lua5.2参考手册—keyring (译) keyrings@163.com
Lua 5.2 参考手册
by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, Waldemar Celes
Copyright © 2011–2013 Lua.org, PUC-Rio. Freely available under the terms of the Lua license.
keyring (译) keyrings@163.com
目录
- 1 – 简介
- 2 – 基本概念
- 3 – 语言
- 4 – 应用程序接口
- 4.1 – 栈
- 4.2 – 栈的大小
- 4.3 – 有效索引与合法索引
- 4.4 – C 闭包
- 4.5 – 注册表
- 4.6 – C 中的错误处理
- 4.7 – C 中的挂起处理
- 4.8 – 函数与类型
- 4.9 – 调试接口
- 5 – The Auxiliary Library
- 6 – Standard Libraries
- 6.1 – Basic Functions
- 6.2 – Coroutine Manipulation
- 6.3 – Modules
- 6.4 – String Manipulation
- 6.5 – Table Manipulation
- 6.6 – Mathematical Functions
- 6.7 – Bitwise Operations
- 6.8 – Input and Output Facilities
- 6.9 – Operating System Facilities
- 6.10 – The Debug Library
- 7 – Lua Standalone
- 8 – Incompatibilities with the Previous Version
- 9 – The Complete Syntax of Lua
索引
1 – 简介
Lua 是一门扩展型程序设计语言，用于辅助一般的过程式编程。当然，它也对面向对象编程、函数式编程和数据驱动式编程提供良好的支持。Lua 作为一门强大、轻量、可嵌入式的脚本语言能供任何需要的程序使用。Lua 是由 clean C（标准 C和C++的一个共通子集）实现的一个库。
作为一门扩展型语言，Lua没有“main”程序的概念：它只能嵌入宿主程序，宿主程序被称为 embedding program 或者简称 host。宿主程序可以调用函数执行一段 Lua 代码，可以读写 Lua 变量，甚至可以注册 C 函数以在Lua代码中调用。通过对 C 函数的使用，Lua足以应付各种领域，完全可以共享一个句法框架而定制不同的程序语言。Lua官方发布版包含一个叫 lua 的宿主程序示例，它是一个利用Lua库写成的完整独立的Lua解释器，一般用于交互和批处理。
Lua 作为自由软件,就像license里面所说，使用它是不需要抵押品的 :)。本手册所描述的实现可以在 Lua 官方网站 www.lua.org 中找到。
就像任何参考手册一样，这份文档某些地方有点枯燥。关于 Lua 背后设计思想的探讨，可以看看 Lua 官方网站上的技术论文。至于Lua编程的详细介绍，请参阅Roberto的书， Programming in Lua（当前最新是第三版）。
2 – 基本概念
本章节介绍 Lua 语言里的基本概念。
2.1 – 值与类型
Lua 是一门动态类型语言。这意味着变量没有类型，只有值。Lua 没有类型定义，由值本身携带自己的类型信息。
Lua 中的所有值均是 first-class values（第一类值）。这意味着所有的值均可存于变量中，也可作为参数传递给其他函数，也可作为函数结果被返回。
Lua中有八种基本类型： nil（空）、boolean（布尔）、number（数字）、string（字符串）、function（函数）、userdata（用户数据）、thread（线程）和 table（表）。
Nil 类型只有值 nil，主要用于标识与其他值的不同；通常代表无意义的值。
Boolean 类型只有两种值 false 和 true。 nil 和 false 都表示条件为假；而其他任何值均表示为真。
Number 表示实数（双精度浮点数）。数字间的运算操作与底层C实现遵循相同的规则，一般就是IEEE754标准。（使用内部其他数字类型如单精度浮点或长整型重新编译一个Lua解释器也非常容易；参见 luaconf.h 文件。）
String 代表字节串。Lua is 8-bit clean：字符串可以包含任意8-bit值，包括 '\0' 。
Lua 可以调用（操作）由 Lua 和 C 写成的 Function （参见 §3.4.9）。
userdata 类型用来将任意 C 数据保存在 Lua 变量中。 一个 userdata 类型的值就是一块原生内存的指针。 userdata 分两类：full userdata 和 light userdata ，前者的内存块由 Lua 管理；后者的内存块由 host 管理。 Userdata 在 Lua 中除了赋值与鉴定测试便没有其他预定义操作了。但是通过使用 metatables（元表），程序员可以为 full userdata 类型的值自定义操作（参见 §2.4）。 Userdata 类型的值不能在 Lua 中创建与修改，只能通过 C API。这样保证了宿主程序能完全掌管其中的数据。
thread 代表独立的执行线程，用于实现 coroutines（协同程序）（参见 §2.6）。不要把 Lua 线程与操作系统线程搞混。Lua 在所有系统上均支持 coroutines，即使系统并不支持 threads 。
table 类型实现了关联数组。也就是说，该数组可以用任何 Lua 值（除了nil 和 NaN）作索引，而不仅限于数字。 Tables 可以包含所有类型的值（除了nil）。任何带 nil 值的键都不是 table 的一部分。相应的，任何不属于 table 的键都关联一个 nil 值。
Table 是 Lua 中唯一的数据组织机制，可以用于表示普通数组、序列、符号表、集合、记录、图、树等等。在表示记录时，Lua 以域作为索引。语言支持以 a.name 来表示 a["name"]，这只是一种语法糖。table 的创建方式有很多种（参见 §3.4.8）。
我们使用 序列 这个词来表示一个递增表，其索引为 {1..n} ，其中的 n 表示序列的长度（参见 §3.4.6）。
与索引一样，table 中的域也可为任意类型。特别的，由于函数也是 first-class values （第一类值/一等公民），所以 table 的域也可包含函数。从而 table 也能携带 methods （参见 §3.4.10）。（译注：这为面向对象提供了基础）
table 的索引遵循语言中定义的原生相等性。表达式 a[i] 与 a[j] 只在 i 与 j 原生相等（指无元方法的相等）时才表示同一个表元素。
Table、function、thread 和 (full) userdata 类型的值均是 对象：变量实际上不含有 值，只是引用 值。赋值、参数传递和函数返回等操控的只是值的引用；这些操作不会做任何性质的拷贝。
库函数 type 可以返回一个给定值的类型（参见 §6.1）。
2.2 – 环境与全局变量
就像在 §3.2 和 §3.3.3 讨论的那样，任何对全局名称 var 的引用均会被翻译成 _ENV.var。此外，任何在局部变量之外编译的程序块均叫作 _ENV（参见 §3.3.2）， 所以 _ENV 本身在程序块中从来都不是全局名称。
尽管存在外部 _ENV 变量用于全局名称的翻译，_ENV 本身其实是一个完全符合命名规则的名称。你完全可以定义新的变量/参数并取这个名字。每一次使用 _ENV 对全局名称的引用在当时对程序是可见的，完全遵循 Lua 通用可见性规则（参见 §3.5）。
任何被存储 _ENV 值的表叫作 environment（环境）.
Lua 维持一个叫 global environment（全局环境）的特殊环境。这个值在 C 注册表（参见 §4.5）表示一个特殊索引。在 Lua 中，变量 _G 被初始化为相同值。
当 Lua 编译一个 chunk（程序块）时，会先将其中 _ENV 的值提升为全局环境（参见 load）。因此，默认情况下，Lua代码中的全局变量一般指的是全局环境里的入口。此外，加载到全局环境下的所有标准库和部分函数均是在该环境下运作。你可以使用 load （或者 loadfile）来加载一个不同环境下的 chunk。
如果你改变了注册表中的全局环境（通过 C 代码或者 debug 库），在改变之后加载的 chunk 将获得新的环境。先前加载的 chunk 虽不受影响，但都要在自身的 _ENV 变量中引用到新环境去。此外，Lua 从不更新变量 _G（存于原始全局环境）。
2.3 – 错误处理
由于 Lua 是一门嵌入式扩展语言，所有的 Lua 行为均源于宿主程序 C 代码对 Lua 库的函数调用（参见 lua_pcall）。无论何时 Lua 程序块发生编译/执行错误，控制权均会交给宿主程序，由宿主程序触发恰当的措施（比如打印一条出错信息）。
Lua 代码可以通过调用 error 函数明确产生错误信息。如果需要在 Lua 中 捕捉错误，可以使用 pcall 或 xpcall 来在 protected mode（保护模式）下调用一个处理函数。
每产生一个错误，就会传播一个包含该错误信息的 error object（错误对象） （也叫 error message（错误消息））。Lua 本身只为错误对象产生一条字符串错误信息，而程序可以为错误对象产生任何形式的错误信息。
当使用 xpcall 或者 lua_pcall时，可以传递一个 message handler（消息处理程序）以在错误出现时调用。这个程序由原始错误消息调用并返回一条新的消息。它在错误离开栈之前被调用，故能收集更多关于错误的信息，比如检查堆栈和创建堆栈回溯。这个消息处理程序在保护模式下被调用，自身也处在保护中，因此，消息处理中出现错误的话将再次调用消息处理（也就是自己）。如果发生了这种循环，将由 Lua 中断并返回一段恰当的消息。
2.4 – 元表与元方法
Lua 中的每个值均能拥有一个metatable（元表）。这个 metatable 就是普通的 Lua table，然后定义了一些原始值在特定操作下的行为。你可以通过在一个值的元表中设置特定的域来改变操作行为。例如，当一个非数字的值要做加法操作，Lua 会查询该值的元表中 "__add" 域中的函数，如果找到一个函数，Lua 则调用该函数来执行加法。
元表中的键命名为 event（事件）；对应的值叫 metamethods（元方法）。在刚才的例子中，事件是 "add"，元方法则是那个执行加法操作的函数。
你可以使用 getmetatable 函数来查询任何值的 metatable（元表）。
你可以使用 setmetatable 函数来替换 table 的 metatable 。但不能在 Lua 中改变其他类型（比如 number、string类型）的 metatable（除非使用 debug 库），必须得使用 C API。
每个 table 和 full userdata 拥有独立的 metatable（虽然多个 table 和 userdata 也可共享它们的 metatable）。而所有其他类型的值，一种类型只有一个 metatable；意味着所有数字只有一个 metatable，所有字符串也一样，等等。默认情况下，单个值是没有 metatable 的，但字符串库为 string 类型设置了一个 metatable（参见 §6.4）。
一个 metatable 控制着一个对象在数学运算、比较、连接、取长和索引等操作的行为表现。它也能定义函数，让 userdata 或者 table 做垃圾收集时调用。当 Lua 需要对某个值执行其中一种操作时，会先检查值的 metatable 中是否有对应的事件。如果有，键对应的值（metamethod 元方法）将决定 Lua 如何执行操作。
Metatable 能控制的操作全列在了下面。各种操作用对应的名字区分。每种操作的键都是操作名前面加上两个下划线 （ '__' ）的字符串。比如 "add" 操作的键就是字符串 "__add"。
这些操作的含义由 Lua 函数来解释更准确，毕竟函数展示了解释器内部如何执行该操作。这里的 Lua 代码仅用作解释说明，实际的行为已经硬编码在解释器中，自然也比这些模拟代码高效。在下面代码使用到的 rawget, tonumber 等等函数可在 §6.1 找到。注意，我们使用下面一行表达式来从给定对象中提取元方法
metatable(obj)[event]
可被解读成
rawget(getmetatable(obj) or {}, event)
这意味着访问一个元方法不会调用其他元方法，而且访问没有 metatable 的对象也不会失败（仅仅导致 nil）。
对于一元操作符 - 和 #，元方法由虚拟的第二参数调用。这个额外的参数仅用于简化 Lua 的内建对象；在将来的版本中可能被移除，因此我们不会在下面讨论。（其实额外参数的大部分使用都是无关紧要的。）
- "add":
+操作。下面的
getbinhandler函数定义了 Lua 如何为二元运算选择处理方法。Lua 首先测试第一个操作数，如果它的类型没有为该运算操作定义处理方法，Lua 就会测试第二个操作数。function getbinhandler (op1, op2, event) return metatable(op1)[event] or metatable(op2)[event] end通过这个函数，
op1 + op2的行为就是function add_event (op1, op2) local o1, o2 = tonumber(op1), tonumber(op2) if o1 and o2 then -- 两个操作数均为数字？ return o1 + o2 -- 这里的 '+' 就是基本加法（ 'add'） else -- 至少有一个操作数不是数字 local h = getbinhandler(op1, op2, "__add") if h then -- 以两个操作数调用处理方法 return (h(op1, op2)) else -- 没有处理方法：默认行为 error(···) end end end - "sub"：
-操作， 行为类似 "add" 操作。 - "mul"：
*操作， 行为类似 "add" 操作。 - "div"：
/操作， 行为类似 "add" 操作。 - "mod"：
%操作， 行为类似 "add" 操作，原始操作是o1 - floor(o1/o2)*o2。 - "pow"：
^（幂）操作。 行为类似 "add" 操作，原始操作是pow函数（来自 C math 库）。 - "unm"： 一元
-操作。function unm_event (op) local o = tonumber(op) if o then -- operand is numeric? return -o -- '-' here is the primitive 'unm' else -- the operand is not numeric. -- Try to get a handler from the operand local h = metatable(op).__unm if h then -- call the handler with the operand return (h(op)) else -- no handler available: default behavior error(···) end end end - "concat"：
..（连接）操作。function concat_event (op1, op2) if (type(op1) == "string" or type(op1) == "number") and (type(op2) == "string" or type(op2) == "number") then return op1 .. op2 -- primitive string concatenation else local h = getbinhandler(op1, op2, "__concat") if h then return (h(op1, op2)) else error(···) end end end - "len"：
#操作。function len_event (op) if type(op) == "string" then return strlen(op) -- primitive string length else local h = metatable(op).__len if h then return (h(op)) -- call handler with the operand elseif type(op) == "table" then return #op -- primitive table length else -- no handler available: error error(···) end end end关于 table 的长度请参阅 §3.4.6。
- "eq"：
==操作。 函数getequalhandler定义了 Lua 如何选择元方法进行相等操作。只有当两个被比较的值类型相同，而且在当前操作下拥有相同的元方法，而且不属于 table 或 full userdata 间的一种时，被选出的元方法才会有效。function getequalhandler (op1, op2) if type(op1) ~= type(op2) or (type(op1) ~= "table" and type(op1) ~= "userdata") then return nil -- different values end local mm1 = metatable(op1).__eq local mm2 = metatable(op2).__eq if mm1 == mm2 then return mm1 else return nil end end"eq" 事件定义如下：
function eq_event (op1, op2) if op1 == op2 then -- primitive equal? return true -- values are equal end -- try metamethod local h = getequalhandler(op1, op2) if h then return not not h(op1, op2) else return false end end注意结果常为 boolean 值。
- "lt"：
<操作。function lt_event (op1, op2) if type(op1) == "number" and type(op2) == "number" then return op1 < op2 -- numeric comparison elseif type(op1) == "string" and type(op2) == "string" then return op1 < op2 -- lexicographic comparison else local h = getbinhandler(op1, op2, "__lt") if h then return not not h(op1, op2) else error(···) end end end注意结果常为 boolean 值。
- "le":
<=操作。function le_event (op1, op2) if type(op1) == "number" and type(op2) == "number" then return op1 <= op2 -- numeric comparison elseif type(op1) == "string" and type(op2) == "string" then return op1 <= op2 -- lexicographic comparison else local h = getbinhandler(op1, op2, "__le") if h then return not not h(op1, op2) else h = getbinhandler(op1, op2, "__lt") if h then return not h(op2, op1) else error(···) end end end end注意，缺少 "le" 元方法时，Lua 就尝试 "lt"，并假定
a <= b等价于not (b < a)。就像其他比较运算符，其结果常为 boolean 值。
- "index"： 索引操作用于访问
table[key]。注意仅当key不在table中时，才会尝试元方法。（当table不是表时，里面没有键，所以会一直尝试元方法。）function gettable_event (table, key) local h if type(table) == "table" then local v = rawget(table, key) -- if key is present, return raw value if v ~= nil then return v end h = metatable(table).__index if h == nil then return nil end else h = metatable(table).__index if h == nil then error(···) end end if type(h) == "function" then return (h(table, key)) -- call the handler else return h[key] -- or repeat operation on it end end - "newindex"： 赋值给索引
table[key] = value。注意仅当key不在table中时，才会尝试元方法。function settable_event (table, key, value) local h if type(table) == "table" then local v = rawget(table, key) -- if key is present, do raw assignment if v ~= nil then rawset(table, key, value); return end h = metatable(table).__newindex if h == nil then rawset(table, key, value); return end else h = metatable(table).__newindex if h == nil then error(···) end end if type(h) == "function" then h(table, key,value) -- call the handler else h[key] = value -- or repeat operation on it end end - "call"： 当 Lua 调用一个值时调用。
function function_event (func, ...) if type(func) == "function" then return func(...) -- primitive call else local h = metatable(func).__call if h then return h(func, ...) else error(···) end end end
2.5 – 垃圾收集
Lua 实行自动内存管理。意味着你不用担心创建新对象时的内存分配，也不用担心对象不再需要时的内存释放。Lua 运行一个 garbage collector（垃圾收集器） 来自动管理内存，收集 dead objects（死亡对象）（指 Lua 不再访问的对象）。Lua 中使用的所有内存（对象）均被自动管理：strings、tables、userdata、functions、threads、internal structures 等等。
Lua 实现了一个增量标记-清除收集器。它使用两个数字来控制垃圾收集周期： garbage-collector pause 和 garbage-collector step multiplier。两者均使用百分点作为计数单元（比如，100在内在表示 为1个百分点）。
garbage-collector pause 控制着收集器在开始新周期前的停顿时长。数字越大收集器越不积极，停顿时间越长。小于100意味着收集器不停顿即开始新周期。200表示收集器在内存使用量达到原来的两倍时才会开启新的周期。
garbage-collector step multiplier 控制收集器与内存分配的相对速度。数字越大收集器越积极，增加的收集量也越大。小于100会让收集器工作的非常慢，甚至造成收集器永远无法结束当前周期。默认值为200，表示收集器以内存分配的两倍速度运行着。
如果你将 step multiplier 设置的非常大（比程序中可能使用到的最大字节数还要大10%），收集器的行为将类似 stop-the-world 收集器（即为了完成一个完整的垃圾收集周期而暂停与主程序的交互）。如果你同时又将 pause 设成200，那收集器就像老版本的Lua一样，等内存使用量达到 Lua 本身使用量的两倍才会做一次完全的垃圾回收。
你可以通过在 C 中调用 lua_gc 或者在 Lua 中调用 collectgarbage 来改变这些数字。你也可以通过这些函数来直接控制垃圾收集器（比如关闭、重启之类）。
作为 Lua 5.2 中的一个实验特性，你可以改变收集器的操作模式，从 incremental 改成generational。generational collector 假设大部分对象存活时间很短，因此收集器只会检测最近创建的新对象。这种行为能降低收集器的使用时间，但会增加内存使用量（逐渐累积老死的对象）。要解决第二个问题，可以让 generational collector 时常执行完整的回收。记住，这只是处于实验阶段的特性，欢迎尝试，看看效果咋样。
2.5.1 – 垃圾收集元方法
你可以为 table 设置垃圾收集元方法，也可使用 C API 为 full userdata 设置（参见 §2.4）。这些元方法也叫 finalizers（析构器）。析构器允许外部资源管理（比如关闭文件、关闭网络、关闭数据库连接或者释放自己的内存）配合 Lua 的垃圾收集。
对象（table 或者 userdata）需要被析构回收时，你必须 标记 它为可析构。只需为对象设置一张有 "__gc 域的元表即可标记对象为可析构。注意一点，如果你为对象先设置了一张没有 __gc 域的元表，然后在元表中新建一个 __gc 域，这个对象是没法被标记的。而在对象被标记后，你可以随意改变元表中的 __gc 域 。
当被标记对象变成垃圾时，垃圾收集器不会立马将它回收，而是由 Lua 将之放进一个列表中。收集结束后，Lua 对列表中每个对象执行下面函数的等价操作：
function gc_event (obj)
local h = metatable(obj).__gc
if type(h) == "function" then
h(obj)
end
end
每个垃圾收集周期的结尾，当前周期内收集到的垃圾对象会以被标记顺序的 反序 调用析构器。这意味着程序里最后被标记的对象最先析构。析构器的执行可能发生在普通代码执行期间的任何时候。
由于被回收了的对象还要被析构器使用，该对象会由 Lua resurrected（复活）。当然，这个复活是短暂的，而且对象的内存会在下一个垃圾收集周期释放掉。然而，如果析构器将对象存在某些全局位置（比如一个全局变量里面），那这就变成永久性复活了。总之，对象的内存只有在完全不可访问的情况下才能被释放；它的析构器也从不会被调用两次。
当关闭一个状态（参见 lua_close），Lua将对所有被标记为可析构的对象以标记的反序来调用析构器。如果刚好有新对象在这个阶段被标记，新对象是不会被析构的。
2.5.2 – 弱表
weak table 指表中元素被 weak references（弱引用）。而垃圾收集器会忽略弱引用，换言之，如果一个对象只有弱引用，垃圾收集器会将之回收。
弱表的键与值均可为 weak。拥有弱键的表，键会被回收而对应的值不会。而键与值都 weak 的话，收集器会将键与值都回收掉。而且，无论键与值谁被回收了，对应的键值对都会从表中移除。table 的 weak 性质受元表的 __mode 域控制。如果 __mode 域是一个含有字符 'k' 的字符串，则 table 中的键就是 weak 的；相应的，如果 __mode 域含有字符 'v'，table 中的值就是 weak 的。
拥有弱键与强值的表也叫 ephemeron table（短命表）。在这种表中，值的可访问性只依赖对应键的可访问性。尤其是键的唯一引用来自它的值时，这个键值对将被移除。
对表的 weak 性质做任何改变只会对下一个垃圾收集周期产生影响。而且，如果你改成更 strong（强）的模式，Lua 在更改真正生效之前仍然会回收表中某些项。
只有那些明确说明的对象才会从弱表中移除。数字和轻量 C 函数之类不受垃圾收集管理，因此不会从弱表中移除（除非它的关联值已被回收了）。虽然字符串受垃圾收集管理，但它们没有明确说明，因此也不会从弱表中移除。
复活的对象（指那些即将析构且只能由析构器访问的对象）在弱表中有一种特殊行为。它们在运行自身析构器之前会从弱值中移除，但只能在下一个垃圾收集周期从弱键中移除，此时才是真正的对象释放。这种行为就允许析构器在弱表中访问与对象相关的属性。
如果一个弱表在当前回收周期位于一个复活对象中，那只能在下一个周期里才能正确清除掉。
2.6 – 协程
Lua 支持 coroutines（协程），也叫 collaborative multithreading（协同多线程）。Lua 中的每个协程都在各自独立的线程中执行。不像多线程系统中的线程，协程只会在显示调用 yield 函数时挂起。
调用 coroutine.create 会创建一个协程，其参数就是协程的主函数。 这个 create 函数仅仅创建新的协程并返回句柄（一个类型为 thread 的对象），而不会启动协程。
调用 coroutine.resume 就开始执行协程。当你首次调用 coroutine.resume，需传入的第一个参数就是coroutine.create 返回的 thread。然后协程从它的主函数第一行开始执行。接下来传入 coroutine.resume 的其他参数均会传递给协程的主函数。协程一旦运行，直到终止或者挂起。
协程有两种方式终止执行：正常退出和异常退出，前者指由它的主函数返回退出（无论最后有没有显示 return），这时coroutine.resume 返回 true，并附带协程主函数的返回值；后者指发生未预防的错误，这时 coroutine.resume 返回 false，并附加一条错误消息。
调用 coroutine.yield 能使协程挂起。当协程发生挂起时，相应的 coroutine.resume 会立刻返回，即使挂起发生在内层函数调用中（这意味着不限于在主函数之内，其他受主函数直接/间接调用的函数之内也能发生）。协程的挂起，使coroutine.resume 同样返回 true，并附带那些传递给 coroutine.yield 的参数值。再次调用 coroutine.yield 将恢复该协程，并从挂起的断点处继续执行，其返回值则是传递给 coroutine.resume 的参数值。
类似 coroutine.create，coroutine.wrap 函数也能创建协程，但返回的不是协程本身，而是一个能重新获得协程的函数。传递给 wrap 函数的任何参数都将作为 coroutine.resume的额外参数。 coroutine.wrap 返回coroutine.resume 返回值中除第一个（boolean型错误代码）之外其他所有值。与 coroutine.resume不同的是， coroutine.wrap 不捕获错误；任何错误都得由调用者来传递。
协程如何运作，思考下面的示例：
function foo (a)
print("foo", a)
return coroutine.yield(2*a)
end
co = coroutine.create(function (a,b)
print("co-body", a, b)
local r = foo(a+1)
print("co-body", r)
local r, s = coroutine.yield(a+b, a-b)
print("co-body", r, s)
return b, "end"
end)
print("main", coroutine.resume(co, 1, 10))
print("main", coroutine.resume(co, "r"))
print("main", coroutine.resume(co, "x", "y"))
print("main", coroutine.resume(co, "x", "y"))
运行结果如下：
co-body 1 10
foo 2
main true 4
co-body r
main true 11 -9
co-body x y
main true 10 end
main false cannot resume dead coroutine
你也可以通过 C API 来创建和操作协程：参阅 lua_newthread、lua_resume、lua_yield 等函数。
3 – 语言
本章介绍 Lua 的词法、语法和句法。换言之，这一章介绍哪些 token（符号）是合法的，它们如何组合，以及组合后的含义。
语言的结构由常用的扩展BNF范式来表示，即 {a} 表示0或多个 a，[a] 表示 a 为可选项。非终结符（一般是语言里的语法成分）保持不变，关键字类似 kword，其余的终结符（语言字符集的基本字符）要像这样 ‘=’ 位于引号中。完整的 Lua 语法可在本手册最末处 §9 找到。
3.1 – 词法规定
Lua 是门 free-form 语言。除了名字与关键字间的分隔符，它将忽略词法元素（符号）之间的空白（包括换行）和注释。
Lua 中的 Names（名字） （也叫 identifiers（标识符））可以是任何字母、数字、下划线组成的非数字开头的字符串。标识符用于命名变量、表的域和标签。
下面是 Lua 保留的 keywords（关键字），不能用作名字：
and break do else elseif end
false for function goto if in
local nil not or repeat return
then true until while
Lua 区分大小写： and 是保留字，但 And 和 AND 则是两个不同的合法名字。一般约定，以下划线开头接大写字母的名字（比如 _VERSION）被保留用于 Lua 内部变量。
下面是其他符号：
+ - * / % ^ #
== ~= <= >= < > =
( ) { } [ ] ::
; : , . .. ...
