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WHITE-BOX TESTING
White-box testing, sometimes called glass-box testing, is a test case design method
that uses the control structure of the procedural design to derive test cases. Using
white-boxtesting methods, the software engineer can derive test cases that (1) guarantee
that all independent paths within a module have been exercised at least once,
(2) exercise all logical decisions ontheir true and false sides, (3) execute all loops at
their boundaries and within their operational bounds, and (4) exercise internal data
structures to ensure their validity.
A reasonable questionmight be posed at this juncture: "Why spend time and energy
worrying about (and testing) logical minutiae when we might better expend effort
It is not possible to
exhaustively test every
programpath because
the number of paths is
simply too large.
White-box tests can be
designed only after a
component-level design
(or source code)
exists. The logical
details of the program
must beavailable.
CHAPTER 17 SOFTWARE TESTING TECHNIQUES
ensuring that program requirements have been met?" Stated another way, why don't
we spend all of our energy on black-box tests? The answer lies in thenature of software
defects (e.g., [JON81]):
• Logic errors and incorrect assumptions are inversely proportional to the probability
that a program path will be executed. Errors tend to creep intoour work
when we design and implement function, conditions, or control that are out
of the mainstream. Everyday processing tends to be well understood (and
well scrutinized), while "special case"processing tends to fall into the cracks.
• We often believe that a logical path is not likely to be executed when, in fact, it
may be executed on a regular basis. The logical flow of a program issometimes
counterintuitive, meaning that our unconscious assumptions about
flow of control and data may lead us to make design errors that are uncovered
only once path testing commences.
•...