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| author | fschmidt@gmail.com <fschmidt@gmail.com@21e917c8-12df-6dd8-5cb6-c86387c605b9> |
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| date | Mon, 06 Oct 2014 22:03:16 +0000 |
| parents | |
| children | bcc3911285a2 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/docs/manual.html Mon Oct 06 22:03:16 2014 +0000 @@ -0,0 +1,10603 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> +<html> + +<head> +<title>Lua 5.2 Reference Manual</title> +<link rel="stylesheet" type="text/css" href="http://www.lua.org/lua.css"> +<link rel="stylesheet" type="text/css" href="http://www.lua.org/manual/manual.css"> +<META HTTP-EQUIV="content-type" CONTENT="text/html; charset=iso-8859-1"> +</head> + +<body> + +<hr> +<h1> +<a href="../../home.html"><img src="../../images/logo.gif" alt="" border="0"></a> +Lua 5.2 Reference Manual +</h1> + +by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, Waldemar Celes +<p> +<small> +Copyright © 2011–2013 Lua.org, PUC-Rio. +Freely available under the terms of the +<a href="../../license.html">Lua license</a>. +</small> +<hr> +<p> + +<a href="contents.html#contents">contents</A> +· +<a href="contents.html#index">index</A> +· +<a href="../">other versions</A> + +<!-- ====================================================================== --> +<p> + +<!-- $Id: manual.of,v 1.103 2013/03/14 18:51:56 roberto Exp $ --> + + +<H2><A NAME="contents">Contents</A></H2> +<UL style="padding: 0"> +<LI><A HREF="manual.html#1">1 – Introduction</A> +<P> +<LI><A HREF="manual.html#2">2 – Basic Concepts</A> +<UL> +<LI><A HREF="manual.html#2.1">2.1 – Values and Types</A> +<LI><A HREF="manual.html#2.2">2.2 – Environments and the Global Environment</A> +<LI><A HREF="manual.html#2.3">2.3 – Error Handling</A> +<LI><A HREF="manual.html#2.4">2.4 – Metatables and Metamethods</A> +<LI><A HREF="manual.html#2.5">2.5 – Garbage Collection</A> +<UL> +<LI><A HREF="manual.html#2.5.1">2.5.1 – Garbage-Collection Metamethods</A> +<LI><A HREF="manual.html#2.5.2">2.5.2 – Weak Tables</A> +</UL> +<LI><A HREF="manual.html#2.6">2.6 – Coroutines</A> +</UL> +<P> +<LI><A HREF="manual.html#3">3 – The Language</A> +<UL> +<LI><A HREF="manual.html#3.1">3.1 – Lexical Conventions</A> +<LI><A HREF="manual.html#3.2">3.2 – Variables</A> +<LI><A HREF="manual.html#3.3">3.3 – Statements</A> +<UL> +<LI><A HREF="manual.html#3.3.1">3.3.1 – Blocks</A> +<LI><A HREF="manual.html#3.3.2">3.3.2 – Chunks</A> +<LI><A HREF="manual.html#3.3.3">3.3.3 – Assignment</A> +<LI><A HREF="manual.html#3.3.4">3.3.4 – Control Structures</A> +<LI><A HREF="manual.html#3.3.5">3.3.5 – For Statement</A> +<LI><A HREF="manual.html#3.3.6">3.3.6 – Function Calls as Statements</A> +<LI><A HREF="manual.html#3.3.7">3.3.7 – Local Declarations</A> +</UL> +<LI><A HREF="manual.html#3.4">3.4 – Expressions</A> +<UL> +<LI><A HREF="manual.html#3.4.1">3.4.1 – Arithmetic Operators</A> +<LI><A HREF="manual.html#3.4.2">3.4.2 – Coercion</A> +<LI><A HREF="manual.html#3.4.3">3.4.3 – Relational Operators</A> +<LI><A HREF="manual.html#3.4.4">3.4.4 – Logical Operators</A> +<LI><A HREF="manual.html#3.4.5">3.4.5 – Concatenation</A> +<LI><A HREF="manual.html#3.4.6">3.4.6 – The Length Operator</A> +<LI><A HREF="manual.html#3.4.7">3.4.7 – Precedence</A> +<LI><A HREF="manual.html#3.4.8">3.4.8 – Table Constructors</A> +<LI><A HREF="manual.html#3.4.9">3.4.9 – Function Calls</A> +<LI><A HREF="manual.html#3.4.10">3.4.10 – Function Definitions</A> +</UL> +<LI><A HREF="manual.html#3.5">3.5 – Visibility Rules</A> +</UL> +<P> +<LI><A HREF="manual.html#4">4 – The Application Program Interface</A> +<UL> +<LI><A HREF="manual.html#4.1">4.1 – The Stack</A> +<LI><A HREF="manual.html#4.2">4.2 – Stack Size</A> +<LI><A HREF="manual.html#4.3">4.3 – Valid and Acceptable Indices</A> +<LI><A HREF="manual.html#4.4">4.4 – C Closures</A> +<LI><A HREF="manual.html#4.5">4.5 – Registry</A> +<LI><A HREF="manual.html#4.6">4.6 – Error Handling in C</A> +<LI><A HREF="manual.html#4.7">4.7 – Handling Yields in C</A> +<LI><A HREF="manual.html#4.8">4.8 – Functions and Types</A> +<LI><A HREF="manual.html#4.9">4.9 – The Debug Interface</A> +</UL> +<P> +<LI><A HREF="manual.html#5">5 – The Auxiliary Library</A> +<UL> +<LI><A HREF="manual.html#5.1">5.1 – Functions and Types</A> +</UL> +<P> +<LI><A HREF="manual.html#6">6 – Standard Libraries</A> +<UL> +<LI><A HREF="manual.html#6.1">6.1 – Basic Functions</A> +<LI><A HREF="manual.html#6.2">6.2 – Coroutine Manipulation</A> +<LI><A HREF="manual.html#6.3">6.3 – Modules</A> +<LI><A HREF="manual.html#6.4">6.4 – String Manipulation</A> +<UL> +<LI><A HREF="manual.html#6.4.1">6.4.1 – Patterns</A> +</UL> +<LI><A HREF="manual.html#6.5">6.5 – Table Manipulation</A> +<LI><A HREF="manual.html#6.6">6.6 – Mathematical Functions</A> +<LI><A HREF="manual.html#6.7">6.7 – Bitwise Operations</A> +<LI><A HREF="manual.html#6.8">6.8 – Input and Output Facilities</A> +<LI><A HREF="manual.html#6.9">6.9 – Operating System Facilities</A> +<LI><A HREF="manual.html#6.10">6.10 – The Debug Library</A> +</UL> +<P> +<LI><A HREF="manual.html#7">7 – Lua Standalone</A> +<P> +<LI><A HREF="manual.html#8">8 – Incompatibilities with the Previous Version</A> +<UL> +<LI><A HREF="manual.html#8.1">8.1 – Changes in the Language</A> +<LI><A HREF="manual.html#8.2">8.2 – Changes in the Libraries</A> +<LI><A HREF="manual.html#8.3">8.3 – Changes in the API</A> +</UL> +<P> +<LI><A HREF="manual.html#9">9 – The Complete Syntax of Lua</A> +</UL> + + +<h1>1 – <a name="1">Introduction</a></h1> + +<p> +Lua is an extension programming language designed to support +general procedural programming with data description +facilities. +It also offers good support for object-oriented programming, +functional programming, and data-driven programming. +Lua is intended to be used as a powerful, lightweight, +embeddable scripting language for any program that needs one. +Lua is implemented as a library, written in <em>clean C</em>, +the common subset of Standard C and C++. + + +<p> +Being an extension language, Lua has no notion of a "main" program: +it only works <em>embedded</em> in a host client, +called the <em>embedding program</em> or simply the <em>host</em>. +The host program can invoke functions to execute a piece of Lua code, +can write and read Lua variables, +and can register C functions to be called by Lua code. +Through the use of C functions, Lua can be augmented to cope with +a wide range of different domains, +thus creating customized programming languages sharing a syntactical framework. +The Lua distribution includes a sample host program called <code>lua</code>, +which uses the Lua library to offer a complete, standalone Lua interpreter, +for interactive or batch use. + + +<p> +Lua is free software, +and is provided as usual with no guarantees, +as stated in its license. +The implementation described in this manual is available +at Lua's official web site, <code>www.lua.org</code>. + + +<p> +Like any other reference manual, +this document is dry in places. +For a discussion of the decisions behind the design of Lua, +see the technical papers available at Lua's web site. +For a detailed introduction to programming in Lua, +see Roberto's book, <em>Programming in Lua</em>. + + + +<h1>2 – <a name="2">Basic Concepts</a></h1> + +<p> +This section describes the basic concepts of the language. + + + +<h2>2.1 – <a name="2.1">Values and Types</a></h2> + +<p> +Lua is a <em>dynamically typed language</em>. +This means that +variables do not have types; only values do. +There are no type definitions in the language. +All values carry their own type. + + +<p> +All values in Lua are <em>first-class values</em>. +This means that all values can be stored in variables, +passed as arguments to other functions, and returned as results. + + +<p> +There are eight basic types in Lua: +<em>nil</em>, <em>boolean</em>, <em>number</em>, +<em>string</em>, <em>function</em>, <em>userdata</em>, +<em>thread</em>, and <em>table</em>. +<em>Nil</em> is the type of the value <b>nil</b>, +whose main property is to be different from any other value; +it usually represents the absence of a useful value. +<em>Boolean</em> is the type of the values <b>false</b> and <b>true</b>. +Both <b>nil</b> and <b>false</b> make a condition false; +any other value makes it true. +<em>Number</em> represents real (double-precision floating-point) numbers. +Operations on numbers follow the same rules of +the underlying C implementation, +which, in turn, usually follows the IEEE 754 standard. +(It is easy to build Lua interpreters that use other +internal representations for numbers, +such as single-precision floats or long integers; +see file <code>luaconf.h</code>.) +<em>String</em> represents immutable sequences of bytes. + +Lua is 8-bit clean: +strings can contain any 8-bit value, +including embedded zeros ('<code>\0</code>'). + + +<p> +Lua can call (and manipulate) functions written in Lua and +functions written in C +(see <a href="#3.4.9">§3.4.9</a>). + + +<p> +The type <em>userdata</em> is provided to allow arbitrary C data to +be stored in Lua variables. +A userdata value is a pointer to a block of raw memory. +There are two kinds of userdata: +full userdata, where the block of memory is managed by Lua, +and light userdata, where the block of memory is managed by the host. +Userdata has no predefined operations in Lua, +except assignment and identity test. +By using <em>metatables</em>, +the programmer can define operations for full userdata values +(see <a href="#2.4">§2.4</a>). +Userdata values cannot be created or modified in Lua, +only through the C API. +This guarantees the integrity of data owned by the host program. + + +<p> +The type <em>thread</em> represents independent threads of execution +and it is used to implement coroutines (see <a href="#2.6">§2.6</a>). +Do not confuse Lua threads with operating-system threads. +Lua supports coroutines on all systems, +even those that do not support threads. + + +<p> +The type <em>table</em> implements associative arrays, +that is, arrays that can be indexed not only with numbers, +but with any Lua value except <b>nil</b> and NaN +(<em>Not a Number</em>, a special numeric value used to represent +undefined or unrepresentable results, such as <code>0/0</code>). +Tables can be <em>heterogeneous</em>; +that is, they can contain values of all types (except <b>nil</b>). +Any key with value <b>nil</b> is not considered part of the table. +Conversely, any key that is not part of a table has +an associated value <b>nil</b>. + + +<p> +Tables are the sole data structuring mechanism in Lua; +they can be used to represent ordinary arrays, sequences, +symbol tables, sets, records, graphs, trees, etc. +To represent records, Lua uses the field name as an index. +The language supports this representation by +providing <code>a.name</code> as syntactic sugar for <code>a["name"]</code>. +There are several convenient ways to create tables in Lua +(see <a href="#3.4.8">§3.4.8</a>). + + +<p> +We use the term <em>sequence</em> to denote a table where +the set of all positive numeric keys is equal to <em>{1..n}</em> +for some integer <em>n</em>, +which is called the length of the sequence (see <a href="#3.4.6">§3.4.6</a>). + + +<p> +Like indices, +the values of table fields can be of any type. +In particular, +because functions are first-class values, +table fields can contain functions. +Thus tables can also carry <em>methods</em> (see <a href="#3.4.10">§3.4.10</a>). + + +<p> +The indexing of tables follows +the definition of raw equality in the language. +The expressions <code>a[i]</code> and <code>a[j]</code> +denote the same table element +if and only if <code>i</code> and <code>j</code> are raw equal +(that is, equal without metamethods). + + +<p> +Tables, functions, threads, and (full) userdata values are <em>objects</em>: +variables do not actually <em>contain</em> these values, +only <em>references</em> to them. +Assignment, parameter passing, and function returns +always manipulate references to such values; +these operations do not imply any kind of copy. + + +<p> +The library function <a href="#pdf-type"><code>type</code></a> returns a string describing the type +of a given value (see <a href="#6.1">§6.1</a>). + + + + + +<h2>2.2 – <a name="2.2">Environments and the Global Environment</a></h2> + +<p> +As will be discussed in <a href="#3.2">§3.2</a> and <a href="#3.3.3">§3.3.3</a>, +any reference to a global name <code>var</code> is syntactically translated +to <code>_ENV.var</code>. +Moreover, every chunk is compiled in the scope of +an external local variable called <code>_ENV</code> (see <a href="#3.3.2">§3.3.2</a>), +so <code>_ENV</code> itself is never a global name in a chunk. + + +<p> +Despite the existence of this external <code>_ENV</code> variable and +the translation of global names, +<code>_ENV</code> is a completely regular name. +In particular, +you can define new variables and parameters with that name. +Each reference to a global name uses the <code>_ENV</code> that is +visible at that point in the program, +following the usual visibility rules of Lua (see <a href="#3.5">§3.5</a>). + + +<p> +Any table used as the value of <code>_ENV</code> is called an <em>environment</em>. + + +<p> +Lua keeps a distinguished environment called the <em>global environment</em>. +This value is kept at a special index in the C registry (see <a href="#4.5">§4.5</a>). +In Lua, the variable <a href="#pdf-_G"><code>_G</code></a> is initialized with this same value. + + +<p> +When Lua compiles a chunk, +it initializes the value of its <code>_ENV</code> upvalue +with the global environment (see <a href="#pdf-load"><code>load</code></a>). +Therefore, by default, +global variables in Lua code refer to entries in the global environment. +Moreover, all standard libraries are loaded in the global environment +and several functions there operate on that environment. +You can use <a href="#pdf-load"><code>load</code></a> (or <a href="#pdf-loadfile"><code>loadfile</code></a>) +to load a chunk with a different environment. +(In C, you have to load the chunk and then change the value +of its first upvalue.) + + +<p> +If you change the global environment in the registry +(through C code or the debug library), +all chunks loaded after the change will get the new environment. +Previously loaded chunks are not affected, however, +as each has its own reference to the environment in its <code>_ENV</code> variable. +Moreover, the variable <a href="#pdf-_G"><code>_G</code></a> +(which is stored in the original global environment) +is never updated by Lua. + + + + + +<h2>2.3 – <a name="2.3">Error Handling</a></h2> + +<p> +Because Lua is an embedded extension language, +all Lua actions start from C code in the host program +calling a function from the Lua library (see <a href="#lua_pcall"><code>lua_pcall</code></a>). +Whenever an error occurs during +the compilation or execution of a Lua chunk, +control returns to the host, +which can take appropriate measures +(such as printing an error message). + + +<p> +Lua code can explicitly generate an error by calling the +<a href="#pdf-error"><code>error</code></a> function. +If you need to catch errors in Lua, +you can use <a href="#pdf-pcall"><code>pcall</code></a> or <a href="#pdf-xpcall"><code>xpcall</code></a> +to call a given function in <em>protected mode</em>. + + +<p> +Whenever there is an error, +an <em>error object</em> (also called an <em>error message</em>) +is propagated with information about the error. +Lua itself only generates errors where the error object is a string, +but