Debugging

Debugging
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Getting to Debugging

Testing:
- The output of a failed test is a symptom and one of the trigger conditions that gives rise to the symptom
Debugging:
- Using the trigger condition to identify and correct the fault

The Debugging Process

Stabilize - Understand the symptom and the trigger condition identified by the test so that the failure can be reproduced.
Localize - Locate the fault.
- Examine the sections of code that are likely to be influenced by the trigger condition.
- Form a hypothesis about the fault.
- Instrument the relevant sections of code.
- Execute the code using the instrumentation.
- Prove or disprove the hypothesis. If proven, go to step 3; otherwise go to step 2.
Correct - Fix the fault.
Verify - Test the fix and run regression tests.
Globalize - Look for and fix similar defects in other parts of the system. Refactor if necessary.

Nerd Humor

(Courtesy of xkcd)

Instrumentation Techniques

Using Debug Code:
- Add temporary output statements that can be used to monitor state information
- Add temporary input statements that can be used to pause the execution
Using a Debugger:
- Use watches to monitor state information
- Set breakpoints to pause the execution
- Execute statements "one at a time" (e.g., step into, step over)