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Showing content from https://docs.wasmtime.dev/api/wasmtime/struct.Caller.html below:

Caller in wasmtime - Rust

pub struct Caller<'a, T: 'static> {  }

A structure representing the caller’s context when creating a function via Func::wrap.

This structure can be taken as the first parameter of a closure passed to Func::wrap or other constructors, and serves two purposes:

• First consumers can use Caller<'_, T> to get access to StoreContextMut<'_, T> and/or get access to T itself. This means that the Caller type can serve as a proxy to the original Store itself and is used to satisfy AsContext and AsContextMut bounds.

• Second a Caller can be used as the name implies, learning about the caller’s context, namely it’s exported memory and exported functions. This allows functions which take pointers as arguments to easily read the memory the pointers point into, or if a function is expected to call malloc in the wasm module to reserve space for the output you can do that.

Host functions which want access to Store-level state are recommended to use this type.

Looks up an export from the caller’s module by the name given.

This is a low-level function that’s typically used to implement passing of pointers or indices between core Wasm instances, where the callee needs to consult the caller’s exports to perform memory management and resolve the references.

For comparison, in components, the component model handles translating arguments from one component instance to another and managing memory, so that callees don’t need to be aware of their callers, which promotes virtualizability of APIs.

If an export with the name provided was found, then it is returned as an Extern. There are a number of situations, however, where the export may not be available:

• The caller instance may not have an export named name
• There may not be a caller available, for example if Func was called directly from host code.

It’s recommended to take care when calling this API and gracefully handling a None return value.

Looks up an exported Extern value by a ModuleExport value.

This is similar to Self::get_export but uses a ModuleExport value to avoid string lookups where possible. ModuleExports can be obtained by calling [Module::get_export_index] on the [Module] that an instance was instantiated with.

This method will search the module for an export with a matching entity index and return the value, if found.

Returns None if there was no export with a matching entity index.

Panics if store does not own this instance.

use std::str;


let module = Module::new(
    store.engine(),
    r#"
        (module
            (import "" "" (func $log_str (param i32 i32)))
            (func (export "foo")
                i32.const 4   ;; ptr
                i32.const 13  ;; len
                call $log_str)
            (memory (export "memory") 1)
            (data (i32.const 4) "Hello, world!"))
    "#,
)?;

let Some(module_export) = module.get_export_index("memory") else {
   anyhow::bail!("failed to find `memory` export in module");
};

let log_str = Func::wrap(&mut store, move |mut caller: Caller<'_, ()>, ptr: i32, len: i32| {
    let mem = match caller.get_module_export(&module_export) {
        Some(Extern::Memory(mem)) => mem,
        _ => anyhow::bail!("failed to find host memory"),
    };
    let data = mem.data(&caller)
        .get(ptr as u32 as usize..)
        .and_then(|arr| arr.get(..len as u32 as usize));
    let string = match data {
        Some(data) => match str::from_utf8(data) {
            Ok(s) => s,
            Err(_) => anyhow::bail!("invalid utf-8"),
        },
        None => anyhow::bail!("pointer/length out of bounds"),
    };
    assert_eq!(string, "Hello, world!");
    println!("{}", string);
    Ok(())
});
let instance = Instance::new(&mut store, &module, &[log_str.into()])?;
let foo = instance.get_typed_func::<(), ()>(&mut store, "foo")?;
foo.call(&mut store, ())?;

Access the underlying data owned by this Store.

Same as Store::data

Returns the underlying Engine this store is connected to.

Perform garbage collection.

Same as Store::gc.

Returns the remaining fuel in the store.

For more information see Store::get_fuel

Set the amount of fuel in this store to be consumed when executing wasm code.

For more information see Store::set_fuel

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