Software bug

A software bug is an error, flaw, mistake, failure, or fault in a computer program that prevents it from working as intended, or produces an incorrect result. Bugs arise from mistakes and errors, made by people, in either a program's source code or its design. A program that contains a large number of bugs, and/or bugs that seriously interfere with its functionality, is said to be buggy. Reports about bugs in a program are referred to as bug reports, also called PRs (problem reports), trouble reports, CRs (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. Other bugs lead to security problems; for example, a common type of bug which allows a buffer overflow may allow a malicious user to execute other programs that are normally not allowed to run.

The results of bugs may be extremely serious. A bug in the code controlling the Therac-25 radiation therapy machine was directly responsible for 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 evidence sufficient to convince a House of Lords enquiry that it may have been caused by a software bug in the aircraft's FADEC. [2] [3]

Etymology

Usage of the term "bug" to describe inexplicable defects has been a part of engineering jargon for many decades; 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 actual 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. 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 1947, not of 1945 [4] [5]. The operators who did find it 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. [6]

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".

Prevention

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 (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

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.

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

Space exploration

• NASA Mariner 1 went off-course during launch, due to a missing 'bar' in its FORTRAN software (July 22, 1962).[7]
• 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).

Medical

• The Therac-25 accidents (1985-1987), quite possibly the most serious computer-related failure ever in terms of human life (three died, at least three more were injured).

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 undetetected 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 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

Traditionally bugs are fixed before a new release. In the first decade of the twenty-first century, as software becomes more complex, sometimes software is released with unknown bugs. Such bugs may just prevent the user from operating the software properly, but often they also produce:

• Operating System instability: Some of these bugs will cause the operating system to crash.
  • Windows will display what is known as the "Blue screen of death," a stop message that lets you know that an error has occurred in either your computer's hardware or software.
  • the Linux kernel has a similar message called "kernel panic." This is displayed when an unstable version of the Linux kernel or a buggy driver is used and an error occurs.
• Though these messages do occur, modern operating systems using the latest Linux kernel and Windows NT kernel (Windows 2000/2003/XP) are known to be able to run without a restart for months and even years.
• Application instability: Many applications will crash because of unknown bugs. Usually when an application crashes, the system is still running.
• Security vulnerabilities or security holes:
  • Many computer systems are able to be infected by viruses. Viruses exploit known vulnerabilities in the system. Viruses are not bugs, though they 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.

In general, all unverified software may have bugs. Although it is often easy to prove that a bug exists in a piece of code, it is very difficult or even impossible to prove that there are no bugs in another piece of code.

To find more about the number of known vulnerabilities a particular software may have at this moment, you can search for security bugs on the Secunia web page.

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
• Deadlock
• Off by one error
• Race hazard
• Loss of precision in type conversion

See also

• Glitch
• ISO 9126, which classifies a bug as either a defect or a nonconformity
• Workaround
• Bug tracker
• Bit rot
• Anti-pattern
• Unusual software bugs (Schroedinbug, Heisenbug, Bohr bug, and Mandelbug)

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)
• 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.