2.1.1 • Public • Published

Easy C/C++ Web Assembly with twr-wasm

Version 2.1.1

twr-wasm is a simple, lightweight and easy to use library for building C/C++ Web Assembly 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 cool features. You can input and print streaming character i/o to a <div> tag, use a <canvas> element as an ANSI terminal, or use a C/C++ 2D drawing api (that is compatible with JavaScript Canvas APIs) to draw to a <canvas> element. You can run blocking C/C++.

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.

View C++ Web Assembly Bouncing Ball Demo

View bouncing balls here

Full Documentation

The full documentation can be found here

Key Features

  • compile and link C/C++ for use with web assembly using clang directly
  • standard C library, libc++. and purpose built APIs available from C/C++
  • TypeScrpt/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

View Live Demos

Name View Live Link Source Link
Bouncing Balls (C++) View bouncing balls Source for balls
Maze (Win32 C Port) View live maze Source for maze
Input from <div> View square demo Source
Mini-Terminal View mini-term demo Source


Clone from github, or use npm install twr-wasm. To compile C/C++, install clang and wasm-ld.

For details see

Hello World

Here is the simplest twr-wasm example.

C code:

#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"]);

Simple <div> i/o

I/O can be directed to or from a <div> or a <canvas> tag. Here is a simple example using a <div> for stdio input and output.

#include <stdio.h>
#include <stdlib.h>
#include "twr-crt.h"

void stdio_div() {
   char inbuf[64];
   int i;

   printf("Square Calculator\n");

   while (1) {
      printf("Enter an integer: ");
      printf("%d squared is %d\n\n",i,i*i);

With an index.html like the following. This time we are using twrWasmModuleAsync which integrates blocking C code into Javascript. twrWasmModuleAsync can also be used to receive key input from a <div> or <canvas> tag.

   <title>stdio-div example</title>
   <div id="twr_iodiv" style="background-color:LightGray;color:DarkGreen" tabindex="0">Loading... <br></div>

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

      let amod;

      try {
         amod = new twrWasmModuleAsync();

         document.getElementById("twr_iodiv").innerHTML ="<br>";

         await amod.loadWasm("./stdio-div.wasm");
         await amod.callC(["stdio_div"]);
catch(ex) {
   amod.divLog("unexpected exception");
   throw ex;


Full Documentation

The full documentation can be found here

Package Sidebar


npm i twr-wasm

Weekly Downloads






Unpacked Size

13.1 MB

Total Files


Last publish


  • twiddlingbits