Re: elaboration circularity detected problem, How to fix?

From: Scott Travak (s.travak_at_j-inet.com)
Date: 09/21/04 

Date: Wed, 22 Sep 2004 06:15:58 +1000

Thanks for that Dennis and Ludovic! The both of you helped me out a lot on
this. From Dennis I realised that I could be splitting the 'Location'
package into two seperate packages "Location_Send' and 'Location_Receive'.
In that case circularity is removed. Ludovic pointed me towards a problem
being in Station.Chicago_Ptr not being initialised. The program works great
now.

Yep the tasks CHICAGO_RECEIVE and DESTINATION are still waiting for input. I
removed some code to make my posted code smaller. My original code controls
all shutdowns to all tasks.

Thanks so much again!

Cheers, Scott

"Ludovic Brenta" <ludovic.brenta@insalien.org> wrote in message
news:87y8j3foto.fsf@insalien.org...
>I had to modify Scott's code just a wee bit and I tried it on two
> compilers, one being GNAT 3.15p on Debian. None of the compilers gave
> any error message, and the program runs as expected. Here is my
> slightly amended version. The changes are:
>
> - place everything into one procedure, with nested packages
>
> - remove Station.Chicago_Ptr, which hides the top-level Chicago_Ptr
> but is different, since its pointers are left uninitialised. In
> fact, this may well be the source of the problem.
>
> Enjoy:
>
> with Ada.Text_Io;
> use Ada.Text_Io;
>
> procedure Circularity is
>
> type Location_Type is (Chicago, Paris, London);
>
> generic
> Centre : Location_Type;
> package Location is
> task type Send_Type is
> end Send_Type;
>
> task type Receive_Type is
> entry Ack_Channel;
> end Receive_Type;
>
> type Send_Ptr_Type is access Send_Type;
> type Receive_Ptr_Type is access Receive_Type;
>
> Send_Ptr : Send_Ptr_Type;
> Receive_Ptr : Receive_Ptr_Type;
> end Location;
>
>
> package Station is
> task Destination is
> entry Signal;
> end Destination;
> end Station;
>
>
> package body Location is
> task body Send_Type is
> Calls : Integer := 2;
> Done : Boolean := False;
> begin
> Put_Line (Location_Type'Image (Centre) & "_SEND alive");
> while Calls > 0 loop
> delay 0.1;
> Put_Line (Location_Type'Image (Centre) &
> " pings the Destination");
> Station.Destination.Signal;
> Calls := Calls - 1;
> end loop;
> Put_Line (Location_Type'Image (Centre) & "_SEND dead");
> end Send_Type;
>
>
> task body Receive_Type is
> Done : Boolean := False;
> begin
> Put_Line (Location_Type'Image (Centre) & "_RECEIVE alive");
> while not Done loop
> select
> accept Ack_Channel do
> Put_Line (Location_Type'Image (Centre) &
> " received an acknowledgement");
> end Ack_Channel;
> else
> delay 0.1;
> end select;
> end loop;
> Put_Line (Location_Type'Image (Centre) & "_RECEIVE dead");
> end Receive_Type;
> end Location;
>
>
> package Chicago_Ptr is new Location (Chicago);
>
>
> package body Station is
> task body Destination is
> Done : Boolean := False;
> begin
> Put_Line ("DESTINATION alive");
> while not Done loop
> select
> accept Signal do
> Put_Line ("DESTINATION: Received signal");
> Chicago_Ptr.Receive_Ptr.Ack_Channel;
> end Signal;
> else
> delay 0.1;
> end select;
> end loop;
> Put_Line ("DESTINATION dead");
> end Destination;
> end Station;
>
> begin
> Chicago_Ptr.Send_Ptr := new Chicago_Ptr.Send_Type;
> Chicago_Ptr.Receive_Ptr := new Chicago_Ptr.Receive_Type;
> end Circularity;
>
>
> The output is:
> ./circularity
> DESTINATION alive
> CHICAGO_SEND alive
> CHICAGO_RECEIVE alive
> CHICAGO pings the Destination
> DESTINATION: Received signal
> CHICAGO received an acknowledgement
> CHICAGO pings the Destination
> DESTINATION: Received signal
> CHICAGO received an acknowledgement
> CHICAGO_SEND dead
>
> The program does not exit; of course, the tasks CHICAGO_RECEIVE and
> DESTINATION are still waiting for input. Hence the program behaves as
> expected, at least for me.
>
> --
> Ludovic Brenta.

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