将JavaScript字符串传递给编译为WebAssembly的Rust函数

Question

我有这个简单的Rust函数：

#[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
    }
}

我正在成功地将此编译为WebAssembly，但是没有设法将operator参数从JS传递给Rust。

调用Rust函数的JS行如下所示：

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

operator是JS String和n1，n2是JS Number。

n1和n2正确传递，可以在编译函数内部读取，所以我猜问题是我如何传递字符串。我想它是作为从JS到WebAssembly的指针传递的，但无法找到有关其工作原理的证据或材料。

我没有使用Emscripten，并希望将其保持独立（编译目标wasm32-unknown-unknown），但我看到他们将编译后的函数包装在Module.cwrap中，也许这可能会有所帮助？

Answer 1

最简单，最惯用的解决方案

大多数人应该使用wasm-bindgen，这使整个过程更多更简单！

低级手动实施

要在JavaScript和Rust之间传输字符串数据，您需要决定

1. 文本的编码：UTF-8（Rust native）或UTF-16（JS native）。
2. 谁将拥有内存缓冲区：JS（调用者）或Rust（被调用者）。
3. 如何表示字符串数据和长度：NUL终止（C风格）或不同长度（Rust风格）。
4. 如果数据和长度分开，如何传达数据和长度。

常用设置

为WASM构建C dylib以帮助它们缩小尺寸非常重要。

<强> Cargo.toml

[package]
name = "quick-maths"
version = "0.1.0"
authors = ["An Devloper <an.devloper@example.com>"]

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

<强> .cargo /配置

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

<强>的package.json

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

我使用的是NodeJS 12.1.0。

<强>执行

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

解决方案1 ​​

我决定：

1. 要将JS字符串转换为UTF-8，这意味着TextEncoder JS API最适合。
2. 调用者应该拥有内存缓冲区。
3. 将长度设为单独的值。
4. 应该使用另一个结构和分配来保存指针和长度。

<强> 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,
        }
    }
}

<强> 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);
  });

<强>执行

$ yarn run example
4200

解决方案2

我决定：

1. 要将JS字符串转换为UTF-8，这意味着TextEncoder JS API最适合。
2. 模块应该拥有内存缓冲区。
3. 将长度设为单独的值。
4. 使用Box<String>作为基础数据结构。这允许Rust代码进一步使用分配。

<强>的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,
        }
    }
}

<强> 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();

<强>执行

$ yarn run example
-101

请注意，此过程可用于其他类型。你＆＃34;只是＆＃34;必须决定如何将数据表示为双方同意然后发送它的一组字节。

另见：

• 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?

Answer 2

WebAssembly程序拥有自己的内存空间。此空间通常由WebAssembly程序本身管理，借助于分配器库，例如wee_alloc。

JavaScript可以查看和修改该内存空间，但无法知道分配器库结构的组织方式。因此，如果我们只是从JavaScript写入WASM内存，那么我们可能会覆盖重要的东西并搞砸了。因此，WebAssembly程序本身必须首先分配内存区域，将其传递给JavaScript，然后JavaScript可以用数据填充该区域。

在下面的例子中我们这样做：在WASM内存空间中分配一个缓冲区，在那里复制UTF-8字节，将缓冲区位置传递给Rust函数，然后释放缓冲区。

锈：

#![feature(allocator_api)]

use std::heap::{Alloc, Heap, Layout};

#[no_mangle]
pub fn alloc(len: i32) -> *mut u8 {
    let mut heap = Heap;
    let layout = Layout::from_size_align(len as usize, 1).expect("!from_size_align");
    unsafe { heap.alloc(layout).expect("!alloc") }
}

#[no_mangle]
pub fn dealloc(ptr: *mut u8, len: i32) {
    let mut heap = Heap;
    let layout = Layout::from_size_align(len as usize, 1).expect("!from_size_align");
    unsafe { heap.dealloc(ptr, layout) }
}

#[no_mangle]
pub fn is_foobar(buf: *const u8, len: i32) -> i32 {
    let js = unsafe { std::slice::from_raw_parts(buf, len as usize) };
    let js = unsafe { std::str::from_utf8_unchecked(js) };
    if js == "foobar" {
        1
    } else {
        0
    }
}

打字稿：

// cf. https://github.com/Microsoft/TypeScript/issues/18099
declare class TextEncoder {constructor (label?: string); encode (input?: string): Uint8Array}
declare class TextDecoder {constructor (utfLabel?: string); decode (input?: ArrayBufferView): string}
// https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/webassembly-js-api/index.d.ts
declare namespace WebAssembly {
  class Instance {readonly exports: any}
  interface ResultObject {instance: Instance}
  function instantiateStreaming (file: Promise<Response>, options?: any): Promise<ResultObject>}

var main: {
  memory: {readonly buffer: ArrayBuffer}
  alloc (size: number): number
  dealloc (ptr: number, len: number): void
  is_foobar (buf: number, len: number): number}

function withRustString (str: string, cb: (ptr: number, len: number) => any): any {
  // Convert the JavaScript string to an array of UTF-8 bytes.
  const utf8 = (new TextEncoder()).encode (str)
  // Reserve a WASM memory buffer for the UTF-8 array.
  const rsBuf = main.alloc (utf8.length)
  // Copy the UTF-8 array into the WASM memory.
  new Uint8Array (main.memory.buffer, rsBuf, utf8.length) .set (utf8)
  // Pass the WASM memory location and size into the callback.
  const ret = cb (rsBuf, utf8.length)
  // Free the WASM memory buffer.
  main.dealloc (rsBuf, utf8.length)
  return ret}

WebAssembly.instantiateStreaming (fetch ('main.wasm')) .then (results => {
  main = results.instance.exports
  // Prints "foobar is_foobar? 1".
  console.log ('foobar is_foobar? ' +
    withRustString ("foobar", function (buf, len) {return main.is_foobar (buf, len)}))
  // Prints "woot is_foobar? 0".
  console.log ('woot is_foobar? ' +
    withRustString ("woot", function (buf, len) {return main.is_foobar (buf, len)}))})

P.S。 The Module._malloc in Emscripten可能在语义上等同于我们上面实现的alloc函数。在“wasm32-unknown-emscripten”目标you can use the Module._malloc with Rust下。

Answer 3

正如Shepmaster所指出的，只有数字可以传递给WebAssembly，因此我们需要将字符串转换为Uint16Array。

为此，我们可以使用str2ab找到的function str2ab(str) { var buf = new ArrayBuffer(str.length*2); // 2 bytes for each char var bufView = new Uint16Array(buf); for (var i=0, strLen=str.length; i < strLen; i++) { bufView[i] = str.charCodeAt(i); } return buf; } 函数：

instance.exports.compute(
    str2ab(operator), 
    n1, n2
);

这现在有效：

pd.rolling()

因为我们将引用传递给无符号整数数组。