Best way for interprocess communication in C++

I'll second Marc's suggestion -- I'd not bother with boost unless you have a portability concern or want to do cool stuff like map standard container types over shared memory (in which case I'd definitely use boost).

Otherwise, message queues and shared memory are pretty simple to deal with.

One Word: Boost.InterProcess. If it really needs to be fast, shared memory is the way to go. You nearly have zero overhead as the operation system does the usual mapping between virtual and physical addresses and no copy is required for the data. You just have to lookout for concurrency issues.

For actually sending commands like shutdown and query, I would use message queues. I previously used localhost network programming to do that, and used manual shared memory allocation, before i knew about boost. Damn if i would need to rewrite the app, I would immediately pick boost. Boost.InterProcess makes this more easy for you. Check it out.

I would use shared memory to store the data, and message queues to send the queries.