Passing ArrayBuffer from C to JS

[linfan68]

I'm having trouble passing a data buffer from C to JS through a callback.

Here is the JS code:

foo({
  // Other values
  onCaptureDone: (a) => {
     trace('============== capture Done CB =============\n')
     trace(`${a.byteLength} ${a[0]} \n`)
   }
 }
);

and C code

xsSlot _onCaptureDone;
xsmcGet(_onCaptureDone, xsArg(0), xsID_onCaptureDone);
static uint8_t buffer[10] = {0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0xFF, 0x11, 0x22, 0x33, 0x44};
xsmcSetArrayBuffer(xsVar(0), buffer, 10);
xsCallFunction1(xsAccess(_onCaptureDone), xsUndefined, xsVar(0));

and the output

============== capture Done CB =============
10 undefined 

And I tried passing string, which works fine:

    // static uint8_t buffer[10] = {0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0xFF, 0x11, 0x22, 0x33, 0x44};
    // xsmcSetArrayBuffer(xsVar(0), buffer, 10);
    xsmcSetString(xsVar(0), "Hello");
    xsCallFunction1(xsAccess(_onCaptureDone), xsUndefined, xsVar(0));

output

============== capture Done CB =============
Hello

Any thoughts?

[phoddie]

At first glance, the code you shared looks reasonable. Today is a busy, but I will take a look when I can.

One question for you. There must always be a JavaScript stack frame when invoking a callback. If your C callback is triggered outside of an existing XS callback, you need to surround your XS in C calls with xsBeginHost / xsEndHost. I don't see that in your C code fragment, but perhaps you didn't include it since it is kind of boiler plate. Here's a typical example:

moddable/modules/io/digital/esp32/digitalbank.c

Lines 382 to 385 in df88825

	xsBeginHost(digital->the);
	xsmcSetInteger(xsResult, triggered);
	xsCallFunction1(xsReference(digital->onReadable), digital->obj, xsResult);
	xsEndHost(digital->the);

[phoddie]

Oh, the problem is in the JavaScript code. ;)

trace(`${a.byteLength} ${a[0]} \n`)`

The contents of an ArrayBuffer cannot be accessed using array notation (e.g. a[0]). That gives undefined. Running this with any engine -- (new ArrayBuffer(10))[0] -- gives undefined. From MDN:

You cannot directly manipulate the contents of an ArrayBuffer; instead, you create one of the typed array objects or a DataView object which represents the buffer in a specific format, and use that to read and write the contents of the buffer

For example, this wraps the ArrayBuffer in a Uint8Array:

a = new Uint8Array(a);
trace(`${a.byteLength} ${a[0]} \n`)