Wasmer Go embedding 1.0 lift-off

Announcing the immediate availability of the Wasmer Go embedding 1.0 version!

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Syrus Akbary

Founder & CEO


February 26, 2021

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We are delighted to announce the release of Wasmer Go embedding 1.0 version.

About 1.5 years ago we first released wasmer-go, the Wasmer embedding for Go. The reception by the community was beyond our expectations, the response to our launch of the fastest WebAssembly runtime for Go was a great way to start! A community of more than 1,300 people gives us the energy to push this project further. We have seen many users from various domains that weren't anticipated. After hundreds of thousands of installations, it's our great pleasure to introduce the 1.0 version, fully rewritten to provide a stable and complete API, better performance, cross-compilation, two compilers, two engines, and many more advanced features!

Improved and simplified API

We have entirely rewritten the project with a new, improved API. All the WebAssembly externals are now supported, which includes Function, Global, Memory, and Table. All of them can be used as imports or as exports. Well, now that's straightforward.

The ImportObject API is now unified and simplified (previously, we had 2 distinct API for historical reasons).

An example is worth a thousand words. Let's consider the following example to illustrate many of the improvements:

import ""

// Create an engine. It's responsible for driving the compilation and the
// execution of a WebAssembly module.
engine := wasmer.NewEngine()

// Create a store, that holds the engine.
store := wasmer.NewStore(engine)

// Create a new module from some WebAssembly in its text representation
// (for the sake of simplicity of the example).
module, _ := wasmer.NewModule(
		  ;; We import a math.sum function.
		  (import "math" "sum" (func $sum (param i32 i32) (result i32)))

		  ;; We export an add_one function.
		  (func (export "add_one") (param $x i32) (result i32)
		    local.get $x
		    i32.const 1
		    call $sum))

// Let's create a new host function for `math.sum`.
function := wasmer.NewFunction(

	// The function signature.
		// Parameters.
		wasmer.NewValueTypes(wasmer.I32, wasmer.I32),
		// Results.

	// The function implementation.
	func(args []wasmer.Value) ([]wasmer.Value, error) {
		x := args[0].I32()
		y := args[1].I32()

		return []wasmer.Value{wasmer.NewI32(x + y)}, nil

// Let's use the new `ImportObject` API…
importObject := wasmer.NewImportObject()

// … to register the `math.sum` function.
		"sum": function,

// Finally, let's instantiate the module, with all the imports.
instance, _ := wasmer.NewInstance(module, importObject)

// And let's call the `add_one` function!
addOne, _ := instance.Exports.GetFunction("add_one")

result, _ := addOne(41)

assert(result, int32(42))

A couple of interesting things can be seen here:

  1. There is an Engine API — more about this in the next section,

  2. Host functions are fully implemented in Go; cgo is no longer necessary when declaring host functions. Moreover, host functions no longer receive the famous context argument — more on that in a second,

  3. Host functions can return multiple-values,

  4. Host functions can return any error à la Go,

  5. The ImportObject API allows to register a collection of “externals”: Function, Memory etc., i.e. any type that implements the IntoExtern interface, to a given namespace (here math),

  6. The Exports API allows to read any kinds of exports, including multiple memories.

The entire API is more idiomatic, and very much simpler to use from our perspective.

A word about host functions

Previously, a host function had to be declared with a cgo mapping:

// extern int32_t sum(void *context, int32_t x, int32_t y);
import "C"

//export sum
func sum(context unsafe.Pointer, x int32, y int32) int32 {
	return x + y

Now, host functions no longer need cgo. That's a big user-experience improvement! A host function can be any Go function or Go lambda function.

Also, host functions can now return errors, which was impossible before. It will automatically translate to a WebAssembly trap (with the new Trap API).

Finally, host functions receive and return slices of values, i.e. it supports multi-values!

func sum(args []wasmer.Value) ([]wasmer.Value, error) {
	x := args[0].I32()
	y := args[1].I32()

	return []wasmer.Value{wasmer.NewI32(x + y)}, nil

Note that host functions manipulate values of type Value to allow a more unified API.

Continuing on the subject of implementing host functions, to create a proper host function, we use the NewFunction function. It is also possible to create a new host function with an attached environment with NewFunctionWithEnvironment which expects a function of the following form:

type MyEnvironment struct {
	shift int32

environment := &MyEnvironment {
	shift: 42,

hostFunction := wasmer.NewFunction(

	// The host function signature.
		wasmer.NewValueTypes(wasmer.I32, wasmer.I32), // two i32 params
		wasmer.NewValueTypes(wasmer.I32), // one i32 result

	// Our environment!

	// The function implementation.
	func(environment interface{}, args []wasmer.Value) ([]wasmer.Value, error) {
		// Cast to our environment type, and do whatever we want!
		env := environment.(*MyEnvironment)
		e := env.shift;
		x := args[0].I32()
		y := args[1].I32()

		return []wasmer.Value{wasmer.NewI32(e + x + y)}, nil

The biggest improvement is that the environment can be anything! No more restriction due to cgo. The user is free to store anything needed.

