Hello, world!

You can also browse this source code online and clone the wasmtime repository to run the example locally:

This example shows off how to instantiate a simple wasm module and interact with it.

Wasm Source

(module
  (func $hello (import "" "hello"))
  (func (export "run") (call $hello))
)

Host Source

//! Small example of how to instantiate a wasm module that imports one function,
//! showing how you can fill in host functionality for a wasm module.

// You can execute this example with `cargo run --example hello`

use wasmtime::*;

struct MyState {
    name: String,
    count: usize,
}

fn main() -> Result<()> {
    // First the wasm module needs to be compiled. This is done with a global
    // "compilation environment" within an `Engine`. Note that engines can be
    // further configured through `Config` if desired instead of using the
    // default like this is here.
    println!("Compiling module...");
    let engine = Engine::default();
    let module = Module::from_file(&engine, "examples/hello.wat")?;

    // After a module is compiled we create a `Store` which will contain
    // instantiated modules and other items like host functions. A Store
    // contains an arbitrary piece of host information, and we use `MyState`
    // here.
    println!("Initializing...");
    let mut store = Store::new(
        &engine,
        MyState {
            name: "hello, world!".to_string(),
            count: 0,
        },
    );

    // Our wasm module we'll be instantiating requires one imported function.
    // the function takes no parameters and returns no results. We create a host
    // implementation of that function here, and the `caller` parameter here is
    // used to get access to our original `MyState` value.
    println!("Creating callback...");
    let hello_func = Func::wrap(&mut store, |mut caller: Caller<'_, MyState>| {
        println!("Calling back...");
        println!("> {}", caller.data().name);
        caller.data_mut().count += 1;
    });

    // Once we've got that all set up we can then move to the instantiation
    // phase, pairing together a compiled module as well as a set of imports.
    // Note that this is where the wasm `start` function, if any, would run.
    println!("Instantiating module...");
    let imports = [hello_func.into()];
    let instance = Instance::new(&mut store, &module, &imports)?;

    // Next we poke around a bit to extract the `run` function from the module.
    println!("Extracting export...");
    let run = instance.get_typed_func::<(), ()>(&mut store, "run")?;

    // And last but not least we can call it!
    println!("Calling export...");
    run.call(&mut store, ())?;

    println!("Done.");
    Ok(())
}

/*
Example of instantiating of the WebAssembly module and invoking its exported
function.

You can build using cmake:

mkdir build && cd build && cmake .. && cmake --build . --target wasmtime-hello
*/

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <wasm.h>
#include <wasmtime.h>

static void exit_with_error(const char *message, wasmtime_error_t *error,
                            wasm_trap_t *trap);

static wasm_trap_t *hello_callback(void *env, wasmtime_caller_t *caller,
                                   const wasmtime_val_t *args, size_t nargs,
                                   wasmtime_val_t *results, size_t nresults) {
  printf("Calling back...\n");
  printf("> Hello World!\n");
  return NULL;
}

int main() {
  int ret = 0;
  // Set up our compilation context. Note that we could also work with a
  // `wasm_config_t` here to configure what feature are enabled and various
  // compilation settings.
  printf("Initializing...\n");
  wasm_engine_t *engine = wasm_engine_new();
  assert(engine != NULL);

  // With an engine we can create a *store* which is a long-lived group of wasm
  // modules. Note that we allocate some custom data here to live in the store,
  // but here we skip that and specify NULL.
  wasmtime_store_t *store = wasmtime_store_new(engine, NULL, NULL);
  assert(store != NULL);
  wasmtime_context_t *context = wasmtime_store_context(store);

  // Read our input file, which in this case is a wasm text file.
  FILE *file = fopen("examples/hello.wat", "r");
  assert(file != NULL);
  fseek(file, 0L, SEEK_END);
  size_t file_size = ftell(file);
  fseek(file, 0L, SEEK_SET);
  wasm_byte_vec_t wat;
  wasm_byte_vec_new_uninitialized(&wat, file_size);
  if (fread(wat.data, file_size, 1, file) != 1) {
    printf("> Error loading module!\n");
    return 1;
  }
  fclose(file);

  // Parse the wat into the binary wasm format
  wasm_byte_vec_t wasm;
  wasmtime_error_t *error = wasmtime_wat2wasm(wat.data, wat.size, &wasm);
  if (error != NULL)
    exit_with_error("failed to parse wat", error, NULL);
  wasm_byte_vec_delete(&wat);

