On Mon, 5 Aug 2002, Kai Germaschewski wrote:
> On Mon, 5 Aug 2002, Patrick Mochel wrote:
>
> > [1]: The reason for the macro is because the driverfs internals
> > have changed enough to be able to support attributes of any type. In
> > order to do this in a type-safe manner, we have a generic object type
> > (struct attribute) that we use. We pass this to an intermediate layer
> > that does a container_of() on that object to obtain the specific
> > attribute type.
>
> Of course that means that it's not really type-safe, since it has no way
> to check whether the object is embedded in the right type of struct, right
> ;) (But I think that's okay, C doesn't have provisions for real
> inheritance)
Well sure, if you want to get technical. ;) I almost said 'mostly'
type-safe, but I decided I would fib a little to make myself sound
stronger. Basically, I think it's as safe as we can get, and it was
type-safe enough for Linus..
> > This means the specific attribute types have an embedded struct
> > attribute in them, making the initializers kinda ugly. I played with
> > anonymous structs and unions to have something that could
> > theoretically work, but they apparently don't like named
> > initializers.
>
> Have you considered just putting in the embedded part via some macro -
> I think that's what NTFS does for compilers which don't support unnamed
> structs.
>
> Basically
>
> #define EMB_ATTRIBUTE \
> int emb1;
> int emb2
>
> struct my_attribute {
> EMB_ATTRIBUTE
> int my1;
> int my2;
> };
It's not that it's unnamed, it's that we want both object types to exist,
and have something to do container_of() on. Yet, have something easy to
declare.
I wanted something like:
struct attribute {
char * name;
mode_t mode;
};
struct device_attribute {
union {
struct attribute attr;
struct {
char * name;
mode_t mode;
};
device_show show;
device_store store;
};
You can access struct device_attribute::name fine, except when using named
initializers. At least with gcc 2.96.
> That'll work with named initializers just fine, so the users don't have to
> deal with ugly DEVICE_ATTR macros, where one forgets if parameter #3 was
> show or store ;) - It follows the common way of hiding away unavoidable
> ugliness in some header.
Bah, tradeoffs. It's not that hard to get the parameters right; and it
won't take long for them to notice ;)
> > [2]: I wanted to consolidate the first two parameters, but I couldn't
> > find a way to stringify ##name (or un-stringify "strname"). Is that
> > even possible?
>
> Why would stringify (include/linux/stringify.h) not work? However, Al Viro
> may get mad at you for generating ungreppable symbols either way ;-)
Uhm, because I'm retarded, and I didn't actually try.
-pat
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