How to write a 'catch all' exeption handler

How can I prevent ALL nasty Delphi expection messages (particularly EAccessViolations) being displayed to users, including during shutdown of my app.  

My Delphi 3 app calls a lot of functions in C++ DLLs involving callbacks and I often get spurious EAccessViolations occuring, particularly during shutdown after I call FreeLibrary.  I suspect I'm getting memory corrputions somewhere and want to be able to handle the errors more gracefully than at present.

I've tried writing a procedure to handle the TApplication.OnException event but exceptions still appear:

procedure TMy.OnApplicationException(Sender: TObject; E: Exception);
begin
  { do nothing, but prevent error message being displayed }
end;

Thanks
andrewjacksonAsked:
Who is Participating?
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

ronit051397Commented:
Exceptions cannot handle AccessViolations.
0
julio011597Commented:
My C experience makes me guess ronit is right, even if the Delphi documentation doesn't say anything about this particular topic.
0
andrewjacksonAuthor Commented:
Thanks for your replies. I take both your points but I'm still surprised by what you say.  

The 'machine code' or whatever it is that the Delphi compiler embeds in the form of an 'exe' file is able to trap this type of exception during run-time because the 'EAccessViolation at ???? etc.' message that appears is very obviously a Borland / Delphi generated message.  Given that Delphi is so object-oriented and provides developers with basically full access to the VCL, WIN32 API etc. I'm puzzled that I can't somehow override the method or whatever it is that Delphi embeds in my 'exe' to trap this error and display that horrible message.

Any nore thoughts?
0
Cloud Class® Course: Microsoft Office 2010

This course will introduce you to the interfaces and features of Microsoft Office 2010 Word, Excel, PowerPoint, Outlook, and Access. You will learn about the features that are shared between all products in the Office suite, as well as the new features that are product specific.

ronit051397Commented:
It catches some AccessViolations but not all of them:

Try this one which does not catch the error:

procedure TForm1.Button1Click(Sender: TObject);
var xxx: pchar;
begin
  try
    strpcopy(xxx, 'test');
  except
  on E: EAccessViolation do
    Showmessage('You violated memory access');
  end;
end;

Now try this one which do catch the error:

procedure TForm1.Button1Click(Sender: TObject);
var xxx: pchar;
begin
  try
    Getmem(xxx,0);
    Strpcopy(xxx, 'test');
    Freemem(xxx);
  except
    on E: EAccessViolation do
    Showmessage('You violated memory access');
  end;
end;

0
gipaCommented:
Try this one.

procedure TfrmMain.AppException(Sender: TObject; E: Exception);
begin
  if (E is EDBEngineError) then begin
    ShowMessage('');
  end else  if (E is EExternalException) then begin
    Abort;
  end else if (E is EAccessViolation) then begin
    Abort;  // you can also use 'Halt' to terminate your appl.
  end else ShowMessage(E.Message);
end;

0

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
gipaCommented:
Ronit

I tested your examples and they do behave as you say, the first exception handler doesn't get called but the second does work properly.  

I then created a generic exception handling procedure (as per my original post and by modifying gipa's suggestion) and assigned this procedure to the Application.OnException event...

procedure TfrmMain.AppException(Sender: TObject; E: Exception);
begin
   MessageBeep(0);
   ShowMEssage('Unknoen exception occurred);
end;

This procedure does get called even for you first example.  However, the application must have already decided at this point that it is aborting because I hear the message beep (several times) but the message box is not displayed which seems to be the norm when a delphi app is shutting down.

So it looks like I am able to at least trap the two exceptions you've provided this way but not been able to do anything more elegant with them!!

Thanks everbody for you help.
0
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Delphi

From novice to tech pro — start learning today.