Literal strings（文字串） 可以放在单双引号内，其中也可包含类似 C 中的转义序列： '\a' (响铃)， '\b' (退格)， '\f' (换页)， '\n' (换行)， '\r' (回车)， '\t' (横向制表)， '\v' (纵向制表)， '\\' (反斜线)， '\"' (双引号)， 和 '\'' (单引号)。 一个反斜线后跟一个换行符（\\n）会让字符串中产生一个换行符号（\n）。转义序列 '\z' 会跳过后面的空白字符，包括换行符；这个在将一段很长的文字串截成多行时非常有用。
文字串中的字节也可通过数字表示出来。使用转义序列 \xXX即可办到， XX 是两个十六进制数字；或者使用转义序列 \ddd，ddd 表示最多三个十进制数字的序列。（注意，如果要在转义之后跟一个数字，那斜线后必须写满三个数字，否则会有歧义。）Lua 中的字符串可以包含任意8-bit值，包括 '\0' 表示的零；
文字串也可由长括号的形式定义。左长括号由两个左方括号（[[）表示，在两个方括号之间可以有 n 个等号（=），等号代表级别。于是，0级左长括号就写作 [[，1级就写作 [=[。相对应的右长括号就是 ]] 和 ]=]。这主要用于长文字串，在相同等级（就是等号一样多）的左右长括号之内，可输入任何文本，不处理转义符，不受分行限制，忽略所有不对等级的嵌套长括号。任何行尾符（回车、换行、回车加换行、换行加回车）统统变成简单的换行。
Any byte in a literal string not explicitly affected by the previous rules represents itself. However, Lua opens files for parsing in text mode, and the system file functions may have problems with some control characters. So, it is safer to represent non-text data as a quoted literal with explicit escape sequences for non-text characters.
为了方便，左长括号后面紧跟的那个换行符将被忽略。例如，一个使用 ASCII 码的系统中（ 'a' 编码为 97，\n 编码为 10， '1' 编码为 49），下面五种形式表示相同的字符串：
a = 'alo\n123"'
a = "alo\n123\""
a = '\97lo\10\04923"'
a = [[alo
123"]]
a = [==[
alo
123"]==]
numerical constant（数值常量） 可以写成普通形式，也可以写成科学计数法（指数部分由字母 'e' 或 'E' 标记）。Lua 也支持十六进制常量，只需前面加上 0x 或者 0X。十六进制也接受二进制指数形式的小数（用字母 'p' 或 'P 标记）。以下是一些合法的数值常量写法：
3 3.0 3.1416 314.16e-2 0.31416E1
0xff 0x0.1E 0xA23p-4 0X1.921FB54442D18P+1
comment（注释） 由两横开始 (--)，可以出现在字符串之外的任何地方。如果跟在 -- 后面的不是一个左长括号，这就是一个 短注释，作用范围直到行末。否则就是一个 长注释，作用范围是同等级的右长括号。长注释常用于临时屏蔽代码块。
3.2 – 变量
变量是存储值的地方。Lua 中有三种：全局变量、局部变量和表的域。
一个单独的名字可以表示为一个全局变量或者一个局部变量（或者一个函数参数，这也是一种局部变量）：
var ::= Name
Name 就是 §3.1所定义的标识符。
任何没有显示声明为 local （参见 §3.3.7）的变量名均可假定为全局变量。局部变量的 lexically scoped（作用域）：定义在函数作用域（参见 §3.5）的局部变量可被其自由访问。
变量首次赋值前，默认值均为 nil。
方括号用于索引表：
var ::= prefixexp ‘[’ exp ‘]’
访问表的域的含义能被元表更改。而访问被索引的变量 t[i] 等价于调用 gettable_event(t,i)。（参见 §2.4 获取 gettable_event 函数的完整描述。该函数未在 Lua 中定义，也不能调用。这里仅仅用于阐述原理。）
语法 var.Name 也仅是 var["Name"] 的语法糖：
var ::= prefixexp ‘.’ Name
对全局变量 x 的访问等价于访问 _ENV.x。 由于按程序块编译所以，_ENV 本身并不是一个全局名称 （参见 §2.2）。
3.3 – 语句
Lua 支持的常见语句类似Pascal或者C。包括赋值语句、控制语句、函数调用和变量声明。
3.3.1 – 语句块
一个语句块就是一堆按顺序执行的语句：
block ::= {stat}
Lua 允许 empty statements（空语句），可以用分号隔开语句，也可用一个或者连续两个分号来作空语句块。
stat ::= ‘;’
左括号表示函数调用和赋值语句。这有时会让 Lua 的语法产生歧义。比如下面这段代码：
a = b + c
(print or io.write)('done')
有两种语法分析方式：
a = b + c(print or io.write)('done')
a = b + c; (print or io.write)('done')
现今的语法分析器经常以第一种方式解析这类写法。即把左括号解释成函数调用。要避免这种歧义，最好是在那些括号开头的语句前面写个分号：
;(print or io.write)('done')
一个语句块也可以用下面的方式写成一条语句（写在 do 和 end 之间）：
stat ::= do block end
显示的语句块对控制变量的作用域很有用。有时也用于在其他语句块中插入 return 语句（参见see §3.3.4）。
3.3.2 – 程序块
chunk（程序块）就是 Lua 中的编译单元。从语法上讲，程序块就是简单的语句块。
chunk ::= block
Lua 将程序块当作一个拥有可变参数的匿名函数来处理（参见 §3.4.10）。所以，程序块可以定义局部变量、接收参数并返回值。此外，这样的匿名函数被编译在一个叫 _ENV （参见 §2.2）的外部局部变量的作用域内。产生的函数常用 _ENV 作为唯一的自由变量，即便根本没用它。（译者注：说白了就是程序块相当于匿名函数，这些函数默认处于 _ENV 的命名空间中，使用时要写成 _ENV.*****。）
程序块可以存在单独的文件中，也可直接写在宿主程序的一个字符串中。执行程序块时，Lua 现将它预编译成虚拟机指令，然后由虚拟机的解释器来执行这些指令。
程序块也可被预编译成二进制形式；详情查阅程序 luac。源码形式和二进制形式的程序可以相互转换；Lua 会自动识别文件类型并做正确处理。
3.3.3 – 赋值
Lua 允许多重赋值。因此，赋值的语法规定等号左边是一串变量，右边是一串表达式。串中的元素用等号隔开：
stat ::= varlist ‘=’ explist
varlist ::= var {‘,’ var}
explist ::= exp {‘,’ exp}
表达式放在 §3.4 讨论。
赋值之前，会先根据左边变量的数量对右边值的数量进行调整。值多了，多余的将被丢弃；值少了，用 nil 补齐。如果右边最后是一个函数调用结尾，则在数量对齐之前用该函数的全部返回值替换它（除非函数调用位于括号内：参见 §3.4）。
赋值语句会首先执行所有表达式（两边），然后做纯粹的赋值。因此，下面这段代码
i = 3
i, a[i] = i+1, 20
把 a[3] 赋值为20而不是影响 a[4]，因为 a[i] 中的 i 在赋值为4之前已经确定（为3）。同样，这一行
x, y = y, x
能用来交换 x 和 y的值，而且
x, y, z = y, z, x
能循环交换 x，y，z 的值。
对全局变量和表的域赋值的含义能通过元表进行更改。对索引变量赋值，即 t[i] = val 等价于 settable_event(t,i,val)。（参见 §2.4 获取 settable_event 函数的完整描述。Lua 中并没有定义该函数，也不能调用。这里仅用于阐述原理。）
对全局变量赋值 x = val 等价于 _ENV.x = val （参见 §2.2）。
3.3.4 – 控制结构
if、while 和 repeat 等结构的意义和语法都很常见：
stat ::= while exp do block end
stat ::= repeat block until exp
stat ::= if exp then block {elseif exp then block} [else block] end
Lua 也有 for 语句，并有两种形式（参见 §3.3.5）。
控制结构中的条件表达式可以返回任何值。只有 false 和 nil 才被当作条件假。其他任何值都被当作条件真（尤其注意数字 0 和空字符串也为真）。
在 repeat–until 循环中，内部语句不在 until 关键字处结束，而是在条件后面结束。所以，条件中可以使用循环体内声明的局部变量。
goto 语句能将程序控制转移到一个标签处。从语法上讲，Lua 中的标签也被当作是语句：
stat ::= goto Name stat ::= label label ::= ‘::’ Name ‘::’
标签在被定义的语句块内全部可见，除非在某个嵌套语句块的嵌套函数内定义了一个同名的标签。一个 goto 只要不进入局部变量作用域，它能跳转到任意可见的标签上。
标签可空语句叫作 void statements，表示它们本身没有任何行为。
break 语句用于终止 while、repeat 或 for 循环，并忽略循环体中剩下的语句：
stat ::= break
break 能结束内层循环。
return 语句用于从函数或程序块（伪装成函数）中返回值。 函数可以返回多个值。所以 return 语句的语法是：
stat ::= return [explist] [‘;’]
return 语句只能写作语句块的最后一句。如果在语句块中间确实需要 return，得显示使用内部语句块。习惯写成 do return end，于是这个return 在内层语句块中成了最后一句（这完全符合前面的规定）。
3.3.5 – For 语句
for 语句有两种形式：数字形式和一般形式。
数字形式的 for 循环，控制量以等差数列变化来重复运行代码块。语法如下：
stat ::= for Name ‘=’ exp ‘,’ exp [‘,’ exp] do block end
block 从 name 等于第一个 exp 开始重复运行。不断以第三个 exp 作为步长直到等于第二个 exp 才停止循环。更准确的说，一条 for 语句类似于：
for v = e1, e2, e3 do block end
等价于如下代码：
do
local var, limit, step = tonumber(e1), tonumber(e2), tonumber(e3)
if not (var and limit and step) then error() end
while (step > 0 and var <= limit) or (step <= 0 and var >= limit) do
local v = var
block
var = var + step
end
end
注意事项：
- 所有三个控制表达式在循环前仅被求值一次。它们的结果必须为数字。
var、limit和step都是不可见变量。这里的名字仅用作解释说明。- 如果省略第三个表达式（步长），则步长默认为 1.
- 可以使用 break 来跳出 for 循环。
- 循环量
v是循环内部的局部变量：不能在 for 循环结束或跳出后继续使用它的值。如果确实需要该值，请在循环退出或跳出之前把值赋给另外的变量。
一般形式的 for 语句通过叫 iterators（迭代器） 的函数工作。每一次迭代，都会调用迭代器来产生一个新值。当新值为 nil 时，循环停止。一般形式的 for 循环语法如下：
stat ::= for namelist in explist do block end
namelist ::= Name {‘,’ Name}
一条 for 语句类似于
for var_1, ···, var_n in explist do block end
等价于如下代码：
do
local f, s, var = explist
while true do
local var_1, ···, var_n = f(s, var)
if var_1 == nil then break end
var = var_1
block
end
end
注意事项：
explist仅被求值一次。其结果有三：一个 iterator（迭代器） 函数，一个 state（状态）和首个 iterator variable（迭代量）的初始值。f、s和var都是不可见变量。这里的名字仅用作解释说明。- 可以使用 break 来跳出 for 循环。
- 循环量
var_i是循环体内的局部变量：不能在 for 循环结束后继续使用它的值。如果确实需要该值，请在循环退出或跳出之前把值赋给另外的变量。
3.3.6 – 函数调用作为语句
为了允许使用可能的副作用，函数调用可当作语句执行：
stat ::= functioncall
这种情况下，所有的返回值均被丢弃。关于函数调用的说明在 §3.4.9。
3.3.7 – 局部声明
局部变量可在语句块内任何地方声明。声明的同时可以进行初始化赋值操作：
stat ::= local namelist [‘=’ explist]
如果做了，初始化赋值操作在语法上等同于多重赋值（see §3.3.3）。否则，所有变量均被初始化为 nil。
程序块也是语句块（参见 §3.3.2），因此局部变量可在任何显示语句块之外的程序块中声明。
局部变量的可见性规则在 §3.5 说明。
3.4 – 表达式
Lua 中的基本表达式如下：
exp ::= prefixexp exp ::= nil | false | true exp ::= Number exp ::= String exp ::= functiondef exp ::= tableconstructor exp ::= ‘...’ exp ::= exp binop exp exp ::= unop exp prefixexp ::= var | functioncall | ‘(’ exp ‘)’
数字和文字串的说明在 §3.1； 变量的说明在 §3.2； 函数定义的介绍在 §3.4.10； 函数调用的说明在 §3.4.9； 表的构造器在 §3.4.8。 由三点（'...'）表示的可变参数表达式只能用于有可变参数的函数中：详见 §3.4.10。
二元运算包括算术运算（§3.4.1）、关系运算（§3.4.3）、逻辑运算（§3.4.4）以及连接运算（§3.4.5）。一元运算包括取负（§3.4.1）、not（§3.4.4）和 取长度 （§3.4.6）。
函数调用和参数可变的表达式均可返回多个值。如果将函数调用作为一条语句使用（参见§3.3.6），则其返回值列表与0对齐，即丢弃所有返回值。如果一个表达式是一串表达式中最后的/唯一的元素，则不会有对齐操作（除非该表达式位于括号内）。在其他情况，Lua 会将返回结果表与1对齐，即丢弃第一个返回值外的其他值；如果没有返回值，Lua 自动添加一个 nil。
以下是一些示例：
f() -- 调整成 返回0 个结果
g(f(), x) -- f() 被调整成返回 1 个结果
g(x, f()) -- g 被传入 x 和所有 f() 返回值
a,b,c = f(), x -- f() 被调整成返回 1 个结果（c 就成了 nil）
a,b = ... -- a 被赋值为可变参数中的第一个，
-- b 为第二个（如果可变参数没有对应参数，a 和 b 都为 nil ）
a,b,c = x, f() -- f() 被调整成返回 2 个结果
a,b,c = f() -- f() 被调整成返回 3 个结果
return f() -- 返回 f() 所有结果
return ... -- 返回可变参数接收到的所有参数
return x,y,f() -- 返回 x、y、和所有 f() 返回值
{f()} -- 为 f() 的所有返回值创建一个列表
{...} -- 为可变参数接收到的所有参数创建一个列表
{f(), nil} -- f() 被调整成返回 1 个结果
任何由括号括起来的表达式只被当成一个值。因此，(f(x,y,z)) 永远都只能是个单值，即使 f 返回了多个值。（(f(x,y,z)) 的值是 f 返回的第一个值；如果 f 没有返回值，则为 nil。）
3.4.1 – 算术运算符
Lua 支持常用算术运算符： +（加）、-（减）、*（乘）、/（除）、%（取模）和 ^（幂）；还有一元-（取负）操作。如果运算对象是数字，或是可转换为数字的字符串（参见 §3.4.2），那就是单纯的数学运算。求幂操作可用于任何指数，比如 x^(-0.5) 计算出 x 平方根的倒数。取模操作的定义是
a % b == a - math.floor(a/b)*b
That is, it is the remainder of a division that rounds the quotient towards minus infinity.
3.4.2 – 强制转换
Lua 在运行时提供字符串与数字之见的自动转换。在 Lua 的词法分析下，任何适用字符串的算术运算都会尝试把字符串转化为数字。（字符串首尾可能有空白和标记。）反过来，任何需要字符串的时候，数字就会被转化成字符串。想要完全掌控数字转化字符串，可以使用 string 库中的 format 函数（参见 string.format）。
3.4.3 – 关系运算符
Lua 中的关系运算符有
== ~= < > <= >=
运算结果为 false 或者 true.
相等判断（==）首先比较运算对象的类型。如果类型不同，则结果为 false。否则才比较对象的值。数字、字符串用常规方式比较。而表、用户数据和线程得靠引用来比较：两个对象只在指向（引用）同一个对象才判为相等。每一次创建的新对象（表、用户数据或者线程）都与前面存在的对象不同。对相同引用的闭包互相相等。而任何可检测出不同（行为不同、定义不同）的闭包自然也不相等。
当然，你可以使用 "eq" 元方法（参见 §2.4）来改变表与用户数据的比较操作。
§3.4.2 的转换规则不适用于相等性判断操作。因此， "0"==0 值为 false。t[0] 和 t["0"] 代表的是表中不同的元素。
~= 操作符刚好与 == 意义相反。
大小比较的规则如下：如果比较参数都为数字，则做数字比较；如果都为字符串，则按字符串比较规则进行；否则，Lua 就尝试使用 "lt" 或 "le" 元方法（参见 §2.4）。a > b 被翻译成 b < a，a >= b 被翻译成 b <= a。
3.4.4 – 逻辑运算符
Lua 中的逻辑运算符有 and、or、和 not。类似控制结构（参见 §3.3.4），所有的逻辑操作符只将 false 和 nil 当作假，其他所有情况都为真。
取反操作 not 只返回 false 或者 true。与操作 and 在第一个参数为 false 或 nil 时返回第一个参数，否则返回第二个参数。或操作 or 在第一个参数不是 nil 和 false 时返回第一个参数。否则返回第二个参数。and 和 or 都使用短路求值法；意思是第二个参数只在必要时才求值。下面是些例子：
10 or 20 --> 10
10 or error() --> 10
nil or "a" --> "a"
nil and 10 --> nil
false and error() --> false
false and nil --> false
false or nil --> nil
10 and 20 --> 20
（本手册中的 --> 符号指代前面表达式的结果。）
3.4.5 – 连接操作
Lua 使用两个点（'..'）表示字符串连接操作。如果操作对象是字符串和数字，则会按照 §3.4.2 提到的规则将他们转化为字符串。不然就调用 __concat 元方法（参见 §2.4）。
3.4.6 – 取长度操作符
取长度的操作符是一元运算符 #。字符串的长度是其字节数（就是一个字符表示一个字节算出的长度）。
程序里可通过 __len 元方法（参见 §2.4）来修改除字符串外所有值的取长度操作行为。
除了已知的 __len 元方法，表 t 的长度只在它是顺序表时有定义。而且其键值要按 {1..n} 递增，则整数 n 就是长度。注意像下面这样的表
{10, 20, nil, 40}
不是顺序表，因为它有第 4 个键，却没有第 3 个键。而非数字键则不影响判断表是否为顺序表。
3.4.7 – 优先级
Lua 中操作符优先级如下表所示，从低到高排列：
or
and
< > <= >= ~= ==
..
