How to resolve function pointers in Rust & WebAssembly

Question

I want to provide a WebAssembly module with an external JavaScript function that accepts a Rust function pointer.

Once this JS module is initialized, it will call the run() function from the .wasm module, and that will in turn call peekaboo:

window.Module = {};

const imports = {
  env: {
    memoryBase: 0,
    tableBase: 0,
    memory: new WebAssembly.Memory({ initial: 256 }),
    table: new WebAssembly.Table({ initial: 4, element: 'anyfunc' })
  }
};

imports.env.peekaboo = function(f) {
  const fn = imports.env.table.get(f);
  return fn(2);
};

fetch('game.wasm')
  .then(response => response.arrayBuffer())
  .then(bytes => WebAssembly.compile(bytes))
  .then(mod => WebAssembly.instantiate(mod, imports))
  .then(mod => {
    mod.exports.run();

    Module.memory = imports.env.memory;
    Module.dealloc_str = mod.exports.dealloc_str;
  });

The examples I've seen indicate that if I import the memory in this way, I should be able to use the table to resolve my function pointer. Here is the Rust code:

#![feature(wasm_import_memory)]
#![wasm_import_memory]

extern "C" {
    fn peekaboo(f: fn(u32) -> u32);
}

fn main() {}

#[no_mangle]
pub fn run() {
    let plus_one = |x: u32| -> u32 { x + 1 };

    unsafe {
        peekaboo(plus_one);
    }
}

Everything compiles fine but when I execute the peekaboo function, the fn variable is null, indicating that the table was unable to find the function pointer. Therefore executing fn(2) blows up with:

Uncaught (in promise) TypeError: fn is not a function

I more or less followed this example but since I'm working in Rust, the translation is not 1-to-1. I suspect that I've overlooked something that's not obvious to me because I'm new to both Rust and WebAssembly. Can anyone spot my mistake?

Stefan · Accepted Answer · 2018-04-28 23:12:47Z

4

It seems so far the WebAssembly backend in Rust doesn't provide a way import or export the (function) table; the index f is just fine, but imports.env.table is not the same table used by the wasm instance (i.e. empty).

Also you should use extern fn in FFI.

If you want to take a look under the hood the Playground provides some nice WebAssembly optimizations, check out this example:

Playground

#![crate_type = "cdylib"]
#![feature(link_args)]
#![allow(unused_attributes)] // link_args actually is used
#![link_args = "--import-memory"]

extern "C" {
    fn peekaboo(f: extern "C" fn(u32) -> u32);
}

#[no_mangle]
pub fn run() {
    extern "C" fn plus_one(x: u32) -> u32 {
        x + 1
    }

    unsafe {
        peekaboo(plus_one);
    }
}

The result should look like this:

(module
  (type $t0 (func))
  (type $t1 (func (param i32) (result i32)))
  (type $t2 (func (param i32)))
  (import "env" "peekaboo" (func $peekaboo (type $t2)))
  (import "env" "memory" (memory $env.memory 17))
  (func $run (export "run") (type $t0)
    (call $peekaboo
      (i32.const 1)))
  (func $playground::run::plus_one::h85275af105f0cc85 (type $t1) (param $p0 i32) (result i32)
    (i32.add
      (get_local $p0)
      (i32.const 1)))
  (table $T0 2 2 anyfunc)
  (elem (i32.const 1) $playground::run::plus_one::h85275af105f0cc85))

If you want to reproduce this locally add this in your Cargo.toml:

[lib]
crate-type = ["cdylib"]

[profile.release]
lto = true

And build with cargo +nightly build --release --target wasm32-unknown-unknown (assuming a rustup setup with a nightly toolchain and wasm32-unknown-unknown target enabled for the nightly toolchain).

edited Apr 28, 2018 at 23:12

answered Dec 12, 2017 at 18:50

Stefan

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7 Comments

Shepmaster Over a year ago

using no_std — The correct way is to generate a dynamic library. I'm not sure why that information has yet to be picked up by the community.

Samo Over a year ago

Ah, these both look promising, thanks! Can I assume that some of the code in this example is irrelevent? e.g. eh_personality()? :)

Sven Marnach Over a year ago

@Samo If you use no_std, you need to provide some symbols otherwise provided by the standard library for the code to link, including the ones you see in the code here. I don't know what items exactly are needed; it depends on your code, but you can simply let the compiler tell you.

Shepmaster Over a year ago

@Stefan You also need to use wasm-gc, which the playground uses.

Stefan Over a year ago

@starwed should be fixed now in the answer... although such things are expected to happen with unstable features :) it also seems wasm-gc isn't needed anymore (at least not for this small example), cdylib and lto are doing a good job.

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