BUG: Unhandled Exception Filter Not Called Inside Debugger

   void main (int argc, char **argv)
   {
      __try
      {
      // all of code normally inside of main or WinMain here...

      }
      __except (MyUnFilter (GetExceptionInformation()))
      {
         OutputDebugString ("executed filter function\n");
      }
   }
				

MORE INFORMATION

The purpose of an unhandled exception filter function is to give a process the opportunity to respond to exceptions that occur outside any __try/__except block. Unhandled exception filters are process-wide; if any thread causes an exception, but does not handle it, the unhandled exception filter will get called. Once called, the filter function can fix the cause of the exception and continue execution, or it can pass the error to the default unhandled exception handler, which terminates the process.

Win32 applications can install a process-wide unhandled exception filter with the SetUnhandledExceptionFilter() API function. The Microsoft Visual C++ C Runtime (CRT) library startup code implements a __try/__except block around the main() function. When an exception is caught by this block, its exception filter function, named _XcptFilter, is called. _XcptFilter does two things: it maps the Win32 exceptions to signals used by the signal() CRT function, and dispatches signals to their appropriate handlers. If a signal's handler is set to SIG_DFL (as is the case by default), _XcptFilter calls the UnhandledExceptionFilter() API.

The UnhandledExceptionFilter() API determines whether the process is being debugged. If it is, then UnhandledExceptionFilter passes the exception first to the debugger, and then is supposed to call the process's unhandled exception filter.

If the process isn't being debugged, then UnhandledExceptionFilter calls the process's unhandled exception filter. Depending on the filter function's return, UnhandledExceptionFilter either continues execution at the point of the exception, or displays a system-modal dialog box informing the user that the application is to be terminated due to an unhandled exception.

Steps to Reproduce Behavior

Compile the following sample code, and then set a breakpoint inside MyUnFilter. Observe that the breakpoint never gets hit.

   /*
      Demonstrates how to use SEH so that unhandled exception filter
      can trap unhandled instructions.
   */ 

   #include <windows.h>
   #include <stdio.h>

   LONG WINAPI MyUnFilter (struct _EXCEPTION_POINTERS *lpExceptionInfo);

   void main (void)
   {
      LPBYTE lpBuff = NULL;

      SetUnhandledExceptionFilter (MyUnFilter);

      *lpBuff = 54;
   }


   LONG WINAPI MyUnFilter (struct _EXCEPTION_POINTERS *lpExceptionInfo)
   {
      static int s_SigCount = 0;

      DWORD dwExceptCode = lpExceptionInfo -> ExceptionRecord ->
                           ExceptionCode;
      LONG  lResult;

      if (dwExceptCode == EXCEPTION_ACCESS_VIOLATION)
      {
         printf ("access violation caught by MyUnFilter\n");

         ++s_SigCount;

         // test case to keep this from going on forever (since we're not
         // fixing the AV)
            if (s_SigCount < 5)
               lResult = EXCEPTION_CONTINUE_EXECUTION;
            else
               lResult = EXCEPTION_EXECUTE_HANDLER;

      }
      else
         lResult = EXCEPTION_CONTINUE_SEARCH;

      return lResult;
   }