Async comunication with javascript

[jcubic]

Here is a question on SO How can I run an interactive program compiled with Emscripten in a web page?

And I came up with a hack that allows to send asynchronous messages from C to javascript and get the value from JS in C. So I can use it with the jQuery Terminal read method.

I've abused the fetch API and created a wrapper over XHR in JS that sends real XHR or my code if the URL is in a special format. The code can be found here

https://gist.github.com/jcubic/87f2b4c5ef567be43796e179ca08c0da

Does a better solution exist? Or will you consider adding this to emscripten to allow for Bidirectional async communication with javascript?

I was thinking about something like

void read_async();

void success(void *text) {
   printf("input: %s\n", text);
   read_async();
}

void fail() {
   printf("fail\n");
}


void read_async() {
   char** args = {"name: "};
   emscripten_async_js("read", args, &success, &fail);
}

int main() {
  read_async();
}

and in JS the code can execute global read function and if returned promise resolve it will execute success in C with data like this:

function read(prompt) {
   display(prompt);
   return new Promise(function(resolve, reject) {
      if (true) {
         resolve("hello");
      } else {
         reject();
   });
}

and it will execute C function success with the string "hello".

[xloem]

It looks like a lot of the js libraries used in emterm could be made more general to allow for interoperation with e.g. xterm as a normal terminal. I started this a little at https://github.com/xloem/emterm . with more dev work from somebody, terminal applications could be running in the browser.

Emterm lets the user change up their js libraries with --js-library .

[gaycodegal]

Synchronous terminal communication is possible now thanks to emscripten asyncify.
Code demonstration
https://github.com/gaycodegal/lua-repl

Live demo
https://gaycodegal.github.io/lua-repl

[TheGeekOnSkates]

Asyncify is great, but how then would I point fgets or getchar or whatever at it? I mean I suppose you could pretend you're building an OS from scratch - no standard library, no standard anything - and re-implement these functions as Asyncify-based stdin... but is that really the best way? If it is, it is. fgets is out, custom hacks like the OP's read_async or whatever is in. Forget anything like GNU Readline. 😆

Another thing I've had limited success with is by hacking the "compiled" Javascript and redefining stdin and stdout (a thing I have to redo EVERY TIME I BUILD - so annoying - sorry). For example, I can redefine console.log to run xterm.js's output function, like I did on one of my projects, but stdin is much much harder. I've tried redefining prompt to return the next character in a buffer, or 0 if there was none, and then use a while-loop in C to wait for input... nope. It hung the main thread. So apparently it's Asyncify or bust? ☹️

There's another option, one I've been trying to avoid but might have to be the way to go: SDL. SDL is great for games, and has keyboard event handling, so maybe if I geek-rigged a terminal in SDL, maybe then I could just use stdin as stdin... Obviously I'd rather not reinvent that wheel, but IMO something really needs to be done to make stdin more usable. Sorry, I know this sounds an awful lot like a rant, but this single issue has kept me from using Emscripten in any meaningful way since I found it 3-4 years ago. That's why I'm taking the time to add to this question/issue/whatever. Thank you for reading, and for all the awesome work you guys do to make WebAssembly accessible to us C programmers. :)

[gaycodegal]

Readline is totally something you can emulate in wasm, I do it for my own project. You just need to write your own headers and define functions with the same API as readline. I didn't bother to implement the history part of readline, as that's just extra JavaScript work, but it's totally doable too