Crouton

Crouton is a C++20 coroutine runtime library that provides some general purpose concurrency utilities, as well as cross-platform event loops, I/O and networking that work the same way on Mac, Linux, Windows and ESP32 microcontrollers.

The cross-platform support is based on the widely-used libuv, mbedTLS and llhttp libraries. (On Apple platforms it can also use the system Network.framework. On ESP32, where libuv is not supported, it uses lwip and FreeRTOS APIs instead.)

A Coroutine is a function that can return partway through, and then later be resumed where it left off. Knuth wrote about them in the 1960s and they remained a curiosity for a long time, but they've since become widely used under the hood of the "async / await" concurrency model used in languages like JavaScript, C#, Rust, Nim, and Swift. Crouton brings this to C++.

Async/await gives you concurrency without the pitfalls of multithreading. You can write code in a linear fashion, "blocking" on slow operations like I/O (see the example below), but the thread doesn't really block: instead your (coroutine) function is suspended and some other function that's finished blocking can resume. When the I/O completes, your function is resumed transparently at the next opportunity.

How is that better than threads? It's safer and easier to reason about. The only places where concurrency happens are well-marked by the co_await and co_yield keywords. You don't need mutexes or atomic variables, and there are far fewer opportunities for race conditions or deadlocks. (There are performance benefits too: no expensive context switches, less stack usage.)

Detailed documentation is being written.

Features

Coroutine library:
- Useful base classes for implementing new coroutine types
- Future, an asynchronous promise type
- Generator, an iterator implementation based on co_yield-ing values
- CoMutex, CoCondition, Blocker: cooperative equivalents of common sync primitives
- AsyncQueue, a producer/consumer queue
- Select, a way to await multiple things in parallel
- Optional coroutine lifecycle tracking, with debugging utilities to dump all existing coroutines and their "call stacks".
Event loops:
- Scheduler, which manages multiple active coroutines on a thread
  - Round-robin scheduling of multiple active coroutines
  - Suspending a coroutine, then waking it when it's ready
  - Scheduling a function to run on the next event-loop iteration, even on a different thread
  - Scheduling a function to run on a background thread-pool
- Task, an independently-running coroutine that can co_yield to give time to others
- Timer, repeating or one-shot
Reactive publish/subscribe framework
- Enables building complex networked data flows out of modular components
- Intrinsically supports backpressure to manage flow control on sockets
- Loosely inspired by Apple's Combine framework
- Double-plus experimental ⚗️
Asynchronous I/O classes:
- DNS lookup
- File I/O
- Filesystem APIs like mkdir and stat
- Abstract asynchronous stream interface
- In-process pipes
- TCP sockets, with or without TLS
- A TCP listener (no TLS support yet)
- URL parser
- HTTP client
- HTTP server (very basic so far)
- WebSocket client and server
- BLIP RPC protocol (optional; under separate license.)
Core classes & APIs:
- General-purpose Error and Result<T> types
- Logging, a very compact library with an API inspired by spdlog.
- Type-safe string formatting, similar to std::format but with a much lower code footprint.
Cross-Platform:
- macOS (builds and passes tests)
  - iOS? ("It's still Darwin…")
- Linux (builds and passes test)
  - Android? ("It's still Linux…")
- ESP32 embedded CPUs (builds and passes tests. Networking works, but filesystem APIs aren't implemented yet.)
- Windows (builds, but I don't have any Windows machines to test on. Help wanted!)

Example

// Simple HTTP client request:
HTTPConnection client("https://example.com");
HTTPRequest req;
HTTPResponse resp = co_await req.sendRequest(req);

cout << int(resp.status()) << " " << resp.statusMessage() << endl;

for (auto &header : resp.headers())
    cout << header.first << " = " << header.second << endl;

ConstBuf body;
do {
    body = co_await resp.readNoCopy();
    cout << string_view(body);
} while (body.len > 0);
cout << endl;

See also demo_server.cc, a simple HTTP and WebSocket server.

An example embedded app is at tests/ESP32.

Status: ☠️EXPERIMENTAL☠️

This is new code, under heavy development! So far, it builds with Clang (Xcode 15) on macOS, GCC 12 on Ubuntu, Visual Studio 17 2022 on Windows, and ESP-IDF 5.1.

The tests run regularly on macOS, and occasionally on Ubuntu (though not in CI.) Test coverage is very limited.

APIs are still in flux. Things get refactored a lot.

Building It

Important: Make sure you checked out the submodules! git submodule update --init --recursive

Prerequisites:

CMake
Clang 15, Xcode 15, or GCC 12
zlib (aka libz)

on macOS:

Install Xcode 15 or later, or at least the command-line tools.
Install CMake; this is most easily done with HomeBrew, by running brew install cmake

on Ubuntu Linux

sudo apt-get install g++ cmake cmake-data zlib1g-dev

Building With CMake

make
make test

The library is libCrouton, in either the build_cmake/debug/ or build_cmake/release/ directory.

Building With Xcode 15+

Before first building with Xcode, you must use CMake to build libuv and mbedTLS:

make xcode_deps

You only need to do this on initial setup, or after those submodules are updated.

Then:

open crouton.xcodeproj
Select the Tests scheme and Run.
To locate the binaries, choose Product > Show Build Folder In Finder

Building For ESP32 Embedded Systems

The ESP-IDF component is at src/io/esp32; please see the README for details. A demo app is at tests/ESP32.

License(s)

Crouton itself is licensed under the Apache 2 license.
- The files in the subdirectory src/io/blip, however, are under the Business Software License since they are adapted from existing BSL-licensed code. These source files are optional and are by default not compiled into the Crouton library. See their README for details and the full license.
- The source file src/io/URL.cc contains some code adapted from tlsuv, which is Apache 2 licensed.
Of the third party code in vendor/:
- Catch2 is licensed under the Boost Software License (but is only linked into the tests)
- libuv is licensed under the MIT license.
- llhttp is licensed under the MIT license.
- mbedtls is licensed under the Apache 2 license.
- spdlog is licensed under the MIT license.

Credits

Crouton code by Jens Alfke (@snej)
Initial inspiration, coroutine knowledge, starting code: Simon Tatham's brilliant tutorial. Lewis Baker's blog posts have also been helpful.
Event loops, I/O, networking: libuv
TLS engine: mbedTLS
HTTP parser: llhttp
Logging: spdlog
Unit tests: Catch2
Extra inspiration and URL parser: tlsuv

couchbaselabs/crouton