+ -
* / %
not # - (unary)
^
通常，可以在表达式中使用括号来改变优先级。连接符（'..'）和求幂（'^'）从右往左结合。其他二元运算符是从左往右的。
3.4.8 – 表的构造器
表的构造器是一对用于创建表的表达式。每执行一次构造器就创建一张新表。构造器可用于创建空表或者创建的同时初始化一些域。构造器的语法如下：
tableconstructor ::= ‘{’ [fieldlist] ‘}’
fieldlist ::= field {fieldsep field} [fieldsep]
field ::= ‘[’ exp ‘]’ ‘=’ exp | Name ‘=’ exp | exp
fieldsep ::= ‘,’ | ‘;’
每一个 [exp1] = exp2 形式的域都将为新表加入一个键为 exp1 值为 exp2 的元素。name = exp 形式的域等价于 ["name"] = exp。而 exp 形式的域等价于 [i] = exp，其中的 i 为从 1 开始的连续整数。其他格式的域不影响计数。比如，
a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 }
等价于
do
local t = {}
t[f(1)] = g
t[1] = "x" -- 1st exp
t[2] = "y" -- 2nd exp
t.x = 1 -- t["x"] = 1
t[3] = f(x) -- 3rd exp
t[30] = 23
t[4] = 45 -- 4th exp
a = t
end
如果列表中最后一个域形式为 exp，而且这个表达式是个函数调用或者参数可变，那么表达式的所有返回值将按顺序插入列表（参见 §3.4.9）。
域的列表可以有一个可选的尾部分隔符，这样方便机器生成代码。
3.4.9 – 函数调用
Lua 中的函数调用语法如下：
functioncall ::= prefixexp args
在函数调用中，首先对 prefixexp 和 args 求值。如果 prefixexp 的值的也是 function（函数），则用相应参数调用该函数。否则，调用 prefixexp 的 "call" 元方法，并将 prefixexp 的值作第一个参数，再跟其他原始参数（参见 §2.4）。
这种形式
functioncall ::= prefixexp ‘:’ Name args
可以用来调用“方法”。 v.name(v,args) 只在 v 仅被求值一次的时候可以写成 v:name(args)，这种写法只是语法糖便利。
参数的语法如下：
args ::= ‘(’ [explist] ‘)’ args ::= tableconstructor args ::= String
所有参数表达式都在函数调用之前被求值。f{fields} 形式的函数调用也只是 f({fields}) 形式的语法糖写法；即参数列表是个新表。而 f'string' （或者 f"string" 或者f[[string]]）一类的调用形式也只是 f('string') 的语法糖写法；即参数表是单一的文字串。
Lua 中将 return functioncall 这种形式叫 tail call（尾调用）。Lua 实现的是完全尾调用（也叫完全尾递归）：在尾调用中，被调用的函数重用调用它的函数的堆栈。因此，程序可执行的嵌套尾调用没有层数限制（即无限尾调用）。而尾调用也会删除任何关于调用它的函数的调试信息。注意，尾调用只能出现在特殊的语法下：return 只有一个函数作为参数；这使得调用函数能明确返回被调函数的结果。所以，下面这些都不是尾调用：
return (f(x)) -- results adjusted to 1
return 2 * f(x)
return x, f(x) -- additional results
f(x); return -- results discarded
return x or f(x) -- results adjusted to 1
3.4.10 – 函数定义
函数定义的语法如下：
functiondef ::= function funcbody funcbody ::= ‘(’ [parlist] ‘)’ block end
下面的语法糖写法可以简化函数的定义：
stat ::= function funcname funcbody
stat ::= local function Name funcbody
funcname ::= Name {‘.’ Name} [‘:’ Name]
语句
function f () body end
转化成
f = function () body end
语句
function t.a.b.c.f () body end
转化成
t.a.b.c.f = function () body end
语句
local function f () body end
转化成
local f; f = function () body end
而不是转化成
local f = function () body end
（这只在函数体内需要引用 f 时才有点区别。）
函数定义是一个可执行表达式，执行结果是个类型为 function的值。Lua 预编译 chunk（程序块）时，它的所有函数体也会被预编译。每当 Lua 执行函数定义，函数才被 instantiated（实例化）（也叫 closed（闭合））。这个函数实例（也叫closure（闭包））才是表达式最终的值。
形参（函数定义的参数）作为局部变量由实参（函数调用时传入的参数）的值初始化：
parlist ::= namelist [‘,’ ‘...’] | ‘...’
调用函数时，实参列表会根据形参列表的长度做调整，除非遇上了形参列表以三点（'...'）结尾的 vararg function（变参函数）。变参函数不会调整实参列表，而是收集所有额外的实参然后通过 vararg expression（变参表达式）（也写成 '...'）传给函数。变参表达式的值就是包含全部额外实参的列表，这类似返回多值的函数。如果变参表达式在另一个表达式中使用，或者是位于一堆表达式的中间，那么它返回的值会被调整成单个元素。而若是该表达式是那堆表达式的最后一个，那就不用做调整了（除非最后的表达式位于括号之内）。
先看下面这个函数定义的例子：
function f(a, b) end
function g(a, b, ...) end
function r() return 1,2,3 end
然后有如下实参到形参和变参表达式的映射关系：
CALL PARAMETERS
f(3) a=3, b=nil
f(3, 4) a=3, b=4
f(3, 4, 5) a=3, b=4
f(r(), 10) a=1, b=10
f(r()) a=1, b=2
g(3) a=3, b=nil, ... --> (nothing)
g(3, 4) a=3, b=4, ... --> (nothing)
g(3, 4, 5, 8) a=3, b=4, ... --> 5 8
g(5, r()) a=5, b=1, ... --> 2 3
return 语句用于返回结果（参见 §3.3.4）。如果执行到函数结尾没发现一个 return 语句，则函数没有结果返回。
一个函数能返回的结果数量因系统不同而限制不同，但肯定大于1000。
colon（冒号） 语法用于定义 methods（方法）。意思是函数有一个额外的隐式参数 self。所以，如下语句
function t.a.b.c:f (params) body end
是如下写法的语法糖（简写）形式
t.a.b.c.f = function (self, params) body end
3.5 – 可见性规则
Lua 是门有词法域的语言。一个局部变量的作用域开始于声明后的第一条语句，结束于包含该声明的最内层语句块中的最后一条非空语句。思考下面的例子：
x = 10 -- 全局变量
do -- 新语句块开始
local x = x -- 新的 'x'，值为 10
print(x) --> 10
x = x+1
do -- 另一个语句块开始
local x = x+1 -- 另一个 'x'
print(x) --> 12
end
print(x) --> 11
end
print(x) --> 10（此为全局的那一个）
留心注意，类似 local x = x 的声明，新声明的 x 还不在作用域内（译者注：作用域开始于声明语句的下一句，不包括声明语句本身），所以后面的 x 还是指那个外层变量。
因为有词法域规则，所以函数可随意访问定义在其作用域内的局部变量。局部变量被更内层的函数使用，则称其为 upvalue，或者是 external local variable（外层局部变量）。
要注意，每执行一次 local 语句都会定义新的局部变量。思考下面的例子：
a = {}
local x = 20
for i=1,10 do
local y = 0
a[i] = function () y=y+1; return x+y end
end
这个循环创建了十个 closures（闭包，这里指十个匿名函数实例）。每一个 closure 使用的都是不同的 y ，相同的 x。
4 – 应用程序接口（API）
本章描述 Lua 中的 C API，就是那些用于宿主程序和 Lua 通讯的 C 函数。所有 API 函数和相关的类型/常量定义都在 lua.h 头文件中。
虽然我们说的是“函数”，但某些 API 其实是以宏的形式提供的。除非特别说明，所有这些宏对它们的参数都只使用一次（除了常为 Lua 状态机的第一个参数），所以不用担心这些宏的展开会隐藏一些副作用。
在大部分 C 库中，Lua 的 API 函数都不会检查参数的有效性和一致性。但你可以在编译 Lua 时通过内置的宏 LUA_USE_APICHECK 来改变这个设定。
4.1 – 栈
Lua 使用一个 virtual stack（虚拟栈） 来与 C 传递值。栈内每个元素都代表一个 Lua 值（nil、数字、字符串等等）。
任何时候 Lua 调用 C 函数，被调函数都会获得一个新栈。该栈独立于以前的栈，也独立于函数在 C 中的栈。这个栈初始时包含传递给 C 函数的所有参数，然后 C 函数把要返回给调用者的结果也压入其中（参见 lua_CFunction）。
为方便起见，API 中大部分查询操作都不严格遵守常规栈的规则。而是可以通过一个 index 来索引栈中任意元素。一个正的索引值表示了栈中的绝对位置（从 1 开始）；负的索引值表示了相对栈顶的偏移量。具体来说，如果栈有 n 个元素，则索引 1 代表第一个元素（就是第一个压入栈的元素，在栈底），索引 n 代表最后一个元素；而索引 -1 也代表最后一个元素（就是栈顶元素），索引 -n 代表第一个元素。
4.2 – 栈大小
与 Lua 交互时，要负责确保一致性。尤其要 负责控制栈的溢出情况。在压栈前可以使用函数 lua_checkstack 来检查栈中是否拥有足够的位置。
Lua 无论何时调用 C，它都确保栈至少拥有 LUA_MINSTACK 个位置。LUA_MINSTACK 被定义为 20。所以，只要不是循环压栈，通常无需担心栈空间。
调用一个返回结果数量不定的 Lua 函数（参见 lua_call）时，Lua 会确保栈空间足够容纳所有返回结果，但不确保还有更多额外空间。所以，在调用了这种函数后，继续压栈前最好使用lua_checkstack 检查一下栈空间。
4.3 – 有效索引与合格索引
API 中的任何能接受栈索引的函数只接受 valid indices（有效索引） 或 acceptable indices（合格索引）。
有效索引 指的是指向栈内真实位置的索引，意思是其值在 1 和栈顶之间（即1 ≤ abs(index) ≤ top）。通常，函数要修改某个索引项的值时就要求有效索引。
除非另有提示，任何接受有效索引的函数同时也接受 pseudo-indices（伪索引），帮助 C 代码访问一些不在栈内的 Lua 值。伪索引可用于访问注册表和 C 函数的 upvalues（参见 §4.4）。
那种不需具体位置只要栈内一个值的函数（比如，查询函数）可以用合格索引进行调用。一个 acceptable index（合格索引） 可为任意有效索引，包括伪索引，也可为栈空间内栈顶之后的任和正索引，意味着索引值最大为栈的大小。（注意，0 从来都不是合格索引。）除非特殊说明，API 中的函数只在合格索引下有效工作。
合格索引在查询栈的时候避免了额外的栈顶判断。例如，一个 C 函数可以查询它的第三个参数而不必先检查是否存在第三个参数，意思是不必检查索引 3 是否有效。
对于那些能用合格索引调用的函数，任何无效索引都被当作成包含一个虚拟类型为 LUA_TNONE 的值，其行为类似一个 nil 值。
4.4 – C Closures（闭包）
创建一个 C 函数时，可能会关联一些值，因此就创建了一个 C closure（参见 lua_pushcclosure），那些值就叫 upvalues，并且可在函数被调用时访问到。
每当调用 C 函数时，它的 upvalue 就被放在特定的伪索引处。这些伪索引由宏 lua_upvalueindex 产生。与函数相关的第一个值在 lua_upvalueindex(1) 处，并依此类推。当 n 大于当前函数中 upvalue 的数量（小于 256）时， lua_upvalueindex(n) 将产生合格但无效的索引值。
4.5 – 注册表
Lua 提供一个 registry（注册表），是一张用于 C 代码保存所需 Lua 值的预定义表。注册表位于伪索引 LUA_REGISTRYINDEX 处，这是一个有效索引。任何 C 库都可以向表中保存数据，只是在选择键的时候要避免和其他库冲突。一般来说，可以用包含库名的字符串，或者用代码中一个 C 对象的地址做成的简易 userdata，或者用代码中创建的任意 Lua 对象。（译者注：键的形式很随意，不局限于字符串，可详细参考 table 类型。）与全局变量一样，以下划线开头跟大写字母的字符串键是为 Lua 保留的。
注册表内的整数键用作引用机制，在辅助库中实现时也使用一些预定义值。因此，整数键不能做他用。
创建一个新的 Lua 状态机时，它的注册表来自于一些预定义的值。这些预定义值作为常量定义在 lua.h，由整数键进行索引。 下面的常量定义：
4.6 – C 中的错误处理
在内部实现中，Lua 使用 C 的 longjmp 机制来处理错误。（如果 Lua 和 C++ 编译，也可以选择使用异常；查阅源码内的 LUAI_THROW。）Lua 面对任何错误（比如内存分配错误、类型错误、语法错误和运行时错误）都会产生一个错误；就是做一个 long jump。在 protected environment（保护环境）下使用 setjmp 可以设置一个恢复点；任何错误都会跳转到最近活动的恢复点上。
如果错误发生在保护环境之外，Lua 会调用一个 panic 函数（参见 lua_atpanic），然后调用 abort来退出宿主程序。panic 函数也可以不返回来避免这种退出（比如，做一个 long jump 跳转到 Lua 之外你自己的恢复点上）。
panic 函数运行起来就像一个消息处理器（参见 §2.3）；尤其是错误消息位于栈顶时。问题在于没法保证栈的空间。向栈内压入任何东西，panic 函数都需要先检查可用空间（参见 §4.2）。
API 中大部分函数都能抛出错误，比如内存分配错误。每个函数的文档内具体指示了它是否能抛出错误。
在 C 函数内可以通过调用 lua_error 来抛出一个错误。
4.7 – C 中的挂起处理
在内部实现中，Lua 使用 C的 longjmp 机制来挂起协程。因此，如果函数 foo 调用一个 API 函数，而该 API 函数挂起了（直接或间接挂起），Lua 没法给 foo 返回任何东西，因为 longjmp已将它的 frame 从 C 栈中移除了。
为避免这类问题，每当穿过 API 调用而挂起时 Lua 都会弹出错误，只除了这三个函数：lua_yieldk、lua_callk 和 lua_pcallk。这三个函数接收一个 continuation function（继续函数）（有一个参数叫 k）用于挂起后继续执行。
我们需要用一些术语来解释这个 continuation。首先，我们有一个由 Lua 调用的 C 函数，它叫 original function（原始函数）。这个原始函数随后在 C API 内调用了前面三个函数中的一个，叫callee function（被调函数），然后当前线程被挂起。（挂起可能发生在被调函数是 lua_yieldk时，或者被调函数是 lua_callk 或 lua_pcallk二者其一而且被自己挂起。）
假定是在执行被调函数时运行的线程被挂起。等线程恢复后，它最终会结束运行被调函数。然而，被调函数此时却没法返回给原始函数，因为它在 C 栈中的 frame 在挂起时给销毁了。于是，Lua 调用当作参数传递给被调函数的 continuation function（继续函数）。顾名思义，继续函数可以继续原始函数的工作。
Lua 将继续函数看作原始函数。继续函数接收来源于原始函数的相同 Lua 栈，在被调函数返回后也（与原始函数）处于一样的状态。（例如，lua_callk 调用一个函数后，它的参数均从栈中移除并被替换成那个函数的返回结果。）继续函数也拥有相同的 upvalue。无论继续函数返回什么值，都会被 Lua 当作是由原始函数返回的一样进行处理。
在 Lua 状态机中，原始函数与它的继续函数唯一的不同就是 lua_getctx 返回结果的不同。
4.8 – 函数与类型
我们在这儿按字母顺序列出 C API 中所有的函数与类型。每一个函数都有一个类似右边这样的标记：[-o, +p, x]
第一个字段，o，表示函数从栈中弹出多少个元素。
第二个字段，p，表示函数向栈压入多少个元素。（所有函数都会在弹出参数后再压入结果。）形如 x|y 这样的字段表示函数可以压入（弹出） x 或 y 个元素，由具体情况决定；而问号 '?' 表示我们无法只通过观察参数来得知函数弹出/压入多少个元素（比如，它们依赖与栈中的内容）。
第三个字段，x，用于告知函数是否抛出错误： '-' 表示从不抛出任何错误；'e' 表示可能抛出错误；'v' 表示有目的地抛出一个错误。
lua_absindex
[-0, +0, –]
int lua_absindex (lua_State *L, int idx);
将合格索引 idx 转化为一个绝对索引（就是不依赖栈顶）。
lua_Alloc
typedef void * (*lua_Alloc) (void *ud,
void *ptr,
size_t osize,
size_t nsize);
The type of the memory-allocation function used by Lua states. The allocator function must provide a functionality similar to realloc, but not exactly the same. Its arguments are ud, an opaque pointer passed to lua_newstate; ptr, a pointer to the block being allocated/reallocated/freed; osize, the original size of the block or some code about what is being allocated; nsize, the new size of the block.
When ptr is not NULL, osize is the size of the block pointed by ptr, that is, the size given when it was allocated or reallocated.
When ptr is NULL, osize encodes the kind of object that Lua is allocating. osize is any of LUA_TSTRING, LUA_TTABLE, LUA_TFUNCTION, LUA_TUSERDATA, or LUA_TTHREAD when (and only when) Lua is creating a new object of that type. When osize is some other value, Lua is allocating memory for something else.
Lua assumes the following behavior from the allocator function:
When nsize is zero, the allocator should behave like free and return NULL.
When nsize is not zero, the allocator should behave like realloc. The allocator returns NULL if and only if it cannot fulfill the request. Lua assumes that the allocator never fails when osize >= nsize.
Here is a simple implementation for the allocator function. It is used in the auxiliary library by luaL_newstate.
static void *l_alloc (void *ud, void *ptr, size_t osize,
size_t nsize) {
(void)ud; (void)osize; /* not used */
if (nsize == 0) {
free(ptr);
return NULL;
}
else
return realloc(ptr, nsize);
}
Note that Standard C ensures that free(NULL) has no effect and that realloc(NULL, size) is equivalent to malloc(size). This code assumes that realloc does not fail when shrinking a block. (Although Standard C does not ensure this behavior, it seems to be a safe assumption.)
lua_arith
[-(2|1), +1, e]
void lua_arith (lua_State *L, int op);
Performs an arithmetic operation over the two values (or one, in the case of negation) at the top of the stack, with the value at the top being the second operand, pops these values, and pushes the result of the operation. The function follows the semantics of the corresponding Lua operator (that is, it may call metamethods).
The value of op must be one of the following constants:
LUA_OPADD: performs addition (+)LUA_OPSUB: performs subtraction (-)LUA_OPMUL: performs multiplication (*)LUA_OPDIV: performs division (/)LUA_OPMOD: performs modulo (%)LUA_OPPOW: performs exponentiation (^)LUA_OPUNM: performs mathematical negation (unary-)
lua_atpanic
[-0, +0, –]
lua_CFunction lua_atpanic (lua_State *L, lua_CFunction panicf);
Sets a new panic function and returns the old one (see §4.6).
lua_call
[-(nargs+1), +nresults, e]
void lua_call (lua_State *L, int nargs, int nresults);
Calls a function.
To call a function you must use the following protocol: first, the function to be called is pushed onto the stack; then, the arguments to the function are pushed in direct order; that is, the first argument is pushed first. Finally you call lua_call; nargs is the number of arguments that you pushed onto the stack. All arguments and the function value are popped from the stack when the function is called. The function results are pushed onto the stack when the function returns. The number of results is adjusted to nresults, unless nresults is LUA_MULTRET. In this case, all results from the function are pushed. Lua takes care that the returned values fit into the stack space. The function results are pushed onto the stack in direct order (the first result is pushed first), so that after the call the last result is on the top of the stack.
Any error inside the called function is propagated upwards (with a longjmp).
The following example shows how the host program can do the equivalent to this Lua code:
a = f("how", t.x, 14)
Here it is in C:
lua_getglobal(L, "f"); /* function to be called */
lua_pushstring(L, "how"); /* 1st argument */
lua_getglobal(L, "t"); /* table to be indexed */
lua_getfield(L, -1, "x"); /* push result of t.x (2nd arg) */
lua_remove(L, -2); /* remove 't' from the stack */
lua_pushinteger(L, 14); /* 3rd argument */
lua_call(L, 3, 1); /* call 'f' with 3 arguments and 1 result */
lua_setglobal(L, "a"); /* set global 'a' */
Note that the code above is "balanced": at its end, the stack is back to its original configuration. This is considered good programming practice.
lua_callk
[-(nargs + 1), +nresults, e]
void lua_callk (lua_State *L, int nargs, int nresults, int ctx,
lua_CFunction k);
This function behaves exactly like lua_call, but allows the called function to yield (see §4.7).
lua_CFunction
typedef int (*lua_CFunction) (lua_State *L);
Type for C functions.
In order to communicate properly with Lua, a C function must use the following protocol, which defines the way parameters and results are passed: a C function receives its arguments from Lua in its stack in direct order (the first argument is pushed first). So, when the function starts, lua_gettop(L) returns the number of arguments received by the function. The first argument (if any) is at index 1 and its last argument is at index lua_gettop(L). To return values to Lua, a C function just pushes them onto the stack, in direct order (the first result is pushed first), and returns the number of results. Any other value in the stack below the results will be properly discarded by Lua. Like a Lua function, a C function called by Lua can also return many results.
As an example, the following function receives a variable number of numerical arguments and returns their average and sum:
static int foo (lua_State *L) {
int n = lua_gettop(L); /* number of arguments */
lua_Number sum = 0;
int i;
for (i = 1; i <= n; i++) {
if (!lua_isnumber(L, i)) {
lua_pushstring(L, "incorrect argument");
lua_error(L);
}
sum += lua_tonumber(L, i);
}
lua_pushnumber(L, sum/n); /* first result */
lua_pushnumber(L, sum); /* second result */
return 2; /* number of results */
}
lua_checkstack
[-0, +0, –]
int lua_checkstack (lua_State *L, int extra);
Ensures that there are at least extra free stack slots in the stack. It returns false if it cannot fulfill the request, because it would cause the stack to be larger than a fixed maximum size (typically at least a few thousand elements) or because it cannot allocate memory for the new stack size. This function never shrinks the stack; if the stack is already larger than the new size, it is left unchanged.
lua_close
[-0, +0, –]
void lua_close (lua_State *L);
Destroys all objects in the given Lua state (calling the corresponding garbage-collection metamethods, if any) and frees all dynamic memory used by this state. On several platforms, you may not need to call this function, because all resources are naturally released when the host program ends. On the other hand, long-running programs that create multiple states, such as daemons or web servers, might need to close states as soon as they are not needed.
lua_compare
[-0, +0, e]
int lua_compare (lua_State *L, int index1, int index2, int op);
Compares two Lua values. Returns 1 if the value at index index1 satisfies op when compared with the value at index index2, following the semantics of the corresponding Lua operator (that is, it may call metamethods). Otherwise returns 0. Also returns 0 if any of the indices is non valid.
The value of op must be one of the following constants:
LUA_OPEQ: compares for equality (==)LUA_OPLT: compares for less than (<)LUA_OPLE: compares for less or equal (<=)
lua_concat
[-n, +1, e]
void lua_concat (lua_State *L, int n);
Concatenates the n values at the top of the stack, pops them, and leaves the result at the top. If n is 1, the result is the single value on the stack (that is, the function does nothing); if n is 0, the result is the empty string. Concatenation is performed following the usual semantics of Lua (see §3.4.5).
lua_copy
[-0, +0, –]
void lua_copy (lua_State *L, int fromidx, int toidx);
Moves the element at index fromidx into the valid index toidx without shifting any element (therefore replacing the value at that position).
lua_createtable
[-0, +1, e]
void lua_createtable (lua_State *L, int narr, int nrec);
Creates a new empty table and pushes it onto the stack. Parameter narr is a hint for how many elements the table will have as a sequence; parameter nrec is a hint for how many other elements the table will have. Lua may use these hints to preallocate memory for the new table. This pre-allocation is useful for performance when you know in advance how many elements the table will have. Otherwise you can use the function lua_newtable.
lua_dump
[-0, +0, e]
int lua_dump (lua_State *L, lua_Writer writer, void *data);
Dumps a function as a binary chunk. Receives a Lua function on the top of the stack and produces a binary chunk that, if loaded again, results in a function equivalent to the one dumped. As it produces parts of the chunk, lua_dump calls function writer (see lua_Writer) with the given data to write them.
