async callback using libuv not working

[shah...@essential.com]

Hello everyone,

I have a node js service that calls into a native C library. The native library, fire events repeatedly and continuously. These events are delivered to a C callback function. My goal is to call a Javascript callback from this native C callback.

Based on my reading, I am using  uv_async_init and uv_async_send to accomplish this goal.

The problem I am encountering is that my native C callback function gets called many times and in there uv_async_send is called many times, but the function passed to uv_async_init is called only once and only when my program exits.

Here is my C code:

=====================================================

#include "jsaudio.h"

#include <iostream>

using namespace v8;

static void recordAudioCallback(int index);

void asyncmsg(uv_async_t* handle);

Callback* cbPeriodic;

uv_async_t async;

uv_loop_t* loop;

NAN_METHOD(createEngine) {

    std::cout << "==> createEngine\n" ;

}

void createAudioRecorder(const Nan::FunctionCallbackInfo<v8::Value>& info) {

    std::cout << "==> createAudioRecorder\n";

}

void startRecording(const Nan::FunctionCallbackInfo<v8::Value>& info) {

    std::cout << "==> startRecording\n";

    cbPeriodic = new Callback(info[0].As<Function>());

    loop = uv_default_loop();

    uv_async_init(loop, &async, asyncmsg);

}

void asyncmsg(uv_async_t* handle) {

  std::cout << "==> asyncmsg \n";

  Nan::HandleScope scope;

  v8::Isolate* isolate = v8::Isolate::GetCurrent();

  Local<Value> argv[] = { v8::String::NewFromUtf8(isolate, "Hello world") };

  cbPeriodic->Call(1, argv);

}

/* This my callback that gets called many times, by a native library(/

static void recordAudioCallback(int index) {

  std::cout << "==> recordAudioCallback " << index << "\n";

  uv_async_send(&async);

}

======================================================

Here is my test node.js code that calls the above native code

const jsAudio = new JsAudio({sampleRate: 48000, bufferSize: 8192})

function Test () {

  jsAudio.createEngine();

  jsAudio.createAudioRecorder();

  jsAudio.startRecording(function(error, result) {

    if (error) {

      console.log('startRecording failed: ' + error);

    } else {

      console.log('startRecording result: ' + result);

    }

  });

}

Test();

var waitTill = new Date(new Date().getTime() + 3 * 1000);

while(waitTill > new Date()){}

jsAudio.shutdown();

console.log('program exit...');

==========================================================

Here is the out:

==> createEngine

==> createAudioRecorder

==> startRecording

==> recordAudioCallback 0

==> recordAudioCallback 1

==> recordAudioCallback 0

==> recordAudioCallback 1

==> recordAudioCallback 0

==> shutdown

program exit...

==> asyncmsg 

startRecording failed: Hello world

===========================================================

Why asyncmsg is called only once! even though recordAudioCallback is called several times! Any why is it called after program exits!

Any help is appreciated.

Regards, Shahriar

[Ben Noordhuis]

It's right there in the documentation for uv_async_send():

libuv will coalesce calls to :c:func:`uv_async_send`, that is, not
every call to it will
yield an execution of the callback. For example: if
:c:func:`uv_async_send` is called 5
times in a row before the callback is called, the callback will
only be called once. If
:c:func:`uv_async_send` is called again after the callback was
called, it will be called
again.

[shah...@essential.com]

Hi,

The problem was these two lines

var waitTill = new Date(new Date().getTime() + 3 * 1000);

while(waitTill > new Date()){}

This loop never allows the javascript main loop to run. I replaced it with a setTimeout and it is working.

Thanks!