[gorset]

> Aside from the mmap() call, you can do all this without even allocating any memory at all.

I think this is the important point when it comes to discussing zero-copy. I've written a custom protobuf implementation for java which can do exactly that.

It's a bit tricky since protobuf supports recursive messages and java's Unsafe is not as powerful as what you can have in C++. My trade-off was to require the caller to pre-allocate messages needed before parsing the data. This works great when working with multi-gigabyte files where you want to process a large number of (possibly nested) messages, but is not as ergonomic as normal protobuf code.

It obviously doesn't come for free, as you need to do a linear scan to find those tag-values, but there are ways to speed that up too, so it becomes very fast in practice.

I'm sure Cap'n Proto and FlatBuffers are faster for some use-cases (I haven't tested), but a very important point for me is to be wire-compatible with protobuf3 and its ecosystem... and still be zero-copy/zero-alloc.