Skip to content

Learn WebAssembly with twr-wasm
Documentation and Examples

Easier C/C++ WebAssembly

Version 2.2.1

twr-wasm is a simple, lightweight and easy to use library for building C/C++ WebAssembly code directly with clang. It solves some common use cases with less work than the more feature rich emscripten.

twr-wasm is easy to understand, and has some great features. You can call blocking functions. You can input and print streaming character i/o to a <div> tag, use a <canvas> element as an ANSI terminal, and use 2D drawing apis (that are compatible with JavaScript Canvas APIs) to draw to a <canvas> element.

twr-wasm allows you to run C/C++ code in a web browser. Legacy code, libraries, full applications, or single functions can be integrated with JavaScript and TypeScript.

twr-wasm is designed to be used with the standard llvm clang compiler and tools.

twr-wasm was previously named tiny-wasm-runtime.

Live WebAssembly Examples and Source

Name View Live Link Source Link
Bouncing Balls (C++) View bouncing balls Source for balls
Maze Gen/Solve (Win32 C Port) View live maze Source for maze
Input/Output with <div> View square demo Source
Mini-Terminal (hello world using <canvas>) View demo Source
CLI using libc++ and <canvas>) View console Source

Key Features

  • compile and link C/C++ for use with WebAssembly using clang directly
  • standard C library, libc++. and purpose built APIs available from C/C++
  • TypeScript/JavaScript classes to load Wasm modules and call C/C++ functions
  • localization support, UTF-8, and windows-1252 support
  • in C/C++, print and get characters to/from <div> tags in your HTML page
  • in C/C++, print and get characters to/from a <canvas> based "terminal"
  • in C/C++ use 2D drawing API compatible with JavaScript Canvas
  • in C/C++, use the "blocking loop" pattern and integrate with Javascript's asynchronous event loop

Hello World

Here is the simplest twr-wasm example.

#include <stdio.h>

void hello() {
    printf("hello, world!\n");
   <title>Hello World</title>
   <div id="twr_iodiv"></div>

   <script type="module">
      import {twrWasmModule} from "twr-wasm";

      const mod = new twrWasmModule();
      await mod.loadWasm("./helloworld.wasm");
      await mod.callC(["hello"]);

On Github


The Wasm Runtime Limitations section explains why a library like twr-wasm is needed to use WebAssembly.

Version 2 vs. 1

  • libc++ built for WebAssembly is included
  • most of the standard C library is now implemented
  • instructions for WebAssembly C/C++ source level debugging
  • version of library with debug symbols provided
  • locale, UTF-8, and windows-1252 support

Version 2 Limitations

  • libc++ not built with exceptions enabled
  • some standard C library functions are not 100% implemented
  • Designed to work with a browser. Not tested with or designed to work with node.js
  • Not all of compile-rt is ported (but most bits you need are)
  • The following non-compatible changes since 1.0 (relatively minor)
    • there is no longer 'twr_' prefixed std c lib functions (use the normal std c lib names)
    • most 'twr_wasm_' prefixed functions have been shortened to 'twr_'.
    • some functions were renamed or changed slightly to be more consistent, but no functionality is lost.

Post Feedback

Please post feedback (it worked for you, didn't work, requests, questions, etc) at