14.

We rigorously define the concept of a bug, explain why a specification is crucial for controlling software bugs, highlight the differences between a specification and an implementation, and discuss a cost-effective means for developing specifications.

What Is a Bug?

This book is about debugging software. In order to discuss the act of debugging, it is important to define precisely what does and does not constitute a bug.

For the purposes of this text, I will define a bug as "program behavior that deviates from its specification." This definition does not include:

• Poor performance, unless a threshold level of performance is included as part of the specification.

• An awkward or inefficient user interface. Although user interface design is an important topic, it's not the subject of this book.

• Lack of features, lack of a particular useful feature, or lack of any feature not included in the program specification (even if it was intended to be in the specification).

The lack-of-features category illustrates an important aspect of our definition of bugs: they are inextricably linked to a program specification. If there is no program specification, then there literally are no bugs. To be sure, there are some generally accepted behavioral qualities expected from any software, e.g., it won't crash, it won't run forever without producing output, etc. Properties like these are implicitly part of the specification of any software. But these properties are the exception; most behavior must be explicitly specified. Because specifications define behavior which defines bugs, we had better discuss what constitutes a specification.

Tip 

The simplest definition of a bug is "program behavior that deviates from its specification."

Intuitively, a program specification is a description of the behavior of a program. Therefore, having some kind of specification is essential to determining when the system is misbehaving. What form would we like this specification to take? First, let's consider how traditional software engineering answers this question.

Tip 

Bugs and program specifications are inextricably linked. Since specifications define behavior, without a specification, bugs are not possible.