Re: [PATCH 08/11] xfsprogs: replace obsolete memalign with posix_memalign

[Jan Tulak <jtulak@xxxxxxxxxx>]

On Tue, Aug 18, 2015 at 10:20 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:

On Tue, Aug 18, 2015 at 09:04:24AM +0200, Jan Tulak wrote:
> I thought about it. However, with memalign from malloc marked obsolete
> (and with posix_memalign having guaranteed alignment restrictions [1]), I
> saw it better
> to use the posix variant everywhere.

Putting a sane wrapper around an nasty library function is just
fine. The memalign wrapper makes sense from this perspective - even
gcc can't tell if variables passed to posix_memalign are correctly
initialised or not, whereas no such problems exist for memalign().

> I could make a wrapper simulating the old memalign behaviour, but I don't
> think it would make sense.

I think it makes more sense than using posix_memalign() everywhere
and then ignoring the return variable that tells you it failed...

> I searched for this, but didn't find any reasonable answer:
> How long can be things in standard libraries marked obsolete before
> removing?

With a wrapper, we don't care.

> [1] man memalign:
>Â Â Â Â On many systems there are alignment restrictions, for example, on buf-
>    fers used for direct block device I/O. POSIX specifies the path-
>Â Â Â Â conf(path,_PC_REC_XFER_ALIGN) call that tells what alignment is needed.
>Â Â Â Â Now one can use posix_memalign() to satisfy this requirement.
>
>    posix_memalign() verifies that alignment matches the requirements
>    detailed above. memalign() may not check that the alignment argument
>Â Â Â Â is correct.

Yes, you can get it wrong with memalign. But we don't, because we
follow the rules for DIO buffer alignment and set it correctly.
Being able to directly control the alignment of the memory buffer is
a reason for using memalign() over posix_memalign(), not the other
way around.

So a wrapper used on all platforms is an acceptable solution?ÂAll right,Â

this explanation makes sense.ÂI will change it that way. The only question

I have now is whether to use posix_memalign on every platform, or whether to

make it platform_memalign and use the old memalign insideÂfor Linux.

Cheers,

Jan

--

Jan Tulak

jtulak@xxxxxxxxxx