Home / Logging in WinDBG

Logging is often useful when debugging software to help understand its behavior.

Consider a problem I was having: Sumatra was crashing because of invalid refcounting of fz_shade object. Refcounting took place only in two functions: fz_keepshade() and fz_dropshade() so my plan of attack was to see who calls those functions and from that hopefully figure out why there is a mismatch.

If I had a logging system with ability to dump callstacks already built into the program, I could modify the program to add logging to those two functions, re-run and look at the logs.

But I didn’t so I used WinDBG as an ad-hoc logger.

The trick is to know the following:

  • .logopen $filename WinDBG command
  • .logclose WinDBG command
  • conditional breakpoints

Under WinDBG I set the following two breakpoints:

  • bp SumatraPDF!fz_dropshade "kb; g"
  • bp SumatraPDF!fz_keepshade "kb; g"

bp creates a new breakpoint on entry to a given function. A string "kb; g" is composed of WinDBG commands executed when a breakpoint is hit. In this case I just dump the callstack with kb and continue execution with g.

Then I used .logopen $filename so that everything WinDBG prints in output window also gets written to a file and voila - an ad-hoc logging without the need to modify and recompile the program.

And yes, after some staring at resulting log I fixed the crash.