The value returned is the error code returned by the last call to the writer; 0 means no errors.
This function does not pop the Lua function from the stack.
lua_error
[-1, +0, v]
int lua_error (lua_State *L);
Generates a Lua error. The error message (which can actually be a Lua value of any type) must be on the stack top. This function does a long jump, and therefore never returns (seeluaL_error).
lua_gc
[-0, +0, e]
int lua_gc (lua_State *L, int what, int data);
Controls the garbage collector.
This function performs several tasks, according to the value of the parameter what:
LUA_GCSTOP: stops the garbage collector.LUA_GCRESTART: restarts the garbage collector.LUA_GCCOLLECT: performs a full garbage-collection cycle.LUA_GCCOUNT: returns the current amount of memory (in Kbytes) in use by Lua.LUA_GCCOUNTB: returns the remainder of dividing the current amount of bytes of memory in use by Lua by 1024.LUA_GCSTEP: performs an incremental step of garbage collection. The step "size" is controlled bydata(larger values mean more steps) in a non-specified way. If you want to control the step size you must experimentally tune the value ofdata. The function returns 1 if the step finished a garbage-collection cycle.LUA_GCSETPAUSE: setsdataas the new value for the pause of the collector (see §2.5). The function returns the previous value of the pause.LUA_GCSETSTEPMUL: setsdataas the new value for the step multiplier of the collector (see §2.5). The function returns the previous value of the step multiplier.LUA_GCISRUNNING: returns a boolean that tells whether the collector is running (i.e., not stopped).LUA_GCGEN: changes the collector to generational mode (see §2.5).LUA_GCINC: changes the collector to incremental mode. This is the default mode.
For more details about these options, see collectgarbage.
lua_getallocf
[-0, +0, –]
lua_Alloc lua_getallocf (lua_State *L, void **ud);
Returns the memory-allocation function of a given state. If ud is not NULL, Lua stores in *ud the opaque pointer passed to lua_newstate.
lua_getctx
[-0, +0, –]
int lua_getctx (lua_State *L, int *ctx);
This function is called by a continuation function (see §4.7) to retrieve the status of the thread and a context information.
When called in the original function, lua_getctx always returns LUA_OK and does not change the value of its argument ctx. When called inside a continuation function, lua_getctxreturns LUA_YIELD and sets the value of ctx to be the context information (the value passed as the ctx argument to the callee together with the continuation function).
When the callee is lua_pcallk, Lua may also call its continuation function to handle errors during the call. That is, upon an error in the function called by lua_pcallk, Lua may not return to the original function but instead may call the continuation function. In that case, a call to lua_getctx will return the error code (the value that would be returned by lua_pcallk); the value ofctx will be set to the context information, as in the case of a yield.
lua_getfield
[-0, +1, e]
void lua_getfield (lua_State *L, int index, const char *k);
Pushes onto the stack the value t[k], where t is the value at the given index. As in Lua, this function may trigger a metamethod for the "index" event (see §2.4).
lua_getglobal
[-0, +1, e]
void lua_getglobal (lua_State *L, const char *name);
Pushes onto the stack the value of the global name.
lua_getmetatable
[-0, +(0|1), –]
int lua_getmetatable (lua_State *L, int index);
Pushes onto the stack the metatable of the value at the given index. If the value does not have a metatable, the function returns 0 and pushes nothing on the stack.
lua_gettable
[-1, +1, e]
void lua_gettable (lua_State *L, int index);
Pushes onto the stack the value t[k], where t is the value at the given index and k is the value at the top of the stack.
This function pops the key from the stack (putting the resulting value in its place). As in Lua, this function may trigger a metamethod for the "index" event (see §2.4).
lua_gettop
[-0, +0, –]
int lua_gettop (lua_State *L);
Returns the index of the top element in the stack. Because indices start at 1, this result is equal to the number of elements in the stack (and so 0 means an empty stack).
lua_getuservalue
[-0, +1, –]
void lua_getuservalue (lua_State *L, int index);
Pushes onto the stack the Lua value associated with the userdata at the given index. This Lua value must be a table or nil.
lua_insert
[-1, +1, –]
void lua_insert (lua_State *L, int index);
Moves the top element into the given valid index, shifting up the elements above this index to open space. This function cannot be called with a pseudo-index, because a pseudo-index is not an actual stack position.
lua_Integer
typedef ptrdiff_t lua_Integer;
The type used by the Lua API to represent signed integral values.
By default it is a ptrdiff_t, which is usually the largest signed integral type the machine handles "comfortably".
lua_isboolean
[-0, +0, –]
int lua_isboolean (lua_State *L, int index);
Returns 1 if the value at the given index is a boolean, and 0 otherwise.
lua_iscfunction
[-0, +0, –]
int lua_iscfunction (lua_State *L, int index);
Returns 1 if the value at the given index is a C function, and 0 otherwise.
lua_isfunction
[-0, +0, –]
int lua_isfunction (lua_State *L, int index);
Returns 1 if the value at the given index is a function (either C or Lua), and 0 otherwise.
lua_islightuserdata
[-0, +0, –]
int lua_islightuserdata (lua_State *L, int index);
Returns 1 if the value at the given index is a light userdata, and 0 otherwise.
lua_isnil
[-0, +0, –]
int lua_isnil (lua_State *L, int index);
Returns 1 if the value at the given index is nil, and 0 otherwise.
lua_isnone
[-0, +0, –]
int lua_isnone (lua_State *L, int index);
Returns 1 if the given index is not valid, and 0 otherwise.
lua_isnoneornil
[-0, +0, –]
int lua_isnoneornil (lua_State *L, int index);
Returns 1 if the given index is not valid or if the value at this index is nil, and 0 otherwise.
lua_isnumber
[-0, +0, –]
int lua_isnumber (lua_State *L, int index);
Returns 1 if the value at the given index is a number or a string convertible to a number, and 0 otherwise.
lua_isstring
[-0, +0, –]
int lua_isstring (lua_State *L, int index);
Returns 1 if the value at the given index is a string or a number (which is always convertible to a string), and 0 otherwise.
lua_istable
[-0, +0, –]
int lua_istable (lua_State *L, int index);
Returns 1 if the value at the given index is a table, and 0 otherwise.
lua_isthread
[-0, +0, –]
int lua_isthread (lua_State *L, int index);
Returns 1 if the value at the given index is a thread, and 0 otherwise.
lua_isuserdata
[-0, +0, –]
int lua_isuserdata (lua_State *L, int index);
Returns 1 if the value at the given index is a userdata (either full or light), and 0 otherwise.
lua_len
[-0, +1, e]
void lua_len (lua_State *L, int index);
Returns the "length" of the value at the given index; it is equivalent to the '#' operator in Lua (see §3.4.6). The result is pushed on the stack.
lua_load
[-0, +1, –]
int lua_load (lua_State *L,
lua_Reader reader,
void *data,
const char *source,
const char *mode);
Loads a Lua chunk (without running it). If there are no errors, lua_load pushes the compiled chunk as a Lua function on top of the stack. Otherwise, it pushes an error message.
The return values of lua_load are:
LUA_OK: no errors;LUA_ERRSYNTAX: syntax error during precompilation;LUA_ERRMEM: memory allocation error;LUA_ERRGCMM: error while running a__gcmetamethod. (This error has no relation with the chunk being loaded. It is generated by the garbage collector.)
The lua_load function uses a user-supplied reader function to read the chunk (see lua_Reader). The data argument is an opaque value passed to the reader function.
The source argument gives a name to the chunk, which is used for error messages and in debug information (see §4.9).
lua_load automatically detects whether the chunk is text or binary and loads it accordingly (see program luac). The string mode works as in function load, with the addition that a NULLvalue is equivalent to the string "bt".
lua_load uses the stack internally, so the reader function should always leave the stack unmodified when returning.
If the resulting function has one upvalue, this upvalue is set to the value of the global environment stored at index LUA_RIDX_GLOBALS in the registry (see §4.5). When loading main chunks, this upvalue will be the _ENV variable (see §2.2).
lua_newstate
[-0, +0, –]
lua_State *lua_newstate (lua_Alloc f, void *ud);
Creates a new thread running in a new, independent state. Returns NULL if cannot create the thread or the state (due to lack of memory). The argument f is the allocator function; Lua does all memory allocation for this state through this function. The second argument, ud, is an opaque pointer that Lua passes to the allocator in every call.
lua_newtable
[-0, +1, e]
void lua_newtable (lua_State *L);
Creates a new empty table and pushes it onto the stack. It is equivalent to lua_createtable(L, 0, 0).
lua_newthread
[-0, +1, e]
lua_State *lua_newthread (lua_State *L);
Creates a new thread, pushes it on the stack, and returns a pointer to a lua_State that represents this new thread. The new thread returned by this function shares with the original thread its global environment, but has an independent execution stack.
There is no explicit function to close or to destroy a thread. Threads are subject to garbage collection, like any Lua object.
lua_newuserdata
[-0, +1, e]
void *lua_newuserdata (lua_State *L, size_t size);
This function allocates a new block of memory with the given size, pushes onto the stack a new full userdata with the block address, and returns this address. The host program can freely use this memory.
lua_next
[-1, +(2|0), e]
int lua_next (lua_State *L, int index);
Pops a key from the stack, and pushes a key–value pair from the table at the given index (the "next" pair after the given key). If there are no more elements in the table, then lua_next returns 0 (and pushes nothing).
A typical traversal looks like this:
/* table is in the stack at index 't' */
lua_pushnil(L); /* first key */
while (lua_next(L, t) != 0) {
/* uses 'key' (at index -2) and 'value' (at index -1) */
printf("%s - %s\n",
lua_typename(L, lua_type(L, -2)),
lua_typename(L, lua_type(L, -1)));
/* removes 'value'; keeps 'key' for next iteration */
lua_pop(L, 1);
}
While traversing a table, do not call lua_tolstring directly on a key, unless you know that the key is actually a string. Recall that lua_tolstring may change the value at the given index; this confuses the next call to lua_next.
See function next for the caveats of modifying the table during its traversal.
lua_Number
typedef double lua_Number;
The type of numbers in Lua. By default, it is double, but that can be changed in luaconf.h. Through this configuration file you can change Lua to operate with another type for numbers (e.g., float or long).
lua_pcall
[-(nargs + 1), +(nresults|1), –]
int lua_pcall (lua_State *L, int nargs, int nresults, int msgh);
Calls a function in protected mode.
Both nargs and nresults have the same meaning as in lua_call. If there are no errors during the call, lua_pcall behaves exactly like lua_call. However, if there is any error,lua_pcall catches it, pushes a single value on the stack (the error message), and returns an error code. Like lua_call, lua_pcall always removes the function and its arguments from the stack.
If msgh is 0, then the error message returned on the stack is exactly the original error message. Otherwise, msgh is the stack index of a message handler. (In the current implementation, this index cannot be a pseudo-index.) In case of runtime errors, this function will be called with the error message and its return value will be the message returned on the stack by lua_pcall.
Typically, the message handler is used to add more debug information to the error message, such as a stack traceback. Such information cannot be gathered after the return of lua_pcall, since by then the stack has unwound.
The lua_pcall function returns one of the following codes (defined in lua.h):
LUA_OK(0): success.LUA_ERRRUN: a runtime error.LUA_ERRMEM: memory allocation error. For such errors, Lua does not call the message handler.LUA_ERRERR: error while running the message handler.LUA_ERRGCMM: error while running a__gcmetamethod. (This error typically has no relation with the function being called. It is generated by the garbage collector.)
lua_pcallk
[-(nargs + 1), +(nresults|1), –]
int lua_pcallk (lua_State *L,
int nargs,
int nresults,
int errfunc,
int ctx,
lua_CFunction k);
This function behaves exactly like lua_pcall, but allows the called function to yield (see §4.7).
lua_pop
[-n, +0, –]
void lua_pop (lua_State *L, int n);
Pops n elements from the stack.
lua_pushboolean
[-0, +1, –]
void lua_pushboolean (lua_State *L, int b);
Pushes a boolean value with value b onto the stack.
lua_pushcclosure
[-n, +1, e]
void lua_pushcclosure (lua_State *L, lua_CFunction fn, int n);
Pushes a new C closure onto the stack.
When a C function is created, it is possible to associate some values with it, thus creating a C closure (see §4.4); these values are then accessible to the function whenever it is called. To associate values with a C function, first these values should be pushed onto the stack (when there are multiple values, the first value is pushed first). Then lua_pushcclosure is called to create and push the C function onto the stack, with the argument n telling how many values should be associated with the function. lua_pushcclosure also pops these values from the stack.
The maximum value for n is 255.
When n is zero, this function creates a light C function, which is just a pointer to the C function. In that case, it never throws a memory error.
lua_pushcfunction
[-0, +1, –]
void lua_pushcfunction (lua_State *L, lua_CFunction f);
Pushes a C function onto the stack. This function receives a pointer to a C function and pushes onto the stack a Lua value of type function that, when called, invokes the corresponding C function.
Any function to be registered in Lua must follow the correct protocol to receive its parameters and return its results (see lua_CFunction).
lua_pushcfunction is defined as a macro:
#define lua_pushcfunction(L,f) lua_pushcclosure(L,f,0)
Note that f is used twice.
lua_pushfstring
[-0, +1, e]
const char *lua_pushfstring (lua_State *L, const char *fmt, ...);
Pushes onto the stack a formatted string and returns a pointer to this string. It is similar to the ANSI C function sprintf, but has some important differences:
- You do not have to allocate space for the result: the result is a Lua string and Lua takes care of memory allocation (and deallocation, through garbage collection).
- The conversion specifiers are quite restricted. There are no flags, widths, or precisions. The conversion specifiers can only be '
%%' (inserts a '%' in the string), '%s' (inserts a zero-terminated string, with no size restrictions), '%f' (inserts alua_Number), '%p' (inserts a pointer as a hexadecimal numeral), '%d' (inserts anint), and '%c' (inserts anintas a byte).
lua_pushglobaltable
[-0, +1, –]
void lua_pushglobaltable (lua_State *L);
Pushes the global environment onto the stack.
lua_pushinteger
[-0, +1, –]
void lua_pushinteger (lua_State *L, lua_Integer n);
Pushes a number with value n onto the stack.
lua_pushlightuserdata
[-0, +1, –]
void lua_pushlightuserdata (lua_State *L, void *p);
Pushes a light userdata onto the stack.
Userdata represent C values in Lua. A light userdata represents a pointer, a void*. It is a value (like a number): you do not create it, it has no individual metatable, and it is not collected (as it was never created). A light userdata is equal to "any" light userdata with the same C address.
lua_pushliteral
[-0, +1, e]
const char *lua_pushliteral (lua_State *L, const char *s);
This macro is equivalent to lua_pushlstring, but can be used only when s is a literal string. It automatically provides the string length.
lua_pushlstring
[-0, +1, e]
const char *lua_pushlstring (lua_State *L, const char *s, size_t len);
Pushes the string pointed to by s with size len onto the stack. Lua makes (or reuses) an internal copy of the given string, so the memory at s can be freed or reused immediately after the function returns. The string can contain any binary data, including embedded zeros.
Returns a pointer to the internal copy of the string.
lua_pushnil
[-0, +1, –]
void lua_pushnil (lua_State *L);
Pushes a nil value onto the stack.
lua_pushnumber
[-0, +1, –]
void lua_pushnumber (lua_State *L, lua_Number n);
Pushes a number with value n onto the stack.
lua_pushstring
[-0, +1, e]
const char *lua_pushstring (lua_State *L, const char *s);
Pushes the zero-terminated string pointed to by s onto the stack. Lua makes (or reuses) an internal copy of the given string, so the memory at s can be freed or reused immediately after the function returns.
Returns a pointer to the internal copy of the string.
If s is NULL, pushes nil and returns NULL.
lua_pushthread
[-0, +1, –]
int lua_pushthread (lua_State *L);
Pushes the thread represented by L onto the stack. Returns 1 if this thread is the main thread of its state.
lua_pushunsigned
[-0, +1, –]
void lua_pushunsigned (lua_State *L, lua_Unsigned n);
Pushes a number with value n onto the stack.
lua_pushvalue
[-0, +1, –]
void lua_pushvalue (lua_State *L, int index);
Pushes a copy of the element at the given index onto the stack.
lua_pushvfstring
[-0, +1, e]
const char *lua_pushvfstring (lua_State *L,
const char *fmt,
va_list argp);
Equivalent to lua_pushfstring, except that it receives a va_list instead of a variable number of arguments.
lua_rawequal
[-0, +0, –]
int lua_rawequal (lua_State *L, int index1, int index2);
Returns 1 if the two values in indices index1 and index2 are primitively equal (that is, without calling metamethods). Otherwise returns 0. Also returns 0 if any of the indices are non valid.
lua_rawget
[-1, +1, –]
void lua_rawget (lua_State *L, int index);
Similar to lua_gettable, but does a raw access (i.e., without metamethods).
lua_rawgeti
[-0, +1, –]
void lua_rawgeti (lua_State *L, int index, int n);
Pushes onto the stack the value t[n], where t is the table at the given index. The access is raw; that is, it does not invoke metamethods.
lua_rawgetp
[-0, +1, –]
void lua_rawgetp (lua_State *L, int index, const void *p);
Pushes onto the stack the value t[k], where t is the table at the given index and k is the pointer p represented as a light userdata. The access is raw; that is, it does not invoke metamethods.
lua_rawlen
[-0, +0, –]
size_t lua_rawlen (lua_State *L, int index);
Returns the raw "length" of the value at the given index: for strings, this is the string length; for tables, this is the result of the length operator ('#') with no metamethods; for userdata, this is the size of the block of memory allocated for the userdata; for other values, it is 0.
lua_rawset
[-2, +0, e]
void lua_rawset (lua_State *L, int index);
Similar to lua_settable, but does a raw assignment (i.e., without metamethods).
lua_rawseti
[-1, +0, e]
void lua_rawseti (lua_State *L, int index, int n);
Does the equivalent of t[n] = v, where t is the table at the given index and v is the value at the top of the stack.
This function pops the value from the stack. The assignment is raw; that is, it does not invoke metamethods.
lua_rawsetp
[-1, +0, e]
void lua_rawsetp (lua_State *L, int index, const void *p);
Does the equivalent of t[k] = v, where t is the table at the given index, k is the pointer p represented as a light userdata, and v is the value at the top of the stack.
This function pops the value from the stack. The assignment is raw; that is, it does not invoke metamethods.
lua_Reader
typedef const char * (*lua_Reader) (lua_State *L,
void *data,
size_t *size);
The reader function used by lua_load. Every time it needs another piece of the chunk, lua_load calls the reader, passing along its data parameter. The reader must return a pointer to a block of memory with a new piece of the chunk and set size to the block size. The block must exist until the reader function is called again. To signal the end of the chunk, the reader must return NULL or set size to zero. The reader function may return pieces of any size greater than zero.
lua_register
[-0, +0, e]
void lua_register (lua_State *L, const char *name, lua_CFunction f);
Sets the C function f as the new value of global name. It is defined as a macro:
#define lua_register(L,n,f) \
(lua_pushcfunction(L, f), lua_setglobal(L, n))
lua_remove
[-1, +0, –]
void lua_remove (lua_State *L, int index);
Removes the element at the given valid index, shifting down the elements above this index to fill the gap. This function cannot be called with a pseudo-index, because a pseudo-index is not an actual stack position.
lua_replace
[-1, +0, –]
void lua_replace (lua_State *L, int index);
Moves the top element into the given valid index without shifting any element (therefore replacing the value at the given index), and then pops the top element.
lua_resume
[-?, +?, –]
int lua_resume (lua_State *L, lua_State *from, int nargs);
Starts and resumes a coroutine in a given thread.
To start a coroutine, you push onto the thread stack the main function plus any arguments; then you call lua_resume, with nargs being the number of arguments. This call returns when the coroutine suspends or finishes its execution. When it returns, the stack contains all values passed to lua_yield, or all values returned by the body function. lua_resume returnsLUA_YIELD if the coroutine yields, LUA_OK if the coroutine finishes its execution without errors, or an error code in case of errors (see lua_pcall).
In case of errors, the stack is not unwound, so you can use the debug API over it. The error message is on the top of the stack.
To resume a coroutine, you remove any results from the last lua_yield, put on its stack only the values to be passed as results from yield, and then call lua_resume.
The parameter from represents the coroutine that is resuming L. If there is no such coroutine, this parameter can be NULL.
lua_setallocf
[-0, +0, –]
void lua_setallocf (lua_State *L, lua_Alloc f, void *ud);
Changes the allocator function of a given state to f with user data ud.
lua_setfield
[-1, +0, e]
void lua_setfield (lua_State *L, int index, const char *k);
Does the equivalent to t[k] = v, where t is the value at the given index and v is the value at the top of the stack.
This function pops the value from the stack. As in Lua, this function may trigger a metamethod for the "newindex" event (see §2.4).
lua_setglobal
[-1, +0, e]
void lua_setglobal (lua_State *L, const char *name);
Pops a value from the stack and sets it as the new value of global name.
lua_setmetatable
[-1, +0, –]
void lua_setmetatable (lua_State *L, int index);
Pops a table from the stack and sets it as the new metatable for the value at the given index.
lua_settable
[-2, +0, e]
void lua_settable (lua_State *L, int index);
Does the equivalent to t[k] = v, where t is the value at the given index, v is the value at the top of the stack, and k is the value just below the top.
This function pops both the key and the value from the stack. As in Lua, this function may trigger a metamethod for the "newindex" event (see §2.4).
lua_settop
[-?, +?, –]
void lua_settop (lua_State *L, int index);
Accepts any index, or 0, and sets the stack top to this index. If the new top is larger than the old one, then the new elements are filled with nil. If index is 0, then all stack elements are removed.
lua_setuservalue
[-1, +0, –]
void lua_setuservalue (lua_State *L, int index);
Pops a table or nil from the stack and sets it as the new value associated to the userdata at the given index.
lua_State
typedef struct lua_State lua_State;
An opaque structure that points to a thread and indirectly (through the thread) to the whole state of a Lua interpreter. The Lua library is fully reentrant: it has no global variables. All information about a state is accessible through this structure.
A pointer to this structure must be passed as the first argument to every function in the library, except to lua_newstate, which creates a Lua state from scratch.
lua_status
[-0, +0, –]
int lua_status (lua_State *L);
Returns the status of the thread L.
The status can be 0 (LUA_OK) for a normal thread, an error code if the thread finished the execution of a lua_resume with an error, or LUA_YIELD if the thread is suspended.
