Callbacks from Threaded Node.js C++ Extension

UPDATE: this guide is bit outdated, new Node (0.6 > ) versions support easier way to access pooled worker threads so extension doesn’t need to create its own. See links in comments.

Writing threaded Node.js extension requires some care. All Javascript in Node.js is executed in single main thread, so you can not simply call the V8 engine directly from your background thread. That would cause segfault. Recommended way to do this is to spawn new thread on background and use the libev events to wake up the main thread to execute the Javascript callbacks.

Node.js framework has lots of ready stuff for implementing extensions, but there is no simple example how to implement this kind extension so here it is.

Add-on Source

Save this source to

#include <queue>

// node headers
#include <v8.h>
#include <node.h>
#include <ev.h>
#include <pthread.h>
#include <unistd.h>
#include <string.h>

using namespace node;
using namespace v8;

// handles required for callback messages
static pthread_t texample_thread;
static ev_async eio_texample_notifier;
Persistent<String> callback_symbol;
Persistent<Object> module_handle;

// message queue
std::queue<int> cb_msg_queue = std::queue<int>();
pthread_mutex_t queue_mutex = PTHREAD_MUTEX_INITIALIZER;

// The background thread
static void* TheThread(void *)
    int i = 0;
    while(true) {
         // fire event every 5 seconds
      // wake up callback
      ev_async_send(EV_DEFAULT_UC_ &eio_texample_notifier);
    return NULL;

// callback that runs the javascript in main thread
static void Callback(EV_P_ ev_async *watcher, int revents)
    HandleScope scope;

    assert(watcher == &eio_texample_notifier);
    assert(revents == EV_ASYNC);

    // locate callback from the module context if defined by script
    // texample = require('texample')
    // texample.callback = function( ... ) { ..
    Local<Value> callback_v = module_handle->Get(callback_symbol);
    if (!callback_v->IsFunction()) {
         // callback not defined, ignore
    Local<Function> callback = Local<Function>::Cast(callback_v);

    // dequeue callback message
    int number = cb_msg_queue.front();

    TryCatch try_catch;

    // prepare arguments for the callback
    Local<Value> argv[1];
    argv[0] = Local<Value>::New(Integer::New(number));

    // call the callback and handle possible exception
    callback->Call(module_handle, 1, argv);

    if (try_catch.HasCaught()) {

// Start the background thread
Handle<Value> Start(const Arguments &args)
    HandleScope scope;

    // start background thread and event handler for callback
    ev_async_init(&eio_texample_notifier, Callback);
    //ev_set_priority(&eio_texample_notifier, EV_MAXPRI);
    ev_async_start(EV_DEFAULT_UC_ &eio_texample_notifier);
    pthread_create(&texample_thread, NULL, TheThread, 0);

    return True();

void Initialize(Handle<Object> target)
    HandleScope scope;

    NODE_SET_METHOD(target, "start", Start);

    callback_symbol = NODE_PSYMBOL("callback");
    // store handle for callback context
    module_handle = Persistent<Object>::New(target);

extern "C" {
static void Init(Handle<Object> target)

NODE_MODULE(texample, Init);

Function walkthrough

  • The Init function gets called when you require('texample') the native module.
  • Initialize function defines module function start that will be called by javascript. It also stores the module handle for locating and calling the script defined callback on right context.
  • Start function initializes the libev event notifier and starts the background thread TheThread
  • Thread TheThread simply loops, sleeps and puts incremental integers to the queue and wakes up the main thread each time.
  • Callback function gets waken up the libev and it locates and calls the javascript function callback


Copy this to the ‘wscript’  file.

def set_options(opt):

def configure(conf):

def build(bld):
  obj = bld.new_task_gen("cxx", "shlib", "node_addon")
  obj.cxxflags = ["-g", "-D_FILE_OFFSET_BITS=64",
                  "-D_LARGEFILE_SOURCE", "-Wall"] = "texample"
  obj.source = ""

Compile the code with node-waf

$ node-waf configure
$ node-waf build


Start node shell and load the native add on module

$ node
> texample = require('./build/default/texample');

Define the callback function that the module will call

> texample.callback = function(i) {
... console.log('Bang: ' + i);
... }

Call start to kick of the background thread

> texample.start();

Wait for 5 seconds, you’ll start seeing your callback getting triggered every 5 seconds.

> Bang: 0
Bang: 1
> Bang: 2

Have fun!