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0 pointsklibertp4y ago0 comments

I think you actually can't pass any non-trivial object to WASM. The only thing you can pass to and get out of WASM (IIRC) is a chunk of linear memory, a buffer. I don't think it's possible to obtain a raw pointer to an object on a JS side - passing that would be very unsafe, but also pretty much useless, unless you knew precisely the memory layout of said object. If you need to expose operations on a more complex objects to WASM, you need to encode the object identity somehow (as a number, or a buffer) and expose an API that will decode the object reference and call the needed function/method on it.

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johannes12343214y ago

When using emscripten you get the full marshalling of objects.

Doing a network request from C++:

   val xhr = val::global("XMLHttpRequest").new_();
  xhr.call<void>("open", std::string("GET"), std::string("http://url"));

https://emscripten.org/docs/api_reference/val.h.html#val-h

Of course this can be thrown out of the JavaScript binding side so that it isn't there anymore, but the marshalling makes the API nice and so many emscripten users will use it, thus it's present, thus you have to trust the wasm library.

How complicated this is without emscripten i don't know, but even then I guess many people will need some marshalling for real life scenarios.

klibertpOP4y ago

As I said, you need to encode object identity (here - as a string to look up in the global namespace) and handle requests for doing things with that object (encoded as a method name + a list of strings as arguments) on the JS side. Strings are probably encoded as pointer + length, and the JS side takes care of locating the object, decoding arguments into JS objects, performing the action, and returning the result, encapsulated in a val class instance.

But, you're right, I forgot about this part of emscripten, my bad :) My use case was passing a 2Mb of image data to WASM, which was simple to do with just WebAssembly.Memory, so I didn't get the chance to use this part of the FFI (I used the part going in the opposite direction[1]). I don't know the details of the val implementation, nor the details of JS-side handling the calls, but the basic principle should be as I said: the only "things" you can pass between JS and WASM are numbers and linear buffers. To do anything above that you need support on the JS side and some kind of encoding/serialization, similarly to what you do in IPC/RPC. EDIT: I also suspect it's possible to exclude/disable val.h with a define flag (I didn't check this though).

[1] https://emscripten.org/docs/api_reference/bind.h.html#_CPPv4...

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