Re: [Boost-users] Mutex synchronised wrapper class for boost::interprocess vector of strings

Subject: Re: [Boost-users] Mutex synchronised wrapper class for boost::interprocess vector of strings
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2013-07-09 16:31:22

El 09/07/2013 12:17, Riskybiz escribió:
> “It's not an easy task. You should catch the exception and then notify
> all connected processes that should disconnected from the segment (as
> only offline growing is supported). After that a process can call "grow"
>
> to try extend the segment. Then, all processes can attach to the newly
> grown segment.
>
> Think of that as a complex binary file that must be extended and
> internal structures changed. All users should close the file, the file
> should be changed by a process and then all users can start using the
> file again.”
>
> OK Ion, thank you for that, I see……..
>
> 1.Can I ask what mechanism you would suggest for notifying the other
> processes to disconnect before the grow() operation?

A condition variable or semaphore could do the work, but all processes 
should check it periodically.

> 2.Considering how to deal with the issue of dynamically growing a shared
> memory vector where there is one producer process and several consumers;
> I hit upon the notion of having the producer process use a std::vector
> to conventionally store data and then using a boost shared memory
> segment to store a pointer to the std::vector.  Consumer processes would
> access the boost shared memory to retrieve the pointer value thus
> enabling access to the std::vector data store in the producer process.
> This, I hoped, might remove the complexities associated with dynamically
> growing a shared memory segment? The big question is: _would it work_?
> Would this be a valid means of interprocess communication?  Or fraught
> with complexities?

That wouldn't work. The memory where std::vector is stored is not 
accesible to other processes. Only memory explicitly shared (via 
managed_shared_memory, or lower level shared_memory_object) is accesible 
to other processes.

I'm afraid that what you are trying to achieve is very complex. I have 
never tried to do it so I'm afraid you won't find many people to guide 
you in this issue. Good luck.

Ion