Debugging is the process of finding and resolving defects or problems within a computer program that prevent correct operation of computer software or a system.
The terms "bug" and "debugging" are popularly attributed to Admiral Grace Hopper in the 1940s. Grace Hopper from FOLDOC While she was working on a Mark II computer at Harvard University, her associates discovered a moth stuck in a relay and thereby impeding operation, whereupon she remarked that they were "debugging" the system. However, the term "bug", in the sense of "technical error", dates back at least to 1878 and Thomas Edison (see software bug for a full discussion). Similarly, the term "debugging" seems to have been used as a term in aeronautics before entering the world of computers. Indeed, in an interview Grace Hopper remarked that she was not coining the term. The moth fit the already existing terminology, so it was saved. A letter from J. Robert Oppenheimer (director of the WWII atomic bomb "Manhattan" project at Los Alamos, NM) used the term in a letter to Dr. Ernest Lawrence at UC Berkeley, dated October 27, 1944,http://bancroft.berkeley.edu/Exhibits/physics/images/bigscience25.jpg regarding the recruitment of additional technical staff.
The Oxford English Dictionary entry for "debug" quotes the term "debugging" used in reference to airplane engine testing in a 1945 article in the Journal of the Royal Aeronautical Society. An article in "Airforce" (June 1945 p. 50) also refers to debugging, this time of aircraft cameras. Hopper's computer bug was found on September 9, 1947. The term was not adopted by computer programmers until the early 1950s. The seminal article by GillS. Gill, The Diagnosis of Mistakes in Programmes on the EDSAC, Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, Vol. 206, No. 1087 (May 22, 1951), pp. 538-554 in 1951 is the earliest in-depth discussion of programming errors, but it does not use the term "bug" or "debugging". In the ACM's digital library, the term "debugging" is first used in three papers from 1952 ACM National Meetings. Robert V. D. Campbell, Evolution of automatic computation, Proceedings of the 1952 ACM national meeting (Pittsburgh), p 29-32, 1952. Alex Orden, Solution of systems of linear inequalities on a digital computer, Proceedings of the 1952 ACM national meeting (Pittsburgh), p. 91-95, 1952. Howard B. Demuth, John B. Jackson, Edmund Klein, N. Metropolis, Walter Orvedahl, James H. Richardson, MANIAC, Proceedings of the 1952 ACM national meeting (Toronto), p. 13-16 Two of the three use the term in quotation marks. By 1963 "debugging" was a common enough term to be mentioned in passing without explanation on page 1 of the CTSS manual. The Compatible Time-Sharing System, M.I.T. Press, 1963
Kidwell's article Stalking the Elusive Computer BugPeggy Aldrich Kidwell, Stalking the Elusive Computer Bug, IEEE Annals of the History of Computing, 1998. discusses the etymology of "bug" and "debug" in greater detail.
Debugging ranges in complexity from fixing simple errors to performing lengthy and tiresome tasks of data collection, analysis, and scheduling updates. The debugging skill of the programmer can be a major factor in the ability to debug a problem, but the difficulty of software debugging varies greatly with the complexity of the system, and also depends, to some extent, on the programming language(s) used and the available tools, such as . Debuggers are software tools which enable the programmer to monitor the execution of a program, stop it, restart it, set , and change values in memory. The term debugger can also refer to the person who is doing the debugging.
Generally, high-level programming languages, such as Java, make debugging easier, because they have features such as exception handling and type checking that make real sources of erratic behaviour easier to spot. In programming languages such as C or assembly, bugs may cause silent problems such as memory corruption, and it is often difficult to see where the initial problem happened. In those cases, memory debugging tools may be needed.
In certain situations, general purpose software tools that are language specific in nature can be very useful. These take the form of static code analysis tools. These tools look for a very specific set of known problems, some common and some rare, within the source code. All such issues detected by these tools would rarely be picked up by a compiler or interpreter, thus they are not syntax checkers, but more semantic checkers. Some tools claim to be able to detect 300+ unique problems. Both commercial and free tools exist in various languages. These tools can be extremely useful when checking very large source trees, where it is impractical to do code walkthroughs. A typical example of a problem detected would be a variable dereference that occurs before the variable is assigned a value. As another example, some such tools perform strong type checking when the language does not require it. Thus, they are better at locating likely errors, versus actual errors. As a result, these tools have a reputation of false positives. The old Unix lint program is an early example.
For debugging electronic hardware (e.g., computer hardware) as well as low-level software (e.g., , ) and firmware, instruments such as , or in-circuit emulators (ICEs) are often used, alone or in combination. An ICE may perform many of the typical software debugger's tasks on low-level software and firmware.
After the bug is reproduced, the input of the program may need to be simplified to make it easier to debug. For example, a bug in a compiler can make it crash when parsing some large source file. However, after simplification of the test case, only few lines from the original source file can be sufficient to reproduce the same crash. Such simplification can be made manually, using a divide-and-conquer approach. The programmer will try to remove some parts of original test case and check if the problem still exists. When debugging the problem in a GUI, the programmer can try to skip some user interaction from the original problem description and check if remaining actions are sufficient for bugs to appear.
After the test case is sufficiently simplified, a programmer can use a debugger tool to examine program states (values of variables, plus the call stack) and track down the origin of the problem(s). Alternatively, tracing can be used. In simple cases, tracing is just a few print statements, which output the values of variables at certain points of program execution.
Despite the challenge of heterogeneity mentioned above, some debuggers have been developed commercially as well as research prototypes. Examples of commercial solutions come from Green Hills Software and Microchip's MPLAB-ICD (for in-circuit debugger). Two examples of research prototype tools are Aveksha
In addition to the typical task of identifying bugs in the system, embedded system debugging also seeks to collect information about the operating states of the system that may then be used to analyze the system: to find ways to boost its performance or to optimize other important characteristics (e.g. energy consumption, reliability, real-time response etc.).
An early example of anti-debugging existed in early versions of Microsoft Word which, if a debugger was detected, produced a message that said, "The tree of evil bears bitter fruit. Now trashing program disk.", after which it caused the floppy disk drive to emit alarming noises with the intent of scaring the user away from attempting it again.