Versus iceoryx (the C++ version, not the Rust-oriented iceoryx2):

TL;DR: So far, it looks super-sweet (as well as mature, already supporting macOS for example). However more of an investment to use than is Flow-IPC, with a central daemon and a special event-loop model. It also doesn't want to do #1 above described (no pointers, no using existing STL-compliant container types).

This guy seems really cool, and it directly addresses at least the major part of need #2 above. You can transmit buffers with near-zero latency, and it'll do the SHM stuff for you. (For capnp specifically one would then implement the required SHM-allocating capnp::MessageBuilder, and off we go. Flow-IPC does give you this part out-of-the-box, granted.) Looking over the examples and overview, it seems like integrating it into an event loop might involve some pretty serious learning of iceoryx's event-loop model + subscribe/publish. There is also a central daemon that needs to run.

Flow-IPC, to me, seems to have a lower-learning/lower-maintenance curve approach to this. There's no central daemon or any equivalent of it. For each asynchronous thing (a transport::Channel, for example, which has receive-oriented methods), you can use one of 2 supplied APIs. The sync_io-style API will let you plug into anything select()/poll()/epoll()-oriented (and has a syntactic-sugar hook for boost.asio loops). If you've got an event loop, it'll be easy to plug Flow-IPC ops right into it - no background threads added thereby. Or, use the async-I/O-style API; then it'll create background threads as needed and call your callback (e.g., on message receipt) from there, leaving it to you to handle it there or by posting the "true" handling onto one of your own threads.

Point being, my impression so far is, using Flow-IPC in this sense is a lower-effort enterprise. It's pretty much just there to plug-in. (I really hope that isn't slander. That's my take so far - as I said, it'll take me a few days to understand these products in-depth.)

Now, in terms of need #1. (I acknowledge, this need is not for every C++ IPC use-case ever. 2 processes collaborating on one native C++ data structure full of SHM-compliant containers and/or pointers =/= done every day. Still, though, if 2 threads in one process can do it easily, why shouldn't they as-easily be able to do it across a process boundary? Right?) If I understand iceoryx's example on this topic (https://iceoryx.io/latest/examples/complexdata/)... I quote: "To implement zero-copy data transfer we use a shared memory approach. This requires that every data structure needs to be entirely contained in the shared memory and must not internally use pointers or references. ... Therefore, most of the STL types cannot be used, but we reimplemented some constructs. This example shows how to send/receive a iox::cxx::vector and how to send/receive a complex data structure containing some of our STL container surrogates."

With Flow-IPC, this does not apply. You can share existing STL-compliant containers, and (if you want) can have raw pointers too. We have tests nesting boost::container string/vector/map guys and our own flow::util::Basic_blob STL-compliant guy and sharing them, no problem. We've provided the necessary allocator and fancy-pointer types. Moreover, with a single line you can do this in jemalloc-allocated SHM; or instead choose a Boost.ipc-backed single-segment SHM. (Depends on what you desire for safety versus simplicity, internally. I am being a bit vague on that here, but it's in the docs, I promise.) I believe this is a pretty good illustration of Flow-IPC's "thing":

- Meat-and-potatoes, do what you want to do in your daily C++, without a major learning curve...
- ...but without sacrificing essential power...
- ...and extensibly, meaning you can modify its behavior in core ways without requiring a massive amount of learning of how Flow-IPC is built.

Versus Mojo IPC:

I really need to understand it better, before I can really comment. So far, it seems like its equivalent of Flow-IPC's sessions = super cool, building up a network of processes that can all talk to each other once in the network. Flow-IPC's sessions are basic: you want process A and B to speak, you establish a session (during this step, one is designated as the session-server and can therefore accept more sessions from that app or other apps)... then from there, you can make channels (and access SHM arenas, if you are using SHM directly as opposed to letting the zero-copy channels do it invisibly). It also has various-language bindings; Flow-IPC is C++... straight up.

That established, I need to understand it better. It looks like it provides super-fast low-level IPC transports (similar to Flow-IPC's unstructured-layer channels) in platform-agnostic fashion - but does not seem to specifically facilitate end-to-end zero-copy transmission of data structures via SHM. I could be completely wrong here, but it actually looks like one could feasibly plug-in Mojo IPC pipes as Flow-IPC Blob_sender/receiver (and/or Native_handle_sender/receiver) concept impl, into Flow-IPC, and get the end-to-end zero-copy goodness.

At least superficially, so far, Flow-IPC again looks like perhaps a more down-to-earth/readily-pluggable effort. (But, still documented out-the-wazoo!)