What is the UTF8ToString equivalent for turning binary data to a Javascript array

Question

I have C code which returns a void* and size_t length to Javascript running in a web worker. In Javascript i have the following

let start = getStartAddress();
let len = getLength();

I know for Strings i can do

let mystring = UTF8ToString(start);

This is documented https://emscripten.org/docs/api_reference/preamble.js.html#conversion-functions-strings-pointers-and-arrays

I am at a total loss as to how i get an Uint8Array that i can send through postMessage.

Where would i find documentation that tells me how to do that?

In the absence of documentation an answer to my question would suffice.

"I am at a total loss as to how i get an Uint8Array that i can send through postMessage." - hang on, what's going on here? Why do you need to use Uint8Array in JS? Do you want the JS to allocate first, or the C code to allocate firsty? What is the technical problem exactly? — Dai
– Dai, Commented Sep 2, 2022 at 10:26
Anyway, have you reviewed the documentation for ArrayBuffer? It can represent UInt8Array arrays and it's a transferrable object which can safely be used with postMessage and other browser APIs without issue. — Dai
– Dai, Commented Sep 2, 2022 at 10:28
Simply there might not be a convenience function for that. But you should be able to access buffer, and make a copy of its contents. I can't tell if there's no simpler way now, but see my experiment with wasm from 4 years ago here: stackoverflow.com/questions/51666736/… - I simply access e.memory.buffer, where e=instance.exports, comes from the instantiated wasm module. — tevemadar
– tevemadar, Commented Sep 2, 2022 at 10:43
However you definitely don't want to transfer the buffer of the module, but make a copy of what you need. And remember while you are making the copy you will likely use a typed array (like most probably a Uint8Array), but what you can transfer is the underlying ArrayBuffer. — tevemadar
– tevemadar, Commented Sep 2, 2022 at 10:48
@Dai I simply want to send the bytes through postMessage which already makes a copy. It seemed Uint8Array was the way to go. As a said, i'm not finding any docs and tapping in the dark. — Mario Theodoridis
– Mario Theodoridis, Commented Sep 2, 2022 at 11:32

Mario Theodoridis · Accepted Answer · 2022-09-02 12:21:59Z

-1

Ok, so i actually found the definition for UTF8ToString in the Javascript file the emscripten compiler created for my .wasm file. I thought it was a native function. It calls UTF8ArrayToString in the same javascript file. Learning from that source code i came up with the following.

function BytesToArray(base, len) {
  let out = new Array(len);
  let i = 0, ptr = base, end = base + len;
  while (ptr < end) {
    out[i] = HEAPU8[ptr];
    i++;
    ptr++;
  }
  return out;
}
let mydata = BytesToArray(start, len);
postMessage(mydata);

I hope the performance isn't too bad.

answered Sep 2, 2022 at 12:21

Mario Theodoridis

2711 silver badge13 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

What is the UTF8ToString equivalent for turning binary data to a Javascript array

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related