Ada scheduling in EDF - scheduled-tasks

Question

For some reason this EDF example doesn't compile. I'm using GNAT I tried it in Windows 8.1, Debian no result.
with Ada.Real_Time, ada.Task_Identification,ada.Dispatching.EDF; use Ada.Real_Time, ada.Task_Identification,ada.Dispatching.EDF;
Procedure exemple_ordon is
Task tache_Periodique;
Task body tache_Periodique is
Echeance: Time_Span := Milliseconds(30); heure: Time;
Begin
heure:= Clock; Set_Deadline(Clock + Echeance);
Loop
heure := heure + Echeance;Delay_Until_And_Set_Deadline(heure,Echeance);
end loop;
End tache_Periodique;
Begin Null; End exemple_ordon ;
The error message:
gnatmake -d -PC:\Users\Awk\default.gpr exemple_ordon.adb
gcc -c -g -O2 -I- -gnatA C:\Users\Awk\exemple_ordon.adb
Edf is not supported in this configuration
compilation abandoned
gnatmake: "C:\Users\Awk\exemple_ordon.adb" compilation error

Answer 1

The message Edf is not supported in this configuration tells the story!
I don’t have access to any supported version of GNAT, but the file a-disedf.ads (the spec of Ada.Dispatching.EDF) in FSF GCC 4.9.0 contains the comment
-- This unit is not implemented in typical GNAT implementations that lie on
-- top of operating systems, because it is infeasible to implement in such
-- environments.
-- If a target environment provides appropriate support for this package,
-- then the Unimplemented_Unit pragma should be removed from this spec and
-- an appropriate body provided.
so it’s possible that AdaCore may provide an implementation for some of the more real-time targets (e.g. VxWorks) for their paying customers.
Do you have access to AdaCore’s academic program (GAP)?
If you really need EDF scheduling, Concurrency in Ada by Burns and Wellings has an example (I have the paperback second edition); you can pick it up cheaply at AbeBooks.
If you ‘just’ need information about general tasking, there are several sources listed at the Ada Information Clearinghouse.
Many people developing high-integrity real time software in Ada use the Ravenscar Profile.