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[#]: subject: (How to write 'Hello World' in WebAssembly)
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[#]: via: (https://opensource.com/article/21/3/hello-world-webassembly)
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[#]: author: (Stephan Avenwedde https://opensource.com/users/hansic99)
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[#]: collector: (lujun9972)
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[#]: translator: (geekpi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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How to write 'Hello World' in WebAssembly
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======
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Get started writing WebAssembly in human-readable text with this
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step-by-step tutorial.
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![Hello World inked on bread][1]
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WebAssembly is a bytecode format that [virtually every browser][2] can compile to its host system's machine code. Alongside JavaScript and WebGL, WebAssembly fulfills the demand for porting applications for platform-independent use in the web browser. As a compilation target for C++ and Rust, WebAssembly enables web browsers to execute code at near-native speed.
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When you talk about a WebAssembly, application, you must distinguish between three states:
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1. **Source code (e.g., C++ or Rust):** You have an application written in a compatible language that you want to execute in the browser.
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2. **WebAssembly bytecode:** You choose WebAssembly bytecode as your compilation target. As a result, you get a `.wasm` file.
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3. **Machine code (opcode):** The browser loads the `.wasm` file and compiles it to the corresponding machine code of its host system.
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WebAssembly also has a text format that represents the binary format in human-readable text. For the sake of simplicity, I will refer to this as **WASM-text**. WASM-text can be compared to high-level assembly language. Of course, you would not write a complete application based on WASM-text, but it's good to know how it works under the hood (especially for debugging and performance optimization).
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This article will guide you through creating the classic _Hello World_ program in WASM-text.
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### Creating the .wat file
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WASM-text files usually end with `.wat`. Start from scratch by creating an empty text file named `helloworld.wat`, open it with your favorite text editor, and paste in:
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```
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(module
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;; Imports from JavaScript namespace
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(import "console" "log" (func $log (param i32 i32))) ;; Import log function
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(import "js" "mem" (memory 1)) ;; Import 1 page of memory (54kb)
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;; Data section of our module
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(data (i32.const 0) "Hello World from WebAssembly!")
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;; Function declaration: Exported as helloWorld(), no arguments
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(func (export "helloWorld")
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i32.const 0 ;; pass offset 0 to log
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i32.const 29 ;; pass length 29 to log (strlen of sample text)
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call $log
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)
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)
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```
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The WASM-text format is based upon S-expressions. To enable interaction, JavaScript functions are imported with the `import` statement, and WebAssembly functions are exported with the `export` statement. For this example, import the `log `function from the `console` module, which takes two parameters of type `i32` as input and one page of memory (64KB) to store the string.
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The string will be written into the `data` section at offset `0`. The `data` section is an overlay of your memory, and the memory is allocated in the JavaScript part.
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Functions are marked with the keyword `func`. The stack is empty when entering a function. Function parameters are pushed onto the stack (here offset and length) before another function is called (see `call $log`). When a function returns an `f32` type (for example), an `f32` variable must remain on the stack when leaving the function (but this is not the case in this example).
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### Creating the .wasm file
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The WASM-text and the WebAssembly bytecode have 1:1 correspondence. This means you can convert WASM-text into bytecode (and vice versa). You already have the WASM-text, and now you want to create the bytecode.
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The conversion can be performed with the [WebAssembly Binary Toolkit][3] (WABT). Make a clone of the repository at that link and follow the installation instructions.
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After you build the toolchain, convert WASM-text to bytecode by opening a console and entering:
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```
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`wat2wasm helloworld.wat -o helloworld.wasm`
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```
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You can also convert bytecode to WASM-text with:
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```
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`wasm2wat helloworld.wasm -o helloworld_reverse.wat`
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```
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A `.wat` file created from a `.wasm` file does not include any function nor parameter names. By default, WebAssembly identifies functions and parameters with their index.
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### Compiling the .wasm file
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Currently, WebAssembly only coexists with JavaScript, so you have to write a short script to load and compile the `.wasm` file and do the function calls. You also need to define the functions you will import in your WebAssembly module.
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Create an empty text file and name it `helloworld.html`, then open your favorite text editor and paste in:
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```
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<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<title>Simple template</title>
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</head>
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<body>
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<script>
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var memory = new WebAssembly.Memory({initial:1});
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function consoleLogString(offset, length) {
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var bytes = new Uint8Array(memory.buffer, offset, length);
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var string = new TextDecoder('utf8').decode(bytes);
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console.log(string);
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};
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var importObject = {
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console: {
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log: consoleLogString
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},
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js : {
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mem: memory
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}
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};
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WebAssembly.instantiateStreaming(fetch('helloworld.wasm'), importObject)
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.then(obj => {
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obj.instance.exports.helloWorld();
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});
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</script>
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</body>
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</html>
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```
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The `WebAssembly.Memory(...)` method returns one page of memory that is 64KB in size. The function `consoleLogString` reads a string from that memory page based on the length and offset. Both objects are passed to your WebAssembly module as part of the `importObject`.
