Re: [Boost-users] [shmem] Help with shmem and fixed segments

From: Berenguer Blasi (bblasi_at_[hidden])
Date: 2006-07-16 04:39:25

Roman,

The OS is Fedora Core 5.

Ion,

Thanks for your time.

I would still update the documentation so that nobody does as I've done
trying to use a multiindex in a shared to find out it's not a good idea
after all the hard work...

Great lib though!

Cheers.

-----Mensaje original-----
De: Roman Neuhauser [mailto:neuhauser_at_[hidden]]
Enviado el: sabado, 15 de julio de 2006 11:55
Para: Berenguer Blasi
CC: boost-users_at_[hidden]
Asunto: Re: [Boost-users] [shmem] Help with shmem and fixed segments

# bblasi_at_[hidden] / 2006-07-15 09:16:34 +0100:
> Ion,
>
> thank you for your reply.
>
> As you suggested mmap returns a different address than the one specified,
> behaving as a named_shared_object under the hood instead of a
> fixed_named_shared_object.
>
> I have used the MAP_FIXED flag for mmap with disastrous results (the man
> pages already discourage you to use this flag). The computer hanged, very
> strange behaviours, etc... It just forces a segment the size you want at
the
> address you specify overwriting anything in it's way. Lots of fun as you
can
> foresee...

    What operating system was that? mmap fails if it cannot satisfy the
    request.

> So as I see it:
>
> 	- You cannot store raw pointers in a shared memory even if you use
> fixed_named_shared_object. They all have to be offset_ptr always as it may
> not map where you want.
> 	- Documentation under 'Using STL containers and mapping the memory at a
> fixed address' should reflect this.
>
> A possible workaround, as you suggested, would be to somehow write down
the
> address where the memory was mapped to so that other processes know about
> it. IMHO this is a named_shared_object in the end, so why use
> fixed_named_shared_object?
>
> I think that due to the inner behavior of mmap as long as you are aware
that
> a fixed_named_shared_object is in fact an
> OCCASIONALY_fixed_named_shared_object everything sould be alright. Notice
> that the same binary run twice on the same box may map to a different
> address each time which may or may not be the one specified! And even if
it
> returns false during creation of the segment you have already screwed your
> box overwritting protected memory areas...
>
> Do you agree or have I got the wrong end of the stick?
>
> (I use Linux Fedora Core 5)
>
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