Date: Fri, 10 Jul 2009 01:26:30 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: kientzle@freebsd.org Cc: svn-src-projects@freebsd.org, des@des.no, src-committers@freebsd.org, rpaulo@freebsd.org, brde@optusnet.com.au Subject: Re: svn commit: r195460 - projects/mesh11s/sys/net80211 Message-ID: <20090710.012630.-875846397.imp@bsdimp.com> In-Reply-To: <4A561477.7000508@freebsd.org> References: <20090709041145.G46111@delplex.bde.org> <86prc9g8yf.fsf@ds4.des.no> <4A561477.7000508@freebsd.org>
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Tim Kientzle <kientzle@FreeBSD.org> writes:
: Dag-Erling Smørgrav wrote:
: > Bruce Evans <brde@optusnet.com.au> writes:
: >> Broken ABIs (e.g., arm?) may add padding at the end of structs,
: >
: > Padding at the end of a struct is *required* to ensure proper alignment
: > of the next element in an array, e.g.
: >
: > struct foo { int i; char c; } bar[2];
: >
: > Assuming a four-byte alignment requirement for int, the compiler *must*
: > add three bytes of padding at the end of struct foo so bar[1].i is
: > correctly aligned.
: >
: > cf. ISO/IEC 9899:1999 §6.7.2.1:
: >
: > 15 There may be unnamed padding at the end of a structure or union.
:
: Everyone (I hope!) expects alignment in the
: case you outlined. But many of us have been
: surprised to see the ARM compiler pad
:
: struct foo {
: char f1[4];
: char f2[1];
: char f3[1];
: };
:
: to 12 bytes. This breaks a lot of traditional
: code that uses structs of char arrays to
: define memory and disk layouts.
:
: I'm not claiming that the ARM compiler is broken,
: just that its padding behavior surprises
: a lot of people.
It isn't a compiler bug. It is an ABI requirement for ARM. At least
for the ABI we use. there are others that might be better, and not
suffer from this surprise. We[*] should investigate them.
Warner
[*] By "we" of course I mean "some smart guy that isn't me" :)
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