Hi Andy,
***Design***
Shmem is limited in scope. Its a low level building block for implementing local inter process Object transfer. AFAICS shmem doesnt concern itself much with the protocols of IPC. It wont scale to distributed communications. Is there scope for proprietary IPC mechanisms , (such as COM, DDE, and CORBA), to be built on top of it ?
You are right. Shmem is focused on classic inter-process mechanisms like shared memory/memory mapped files and it supposes ABI-compatible C++ clients. However, it can be used to build complex binary-serialized messages through network. But I haven't put any effort in distributed approach, mainly because I would be reinventing the wheel. I think that for distributed approach you could use boost::serialization over TCP/IP. Shmem is focused on raw, maximum efficiency transport between processes.
The restrictions placed on the types of objects that can be placed in shared memory are understandable, but heavy and presumably cant be checked?.
Well, I could put some is_polymorphic<> checks. However, I don't know how to detect member pointers and references.
Now there Must be a special definition of an shareable-object outside C++ class eg like a COM interface. In fact some evidence that this is occurring with classes such as basic_named_shared_object. However should these objects not be designed for distributed environment as well as local one? If shmem doesnt provide protocols for building objects that can be passed over a network and in a scaleable way then objects using its mechanisms direct will be of limited use, so larger picture should be taken into account Now.
*** Offset Pointers ***
Couldnt pointer_offsets be a class?. This would mean overloadable functions, type safety, rather than ptrdiff_t which I'm guessing is an int or something?
That seems a good idea. You prefer some kind of handle or identifier that can identify an object in shared memory that can be passed between processes. I think it's a good idea to encapsulate it since in a multi-segment architecture, the unique identifier of an object in a multi-segment group could be a more complex object. I would need to change get_offset_from_address()/get_address_from_object functions of named_shared_object to a get_handle_from_address/get_address_from_handle approach. So you can pass the handle of an object through an IPC mechanism instead of raw ptrdiff-s.
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*Documentation*
It seems a lot of effort has been put into documentation. Its noted that Ion is not a native English speaker, but it reads OK in spite of that. It probably needs to be better organised though. Concepts and Definitions section should be split into Concepts section and Definitions section. Traditionally definitions are put near to start of docs so user knows where they are before defined words are used. Definitions should be expanded A Lot ... to include many more entities such as mutex, semaphore, shared-memory, named-shared-memory,offset-pointer, process etc etc etc.
I will try to improve that.
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**Construction ***
On construction issue , only because its been brought to light recently.
Ready to implement RAII as you know.
***Should it be a boost library**
You betcha. Yes indeed It should definitely be a boost library. C++ needs this type of library badly. It is all very low level however and I would like to see a higher level abstraction for designing objects that can be passed about over a network too though and shmem objects should then try to conform to that, but that shmems role would be maybe as a building block for that. Needs to be discussed. If it is in Docs.. Sorry I missed it!
The idea of Shmem is to start building a better shared message queue, and a pipe-like stream. After that, maybe a interprocess datagram/stream mechanism similar to sockets. A Solaris doors like mechanism would be fine too. And apart from this, I want to do some research with a multi-segment Shmem architecture so that we can allocate new shared memory segments automatically when a segment is full of data. That would require also a special pointer type to be able to point from one segment to other when every segment can be mapped in a different address in each process. This can be a good base for multi mapped-file data-bases. A lot of work to do, as you can see. Ion