A word about exported functions

The Exports API provides the following methods: GetFunction, GetMemory, GetGlobal and GetMemory, to get an export by its name.

GetFunction returns an exported function with a native Go API, i.e. a function that can be invoked as addOne(42). It's actually an alias of GetRawFunction(name).Native() (including error handling):

  • A raw function is represented by the type Function,
  • A native function is represented by the type NativeFunction.

The Function type provides more information about the function itself, whilst NativeFunction serves the purpose of being easy to use as any Go functions.

addOne, _ := instance.Exports.GetRawFunction("add_one")


result, _ := addOne.Call(41)
// or
addOneNative := addOne.Native()
result, _ := addOneNative(41)

This example illustrates that even if the new API is more idiomatic, it's not without being less expressive or losing features.

Compilers and Engines

Let's talk for a moment about compilers and engines. Compilers aim at compiling WebAssembly modules into executable code. Engines drive the compilation and the execution of the WebAssembly modules. This design provides unique flexibility which allows Wasmer to be used in various contexts.

Cranelift has new companions: Singlepass and LLVM!

The Wasmer runtime provides 3 compilers to compile the WebAssembly modules into executable codes:

  • Singlepass: Super fast compilation times, slow execution times. Not prone to JIT-bombs,

  • Cranelift: Fast compilation times, fast execution times,

  • LLVM: Slow compilation times, very fast execution times (close to native).

Previously, wasmer-go was providing only one compiler: Cranelift. However, a non-negligible population of our users run small WebAssemby modules. In that specific case, execution time is not an issue since it will always be quick, however we must improve the compilation time. And that's why we are happy to announce that wasmer-go now embeds the Singlepass compiler too by default!

Double announcement: When execution performance really matters to you, you can now use the LLVM compiler too! Even if the default libwasmers embedded inside wasmer-go does not provide a support for LLVM yet (we are working on it), the entire API already supports it. It is really easy to use your own custom libwasmer that includes LLVM and build against it (see below about the custom_wasmer_runtime tag).

Cranelift will continue to be the default compiler. To change that behaviour, one needs to create a new configuration for the engine and use Config.UseSinglepassCompiler or Config.UseLLVMCompiler:

// Configure the engine to use the Singlepass compiler.
config := wasmer.NewConfig().UseSinglepassCompiler()
engine := wasmer.NewEngineWithConfig(config)

// Compile the WebAssembly module.
store := wasmer.NewStore(engine)
module, _ := wasmer.NewModule(store, wasmBytes)

// Instantiate it, and something with it.
instance, _ := wasmer.NewInstance(module, wasmer.NewImportObject())

It's that simple!

Note: The new IsCompilerAvailable function might be your best friend to test whether a compiler is available.

JIT and Native engines

The Wasmer runtime also provides 3 engines to compile and to execute WebAssembly modules. Let's only keep 2 engines in this article. In a nutshell:

  • The JIT engine stores the executable code in memory,

  • The Native engine stores the executable code in a native shared library object (.so, .dylib, or .dll files depending on the Operating System it runs),

Previously, wasmer-go only provided the JIT engine. Now, it's our pleasure to announce that the Native engine is now also part of the family!

Note: The Native engine doesn't work with the Singlepass compiler yet.

The difference between the engines is in how the executable code is stored, especially when serializing and deserializing a compiled WebAssembly module; let's see:

// Configure the engine to use the Cranelift compiler with the Native
// engine.
config := wasmer.NewConfig().UseCraneliftCompiler().UseNativeEngine()
engine := wasmer.NewEngineWithConfig(config)

// Compile the WebAssembly module.
store := wasmer.NewStore(engine)
module, _ := wasmer.NewModule(store, wasmBytes)

// It's time to save this compiled WebAssembly module!
serializedModule, _ := module.Serialize()
_ := ioutil.WriteFile("", serializedModule, 0644)

// … later… in another galaxy, far far away…

serializedModule, _ := ioutil.ReadFile("")
module, _ := wasmer.DeserializeModule(store, serializedModule)

// And instantiate it!
instance, _ := wasmer.NewInstance(module, wasmer.NewImportObject())

Deserializing a compiled WebAssembly module with the Native engine is generally faster as it requires fewer operations.


The engines and the compilers have a new, very interesting feature: They can cross-compile. This means that from machine A with a certain CPU, it is possible to compile for another machine B with a different CPU. To do this we are introducing the Target, Triple and CpuFeatures API.

