How to read the real `byteOffset`?

[zie1ony]

I try to read an array of bytes on my host:

@external("env", "read_bytes")
declare function read_bytes(
  ptr: i32,
  size: i32,
): void;

export function call(): void {
    var value = new Uint8Array(3);
    value[0] = 1;
    value[1] = 2;
    value[2] = 3;
    read_bytes(
        value.byteOffset, // returns 0
        value.byteLength
    );

    var value2 = new Uint8Array(3);
    value2[0] = 4;
    value2[1] = 5;
    value2[2] = 6;
    read_bytes(
        value2.byteOffset,  // returns 0
        value2.byteLength
    );
}

I expected byteOffset to return a point in WASM memory where structure starts, but it doesn't work like that.

I have the solution, that works fine in Rust:

let bytes: [u8; 32] = [255u8; 32];
let ptr = bytes.as_ptr();
let size = bytes.len();

unsafe {
    read_bytes(ptr, size);
}

Is it possible to have something similar in AssemblyScript?

[MaxGraey]

byteOffset is not pointer to backing storage of TypedArrays. You should use value.dataStart or changetype<usize>(value.buffer) + value.byteOffset instead.

[zie1ony]

Great, value.dataStart works as expected.

I see in docs https://docs.assemblyscript.org/details/memory#arraybufferview-layout there should be also dataLength method, but it doesn't compile.

ERROR TS2339: Property 'dataLength' does not exist on type '~lib/arraybuffer/ArrayBufferView'.

I ended up with:

read_bytes(
    value.dataStart,
    value.byteLength
);

[MaxGraey]

Yes. That chapter required updates.

[zie1ony]

I have follow up question here. How to reproduce Uint8Array given a pointer and a size? I think of something like load<Uint8Array>(value.dataStart, value.byteLength), but value.byteLength needs to be know at compile size. And I got lost :(

[MaxGraey]

// for Uint8Array

const arr = Uint8Array.wrap(
  changetype<ArrayBuffer>(value.dataStart),
  value.byteOffset, // or `0`
  value.byteLength // same value.byteLength >> alignof<u8>() or value.length
)

// for Uint32Array

const arr = Uint32Array.wrap(
  changetype<ArrayBuffer>(value.dataStart),
  value.byteOffset, // or `0`
  value.byteLength >> alignof<u32>()  // or value.length
)

// and etc

<TypedArray>.wrap in docs

[zie1ony]

Thank you one more time.
I managed to write smart contract for CasperLabs platform using AssemblyScript and your help.
https://github.com/zie1ony/casperlabs-assemblyscript-transfer-to-account

[MaxGraey]

Nice!