Re: confused by exception handling in VC 2008

From:

"Doug Harrison [MVP]" <dsh@mvps.org>

Newsgroups:

microsoft.public.vc.mfc

Date:

Mon, 08 Dec 2008 14:19:54 -0600

Message-ID:

<g10rj4970f7btk60j7aptei1k0mjprepn8@4ax.com>

On Mon, 8 Dec 2008 11:28:21 -0800 (PST), max <maxxx126@gmail.com> wrote:

I am using VC 2008 and I was trying the __try exception handling and I
bumped into the following 3 cases:

1)
__try {
       char *p = 0;
       *p = '\0';
       AfxMessageBox("no exception caught");
   }
   __except (EXCEPTION_EXECUTE_HANDLER){
       AfxMessageBox("exception caught");
   }

the message "exception caught" is displayed as expected.

It's expected because memory location zero (actually the bottom 64KB, IIRC)
is never mapped into the process, precisely so that the hardware can detect
the usage of invalid pointers to this area.

2)
__try {
       int v[5];

       v[10] = 1; // This vector access is out of range
       AfxMessageBox("no exception");
   }
   __except (EXCEPTION_EXECUTE_HANDLER){
       AfxMessageBox("exception caught");
   }

no exception raised, the message "no exception" is displayed !

That's not surprising. Array access is not checked. The only way an
exception will be thrown here is if you refer to memory that isn't mapped
or if mapped, isn't committed (and can't be dynamically committed).
However, the compiler's /RTC option may help detect this sort of buffer
overrun.

3)
__try {
       char *p = 0;
       int v[5];

       v[10] = 1; // This vector access is out of range
       *p = '\0';
       AfxMessageBox("no exception");
   }
   __except (EXCEPTION_EXECUTE_HANDLER){
       AfxMessageBox("exception caught");
   }

In this case the application crashes, the exception handler failed.

Any ideas for the behaviour of cases 2) and 3) ?

Not sure what (3) was supposed to elucidate. You already knew that
dereferencing a null pointer throws an exception.

--
Doug Harrison
Visual C++ MVP