Re: Newbie: Task parametering

[Andrew Price <whatsfivecentsbuy@gmail.com>]

jimmaureenrogers@worldnet.att.net wrote:
> Andrew Price wrote:
> 
>>Hello all,
>>
>>I need to initialise some tasks with a few parameters when I instantiate
>>them. After looking at
>>http://www.adaic.com/docs/95style/html/sec_6/6-1-3.html it seems the
>>best way to do this is using an access type discriminant to pass a
>>record to the tasks.
>>
>>With that in mind, I created this record type;
>>
>>type Task_Data is
>>       record
>>       Name : aliased String(1..8);
>>       Period : aliased Duration;
>>       Repetitions : aliased Integer;
>>    end record;
>>
>>
>>and then in the main part of my program I use it like this;
>>
>>    type Task_Handle is access A_Task;  -- used to point to the task
>>    Another_Task : Task_Handle;
>>    type Data_Handle is access Task_Data; -- used to point to its data
>>    Data_For_Task : Data_Handle;
>>
>>begin  -- The main prog will now spawn three of the above tasks
>>
>>    -- Parameterise the first task and then start it.
>>    Data_For_Task := new Task_Data;
>>    Data_For_Task.Name := "Task A  ";
>>    Data_For_Task.Period := 0.5;
>>    Data_For_Task.Repetitions := 5;
>>    Another_Task := new A_Task(Data_For_Task);
>>
>>So while this approach works, I can't help but feel that I'm going about
>>this in a 'long winded' way (especially coming from a mostly C background).
>>
>>Is there are neater way of putting the data into the Task_Data records?
>>Or is there an better approach to the whole task parameterisation I
>>should consider?
>>
>>Now is probably a good time to add the disclaimer that this is part of a
>>lab work exercise for a course I'm currently undertaking at university.
> 
> 
> There is a little roblem with your approach.
> 
> It does not actually initialize the task. The task begins running
> before the values are set. You are likely to encounter a race
> condition with your approach.
> 
> True initialization can be achieved by providing a discriminant
> to the task.
> 
> On the other hand, you may not need a true initialization.
> Instead, you may simply want the task to begin by accepting
> an entry that passes in all the values you need to set before
> the task does anything else.
> 
> Read about the concepts of discriminants and task entries.
> You should then be able to decide which approach best fits
> your needs.
> 
> Jim Rogers
> 

Thanks for the input Jim and others.

I thought that the task would not start running until the statement

Another_Task := new A_Task(Data_For_Task);

which occurs after the values in the Data_For_Task record have been set?


I originally wanted to just use multiple discriminants to initialise the 
  task however I ran into compilation errors because "discriminants must 
have a discrete or access type" and I wanted to include a string among 
the initialising variables.

 From what I have read it seems that maybe I would be better to make use 
of a task entry to set up each task, which I have done and it seems much 
neater for me.

Thanks again.

Here is what I ended up doing if anyone is interested (I'm sure there 
are probably many other improvements that could be made but I'm much 
happier with this than the first effort)

task type A_Task is
       entry Start(Name: String; Period : Duration; Repetitions : Integer);
    end A_Task;

    task body A_Task is  -- and this is the task's code
       Myname : String(1..8);
       Myperiod : Duration;
       myrepetitions : integer;
       Next_Time : Time := clock;
       Iteration : Integer := 0;
    begin               -- It just diplays the task name and some data 
from the discriminant
       accept Start(Name: String; Period : Duration; Repetitions : 
Integer) do
          Myname := Name;
          Myperiod := Period;
          Myrepetitions := Repetitions;
       end;

       while Iteration < myRepetitions loop
          Display_Lock.Seize;
          Put("Hello from ");
          Put(Image(Current_Task));   -- from Task_Identification package
          Put(" with name ");
          Put(myName);
          New_Line;
          Display_Lock.Release;
          Iteration := Iteration + 1;
          Next_Time := Next_Time + myPeriod;
          delay until Next_Time;
       end loop;
    exception
       when others =>
          Put("unexpected exception in task ");
          Put_line(Image(Current_Task));
    end A_Task;

    type Task_Handle is access A_Task;  -- used to point to the task
    Another_Task : Task_Handle;

begin  -- The main prog will now spawn three of the above tasks

    -- Start first task
    Another_Task := new A_Task;
    Set_Priority(30,Another_Task'Identity);
    Another_Task.Start(Name => "Task A  ", Period => 0.5, Repetitions => 5);