Your First C Web Assembly Program

You can find all of the code for this section in the folder examples\helloworld.

step 1: Create the C code

Create a file helloworld.c

#include <stdio.h>

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

step 2: create the HTML

Create a file index.html

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

   <script type="importmap">
      "imports": {
      "tiny-wasm-runtime": "../../lib-js/index.js",
      "whatkey": "../../lib-js/whatkey.js"

   <div id="twr_iodiv"></div>

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

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

The two relative paths in the importmap section need to be updated to point to the location where you installed tiny-wasm-runtime/lib-js. The paths above are correct if your file is in an example subfolder.

step 3: compile your C code to create your .wasm file

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

The path to twa.a and to include may need to be updated. The above path is correct if your code is in an example subfolder.

step 4: run your program

The two easiest ways to run your index.html web page locally are:

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 project folder where your index.html resides. Execute with the shell command python

VS Code launch.json

Alternately, you can launch chrome without a local web server. Add an entry similar to this to your launch.json. Adjust the file and cwd lines to be correct for your project.

   "name": "hello",
   "type": "chrome",
   "request": "launch",
   "runtimeArgs": [
   "file": "${workspaceFolder}/examples/index.html",
   "cwd": "${workspaceFolder}/examples/"

See live version

Here is a link to the helloworld function running.

Next steps after hello world

A good way to get your own code up and running is to copy one of the tiny-wasm-runtime/examples, get it to build and run, then start modifying it.

The example makefiles prove a more practical way to configure clang and wasm-ld.

Hello World uses the tiny-wasm-runtime class twrWasmModule. If you wish to use C blocking functions, such as twr_getc32 or twr_sleep, you can 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.