From: | Andres Freund <andres(at)anarazel(dot)de> |
---|---|
To: | pgsql-hackers(at)lists(dot)postgresql(dot)org,Antonin Houska <ah(at)cybertec(dot)at>,pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: xl_heap_header alignment? |
Date: | 2020-07-21 18:02:11 |
Message-ID: | 5A0F3128-D990-4431-9E97-793CD99FF11E@anarazel.de |
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Lists: | pgsql-hackers |
Hi,
On July 21, 2020 10:45:37 AM PDT, Antonin Houska <ah(at)cybertec(dot)at> wrote:
>I don't quite understand this part of the comment of the xl_heap_header
>structure:
>
>* NOTE: t_hoff could be recomputed, but we may as well store it because
> * it will come for free due to alignment considerations.
>
>What are the alignment considerations? The WAL code does not appear to
>assume
>any alignment, and therefore it uses memcpy() to copy the structure
>into a
>local variable before accessing its fields. For example,
>heap_xlog_insert().
Unless you declare them as packed, structs will add padding to align members correctly (if, and only if, the whole struct is stored well aligned).
Regards,
Andres
--
Sent from my Android device with K-9 Mail. Please excuse my brevity.
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