javascript function callback in C code using Emscripten

Question

The task is to call a javascript function as a callback in order to show a progress of while-loop operation. E.g. JS:

var my_js_fn = function(curstate, maxstate){//int variables
console.log(curstate.toString() + " of " + maxstate.toString());
}

C pseudocode:

int smth_that_calls_my_fn(int i, int max) {
/*
the_magic to call my_js_fn()
*/
}
    int main(){
    //....
        while (i < max){
        smth_that_calls_my_fn(i,max);
        }
    //....
    return 0;
    }

How can I link smth_that_calls_my_fn and my_js_fn ?

You can run inline javascript (doc). Or you can implement C API in JS (doc). — Andrey
– Andrey, Commented Aug 5, 2015 at 10:47

Charles Ofria · Accepted Answer · 2015-08-05 15:40:29Z

5

The magic you're looking for is pretty simple -- you need to use the EM_ASM_ARGS macro.

Specifically, it can look like

int smth_that_calls_my_fn(int i, int max) {
  EM_ASM_ARGS({ my_js_fn($0, $1); }, i, max);
}

Make sure that you #include <emscripten.h> in your C file so that this macro exists.

The EM_ASM_ARGS macro takes JavaScript code (in braces) as the first argument, and then any other parameters you want to pass in. In the JS code, $0 is the first argument to follow, $1 the next and so on.

I just wrote a blog entry going into detail on this topic if you want more information: http://devosoft.org/an-introduction-to-web-development-with-emscripten/

edited Aug 5, 2015 at 15:40

answered Aug 5, 2015 at 15:29

Charles Ofria

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