Stream: general

Topic: How to write wit ?

celine santosh (Mar 26 2025 at 13:32):

Hi

I am trying to create a component using cargo component. In my lib.rs I have a function which I export with signature
fn compute_result(&mut self, out: &mut [u8])

I am confused how to write this in wit. Self comes in Resource I think . I don't have resources.
Also how to write the second parameter which is array.

Colin D Murphy (Mar 26 2025 at 18:12):

Here is an example I'm working on:
https://github.com/cdmurph32/c2pa-component/blob/v24/wit/cai.wit

It has a couple of resources. The self is implied.

celine santosh (Mar 26 2025 at 18:41):

Thanks.
What about the second parameter which is an array.
I am not able to write the corresponding wit for it. I can only see list<u8> which is vec in rust.

Colin D Murphy (Mar 27 2025 at 13:47):

That might be best implemented as an output stream depending on how big you intend it to be. Is there a reason it needs to be a pointer to an array?

Colin D Murphy (Mar 27 2025 at 13:51):

In your implementation on either side of the wit interface you can have an array.

celine santosh (Mar 27 2025 at 14:53):

This is my implementation in lib.rs

fn result(&mut self, out: &mut [u8]) {
        let hash = self.hasher.finalize_reset();
        copy_slices!(hash.as_slice(), out)

    }

It's a function in snow framework. I want to override this function, so I am trying to implement the same thing with same function signature.

celine santosh (Mar 27 2025 at 14:54):

So I was stuck at a point where I am writing the wit for it, because of second parameter. It's slice , sorry I thought it was an array.

Joel Dice (Mar 27 2025 at 15:39):

The component model is designed around "shared-nothing" composition, meaning components do not have access to each others' memory. Consequently, WIT interfaces do not support passing pointers, and a slice is basically just a fat pointer. The callee couldn't do anything with such a pointer, since it's relative to a memory it doesn't have access to.

If you need to return a list of bytes to the caller, you'll want to model that as %result: func() -> list<u8> (which translates to fn result(&self) -> Vec<u8>; in Rust). The host will take care of copying the contents of that list<u8> from the callee's memory to the caller's memory.

celine santosh (Mar 30 2025 at 15:20):

Thanks
So even if I compose both wasm components together with wac plug , both components will have isolated memory right?

Joel Dice (Mar 31 2025 at 14:41):

Yes, correct. You also have the option of "shared-everything" module linking using wasm-tools component link if you prefer. This document describes the various way static libraries, modules, and components can be composed.

Last updated: Apr 07 2025 at 20:03 UTC