Asyncify + EM_ASM + async/await

[davlhd]

Hello, I want to run a function that has await inside it:

async function loadStuff() {
    await stuffLoading();
}

if I use emscripten_run_script its telling me that await is only valid in async function

EM_ASM(
    await loadStuff();
)

Gives me an error SyntaxError: Unexpected token

How do I call async functions?

[kripken]

I think the immediate problem here is that we emit an EM_ASM in a JS function (can look in the output JS to see) but it must be marked as an async function as that error message says.

The larger issue is that even if we marked it as such, the C code underneath it is not async, at least not unless we built with Asyncify or such. We can probably get that working together, if someone's interested to look into it.

[stale]

This issue has been automatically marked as stale because there has been no activity in the past year. It will be closed automatically if no further activity occurs in the next 7 days. Feel free to re-open at any time if this issue is still relevant.

[kripken]

Renamed, let's keep this open if someone wants to look at Asyncify + EM_ASM + await in JS.

[jrynkiew]

I tried this example from the Emscripten Asyncify documentation available here: https://emscripten.org/docs/porting/asyncify.html

In the example they use EM_JS, but turns out you can use EM_ASM just as well.

So I run this is in synchroneous single threaded C++ app.

#if defined(__EMSCRIPTEN__)
EM_ASM({
    Asyncify.handleAsync(async () => {
        let resp = await window.antenna.iotx.sendTransfer({
            to: "io1098h946aa5us8h4fql6zptv9kjqn7cu9x9uzm0",
            from: window.antenna.iotx.accounts[0].address,
            value: toRau("1", "Iotx"),
            gasLimit: "100000",
            gasPrice: toRau("1", "Qev")
        });
    });
});
#endif

Then in the compiler flags you need to add

-s ASYNCIFY -s 'ASYNCIFY_IMPORTS=["emscripten_asm_const_int"]'

I checked this, and it works. You just need to be aware that the underlying C code will need to be multi-threaded, or you need to handle worker threads in emscripten is some other way, otherwise the entire application will pause and wait for the response from the await function.

This is just an example I wrote to await a blockchain transaction approval from a soft wallet, but it should work for all other uses just as well

Alternatively you can use a standard JavaScript function call from C/C++

#if defined(__EMSCRIPTEN__)
EM_ASM({
    testTransaction();
});

and in your HTML file between the <script></script> tags specify the function as asynchronous

<script type="text/javascript">
async function testTransaction()
{
    let resp = await window.antenna.iotx.sendTransfer({
        to: "io1098h946aa5us8h4fql6zptv9kjqn7cu9x9uzm0",
        from: window.antenna.iotx.accounts[0].address,
        value: toRau("1", "Iotx"),
        gasLimit: "100000",
        gasPrice: toRau("1", "Qev")
    });
} 
</script>

this time you don't need to specify the compiler flags ASYNCIFY or ASYNCIFY_IMPORTS, and will work,

However, just because the JS is asynch, doesn't mean that your C/C++ code will behave the same. C will just pause and wait for the asynch to complete. If you want the underlying C/C++ code to be asynch, you will need to work with multi-threading

This is just what I found in some experiments I did on this topic

[RReverser]

I believe this is resolved since Asyncify.handleAsync was added (which was after the original issue date).