wasmtime | A fast and secure runtime for WebAssembly | Binary Executable Format library

Let me explain a little bit about what I'm doing. I have an application that I'm embedding a wasm engine (specifically wasmtime) and writing the wasm in Rust. The target is not a browser so I'm not using wasm-bindgen. I'm trying to import two functions from the host environment. I've gotten it to work but with a bit of a hack and I'd like to remove the hack. Here's the problem I'm having. I've been able to figure out how to get Rust to generate the import statements in the wasm output. The problem is that if those functions aren't used the import statements in the wasm aren't included. Wasmtime seem to require that the number of imports passed to the wasm instance match the number of import statements. Makes sense that if the wasm module is expecting imports it's a problem if you don't provide them. The problem is, it's also an error if you provide imports and they're not used.

So I see two ways to fix that, either I need to figure out how to get Rust to include the import statements even if they're not used (right now the hack is to call the functions but ignore the results and it's basically a no-op) or to find a way to introspect the wasm an figure out what kind of imports it's expecting and only pass what it's asking for.

My question is is that the correct approach and if so how do you do that?

Given I want to sum the first n terms of a series 1,2,3,.. with the following function in Rust

I've been handwriting some WebAssembly modules to learn how they work. Right now I'm trying to understand how imports and exports work, and how to perform IO.

The module is as follows:

part of the talk of Lin Clark in https://hacks.mozilla.org/2019/03/standardizing-wasi-a-webassembly-system-interface/:

It also gives us sandboxing because the host can choose which wasi-core functions to pass in — so, which system calls to allow — on a program-by-program basis. This preserves security.

she says host can choose which system calls pass to each wasm module. for example read() system call passes to module A and write() system call to module B.

is it implemented in wasmtime or lucet or other runtimes? or is it just a dream without implementation in real world?

I have been trying to pass a string to a Rust function (compiled to Wasm), however for what I understood, right now there is no way to pass strings directy, because the "str" is not a type in the "FFI world" (at least that's what the rust compiler is saying): = help: consider using `*const u8` and a length instead

So what I did was changing the function to this form (instead of using a simple &str type):

I would like to create a callback function that has access to the argument instanceContext, but I am not sure how to fix the code. The failing code is instanceContext.instances.len().

I have tried using lifetimes, move, Rc, Arc, Box, RefCell, but nothing seems to work.

The goal is to have multiple Instances which will have callbacks that will make it possible for one Instance to call/modify another Instance, through the InstanceContext.

During my experimentation I've been getting the following errors:

• may outlive borrowed value
• borrowed value does not live long enough
• closure is FnOnce because it moves the variable
• this closure implements FnOnce, not Fn

I'm trying to call a WASI function (fd_write) directly in a C program using wasi-sdk. This is the the library (lib.c):