tk's blog

用 C 扩展写 Lua closure

tk posted @ May 30, 2012 07:30:28 PM in Lua , 4093 阅读

这个是《programming in lua》中的有的一个例子,通过 closure(闭包)来实现一个 tuple(元组)。


这里的实现将元组表示为函数,元组的值存放在函数绑定的 upvalue 中。大概的 Lua 代码示意如下:

local tuple = {}
    local t = {...}
    return function(index)
        if index then
            if type(index) ~= "number" then error("invalid index type") end
            if index < 0 then error("index is out of range") end
            return t[index]            
            return unpack(t)

闭包工厂 中的 table t 就是用于存放元组内容的 upvalue,这个 tuple 的使用如下:

local x =, "hi", {}, 4)


10 hi table: 0041B870 4
下面使用 Lua 的 C 扩展来实现这个 tuple:
#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>

static int
t_tuple(lua_State *L) 
    int op = luaL_optint(L, 1, 0);
    if (op == 0) { /* 无参数? */
        int i;
        /* 将所有合法 upvalue 压入栈中 */
        for (i = 1; !lua_isnone(L, lua_upvalueindex(i)); i++)
            lua_pushvalue(L, lua_upvalueindex(i));

        return i - 1; /* 栈中值的数量 */
    else { /* 获取 'op' 字段 */
        luaL_argcheck(L, 0 < op, 1, "index out of range");
        if (lua_isnone(L, lua_upvalueindex(op))) { /* 无此字段 */
            lua_pushnil(L);  /* 返回 nil 表明不存在 */
            return 1;
        lua_pushvalue(L, lua_upvalueindex(op));
        return 1;

static int 
t_new(lua_State *L)
    lua_pushcclosure(L, t_tuple, lua_gettop(L));
    return 1;

static const struct luaL_Reg tuplelib[] = {
    {"new", t_new},
    {NULL, NULL}

int luaopen_tuple(lua_State *L)
    luaL_register(L, "tuple", tuplelib);
    return 1;

编译成“tuple.dll”(windows 下)然后仍然用上面的 tuple 使用代码测试,不过要先“require 'tuple'”来加载这个扩展。DLL 中的需要导出 lutopen_tuple 这个函数符号。

t_new 是创建元组的工厂函数。将需要作为 upvalue 的参数实现压入栈中,然后调用 lua_pushcclosure 来创建一个基于 t_tuple 的 closure 。lua_gettop 返回栈的大小,在这里也就等于已经压入栈中作为 upvalue 的数量。lua_upvalueindex 是一个用来索引 upvalue 的宏,当索引一个不存在的 upvalue ,结果会是一个类型为 LUA_TNONE 的伪值(pseudo-value),当索引超出当前栈范围时候,也会得到这样的 LUA_TNONE 的伪值。不能以负数来调用 lua_upvalueindex ,所以做了范围检查,luaL_argcheck(L, 0 < op, 1, "index out of range") 第二个参数是个条件表达式,第三个是检查的参数数量,最后一个是检查失败传递的消息。

用于检测可选参数的函数是 luaL_optint ,它允许参数不存在,如果不存在,则返回一个指定的默认值,也就是最后一个参数。

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