Embedded Muse 80 Copyright 2003 TGG January 25, 2003

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CONTENTS:
- Editor�s Notes
- A Fascinating Study
- Jobs
- Thought for the Week
- About The Embedded Muse

Editor�s Notes

I wrote a three-article series in Embedded Systems Programming about watchdog timers. The first appeared this month and has generated a tremendous amount of reader feedback. So, if watchdogs and/or reliable firmware are important to you, check it out in the print edition of the magazine or at www.embedded.com.

A Fascinating Study

Researchers at Stanford have just released a paper detailing their use of automated tools to look for redundant code in 1.6 million lines of Linux. �Redundant� is defined as:
- Idempotent operations (like assigning a variable to itself)
- Values assigned to variables that are not subsequently used
- Dead code
- Redundant conditionals

(The authors did eliminate the oddball cases where these operations are important, like in setting memory mapped I/O.)

They found that redundancies, even when harmless, strongly correlate with bugs. Even when the extra code causes no problems, odds are high that other, real, errors will be found within a few lines of the redundant operations.

In their words �The results indicate that (1) files with redundant errors are good audit candidates and (2) redundancy correlates with confused programmers who will probably make a series of mistakes.�

Block-copied code is often suspect, as the developer neglects to change things needed for the code�s new use. Another common problem area: error handlers, which are tough to test, and are, in data I�ve gathered, a huge source of problems in deployed systems.

The authors note that their use of lint has long produced warnings about unused variables and return codes, which they�ve always treated as harmless stylistic issues. Now it�s clear that lint is indeed signaling something that may be critically important.

The study makes me wonder if compilers that optimize out dead code to reduce memory needs aren�t in fact doing us a disservice. Perhaps they should error and exit instead.

Get the study at www.stanford.edu/~engler/p401-xie.pdf.

Jobs

Thought for the Week

The first 90% of the code accounts for the first 90% of the development time. The remaining 10% of the code accounts for the other 90% of the development time.