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	Mon, 17 Jun 2019 14:13:12 -0700 (PDT)
Subject:	Re: malloc() returns pointer to already allocated memory
To:	djgpp AT delorie DOT com
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From:	"J.W. Jagersma (jwjagersma AT gmail DOT com) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
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On 2019-06-17 19:08, Eli Zaretskii (eliz AT gnu DOT org) [via 
djgpp AT delorie DOT com] wrote:
>> From: "J.W. Jagersma (jwjagersma AT gmail DOT com) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
>> Date: Mon, 17 Jun 2019 18:46:24 +0200
>>
>>> 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.
> 
> How can it?  sbrk cannot possibly cause this because your program
> doesn't sound like one that would ever release sbrk'ed memory back to
> the DPMI host.  And malloc just fills the buffer it hands to you,
> there's no effect of this flag on the address of that buffer.

The zero-fill is performed at the end of sbrk 
(src/libc/crt0/crt0.S:614), so a bug like this has to originate before 
that point; either in sbrk itself, or the dpmi host. If at any point 
before the zero-fill the same pointer is returned twice (or possibly 
different pointers mapped to the same memory), you wouldn't be able to 
detect that.

>> With this flag enabled, the test does complete successfully on
>> cwsdpmi.
> 
> Which probably means your detection of "already allocated" is flawed
> in some sense.  All this flag does is wipe out any possible garbage in
> the memory you get from malloc, left-overs from previous allocations;
> the flag AFAIR has no other effects on the algorithms of malloc.

If you never free anything, as the test program does, you wouldn't 
expect there to be any leftovers from previous allocations.
However I have written a new test case (attached) which saves and checks 
previously allocated pointers for aliasing. It's been running for a 
while and so far failed to find any. So it appears that the "clobbering" 
I was initially seeing is an artifact of the way cwsdpmi allocates new 
pages, apparently it sporadically clones some data from other pages.

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