Background and Concepts

Web Assembly

WebAssembly is a simple machine model and executable format with an extensive specification.

Although it has currently gathered attention in the JavaScript and web communities in general, it makes no assumptions about its host environment. Thus, it makes sense to think that wasm will become an important "portable executable" format used in a variety of contexts in the near future (we will dedicate some time to take a closer look at wasm's portability features further in the book).

As of today, however, wasm is mostly related to JavaScript, which comes in many flavors (including both browsers and Node.js). Due to JS being widespread and easy to access we will focus mostly on using these platforms to run Rust-generated wasm, but other interpreters are probably going to be released in the near future.

As a programming language, WebAssembly is comprised of two formats: The binary format and the text format. Both represent a common structure, albeit in different ways. The text format (generally called wat) uses S-expressions, which bears some resemblance to languages like Clojure or Racket. The binary format wasm is a lower level format, being itself the assembly code which is run by the interpreters.

For reference, here is a factorial function in wat:

  (func $fac (param f64) (result f64)
    get_local 0
    f64.const 1
    if (result f64)
      f64.const 1
      get_local 0
      get_local 0
      f64.const 1
      call $fac
  (export "fac" (func $fac)))

If you're curious about how a wasm file looks like you can use wat2wasm demo with the above code.

WebAssembly has a very simple memory model. At the moment, a wasm module has access to a single "linear memory", which is essentially a flat array of a fixed numeric type. This memory can be grown by a multiple of the page size (64K), and cannot be shrunk.