Re: [Newbie][Interprocess] Placing a boost interprocess vector in an existing (not managed) shared memory

From: degski (degski_at_[hidden])
Date: 2020-02-26 12:55:53

On Thu, 13 Feb 2020 at 22:28, Ion Gaztañaga via Boost-users <
boost-users_at_[hidden]> wrote:

> On 30/01/2020 9:19, Gaier, Bjoern via Boost-users wrote:
> > Hello Boost-Mailing-List,
> >
> > I have a question about the boost interprocess library - and I hope I
> > can explain it correctly.
> >
> > I looked at the "Quick guide for the impatient" about the interprocess
> > library - but I'm not sure if I can use this for our purpose.
> >
> > The main goal is to insert a STL vector (or different container) in a
> > predefined portion of an already existing shared memory. The main
> > problem is, that we cannot use the managed_shared_memory code from
> > boost, because we rely on a different API to allocate the shared memory
> > between Windows and other devices. Also the container should be placed
> > at a fixed offset inside the created shared memory.
> >
> > I'm a complete newbie with boost and the interprocess library, that is
> > why I'm not sure which interface I have to provide for the interprocess
> > vector to be setup correctly. I hoped I could do something like:
> >
> > new (srh_mem_base + offset) boost::interprocess::vector<allocator>;
> >
> > I tried looking into the headers of the library to get an idea of that,
> > but I honestly got lost.
> >
> > I'm really sorry if this a stupid or obvious question - and I thank for
> > any help in advance!
>
> Unfortunatelly does not going to work. A shared memory vector expects a
> shared memory allocator that relies on the segment_manager class.

In the specific case of a vector, wouldn't it be simpler to allocate
directly (a wrapper around VM) from Virtual Memory(i.e. without the segment
manager, as there don't have to be segments, and there is nothing to
manage), and there is nothing to relocate (ever, if you make the virtual
reserved space adequately large)?

This is just an idea I've been thinking about (for my own purposes). What
would be against this approach?

degski

> That
> class expects a managed shared memory (or mapped file) that resolves
> allocation requests looking for free space in the managed shared memory
> segment.
>
> You would need to define a new allocator and managed shared memory  (as
> you need to handle the allocation requests from the vector) that
> fulfills your needs. Not an easy task at all.
>
> For fixed offsets, you'd better avoid Interprocess and use an array or
> similar with no dynamic allocations.
>
> Best,
>
> Ion
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