You can only call functions in threads with status LUA_OK. You can resume threads with status LUA_OK (to start a new coroutine) or LUA_YIELD (to resume a coroutine).
lua_toboolean
[-0, +0, –]
int lua_toboolean (lua_State *L, int index);
Converts the Lua value at the given index to a C boolean value (0 or 1). Like all tests in Lua, lua_toboolean returns true for any Lua value different from false and nil; otherwise it returns false. (If you want to accept only actual boolean values, use lua_isboolean to test the value's type.)
lua_tocfunction
[-0, +0, –]
lua_CFunction lua_tocfunction (lua_State *L, int index);
Converts a value at the given index to a C function. That value must be a C function; otherwise, returns NULL.
lua_tointeger
[-0, +0, –]
lua_Integer lua_tointeger (lua_State *L, int index);
Equivalent to lua_tointegerx with isnum equal to NULL.
lua_tointegerx
[-0, +0, –]
lua_Integer lua_tointegerx (lua_State *L, int index, int *isnum);
Converts the Lua value at the given index to the signed integral type lua_Integer. The Lua value must be a number or a string convertible to a number (see §3.4.2); otherwise,lua_tointegerx returns 0.
If the number is not an integer, it is truncated in some non-specified way.
If isnum is not NULL, its referent is assigned a boolean value that indicates whether the operation succeeded.
lua_tolstring
[-0, +0, e]
const char *lua_tolstring (lua_State *L, int index, size_t *len);
Converts the Lua value at the given index to a C string. If len is not NULL, it also sets *len with the string length. The Lua value must be a string or a number; otherwise, the function returnsNULL. If the value is a number, then lua_tolstring also changes the actual value in the stack to a string. (This change confuses lua_next when lua_tolstring is applied to keys during a table traversal.)
lua_tolstring returns a fully aligned pointer to a string inside the Lua state. This string always has a zero ('\0') after its last character (as in C), but can contain other zeros in its body. Because Lua has garbage collection, there is no guarantee that the pointer returned by lua_tolstring will be valid after the corresponding value is removed from the stack.
lua_tonumber
[-0, +0, –]
lua_Number lua_tonumber (lua_State *L, int index);
Equivalent to lua_tonumberx with isnum equal to NULL.
lua_tonumberx
[-0, +0, –]
lua_Number lua_tonumberx (lua_State *L, int index, int *isnum);
Converts the Lua value at the given index to the C type lua_Number (see lua_Number). The Lua value must be a number or a string convertible to a number (see §3.4.2); otherwise,lua_tonumberx returns 0.
If isnum is not NULL, its referent is assigned a boolean value that indicates whether the operation succeeded.
lua_topointer
[-0, +0, –]
const void *lua_topointer (lua_State *L, int index);
Converts the value at the given index to a generic C pointer (void*). The value can be a userdata, a table, a thread, or a function; otherwise, lua_topointer returns NULL. Different objects will give different pointers. There is no way to convert the pointer back to its original value.
Typically this function is used only for debug information.
lua_tostring
[-0, +0, e]
const char *lua_tostring (lua_State *L, int index);
Equivalent to lua_tolstring with len equal to NULL.
lua_tothread
[-0, +0, –]
lua_State *lua_tothread (lua_State *L, int index);
Converts the value at the given index to a Lua thread (represented as lua_State*). This value must be a thread; otherwise, the function returns NULL.
lua_tounsigned
[-0, +0, –]
lua_Unsigned lua_tounsigned (lua_State *L, int index);
Equivalent to lua_tounsignedx with isnum equal to NULL.
lua_tounsignedx
[-0, +0, –]
lua_Unsigned lua_tounsignedx (lua_State *L, int index, int *isnum);
Converts the Lua value at the given index to the unsigned integral type lua_Unsigned. The Lua value must be a number or a string convertible to a number (see §3.4.2); otherwise,lua_tounsignedx returns 0.
If the number is not an integer, it is truncated in some non-specified way. If the number is outside the range of representable values, it is normalized to the remainder of its division by one more than the maximum representable value.
If isnum is not NULL, its referent is assigned a boolean value that indicates whether the operation succeeded.
lua_touserdata
[-0, +0, –]
void *lua_touserdata (lua_State *L, int index);
If the value at the given index is a full userdata, returns its block address. If the value is a light userdata, returns its pointer. Otherwise, returns NULL.
lua_type
[-0, +0, –]
int lua_type (lua_State *L, int index);
Returns the type of the value in the given valid index, or LUA_TNONE for a non-valid (but acceptable) index. The types returned by lua_type are coded by the following constants defined inlua.h: LUA_TNIL, LUA_TNUMBER, LUA_TBOOLEAN, LUA_TSTRING, LUA_TTABLE, LUA_TFUNCTION, LUA_TUSERDATA, LUA_TTHREAD, and LUA_TLIGHTUSERDATA.
lua_typename
[-0, +0, –]
const char *lua_typename (lua_State *L, int tp);
Returns the name of the type encoded by the value tp, which must be one the values returned by lua_type.
lua_Unsigned
typedef unsigned long lua_Unsigned;
The type used by the Lua API to represent unsigned integral values. It must have at least 32 bits.
By default it is an unsigned int or an unsigned long, whichever can hold 32-bit values.
lua_upvalueindex
[-0, +0, –]
int lua_upvalueindex (int i);
Returns the pseudo-index that represents the i-th upvalue of the running function (see §4.4).
lua_version
[-0, +0, v]
const lua_Number *lua_version (lua_State *L);
Returns the address of the version number stored in the Lua core. When called with a valid lua_State, returns the address of the version used to create that state. When called with NULL, returns the address of the version running the call.
lua_Writer
typedef int (*lua_Writer) (lua_State *L,
const void* p,
size_t sz,
void* ud);
The type of the writer function used by lua_dump. Every time it produces another piece of chunk, lua_dump calls the writer, passing along the buffer to be written (p), its size (sz), and thedata parameter supplied to lua_dump.
The writer returns an error code: 0 means no errors; any other value means an error and stops lua_dump from calling the writer again.
lua_xmove
[-?, +?, –]
void lua_xmove (lua_State *from, lua_State *to, int n);
Exchange values between different threads of the same state.
This function pops n values from the stack from, and pushes them onto the stack to.
lua_yield
[-?, +?, –]
int lua_yield (lua_State *L, int nresults);
This function is equivalent to lua_yieldk, but it has no continuation (see §4.7). Therefore, when the thread resumes, it returns to the function that called the function calling lua_yield.
lua_yieldk
[-?, +?, –]
int lua_yieldk (lua_State *L, int nresults, int ctx, lua_CFunction k);
Yields a coroutine.
This function should only be called as the return expression of a C function, as follows:
return lua_yieldk (L, n, i, k);
When a C function calls lua_yieldk in that way, the running coroutine suspends its execution, and the call to lua_resume that started this coroutine returns. The parameter nresults is the number of values from the stack that are passed as results to lua_resume.
When the coroutine is resumed again, Lua calls the given continuation function k to continue the execution of the C function that yielded (see §4.7). This continuation function receives the same stack from the previous function, with the results removed and replaced by the arguments passed to lua_resume. Moreover, the continuation function may access the value ctx by callinglua_getctx.
4.9 – The Debug Interface
Lua has no built-in debugging facilities. Instead, it offers a special interface by means of functions and hooks. This interface allows the construction of different kinds of debuggers, profilers, and other tools that need "inside information" from the interpreter.
lua_Debug
typedef struct lua_Debug {
int event;
const char *name; /* (n) */
const char *namewhat; /* (n) */
const char *what; /* (S) */
const char *source; /* (S) */
int currentline; /* (l) */
int linedefined; /* (S) */
int lastlinedefined; /* (S) */
unsigned char nups; /* (u) number of upvalues */
unsigned char nparams; /* (u) number of parameters */
char isvararg; /* (u) */
char istailcall; /* (t) */
char short_src[LUA_IDSIZE]; /* (S) */
/* private part */
other fields
} lua_Debug;
A structure used to carry different pieces of information about a function or an activation record. lua_getstack fills only the private part of this structure, for later use. To fill the other fields oflua_Debug with useful information, call lua_getinfo.
The fields of lua_Debug have the following meaning:
source: the source of the chunk that created the function. Ifsourcestarts with a '@', it means that the function was defined in a file where the file name follows the '@'. Ifsourcestarts with a '=', the remainder of its contents describe the source in a user-dependent manner. Otherwise, the function was defined in a string wheresourceis that string.short_src: a "printable" version ofsource, to be used in error messages.linedefined: the line number where the definition of the function starts.lastlinedefined: the line number where the definition of the function ends.what: the string"Lua"if the function is a Lua function,"C"if it is a C function,"main"if it is the main part of a chunk.currentline: the current line where the given function is executing. When no line information is available,currentlineis set to -1.name: a reasonable name for the given function. Because functions in Lua are first-class values, they do not have a fixed name: some functions can be the value of multiple global variables, while others can be stored only in a table field. Thelua_getinfofunction checks how the function was called to find a suitable name. If it cannot find a name, thennameis set toNULL.namewhat: explains thenamefield. The value ofnamewhatcan be"global","local","method","field","upvalue", or""(the empty string), according to how the function was called. (Lua uses the empty string when no other option seems to apply.)istailcall: true if this function invocation was called by a tail call. In this case, the caller of this level is not in the stack.nups: the number of upvalues of the function.nparams: the number of fixed parameters of the function (always 0 for C functions).isvararg: true if the function is a vararg function (always true for C functions).
lua_gethook
[-0, +0, –]
lua_Hook lua_gethook (lua_State *L);
Returns the current hook function.
lua_gethookcount
[-0, +0, –]
int lua_gethookcount (lua_State *L);
Returns the current hook count.
lua_gethookmask
[-0, +0, –]
int lua_gethookmask (lua_State *L);
Returns the current hook mask.
lua_getinfo
[-(0|1), +(0|1|2), e]
int lua_getinfo (lua_State *L, const char *what, lua_Debug *ar);
Gets information about a specific function or function invocation.
To get information about a function invocation, the parameter ar must be a valid activation record that was filled by a previous call to lua_getstack or given as argument to a hook (seelua_Hook).
To get information about a function you push it onto the stack and start the what string with the character '>'. (In that case, lua_getinfo pops the function from the top of the stack.) For instance, to know in which line a function f was defined, you can write the following code:
lua_Debug ar;
lua_getglobal(L, "f"); /* get global 'f' */
lua_getinfo(L, ">S", &ar);
printf("%d\n", ar.linedefined);
Each character in the string what selects some fields of the structure ar to be filled or a value to be pushed on the stack:
- '
n': fills in the fieldnameandnamewhat; - '
S': fills in the fieldssource,short_src,linedefined,lastlinedefined, andwhat; - '
l': fills in the fieldcurrentline; - '
t': fills in the fieldistailcall; - '
u': fills in the fieldsnups,nparams, andisvararg; - '
f': pushes onto the stack the function that is running at the given level; - '
L': pushes onto the stack a table whose indices are the numbers of the lines that are valid on the function. (A valid line is a line with some associated code, that is, a line where you can put a break point. Non-valid lines include empty lines and comments.)
This function returns 0 on error (for instance, an invalid option in what).
lua_getlocal
[-0, +(0|1), –]
const char *lua_getlocal (lua_State *L, lua_Debug *ar, int n);
Gets information about a local variable of a given activation record or a given function.
In the first case, the parameter ar must be a valid activation record that was filled by a previous call to lua_getstack or given as argument to a hook (see lua_Hook). The index n selects which local variable to inspect; see debug.getlocal for details about variable indices and names.
lua_getlocal pushes the variable's value onto the stack and returns its name.
In the second case, ar should be NULL and the function to be inspected must be at the top of the stack. In this case, only parameters of Lua functions are visible (as there is no information about what variables are active) and no values are pushed onto the stack.
Returns NULL (and pushes nothing) when the index is greater than the number of active local variables.
lua_getstack
[-0, +0, –]
int lua_getstack (lua_State *L, int level, lua_Debug *ar);
Gets information about the interpreter runtime stack.
This function fills parts of a lua_Debug structure with an identification of the activation record of the function executing at a given level. Level 0 is the current running function, whereas leveln+1 is the function that has called level n (except for tail calls, which do not count on the stack). When there are no errors, lua_getstack returns 1; when called with a level greater than the stack depth, it returns 0.
lua_getupvalue
[-0, +(0|1), –]
const char *lua_getupvalue (lua_State *L, int funcindex, int n);
Gets information about a closure's upvalue. (For Lua functions, upvalues are the external local variables that the function uses, and that are consequently included in its closure.)lua_getupvalue gets the index n of an upvalue, pushes the upvalue's value onto the stack, and returns its name. funcindex points to the closure in the stack. (Upvalues have no particular order, as they are active through the whole function. So, they are numbered in an arbitrary order.)
Returns NULL (and pushes nothing) when the index is greater than the number of upvalues. For C functions, this function uses the empty string "" as a name for all upvalues.
lua_Hook
typedef void (*lua_Hook) (lua_State *L, lua_Debug *ar);
Type for debugging hook functions.
Whenever a hook is called, its ar argument has its field event set to the specific event that triggered the hook. Lua identifies these events with the following constants: LUA_HOOKCALL,LUA_HOOKRET, LUA_HOOKTAILCALL, LUA_HOOKLINE, and LUA_HOOKCOUNT. Moreover, for line events, the field currentline is also set. To get the value of any other field in ar, the hook must call lua_getinfo.
For call events, event can be LUA_HOOKCALL, the normal value, or LUA_HOOKTAILCALL, for a tail call; in this case, there will be no corresponding return event.
While Lua is running a hook, it disables other calls to hooks. Therefore, if a hook calls back Lua to execute a function or a chunk, this execution occurs without any calls to hooks.
Hook functions cannot have continuations, that is, they cannot call lua_yieldk, lua_pcallk, or lua_callk with a non-null k.
Hook functions can yield under the following conditions: Only count and line events can yield and they cannot yield any value; to yield a hook function must finish its execution callinglua_yield with nresults equal to zero.
lua_sethook
[-0, +0, –]
int lua_sethook (lua_State *L, lua_Hook f, int mask, int count);
Sets the debugging hook function.
Argument f is the hook function. mask specifies on which events the hook will be called: it is formed by a bitwise or of the constants LUA_MASKCALL, LUA_MASKRET, LUA_MASKLINE, andLUA_MASKCOUNT. The count argument is only meaningful when the mask includes LUA_MASKCOUNT. For each event, the hook is called as explained below:
- The call hook: is called when the interpreter calls a function. The hook is called just after Lua enters the new function, before the function gets its arguments.
- The return hook: is called when the interpreter returns from a function. The hook is called just before Lua leaves the function. There is no standard way to access the values to be returned by the function.
- The line hook: is called when the interpreter is about to start the execution of a new line of code, or when it jumps back in the code (even to the same line). (This event only happens while Lua is executing a Lua function.)
- The count hook: is called after the interpreter executes every
countinstructions. (This event only happens while Lua is executing a Lua function.)
A hook is disabled by setting mask to zero.
lua_setlocal
[-(0|1), +0, –]
const char *lua_setlocal (lua_State *L, lua_Debug *ar, int n);
Sets the value of a local variable of a given activation record. Parameters ar and n are as in lua_getlocal (see lua_getlocal). lua_setlocal assigns the value at the top of the stack to the variable and returns its name. It also pops the value from the stack.
Returns NULL (and pops nothing) when the index is greater than the number of active local variables.
lua_setupvalue
[-(0|1), +0, –]
const char *lua_setupvalue (lua_State *L, int funcindex, int n);
Sets the value of a closure's upvalue. It assigns the value at the top of the stack to the upvalue and returns its name. It also pops the value from the stack. Parameters funcindex and n are as in the lua_getupvalue (see lua_getupvalue).
Returns NULL (and pops nothing) when the index is greater than the number of upvalues.
lua_upvalueid
[-0, +0, –]
void *lua_upvalueid (lua_State *L, int funcindex, int n);
Returns an unique identifier for the upvalue numbered n from the closure at index funcindex. Parameters funcindex and n are as in the lua_getupvalue (see lua_getupvalue) (butn cannot be greater than the number of upvalues).
These unique identifiers allow a program to check whether different closures share upvalues. Lua closures that share an upvalue (that is, that access a same external local variable) will return identical ids for those upvalue indices.
lua_upvaluejoin
[-0, +0, –]
void lua_upvaluejoin (lua_State *L, int funcindex1, int n1,
int funcindex2, int n2);
Make the n1-th upvalue of the Lua closure at index funcindex1 refer to the n2-th upvalue of the Lua closure at index funcindex2.
5 – The Auxiliary Library
The auxiliary library provides several convenient functions to interface C with Lua. While the basic API provides the primitive functions for all interactions between C and Lua, the auxiliary library provides higher-level functions for some common tasks.
All functions and types from the auxiliary library are defined in header file lauxlib.h and have a prefix luaL_.
All functions in the auxiliary library are built on top of the basic API, and so they provide nothing that cannot be done with that API. Nevertheless, the use of the auxiliary library ensures more consistency to your code.
Several functions in the auxiliary library use internally some extra stack slots. When a function in the auxiliary library uses less than five slots, it does not check the stack size; it simply assumes that there are enough slots.
Several functions in the auxiliary library are used to check C function arguments. Because the error message is formatted for arguments (e.g., "bad argument #1"), you should not use these functions for other stack values.
Functions called luaL_check* always throw an error if the check is not satisfied.
5.1 – Functions and Types
Here we list all functions and types from the auxiliary library in alphabetical order.
luaL_addchar
[-?, +?, e]
void luaL_addchar (luaL_Buffer *B, char c);
Adds the byte c to the buffer B (see luaL_Buffer).
luaL_addlstring
[-?, +?, e]
void luaL_addlstring (luaL_Buffer *B, const char *s, size_t l);
Adds the string pointed to by s with length l to the buffer B (see luaL_Buffer). The string can contain embedded zeros.
luaL_addsize
[-?, +?, e]
void luaL_addsize (luaL_Buffer *B, size_t n);
Adds to the buffer B (see luaL_Buffer) a string of length n previously copied to the buffer area (see luaL_prepbuffer).
luaL_addstring
[-?, +?, e]
void luaL_addstring (luaL_Buffer *B, const char *s);
Adds the zero-terminated string pointed to by s to the buffer B (see luaL_Buffer). The string cannot contain embedded zeros.
luaL_addvalue
[-1, +?, e]
void luaL_addvalue (luaL_Buffer *B);
Adds the value at the top of the stack to the buffer B (see luaL_Buffer). Pops the value.
This is the only function on string buffers that can (and must) be called with an extra element on the stack, which is the value to be added to the buffer.
luaL_argcheck
[-0, +0, v]
void luaL_argcheck (lua_State *L,
int cond,
int arg,
const char *extramsg);
Checks whether cond is true. If not, raises an error with a standard message.
luaL_argerror
[-0, +0, v]
int luaL_argerror (lua_State *L, int arg, const char *extramsg);
Raises an error with a standard message that includes extramsg as a comment.
This function never returns, but it is an idiom to use it in C functions as return luaL_argerror(args).
luaL_Buffer
typedef struct luaL_Buffer luaL_Buffer;
Type for a string buffer.
A string buffer allows C code to build Lua strings piecemeal. Its pattern of use is as follows:
- First declare a variable
bof typeluaL_Buffer. - Then initialize it with a call
luaL_buffinit(L, &b). - Then add string pieces to the buffer calling any of the
luaL_add*functions. - Finish by calling
luaL_pushresult(&b). This call leaves the final string on the top of the stack.
If you know beforehand the total size of the resulting string, you can use the buffer like this:
- First declare a variable
bof typeluaL_Buffer. - Then initialize it and preallocate a space of size
szwith a callluaL_buffinitsize(L, &b, sz). - Then copy the string into that space.
- Finish by calling
luaL_pushresultsize(&b, sz), whereszis the total size of the resulting string copied into that space.
During its normal operation, a string buffer uses a variable number of stack slots. So, while using a buffer, you cannot assume that you know where the top of the stack is. You can use the stack between successive calls to buffer operations as long as that use is balanced; that is, when you call a buffer operation, the stack is at the same level it was immediately after the previous buffer operation. (The only exception to this rule is luaL_addvalue.) After calling luaL_pushresult the stack is back to its level when the buffer was initialized, plus the final string on its top.
luaL_buffinit
[-0, +0, –]
void luaL_buffinit (lua_State *L, luaL_Buffer *B);
Initializes a buffer B. This function does not allocate any space; the buffer must be declared as a variable (see luaL_Buffer).
luaL_buffinitsize
[-?, +?, e]
char *luaL_buffinitsize (lua_State *L, luaL_Buffer *B, size_t sz);
Equivalent to the sequence luaL_buffinit, luaL_prepbuffsize.
luaL_callmeta
[-0, +(0|1), e]
int luaL_callmeta (lua_State *L, int obj, const char *e);
Calls a metamethod.
If the object at index obj has a metatable and this metatable has a field e, this function calls this field passing the object as its only argument. In this case this function returns true and pushes onto the stack the value returned by the call. If there is no metatable or no metamethod, this function returns false (without pushing any value on the stack).
luaL_checkany
[-0, +0, v]
void luaL_checkany (lua_State *L, int arg);
Checks whether the function has an argument of any type (including nil) at position arg.
luaL_checkint
[-0, +0, v]
int luaL_checkint (lua_State *L, int arg);
Checks whether the function argument arg is a number and returns this number cast to an int.
luaL_checkinteger
[-0, +0, v]
lua_Integer luaL_checkinteger (lua_State *L, int arg);
Checks whether the function argument arg is a number and returns this number cast to a lua_Integer.
luaL_checklong
[-0, +0, v]
long luaL_checklong (lua_State *L, int arg);
Checks whether the function argument arg is a number and returns this number cast to a long.
luaL_checklstring
[-0, +0, v]
const char *luaL_checklstring (lua_State *L, int arg, size_t *l);
Checks whether the function argument arg is a string and returns this string; if l is not NULL fills *l with the string's length.
This function uses lua_tolstring to get its result, so all conversions and caveats of that function apply here.
luaL_checknumber
[-0, +0, v]
lua_Number luaL_checknumber (lua_State *L, int arg);
Checks whether the function argument arg is a number and returns this number.
luaL_checkoption
[-0, +0, v]
int luaL_checkoption (lua_State *L,
int arg,
const char *def,
const char *const lst[]);
Checks whether the function argument arg is a string and searches for this string in the array lst (which must be NULL-terminated). Returns the index in the array where the string was found. Raises an error if the argument is not a string or if the string cannot be found.
If def is not NULL, the function uses def as a default value when there is no argument arg or when this argument is nil.
This is a useful function for mapping strings to C enums. (The usual convention in Lua libraries is to use strings instead of numbers to select options.)
luaL_checkstack
[-0, +0, v]
void luaL_checkstack (lua_State *L, int sz, const char *msg);
Grows the stack size to top + sz elements, raising an error if the stack cannot grow to that size. msg is an additional text to go into the error message (or NULL for no additional text).
luaL_checkstring
[-0, +0, v]
const char *luaL_checkstring (lua_State *L, int arg);
Checks whether the function argument arg is a string and returns this string.
