Callbacks using a 3rd party library in C

[Michael Roth <mroth@...>]

Hello,

I need some help or ideas for the following problem:

Assume a third party C library to wich I can register any number of 
callbacks that this library will sometimes call, like:

foo * f = foo_open(...);

foo_reg_cb(f, "nameA", <void *userdata>, <void *callback(void *)> );
foo_reg_cb(f, "nameB", <void *userdata>, <void *callback(void *)> );
...

I can register as many callbacks as I would like. Every callback is 
given a unique name.

Now, somewhere later, the foo-library will use the callbacks during a 
heavy computation. The call to the libray is somthing like:


int l_foo_open(lua_State * L)
{
  foo * f = foo_open(lua_tostring(L, 1), ...);
  lua_pushlightuserdata(L, f);
  return 1;
}

int l_foo_compute(lua_State * L)
{
  foo * f = (foo *)lua_touserdata(L, 1);
  foo_compute(f);
  return 0;
}


Now the problem:

How can I write a wrapper arround foo_reg_cb() to register lua 
functions? This will not work:


static int callback_wrapper(void *userdata)
{
  lua_State * L = (lua_State * userdata);
  /* PROBLEM: how to find the the function to call ???
  lua_call(L, 0, 0);
  return 0;
}

int l_foo_reg_cb(lua_State * L)
{
  foo * f = (foo *)lua_touserdata(L, 1);
  char * name = lua_tostring(L, 2);
  /* PROBLEM: What todo with the 3rd arg, the lua callback function? */
  foo_reg_cb(f, name, L, callback_wrapper);
  return 0;
}


How should I deal with this problem? I don't like the idea of a global 
lua_State and pass the function itself or some reference to the wrapper, 
because then it won't be possible to use this binding with two or more 
different lua-States in the same host.

The Problem is, that there is not an fixed number of callbacks, so that 
 I can't write N callback_wrappers.

Any Ideas?

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