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From: chewy@apple.com (Paul Snively)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Tail patches
Message-ID: <5212@internal.Apple.COM>
Date: 16 Nov 89 00:11:49 GMT
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
Lines: 69
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>

In article <26693@santra.UUCP> jmunkki@kampi.hut.fi (Juri Munkki) writes:
> Paul Snively continues:
> >In particular, I can think of some really evil compatibility problems 
that 
> >your suggested fix could provoke; that's one reason that we aren't 
likely 
> >to use it in the future.
> 
> Please tell us what the problems are so that we can work them out.

The most obvious that come to my mind are: some software, such as 
debuggers, look directly at the trap dispatching tables because they have 
to.  Other software does either for (silly) performance reasons, or to 
avoid being lied to by _GetTrapAddress.  The problem here is that if 
_GetTrapAddress lies to you (as it DOES under certain conditions with 
certain vendors' hardware/software) then you have no other recourse.

And there's no way to "work them out" without completely throwing out the 
current trap<->address schtick and starting over.  Fat chance.

In article <26693@santra.UUCP> jmunkki@kampi.hut.fi (Juri Munkki) writes:
> I find it very interesting that you find that you can just say "it can't 
be done"
> without explaining yourself. Just because you work at Apple doesn't mean
> that you are right just because you say so. Just because I say I'm right
> doesn't mean I'm right, but you have to prove me wrong first!

Spare me, please.  I had only hoped to save some time/bandwidth by not 
rehashing something that Macintosh Developer Technical Support (and 
others) have said time and time again: don't tail patch; you'll break 
things in ways that are far too subtle and bizarre to fully explain.  The 
fact that I work at Apple may not confer 100% accuracy, but it IS my job 
to say things like "don't do that because it won't work," and it IS in 
your best interests to listen to me and the others who are in this 
(unfortunate) position, because far more often than not, we know what we 
are talking about.

Incidentally, when you say something is true, I am certainly under no 
obligation to prove you wrong; the burden of proof lies on the person 
making the statement, if indeed anything is to be proven (as opposed to 
being accepted as a statement of personal opinion).

In article <26693@santra.UUCP> jmunkki@kampi.hut.fi (Juri Munkki) writes:
> The discussion on tail patching and come-from patching has always been
> theoretical. I've never really believed that Apple would ever fix 
anything
> that they have decided to break... So, let's say that everything I write
> is just theoretical. That way no one will get offended, right?

Perhaps no one will become offended, but will the writing then have any 
semantic value whatsoever?  I fully expect to offend some people, but if 
the other side-effect is to have more robust Macintosh software that 
follows more of the rules, then that's a price I'll pay in spades.

And I'd be interested in any examples of "something that we have decided to break," let alone something that we have decided to break and then not fix.

The discussion about tail-patching and come-from code is most certainly 
NOT theoretical; many people tail-patching have said that they are doing 
this, and many of us here at Apple have said that we're not saying "this 
is a bad idea in theory."  It is the WRONG thing to do and it DOES break 
software, period, no ifs, ands, or buts, read my lips, for the last time: 
TAIL-PATCHING BREAKS MACINTOSH SOFTWARE, SO DON'T DO IT.

__________________________________________________________________________
Just because I work for Apple Computer, Inc. doesn't mean that they 
believe what I believe or vice-versa.
__________________________________________________________________________
C++ -- The language in which only friends can access your private members.
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