POSTMORTEM DEBUGGING in VS .NET

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8 POSTMORTEM DEBUGGING
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Debugging Dump Files
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Now that you have been presented with one or more dump files and tasked with the investigation of finding the root cause of the faulty process, what can actually be done using these dump files Can you dump memory, look at handles, or step through code Remember that a dump file is simply a static snapshot of the state of a process As such, setting break points and stepping through code is not possible Using dump files can be best viewed as manual debugging By manual, we mean that simply by looking at the state of an application, you will need to manually construct theories about what code has executed to get the application into that state It should be evident that constructing code execution by state analysis is a much harder proposition than engaging in a live debug session Nevertheless, plenty of the debugger commands that massage application state into a more digestible form still work when using dump files, and in most cases, with enough patience, the root cause can be found
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8
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Postmortem Debugging
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Before we take a closer look at the dump files generated in the previous section, there are two critical pieces of information that need to be brought up: symbol files and the data access layer Because dump files contain no symbolic information, it is critical that symbol files be available when analyzing a dump file We ve already seen the most common symbol commands in action throughout this book The next critical piece of information is the CLR data access layer (DAC), which is heavily used by the SOS debugger extension to provide you with all the data needed during a debug session
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The Data Access Layer
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Contrary to the world of native debugging where a lot can be gleaned from looking at raw memory, in the managed code world, SOS relies heavily on the CLR to provide the debug output and results that we have come to expect For SOS to properly interpret (or translate ) the raw data that is passed to it, SOS actually makes calls into the CLR (ie, executing CLR code) to help with this process The component in the CLR responsible for this functionality is the Data Access Layer (DAC) and is implemented in mscordacwksdll Now, because the CLR is continuously undergoing enhancements, the underlying DAC also changes with each version (including hotfixes) You can easily verify this by looking in the installation folder for each of the NET versions installed on your machine For example, on my machine, I can see that the mscordacwksdll is located in the following folder:
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c:\Windows\MicrosoftNET\Framework\v2050727
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On a machine running Visual Studio 2010 CTP, however, the file is located in the following folders, indicating that a new version of mscordacwksdll has been shipped with CLR 40:
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Because the debugger requires this component during its operation, being able to tell the debugger where the file is located is crucial During live debugging, you typically do not have to worry about it because SOS is smart enough and locates the file from the same location where the version of the runtime currently being debugged is located During postmortem (or dump file) debugging on the other hand, the version of the CLR used in the application may be different than what is available on the machine from where you load and debug the dump file
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Dump File Fundamentals
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Again, it is important to reiterate that the SOS debugger extension calls into the mscordacwksdll, which actually executes CLR code, and thus it is critically important that the debugger use the correct version Because having the correct version is the key to success, Microsoft publishes most all of the mscordacwksdll that it has shipped on the Microsoft public symbol server As long as you point the debuggers symbol path to the public symbol server (using symfix or related commands), the debugger will be able to find the file There are scenarios, however, where you still may have to explicitly tell the SOS extension where to find the file These scenarios include cases where the file isn t available on the public symbol server (rare) or the file wasn t installed to the same location as on the machine where the dump file was generated To aid in these scenarios, the cordll command can be used The cordll command controls the way in which mscordacwksdll is loaded and can be a lifesaver when dealing with potential mismatches Table 8-4 details the switches available to the cordll command Table 8-4 Cordll Switches
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