Let's say we are on a aarch64-unknown-linux-gnu machine, and we want to compile for an x86_64-apple-darwin machine:

// Let's declare the triple of the machine we want to compile for.
triple, _ := wasmer.NewTriple("x86_64-apple-darwin")

// Let's configure the CPU features.
cpuFeatures := wasmer.NewCpuFeatures()

// Finally, a Target is a pair composed of a Triple and CPU features.
target := wasmer.NewTarget(triple, cpuFeatures)

After that, we pass the Target to the Config API, we compile the WebAssembly module, and save it:

// Compile for a specific target.
config := wasmer.NewConfig().UseTarget(target)

// Create the engine with a specific configuration.
engine := wasmer.NewEngineWithConfig(config)
store := wasmer.NewStore(engine)

// Let's compile the module for the given target.
module, _ := wasmer.NewModule(store, wasmBytes)

// Serialize the compiled module.
ioutil.Writefile("my_wasm_module.wjit", module.Serialize(), 0644)

The serialized compiled module can be then deserialized on the targeted machine, instantiated, and executed!

That way, it is easier to pre-compile any WebAssembly modules for a variety of machines!


The wasmer-go package now features WASI (The WebAssembly System Interface) support with all the snapshot previews (that is, all the versions).

First, the GetWasiVersion function can be used to know which version of WASI a WebAssembly module is using or whether it's not using WASI at all:

module, _ := wasmer.NewModule(store, wasmBytes)


Second, to setup WASI, we start by creating a WasiEnvironment with the help of the NewWasiStateBuilder API. We then use the generated WasiEnvironment object to generate an ImportObject. This contains all the imports that “bridge” the WebAssembly module to the host to make WASI a reality. Of course, it's possible to use this ImportObject to import your own host functions, memories etc., just like with any other ImportObject. WasiEnvironment is also responsible for redirecting the stdout and stderr streams if they are captured.

Let's see this with an example. We want to execute this Rust program, that prints its arguments, its environment variables, and that lists the contents of its current working directory.

engine := wasmer.NewEngine()
store := wasmer.NewStore(engine)
module, _ := wasmer.NewModule(store, wasmBytes)

// We specify the program name: `test-program`. We also specify the
// program is invoked with the `--test` argument, in addition to two
// environment variables: `COLOR` and `APP_SHOULD_LOG`. Finally, we map
// the `the_host_current_directory` to the current directory.
wasiEnv, _ := wasmer.NewWasiStateBuilder("wasi-test-program").
	Environment("COLOR", "true")
	Environment("APP_SHOULD_LOG", "false")
	MapDirectory("the_host_current_directory", ".").

// Get the import object (WASI version is auto-detected)!
importObject, _ := wasiEnv.GenerateImportObject(store, module)

// Finally, let's instantiate the module.
instance, _ := wasmer.NewInstance(module, importObject)

At this step, the WebAssembly module is ready to be executed. Which exported function should be called from the instance to start the program? Think no longer, and let's use Exports.GetWasiStartFunction:

start, _ := instance.Exports.GetWasiStartFunction()

Did you notice that stdout is expected to be captured, as defined by CaptureStdout above? Well, here is its content:

stdout := string(wasiEnv.ReadStdout())


It prints:

Found program name: `wasi_test_program`
Found 1 arguments: --test
Found 2 environment variables: COLOR=true, APP_SHOULD_LOG=false
Found 1 preopened directories: DirEntry("/the_host_current_directory")

If CaptureStdout isn't called, or if InheritStdout is called (its opposite function), the stdout stream from Go will be used transparently.

Ready to use on major platforms and architectures

One of the promises of WebAssembly is its universality. By design, it aims at being run anywhere.

wasmer-go embeds the Wasmer runtime as native shared object library (.so, .dylib etc.) for the following platforms:

  • Linux on amd64,
  • Linux on arm64,
  • Darwin on amd64.

More is coming very soon, like Windows, Linux with musl, and more!

If you want to use a custom configuration of Wasmer, we've got you covered with the custom_wasmer_runtime build tag.

$ # Configure cgo.
$ export CGO_CFLAGS="-I/path/to/include/"
$ export CGO_LDFLAGS="-Wl,-rpath,/path/to/lib/ -L/path/to/lib/ -lwasmer_go"
$ # Run the tests to check everything is correct.
$ go test -tags custom_wasmer_runtime


The 1.0 version is more than performance improvements: it provides a stable and powerful API that fulfills more people's needs. We believe that the new API design, the 2 compilers, and the 2 engines are great improvements that provide more power and flexibility than ever before.

And with the help of the new cross-compilation API, we believe that it's now easier than ever to execute WebAssembly anywhere.

Documentation and examples have been meticulously written to help users new to WebAssembly, as well as advanced users. We believe it will facilitate further usage of WebAssembly in the Go ecosystem.

Join a community of more than 1300 Go and WebAssembly passionate developers!.

About the Author

Syrus Akbary is an enterpreneur and programmer. Specifically known for his contributions to the field of WebAssembly. He is the Founder and CEO of Wasmer, an innovative company that focuses on creating developer tools and infrastructure for running Wasm

Syrus Akbary avatar
Syrus Akbary
Syrus Akbary

Founder & CEO

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