Compiling from C/C++ to WebAssembly

When you’ve written a code module in a language like C/C++, you can then compile it into WebAssembly using a tool like Emscripten. Let’s look at how it works.

Emscripten environment setup

First, let's set up the required development environment.

Prerequisites

There are some prerequisites that need installing on your computer. Make sure these are present first:

Git — On Linux and OS X this is likely already present. On Windows download the Git for Windows installer.
CMake — On Linux and OS X, install this using a package manager like apt-get or brew to make sure the dependencies and paths are handled correctly. On Windows, use the CMake installer.
Host system compiler — On Linux, install GCC. On OS X, install Xcode. On Windows, install Visual Studio Community 2015 with Update 3 or newer. Note that the recently-released VS 2017 won't work — it isn't supported by Emscripten yet.
Python 2.7.x — On Linux and OS X, this is most likely provided out of the box. See this Beginners guide for instructions. On Windows, get an installer from the Python homepage.

Note: On Windows you may require pywin32. In order to reduce errors while installing pywin32, please run the installer from CMD as admin.

Compiling Emscripten

Next, you need to compile Emscripten from source. Run the following commands (inside the parent directory where you want to store Emscripten) to automate this using the Emscripten SDK:

git clone https://github.com/juj/emsdk.git
cd emsdk
# on Linux or Mac OS X
./emsdk install --build=Release sdk-incoming-64bit binaryen-master-64bit
./emsdk activate --global --build=Release sdk-incoming-64bit binaryen-master-64bit
# on Windows
emsdk install --build=Release sdk-incoming-64bit binaryen-master-64bit
emsdk activate --global --build=Release sdk-incoming-64bit binaryen-master-64bit

The install step can take a while, so it’s a good place to go and get a cup of tea. The install step sets up all the environment variables, etc., needed to run Emscripten.

Note: The --global flag causes the required PATH variables to be set globally, so this and all future terminal/command prompt windows will have them set. If you want to just activate Emscripten for the current window, omit this flag.

Note: It's a good idea to pull the latest emscripten code (run git pull inside the emsdk directory) and then rerun the install and activate commands shown each time you intend to use Emscripten, to make sure you have the latest features.

Now from inside the emsdk directory, enter the following command to enter an Emscripten compiler environment from which you can compile C examples to asm.js/wasm:

# on Linux or Mac OS X
source ./emsdk_env.sh
# on Windows
emsdk_env.bat

Compiling an example

With the environment set up, let's look at how to use it to compile a C example to Emscripten. There are a number of options available when compiling with Emscripten, but the main two scenarios we'll cover are:

Compiling to wasm and creating HTML to run our code in, plus all the JavaScript "glue" code needed to run the wasm in the web environment.
Compiling to wasm and just creating the JavaScript.

We will look at both below.

Creating HTML and JavaScript

This is the simplest case we'll look at, whereby you get emscripten to generate everything you need to run your code, as WebAssembly, in the browser.

First we need an example to compile. Take a copy of the following simple C example, and save it in a file called hello.c in a new directory on your local drive:
```
#include <stdio.h>
int main(int argc, char ** argv) {
  printf("Hello World\n");
}
```
Now, using the terminal window you used to enter the Emscripten compiler environment, navigate to the same directory as your hello.c file, and run the following command:
```
emcc hello.c -s WASM=1 -o hello.html
```

The options we’ve passed in with the command are as follows:

-s WASM=1 — Specifies that we want wasm output. If we don’t specify this, Emscripten will just output asm.js, as it does by default.
-o hello.html — Specifies that we want Emscripten to generate an HTML page to run our code in (and a filename to use), as well as the wasm module and the JavaScript "glue" code to compile and instantiate the wasm so it can be used in the web environment.

At this point in your source directory you should have:

The binary wasm module code (hello.wasm)
A JavaScript file containing glue code to translate between the native C functions, and JavaScript/wasm (hello.js)
An HTML file to load, compile, and instantiate your wasm code, and display its output in the browser (hello.html)

Running your example

Now all that remains is for you to load the resulting hello.html in a browser that supports WebAssembly. It is enabled by default in Firefox 52+ and Chrome 57+/latest Opera (you can also run wasm code in Firefox 47+ by enabling the javascript.options.wasm flag in about:config, or Chrome (51+) and Opera (38+) by going to chrome://flags and enabling the Experimental WebAssembly flag.)

