Debugging

Debugging

Discussion
Ask a question about 'Debugging'
Start a new discussion about 'Debugging'
Answer questions from other users
Full Discussion Forum
 
Encyclopedia
Debugging is a methodical process of finding and reducing the number of bugs, or defects, in a computer program
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...

 or a piece of electronic hardware
Electronic hardware
Electronic hardware refers to interconnected electronic components which perform analog and/or logic operations on received and locally stored information to produce as output and/or store resulting new information and/or to provide control for output actuator mechanisms.Electronic hardware can...

, 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. Many books have been written about debugging (see below: Further reading), as it involves numerous aspects, including: interactive debugging, control flow
Control flow
In computer science, control flow refers to the order in which the individual statements, instructions, or function calls of an imperative or a declarative program are executed or evaluated....

, integration testing
Integration testing
Integration testing is the phase in software testing in which individual software modules are combined and tested as a group. It occurs after unit testing and before validation testing...

, log file
Log file
The term log file can refer to:*Text saved by a computer operating system to recored its activities, such as by the Unix syslog facility*Output produced by a data loggerAlso see Wikibooks chapter...

s, monitoring (application, system
System Monitoring
A system monitor in systems engineering is a process within a distributed system for collecting and storing state data.- Overview :The configuration for the system monitor takes two forms:# configuration data for the monitor application itself, and...

), memory dumps, profiling, Statistical Process Control
Statistical process control
Statistical process control is the application of statistical methods to the monitoring and control of a process to ensure that it operates at its full potential to produce conforming product. Under SPC, a process behaves predictably to produce as much conforming product as possible with the least...

, and special design tactics to improve detection while simplifying changes.

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. 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 meaning of technical error dates back at least to 1878 and Thomas Edison
Thomas Edison
Thomas Alva Edison was an American inventor and businessman. He developed many devices that greatly influenced life around the world, including the phonograph, the motion picture camera, and a long-lasting, practical electric light bulb. In addition, he created the world’s first industrial...

 (see software bug
Software bug
A software bug is the common term used to describe an error, flaw, mistake, failure, or fault in a computer program or system that produces an incorrect or unexpected result, or causes it to behave in unintended ways. Most bugs arise from mistakes and errors made by people in either a program's...

 for a full discussion), and "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.

The Oxford English Dictionary
Oxford English Dictionary
The Oxford English Dictionary , published by the Oxford University Press, is the self-styled premier dictionary of the English language. Two fully bound print editions of the OED have been published under its current name, in 1928 and 1989. The first edition was published in twelve volumes , and...

 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, Hopper's bug was found on September 9, 1947. The term was not 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 does not use the term "bug" or "debugging".
In the ACM
Association for Computing Machinery
The Association for Computing Machinery is a learned society for computing. It was founded in 1947 as the world's first scientific and educational computing society. Its membership is more than 92,000 as of 2009...

'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.

Scope


As software and electronic systems have become generally more complex, the various common debugging techniques have expanded with more methods to detect anomalies, assess impact, and schedule software patches or full updates to a system. The words "anomaly" and "discrepancy" can be used, as being more neutral terms, to avoid the words "error" and "defect" or "bug" where there might be an implication that all so-called errors, defects or bugs must be fixed (at all costs). Instead, an impact assessment
Impact assessment
Impact assessment is "a process aimed at structuring and supporting the development of policies. It identifies and assesses the problem at stake and the objectives pursued. It identifies the main options for achieving the objective and analyses their likely impacts in the economic, environmental...

 can be made to determine if changes to remove an anomaly (or discrepancy) would be cost-effective for the system, or perhaps a scheduled new release might render the change(s) unnecessary. Not all issues are life-critical or mission-critical in a system. Also, it is important to avoid the situation where a change might be more upsetting to users, long-term, than living with the known problem(s) (where the "cure would be worse than the disease"). Basing decisions of the acceptability of some anomalies can avoid a culture of a "zero-defects" mandate, where people might be tempted to deny the existence of problems so that the result would appear as zero defects. Considering the collateral issues, such as the cost-versus-benefit impact assessment, then broader debugging techniques will expand to determine the frequency of anomalies (how often the same "bugs" occur) to help assess their impact to the overall system.

Tools


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
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....

(s) used and the available tools, such as debugger
Debugger
A debugger or debugging tool is a computer program that is used to test and debug other programs . The code to be examined might alternatively be running on an instruction set simulator , a technique that allows great power in its ability to halt when specific conditions are encountered but which...

s
. Debuggers are software tools which enable the programmer
Programmer
A programmer, computer programmer or coder is someone who writes computer software. The term computer programmer can refer to a specialist in one area of computer programming or to a generalist who writes code for many kinds of software. One who practices or professes a formal approach to...

 to monitor the execution
Execution (computers)
Execution in computer and software engineering is the process by which a computer or a virtual machine carries out the instructions of a computer program. The instructions in the program trigger sequences of simple actions on the executing machine...

 of a program, stop it, re-start it, set breakpoint
Breakpoint
In software development, a breakpoint is an intentional stopping or pausing place in a program, put in place for debugging purposes. It is also sometimes simply referred to as a pause....

s, and change values in memory. The term debugger can also refer to the person who is doing the debugging.

