path: root/md/writeup/web_assembly_sdl_example.md

title: WebAssembly SDL example
keywords:c,webassembly,js,SDL

# WebAssembly SDL example

## Intro

There is many examples of how to compile SDL for webassembly, but many of them isnt 
minimal as necessary, so here tried to make just bare minimum for application
that can be compiled for 2 targets webassembly and pc.

## How things works

Webassembly is virtual machine that you can run in your browser, it allows you to run compiled code.
Its not 100% like real pc virtual machine and more oriented on web. But its close enough to just
port and run you C/C++ or any other language that supports LLVM as backend. 

There need to be included emscripten header that gives callback for javascript to be runned.

```c
#include <emscripten/emscripten.h>
```

And as all this going to be compiled to webassembly byte code then you can _just_ load compiled file.
There is many ways how wasm bytecode going to be runned. We registed our code so its runes specified
number of frames per second. Check functions docs for better description. 

```c
void emscripten_set_main_loop(em_callback_func func, int fps, int simulate_infinite_loop)
```

Porting requiring to define loop that will be registered for webassembly engine and thats all. 
There need to be covered more things if there is need to use some fonts,images or files. But for
now its minimal example that will draw SDL window in your browser ;]

## Demo

This example compilable for PC target and for webassembly. Enjoy playing with it.

<script type='text/javascript'>
	var Module = {};
	fetch('/demo/sdl/index.wasm')
	.then(response =>
		response.arrayBuffer()
	).then(buffer => {
		Module.canvas = document.getElementById("canvas");
		Module.wasmBinary = buffer;
		var script = document.createElement('script');
		script.src = "/demo/sdl/index.js";
		script.onload = function() {
		console.log("Emscripten boilerplate loaded.")
		}
		document.body.appendChild(script);
	});
</script><canvas id="canvas" style="width:100%; height:100%;"></canvas>


## Source


```c
#include <stdio.h>
#include <stdlib.h>

#if __EMSCRIPTEN__
#include <emscripten/emscripten.h>
#include <SDL2/SDL.h>
#include <SDL/SDL_ttf.h>
#else
#include <SDL2/SDL.h>
#include <SDL2/SDL_ttf.h>
#endif

#define SCREEN_WIDTH  200
#define SCREEN_HEIGHT 200

SDL_Window *window = NULL;
SDL_Renderer *renderer = NULL;

#define MAX(a,b) ((a) > (b) ? a : b)
#define MIN(a,b) ((a) < (b) ? a : b)

int posX=0;
int posY=0;
int sizeW=20;
int sizeH=20;

static int quit = 0;

void render()
{
	
	SDL_Rect r_scr;
	r_scr.x = posX;
	r_scr.y = posY;
	r_scr.w = sizeW;
	r_scr.h = sizeH;
	SDL_SetRenderDrawColor( renderer, 0x00, 0x00, 0x00, 0xFF );
	SDL_RenderClear(renderer);

	SDL_SetRenderDrawColor(renderer, 0xff, 0xff, 0x00, 0x00);
	SDL_RenderDrawRect(renderer, &r_scr);
	SDL_RenderPresent(renderer);
}

#if __EMSCRIPTEN__
void main_tick() {
#else
int main_tick() {
#endif
	
	SDL_Event event;

		while (SDL_PollEvent(&event))
		{
			switch (event.type)
			{
			case SDL_QUIT:
			{
				quit = 1;
				break;
			}
			case SDL_KEYDOWN:
			{
				switch (event.key.keysym.sym)
				{
				case SDLK_UP:
				{
					if (posY>=20)
					{
						posY-=20;
					}
					break;
				}
				case SDLK_DOWN:
				{
					if (posY+sizeH<SCREEN_HEIGHT)
					{
						posY += 20;
					}
					break;
				}
				case SDLK_LEFT:
				{
					if (posX>=20)
					{
						posX-=20;
					}
					break;
				}
				case SDLK_RIGHT:
				{
					if (posX+sizeW<SCREEN_WIDTH)
					{
						posX+=20;
					}
					break;
				}
				}
				break;
			}
			}
			
		}
	

	render();
	SDL_UpdateWindowSurface(window);

#if !__EMSCRIPTEN__
	return 0;
#endif
}

void main_loop()
{

#if __EMSCRIPTEN__
	emscripten_set_main_loop(main_tick, -1, 1);
#else
	while (0 == quit)
	{
		main_tick();
	}
#endif
}

int main()
{
	SDL_Init(SDL_INIT_VIDEO);

	window = SDL_CreateWindow(
		"WEBASM",
		SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
		SCREEN_WIDTH, SCREEN_HEIGHT,
		SDL_WINDOW_SHOWN | SDL_WINDOW_OPENGL);

	renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);
	SDL_SetRenderDrawColor(renderer, 0xff, 0xff, 0xff, 0xff);

	main_loop();

	SDL_DestroyRenderer(renderer);
	SDL_DestroyWindow(window);
	SDL_Quit();

	return 0;
}
```


```js
<html>
    <head>
    	<meta charset="utf-8">
    	<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
    </head>
    <body>
    	<script type='text/javascript'>
	      var Module = {};
	      fetch('index.wasm')
	        .then(response =>
	          response.arrayBuffer()
	        ).then(buffer => {
	          Module.canvas = document.getElementById("canvas");
	          Module.wasmBinary = buffer;
	          var script = document.createElement('script');
	          script.src = "index.js";
	          script.onload = function() {
	            console.log("Emscripten boilerplate loaded.")
	          }
	          document.body.appendChild(script);
	        });
	    </script>
	    <canvas id="canvas" style="width:100%; height:100%"></canvas>

	</body>
</html>
```

```makefile
CC=gcc
CFALGS=
LDFLAGS=-lSDL2 -lSDL2_ttf

EM_ENV=LLVM=/usr/bin NODE_JS=node EMSCRIPTEN_ROOT=/usr/lib/emscripten
EM_CC=emcc
EM_CFLAGS=-s WASM=1 -O3
EM_LDFALGS=-s USE_SDL=2 -s USE_SDL_IMAGE=2 -s SDL2_IMAGE_FORMATS='["png"]' -s USE_SDL_TTF=2

pc:
	$(CC) $(CFLAGS) $(LDFLAGS) main.c -o main

em:
	$(EM_CC) main.c $(EM_CFLAGS) $(EM_LDFALGS) -o index.js -s EXTRA_EXPORTED_RUNTIME_METHODS='["ccall", "cwrap"]'  
```

## Links

1. https://wiki.libsdl.org/CategoryAPI
2. https://developer.mozilla.org/en-US/docs/WebAssembly/C_to_wasm
3. https://github.com/kvark/wasm-triangle/blob/master/src/main.rs
4. https://github.com/belen-albeza/space-shooter-wasm/blob/master/src/main.c
5. https://kripken.github.io/emscripten-site/docs/api_reference/emscripten.h.html