Time travel debugging

Stepping back in time through source code

Time travel debugging or time traveling debugging is the process of stepping back in time through source code to understand what is happening during execution of a computer program.[1] Typically, debugging and debuggers, tools that assist a user with the process of debugging, allow users to pause the execution of running software and inspect the current state of the program.[2] Users can then step forward in time, stepping into or over statements and proceeding in a forward direction.[3] Interactive debuggers include the ability to modify code and step forward based on updated information.[4] Reverse debugging tools allow users to step backwards in time through the steps that resulted in reaching a particular point in the program. Time traveling debuggers provide these features and also allow users to interact with the program, changing the history if desired, and watch how the program responds.[5]

## Characteristics supporting bi-directional travel

[edit]

There are several characteristics that support the ability to move backwards as well as forwards in time.

* Selecting a purely functional programming language helps due to the self-contained nature of pure functions. Pure functions have no side effects and depend only on the information explicitly provided to the function, providing a repeatable, reliable, replayable path through the code.
* Languages and debuggers that enable hot swapping, the ability to modify code as the code is running, provide some of the requirements necessary to rewind, and potentially rewrite execution.[6][7]
* Tools based on the GNU debugger (GDB), available for compatible languages such as C, C++, Go, and Fortran are capable of reverse debugging, but the effort significantly slows interaction.[8]
* Programming languages intended for reversible computing inherently support stepping a program backwards via uncomputation if garbage bits are retained. This may require instruction set support.

## Time traveling debuggers

[edit]

Examples of debuggers with the ability to step backwards:

Language | Debuggers

|

C++ | rr for x86 Linux, Undo UDB for Linux[9]
R | provDebugR[10]
Python | PyTrace[11]
JavaScript | Wallaby.js,[12] Meiosis Tracer[13]
C# | RevDeBug
Java | RevDeBug for C# and Java,[14] WhyLine for Java,[15] Undo UDB
Elm | Elm Debugger, Elm Reactor[16]
OCaml | ocamldebug
Go | Undo UDB for Linux[17]
Rust | Undo UDB for Linux[18]
Windows Native | Microsoft Time Travel Debugging (TTD) Tool[19] for native Windows software (x86, x64, ARM, ARM64[20]),

eShard esReverse Full System Time Travel Analysis for Windows[21]

Linux Native | eShard esReverse Full System Time Travel Analysis for Linux[22]
Android | eShard esReverse Full System Time Travel Analysis for Android[23]

## See also

[edit]

* Interactive computing
* List of purely functional programming languages
* Record and replay debugging

## References

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1. ^ "Time Travel Debugging in WinDbg Preview!". Debugging Tools for Windows. Retrieved 2018-05-08.
2. ^ Telles, Matthew; Hsieh, Yuan (2001-04-01). The Science of Debugging. Coriolis Group Books. ISBN 9781576109175.
3. ^ "Reverse debugging, time travel debugging". undo.io. 9 January 2013. Retrieved 2018-05-08.
4. ^ "Interactive Debugging With Node.js - DZone Web Dev". dzone.com. Retrieved 2018-05-08.
5. ^ "Elm's Time Travelling Debugger". debug.elm-lang.org. Retrieved 2018-05-08.
6. ^ "interactive programming". elm-lang.org. Retrieved 2018-05-08.
7. ^ "Hot reloading and time travel debugging: what are they?". Code Cartoons. 2015-10-21. Retrieved 2018-05-08.
8. ^ "6 Things You Should Know About Time Travel Debugging". undo.io. 3 January 2015. Retrieved 2018-05-08.
9. ^ "UDB - Time Travel Debugger for C/C++".
10. ^ "ProvTools/provDebugR". github.com. 2018-07-31. Retrieved 2018-07-31.
11. ^ "PyTrace Time Travel Debugger for Python".
12. ^ "Wallaby.js Introduction: Time Travel Debugger". wallabyjs.com. Retrieved 2020-02-27.
13. ^ Donut, Fox (2018-04-29). "You Don't Need Redux, MobX, RxJS, Cerebral". Fox Donut. Retrieved 2018-05-24.
14. ^ "About RevDeBug". revdebug.com. Retrieved 2020-03-28.
15. ^ "Whyline for Java". cs.cmu.edu. Retrieved 2021-01-21.
16. ^ "time travel made easy". elm-lang.org. Retrieved 2018-05-08.
17. ^ "Greg Law on Debugging, Record & Replay of Data, and Hyper-Observability".
18. ^ "Greg Law on Debugging, Record & Replay of Data, and Hyper-Observability".
19. ^ DOMARS. "Time Travel Debugging - Overview". docs.microsoft.com. Retrieved 2018-05-08.
20. ^ "Get WinDbg Preview". Microsoft.
21. ^ "Full System Time Travel Analysis for Windows".
22. ^ "Full System Time Travel Analysis for Linux".
23. ^ "Full System Time Travel Analysis for Android".

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