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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.
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 5Chinook 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 LordsFADEC. 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 inquiry that it may have been caused by a software bug in the aircraft’s
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.
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
