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	Thu, 03 Sep 2026 10:45:50 -0700 (PDT)
Message-ID:	<73ea67bd-ae77-4338-8698-e755bee80959@gmail.com>
Date:	Thu, 3 Sep 2026 13:45:48 -0400
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Subject:	Re: memalign broken (again)
To:	djgpp-workers AT delorie DOT com
References:	<xn1pbaxnji DOT fsf AT envy DOT delorie DOT com>
From:	"Frank Sapone (emoaddict15 AT gmail DOT com) [via djgpp-workers AT delorie DOT com]" <djgpp-workers AT delorie DOT com>
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On 9/3/2026 1:17 PM, DJ Delorie (dj AT delorie DOT com) [via 
djgpp-workers AT delorie DOT com] wrote:
> "Frank Sapone (emoaddict15 AT gmail DOT com) [via djgpp-workers AT delorie DOT com]"
> <djgpp-workers AT delorie DOT com> writes:
>>> @@ -141,7 +142,9 @@ void *nmemalign(size_t alignment, size_t size)
>>>                /* two or more chunks to release */
>>>                DBGPRTM("  Complex case, release multiple chunks");
>>>                DBGEOLN;
>>> -            nfree(PTR(__nmalloc_split(&m, alignment - misalign)));
>>> +            malign = (char *)(((ulong)minit + XTRA - 1) & ~(alignment - 1));
>>> +            m1 = MEMBLKp(malign);
>>> +            nfree(PTR(__nmalloc_split(&m, (ulong)m1 - (ulong)m)));
>>>                return nrealloc(PTR(m), size);
>>>             }
>>>          } /* alignment > ALIGN */
>> Bringing this up again.  I have had a small handful of complaints about
>> combinelo: memory fouled on some machines for my Quake DOS port.  This
>> was never tested and vetted, but the patch looks and sounds reasonable.
> I assume you *did* test it with your port?
>
> My thoughts after years of fixing glibc malloc bugs... ;-)
>
> If the minimum chunk size is 3 (2 overhead, 1 data), and you want to
> align to something more than 3 chunks, the worst case is the front-extra
> is slightly less than a chunk (2 units, in this case).  To fix that, you
> have to move the aligned pointer by an extra "alignment", making it
> 2+alignment past the original allocation.
>
> Example: alignment is 64, ALIGN is 8, size is 1024, and the returned
> pointer is 0xff8 (chunk at 0xff8 - 0x10 = 0xfe8)
>
> The next valid chunk after the above (if smallest split) would be
> 0x1000/0x1010 but 0x1010 is not sufficiently aligned.
>
> The next aligned address after that is 0x1040 (chunk 0x1030, so 0x1030/0x1040)
>
> So the needed adjustment would be 0x1030/0x1040 - 0xfe8/0xff8 = 0x48 (72).
>
> malign in the above would be (0xff8 + (16+64) - 1) & ~ 63 or 0x1040
> m1 would thus be 0x1040 - 0x10 = 0x1030.
>
> (note: I did a full analysis before I remembered that XTRA included
> "alignment" and was going to claim the patch was wrong, so my confidence
> in this new analysis is not 100% ;)

Hey DJ,

I admit I did NOT test it, yet.  I'd have to recompile the libc for this 
and never made time to do so.  For the users with the memory fouled 
errors I don't always hear back from them.  They just send me 
screenshots complaining about it on 486 machines (which are not 
appropriate for Quake 1... but...).  So who knows?

Frank

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