Function Example_storedata

example_test.go:776–842

An example of using [NewStoreWithData] to associate arbitrary data with a [Store], accessible in a callback function via [Caller.Data]

()

// An example of using [NewStoreWithData] to associate arbitrary data with a [Store], accessible in
// a callback function via [Caller.Data]
func Example_storedata() {
	// First, create an engine. This'll hold our linker and compiled module.
	engine := wasmtime.NewEngine()

	// Create our wasm module in text format (wat)
	wasm, err := wasmtime.Wat2Wasm(`
	(module
	  (import "" "hello" (func $hello (param i32)))
	  (import "session" "get_request_id" (func $get_req_id (result i32)))
	  (func (export "run")
	    (call $hello (call $get_req_id))
	  )
	)
	`)
	if err != nil {
		log.Fatal(err)
	}

	// Once we have our binary `wasm` we can compile that into a `*Module`
	// which represents compiled JIT code.
	module, err := wasmtime.NewModule(engine, wasm)
	if err != nil {
		log.Fatal(err)
	}

	// Our wasm module imports two functions, which we're going to implement on the linker.
	linker := wasmtime.NewLinker(engine)

	// Implement host function session::get_request_id
	linker.FuncWrap("session", "get_request_id", func(c *wasmtime.Caller) int32 {
		data := c.Data().(*sessionData)
		return data.request_id
	})

	// Implement host function ::hello
	linker.FuncWrap("", "hello", func(arg0 int32) {
		fmt.Printf("Hello request %d\n", arg0)
	})

	// Now, we're going to create 5 instances of our module using this linker, and each instance
	// will have its own request id.
	for i := 0; i < 5; i++ {
		store := wasmtime.NewStoreWithData(engine, &sessionData{
			request_id: int32(i),
		})

		instance, err := linker.Instantiate(store, module)
		if err != nil {
			log.Fatal(err)
		}

		// After we've instantiated we can lookup our `run` function and call
		// it.
		run := instance.GetFunc(store, "run")
		_, err = run.Call(store)
		if err != nil {
			log.Fatal(err)
		}