Contributor(s): Valerie Silverthorne, Matthew Heusser and Tom Davis

Debugging, in computer programming and engineering, is a multistep process that involves identifying a problem, isolating the source of the problem, and then either correcting the problem or determining a way to work around it. The final step of debugging is to test the correction or workaround and make sure it works.

In software development, debugging involves locating and correcting code errors in a computer program. Debugging is part of the software testing process and is an integral part of the entire software development lifecycle. The debugging process starts as soon as code is written and continues in successive stages as code is combined with other units of programming to form a software product. In a large program that has thousands and thousands of lines of code, the debugging process can be made easier by using strategies such as unit tests, code reviews and pair programming.

Once an error has been identified, it is necessary to actually find the error in the code. At this point, it can be useful to look at the code's logging and use a stand-alone debugger tool or the debugging component of an integrated development environment (IDE). Invariably, the bugs in the functions that get most use are found and fixed first. In some cases, the module that presents the problem is obvious, while the line of code itself is not. In that case, unit tests -- such as JUnit and xUnit, which allow the programmer to run a specific function with specific inputs -- can be helpful in debugging.

The standard practice is to set up a "breakpoint" and run the program until that breakpoint, at which time program execution stops. The debugging component of an IDE typically provides the programmer with the capability to view memory and see variables, run the program to the next breakpoint, execute just the next line of code, and, in some cases, change the value of variables or even change the contents of the line of code about to be executed.

Source code analyzers, which include security, common code errors and complexity analyzers, can also be helpful in debugging. A complexity analyzer can find modules that are so intricate as to be hard to understand and test. Some tools can actually analyze a test run to see what lines of code were not executed, which can aid in debugging. Other debugging tools include advanced logging and simulators that allow the programmer to model how an app on a mobile device will display and behave.

Finding and removing
software errors.

Some tools, especially open source tools and scripting languages, do not run in an IDE and require a more manual approach to debugging. Such techniques include dropping values to a log, extensive "print" statements added during code execution or hard-coded "wait" commands that simulate a breakpoint by waiting for keyboard input at specific times.

The use of the word bug as a synonym for error originated in engineering. The term's application to computing and the inspiration for using the word debugging as a synonym for troubleshooting has been attributed to Admiral Grace Hopper, a pioneer in computer programming, who was also known for her dry sense of humor. When an actual bug (a moth) got caught between electrical relays and caused a problem in the U.S. Navy's first computer, Admiral Hopper and her team "debugged" the computer and saved the moth. It now resides in the Smithsonian Museum.

This was last updated in October 2016

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