This function uses lua_tolstring to get its result, so all conversions and caveats of that function apply here.
luaL_checktype
[-0, +0, v]
void luaL_checktype (lua_State *L, int arg, int t);
Checks whether the function argument arg has type t. See lua_type for the encoding of types for t.
luaL_checkudata
[-0, +0, v]
void *luaL_checkudata (lua_State *L, int arg, const char *tname);
Checks whether the function argument arg is a userdata of the type tname (see luaL_newmetatable) and returns the userdata address (see lua_touserdata).
luaL_checkunsigned
[-0, +0, v]
lua_Unsigned luaL_checkunsigned (lua_State *L, int arg);
Checks whether the function argument arg is a number and returns this number cast to a lua_Unsigned.
luaL_checkversion
[-0, +0, –]
void luaL_checkversion (lua_State *L);
Checks whether the core running the call, the core that created the Lua state, and the code making the call are all using the same version of Lua. Also checks whether the core running the call and the core that created the Lua state are using the same address space.
luaL_dofile
[-0, +?, e]
int luaL_dofile (lua_State *L, const char *filename);
Loads and runs the given file. It is defined as the following macro:
(luaL_loadfile(L, filename) || lua_pcall(L, 0, LUA_MULTRET, 0))
It returns false if there are no errors or true in case of errors.
luaL_dostring
[-0, +?, –]
int luaL_dostring (lua_State *L, const char *str);
Loads and runs the given string. It is defined as the following macro:
(luaL_loadstring(L, str) || lua_pcall(L, 0, LUA_MULTRET, 0))
It returns false if there are no errors or true in case of errors.
luaL_error
[-0, +0, v]
int luaL_error (lua_State *L, const char *fmt, ...);
Raises an error. The error message format is given by fmt plus any extra arguments, following the same rules of lua_pushfstring. It also adds at the beginning of the message the file name and the line number where the error occurred, if this information is available.
This function never returns, but it is an idiom to use it in C functions as return luaL_error(args).
luaL_execresult
[-0, +3, e]
int luaL_execresult (lua_State *L, int stat);
This function produces the return values for process-related functions in the standard library (os.execute and io.close).
luaL_fileresult
[-0, +(1|3), e]
int luaL_fileresult (lua_State *L, int stat, const char *fname);
This function produces the return values for file-related functions in the standard library (io.open, os.rename, file:seek, etc.).
luaL_getmetafield
[-0, +(0|1), e]
int luaL_getmetafield (lua_State *L, int obj, const char *e);
Pushes onto the stack the field e from the metatable of the object at index obj. If the object does not have a metatable, or if the metatable does not have this field, returns false and pushes nothing.
luaL_getmetatable
[-0, +1, –]
void luaL_getmetatable (lua_State *L, const char *tname);
Pushes onto the stack the metatable associated with name tname in the registry (see luaL_newmetatable).
luaL_getsubtable
[-0, +1, e]
int luaL_getsubtable (lua_State *L, int idx, const char *fname);
Ensures that the value t[fname], where t is the value at index idx, is a table, and pushes that table onto the stack. Returns true if it finds a previous table there and false if it creates a new table.
luaL_gsub
[-0, +1, e]
const char *luaL_gsub (lua_State *L,
const char *s,
const char *p,
const char *r);
Creates a copy of string s by replacing any occurrence of the string p with the string r. Pushes the resulting string on the stack and returns it.
luaL_len
[-0, +0, e]
int luaL_len (lua_State *L, int index);
Returns the "length" of the value at the given index as a number; it is equivalent to the '#' operator in Lua (see §3.4.6). Raises an error if the result of the operation is not a number. (This case only can happen through metamethods.)
luaL_loadbuffer
[-0, +1, –]
int luaL_loadbuffer (lua_State *L,
const char *buff,
size_t sz,
const char *name);
Equivalent to luaL_loadbufferx with mode equal to NULL.
luaL_loadbufferx
[-0, +1, –]
int luaL_loadbufferx (lua_State *L,
const char *buff,
size_t sz,
const char *name,
const char *mode);
Loads a buffer as a Lua chunk. This function uses lua_load to load the chunk in the buffer pointed to by buff with size sz.
This function returns the same results as lua_load. name is the chunk name, used for debug information and error messages. The string mode works as in function lua_load.
luaL_loadfile
[-0, +1, e]
int luaL_loadfile (lua_State *L, const char *filename);
Equivalent to luaL_loadfilex with mode equal to NULL.
luaL_loadfilex
[-0, +1, e]
int luaL_loadfilex (lua_State *L, const char *filename,
const char *mode);
Loads a file as a Lua chunk. This function uses lua_load to load the chunk in the file named filename. If filename is NULL, then it loads from the standard input. The first line in the file is ignored if it starts with a #.
The string mode works as in function lua_load.
This function returns the same results as lua_load, but it has an extra error code LUA_ERRFILE if it cannot open/read the file or the file has a wrong mode.
As lua_load, this function only loads the chunk; it does not run it.
luaL_loadstring
[-0, +1, –]
int luaL_loadstring (lua_State *L, const char *s);
Loads a string as a Lua chunk. This function uses lua_load to load the chunk in the zero-terminated string s.
This function returns the same results as lua_load.
Also as lua_load, this function only loads the chunk; it does not run it.
luaL_newlib
[-0, +1, e]
void luaL_newlib (lua_State *L, const luaL_Reg *l);
Creates a new table and registers there the functions in list l. It is implemented as the following macro:
(luaL_newlibtable(L,l), luaL_setfuncs(L,l,0))
luaL_newlibtable
[-0, +1, e]
void luaL_newlibtable (lua_State *L, const luaL_Reg l[]);
Creates a new table with a size optimized to store all entries in the array l (but does not actually store them). It is intended to be used in conjunction with luaL_setfuncs (seeluaL_newlib).
It is implemented as a macro. The array l must be the actual array, not a pointer to it.
luaL_newmetatable
[-0, +1, e]
int luaL_newmetatable (lua_State *L, const char *tname);
If the registry already has the key tname, returns 0. Otherwise, creates a new table to be used as a metatable for userdata, adds it to the registry with key tname, and returns 1.
In both cases pushes onto the stack the final value associated with tname in the registry.
luaL_newstate
[-0, +0, –]
lua_State *luaL_newstate (void);
Creates a new Lua state. It calls lua_newstate with an allocator based on the standard C realloc function and then sets a panic function (see §4.6) that prints an error message to the standard error output in case of fatal errors.
Returns the new state, or NULL if there is a memory allocation error.
luaL_openlibs
[-0, +0, e]
void luaL_openlibs (lua_State *L);
Opens all standard Lua libraries into the given state.
luaL_optint
[-0, +0, v]
int luaL_optint (lua_State *L, int arg, int d);
If the function argument arg is a number, returns this number cast to an int. If this argument is absent or is nil, returns d. Otherwise, raises an error.
luaL_optinteger
[-0, +0, v]
lua_Integer luaL_optinteger (lua_State *L,
int arg,
lua_Integer d);
If the function argument arg is a number, returns this number cast to a lua_Integer. If this argument is absent or is nil, returns d. Otherwise, raises an error.
luaL_optlong
[-0, +0, v]
long luaL_optlong (lua_State *L, int arg, long d);
If the function argument arg is a number, returns this number cast to a long. If this argument is absent or is nil, returns d. Otherwise, raises an error.
luaL_optlstring
[-0, +0, v]
const char *luaL_optlstring (lua_State *L,
int arg,
const char *d,
size_t *l);
If the function argument arg is a string, returns this string. If this argument is absent or is nil, returns d. Otherwise, raises an error.
If l is not NULL, fills the position *l with the result's length.
luaL_optnumber
[-0, +0, v]
lua_Number luaL_optnumber (lua_State *L, int arg, lua_Number d);
If the function argument arg is a number, returns this number. If this argument is absent or is nil, returns d. Otherwise, raises an error.
luaL_optstring
[-0, +0, v]
const char *luaL_optstring (lua_State *L,
int arg,
const char *d);
If the function argument arg is a string, returns this string. If this argument is absent or is nil, returns d. Otherwise, raises an error.
luaL_optunsigned
[-0, +0, v]
lua_Unsigned luaL_optunsigned (lua_State *L,
int arg,
lua_Unsigned u);
If the function argument arg is a number, returns this number cast to a lua_Unsigned. If this argument is absent or is nil, returns u. Otherwise, raises an error.
luaL_prepbuffer
[-?, +?, e]
char *luaL_prepbuffer (luaL_Buffer *B);
Equivalent to luaL_prepbuffsize with the predefined size LUAL_BUFFERSIZE.
luaL_prepbuffsize
[-?, +?, e]
char *luaL_prepbuffsize (luaL_Buffer *B, size_t sz);
Returns an address to a space of size sz where you can copy a string to be added to buffer B (see luaL_Buffer). After copying the string into this space you must call luaL_addsize with the size of the string to actually add it to the buffer.
luaL_pushresult
[-?, +1, e]
void luaL_pushresult (luaL_Buffer *B);
Finishes the use of buffer B leaving the final string on the top of the stack.
luaL_pushresultsize
[-?, +1, e]
void luaL_pushresultsize (luaL_Buffer *B, size_t sz);
Equivalent to the sequence luaL_addsize, luaL_pushresult.
luaL_ref
[-1, +0, e]
int luaL_ref (lua_State *L, int t);
Creates and returns a reference, in the table at index t, for the object at the top of the stack (and pops the object).
A reference is a unique integer key. As long as you do not manually add integer keys into table t, luaL_ref ensures the uniqueness of the key it returns. You can retrieve an object referred by reference r by calling lua_rawgeti(L, t, r). Function luaL_unref frees a reference and its associated object.
If the object at the top of the stack is nil, luaL_ref returns the constant LUA_REFNIL. The constant LUA_NOREF is guaranteed to be different from any reference returned by luaL_ref.
luaL_Reg
typedef struct luaL_Reg {
const char *name;
lua_CFunction func;
} luaL_Reg;
Type for arrays of functions to be registered by luaL_setfuncs. name is the function name and func is a pointer to the function. Any array of luaL_Reg must end with an sentinel entry in which both name and func are NULL.
luaL_requiref
[-0, +1, e]
void luaL_requiref (lua_State *L, const char *modname,
lua_CFunction openf, int glb);
Calls function openf with string modname as an argument and sets the call result in package.loaded[modname], as if that function has been called through require.
If glb is true, also stores the result into global modname.
Leaves a copy of that result on the stack.
luaL_setfuncs
[-nup, +0, e]
void luaL_setfuncs (lua_State *L, const luaL_Reg *l, int nup);
Registers all functions in the array l (see luaL_Reg) into the table on the top of the stack (below optional upvalues, see next).
When nup is not zero, all functions are created sharing nup upvalues, which must be previously pushed on the stack on top of the library table. These values are popped from the stack after the registration.
luaL_setmetatable
[-0, +0, –]
void luaL_setmetatable (lua_State *L, const char *tname);
Sets the metatable of the object at the top of the stack as the metatable associated with name tname in the registry (see luaL_newmetatable).
luaL_testudata
[-0, +0, e]
void *luaL_testudata (lua_State *L, int arg, const char *tname);
This function works like luaL_checkudata, except that, when the test fails, it returns NULL instead of throwing an error.
luaL_tolstring
[-0, +1, e]
const char *luaL_tolstring (lua_State *L, int idx, size_t *len);
Converts any Lua value at the given index to a C string in a reasonable format. The resulting string is pushed onto the stack and also returned by the function. If len is not NULL, the function also sets *len with the string length.
If the value has a metatable with a "__tostring" field, then luaL_tolstring calls the corresponding metamethod with the value as argument, and uses the result of the call as its result.
luaL_traceback
[-0, +1, e]
void luaL_traceback (lua_State *L, lua_State *L1, const char *msg,
int level);
Creates and pushes a traceback of the stack L1. If msg is not NULL it is appended at the beginning of the traceback. The level parameter tells at which level to start the traceback.
luaL_typename
[-0, +0, –]
const char *luaL_typename (lua_State *L, int index);
Returns the name of the type of the value at the given index.
luaL_unref
[-0, +0, –]
void luaL_unref (lua_State *L, int t, int ref);
Releases reference ref from the table at index t (see luaL_ref). The entry is removed from the table, so that the referred object can be collected. The reference ref is also freed to be used again.
If ref is LUA_NOREF or LUA_REFNIL, luaL_unref does nothing.
luaL_where
[-0, +1, e]
void luaL_where (lua_State *L, int lvl);
Pushes onto the stack a string identifying the current position of the control at level lvl in the call stack. Typically this string has the following format:
chunkname:currentline:
Level 0 is the running function, level 1 is the function that called the running function, etc.
This function is used to build a prefix for error messages.
6 – Standard Libraries
The standard Lua libraries provide useful functions that are implemented directly through the C API. Some of these functions provide essential services to the language (e.g., type andgetmetatable); others provide access to "outside" services (e.g., I/O); and others could be implemented in Lua itself, but are quite useful or have critical performance requirements that deserve an implementation in C (e.g., table.sort).
All libraries are implemented through the official C API and are provided as separate C modules. Currently, Lua has the following standard libraries:
- basic library (§6.1);
- coroutine library (§6.2);
- package library (§6.3);
- string manipulation (§6.4);
- table manipulation (§6.5);
- mathematical functions (§6.6) (sin, log, etc.);
- bitwise operations (§6.7);
- input and output (§6.8);
- operating system facilities (§6.9);
- debug facilities (§6.10).
Except for the basic and the package libraries, each library provides all its functions as fields of a global table or as methods of its objects.
To have access to these libraries, the C host program should call the luaL_openlibs function, which opens all standard libraries. Alternatively, the host program can open them individually by using luaL_requiref to call luaopen_base (for the basic library), luaopen_package (for the package library), luaopen_coroutine (for the coroutine library), luaopen_string(for the string library), luaopen_table (for the table library), luaopen_math (for the mathematical library), luaopen_bit32 (for the bit library), luaopen_io (for the I/O library),luaopen_os (for the Operating System library), and luaopen_debug (for the debug library). These functions are declared in lualib.h.
6.1 – Basic Functions
The basic library provides core functions to Lua. If you do not include this library in your application, you should check carefully whether you need to provide implementations for some of its facilities.
assert (v [, message])
Issues an error when the value of its argument v is false (i.e., nil or false); otherwise, returns all its arguments. message is an error message; when absent, it defaults to "assertion failed!"
collectgarbage ([opt [, arg]])
This function is a generic interface to the garbage collector. It performs different functions according to its first argument, opt:
- "
collect": performs a full garbage-collection cycle. This is the default option. - "
stop": stops automatic execution of the garbage collector. The collector will run only when explicitly invoked, until a call to restart it. - "
restart": restarts automatic execution of the garbage collector. - "
count": returns the total memory in use by Lua (in Kbytes) and a second value with the total memory in bytes modulo 1024. The first value has a fractional part, so the following equality is always true:k, b = collectgarbage("count") assert(k*1024 == math.floor(k)*1024 + b)(The second result is useful when Lua is compiled with a non floating-point type for numbers.)
- "
step": performs a garbage-collection step. The step "size" is controlled byarg(larger values mean more steps) in a non-specified way. If you want to control the step size you must experimentally tune the value ofarg. Returns true if the step finished a collection cycle. - "
setpause": setsargas the new value for the pause of the collector (see §2.5). Returns the previous value for pause. - "
setstepmul": setsargas the new value for the step multiplier of the collector (see §2.5). Returns the previous value for step. - "
isrunning": returns a boolean that tells whether the collector is running (i.e., not stopped). - "
generational": changes the collector to generational mode. This is an experimental feature (see §2.5). - "
incremental": changes the collector to incremental mode. This is the default mode.
dofile ([filename])
Opens the named file and executes its contents as a Lua chunk. When called without arguments, dofile executes the contents of the standard input (stdin). Returns all values returned by the chunk. In case of errors, dofile propagates the error to its caller (that is, dofile does not run in protected mode).
error (message [, level])
Terminates the last protected function called and returns message as the error message. Function error never returns.
Usually, error adds some information about the error position at the beginning of the message, if the message is a string. The level argument specifies how to get the error position. With level 1 (the default), the error position is where the error function was called. Level 2 points the error to where the function that called error was called; and so on. Passing a level 0 avoids the addition of error position information to the message.
_G
A global variable (not a function) that holds the global environment (see §2.2). Lua itself does not use this variable; changing its value does not affect any environment, nor vice-versa.
getmetatable (object)
If object does not have a metatable, returns nil. Otherwise, if the object's metatable has a "__metatable" field, returns the associated value. Otherwise, returns the metatable of the given object.
ipairs (t)
If t has a metamethod __ipairs, calls it with t as argument and returns the first three results from the call.
Otherwise, returns three values: an iterator function, the table t, and 0, so that the construction
for i,v in ipairs(t) do body end
will iterate over the pairs (1,t[1]), (2,t[2]), ..., up to the first integer key absent from the table.
load (ld [, source [, mode [, env]]])
Loads a chunk.
If ld is a string, the chunk is this string. If ld is a function, load calls it repeatedly to get the chunk pieces. Each call to ld must return a string that concatenates with previous results. A return of an empty string, nil, or no value signals the end of the chunk.
If there are no syntactic errors, returns the compiled chunk as a function; otherwise, returns nil plus the error message.
If the resulting function has upvalues, the first upvalue is set to the value of env, if that parameter is given, or to the value of the global environment. (When you load a main chunk, the resulting function will always have exactly one upvalue, the _ENV variable (see §2.2). When you load a binary chunk created from a function (see string.dump), the resulting function can have arbitrary upvalues.)
source is used as the source of the chunk for error messages and debug information (see §4.9). When absent, it defaults to ld, if ld is a string, or to "=(load)" otherwise.
The string mode controls whether the chunk can be text or binary (that is, a precompiled chunk). It may be the string "b" (only binary chunks), "t" (only text chunks), or "bt" (both binary and text). The default is "bt".
loadfile ([filename [, mode [, env]]])
Similar to load, but gets the chunk from file filename or from the standard input, if no file name is given.
next (table [, index])
Allows a program to traverse all fields of a table. Its first argument is a table and its second argument is an index in this table. next returns the next index of the table and its associated value. When called with nil as its second argument, next returns an initial index and its associated value. When called with the last index, or with nil in an empty table, next returns nil. If the second argument is absent, then it is interpreted as nil. In particular, you can use next(t) to check whether a table is empty.
The order in which the indices are enumerated is not specified, even for numeric indices. (To traverse a table in numeric order, use a numerical for.)
The behavior of next is undefined if, during the traversal, you assign any value to a non-existent field in the table. You may however modify existing fields. In particular, you may clear existing fields.
pairs (t)
If t has a metamethod __pairs, calls it with t as argument and returns the first three results from the call.
Otherwise, returns three values: the next function, the table t, and nil, so that the construction
for k,v in pairs(t) do body end
will iterate over all key–value pairs of table t.
See function next for the caveats of modifying the table during its traversal.
pcall (f [, arg1, ···])
Calls function f with the given arguments in protected mode. This means that any error inside f is not propagated; instead, pcall catches the error and returns a status code. Its first result is the status code (a boolean), which is true if the call succeeds without errors. In such case, pcall also returns all results from the call, after this first result. In case of any error, pcall returnsfalse plus the error message.
print (···)
Receives any number of arguments and prints their values to stdout, using the tostring function to convert each argument to a string. print is not intended for formatted output, but only as a quick way to show a value, for instance for debugging. For complete control over the output, use string.format and io.write.
rawequal (v1, v2)
Checks whether v1 is equal to v2, without invoking any metamethod. Returns a boolean.
rawget (table, index)
Gets the real value of table[index], without invoking any metamethod. table must be a table; index may be any value.
rawlen (v)
Returns the length of the object v, which must be a table or a string, without invoking any metamethod. Returns an integer number.
rawset (table, index, value)
Sets the real value of table[index] to value, without invoking any metamethod. table must be a table, index any value different from nil and NaN, and value any Lua value.
This function returns table.
select (index, ···)
If index is a number, returns all arguments after argument number index; a negative number indexes from the end (-1 is the last argument). Otherwise, index must be the string "#", andselect returns the total number of extra arguments it received.
setmetatable (table, metatable)
Sets the metatable for the given table. (You cannot change the metatable of other types from Lua, only from C.) If metatable is nil, removes the metatable of the given table. If the original metatable has a "__metatable" field, raises an error.
This function returns table.
tonumber (e [, base])
When called with no base, tonumber tries to convert its argument to a number. If the argument is already a number or a string convertible to a number (see §3.4.2), then tonumber returns this number; otherwise, it returns nil.
When called with base, then e should be a string to be interpreted as an integer numeral in that base. The base may be any integer between 2 and 36, inclusive. In bases above 10, the letter 'A' (in either upper or lower case) represents 10, 'B' represents 11, and so forth, with 'Z' representing 35. If the string e is not a valid numeral in the given base, the function returns nil.
tostring (v)
Receives a value of any type and converts it to a string in a reasonable format. (For complete control of how numbers are converted, use string.format.)
If the metatable of v has a "__tostring" field, then tostring calls the corresponding value with v as argument, and uses the result of the call as its result.
type (v)
Returns the type of its only argument, coded as a string. The possible results of this function are "nil" (a string, not the value nil), "number", "string", "boolean", "table", "function", "thread", and "userdata".
_VERSION
A global variable (not a function) that holds a string containing the current interpreter version. The current contents of this variable is "Lua 5.2".
xpcall (f, msgh [, arg1, ···])
This function is similar to pcall, except that it sets a new message handler msgh.
6.2 – Coroutine Manipulation
The operations related to coroutines comprise a sub-library of the basic library and come inside the table coroutine. See §2.6 for a general description of coroutines.
coroutine.create (f)
Creates a new coroutine, with body f. f must be a Lua function. Returns this new coroutine, an object with type "thread".
coroutine.resume (co [, val1, ···])
Starts or continues the execution of coroutine co. The first time you resume a coroutine, it starts running its body. The values val1, ... are passed as the arguments to the body function. If the coroutine has yielded, resume restarts it; the values val1, ... are passed as the results from the yield.
If the coroutine runs without any errors, resume returns true plus any values passed to yield (if the coroutine yields) or any values returned by the body function (if the coroutine terminates). If there is any error, resume returns false plus the error message.
coroutine.running ()
Returns the running coroutine plus a boolean, true when the running coroutine is the main one.
coroutine.status (co)
Returns the status of coroutine co, as a string: "running", if the coroutine is running (that is, it called status); "suspended", if the coroutine is suspended in a call to yield, or if it has not started running yet; "normal" if the coroutine is active but not running (that is, it has resumed another coroutine); and "dead" if the coroutine has finished its body function, or if it has stopped with an error.
coroutine.wrap (f)
Creates a new coroutine, with body f. f must be a Lua function. Returns a function that resumes the coroutine each time it is called. Any arguments passed to the function behave as the extra arguments to resume. Returns the same values returned by resume, except the first boolean. In case of error, propagates the error.
coroutine.yield (···)
Suspends the execution of the calling coroutine. Any arguments to yield are passed as extra results to resume.
