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A software bug is an error, flaw, mistake, failure, or fault in a computer program that prevents it from behaving as intended (e.g., producing an incorrect result). Most bugs arise from mistakes and errors made by people in either a program's source code or its design, and a few are caused by compilers producing incorrect code. A program that contains a large number of bugs, and/or bugs that seriously interfere with its functionality, is said to be buggy. Reports detailing bugs in a program are commonly known as bug reports, fault reports, problem reports, trouble reports, change requests, and so forth.
Bugs can have a wide variety of effects, with varying levels of inconvenience to the user of the program. Some bugs have only a subtle effect on the program's functionality, and may thus lie undetected for a long time. More serious bugs may cause the program to crash or freeze leading to a denial of service. Others qualify as security bugs and might for example enable a malicious user to bypass access controls in order to obtain unauthorized privileges.
The results of bugs may be extremely serious. A bug in the code controlling the Therac-25 radiation therapy machine was directly responsible for some patient deaths and in 1996, the European Space Agency's US$1 billion prototype Ariane 5 rocket was destroyed less than a minute after launch, due to a bug in the on-board guidance computer program. In June 1994, a Royal Air Force Chinook crashed into the Mull of Kintyre, killing 29. This was initially dismissed as pilot error, but an investigation by Computer Weekly uncovered sufficient evidence to convince a House of Lords enquiry that it may have been caused by a software bug in the aircraft's FADEC.  
The concept that software might contain errors dates back to 1842 in Ada Byron's notes on the analytical engine in which she speaks of the difficulty of preparing program 'cards' for Charles Babbage's Analytical engine:
Usage of the term "bug" to describe inexplicable defects has been a part of engineering jargon for many decades and predates computers and computer software; it may have originally been used in hardware engineering to describe mechanical malfunctions. For instance, Thomas Edison wrote the following words in a letter to an associate in 1878:
Problems with radar electronics during World War II were referred to as bugs (or glitches), and there is additional evidence that the usage dates back much earlier.
The invention of the term is often erroneously attributed to Grace Hopper, who publicized the cause of a malfunction in an early electromechanical computer. A typical version of the story is given by this quote:
Hopper was not actually the one who found the insect, as she readily acknowledged. And the date was September 9, 1947, not of 1945  . The operators who did find it (including William "Bill" Burke, later of the Naval Weapons Laboratory, Dahlgren Va. ), were familiar with the engineering term and, amused, kept the insect with the notation "First actual case of bug being found." Hopper loved to recount the story. 
While it is certain that the Mark II operators did not coin the term "bug", it has been suggested that they did coin the related term, "debug".
It can be psychologically difficult for some engineers to accept that their design contains bugs. They may hide behind euphemisms like "issues" or "unplanned/unexpected/undocumented 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. 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 style
- Bugs are often created by typos that are not caught by the compiler. Some innovations to programming style such as indentation, clearly-distinguished variable names, vertically aligning similar blocks, and so forth, are designed to make these bugs less likely, or easier to spot. In curly bracket programming languages, it has become common for style documents to require that even where optional, curly brackets be placed after all control flow constructs. This prevents program-flow bugs which can be very time-consuming to track down, such as where a terminating semicolon is introduced at the end of the construct (a common typo); where another line is added before the first; or where the following line may be removed by the preprocessor. Other examples of using style to prevent bugs include placing constants on the left hand side in comparisons, (which causes a syntax error in the case of the common typo of replacing the comparison operator "==" with the assignment operator "="); placing a comma after even the last element of a list, and the last line in a block, where these are normally optional; etc.
- 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.
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 find the point at which the program went astray. Sometimes, a bug is not a single flawed instruction, but represents an error of thinking or planning on the part of the programmer. Such logic errors require a section of the program to be overhauled or rewritten.
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 code analysis by abstract interpretation have already made significant achievements, while still remaining much of a work in progress.
Famous computer bugs
- NASA Mariner 1 went off-course during launch, due to a missing 'bar' in its FORTRAN software (July 22, 1962).
- NASA Apollo 11 landing problem (July 20, 1969).
- NASA Voyager 2 (January 25, 1986).
- Phobos 1 lost (September 10, 1988).
- ESA Ariane 5 Flight 501 self-destruction 40 seconds after takeoff (June 4, 1996).
