Re: Migrating to larger numbers
H. Peter Anvin (hpa@transmeta.com)
Fri, 28 May 1999 17:40:25 -0700
Guest section DW wrote:
>
> > dev_t: 32 bits minimum, suggest 64 bits
>
> > Recall that uid_t, gid_t and dev_t both have -1 (0xffff) reserved
> > in a 16-bit environment;
>
> This suggests a particular way of going to a larger dev_t.
>
> I have been using (and am using a kernel like this right now):
>
> static inline kdev_t to_kdev_t(dev_t dev)
> {
> int major, minor;
>
> major = (dev >> 32);
> if (!major) {
> major = (dev >> 16);
> if (!major) {
> major = (dev >> 8);
> minor = (dev & 0xff);
> } else
> minor = (dev & 0xffff);
> } else
> minor = (dev & 0xffffffff);
>
> return MKDEV(major, minor);
> }
>
> I am not aware of any disadvantages of this system,
> although, now that I think about it, the fact that
> multiple representations yield the same (major,minor)
> has both advantages and disadvantages.
> So far I have not encountered real problems.
A much worse error is that it relies on zero, not -1. Zero is not a
reserved device number; in fact, it has special meaning.
-hpa
--
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