6.3 – Modules
The package library provides basic facilities for loading modules in Lua. It exports one function directly in the global environment: require. Everything else is exported in a table package.
require (modname)
Loads the given module. The function starts by looking into the package.loaded table to determine whether modname is already loaded. If it is, then require returns the value stored atpackage.loaded[modname]. Otherwise, it tries to find a loader for the module.
To find a loader, require is guided by the package.searchers sequence. By changing this sequence, we can change how require looks for a module. The following explanation is based on the default configuration for package.searchers.
First require queries package.preload[modname]. If it has a value, this value (which should be a function) is the loader. Otherwise require searches for a Lua loader using the path stored in package.path. If that also fails, it searches for a C loader using the path stored in package.cpath. If that also fails, it tries an all-in-one loader (see package.searchers).
Once a loader is found, require calls the loader with two arguments: modname and an extra value dependent on how it got the loader. (If the loader came from a file, this extra value is the file name.) If the loader returns any non-nil value, require assigns the returned value to package.loaded[modname]. If the loader does not return a non-nil value and has not assigned any value to package.loaded[modname], then require assigns true to this entry. In any case, require returns the final value of package.loaded[modname].
If there is any error loading or running the module, or if it cannot find any loader for the module, then require raises an error.
package.config
A string describing some compile-time configurations for packages. This string is a sequence of lines:
- The first line is the directory separator string. Default is '
\' for Windows and '/' for all other systems. - The second line is the character that separates templates in a path. Default is '
;'. - The third line is the string that marks the substitution points in a template. Default is '
?'. - The fourth line is a string that, in a path in Windows, is replaced by the executable's directory. Default is '
!'. - The fifth line is a mark to ignore all text before it when building the
luaopen_function name. Default is '-'.
package.cpath
The path used by require to search for a C loader.
Lua initializes the C path package.cpath in the same way it initializes the Lua path package.path, using the environment variable LUA_CPATH_5_2 or the environment variableLUA_CPATH or a default path defined in luaconf.h.
package.loaded
A table used by require to control which modules are already loaded. When you require a module modname and package.loaded[modname] is not false, require simply returns the value stored there.
This variable is only a reference to the real table; assignments to this variable do not change the table used by require.
package.loadlib (libname, funcname)
Dynamically links the host program with the C library libname.
If funcname is "*", then it only links with the library, making the symbols exported by the library available to other dynamically linked libraries. Otherwise, it looks for a function funcnameinside the library and returns this function as a C function. So, funcname must follow the lua_CFunction prototype (see lua_CFunction).
This is a low-level function. It completely bypasses the package and module system. Unlike require, it does not perform any path searching and does not automatically adds extensions.libname must be the complete file name of the C library, including if necessary a path and an extension. funcname must be the exact name exported by the C library (which may depend on the C compiler and linker used).
This function is not supported by Standard C. As such, it is only available on some platforms (Windows, Linux, Mac OS X, Solaris, BSD, plus other Unix systems that support the dlfcnstandard).
package.path
The path used by require to search for a Lua loader.
At start-up, Lua initializes this variable with the value of the environment variable LUA_PATH_5_2 or the environment variable LUA_PATH or with a default path defined in luaconf.h, if those environment variables are not defined. Any ";;" in the value of the environment variable is replaced by the default path.
package.preload
A table to store loaders for specific modules (see require).
This variable is only a reference to the real table; assignments to this variable do not change the table used by require.
package.searchers
A table used by require to control how to load modules.
Each entry in this table is a searcher function. When looking for a module, require calls each of these searchers in ascending order, with the module name (the argument given to require) as its sole parameter. The function can return another function (the module loader) plus an extra value that will be passed to that loader, or a string explaining why it did not find that module (ornil if it has nothing to say).
Lua initializes this table with four searcher functions.
The first searcher simply looks for a loader in the package.preload table.
The second searcher looks for a loader as a Lua library, using the path stored at package.path. The search is done as described in function package.searchpath.
The third searcher looks for a loader as a C library, using the path given by the variable package.cpath. Again, the search is done as described in function package.searchpath. For instance, if the C path is the string
"./?.so;./?.dll;/usr/local/?/init.so"
the searcher for module foo will try to open the files ./foo.so, ./foo.dll, and /usr/local/foo/init.so, in that order. Once it finds a C library, this searcher first uses a dynamic link facility to link the application with the library. Then it tries to find a C function inside the library to be used as the loader. The name of this C function is the string "luaopen_" concatenated with a copy of the module name where each dot is replaced by an underscore. Moreover, if the module name has a hyphen, its prefix up to (and including) the first hyphen is removed. For instance, if the module name is a.v1-b.c, the function name will be luaopen_b_c.
The fourth searcher tries an all-in-one loader. It searches the C path for a library for the root name of the given module. For instance, when requiring a.b.c, it will search for a C library for a. If found, it looks into it for an open function for the submodule; in our example, that would be luaopen_a_b_c. With this facility, a package can pack several C submodules into one single library, with each submodule keeping its original open function.
All searchers except the first one (preload) return as the extra value the file name where the module was found, as returned by package.searchpath. The first searcher returns no extra value.
package.searchpath (name, path [, sep [, rep]])
Searches for the given name in the given path.
A path is a string containing a sequence of templates separated by semicolons. For each template, the function replaces each interrogation mark (if any) in the template with a copy of namewherein all occurrences of sep (a dot, by default) were replaced by rep (the system's directory separator, by default), and then tries to open the resulting file name.
For instance, if the path is the string
"./?.lua;./?.lc;/usr/local/?/init.lua"
the search for the name foo.a will try to open the files ./foo/a.lua, ./foo/a.lc, and /usr/local/foo/a/init.lua, in that order.
Returns the resulting name of the first file that it can open in read mode (after closing the file), or nil plus an error message if none succeeds. (This error message lists all file names it tried to open.)
6.4 – String Manipulation
This library provides generic functions for string manipulation, such as finding and extracting substrings, and pattern matching. When indexing a string in Lua, the first character is at position 1 (not at 0, as in C). Indices are allowed to be negative and are interpreted as indexing backwards, from the end of the string. Thus, the last character is at position -1, and so on.
The string library provides all its functions inside the table string. It also sets a metatable for strings where the __index field points to the string table. Therefore, you can use the string functions in object-oriented style. For instance, string.byte(s,i) can be written as s:byte(i).
The string library assumes one-byte character encodings.
string.byte (s [, i [, j]])
Returns the internal numerical codes of the characters s[i], s[i+1], ..., s[j]. The default value for i is 1; the default value for j is i. These indices are corrected following the same rules of function string.sub.
Numerical codes are not necessarily portable across platforms.
string.char (···)
Receives zero or more integers. Returns a string with length equal to the number of arguments, in which each character has the internal numerical code equal to its corresponding argument.
Numerical codes are not necessarily portable across platforms.
string.dump (function)
Returns a string containing a binary representation of the given function, so that a later load on this string returns a copy of the function (but with new upvalues).
string.find (s, pattern [, init [, plain]])
Looks for the first match of pattern in the string s. If it finds a match, then find returns the indices of s where this occurrence starts and ends; otherwise, it returns nil. A third, optional numerical argument init specifies where to start the search; its default value is 1 and can be negative. A value of true as a fourth, optional argument plain turns off the pattern matching facilities, so the function does a plain "find substring" operation, with no characters in pattern being considered magic. Note that if plain is given, then init must be given as well.
If the pattern has captures, then in a successful match the captured values are also returned, after the two indices.
string.format (formatstring, ···)
Returns a formatted version of its variable number of arguments following the description given in its first argument (which must be a string). The format string follows the same rules as the ANSI C function sprintf. The only differences are that the options/modifiers *, h, L, l, n, and p are not supported and that there is an extra option, q. The q option formats a string between double quotes, using escape sequences when necessary to ensure that it can safely be read back by the Lua interpreter. For instance, the call
string.format('%q', 'a string with "quotes" and \n new line')
may produce the string:
"a string with \"quotes\" and \
new line"
Options A and a (when available), E, e, f, G, and g all expect a number as argument. Options c, d, i, o, u, X, and x also expect a number, but the range of that number may be limited by the underlying C implementation. For options o, u, X, and x, the number cannot be negative. Option q expects a string; option s expects a string without embedded zeros. If the argument to option sis not a string, it is converted to one following the same rules of tostring.
string.gmatch (s, pattern)
Returns an iterator function that, each time it is called, returns the next captures from pattern over the string s. If pattern specifies no captures, then the whole match is produced in each call.
As an example, the following loop will iterate over all the words from string s, printing one per line:
s = "hello world from Lua"
for w in string.gmatch(s, "%a+") do
print(w)
end
The next example collects all pairs key=value from the given string into a table:
t = {}
s = "from=world, to=Lua"
for k, v in string.gmatch(s, "(%w+)=(%w+)") do
t[k] = v
end
For this function, a caret '^' at the start of a pattern does not work as an anchor, as this would prevent the iteration.
string.gsub (s, pattern, repl [, n])
Returns a copy of s in which all (or the first n, if given) occurrences of the pattern have been replaced by a replacement string specified by repl, which can be a string, a table, or a function.gsub also returns, as its second value, the total number of matches that occurred. The name gsub comes from Global SUBstitution.
If repl is a string, then its value is used for replacement. The character % works as an escape character: any sequence in repl of the form %d, with d between 1 and 9, stands for the value of the d-th captured substring. The sequence %0 stands for the whole match. The sequence %% stands for a single %.
If repl is a table, then the table is queried for every match, using the first capture as the key.
If repl is a function, then this function is called every time a match occurs, with all captured substrings passed as arguments, in order.
In any case, if the pattern specifies no captures, then it behaves as if the whole pattern was inside a capture.
If the value returned by the table query or by the function call is a string or a number, then it is used as the replacement string; otherwise, if it is false or nil, then there is no replacement (that is, the original match is kept in the string).
Here are some examples:
x = string.gsub("hello world", "(%w+)", "%1 %1")
--> x="hello hello world world"
x = string.gsub("hello world", "%w+", "%0 %0", 1)
--> x="hello hello world"
x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1")
--> x="world hello Lua from"
x = string.gsub("home = $HOME, user = $USER", "%$(%w+)", os.getenv)
--> x="home = /home/roberto, user = roberto"
x = string.gsub("4+5 = $return 4+5$", "%$(.-)%$", function (s)
return load(s)()
end)
--> x="4+5 = 9"
local t = {name="lua", version="5.2"}
x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t)
--> x="lua-5.2.tar.gz"
string.len (s)
Receives a string and returns its length. The empty string "" has length 0. Embedded zeros are counted, so "a\000bc\000" has length 5.
string.lower (s)
Receives a string and returns a copy of this string with all uppercase letters changed to lowercase. All other characters are left unchanged. The definition of what an uppercase letter is depends on the current locale.
string.match (s, pattern [, init])
Looks for the first match of pattern in the string s. If it finds one, then match returns the captures from the pattern; otherwise it returns nil. If pattern specifies no captures, then the whole match is returned. A third, optional numerical argument init specifies where to start the search; its default value is 1 and can be negative.
string.rep (s, n [, sep])
Returns a string that is the concatenation of n copies of the string s separated by the string sep. The default value for sep is the empty string (that is, no separator).
string.reverse (s)
Returns a string that is the string s reversed.
string.sub (s, i [, j])
Returns the substring of s that starts at i and continues until j; i and j can be negative. If j is absent, then it is assumed to be equal to -1 (which is the same as the string length). In particular, the call string.sub(s,1,j) returns a prefix of s with length j, and string.sub(s, -i) returns a suffix of s with length i.
If, after the translation of negative indices, i is less than 1, it is corrected to 1. If j is greater than the string length, it is corrected to that length. If, after these corrections, i is greater than j, the function returns the empty string.
string.upper (s)
Receives a string and returns a copy of this string with all lowercase letters changed to uppercase. All other characters are left unchanged. The definition of what a lowercase letter is depends on the current locale.
6.4.1 – Patterns
Character Class:
A character class is used to represent a set of characters. The following combinations are allowed in describing a character class:
- x: (where x is not one of the magic characters
^$()%.[]*+-?) represents the character x itself. .: (a dot) represents all characters.%a: represents all letters.%c: represents all control characters.%d: represents all digits.%g: represents all printable characters except space.%l: represents all lowercase letters.%p: represents all punctuation characters.%s: represents all space characters.%u: represents all uppercase letters.%w: represents all alphanumeric characters.%x: represents all hexadecimal digits.%x: (where x is any non-alphanumeric character) represents the character x. This is the standard way to escape the magic characters. Any punctuation character (even the non magic) can be preceded by a '%' when used to represent itself in a pattern.[set]: represents the class which is the union of all characters in set. A range of characters can be specified by separating the end characters of the range, in ascending order, with a '-', All classes%x described above can also be used as components in set. All other characters in set represent themselves. For example,[%w_](or[_%w]) represents all alphanumeric characters plus the underscore,[0-7]represents the octal digits, and[0-7%l%-]represents the octal digits plus the lowercase letters plus the '-' character.The interaction between ranges and classes is not defined. Therefore, patterns like
[%a-z]or[a-%%]have no meaning.[^set]: represents the complement of set, where set is interpreted as above.
For all classes represented by single letters (%a, %c, etc.), the corresponding uppercase letter represents the complement of the class. For instance, %S represents all non-space characters.
The definitions of letter, space, and other character groups depend on the current locale. In particular, the class [a-z] may not be equivalent to %l.
Pattern Item:
A pattern item can be
- a single character class, which matches any single character in the class;
- a single character class followed by '
*', which matches 0 or more repetitions of characters in the class. These repetition items will always match the longest possible sequence; - a single character class followed by '
+', which matches 1 or more repetitions of characters in the class. These repetition items will always match the longest possible sequence; - a single character class followed by '
-', which also matches 0 or more repetitions of characters in the class. Unlike '*', these repetition items will always match the shortest possible sequence; - a single character class followed by '
?', which matches 0 or 1 occurrence of a character in the class; %n, for n between 1 and 9; such item matches a substring equal to the n-th captured string (see below);%bxy, where x and y are two distinct characters; such item matches strings that start with x, end with y, and where the x and y are balanced. This means that, if one reads the string from left to right, counting +1 for an x and -1 for a y, the ending y is the first y where the count reaches 0. For instance, the item%b()matches expressions with balanced parentheses.%f[set], a frontier pattern; such item matches an empty string at any position such that the next character belongs to set and the previous character does not belong to set. The set set is interpreted as previously described. The beginning and the end of the subject are handled as if they were the character '\0'.
Pattern:
A pattern is a sequence of pattern items. A caret '^' at the beginning of a pattern anchors the match at the beginning of the subject string. A '$' at the end of a pattern anchors the match at the end of the subject string. At other positions, '^' and '$' have no special meaning and represent themselves.
Captures:
A pattern can contain sub-patterns enclosed in parentheses; they describe captures. When a match succeeds, the substrings of the subject string that match captures are stored (captured) for future use. Captures are numbered according to their left parentheses. For instance, in the pattern "(a*(.)%w(%s*))", the part of the string matching "a*(.)%w(%s*)" is stored as the first capture (and therefore has number 1); the character matching "." is captured with number 2, and the part matching "%s*" has number 3.
As a special case, the empty capture () captures the current string position (a number). For instance, if we apply the pattern "()aa()" on the string "flaaap", there will be two captures: 3 and 5.
6.5 – Table Manipulation
This library provides generic functions for table manipulation. It provides all its functions inside the table table.
Remember that, whenever an operation needs the length of a table, the table should be a proper sequence or have a __len metamethod (see §3.4.6). All functions ignore non-numeric keys in tables given as arguments.
For performance reasons, all table accesses (get/set) performed by these functions are raw.
table.concat (list [, sep [, i [, j]]])
Given a list where all elements are strings or numbers, returns the string list[i]..sep..list[i+1] ··· sep..list[j]. The default value for sep is the empty string, the default for iis 1, and the default for j is #list. If i is greater than j, returns the empty string.
table.insert (list, [pos,] value)
Inserts element value at position pos in list, shifting up the elements list[pos], list[pos+1], ···, list[#list]. The default value for pos is #list+1, so that a calltable.insert(t,x) inserts x at the end of list t.
table.pack (···)
Returns a new table with all parameters stored into keys 1, 2, etc. and with a field "n" with the total number of parameters. Note that the resulting table may not be a sequence.
table.remove (list [, pos])
Removes from list the element at position pos, returning the value of the removed element. When pos is an integer between 1 and #list, it shifts down the elements list[pos+1], list[pos+2], ···, list[#list] and erases element list[#list]; The index pos can also be 0 when #list is 0, or #list + 1; in those cases, the function erases the elementlist[pos].
The default value for pos is #list, so that a call table.remove(t) removes the last element of list t.
table.sort (list [, comp])
Sorts list elements in a given order, in-place, from list[1] to list[#list]. If comp is given, then it must be a function that receives two list elements and returns true when the first element must come before the second in the final order (so that not comp(list[i+1],list[i]) will be true after the sort). If comp is not given, then the standard Lua operator < is used instead.
The sort algorithm is not stable; that is, elements considered equal by the given order may have their relative positions changed by the sort.
table.unpack (list [, i [, j]])
Returns the elements from the given table. This function is equivalent to
return list[i], list[i+1], ···, list[j]
By default, i is 1 and j is #list.
6.6 – Mathematical Functions
This library is an interface to the standard C math library. It provides all its functions inside the table math.
math.abs (x)
Returns the absolute value of x.
math.acos (x)
Returns the arc cosine of x (in radians).
math.asin (x)
Returns the arc sine of x (in radians).
math.atan (x)
Returns the arc tangent of x (in radians).
math.atan2 (y, x)
Returns the arc tangent of y/x (in radians), but uses the signs of both parameters to find the quadrant of the result. (It also handles correctly the case of x being zero.)
math.ceil (x)
Returns the smallest integer larger than or equal to x.
math.cos (x)
Returns the cosine of x (assumed to be in radians).
math.cosh (x)
Returns the hyperbolic cosine of x.
math.deg (x)
Returns the angle x (given in radians) in degrees.
math.exp (x)
Returns the value ex.
math.floor (x)
Returns the largest integer smaller than or equal to x.
math.fmod (x, y)
Returns the remainder of the division of x by y that rounds the quotient towards zero.
math.frexp (x)
Returns m and e such that x = m2e, e is an integer and the absolute value of m is in the range [0.5, 1) (or zero when x is zero).
math.huge
The value HUGE_VAL, a value larger than or equal to any other numerical value.
math.ldexp (m, e)
Returns m2e (e should be an integer).
math.log (x [, base])
Returns the logarithm of x in the given base. The default for base is e (so that the function returns the natural logarithm of x).
math.max (x, ···)
Returns the maximum value among its arguments.
math.min (x, ···)
Returns the minimum value among its arguments.
math.modf (x)
Returns two numbers, the integral part of x and the fractional part of x.
math.pi
The value of π.
math.pow (x, y)
Returns xy. (You can also use the expression x^y to compute this value.)
math.rad (x)
Returns the angle x (given in degrees) in radians.
math.random ([m [, n]])
This function is an interface to the simple pseudo-random generator function rand provided by Standard C. (No guarantees can be given for its statistical properties.)
When called without arguments, returns a uniform pseudo-random real number in the range [0,1). When called with an integer number m, math.random returns a uniform pseudo-random integer in the range [1, m]. When called with two integer numbers m and n, math.random returns a uniform pseudo-random integer in the range [m, n].
math.randomseed (x)
Sets x as the "seed" for the pseudo-random generator: equal seeds produce equal sequences of numbers.
math.sin (x)
Returns the sine of x (assumed to be in radians).
math.sinh (x)
Returns the hyperbolic sine of x.
math.sqrt (x)
Returns the square root of x. (You can also use the expression x^0.5 to compute this value.)
math.tan (x)
Returns the tangent of x (assumed to be in radians).
math.tanh (x)
Returns the hyperbolic tangent of x.
6.7 – Bitwise Operations
This library provides bitwise operations. It provides all its functions inside the table bit32.
Unless otherwise stated, all functions accept numeric arguments in the range (-251,+251); each argument is normalized to the remainder of its division by 232 and truncated to an integer (in some unspecified way), so that its final value falls in the range [0,232 - 1]. Similarly, all results are in the range [0,232 - 1]. Note that bit32.bnot(0) is 0xFFFFFFFF, which is different from -1.
bit32.arshift (x, disp)
Returns the number x shifted disp bits to the right. The number disp may be any representable integer. Negative displacements shift to the left.
This shift operation is what is called arithmetic shift. Vacant bits on the left are filled with copies of the higher bit of x; vacant bits on the right are filled with zeros. In particular, displacements with absolute values higher than 31 result in zero or 0xFFFFFFFF (all original bits are shifted out).
bit32.band (···)
Returns the bitwise and of its operands.
bit32.bnot (x)
Returns the bitwise negation of x. For any integer x, the following identity holds:
assert(bit32.bnot(x) == (-1 - x) % 2^32)
bit32.bor (···)
Returns the bitwise or of its operands.
bit32.btest (···)
Returns a boolean signaling whether the bitwise and of its operands is different from zero.
bit32.bxor (···)
Returns the bitwise exclusive or of its operands.
bit32.extract (n, field [, width])
Returns the unsigned number formed by the bits field to field + width - 1 from n. Bits are numbered from 0 (least significant) to 31 (most significant). All accessed bits must be in the range [0, 31].
The default for width is 1.
bit32.replace (n, v, field [, width])
Returns a copy of n with the bits field to field + width - 1 replaced by the value v. See bit32.extract for details about field and width.
bit32.lrotate (x, disp)
Returns the number x rotated disp bits to the left. The number disp may be any representable integer.
For any valid displacement, the following identity holds:
assert(bit32.lrotate(x, disp) == bit32.lrotate(x, disp % 32))
In particular, negative displacements rotate to the right.
bit32.lshift (x, disp)
Returns the number x shifted disp bits to the left. The number disp may be any representable integer. Negative displacements shift to the right. In any direction, vacant bits are filled with zeros. In particular, displacements with absolute values higher than 31 result in zero (all bits are shifted out).
For positive displacements, the following equality holds:
assert(bit32.lshift(b, disp) == (b * 2^disp) % 2^32)
bit32.rrotate (x, disp)
Returns the number x rotated disp bits to the right. The number disp may be any representable integer.
For any valid displacement, the following identity holds:
assert(bit32.rrotate(x, disp) == bit32.rrotate(x, disp % 32))
In particular, negative displacements rotate to the left.
bit32.rshift (x, disp)
Returns the number x shifted disp bits to the right. The number disp may be any representable integer. Negative displacements shift to the left. In any direction, vacant bits are filled with zeros. In particular, displacements with absolute values higher than 31 result in zero (all bits are shifted out).
