A software bug (or just “bug”) is an error, flaw, mistake, failure, fault or “undocumented feature” in a computer program that prevents it from behaving as intended (e.g., producing an incorrect or unexpected 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 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.
In 2002, a study commissioned by the US Department of Commerce' National Institute of Standards and Technology concluded that software bugs, or errors, are so prevalent and so detrimental that they cost the US economy an estimated $59 billion annually, or about 0.6 percent of the gross domestic product.
...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.
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 glitches [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).
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.
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.
There are various reasons for such a list:
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.