High Integrity Software

Jack Ganssle writes "High Integrity Software: the title alone got me interested in this book by John Barnes. Subtitled "The SPARK Approach to Safety and Security," the book is a description of the SPARK programming language's syntax and rationale. The very first page quotes C.A.R Hoare's famous and profound statement:'There are two ways of constructing a software design. One way is to make it so simple that there are obviously no deficiencies. And the other way is to make it so complicated that there are no obvious deficiencies.' This meme has always rung true, and is one measure I use when looking at the quality of code. It's the basis of the SPARK philosophy." Read on for more of Ganssle's review of the book, and some more on the SPARK language. High Integrity Software author John Barnes pages 448 publisher Addison-Wesley rating 8 reviewer Jack Ganssle ISBN 0321136160 summary The book describes a language that insures programs are inherently correct.

What is SPARK? It's a language, a subset of Ada that will run on any Ada compiler, with extensions that automated tools can analyze to prove the correctness of programs. As the author says in his Preface, "I would like my programs to work without spending ages debugging the wretched things." SPARK is designed to minimize debugging time (which averages 50% of a project's duration in most cases).

SPARK relies on Ada's idea of "programming by contract," which separates the ability to describe a software interface (the contract) from its implementation (the code). This permits each to be compiled and analyzed separately.

It specifically attempts to insure the program is correct as built, in contrast to modern Agile methods which stress cranking a lot of code fast and then making it work via testing. Though Agility is appealing in some areas, I believe that, especially for safety critical system, focus on careful design and implementation beats a code-centric view hands down.

SPARK mandates adding numerous instrumentation constructs to the code for the sake of analysis. An example from the book:

Procedure Add(X: In Integer);

--#global in out Total;

--#post Total=Total~ + X;

--#pre X > 0;

The procedure definition statement is pure Ada, but the following three statements SPARK-specific tags. The first tells the analysis tool that the only global used is Total, and that it's both an input and output variable. The next tag tells the tool how the procedure will use and modify Total. Finally a precondition is specified for the passed argument X.

Wow! Sounds like a TON of work! Not only do we have to write all of the normal code, we're also constructing an almost parallel pseudo-execution stream for the analysis tool. But isn't this what we do (much more crudely) when building unit tests? In effect we're putting the system specification into the code, in a clear manner that the tool can use to automatically check against the code. What a powerful and interesting idea!

And it's similar to some approaches we already use, like strong typing and function prototyping (though God knows C mandates nothing and encourages any level of software anarchy).

There's no dynamic memory usage in SPARK -- not that malloc() is inherently evil, but because use of those sorts of constructs can't be automatically analyzed. SPARK's philosophy is one of provable correctness. Again -- WOW!

SPARK isn't perfect, of course. It's possible for a code terrorist to cheat the language, defining, for instance, that all globals are used everywhere as in and out parameters. A good program of code inspections would serve as a valuable deterrent to lazy abuse. And it is very wordy; in some cases the excess of instrumentation seems to make the software less readable. Yet SPARK is still concise compared to, say, the specifications document. Where C allows a starkness that makes code incomprehensible, SPARK lies in a domain between absolute computerese and some level of embedded specification.

The book has some flaws: it assumes the reader knows Ada, or can at least stumble through the language. That's not a valid assumption any more. And I'd like to see real-life examples of SPARK's successes, though there's more info on that at www.sparkada.com.

I found myself making hundreds of comments and annotations in the book, underlining powerful points and turning down corners of pages I wanted to reread and think about more deeply.

A great deal of the book covers SPARK's syntax and the use of the automated analysis tools. If you're not planning to actually use the language, your eyes may glaze over in these chapters. But Part 1 of the tome, the first 80 pages which describes the philosophy and fundamentals of the language and the tools, is breathtaking. I'd love to see Mr. Barnes publish just this section as a manifesto of sorts, a document for advocates of great software to rally around. For I fear the real issue facing software development today is a focus on code ueber alles, versus creating provably correct code from the outset.

You can purchase High Integrity Software from bn.com. Slashdot welcomes readers' book reviews -- to see your own review here, carefully read the book review guidelines, then visit the submission page.

Comments

References to the story

[SamiousHaze]

In General, if you want info, RTFA. However for those of you who just want some links to check things out quickly:

Hereis a PDF that contains samble chapters of the book reviewed.

Also from the same site is the following text and links for those of you wanting "real world examples":

"Industrial Experience with SPARK (PDF 234kb) Dr. Roderick Chapman, Praxis Critical Systems Limted. Presented at ACM SigAda 2000 conference. This paper discusses three large, real-world projects (C130J, SHOLIS and the MULTOS CA) where SPARK has made a contribution to meeting stringent software engineering standards. "

Eurofighter

[Anonymous Coward]

SPARK is used heavily in the safety critical software in the Eurofighter amongst other projects. It is a complete pain to type all of the annotation, takes forever to run the tool and it very rarely comes up with any real problems in the code. I would pay good money never to have to go near it again. It was used to meet contractual requirements, not engineering requirements.

One neat trick is to generate a large proportion of the annotation from the output error messages. Sort of defeats using the tool though but since it doesn't find much anyway the time freed up can be used to do some real testing.

Re:question

[deepchasm]

Yes, they do.

SPARK Ada came from Praxis Critical Systems. (http://www.praxis-cs.co.uk/). Go take a look. You can read about how SPARK Ada is used in things like aircraft, and (increasingly) in the automotive industry.

Re:public class interfaces

[William Tanksley]

Design By Contract was not invented by SPARK; the name was popularised by Bertrand Meyer, who added it to his "Eiffel" language.

Anyhow, DBC is totally distinct from object orientation. In DBC, each component in your software comes with a "contract" that states "if I am called when the _preconditions_ are true, I promise that after I run the _postconditions_ will be true."

The preconditions and postconditions are a group of logical statements, hopefully ones which are useful to your program :-).

Let me give a little example.

function: sqrt( x )
preconditions:
- integer (x)
- positive (x)
postconditions:
- result > 0
- result * result x

Do you see what's happening there? Without knowing /anything/ about how sqrt is computed, you can use it in powerful ways. Preconditions and postconditions can't always be as informative as the ones above are (the ones above define everything about the integer sqrt function), but they can give useful information.

Adding in object orientation support to DBC is a little more complex, but I won't go into that unless asked.

Traditional DBC systems, including Eiffel, couldn't verify your contracts, so most of them would translate the contracts into code, and include that code in the executable; if a contract failed, the code would throw an exception or otherwise fail. SPARK is interesting because it can detect contract failures without running the code; it can also detect when your contracts fail to promise enough.

-Billy

Re:Proving sqrt() correctness?

[Suslik]

To prove the correctness of an (Integer) square root routine you would specify something like:

procedure Sqrt(X : in Integer; Y : out Integer)
--# derives Y from X;
--# pre X >= 0;
--# post (Y*Y <= X) and ((Y+1)*(Y+1) > X);

i.e. Y, your sqrt, is no more than X when squared, but increase it by 1 and it is more than X. You require X to be non-negative.

Assuming that your implementation implements an initial guess at Y and then repeatedly increments it, you would specify a loop invariant that shows that your guess at Y (say 'Z') is such that (Z+1)*(Z+1) For more information on what's practicable in a customer-specified system, read the peer-reviewed publications...

Disclaimer: SPARK hacker for 6 years