line);
   end select;
end Test_ATC;

Actually I was almost certain that this were *not* required to work. I only
wished to know if they considered it as nice to have working or not. (:-))

There is no way Ada could abort I/O if the OS does not allow this. Second
to Get_Line, or likely the first wanted case is canceling blocking socket
read. I would give 98% that it never will work with ATC.

>>> Isn't pragma Restrictions (No_Select_Statements) enough?
>>> Or is it too strong, and a new restriction specifically
>>> for ATC would be better?
>> 
>> I would prefer pragma Cancelable put on a task. If a task
>> is not cancelable then abort would raise Tasking_Error,
>> and an ATC in the task body would be a compile-time error.
> 
> I will leave it to the language experts to comment on that 
> suggestion. I assume it would have to forbid ATCs in subprograms 
> called from the task, too, which would require some form of 
> subprogram-level contract that the subprogram body executes no ATCs.

Or to constrain its use like with the selective accept.

> What about ctrl-C, that is, process abort from the operating 
> system? I don't think that users would be happy if Ada applications 
> could not be aborted with ctrl-C. Do abort-deferred operations now 
> defer ctrl-C, too?

The meaning of ctrl-C depends on the OS. Ada standard cannot mandate that
pressing ctrl-C is equivalent to aborting the environment task in the sense
of "abort" statement. Then usually OS have efficient means to kill a
process preemptively bypassing any "abort deferred" stuff.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de