Calling a callback sent from lua to c

example.c

// taken from here https://stackoverflow.com/a/21947358 and made into single file
// I was using luajit like this from my own game's code gcc example.c -L ./luajit/LuaJIT-2.0.5/src/ -lluajit

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

#include "./luajit/LuaJIT-2.0.5/src/lua.h"
#include "./luajit/LuaJIT-2.0.5/src/lauxlib.h"
#include "./luajit/LuaJIT-2.0.5/src/lualib.h"

int callback_reference = 0;

int lua_registerCallback( lua_State *L ) {
  callback_reference = luaL_ref( L, LUA_REGISTRYINDEX );

  return 0;
}

void call_callback( lua_State *L ) {
  lua_rawgeti( L, LUA_REGISTRYINDEX, callback_reference );
  lua_pushvalue( L, 1 );

  if ( 0 != lua_pcall( L, 0, 0, 0 ) ) {
    printf("Failed to call the callback!\n %s\n", lua_tostring( L, -1 ) );
    return;
  }

  callback_reference = luaL_ref( L, LUA_REGISTRYINDEX );
}

char* luaCode =
  "function MyCallback()\n"
  "  print(\"Hello World!\")\n"
  "end\n"
  "RegisterCallback( MyCallback )\n";


int main( void ) {
  lua_State *L = lua_open();
  luaL_openlibs( L );

  lua_pushcfunction( L, lua_registerCallback );
  lua_setglobal( L, "RegisterCallback" );

  if ( 0 != luaL_dostring( L, luaCode ) ) {
    printf("Failed to load calback.lua!\n %s",
      lua_tostring( L, -1 ) );
    lua_close( L );
    return 1;
  }

  call_callback( L );
  call_callback( L );

  luaL_unref( L, LUA_REGISTRYINDEX, callback_reference );

  lua_close( L );

  return 0;
}