svn commit: r195460 - projects/mesh11s/sys/net80211

[M. Warner Losh]

In message: <4A561477.7000508 at freebsd.org>
            Tim Kientzle <kientzle at FreeBSD.org> writes:
: Dag-Erling Smørgrav wrote:
: > Bruce Evans <brde at 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" :)