Everything about Debugging totally explained
Debugging is a methodical process of finding and reducing the number of
bugs, or defects, in a
computer program or a piece of
electronic hardware thus making it behave as expected. Debugging tends to be harder when various subsystems are
tightly coupled, as changes in one may cause bugs to emerge in another.
Origin
There is some controversy over the origin of the term "debugging."
The terms "bug" and "debugging" are both popularly attributed to
Admiral Grace Hopper in the 1940s, but the term "bug" dates back at least to 1878 and
Thomas Edison (see the
Software bug article for a full discussion), and "debugging" seems to have been used as a term in aeronautics before entering the world of computers.
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, but Hopper's
bug wasn't found until 1947 and the term wasn't adopted by computer programmers until the early 1950s.
The seminal article by Gill in 1951 is the earliest in-depth discussion of programming errors, but it doesn't 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. 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.
Kidwell's article
Stalking the Elusive Computer Bug discusses the etymology of "bug" and "debug" in greater detail.
Tools
Debugging is, in general, a cumbersome and tiring task. The debugging skill of the programmer is probably the biggest factor in the ability to debug a problem, but the difficulty of software debugging varies greatly with the
programming language used and the available tools, such as
debuggers. Debuggers are software tools which enable the
programmer to monitor the
execution of a program, stop it, re-start it, set
breakpoints, change values in memory and even, in some cases, go back in time. 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've features such as
exception handling that make real sources of erratic behaviour easier to spot. In lower-level programming languages such as
C or
assembly, bugs may cause silent problems such as
memory corruption, and it's often difficult to see where the initial problem happened. In those cases,
memory debugger 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're 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's 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. Another example would be to perform strong type checking when the language doesn't require such. Thus, they're 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 (for example,
computer hardware) as well as low-level software (for example,
BIOSes,
device drivers) and
firmware, instruments such as
oscilloscopes,
logic analyzers 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.
Debugging process
The debugging is started by trying to reproduce the problem. This can be a non-trivial task, for example in case of parallel processes or some
unusual software bugs. Also specific user environment and usage history can make it difficult to reproduce the problem.
After the bug is reproduced, the input of the program needs 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
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
GUI, the programmer will try to skip some user interaction from the original problem description and check if remaining actions are sufficient for bug to appear. To automate test case simplification,
delta debugging methods can be used.
After the test case is sufficiently simplified, a programmer can use
debugger to examine program states (values of variables,
call stack) and track down the origin of the problem. Alternatively a
tracing can be used. In simple case the tracing is just a few print statements, which print out the values of variables in certain points of program execution.
Remote debugging is the process of debugging a program running on a system different than the debugger. To start remote debugging, debugger connects to a remote system over a network. Once connected, debugger can control the execution of the program on the remote system and retrieve information about its state.
Quotes
- The most effective debugging tool is still careful thought, coupled with judiciously placed print statements.
- Brian Kernighan, "Unix for Beginners" (1979)
- Everyone knows that debugging is twice as hard as writing a program in the first place. So if you're as clever as you can be when you write it, how will you ever debug it?
- Brian Kernighan, "The Elements of Programming Style", 2nd edition, chapter 2
Further Information
Get more info on 'Debugging'.
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