programs may generate errors with +any value for the error object. + + +<p> +When you use <a href="#pdf-xpcall"><code>xpcall</code></a> or <a href="#lua_pcall"><code>lua_pcall</code></a>, +you may give a <em>message handler</em> +to be called in case of errors. +This function is called with the original error message +and returns a new error message. +It is called before the error unwinds the stack, +so that it can gather more information about the error, +for instance by inspecting the stack and creating a stack traceback. +This message handler is still protected by the protected call; +so, an error inside the message handler +will call the message handler again. +If this loop goes on, Lua breaks it and returns an appropriate message. + + + + + +<h2>2.4 – <a name="2.4">Metatables and Metamethods</a></h2> + +<p> +Every value in Lua can have a <em>metatable</em>. +This <em>metatable</em> is an ordinary Lua table +that defines the behavior of the original value +under certain special operations. +You can change several aspects of the behavior +of operations over a value by setting specific fields in its metatable. +For instance, when a non-numeric value is the operand of an addition, +Lua checks for a function in the field "<code>__add</code>" of the value's metatable. +If it finds one, +Lua calls this function to perform the addition. + + +<p> +The keys in a metatable are derived from the <em>event</em> names; +the corresponding values are called <em>metamethods</em>. +In the previous example, the event is <code>"add"</code> +and the metamethod is the function that performs the addition. + + +<p> +You can query the metatable of any value +using the <a href="#pdf-getmetatable"><code>getmetatable</code></a> function. + + +<p> +You can replace the metatable of tables +using the <a href="#pdf-setmetatable"><code>setmetatable</code></a> function. +You cannot change the metatable of other types from Lua +(except by using the debug library); +you must use the C API for that. + + +<p> +Tables and full userdata have individual metatables +(although multiple tables and userdata can share their metatables). +Values of all other types share one single metatable per type; +that is, there is one single metatable for all numbers, +one for all strings, etc. +By default, a value has no metatable, +but the string library sets a metatable for the string type (see <a href="#6.4">§6.4</a>). + + +<p> +A metatable controls how an object behaves in arithmetic operations, +order comparisons, concatenation, length operation, and indexing. +A metatable also can define a function to be called +when a userdata or a table is garbage collected. +When Lua performs one of these operations over a value, +it checks whether this value has a metatable with the corresponding event. +If so, the value associated with that key (the metamethod) +controls how Lua will perform the operation. + + +<p> +Metatables control the operations listed next. +Each operation is identified by its corresponding name. +The key for each operation is a string with its name prefixed by +two underscores, '<code>__</code>'; +for instance, the key for operation "add" is the +string "<code>__add</code>". + + +<p> +The semantics of these operations is better explained by a Lua function +describing how the interpreter executes the operation. +The code shown here in Lua is only illustrative; +the real behavior is hard coded in the interpreter +and it is much more efficient than this simulation. +All functions used in these descriptions +(<a href="#pdf-rawget"><code>rawget</code></a>, <a href="#pdf-tonumber"><code>tonumber</code></a>, etc.) +are described in <a href="#6.1">§6.1</a>. +In particular, to retrieve the metamethod of a given object, +we use the expression + +<pre> + metatable(obj)[event] +</pre><p> +This should be read as + +<pre> + rawget(getmetatable(obj) or {}, event) +</pre><p> +This means that the access to a metamethod does not invoke other metamethods, +and access to objects with no metatables does not fail +(it simply results in <b>nil</b>). + + +<p> +For the unary <code>-</code> and <code>#</code> operators, +the metamethod is called with a dummy second argument. +This extra argument is only to simplify Lua's internals; +it may be removed in future versions and therefore it is not present +in the following code. +(For most uses this extra argument is irrelevant.) + + + +<ul> + +<li><b>"add": </b> +the <code>+</code> operation. + + + +<p> +The function <code>getbinhandler</code> below defines how Lua chooses a handler +for a binary operation. +First, Lua tries the first operand. +If its type does not define a handler for the operation, +then Lua tries the second operand. + +<pre> + function getbinhandler (op1, op2, event) + return metatable(op1)[event] or metatable(op2)[event] + end +</pre><p> +By using this function, +the behavior of the <code>op1 + op2</code> is + +<pre> + function add_event (op1, op2) + local o1, o2 = tonumber(op1), tonumber(op2) + if o1 and o2 then -- both operands are numeric? + return o1 + o2 -- '+' here is the primitive 'add' + else -- at least one of the operands is not numeric + local h = getbinhandler(op1, op2, "__add") + if h then + -- call the handler with both operands + return (h(op1, op2)) + else -- no handler available: default behavior + error(···) + end + end + end +</pre><p> +</li> + +<li><b>"sub": </b> +the <code>-</code> operation. + +Behavior similar to the "add" operation. +</li> + +<li><b>"mul": </b> +the <code>*</code> operation. + +Behavior similar to the "add" operation. +</li> + +<li><b>"div": </b> +the <code>/</code> operation. + +Behavior similar to the "add" operation. +</li> + +<li><b>"mod": </b> +the <code>%</code> operation. + +Behavior similar to the "add" operation, +with the operation +<code>o1 - floor(o1/o2)*o2</code> as the primitive operation. +</li> + +<li><b>"pow": </b> +the <code>^</code> (exponentiation) operation. + +Behavior similar to the "add" operation, +with the function <code>pow</code> (from the C math library) +as the primitive operation. +</li> + +<li><b>"unm": </b> +the unary <code>-</code> operation. + + +<pre> + 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 +</pre><p> +</li> + +<li><b>"concat": </b> +the <code>..</code> (concatenation) operation. + + +<pre> + 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 +</pre><p> +</li> + +<li><b>"len": </b> +the <code>#</code> operation. + + +<pre> + 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 +</pre><p> +See <a href="#3.4.6">§3.4.6</a> for a description of the length of a table. +</li> + +<li><b>"eq": </b> +the <code>==</code> operation. + +The function <code>getequalhandler</code> defines how Lua chooses a metamethod +for equality. +A metamethod is selected only when both values +being compared have the same type +and the same metamethod for the selected operation, +and the values are either tables or full userdata. + +<pre> + 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 +</pre><p> +The "eq" event is defined as follows: + +<pre> + 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 +</pre><p> +Note that the result is always a boolean. +</li> + +<li><b>"lt": </b> +the <code><</code> operation. + + +<pre> + 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 +</pre><p> +Note that the result is always a boolean. +</li> + +<li><b>"le": </b> +the <code><=</code> operation. + + +<pre> + 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 +</pre><p> +Note that, in the absence of a "le" metamethod, +Lua tries the "lt", assuming that <code>a <= b</code> is +equivalent to <code>not (b < a)</code>. + + +<p> +As with the other comparison operators, +the result is always a boolean. +</li> + +<li><b>"index": </b> +The indexing access <code>table[key]</code>. +Note that the metamethod is tried only +when <code>key</code> is not present in <code>table</code>. +(When <code>table</code> is not a table, +no key is ever present, +so the metamethod is always tried.) + + +<pre> + 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 +</pre><p> +</li> + +<li><b>"newindex": </b> +The indexing assignment <code>table[key] = value</code>. +Note that the metamethod is tried only +when <code>key</code> is not present in <code>table</code>. + + +<pre> + 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 +</pre><p> +</li> + +<li><b>"call": </b> +called when Lua calls a value. + + +<pre> + 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 +</pre><p> +</li> + +</ul> + + + + +<h2>2.5 – <a name="2.5">Garbage Collection</a></h2> + +<p> +Lua performs automatic memory management. +This means that +you have to worry neither about allocating memory for new objects +nor about freeing it when the objects are no longer needed. +Lua manages memory automatically by running +a <em>garbage collector</em> to collect all <em>dead objects</em> +(that is, objects that are no longer accessible from Lua). +All memory used by Lua is subject to automatic management: +strings, tables, userdata, functions, threads, internal structures, etc. + + +<p> +Lua implements an incremental mark-and-sweep collector. +It uses two numbers to control its garbage-collection cycles: +the <em>garbage-collector pause</em> and +the <em>garbage-collector step multiplier</em>. +Both use percentage points as units +(e.g., a value of 100 means an internal value of 1). + + +<p> +The garbage-collector pause +controls how long the collector waits before starting a new cycle. +Larger values make the collector less aggressive. +Values smaller than 100 mean the collector will not wait to +start a new cycle. +A value of 200 means that the collector waits for the total memory in use +to double before starting a new cycle. + + +<p> +The garbage-collector step multiplier +controls the relative speed of the collector relative to +memory allocation. +Larger values make the collector more aggressive but also increase +the size of each incremental step. +Values smaller than 100 make the collector too slow and +can result in the collector never finishing a cycle. +The default is 200, +which means that the collector runs at "twice" +the speed of memory allocation. + + +<p> +If you set the step multiplier to a very large number +(larger than 10% of the maximum number of +bytes that the program may use), +the collector behaves like a stop-the-world collector. +If you then set the pause to 200, +the collector behaves as in old Lua versions, +doing a complete collection every time Lua doubles its +memory usage. + + +<p> +You can change these numbers by calling <a href="#lua_gc"><code>lua_gc</code></a> in C +or <a href="#pdf-collectgarbage"><code>collectgarbage</code></a> in Lua. +You can also use these functions to control +the collector directly (e.g., stop and restart it). + + +<p> +As an experimental feature in Lua 5.2, +you can change the collector's operation mode +from incremental to <em>generational</em>. +A <em>generational collector</em> assumes that most objects die young, +and therefore it traverses only young (recently created) objects. +This behavior can reduce the time used by the collector, +but also increases memory usage (as old dead objects may accumulate). +To mitigate this second problem, +from time to time the generational collector performs a full collection. +Remember that this is an experimental feature; +you are welcome to try it, +but check your gains. + + + +<h3>2.5.1 – <a name="2.5.1">Garbage-Collection Metamethods</a></h3> + +<p> +You can set garbage-collector metamethods for tables +and, using the C API, +for full userdata (see <a href="#2.4">§2.4</a>). +These metamethods are also called <em>finalizers</em>. +Finalizers allow you to coordinate Lua's garbage collection +with external resource management +(such as closing files, network or database connections, +or freeing your own memory). + + +<p> +For an object (table or userdata) to be finalized when collected, +you must <em>mark</em> it for finalization. + +You mark an object for finalization when you set its metatable +and the metatable has a field indexed by the string "<code>__gc</code>". +Note that if you set a metatable without a <code>__gc</code> field +and later create that field in the metatable, +the object will not be marked for finalization. +However, after an object is marked, +you can freely change the <code>__gc</code> field of its metatable. + + +<p> +When a marked object becomes garbage, +it is not collected immediately by the garbage collector. +Instead, Lua puts it in a list. +After the collection, +Lua does the equivalent of the following function +for each object in that list: + +<pre> + function gc_event (obj) + local h = metatable(obj).__gc + if type(h) == "function" then + h(obj) + end + end +</pre> + +<p> +At the end of each garbage-collection cycle, +the finalizers for objects are called in +the reverse order that they were marked for collection, +among those collected in that cycle; +that is, the first finalizer to be called is the one associated +with the object marked last in the program. +The execution of each finalizer may occur at any point during +the execution of the regular code. + + +<p> +Because the object being collected must still be used by the finalizer, +it (and other objects accessible only through it) +must be <em>resurrected</em> by Lua. +Usually, this resurrection is transient, +and the object memory is freed in the next garbage-collection cycle. +However, if the finalizer stores the object in some global place +(e.g., a global variable), +then there is a permanent resurrection. +In any case, +the object memory is freed only when it becomes completely inaccessible; +its finalizer will never be called twice. + + +<p> +When you close a state (see <a href="#lua_close"><code>lua_close</code></a>), +Lua calls the finalizers of all objects marked for finalization, +following the reverse order that they were marked. +If any finalizer marks new objects for collection during that phase, +these new objects will not be finalized. + + + + + +<h3>2.5.2 – <a name="2.5.2">Weak Tables</a></h3> + +<p> +A <em>weak table</em> is a table whose elements are +<em>weak references</em>. +A weak reference is ignored by the garbage collector. +In other words, +if the only references to an object are weak references, +then the garbage collector will collect that object. + + +<p> +A weak table can have weak keys, weak values, or both. +A table with weak keys allows the collection of its keys, +but prevents the collection of its values. +A table with both weak keys and weak values allows the collection of +both keys and values. +In any case, if either the key or the value is collected, +the whole pair is removed from the table. +The weakness of a table is controlled by the +<code>__mode</code> field of its metatable. +If the <code>__mode</code> field is a string containing the character '<code>k</code>', +the keys in the table are weak. +If <code>__mode</code> contains '<code>v</code>', +the values in the table are weak. + + +<p> +A table with weak keys and strong values +is also called an <em>ephemeron table</em>. +In an ephemeron table, +a value is considered reachable only if its key is reachable. +In particular, +if the only reference to a key comes through its value, +the pair is removed. + + +<p> +Any change in the weakness of a table may take effect only +at the next collect cycle. +In particular, if you change the weakness to a stronger mode, +Lua may still collect some items from that table +before the change takes effect. + + +<p> +Only objects that have an explicit construction +are removed from