Path: utzoo!attcan!uunet!snorkelwacker!apple!voder!dtg.nsc.com!mcdan
From: mcdan@dtg.nsc.com (Eric McDaniel)
Newsgroups: comp.sys.mac.programmer
Subject: Trap patch questions
Message-ID: <1101@haydn.nsc.com>
Date: 5 Jun 90 16:04:41 GMT
Organization: National Semiconductor, Santa Clara
Lines: 32

Reply-To: mcdan@dtg.nsc.com
Distribution: 

I am trying to write a patch to display a dialog box
under certain circumstances when an application is
launched, and am having a few problems:

1.  At first I tried patching the _Launch and _Chain
traps, but some launching utilities (notably OnCue)
apparently don't use these traps, as my code was not
always executed.  I now patch _LoadSeg and check if
the segment number being loaded is #1 before calling
my code. It works, but seems less efficient since _LoadSeg
can be called any number of times within an application.

Any suggestions?  How do OnCue and similar utilities
work?

2.  When I call ShowWindow to display the dialog box,
I consistently get some sort of system error.  I checked
the DialogPtr and all associated handles, and everything
seems to be valid and on the system heap.  I'm guessing
the problem has something to do with setting up A5 or
the GrafPort correctly.

Could my use of a 'tail-patch' be the problem, and if
so, how else could I write this thing?

I'mm using Lightspeed C 3.01 on a Mac Plus, if that makes
any difference.-- 
Eric McDaniel
mcdan@dtg.nsc.com