Lesson #12

Basic Testing and Debugging Techniques


In this lesson I will cover the following topics

  1. The purpose of testing.

  2. Black box vs white box testing.

  3. The use of debugging printfs to locate bugs.


The purpose of testing

Once you have gotten your program to compile (sometimes not an easy thing to do) you are ready to try it out. Before releasing your program to the world, you need to test it. Programs rarely work properly the first time. Complex programs never do.

People sometimes think that the purpose of testing is to verify that a program is correct. That is wrong. The purpose of testing is to find bugs. If you run a test on your program and the test does not find a bug, then the test failed. You are a good programmer if your programs always work. You are a good tester if you can cause programs to malfunction. Testing and programming are inherently in conflict. This means that programmers often make poor testers. Programmers don't want to hurt their programs. Yet testing is about making software suffer.

This attitude tends to surprise some people. But it really isn't all that surprising. Doctors use the same approach. If you get tested for HIV infection the test is said to be "positive" if you are infected. Of course that is not good news. You want the test to be negative. Yet from the point of view of testing, the test was a success if it finds infection. It's the same with software testing. If you run a test and the program malfunctions, the test was positive. You found a bug. That is not good news, of course, but it does mean that your testing was worthwhile.

All complex software has bugs. In theory, programmers should greet bug reports with happiness. Each bug report gives them the opportunity to fix the bug and make the program better. The proper response when a user tells you about a bug in your software is, "thank you". Even if the problem is not really a bug in your software but just a misunderstanding on the part of the user, a bug may still be present. Perhaps your user interface is confusing (I'd call that an interface bug). Maybe your documentation is misleading (documentation bug). Maybe the user isn't qualified to use the program (training or marketing bug). No matter what the problem is, it's a bug of some kind and the user's report gives you the opportunity to fix it.

When you test your own software don't pull your punches. Make your tests hard. Make your program squirm. Make it writh. And when your program fails, give a sinister laugh, wring your hands, and then torture it some more. Once you've identified and fixed all the bugs you find this way you'll have a top quality program.

Some years ago the luggage company, American Tourister, ran a TV ad that showed a gorilla throwing a few suitcases around. The gorilla slammed the suitcases against the side of its cage, jumped up and down on them, and did all manner of horrible things to them. The suitcases worked fine afterward. The ad closed by saying, "You might not be this hard on your suitcases, but aren't you glad to know they can take it?" That is exactly what software testing is about. Beat on your program. Make sure it can take it. The end user will be grateful.

Black box testing

You can test software without knowing anything about how it was written. If you know the specification for a piece of software you have all the information you need. If you know what the software is supposed to do, you can construct tests to verify that it actually does it. This approach to testing is called "black box" testing because you treat the software like a black box. You know what it's supposed to produce but you have no idea about its inner workings.

Serious black box testing starts with the specification of the program. Highlight every line in the specification that says things like, "the output is..." or, "the result should be..." Then come up with a test case to exercise that situation. Verify that the output is as it should be. Try inputs that are illegal or out of bounds and see what happens. Try inputs that seem to be legal but that don't make any sense. Try inputs that are perfectly reasonably, just very large. It is sometimes quite easy to get programs to screw up.

You can do a little black box testing on any piece of software you have around. Consider your word processor. Let's check out the section that handles margin settings. First, read the word processor's documentation and verify that all the features it claims it has actually work as documented. In addition, try things like

  1. The normal case: left and right margins at one inch.
  2. How about zero sized margins? What does that do?
  3. How about negative sized margins? Does the program freak out?
  4. Make the left margin so large that it overlaps the right margin.
  5. Make the left margin larger than the width of the document window.
  6. ???

With a little thought you can probably come up with some other interesting cases to try. After you've throughly explored margins, move on to some other feature. It takes time. Good testing is hard and requires thought. Yet if you do test your word processor carefully, you'll probably find a bug before too long. Most major programs have lots of bugs.

The good thing about black box testing is that anybody can do it. Most software vendors release "beta" test versions of their software for ordinary users to exercise. Beta software is usually pretty close to correct—it has already passed the vendor's internal "alpha" testing phase—but it may still have quite a few bugs in it. However, by exposing the software to a large number of people the vendor hopes to find and fix many more bugs. The idea is not to necessarily fix every bug, but to at least fix the ones users are likely to encounter when they use the program.

White box testing

To do white box testing you need access to the original source code of the program itself. The idea here is to design your test cases taking into account how the program is structured and organized. For example, you should come up with enough test cases so that every line of your program gets exercised. Black box testers can't do that because they don't necessarily know about all the various parts of your program. In particular, a black box tester might not know about all the error conditions your program checks. However, with access to the program source, you can see all the error checks. Design test cases to verify that all your different error handling works. (As a side note: testing error handling is hard because it's not always very feasible to generate every possible error condition on demand. Consequently many programs tend to screw up while they are handling errors).

Actually it turns out that just exercising every line of code is not enough. It is possible to write a program such that it has bugs and yet such that a set of test cases that exercise every line works fine. In fact, it is almost impossible to be 100% sure that your program is bug free. Even if your program has never failed, there might still be a bug in it that you haven't encountered yet. Some people dream of the day when it will be possible to prove software correct mathematically. This is not that day.

However, it turns out that a good way to test your software is to use McCabe's metric for software complexity. Count the number of if statements, cases in switch statements, and loops. Compound conditions (that use || or &&) should have each of their subconditions counted separately. Say the total is N. You need to come up with N+1 test cases that together exercise every line in your program and that exercise both directions of every condition. Let's look at the prime number testing example again.

