Archive for November, 2009

A nail for a fuse

Friday, November 27th, 2009 Michael Barr

If I were to search my soul, I’d have to admit that the use of assertions has helped me more than any other single technique, even more than my favorite state machines. But, the use of assertions, simple as they are, is surrounded by so many misconceptions and misunderstandings that it’s difficult to know where to start. The discussion around the recent Jack Ganssle’s article “The Use of Assertions” shows many of the misunderstandings.

I suppose that the main difficulties in understanding assertions lay in the fact that while the implementation of assertions is trivial, the effective use of assertions requires a paradigm shift in the view of software construction and the nature of software errors in particular.

Perhaps the most important point to understand about assertions is that they neither handle nor prevent errors, in the same way as fuses in electrical circuits don’t prevent accidents or abuse. In fact, a fuse is an intentionally introduced weak spot in the circuit that is designed to fail sooner than anything else, so actually the whole circuit with a fuse is less robust than without it.

I believe that the analogy between assertions and fuses (which, by the way has been originally proposed by Niall Murphy in a private conversation at one of the Embedded Systems Conferences) is accurate and valuable, because it helps in making the paradigm shift in understanding many aspects of using assertions. Here I’d only like to elaborate just two aspects.

First, the analogy to fuses correctly suggests that assertions work best in the “weakest” spots. Such “weak spots” are often found at the interface between components (e.g., preconditions in a function) but there are many others. The best assertions are those that protect the most of the system. In other words, the best assertions catch errors that would have the most impact on the rest of the system.

The second important implication of the fuse analogy is the issue of disabling assertions in the production code. As the comments to the aforementioned article suggest, most engineers tend to disable assertions before shipping the code, especially in the safety critical products. I believe that this is exactly backwards.

I understand that the standard “assert.h” header file is designed to use assertions only in a debug build, so the macro assert() compiles to nothing when the symbol NDEBUG is defined. I strongly suggest rethinking this philosophy, because disabling assertions in the release configuration is like using nails, paper clips, or coins for fuses. Just imagine finding a nail in place of a fuse in a hospital’s operating room or in a dashboard of an airliner? What would you think of this sort of “repairs”?

Yet, by disabling assertions in our code we do exactly this.

I believe it is very important to understand that assertions have a very important role to play, especially in the filed and especially in the mission-critical systems, because they add additional safety layer in the software. Perhaps the biggest fallacy of our profession is the na├»ve optimism that our software will not fail. In a nutshell we somehow believe that when we stop checking for errors, they will stop occurring. After all–we don’t see them anymore. But this is not how computer systems work. An error, no matter how small, can cause catastrophic failure. With software, there are no “small” errors. Our software is either in complete control over the machine or it isn’t. Assertions help us know when we lose control.

So what do I suggest we do when the assertion fires in the filed? The proper course of action requires a lot of thinking and sometimes a lot of work. In safety-critical systems software failure should be part of the fault-tree analysis. Sometimes, reaching a fail-safe state requires some redundancy in the hardware. In any case, the assertion failures should be extensively tested.

But this is really the best we can do.