Is it possible to submit HTTP requests with WebAssembly?

Question

I'm trying to submit a simple HTTP GET request in WebAssembly. For this purpose, I wrote this program (copied from Emscripten site with slight modifications):

#include <stdio.h>
#include <string.h>
#ifdef __EMSCRIPTEN__
#include <emscripten/fetch.h>
#include <emscripten.h>
#endif


void downloadSucceeded(emscripten_fetch_t *fetch) {
  printf("Finished downloading %llu bytes from URL %s.\n", fetch->numBytes, fetch->url);
  // The data is now available at fetch->data[0] through fetch->data[fetch->numBytes-1];
  emscripten_fetch_close(fetch); // Free data associated with the fetch.
}

void downloadFailed(emscripten_fetch_t *fetch) {
  printf("Downloading %s failed, HTTP failure status code: %d.\n", fetch->url, fetch->status);
  emscripten_fetch_close(fetch); // Also free data on failure.
}

unsigned int EMSCRIPTEN_KEEPALIVE GetRequest() {
  emscripten_fetch_attr_t attr;
  emscripten_fetch_attr_init(&attr);
  strcpy(attr.requestMethod, "GET");
  attr.attributes = EMSCRIPTEN_FETCH_LOAD_TO_MEMORY;
  attr.onsuccess = downloadSucceeded;
  attr.onerror = downloadFailed;
  emscripten_fetch(&attr, "http://google.com");
  return 1;
}

When I compile it using $EMSCRIPTEN/emcc main.c -O1 -s MODULARIZE=1 -s WASM=1 -o main.js --emrun -s FETCH=1 I get the error

ERROR:root:FETCH not yet compatible with wasm (shared.make_fetch_worker is asm.js-specific)

Is there a way to run HTTP requests from WebAssembly? If yes, how can I do it?

Update 1: The following code attempts to send a GET request, but fails due to CORS issues.

#include <stdio.h>
#include <string.h>
#ifdef __EMSCRIPTEN__
#include <emscripten/fetch.h>
#include <emscripten.h>
#endif

unsigned int EMSCRIPTEN_KEEPALIVE GetRequest() {
  EM_ASM({
    var xhr = new XMLHttpRequest();
    xhr.open("GET", "http://google.com");
    xhr.send();
  });
  return 1;
}

Answer 1

No, you cannot execute HTTP request from WebAssembly (or access DOM, or any other browser APIs). WebAssembly by itself doesn’t have any access to its host environment, hence it doesn’t have any built in IO capabilities.

You can however export functions from WebAssembly, and import functions from the host environment. This will allow you to make HTTP requests indirectly via the host.