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Why is code so fault tolerant?

December 22nd, 2008

All professional developers eventually encounter a program containing a fault that appears to be so devastating that the program could not possibly perform its intended task, yet the program has been and continues to function more or less as expected.  In my case the program was a cpu instruction set emulator (for a Z80 written in Fortran) that I had written and the fault was a copy-and-past editing mistake that resulted in one of the subtract instructions behaving like the equivalent addition instruction.  The emulator was used to  execute CP/M and various applications (on a minicomputer that did not have any desktop office applications).  I was astounded that CP/M booted and appeared to work correctly, along with various applications (apart from the one exhibiting behavior differences that resulted in me tracking down this fault).

My own continuing experience with apparently fatal faults, in mine and other peoples code, lead me to the conclusion that researchers should be putting most of their effort into trying to figure out why so much software does such a good job of behaving in an acceptable manner while containing so many faults (of various apparent seriousness).  Proving software correctness is an expensive and time consuming dead-end for all but a few specialist applications.

One way for developers to vividly see how robust most software is to random faults is to use a mutation tool on the source.  Such tools introduce faults into code with the aim of checking the thoroughness of a set of test cases.  It is a sobering experience to see how many mutations fail to have any noticeable effect on a programs external behavior.

One group of researchers took this mutation idea to an extreme by changing all less-than operators in for-loops into less-than-or-equals operators. They found that only a handful of the changes prevented the recompiled programs being at all useful to users. While some of the changes produced output that was obviously incorrect, it was still possible to use much of the original functionality.

What is it about the shape of most code that allows it to continue to function in the presence of faults? It is time faults were acknowledged as a fact of life in all actively developed systems and that we should concentrate on developing techniques to help ensure that software containing them continues to behave as intended, rather than the unsophisticated zero-tolerance approach that has held sway for so long.

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