Re: RFC: Prototype for a user threading library in Ada

[Anh Vo <anhvofrcaus@gmail.com>]

On Saturday, June 25, 2016 at 11:15:48 PM UTC-7, Dmitry A. Kazakov wrote:
> On 2016-06-26 05:21, Randy Brukardt wrote:
> > "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> > news:nkl8bm$19q7$1@gioia.aioe.org...
> >> On 2016-06-24 02:38, rieachus@comcast.net wrote:
> >>> I don't get it.  If this is your "motivation":
> >>>
> >>>> The motivation is a two-liner. Let you have some consumer of data:
> >>>>
> >>>>    procedure Write (Buffer : String; Last : out Integer);
> >>>>
> >>>> It may take less than the whole string when called, but will take more
> >>>> data later. So, the parameter Last. Now you want to write a program in a
> >>>> *normal* way:
> >>>>
> >>>>    Write ("This");
> >>>>    Write ("That");
> >>>>
> >>>> That's it.
> >>>
> >>> You may want to make your Last parameter in or in out, but that's a
> >>> detail.
> >>
> >> It is not a detail. The caller of Write does not know how much data the
> >> transport layer is ready to accept. That is the nature of non-blocking
> >> I/O. Write takes as much data it can and tells through Last where the
> >> caller must continue *later*.
> >>
> >> A blocking busy-waiting wrapper looks this way:
> >>
> >>    procedure Write (Buffer : String) is
> >>       First : Integer := Buffer'First;
> >>       Last  : Integer;
> >>    begin
> >>       loop
> >>          Write (Buffer (First..Buffer'Last), Last);
> >>          exit when Last = Buffer'Last;
> >>          First := Last + 1;
> >>       end loop;
> >>    end Write;
> >
> > You forgot the "delay 0.0;"
> 
> I didn't. With yielding processor it would no more be busy-waiting.
> 
> > That, combined with proper tasking runtimes, would seem to provide better
> > results than doing all one thing (task = thread) or all othe ther (some
> > fancy coroutine system).
> 
> Not really. The whole point is that in the imaginary case under 
> consideration you don't need a timer event in order to wake Write up. I 
> presume that there is an I/O event that tells this:
> 
>     procedure Write (Buffer : String) is
>        First : Integer := Buffer'First;
>        Last  : Integer;
>     begin
>        loop
>           Write (Buffer (First..Buffer'Last), Last);
>           exit when Last = Buffer'Last;
>           First := Last + 1;
>           Output_Buffer.Wait_For_State (Not_Full); -- A PO's entry call
>        end loop;
>     end Write;
 
This procedure works correctly if the entire buffer is written. However, it will not work as intended if partial buffer is written first. Thus, the exit statement will never be true. One of the fix is to replace it, after First is recalculated, with "exit when First > Buffer'Last" statement.

Anh Vo