6.4. Testing and Debugging

Correct programs behave as we intend them to behave. Validation is the process of building our confidence in correct program behavior.

6.4.1. Validation

There are many ways to increase that confidence. Social methods, formal methods, and testing are three. The latter is our main focus, but let’s first consider the other two.

Social methods involve developing programs with other people, relying on their assistance to improve correctness. Some good techniques include the following:

  • Code walkthrough. In the walkthrough approach, the programmer presents the documentation and code to a reviewing team, and the team gives comments. This is an informal process. The focus is on the code rather than the coder, so hurt feelings are easier to avoid. However, the team may not get as much assurance that the code is correct.

  • Code inspection. Here, the review team drives the code review process. Some, though not necessarily very much, team preparation beforehand is useful. They define goals for the review process and interact with the coder(s) to understand where there may be quality problems. Again, making the process as blameless as possible is important.

  • Pair programming. The most informal approach to code review is through pair programming, in which code is developed by a pair of engineers: the driver who writes the code, and the observer who watches. The role of the observer is be a critic, to think about potential errors, and to help navigate larger design issues. It’s usually better to have the observer be the engineer with the greater experience with the coding task at hand. The observer reviews the code, serving as the devil’s advocate that the driver must convince. When the pair is developing specifications, the observer thinks about how to make specs clearer or shorter. Pair programming has other benefits. It is often more fun and educational to work with a partner, and it helps focus both partners on the task. If you are just starting to work with another programmer, pair programming is a good way to understand how your partner thinks and to establish common vocabulary. It is a good idea for partners to trade off roles, too.

These social techniques for code review can be remarkably effective. In one study conducted at IBM (Jones, 1991), code inspection found 65% of the known coding errors and 25% of the known documentation errors, whereas testing found only 20% of the coding errors and none of the documentation errors. The code inspection process may be more effective than walkthroughs. One study (Fagan, 1976) found that code inspections resulted in code with 38% fewer failures, compared to code walkthroughs.

Thorough code review can be expensive, however. Jones found that preparing for code inspection took one hour per 150 lines of code, and the actual inspection covered 75 lines of code per hour. Having up to three people on the inspection team improves the quality of inspection; beyond that, more inspectors doesn’t seem to help. Spending a lot of time preparing for inspection did not seem to be useful, either. Perhaps this is because much of the value of inspection lies in the interaction with the coders.

Formal methods use the power of mathematics and logic to validate program behavior. Verification uses the program code and its specifications to construct a proof that the program behaves correctly on all possible inputs. There are research tools available to help with program verification, often based on automated theorem provers, as well as research languages that are designed for program verification. Verification tends to be expensive and to require thinking carefully about and deeply understanding the code to be verified. So in practice, it tends to be applied to code that is important and relatively short. Verification is particularly valuable for critical systems where testing is less effective. Because their execution is not deterministic, concurrent programs are hard to test, and sometimes subtle bugs can only be found by attempting to verify the code formally. In fact, tools to help prove programs correct have been getting increasingly effective and some large systems have been fully verified, including compilers, processors and processor emulators, and key pieces of operating systems.

Testing involves actually executing the program on sample inputs to see whether the behavior is as expected. By comparing the actual results of the program with the expected results, we find out whether the program really works on the particular inputs we try it on. Testing can never provide the absolute guarantees that formal methods do, but it is significantly easier and cheaper to do. It is also the validation methodology with which you are probably most familiar. Testing is a good, cost-effective way of building confidence in correct program behavior.

6.4.2. Debugging

When testing reveals an error, we usually say that the program is “buggy”. But the word “bug” suggests something that wandered into a program. Better terminology would be that there are

  • faults, which are the result of human errors in software systems, and

  • failures, which are violations of requirements.

Some faults might never appear to an end user of a system, but failures are those faults that do. A fault might result because an implementation doesn’t match design, or a design doesn’t match the requirements.

Debugging is the process of discovering and fixing faults. Testing clearly is the “discovery” part, but fixing can be more complicated. Debugging can be a task that takes even more time than an original implementation itself! So you would do well to make it easy to debug your programs from the start. Write good specifications for each function. Document the AF and RI for each data abstraction. Keep modules small, and test them independently.

Inevitably, though, you will discover faults in your programs. When you do, approach them as a scientist by employing the scientific method:

  • evaluate the data that are available;

  • formulate a hypothesis that might explain the data;

  • design a repeatable experiment to test that hypothesis; and

  • use the result of that experiment to refine or refute your hypothesis.

Often the crux of this process is finding the simplest, smallest input that triggers a fault. That’s not usually the original input for which we discover a fault. So some initial experimentation might be needed to find a minimal test case.

Never be afraid to write additional code, even a lot of additional code, to help you find faults. Functions like to_string or format can be invaluable in understanding computations, so writing them up front before any faults are detected is completely worthwhile.

When you do discover the source of a fault, be extra careful in fixing it. It is tempting to slap a quick fix into the code and move on. This is quite dangerous. Far too often, fixing a fault just introduces a new (and unknown) fault! If a bug is difficult to find, it is often because the program logic is complex and hard to reason about. Think carefully about why the fault could have been introduced in the first place, and about how you might prevent similar faults in the future.