https://github.com/kripken/emscripten/pull/1405 example

gistfile1.c

#include <stdio.h>
#include <stdlib.h>
#include <SDL/SDL.h>

#ifdef EMSCRIPTEN
#include <emscripten/emscripten.h>
// bug - SDL_GetKeyboardState doesn't return scancodes, it returns keycodes, so acts exactly like
// SDL_GetKeyState instead
#define SDL_GetKeyState SDL_GetKeyboardState
#endif

void loop()
{
   unsigned i;
#ifdef EMSCRIPTEN
   SDL_Event e;
   while (SDL_PollEvent(&e));
#else
   // emscripten should also have PumpEvents actually run all the events, but that's another issue
   SDL_PumpEvents();
#endif

   int num;
   const Uint8 *keys = SDL_GetKeyState(&num);
   if (keys[SDLK_ESCAPE])
      exit(0);
   for (i = 0; i < 256; i++)
      if (keys[i] != 0)
         printf("key %u\n", i);
}

int main(int argc, char *argv[])
{
   SDL_Init(SDL_INIT_EVERYTHING);
   SDL_SetVideoMode(600, 400, 32, SDL_SWSURFACE);

#ifdef EMSCRIPTEN
   emscripten_set_main_loop(loop, 0, 0);
#else
   while (1)
      loop();
#endif
   return 0;
}