Re: [RFC] lazy allocation of struct block_device

[Anton Altaparmakov]

At 22:26 01/09/2001, Jamie Lokier wrote:
>[e-mail] wrote:
> >[...]
> > However, a union is not so bad. It seems a pity to avoid unions
> > and waste 4 bytes for every inode with separate i_bdev and i_cdev
> > instead of a single i_bcdev.
>
>Please, a union of different pointer types is much nicer.  You can have
>i_bdev and i_cdev without wasting any bytes.
>
>This form works with GCC 2.96:
>
>                 union {
>                         struct char_device * i_cdev;
>                         struct block_device * i_bdev;
>                 };

It sure does. One of the nicest new gcc features IMHO!

>If you're using a really old compiler that doesn't support anonymous unions,
>(GCC 2.95 might be in this category, I'm not sure),

GCC 2.95 does NOT support this. I only found out after I had people 
complain to me that Linux-NTFS userspace tools would not compile for them 
and it turned out they were using gcc-2.95... and I 2.96. - I have since 
then verified this myself:

egcs and gcc up to 2.95 do not support unnamed structs/unions.

gcc-2.96 and gcc-3.0 support them fine.

>  then you'll need this:
>
>         #define i_bdev __i_bcdev_union.i_bdev
>         #define i_cdev __i_bcdev_union.i_cdev
>                 union {
>                         struct char_device * i_cdev;
>                         struct block_device * i_bdev;
>                 } __i_bcdev_union;

Neat trick! Thanks! I was wondering what to do with NTFS TNG driver (which 
uses unnamed structs/unions extensively) and this just might solve my 
problems without having to rewrite half the driver... (-;

Best regards,

         Anton