Doing the Emscripten Tutorials - Why does int_sqrt(25) not return 5 in this code?

Question

What are the details of your problem?

So far using Emscripten, I've converted this C code to a.out.wasm and a.out.js:

#include <math.h>
#include <emscripten.h>

int int_sqrt(int x) {
  return sqrt(x);
}

I require'd the output and I'm trying to test out using the int_sqrt() function in Node by using cwrap() in this code:

a = require('./a.out');

int_sqrt = a.onRuntimeInitialized = () => {
    return a.cwrap('int_sqrt', 'number', ['number']);

};
console.log(int_sqrt(25));

The output I'm getting is [Function (anonymous)] even though I think I'm passing in 25 to the "cwrapped" int_sqrt function.

My question is: How can I get this to let me use the int_sqrt() function for the rest of my code and have it give the correct output without putting all my code inside the initializing function?

---

What did you try and what were you expecting?

I'm new to programming and I'm still trying to learn concepts so I would really appreciate it if someone could explain it to me. I think it might have to do with scope? But I'm not sure.

Following the code exactly off of the tutorials:

int_sqrt = Module.cwrap('int_sqrt', 'number', ['number'])
int_sqrt(12)
int_sqrt(28)

would give me an error of Aborted(Assertion failed: native function called before runtime initialization) Error. So I edited it to use Module.onRuntimeInitialized on someone else's recommendation from here.

Using int_sqrt() inside of the scope like this seems to work and returns 5.

a = require('./a.out');

a.onRuntimeInitialized = () => {
     int_sqrt = a.cwrap('int_sqrt', 'number', ['number']);
     console.log(int_sqrt(25));
};

But it doesn't let the rest of my code use int_sqrt() properly so I'm wondering how can I get it to.

Thank you if someone could explain it.

Answer 1

I don't see anything in the documentation for onRuntimeInitialized saying that it returns what the callback it calls returns. From what you've described, apparently it doesn't.

Using int_sqrt() inside of the scope like this seems to work and returns 5...But it doesn't let the rest of my code use int_sqrt() properly so I'm wondering how can I get it to.

Since you have to wait for the runtime to be initialized, you'll have to wait for that callback.

Assuming Emscripten works with JavaScript's native modules ("type": "module" in package.json, details), you could switch to those, wrap the callback in a promise, and use await at the top level to wait for the promise to settle.

Here's a sketch:

import a from "./a.out'";

const { int_sqrt } = await new Promise((resolve) => {
    a.onRuntimeInitialized = () => {
        try {
            const int_sqrt = a.cwrap("int_sqrt", "number", ["number"]);
            resolve({ int_sqrt });
        } catch (error) {
            reject(error);
        }
    };
});

// ...use `int_sqrt` here...

If you need to do other functions, just add them to that template:

import a from "./a.out'";

const { int_sqrt, int_mult } = await new Promise((resolve, reject) => {
    a.onRuntimeInitialized = () => {
        try {
            const int_sqrt = a.cwrap("int_sqrt", "number", ["number"]);
            const int_mult = a.cwrap("int_mult", "number", ["number"]);
            resolve({ int_sqrt, int_mult });
        } catch (error) {
            reject(error);
        }
    };
});

// ...use `int_sqrt` and `int_mult` here...

You could make that a module that exports the functions, which are then used by other modules; for instance, at the end of the above:

export { int_sqrt, int_mult };

Then in another module:

import { int_sqrt, int_mult } from "./the-module-above.js";