If everything has worked as planned, you should see "Hello world" output in the Emscripten console appearing on the web page, and your browser’s JavaScript console. Congratulations, you’ve just compiled C to WebAssembly and run it in your browser!

Using a custom HTML template

Sometimes you will want to use a custom HTML template. Let's look at how we can do this.

First of all, save the following C code in a file called hello2.c, in a new directory:
```
#include <stdio.h>
int main(int argc, char ** argv) {
    printf("Hello World\n");
}
```
Search for the file shell_minimal.html in your emsdk repo. Copy it into a sub-directory called html_template inside your previous new directory.
Now navigate into your new directory (again, in your Emscripten compiler environment terminal window), and run the following command:
```
emcc -o hello2.html hello2.c -O3 -s WASM=1 --shell-file html_template/shell_minimal.html
```
The options we've passed are slightly different this time:
- We've specified -o hello2.html, meaning that the compiler will still output the JavaScript glue code and .html.
- We've also specified --shell-file html_template/shell_minimal.html — this provides the path to the HTML template you want to use to create the HTML you will run your example through.
Now let's run this example. The above command will have generated hello2.html, which will have much the same content as the template with some glue code added into load the generated wasm, run it, etc. Open it in your browser and you'll see much the same output as the last example.

Note: You could specify outputting just JavaScript rather than the full HTML by specifying a .js file instead of an HTML file in the -o flag, e.g. emcc -o hello2.js hello2.c -O3 -s WASM=1. You could then build your custom HTML complete from scratch. This isn't recommended however — Emscripten requires a large variety of JavaScript "glue" code to handle memory allocation, memory leaks, and a host of other problems, which is already included in the provided template. It is easier to use that than having to write it all out yourself, although as you become more experienced at what it all does, you'll be able to create your own customized versions for your needs.

Calling a custom function defined in C

If you have a function defined in your C code that you want to call as needed from JavaScript, you can do this using the Emscripten ccall() function, and the EMSCRIPTEN_KEEPALIVE declaration (which adds your functions to the exported functions list (see Why do functions in my C/C++ source code vanish when I compile to JavaScript, and/or I get No functions to process?)). Let's look at how this works.

To start with, save the following code as hello3.c in a new directory:
```
#include <stdio.h>
#include <emscripten/emscripten.h>
int main(int argc, char ** argv) {
    printf("Hello World\n");
}
#ifdef __cplusplus
extern "C" {
#endif
void EMSCRIPTEN_KEEPALIVE myFunction(int argc, char ** argv) {
  printf("MyFunction Called\n");
}
#ifdef __cplusplus
}
#endif
```
By default, Emscripten-generated code always just calls the main() function, and other functions are eliminated as dead code. Putting EMSCRIPTEN_KEEPALIVE before a function name stops this from happening. You also need to import the emscripten.h library to use EMSCRIPTEN_KEEPALIVE.

Note: We are including the #ifdef blocks so that if you are trying to include this in C++ code, the example will still work. Due to C versus C++ name mangling rules, this would otherwise break, but here we are setting it so that it treats it as an external C function if you are using C++.
Now add html_template/shell_minimal.html into this new directory too, just for convenience (you'd obviously put this in a central place in your real dev environment).
Now let's run the compilation step again. From inside your latest directory (and while inside your Emscripten compiler environment terminal window), compile your C code with this:
```
emcc -o hello3.html hello3.c -O3 -s WASM=1 --shell-file html_template/shell_minimal.html
```
If you load the example in your browser again, you'll see the same thing as before!
Now we need to run our new myFunction() function from JavaScript. First of all, let's add a <button> as shown below, just above the first opening <script type='text/javascript'> tag.
```
<button class="mybutton">Run myFunction</button>
```

Now add the following code inside the last <script> element (just above the closing </script> tag):

document.querySelector('.mybutton').addEventListener('click', function(){
  alert('check console');
  var result = Module.ccall('myFunction', // name of C function 
                             null, // return type
                             null, // argument types
                             null); // arguments
});

This illustrates how ccall() is used to call the exported function.