Using WebAssembly from Rust

This document shows an example of how to embed Wasmtime using the Rust API to execute a simple wasm program. Be sure to also check out the full API documentation for a full listing of what the wasmtime crate has to offer.

Creating the WebAssembly to execute

We'll just assume that you've already got a wasm file on hand for the rest of this tutorial. To make things simple we'll also just assume you've got a hello.wat file which looks like this:

(module
  (func (export "answer") (result i32)
     i32.const 42
  )
)

Here we're just exporting one function which returns an integer that we'll read from Rust.

Hello, World!

First up let's create a rust project

cargo new --bin wasmtime_hello
cd wasmtime_hello

Next you'll want to add hello.wat to the root of your project.

We will be using the wasmtime crate to run the wasm file. Please execute the command cargo add wasmtime to use the latest version of the crate. The dependencies block in the Cargo.toml file will appear as follows:

[dependencies]
wasmtime = "19.0.0"

Next up let's write the code that we need to execute this wasm file. The simplest version of this looks like so:

extern crate wasmtime;
use std::error::Error;
use wasmtime::*;

fn main() -> Result<(), Box<dyn Error>> {
    // An engine stores and configures global compilation settings like
    // optimization level, enabled wasm features, etc.
    let engine = Engine::default();

if false {
    // We start off by creating a `Module` which represents a compiled form
    // of our input wasm module. In this case it'll be JIT-compiled after
    // we parse the text format.
    let module = Module::from_file(&engine, "hello.wat")?;
}
let module = Module::new(&engine, r#"(module (func (export "answer") (result i32) i32.const 42))"#)?;

    // A `Store` is what will own instances, functions, globals, etc. All wasm
    // items are stored within a `Store`, and it's what we'll always be using to
    // interact with the wasm world. Custom data can be stored in stores but for
    // now we just use `()`.
    let mut store = Store::new(&engine, ());

    // With a compiled `Module` we can then instantiate it, creating
    // an `Instance` which we can actually poke at functions on.
    let instance = Instance::new(&mut store, &module, &[])?;

    // The `Instance` gives us access to various exported functions and items,
    // which we access here to pull out our `answer` exported function and
    // run it.
    let answer = instance.get_func(&mut store, "answer")
        .expect("`answer` was not an exported function");

    // There's a few ways we can call the `answer` `Func` value. The easiest
    // is to statically assert its signature with `typed` (in this case
    // asserting it takes no arguments and returns one i32) and then call it.
    let answer = answer.typed::<(), i32>(&store)?;

    // And finally we can call our function! Note that the error propagation
    // with `?` is done to handle the case where the wasm function traps.
    let result = answer.call(&mut store, ())?;
    println!("Answer: {:?}", result);
    Ok(())
}

We can build and execute our example with cargo run. Note that by depending on wasmtime you're depending on a JIT compiler, so it may take a moment to build all of its dependencies:

$ cargo run
  Compiling ...
  ...
   Finished dev [unoptimized + debuginfo] target(s) in 42.32s
    Running `wasmtime_hello/target/debug/wasmtime_hello`
Answer: 42

and there we go! We've now executed our first WebAssembly in wasmtime and gotten the result back.

Importing Host Functionality

What we've just seen is a pretty small example of how to call a wasm function and take a look at the result. Most interesting wasm modules, however, are going to import some functions to do something a bit more interesting. For that you'll need to provide imported functions from Rust for wasm to call!

Let's take a look at a wasm module which imports a logging function as well as some simple arithmetic from the environment.

(module
  (import "" "log" (func $log (param i32)))
  (import "" "double" (func $double (param i32) (result i32)))
  (func (export "run")
    i32.const 0
    call $log
    i32.const 1
    call $log
    i32.const 2
    call $double
    call $log
  )
)

This wasm module will call our "log" import a few times and then also call the "double" import. We can compile and instantiate this module with code that looks like this:

extern crate wasmtime;
use std::error::Error;
use wasmtime::*;

struct Log {
    integers_logged: Vec<u32>,
}

fn main() -> Result<(), Box<dyn Error>> {
    let engine = Engine::default();
if false {
    let module = Module::from_file(&engine, "hello.wat")?;
}
let module = Module::new(&engine, r#"(module (import "" "log" (func $log (param i32))) (import "" "double" (func $double (param i32) (result i32))) (func (export "run") i32.const 0 call $log i32.const 1 call $log i32.const 2 call $double call $log))"#)?;

    // For host-provided functions it's recommended to use a `Linker` which does
    // name-based resolution of functions.
    let mut linker = Linker::new(&engine);

    // First we create our simple "double" function which will only multiply its
    // input by two and return it.
    linker.func_wrap("", "double", |param: i32| param * 2)?;

    // Next we define a `log` function. Note that we're using a
    // Wasmtime-provided `Caller` argument to access the state on the `Store`,
    // which allows us to record the logged information.
    linker.func_wrap("", "log", |mut caller: Caller<'_, Log>, param: u32| {
        println!("log: {}", param);
        caller.data_mut().integers_logged.push(param);
    })?;

    // As above, instantiation always happens within a `Store`. This means to
    // actually instantiate with our `Linker` we'll need to create a store. Note
    // that we're also initializing the store with our custom data here too.
    //
    // Afterwards we use the `linker` to create the instance.
    let data = Log { integers_logged: Vec::new() };
    let mut store = Store::new(&engine, data);
    let instance = linker.instantiate(&mut store, &module)?;

    // Like before, we can get the run function and execute it.
    let run = instance.get_typed_func::<(), ()>(&mut store, "run")?;
    run.call(&mut store, ())?;

    // We can also inspect what integers were logged:
    println!("logged integers: {:?}", store.data().integers_logged);

    Ok(())
}

Note that there's a number of ways to define a Func, be sure to consult its documentation for other ways to create a host-defined function.