Win32 Programming Lesson 25: Unhandled Exceptions Bet you’ve never encountered one of those, eh?

Where are we?  Covered exception handling in depth…  But there’s more! Yay! And an assignment, how lucky you are!

Applications don’t have to handle  What happens if the outermost exception handler returns EXCEPTION_CONTINUE_SEARCH?  Well, then you have an unhandled exception  Handled by the OS… But you have one last chance to catch it before this… it’s a million to one shot, but it just might work!

SetUnhandledExceptionFilter  PTOP_LEVEL_EXCEPTION_FILTER SetUnhandledExceptionFilter(PTOP_LEVEL_E XCEPTION_FILTER pTopLevelExceptionFilter);  Usually called during initialization  Function called declared like this: LONG WINAPI TopLevelUnhandledExceptionFilter(PEXCEPTION_ POINTERS pExceptionInfo) Note returns address of previously installed filter

Return Values  EXCEPTION_EXECUTE_HANDLER: Terminate, executing finally blocks  EXCEPTION_CONTINUE_EXECUTION: Continue at the instruction that raised the exception  EXCEPTION_CONTINUE_SEARCH: Really don’t handle the exception…

UnhandledExceptionFilter  Called automatically when we return EXCEPTION_CONTINUE_SEARCH  Turns things over to Windows Error Reporting (WER) eventually…  But first, it does the following in order

Steps 1. Allow write access to a resource (EXE and DLL) and continue 2. Notify the debugger of an unhandled exception 3. Notify your global filter function 4. Notify the debugger of an unhandled exception (again!) 5. Terminate the process

WER Linkage  When the exception is passed to Windows WER gets a call from an Advanced Local Procedure Call (ALPC)  Blocks all local thread execution  Passed to a dedicated service WerSvc… spawns WerFault.exe  This executable handled the functionality of reporting etc.  Configurable behavior via Registry

Just In Time Debugging  Nice, because we can debug without restarting our application and reproducing the fault in a debugger  Configured through the registry  Called CreateProcess (How? Anything we should think about?)  Aside: don’t have to wait for a blowup… can always call vsjitdebugger.exe –p PID

Vectored Exception Handling  SEH has one fundamental structural property…  PVOID AddVectoredExceptionHandler( ULONG bFirstInTheList, PVECTORED_EXCEPTION_HANDLER pfnHandler);  Called before SEH handling  Remove by ULONG RemoveVectoredExceptionHandler(PVOID Handler)

Vectored Continue Handler  Called when we’re continuing up *after* everything else  Can return EXCEPTION_CONTINUE_EXECUTION

C++ try and catch  Suddenly, we should think back about C++ SEH  That is, try/catch blocks  Internally implemented by __try and __except blocks…  Should use the C++ mechanism whenever possible, as it knows about the underlying language issues

Exceptions and the Debugger  Let’s look at the options…

Assignment  Construct an “error handling experimenters lab” that lets you combine these things together in a GUI and tells you what order things get processed in.  Output should show me how things get executed… there’s room for some *very* imaginative programming here