how to callback a lua function from a c function

Question

I have a C function (A) test_callback accepting a pointer to a function(B) as the parameter and A will "callback" B.

//typedef int(*data_callback_t)(int i);
int test_callback(data_callback_t f)
{
    f(3);   
}


int datacallback(int a )
{
    printf("called back %d\n",a);
    return 0;
}


//example 
test_callback(datacallback); // print : called back 3

Now, I want to wrap test_callback so that they can be called from lua, suppose the name is lua_test_callback ;and also the input parameter to it would be a lua function. How should I achieve this goal?

function lua_datacallback (a )
    print "hey , this is callback in lua" ..a
end


lua_test_callback(lua_datacallback)  //expect to get "hey this is callback in lua 3 "

EDIT:

This link provide a way to store the callback function for later use .

//save function for later use 
callback_function = luaL_ref(L,LUA_REGISTRYINDEX);


//retrive function and call it 
lua_rawgeti(L,LUA_REGISTRYINDEX,callback_function);
//push the parameters and call it
lua_pushnumber(L, 5); // push first argument to the function
lua_pcall(L, 1, 0, 0); // call a function with one argument and no return values

sbk · Accepted Answer · 2010-04-22 16:29:50Z

9

I'm not sure I understand your question, if you are asking what would lua_test_callback look in C, it should be something like this

int lua_test_callback(lua_State* lua)
{
    if (lua_gettop(lua) == 1 && // make sure exactly one argument is passed
       lua_isfunction(lua, -1)) // and that argument (which is on top of the stack) is a function
    {
        lua_pushnumber(lua, 3); // push first argument to the function
        lua_pcall(lua, 1, 0, 0); // call a function with one argument and no return values
    }
    return 0; // no values are returned from this function
}

You cannot just wrap test_callback, you need entirely different implementation to call Lua functions.

(edit: changed lua_call to lua_pcall as suggested by Nick. I still omitted any error handling for brevity)

edited Apr 22, 2010 at 16:29

answered Apr 22, 2010 at 8:51

sbk

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2 Comments

Nick Van Brunt Over a year ago

It is preferable to use lua_pcall which will push an error onto the stack rather than just crashing.

topright gamedev Over a year ago

Nice correct example. But in practice it is better to use common, well-tested interface, despite of writing such lua_test_callback() functions again and again.

topright gamedev · Accepted Answer · 2010-04-22 18:08:22Z

The convenient way of doing calls to different Lua functions with different signatures:

A. Make a class that will maintain Lua state safely and will provide easy interface. Do not write calls to Lua functions from scratch (with a lot of push/pop work and asserts) again and again - just use this class interface. This is safe, fast and convenient approach.

B. Define push and pop methods to push/pop arguments on/from Lua stack:

template<typename T> void push(T argument);
template<typename T> void get(const int index, T& return_value);

template<> void State::push(bool arg)
{
  lua_pushboolean (lua_state, arg ? 1 : 0);
}

template<> void State::push(float arg)
{
  lua_pushnumber (lua_state, arg);
}

template<> void State::push(int arg)
{
  lua_pushnumber (lua_state, arg);
}

// ...
template<> void State::get(const int index, bool& ret)
{
      if (!lua_isboolean(lua_state, index)) { ... }
      ret = lua_toboolean(lua_state, index) != 0;
}

C. Define functions to call Lua functions:

// Call function that takes 1 argument and returns nothing
template <typename A1>
void call(const char * funcName, A1 arg1)
{
  lua_getfield (lua_state, LUA_GLOBALSINDEX, funcName);       // push global function f on stack
  push (arg1);                                            // push first argument on stack
  assert_call(    lua_pcall(lua_state, 1, 0, this->err_h) );      // call function taking 1 argument and getting no return value
}


// call function that takes 2 argument and returns 1 value
template <typename R1, typename A1, typename A2>
void callr1(const char * funcName, R1& res, A1 arg1, A2 arg2)
{
  lua_getfield (lua_state, LUA_GLOBALSINDEX, funcName);       // push global function f on stack
  push (arg1);                                            // push first argument on stack
  push (arg2);
  assert_call(    lua_pcall(lua_state, 2, 1, this->err_h) );      // call function taking 2 arguments and getting 1 return value
  get  (-1, res);
  lua_pop(lua_state, 1);
}

D. Set error handler (lua_pcall will call this Lua function if error)

void setErrorHandler(const char * funcName)
{
  lua_getfield (lua_state, LUA_GLOBALSINDEX, funcName);
  this->err_h = lua_gettop(lua_state);
}

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