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From: osborn@ux1.lbl.gov (James R Osborn)
Newsgroups: comp.sys.mac.programmer
Subject: Toolbox Patching
Summary: What is the preferred method?
Keywords: patch
Message-ID: <13613@dog.ee.lbl.gov>
Date: 28 May 91 17:31:32 GMT
References: <1991May28.163618.8353@crash.cts.com>
Reply-To: osborn@ux1.lbl.gov (James R Osborn)
Organization: Lawrence Berkeley Laboratory
Lines: 49
X-Local-Date: Tue, 28 May 91 10:31:32 PDT


Just what exactly is the preferred method for applying a
tail patch to a Toolbox routine?  I've read all sorts of
stuff that waves its (metaphorically speaking) hands and
never explicitly says how to do it.  Maybe I've missed a
tech note that describes the process?

Here is something that I have done that worked but seems
incredibly unelegant and kludgey:

using an init (this part seems ok to me):

load the patch routine into the system heap with the init
DetachResource it
HNoPurge it
HLock it
save original trap address in the patch routine's header
set the trap address to point at the patch

then in the patch:

the patch routine gets passed the appropriate arguments
  for the trap being patched
do my preprocessing, whatever it is
set trap address to original trap address (from header)
call trap, passing appropriate arguments, and saving
  result (if necessary)
set trap address back to this patch
return result (if necessary)


Why did I do it this way?  Because I used C and was too lazy
to figure out how to call the trap routine directly myself
using assembly.  I presume the preferred method is to push
the appropriate arguments onto the stack (pascal fashion)
and then JSR to the original trap?  Then I wouldn't be
SetTrapAddress'ing all over the place.

Would my psuedo-code above then be preferred?  (Or am I totally
or even partially blowing it?  8^)

-- James

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| James R. Osborn              | It just goes to show you it's always |
| Lawrence Berkeley Laboratory | something.  Either it's baffling     |
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