weak tables. +Values, such as numbers and light C functions, +are not subject to garbage collection, +and therefore are not removed from weak tables +(unless its associated value is collected). +Although strings are subject to garbage collection, +they do not have an explicit construction, +and therefore are not removed from weak tables. + + +<p> +Resurrected objects +(that is, objects being finalized +and objects accessible only through objects being finalized) +have a special behavior in weak tables. +They are removed from weak values before running their finalizers, +but are removed from weak keys only in the next collection +after running their finalizers, when such objects are actually freed. +This behavior allows the finalizer to access properties +associated with the object through weak tables. + + +<p> +If a weak table is among the resurrected objects in a collection cycle, +it may not be properly cleared until the next cycle. + + + + + + + +<h2>2.6 – <a name="2.6">Coroutines</a></h2> + +<p> +Lua supports coroutines, +also called <em>collaborative multithreading</em>. +A coroutine in Lua represents an independent thread of execution. +Unlike threads in multithread systems, however, +a coroutine only suspends its execution by explicitly calling +a yield function. + + +<p> +You create a coroutine by calling <a href="#pdf-coroutine.create"><code>coroutine.create</code></a>. +Its sole argument is a function +that is the main function of the coroutine. +The <code>create</code> function only creates a new coroutine and +returns a handle to it (an object of type <em>thread</em>); +it does not start the coroutine. + + +<p> +You execute a coroutine by calling <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>. +When you first call <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>, +passing as its first argument +a thread returned by <a href="#pdf-coroutine.create"><code>coroutine.create</code></a>, +the coroutine starts its execution, +at the first line of its main function. +Extra arguments passed to <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> are passed on +to the coroutine main function. +After the coroutine starts running, +it runs until it terminates or <em>yields</em>. + + +<p> +A coroutine can terminate its execution in two ways: +normally, when its main function returns +(explicitly or implicitly, after the last instruction); +and abnormally, if there is an unprotected error. +In the first case, <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> returns <b>true</b>, +plus any values returned by the coroutine main function. +In case of errors, <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> returns <b>false</b> +plus an error message. + + +<p> +A coroutine yields by calling <a href="#pdf-coroutine.yield"><code>coroutine.yield</code></a>. +When a coroutine yields, +the corresponding <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> returns immediately, +even if the yield happens inside nested function calls +(that is, not in the main function, +but in a function directly or indirectly called by the main function). +In the case of a yield, <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> also returns <b>true</b>, +plus any values passed to <a href="#pdf-coroutine.yield"><code>coroutine.yield</code></a>. +The next time you resume the same coroutine, +it continues its execution from the point where it yielded, +with the call to <a href="#pdf-coroutine.yield"><code>coroutine.yield</code></a> returning any extra +arguments passed to <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>. + + +<p> +Like <a href="#pdf-coroutine.create"><code>coroutine.create</code></a>, +the <a href="#pdf-coroutine.wrap"><code>coroutine.wrap</code></a> function also creates a coroutine, +but instead of returning the coroutine itself, +it returns a function that, when called, resumes the coroutine. +Any arguments passed to this function +go as extra arguments to <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>. +<a href="#pdf-coroutine.wrap"><code>coroutine.wrap</code></a> returns all the values returned by <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>, +except the first one (the boolean error code). +Unlike <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>, +<a href="#pdf-coroutine.wrap"><code>coroutine.wrap</code></a> does not catch errors; +any error is propagated to the caller. + + +<p> +As an example of how coroutines work, +consider the following code: + +<pre> + 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")) +</pre><p> +When you run it, it produces the following output: + +<pre> + 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 +</pre> + +<p> +You can also create and manipulate coroutines through the C API: +see functions <a href="#lua_newthread"><code>lua_newthread</code></a>, <a href="#lua_resume"><code>lua_resume</code></a>, +and <a href="#lua_yield"><code>lua_yield</code></a>. + + + + + +<h1>3 – <a name="3">The Language</a></h1> + +<p> +This section describes the lexis, the syntax, and the semantics of Lua. +In other words, +this section describes +which tokens are valid, +how they can be combined, +and what their combinations mean. + + +<p> +Language constructs will be explained using the usual extended BNF notation, +in which +{<em>a</em>} means 0 or more <em>a</em>'s, and +[<em>a</em>] means an optional <em>a</em>. +Non-terminals are shown like non-terminal, +keywords are shown like <b>kword</b>, +and other terminal symbols are shown like ‘<b>=</b>’. +The complete syntax of Lua can be found in <a href="#9">§9</a> +at the end of this manual. + + + +<h2>3.1 – <a name="3.1">Lexical Conventions</a></h2> + +<p> +Lua is a free-form language. +It ignores spaces (including new lines) and comments +between lexical elements (tokens), +except as delimiters between names and keywords. + + +<p> +<em>Names</em> +(also called <em>identifiers</em>) +in Lua can be any string of letters, +digits, and underscores, +not beginning with a digit. +Identifiers are used to name variables, table fields, and labels. + + +<p> +The following <em>keywords</em> are reserved +and cannot be used as names: + + +<pre> + and break do else elseif end + false for function goto if in + local nil not or repeat return + then true until while +</pre> + +<p> +Lua is a case-sensitive language: +<code>and</code> is a reserved word, but <code>And</code> and <code>AND</code> +are two different, valid names. +As a convention, names starting with an underscore followed by +uppercase letters (such as <a href="#pdf-_VERSION"><code>_VERSION</code></a>) +are reserved for variables used by Lua. + + +<p> +The following strings denote other tokens: + +<pre> + + - * / % ^ # + == ~= <= >= < > = + ( ) { } [ ] :: + ; : , . .. ... +</pre> + +<p> +<em>Literal strings</em> +can be delimited by matching single or double quotes, +and can contain the following C-like escape sequences: +'<code>\a</code>' (bell), +'<code>\b</code>' (backspace), +'<code>\f</code>' (form feed), +'<code>\n</code>' (newline), +'<code>\r</code>' (carriage return), +'<code>\t</code>' (horizontal tab), +'<code>\v</code>' (vertical tab), +'<code>\\</code>' (backslash), +'<code>\"</code>' (quotation mark [double quote]), +and '<code>\'</code>' (apostrophe [single quote]). +A backslash followed by a real newline +results in a newline in the string. +The escape sequence '<code>\z</code>' skips the following span +of white-space characters, +including line breaks; +it is particularly useful to break and indent a long literal string +into multiple lines without adding the newlines and spaces +into the string contents. + + +<p> +A byte in a literal string can also be specified by its numerical value. +This can be done with the escape sequence <code>\x<em>XX</em></code>, +where <em>XX</em> is a sequence of exactly two hexadecimal digits, +or with the escape sequence <code>\<em>ddd</em></code>, +where <em>ddd</em> is a sequence of up to three decimal digits. +(Note that if a decimal escape is to be followed by a digit, +it must be expressed using exactly three digits.) +Strings in Lua can contain any 8-bit value, including embedded zeros, +which can be specified as '<code>\0</code>'. + + +<p> +Literal strings can also be defined using a long format +enclosed by <em>long brackets</em>. +We define an <em>opening long bracket of level <em>n</em></em> as an opening +square bracket followed by <em>n</em> equal signs followed by another +opening square bracket. +So, an opening long bracket of level 0 is written as <code>[[</code>, +an opening long bracket of level 1 is written as <code>[=[</code>, +and so on. +A <em>closing long bracket</em> is defined similarly; +for instance, a closing long bracket of level 4 is written as <code>]====]</code>. +A <em>long literal</em> starts with an opening long bracket of any level and +ends at the first closing long bracket of the same level. +It can contain any text except a closing bracket of the proper level. +Literals in this bracketed form can run for several lines, +do not interpret any escape sequences, +and ignore long brackets of any other level. +Any kind of end-of-line sequence +(carriage return, newline, carriage return followed by newline, +or newline followed by carriage return) +is converted to a simple newline. + + +<p> +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. + + +<p> +For convenience, +when the opening long bracket is immediately followed by a newline, +the newline is not included in the string. +As an example, in a system using ASCII +(in which '<code>a</code>' is coded as 97, +newline is coded as 10, and '<code>1</code>' is coded as 49), +the five literal strings below denote the same string: + +<pre> + a = 'alo\n123"' + a = "alo\n123\"" + a = '\97lo\10\04923"' + a = [[alo + 123"]] + a = [==[ + alo + 123"]==] +</pre> + +<p> +A <em>numerical constant</em> can be written with an optional fractional part +and an optional decimal exponent, +marked by a letter '<code>e</code>' or '<code>E</code>'. +Lua also accepts hexadecimal constants, +which start with <code>0x</code> or <code>0X</code>. +Hexadecimal constants also accept an optional fractional part +plus an optional binary exponent, +marked by a letter '<code>p</code>' or '<code>P</code>'. +Examples of valid numerical constants are + +<pre> + 3 3.0 3.1416 314.16e-2 0.31416E1 + 0xff 0x0.1E 0xA23p-4 0X1.921FB54442D18P+1 +</pre> + +<p> +A <em>comment</em> starts with a double hyphen (<code>--</code>) +anywhere outside a string. +If the text immediately after <code>--</code> is not an opening long bracket, +the comment is a <em>short comment</em>, +which runs until the end of the line. +Otherwise, it is a <em>long comment</em>, +which runs until the corresponding closing long bracket. +Long comments are frequently used to disable code temporarily. + + + + + +<h2>3.2 – <a name="3.2">Variables</a></h2> + +<p> +Variables are places that store values. +There are three kinds of variables in Lua: +global variables, local variables, and table fields. + + +<p> +A single name can denote a global variable or a local variable +(or a function's formal parameter, +which is a particular kind of local variable): + +<pre> + var ::= Name +</pre><p> +Name denotes identifiers, as defined in <a href="#3.1">§3.1</a>. + + +<p> +Any variable name is assumed to be global unless explicitly declared +as a local (see <a href="#3.3.7">§3.3.7</a>). +Local variables are <em>lexically scoped</em>: +local variables can be freely accessed by functions +defined inside their scope (see <a href="#3.5">§3.5</a>). + + +<p> +Before the first assignment to a variable, its value is <b>nil</b>. + + +<p> +Square brackets are used to index a table: + +<pre> + var ::= prefixexp ‘<b>[</b>’ exp ‘<b>]</b>’ +</pre><p> +The meaning of accesses to table fields can be changed via metatables. +An access to an indexed variable <code>t[i]</code> is equivalent to +a call <code>gettable_event(t,i)</code>. +(See <a href="#2.4">§2.4</a> for a complete description of the +<code>gettable_event</code> function. +This function is not defined or callable in Lua. +We use it here only for explanatory purposes.) + + +<p> +The syntax <code>var.Name</code> is just syntactic sugar for +<code>var["Name"]</code>: + +<pre> + var ::= prefixexp ‘<b>.</b>’ Name +</pre> + +<p> +An access to a global variable <code>x</code> +is equivalent to <code>_ENV.x</code>. +Due to the way that chunks are compiled, +<code>_ENV</code> is never a global name (see <a href="#2.2">§2.2</a>). + + + + + +<h2>3.3 – <a name="3.3">Statements</a></h2> + +<p> +Lua supports an almost conventional set of statements, +similar to those in Pascal or C. +This set includes +assignments, control structures, function calls, +and variable declarations. + + + +<h3>3.3.1 – <a name="3.3.1">Blocks</a></h3> + +<p> +A block is a list of statements, +which are executed sequentially: + +<pre> + block ::= {stat} +</pre><p> +Lua has <em>empty statements</em> +that allow you to separate statements with semicolons, +start a block with a semicolon +or write two semicolons in sequence: + +<pre> + stat ::= ‘<b>;</b>’ +</pre> + +<p> +Function calls and assignments +can start with an open parenthesis. +This possibility leads to an ambiguity in Lua's grammar. +Consider the following fragment: + +<pre> + a = b + c + (print or io.write)('done') +</pre><p> +The grammar could see it in two ways: + +<pre> + a = b + c(print or io.write)('done') + + a = b + c; (print or io.write)('done') +</pre><p> +The current parser always sees such constructions +in the first way, +interpreting the open parenthesis +as the start of the arguments to a call. +To avoid this ambiguity, +it is a good practice to always precede with a semicolon +statements that start with a parenthesis: + +<pre> + ;(print or io.write)('done') +</pre> + +<p> +A block can be explicitly delimited to produce a single statement: + +<pre> + stat ::= <b>do</b> block <b>end</b> +</pre><p> +Explicit blocks are useful +to control the scope of variable declarations. +Explicit blocks are also sometimes used to +add a <b>return</b> statement in the middle +of another block (see <a href="#3.3.4">§3.3.4</a>). + + + + + +<h3>3.3.2 – <a name="3.3.2">Chunks</a></h3> + +<p> +The unit of compilation of Lua is called a <em>chunk</em>. +Syntactically, +a chunk is simply a block: + +<pre> + chunk ::= block +</pre> + +<p> +Lua handles a chunk as the body of an anonymous function +with a variable number of arguments +(see <a href="#3.4.10">§3.4.10</a>). +As such, chunks can define local variables, +receive arguments, and return values. +Moreover, such anonymous function is compiled as in the +scope of an external local variable called <code>_ENV</code> (see <a href="#2.2">§2.2</a>). +The resulting function always has <code>_ENV</code> as its only upvalue, +even if it does not use that variable. + + +<p> +A chunk can be stored in a file or in a string inside the host program. +To execute a chunk, +Lua first precompiles the chunk into instructions for a virtual machine, +and then it executes the compiled code +with an interpreter for the virtual machine. + + +<p> +Chunks can also be precompiled into binary form; +see program <code>luac</code> for details. +Programs in source and compiled forms are interchangeable; +Lua automatically detects the file type and acts accordingly. + + + + + + +<h3>3.3.3 – <a name="3.3.3">Assignment</a></h3> + +<p> +Lua allows multiple assignments. +Therefore, the syntax for assignment +defines a list of variables on the left side +and a list of expressions on the right side. +The elements in