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February 14, 1991
Communications of the ACM
Association for Computing Machinery
11 West 42nd St.
New York, NY 10036
The overview article, "Software Safety in Embedded Computer Systems", by Nancy Leveson in the February, 1991, issue of the Communications of the ACM provides some valuable insights into some of the problems of developing safe software for the ever-increasing number of software-intensive online and control systems. I was amazed, however, that I could not find the word redundancy at any place in this relatively long article. Engineered redundancy is the fundamental tool for reducing the errors in communications systems, and also forms the basis for many of the techniques Dr. Leveson recommends for software practise.
While much remains to be done in providing proper specifications for software systems to avoid high-level hazards, there are a number of low-level hazards for which redundancy tools already exist, but which are not currently being used. We refer specifically to programming errors which cannot currently be caught by a compiler's typing system, but can be caught by a proper system of run-time type checking. Reducing these hazards does not require millions of dollars of R&D for "specification languages" or theorem-provers, but simply requiring that existing run-time type-checks always be enabled.
Dr. Leveson does not discuss particular programming languages, but since the Ada language is currently required for virtually all defense-related "mission-critical" systems, it is pertinent to look at the redundancy built into this language. Ada's highly-sophisticated typing system provides redundancy in the form of a "homomorphic computation" that can be performed at compile time to determine if a program type-checks. Furthermore, Ada provides additional run-time redundancy in the form of subrange checks, array bounds checks, pointer dereferencing checks, and variant record (aka union) discriminant checks. Unfortunately, these checks are often not used because 1) the precise specification of types and subtypes in Ada is a tedious and frustrating activity that is often rewarded by poor execution performance and bad software productivity metrics; and 2) the speed of an embedded system is often at such a premium that management and/or the customer requires that the run-time checking be suppressed.
It was intended that sophisticated Ada compilers would be able to optimize redundant run-time checks away, whenever the compiler could prove mathematically that the checks would always be satisfied. Unfortunately, the reliability of optimizing compilers is so suspect, that some certification organizations require that critical software be compiled with optimization turned off (viz. Ada-9X revision request 729). Since the certification organizations do not simultaneously require that run-time checks be turned on, we achieve a most undesirable situation of no optimization and no run-time checking. The tradition of benchmarking with all run-time checking disabled also leads to unrealistic performance expectations, and to the virtual certainty that performance specifications can only be met by turning all run-time checking off.
Even assuming that Ada is used as intended, with full typing and type-checking enabled, Ada fails miserably at reducing the next higher level of hazards--the provable meeting of specifications. Even though one of the goals of Ada was to enable a computer to construct (with human help) mathematical proofs that certain programs could meet certain specifications, the sheer complexity of the Ada language made the achievement of this goal so expensive that it cannot be routinely achieved. The syntax and semantics of Ada are so complex, that ACM Ada Letters is filled with "Dear Ada" letters which illuminate some incredibly abstruse part of the language. The only hope for achieving some level of mathematical certainty about program correctness is to require that the complexity of the language itself not get in the way of understanding the complexity of a program in the language. This means that the language should be amenable to a short denotational semantics, as has been achieved for languages such as ML and Scheme.
In engineering disciplines other than software development, reliability can often be improved by reducing the number of parts to a minimum, since a greater number of parts means that there is more to fail. The KISS rule ("keep it simple, stupid") has been proven over and over to reduce errors, by allowing the engineer to focus on the entire system, rather than becoming overloaded with the complexity of one of its parts. The distressing trend of "reusability" requirements to force the use of software in unfamiliar environments is more likely to decrease, rather than increase, the reliability of the resulting systems.
To summarize: we already have within our grasp many tools to improve reliability, but due to improper organizational constraints, we are often unable or unwilling to use them. With apologies to Pogo, "we have met the enemy, and he is us".
Henry G. Baker, Ph.D.