Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!snorkelwacker!bloom-beacon!eru!hagbard!sunic!nuug!ifi!pers
From: pers@ifi.uio.no (Per Siljubergs}sen)
Newsgroups: comp.sys.mac.programmer
Subject: Access global variables from trap patches? (Summary)
Summary: Store reference to globals inside code
Message-ID: <1990Oct4.062049.17455@ifi.uio.no>
Date: 4 Oct 90 06:20:49 GMT
References: <1990Sep24.231618.29949@ifi.uio.no>
Organization: Dept. of Informatics, University of Oslo, Norway
Lines: 64
Nntp-Posting-Host: gille.ifi.uio.no
Originator: pers@gille.ifi.uio.no


Thanks to all of you who helped me out with this. 

In article <1990Sep24.231618.29949@ifi.uio.no> pers@ifi.uio.no (Per Siljubergs}sen) writes:

   >The usual tips regarding stand-alone code don't seem to
   >help me. Say one of my patches need access to the globals:
   >
   >   1) The patch can't get it's hands on my A5 world or a
   >      handle to the globals through the parameters, because
   >      they (the parameters) must be the same as the trap.
   >
   >   2) I can't set up A5 before the patch gets called. I have
   >      no way to predict when the system or the applications
   >      will call the trap.

Most of the answers I got, recommended to store a reference to the
globals inside the stand-alone code segment. Here follows a little
subset of comments (in received order):

David Shayer <das@apple.com> writes:

   >The way I would probably do it is to store the A5 inside my code.
   >This is way DTS usually recommends.  With a little assembly code,
   >you can set up space in your code, and have it set to zero.  Also,
   >set up an assembly routine which returns the address of this space.
   >If its zero, you know its the first time through, and you have to
   >set up the A5 world.  Otherwise just set A5 to the stored value.

Mark Valence <kurash@chocorua.dartmouth.edu> writes:

   >   ... set aside a longword inside your patch.  This
   >way, the INIT that installs the patch can store a pointer
   >to your global variables in the space you provided.  This
   >gets around the problem of the instruction cache because
   >you are writing to that memory location before the code
   >is run (so it's not in the cache), and once you've
   >written to it, it never changes (so the instruction
   >cache will always be valid).

Larry Rosenstein <lsr@apple.com> writes:

   >>   3) System 7.0 IPC? Is it possible? Even if it is, I would
   >>      like this to work with System 6.x.x too.
   >
   >Actually in System 7 you can use the Gestalt manager to register your A5
   >value under a 4-byt signature.  Each piece of code can then look up the
   >value of A5 using that signature.
   [My comment: This works with System 6.0.4 and later.]

Andrew D. Sonenblick <sonenbli@oxy.edu> writes:

   >I use A4--and Think C makes this very simple for me.  All I have to do
   >is set it up to the the value A0 had the first time my init was called..
   >and yes, I do save this to my own code.  Apple said this isnt that bad
   >and shouldnt be a problem.  I have done this and it works on all
   >processors and all systems...


-Per

----------
Per Siljubergsaasen, Dept. of Informatics, University of Oslo, Norway
pers@ifi.uio.no