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Embedded Ada Journey Extras - Ravenscar/SPARK


If your google something about Ada programming language you will encounter yourself with the following terms Ravenscar Profile and SPARK. So what are these terms:

SPARK

It’s a subset of the ADA language specifically designed for engineering high-reliability applications, with strong features to guarantee reliability, safety, and security with formal methods. It uses contracts (pre- and postconditions) to describe the specification of components in a form that is suitable for both static and dynamic verification.

For example, a simple Ada program:

procedure Swap_X_And_Y ( X,Y: in out Integer);

What are the restrictions of the procedure? X and Y can virtually be anything. But in SPARK:

procedure Swap_X_And_Y (X,Y: in out Integer) 
with
  Globals => (In_Out => (X, Y)),
  --  Swap_X_And_Y modifies the global variables X and Y.
  Depends => (X => Y, Y => X);
  --  The final value of X depends only on the initial value of Y 
  --  and the final value of Y depends only on the initial value of X.

In this case it’s explicit that X and Y are global variables and that the value of X will depend only on Y and viceversa.

Ravenscar Profile

As the name suggests, Ravenscar is a profile unlike SPARK that is a programming language. Ravenscar can be seen as a set of restrictions specified as compiler directives. The motivation for Ravenscar profile primarily comes from the need to be able to verify concurrent real-time programs, and to have these programs implemented reliably and efficiently.

The ravenscar profile is based on the following pragmas

pragma Task_Dispatching_Policy (FIFO_Within_Priorities);
pragma Locking_Policy (Ceiling_Locking);
pragma Detect_Blocking;
pragma Restrictions (
                 No_Abort_Statements,
                 No_Dynamic_Attachment,
                 No_Dynamic_Priorities,
                 No_Implicit_Heap_Allocations,
                 No_Local_Protected_Objects,
                 No_Local_Timing_Events,
                 No_Protected_Type_Allocators,
                 No_Relative_Delay,
                 No_Requeue_Statements,
                 No_Select_Statements,
                 No_Specific_Termination_Handlers,
                 No_Task_Allocators,
                 No_Task_Hierarchy,
                 No_Task_Termination,
                 Simple_Barriers,
                 Max_Entry_Queue_Length => 1,
                 Max_Protected_Entries  => 1,
                 Max_Task_Entries       => 0,
                 No_Dependence => Ada.Asynchronous_Task_Control,
                 No_Dependence => Ada.Calendar,
                 No_Dependence => Ada.Execution_Time.Group_Budget,
                 No_Dependence => Ada.Execution_Time.Timers,
                 No_Dependence => Ada.Task_Attributes);

Whenever you want to use the Ravenscar profile you just uses the compiler directive

pragma Profile (Ravenscar);

More information about Ravenscar Profile can be found here

SPARK + Ravenscar

It seems that it’s possible to work with SPARK and Ravenscar at the same time. This post entry on Adacore blog show some functionalities of SPARK 2016 that has support for Ravenscar. Also the is an official documentation of the SPARK Ravenscar Profile given by the SPARK team. These documentation can be found here.


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