Crate wasmi

Expand description

The wasmi virtual machine definitions.

These closely mirror the WebAssembly specification definitions. The overall structure is heavily inspired by the wasmtime virtual machine architecture.

Example

The following example shows a “Hello, World!”-like example of creating a Wasm module from some initial .wat contents, defining a simple host function and calling the exported Wasm function.

The example was inspired by Wasmtime’s API example.

use anyhow::{anyhow, Result};
use wasmi::*;

fn main() -> Result<()> {
    // First step is to create the Wasm execution engine with some config.
    // In this example we are using the default configuration.
    let engine = Engine::default();
    let wat = r#"
        (module
            (import "host" "hello" (func $host_hello (param i32)))
            (func (export "hello")
                (call $host_hello (i32.const 3))
            )
        )
    "#;
    // Wasmi does not yet support parsing `.wat` so we have to convert
    // out `.wat` into `.wasm` before we compile and validate it.
    let wasm = wat::parse_str(&wat)?;
    let module = Module::new(&engine, &mut &wasm[..])?;

    // All Wasm objects operate within the context of a `Store`.
    // Each `Store` has a type parameter to store host-specific data,
    // which in this case we are using `42` for.
    type HostState = u32;
    let mut store = Store::new(&engine, 42);
    let host_hello = Func::wrap(&mut store, |caller: Caller<'_, HostState>, param: i32| {
        println!("Got {param} from WebAssembly");
        println!("My host state is: {}", caller.host_data());
    });

    // In order to create Wasm module instances and link their imports
    // and exports we require a `Linker`.
    let mut linker = <Linker<HostState>>::new();
    // Instantiation of a Wasm module requires defining its imports and then
    // afterwards we can fetch exports by name, as well as asserting the
    // type signature of the function with `get_typed_func`.
    //
    // Also before using an instance created this way we need to start it.
    linker.define("host", "hello", host_hello)?;
    let instance = linker
        .instantiate(&mut store, &module)?
        .start(&mut store)?;
    let hello = instance
        .get_export(&store, "hello")
        .and_then(Extern::into_func)
        .ok_or_else(|| anyhow!("could not find function \"hello\""))?
        .typed::<(), (), _>(&mut store)?;

    // And finally we can call the wasm!
    hello.call(&mut store, ())?;

    Ok(())
}