Using multi-value

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

This example shows off how to interact with a wasm module that uses multi-value exports and imports.

Wasm Source

(module
  (func $f (import "" "f") (param i32 i64) (result i64 i32))

  (func $g (export "g") (param i32 i64) (result i64 i32)
    (call $f (local.get 0) (local.get 1))
  )

  (func $round_trip_many
    (export "round_trip_many")
    (param i64 i64 i64 i64 i64 i64 i64 i64 i64 i64)
    (result i64 i64 i64 i64 i64 i64 i64 i64 i64 i64)

    local.get 0
    local.get 1
    local.get 2
    local.get 3
    local.get 4
    local.get 5
    local.get 6
    local.get 7
    local.get 8
    local.get 9)
)

Host Source

//! This is an example of working with multi-value modules and dealing with
//! multi-value functions.
//!
//! Note that the `Func::wrap*` interfaces cannot be used to return multiple
//! values just yet, so we need to use the more dynamic `Func::new` and
//! `Func::call` methods.

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

use anyhow::Result;

fn main() -> Result<()> {
    use wasmtime::*;

    println!("Initializing...");
    let engine = Engine::default();
    let mut store = Store::new(&engine, ());

    // Compile.
    println!("Compiling module...");
    let module = Module::from_file(&engine, "examples/multi.wat")?;

    // Create a host function which takes multiple parameters and returns
    // multiple results.
    println!("Creating callback...");
    let callback_func = Func::wrap(&mut store, |a: i32, b: i64| -> (i64, i32) {
        (b + 1, a + 1)
    });

    // Instantiate.
    println!("Instantiating module...");
    let instance = Instance::new(&mut store, &module, &[callback_func.into()])?;

    // Extract exports.
    println!("Extracting export...");
    let g = instance.get_typed_func::<(i32, i64), (i64, i32)>(&mut store, "g")?;

    // Call `$g`.
    println!("Calling export \"g\"...");
    let (a, b) = g.call(&mut store, (1, 3))?;

    println!("Printing result...");
    println!("> {a} {b}");

    assert_eq!(a, 4);
    assert_eq!(b, 2);

    // Call `$round_trip_many`.
    println!("Calling export \"round_trip_many\"...");
    let round_trip_many = instance
        .get_typed_func::<
        (i64, i64, i64, i64, i64, i64, i64, i64, i64, i64),
        (i64, i64, i64, i64, i64, i64, i64, i64, i64, i64),
        >
        (&mut store, "round_trip_many")?;
    let results = round_trip_many.call(&mut store, (0, 1, 2, 3, 4, 5, 6, 7, 8, 9))?;

    println!("Printing result...");
    println!("> {results:?}");
    assert_eq!(results, (0, 1, 2, 3, 4, 5, 6, 7, 8, 9));

    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-multi

Also note that this example was taken from
https://github.com/WebAssembly/wasm-c-api/blob/master/example/multi.c
originally
*/

#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <wasm.h>
#include <wasmtime.h>

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

// A function to be called from Wasm code.
wasm_trap_t *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("> %" PRIu32 " %" PRIu64 "\n", args[0].of.i32, args[1].of.i64);
  printf("\n");

  results[0] = args[1];
  results[1] = args[0];
  return NULL;
}

// A function closure.
wasm_trap_t *closure_callback(void *env, wasmtime_caller_t *caller,
                              const wasmtime_val_t *args, size_t nargs,
                              wasmtime_val_t *results, size_t nresults) {
  int i = *(int *)env;
  printf("Calling back closure...\n");
  printf("> %d\n", i);

  results[0].kind = WASMTIME_I32;
  results[0].of.i32 = (int32_t)i;
  return NULL;
}

int main(int argc, const char *argv[]) {
  // Initialize.
  printf("Initializing...\n");
  wasm_engine_t *engine = wasm_engine_new();
  wasmtime_store_t *store = wasmtime_store_new(engine, NULL, NULL);
  wasmtime_context_t *context = wasmtime_store_context(store);

  // Load our input file to parse it next
  FILE *file = fopen("examples/multi.wat", "r");
  if (!file) {
    printf("> Error loading file!\n");
    return 1;
  }
  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 binary;
  wasmtime_error_t *error = wasmtime_wat2wasm(wat.data, wat.size, &binary);
  if (error != NULL)
    exit_with_error("failed to parse wat", error, NULL);
  wasm_byte_vec_delete(&wat);

  // Compile.
  printf("Compiling module...\n");
  wasmtime_module_t *module = NULL;
  error =
      wasmtime_module_new(engine, (uint8_t *)binary.data, binary.size, &module);
  if (error)
    exit_with_error("failed to compile module", error, NULL);
  wasm_byte_vec_delete(&binary);

