How to access the instance's memory in a shared host function?

[eigenein]

👋 Hi team,

I'm instantiating a long-lived Engine and Linker, and then I'm adding some host functions to the linker (Rust API). Some of these functions should receive a string as a parameter, so an offset and a length in a guest's memory, for example:

let mut linker = wasmtime::Linker::new(&engine);
linker.func_wrap("logging", "info",
    |mut caller: Caller<'_, ()>, (offset, size): (u32, u32)| {
        // How do I access instance's memory here and read from it?
    },
)?;

I could use a closure as suggested in #2491 (comment), but I'm going to support a plugin system, and a shared host function may be called from any plugin. Also, Store is short-lived, and I wouldn't like to re-instantiate a Linker together with the closures for every host-to-guest call

Should I put a reference to Instance or to the instance's Memory into the instance's Store? That smells a little, since Caller already contains Store and all the memories are already owned by Store – so that would effectively be making a loop of references. Am I missing something?

[alexcrichton]

There's an example for this on the documentation of Func::wrap where the gist of it is:

    let mem = match caller.get_export("memory") {
        Some(Extern::Memory(mem)) => mem,
        _ => anyhow::bail!("failed to find host memory"),
    };

One option is to store a Memory in the T of Store<T> (i.e. replace the () you're using currently), but the easiest option is probably to call caller.get_export(...)

[eigenein]

@alexcrichton Thank you, just exactly what I needed and completely overlooked!