Re: calloc question about clearing allocated memory

[Vicente Bergas <vicencb@gmail.com>]

Thanks for the explanation, now it is clear!

On Tue, Jun 27, 2017 at 3:13 PM, Szabolcs Nagy <nsz@port70.net> wrote:
> * Vicente Bergas <vicencb@gmail.com> [2017-06-26 23:30:53 +0200]:
>> Hello,
>> when compiling a program against the musl libc and running it under
>> valgrind, then it reports lots of uninitialized value errors.
>> I have tracked them down to only two sources:
>> http://git.musl-libc.org/cgit/musl/tree/src/string/strlen.c?id=e6def544#n15
>> http://git.musl-libc.org/cgit/musl/tree/src/malloc/malloc.c?id=e6def544#n375
>>
>> The first one about strlen is clearly an optimization everybody would
>> like to have, but the second one in __malloc0 is not that obvious.
>> Please, could that be explained: why is *z read just before being
>> cleared?
>> In fact it is counter-intuitive and looks like an overhead.
>> In case there is a reason for it, it would be nice to put an
>> explanatory comment there. Otherwise, that extra read could be
>> removed to make valgrind happy.
>
> this should be the faq of the month..
>
> a zero page does not take up resources, however when
> it is written (even if that's just 0) the page gets
> dirty and it does take up resources (and the write
> operation would cause a page fault introducing
> significant delays)
>
> so if(*z) *z=0; should be an obvious optimization.
> that said the current code is not optimal and an
> improvement was just posted:
> http://www.openwall.com/lists/musl/2017/06/26/1
> but that won't fix the valgrind issue: to fix that
> valgrind just needs to be taught not to report
> uninitialized error for calloced memory.
>
>>
>> Regards,
>>   Vicenç.
>>
>> P.S.: Please, CC me as I am not subscribed to the list.