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Go embedding of Wasmtime

A Bytecode Alliance project

CI status Documentation Code Coverage


go get -u

Be sure to check out the API documentation!

This Go library uses CGO to consume the C API of the Wasmtime project which is written in Rust. Precompiled binaries of Wasmtime are checked into this repository on tagged releases so you won't have to install Wasmtime locally, but it means that this project only works on Linux x86_64, macOS x86_64 , and Windows x86_64 currently. Building on other platforms will need to arrange to build Wasmtime and use CGO_* env vars to compile correctly.

This project has been tested with Go 1.13 or later.

If you are a bazel user, add following to your WORKSPACE file:

    name = "com_github_bytecodealliance_wasmtime_go",
    importpath = "",
    version = "v3.0.2",


A "Hello, world!" example of using this package looks like:

package main

import (

func main() {
    // Almost all operations in wasmtime require a contextual `store`
    // argument to share, so create that first
    store := wasmtime.NewStore(wasmtime.NewEngine())

    // Compiling modules requires WebAssembly binary input, but the wasmtime
    // package also supports converting the WebAssembly text format to the
    // binary format.
    wasm, err := wasmtime.Wat2Wasm(`
        (import "" "hello" (func $hello))
        (func (export "run")
          (call $hello))

    // Once we have our binary `wasm` we can compile that into a `*Module`
    // which represents compiled JIT code.
    module, err := wasmtime.NewModule(store.Engine, wasm)

    // Our `hello.wat` file imports one item, so we create that function
    // here.
    item := wasmtime.WrapFunc(store, func() {
        fmt.Println("Hello from Go!")

    // Next up we instantiate a module which is where we link in all our
    // imports. We've got one import so we pass that in here.
    instance, err := wasmtime.NewInstance(store, module, []wasmtime.AsExtern{item})

    // After we've instantiated we can lookup our `run` function and call
    // it.
    run := instance.GetFunc(store, "run")
    if run == nil {
        panic("not a function")
    _, err = run.Call(store)

func check(e error) {
    if e != nil {


So far this extension has been written by folks who are primarily Rust programmers, so it's highly likely that there's some faux pas in terms of Go idioms. Feel free to send a PR to help make things more idiomatic if you see something!

To work on this extension locally you'll first want to clone the project:

$ git clone

Next up you'll want to have a local Wasmtime build available.

You'll need to build at least the wasmtime-c-api crate, which, at the time of this writing, would be:

$ cargo build -p wasmtime-c-api

Once you've got that you can set up the environment of this library with:

$ ./ci/ /path/to/wasmtime

This will create a build directory which has the compiled libraries and header files. Next up you can run normal commands such as:

$ go test

And after that you should be good to go!