both lists are separated by commas: + +<pre> + stat ::= varlist ‘<b>=</b>’ explist + varlist ::= var {‘<b>,</b>’ var} + explist ::= exp {‘<b>,</b>’ exp} +</pre><p> +Expressions are discussed in <a href="#3.4">§3.4</a>. + + +<p> +Before the assignment, +the list of values is <em>adjusted</em> to the length of +the list of variables. +If there are more values than needed, +the excess values are thrown away. +If there are fewer values than needed, +the list is extended with as many <b>nil</b>'s as needed. +If the list of expressions ends with a function call, +then all values returned by that call enter the list of values, +before the adjustment +(except when the call is enclosed in parentheses; see <a href="#3.4">§3.4</a>). + + +<p> +The assignment statement first evaluates all its expressions +and only then are the assignments performed. +Thus the code + +<pre> + i = 3 + i, a[i] = i+1, 20 +</pre><p> +sets <code>a[3]</code> to 20, without affecting <code>a[4]</code> +because the <code>i</code> in <code>a[i]</code> is evaluated (to 3) +before it is assigned 4. +Similarly, the line + +<pre> + x, y = y, x +</pre><p> +exchanges the values of <code>x</code> and <code>y</code>, +and + +<pre> + x, y, z = y, z, x +</pre><p> +cyclically permutes the values of <code>x</code>, <code>y</code>, and <code>z</code>. + + +<p> +The meaning of assignments to global variables +and table fields can be changed via metatables. +An assignment to an indexed variable <code>t[i] = val</code> is equivalent to +<code>settable_event(t,i,val)</code>. +(See <a href="#2.4">§2.4</a> for a complete description of the +<code>settable_event</code> function. +This function is not defined or callable in Lua. +We use it here only for explanatory purposes.) + + +<p> +An assignment to a global variable <code>x = val</code> +is equivalent to the assignment +<code>_ENV.x = val</code> (see <a href="#2.2">§2.2</a>). + + + + + +<h3>3.3.4 – <a name="3.3.4">Control Structures</a></h3><p> +The control structures +<b>if</b>, <b>while</b>, and <b>repeat</b> have the usual meaning and +familiar syntax: + + + + +<pre> + stat ::= <b>while</b> exp <b>do</b> block <b>end</b> + stat ::= <b>repeat</b> block <b>until</b> exp + stat ::= <b>if</b> exp <b>then</b> block {<b>elseif</b> exp <b>then</b> block} [<b>else</b> block] <b>end</b> +</pre><p> +Lua also has a <b>for</b> statement, in two flavors (see <a href="#3.3.5">§3.3.5</a>). + + +<p> +The condition expression of a +control structure can return any value. +Both <b>false</b> and <b>nil</b> are considered false. +All values different from <b>nil</b> and <b>false</b> are considered true +(in particular, the number 0 and the empty string are also true). + + +<p> +In the <b>repeat</b>–<b>until</b> loop, +the inner block does not end at the <b>until</b> keyword, +but only after the condition. +So, the condition can refer to local variables +declared inside the loop block. + + +<p> +The <b>goto</b> statement transfers the program control to a label. +For syntactical reasons, +labels in Lua are considered statements too: + + + +<pre> + stat ::= <b>goto</b> Name + stat ::= label + label ::= ‘<b>::</b>’ Name ‘<b>::</b>’ +</pre> + +<p> +A label is visible in the entire block where it is defined, +except +inside nested blocks where a label with the same name is defined and +inside nested functions. +A goto may jump to any visible label as long as it does not +enter into the scope of a local variable. + + +<p> +Labels and empty statements are called <em>void statements</em>, +as they perform no actions. + + +<p> +The <b>break</b> statement terminates the execution of a +<b>while</b>, <b>repeat</b>, or <b>for</b> loop, +skipping to the next statement after the loop: + + +<pre> + stat ::= <b>break</b> +</pre><p> +A <b>break</b> ends the innermost enclosing loop. + + +<p> +The <b>return</b> statement is used to return values +from a function or a chunk (which is a function in disguise). + +Functions can return more than one value, +so the syntax for the <b>return</b> statement is + +<pre> + stat ::= <b>return</b> [explist] [‘<b>;</b>’] +</pre> + +<p> +The <b>return</b> statement can only be written +as the last statement of a block. +If it is really necessary to <b>return</b> in the middle of a block, +then an explicit inner block can be used, +as in the idiom <code>do return end</code>, +because now <b>return</b> is the last statement in its (inner) block. + + + + + +<h3>3.3.5 – <a name="3.3.5">For Statement</a></h3> + +<p> + +The <b>for</b> statement has two forms: +one numeric and one generic. + + +<p> +The numeric <b>for</b> loop repeats a block of code while a +control variable runs through an arithmetic progression. +It has the following syntax: + +<pre> + stat ::= <b>for</b> Name ‘<b>=</b>’ exp ‘<b>,</b>’ exp [‘<b>,</b>’ exp] <b>do</b> block <b>end</b> +</pre><p> +The <em>block</em> is repeated for <em>name</em> starting at the value of +the first <em>exp</em>, until it passes the second <em>exp</em> by steps of the +third <em>exp</em>. +More precisely, a <b>for</b> statement like + +<pre> + for v = <em>e1</em>, <em>e2</em>, <em>e3</em> do <em>block</em> end +</pre><p> +is equivalent to the code: + +<pre> + do + local <em>var</em>, <em>limit</em>, <em>step</em> = tonumber(<em>e1</em>), tonumber(<em>e2</em>), tonumber(<em>e3</em>) + if not (<em>var</em> and <em>limit</em> and <em>step</em>) then error() end + while (<em>step</em> > 0 and <em>var</em> <= <em>limit</em>) or (<em>step</em> <= 0 and <em>var</em> >= <em>limit</em>) do + local v = <em>var</em> + <em>block</em> + <em>var</em> = <em>var</em> + <em>step</em> + end + end +</pre><p> +Note the following: + +<ul> + +<li> +All three control expressions are evaluated only once, +before the loop starts. +They must all result in numbers. +</li> + +<li> +<code><em>var</em></code>, <code><em>limit</em></code>, and <code><em>step</em></code> are invisible variables. +The names shown here are for explanatory purposes only. +</li> + +<li> +If the third expression (the step) is absent, +then a step of 1 is used. +</li> + +<li> +You can use <b>break</b> to exit a <b>for</b> loop. +</li> + +<li> +The loop variable <code>v</code> is local to the loop; +you cannot use its value after the <b>for</b> ends or is broken. +If you need this value, +assign it to another variable before breaking or exiting the loop. +</li> + +</ul> + +<p> +The generic <b>for</b> statement works over functions, +called <em>iterators</em>. +On each iteration, the iterator function is called to produce a new value, +stopping when this new value is <b>nil</b>. +The generic <b>for</b> loop has the following syntax: + +<pre> + stat ::= <b>for</b> namelist <b>in</b> explist <b>do</b> block <b>end</b> + namelist ::= Name {‘<b>,</b>’ Name} +</pre><p> +A <b>for</b> statement like + +<pre> + for <em>var_1</em>, ···, <em>var_n</em> in <em>explist</em> do <em>block</em> end +</pre><p> +is equivalent to the code: + +<pre> + do + local <em>f</em>, <em>s</em>, <em>var</em> = <em>explist</em> + while true do + local <em>var_1</em>, ···, <em>var_n</em> = <em>f</em>(<em>s</em>, <em>var</em>) + if <em>var_1</em> == nil then break end + <em>var</em> = <em>var_1</em> + <em>block</em> + end + end +</pre><p> +Note the following: + +<ul> + +<li> +<code><em>explist</em></code> is evaluated only once. +Its results are an <em>iterator</em> function, +a <em>state</em>, +and an initial value for the first <em>iterator variable</em>. +</li> + +<li> +<code><em>f</em></code>, <code><em>s</em></code>, and <code><em>var</em></code> are invisible variables. +The names are here for explanatory purposes only. +</li> + +<li> +You can use <b>break</b> to exit a <b>for</b> loop. +</li> + +<li> +The loop variables <code><em>var_i</em></code> are local to the loop; +you cannot use their values after the <b>for</b> ends. +If you need these values, +then assign them to other variables before breaking or exiting the loop. +</li> + +</ul> + + + + +<h3>3.3.6 – <a name="3.3.6">Function Calls as Statements</a></h3><p> +To allow possible side-effects, +function calls can be executed as statements: + +<pre> + stat ::= functioncall +</pre><p> +In this case, all returned values are thrown away. +Function calls are explained in <a href="#3.4.9">§3.4.9</a>. + + + + + +<h3>3.3.7 – <a name="3.3.7">Local Declarations</a></h3><p> +Local variables can be declared anywhere inside a block. +The declaration can include an initial assignment: + +<pre> + stat ::= <b>local</b> namelist [‘<b>=</b>’ explist] +</pre><p> +If present, an initial assignment has the same semantics +of a multiple assignment (see <a href="#3.3.3">§3.3.3</a>). +Otherwise, all variables are initialized with <b>nil</b>. + + +<p> +A chunk is also a block (see <a href="#3.3.2">§3.3.2</a>), +and so local variables can be declared in a chunk outside any explicit block. + + +<p> +The visibility rules for local variables are explained in <a href="#3.5">§3.5</a>. + + + + + + + +<h2>3.4 – <a name="3.4">Expressions</a></h2> + +<p> +The basic expressions in Lua are the following: + +<pre> + exp ::= prefixexp + exp ::= <b>nil</b> | <b>false</b> | <b>true</b> + exp ::= Number + exp ::= String + exp ::= functiondef + exp ::= tableconstructor + exp ::= ‘<b>...</b>’ + exp ::= exp binop exp + exp ::= unop exp + prefixexp ::= var | functioncall | ‘<b>(</b>’ exp ‘<b>)</b>’ +</pre> + +<p> +Numbers and literal strings are explained in <a href="#3.1">§3.1</a>; +variables are explained in <a href="#3.2">§3.2</a>; +function definitions are explained in <a href="#3.4.10">§3.4.10</a>; +function calls are explained in <a href="#3.4.9">§3.4.9</a>; +table constructors are explained in <a href="#3.4.8">§3.4.8</a>. +Vararg expressions, +denoted by three dots ('<code>...</code>'), can only be used when +directly inside a vararg function; +they are explained in <a href="#3.4.10">§3.4.10</a>. + + +<p> +Binary operators comprise arithmetic operators (see <a href="#3.4.1">§3.4.1</a>), +relational operators (see <a href="#3.4.3">§3.4.3</a>), logical operators (see <a href="#3.4.4">§3.4.4</a>), +and the concatenation operator (see <a href="#3.4.5">§3.4.5</a>). +Unary operators comprise the unary minus (see <a href="#3.4.1">§3.4.1</a>), +the unary <b>not</b> (see <a href="#3.4.4">§3.4.4</a>), +and the unary <em>length operator</em> (see <a href="#3.4.6">§3.4.6</a>). + + +<p> +Both function calls and vararg expressions can result in multiple values. +If a function call is used as a statement (see <a href="#3.3.6">§3.3.6</a>), +then its return list is adjusted to zero elements, +thus discarding all returned values. +If an expression is used as the last (or the only) element +of a list of expressions, +then no adjustment is made +(unless the expression is enclosed in parentheses). +In all other contexts, +Lua adjusts the result list to one element, +either discarding all values except the first one +or adding a single <b>nil</b> if there are no values. + + +<p> +Here are some examples: + +<pre> + f() -- adjusted to 0 results + g(f(), x) -- f() is adjusted to 1 result + g(x, f()) -- g gets x plus all results from f() + a,b,c = f(), x -- f() is adjusted to 1 result (c gets nil) + a,b = ... -- a gets the first vararg parameter, b gets + -- the second (both a and b can get nil if there + -- is no corresponding vararg parameter) + + a,b,c = x, f() -- f() is adjusted to 2 results + a,b,c = f() -- f() is adjusted to 3 results + return f() -- returns all results from f() + return ... -- returns all received vararg parameters + return x,y,f() -- returns x, y, and all results from f() + {f()} -- creates a list with all results from f() + {...} -- creates a list with all vararg parameters + {f(), nil} -- f() is adjusted to 1 result +</pre> + +<p> +Any expression enclosed in parentheses always results in only one value. +Thus, +<code>(f(x,y,z))</code> is always a single value, +even if <code>f</code> returns several values. +(The value of <code>(f(x,y,z))</code> is the first value returned by <code>f</code> +or <b>nil</b> if <code>f</code> does not return any values.) + + + +<h3>3.4.1 – <a name="3.4.1">Arithmetic Operators</a></h3><p> +Lua supports the usual arithmetic operators: +the binary <code>+</code> (addition), +<code>-</code> (subtraction), <code>*</code> (multiplication), +<code>/</code> (division), <code>%</code> (modulo), and <code>^</code> (exponentiation); +and unary <code>-</code> (mathematical negation). +If the operands are numbers, or strings that can be converted to +numbers (see <a href="#3.4.2">§3.4.2</a>), +then all operations have the usual meaning. +Exponentiation works for any exponent. +For instance, <code>x^(-0.5)</code> computes the inverse of the square root of <code>x</code>. +Modulo is defined as + +<pre> + a % b == a - math.floor(a/b)*b +</pre><p> +That is, it is the remainder of a division that rounds +the quotient towards minus infinity. + + + + + +<h3>3.4.2 – <a name="3.4.2">Coercion</a></h3> + +<p> +Lua provides automatic conversion between +string and number values at run time. +Any arithmetic operation applied to a string tries to convert +this string to a number, following the rules of the Lua lexer. +(The string may have leading and trailing spaces and a sign.) +Conversely, whenever a number is used where a string is expected, +the number is converted to a string, in a reasonable format. +For complete control over how numbers are converted to strings, +use the <code>format</code> function from the string library +(see <a href="#pdf-string.format"><code>string.format</code></a>). + + + + + +<h3>3.4.3 – <a name="3.4.3">Relational Operators</a></h3><p> +The relational operators in Lua are + +<pre> + == ~= < > <= >= +</pre><p> +These operators always result in <b>false</b> or <b>true</b>. + + +<p> +Equality (<code>==</code>) first compares the type of its operands. +If the types are different, then the result is <b>false</b>. +Otherwise, the values of the operands are compared. +Numbers and strings are compared in the usual way. +Tables, userdata, and threads +are compared by reference: +two objects are considered equal only if they are the same object. +Every time you create a new object +(a table, userdata, or thread), +this new object is different from any previously existing object. +Closures with the same reference are always equal. +Closures with any detectable difference +(different behavior, different definition) are always different. + + +<p> +You can change the way that Lua compares tables and userdata +by using the "eq" metamethod (see <a href="#2.4">§2.4</a>). + + +<p> +The conversion rules of <a href="#3.4.2">§3.4.2</a> +do not apply to equality comparisons. +Thus, <code>"0"==0</code> evaluates to <b>false</b>, +and <code>t[0]</code> and <code>t["0"]</code> denote different +entries in a table. + + +<p> +The operator <code>~=</code> is exactly the negation of equality (<code>==</code>). + + +<p> +The order operators work as follows. +If both arguments are numbers, then they are compared as such. +Otherwise, if both arguments are strings, +then their values are compared according to the current locale. +Otherwise, Lua tries to call the "lt" or the "le" +metamethod (see <a href="#2.4">§2.4</a>). +A comparison <code>a > b</code> is translated to <code>b < a</code> +and <code>a >= b</code> is translated to <code>b <= a</code>. + + + + + +<h3>3.4.4 – <a name="3.4.4">Logical Operators</a></h3><p> +The logical operators in Lua are +<b>and</b>, <b>or</b>, and <b>not</b>. +Like the control structures (see <a href="#3.3.4">§3.3.4</a>), +all logical operators consider both <b>false</b> and <b>nil</b> as false +and anything else as true. + + +<p> +The negation operator <b>not</b> always returns <b>false</b> or <b>true</b>. +The conjunction operator <b>and</b> returns its first argument +if this value is <b>false</b> or <b>nil</b>; +otherwise, <b>and</b> returns its second argument. +The disjunction operator <b>or</b> returns its first argument +if this value is different from <b>nil</b> and <b>false</b>; +otherwise, <b>or</b> returns its second argument. +Both <b>and</b> and <b>or</b> use short-cut evaluation; +that is, +the second operand is evaluated only if necessary. +Here are some examples: + +<pre> + 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 +</pre><p> +(In this manual, +<code>--></code> indicates the result of the preceding expression.) + + + + + +<h3>3.4.5 – <a name="3.4.5">Concatenation</a></h3><p> +The string concatenation operator in Lua is +denoted by two dots ('<code>..