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_shadeobject. Refcounting took place only in two functions:
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 $filenameWinDBG 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
kband continue execution with
Then I used
.logopen $filenameso 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.
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