Stream: general

Topic: Consuming wasi components with the wasmtime c api

Jean (Sep 12 2025 at 19:37):

Hi!

Like the title says, I'm trying to run a wasi component in a host program by using the wasmtime c-API. I created a component following the example in the component book:

package docs:adder@0.1.0;

interface add {
    add: func(x: u32, y: u32) -> u32;
}

world adder {
    export add;
}

And I'm trying to access the add function with the following code but it fails.

// Finding the add interface
wasmtime_component_export_index_t *interface_export = wasmtime_component_instance_get_export_index(&instance, context, nullptr, "add", 3);
if (interface_export == nullptr) {
    LOGE("Failed to find interface export 'add'");
    return;
}

The documentation for the c-api only shows how to load and run a module. Does anyone know where I can find info on how to load a component's world, interface, functions, etc...?

Alex Crichton (Sep 12 2025 at 20:04):

you'll need to lookup the instance named docs:adder/add@0.1.0 and then lookup the add function from that instance

Jean (Sep 13 2025 at 10:27):

That didn't work neither. I tried to create a simple rust host that would run the same component based on this example but I get the same error "Cannot get docs:adder@0.1.0 interface" from this :

let interface_idx = instance
        .get_export_index(&mut store, None, "docs:adder/add@0.1.0")
        .expect("Cannot get `docs:adder/add@0.1.0` interface");

I tried with "adder" and "add", but that didn't work neither.
When looking at this example instead, I see the snippet uses bindgen. Are component makers supposed to bundle their wit files with the wasm file so host applications can read them?

Alex Crichton (Sep 15 2025 at 14:01):

Could you share a reproduction of this?

Are component makers supposed to bundle their wit files with the wasm file so host applications can read them?

A WIT can be inferred from a wasm, so no, that's not required. The host would typically be originally built with a WIT, however, but that would be independent from wasms loaded at runtime.

Jean (Sep 15 2025 at 17:38):

Right, that's actually what I ended up doing. My host library defines the wit and the wasi component implements it. Thanks!

Last updated: Dec 06 2025 at 05:03 UTC