  // Now that we've got our binary webassembly we can compile our module.
  printf("Compiling module...\n");
  wasmtime_module_t *module = NULL;
  error = wasmtime_module_new(engine, (uint8_t *)wasm.data, wasm.size, &module);
  wasm_byte_vec_delete(&wasm);
  if (error != NULL)
    exit_with_error("failed to compile module", error, NULL);

  // Next up we need to create the function that the wasm module imports. Here
  // we'll be hooking up a thunk function to the `hello_callback` native
  // function above. Note that we can assign custom data, but we just use NULL
  // for now).
  printf("Creating callback...\n");
  wasm_functype_t *hello_ty = wasm_functype_new_0_0();
  wasmtime_func_t hello;
  wasmtime_func_new(context, hello_ty, hello_callback, NULL, NULL, &hello);

  // With our callback function we can now instantiate the compiled module,
  // giving us an instance we can then execute exports from. Note that
  // instantiation can trap due to execution of the `start` function, so we need
  // to handle that here too.
  printf("Instantiating module...\n");
  wasm_trap_t *trap = NULL;
  wasmtime_instance_t instance;
  wasmtime_extern_t import;
  import.kind = WASMTIME_EXTERN_FUNC;
  import.of.func = hello;
  error = wasmtime_instance_new(context, module, &import, 1, &instance, &trap);
  if (error != NULL || trap != NULL)
    exit_with_error("failed to instantiate", error, trap);

  // Lookup our `run` export function
  printf("Extracting export...\n");
  wasmtime_extern_t run;
  bool ok = wasmtime_instance_export_get(context, &instance, "run", 3, &run);
  assert(ok);
  assert(run.kind == WASMTIME_EXTERN_FUNC);

  // And call it!
  printf("Calling export...\n");
  error = wasmtime_func_call(context, &run.of.func, NULL, 0, NULL, 0, &trap);
  if (error != NULL || trap != NULL)
    exit_with_error("failed to call function", error, trap);

  // Clean up after ourselves at this point
  printf("All finished!\n");
  ret = 0;

  wasmtime_module_delete(module);
  wasmtime_store_delete(store);
  wasm_engine_delete(engine);
  return ret;
}

static void exit_with_error(const char *message, wasmtime_error_t *error,
                            wasm_trap_t *trap) {
  fprintf(stderr, "error: %s\n", message);
  wasm_byte_vec_t error_message;
  if (error != NULL) {
    wasmtime_error_message(error, &error_message);
    wasmtime_error_delete(error);
  } else {
    wasm_trap_message(trap, &error_message);
    wasm_trap_delete(trap);
  }
  fprintf(stderr, "%.*s\n", (int)error_message.size, error_message.data);
  wasm_byte_vec_delete(&error_message);
  exit(1);
}

#include <fstream>
#include <iostream>
#include <sstream>
#include <wasmtime.hh>

using namespace wasmtime;

std::string readFile(const char *name) {
  std::ifstream watFile;
  watFile.open(name);
  std::stringstream strStream;
  strStream << watFile.rdbuf();
  return strStream.str();
}

int main() {
  // First the wasm module needs to be compiled. This is done with a global
  // "compilation environment" within an `Engine`. Note that engines can be
  // further configured through `Config` if desired instead of using the
  // default like this is here.
  std::cout << "Compiling module\n";
  Engine engine;
  auto module =
      Module::compile(engine, readFile("examples/hello.wat")).unwrap();

  // After a module is compiled we create a `Store` which will contain
  // instantiated modules and other items like host functions. A Store
  // contains an arbitrary piece of host information, and we use `MyState`
  // here.
  std::cout << "Initializing...\n";
  Store store(engine);

  // Our wasm module we'll be instantiating requires one imported function.
  // the function takes no parameters and returns no results. We create a host
  // implementation of that function here.
  std::cout << "Creating callback...\n";
  Func host_func =
      Func::wrap(store, []() { std::cout << "Calling back...\n"; });

  // Once we've got that all set up we can then move to the instantiation
  // phase, pairing together a compiled module as well as a set of imports.
  // Note that this is where the wasm `start` function, if any, would run.
  std::cout << "Instantiating module...\n";
  auto instance = Instance::create(store, module, {host_func}).unwrap();

  // Next we poke around a bit to extract the `run` function from the module.
  std::cout << "Extracting export...\n";
  auto run = std::get<Func>(*instance.get(store, "run"));

  // And last but not least we can call it!
  std::cout << "Calling export...\n";
  run.call(store, {}).unwrap();

  std::cout << "Done\n";
}

Wasmtime

Hello, world!

Wasm Source

Host Source