Built-in way to pass JS objects to C++ using emscripten

Question

Is there a simple built-in way to pass JS objects to C++?

I tried doing it the obvious way:

echo.cpp:

#include <iostream>

#include <emscripten.h>
#include <emscripten/val.h>

using emscripten::val;

#ifdef __cplusplus
extern "C"  {
#endif

EMSCRIPTEN_KEEPALIVE void echo(val x){
    val::global("console").call<void>("log", x);
}

int main(int argc, char **argv){
    return 0;
}


#ifdef __cplusplus
}
#endif

script.mjs:

import initEM from "./echo.mjs";

var mod = await initEM();

export function echo(x){
  mod.ccall("echo", "void", ["object"], [x]);
}

echo({attr: 9});

Compiled using:

emcc ./echo.cpp -o ./echo.mjs \
  -sEXPORTED_FUNCTIONS=_main,_echo \
  -sEXPORTED_RUNTIME_METHODS=ccall,cwrap,registeredTypes \
  -lembind --bind

But I got an error:

Uncaught TypeError: Cannot read properties of undefined (reading 'refcount')
    at __emval_incref (echo.mjs:2622:29)
    at echo.wasm:0x1957
    at echo.wasm:0x1842
    at echo.wasm:0x121c
    at echo.wasm:0x110f
    at echo.wasm:0x104a
    at echo.mjs:1639:22
    at Object.ccall (echo.mjs:845:18)
    at echo (script.mjs:6:7)
    at script.mjs:9:1

After some trial and error, I got it to work:

echo.cpp:

#include <iostream>

#include <emscripten.h>
#include <emscripten/val.h>

using emscripten::val;
using emscripten::internal::EM_VAL;

#ifdef __cplusplus
extern "C"  {
#endif

EMSCRIPTEN_KEEPALIVE void echo(EM_VAL x_ptr){
    // converts it to object from the pointer
    val x = val::take_ownership(x_ptr);
    val::global("console").call<void>("log", x);
}

int main(int argc, char **argv){
    return 0;
}


#ifdef __cplusplus
}
#endif

script.mjs:

import initEM from "./echo.mjs";

var mod = await initEM();

let objToC = null;
for(let tp of Object.values(mod.registeredTypes)){
  if(tp.name=="emscripten::val"){
    // turns it into a pointer (I think)
    objToC = (v) => tp.toWireType(null, v);
    break;
  }
}
if(objToC==null){
  throw new ReferenceError("val.toWireType not found");
}

export function echo(x){
  mod.ccall("echo", "void", ["number"], [objToC(x)]);
}

echo({attr: 9});

(Compiled using same thing as the other one)

Questions:

1. Why isn't this already in the ccall / cwrap functions?
2. Why doesn't emscripten expose the val.toWireType as an attribute of the module object (ie why do I have to loop through all types to find it), or is there something I've missed?

Answer 1

Via the docs :

EMSCRIPTEN_BINDINGS(my_module) {
  function("lerp", &lerp);
}

my_module is just a (globally) unique name you have to add, it doesn't have any other purpose.

void echo(EM_VAL x_ptr){
  // converts it to object from the pointer
  val x = val::take_ownership(x_ptr);
  val::global("console").call<void>("log", x);
}
EMSCRIPTEN_BINDINGS(my_bindings) {
  function("echo", echo);
}

now you can invoke echo from JS without using ccall :

Module.echo({addr: 9});

note that this doesn't work well from a webworker; the registration of the echo method in Module is done as part of WASM initialization, and only in the initializing thread.

While EMSCRITEN_BINDINGS looks magical, it basically just makes a static global function and calls it at static construction time. It is

function("echo", echo);

that does all of the work; it determines the arguments of echo , and builds a JS wrapper that converts arguments and calls the C++ function echo , using the name "echo" .