Why are there Bugs in Software?

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(Continued from previous part...)

Preventing bugs

It can be psychologically difficult for some engineers to accept that their design contains bugs. They may hide behind euphemisms like "issues" or "unplanned features". This is also true of corporate software where a fix for a bug is often called "a reliability enhancement".

Bugs are a consequence of the nature of the programming task. Some bugs arise from simple oversights made when computer programmers write source code carelessly or transcribe data incorrectly. Many off-by-one errors fall into this category. Other bugs arise from unintended interactions between different parts of a computer program. This happens because computer programs are often complex, often having been programmed by several different people over a great length of time, so that programmers are unable to mentally keep track of every possible way in which different parts can interact (the so-called hrair limit). Many race condition bugs fall into this category.

The computer software industry has put a great deal of effort into finding methods for preventing programmers from inadvertently introducing bugs while writing software. These include:

Programming techniques
Bugs often create inconsistencies in the internal data of a running program. Programs can be written to check the consistency of their own internal data while running. If an inconsistency is encountered, the program can immediately halt, so that the bug can be located and fixed. Alternatively, the program can simply inform the user, attempt to correct the inconsistency, and continue running.
Development methodologies
There are several schemes for managing programmer activity, so that fewer bugs are produced. Many of these fall under the discipline of software engineering (which addresses software design issues as well.) For example, formal program specifications are used to state the exact behavior of programs, so that design bugs can be eliminated. Programming language support
Programming languages often include features which help programmers deal with bugs, such as exception handling. In addition, many recently-invented languages have deliberately excluded features which can easily lead to bugs. For example, the Java programming language does not support pointer arithmetic.

Debugging

Finding and fixing bugs, or "debugging", has always been a major part of computer programming. Maurice Wilkes, an early computing pioneer, described his realization in the late 1940s that much of the rest of his life would be spent finding mistakes in his own programs. As computer programs grow more complex, bugs become more common and difficult to fix. Often programmers spend more time and effort finding and fixing bugs than writing new code.
Usually, the most difficult part of debugging is locating the erroneous part of the source code. Once the mistake is found, correcting it is usually easy. Programs known as debuggers exist to help programmers locate bugs. However, even with the aid of a debugger, locating bugs is something of an art.
Typically, the first step in locating a bug is finding a way to reproduce it easily. Once the bug is reproduced, the programmer can use a debugger or some other tool to monitor the execution of the program in the faulty region, and (eventually) find the problem. However, it is not always easy to reproduce bugs. Some bugs are triggered by inputs to the program which may be difficult for the programmer to re-create. One cause of the Therac-25 radiation machine deaths was a bug that occurred only when the machine operator very rapidly entered a treatment plan; it took days of practice to become able to do this, so the bug did not manifest in testing or when the manufacturer attempted to duplicate it. Other bugs may disappear when the program is run with a debugger; these are heisenbugs (humorously named after the Heisenberg uncertainty principle.)
Debugging is still a tedious task requiring considerable manpower. Since the 1990s, particularly following the Ariane 5 Flight 501 disaster, there has been a renewed interest in the development of effective automated aids to debugging. For instance, methods of static analysis by abstract interpretation have already made significant achievements, while still remaining much of a work in progress.

(Continued on next part...)

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