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.


  (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)


Example of instantiating of the WebAssembly module and invoking its exported

You can compile and run this example on Linux with:

   cargo build --release -p wasmtime-c-api
   cc examples/multi.c \
       -I crates/c-api/include \
       -I crates/c-api/wasm-c-api/include \
       target/release/libwasmtime.a \
       -lpthread -ldl -lm \
       -o multi

Note that on Windows and macOS the command will be similar, but you'll need
to tweak the `-lpthread` and such annotations.

You can also build using cmake:

mkdir build && cd build && cmake .. && cmake --build . --target wasmtime-multi

Also note that this example was taken from

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <inttypes.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);

  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.
  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(, file_size, 1, file) != 1) {
    printf("> Error loading module!\n");
    return 1;

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

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

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

  // 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);

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

  // All done.
  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);
  } else {
    wasm_trap_message(trap, &error_message);
  fprintf(stderr, "%.*s\n", (int) error_message.size,;