This article explains the concepts behind how WebAssembly works including its goals, the problems it solves, and how it runs inside the web browser's rendering engine.
What is WebAssembly?
WebAssembly is a new type of code that can be run in modern web browsers and provides new features and major gains in performance. It is not primarily intended to be written by hand, rather it is designed to be an effective compilation target for low-level source languages like C, C++, Rust, etc.
This has huge implications for the web platform — it provides a way to run code written in multiple languages on the web at near-native speed, with client apps running on the web that previously couldn’t have done so.
WebAssembly is being created as an open standard inside the W3C WebAssembly Community Group with the following goals:
- Be fast, efficient, and portable — WebAssembly code can be executed at near-native speed across different platforms by taking advantage of common hardware capabilities.
- Be readable and debuggable — WebAssembly is a low-level assembly language, but it does have a human-readable text format (the specification for which is still being finalized) that allows code to be written, viewed, and debugged by hand.
- Keep secure — WebAssembly is specified to be run in a safe, sandboxed execution environment. Like other web code, it will enforce the browser's same-origin and permissions policies.
- Don't break the web — WebAssembly is designed so that it plays nicely with other web technologies and maintains backwards compatibility.
How does WebAssembly fit into the web platform?
The web platform can be thought of as having two parts:
- A set of Web APIs that the Web app can call to control web browser/device functionality and make things happen (DOM, CSSOM, WebGL, IndexedDB, Web Audio API, etc.).
- WebAssembly is a low-level assembly-like language with a compact binary format that runs with near-native performance and provides languages with low-level memory models such as C++ and Rust with a compilation target so that they can run on the web. (Note that WebAssembly has the high-level goal of supporting languages with garbage-collected memory models in the future.)
WebAssembly key concepts
- Module: Represents a WebAssembly binary that has been compiled by the browser into executable machine code. A Module is stateless and thus, like a Blob, can be explicitly shared between windows and workers (via
postMessage()). A Module declares imports and exports just like an ES2015 module.
- Memory: A resizable ArrayBuffer that contains the linear array of bytes read and written by WebAssembly’s low-level memory access instructions.
- Table: A resizable typed array of references (e.g. to functions) that could not otherwise be stored as raw bytes in Memory (for safety and portability reasons).
- Instance: A Module paired with all the state it uses at runtime including a Memory, Table, and set of imported values. An Instance is like an ES2015 module that has been loaded into a particular global with a particular set of imports.
In the future, WebAssembly modules will be loadable just like ES2015 modules (using
How do I use WebAssembly in my app?
Above we talked about the raw primitives that WebAssembly adds to the Web platform: a binary format for code and APIs for loading and running this binary code. Now let’s talk about how we can use these primitives in practice.
The WebAssembly ecosystem is at a nascent stage; more tools will undoubtedly emerge going forward. Right now, there are three main entry points:
- Porting a C/C++ application with Emscripten.
- Writing or generating WebAssembly directly at the assembly level.
- Writing a Rust application and targetting WebAssembly as its output.
Let’s talk about these options:
Porting from C/C++
Two of the many options for creating WASM code are an online wasm assembler or Emscripten. There are a number of online WASM assembler choices, such as:
These are great resources for people who are trying to figure out where to start, but they lack some of the tooling and optimizations of Emscripten.
In a nutshell, the process works as follows:
- Emscripten first feeds the C/C++ into clang+LLVM — a mature open-source C/C++ compiler toolchain, shipped as part of XCode on OSX for example.
- Emscripten transforms the compiled result of clang+LLVM into a .wasm binary.
Note: There are future plans to allow WebAssembly to call Web APIs directly.
<textarea>. If the application uses OpenGL, the HTML also contains a
<canvas> element that is used as the rendering target. It’s very easy to modify the Emscripten output and turn it into whatever web app you require.
Writing WebAssembly directly
In the same fashion as physical assembly languages, the WebAssembly binary format has a text representation — the two have a 1:1 correspondence. You can write or generate this format by hand and then convert it into the binary format with any of several WebAssemby text-to-binary tools.
For a simple guide on how to do this, see our Converting WebAssembly text format to wasm article.
Writing Rust Targetting WebAssembly
Since the end of November 2017, Rust Nightly has supported targetting WebAssembly as its output, without requiring Emscripten to port it over. More details here.
This article has given you an explanation of what WebAssembly is, why it is so useful, how it fits into the web, and how you can make use of it.