The end is nigh (for my process) (16 Jun 2007)

How can you tell that you're the control freak type of Windows programmer? Easy: You feel that irresistible urge to install top-level exception handlers which report application crashes to the end user and provide useful options on how to proceed, such as to report the issue to the software vendor, save the currently loaded data, inspect the issue in more detail, or call the police.

In fact, this is pretty much what Windows Error Reporting is all about, only that the crash reports are sent to Microsoft first (to their Winqual site, that is), from where ISVs can then download them for further analysis. Oh, and the other difference is that Microsoft dropped the "call the police" feature in order to get Vista done in time.

One of the applications that I'm working on already had its own top-level crash handler which performed some of the services also provided by Windows Error Reporting. It was about time to investigate Microsoft's offerings in this area and see how they can replace or augment the existing crash handler code.

The first option I looked at was the ReportFault API. Microsoft's documentation says that the function is obsolete, and we should rather use a different set of APIs collectively called the "WER functions". However, understanding them requires a lot more brain calories than the trivial ReportFault call which you can simply drop into an exception filter, and you're done.

The required code is pretty trivial and looks roughly like this:

int filter_exception(EXCEPTION_POINTERS *exc_ptr)
{
  EFaultRepRetVal repret = ReportFault(exc_ptr, 0);
  switch (repret)
  {
         // decode return value...
         //
  }
  return ret;
  return EXCEPTION_EXECUTE_HANDLER;
}

void main(void)
{
  __try {
    int *foo = (int *)0;
    *foo = 42;
  } __except(filter_exception(GetExceptionInformation())) {
    _tprintf(__T("Nothing to see here, move on, process is still alive!\n"));
  }
  Sleep(5000);
}

Sequence of events:

• A structured exception block is established using __try and __except.
• Code provokes an access violation.
• The exception filter filter_exception is consulted by the exception handling infrastructure to find out how to proceed with the exception.
• The filter calls ReportFault to display the crash dialog as shown above, and to give the user options to debug the problem, ignore it, or report it to Microsoft.
• After performing its menial reporting duties, the exception filter returns EXCEPTION_EXECUTE_HANDLER to indicate that its associated exception handler should be called.

That exception handler is, in fact, essentially the _tprintf statement which spreads the good news about the process still being alive.

On XP, that is. On Vista, the _tprintf statement may actually never execute. You'll still get a nice reporting dialog, such as the one in the screenshot to the right, but when you click the "Close program" button, the calling process will be terminated immediately, i.e. the call to ReportFault never really returns to the caller!

I debugged into ReportFault and found that it spawns off a process called wermgr.exe which performs the actual work. My current hypothesis is that it is that wermgr.exe process which terminates the calling process if the user chooses "Close program".

If you want to try it yourself, click here to download the demo code. To compile, simply run it through cl.exe:

  cl.exe reportfault.cpp

Now, I can't really complain - the fact that a program is closed after hitting the "Close program" button is hardly surprising. Still, the behavior differs from the old XP dialog. Worse, it is in fact inconsistent. What I just described is the behavior that I found with the default error reporting settings in Vista. By default, Vista "checks for solutions automatically" and doesn't ask the user what to do when a crash occurs. This can be changed in the "Problem Reports and Solutions" control panel:

After changing the report settings as shown above ("Ask me"), the error reporting dialog looks like this:

And when I click on "Close program" now, guess what - the process does not terminate, and the _tprintf statement in my exception handler is executed, just like on XP. So that "Close program" button can mean two different things on Vista!

But it's not just this inconsistency which bugged me. I also don't like the idea of letting the error reporting dialog pull the rug from under my feet. Sure, I'd like to use the dialog's services, but when it returns, I want to make my own decisions about how to proceed. For example, I could try and save the currently loaded data, or I could add my own special reporting. Or call the police.

But ReportFault won't let me do all that on Vista. And so I set out to burn those extra brain calories anyway and learn about the new WER APIs which were introduced with Windows Vista.

And burn calories I did, oh yes. More on this hopefully soon.

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