Re: How to: communication between multiple tasks using protected objects - with no polling?

[Esa Riihonen <esa.riihonen.er@gmail.com>]

keskiviikko 21. tammikuuta 2015 2.47.29 UTC+2 Jeffrey Carter kirjoitti:

... snip ...

> You should also revisit your design in light of Ada tasking capabilities. The
> choice of multiple tasks that communicate through queues may not be the best choice.

That is totally correct.

... snip ...

> You can do
> 
> select
>    PO1.Get (...);
> then abort
>    PO2.Get (...);
> end select;
> 
> This might, however, result in both entries being executed.

If I understand correctly this will indeed work when selecting between two PO entries. And it wouldn't matter if both got executed (although I just now can't see how this could happen). However, there seems not to be a natural way to expand this for more than those two. This seems to be syntactically correct (compiles):

 select
    PO1.Get (...);
 then abort
    select
       PO2.Get (...);
    then abort
       PO3.Get (...);
    end select
 end select;

But I'm not sure whether it would actually do what I want it to do. That is: suspend there until any of the entry barriers becomes true and then exit the whole outermost select statement?

Even if it works I have a (minor) concern that it is not 'pretty' as it doesn't reflect the mutual equality of the entries - and thus has a feel of 'hack' there ;)

> Another approach might be
> 
> type Q_ID is (Q1, Q2, ...);
> 
> type Q_Item (Q : Q_ID := Q_ID'First) is record
>    case Q is
>    when Q1 =>
>       Item_1 : Q1_Item;
>    when Q2 =>
>       Item_2 : Q2:Item;
>    ...
>    end case;
> end record;
> 
> protected Qs is
>    entry Get (Item : out Q_Item);
> private -- Qs
>    Q_1 : Q1_Q;
>    Q_2 : Q2_Q;
>    ...
> end Qs;
> 
> protected body Qs is
>    entry Get (Item : out Q_Item) when
>       not Q_1.Is_Empty or not Q_2.Is_Empty or ...
>    is
>       -- Empty declarative part
>    begin -- Get
>       if not Q_1.Is_Empty then
>          Item := (Q => Q1, Item_1 => Q_1.Get);
>       elsif not Q_2.Is_Empty then
>          item := (Q => Q2, Item_2 => Q_2.Get;
>       ...
>       end if;
>    end Get;
> end Qs;

Thanks for that! I'm not sure I did understand it thoroughly at first reading though I did say that I did read 'the book' - but surely it didn't all sink in in the first reading ;)

> if your system is actually many tasks adding items to queues and one waiting for
> something on one of those queues, then it might be best to have the waiting task
> have entries, and put forwarder tasks between the queues and the waiting task.

'Forwarder tasks' - definitely might be something I will study at this stage.

Thanks a lot.

> -- 
> Jeff Carter
> "Ada has made you lazy and careless. You can write programs in C that
> are just as safe by the simple application of super-human diligence."
> E. Robert Tisdale
> 72

I really like the Tisdale quote - gonna steal it ;)