330 lines
7.6 KiB
Markdown
330 lines
7.6 KiB
Markdown
## Before starting
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This file is a presentation.
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Better get the point tools here: https://git.baguette.netlib.re/Baguette/pointtools
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TODO: Explain the problem
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TODO: Explain the solution
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TODO: Explain why LibIPC
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TODO: Explain how LibIPC
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TODO: Explain other possible implementations
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TODO: Explain future of LibIPC
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TODO: Explain what can be done right now
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TODO: Explain what I actually do with it
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TODO: Explain LibIPC isn't a silver bullet but fine for what I want
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Have fun!
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## Programming problems
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Libraries
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* change often = hard to follow
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* don't often provide a high-level interface
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* each library is coded in its own way
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* availability vary depending on the language
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Example: libraries to access databases
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* languages often have their own implementation
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* functions may vary from a language to another
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## Infrastructure problems
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Infrastructure
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* Always need to install all required libraries
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* No clear way to sandbox part of an application
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## What solution?
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MAKE APPLICATIONS, NOT LIBRARIES
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* apps talking to apps
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Create an abstraction for
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libraries
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* languages, implementations, code modifications in general
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network code
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* networking: performed by dedicated services
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- example: TCPd, UDPd, TLSd, HTTPd...
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- LibIPC is independant from protocols and formats
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* applications think communications are local
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## In practice
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usage must be simple
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1. init connection or service
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2. loop over events
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#pause
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events are simple and high level
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1. connection and disconnection
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2. message received and sent
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#pause
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message have a simple format: length + value
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#pause
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And that's it.
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## When
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LibIPC is useful when the app:
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- cannot be a simple shell command
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- needs a bidirectional communication
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- is an abstraction over a library
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## Available libraries
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* DBUS
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* libevent
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* even more complicated stuff
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* RPC-style, like Corba
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#pause
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* ... or bare libc api
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* shared memory
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* pipes
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* sockets (unix, inet, inet6)
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## DBUS
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* not well suited for our needs
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(a polite way to say: what a bloody mess)
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Is it designed *NOT* to be used?
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* over-engineered
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* complex
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* documentation isn't great
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* no code example
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## DBUS (bonus page!)
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DBUS feels obsolete: a big chunk of the documentation is
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about message format. Just use CBOR already!
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#pause
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They even admit they did a poor job on the C part:
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> There is a low-level C binding, but that is probably too detailed
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> and cumbersome for anything but writing other bindings.
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#pause
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Oh. And C++. YOU SHALL NOT PASS!
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This is a Linux requirement nowadays, wth?
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## libevent
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* works with epoll and kqueue
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* great performances
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* works on Linux and *BSD
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* a bit complicated
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## Bare libc api
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shared memory and semaphores
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* (kinda) complicated api
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* not about exchanging messages
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pipes, sockets
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* lack a conventional message format
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... but that's about it
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* Great to start with!
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All have great performances to exchange data.
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What is slow is the function to _wait_ for new events.
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## LibIPC's choice
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Unix sockets
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- fast, simple, reliable, bidirectional
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- remote connections will have their own service (ex: TCPd)
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Dumbest possible message format
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- length + value
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- build your own format on top of it!
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Wait on file descriptors with poll(2)
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- slow, but available everywhere
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- may upgrade to libevent
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## LibIPC history (1/2)
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1. based on pipes
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* because we gotta go fast!
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* ... but implementation was a bit of a mess
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#pause
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2. rewrite to work with unix sockets
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* performances are excellent, no need for _absolute best_
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* way nicer implementation
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* select(2) for listening on file descriptors
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#pause
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* ... wait, does select(2) support more than 1024 connections?
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## LibIPC history (2/2)
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3. rewrite using poll(2)
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* many bugfixes later, way more tested than before
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* implementation was (kinda) production-ready
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* implementation was simple: < 2000 lines of C code
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Still wasn't as simple as I wanted
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## Current implementation of libIPC
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Written in Zig
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bindings available in Crystal
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* as well as fancy mappings: JSON and CBOR class serialization
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#pause
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epoll (Linux) and kqueue (*BSD) were avoided
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* 'cause callbacks hell => harder to read and to write code
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#pause
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* but we need them for better performances with many connections
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though, API should stay the same for non threaded applications (simple implementation)
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#pause
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LibIPC doesn't handle parallelism, yet
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## How libIPC works
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LibIPC has a high level API for the user
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1. init a connection (client) or create an unix socket (service)
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example: ipc_service_init (context, "service")
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#pause
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2. loop, wait for events
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listening to file descriptors (libIPC ones or not)
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example:
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while(1) {
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wait_event (context, &event, &timer)
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switch (event.type) {
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case IPC_CONNECTION : ...
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case IPC_DISCONNECTION : ...
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case IPC_MESSAGE: {
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struct ipc_message *m = event.m;
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...
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}
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}
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}
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## How libIPC works
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3. send messages
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```c
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struct ipc_message m
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m.payload = ...
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m.length = strlen(m.payload)
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m.fd = event.fd
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m.type = ...
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m.user_type = ...
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ipc_write (context, &m)
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```
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#pause
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4. add and remove fd from the context
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ipc_add_fd (context, fd)
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ipc_del_fd (context, fd)
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## How libIPC works
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LibIPC also helps to create "protocol daemons" like TCPd with
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automatic switching between file descriptors
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LibIPC takes callbacks to obtain libipc payloads inside arbitrary message structure
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Example: websocketd.
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Clients exchange data with a libipc service through websockets messages.
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websocketd binds both the client and its service file descriptors,
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then provides the libipc a callback to extract libipc messages from
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the websocket messages sent by the client.
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Same thing the other way.
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ipc_switching_callbacks (context, client_fd, cb_in, cb_out)
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## libIPC internal structures
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Main goal: simplest possible structures
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Examples:
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Message
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struct ipc_message {
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char type; => Internal message type, used by protocol daemons.
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char user_type; => User-defined message type (arbitrary).
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int fd; => File descriptor concerned about this message.
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uint32_t length; => Payload length.
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char *payload; => Actual payload.
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};
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Context of the whole networking state
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struct ipc_ctx {
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struct ipc_connection_info *cinfos; => Keeps track of connections.
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struct pollfd *pollfd; => List of "pollfd" structures within cinfos,
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so we can pass it to poll(2).
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size_t size; => Size of the connection list.
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struct ipc_messages tx; => Messages to send.
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struct ipc_switchings switchdb; => Relations between fd.
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};
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## Future of libIPC
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LibIPC will be rewritten in Zig
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* simpler to read and write
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* data structures are simpler to create with comptime
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* less code redundancy (more generic functions)
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* already existing error management as written in current libIPC
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* already existing loggin system
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* no more C's pitfalls
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* way better at exposing bugs
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* thanks to a better type system
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* way safer: cannot ignore errors
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* so I won't
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* simpler to cross-compile: same standard library for every OSs
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* simpler (and more secure) memory management
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Also: this won't change existing bindings! No excuses!
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## Why not use it?
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Current limitations
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* performances (libIPC is based on poll(2), not epoll nor kqueue)
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* it really isn't an issue until you have hundreds or thousands of clients
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* parallelism is not permitted
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nothing in libIPC is thread-safe
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The future Zig implementation will overcome these issues.
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## Questions?
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Ask! `karchnu at karchnu.fr`
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