</code>'). +If both operands are strings or numbers, then they are converted to +strings according to the rules mentioned in <a href="#3.4.2">§3.4.2</a>. +Otherwise, the <code>__concat</code> metamethod is called (see <a href="#2.4">§2.4</a>). + + + + + +<h3>3.4.6 – <a name="3.4.6">The Length Operator</a></h3> + +<p> +The length operator is denoted by the unary prefix operator <code>#</code>. +The length of a string is its number of bytes +(that is, the usual meaning of string length when each +character is one byte). + + +<p> +A program can modify the behavior of the length operator for +any value but strings through the <code>__len</code> metamethod (see <a href="#2.4">§2.4</a>). + + +<p> +Unless a <code>__len</code> metamethod is given, +the length of a table <code>t</code> is only defined if the +table is a <em>sequence</em>, +that is, +the set of its positive numeric keys is equal to <em>{1..n}</em> +for some integer <em>n</em>. +In that case, <em>n</em> is its length. +Note that a table like + +<pre> + {10, 20, nil, 40} +</pre><p> +is not a sequence, because it has the key <code>4</code> +but does not have the key <code>3</code>. +(So, there is no <em>n</em> such that the set <em>{1..n}</em> is equal +to the set of positive numeric keys of that table.) +Note, however, that non-numeric keys do not interfere +with whether a table is a sequence. + + + + + +<h3>3.4.7 – <a name="3.4.7">Precedence</a></h3><p> +Operator precedence in Lua follows the table below, +from lower to higher priority: + +<pre> + or + and + < > <= >= ~= == + .. + + - + * / % + not # - (unary) + ^ +</pre><p> +As usual, +you can use parentheses to change the precedences of an expression. +The concatenation ('<code>..</code>') and exponentiation ('<code>^</code>') +operators are right associative. +All other binary operators are left associative. + + + + + +<h3>3.4.8 – <a name="3.4.8">Table Constructors</a></h3><p> +Table constructors are expressions that create tables. +Every time a constructor is evaluated, a new table is created. +A constructor can be used to create an empty table +or to create a table and initialize some of its fields. +The general syntax for constructors is + +<pre> + tableconstructor ::= ‘<b>{</b>’ [fieldlist] ‘<b>}</b>’ + fieldlist ::= field {fieldsep field} [fieldsep] + field ::= ‘<b>[</b>’ exp ‘<b>]</b>’ ‘<b>=</b>’ exp | Name ‘<b>=</b>’ exp | exp + fieldsep ::= ‘<b>,</b>’ | ‘<b>;</b>’ +</pre> + +<p> +Each field of the form <code>[exp1] = exp2</code> adds to the new table an entry +with key <code>exp1</code> and value <code>exp2</code>. +A field of the form <code>name = exp</code> is equivalent to +<code>["name"] = exp</code>. +Finally, fields of the form <code>exp</code> are equivalent to +<code>[i] = exp</code>, where <code>i</code> are consecutive numerical integers, +starting with 1. +Fields in the other formats do not affect this counting. +For example, + +<pre> + a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 } +</pre><p> +is equivalent to + +<pre> + 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 +</pre> + +<p> +If the last field in the list has the form <code>exp</code> +and the expression is a function call or a vararg expression, +then all values returned by this expression enter the list consecutively +(see <a href="#3.4.9">§3.4.9</a>). + + +<p> +The field list can have an optional trailing separator, +as a convenience for machine-generated code. + + + + + +<h3>3.4.9 – <a name="3.4.9">Function Calls</a></h3><p> +A function call in Lua has the following syntax: + +<pre> + functioncall ::= prefixexp args +</pre><p> +In a function call, +first prefixexp and args are evaluated. +If the value of prefixexp has type <em>function</em>, +then this function is called +with the given arguments. +Otherwise, the prefixexp "call" metamethod is called, +having as first parameter the value of prefixexp, +followed by the original call arguments +(see <a href="#2.4">§2.4</a>). + + +<p> +The form + +<pre> + functioncall ::= prefixexp ‘<b>:</b>’ Name args +</pre><p> +can be used to call "methods". +A call <code>v:name(<em>args</em>)</code> +is syntactic sugar for <code>v.name(v,<em>args</em>)</code>, +except that <code>v</code> is evaluated only once. + + +<p> +Arguments have the following syntax: + +<pre> + args ::= ‘<b>(</b>’ [explist] ‘<b>)</b>’ + args ::= tableconstructor + args ::= String +</pre><p> +All argument expressions are evaluated before the call. +A call of the form <code>f{<em>fields</em>}</code> is +syntactic sugar for <code>f({<em>fields</em>})</code>; +that is, the argument list is a single new table. +A call of the form <code>f'<em>string</em>'</code> +(or <code>f"<em>string</em>"</code> or <code>f[[<em>string</em>]]</code>) +is syntactic sugar for <code>f('<em>string</em>')</code>; +that is, the argument list is a single literal string. + + +<p> +A call of the form <code>return <em>functioncall</em></code> is called +a <em>tail call</em>. +Lua implements <em>proper tail calls</em> +(or <em>proper tail recursion</em>): +in a tail call, +the called function reuses the stack entry of the calling function. +Therefore, there is no limit on the number of nested tail calls that +a program can execute. +However, a tail call erases any debug information about the +calling function. +Note that a tail call only happens with a particular syntax, +where the <b>return</b> has one single function call as argument; +this syntax makes the calling function return exactly +the returns of the called function. +So, none of the following examples are tail calls: + +<pre> + 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 +</pre> + + + + +<h3>3.4.10 – <a name="3.4.10">Function Definitions</a></h3> + +<p> +The syntax for function definition is + +<pre> + functiondef ::= <b>function</b> funcbody + funcbody ::= ‘<b>(</b>’ [parlist] ‘<b>)</b>’ block <b>end</b> +</pre> + +<p> +The following syntactic sugar simplifies function definitions: + +<pre> + stat ::= <b>function</b> funcname funcbody + stat ::= <b>local</b> <b>function</b> Name funcbody + funcname ::= Name {‘<b>.</b>’ Name} [‘<b>:</b>’ Name] +</pre><p> +The statement + +<pre> + function f () <em>body</em> end +</pre><p> +translates to + +<pre> + f = function () <em>body</em> end +</pre><p> +The statement + +<pre> + function t.a.b.c.f () <em>body</em> end +</pre><p> +translates to + +<pre> + t.a.b.c.f = function () <em>body</em> end +</pre><p> +The statement + +<pre> + local function f () <em>body</em> end +</pre><p> +translates to + +<pre> + local f; f = function () <em>body</em> end +</pre><p> +not to + +<pre> + local f = function () <em>body</em> end +</pre><p> +(This only makes a difference when the body of the function +contains references to <code>f</code>.) + + +<p> +A function definition is an executable expression, +whose value has type <em>function</em>. +When Lua precompiles a chunk, +all its function bodies are precompiled too. +Then, whenever Lua executes the function definition, +the function is <em>instantiated</em> (or <em>closed</em>). +This function instance (or <em>closure</em>) +is the final value of the expression. + + +<p> +Parameters act as local variables that are +initialized with the argument values: + +<pre> + parlist ::= namelist [‘<b>,</b>’ ‘<b>...</b>’] | ‘<b>...</b>’ +</pre><p> +When a function is called, +the list of arguments is adjusted to +the length of the list of parameters, +unless the function is a <em>vararg function</em>, +which is indicated by three dots ('<code>...</code>') +at the end of its parameter list. +A vararg function does not adjust its argument list; +instead, it collects all extra arguments and supplies them +to the function through a <em>vararg expression</em>, +which is also written as three dots. +The value of this expression is a list of all actual extra arguments, +similar to a function with multiple results. +If a vararg expression is used inside another expression +or in the middle of a list of expressions, +then its return list is adjusted to one element. +If the expression is used as the last element of a list of expressions, +then no adjustment is made +(unless that last expression is enclosed in parentheses). + + +<p> +As an example, consider the following definitions: + +<pre> + function f(a, b) end + function g(a, b, ...) end + function r() return 1,2,3 end +</pre><p> +Then, we have the following mapping from arguments to parameters and +to the vararg expression: + +<pre> + 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 +</pre> + +<p> +Results are returned using the <b>return</b> statement (see <a href="#3.3.4">§3.3.4</a>). +If control reaches the end of a function +without encountering a <b>return</b> statement, +then the function returns with no results. + + +<p> + +There is a system-dependent limit on the number of values +that a function may return. +This limit is guaranteed to be larger than 1000. + + +<p> +The <em>colon</em> syntax +is used for defining <em>methods</em>, +that is, functions that have an implicit extra parameter <code>self</code>. +Thus, the statement + +<pre> + function t.a.b.c:f (<em>params</em>) <em>body</em> end +</pre><p> +is syntactic sugar for + +<pre> + t.a.b.c.f = function (self, <em>params</em>) <em>body</em> end +</pre> + + + + + + +<h2>3.5 – <a name="3.5">Visibility Rules</a></h2> + +<p> + +Lua is a lexically scoped language. +The scope of a local variable begins at the first statement after +its declaration and lasts until the last non-void statement +of the innermost block that includes the declaration. +Consider the following example: + +<pre> + x = 10 -- global variable + do -- new block + local x = x -- new 'x', with value 10 + print(x) --> 10 + x = x+1 + do -- another block + local x = x+1 -- another 'x' + print(x) --> 12 + end + print(x) --> 11 + end + print(x) --> 10 (the global one) +</pre> + +<p> +Notice that, in a declaration like <code>local x = x</code>, +the new <code>x</code> being declared is not in scope yet, +and so the second <code>x</code> refers to the outside variable. + + +<p> +Because of the lexical scoping rules, +local variables can be freely accessed by functions +defined inside their scope. +A local variable used by an inner function is called +an <em>upvalue</em>, or <em>external local variable</em>, +inside the inner function. + + +<p> +Notice that each execution of a <b>local</b> statement +defines new local variables. +Consider the following example: + +<pre> + a = {} + local x = 20 + for i=1,10 do + local y = 0 + a[i] = function () y=y+1; return x+y end + end +</pre><p> +The loop creates ten closures +(that is, ten instances of the anonymous function). +Each of these closures uses a different <code>y</code> variable, +while all of them share the same <code>x</code>. + + + + + +<h1>4 – <a name="4">The Application Program Interface</a></h1> + +<p> + +This section describes the C API for Lua, that is, +the set of C functions available to the host program to communicate +with Lua. +All API functions and related types and constants +are declared in the header file <a name="pdf-lua.h"><code>lua.h</code></a>. + + +<p> +Even when we use the term "function", +any facility in the API may be provided as a macro instead. +Except where stated otherwise, +all such macros use each of their arguments exactly once +(except for the first argument, which is always a Lua state), +and so do not generate any hidden side-effects. + + +<p> +As in most C libraries, +the Lua API functions do not check their arguments for validity or consistency. +However, you can change this behavior by compiling Lua +with the macro <a name="pdf-LUA_USE_APICHECK"><code>LUA_USE_APICHECK</code></a> defined. + + + +<h2>4.1 – <a name="4.1">The Stack</a></h2> + +<p> +Lua uses a <em>virtual stack</em> to pass values to and from C. +Each element in this stack represents a Lua value +(<b>nil</b>, number, string, etc.). + + +<p> +Whenever Lua calls C, the called function gets a new stack, +which is independent of previous stacks and of stacks of +C functions that are still active. +This stack initially contains any arguments to the C function +and it is where the C function pushes its results +to be returned to the caller (see <a href="#lua_CFunction"><code>lua_CFunction</code></a>). + + +<p> +For convenience, +most query operations in the API do not follow a strict stack discipline. +Instead, they can refer to any element in the stack +by using an <em>index</em>: +A positive index represents an absolute stack position +(starting at 1); +a negative index represents an offset relative to the top of the stack. +More specifically, if the stack has <em>n</em> elements, +then index 1 represents the first element +(that is, the element that was pushed onto the stack first) +and +index <em>n</em> represents the last element; +index -1 also represents the last element +(that is, the element at the top) +and index <em>-n</em> represents the first element. + + + + + +<h2>4.2 – <a name="4.2">Stack Size</a></h2> + +<p> +When you interact with the Lua API, +you are responsible for ensuring consistency. +In particular, +<em>you are responsible for controlling stack overflow</em>. +You can use the function <a href="#lua_checkstack"><code>lua_checkstack</code></a> +to ensure that the stack has extra slots when pushing new elements. + + +<p> +Whenever Lua calls C, +it ensures that the stack has at least <a name="pdf-LUA_MINSTACK"><code>LUA_MINSTACK</code></a> extra slots. +<code>LUA_MINSTACK</code> is defined as 20, +so that usually you do not have to worry about stack space +unless your code has loops pushing elements onto the stack. + + +<p> +When you call a Lua function +without a fixed number of results (see <a href="#lua_call"><code>lua_call</code></a>), +Lua ensures that the stack has enough size for all results, +but it does not ensure any extra space. +So, before pushing anything in the stack after such a call +you should use <a href="#lua_checkstack"><code>lua_checkstack</code></a>. + + + + + +<h2>4.3 – <a name="4.3">Valid and Acceptable Indices</a></h2> + +<p> +Any function in the API that receives stack indices +works only with <em>valid indices</em> or <em>acceptable indices</em>. + + +<p> +A <em>valid index</em> is an index that refers to a +real position within the stack, that is, +its position lies between 1 and the stack top +(<code>1 ≤ abs(index) ≤ top</code>). + +Usually, functions that can modify the value at an index +require valid indices. + + +<p> +Unless otherwise noted, +any function that accepts valid indices also accepts <em>pseudo-indices</em>, +which represent some Lua values that are accessible to C code +but which are not in the stack. +Pseudo-indices are used to access the registry +and the