CODEWITHSUNDEEP
rustemscriptenwebassembly
Andrei CACIO
Andrei CACIO

Reputation: 2119

Obtaining JavaScript import object entries from a WebAssembly .wasm module

I want to understand what a Rust program actually exports when it is compiled to a wasm file so I can provide a valid importObject to the instantiate function:

WebAssembly.instantiate(bufferSource, importObject);

As far as I understand, the only way to do this is by exporting an s-syntax like file of the compiled code. I can't find how to do this in their docs or through web searches.

Upvotes: 5

Views: 2295

Answers (1)

JF Bastien
JF Bastien

Reputation: 6843

You can use a tool such as wabt's wasm2wast to translate a .wasm file to the equivalent .wast. That would do what you ask for.

However, you don't necessarily need to do this! The JavaScript API gives you most of what you want:

let arrayBuffer = ...; // Somehow get your .wasm file into an ArrayBuffer. XHR, from a string, or what have you.
let module = WebAssembly.Module(arrayBuffer); // This is the synchronous API! Only use it for testing / offline things.

let importObject = {};
for (let imp of WebAssembly.Module.imports(module)) {
    if (typeof importObject[imp.module] === "undefined")
        importObject[imp.module] = {};
    switch (imp.kind) {
    case "function": importObject[imp.module][imp.name] = () => {}; break;
    case "table": importObject[imp.module][imp.name] = new WebAssembly.Table({ initial: ???, maximum: ???, element: "anyfunc" }); break;
    case "memory": importObject[imp.module][imp.name] = new WebAssembly.Memory({ initial: ??? }); break;
    case "global": importObject[imp.module][imp.name] = 0; break;
    }
}

Note that Table and Memory initial / maximum are currently guesses! I'm proposing that we add the missing information to the JS API. I think at the next WebAssembly meeting may be a good time to discuss such an addition.

Upvotes: 5

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