PROGRAM VALIDATION AND TESTING :
Complex systems need testing to ensure that they work as they are intended. But bugs can be subtle, particularly in embedded systems, where specialized hardware and real-time responsiveness make programming more challenging. Fortunately, there are many available techniques for software testing that can help us generate a comprehensive set of tests to ensure that our system works properly.
The first question we must ask ourselves is how much testing is enough. Clearly, we cannot test the program for every possible combination of inputs. Because we cannot implement an infinite number of tests, we naturally ask ourselves what a reasonable standard of thoroughness is. One of the major contributions of software testing is to provide us with standards of thoroughness that make sense. Following these standards does not guarantee that we will find all bugs.
The two major types of testing strategies:
Black-box methods generate tests without looking at the internal structure of the program.
Clear-box (also known as white-box) methods generate tests based on the program structure.
The control/data flow graph extracted from a program’s source code is an important tool in developing clear-box tests for the program. To adequately test the program, we must exercise both its control and data operations.
In order to execute and evaluate these tests, we must be able to control variables in the program and observe the results of computations, much as in manufacturing testing. In general, we may need to modify the program to make it more testable. By adding new inputs and outputs, we can usually substantially reduce the effort required to find and execute the test.
No matter what we are testing, we must accomplish the following three things in a test:
· Provide the program with inputs that exercise the test we are interested in.
· Execute the program to perform the test.
· Examine the outputs to determine whether the test was successful.
Black-box tests are generated without knowledge of the code being tested. When used alone, black-box tests have a low probability of finding all the bugs in a program. But when used in conjunction with clear-box tests they help provide a well-rounded test set, since black-box tests are likely to uncover errors that are unlikely to be found by tests extracted from the code structure.
Black-box tests can really work. For instance, when asked to test an instrument whose front panel was run by a microcontroller, one acquaintance of the author used his hand to depress all the buttons simultaneously. The front panel immediately locked up. This situation could occur in practice if the instrument were placed face-down on a table, but discovery of this bug would be very unlikely via clear-box tests.
One important technique is to take tests directly from the specification for the code under design. The specification should state which outputs are expected for certain inputs. Tests should be created that provide specified outputs and evaluate whether the results also satisfy the inputs.
We can’t test every possible input combination, but some rules of thumb help us select reasonable sets of inputs. When an input can range across a set of values, it is a very good idea to test at the ends of the range. For example, if an input must be between 1 and 10, 0, 1, 10, and 11 are all important values to test. We should be sure to consider tests both within and outside the range, such as, testing values within the range and outside the range. We may want to consider tests well outside the valid range as well as boundary-condition tests.
Random tests form one category of black-box test. Random values are generated with a given distribution. The expected values are computed independently of the system, and then the test inputs are applied. A large number of tests must be applied for the results to be statistically significant, but the tests are easy to generate. Another scenario is to test certain types of data values. For example, integer valued inputs can be generated at interesting values such as 0, 1, and values near the maximum end of the data range. Illegal values can be tested as well.
Regression tests form an extremely important category of tests. When tests are created during earlier stages in the system design or for previous versions of the system, those tests should be saved to apply to the later versions of the system. Clearly, unless the system specification changed, the new system should be able to pass old tests. In some cases old bugs can creep back into systems, such as when an old version of a software module is inadvertently installed. In other cases regression tests simply exercise the code in different ways than would be done for the current version of the code and therefore possibly exercise different bugs.
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