Multithreaded Embedding

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

This example demonstrates using Wasmtime in multithreaded runtimes.

threads.wat

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

threads.c

/*
Example of instantiating of the WebAssembly module and invoking its exported
function in a separate thread.

You can build using cmake:

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

#ifndef _WIN32

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

#define own

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

const int N_THREADS = 10;
const int N_REPS = 3;

#if defined(__linux__)
#define _GNU_SOURCE
#include <sys/syscall.h>
uint64_t get_thread_id() { return (uint64_t)syscall(SYS_gettid); }

#elif defined(__APPLE__)
#include <pthread.h>
uint64_t get_thread_id() {
  uint64_t tid;
  pthread_threadid_np(NULL, &tid);
  return tid;
}

#endif

// A function to be called from Wasm code.
own wasm_trap_t *callback(const wasm_val_vec_t *args, wasm_val_vec_t *results) {
  printf("> Thread %lu running\n", (uint64_t)get_thread_id());
  return NULL;
}

typedef struct {
  wasm_engine_t *engine;
  wasm_shared_module_t *module;
  int id;
} thread_args;

void *run(void *args_abs) {
  thread_args *args = (thread_args *)args_abs;

  // Rereate store and module.
  own wasm_store_t *store = wasm_store_new(args->engine);
  own wasm_module_t *module = wasm_module_obtain(store, args->module);

  // Run the example N times.
  for (int i = 0; i < N_REPS; ++i) {
    usleep(100000);

    // Create imports.
    own wasm_functype_t *func_type = wasm_functype_new_0_0();
    own wasm_func_t *func = wasm_func_new(store, func_type, callback);
    wasm_functype_delete(func_type);

    // Instantiate.
    wasm_extern_t *imports[] = {
        wasm_func_as_extern(func),
    };
    wasm_extern_vec_t imports_vec = WASM_ARRAY_VEC(imports);
    own wasm_instance_t *instance =
        wasm_instance_new(store, module, &imports_vec, NULL);
    if (!instance) {
      printf("> Error instantiating module!\n");
      return NULL;
    }

    wasm_func_delete(func);

    // Extract export.
    own wasm_extern_vec_t exports;
    wasm_instance_exports(instance, &exports);
    if (exports.size == 0) {
      printf("> Error accessing exports!\n");
      return NULL;
    }
    const wasm_func_t *run_func = wasm_extern_as_func(exports.data[0]);
    if (run_func == NULL) {
      printf("> Error accessing export!\n");
      return NULL;
    }

    wasm_instance_delete(instance);

    // Call.
    wasm_val_vec_t args_vec = WASM_EMPTY_VEC;
    wasm_val_vec_t results_vec = WASM_EMPTY_VEC;
    if (wasm_func_call(run_func, &args_vec, &results_vec)) {
      printf("> Error calling function!\n");
      return NULL;
    }

    wasm_extern_vec_delete(&exports);
  }

  wasm_module_delete(module);
  wasm_store_delete(store);

  free(args_abs);

  return NULL;
}

int main(int argc, const char *argv[]) {
  // Initialize.
  wasm_engine_t *engine = wasm_engine_new();

  // Load our input file to parse it next
  FILE *file = fopen("examples/threads.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 and share.
  own wasm_store_t *store = wasm_store_new(engine);
  own wasm_module_t *module = wasm_module_new(store, &binary);
  if (!module) {
    printf("> Error compiling module!\n");
    return 1;
  }

  wasm_byte_vec_delete(&binary);

  own wasm_shared_module_t *shared = wasm_module_share(module);

  wasm_module_delete(module);
  wasm_store_delete(store);

  // Spawn threads.
  pthread_t threads[N_THREADS];
  for (int i = 0; i < N_THREADS; i++) {
    thread_args *args = malloc(sizeof(thread_args));
    args->engine = engine;
    args->module = shared;
    printf("Initializing thread %d...\n", i);

    // Guarantee at least 2MB of stack to allow running Cranelift in debug mode
    // on CI.
    pthread_attr_t attrs;
    pthread_attr_init(&attrs);
    pthread_attr_setstacksize(&attrs, 2 << 20);
    pthread_create(&threads[i], &attrs, &run, args);
    pthread_attr_destroy(&attrs);
  }

  for (int i = 0; i < N_THREADS; i++) {
    printf("Waiting for thread: %d\n", i);
    pthread_join(threads[i], NULL);
  }

  wasm_shared_module_delete(shared);
  wasm_engine_delete(engine);

  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, error_message.data);
  wasm_byte_vec_delete(&error_message);
  exit(1);
}

#else
// TODO implement example for Windows
int main(int argc, const char *argv[]) { return 0; }
#endif // _WIN32