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From: Simon Wright <simon@pushface.org>
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Subject: Re: Custom Storage Pool questions
Date: Wed, 15 Sep 2021 21:40:38 +0100
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

>> Mind, I don't quite see how to actually access the header info for a
>> particular allocation ...
>
> By subtracting a fixed offset from Pointer.all'Address. The offset is
> to be determined, because X'Address lies.

Obvs.

But my main point was, the blog which was Jere's original problem is in
fact (now?) wrong.

Anyway, the Gnatcoll solution is here ...
https://github.com/AdaCore/gnatcoll-core/blob/master/src/gnatcoll-storage_pools-headers.adb#L114