Categories
Experience Reports Guide

Using dump files to guide testing

You don’t need to understand code to make use of dump files.

One tool that I’ve frequently used throughout my testing career (and also development) is WinDbg. I was a little surprised when I realised that very few other people use it so I thought that I’d share a little about why I use it and how to get going.

What can you do?

  • See the code path in a crash dump
  • View data in memory
  • View threads that are running when software is in a hang
  • Many more things that I’ve yet to try

This can be especially useful if you’re tasked with reproducing a crash reported by a customer and (as is unfortunately common) they say “I wasn’t doing anything”.

What you’ll need:

  • WinDbg or WinDbg Preview
  • Access to symbols files for your software (developers can probably help set you up).
  • A dump file that you want to look at (more on this later)

This is part of Debugging Tools for Windows. You can download it for free from Microsoft. There’s a newer “preview” version that is quite neat plus the older one that I’m more accustomed to using as part of the Windows 10 SDK. Both are linked from here:

https://docs.microsoft.com/en-us/windows-hardware/drivers/debugger/debugger-download-tools

If the link is broken, Google for WinDbg and you should find it.

Obtaining dump files

Hopefully your software outputs crash dumps but if not, you can add some registry keys to ensure that they are generated in a known location. Even if your software does create minidumps, you may value full dumps more:

  1. Open regedit and access: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Windows Error Reporting\LocalDump
  2. Create the following:
    1. DumpFolder – REG_EXPAND_SZ – %LOCALAPPDATA%\CrashDumps
    2. DumpCount – REG_DWORD – 5
    3. DumpType – REG_DWORD – 2

For more see: https://docs.microsoft.com/en-us/windows/win32/wer/collecting-user-mode-dumps

If you want to create a dump file of running software, for example to inspect memory or dig into a hang then you can use task manager. Just right click on the process and click “Create dump file”.

Note that if you are using a 32bit application then you’ll need the 32bit task manager, e.g. c:\windows\syswow64\Taskmgr.exe

There’s also tools out there that will generate dump files on demand.

Symbols

The next thing that you’ll need to do is setup symbols path. These will help turn the 0s and 1s in a minidump into more readable strings. Create yourself a folder for the symbols, for example c:\symbols. Then in WinDbg you’ll need to set the path. In the old version open the File menu and you should see an option. For the Preview version go to Settings then Debugging.

SRV*c:\symbols\*https://msdl.microsoft.com/download/symbols;SRV*c:\symbols\*<YOURPATHHERE>

You may have to add a few paths in there but hopefully you get the idea.

Analysing crash dumps to get a call stack

Now on to the juicy part, analysing a crash dump. You can open it from the File menu.

From the View menu, you should be able to see the option for a stack / call stack. Bring that up whilst the dump is being loaded.

Now run the following commands (they take a few minutes):

.ecxr

!analyze -v

This should tell you a chunk of information about the crash. Based on this you can

An example of using this information

When looking for an example I found a crash dump from a game that I made many years ago. I have absolutely no idea what might of caused it so hopefully now I can figure out why.

My WinDbg analysis included the following:

System.NullReferenceException

This tells me that it tried using an object that didn’t exist. Either it hasn’t been set or has been deleted but is still in use.

007edc0c 081abfb6 X_Orbtek_II!X_Orbtek_360.XOrbtek.UnloadContent+0x6
007edc10 081abe73 Microsoft_Xna_Framework_Game!Microsoft.Xna.Framework.Game.DeviceDisposing+0x1b
007edc20 081abe43 Microsoft_Xna_Framework_Game!Microsoft.Xna.Framework.GraphicsDeviceManager.OnDeviceDisposing+0x13
007edc28 081abe15 Microsoft_Xna_Framework_Game!Microsoft.Xna.Framework.GraphicsDeviceManager.HandleDisposing+0x15
007edc34 081ab7cf Microsoft_Xna_Framework_Graphics!Microsoft.Xna.Framework.Graphics.GraphicsDevice.~GraphicsDevice+0x2f
007edc40 081ab72f Microsoft_Xna_Framework_Graphics!Microsoft.Xna.Framework.Graphics.GraphicsDevice.Dispose+0x1f
007edc60 081ab543 Microsoft_Xna_Framework_Game!Microsoft.Xna.Framework.GraphicsDeviceManager.Dispose+0x133
007edc74 081ab3f5 Microsoft_Xna_Framework_Game!Microsoft.Xna.Framework.GraphicsDeviceManager.System.IDisposable.Dispose+0x15
007edc80 081ab0e3 Microsoft_Xna_Framework_Game!Microsoft.Xna.Framework.Game.Dispose+0xb3
007edcb4 081ab015 Microsoft_Xna_Framework_Game!Microsoft.Xna.Framework.Game.Dispose+0x15

From this I can tell that the game was being closed. It has happened when unloading content so likely its tried to . It may be possible dig deeper. As you gain more skill with the software it is possible to learn more about what was in memory to understand at exactly what point it crashed.

And here’s the key part

If I was trying to reproduce this crash, I can take the knowledge learnt from the crash dump to guide how I will try and get to the bottom of it. Unfortunately the example dump I picked is a tricky one but I could maybe come up with something like “Explore exiting the game with different assets loaded to discover the source of the crash”.

A 10-20 snoop in the dump file might save me a huge chunk of time in trying to reproduce a crash. Obviously I can’t share real examples from my professional life in any detail but knowing that mouse over a control caused a “random” crash or that the software crashed after hitting “Save” and the top of the call stack was “MyApp!MyApp.FileIo.SaveFile.ApplyTextOverlay” then I can focus on that area.

There’s loads more that you can do but hopefully this has been useful!

Leave a Reply

Your email address will not be published.