upvalues of a C function (see <a href="#4.4">§4.4</a>). + + +<p> +Functions that do not need a specific stack position, +but only a value in the stack (e.g., query functions), +can be called with acceptable indices. +An <em>acceptable index</em> can be any valid index, +including the pseudo-indices, +but it also can be any positive index after the stack top +within the space allocated for the stack, +that is, indices up to the stack size. +(Note that 0 is never an acceptable index.) +Except when noted otherwise, +functions in the API work with acceptable indices. + + +<p> +Acceptable indices serve to avoid extra tests +against the stack top when querying the stack. +For instance, a C function can query its third argument +without the need to first check whether there is a third argument, +that is, without the need to check whether 3 is a valid index. + + +<p> +For functions that can be called with acceptable indices, +any non-valid index is treated as if it +contains a value of a virtual type <a name="pdf-LUA_TNONE"><code>LUA_TNONE</code></a>, +which behaves like a nil value. + + + + + +<h2>4.4 – <a name="4.4">C Closures</a></h2> + +<p> +When a C function is created, +it is possible to associate some values with it, +thus creating a <em>C closure</em> +(see <a href="#lua_pushcclosure"><code>lua_pushcclosure</code></a>); +these values are called <em>upvalues</em> and are +accessible to the function whenever it is called. + + +<p> +Whenever a C function is called, +its upvalues are located at specific pseudo-indices. +These pseudo-indices are produced by the macro +<a href="#lua_upvalueindex"><code>lua_upvalueindex</code></a>. +The first value associated with a function is at position +<code>lua_upvalueindex(1)</code>, and so on. +Any access to <code>lua_upvalueindex(<em>n</em>)</code>, +where <em>n</em> is greater than the number of upvalues of the +current function (but not greater than 256), +produces an acceptable but invalid index. + + + + + +<h2>4.5 – <a name="4.5">Registry</a></h2> + +<p> +Lua provides a <em>registry</em>, +a predefined table that can be used by any C code to +store whatever Lua values it needs to store. +The registry table is always located at pseudo-index +<a name="pdf-LUA_REGISTRYINDEX"><code>LUA_REGISTRYINDEX</code></a>, +which is a valid index. +Any C library can store data into this table, +but it should take care to choose keys +that are different from those used +by other libraries, to avoid collisions. +Typically, you should use as key a string containing your library name, +or a light userdata with the address of a C object in your code, +or any Lua object created by your code. +As with global names, +string keys starting with an underscore followed by +uppercase letters are reserved for Lua. + + +<p> +The integer keys in the registry are used by the reference mechanism, +implemented by the auxiliary library, +and by some predefined values. +Therefore, integer keys should not be used for other purposes. + + +<p> +When you create a new Lua state, +its registry comes with some predefined values. +These predefined values are indexed with integer keys +defined as constants in <code>lua.h</code>. +The following constants are defined: + +<ul> +<li><b><a name="pdf-LUA_RIDX_MAINTHREAD"><code>LUA_RIDX_MAINTHREAD</code></a>: </b> At this index the registry has +the main thread of the state. +(The main thread is the one created together with the state.) +</li> + +<li><b><a name="pdf-LUA_RIDX_GLOBALS"><code>LUA_RIDX_GLOBALS</code></a>: </b> At this index the registry has +the global environment. +</li> +</ul> + + + + +<h2>4.6 – <a name="4.6">Error Handling in C</a></h2> + +<p> +Internally, Lua uses the C <code>longjmp</code> facility to handle errors. +(You can also choose to use exceptions if you compile Lua as C++; +search for <code>LUAI_THROW</code> in the source code.) +When Lua faces any error +(such as a memory allocation error, type errors, syntax errors, +and runtime errors) +it <em>raises</em> an error; +that is, it does a long jump. +A <em>protected environment</em> uses <code>setjmp</code> +to set a recovery point; +any error jumps to the most recent active recovery point. + + +<p> +If an error happens outside any protected environment, +Lua calls a <em>panic function</em> (see <a href="#lua_atpanic"><code>lua_atpanic</code></a>) +and then calls <code>abort</code>, +thus exiting the host application. +Your panic function can avoid this exit by +never returning +(e.g., doing a long jump to your own recovery point outside Lua). + + +<p> +The panic function runs as if it were a message handler (see <a href="#2.3">§2.3</a>); +in particular, the error message is at the top of the stack. +However, there is no guarantees about stack space. +To push anything on the stack, +the panic function should first check the available space (see <a href="#4.2">§4.2</a>). + + +<p> +Most functions in the API can throw an error, +for instance due to a memory allocation error. +The documentation for each function indicates whether +it can throw errors. + + +<p> +Inside a C function you can throw an error by calling <a href="#lua_error"><code>lua_error</code></a>. + + + + + +<h2>4.7 – <a name="4.7">Handling Yields in C</a></h2> + +<p> +Internally, Lua uses the C <code>longjmp</code> facility to yield a coroutine. +Therefore, if a function <code>foo</code> calls an API function +and this API function yields +(directly or indirectly by calling another function that yields), +Lua cannot return to <code>foo</code> any more, +because the <code>longjmp</code> removes its frame from the C stack. + + +<p> +To avoid this kind of problem, +Lua raises an error whenever it tries to yield across an API call, +except for three functions: +<a href="#lua_yieldk"><code>lua_yieldk</code></a>, <a href="#lua_callk"><code>lua_callk</code></a>, and <a href="#lua_pcallk"><code>lua_pcallk</code></a>. +All those functions receive a <em>continuation function</em> +(as a parameter called <code>k</code>) to continue execution after a yield. + + +<p> +We need to set some terminology to explain continuations. +We have a C function called from Lua which we will call +the <em>original function</em>. +This original function then calls one of those three functions in the C API, +which we will call the <em>callee function</em>, +that then yields the current thread. +(This can happen when the callee function is <a href="#lua_yieldk"><code>lua_yieldk</code></a>, +or when the callee function is either <a href="#lua_callk"><code>lua_callk</code></a> or <a href="#lua_pcallk"><code>lua_pcallk</code></a> +and the function called by them yields.) + + +<p> +Suppose the running thread yields while executing the callee function. +After the thread resumes, +it eventually will finish running the callee function. +However, +the callee function cannot return to the original function, +because its frame in the C stack was destroyed by the yield. +Instead, Lua calls a <em>continuation function</em>, +which was given as an argument to the callee function. +As the name implies, +the continuation function should continue the task +of the original function. + + +<p> +Lua treats the continuation function as if it were the original function. +The continuation function receives the same Lua stack +from the original function, +in the same state it would be if the callee function had returned. +(For instance, +after a <a href="#lua_callk"><code>lua_callk</code></a> the function and its arguments are +removed from the stack and replaced by the results from the call.) +It also has the same upvalues. +Whatever it returns is handled by Lua as if it were the return +of the original function. + + +<p> +The only difference in the Lua state between the original function +and its continuation is the result of a call to <a href="#lua_getctx"><code>lua_getctx</code></a>. + + + + + +<h2>4.8 – <a name="4.8">Functions and Types</a></h2> + +<p> +Here we list all functions and types from the C API in +alphabetical order. +Each function has an indicator like this: +<span class="apii">[-o, +p, <em>x</em>]</span> + + +<p> +The first field, <code>o</code>, +is how many elements the function pops from the stack. +The second field, <code>p</code>, +is how many elements the function pushes onto the stack. +(Any function always pushes its results after popping its arguments.) +A field in the form <code>x|y</code> means the function can push (or pop) +<code>x</code> or <code>y</code> elements, +depending on the situation; +an interrogation mark '<code>?</code>' means that +we cannot know how many elements the function pops/pushes +by looking only at its arguments +(e.g., they may depend on what is on the stack). +The third field, <code>x</code>, +tells whether the function may throw errors: +'<code>-</code>' means the function never throws any error; +'<code>e</code>' means the function may throw errors; +'<code>v</code>' means the function may throw an error on purpose. + + + +<hr><h3><a name="lua_absindex"><code>lua_absindex</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_absindex (lua_State *L, int idx);</pre> + +<p> +Converts the acceptable index <code>idx</code> into an absolute index +(that is, one that does not depend on the stack top). + + + + + +<hr><h3><a name="lua_Alloc"><code>lua_Alloc</code></a></h3> +<pre>typedef void * (*lua_Alloc) (void *ud, + void *ptr, + size_t osize, + size_t nsize);</pre> + +<p> +The type of the memory-allocation function used by Lua states. +The allocator function must provide a +functionality similar to <code>realloc</code>, +but not exactly the same. +Its arguments are +<code>ud</code>, an opaque pointer passed to <a href="#lua_newstate"><code>lua_newstate</code></a>; +<code>ptr</code>, a pointer to the block being allocated/reallocated/freed; +<code>osize</code>, the original size of the block or some code about what +is being allocated; +<code>nsize</code>, the new size of the block. + + +<p> +When <code>ptr</code> is not <code>NULL</code>, +<code>osize</code> is the size of the block pointed by <code>ptr</code>, +that is, the size given when it was allocated or reallocated. + + +<p> +When <code>ptr</code> is <code>NULL</code>, +<code>osize</code> encodes the kind of object that Lua is allocating. +<code>osize</code> is any of +<a href="#pdf-LUA_TSTRING"><code>LUA_TSTRING</code></a>, <a href="#pdf-LUA_TTABLE"><code>LUA_TTABLE</code></a>, <a href="#pdf-LUA_TFUNCTION"><code>LUA_TFUNCTION</code></a>, +<a href="#pdf-LUA_TUSERDATA"><code>LUA_TUSERDATA</code></a>, or <a href="#pdf-LUA_TTHREAD"><code>LUA_TTHREAD</code></a> when (and only when) +Lua is creating a new object of that type. +When <code>osize</code> is some other value, +Lua is allocating memory for something else. + + +<p> +Lua assumes the following behavior from the allocator function: + + +<p> +When <code>nsize</code> is zero, +the allocator should behave like <code>free</code> +and return <code>NULL</code>. + + +<p> +When <code>nsize</code> is not zero, +the allocator should behave like <code>realloc</code>. +The allocator returns <code>NULL</code> +if and only if it cannot fulfill the request. +Lua assumes that the allocator never fails when +<code>osize >= nsize</code>. + + +<p> +Here is a simple implementation for the allocator function. +It is used in the auxiliary library by <a href="#luaL_newstate"><code>luaL_newstate</code></a>. + +<pre> + 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); + } +</pre><p> +Note that Standard C ensures +that <code>free(NULL)</code> has no effect and that +<code>realloc(NULL, size)</code> is equivalent to <code>malloc(size)</code>. +This code assumes that <code>realloc</code> does not fail when shrinking a block. +(Although Standard C does not ensure this behavior, +it seems to be a safe assumption.) + + + + + +<hr><h3><a name="lua_arith"><code>lua_arith</code></a></h3><p> +<span class="apii">[-(2|1), +1, <em>e</em>]</span> +<pre>void lua_arith (lua_State *L, int op);</pre> + +<p> +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). + + +<p> +The value of <code>op</code> must be one of the following constants: + +<ul> + +<li><b><a name="pdf-LUA_OPADD"><code>LUA_OPADD</code></a>: </b> performs addition (<code>+</code>)</li> +<li><b><a name="pdf-LUA_OPSUB"><code>LUA_OPSUB</code></a>: </b> performs subtraction (<code>-</code>)</li> +<li><b><a name="pdf-LUA_OPMUL"><code>LUA_OPMUL</code></a>: </b> performs multiplication (<code>*</code>)</li> +<li><b><a name="pdf-LUA_OPDIV"><code>LUA_OPDIV</code></a>: </b> performs division (<code>/</code>)</li> +<li><b><a name="pdf-LUA_OPMOD"><code>LUA_OPMOD</code></a>: </b> performs modulo (<code>%</code>)</li> +<li><b><a name="pdf-LUA_OPPOW"><code>LUA_OPPOW</code></a>: </b> performs exponentiation (<code>^</code>)</li> +<li><b><a name="pdf-LUA_OPUNM"><code>LUA_OPUNM</code></a>: </b> performs mathematical negation (unary <code>-</code>)</li> + +</ul> + + + + +<hr><h3><a name="lua_atpanic"><code>lua_atpanic</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>lua_CFunction lua_atpanic (lua_State *L, lua_CFunction panicf);</pre> + +<p> +Sets a new panic function and returns the old one (see <a href="#4.6">§4.6</a>). + + + + + +<hr><h3><a name="lua_call"><code>lua_call</code></a></h3><p> +<span class="apii">[-(nargs+1), +nresults, <em>e</em>]</span> +<pre>void lua_call (lua_State *L, int nargs, int nresults);</pre> + +<p> +Calls a function. + + +<p> +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 <a href="#lua_call"><code>lua_call</code></a>; +<code>nargs</code> 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 <code>nresults</code>, +unless <code>nresults</code> is <a name="pdf-LUA_MULTRET"><code>LUA_MULTRET</code></a>. +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. + + +<p> +Any error inside the called function is propagated upwards +(with a <code>longjmp</code>). + + +<p> +The following example shows how the host program can do the +equivalent to this Lua code: + +<pre> + a = f("how", t.x, 14) +</pre><p> +Here it is in C: + +<pre> + 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' */ +</pre><p> +Note that the code above is "balanced": +at its end, the stack is back to its original configuration. +This is considered good programming practice. + + + + + +<hr><h3><a name="lua_callk"><code>lua_callk</code></a></h3><p> +<span class="apii">[-(nargs + 1), +nresults, <em>e</em>]</span> +<pre>void lua_callk (lua_State *L, int nargs, int nresults, int ctx, + lua_CFunction k);</pre> + +<p> +This function behaves exactly like <a href="#lua_call"><code>lua_call</code></a>, +but allows the called function to yield (see <a href="#4.7">§4.7</a>). + + + + + +<hr><h3><a name="lua_CFunction"><code>lua_CFunction</code></a></h3> +<pre>typedef int (*lua_CFunction) (lua_State *L);</pre> + +<p> +Type for C functions. + + +<p> +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, +<code>lua_gettop(L)</code> 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 <code>lua_gettop(L)</code>. +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. + + +<p> +As an example, the following function receives a variable number +of numerical arguments and returns their average and sum: + +<pre> + 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 */ + } +</pre> + + + + +<hr><h3><a name="lua_checkstack"><code>lua_checkstack</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_checkstack (lua_State *L, int extra);</pre> + +<p> +Ensures that there are at least <code>extra</code> 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. + + + + + +<hr><h3><a name="lua_close"><code>lua_close</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>void