Asyncify lets synchronous C or C++ code interact with asynchronous JavaScript. This allows things like:

  • A synchronous call in C that yields to the event loop, which allows browser events to be handled.
  • A synchronous call in C that waits for an asynchronous operation in JS to complete.

Asyncify automatically transforms your compiled code into a form that can be paused and resumed, and handles pausing and resuming for you, so that it is asynchronous (hence the name “Asyncify”) even though you wrote it in a normal synchronous way.

See the Asyncify introduction blogpost for general background and details of how it works internally. The following expands on the Emscripten examples from that post.


This post talks about Asyncify using the new LLVM wasm backend. There was an older Asyncify implementation for the old fastcomp backend. The two algorithms and implementations are entirely separate, so if you are using fastcomp, these docs may not be accurate - you should upgrade to the wasm backend and new Asyncify!

Sleeping / yielding to the event loop

Let’s begin with the example from that blogpost:

// example.cpp
#include <emscripten.h>
#include <stdio.h>

// start_timer(): call JS to set an async timer for 500ms
EM_JS(void, start_timer, (), {
  Module.timer = false;
  setTimeout(function() {
    Module.timer = true;
  }, 500);

// check_timer(): check if that timer occurred
EM_JS(bool, check_timer, (), {
  return Module.timer;

int main() {
  // Continuously loop while synchronously polling for the timer.
  while (1) {
    if (check_timer()) {
      printf("timer happened!\n");
      return 0;

You can compile that with

emcc -O3 example.cpp -s ASYNCIFY


It’s very important to optimize (-O3 here) when using Asyncify, as unoptimized builds are very large.

And you can run it with

nodejs a.out.js

You should then see something like this:

timer happened!

The code is written with a straightforward loop, which does not exit while it is running, which normally would not allow async events to be handled by the browser. With Asyncify, those sleeps actually yield to the browser’s main event loop, and the timer can happen!

Making async Web APIs behave as if they were synchronous

Aside from emscripten_sleep and the other standard sync APIs Asyncify supports, you can also add your own functions. To do so, you must create a JS function that is called from wasm (since Emscripten controls pausing and resuming the wasm from the JS runtime). One way to do that is with a JS library function; another is to use EM_JS, which we’ll use in this next example:

// example.c
#include <emscripten.h>
#include <stdio.h>

EM_JS(void, do_fetch, (), {
  Asyncify.handleSleep(function(wakeUp) {
    out("waiting for a fetch");
    fetch("a.html").then(response => {
      out("got the fetch response");
      // (normally you would do something with the fetch here)

int main() {

The async operation happens in the EM_JS function do_fetch(), which calls Asyncify.handleSleep. It gives that function the code to be run, and gets a wakeUp function that it calls in the asynchronous future at the right time. After we call wakeUp() the compiled C code resumes normally.

In this example the async operation is a fetch, which means we need to wait for a Promise. While that is async, note how the C code in main() is completely synchronous!

To run this example, first compile it with

./emcc example.c -O3 -o a.html -s ASYNCIFY -s 'ASYNCIFY_IMPORTS=["do_fetch"]'

Note that you must tell the compiler that do_fetch() can do an asynchronous operation, using ASYNCIFY_IMPORTS, otherwise it won’t instrument the code to allow pausing and resuming; see more details later down.

To run this, you must run a local webserver and then browse to http://localhost:8000/a.html. You will see something like this:

waiting for a fetch
got the fetch response

That shows that the C code only continued to execute after the async JS completed.


As in the above example, you can add JS functions that do an async operation but look synchronous from the perspective of C. The key thing is to add such methods to ASYNCIFY_IMPORTS, regardless of whether the JS function is from a JS library or EM_JS. That list of imports is the list of imports to the wasm module that the Asyncify instrumentation must be aware of. Giving it that list tells it that all other JS calls will not do an async operation, which lets it not add overhead where it isn’t needed.

