From: Junio C Hamano <gitster@pobox.com>
Date: Fri, 23 Sep 2022 22:04:10 GMT
Subject: Re: [PATCH] read-cache: avoid misaligned reads in index v4
Message-ID: <xmqqtu4xsq79.fsf@gitster.g>
In-Reply-To: <Yy4nkEnhuzt2iH+R@coredump.intra.peff.net>

Jeff King <peff@peff.net> writes:

> Here (and elsewhere), you can assume that the offsetof() "sec" in
> cache_time is 0, for two reasons:
>
>   - I didn't look up chapter and verse, but I'm pretty sure the standard
>     does guarantee that the first field of a struct is at the beginning.

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf

6.7.2.1 #13 (page 103)

    Within a structure object, the non-bit-field members and the
    units in which bit-fields reside have addresses that increase in
    the order in which they are declared. A pointer to a structure
    object, suitably converted, points to its initial member (or if
    that member is a bit-field, then to the unit in which it
    resides), and vice versa. There may be unnamed padding within a
    structure object, but not at its beginning.

As an initial padding is forbidden, the first member's offset is zero.