Passing byte array from non-main-thread C++ to main-thread JS

[Hanns Holger Rutz]

Hi there,

I wonder what is the best way to pass a byte array from non-main-thread
C++ to main-thread JS. I'm hesitating to call MAIN_THREAD_SYNC_EM_ASM
because I don't want to block the C++ thread. So the next two options
are EM_ASM and MAIN_THREAD_ASYNC_EM_ASM. In the asynchronous case,
obviously I would have to malloc in C++ and free in JS. Is that
possible? In the EM_ASM case, JS doesn't have access to any regular
objects from the main JS environment. So I would have to copy the data
and defer to the main JS thread? I suppose there is one worker.js that I
would have to listen to from main, and use postMessage from within EM_ASM?

Also the data is bounded by 8K, could I perhaps use a different
approach, like use a constant size array direct passing somehow?

best, .h.h.

[Sam Clegg]

I think MAIN_THREAD_ASYNC_EM_ASM is what you want.     malloc and free work both in C++ and in JS just the same and across threads just fine.

Alternatively you could call the C++ function on the main thread using `emscripten_async_run_in_main_runtime_thread` and then once that function is running on the main thread call out ot EM_ASM or other JS library function as needed.

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[Hanns Holger Rutz]

hi sam,

thanks for your thought; in the end I sticked to MAIN_THREAD_EM_ASM,
because it meant I could just pass on a stack allocated structure. so
far there wasn't any perceivable performance problem that would require
me to switch to asynchronous invocation.

best, .h.h.

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