lua_close (lua_State *L);</pre> + +<p> +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. + + + + + +<hr><h3><a name="lua_compare"><code>lua_compare</code></a></h3><p> +<span class="apii">[-0, +0, <em>e</em>]</span> +<pre>int lua_compare (lua_State *L, int index1, int index2, int op);</pre> + +<p> +Compares two Lua values. +Returns 1 if the value at index <code>index1</code> satisfies <code>op</code> +when compared with the value at index <code>index2</code>, +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. + + +<p> +The value of <code>op</code> must be one of the following constants: + +<ul> + +<li><b><a name="pdf-LUA_OPEQ"><code>LUA_OPEQ</code></a>: </b> compares for equality (<code>==</code>)</li> +<li><b><a name="pdf-LUA_OPLT"><code>LUA_OPLT</code></a>: </b> compares for less than (<code><</code>)</li> +<li><b><a name="pdf-LUA_OPLE"><code>LUA_OPLE</code></a>: </b> compares for less or equal (<code><=</code>)</li> + +</ul> + + + + +<hr><h3><a name="lua_concat"><code>lua_concat</code></a></h3><p> +<span class="apii">[-n, +1, <em>e</em>]</span> +<pre>void lua_concat (lua_State *L, int n);</pre> + +<p> +Concatenates the <code>n</code> values at the top of the stack, +pops them, and leaves the result at the top. +If <code>n</code> is 1, the result is the single value on the stack +(that is, the function does nothing); +if <code>n</code> is 0, the result is the empty string. +Concatenation is performed following the usual semantics of Lua +(see <a href="#3.4.5">§3.4.5</a>). + + + + + +<hr><h3><a name="lua_copy"><code>lua_copy</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>void lua_copy (lua_State *L, int fromidx, int toidx);</pre> + +<p> +Moves the element at index <code>fromidx</code> +into the valid index <code>toidx</code> +without shifting any element +(therefore replacing the value at that position). + + + + + +<hr><h3><a name="lua_createtable"><code>lua_createtable</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>void lua_createtable (lua_State *L, int narr, int nrec);</pre> + +<p> +Creates a new empty table and pushes it onto the stack. +Parameter <code>narr</code> is a hint for how many elements the table +will have as a sequence; +parameter <code>nrec</code> 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 <a href="#lua_newtable"><code>lua_newtable</code></a>. + + + + + +<hr><h3><a name="lua_dump"><code>lua_dump</code></a></h3><p> +<span class="apii">[-0, +0, <em>e</em>]</span> +<pre>int lua_dump (lua_State *L, lua_Writer writer, void *data);</pre> + +<p> +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, +<a href="#lua_dump"><code>lua_dump</code></a> calls function <code>writer</code> (see <a href="#lua_Writer"><code>lua_Writer</code></a>) +with the given <code>data</code> +to write them. + + +<p> +The value returned is the error code returned by the last +call to the writer; +0 means no errors. + + +<p> +This function does not pop the Lua function from the stack. + + + + + +<hr><h3><a name="lua_error"><code>lua_error</code></a></h3><p> +<span class="apii">[-1, +0, <em>v</em>]</span> +<pre>int lua_error (lua_State *L);</pre> + +<p> +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 +(see <a href="#luaL_error"><code>luaL_error</code></a>). + + + + + +<hr><h3><a name="lua_gc"><code>lua_gc</code></a></h3><p> +<span class="apii">[-0, +0, <em>e</em>]</span> +<pre>int lua_gc (lua_State *L, int what, int data);</pre> + +<p> +Controls the garbage collector. + + +<p> +This function performs several tasks, +according to the value of the parameter <code>what</code>: + +<ul> + +<li><b><code>LUA_GCSTOP</code>: </b> +stops the garbage collector. +</li> + +<li><b><code>LUA_GCRESTART</code>: </b> +restarts the garbage collector. +</li> + +<li><b><code>LUA_GCCOLLECT</code>: </b> +performs a full garbage-collection cycle. +</li> + +<li><b><code>LUA_GCCOUNT</code>: </b> +returns the current amount of memory (in Kbytes) in use by Lua. +</li> + +<li><b><code>LUA_GCCOUNTB</code>: </b> +returns the remainder of dividing the current amount of bytes of +memory in use by Lua by 1024. +</li> + +<li><b><code>LUA_GCSTEP</code>: </b> +performs an incremental step of garbage collection. +The step "size" is controlled by <code>data</code> +(larger values mean more steps) in a non-specified way. +If you want to control the step size +you must experimentally tune the value of <code>data</code>. +The function returns 1 if the step finished a +garbage-collection cycle. +</li> + +<li><b><code>LUA_GCSETPAUSE</code>: </b> +sets <code>data</code> as the new value +for the <em>pause</em> of the collector (see <a href="#2.5">§2.5</a>). +The function returns the previous value of the pause. +</li> + +<li><b><code>LUA_GCSETSTEPMUL</code>: </b> +sets <code>data</code> as the new value for the <em>step multiplier</em> of +the collector (see <a href="#2.5">§2.5</a>). +The function returns the previous value of the step multiplier. +</li> + +<li><b><code>LUA_GCISRUNNING</code>: </b> +returns a boolean that tells whether the collector is running +(i.e., not stopped). +</li> + +<li><b><code>LUA_GCGEN</code>: </b> +changes the collector to generational mode +(see <a href="#2.5">§2.5</a>). +</li> + +<li><b><code>LUA_GCINC</code>: </b> +changes the collector to incremental mode. +This is the default mode. +</li> + +</ul> + +<p> +For more details about these options, +see <a href="#pdf-collectgarbage"><code>collectgarbage</code></a>. + + + + + +<hr><h3><a name="lua_getallocf"><code>lua_getallocf</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>lua_Alloc lua_getallocf (lua_State *L, void **ud);</pre> + +<p> +Returns the memory-allocation function of a given state. +If <code>ud</code> is not <code>NULL</code>, Lua stores in <code>*ud</code> the +opaque pointer passed to <a href="#lua_newstate"><code>lua_newstate</code></a>. + + + + + +<hr><h3><a name="lua_getctx"><code>lua_getctx</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_getctx (lua_State *L, int *ctx);</pre> + +<p> +This function is called by a continuation function (see <a href="#4.7">§4.7</a>) +to retrieve the status of the thread and a context information. + + +<p> +When called in the original function, +<a href="#lua_getctx"><code>lua_getctx</code></a> always returns <a href="#pdf-LUA_OK"><code>LUA_OK</code></a> +and does not change the value of its argument <code>ctx</code>. +When called inside a continuation function, +<a href="#lua_getctx"><code>lua_getctx</code></a> returns <a href="#pdf-LUA_YIELD"><code>LUA_YIELD</code></a> and sets +the value of <code>ctx</code> to be the context information +(the value passed as the <code>ctx</code> argument +to the callee together with the continuation function). + + +<p> +When the callee is <a href="#lua_pcallk"><code>lua_pcallk</code></a>, +Lua may also call its continuation function +to handle errors during the call. +That is, upon an error in the function called by <a href="#lua_pcallk"><code>lua_pcallk</code></a>, +Lua may not return to the original function +but instead may call the continuation function. +In that case, a call to <a href="#lua_getctx"><code>lua_getctx</code></a> will return the error code +(the value that would be returned by <a href="#lua_pcallk"><code>lua_pcallk</code></a>); +the value of <code>ctx</code> will be set to the context information, +as in the case of a yield. + + + + + +<hr><h3><a name="lua_getfield"><code>lua_getfield</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>void lua_getfield (lua_State *L, int index, const char *k);</pre> + +<p> +Pushes onto the stack the value <code>t[k]</code>, +where <code>t</code> is the value at the given index. +As in Lua, this function may trigger a metamethod +for the "index" event (see <a href="#2.4">§2.4</a>). + + + + + +<hr><h3><a name="lua_getglobal"><code>lua_getglobal</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>void lua_getglobal (lua_State *L, const char *name);</pre> + +<p> +Pushes onto the stack the value of the global <code>name</code>. + + + + + +<hr><h3><a name="lua_getmetatable"><code>lua_getmetatable</code></a></h3><p> +<span class="apii">[-0, +(0|1), –]</span> +<pre>int lua_getmetatable (lua_State *L, int index);</pre> + +<p> +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. + + + + + +<hr><h3><a name="lua_gettable"><code>lua_gettable</code></a></h3><p> +<span class="apii">[-1, +1, <em>e</em>]</span> +<pre>void lua_gettable (lua_State *L, int index);</pre> + +<p> +Pushes onto the stack the value <code>t[k]</code>, +where <code>t</code> is the value at the given index +and <code>k</code> is the value at the top of the stack. + + +<p> +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 <a href="#2.4">§2.4</a>). + + + + + +<hr><h3><a name="lua_gettop"><code>lua_gettop</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_gettop (lua_State *L);</pre> + +<p> +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). + + + + + +<hr><h3><a name="lua_getuservalue"><code>lua_getuservalue</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>void lua_getuservalue (lua_State *L, int index);</pre> + +<p> +Pushes onto the stack the Lua value associated with the userdata +at the given index. +This Lua value must be a table or <b>nil</b>. + + + + + +<hr><h3><a name="lua_insert"><code>lua_insert</code></a></h3><p> +<span class="apii">[-1, +1, –]</span> +<pre>void lua_insert (lua_State *L, int index);</pre> + +<p> +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. + + + + + +<hr><h3><a name="lua_Integer"><code>lua_Integer</code></a></h3> +<pre>typedef ptrdiff_t lua_Integer;</pre> + +<p> +The type used by the Lua API to represent signed integral values. + + +<p> +By default it is a <code>ptrdiff_t</code>, +which is usually the largest signed integral type the machine handles +"comfortably". + + + + + +<hr><h3><a name="lua_isboolean"><code>lua_isboolean</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_isboolean (lua_State *L, int index);</pre> + +<p> +Returns 1 if the value at the given index is a boolean, +and 0 otherwise. + + + + + +<hr><h3><a name="lua_iscfunction"><code>lua_iscfunction</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_iscfunction (lua_State *L, int index);</pre> + +<p> +Returns 1 if the value at the given index is a C function, +and 0 otherwise. + + + + + +<hr><h3><a name="lua_isfunction"><code>lua_isfunction</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_isfunction (lua_State *L, int index);</pre> + +<p> +Returns 1 if the value at the given index is a function +(either C or Lua), and 0 otherwise. + + + + + +<hr><h3><a name="lua_islightuserdata"><code>lua_islightuserdata</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_islightuserdata (lua_State *L, int index);</pre> + +<p> +Returns 1 if the value at the given index is a light userdata, +and 0 otherwise. + + + + + +<hr><h3><a name="lua_isnil"><code>lua_isnil</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_isnil (lua_State *L, int index);</pre> + +<p> +Returns 1 if the value at the given index is <b>nil</b>, +and 0 otherwise. + + + + + +<hr><h3><a name="lua_isnone"><code>lua_isnone</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_isnone (lua_State *L, int index);</pre> + +<p> +Returns 1 if the given index is not valid, +and 0 otherwise. + + + + + +<hr><h3><a name="lua_isnoneornil"><code>lua_isnoneornil</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_isnoneornil (lua_State *L, int index);</pre> + +<p> +Returns 1 if the given index is not valid +or if the value at this index is <b>nil</b>, +and 0 otherwise. + + + + + +<hr><h3><a name="lua_isnumber"><code>lua_isnumber</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_isnumber (lua_State *L, int index);</pre> + +<p> +Returns 1 if the value at the given index is a number +or a string convertible to a number, +and 0 otherwise. + + + + + +<hr><h3><a name="lua_isstring"><code>lua_isstring</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_isstring (lua_State *L, int index);</pre> + +<p> +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. + + + + + +<hr><h3><a name="lua_istable"><code>lua_istable</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_istable (lua_State *L, int index);</pre> + +<p> +Returns 1 if the value at the given index is a table, +and 0 otherwise. + + + + + +<hr><h3><a name="lua_isthread"><code>lua_isthread</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_isthread (lua_State *L, int index);</pre> + +<p> +Returns 1 if the value at the given index is a thread, +and 0 otherwise. + + + + + +<hr><h3><a name="lua_isuserdata"><code>lua_isuserdata</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_isuserdata (lua_State *L, int index);</pre> + +<p> +Returns 1 if the value at the given index is a userdata +(either full or light), and 0 otherwise. + + + + + +<hr><h3><a name="lua_len"><code>lua_len</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>void lua_len (lua_State *L, int index);</pre> + +<p> +Returns the "length" of the value at the given index; +it is equivalent to the '<code>#</code>' operator in Lua (see <a href="#3.4.6">§3.4.6</a>). +The result is pushed on the stack. + + + + + +<hr><h3><a name="lua_load"><code>lua_load</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>int lua_load (lua_State *L, + lua_Reader reader, + void *data, + const char *source, + const char *mode);</pre> + +<p> +Loads a Lua chunk (without running it). +If there are no errors, +<code>lua_load</code> pushes the compiled chunk as a Lua +function on top of the stack. +Otherwise, it pushes an error message. + + +<p> +The return values of <code>lua_load</code> are: + +<ul> + +<li><b><a href="#pdf-LUA_OK"><code>LUA_OK</code></a>: </b> no errors;</li> + +<li><b><a name="pdf-LUA_ERRSYNTAX"><code>LUA_ERRSYNTAX</code></a>: </b> +syntax error during precompilation;</li> + +<li><b><a href="#pdf-LUA_ERRMEM"><code>LUA_ERRMEM</code></a>: </b> +memory allocation error;</li> + +<li><b><a href="#pdf-LUA_ERRGCMM"><code>LUA_ERRGCMM</code></a>: </b> +error while running a <code>__gc</code> metamethod. +(This error has no relation with the chunk being loaded. +It is generated by the garbage collector.) +</li> + +</ul> + +<p> +The <code>lua_load</code> function uses a user-supplied <code>reader</code> function +to read the chunk (see <a href="#lua_Reader"><code>lua_Reader</code></a>). +The <code>data</code> argument is an opaque value passed to the reader function. + + +<p> +The <code>source</code> argument gives a name to the chunk, +which is used for error messages and in debug information (see <a href="#4.9">§4.9</a>). + + +<p> +<code>lua_load</code> automatically detects whether the chunk is text or binary +and loads it accordingly (see program <code>luac</code>). +The string <code>mode</code> works as in function <a href="#pdf-load"><code>load</code></a>, +with the addition that +a <code>NULL</code> value is equivalent to the string "<code>bt</code>". + + +<p> +<code>lua_load</code> uses the stack internally, +so the reader function should always leave the stack +unmodified when returning. + + +<p> +If the resulting function has one upvalue, +this upvalue is set to the value of the global environment +stored at index <code>LUA_RIDX_GLOBALS</code> in the registry (see <a href="#4.5">§4.5</a>). +When loading main chunks, +this upvalue will be the <code>_ENV</code> variable (see <a href="#2.2">§2.2</a>). + + + + + +<hr><h3><a name="lua_newstate"><code>lua_newstate</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>lua_State *lua_newstate (lua_Alloc f, void *ud);</pre> + +<p> +Creates a new thread running in a