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Learning Rust Part 16 - Interoperability

30 Oct 2024

Introduction

Rust’s FFI (Foreign Function Interface) capabilities and rich library support enable it to integrate seamlessly with other languages like C, C++, and Python. Rust can also produce shared libraries and handle various data interchange formats such as JSON, Protobuf, and MsgPack, making it a great choice for cross-language applications and APIs. This post covers essential tools and techniques for interfacing Rust with other languages.

FFI with C and C++

Rust’s FFI makes it possible to interact directly with C libraries, letting Rust leverage existing C code or integrate with languages like C++. The extern keyword and the libc crate facilitate this interoperability.

Calling C Functions from Rust

To call a C function, define an extern block and use #[link] to specify the library. Here’s an example with the C sqrt function from the math library:

extern "C" {
    fn sqrt(x: f64) -> f64;
}

fn main() {
    let x = 25.0;
    unsafe {
        println!("sqrt({}) = {}", x, sqrt(x));
    }
}

Exposing Rust Functions to C

To expose Rust functions for use in C, use #[no_mangle] and declare the function as extern "C". This prevents Rust from altering the function name.

#[no_mangle]
pub extern "C" fn rust_add(a: i32, b: i32) -> i32 {
    a + b
}

Interfacing with C++ using the cxx crate

The cxx crate provides an interface for calling C++ code from Rust and vice versa, handling C++ types like std::string and std::vector.

Add cxx to Cargo.toml and define a C++ bridge file (bridge.rs):

#[cxx::bridge]
mod ffi {
    extern "C++" {
        include!("example.h");
        fn cpp_function(x: i32) -> i32;
    }
}

fn main() {
    let result = ffi::cpp_function(42);
    println!("Result from C++: {}", result);
}

Rust and Python Interfacing with pyo3

The pyo3 crate allows Rust to execute Python code, call Python functions, and even create Python modules directly from Rust.

Calling Python Code from Rust

Use pyo3 to execute Python code within Rust. First, add pyo3 to Cargo.toml:

[dependencies]
pyo3 = { version = "0.15", features = ["extension-module"] }

Then, write a Rust function that interacts with Python:

use pyo3::prelude::*;

fn main() -> PyResult<()> {
    Python::with_gil(|py| {
        let sys = py.import("sys")?;
        let version: String = sys.get("version")?.extract()?;
        println!("Python version: {}", version);
        Ok(())
    })
}

Building a Python Module in Rust

Rust can also create native Python modules. Annotate functions with #[pyfunction] and use #[pymodule] to define the module.

use pyo3::prelude::*;

#[pyfunction]
fn sum_as_string(a: i64, b: i64) -> PyResult<String> {
    Ok((a + b).to_string())
}

#[pymodule]
fn my_rust_module(py: Python, m: &PyModule) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(sum_as_string, m)?)?;
    Ok(())
}

Build this as a shared library, and it can be imported into Python just like a native module.

Building Shared Libraries

Rust can produce shared libraries (e.g., .dll on Windows, .so on Linux, and .dylib on macOS), making it easy to share Rust code across multiple languages.

Compiling Rust to a Shared Library

To build a Rust project as a shared library, set the crate-type in Cargo.toml:

[lib]
crate-type = ["cdylib"]

Then build the library with:

cargo build --release

This generates a .dll, .so, or .dylib file, depending on your operating system, which other languages can link to and use.

Using the Shared Library

From another language, import the shared library and call its functions. For instance, in Python, you can use ctypes to load and call functions from the Rust shared library:

import ctypes

lib = ctypes.CDLL('./target/release/libmy_rust_lib.so')
result = lib.rust_add(10, 20)
print(f"Result from Rust: {result}")

Using Rust with Other Languages

Rust can interface with languages like JavaScript, Ruby, and Go by using FFI or compiling Rust to WebAssembly or shared libraries.

WebAssembly (Wasm) for JavaScript Interoperability

WebAssembly allows Rust code to run in the browser or JavaScript environments. Using wasm-bindgen, Rust functions can be exposed to JavaScript.

Add wasm-bindgen to Cargo.toml:

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub fn greet(name: &str) -> String {
    format!("Hello, {}!", name)
}

Build the Rust code as WebAssembly and import it in JavaScript, making Rust interoperable with frontend applications.

Data Interchange Formats (JSON, Protobuf, MsgPack)

Rust supports serialization formats that allow data interchange with other systems and languages.

JSON with serde_json

The serde_json crate is the standard for JSON serialization and deserialization in Rust.

use serde::{Serialize, Deserialize};
use serde_json;

#[derive(Serialize, Deserialize)]
struct User {
    id: u32,
    name: String,
}

fn main() -> serde_json::Result<()> {
    let user = User { id: 1, name: "Alice".to_string() };
    let json = serde_json::to_string(&user)?;
    println!("Serialized JSON: {}", json);

    let deserialized: User = serde_json::from_str(&json)?;
    println!("Deserialized: {:?}", deserialized);
    Ok(())
}

Protobuf with prost

Google’s Protocol Buffers (Protobuf) is a fast, language-agnostic format used for efficient data serialization. Rust’s prost crate generates Rust types from .proto files.

Define a .proto file for your data structures and use prost to generate Rust types.

use prost::Message;

#[derive(Message)]
struct User {
    #[prost(uint32, tag = "1")]
    pub id: u32,
    #[prost(string, tag = "2")]
    pub name: String,
}

MsgPack with rmp-serde

MsgPack is a compact, binary format for data serialization, providing efficiency for high-performance applications. rmp-serde allows Rust to serialize and deserialize MsgPack data using serde.

use serde::{Serialize, Deserialize};
use rmp_serde::{to_vec, from_slice};

#[derive(Serialize, Deserialize, Debug)]
struct User {
    id: u32,
    name: String,
}

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let user = User { id: 1, name: "Alice".to_string() };
    let msgpack = to_vec(&user)?; // Serialize to MsgPack

    let deserialized: User = from_slice(&msgpack)?; // Deserialize
    println!("Deserialized: {:?}", deserialized);
    Ok(())
}

Summary

Rust’s interoperability capabilities make it ideal for building cross-language applications. Whether through FFI, shared libraries, or data interchange formats like JSON and Protobuf, Rust can integrate seamlessly with various ecosystems, enabling it to act as a high-performance backend or computational layer in multi-language projects.

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