async javascript library call from C code finishes too late

Question

I am using emscripten to compile a C program which makes use of a JavaScript library I have written, and I'm linking them together with the --js-library flag. My C code calls a function in the library which has a Promise in it, and in the callback I want it to write the given result to some memory which has been passed into the JS function from the C caller. Here is an example (I omitted the error-catching catch part of the Promise for brevity):

lib.js

mergeInto(LibraryManager.library, {
  compute_js: function(input, out_buf) {
    do_promise_computation(
      input
    ).then(function(result){
       Module.print("Promise Returned fully");
       for (var i = 0; i < 8; i++) {
         var num = result[i]
         {{{makeSetValue('out_buf+(i*4)', 0, 'num', 'i32')}}}
       }
  });
}

program.c

#include <stdio.h>
#include <stdint.h>
#include <emscripten.h>

extern void compute_js(int32_t, int64_t*);

int main() {
  int32_t input = 1234;
  int64_t out_buf[4];
  int64_t* out_ptr = (int64_t*)(&out_buf);
  printf("Calling Javascript\n");
  compute_js(input, out_ptr);
  printf("%lld\n", out_buf[0]);
  printf("%lld\n", out_buf[1]);
}

In other functions, without Promises, that I've written for the library, I have been able to successfully write the data to the buffer given from the C code, but this time, the output comes back as follows:

Calling Javascript
0
0
Promise Returned fully

But I want to be able to wait for the result from the Promise and then write it into the C code, so that Promise Returned Fully comes before the printf statements in the C code which are after the JavaScript call. Is there a way to do this with emscripten?

Answer 1

Use ASYNCIFY_FUNCTIONS and _emscripten_async_resume.

https://kripken.github.io/emscripten-site/docs/porting/asyncify.html