Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!tut.cis.ohio-state.edu!att!cbnewsk!ech
From: ech@cbnewsk.ATT.COM (ned.horvath)
Newsgroups: comp.sys.mac.programmer
Subject: Re: HLock and Think C 4.0
Message-ID: <1553@cbnewsk.ATT.COM>
Date: 24 Nov 89 02:05:13 GMT
References: <3997@sbcs.sunysb.edu>
Organization: AT&T Bell Laboratories
Lines: 34

From article <3997@sbcs.sunysb.edu>, by vallon@sbcs.sunysb.edu (Justin Vallon):

! Yes, it is safe to do a BlockMove without locking the handle.  One of IM's
! appendicies is a list of routines that move/purge memory.  BlockMove isn't
! in it, so you can use it without locking.
! 
! However, you seem to have discovered the problem.  The first call to your
! ANSI library causes a LoadSeg (see Segment Loader for details), since the
! library (is presumably) in an unloaded segment.  Notice that LoadSeg is in
! the list of routines that may move/purge memory.  Memory is moved, and the
! address to your string is now pointing into never-never land.  The second
! call succeeds because the segment has been loaded, and LoadSeg is not
! called.

! Solution:  (1)  Make sure that all "safe" routines (ie:  ones that do not
! move/purge memory) are always loaded.  This insures that a disasterous
! LoadSeg is not called when you are vulnerable.  (2)  Be paranoid, and
! Lock everything all the time.  This sort of defeats the purpose, and it's
! sort of like stopping at a green light, just in case.

! (1) is probably a better solution.  I usually leave the libraries in the
! main segment, so that stuff like this doesn't happen.

This is difficult when linked with the full ANSI library in TC4 (around 24K
I think).  However, it is very sensible to preload the segment(s) containing
MacTraps and ANSI.  You can do this explicitly: touch the library, select
the Get Info command, and note the segment number.  In ResEdit, open the 
built application, open CODE, touch the segment, and do a Get Info.  Set
the Preload bit.

An indirect method is to simply make a call on the library early, and never
UnloadSeg any library segment.  A dummy BlockMove call would appear to do it!

=Ned Horvath=