Path: utzoo!attcan!uunet!mailrus!accuvax.nwu.edu!bob
From: bob@accuvax.nwu.edu (Bob Hablutzel)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Tail patches
Message-ID: <1629@accuvax.nwu.edu>
Date: 19 Nov 89 15:18:35 GMT
References: <1459@sequent.cs.qmc.ac.uk> <36250@apple.Apple.COM> <5056@internal.Apple.COM> <1989Nov7.212837.5146@oracle.com> <26596@santra.UUCP> <5148@internal.Apple.COM> <26693@santra.UUCP> <5250@internal.Apple.COM> <26788@santra.UUCP>
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Reply-To: bob@accuvax.nwu.edu (Bob Hablutzel)
Organization: Northwestern Univ. Evanston, Il.
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In article <26788@santra.UUCP> jmunkki@kampi.hut.fi (Juri Munkki) writes:
>(comments on my scheme to make comefroms and tailpatches co-exist:)
>In article <5250@internal.Apple.COM> lsr@Apple.COM (Larry Rosenstein) writes:
>>Suppose trap X needs a come-from patch to fix a bug.  You propose that 
>>when you make a call to X, the trap dispatcher branch directly to the 
>>come-from patch.  The patch can do its thing, and then branch to the 
>>address defined in the trap table.  This allows the come-from trap to get 
>>control first, when there's no possibility of the stack being messed up.
>
>Right. The trap dispatcher is not changed. The only new things are
>a patched SetTrapAddress/GetTrapAddress (and the N-versions of those)
>and a new secondary dispatch table.
>

An INIT I wrote, FreshStart, does do come-from patches, and impliments exactly
this notion. Any patch to the 8 or so traps that I'm worried about don't
actually change the trap dispatch table, but rather the internal variable I
keep for the "original" trap address. (It's initialized when I put in my patch
in the first place). It seems to work OK for INITs (FreshStart is an INIT
manager, and so it running the INITs. I know if a patch is for an INIT or not).
I still have problems with applications that tail-patch. MPW, for example.

Notice, though, that my code has to have some notion of my patches; this is
not a general purpose fix, and I'd be happier if I didn't have to do it. (It
added two patches I had to make JUST FOR COMPATABILITY!) I agree with Apple
(and I agreed with Apple _BEFORE_ I took a job there): tail patching is a
pain in the ass.

Bob Hablutzel	(Wildwood Software, RIP)	BOB@NUACC.ACNS.NWU.EDU
Disclaimer:	As of tomorrow, I work for Apple. I speak for myself.
If you care:	I'll still work on FreshStart.