Returning values

You can also return values from async JS functions. Here is an example:

// example.c
#include <emscripten.h>
#include <stdio.h>

EM_JS(int, get_digest_size, (const char* str), {
  // Note how we return the output of handleSleep() here.
  return Asyncify.handleSleep(function(wakeUp) {
    const text = UTF8ToString(str);
    const encoder = new TextEncoder();
    const data = encoder.encode(text);
    out("ask for digest for " + text);
    window.crypto.subtle.digest("SHA-256", data).then(digestValue => {
      out("got digest of length " + digestValue.byteLength);
      // Return the value by sending it to wakeUp(). It will then be returned
      // from handleSleep() on the outside.

int main() {
  const char* silly = "some silly text";
  printf("%s's digest size is: %d\n", silly, get_digest_size(silly));
  return 0;

You can build this with

../emcc example.c -s ASYNCIFY=1 -s 'ASYNCIFY_IMPORTS=["get_digest_size"]' -o a.html -O2

This example calls the Promise-returning window.crypto.subtle() API (the example is based off of this MDN example ). Note how we pass the value to be returned into wakeUp(). We must also return the value returned from handleSleep(). The calling C code then gets it normally, after the Promise completes.


As mentioned earlier, unoptimized builds with Asyncify can be large and slow. Build with optimizations (say, -O3) to get good results.

Asyncify adds overhead, both code size and slowness, because it instruments code to allow unwinding and rewinding. That overhead is usually not extreme, something like 50% or so. Asyncify achieves that by doing a whole-program analysis to find functions need to be instrumented and which do not - basically, which can call something that reaches one of ASYNCIFY_IMPORTS. That analysis avoids a lot of unnecessary overhead, however, it is limited by indirect calls, since it can’t tell where they go - it could be anything in the function table (with the same type).

If you know that indirect calls are never on the stack when unwinding, then you can tell Asyncify to ignore indirect calls using ASYNCIFY_IGNORE_INDIRECT.

If you know that some indirect calls matter and others do not, then you can provide a manual list of functions to Asyncify:

  • ASYNCIFY_BLACKLIST is a list of functions that do not unwind the stack. Asyncify will do it’s normal whole-program analysis under the assumption that those do not unwind.
  • ASYNCIFY_WHITELIST is a list of the only functions that can unwind the stack. Asyncify will instrument those and no others.

For more details see settings.js. Note that the manual settings mentioned here are error-prone - if you don’t get things exactly right, your application can break. If you don’t absolutely need maximal performance, it’s usually ok to use the defaults.

Potential problems

Stack overflows

If you see an exception thrown from an asyncify_* API, then it may be a stack overflow. You can increase the stack size with the ASYNCIFY_STACK_SIZE option.


While waiting on an asynchronous operation browser events can happen. That is often the point of using Asyncify, but unexpected events can happen too. For example, if you just want to pause for 100ms then you can call emscripten_sleep(100), but if you have any event listeners, say for a keypress, then if a key is pressed the handler will fire. If that handler calls into compiled code, then it can be confusing, since it starts to look like coroutines or multithreading, with multiple executions interleaved.

It is not safe to start an async operation while another is already running. The first must complete before the second begins.

Such interleaving may also break assumptions in your codebase. For example, if a function uses a global and assumes nothing else can modify it until it returns, but if that function sleeps and an event causes other code to change that global, then bad things can happen.

Starting to rewind with compiled code on the stack

The examples above show wakeUp() being called from JS (after a callback, typically), and without any compiled code on the stack. If there were compiled code on the stack, then that could interfere with properly rewinding and resuming execution, in confusing ways, and therefore an assertion will be thrown in a build with ASSERTIONS.

(Specifically, the problem there is that while rewinding will work properly, if you later unwind again, that unwinding will also unwind through that extra compiled code that was on the stack - causing a later rewind to behave badly.)

A simple workaround you may find useful is to do a setTimeout of 0, replacing wakeUp() with setTimeout(wakeUp, 0);. That will run wakeUp in a later callback, when nothing else is on the stack.

Migrating from older APIs

If you have code uses the old Emterpreter-Async API, or the old Asyncify, then the new API is somewhat different, and you may need some minor changes:

  • The Emterpreter had “yielding” as a concept, but it isn’t needed in Asyncify. You can replace emscripten_sleep_with_yield() calls with emscripten_sleep().
  • The JS API is different. See notes above on Asyncify.handleSleep(), and see src/library_async.js for more examples.