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 covert text format files to the .wasm assembly format.

A first look at the text format

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

(module
  (func $i (import "imports" "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 in turn 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 the tool up.
3. Once you’ve got the tool built, add the /wabt/out directory to your system PATH.
4. Next, execute the wast2wasm program, passing it the path to the input file, followed by an -o parameter, followed by the path to the output file:

   wast2wasm simple.wat -o simple.wasm

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

Viewing the assembly output

Because the output file is assembly-based, it can’t be viewed in a normal text editor. You can however view it using the wast2wasm tool’s -v option. Try this:

wast2wasm simple.wat -v

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

See also

• Understanding WebAssembly text format — detailed explanation of the text format syntax.
• Compiling from C/C++ to WebAssembly — tools like Binaryen/Emscripten both compile your source code to wasm, and create the API code needed to run the module in a JavaScript context. Find out more about how to use them.
• Using the WebAssembly JavaScript API — read this if you want to find out more about how the WebAssembly API code works.
• Text format — more explanation of the text format, on the WebAssembly GitHub repo.