Path: utzoo!utgpu!attcan!uunet!samsung!usc!apple!marc
From: marc@Apple.COM (Mark Dawson)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Tail patches
Message-ID: <36250@apple.Apple.COM>
Date: 6 Nov 89 15:40:39 GMT
References: <1459@sequent.cs.qmc.ac.uk>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 57

In article <1459@sequent.cs.qmc.ac.uk> jeremyr@cs.qmc.ac.uk (Jeremy Roussak) writes:
>
>I hope the net will pardon my ignorance, but I have seen a few comments
>recently about "tail patching" of traps and have a couple of questions:
>
>1. What exactly are "tail patches"?
>2. If they are what I suspect they are, why are they bad?
>

1.  There are times that you may wish to replace or modify a Mac trap call
   (Say for example, _Write).  Suppose you have this horribly gross program
   that passes the volume name, instead of the drive # to the _Write call.
   You could write an intercept routine that would decode this name into the
   correct drive # and drive refnum for the "real" system _Write call.  It
   would then call the system _Write (This would be a "head" patch).  You
   could direct the system _Write to return to another piece of code (the
   "tail" patch) to do some special cleanup if there was some error.

   The problem with the "tail" patch is that there is some possibility that
   an Apple patch to that routine (to fix some bug/enhance it)  might be
   checking the return address to see if it should do something.  Since your
   return address would be on the stack first, this check would fail.  Before
   I worked at Apple I did use both the "head" and "tail" patches; I didn't see
   anyway around them.  I also never ran into this "return address check".  I
   am always crossing my fingers, hoping this case will never happen (it would
   REALLY break my code).   

   Someone else in Apple will have to comment on if they know of any of patches
that DO check the return addresses.  I don't know of any, but I don't claim to 
know the whole system.    Below is a snippet of code to do head & tail patches.
   
; 
; get address of system trap
;
   move.l	WRITE_TRAP,D0		; WRITE_TRAP = $A003 (_Write)
   _GetTrapAddress
   lea  SysTrapAdd,A1
   move.l A0,(A1)                       ; save system trap @
;
; make system _Write goto my trap
;   
   move.l      WRITE_TRAP,D0            ; WRITE_TRAP = $A003 (_Write)
   lea	MyWriteTrap,A0
   _SetTrapAddress 
....
....
;
; My write trap...does translation before & after call
;
MyWriteTrap
	bsr  DoDecode  			; adjust parm block
  	pea  AfterWrite			; write to return to	
	move.l	SysTrapAdd,-(sp)
        RTS				; goto system routine	
AfterWrite
       bsr  DoCleanUp
       RTS				; return to system