Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!well!alcmist
From: alcmist@well.UUCP (Frederick Wamsley)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Pointer arithmetic in C w/ or w/o locked handles
Message-ID: <5744@well.UUCP>
Date: 20 Apr 88 05:22:47 GMT
References: <1988Apr15.125337.100@mntgfx.mentor.com> <10897@tut.cis.ohio-state.edu>
Reply-To: alcmist@well.UUCP (Frederick Wamsley)
Organization: Whole Earth 'Lectronic Link, Sausalito, CA
Lines: 35
Keywords: pointer C arithmetic handles

In article <10897@tut.cis.ohio-state.edu> paul@morganucodon.cis.ohio-state.edu (Paul Placeway) writes:
>In article <1988Apr15.125337.100@mntgfx.mentor.com> tomc@mntgfx.mentor.com (Tom Carstensen) writes:
><     ...
>
><     HUnlock(hdl);
><     length = (long) (ptr - *hdl);
>< ...
>< If I move the length calculation before I unlock the 
>< handle, then length becomes 8, and I'm ok.
>
>I would be _really_ supprised if something like this continued to work
>correctly (assuming that it does allready; which it seems not to).
>
>Personally, I wouldn't HUnlock the handle until you are done with it
>(ie. _after_ the length calculation).  It is possible (although currently
>unlikely) that the block that the handle points to could be moved
>between the HUnlock and the last derefrence.  
Memory blocks can only move if you call a trap in the (documented) set
which use the Memory Manager directly or indirectly, or if you make
an intersegment call (which can potentially call _LoadSeg).

The only problem with dereferencing a handle to unlocked memory under
safe conditions is a code maintenance problem.  If you or someone else
accidentally changes the code so that it can cause memory motion, you
suddenly have an intermittent bug.

Of course keeping everything locked while in use will prevent a nasty
kind of bug, but it's unfriendly to keep anything locked longer than
necessary, even if you call MoveHHi first to keep it from fragmenting the 
heap.

Wistfully thinking about hardware memory management, I remain
-- 
Fred Wamsley  {dual,hplabs}!well!alcmist;well!alcmist@lll-crg.arpa;
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