How to debug a crash that only occurs on application shutdown? (Delphi)

Question

So, after some recent changes we discovered that one of our oldest applications is sometimes crashing on shutdown. This manifests itself either in the form of "Runtime error 216" messages or in a message from Windows Error Reporting that the application has stopped working. The application is already emitting OutputDebugString-messages at every turn and AFAICT all of our own code gets executed correctly to completion. All destructors are called as are all finalization sections and class destructors, none of which are raising any exceptions.

Also, neither madExcept nor FastMM4's Full Debug Mode seem to have anything to complain about (though that might be a false conclusion because the crash might happen even before those components' own finalization code runs).

So, what would you do? Where would you start?

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This question is supposed to be more about the general approach to this class of problems than about the specific instance I'm currently facing so I'm deliberately leaving out details. Feel free to ask if you think they might be relevant to the choice of debugging approach and I will add them later.

Answer 1

A runtime error 216 means that you have an Av (access violation) and SysUtils has already stopped translating those errors to exceptions.

First try: Build with debug DCU's and look in the unit system where the error is raised, set a breakpoint there. hopefully you can catch it in the debugger and work from there.

You probably have a memory bug (dangling pointer, null reference etc etc use of s string constant in an already finalized unit) and the best trick is to check the finalizations after sysutils is finalized. You can do this by building WITH debug dcu's, setting the break point to the finalization in sysutils and start stepping through the code until the error occurs.