Passing JS function to Emscripten-generated code

Question

I have a piece of C++ code converted to JavaScript via Emscripten. I would like the converted C++ code to call back to the JavaScript code that calls it. Something like:

Answer 1

Here's what I have gathered from several posts and by looking at Emscripten bundled code:

In C++:

#include 
#include 

extern "C" {
  void registerCallback(void(*back)(const char*));
  void triggerCallback(char* message); // for invoking it from JS, just for this example
}

// global
std::function gCallback;

void registerCallback(void(*back)(const char*)){
    gCallback = back;
}

void triggerCallback(char* message){
  if (gCallback) {
    gCallback(message);
  } else {
    std::cerr << "Cannot pass '"<< message <<"' to undefined callback\n";
  }
}

An important thing, which was missing in other posts, is to compile C++ with RESERVED_FUNCTION_POINTERS=... flag, e.g.:

em++ -std=c++11 -s RESERVED_FUNCTION_POINTERS=20 source.cpp -s EXPORTED_FUNCTIONS="['_registerCallback','_triggerCallback']" -o try.html

After loading try.html into a browser, you can execute the following JS code in its console:

// Register a callback function
function callback(text){ alert("In JS: "+Pointer_stringify(text)); }
var cb = Runtime.addFunction(callback);
_registerCallback(cb);

// Invoke it with some "C string"
var jsStr = "XOXOXO";
var cStr = allocate(intArrayFromString(jsStr), 'i8', ALLOC_NORMAL)
_triggerCallback(cStr);

// Free Emscripten heap and release the function pointer
_free(cStr);
Runtime.removeFunction(cb);

You should see an alert with "In JS: XOXOXO".