Problems using std::future, std::launch::async

[zajo]

I'm trying to get this simple test to work (on Node) and it seems to deadlock:

int main()
{
    std::future<int> fut = std::async(std::launch::async, [] { return 42; });
    return fut.get();
}

Is this supported at all? I'm compiling/linking with the following options:

WASM=1
USE_PTHREADS=1
PROXY_TO_PTHREAD=1
DISABLE_EXCEPTION_CATCHING=0

[sbc100]

We have no tests for std::async so I guess its not surprising that its not working yet :( If you would like work on this I think other would be excited to see it happen.

[kleisauke]

Try setting -s PTHREAD_POOL_SIZE=2, see: #8988.

[zajo]

Thanks for the suggestion but it did not help. Reading the documentation on PTHREAD_POOL_SIZE, is it needed even under PROXY_TO_PTHREAD?

[kleisauke]

I tried this test:

#include <future>
#include <iostream>

int main() {
    std::future<int> fut = std::async(std::launch::async, [] {
        return 42;
    });
    std::cout << fut.get() << std::endl;
    return 0;
}

I compiled it with:

emcc test.cpp -std=c++11 -s ASSERTIONS=0 -s USE_PTHREADS=1 -s PTHREAD_POOL_SIZE=1 -o test.js

^{(note that I've set PTHREAD_POOL_SIZE to 1, which simply works, so perhaps #8988 is outdated)}

And ran it with Node.js like this:

node --experimental-wasm-threads -e "require('./test.js')"

This prints the expected 42, although the runtime did not exits (I needed to hit Ctrl + C). Perhaps that's the problem?

[kleisauke]

... If that's the problem, try setting -s EXIT_RUNTIME=1 to allow the runtime to quit when main() is finished.

(Pressed the comment button too early)

[zajo]

Thanks, yes that was the problem. In fact, it didn't even need PTHREAD_POOL_SIZE. Is this due to PROXY_TO_THREAD?

[kleisauke]

I think so. Doing pthread_create() calls within a thread (i.e. -s PROXY_TO_PTHREAD=1) can probably be executed synchronously without having to yield back to the main thread.

[zajo]

Thanks for the help!