static int cf_tolua(lua_State *L) {
    lua_pushliteral(L, "hello");
    return 1;
}

static int cf_fromlua(lua_State *L) {
    const char *str = lua_tostring(L, 1); // first argument passed in; second would be at index 2
    printf("%s\n", str);
    return 0;
}

static const luaL_Reg my_funcs[] = {
    { "tolua", cf_tolua },
    { "fromlua", cf_fromlua },
    { NULL, NULL }
};

// run this before running your scripts
void register_my_funcs(void) {
    lua_getglobal(L, "package");
    lua_getfield(L, -1, "loaded");
    lua_newtable(L);
    luaL_register(L, NULL, my_funcs);
    lua_setfield(L, -2, "cfuncs");
}