Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hoptoad!tim
From: tim@hoptoad.uucp (Tim Maroney)
Newsgroups: comp.sys.mac.programmer
Subject: Re: How to do a LEGAL tail-patch
Message-ID: <6541@hoptoad.uucp>
Date: 15 Feb 89 23:17:18 GMT
References: <1272@ccnysci.UUCP>
Reply-To: tim@hoptoad.UUCP (Tim Maroney)
Organization: Eclectic Software, San Francisco
Lines: 25

In article <1272@ccnysci.UUCP> alexis@ccnysci.UUCP (Alexis Rosen) writes:
>
>I've been thinking about this for a couple of days now. Here's my scheme.
>Does anyone see any flaws?
>
>Procedure MyPatch
>SetTrapAddress to original trap address
>Trap to original trap using A-line instruction
>SetTrapAddress back to @MyPatch (&MyPatch for C people)
>Fixup stack and return

Nope, doesn't work.  Imagine that GetResource checks to see if it's
being called from GetFontInfo by looking at the return address.  The
reason you have to avoid tail patching is because if you just call the
old GetResource from inside your patch of GetResource, the return
address is inside your patch instead of inside GetFontInfo.

Your approach has exactly the same problem.  When your patch code calls
the trap, the return address is inside your code, not inside
GetFontInfo, the real caller.
-- 
Tim Maroney, Consultant, Eclectic Software, sun!hoptoad!tim
"Its failings notwithstanding, there is much to be said in favor
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 keeps us in touch with the ignorance of the community." -- Oscar Wilde