From: Joe Perches
> On Wed, 2014-05-28 at 17:11 -0500, Cody P Schafer wrote:
> > On Wed, May 28, 2014 at 5:05 PM, Cody P Schafer <d...@codyps.com> wrote:
> > > On Wed, May 28, 2014 at 3:45 AM, David Laight <david.lai...@aculab.com>
> > > wrote:
> > >> From: Cody P Schafer
> > >>> Rather manually specifying the size of the integer to be converted, key
> > >>> off of the type size. Reduces duplicate size info and the occurance of
> > >>> certain types of bugs (using the wrong sized conversion).
> > >> ...
> > >>> +#define be_to_cpu(v) \
> > >>> + __builtin_choose_expr(sizeof(v) == sizeof(uint8_t) , v, \
> > >>> + __builtin_choose_expr(sizeof(v) == sizeof(uint16_t),
> > >>> be16_to_cpu(v), \
> > >>> + __builtin_choose_expr(sizeof(v) == sizeof(uint32_t),
> > >>> be32_to_cpu(v), \
> > >>> + __builtin_choose_expr(sizeof(v) == sizeof(uint64_t),
> > >>> be64_to_cpu(v), \
> > >>> + (void)0))))
> > >> ...
> > >>
> > >> I'm not at all sure that using the 'size' of the constant will reduce
> > >> the number of bugs - it just introduces a whole new category of bugs.
> > >
> > > Certainly, if you mis-size the argument (and thus have missized one of
> > > the variables containing the be value, probably a bug anyhow), there
> > > will be problems.
> > >
> > > I put this interface together because of an actual bug I wrote into
> > > the initial code of the hv_24x7 driver (resized a struct member
> > > without adjusting the be*_to_cpu() sizing).
> > > Having this "auto sizing" macro means I can avoid encoding the size of
> > > a struct field in multiple places.
> >
> > To clarify, the point I'm making here is that this simply cuts out 1
> > more place we can screw up endianness conversion sizing.
>
> It does screw up other types when you do things like:
>
> u8 foo = some_function();
>
> cpu_to_be(foo + 1);
>
> the return value is sizeof(int) not u8
And you'd have to be very, very careful with constants:
#define SOME_CONST xxxxx
cp_to_be(SOME_CONST)
isn't likely to be the right size.
David
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