Software bug

A software bug (or just "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.

Effects

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 in the 1980s. 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 inquiry that it may have been caused by a software bug in the aircraft's engine control computer.

Etymology

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:

...an analyzing process must equally have been performed in order to furnish the Analytical Engine with the necessary operative data; and that herein may also lie a possible source of error. Granted that the actual mechanism is unerring in its processes, the cards may give it wrong orders.

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:

It has been just so in all of my inventions. The first step is an intuition, and comes with a burst, then difficulties arise—this thing gives out and [it is] then that "Bugs"—as such little faults and difficulties are called—show themselves and months of intense watching, study and labor are requisite before commercial success or failure is certainly reached.

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.

Photo of what is possibly the first real bug found in a computer.

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:

In 1946, when Hopper was released from active duty, she joined the Harvard Faculty at the Computation Laboratory where she continued her work on the Mark II and Mark III. Operators traced an error in the Mark II to a moth trapped in a relay, coining the term bug. This bug was carefully removed and taped to the log book September 9th 1945 [sic]. Stemming from the first bug, today we call errors or glitch's [sic] in a program a bug.

Hopper was not actually the one who found the insect, as she readily acknowledged. And the date was September 9, but in 1947, not 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". Even this is unlikely, since the Oxford English Dictionary entry for "debug" contains a use of "debugging" in the context of airplane engines in 1945 (see the debugging article for more).

Prevention

Bugs are a consequence of the nature of human factors in the programming task. They arise from oversights made by computer programmers during design, coding and data entry. For example: In creating a relatively simple program to sort a list of words into alphabetical order, one's design might fail to consider what should happen when a word contains a hyphen. Perhaps, when converting the abstract design into the chosen programming lanugage, one might inadvertently create an off-by-one error and fail to sort the last word in the list. Finally, when typing the resulting program into the computer, one might accidentally type a '+' where a '-' was intended, perhaps resulting in the words being sorted into reverse alphabetical order. More complex bugs can arise from unintended interactions between different parts of a computer program. This frequently occurs because computer programs can be complex - millions of lines long in some cases - often having been programmed by many people over a great length of time, so that programmers are unable to mentally track every possible way in which parts can interact. Another category of bug called a race condition comes about when a program is running in more than one thread.

The software industry has put much effort into finding methods for preventing programmers from inadvertently introducing bugs while writing software. These include:

Programming style

While typos in the program code most likely are caught by the compiler, a bug usually appears when the programmer makes a logic error. Various innovations in programming style and defensive programming are designed to make these bugs less likely, or easier to spot.

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

The typical bug history (GNU Classpath project data). A bug, submitted by the user, is unconfirmed. A reproduced bug is a confirmed bug. The confirmed bugs are later fixed. Bugs, belonging to other categories (unreproducible, will not be fixed, etc) are usually in the minority

Main article: 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. It is not uncommon for a bug in one section of a program to cause failures in a completely different section, thus making it especially difficult to track (for example, an error in a graphic rendering routine causing a file I/O routine to fail); this is most commonly caused by errors that lead to the corruption of program instructions or variables in memory.

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.

Managing bugs

It is common practice for software to be released with known bugs that are considered non-critical. 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. Most big software projects maintain a list of "known bugs". This list inform users about bugs that are not fixed in the current release, or not fixed at all, and often a workaround is offered additionally.

There are various reasons for such a list:

• The developers often don't have time to fix all non-severe bugs.
• The bug could be fixed in a new version or patch that is not yet released.
• The changes to the code required to fix the bug would be large, and would bring with them the chance of introducing other bugs into the system.

Given the above, it is often considered impossible to write completely bug-free software of any real complexity. So bugs are categorized by severity, and low-severity non-critical bugs are tolerated, as they do not impact the proper operation of the system, for the majority of users. NASA's SATC managed to reduce number of errors to fewer than 0.1 per 1000 lines of code (SLOC) but this was not felt to be feasible for any real world projects.

One school of thought, popularized by Eric S. Raymond as Linus's Law in his essay The Cathedral and the Bazaar, holds that popular open-source software holds a better chance of having few or no bugs than other software, because "given enough eyeballs, all bugs are shallow". This assertion has been disputed, however: computer security specialist Elias Levy wrote that "it is easy to hide vulnerabilities in complex, little understood and undocumented source code," because, "even if people are reviewing the code, that doesn't mean they're qualified to do so."

Famous computer bugs

Space exploration

• NASA Mariner 1 went off-course during launch, due to a missing overbar in the specifications for its FORTRAN software (July 22, 1962).[2]
• 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 entry of momentum data in imperial units instead of the metric system (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).
• NASA Mars Global Surveyor battery failure was the result of a series of events linked to a computer error made five months before (November 2, 2006).[3]

Medical

• The Therac-25 accidents (1985-1987), which caused at least five deaths.

Computing

• 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.

Telecommunications

• AT&T long distance network crash (January 15, 1990), documented in Bruce Sterling's The Hacker Crackdown.

Military

• The software error of a MIM-104 Patriot, which ultimately contributed to the deaths of 28 Americans in Dhahran, 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.

Video games

• The Missingno. and Glitch City bugs, found in the Pokémon series
• The Minus World in NES version of Super Mario Brothers

Security vulnerabilities

Malicious software may attempt to exploit known vulnerabilities in a 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.

Common types of computer bugs

• Divide by zero
• NULL pointer dereference
• 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
• Deadlock
• Off by one error
• Race condition
• Loss of precision in type conversion

Bugs in popular culture

• In the 1968 novel 2001: A Space Odyssey (and the corresponding film), a spaceship's onboard computer, HAL 9000, is programmed with two conflicting objectives: to fully disclose all its information, and to keep the true purpose of the flight secret from the crew. This conflict causes HAL to eventually try to kill all the crew members (since, if there were no crew, there would be no contradiction).
• In the 1984 song 99 Red Balloons (though not in the original German version), "bugs in the software" lead to a computer mistaking a group of balloons for a nuclear missile and starting a nuclear war.
• The 2004 novel The Bug, by Ellen Ullman, is about a programmer's attempt to find an elusive bug in a database application.

See also

• Glitch
• ISO 9126, which classifies a bug as either a defect or a nonconformity
• Workaround
• Bug tracking system
• Bit rot
• Anti-pattern
• Unusual software bugs (schroedinbug, heisenbug, Bohr bug, and mandelbug)
• Hysterical raisins, or hard-to-explain features that are kept for backward compatibility

External links

• 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)
• Rates of Design Failure
• 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. (Packages for Fedora available)