Re: [boost] [Review] Shmem, Serialization ad I/O streams

From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2006-02-12 09:14:28

Hi Reece,

> If process A is writing data to that block, process B should be aware of 
> that and not start reading until process A signals that all the data is 
> written (especially if process A is writing the data incrementally). 
> Similarly, process A should not be allowed to write to that memory while 
> process B is reading from it.
> 
> With this in place, transfering C++ objects across processes becomes easy if 
> you know the type of object being passed. Boost already has a mechanism to 
> do this (serialization), so you could make use of the Boost.Serialization 
> library to persist the objects to a shmem memory block rather than a file. 
> Therefore, the only thing that the shmem needs to provide is a serialization 
> archive that works for its shared memory resources.

You can do that, or you can construct the object directly in shared 
memory if both process share the same ABI. In this case there is no 
serialization. But this approach is more likely when you want to share 
some complex data (for example, a data-base) between all processes.

> What would be nice is if the I/O part of the serialization could be seperate 
> from the archive format. (I am not sure this is possible with the current 
> serialization library). That way, you could do:
> 
>    object --> binary archive --> shmem | shmem --> binary archive --> object
> 
> or
> 
>    object --> xml archive --> shmem | shmem --> xml archive --> object
> 
> where shmem provides what is needed by the different archives in terms of 
> I/O.

Shmem offers two general stream-like classes bufferstream and 
vectorstream. The first one directly serializes data via operator << to 
an user provided buffer (which can be a shared memory buffer). You are 
protected against buffer overflows with classic iostream errors 
managing. The second one serializes data via operator << to any 
character-vector (a shared memory boost::shmem::vector<> for example. 
The difference is that vectorstream reallocates the vector as you insert 
data. Take a look at the Shmem documentation in this address:

http://ice.prohosting.com/newfunk/boost/libs/shmem/doc/html/shmem/streams.html

> If shmem provides Boost.Serialization support and iostreams support then it 
> would be easy to leverage those libraries to provide the high-level passing 
> of objects you describe.

I agree. I would need to see what do I need to have Serialization 
support what are the requirements of a Serialization archive.

Regards,

Ion