Using embind + pthreads - deallocating shared_ptr from other thread #19299

ricardperez · 2023-05-05T10:19:01Z

ricardperez
May 5, 2023

Hello everybody,

I am facing a problem when allocating a C++ object from JS, which is handled by a std::shared_ptr and it is being deallocated from a std::thread (reaching ref count 0).

Let me introduce you with the most basic example to reproduce such problem:

lib.cpp

#include <thread>
#include <emscripten.h>
#include <emscripten/bind.h>

using namespace emscripten;

class Base {};
class Derived : public Base {};

EMSCRIPTEN_KEEPALIVE void launch(std::shared_ptr<Base> x) {
    static std::thread t;
    t = std::thread([x = std::move(x)]{
        // note how x ownership is being transferred to the lambda
        // which when deallocated will decrease its ref count, in this
        // case triggering the destructor
    });
}

EMSCRIPTEN_BINDINGS(my_module) {
    class_<Base>("Base")
    .smart_ptr<std::shared_ptr<Base>>("std::shared_ptr<Base>");

    class_<Derived, base<Base>>("Derived")
    .smart_ptr_constructor("std::shared_ptr<Derived>", &std::make_shared<Derived>);

    function("launch", &launch);
}

int main() {
    return 0;
}

worker.js

importScripts('lib.js');

function doLaunch() {
    var x = new Module.Derived();
    Module.launch(x);
    x.delete();
}

self.addEventListener('message', (event) => {
    if (event.data === 'launch') {
        doLaunch();
    }
});

index.html

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Hello World</title>
</head>
<body>
    <button>Launch</button>
    <script>
        const worker = new Worker('worker.js');
        const button = document.querySelector('button');
        button.addEventListener('click', () => {
          worker.postMessage('launch');
        });
    </script>
</body>
</html>

And I would be building and serving the app with the commands:

em++ lib.cpp -lembind -o lib.js -s USE_PTHREADS=1 -s PROXY_TO_PTHREAD=1 -s ALLOW_MEMORY_GROWTH=1 -s EXIT_RUNTIME=0 -O0 -g3
emrun index.html

Demo

This is the error when clicking the button and logic is triggered:

What I've tried so far

Using the derived type directly

I've noticed that if my C++ function consumes a std::shared_ptr<Derived> instead of a std::shared_ptr<Base> then everything goes fine. The destructor is being called and there's no error.

Somehow, the problem happens when the shared_ptr reference count reaches 0 and it needs to invoke the destructor of its base class, I guess.

And by the way, I can't change my application functions to receive the derived types.

Explicitly decreasing the ref count from the main thread

I tried to call .reset() to the shared pointer from inside a function passed to emscripten_sync_run_in_main_runtime_thread. This is forcing the deallocation to be triggered from the main runtime thread, which has no problem in calling the destructor properly.

But again this is something I would like to avoid since I could not be sure of how many places in my application I'd need to modify.

sbc100 · 2023-05-09T20:11:21Z

sbc100
May 9, 2023
Maintainer

Duplicate of #19311

0 replies

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Using embind + pthreads - deallocating shared_ptr from other thread #19299

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Using embind + pthreads - deallocating shared_ptr from other thread #19299

ricardperez May 5, 2023

Demo

What I've tried so far

Using the derived type directly

Explicitly decreasing the ref count from the main thread

Replies: 1 comment

sbc100 May 9, 2023 Maintainer

ricardperez
May 5, 2023

sbc100
May 9, 2023
Maintainer