mymodule.c

#include <lua.h>
#include <lauxlib.h>
#include <stdio.h>

static int greeting_key = LUA_NOREF;

static int mymodule_greet(lua_State *L)
{
    const char *greeting;

    if(greeting_key == LUA_NOREF) {
        greeting = "Hello!";
    } else {
        lua_rawgeti(L, LUA_REGISTRYINDEX, greeting_key);
        greeting = lua_tostring(L, -1);
        lua_pop(L, -1);
    }
    printf("%s\n", greeting);
    return 0;
}

static int mymodule_configure(lua_State *L)
{
    lua_pushvalue(L, 2);
    greeting_key = luaL_ref(L, LUA_REGISTRYINDEX);
    lua_pushvalue(L, 1);
    return 1;
}

int luaopen_mymodule(lua_State *L)
{
    lua_newtable(L);

    lua_pushcfunction(L, mymodule_greet);
    lua_setfield(L, -2, "greet");

    lua_newtable(L);
    lua_pushvalue(L, -1);
    lua_setmetatable(L, -3);

    lua_pushcfunction(L, mymodule_configure);
    lua_setfield(L, -2, "__call");

    lua_pop(L, 1);

    return 1;
}