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From: "=?iso-8859-1?B?RWdpbCBI+HZpaw==?=" <egilhovik@hotmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Newbie: Code isn't executed
Date: 4 Aug 2005 07:48:26 -0700
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Date: 2005-08-04T07:48:26-07:00
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Thomas Ruschival wrote:
> Hello Ada group
> I am a student learning Ada at university. I played with some tasks
> that want to join another task for a rendevouz, its a modiefied exaple
> from the lectures. Basically I let 2 tasks sleep a different amount of
> time then wake up and execute a piece of code with another task during
> a rendevouz, I count the number how often a task got executed.
> Somehow a piece of code never gets executed - neither me nor nor my
> tutor has a clue why -- here is the code:
>
> Thanks for _ANY_ comments - I know this isn't the best way of coding -
> but it was just meant as an example until strange things happened
>
> Thomas
>

Adding an exception handler in the Susan task reveals a
CONSTRAINT_ERROR when Total becomes 0. You should change Total to be of
type Natural instead of Positive (which can never be 0).

Also you have a possible race condition when using the Partner variable
which could give you wrong statistics. See comments in your code below.

Ada.Text_IO provides a Put_Line procedure so you can write
   Ada.Text_IO.Put_Line("...");
instead of
   Ada.Text_IO.Put("...");
   Ada.Text_IO.New_Line;


>
> with Ada.Text_IO;
> use Ada.Text_IO;
>
> procedure myfirst
> is
>    type Name is (Peter,Michael);
>    type Statistics is array(Name) of Integer;
>    Partner : Name;
>    Stat : Statistics :=(others => 0);
>    Total: Positive :=1000;

Should be Total : Natural;

>
>    task Michael_task;
>    task Peter_task;
>    task Susan is
>       entry Dinner;


To avoid a possible race condition you probably want peter and michael
to tell susan who they are directly in the rendezvous, and not use the
"global" variable Partner:

entry Dinner( Partner : Name );

>    end Susan;
>
>
>    task body Peter_task
>    is
>    begin
>       while Total > 0 loop
>          Put("Peters turn");
>          New_Line;
>          Partner := Peter;
>          Susan.Dinner;

Susan.Dinner(Peter);

>          delay 0.002;
>       end loop;
>    end Peter_task;
>
>    task body Michael_task is
>    begin
>       while Total > 0 loop
>          Put("Michaels turn");
>          New_Line;
>          Partner := Michael;
>          Susan.Dinner;

Susan.Dinner(Michael);

>          delay 0.001;
>       end loop;
>    end Michael_task;
>
>
>    task body Susan
>    is
>    begin
>
>       while Total > 0 loop
>          Put("susans turn");
>          New_Line;
>          accept Dinner do

accept Dinner( Parnter : Name ) do

>             stat(Partner) := stat(Partner)+1;
>             Put("Susan is having dinner with...."& Name'Image(Partner));
>             New_Line;
>          end Dinner;
>          Total := Total-1;
>       end loop;
>
> -- THIS WILL NEVER BE EXECUTED --- WHY ??????????????????????
>       Put("Statistics:");
>       New_Line;
>       Put("---------------------------------------------");
>       New_Line;
>       Put("Peter:  "&Integer'Image(Stat(Peter)));
>       New_Line;
>       Put("Michael:  "&Integer'Image(Stat(Michael)));
>       New_Line;
>       Put("---------------------------------------------");
>       New_Line;
> 
>    end Susan;
> 
> 
> begin -- Procedure
>    null;
> end myfirst;