Semaphores in Ada

Question

I have been given the following code and asked to implement a semaphore.

with Ada.Text_IO; use Ada.Text_IO;
with Id_Dispenser;
with Semaphores; use Semaphores;
procedure Philos is
 No_of_Philos : constant Positive := 5;
 Meditation : constant Duration := 0.0;
 type Table_Ix is mod No_of_Philos;
 Forks : array (Table_Ix) of Binary_Semaphore (Initially_Available => True);
 package Index_Dispenser is new Id_Dispenser (Element => Table_Ix);
 use Index_Dispenser;
 task type Philo;
 task body Philo is
  Philo_Nr : Table_Ix;
 begin
  Dispenser.Draw_Id (Id => Philo_Nr);
  Put_Line (“Philosopher” & Table_Ix’Image (Philo_Nr) & “ looks for forks.”);
  Forks (Philo_Nr).Wait; delay Meditation; Forks (Philo_Nr + 1).Wait;
  Put_Line (“Philosopher” & Table_Ix’Image (Philo_Nr) & “ eats.”);
  Forks (Philo_Nr).Signal; Forks (Philo_Nr + 1).Signal;
  Put_Line (“Philosopher” & Table_Ix’Image (Philo_Nr) & “ dropped forks.”);
 end Philo;
 Table : array (Table_Ix) of Philo; pragma Unreferenced (Table);
begin
 null;
end Philos;

The task requires a Semaphores package and a package Id_Dispenser. I am very new to Ada, but what is meant by a package? Does this mean both specification and body or just one, and how shall I go about implementing this?

Answer 1

What is meant by a package?

As suggested here, an Ada package provides for modular programming to support encapsulation.

The task requires a Semaphores package.

To this end, Ada offers protected types "which encapsulate and provide synchronized access to the private data of objects of the type without the introduction of an additional task." Additional discussion and examples may be found here.

In the context of the dining philosophers problem, this complete example is worth reading; it is included in the GNAT community edition in share/examples/gnat/simple_project. In particular, the package Chop exports protected type Stick; each instance of Stick has an entry Pick_Up and procedure Put_Down. The package Room can then hold an array of utensils available to the diners, corresponding to Forks in your fragment.

Sticks : array (Table_Type) of Chop.Stick;

Answer 2

As to "what is a package", check out the Packages section in the Ada Wikibook.

All packages have a specification part. Most also have a body (and if the spec promises one, for example by declaring a subprogram, there must actually be one).

You can find discussion of implementing a semaphore in the Wikibook section on Tasking, including code for a Semaphore_Protected_Type.

In your case, you need

package Semaphores is
   protected type Binary_Semaphore (Initially_Available : Boolean) is
      entry Wait;
      procedure Signal;
   private
      Available : Boolean := Initially_Available;
   end Binary_Semaphore;
   ...
end Semaphores;