White Box Techniques

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White Box Testing is a general label applied to design techniques used when the tester/test designer knows and takes into account the software's code, design, and other aspects of its structure. This is not a properly a skill. It is a label for techniques that incorporate many skills within the broader skill of designing meaningful tests.

A context where I used this skill:
When I was a developer-‐‑tester (now called an Agile developer), I iteratively built design, code, and test information. Yes I did the code and then tested, so I was not pure Agile TDD, but every line of code had tests for it. This gave me statement coverage.

Also, each "IF" test had a truth table from which I build more test cases. This gave me decision and branch coverage. Well with the coverage based white box testing, my code had few bugs than other developers and I was invited back during system integration to "help" when other developers were not (I had job security).

How I'd recommend someone learn this skill:
Understand what white box testing means as a technique, and then learn how to either read or write code. Here are the classic white box code coverage techniques:
➔ Statement Testing
➔ Branch Testing
➔ Decision Testing
➔ Branch Condition Testing
➔ Branch Condition Combination Testing
➔ Modified Condition Decision Coverage (MCDC) Testing
➔ Data Flow Testing

Each of these have coverage measures associated with them. There are subtle little problems in coverage measurement and a coverage can "mislead" and miss bugs. For example consider the line of code

x = y/z

I can test this with setting y = 4 and z = 2. My answer is 2 (this is the oracle), but consider there is still a bug here possibly. What if I set z = 0? What does the computer do with something that is math undefined? Is there a "IF" test or some language feature to protect z from being zero? So there is more to this skill-‐‑technique than just coverage.

Additional resources:
"Software Test Attacks to Break Embedded and Mobile Software", Jon Hagar, 2013 Ch 2 developer attacks, CRC press

By: Peter Walen

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