Generally, high-level programming language
High-level programming language
A high-level programming language is a programming language with strong abstraction from the details of the computer. In comparison to low-level programming languages, it may use natural language elements, be easier to use, or be from the specification of the program, making the process of...

s, such as Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...

, make debugging easier, because they have features such as exception handling
Exception handling
Exception handling is a programming language construct or computer hardware mechanism designed to handle the occurrence of exceptions, special conditions that change the normal flow of program execution....

 that make real sources of erratic behaviour easier to spot. In programming languages such as C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

 or assembly
Assembly language
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...

, bugs may cause silent problems such as memory corruption
Memory corruption
Memory corruption happens when the contents of a memory location are unintentionally modified due to programming errors; this is known as violating memory safety. When the corrupted memory contents are used later in the computer program, it leads either to program crash or to strange and bizarre...

, and it is 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 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. Another example would be to perform strong type checking when the language does not require such. 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
Lint programming tool
In computer programming, lint was the name originally given to a particular program that flagged some suspicious and non-portable constructs in C language source code. The term is now applied generically to tools that flag suspicious usage in software written in any computer language...

program is an early example.

For debugging electronic hardware (e.g., computer hardware
Computer hardware
Personal computer hardware are component devices which are typically installed into or peripheral to a computer case to create a personal computer upon which system software is installed including a firmware interface such as a BIOS and an operating system which supports application software that...

) as well as low-level software (e.g., BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....

es, device driver
Device driver
In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....

s) and firmware
Firmware
In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices...

, instruments such as oscilloscope
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...

s, logic analyzer
Logic analyzer
A logic analyzer is an electronic instrument which displays signals in a digital circuit. A logic analyzer may convert the captured data into timing diagrams, protocol decodes, state machine traces, assembly language, or correlate assembly with source-level software.Presently, there are three...

s or in-circuit emulators (ICEs)
In-circuit emulator
An in-circuit emulator is a hardware device used to debug the software of an embedded system. It was historically in the form of bond-out processor which has many internal signals brought out for the purpose of debugging...

 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
Firmware
In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices...

.

Typical debugging process


Normally the first step in debugging is to attempt to reproduce the problem. This can be a non-trivial task, for example as with parallel processes
Parallel processing
Parallel processing is the ability to carry out multiple operations or tasks simultaneously. The term is used in the contexts of both human cognition, particularly in the ability of the brain to simultaneously process incoming stimuli, and in parallel computing by machines.-Parallel processing by...

 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 may need to be simplified to make it easier to debug. For example, a bug in a compiler can make it crash
Crash (computing)
A crash in computing is a condition where a computer or a program, either an application or part of the operating system, ceases to function properly, often exiting after encountering errors. Often the offending program may appear to freeze or hang until a crash reporting service documents...

 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
Divide and conquer algorithm
In computer science, divide and conquer is an important algorithm design paradigm based on multi-branched recursion. A divide and conquer algorithm works by recursively breaking down a problem into two or more sub-problems of the same type, until these become simple enough to be solved directly...

 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
Graphical user interface
In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...

, 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
Debugger
A debugger or debugging tool is a computer program that is used to test and debug other programs . The code to be examined might alternatively be running on an instruction set simulator , a technique that allows great power in its ability to halt when specific conditions are encountered but which...

 tool to examine program states (values of variables, plus the call stack
Call stack
In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, control stack, run-time stack, or machine stack, and is often shortened to just "the stack"...

) and track down the origin of the problem(s). Alternatively, tracing
Tracing (software)
In software engineering, tracing is a specialized use of logging to record information about a program's execution. This information is typically used by programmers for debugging purposes, and additionally, depending on the type and detail of information contained in a trace log, by experienced...

 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.

Techniques

  • Print (or tracing) debugging is the act of watching (live or recorded) trace statements, or print statements, that indicate the flow of execution of a process. This is sometimes called , due to the use of the printf
    Printf
    Printf format string refers to a control parameter used by a class of functions typically associated with some types of programming languages. The format string specifies a method for rendering an arbitrary number of varied data type parameter into a string...

     statement in C.

  • 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.

  • Post-mortem debugging is debugging of the program after it has already crashed
    Crash (computing)
    A crash in computing is a condition where a computer or a program, either an application or part of the operating system, ceases to function properly, often exiting after encountering errors. Often the offending program may appear to freeze or hang until a crash reporting service documents...