For positive displacements, the following equality holds:
assert(bit32.rshift(b, disp) == math.floor(b % 2^32 / 2^disp))
This shift operation is what is called logical shift.
6.8 – Input and Output Facilities
The I/O library provides two different styles for file manipulation. The first one uses implicit file descriptors; that is, there are operations to set a default input file and a default output file, and all input/output operations are over these default files. The second style uses explicit file descriptors.
When using implicit file descriptors, all operations are supplied by table io. When using explicit file descriptors, the operation io.open returns a file descriptor and then all operations are supplied as methods of the file descriptor.
The table io also provides three predefined file descriptors with their usual meanings from C: io.stdin, io.stdout, and io.stderr. The I/O library never closes these files.
Unless otherwise stated, all I/O functions return nil on failure (plus an error message as a second result and a system-dependent error code as a third result) and some value different from nil on success. On non-Posix systems, the computation of the error message and error code in case of errors may be not thread safe, because they rely on the global C variable errno.
io.close ([file])
Equivalent to file:close(). Without a file, closes the default output file.
io.flush ()
Equivalent to io.output():flush().
io.input ([file])
When called with a file name, it opens the named file (in text mode), and sets its handle as the default input file. When called with a file handle, it simply sets this file handle as the default input file. When called without parameters, it returns the current default input file.
In case of errors this function raises the error, instead of returning an error code.
io.lines ([filename ···])
Opens the given file name in read mode and returns an iterator function that works like file:lines(···) over the opened file. When the iterator function detects the end of file, it returns nil(to finish the loop) and automatically closes the file.
The call io.lines() (with no file name) is equivalent to io.input():lines(); that is, it iterates over the lines of the default input file. In this case it does not close the file when the loop ends.
In case of errors this function raises the error, instead of returning an error code.
io.open (filename [, mode])
This function opens a file, in the mode specified in the string mode. It returns a new file handle, or, in case of errors, nil plus an error message.
The mode string can be any of the following:
- "
r": read mode (the default); - "
w": write mode; - "
a": append mode; - "
r+": update mode, all previous data is preserved; - "
w+": update mode, all previous data is erased; - "
a+": append update mode, previous data is preserved, writing is only allowed at the end of file.
The mode string can also have a 'b' at the end, which is needed in some systems to open the file in binary mode.
io.output ([file])
Similar to io.input, but operates over the default output file.
io.popen (prog [, mode])
This function is system dependent and is not available on all platforms.
Starts program prog in a separated process and returns a file handle that you can use to read data from this program (if mode is "r", the default) or to write data to this program (if mode is"w").
io.read (···)
Equivalent to io.input():read(···).
io.tmpfile ()
Returns a handle for a temporary file. This file is opened in update mode and it is automatically removed when the program ends.
io.type (obj)
Checks whether obj is a valid file handle. Returns the string "file" if obj is an open file handle, "closed file" if obj is a closed file handle, or nil if obj is not a file handle.
io.write (···)
Equivalent to io.output():write(···).
file:close ()
Closes file. Note that files are automatically closed when their handles are garbage collected, but that takes an unpredictable amount of time to happen.
When closing a file handle created with io.popen, file:close returns the same values returned by os.execute.
file:flush ()
Saves any written data to file.
file:lines (···)
Returns an iterator function that, each time it is called, reads the file according to the given formats. When no format is given, uses "*l" as a default. As an example, the construction
for c in file:lines(1) do body end
will iterate over all characters of the file, starting at the current position. Unlike io.lines, this function does not close the file when the loop ends.
In case of errors this function raises the error, instead of returning an error code.
file:read (···)
Reads the file file, according to the given formats, which specify what to read. For each format, the function returns a string (or a number) with the characters read, or nil if it cannot read data with the specified format. When called without formats, it uses a default format that reads the next line (see below).
The available formats are
- "
*n": reads a number; this is the only format that returns a number instead of a string. - "
*a": reads the whole file, starting at the current position. On end of file, it returns the empty string. - "
*l": reads the next line skipping the end of line, returning nil on end of file. This is the default format. - "
*L": reads the next line keeping the end of line (if present), returning nil on end of file. - number: reads a string with up to this number of bytes, returning nil on end of file. If number is zero, it reads nothing and returns an empty string, or nil on end of file.
file:seek ([whence [, offset]])
Sets and gets the file position, measured from the beginning of the file, to the position given by offset plus a base specified by the string whence, as follows:
- "
set": base is position 0 (beginning of the file); - "
cur": base is current position; - "
end": base is end of file;
In case of success, seek returns the final file position, measured in bytes from the beginning of the file. If seek fails, it returns nil, plus a string describing the error.
The default value for whence is "cur", and for offset is 0. Therefore, the call file:seek() returns the current file position, without changing it; the call file:seek("set") sets the position to the beginning of the file (and returns 0); and the call file:seek("end") sets the position to the end of the file, and returns its size.
file:setvbuf (mode [, size])
Sets the buffering mode for an output file. There are three available modes:
- "
no": no buffering; the result of any output operation appears immediately. - "
full": full buffering; output operation is performed only when the buffer is full or when you explicitlyflushthe file (seeio.flush). - "
line": line buffering; output is buffered until a newline is output or there is any input from some special files (such as a terminal device).
For the last two cases, size specifies the size of the buffer, in bytes. The default is an appropriate size.
file:write (···)
Writes the value of each of its arguments to file. The arguments must be strings or numbers.
In case of success, this function returns file. Otherwise it returns nil plus a string describing the error.
6.9 – Operating System Facilities
This library is implemented through table os.
os.clock ()
Returns an approximation of the amount in seconds of CPU time used by the program.
os.date ([format [, time]])
Returns a string or a table containing date and time, formatted according to the given string format.
If the time argument is present, this is the time to be formatted (see the os.time function for a description of this value). Otherwise, date formats the current time.
If format starts with '!', then the date is formatted in Coordinated Universal Time. After this optional character, if format is the string "*t", then date returns a table with the following fields:year (four digits), month (1–12), day (1–31), hour (0–23), min (0–59), sec (0–61), wday (weekday, Sunday is 1), yday (day of the year), and isdst (daylight saving flag, a boolean). This last field may be absent if the information is not available.
If format is not "*t", then date returns the date as a string, formatted according to the same rules as the ANSI C function strftime.
When called without arguments, date returns a reasonable date and time representation that depends on the host system and on the current locale (that is, os.date() is equivalent toos.date("%c")).
On non-Posix systems, this function may be not thread safe because of its reliance on C function gmtime and C function localtime.
os.difftime (t2, t1)
Returns the number of seconds from time t1 to time t2. In POSIX, Windows, and some other systems, this value is exactly t2-t1.
os.execute ([command])
This function is equivalent to the ANSI C function system. It passes command to be executed by an operating system shell. Its first result is true if the command terminated successfully, or nilotherwise. After this first result the function returns a string and a number, as follows:
- "
exit": the command terminated normally; the following number is the exit status of the command. - "
signal": the command was terminated by a signal; the following number is the signal that terminated the command.
When called without a command, os.execute returns a boolean that is true if a shell is available.
os.exit ([code [, close])
Calls the ANSI C function exit to terminate the host program. If code is true, the returned status is EXIT_SUCCESS; if code is false, the returned status is EXIT_FAILURE; if code is a number, the returned status is this number. The default value for code is true.
If the optional second argument close is true, closes the Lua state before exiting.
os.getenv (varname)
Returns the value of the process environment variable varname, or nil if the variable is not defined.
os.remove (filename)
Deletes the file (or empty directory, on POSIX systems) with the given name. If this function fails, it returns nil, plus a string describing the error and the error code.
os.rename (oldname, newname)
Renames file or directory named oldname to newname. If this function fails, it returns nil, plus a string describing the error and the error code.
os.setlocale (locale [, category])
Sets the current locale of the program. locale is a system-dependent string specifying a locale; category is an optional string describing which category to change: "all", "collate","ctype", "monetary", "numeric", or "time"; the default category is "all". The function returns the name of the new locale, or nil if the request cannot be honored.
If locale is the empty string, the current locale is set to an implementation-defined native locale. If locale is the string "C", the current locale is set to the standard C locale.
When called with nil as the first argument, this function only returns the name of the current locale for the given category.
This function may be not thread safe because of its reliance on C function setlocale.
os.time ([table])
Returns the current time when called without arguments, or a time representing the date and time specified by the given table. This table must have fields year, month, and day, and may have fields hour (default is 12), min (default is 0), sec (default is 0), and isdst (default is nil). For a description of these fields, see the os.date function.
The returned value is a number, whose meaning depends on your system. In POSIX, Windows, and some other systems, this number counts the number of seconds since some given start time (the "epoch"). In other systems, the meaning is not specified, and the number returned by time can be used only as an argument to os.date and os.difftime.
os.tmpname ()
Returns a string with a file name that can be used for a temporary file. The file must be explicitly opened before its use and explicitly removed when no longer needed.
On POSIX systems, this function also creates a file with that name, to avoid security risks. (Someone else might create the file with wrong permissions in the time between getting the name and creating the file.) You still have to open the file to use it and to remove it (even if you do not use it).
When possible, you may prefer to use io.tmpfile, which automatically removes the file when the program ends.
6.10 – The Debug Library
This library provides the functionality of the debug interface (§4.9) to Lua programs. You should exert care when using this library. Several of its functions violate basic assumptions about Lua code (e.g., that variables local to a function cannot be accessed from outside; that userdata metatables cannot be changed by Lua code; that Lua programs do not crash) and therefore can compromise otherwise secure code. Moreover, some functions in this library may be slow.
All functions in this library are provided inside the debug table. All functions that operate over a thread have an optional first argument which is the thread to operate over. The default is always the current thread.
debug.debug ()
Enters an interactive mode with the user, running each string that the user enters. Using simple commands and other debug facilities, the user can inspect global and local variables, change their values, evaluate expressions, and so on. A line containing only the word cont finishes this function, so that the caller continues its execution.
Note that commands for debug.debug are not lexically nested within any function and so have no direct access to local variables.
debug.gethook ([thread])
Returns the current hook settings of the thread, as three values: the current hook function, the current hook mask, and the current hook count (as set by the debug.sethook function).
debug.getinfo ([thread,] f [, what])
Returns a table with information about a function. You can give the function directly or you can give a number as the value of f, which means the function running at level f of the call stack of the given thread: level 0 is the current function (getinfo itself); level 1 is the function that called getinfo (except for tail calls, which do not count on the stack); and so on. If f is a number larger than the number of active functions, then getinfo returns nil.
The returned table can contain all the fields returned by lua_getinfo, with the string what describing which fields to fill in. The default for what is to get all information available, except the table of valid lines. If present, the option 'f' adds a field named func with the function itself. If present, the option 'L' adds a field named activelines with the table of valid lines.
For instance, the expression debug.getinfo(1,"n").name returns a table with a name for the current function, if a reasonable name can be found, and the expressiondebug.getinfo(print) returns a table with all available information about the print function.
debug.getlocal ([thread,] f, local)
This function returns the name and the value of the local variable with index local of the function at level f of the stack. This function accesses not only explicit local variables, but also parameters, temporaries, etc.
The first parameter or local variable has index 1, and so on, until the last active variable. Negative indices refer to vararg parameters; -1 is the first vararg parameter. The function returns nil if there is no variable with the given index, and raises an error when called with a level out of range. (You can call debug.getinfo to check whether the level is valid.)
Variable names starting with '(' (open parenthesis) represent internal variables (loop control variables, temporaries, varargs, and C function locals).
The parameter f may also be a function. In that case, getlocal returns only the name of function parameters.
debug.getmetatable (value)
Returns the metatable of the given value or nil if it does not have a metatable.
debug.getregistry ()
Returns the registry table (see §4.5).
debug.getupvalue (f, up)
This function returns the name and the value of the upvalue with index up of the function f. The function returns nil if there is no upvalue with the given index.
debug.getuservalue (u)
Returns the Lua value associated to u. If u is not a userdata, returns nil.
debug.sethook ([thread,] hook, mask [, count])
Sets the given function as a hook. The string mask and the number count describe when the hook will be called. The string mask may have the following characters, with the given meaning:
- '
c': the hook is called every time Lua calls a function; - '
r': the hook is called every time Lua returns from a function; - '
l': the hook is called every time Lua enters a new line of code.
With a count different from zero, the hook is called after every count instructions.
When called without arguments, debug.sethook turns off the hook.
When the hook is called, its first parameter is a string describing the event that has triggered its call: "call" (or "tail call"), "return", "line", and "count". For line events, the hook also gets the new line number as its second parameter. Inside a hook, you can call getinfo with level 2 to get more information about the running function (level 0 is the getinfo function, and level 1 is the hook function).
debug.setlocal ([thread,] level, local, value)
This function assigns the value value to the local variable with index local of the function at level level of the stack. The function returns nil if there is no local variable with the given index, and raises an error when called with a level out of range. (You can call getinfo to check whether the level is valid.) Otherwise, it returns the name of the local variable.
See debug.getlocal for more information about variable indices and names.
debug.setmetatable (value, table)
Sets the metatable for the given value to the given table (which can be nil). Returns value.
debug.setupvalue (f, up, value)
This function assigns the value value to the upvalue with index up of the function f. The function returns nil if there is no upvalue with the given index. Otherwise, it returns the name of the upvalue.
debug.setuservalue (udata, value)
Sets the given value as the Lua value associated to the given udata. value must be a table or nil; udata must be a full userdata.
Returns udata.
debug.traceback ([thread,] [message [, level]])
If message is present but is neither a string nor nil, this function returns message without further processing. Otherwise, it returns a string with a traceback of the call stack. An optionalmessage string is appended at the beginning of the traceback. An optional level number tells at which level to start the traceback (default is 1, the function calling traceback).
debug.upvalueid (f, n)
Returns an unique identifier (as a light userdata) for the upvalue numbered n from the given function.
These unique identifiers allow a program to check whether different closures share upvalues. Lua closures that share an upvalue (that is, that access a same external local variable) will return identical ids for those upvalue indices.
debug.upvaluejoin (f1, n1, f2, n2)
Make the n1-th upvalue of the Lua closure f1 refer to the n2-th upvalue of the Lua closure f2.
7 – Lua Standalone
Although Lua has been designed as an extension language, to be embedded in a host C program, it is also frequently used as a standalone language. An interpreter for Lua as a standalone language, called simply lua, is provided with the standard distribution. The standalone interpreter includes all standard libraries, including the debug library. Its usage is:
lua [options] [script [args]]
The options are:
-e stat: executes string stat;-l mod: "requires" mod;-i: enters interactive mode after running script;-v: prints version information;-E: ignores environment variables;--: stops handling options;-: executesstdinas a file and stops handling options.
After handling its options, lua runs the given script, passing to it the given args as string arguments. When called without arguments, lua behaves as lua -v -i when the standard input (stdin) is a terminal, and as lua - otherwise.
When called without option -E, the interpreter checks for an environment variable LUA_INIT_5_2 (or LUA_INIT if it is not defined) before running any argument. If the variable content has the format @filename, then lua executes the file. Otherwise, lua executes the string itself.
When called with option -E, besides ignoring LUA_INIT, Lua also ignores the values of LUA_PATH and LUA_CPATH, setting the values of package.path and package.cpath with the default paths defined in luaconf.h.
All options are handled in order, except -i and -E. For instance, an invocation like
$ lua -e'a=1' -e 'print(a)' script.lua
will first set a to 1, then print the value of a, and finally run the file script.lua with no arguments. (Here $ is the shell prompt. Your prompt may be different.)
Before starting to run the script, lua collects all arguments in the command line in a global table called arg. The script name is stored at index 0, the first argument after the script name goes to index 1, and so on. Any arguments before the script name (that is, the interpreter name plus the options) go to negative indices. For instance, in the call
$ lua -la b.lua t1 t2
the interpreter first runs the file a.lua, then creates a table
arg = { [-2] = "lua", [-1] = "-la",
[0] = "b.lua",
[1] = "t1", [2] = "t2" }
and finally runs the file b.lua. The script is called with arg[1], arg[2], ... as arguments; it can also access these arguments with the vararg expression '...'.
In interactive mode, if you write an incomplete statement, the interpreter waits for its completion by issuing a different prompt.
In case of unprotected errors in the script, the interpreter reports the error to the standard error stream. If the error object is a string, the interpreter adds a stack traceback to it. Otherwise, if the error object has a metamethod __tostring, the interpreter calls this metamethod to produce the final message. Finally, if the error object is nil, the interpreter does not report the error.
When finishing normally, the interpreter closes its main Lua state (see lua_close). The script can avoid this step by calling os.exit to terminate.
To allow the use of Lua as a script interpreter in Unix systems, the standalone interpreter skips the first line of a chunk if it starts with #. Therefore, Lua scripts can be made into executable programs by using chmod +x and the #! form, as in
#!/usr/local/bin/lua
(Of course, the location of the Lua interpreter may be different in your machine. If lua is in your PATH, then
#!/usr/bin/env lua
is a more portable solution.)
8 – Incompatibilities with the Previous Version
Here we list the incompatibilities that you may find when moving a program from Lua 5.1 to Lua 5.2. You can avoid some incompatibilities by compiling Lua with appropriate options (see fileluaconf.h). However, all these compatibility options will be removed in the next version of Lua. Similarly, all features marked as deprecated in Lua 5.1 have been removed in Lua 5.2.
8.1 – Changes in the Language
- The concept of environment changed. Only Lua functions have environments. To set the environment of a Lua function, use the variable
_ENVor the functionload.C functions no longer have environments. Use an upvalue with a shared table if you need to keep shared state among several C functions. (You may use
luaL_setfuncsto open a C library with all functions sharing a common upvalue.)To manipulate the "environment" of a userdata (which is now called user value), use the new functions
lua_getuservalueandlua_setuservalue. - Lua identifiers cannot use locale-dependent letters.
- Doing a step or a full collection in the garbage collector does not restart the collector if it has been stopped.
- Weak tables with weak keys now perform like ephemeron tables.
- The event tail return in debug hooks was removed. Instead, tail calls generate a special new event, tail call, so that the debugger can know that there will not be a corresponding return event.
- Equality between function values has changed. Now, a function definition may not create a new value; it may reuse some previous value if there is no observable difference to the new function.
8.2 – Changes in the Libraries
- Function
moduleis deprecated. It is easy to set up a module with regular Lua code. Modules are not expected to set global variables. - Functions
setfenvandgetfenvwere removed, because of the changes in environments. - Function
math.log10is deprecated. Usemath.logwith 10 as its second argument, instead. - Function
loadstringis deprecated. Useloadinstead; it now accepts string arguments and are exactly equivalent toloadstring. - Function
table.maxnis deprecated. Write it in Lua if you really need it. - Function
os.executenow returns true when command terminates successfully and nil plus error information otherwise. - Function
unpackwas moved into the table library and therefore must be called astable.unpack. - Character class
%zin patterns is deprecated, as now patterns may contain '\0' as a regular character. - The table
package.loaderswas renamedpackage.searchers. - Lua does not have bytecode verification anymore. So, all functions that load code (
loadandloadfile) are potentially insecure when loading untrusted binary data. (Actually, those functions were already insecure because of flaws in the verification algorithm.) When in doubt, use themodeargument of those functions to restrict them to loading textual chunks. - The standard paths in the official distribution may change between versions.
8.3 – Changes in the API
- Pseudoindex
LUA_GLOBALSINDEXwas removed. You must get the global environment from the registry (see §4.5). - Pseudoindex
LUA_ENVIRONINDEXand functionslua_getfenv/lua_setfenvwere removed, as C functions no longer have environments. - Function
luaL_registeris deprecated. UseluaL_setfuncsso that your module does not create globals. (Modules are not expected to set global variables anymore.) - The
osizeargument to the allocation function may not be zero when creating a new block, that is, whenptrisNULL(seelua_Alloc). Use only the testptr == NULLto check whether the block is new. - Finalizers (
__gcmetamethods) for userdata are called in the reverse order that they were marked for finalization, not that they were created (see §2.5.1). (Most userdata are marked immediately after they are created.) Moreover, if the metatable does not have a__gcfield when set, the finalizer will not be called, even if it is set later. luaL_typerrorwas removed. Write your own version if you need it.- Function
lua_cpcallis deprecated. You can simply push the function withlua_pushcfunctionand call it withlua_pcall. - Functions
lua_equalandlua_lessthanare deprecated. Use the newlua_comparewith appropriate options instead. - Function
lua_objlenwas renamedlua_rawlen. - Function
lua_loadhas an extra parameter,mode. PassNULLto simulate the old behavior. - Function
lua_resumehas an extra parameter,from. PassNULLor the thread doing the call.
9 – The Complete Syntax of Lua
Here is the complete syntax of Lua in extended BNF. (It does not describe operator precedences.)
chunk ::= block
block ::= {stat} [retstat]
stat ::= ‘;’ |
varlist ‘=’ explist |
functioncall |
label |
break |
goto Name |
do block end |
while exp do block end |
repeat block until exp |
if exp then block {elseif exp then block} [else block] end |
for Name ‘=’ exp ‘,’ exp [‘,’ exp] do block end |
for namelist in explist do block end |
function funcname funcbody |
local function Name funcbody |
local namelist [‘=’ explist]
retstat ::= return [explist] [‘;’]
label ::= ‘::’ Name ‘::’
funcname ::= Name {‘.’ Name} [‘:’ Name]
varlist ::= var {‘,’ var}
var ::= Name | prefixexp ‘[’ exp ‘]’ | prefixexp ‘.’ Name
namelist ::= Name {‘,’ Name}
explist ::= exp {‘,’ exp}
exp ::= nil | false | true | Number | String | ‘...’ | functiondef |
prefixexp | tableconstructor | exp binop exp | unop exp
prefixexp ::= var | functioncall | ‘(’ exp ‘)’
functioncall ::= prefixexp args | prefixexp ‘:’ Name args
args ::= ‘(’ [explist] ‘)’ | tableconstructor | String
functiondef ::= function funcbody
funcbody ::= ‘(’ [parlist] ‘)’ block end
parlist ::= namelist [‘,’ ‘...’] | ‘...’
tableconstructor ::= ‘{’ [fieldlist] ‘}’
fieldlist ::= field {fieldsep field} [fieldsep]
field ::= ‘[’ exp ‘]’ ‘=’ exp | Name ‘=’ exp | exp
fieldsep ::= ‘,’ | ‘;’
binop ::= ‘+’ | ‘-’ | ‘*’ | ‘/’ | ‘^’ | ‘%’ | ‘..’ |
‘<’ | ‘<=’ | ‘>’ | ‘>=’ | ‘==’ | ‘~=’ |
and | or
unop ::= ‘-’ | not | ‘#’
Last update: Thu Mar 21 13:01:53 BRT 2013
中文版最近更新: 2013-08-12 13:04
