如何在 WebAssembly 中 return 来自 Rust 的字符串（或类似字符串）？

Question

我用这个 Rust 代码创建了一个小的 Wasm 文件：

#[no_mangle]
pub fn hello() -> &'static str {
    "hello from rust"
}

它构建并且可以从 JS 调用 hello 函数：

<!DOCTYPE html>
<html>
<body>
  <script>
    fetch('main.wasm')
    .then(response => response.arrayBuffer())
    .then(bytes => WebAssembly.instantiate(bytes, {}))
    .then(results => {
      alert(results.instance.exports.hello());
    });
  </script>
</body>
</html>

我的问题是 alert 显示 "undefined"。如果我 return a i32，它会工作并显示 i32。我也试过 return a String 但它不起作用（它仍然显示 "undefined"）。

有没有办法在 WebAssembly 中 return 来自 Rust 的字符串？我应该使用什么类型？

Answer 1

您不能直接 return Rust String 或 &str。而是分配 return 一个包含数据的原始字节指针，然后必须在 JavaScript 端将其编码为 JS 字符串。

你可以看看 SHA1 例子 here。

感兴趣的函数在

demos/bundle.js - copyCStr
demos/sha1/sha1-digest.rs - digest

更多示例：https://www.hellorust.com/demos/sha1/index.html

Answer 2

WebAssembly 仅支持少数 numeric types，这是可以通过导出函数 return 编辑的所有内容。

当您编译为 WebAssembly 时，您的字符串将保存在模块的线性内存中。为了从主机 JavaScript 中读取此字符串，您需要 return 对其在内存中的位置的引用，以及字符串的长度，即两个整数。这允许您从内存中读取字符串。

无论您将哪种语言编译成 WebAssembly，都可以使用相同的技术。提供了问题的详细背景。

对于 Rust，您需要使用外部函数接口 (FFI)，使用 CString 类型，如下所示：

use std::ffi::CString;
use std::os::raw::c_char;

static HELLO: &'static str = "hello from rust";

#[no_mangle]
pub fn get_hello() -> *mut c_char {
    let s = CString::new(HELLO).unwrap();
    s.into_raw()
}

#[no_mangle]
pub fn get_hello_len() -> usize {
    HELLO.len()
}

上面的代码导出了两个函数，get_hello return 是对字符串的引用，get_hello_len return 是它的长度。

将上述代码编译成wasm模块后，可以按如下方式访问字符串：

const res = await fetch('chip8.wasm');
const buffer = await res.arrayBuffer();
const module = await WebAssembly.compile(buffer);
const instance = await WebAssembly.instantiate(module);

// obtain the module memory
const linearMemory = instance.exports.memory;

// create a buffer starting at the reference to the exported string
const offset = instance.exports.get_hello();
const stringBuffer = new Uint8Array(linearMemory.buffer, offset,
  instance.exports.get_hello_len());

// create a string from this buffer
let str = '';
for (let i=0; i<stringBuffer.length; i++) {
  str += String.fromCharCode(stringBuffer[i]);
}

console.log(str);

C 等价物 can be seen in action in a WasmFiddle.

Answer 3

我看到的大多数例子都将字符串复制了两次。首先在 WASM 端，进入 CString 或将 Vec 缩小到其容量，然后在 JS 端解码 UTF-8.

鉴于我们经常为了速度而使用 WASM，我试图实现一个可以重用 Rust 向量的版本。

use std::collections::HashMap;

/// Byte vectors shared with JavaScript.
///
/// A map from payload's memory location to `Vec<u8>`.
///
/// In order to deallocate memory in Rust we need not just the memory location but also it's size.
/// In case of strings and vectors the freed size is capacity.
/// Keeping the vector around allows us not to change it's capacity.
///
/// Not thread-safe (assuming that we're running WASM from the single JavaScript thread).
static mut SHARED_VECS: Option<HashMap<u32, Vec<u8>>> = None;

extern "C" {
    fn console_log(rs: *const u8);
    fn console_log_8859_1(rs: *const u8);
}

#[no_mangle]
pub fn init() {
    unsafe { SHARED_VECS = Some(HashMap::new()) }
}

#[no_mangle]
pub fn vec_len(payload: *const u8) -> u32 {
    unsafe {
        SHARED_VECS
            .as_ref()
            .unwrap()
            .get(&(payload as u32))
            .unwrap()
            .len() as u32
    }
}

pub fn vec2js<V: Into<Vec<u8>>>(v: V) -> *const u8 {
    let v = v.into();
    let payload = v.as_ptr();
    unsafe {
        SHARED_VECS.as_mut().unwrap().insert(payload as u32, v);
    }
    payload
}

