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Re: [virt-tools-list] [osinfo PATCH] Mark the PrimaryVolumeDescriptor struct as packed



On Fri, Jan 06, 2012 at 06:52:04AM -0700, Eric Blake wrote:
> On 01/06/2012 05:21 AM, Christophe Fergeau wrote:
> > This tells gcc it shouldn't add some padding between the struct
> > members. which is important because we rely on this structure size
> > being exactly 2048. One of its members is guint8 ignored2[246]; (only
> > a 2 byte multiple) so all the fields of the struct won't be aligned
> > on a 8 byte boundary, so gcc could do some unexpected things on
> > some arch.
> > ---
> >  osinfo/osinfo_media.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/osinfo/osinfo_media.c b/osinfo/osinfo_media.c
> > index 2366993..2a80205 100644
> > --- a/osinfo/osinfo_media.c
> > +++ b/osinfo/osinfo_media.c
> > @@ -44,7 +44,7 @@ struct _PrimaryVolumeDescriptor {
> >      guint8 ignored2[246];
> >      gchar  publisher[MAX_PUBLISHER]; /* Publisher ID */
> >      guint8 ignored3[1602];
> > -};
> > +} __attribute__ ((packed));;
> 
> When a struct must be an exact size, it's probably worth adding a
> compile-time verification, so that you get a compilation failure if any
> compiler ever picks the wrong size.  If you used gnulib, it would be
> fairly trivial to use "verify.h" for this purpose, but even without
> gnulib, you can do:
> 
> char dummy[sizeof(struct _PrimaryVolumeDescriptor) == 2048 ? 1 : -1];

The gnulib  verify.h header is pretty much self-contained & license
compatible, so we could just copy it across to our code without
any pain.


Daniel
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