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Before you can run this example, you may have to allow Firefox to access files from this directory by typing `about:config` in the address line and setting `privacy.file_unique_origin` to `true`:
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![Firefox setting][4]
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(Stephan Avenwedde, [CC BY-SA 4.0][5])
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> **Caution:** This will make you vulnerable to the [CVE-2019-11730][6] security issue.
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Now, open `helloworld.html` in Firefox and enter **Ctrl**+**K** to open the developer console.
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![Debugger output][7]
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(Stephan Avenwedde, [CC BY-SA 4.0][5])
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### Learn more
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This Hello World example is just one of the detailed tutorials in MDN's [Understanding WebAssembly text format][8] documentation. If you want to learn more about WebAssembly and how it works under the hood, take a look at these docs.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/3/hello-world-webassembly
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作者：[Stephan Avenwedde][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/hansic99
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/helloworld_bread_lead.jpeg?itok=1r8Uu7gk (Hello World inked on bread)
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[2]: https://developer.mozilla.org/en-US/docs/WebAssembly#browser_compatibility
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[3]: https://github.com/webassembly/wabt
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[4]: https://opensource.com/sites/default/files/uploads/firefox_setting.png (Firefox setting)
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[5]: https://creativecommons.org/licenses/by-sa/4.0/
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[6]: https://www.mozilla.org/en-US/security/advisories/mfsa2019-21/#CVE-2019-11730
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[7]: https://opensource.com/sites/default/files/uploads/debugger_output.png (Debugger output)
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[8]: https://developer.mozilla.org/en-US/docs/WebAssembly/Understanding_the_text_format
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@ -0,0 +1,165 @@
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[#]: subject: (How to write 'Hello World' in WebAssembly)
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[#]: via: (https://opensource.com/article/21/3/hello-world-webassembly)
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[#]: author: (Stephan Avenwedde https://opensource.com/users/hansic99)
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[#]: collector: (lujun9972)
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[#]: translator: (geekpi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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如何在 WebAssembly 中写 “Hello World”？
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======
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通过这个分步教程，开始用人类可读的文本编写 WebAssembly。
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![Hello World inked on bread][1]
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WebAssembly 是一种字节码格式，[几乎所有的浏览器][2]都可以将它编译成其主机系统的机器代码。除了 JavaScript 和 WebGL 之外，WebAssembly 还满足了将应用移植到浏览器中以实现平台独立的需求。作为 C++ 和 Rust 的编译目标，WebAssembly 使 Web 浏览器能够以接近原生的速度执行代码。
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当你谈论 WebAssembly、应用时，你必须区分三种状态：
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1. **源码（如 C++ 或 Rust）：** 你有一个用兼容语言编写的应用，你想把它在浏览器中执行。
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2. **WebAssembly 字节码：** 你选择 WebAssembly 字节码作为编译目标。最后，你得到一个 `.wasm` 文件。
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3. **机器码（opcode）：** 浏览器加载 `.wasm` 文件，并将其编译成主机系统的相应机器码。
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WebAssembly 还有一种文本格式，用人类可读的文本表示二进制格式。为了简单起见，我将其称为 **WASM-text**。WASM-text 可以比作高级汇编语言。当然，你不会基于 WASM-text 来编写一个完整的应用，但了解它的底层工作原理是很好的（特别是对于调试和性能优化）。
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本文将指导你在 WASM-text 中创建经典的 _Hello World_ 程序。
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### 创建 .wat 文件
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WASM-text 文件通常以 `.wat` 结尾。第一步创建一个名为 `helloworld.wat` 的空文本文件，用你最喜欢的文本编辑器打开它，然后粘贴进去：
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```
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(module
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;; Imports from JavaScript namespace
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(import "console" "log" (func $log (param i32 i32))) ;; Import log function
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(import "js" "mem" (memory 1)) ;; Import 1 page of memory (54kb)
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;; Data section of our module
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(data (i32.const 0) "Hello World from WebAssembly!")
