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From: fgd3@jc3b21.UUCP (Fabbian G. Dufoe)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Catching free memory violations
Message-ID: <694@jc3b21.UUCP>
Date: 28 Jul 89 19:44:05 GMT
References: <7408@ecsvax.UUCP>
Organization: St. Petersburg Jr. College, FL
Lines: 24

In article <7408@ecsvax.UUCP> utoddl@ecsvax.UUCP (Todd M. Lewis) asks about
clearing free memory periodically as part of an effort to catch programs
which use memory they don't own.  This would work so long as you keep two
things in mind.

     (1) The pointer to the next chunk of free memory occupies the first
     eight bytes of the current chunk.  You can't free that.

     (2) If a program is writing on memory it hasn't allocated it may have
     clobbered one of those pointers.  Consquently, you can't walk the
     system memory free list with any confidence.

Those points aside, you could check whatever memory a chunk contains beyond
the next chunk pointer to be sure that was zero.  However, any application
might allocate the memory, write on it, and free it without clearing it.
So finding non-zero values in free memory wouldn't mean you had discovered
a program which was writing on memory it didn't own.

--Fabbian Dufoe
  350 Ling-A-Mor Terrace South
  St. Petersburg, Florida  33705
  813-823-2350

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