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Create and Run WebAssembly Hello World

This section shows you, step by step, how to to create a C "hello world" program for WebAssembly (Wasm) with twr-wasm, C, HTML, and JavaScript.

You will learn how to:

  • Create the helloworld.c file
  • Create the index.html file
  • Compile the helloworld.c code with clang
  • Link the helloworld.o and twr.a files with wasm-ld to create a helloworld.wasm file
  • Set the needed library and include paths to allow the twr-wasm libraries to be discovered
  • Create an optional Makefile
  • Execute the "hello world" program using a local web server or directly with VS Code and Chrome

You can find code for a hello world example in the folder examples\helloworld. It is similar, but not identical to this walk through. The primary differences are the paths for lib-c, lib-js, and include.

Step 0: Installation

  • prerequisites: install clang, wasm-ld, and python or VS Code (or both)
  • Create a folder for your project, such as hello-proj
  • cd into hello-proj
  • npm install twr-wasm
  • your folder structure should now look similar to this:

Step 1: Create the C code

Create a file helloworld.c in hello-proj

#include <stdio.h>

void hello() {
   printf("hello world\n");

Step 2: Create the HTML

Create a file index.html in hello-proj

<!doctype html>
   <title>Hello World</title>

   <script type="importmap">
      "imports": {
      "twr-wasm": "./node_modules/twr-wasm/lib-js/index.js"

   <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"]);

This example uses Import Maps, which are used when not using a bundler like WebPack or Parcel. For smaller projects, this can be simpler with a more clear debugging and development environment. This is the approach we will use for this example (no bundler).

The path in the importmap section of index.html should point to the location where you installed twr-wasm/lib-js. The path above is correct for this project example with the indicated folder structure.

Step 3: Compile your C code to create your .wasm file

cd hello-proj
clang --target=wasm32 -nostdinc -nostdlib -isystem ./node_modules/twr-wasm/include -c  helloworld.c -o helloworld.o
wasm-ld  helloworld.o ./node_modules/twr-wasm/lib-c/twr.a -o helloworld.wasm  --no-entry --initial-memory=131072 --max-memory=131072 --export=hello 

The path to twr.a and to include should match your installation. The above path is correct for this example.

As an alternate to executing clang and wasm-ld from the shell, here is a Makefile that will work for this example:

CC := clang
TWRCFLAGS := --target=wasm32 -nostdinc -nostdlib -isystem  ./node_modules/twr-wasm/include
CFLAGS := -c -Wall -O3 $(TWRCFLAGS)
CFLAGS_DEBUG := -c -Wall -g -O0  $(TWRCFLAGS)

.PHONY: default

default: helloworld.wasm

helloworld.o: helloworld.c
    $(CC) $(CFLAGS)  $< -o $@

helloworld.wasm: helloworld.o 
    wasm-ld  helloworld.o ./node_modules/twr-wasm/lib-c/twr.a -o helloworld.wasm \
        --no-entry --initial-memory=131072 --max-memory=131072 \

Copy the above into a file named Makefile and execute with make (or mingw32-make in windows).

Step 4: Load and execute your web page

The two easiest ways to load and execute your index.html web page locally are:

Option A: Run a local web Server

You can run a local server to view your helloworld program.

  • Copy the file from the examples folder to your hello-proj folder (where your index.html resides).
  • Execute with the shell command python
  • Open your web browser and browse to http://localhost:8000/index.html
  • You should see 'Hello World' in the browser window!

At this pont your folder structure should look like this:


Option B: VS Code launch.json

Alternately, you can launch chrome without a local web server. Add an entry similar to the following to hello-proj\.vscode\launch.json. This assumes your workspaceFolder is hello-proj.

    "configurations": [
        "name": "Launch Chrome Hello, World!",
        "request": "launch",
        "type": "chrome",
        "runtimeArgs": [
         "file": "${workspaceFolder}/index.html",
         "cwd": "${workspaceFolder}/",

Once you have created this file, you:

  • select the Run and Debug icon on left
  • Select the green play icon at the top, with "Launch Chrome Hello, World!" selected
  • Chrome should launch, and you should see 'Hello World' in the browser window!

--autoplay-policy=no-user-gesture-required and --enable-features=SharedArrayBuffer are not required for this simple "hello world" example, but will be needed if you request user input or you are using twrWasModuleAsync.

See live version

You can find a live link to hello world on this page.

Next steps after hello world

A good way to get your own code up and running is to copy one of the examples, get it to build and run, then start modifying it. Note you will need to modify the paths for include, lib-js, lib-c, etc. based on your project structure. The examples are all setup with relative paths assuming the folder structure twr-wasm\examples\<example>

The examples include MakeFiles.

"Hello World" uses the twr-wasm class twrWasmModule. If you wish to use C blocking functions, such as twr_getc32 or twr_sleep, you should use twrWasmModuleAsync. This square calculator example shows how to do this.

If you wish to build an app that makes non-block calls into C, the balls example shows how to do this. The maze example uses a combination of blocking and non-blocking C functions.