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        Mon, 17 Jun 2019 09:47:30 -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> <83tvcoth73 DOT fsf AT gnu DOT org>
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Date: Mon, 17 Jun 2019 18:46:24 +0200
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On 2019-06-17 16:33, Eli Zaretskii (eliz AT gnu DOT org) [via 
djgpp AT delorie DOT com] wrote:
>> From: Rod Pemberton <invalid AT lkntrgzxc DOT com>
>> Date: Mon, 17 Jun 2019 01:05:38 -0400
>>
>> 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.
> 
> Right.  There's a startup flag that a program could optionally set to
> request that allocated memory be filled with a specific fixed value,
> then tests for clobbering could work.  See the documentation of
> _crt0_startup_flags in the libc manual.
> 

If the issue is caused by the dpmi host or sbrk(), then I think enabling 
the _CRT0_FLAG_FILL_SBRK_MEMORY flag would mask this bug. With this flag 
enabled, the test does complete successfully on cwsdpmi. It also takes 
much longer, probably because it's filling up all 4GBs of virtual memory.