Debugging by Thinking

7.7 Program slice strategy 195

7.7 Program slice strategy

7.7.1

Program slice assumptions

The program slice strategy assumes that a tool is available that will perform

control and data-flow analysis on the application and provide the slicing

information. Doing this analysis by hand can be quite tedious. If the vari-

able that has an incorrect value is a local variable, intraprocedural analysis

may suffice. Otherwise, interprocedural analysis is needed.

A program slice of a program, with respect to a specific location in a

program and a set of variables referenced at that location, is the set of state-

ments and predicates in the program that may affect the value of the vari-

ables at that location.

Intraprocedural analysis is performed within the scope of an individual

procedure (C++ function, Java method), lnterprocedural analysis is per-

formed across the boundaries of individual procedures (C++ functions, Java

methods) and is performed on all of the procedures in an executable pro-

gram. It can sometimes make sense to perform interprocedural analyses

within an intermediate level, such as a library or a Java package.

7.7.2

Program slice control structure

The next code sample shows the control structure for the program slice

strategy.

Identify a variable that has an incorrect value

Generate a slice of the application that shows all the

calculations that contributed to the value of

that variable, considering both control flow

and data flow

Do while there are more unexamined contributing

variables

Observe the values taken on by one of the variables

If any of the values of this variable are invalid,

Then

repeat the slicing process on this variable

End-if

End-do

The code segments that have all valid input values but

which contribute invalid values are the cause of the bug

I Chapter 7