From b83689a1bffa4b0be38a0cd1a88c561eed9d3b29 Mon Sep 17 00:00:00 2001 From: DarkSun Date: Wed, 17 Mar 2021 05:06:47 +0800 Subject: [PATCH] =?UTF-8?q?=E9=80=89=E9=A2=98[tech]:=2020210316=20How=20to?= =?UTF-8?q?=20write=20'Hello=20World'=20in=20WebAssembly?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit sources/tech/20210316 How to write -Hello World- in WebAssembly.md --- ...w to write -Hello World- in WebAssembly.md | 165 ++++++++++++++++++ 1 file changed, 165 insertions(+) create mode 100644 sources/tech/20210316 How to write -Hello World- in WebAssembly.md diff --git a/sources/tech/20210316 How to write -Hello World- in WebAssembly.md b/sources/tech/20210316 How to write -Hello World- in WebAssembly.md new file mode 100644 index 0000000000..986b7fc3b7 --- /dev/null +++ b/sources/tech/20210316 How to write -Hello World- in WebAssembly.md @@ -0,0 +1,165 @@ +[#]: subject: (How to write 'Hello World' in WebAssembly) +[#]: via: (https://opensource.com/article/21/3/hello-world-webassembly) +[#]: author: (Stephan Avenwedde https://opensource.com/users/hansic99) +[#]: collector: (lujun9972) +[#]: translator: ( ) +[#]: reviewer: ( ) +[#]: publisher: ( ) +[#]: url: ( ) + +How to write 'Hello World' in WebAssembly +====== +Get started writing WebAssembly in human-readable text with this +step-by-step tutorial. +![Hello World inked on bread][1] + +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. + +When you talk about a WebAssembly, application, you must distinguish between three states: + + 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. + 2. **WebAssembly bytecode:** You choose WebAssembly bytecode as your compilation target. As a result, you get a `.wasm` file. + 3. **Machine code (opcode):** The browser loads the `.wasm` file and compiles it to the corresponding machine code of its host system. + + + +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). + +This article will guide you through creating the classic _Hello World_ program in WASM-text. + +### Creating the .wat file + +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: + + +``` +(module +    ;; Imports from JavaScript namespace +    (import  "console"  "log" (func  $log (param  i32  i32))) ;; Import log function +    (import  "js"  "mem" (memory  1)) ;; Import 1 page of memory (54kb) +    +    ;; Data section of our module +    (data (i32.const 0) "Hello World from WebAssembly!") +    +    ;; Function declaration: Exported as helloWorld(), no arguments +    (func (export  "helloWorld") +        i32.const 0  ;; pass offset 0 to log +        i32.const 29  ;; pass length 29 to log (strlen of sample text) +        call  $log +        ) +) +``` + +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. + +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. + +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). + +### Creating the .wasm file + +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. + +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. + +After you build the toolchain, convert WASM-text to bytecode by opening a console and entering: + + +``` +`wat2wasm helloworld.wat -o helloworld.wasm` +``` + +You can also convert bytecode to WASM-text with: + + +``` +`wasm2wat helloworld.wasm -o helloworld_reverse.wat` +``` + +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. + +### Compiling the .wasm file + +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. + +Create an empty text file and name it `helloworld.html`, then open your favorite text editor and paste in: + + +``` +<!DOCTYPE  html> +<html> +  <head> +    <meta  charset="utf-8"> +    <title>Simple template</title> +  </head> +  <body> +    <script> +    +      var memory = new  WebAssembly.Memory({initial:1}); + +      function  consoleLogString(offset, length) { +        var  bytes = new  Uint8Array(memory.buffer, offset, length); +        var  string = new  TextDecoder('utf8').decode(bytes); +        console.log(string); +      }; + +      var  importObject = { +        console: { +          log:  consoleLogString +        }, +        js : { +          mem:  memory +        } +      }; +      +      WebAssembly.instantiateStreaming(fetch('helloworld.wasm'), importObject) +      .then(obj  => { +        obj.instance.exports.helloWorld(); +      }); +      +    </script> +  </body> +</html> +``` + +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`. + +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`: + +![Firefox setting][4] + +(Stephan Avenwedde, [CC BY-SA 4.0][5]) + +> **Caution:** This will make you vulnerable to the [CVE-2019-11730][6] security issue. + +Now, open `helloworld.html` in Firefox and enter **Ctrl**+**K** to open the developer console. + +![Debugger output][7] + +(Stephan Avenwedde, [CC BY-SA 4.0][5]) + +### Learn more + +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. + +-------------------------------------------------------------------------------- + +via: https://opensource.com/article/21/3/hello-world-webassembly + +作者：[Stephan Avenwedde][a] +选题：[lujun9972][b] +译者：[译者ID](https://github.com/译者ID) +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 +[b]: https://github.com/lujun9972 +[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) +[2]: https://developer.mozilla.org/en-US/docs/WebAssembly#browser_compatibility +[3]: https://github.com/webassembly/wabt +[4]: https://opensource.com/sites/default/files/uploads/firefox_setting.png (Firefox setting) +[5]: https://creativecommons.org/licenses/by-sa/4.0/ +[6]: https://www.mozilla.org/en-US/security/advisories/mfsa2019-21/#CVE-2019-11730 +[7]: https://opensource.com/sites/default/files/uploads/debugger_output.png (Debugger output) +[8]: https://developer.mozilla.org/en-US/docs/WebAssembly/Understanding_the_text_format