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Software testing is the process of running a program to find errors, and to determine whether or not it is fit for its intended purpose. A software process is similar to a physical process in that inputs are received and outputs produced. The difference is in the many (sometimes bizarre) ways in which software can fail.
Detecting all of the possible ways in which software can fail is often impossible. Unlike physical systems, bugs in software systems are purely down to design errors. They are not caused by defects in materials, nor are they introduced during the manufacturing process. Bugs occur in all software projects of any size, not because the programmers are incompetent, but because of the sheer complexity of such a project.
Because software logic can exhibit such complexity, testing boundary values alone is not sufficient to prove that a program will behave as expected in all circumstances, and far more comprehensive test data must be provided. On the other hand, it is not feasible to test a program of any size under all possible conditions that it might encounter. Even exhaustively testing a simple program to add together two 32-bit integers would take a very long time
Realistically, the test data should include typical values that will require all possible branches of execution to occur at least once. It should include a range of atypical (but valid) data that will test the program's ability to deal with unusual circumstances, and erroneous, incomplete or missing data that will trigger the program's error routines.
In more complex projects, testing may result in changes to the coding of a program to cure an existing problem, which can inadvertently introduce new problems. In this scenario, the only way to be sure of the program's correctness is to begin testing again from scratch. The level of complexity that can be managed in terms of software production is often limited by the effectiveness of the techniques that are used to test and debug programs.
The results of software errors can in some cases be disastrous, as we have seen. Bugs in critical software systems have caused aircraft to crash, and halted trading on the stock market. The reliability of software can literally be a matter of life and death, so it must perform as expected under the conditions specified.
The main purpose of testing is to find and fix problems before the software is deployed. Depending on the nature of the software project, testing may also be used to determine other factors, such as the usability of the software, how well it is documented, and how readily it can adapt to future requirements.
A general definition of dependable software is that it engenders a high probability of fault-free operation, and does not fail in unexpected or catastrophic ways. Simply performing a large number of tests does not guarantee the reliability of a program. It simply proves that the software works correctly under the test conditions. On the other hand, just one failed test is enough to prove that the software does not work correctly.
Given that a negative test is one that is intended to break the software, the goal is to produce a program that has sufficient exception handling capabilities to survive a reasonable number of negative tests. Because testing software often requires significant time and effort, the testability of a piece of software is an important consideration in the design stage of a project.