Path: utzoo!attcan!uunet!husc6!uwvax!oddjob!tank!nucsrl!bob
From: bob@eecs.nwu.edu (Bob Hablutzel)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Multifinder: how do I patch GetNextEvent?
Message-ID: <10050030@eecs.nwu.edu>
Date: 14 Oct 88 13:29:31 GMT
References: <747@ttrdf.UUCP>
Organization: Northwestern U, Evanston IL, USA
Lines: 55

> In article <10050025@eecs.nwu.edu> bob@eecs.nwu.edu (Bob Hablutzel) writes:
> >>What I'm still trying to figure out is this: Why does Apple persist in
> >>forbidding a very useful type of patch?
> >
> >The reason is that 
>   [very lucid explanation not repeated here.]
>
> Thank you, Bob. Now I understand. But I still don't like it :-(

You're welcome.

> There are some traps (like GetNextEvent) which are usefully patched at
> the end. There are lots of people (like me) who want to accomplish something
> that can only be accomplished by tail patching this trap. So our application
> works today, but may someday cause strange failures in other areas.

Well, there actually is another way, if you don't mind getting your feet
a little wet (assembler, or at least glue routines).

There exists a low memory global variables jGNEFilter, which, if not nil,
contains the address of a function to call before returning from _GetNextEvent.
_GetNextEvent calls the routines with A1 (I think) pointing to the event
record for the _GetNextEvent call. You can play with this record in any
way you like.

> As I understand it (now), the only traps which must not be tail patched are
> those which:

> 1. Are relatively small, and
> 2. Are called from other (bigger) traps.

This seems right to me, although one can never tell. We may be missing 
something. But, like I say, it seems right.

> Is it thus possible for Apple to classify traps according to these criteria,
> and to declare some traps as safe for tail patching? I would hope that
> GetNextEvent could be declared safe. Anyone from Apple want to comment on
> this issue?

While it may be possible, I doubt Apple will do it. They have to leave 
their options open.

> I guess it's unavoidable, but it's still annoying that subtle new rules keep
> popping up, documented in obscure places:
> 1. Avoid tail patches.
> 2. Call GetNextEvent three or four times before opening a window.
> 3. Don't use the application events for application events.

Did I miss something? I thought the only application event retroactively
nonexistant was APP4. (Personally, I don't know why Apple didn't take
over network events for multifinder events, since network events don't
exist anymore anyhow. Sigh). But, like you say, the documentation is sometimes
obscure. :-)

Bob Hablutzel		BOB@NUACC.ACNS.NWU.EDU