Concurrency and interfaces

Hi everybody,

I try to explore the Ada05 interfaces in the context of concurrency. My original question was to explore different solutions for the following problem:

How to connect a task (T2) to other tasks (T11 or T12) having different functionalities depending on a given context.

I abstracted the following code :

package Test_Interfaces is

type Int1 is synchronized interface;
procedure Init (I1 : in out Int1) is abstract;

type Int1_Ptr is access Int1'Class;

type Int2 is synchronized interface;
procedure Process(I1 : in out Int2) is abstract;

task type T11 is new Int1 with
entry Init;
end T11;

task type T12 is new Int1 and Int2 with
entry Init;
entry Process; -- Some additional functionalities
end T12;

task type T2 is
entry Connect (T : Int1'Class);
end T2;
end Test_Interfaces;

with Text_Io;
use Text_Io;

package body Test_Interfaces is

task body T11 is
begin
accept Init do
Put ("Init T11"); New_Line;
end Init;
loop
delay 0.5;
Put ("Process T11"); New_Line;
end loop;
end T11;

task body T12 is
begin
accept Init do
Put ("Init T12"); New_Line;
end Init;
loop
select
delay 0.1;
Put ("Process T12"); New_Line;
or
accept Process; -- some extra processing
end select;
end loop;
end T12;

task body T2 is
Int_Task : Int1_Ptr;
begin
accept Connect (T : Int1'Class) do
Put("External task connection"); New_Line;
Int_Task := new Int1'Class'(T);
end Connect;
Int_Task.Init;
end T2;

end Test_Interfaces;

Then the main program:

with Test_Interfaces;
use Test_Interfaces;

procedure test_tasking is Task11 : T11;
Task12 : T12;
Task2 : T2;
begin
Task2.Connect (Task12);
end test_tasking;

In this case, the program is blocked in the Connect call.

When Task11 is used instead Task12, I get a PROGRAM ERROR : unhanded signal. I dont understand why these two situations are not symmetrical and give rise to different behaviors.

When I replace the declaration of Int_Task in task body T2 by Int_Task : access Int1'class; the builder raises the following error:

undefined reference to 'test_interface__t2___master'

When I replace Int1'class by Int1_Ptr :

....
task type T2 is
entry Connect (T : Int1_Ptr);
end T2;
....

task body T2 is
Int_Task : Int1_Ptr;
begin
accept Connect (T : Int1_Ptr) do
Put("External task connection"); New_Line;
Int_Task := T;
end Connect;
Int_Task.Init;
end T2;

....
with Test_Interfaces;
use Test_Interfaces;

procedure test_tasking is Task11 : Int1_Ptr := new T11;
Task12 : Int1_Ptr := new T12;
Task2 : T2;
begin
Task2.Connect (Task11); -- or Task2.Connect(Task12);
end test_tasking;

all runs correctly.

I suspect the conjunction of a faulty construct at the level of the affectation Int_Task := new Int1'Class'(T); incorrectly handled by the compiler but i don't really understand why this construct is illegal.

I conclude that there is no way to hide the use of the pointer Int1_Ptr and probably no way to do that statically.

Any idea?

Philippe Tarroux
philippe.tarroux@xxxxxxxx

OS : WXP
Ada compiler : gcc 4.1.3 20070403 for GNAT GPL 2007 (20070402)






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