WebAssembly return string use C

Question

How can I return a JavaScript string from a WebAssembly function?

https://dev.to/azure/passing-strings-from-c-to-javascript-in-web-assembly-1p01 - not working

C

#include <stdio.h>
#include <string.h>

void jsPrintString(const char *s, uint16_t len);

void print() {
  const char* str = "Hello from C++!";
  jsPrintString(str, strlen(str));
}

Compiler:

emcc -Os start.c  -s STANDALONE_WASM -s EXPORTED_FUNCTIONS="['_hello']" -s ERROR_ON_UNDEFINED_SYMBOLS=0 -Wl,--no-entry -o "test.wasm"

Javascript:

const memory = new WebAssembly.Memory({ initial: 1 });

    function handlePrintString (offset, length) {
      console.log(offset, length);
      const bytes = new Uint8Array(memory.buffer, offset, length);
      const string = new TextDecoder('utf8').decode(bytes);
      console.log(string); //empty ???????????????????????????????????
    }

    const importObject = {
      env: {
        jsPrintString: handlePrintString,
        memory: memory
      },

      js: { mem: memory }
    };

    WebAssembly.instantiateStreaming(fetch('/js/test.wasm'), importObject)
      .then(obj => {
        console.log(obj.instance.exports);
        console.log(obj.instance.exports.print());
      });

memory ArrayBuffer [0,0,0,0,0,0,0,0,0,0,0.....] ???

Answer 1

How can I return a JavaScript string from a WebAssembly function?

You cannot. You have to copy it to memory shared with JavaScript.

void hello(char *array) {
    const my_string = "Hello from C++!";
    memcpy(array, my_string, strlen(my_string));
}

  (async () => {
    const response = await fetch('/js/test.wasm');
    const bytes = await response.arrayBuffer();
    const results = await WebAssembly.instantiate(bytes, {});
    const { instance: { exports: { memory, hello } }} = results;
    const array = new Uint8Array(memory.buffer, 0, 15);
    hello(array.byteOffset);
    console.log(new TextDecoder('utf8').decode(array)); // Hello from C++!
  })();