Re: [RFC 01/32] fs: introduce new 'struct inode_time'

[Geert Uytterhoeven]

On Sat, May 31, 2014 at 4:54 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Saturday 31 May 2014 10:39:02 Andreas Schwab wrote:
>> Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> writes:
>>
>> > Hi Arnd,
>> >
>> > On Fri, May 30, 2014 at 10:01 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> >> + * The variant using bit fields is less efficient to access, but
>> >> + * small and has a wider range as the 32-bit one, plus it keeps
>> >> + * the signedness of the original timespec.
>> >> + */
>> >> +struct inode_time {
>> >> +       long long       tv_sec  : 34;
>> >> +       int             tv_nsec : 30;
>> >> +};
>> >
>> > Don't you need 31 bits for tv_nsec, to accommodate for the sign bit?
>> > I know you won't really store negative numbers there, but storing a large
>> > positive number will become negative on read out, won't it?
>>
>> Only if the int bitfield is signed.  Bitfields are weird, aren't they?

According to 6.7.2#5 (thanks for the reference), this is implementation defined.

> It was a mistake on my side, as I didn't know about that rule and
> meant write 'unsigned int' really. Also, I always have a bad feeling

IC, but the comment said "plus it keeps the signedness".
So it doesn't keep the signedness for the tv_nsec field.

> about using bitfields in general.

Hehe...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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