The lua C api provides a function lua_pushcfunction which allows you to take a function pointer int (*)(lua_State *) and pass it to lua, where scripts can use it like a lua function. When a script calls such a function, your function pointer is called, and the arguments appear on the stack. You have to explicitly use lua_istring etc. to test and obtain the arguments, when using the pure C api.

int
is_pythag(lua_State * L) {
  int a = luaL_checkinteger(L, 1);
  int b = luaL_checkinteger(L, 2);
  int c = luaL_checkinteger(L, 3);

  lua_pushboolean(L, (a * a) + (b * b) == (c * c));
  return 1;
}

  // ...

  lua_pushcfunction(L, &is_pythag);

When using primer, you can automate that to some extent, and pass C++

// functions directly to lua.

primer::result
new_is_pythag(lua_State * L, int a, int b, int c) {
  primer::push(L, (a * a) + (b * b) == (c * c));
  return 1;
}

  // ...

  lua_pushcfunction(L, PRIMER_ADAPT(&new_is_pythag));

PRIMER_ADAPT is used to "convert" the primer signature primer::result (*)(lua_State *, ...) to the lua signature int (*)(lua_State *). PRIMER_ADAPT generates the boiler-plate which reads the arguments off of the stack, using primer::read to do the work. That is: If you implement primer::read for your custom data types, then they can be arguments to functions which are adapted this way. See the reference pages for details.