Converting WebAssembly text format to Wasm

WebAssembly has an S-expression-based textual representation, an intermediate form designed to be exposed in text editors, browser developer tools, etc. This article explains a little bit about how it works, and how to use available tools to convert text format files to the Wasm format.

Note: Text format files are usually saved with a .wat extension. Historically, a .wast extension was also used, however that's now used for the scripting language used by the WebAssembly test suite.

A first look at the text format

Let's look at an example of this — the following program imports a function called imported_func from a module called my_namespace, and exports a function called exported_func:

wasm
(module
  (func $i (import "my_namespace" "imported_func") (param i32))
  (func (export "exported_func")
    i32.const 42
    call $i
  )
)

The WebAssembly function exported_func is exported for use in our environment (e.g. the web app in which we are using our WebAssembly module). When it is called, it calls an imported JavaScript function called imported_func, which is run with the value (42) provided as a parameter.

Converting the text .wat into a binary .wasm file

Let's have a go at converting the above .wat text representation example into .wasm assembly format.

  1. To start with, make a copy of the above listing inside a text file; call it simple.wat.

  2. We need to assemble this textual representation into the assembly language the browser actually reads before we can use it. To do this, we can use the wabt tool, which includes compilers to convert between WebAssembly's text representation and Wasm, and vice versa, plus more besides. Go to https://github.com/webassembly/wabt — follow the instructions on this page to set up the tool.

  3. Once you've got the tool built, add the /wabt/out/clang/Debug directory to your system PATH.

  4. Next, execute the wat2wasm program, passing it the path to the input file, followed by an -o parameter, followed by the path to the output file:

    bash
    wat2wasm simple.wat -o simple.wasm

This will convert the Wasm into a file called simple.wasm, which contains the .wasm assembly code.

Note: You can also convert the assembly back into the text representation using the wasm2wat tool; for example wasm2wat simple.wasm -o text.wat.

Viewing the assembly output

Because the output file is assembly-based, it can't be viewed in a normal text editor. However, you can view it using the wat2wasm tool's -v option. Try this:

bash
wat2wasm simple.wat -v

This will give you an output in your terminal in the following way:

several strings of binary with textual descriptions beside them. For example: 0000008: 01 ; section code

See also