Passing a JavaScript string to a Rust function compiled to WebAssembly

Question

I have this simple Rust function:

#[no_mangle]
pub fn compute(operator: &str, n1: i32, n2: i32) -> i32 {
    match operator {
        "SUM" => n1 + n2,
        "DIFF" => n1 - n2,
        "MULT" => n1 * n2,
        "DIV" => n1 / n2,
        _ => 0
    }
}

I am compiling this to WebAssembly successfully, but don't manage to pass the operator parameter from JS to Rust.

The JS line which calls the Rust function looks like this:

instance.exports.compute(operator, n1, n2);

operator is a JS String and n1, n2 are JS Numbers.

n1 and n2 are passed properly and can be read inside the compiled function so I guess the problem is how I pass the string around. I imagine it is passed as a pointer from JS to WebAssembly but can't find evidence or material about how this works.

I am not using Emscripten and would like to keep it standalone (compilation target wasm32-unknown-unknown), but I see they wrap their compiled functions in Module.cwrap, maybe that could help?

Answer 1

Easiest and most idiomatic solution

Most people should use wasm-bindgen, which makes this whole process much simpler!

Low-level manual implementation

To transfer string data between JavaScript and Rust, you need to decide

1. The encoding of the text: UTF-8 (Rust native) or UTF-16 (JS native).
2. Who will own the memory buffer: the JS (caller) or Rust (callee).
3. How to represent the strings data and length: NUL-terminated (C-style) or distinct length (Rust-style).
4. How to communicate the data and length, if they are separate.

Common setup

It's important to build C dylibs for WASM to help them be smaller in size.

Cargo.toml

[package]
name = "quick-maths"
version = "0.1.0"
authors = ["An Devloper <[email protected]>"]

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

.cargo/config

[target.wasm32-unknown-unknown]
rustflags = [
    "-C", "link-args=--import-memory",
]

package.json

{
  "name": "quick-maths",
  "version": "0.1.0",
  "main": "index.js",
  "author": "An Devloper <[email protected]>",
  "license": "MIT",
  "scripts": {
    "example": "node ./index.js"
  },
  "dependencies": {
    "fs-extra": "^8.0.1",
    "text-encoding": "^0.7.0"
  }
}

I'm using NodeJS 12.1.0.

Execution

$ rustup component add rust-std --target wasm32-unknown-unknown
$ cargo build --release --target wasm32-unknown-unknown

Solution 1

I decided:

1. To convert JS strings to UTF-8, which means that the TextEncoder JS API is the best fit.
2. The caller should own the memory buffer.
3. To have the length be a separate value.
4. Another struct and allocation should be made to hold the pointer and length.

lib/src.rs

// A struct with a known memory layout that we can pass string information in
#[repr(C)]
pub struct JsInteropString {
    data: *const u8,
    len: usize,
}

// Our FFI shim function    
#[no_mangle]
pub unsafe extern "C" fn compute(s: *const JsInteropString, n1: i32, n2: i32) -> i32 {
    // Check for NULL (see corresponding comment in JS)
    let s = match s.as_ref() {
        Some(s) => s,
        None => return -1,
    };

    // Convert the pointer and length to a `&[u8]`.
    let data = std::slice::from_raw_parts(s.data, s.len);

    // Convert the `&[u8]` to a `&str`    
    match std::str::from_utf8(data) {
        Ok(s) => real_code::compute(s, n1, n2),
        Err(_) => -2,
    }
}

// I advocate that you keep your interesting code in a different
// crate for easy development and testing. Have a separate crate
// with the FFI shims.
mod real_code {
    pub fn compute(operator: &str, n1: i32, n2: i32) -> i32 {
        match operator {
            "SUM"  => n1 + n2,
            "DIFF" => n1 - n2,
            "MULT" => n1 * n2,
            "DIV"  => n1 / n2,
            _ => 0,
        }
    }
}

index.js

const fs = require('fs-extra');
const { TextEncoder } = require('text-encoding');

// Allocate some memory.
const memory = new WebAssembly.Memory({ initial: 20, maximum: 100 });

// Connect these memory regions to the imported module
const importObject = {
  env: { memory }
};

// Create an object that handles converting our strings for us
const memoryManager = (memory) => {
  var base = 0;

  // NULL is conventionally at address 0, so we "use up" the first 4
  // bytes of address space to make our lives a bit simpler.
  base += 4;

  return {
    encodeString: (jsString) => {
      // Convert the JS String to UTF-8 data
      const encoder = new TextEncoder();
      const encodedString = encoder.encode(jsString);

      // Organize memory with space for the JsInteropString at the
      // beginning, followed by the UTF-8 string bytes.
      const asU32 = new Uint32Array(memory.buffer, base, 2);
      const asBytes = new Uint8Array(memory.buffer, asU32.byteOffset + asU32.byteLength, encodedString.length);

