How to use lua_yield and lua_sethook to step into code line by line?

Question

I'll want to implement a debugger to navigate through a lua script step by step. The script is embedded in a win32 c++ enviroment, created with the main gui thread and called by gui interaction like buttons, timers. A second thread also calls the script using CSingleLock protection.

First, I tried this:

lua_State* start() {
  lua_State* L = luaL_newstate(); 
  luaL_loadfile(L, "script.lua");
  lua_pcall(L, 0, LUA_MULTRET, 0);
  int count(0);
  lua_sethook(L, trace, LUA_MASKLINE, count);
  return L;
}

void trace(lua_State *L, lua_Debug *ar) {
  // display debug info 
  while (blocked) {}
}

Here are some example calls to the script:

// called repeatedly by the gui
void timer() {
   lua_lock();
   lua_getglobal(L,"timer"); 
   lua_pcall(L, 0, 0, 0);
   lua_unlock();
}

// called often by another thread
int gettemperature() {  
  lua_lock();
  lua_getglobal(L,"gettemperature");
  lua_pcall(L, 0, 1, 0);
  int temperature = lua_checknumber(L, -1);
  lua_unlock();
  return temperature;
}

Because the while loop blocks the gui I tried to use coroutines:

lua_State* start() {
   lua_State* L = luaL_newstate(); 
   lua_State* co = luaL_newthread(L); 
   luaL_loadfile(co, "script.lua");
   lua_resume(co, L, 0);
   int count(0);
   lua_sethook(L, trace, LUA_MASKLINE, count);
   return co;
}

and

void trace(lua_State *L, lua_Debug *ar) {
   // display debug info 
   lua_yieldk(L, 0, 0, 0);
}

A next step button press should reactivate the script using lua_resume. Unfortunatley there's this error: "attempt to yield from outside a coroutine".