Ecosyste.ms: Awesome

An open API service indexing awesome lists of open source software.

Awesome Lists | Featured Topics | Projects

https://github.com/pcwalton/cxx-async

Simple interoperability between C++ coroutines and asynchronous Rust
https://github.com/pcwalton/cxx-async

Last synced: 2 days ago
JSON representation

Simple interoperability between C++ coroutines and asynchronous Rust

Awesome Lists containing this project

README 

        
# `cxx-async`



## Overview



`cxx-async` is a Rust crate that extends the [`cxx`](http://cxx.rs/) library to provide

interoperability between asynchronous Rust code using `async`/`await` and [C++20 coroutines] using

`co_await`. If your C++ code is asynchronous, `cxx-async` can provide a more convenient, and

potentially more efficient, alternative to callbacks. You can freely convert between C++ coroutines

and Rust futures and/or streams and await one from the other.



It's important to emphasize what `cxx-async` isn't: it isn't a C++ binding to Tokio or any other

Rust I/O library. Nor is it a Rust binding to `boost::asio` or similar. Such bindings could in

principle be layered on top of `cxx-async` if desired, but this crate doesn't provide them out of

the box. (Note that this is a tricky problem even in theory, since Rust async I/O code is generally

tightly coupled to a single library such as Tokio, in much the same way C++ async I/O code tends to

be tightly coupled to libraries like `boost::asio`.) If you're writing server code, you can still

use `cxx-async`, but you will need to ensure that both the Rust and C++ sides run separate I/O

executors.



`cxx-async` aims for compatibility with popular C++ coroutine support libraries. Right now, both

the lightweight [`cppcoro`](https://github.com/lewissbaker/cppcoro) and the more comprehensive

[Folly](https://github.com/facebook/folly/) are supported. Pull requests are welcome to support

others.



## Quick tutorial



To use `cxx-async`, first start by adding `cxx` to your project. Then add the following to your

`Cargo.toml`:



```toml

[dependencies]

cxx-async = "0.1"

```



Now, inside your `#[cxx::bridge]` module, declare a future type and some methods like so:



```rust

#[cxx::bridge]

mod ffi {

    // Declare type aliases for each of the future types you wish to use here. Then declare

    // async C++ methods that you wish Rust to call. Make sure they return one of the future

    // types you declared.

    unsafe extern "C++" {

        type RustFutureString = crate::RustFutureString;



        fn hello_from_cpp() -> RustFutureString;

    }



    // Async Rust methods that you wish C++ to call go here. Again, make sure they return one of the

    // boxed future types you declared above.

    extern "Rust" {

        fn hello_from_rust() -> Box;

    }

}

```



After the `#[cxx::bridge]` block, define the future types using the `#[cxx_async::bridge]`

attribute:



```rust

// The inner type is the Rust type that this future yields.

#[cxx_async::bridge]

unsafe impl Future for RustFutureString {

    type Output = String;

}

```



Now, in your C++ file, make sure to `#include` the right headers:



```cpp

#include "rust/cxx.h"

#include "rust/cxx_async.h"

#include "rust/cxx_async_cppcoro.h"  // Or cxx_async_folly.h, as appropriate.

```



And add a call to the `CXXASYNC_DEFINE_FUTURE` macro to define the C++ side of the future:



```cpp

// The first argument is the C++ type that the future yields, and the second argument is the

// fully-qualified name of the future, with `::` namespace separators replaced with commas. (For

// instance, if your future is named `mycompany::myproject::RustFutureString`, you might write

// `CXXASYNC_DEFINE_FUTURE(rust::String, mycompany, myproject, RustFutureString);`. The first

// argument is the C++ type that `cxx` maps your Rust type to: in this case, `String` maps to

// `rust::String`, so we supply `rust::String` here.

//

// This macro must be invoked at the top level, not in a namespace.

CXXASYNC_DEFINE_FUTURE(rust::String, RustFutureString);

```



You're all set! Now you can define asynchronous C++ code that Rust can call:



```cpp

RustFutureString hello_from_cpp() {

    co_return std::string("Hello world!");

}

```



On the Rust side:



```rust

async fn call_cpp() -> String {

    // This returns a Result (with the error variant populated if C++ threw an exception), so you

    // need to unwrap it:

    ffi::hello_from_cpp().await.unwrap()

}

```



And likewise, define some asynchronous Rust code that C++ can call:



```rust

use cxx_async::CxxAsyncResult;

fn hello_from_rust() -> RustFutureString {

    // You can instead use `fallible` if your async block returns a Result.

    RustFutureString::infallible(async { "Hello world!".to_owned() })

}

```



Over on the C++ side:



```cpp

cppcoro::task call_rust() {

    co_return hello_from_rust();

}

```



That's it! You should now be able to freely await futures on either side. An analogous procedure can

be followed to wrap C++ coroutines that yield values with `co_yield` in Rust streams.



## Installation notes



You will need a C++ compiler that implements the coroutines TS, which generally coincides with

support for C++20. Some C++ compilers (e.g. Apple clang 13.0.0) that implement the coroutines TS

crash when compiling Folly. It's also recommended to use `libc++` instead of `libstdc++`, as the

former has more complete support for coroutines.



### Folly installation



Usage of `cxx-async` with Folly requires that Folly have been built with coroutine support. Many

distributions of Folly (e.g.  the one in Homebrew) don't have coroutine support enabled; a common

symptom of this is a linker error mentioning a missing symbol

`folly::resumeCoroutineWithNewAsyncStackRoot`.



To build Folly with coroutine support on macOS, try:



```sh

$ git clone https://github.com/facebook/folly.git

$ cd folly

$ ./build.sh --install-dir=/usr/local --extra-cmake-defines \

    '{"CXX_STD": "c++20", "CMAKE_CXX_FLAGS": "-fcoroutines-ts"}'

```



Note that, depending on how your permissions are set up, you might get an error like `Permission denied: '/usr/local/.built-by-getdeps'`; this is harmless.



## Code of conduct



`cxx-async` follows the same Code of Conduct as Rust itself. Reports can be made to the crate

authors.



## License



Licensed under either of Apache License, Version 2.0 or MIT license at your option.



Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in

this project by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without

any additional terms or conditions.



[C++20 coroutines]: https://en.cppreference.com/w/cpp/language/coroutines