Add emscripten_queue_microtask() API to enable use case described in this thread: #25670 (comment).

Author: juj

ChangeLog.md

@@ -20,6 +20,8 @@ See docs/process.md for more on how version tagging works.
 
 4.0.20 (in development)
 -----------------------
+- Added new `emscripten_queue_microtask()` API to call the JS `queueMicrotask()`
+  function.
 
 4.0.19 - 11/04/25
 -----------------

src/lib/libhtml5.js

@@ -2374,6 +2374,12 @@ var LibraryHTML5 = {
     return requestAnimationFrame(tick);
   },
 
+  emscripten_queue_microtask: (cb, userData) => {
+    queueMicrotask(() => {
+      {{{ makeDynCall('vp', 'cb') }}}(userData);
+    });
+  },
+
   emscripten_get_device_pixel_ratio__proxy: 'sync',
   emscripten_get_device_pixel_ratio: () => {
 #if ENVIRONMENT_MAY_BE_NODE || ENVIRONMENT_MAY_BE_SHELL

src/lib/libsigs.js

@@ -732,6 +732,7 @@ sigs = {
   emscripten_promise_resolve__sig: 'vpip',
   emscripten_promise_then__sig: 'ppppp',
   emscripten_random__sig: 'f',
+  emscripten_queue_microtask__sig: 'vp',
   emscripten_request_animation_frame__sig: 'ipp',
   emscripten_request_animation_frame_loop__sig: 'vpp',
   emscripten_request_fullscreen__sig: 'ipi',

system/include/emscripten/html5.h

@@ -466,6 +466,8 @@ int emscripten_request_animation_frame(bool (*cb)(double time, void *userData),
 void emscripten_cancel_animation_frame(int requestAnimationFrameId);
 void emscripten_request_animation_frame_loop(bool (*cb)(double time, void *userData), void *userData);
 
+int emscripten_queue_microtask(void (*cb)(void *userData), void *userData);
+
 double emscripten_date_now(void);
 double emscripten_performance_now(void);
 

test/emscripten_queue_microtask.c

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2025 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+// Test emscripten_queue_microtask() behavior
+
+#include <stdio.h>
+#include <assert.h>
+#include <emscripten.h>
+#include <emscripten/html5.h>
+
+void cb(void *userData) {
+  printf("cb\n");
+  assert(userData == (void*)42);
+  emscripten_force_exit(0);
+}
+
+int main() {
+  emscripten_queue_microtask(cb, (void*)42);
+  emscripten_exit_with_live_runtime();
+  return 99; // We won't reach here, but return non-zero value to guard against refactors that might exit() with this value.
+}

test/test_browser.py

@@ -5016,6 +5016,13 @@ def test_emscripten_request_animation_frame_loop(self):
   def test_request_animation_frame(self):
     self.btest_exit('test_request_animation_frame.c')
 
+  def test_emscripten_queue_microtask(self):
+    self.btest_exit('emscripten_queue_microtask.c')
+
+  @requires_shared_array_buffer
+  def test_emscripten_queue_microtask_pthread(self):
+    self.btest_exit('emscripten_queue_microtask.c', cflags=['-pthread', '-sPROXY_TO_PTHREAD'])
+
   @requires_shared_array_buffer
   def test_emscripten_set_timeout(self):
     self.btest_exit('emscripten_set_timeout.c', cflags=['-pthread', '-sPROXY_TO_PTHREAD'])