SPARK (programming language)

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SPARK
Influenced by Ada, HAL/S

SPARK is a formally-defined computer programming language based on the Ada programming language, intended to be secure and to support the development of high integrity software used in applications and systems where predictable and highly reliable operation is essential either for reasons of safety (e.g., avionics in aircraft/spacecraft, or medical systems and process control software in nuclear powerplants) or for business integrity (for example financial software for banking and insurance companies).

It is claimed that "for most high integrity systems today the topmost implementation requirement is often the use of Ada/SPARK for critical system components"[citation needed].

There are two versions of the SPARK language; one based on Ada 83 and the other on Ada 95. The SPARK language consists of a highly restricted, well-defined subset of the Ada language that uses annotated meta information (in the form of Ada comments) that describe desired component behavior and individual runtime requirements, thereby optionally facilitating mandatory use of Design by Contract principles to accurately formalize and validate expected runtime behavior.

Because the annotations are in commentary, all SPARK programs are generally also valid Ada programs and can be compiled by an appropriate Ada compiler. The most recent revision of the implementation, RavenSPARK, includes the Ravenscar tasking profile which aims to support concurrency in high integrity applications. The formal, unambiguous, definition of SPARK allows and encourages a variety of static analysis techniques to be applied to SPARK programs.

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[edit] Technical overview

SPARK aims to exploit the strengths of Ada while trying to eliminate all its potential ambiguities and insecurities. SPARK programs are by design meant to be unambiguous, and their behavior is required to be unaffected by the choice of Ada compiler. These goals are achieved partly by omitting some of Ada's more problematic features (such as unrestricted parallel tasking) and partly by introducing annotations or "formal comments" which encode the application designer's intentions and requirements for certain components of a program.

The combination of these approaches is meant to allow SPARK to meet its design objectives, which are:

  • logical soundness
  • rigorous formal definition
  • simple semantics
  • security
  • expressive power
  • verifiability
  • bounded resource (space and time) requirements.
  • minimal runtime system requirements

SPARK being an 'annotated subset' of Ada, programs written in SPARK can be compiled by any Ada compiler ('subset' of Ada implies that not all Ada features may be used).

'Annotated' means that certain annotations in form of Ada comments (i.e., ignored by the Ada compiler) are evaluated by an additional tool called the 'SPARK Examiner' which is meant to ensure strict enforcement of the requirements expressed via the aforementioned annotations to analyze the corresponding SPARK/Ada program for its correctness before passing it to an Ada compiler to compile the source code.

[edit] Tool support

Praxis SPARK Toolset
Developed by Praxis High Integrity Systems
Latest release 7.5 / June 2007
OS Linux, Microsoft Windows, OpenVMS, Solaris, Mac OS X
Genre Programming language
License The Language Definition and Formal Semantics are freely available, with changes controlled by Praxis HIS. The SPARK Toolset is commercially licensed, with a capacity-limited version included in the book by John Barnes and available for download via http://www.sparkada.com
Website http://www.sparkada.com/

The 'SPARK Examiner' (part of the 'SPARK Toolset') performs two kinds of static analysis. The first, made up of language conformance checks and flow analysis, checks that the program is "well-formed" and is consistent with the design information included in its annotations. This stage can be incorporated into the coding phase of the development process. After these checks the source is known to be free from erroneous behaviour and free from conditional and unconditional data flow errors (e.g., use of uninitialised data) on a system-wide basis (including abstract state in package bodies).

The second, optional, analysis is verification: showing by proof that the SPARK program has certain specified properties. The most straightforward is a proof that the code is exception free; this adds the Constraint_Error exception to the list of possible errors eliminated by SPARK. This proof can also be used to demonstrate, unequivocally, that the code maintains important safety or security properties. It can also be used to show conformance with some suitable specification.

[edit] Annotation examples

Consider the Ada subprogram specification below:

procedure Increment (X : in out Counter_Type);

What does this subprogram actually do? In pure Ada, it could do virtually anything — it might increment the X by one or one thousand; or it might set some global counter to X and return the original value of the counter in X; or it might do absolutely nothing with X at all.

With SPARK, annotations are added to the code to provide additional information regarding what a subprogram actually does. For example, we may alter the above specification to say:

procedure Increment (X : in out Counter_Type);
--# derives X from X;

or

procedure Increment (X : in out Counter_Type);
--# global Count;
--# derives
--#   Count from Count, X &
--#   X from ;

The first of these specifications tells us that the Increment procedure does not update or read from any global variables and that the only data item used in calculating the new value of X is X itself. The second set of annotations tells us that Increment will use some global variable called "Count" in the same package as Increment and that the exported value of Count is dependent on the imported values of Count and X, but that exported value of X does not depend on any variables at all — it will be derived simply from constant data.

If the Examiner is then run on the specification and corresponding body of a subprogram, it will analyse the body of the subprogram to build up a model of the information flow. This model is then compared against that which has been specified by the annotations and any discrepancies reported to the user.

We can further extend these specifications by asserting various properties that either need to hold when a subprogram is called (preconditions) or that will hold once execution of the subprogram has completed (postconditions). For example, we could say the following:

procedure Increment (X : in out Counter_Type);
--# derives X from X;
--# pre X < Counter_Type'Last;
--# post X = X~ + 1;

This specification now says that not only is X only derived from itself, but that before Increment is called X must be strictly less than the last possible value of its type and that afterwards X will be equal to the initial value of X plus one — no more and no less.

[edit] Verification Conditions

The Examiner can be requested to generate a set of Verification Conditions or VCs. VCs are used to attempt to establish certain properties hold for a given subprogram. At a minimum, the Examiner will generate VCs attempting to establish that the following run-time errors cannot occur within a subprogram:

  • array index out of range
  • type range violation
  • division by zero
  • numerical overflow.

If a postcondition is added to the specification, the Examiner will also generate VCs that require the user to show that the postcondition will hold for all possible paths through the subprogram.

Discharging these proof obligations is performed using the SPADE Simplifier (an automated theorem prover) and the SPADE Proof Checker (a manual theorem prover, used for those VCs too thorny for the Simplifier to automatically discharge).

[edit] History

The first version of SPARK (based on Ada 83) was produced at the University of Southampton (with UK Ministry of Defence sponsorship) by Bernard Carré and Trevor Jennings. Subsequently the language was progressively extended and refined, first by Program Validation Limited and then by Praxis Critical Systems Limited. In 2004, Praxis Critical Systems Limited changed its name to Praxis High Integrity Systems Limited.

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