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;; Function declaration: Exported as helloWorld(), no arguments
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(func (export "helloWorld")
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i32.const 0 ;; pass offset 0 to log
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i32.const 29 ;; pass length 29 to log (strlen of sample text)
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call $log
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)
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)
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```
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WASM-text 格式是基于 S 表达式的。为了实现交互，JavaScript 函数用 `import` 语句导入，WebAssembly 函数用 `export` 语句导出。在这个例子中，从 `console` 模块中导入 `log` 函数，它需要两个类型为 `i32` 的参数作为输入，以及一页内存（64KB）来存储字符串。
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字符串将被写入偏移量 `0` 的 `data` 部分。`data` 部分是你的内存的覆盖，内存是在 JavaScript 部分分配的。
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函数用关键字 `func` 标记。当进入函数时，栈是空的。在调用另一个函数之前，函数参数会被压入栈中（这里是偏移量和长度）（见 `call $log`）。当一个函数返回一个 `f32` 类型时（例如），当离开函数时，一个 `f32` 变量必须保留在栈中（但在本例中不是这样）。
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### 创建 .wasm 文件
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WASM-text 和 WebAssembly 字节码有 1:1 的对应关系，这意味着你可以将 WASM-text 转换成字节码（反之亦然）。你已经有了 WASM-text，现在将创建字节码。
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转换可以通过 [WebAssembly Binary Toolkit][3]（WABT）来完成。从链接克隆仓库，并按照安装说明进行安装。
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建立工具链后，打开控制台并输入以下内容，将 WASM-text 转换为字节码：
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```
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`wat2wasm helloworld.wat -o helloworld.wasm`
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```
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你也可以用以下方法将字节码转换为 WASM-text：
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```
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`wasm2wat helloworld.wasm -o helloworld_reverse.wat`
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```
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一个从 `.wasm` 文件创建的 `.wat` 文件不包括任何函数或参数名称。默认情况下，WebAssembly 用它们的索引来识别函数和参数。
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### 编译 .wasm 文件
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目前，WebAssembly 只与 JavaScript 共存，所以你必须编写一个简短的脚本来加载和编译 `.wasm` 文件并进行函数调用。你还需要在 WebAssembly 模块中定义你要导入的函数。
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创建一个空的文本文件，并将其命名为 `helloworld.html`，然后打开你喜欢的文本编辑器并粘贴进去：
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```
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<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<title>Simple template</title>
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</head>
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<body>
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<script>
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var memory = new WebAssembly.Memory({initial:1});
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function consoleLogString(offset, length) {
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var bytes = new Uint8Array(memory.buffer, offset, length);
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var string = new TextDecoder('utf8').decode(bytes);
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console.log(string);
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};
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var importObject = {
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console: {
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log: consoleLogString
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},
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js : {
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mem: memory
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}
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};
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WebAssembly.instantiateStreaming(fetch('helloworld.wasm'), importObject)
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.then(obj => {
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obj.instance.exports.helloWorld();
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});
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</script>
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</body>
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</html>
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```
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`WebAssembly.Memory(...)` 方法返回一个大小为 64KB 的内存页。函数 `consoleLogString` 根据长度和偏移量从该内存页读取一个字符串。这两个对象作为 `importObject` 的一部分传递给你的 WebAssembly 模块。
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在你运行这个例子之前，你可能必须允许 Firefox 从这个目录中访问文件，在地址栏输入 `about:config`，并将 `privacy.file_unique_origin` 设置为 `true`：
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![Firefox setting][4]
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（Stephan Avenwedde, [CC BY-SA 4.0][5]）
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> **注意：** 这样做会使你容易受到 [CVE-2019-11730][6] 安全问题的影响。
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现在，在 Firefox 中打开 `helloworld.html`，按下 **Ctrl**+**K** 打开开发者控制台。
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![Debugger output][7]
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（Stephan Avenwedde, [CC BY-SA 4.0][5]）
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### 了解更多
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这个 Hello World 的例子只是 MDN 的[了解 WebAssembly 文本格式][8]文档中的教程之一。如果你想了解更多关于 WebAssembly 的知识以及它的工作原理，可以看看这些文档。
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/3/hello-world-webassembly
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作者：[Stephan Avenwedde][a]
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选题：[lujun9972][b]
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||||
译者：[geekpi](https://github.com/geekpi)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]: https://opensource.com/users/hansic99
|
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[b]: https://github.com/lujun9972
|
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/helloworld_bread_lead.jpeg?itok=1r8Uu7gk (Hello World inked on bread)
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[2]: https://developer.mozilla.org/en-US/docs/WebAssembly#browser_compatibility
|
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[3]: https://github.com/webassembly/wabt
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[4]: https://opensource.com/sites/default/files/uploads/firefox_setting.png (Firefox setting)
|
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[5]: https://creativecommons.org/licenses/by-sa/4.0/
|
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[6]: https://www.mozilla.org/en-US/security/advisories/mfsa2019-21/#CVE-2019-11730
|
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[7]: https://opensource.com/sites/default/files/uploads/debugger_output.png (Debugger output)
|
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[8]: https://developer.mozilla.org/en-US/docs/WebAssembly/Understanding_the_text_format
|
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Block a user