Re: [PATCH 7/7] nfsd: nfs4xdr decode_stateid helper function

J. Bruce Fields
2008-08-13T20:17:37Z

On Wed, Aug 13, 2008 at 03:35:53PM -0400, Trond Myklebust wrote:
> On Wed, 2008-08-13 at 15:11 -0400, J. Bruce Fields wrote:
> > On Wed, Aug 13, 2008 at 02:59:52PM -0400, Trond Myklebust wrote:
> > > On Wed, 2008-08-13 at 14:30 -0400, J. Bruce Fields wrote:
> > > > On Wed, Aug 13, 2008 at 01:59:09PM -0400, Trond Myklebust wrote:
> > > > > Which is a good reason for ditching the entire confusing typedef, and
> > > > > replacing it with a packed structure instead:
> > > > >
> > > > > struct stateid {
> > > > > __be32 generation;
> > > > > char opaque[12];
> > > > > } __attribute__((packed));
> > > >
> > > > So without the ((packed)), all arrays get aligned to 8-byte boundaries
> > > > on 64-bit machines? (What do I need to read to catch up here?)
> > >
> > > A quick google showed up:
> > >
> > > http://sig9.com/articles/gcc-packed-structures
> > >
> > > In any case, yes, the idea behind the packed attribute is to turn off
> > > the field alignment.
> >
> > Yeah, I was more curious about how to decide when it's necessary. (Why
> > didn't we need it before? Is an embedded struct always aligned as if
> > the fields of the embedded struct were declared directly in the
> > containing struct? Or should we really just be using the packed
> > attribute *any* time we depend on that alignment, even if it seems
> > obvious the compiler wouldn't need to add padding?)
>
> The advantage of having it packed like the above is that you can still
> use WRITEMEM() to write out the whole structure in one fell swoop.

Right, I understand. But the code has been doing exactly that (a
WRITEMEM of the whole thing) since the beginning, so I wondered if there
was some reason you thought the switch to the extra char opaque[12] in
particular was something that was likely to trigger the addition of
padding.

Sounds instead like your policy would be just to declare any struct
"packed" if we might depend on the absence of padding, without making
any assumptions about what compilers might do. Which is fine.

--b.

> If you don't specify 'packed', then the C standard allows the compiler
> to add padding between the fields in order align them. I'm not sure
> that compilers will usually do that for a 'char[]' field, but they
> will definitely for the integer types.