Stream: wasmtime

Topic: call function from component file.

view this post on Zulip Ludea (Mar 04 2026 at 16:08):

Hi, I get a component file with this wit file

package mycomponent:remote@0.1.0;

interface myinterface {
  myfunction: async func() -> result<string, string>;
world myworld {
    export myinterface;
}}

I use wasip3 target to build it. Everything is fine

I call my component like

let mut linker = Linker::new(&self.engine);
    wasmtime_wasi::p2::add_to_linker_async(&mut linker)?;

    let wasi = WasiCtx::builder().build();
    let state = ComponentRunStates {
      wasi_ctx: wasi,
      resource_table: ResourceTable::new(),
    };
    let mut store = Store::new(&self.engine, state);
    let plugin_full_path = plugin_dir.display().to_string() + "/" + &plugin + ".wasm";
    let component = Component::from_file(&self.engine, plugin_full_path)?;
    let instance = linker.instantiate_async(&mut store, &component).await?;
    let mut result = [Val::String(String::new())];
     instance.get_func(&mut store, "myfunction" ) {
    ...
     }

But get function is not found.
I miss something?

view this post on Zulip Ludea (Mar 04 2026 at 16:12):

Maybe async fn are not yet supported?

view this post on Zulip Joel Dice (Mar 04 2026 at 16:16):

You need to look up the myinterface export first, then get the myfunction export from that. Let me see if I can find an example.

view this post on Zulip Joel Dice (Mar 04 2026 at 16:20):

Here's a example (although it doesn't involve an async-typed function): https://github.com/bytecodealliance/wasmtime/blob/8adf03d4557acaa8384ec084946614d3f87c39ff/tests/all/component_model/func.rs#L3645-L3650

view this post on Zulip Ludea (Mar 04 2026 at 16:29):

Thanks will try when go back to office

view this post on Zulip Ludea (Mar 05 2026 at 14:55):

get_exports_name name arg is instance name?

view this post on Zulip Ludea (Mar 05 2026 at 14:56):

 let instance_index = instance.get_export_index(&mut store, None, "i").unwrap(); //here

view this post on Zulip Joel Dice (Mar 05 2026 at 16:31):

The last parameter to get_export_index is the exported instance name, yes. In the example I linked to, see the WAT component being used, which exports an instance named i which exports a function named new and a type named r.

view this post on Zulip Ludea (Mar 05 2026 at 17:46):

How to get instance name?
I don't use wat component, so I instanciate through wasmtime and linker

view this post on Zulip Ludea (Mar 05 2026 at 18:30):

If I use myinterface as instance naming, instance is not found

view this post on Zulip Joel Dice (Mar 05 2026 at 19:07):

In the case of the WIT you posted above, I'd expect the instance name to be mycomponent:remote/myinterface@0.1.0. You can check for sure using e.g. wasm-tools print $your_wasm_file and looking at the exports near the bottom of the output.

view this post on Zulip Ludea (Mar 06 2026 at 12:19):

Awesome it works as expected.
Thanks for the help !
I get another issue :

thread 'tokio-runtime-worker' (14529) panicked at src/plugins.rs:74:62:


called `Result::unwrap()` on an `Err` value: error while executing at wasm backtrace:
    0:  0x50229 - cascette.wasm!abort
    1:  0x488d4 - cascette.wasm!std[47f6073b99490d06]::sys::pal::wasi::helpers::abort_internal
    2:  0x4934c - cascette.wasm!std[47f6073b99490d06]::process::abort
    3:  0x4bc1a - cascette.wasm!__rustc[1e24dcc63095d017]::__rust_abort
    4:  0x4b0ed - cascette.wasm!__rustc[1e24dcc63095d017]::__rust_start_panic
    5:  0x4bbbf - cascette.wasm!__rustc[1e24dcc63095d017]::rust_panic
    6:  0x4b631 - cascette.wasm!std[47f6073b99490d06]::panicking::panic_with_hook
    7:  0x4b323 - cascette.wasm!std[47f6073b99490d06]::panicking::panic_handler::{closure#0}
    8:  0x4b2b6 - cascette.wasm!std[47f6073b99490d06]::sys::backtrace::__rust_end_short_backtrace::<std[47f6073b99490d06]::panicking::panic_handler::{closure#0}, !>
    9:  0x4bc53 - cascette.wasm!__rustc[1e24dcc63095d017]::rust_begin_unwind
   10:  0x592e8 - cascette.wasm!core[b849f7044ded83d4]::panicking::panic_fmt
   11:  0x242fa - cascette.wasm!js_sys[a79c64c7964765fd]::global::get_global_object::GLOBAL_THIS::init
   12:  0x23e71 - cascette.wasm!js_sys[a79c64c7964765fd]::global::get_global_object
   13:  0x23c53 - cascette.wasm!js_sys[a79c64c7964765fd]::global
   14:  0x2393c - cascette.wasm!web_sys[e52c455de79da0e4]::window
   15:   0xf6be - cascette.wasm!<cascette_protocol[2eb4d3558c719011]::cache::wasm::ProtocolCache>::get_storage
   16:  0x106a2 - cascette.wasm!<cascette_protocol[2eb4d3558c719011]::cache::wasm::ProtocolCache>::new
   17:   0xe2a1 - cascette.wasm!<cascette_protocol[2eb4d3558c719011]::client::RibbitTactClient>::new
   18:   0x262d - cascette.wasm!cascette[5108c410898490c6]::exports::sparus::cascette::sparus::_export_regions_cabi::<cascette[5108c410898490c6]::CascetteComponent>::{closure#0}
   19:   0xd5d3 - cascette.wasm!<wit_bindgen[7aa71dee9b763b42]::rt::async_support::FutureState>::callback

Caused by:
    wasm trap: wasm `unreachable` instruction executed

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I guess I use wrong function to call an async function? Or it's not yet implemented?

Last updated: Mar 23 2026 at 18:16 UTC