#[no_mangle]
pub extern "C" fn free_vec(payload: *const u8) {
    unsafe {
        SHARED_VECS.as_mut().unwrap().remove(&(payload as u32));
    }
}

#[no_mangle]
pub fn start() {
    unsafe {
        console_log(vec2js(format!("Hello again!")));
        console_log_8859_1(vec2js(b"ASCII string." as &[u8]));
    }
}

和 JavaScript 部分：

(function (iif) {

  function rs2js (mod, rs, utfLabel = 'utf-8') {
    const view = new Uint8Array (mod.memory.buffer, rs, mod.vec_len (rs))
    const utf8dec = new TextDecoder (utfLabel)
    const utf8 = utf8dec.decode (view)
    mod.free_vec (rs)
    return utf8}

  function loadWasm (cache) {
    // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/instantiateStreaming
    WebAssembly.instantiateStreaming (fetch ('main.wasm', {cache: cache ? "default" : "no-cache"}), {env: {
      console_log: function (rs) {if (window.console) console.log ('main]', rs2js (iif.main, rs))},
      console_log_8859_1: function (rs) {if (window.console) console.log ('main]', rs2js (iif.main, rs, 'iso-8859-1'))}
    }}) .then (results => {
      const exports = results.instance.exports
      exports.init()
      iif.main = exports
      iif.main.start()})}

  // Hot code reloading.
  if (window.location.hostname == '127.0.0.1' && window.location.port == '43080') {
    window.setInterval (
      function() {
        // Check if the WASM was updated.
        fetch ('main.wasm.lm', {cache: "no-cache"}) .then (r => r.text()) .then (lm => {
          lm = lm.trim()
          if (/^\d+$/.test (lm) && lm != iif.lm) {
            iif.lm = lm
            loadWasm (false)}})},
      200)
  } else loadWasm (true)

} (window.iif = window.iif || {}))

此处的 trade-off 是我们在 WASM 中使用 HashMap，这可能会增加大小，除非已经需要 HashMap。

一个有趣的替代方法是使用 tables 与 JavaScript 共享（有效载荷、长度、容量）三元组，并在需要释放字符串时取回它。但是我还不知道怎么用这些表格。

P.S。有时我们不想首先分配 Vec。
在这种情况下，我们可以将内存跟踪移动到 JavaScript:

extern "C" {
    fn new_js_string(utf8: *const u8, len: i32) -> i32;
    fn console_log(js: i32);
}

fn rs2js(rs: &str) -> i32 {
    assert!(rs.len() < i32::max_value() as usize);
    unsafe { new_js_string(rs.as_ptr(), rs.len() as i32) }
}

#[no_mangle]
pub fn start() {
    unsafe {
        console_log(rs2js("Hello again!"));
    }
}

(function (iif) {
  function loadWasm (cache) {
    WebAssembly.instantiateStreaming (fetch ('main.wasm', {cache: cache ? "default" : "no-cache"}), {env: {
      new_js_string: function (utf8, len) {
        const view = new Uint8Array (iif.main.memory.buffer, utf8, len)
        const utf8dec = new TextDecoder ('utf-8')
        const decoded = utf8dec.decode (view)
        let stringId = iif.lastStringId
        while (typeof iif.strings[stringId] !== 'undefined') stringId += 1
        if (stringId > 2147483647) {  // Can't easily pass more than that through WASM.
          stringId = -2147483648
          while (typeof iif.strings[stringId] !== 'undefined') stringId += 1
          if (stringId > 2147483647) throw new Error ('Out of string IDs!')}
        iif.strings[stringId] = decoded
        return iif.lastStringId = stringId},
      console_log: function (js) {
        if (window.console) console.log ('main]', iif.strings[js])
        delete iif.strings[js]}
    }}) .then (results => {
      iif.main = results.instance.exports
      iif.main.start()})}

  loadWasm (true)
} (window.iif = window.iif || {strings: {}, lastStringId: 1}))

如何在 WebAssembly 中 return 来自 Rust 的字符串（或类似字符串）？

How to return a string (or similar) from Rust in WebAssembly?

rust

webassembly