    . Related techniques often include various tracing techniques (for example,) and/or analysis of memory dump (or core dump
    Core dump
    In computing, a core dump consists of the recorded state of the working memory of a computer program at a specific time, generally when the program has terminated abnormally...

    ) of the crashed process. The dump of the process could be obtained automatically by the system (for example, when process has terminated due to an unhandled exception), or by a programmer-inserted instruction, or manually by the interactive user.

  • Delta Debugging - technique of automating test case simplification.


  • Saff Squeeze - technique of isolating failure within the test using progressive inlining of parts of the failing test.

Debugging for embedded systems


In contrast to the general purpose computer software design environment, a primary characteristic of embedded environments is the sheer number of different platforms available to the developers (CPU architectures, vendors, operating systems and their variants). Embedded systems are, by definition, not general-purpose designs: they are typically developed for a single task (or small range of tasks), and the platform is chosen specifically to optimize that application. Not only does this fact make life tough for embedded system developers, it also makes debugging and testing of these systems harder as well, since different debugging tools are needed in different platforms.
Put simply, embedded systems debuggers have two key requirements.
  • to identify and fix bugs in the system (e.g. logical or synchronization problems in the code, or a design error in the hardware);
  • 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.).

Anti-debugging


Anti-debugging is "the implementation of one or more techniques within computer code that hinders attempts at reverse engineering
Reverse engineering
Reverse engineering is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation...

 or debugging a target process". It is actively used in legitimate copy-protection schemas
Copy protection
Copy protection, also known as content protection, copy obstruction, copy prevention and copy restriction, refer to techniques used for preventing the reproduction of software, films, music, and other media, usually for copyright reasons.- Terminology :Media corporations have always used the term...

, but is also used by malware
Malware
Malware, short for malicious software, consists of programming that is designed to disrupt or deny operation, gather information that leads to loss of privacy or exploitation, or gain unauthorized access to system resources, or that otherwise exhibits abusive behavior...

 to complicate its detection and elimination. Techniques used in anti-debugging include:
  • API-based: check for the existence of a debugger using system information
  • Exception-based: check to see if exceptions are interfered with
  • Process and thread blocks: check whether process and thread blocks have been manipulated
  • Modified code: check for code modifications made by a debugger handling software breakpoints
  • Hardware- and register-based: check for hardware breakpoints and CPU registers
  • Timing and latency: check the time taken for the execution of instructions
  • Detecting and penalizing debugger

See also


  • Assertion (computing)
    Assertion (computing)
    In computer programming, an assertion is a predicate placed in a program to indicate that the developer thinks that the predicate is always true at that place.For example, the following code contains two assertions:...

  • Debugger
    Debugger
    A debugger or debugging tool is a computer program that is used to test and debug other programs . The code to be examined might alternatively be running on an instruction set simulator , a technique that allows great power in its ability to halt when specific conditions are encountered but which...

  • Magic debug values
  • Software bug
    Software bug
    A software bug is the common term used to describe an error, flaw, mistake, failure, or fault in a computer program or system that produces an incorrect or unexpected result, or causes it to behave in unintended ways. Most bugs arise from mistakes and errors made by people in either a program's...

  • Software testing
    Software testing
    Software testing is an investigation conducted to provide stakeholders with information about the quality of the product or service under test. Software testing can also provide an objective, independent view of the software to allow the business to appreciate and understand the risks of software...


Further reading

  • David J. Agans: Debugging: The Nine Indispensable Rules for Finding Even the Most Elusive Software and Hardware Problems, AMACOM, 2002. ISBN 0-8144-7168-4
  • Bill Blunden: Software Exorcism: A Handbook for Debugging and Optimizing Legacy Code, APress, 2003. ISBN 1-59059-234-4
  • Ann R. Ford, Toby J. Teorey: Practical Debugging in C++, Prentice Hall, 2002. ISBN 0-13-065394-2
  • Thorsten Grötker, Ulrich Holtmann, Holger Keding, Markus Wloka, The Developer's Guide to Debugging, Springer, 2008. ISBN 1-40205-539-0
  • Robert C. Metzger: Debugging by Thinking : A Multidisciplinary Approach, Digital Press, 2003. ISBN 1-55558-307-5
  • Glenford J Myers: *The Art of Software Testing, John Wiley & Sons inc, 2004. ISBN 0-471-04328-1
  • John Robbins: Debugging Applications, Microsoft Press, 2000. ISBN 0-7356-0886-5
  • Matthew A. Telles, Yuan Hsieh: The Science of Debugging, The Coriolis Group, 2001. ISBN 1-57610-917-8
  • Dmitry Vostokov: Memory Dump Analysis Anthology, Volume 1, OpenTask, 2008. ISBN 978-0-9558328-0-2
  • Andreas Zeller: Why Programs Fail: A Guide to Systematic Debugging, Morgan Kaufmann, 2005. ISBN 1-55860-866-4

External links