Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!bbn!gatech!mcnc!ecsvax!utoddl
From: utoddl@ecsvax.UUCP (Todd M. Lewis)
Newsgroups: comp.sys.amiga.tech
Subject: Catching free memory violations
Keywords: FreeMem monitor debug memory spam
Message-ID: <7408@ecsvax.UUCP>
Date: 27 Jul 89 17:37:43 GMT
Organization: UNC Educational Computing Service
Lines: 29


I have a program which seems to use memory it doesn't own
from time to time, and I'd like some way to catch it in the act.
I've been thinking about a method to see if/when it messes up free
memory, and I'd like to know why this wouldn't work:

    * Zap FreeMem() to zero out memory whenever it is freed.
    * Zero out all memory which is already freed.
    * Periodically scan all free memory to see if it is still
       zero'd out.  If it isn't, report the address range that
       has been munged (so you know about it) and optionally 
       zero that RAM (and see who dies:).

Because of the overhead this would put on the system you probably
wouldn't want to run it all the time, but during software development
and testing this could be handy to have around.  It wouldn't catch
writing over another program's data/code, but it could catch some
illegal uses of free RAM.

What I'd like to know is:
    1   Would this work?  What are the pitfalls of this method?
    2   Has it already been done?
    3   Would anybody else be interested it this?

Todd M. Lewis, utoddl@ecsvax.uncecs.edu, utoddl@ecsvax.bitnet
  To quote Eugene H. Spafford, "Crisis and Aftermath", Communications of
  the ACM, vol. 32, no. 6, (June 1989), p. 684:
  "It is curious that this many people [...] would assume to know the
   intent of the author based on the observed behavior of the program."