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	Mon, 17 Jun 2019 09:26:48 -0700 (PDT)
Subject:	Re: malloc() returns pointer to already allocated memory
To:	djgpp AT delorie DOT com
References:	<158e5d20-0a90-4beb-de48-da328379d8fb AT gmail DOT com>
	<qe76u1$1kj8$1 AT gioia DOT aioe DOT org>
From:	"J.W. Jagersma (jwjagersma AT gmail DOT com) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
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Date:	Mon, 17 Jun 2019 18:25:42 +0200
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On 2019-06-17 07:05, Rod Pemberton wrote:
> Second problem is you don't call memset() prior to using memory, nor
> free() after you're done using it.  Of course, calling memset() would
> prevent your method of "clobber" detection from working.  But, not
> calling memset() means you don't know if the magic clobber value is:
> a) from you setting it within your program, or
> b) from some random garbage values in memory.

The probability of one 32-bit memory address having a particular value 
is 1/(2^32), so about 1 in 4 billion. for a machine with, say, 256MB 
memory, you'd expect to find this value roughly once every 64 runs (IF 
all memory is initialized to perfectly random values, which it isn't. 
You're far more likely to find all ones or all zeroes on a clean boot). 
Clearly that isn't the case here, the clobber is detected every single time.
Clearing allocated memory by memset(), you wouldn't be able to find 
clobbered memory unless you kept track of all allocated pointers. Which 
requires a large table... allocated with malloc(). If malloc is bugged, 
you'd end up clobbering this table too.
Initially I did use a fixed magic value, but that causes false-positives 
on subsequent runs since memory isn't reset to zero. Using 
randomly-sized allocations turned out to be unnecessary since it happens 
with fixed size allocations too.

> Finally, you didn't report which version of DJGPP, or CWSDPMI, and
> whether or not your code is operating in a Windows 98/SE/ME/XP etc
> console.  When operating in a Windows console, CWSDPMI is not being
> used.  The Windows DPMI host is being used.

I'm testing on a Pentium 3 machine running FreeDOS, with cwsdpmi r7 and 
hdpmi32 v3.17. I'm cross-compiling from mingw64 with gcc 9.1, using the 
precompiled libraries from djcrx205.zip.

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