Exporting function for host environment

Swift 5.10 or earlier

You can expose a Swift function for host environment using special attribute and linker option.

// File name: lib.swift
@_cdecl("add")
func add(_ lhs: Int, _ rhs: Int) -> Int {
    return lhs + rhs
}

You need to compile the Swift code with linker option --export.

To call the exported function as a library multiple times, you need to:

1. Compile it as a WASI reactor execution model. The default execution model is command, so you need to pass -mexec-model=reactor to linker.
2. Call _initialize function before interacting with the instance.

$ swiftc \
    -target wasm32-unknown-wasi \
    -parse-as-library \
    lib.swift -o lib.wasm \
    -Xlinker --export=add \
    -Xclang-linker -mexec-model=reactor

Then, you can use the exported function from host environment.

// File name: main.mjs
import { WASI, File, OpenFile, ConsoleStdout } from "@bjorn3/browser_wasi_shim";
import fs from "fs/promises";

// Instantiate a new WASI Instance
// See https://github.com/bjorn3/browser_wasi_shim/ for more detail about constructor options
let wasi = new WASI([], [],
  [
    new OpenFile(new File([])), // stdin
    ConsoleStdout.lineBuffered(msg => console.log(`[WASI stdout] ${msg}`)),
    ConsoleStdout.lineBuffered(msg => console.warn(`[WASI stderr] ${msg}`)),
  ],
  { debug: false }
);

const wasmBinary = await fs.readFile("lib.wasm");

// Instantiate the WebAssembly file
const { instance } = await WebAssembly.instantiate(wasmBinary, {
  wasi_snapshot_preview1: wasi.wasiImport,
});
// Initialize the instance by following WASI reactor ABI
wasi.initialize(instance);
// Get the exported function
const addFn = instance.exports.add;
console.log("2 + 3 = " + addFn(2, 3))

If you use SwiftPM package, you can omit linker flag using clang's __atribute__. Please see swiftwasm/JavaScriptKit#91 for more detail info

Swift 6.0 or later

If you use Swift 6.0 or later, you can use @_expose(wasm, "add") and omit the --export linker flag.

// File name: lib.swift
@_expose(wasm, "add")
@_cdecl("add") // This is still required to call the function with C ABI
func add(_ lhs: Int, _ rhs: Int) -> Int {
    return lhs + rhs
}

Then you can compile the Swift code with the following command without --export linker flag.

$ swiftc \
    -target wasm32-unknown-wasi \
    -parse-as-library \
    lib.swift -o lib.wasm \
    -Xclang-linker -mexec-model=reactor