String concat from external function will get RuntimeError: unreachable

[echoloyuk]

I use the external function from JS host, to concat a string in the assemblyscript function, will got RuntimeError: unreachable error. the demo code shows below:

node.js code:

const loader = require("@assemblyscript/loader");
const fs = require("fs");
const myImports = {
  env: {
    abort(_msg, _file, line, column) {},
  },
  index: {
    getSth: function () {
      return wasm.__allocString('world');
    }
  }
};
const wasm = loader.instantiateSync(
  fs.readFileSync("build/untouched.wasm"),
  myImports
);
// execute the wasm.test2()
console.log(wasm.__getString(wasm.test2()));

index.ts

declare function getSth(): string;
export function test2(): string {
  return 'hello' + getSth();
}

when executed, get the error below:

wasm://wasm/00014e42:29

^

RuntimeError: unreachable
    at ~lib/rt/pure/decrement (wasm-function[28]:45)
    at ~lib/rt/pure/__release (wasm-function[29]:13)
    at assembly/index/test2 (wasm-function[65]:71)
    at Object.wrap [as test2] (/Users/mac/myProject/assemblyscript/as2/node_modules/@assemblyscript/loader/index.js:277:12)
    at Object.<anonymous> (/Users/mac/myProject/assemblyscript/as2/node.js:41:35)
    at Module._compile (module.js:652:30)
    at Object.Module._extensions..js (module.js:663:10)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)
    at Function.Module._load (module.js:497:3)

Is there are sth wrong with me?

Btw, when I change return 'hello' + getSth(); to return getSth();, the function works well, and console the world. I read the document then and knowns that __allocString will return the header points of the string. Is there any ways to let ptr to string? I v try 'hello' + getSth().toString(), but is still useless.

[echoloyuk]

and return 'hello'.concat(getSth().toString()) is the same error. with declare function getSth(): i32

[dcodeIO]

Looks like getSth is missing to retain a reference:

    getSth: function () {
      return wasm.__retain(wasm.__allocString('world'));
    }

[MaxGraey]

Yes. Just use wasm.__retain after __allocString / __alloc and don't forget __release.

See working fiddle: https://webassembly.studio/?f=vt3k47rzb8

[dcodeIO]

In this specific example a __release is not necessary since the value is returned, and the convention is that a function call retains managed returns for the caller, hence the ref will already be released by the calling code in Wasm.

[echoloyuk]

Cool! It works when I add __retain . Thank you so much.

As a beginner, I do have a lot to learn. I reading the document for many times, but I still don't realize the problem caused by memory collection until you remind me. The most intuitive feeling for me is that AssemblyScript is just LOOK like TypeScript in syntax, and there are many differences in actual use. In the process of learning, I encountered many problems and have spent a lot of time trying and guessing what goes wrong.

[stale]

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