WebAssembly in Action

Author of the book "WebAssembly in Action"
Save 40% with the code: ggallantbl
The book's original source code can be downloaded from the Manning website and GitHub. The GitHub repository includes an updated-code branch that has been adjusted to work with the latest version of Emscripten (currently version 3.1.44).

Thursday, December 14, 2017

WebAssembly – Using Emscripten to create a bare bones module

In my last blog post, An Introduction to WebAssembly, I showed you some of the basics of working with a WebAssembly module by using Emscripten.

This article is the result of some research and experiments to see if it’s possible to build a WebAssembly module using Emscripten but to not have any of the plumbing code.

For example, if we were to build the following C file using the following command line, the result would be an HTML file that is 101 KB, a JS file that is 80.2 KB, and a wasm file that is 9.4 KB!

#include <stdio.h>
#include "../emscripten/emscripten.h"

int main() { return 0; }

int EMSCRIPTEN_KEEPALIVE add(int x, int y) { return x + y; }

emcc test.c -s WASM=1 -s NO_EXIT_RUNTIME=1 -O1 -o hello.html

Having all of that plumbing is very convenient because it lets you start playing around with WebAssembly modules right away but what if we want just the bare minimum and will handle the HTML and JavaScript ourselves?

Fortunately, there is a way to tell Emscripten to output just the bare bones wasm file.

Let's first strip the C file down to just the bare minimum. In this case, we only want the add method:

int add(int x, int y) { return x + y; }

If we use the following command line, we will get just the wasm file and it’s only 202 bytes!

emcc test.c -s WASM=1 -s SIDE_MODULE=1 -O1 -o test.wasm

The SIDE_MODULE flag tells Emscripten to compile only our methods and nothing else which means you will also not have access to things like printf or malloc.

If not specified, the default optimization flag used is -O0 (capital letter o and the number 0) but that results in the following error being thrown when you try to load the module:

LinkError: import object field 'DYNAMICTOP_PTR' is not a Number

Adding any optimization flag other than -O0 will fix the issue so we went with -O1 (capital letter o and the number 1) for this example.

One thing to note, however, is that the O appears to be case sensitive. The various optimization flags that are available can be found here: https://kripken.github.io/emscripten-site/docs/optimizing/Optimizing-Code.html

We also need to specify the name of the output file in the command line because, if we don't, Emscripten will output the file with the name a.out.wasm.

Because we've decided not to use Emscripten's plumbing code, we need to write our own HTML and JavaScript. The following is some example HTML and JavaScript to load in the module:

<!DOCTYPE html>
<meta charset="utf-8"/>
<input type="button" value="test" onclick="javascript:OnClickTest();" />

<script type="text/javascript">
var gModule = null;

var importObject = {
'env': {
'memoryBase': 0,
'tableBase': 0,
'memory': new WebAssembly.Memory({initial: 256}),
'table': new WebAssembly.Table({initial: 0, element: 'anyfunc'})

fetch('test.wasm').then(response =>
).then(bytes =>
WebAssembly.instantiate(bytes, importObject)
).then(results => {
gModule = results.instance; // Hold onto the module's instance so that we can reuse it

function OnClickTest(){
alert(gModule.exports._add(1, 2));

One thing that you may have noticed that is different with our call to the C method, as compared to what we did in the previous blog post, is that we're not using Module.ccall or Module.cwrap. Those are Emscripten helper methods. Here, we're calling the C method directly.

Something else to be aware of here is that your JavaScript will need to include an underscore character before the method name. For example, in our case, our method is called add. When you call the add method in JavaScript you would use _add(1, 2); rather than add(1, 2);

Although this approach might not be a solution that will work in every situation, given that you don't have access to things like malloc, it might be a useful way of creating a helper library if you're doing things like number crunching and don't need all of the overhead that comes with Emscripten's plumbing.

A New Book

I’ve been honored with an opportunity to write a book about WebAssembly.

If you enjoyed this article and would like to know more about WebAssembly, I welcome you to check out my book: WebAssembly in Action

No comments:

Post a Comment