      // Copy the UTF-8 into the WASM memory.
      asBytes.set(encodedString);

      // Assign the data pointer and length values.
      asU32[0] = asBytes.byteOffset;
      asU32[1] = asBytes.length;

      // Update our memory allocator base address for the next call
      const originalBase = base;
      base += asBytes.byteOffset + asBytes.byteLength;

      return originalBase;
    }
  };
};

const myMemory = memoryManager(memory);

fs.readFile('./target/wasm32-unknown-unknown/release/quick_maths.wasm')
  .then(bytes => WebAssembly.instantiate(bytes, importObject))
  .then(({ instance }) => {
    const argString = "MULT";
    const argN1 = 42;
    const argN2 = 100;

    const s = myMemory.encodeString(argString);
    const result = instance.exports.compute(s, argN1, argN2);

    console.log(result);
  });

Execution

$ yarn run example
4200

Solution 2

I decided:

1. To convert JS strings to UTF-8, which means that the TextEncoder JS API is the best fit.
2. The module should own the memory buffer.
3. To have the length be a separate value.
4. To use a Box<String> as the underlying data structure. This allows the allocation to be further used by Rust code.

src/lib.rs

// Very important to use `transparent` to prevent ABI issues
#[repr(transparent)]
pub struct JsInteropString(*mut String);

impl JsInteropString {
    // Unsafe because we create a string and say it's full of valid
    // UTF-8 data, but it isn't!
    unsafe fn with_capacity(cap: usize) -> Self {
        let mut d = Vec::with_capacity(cap);
        d.set_len(cap);
        let s = Box::new(String::from_utf8_unchecked(d));
        JsInteropString(Box::into_raw(s))
    }

    unsafe fn as_string(&self) -> &String {
        &*self.0
    }

    unsafe fn as_mut_string(&mut self) -> &mut String {
        &mut *self.0
    }

    unsafe fn into_boxed_string(self) -> Box<String> {
        Box::from_raw(self.0)
    }

    unsafe fn as_mut_ptr(&mut self) -> *mut u8 {
        self.as_mut_string().as_mut_vec().as_mut_ptr()
    }
}

#[no_mangle]
pub unsafe extern "C" fn stringPrepare(cap: usize) -> JsInteropString {
    JsInteropString::with_capacity(cap)
}

#[no_mangle]
pub unsafe extern "C" fn stringData(mut s: JsInteropString) -> *mut u8 {
    s.as_mut_ptr()
}

#[no_mangle]
pub unsafe extern "C" fn stringLen(s: JsInteropString) -> usize {
    s.as_string().len()
}

#[no_mangle]
pub unsafe extern "C" fn compute(s: JsInteropString, n1: i32, n2: i32) -> i32 {
    let s = s.into_boxed_string();
    real_code::compute(&s, n1, n2)
}

mod real_code {
    pub fn compute(operator: &str, n1: i32, n2: i32) -> i32 {
        match operator {
            "SUM"  => n1 + n2,
            "DIFF" => n1 - n2,
            "MULT" => n1 * n2,
            "DIV"  => n1 / n2,
            _ => 0,
        }
    }
}

index.js

const fs = require('fs-extra');
const { TextEncoder } = require('text-encoding');

class QuickMaths {
  constructor(instance) {
    this.instance = instance;
  }

  difference(n1, n2) {
    const { compute } = this.instance.exports;
    const op = this.copyJsStringToRust("DIFF");
    return compute(op, n1, n2);
  }

  copyJsStringToRust(jsString) {
    const { memory, stringPrepare, stringData, stringLen } = this.instance.exports;

    const encoder = new TextEncoder();
    const encodedString = encoder.encode(jsString);

    // Ask Rust code to allocate a string inside of the module's memory
    const rustString = stringPrepare(encodedString.length);

    // Get a JS view of the string data
    const rustStringData = stringData(rustString);
    const asBytes = new Uint8Array(memory.buffer, rustStringData, encodedString.length);

    // Copy the UTF-8 into the WASM memory.
    asBytes.set(encodedString);

    return rustString;
  }
}

async function main() {
  const bytes = await fs.readFile('./target/wasm32-unknown-unknown/release/quick_maths.wasm');
  const { instance } = await WebAssembly.instantiate(bytes);
  const maffs = new QuickMaths(instance);

  console.log(maffs.difference(100, 201));
}

main();

Execution

$ yarn run example
-101

---

Note that this process can be used for other types. You "just" have to decide how to represent data as a set of bytes that both sides agree on then send it across.

See also:

• Using the WebAssembly JavaScript API
• TextEncoder API
• Uint8Array / Uint32Array / TypedArray
• WebAssembly.Memory
• Hello, Rust! — Import memory buffer
• How to return a string (or similar) from Rust in WebAssembly?