++it18 – Time Travel Debug

This talk presented a very interesting object – the debugger the could go backward.  But I don’t want to steal the scene, since Paolo Severini from Microsoft explained everything quite well in his talk. Here you can find the slides. Nedless to say errors are mine.


Everyone knows that debugging is twice as hard as writing the code in the first place. So if you’re as clever as you can be when you write it, how long will it take to debug it?

Debugging could be both hard and time consuming. Bugs can be difficult to replicate, especially those involving memory corruption, race conditions, or those that happen randomly or intermittently.

More often than not, it can be hard to spot the cause from logs or crash dumps. Also step by step debugging may require several restarts.

When doing step-by-step debugging is quite common something like:

step into, step over, step over, step over, step ov.. @!# I should have stepped into!.

The more information we can get from debugging tools the better. A tool that could tell “what just happened” would be very very useful.

Reverse debugging is such a tool. The idea is to record state by state everything happens in the CPU that’s executing the code and then the reverse debugger allows the programmer to play back and forth to spot the problem.

First ideas of such a tool dates back to 70s [NdM. I keep noticing that most of the computer science has been invented back in the 60-70s years. They just lacked the CPU resource for practical applications].

There are several tools for reverse debugging native code:

In this speech I will present TTD.

TTD is the reverse debugger for Windows. It is fully integrated with WinDbg. TTD can be used for all user-mode processes, for multithread and multicore

In order to use TTD, you need to set WinDbg to record data. This option needs to be explicitly enabled since has two main drawbacks – first all the program execution is slowed down and then quite a large amount of data is produced. You can expect anywhere between 5 and 50 Mb/s.

After recording has started, debugging works as expected, plus you can step back and forth and also continue backward or forward.

Take note that TTD works only in the latest versions fo Windows 10.

Interestingly you can perform queries on data collected during execution. Every line in the database matches an assembly instruction executed. So you can query how many times some instruction has been executed, how many exceptions have been thrown and so on.

[NdM: A working demo follows, unfortunately the time is quite constrained. A sample program to recursively compute the byte size of file in a directory tree is presented. The program just crashes when executed. So it is executed in WinDbg with TTD enabled. The execution halts as usual in the middle of exceptionland where nothing can be understood unless you speak advanced assembly. By pressing back a couple of time the debugger, going backwards, reaches the last function executed, the one who crashed. At this point Paolo places a breakpoint at the beginning of the function and – with a good implementation of the WOW effect – runs the program backwards. The execution back-reaches the function beginning and can be step forward to better understand why it crashed]


The presentation has been very interesting to show the tool capabilities. I tried an evaluation of UndoDB back in 2006, but it was as uncomfortable as it could be since it had to be used from command line. WinDbg+TTD is light years ahead in terms of usability.

Also I found very interesting the data query feature. Having the db of the program execution states allows the programmer to get a number of important statistics and events without the need of adding instrumentation or specific breakpoints.

The drawback is the execution time, but I guess that without a specific hardware support this is the price we have to pay still for some time in the future.

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