[U-Boot] [PATCH] fat: fix unaligned errors

[Mike Frysinger]

On Friday 02 January 2009 18:48:50 Wolfgang Denk wrote:
> In message <200901021742.05363.vapier at gentoo.org> you wrote:
> > > > +__u8 get_vfatname_block[MAX_CLUSTSIZE] __attribute__
> > > > ((aligned(sizeof(__u16))));
> > >
> > > ...
> > >
> > > > +__u8 get_dentfromdir_block[MAX_CLUSTSIZE] __attribute__
> > > > ((aligned(sizeof(__u32))));
> > >
> > > ...
> > >
> > > > +__u8 do_fat_read_block[MAX_CLUSTSIZE] __attribute__
> > > > ((aligned(sizeof(__u32))));
> > >
> > > What makes you sure that a 16 resp. 32 bit alignment is sufficient,
> > > and that gcc does not decide to align such structures even stricter?
> >
> > read the structures in question.  the largest member is 16bit/32bit.  i
> > really dont know what other alignment gcc could randomly generate.
>
> It is not (only) the size of the "largest" member in a struct which
> determins global alignment, but also the size of the struct itself.
>
> See for example
> http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Type-Attributes.html
>
> Read especially this part:
>
> 	In the example above, if the size of each short is 2 bytes,
> 	then the size of the entire struct S type is 6 bytes. The
> 	smallest power of two which is greater than or equal to that
> 	is 8, so the compiler sets the alignment for the entire struct
> 	S type to 8 bytes.

i was not aware of this, thanks for the highlight

> > > Wouldn't it make more sense to use "__alignof__ ()" here to be on the
> > > safe side?
> >
> > i found no such alignment attribute in the gcc docs which is why i
> > suggested Bryan use aligned(sizeof(...)).
> > http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Function-Attributes.html
> > http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Variable-Attributes.html
> > http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Type-Attributes.html
>
> Maybe you should have searched for the (IMHO pretty obvious) term
> "Alignment" as well, i.e. look at
> http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Alignment.html#Alignment
>
> Quote:
>
> 	The keyword __alignof__ allows you to inquire about how an
> 	object is aligned, or the minimum alignment usually required
> 	by a type. Its syntax is just like sizeof.

well i guess we cant all be "Wolfgang Denk"s
-mike
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