Re: [Boost-users] How to keep a pointer on a shared data in a boost shared memory segment

Subject: Re: [Boost-users] How to keep a pointer on a shared data in a boost shared memory segment
From: Gavin Lambert (boost_at_[hidden])
Date: 2019-03-21 01:38:26

On 21/03/2019 01:03, jucin loic wrote:
> How to keep a pointer on a shared data in a boost shared memory segment 
> ? I have a function which returns 
> shm.construct<SharedData>(_nameSeg.c_str())(innerDataAllocator); But 
> outside of the function, the object pointed by SharedData is inaccessible
> My general problem is that I want to have way to get, and set data in 
> shared memory, but without, each time, having to :
> -get shm
> -find object in shm based on its name
> -construct allocators.
> So I would like to store 1) allocators and 2)a pointer to the shared object
> How could I do this ?

In general it makes no sense to store pointers inside shared memory, 
because it is very likely that each process maps the shared memory to 
different addresses, so a pointer would point to invalid memory in all 
but one process.

To solve this, you store indexes or offsets relative to the base of the 
shared memory itself (or some other known field within the shared 
memory) instead of a "real" pointer.

Some compilers support a so-called "based pointer", which let you use 
offsets as if they were pointers, but this is not portable.  You can 
accomplish something similar in portable code by using a smart pointer 
wrapper type.