Debugging

1
Debugging

• Types of Errors
• Debugging Strategies

2
Agenda

• Administrivia
• Course Information
• Lab 1 Turnin Again
• Homework 1
• Lecture
• Debugging

3
Definitions

• Error
• Physical mistake in the code
• Fault
• Manifestation of the error
• Failure
• Inability of the program to behave in the
  intended manner
• Debugging
• Finding and fixing error

4
Errors

• Classification of Errors
• Structural
• Data
• Functionality as implemented
• Implementation
• Integration
• Functional requirements
• Test definition or execution
• System, software architecture
• Other

5
Thoughts on Errors
80 of errors are found in 20 of a projects
routines Endres 1975, Gremillions 1984, Boehm
1987

• Scope of most errors is fairly limited
• Many errors are outside the domain of
  construction
• Most implementation errors are the programmers
  fault
• Clerical errors are common source of problems
• Misunderstanding the design is a recurring theme
• Most errors are easy to fix
• Industry average 15 50 errors/1000 lines of
  code

6
Debugging

• Process of identifying root cause of an error
  and correcting it
• May occupy up to 50 of project time
• Is not a quality improvement measure
• Is a skill to be learned/practiced/improved

7
Debugging Techniques

• Use a tool
• gdb, visual debugger
• Step through your program
• Insert printfs
• Use assertions to predict intended behavior
• Look at specific paths of execution

8
Finding Easy Bugs (K P)

• Look for familiar patterns
• Examine the most recent change
• Dont make the same mistake twice
• Debug it now, not later
• Get a stack trace
• Read before typing
• Explain your code to someone else

9
Tracking Down Hard Bugs (KP)

• Make the bug reproducible
• Divide and conquer
• Study the pattern in failing examples
• Display output to localize search
• Write self-checking code
• Write a log file
• Draw a picture
• Use tools
• Keep records