Returns struct

lib.c

typedef struct {
    unsigned char data1;
    unsigned char data2;
} MyResult;

MyResult myfunc(unsigned char data1) {
    MyResult result;
    result.data1 = data1;
    result.data2 = 22;
    return result;
}

Makefile

CC = /opt/wasi-sdk/bin/clang
CFLAGS =
LDFLAGS  = -mexec-model=reactor
LDFLAGS += -Wl,--export-all

prog.wasm: lib.c Makefile
	$(CC) $(LDFLAGS) -o $@ $<

run.mjs

import fs from "fs/promises";

const mod = await WebAssembly.compile(await fs.readFile(new URL("./prog.wasm", import.meta.url)));
const instance = await WebAssembly.instantiate(mod, {});
const { myfunc, memory } = instance.exports;

console.log(new Uint8Array(memory.buffer, 65535, 2)); // Uint8Array(2) [ 0, 0 ]
myfunc(65535, 32);
console.log(new Uint8Array(memory.buffer, 65535, 2)); // Uint8Array(2) [ 32, 22 ]

console.log(new Uint8Array(memory.buffer, 66000, 2)); // Uint8Array(2) [ 0, 0 ]
myfunc(66000, 64);
console.log(new Uint8Array(memory.buffer, 66000, 2)); // Uint8Array(2) [ 64, 22 ]