  // Create external print functions.
  printf("Creating callback...\n");
  wasm_functype_t *callback_type =
      wasm_functype_new_2_2(wasm_valtype_new_i32(), wasm_valtype_new_i64(),
                            wasm_valtype_new_i64(), wasm_valtype_new_i32());
  wasmtime_func_t callback_func;
  wasmtime_func_new(context, callback_type, callback, NULL, NULL,
                    &callback_func);
  wasm_functype_delete(callback_type);

  // Instantiate.
  printf("Instantiating module...\n");
  wasmtime_extern_t imports[1];
  imports[0].kind = WASMTIME_EXTERN_FUNC;
  imports[0].of.func = callback_func;
  wasmtime_instance_t instance;
  wasm_trap_t *trap = NULL;
  error = wasmtime_instance_new(context, module, imports, 1, &instance, &trap);
  if (error != NULL || trap != NULL)
    exit_with_error("failed to instantiate", error, trap);
  wasmtime_module_delete(module);

  // Extract export.
  printf("Extracting export...\n");
  wasmtime_extern_t run;
  bool ok = wasmtime_instance_export_get(context, &instance, "g", 1, &run);
  assert(ok);
  assert(run.kind == WASMTIME_EXTERN_FUNC);

  // Call.
  printf("Calling export...\n");
  wasmtime_val_t args[2];
  args[0].kind = WASMTIME_I32;
  args[0].of.i32 = 1;
  args[1].kind = WASMTIME_I64;
  args[1].of.i64 = 2;
  wasmtime_val_t results[2];
  error = wasmtime_func_call(context, &run.of.func, args, 2, results, 2, &trap);
  if (error != NULL || trap != NULL)
    exit_with_error("failed to call run", error, trap);

  // Print result.
  printf("Printing result...\n");
  printf("> %" PRIu64 " %" PRIu32 "\n", results[0].of.i64, results[1].of.i32);

  assert(results[0].kind == WASMTIME_I64);
  assert(results[0].of.i64 == 2);
  assert(results[1].kind == WASMTIME_I32);
  assert(results[1].of.i32 == 1);

  // Shut down.
  printf("Shutting down...\n");
  wasmtime_store_delete(store);
  wasm_engine_delete(engine);

  // All done.
  printf("Done.\n");
  return 0;
}

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);
}

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

You can build the example using CMake:

mkdir build && (cd build && cmake .. && \
  cmake --build . --target wasmtime-multi-cpp)

And then run it:

build/wasmtime-multi-cpp
*/

#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() {
  std::cout << "Initializing...\n";
  Engine engine;
  Store store(engine);

  std::cout << "Compiling module...\n";
  auto wat = readFile("examples/multi.wat");
  Module module = Module::compile(engine, wat).unwrap();

  std::cout << "Creating callback...\n";
  Func callback_func = Func::wrap(
      store, [](int32_t a, int64_t b) -> std::tuple<int64_t, int32_t> {
        // Rust example adds 1 to each argument but flips order.
        return std::make_tuple(b + 1, a + 1);
      });

  std::cout << "Instantiating module...\n";
  Instance instance = Instance::create(store, module, {callback_func}).unwrap();

  std::cout << "Extracting export...\n";
  Func g = std::get<Func>(*instance.get(store, "g"));

  std::cout << "Calling export \"g\"...\n";
  // Provide (i32=1, i64=3) like the Rust example
  auto results = g.call(store, {Val(int32_t(1)), Val(int64_t(3))}).unwrap();

  std::cout << "Printing result...\n";
  std::cout << "> " << results[0].i64() << " " << results[1].i32() << "\n";

  std::cout << "Calling export \"round_trip_many\"...\n";
  Func round_trip_many =
      std::get<Func>(*instance.get(store, "round_trip_many"));
  auto many_results =
      round_trip_many
          .call(store, {Val(int64_t(0)), Val(int64_t(1)), Val(int64_t(2)),
                        Val(int64_t(3)), Val(int64_t(4)), Val(int64_t(5)),
                        Val(int64_t(6)), Val(int64_t(7)), Val(int64_t(8)),
                        Val(int64_t(9))})
          .unwrap();
  std::cout << "Printing result...\n";
  std::cout << "> (";
  for (size_t i = 0; i < many_results.size(); i++) {
    if (i)
      std::cout << ", ";
    std::cout << many_results[i].i64();
  }
  std::cout << ")\n";
  return 0;
}