Re: 2.2/2.4/2.6 VMs: do malloc() ever return NULL?

Oliver
2003-11-25T23:40:40Z

I thought it did return NULL, now... Before that i didn't check for NULL :)

--Oliver

Ihar 'Philips' Filipau wrote:

> Richard B. Johnson wrote:
>
>>
>> As documented, malloc() will never fail as long as there
>> is still address space (not memory) available. This is
>> the required nature of the over-commit strategy. This is
>> necessary because many programs never even touch all the
>> memory they allocate.
>>
>
> We are reading different mans? My man malloc(3) clearly states that
> malloc() can return NULL. (*)
>
> May I ask you one question? Did you were ever doing once graceful
> failure of application under memory pressure? Looks like not.
>
> I can guess why sendmail allocates memory it never touches - memory
> pools. There are situations where you really cannot fail - and memory
> allocation failures are really nasty. Do you wanna to lose your e-mails?
> No? So then think twice, while implementing lazy allocators.
>
> So from my tests I see that by default Linux is not safe. You allocate
> memory - malloc() != NULL. Then later you try to write to this memory
> and you get killed by oom_killer. What is the point of this? Your
> reasoning doesn't sound to me.
>
> Memory pools used by applications exactly to make grace error
> handling under memory pressure - but it looks like this stuff under
> Linux gets no testing at all. And default settings could make from
> simple bug complete disaster.
>
> > You can turn OFF over-commit by doing:
> >
> > echo "2" >proc/sys/vm/overcommit_memory
> >
> > However, you will probably find that many programs fail
> > or seg-fault when normally they wouldn't. So, if you don't
> > mind restarting sendmail occasionally, then turn off over-commit.
> >
>
> I shall try overcommit_memory == 2 tomorrow and say what I see.
>
> P.S. For example application I have ported right now to kernel space has
> a limitiation - it must never ever allocate memory: memory consumption
> is known, protocol just have no situation like ENOMEM - it _must_ fail
> to initialize on start-up. No - not to being killed by oom_killer in
> middle of processing. think carrier grade and/or just good programming
> technics.
>
> (*) Great optimization opportunities: remove from all programmes checks
> of the return value if malloc(). As by your words - why not?
>