- NASA Mars Climate Orbiter destroyed due to incorrect orbit insertion (September 23, 1999).
- Mars Polar Lander lost (December 3, 1999).
- NASA Mars Rover freezes due to too many open files in flash memory (January 21, 2004).
- The Therac-25 accidents (1985-1987), quite possibly the most serious computer-related failure ever in terms of human life (at least five died).
- The year 2000 problem, popularly known as the "Y2K bug", spawned fears of worldwide economic collapse and an industry of consultants providing last-minute fixes.
- The Pentium FDIV bug.
Electric power transmission
- The 2003 North America blackout was triggered by a local outage that went undetected due to a race condition in General Electric Energy's XA/21 monitoring software.
- AT&T long distance network crash (January 15, 1990), documented in Bruce Sterling's The Hacker Crackdown.
- The MIM-104 Patriot bug, which resulted in the deaths of 28 Americans in Dharan, Saudi Arabia (February 25, 1991).
- Chinook crash on Mull of Kintyre: the cause of this event remains a mystery, but strong suspicions have been raised that software problems were a contributory factor.
Computer and video games
- The Missingno. and Glitch City bugs, found in the Pokémon series
- The Minus world in NES version of Super Mario Brothers
Modern bugs and security holes
While people would like bugs to be fixed before a new release, it is common practice for software to be released with known, but considered non-critical, bugs. While software products contain an unknown number of unknown bugs when shipped, measurements during the testing may provide a statistically reliable estimate of the number of likely bugs remaining.
Also known as security holes. Many computer systems are able to be infected by viruses. Viruses exploit known vulnerabilities in the system - which may or may not be bugs. Viruses are not bugs in themselves - they are typically programs that are doing precisely what they were designed to do. However, viruses are occasionally referred to as such in the popular press.
Although all operating systems are vulnerable to viruses, most virus writers only target (write viruses for) operating systems with large userbases, such as various Windows versions, so as to maximize the virus distribution and damages caused by the virus.
Common types of computer bugs
- Divide by zero
- Infinite loops
- Arithmetic overflow or underflow
- Exceeding array bounds
- Using an uninitialized variable
- Accessing memory not owned (Access violation)
- Memory leak or Handle leak
- Stack overflow or underflow
- Buffer overflow
- Off by one error
- Race hazard
- Loss of precision in type conversion
- ISO 9126, which classifies a bug as either a defect or a nonconformity
- Bug tracker
- Bit rot
- Unusual software bugs (Schroedinbug, Heisenbug, Bohr bug, and Mandelbug)
- ^ Edison to Puskas, 13 November 1878, Edison papers, Edison National Laboratory, U.S. National Park Service, West Orange, N.J., cited in Thomas P. Hughes, American Genesis: A History of the American Genius for Invention, Penguin Books, 1989, ISBN 0-14-009741-4, on page 75.
- ^ Danis, Sharron Ann: "Rear Admiral Grace Murray Hopper"
- ^ IEEE Annals of the History of Computing, Vol 22 Issue 1, 2000
- Collection of Software Bugs (Thomas Huckle, TU München)
- Computer-Related Incidents with Commercial Aircraft (Peter B. Ladkin et al., Universität Bielefeld)
- An Investigation of the Therac-25 Accidents (Nancy Leveson, University of Washington and Clark S. Turner, University of California at Irvine)
- Fatal Dose: Radiation Deaths linked to AECL Computer Errors (Barbara Wade Rose, Canadian Coalition for Nuclear Responsibility)
- Software Horror Stories (Nachum Dershowitz)
- Software Does Not Fail (Paul Niquette]
- Picture of the "first computer bug" The error of this term is elaborated above. (Naval Historical Center)
- Page from 1947 log book with "first actual case of bug being found" (moth) (National Museum of American History)
- The First Computer Bug! An email from 1981 about Adm. Hopper's bug
- How to Report Bugs Effectively (Simon G. Tatham)
- Bug Tracking Basics: A beginner’s guide to reporting and tracking defects (Mitch Allen)
- History's Worst Software Bugs
- Bug Isolation Project - This project is to track bugs of popular open source software. Everyone can participate if he/she has Fedora Core 5 installed.
- Demonstration of a bug in Google Maps.