#include <stdio.h>

int main(void)
  int number;
  int i;

  printf("Enter a number: ");
  scanf("%d", &number);

  for (i = 2; i < number; i++) {
    if (number % i == 0) {
      printf("The number, %d, is not prime.\n", number);
      return 1;

  printf("The number, %d, is prime!\n", number);
  return 0;

This program has one loop and one if statement. Thus we need to come up with 2+1 = 3 test cases. Those three cases need to exercise every line of code and every direction of each condition. I believe these three will do it:

  1. The for loop never executes. Choose number <= 2 initially.
  2. The for loop executes, but the if statement never triggers. Make number a prime.
  3. The for loop executes and the if statement triggers. Make number a non-prime.

For this simple program that about does it. Yet even here you can see the virtue of white box testing. A block box tester might not realize the significance of choosing number <= 2.

P.S. The program above has a bug. If you enter 1 you will be told that it is a prime number. However, that is wrong.

McCabe's rule is handy and useful, but it isn't perfect. Remember: no matter how much you test a program you can never be sure that it is bug free. The best you can say is that all known bugs have been fixed. Most likely there are others lurking in your program.

Fixing bugs

Finding bugs is one thing. Fixing them is another. When the program is simple it is often easy to figure out why it doesn't work. However, most of the time it is not clear why the program isn't working. Here's how it usually goes.

  1. You get a bug report about your program. Perhaps you found the bug yourself or perhaps someone on the testing team found it.

  2. If someone other than you found the bug, you should first try to reproduce it. Fixing bugs you can't see is extremely difficult. If you can't reproduce the bug your first priority is to work with the person who reported the bug to figure out just how you can reproduce it.

  3. Sometimes you will get a blinding flash of insight at this point and you will just know what the problem is. That doesn't usually happen.

  4. Try to figure out just when the program goes wrong. Sometimes the manifestation of a bug occurs long after the error actually occured. Your goal is to be able to point your finger at one line in your program and say, "here the incorrect thing happens." This step is often difficult.

  5. Often how to fix the problem is obvious once you know exactly where it occurs. Alas, sometimes that is not true. Sometimes you might be able to point at the spot where bad things happen, but not have any idea why they are happening. Such bugs are the most difficult to handle.

  6. If all else fails and if you have a general idea of where the bug is located, try cleaning your program up. Improve the style, update the comments, and reorganize sections that seem awkward. Bugs most often show up in poorly organized programs. Cleaning up such programs often mysteriously fixes bugs. Believe me: it actually works!

Debugging printfs

Let me focus right now on step #4: locating where things go wrong. One of the easiest ways to find out what your program is doing is to insert extra "debugging" printf statements in it to print out the values of various variables. With a little thought you can get a lot of valuable information about how the program is working (or not working) using this technique.

As a simple example, consider one of the filter programs from the exercises in Lesson #9. Here is the program that only prints the first 40 characters from each line of input. I modified this version so that it fails to print the '\n' character when it sets the char_count variable to zero. As a result of this error the output of the program is on one long line. Try it out and see the effect for yourself.

#include <stdio.h>

int main(void)
  int ch;
  int char_count = 0;  // The number of characters I have passed.

  // Read the charactes at stdin, one character at a time.
  while ((ch = getchar()) != EOF) {

    // If I have come to the end of the line, get ready for the next.
    if (ch == '\n') {
      char_count = 0;

    // Otherwise, print this character only if I am supposed to.
    else {
      if (char_count < 40) {

  return 0;

Now imagine that you wrote this program yourself and, for whatever reason, just can't find the error by inspection. Instead you might wonder if the program is ever setting char_count to zero. To check for that you could edit the if statement like this

if (ch == '\n') {
  printf("(debug: processing incoming newline)");
  char_count = 0;

This way the program will tell you explicitly whenever it sees a newline character in ch. The nature of text printed makes it clear that the message is for debugging purposes only. In the final program the printf would be removed. Try running the program with this extra printf installed. Does the output make more sense? It would be much easier to interpret what is happening with the help of this extra information. If the debugging message never appeared that would tell you something too. In this case it would tell you that the condition in the if statement is never true. That isn't what you would have expected, but knowing that would make it easier to track down the real problem.

Inserting debugging printfs into your program is sometimes called "instrumenting the program for debugging." If you are trying to track down a difficult bug you will probably want to include quite a few debugging printfs. That is fine. This technique, although crude, is often highly effective. It is one of the oldest debugging techniques in the book, but it is still used frequently.


  1. The purpose of testing is to find bugs. The purpose of testing is note to prove that the program is correct. In fact, testing attempts to do just the opposite---prove that the program is wrong. If you take this attitude when you do your testing, your testing will be more effective and, in the long run, your programs will have fewer bugs.

  2. Black box testing is when the tester has no idea how the program works internally. Instead the tester only knows what the program is supposed to do. White box testing is when the tester does know how the program is organized and uses that information to design test cases. When doing white box tests, McCabe's rule is a good guide to selecting test cases. McCabe's rule states that you should have N+1 test cases where N is the number of conditions in the program. The test cases should be such that every line of code is exercised and every condition is forced in both directions. Following McCabe's rule will not gaurentee a correct program, but it will find more bugs than other, less formal approaches usually do.

  3. A tried and true technique for locating the exact place where a bug occurs is to add extra "debugging printf" statements to the program. The output produced by these extra statements can provide you with quite a bit of information about what the program is doing (or not doing).

© Copyright 2003 by Peter C. Chapin.
Last Revised: July 20, 2003