On Tue, Aug 23, 2016 at 01:34:05PM -0700, Joe Perches wrote:
> On Tue, 2016-08-23 at 21:09 +0100, Al Viro wrote:
> > On Tue, Aug 23, 2016 at 11:24:06AM -0700, David Miller wrote:
> > ... and then we can file a bug report against the sodding compiler. Note
> > that
> > struct ethtool_wolinfo {
> > __u32 cmd;
> > __u32 supported;
> > __u32 wolopts;
> > __u8 sopass[SOPASS_MAX]; // 6, actually
> > };
> > is not going to *have* padding. Not on anything even remotely sane.
> > If array of 6 char as member of a struct requires 64bit alignment on some
> > architecture, I would really like some of what the designers of that ABI
> > must have been smoking.
>
> try this on x86-64
>
> $ pahole -C ethtool_wolinfo vmlinux
> struct ethtool_wolinfo {
> __u32 cmd; /* 0 4 */
> __u32 supported; /* 4 4 */
> __u32 wolopts; /* 8 4 */
> __u8 sopass[6]; /* 12 6 */
>
> /* size: 20, cachelines: 1, members: 4 */
> /* padding: 2 */
> /* last cacheline: 20 bytes */
> };
That would be padding after the structure elements.
I think what was meant is that it won't add padding in the middle of the
structure due to alignment, ie it isn't doing:
struct ethtool_wolinfo {
__u32 cmd; /* 0 4 */
__u32 supported; /* 4 4 */
__u32 wolopts; /* 8 4 */
<4 bytes padding here>
__u8 sopass[6]; /* 16 6 */
};
which would have 4 bytes of padding in the middle between wolopts
and sopass.
I would not think it is the compilers job to worry about what is after
your structure elements, since you shouldn't be going there.
--
Len Sorensen
