how to build capnp message on the shared momery?

[fengmao31]

If I want to use capnp with shared memory, how can I serialized_message on the shared momery.

[kentonv]

You would need to write a custom implementation (subclass) of capnp::MessageBuilder which allocates segments directly into your shared memory. You may also need a custom implementation of capnp::MessageReader to read it.

[fengmao31]

1. If I want to request a shared momery block, how to compute the size of the Builder?

2. I read the implainment of MessageReader and messageToFlatArray(), I found it looks like the copy from MessageReader to the
   kj::Array in messageToFlatArray() is not necessary and maybe move is better？
   

[kentonv]

messageToFlatArray() always makes a copy. You probably shouldn't be using that function if you're trying to use shared memory. You should have your custom MessageBuilder allocate segments directly in the shared memory area.

[fengmao31]

This is exactly where I'm puzzled; I don't know how to calculate the size of the MessageBuilder class.

[kentonv]

You aren't expected to know the size upfront. Messages are allocated in multiple segments -- additional segments are added only as needed. So you'd make a guess for the first segment size, and then if the message grows bigger than that, the system will call back to your implementation to allocate additional segments. Generally I'd use a strategy like allocating each segment to be double the size of the previous.