new, independent state. +Returns <code>NULL</code> if cannot create the thread or the state +(due to lack of memory). +The argument <code>f</code> is the allocator function; +Lua does all memory allocation for this state through this function. +The second argument, <code>ud</code>, is an opaque pointer that Lua +passes to the allocator in every call. + + + + + +<hr><h3><a name="lua_newtable"><code>lua_newtable</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>void lua_newtable (lua_State *L);</pre> + +<p> +Creates a new empty table and pushes it onto the stack. +It is equivalent to <code>lua_createtable(L, 0, 0)</code>. + + + + + +<hr><h3><a name="lua_newthread"><code>lua_newthread</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>lua_State *lua_newthread (lua_State *L);</pre> + +<p> +Creates a new thread, pushes it on the stack, +and returns a pointer to a <a href="#lua_State"><code>lua_State</code></a> 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. + + +<p> +There is no explicit function to close or to destroy a thread. +Threads are subject to garbage collection, +like any Lua object. + + + + + +<hr><h3><a name="lua_newuserdata"><code>lua_newuserdata</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>void *lua_newuserdata (lua_State *L, size_t size);</pre> + +<p> +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. + + + + + +<hr><h3><a name="lua_next"><code>lua_next</code></a></h3><p> +<span class="apii">[-1, +(2|0), <em>e</em>]</span> +<pre>int lua_next (lua_State *L, int index);</pre> + +<p> +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 <a href="#lua_next"><code>lua_next</code></a> returns 0 (and pushes nothing). + + +<p> +A typical traversal looks like this: + +<pre> + /* 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); + } +</pre> + +<p> +While traversing a table, +do not call <a href="#lua_tolstring"><code>lua_tolstring</code></a> directly on a key, +unless you know that the key is actually a string. +Recall that <a href="#lua_tolstring"><code>lua_tolstring</code></a> may change +the value at the given index; +this confuses the next call to <a href="#lua_next"><code>lua_next</code></a>. + + +<p> +See function <a href="#pdf-next"><code>next</code></a> for the caveats of modifying +the table during its traversal. + + + + + +<hr><h3><a name="lua_Number"><code>lua_Number</code></a></h3> +<pre>typedef double lua_Number;</pre> + +<p> +The type of numbers in Lua. +By default, it is double, but that can be changed in <code>luaconf.h</code>. +Through this configuration file you can change +Lua to operate with another type for numbers (e.g., float or long). + + + + + +<hr><h3><a name="lua_pcall"><code>lua_pcall</code></a></h3><p> +<span class="apii">[-(nargs + 1), +(nresults|1), –]</span> +<pre>int lua_pcall (lua_State *L, int nargs, int nresults, int msgh);</pre> + +<p> +Calls a function in protected mode. + + +<p> +Both <code>nargs</code> and <code>nresults</code> have the same meaning as +in <a href="#lua_call"><code>lua_call</code></a>. +If there are no errors during the call, +<a href="#lua_pcall"><code>lua_pcall</code></a> behaves exactly like <a href="#lua_call"><code>lua_call</code></a>. +However, if there is any error, +<a href="#lua_pcall"><code>lua_pcall</code></a> catches it, +pushes a single value on the stack (the error message), +and returns an error code. +Like <a href="#lua_call"><code>lua_call</code></a>, +<a href="#lua_pcall"><code>lua_pcall</code></a> always removes the function +and its arguments from the stack. + + +<p> +If <code>msgh</code> is 0, +then the error message returned on the stack +is exactly the original error message. +Otherwise, <code>msgh</code> is the stack index of a +<em>message handler</em>. +(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 <a href="#lua_pcall"><code>lua_pcall</code></a>. + + +<p> +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 <a href="#lua_pcall"><code>lua_pcall</code></a>, +since by then the stack has unwound. + + +<p> +The <a href="#lua_pcall"><code>lua_pcall</code></a> function returns one of the following codes +(defined in <code>lua.h</code>): + +<ul> + +<li><b><a name="pdf-LUA_OK"><code>LUA_OK</code></a> (0): </b> +success.</li> + +<li><b><a name="pdf-LUA_ERRRUN"><code>LUA_ERRRUN</code></a>: </b> +a runtime error. +</li> + +<li><b><a name="pdf-LUA_ERRMEM"><code>LUA_ERRMEM</code></a>: </b> +memory allocation error. +For such errors, Lua does not call the message handler. +</li> + +<li><b><a name="pdf-LUA_ERRERR"><code>LUA_ERRERR</code></a>: </b> +error while running the message handler. +</li> + +<li><b><a name="pdf-LUA_ERRGCMM"><code>LUA_ERRGCMM</code></a>: </b> +error while running a <code>__gc</code> metamethod. +(This error typically has no relation with the function being called. +It is generated by the garbage collector.) +</li> + +</ul> + + + + +<hr><h3><a name="lua_pcallk"><code>lua_pcallk</code></a></h3><p> +<span class="apii">[-(nargs + 1), +(nresults|1), –]</span> +<pre>int lua_pcallk (lua_State *L, + int nargs, + int nresults, + int errfunc, + int ctx, + lua_CFunction k);</pre> + +<p> +This function behaves exactly like <a href="#lua_pcall"><code>lua_pcall</code></a>, +but allows the called function to yield (see <a href="#4.7">§4.7</a>). + + + + + +<hr><h3><a name="lua_pop"><code>lua_pop</code></a></h3><p> +<span class="apii">[-n, +0, –]</span> +<pre>void lua_pop (lua_State *L, int n);</pre> + +<p> +Pops <code>n</code> elements from the stack. + + + + + +<hr><h3><a name="lua_pushboolean"><code>lua_pushboolean</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>void lua_pushboolean (lua_State *L, int b);</pre> + +<p> +Pushes a boolean value with value <code>b</code> onto the stack. + + + + + +<hr><h3><a name="lua_pushcclosure"><code>lua_pushcclosure</code></a></h3><p> +<span class="apii">[-n, +1, <em>e</em>]</span> +<pre>void lua_pushcclosure (lua_State *L, lua_CFunction fn, int n);</pre> + +<p> +Pushes a new C closure onto the stack. + + +<p> +When a C function is created, +it is possible to associate some values with it, +thus creating a C closure (see <a href="#4.4">§4.4</a>); +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 <a href="#lua_pushcclosure"><code>lua_pushcclosure</code></a> +is called to create and push the C function onto the stack, +with the argument <code>n</code> telling how many values should be +associated with the function. +<a href="#lua_pushcclosure"><code>lua_pushcclosure</code></a> also pops these values from the stack. + + +<p> +The maximum value for <code>n</code> is 255. + + +<p> +When <code>n</code> is zero, +this function creates a <em>light C function</em>, +which is just a pointer to the C function. +In that case, it never throws a memory error. + + + + + +<hr><h3><a name="lua_pushcfunction"><code>lua_pushcfunction</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>void lua_pushcfunction (lua_State *L, lua_CFunction f);</pre> + +<p> +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 <code>function</code> that, +when called, invokes the corresponding C function. + + +<p> +Any function to be registered in Lua must +follow the correct protocol to receive its parameters +and return its results (see <a href="#lua_CFunction"><code>lua_CFunction</code></a>). + + +<p> +<code>lua_pushcfunction</code> is defined as a macro: + +<pre> + #define lua_pushcfunction(L,f) lua_pushcclosure(L,f,0) +</pre><p> +Note that <code>f</code> is used twice. + + + + + +<hr><h3><a name="lua_pushfstring"><code>lua_pushfstring</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>const char *lua_pushfstring (lua_State *L, const char *fmt, ...);</pre> + +<p> +Pushes onto the stack a formatted string +and returns a pointer to this string. +It is similar to the ANSI C function <code>sprintf</code>, +but has some important differences: + +<ul> + +<li> +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). +</li> + +<li> +The conversion specifiers are quite restricted. +There are no flags, widths, or precisions. +The conversion specifiers can only be +'<code>%%</code>' (inserts a '<code>%</code>' in the string), +'<code>%s</code>' (inserts a zero-terminated string, with no size restrictions), +'<code>%f</code>' (inserts a <a href="#lua_Number"><code>lua_Number</code></a>), +'<code>%p</code>' (inserts a pointer as a hexadecimal numeral), +'<code>%d</code>' (inserts an <code>int</code>), and +'<code>%c</code>' (inserts an <code>int</code> as a byte). +</li> + +</ul> + + + + +<hr><h3><a name="lua_pushglobaltable"><code>lua_pushglobaltable</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>void lua_pushglobaltable (lua_State *L);</pre> + +<p> +Pushes the global environment onto the stack. + + + + + +<hr><h3><a name="lua_pushinteger"><code>lua_pushinteger</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>void lua_pushinteger (lua_State *L, lua_Integer n);</pre> + +<p> +Pushes a number with value <code>n</code> onto the stack. + + + + + +<hr><h3><a name="lua_pushlightuserdata"><code>lua_pushlightuserdata</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>void lua_pushlightuserdata (lua_State *L, void *p);</pre> + +<p> +Pushes a light userdata onto the stack. + + +<p> +Userdata represent C values in Lua. +A <em>light userdata</em> represents a pointer, a <code>void*</code>. +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. + + + + + +<hr><h3><a name="lua_pushliteral"><code>lua_pushliteral</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>const char *lua_pushliteral (lua_State *L, const char *s);</pre> + +<p> +This macro is equivalent to <a href="#lua_pushlstring"><code>lua_pushlstring</code></a>, +but can be used only when <code>s</code> is a literal string. +It automatically provides the string length. + + + + + +<hr><h3><a name="lua_pushlstring"><code>lua_pushlstring</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>const char *lua_pushlstring (lua_State *L, const char *s, size_t len);</pre> + +<p> +Pushes the string pointed to by <code>s</code> with size <code>len</code> +onto the stack. +Lua makes (or reuses) an internal copy of the given string, +so the memory at <code>s</code> can be freed or reused immediately after +the function returns. +The string can contain any binary data, +including embedded zeros. + + +<p> +Returns a pointer to the internal copy of the string. + + + + + +<hr><h3><a name="lua_pushnil"><code>lua_pushnil</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>void lua_pushnil (lua_State *L);</pre> + +<p> +Pushes a nil value onto the stack. + + + + + +<hr><h3><a name="lua_pushnumber"><code>lua_pushnumber</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>void lua_pushnumber (lua_State *L, lua_Number n);</pre> + +<p> +Pushes a number with value <code>n</code> onto the stack. + + + + + +<hr><h3><a name="lua_pushstring"><code>lua_pushstring</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>const char *lua_pushstring (lua_State *L, const char *s);</pre> + +<p> +Pushes the zero-terminated string pointed to by <code>s</code> +onto the stack. +Lua makes (or reuses) an internal copy of the given string, +so the memory at <code>s</code> can be freed or reused immediately after +the function returns. + + +<p> +Returns a pointer to the internal copy of the string. + + +<p> +If <code>s</code> is <code>NULL</code>, pushes <b>nil</b> and returns <code>NULL</code>. + + + + + +<hr><h3><a name="lua_pushthread"><code>lua_pushthread</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>int lua_pushthread (lua_State *L);</pre> + +<p> +Pushes the thread represented by <code>L</code> onto the stack. +Returns 1 if this thread is the main thread of its state. + + + + + +<hr><h3><a name="lua_pushunsigned"><code>lua_pushunsigned</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>void lua_pushunsigned (lua_State *L, lua_Unsigned n);</pre> + +<p> +Pushes a number with value <code>n</code> onto the stack. + + + + + +<hr><h3><a name="lua_pushvalue"><code>lua_pushvalue</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>void lua_pushvalue (lua_State *L, int index);</pre> + +<p> +Pushes a copy of the element at the given index +onto the stack. + + + + + +<hr><h3><a name="lua_pushvfstring"><code>lua_pushvfstring</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>const char *lua_pushvfstring (lua_State *L, + const char *fmt, + va_list argp);</pre> + +<p> +Equivalent to <a href="#lua_pushfstring"><code>lua_pushfstring</code></a>, except that it receives a <code>va_list</code> +instead of a variable number of arguments. + + + + + +<hr><h3><a name="lua_rawequal"><code>lua_rawequal</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_rawequal (lua_State *L, int index1, int index2);</pre> + +<p> +Returns 1 if the two values in indices <code>index1</code> and +<code>index2</code> are primitively equal +(that is, without calling metamethods). +Otherwise returns 0. +Also returns 0 if any of the indices are non valid. + + + + + +<hr><h3><a name="lua_rawget"><code>lua_rawget</code></a></h3><p> +<span class="apii">[-1, +1, –]</span> +<pre>void lua_rawget (lua_State *L, int index);</pre> + +<p> +Similar to <a href="#lua_gettable"><code>lua_gettable</code></a>, but does a raw access +(i.e., without metamethods). + + + + + +<hr><h3><a name="lua_rawgeti"><code>lua_rawgeti</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>void lua_rawgeti (lua_State *L, int index, int n);</pre> + +<p> +Pushes onto the stack the value <code>t[n]</code>, +where <code>t</code> is the table at the given index. +The access is raw; +that is, it does not invoke metamethods. + + + + + +<hr><h3><a name="lua_rawgetp"><code>lua_rawgetp</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>void lua_rawgetp (lua_State *L, int index, const void *p);</pre> + +<p> +Pushes onto the stack the value <code>t[k]</code>, +where <code>t</code> is the table at the given index and +<code>k</code> is the pointer <code>p</code> represented as a light userdata. +The access is raw; +that is, it does not invoke metamethods. + + + + + +<hr><h3><a name="lua_rawlen"><code>lua_rawlen</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>size_t lua_rawlen (lua_State *L, int index);</pre> + +<p> +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 ('<code>#</code>') +with no metamethods; +for userdata, this is the size of the block of memory allocated +for the userdata; +for other values, it is 0. + + + + + +<hr><h3><a name="lua_rawset"><code>lua_rawset</code></a></h3><p> +<span class="apii">[-2, +0, <em>e</em>]</span> +<pre>void lua_rawset (lua_State *L, int index);</pre> + +<p> +Similar to <a href="#lua_settable"><code>lua_settable</code></a>, but does a raw assignment +(i.e., without metamethods). + + + + + +<hr><h3><a name="lua_rawseti"><code>lua_rawseti</code></a></h3><p> +<span class="apii">[-1, +0, <em>e</em>]</span> +<pre>void lua_rawseti (lua_State *L, int index, int n);</pre> + +<p> +Does the equivalent of <code>t[n] = v</code>, +where <code>t</code> is the table at the given index +and <code>v</code> is the value at the top of the stack. + + +<p> +This function pops the value from the stack. +The assignment is raw; +that is, it does not invoke metamethods. + + + + + +<hr><h3><a name="lua_rawsetp"><code>lua_rawsetp</code></a></h3><p> +<span class="apii">[-1, +0, <em>e</em>]</span> +<pre>void lua_rawsetp (lua_State *L, int index, const void *p);</pre> + +<p> +Does the equivalent of <code>t[k] = v</code>, +where <code>t</code> is the table at the given index, +<code>k</code> is the pointer <code>p</code> represented as a light userdata, +and <code>v</code> is the value at the top of the stack. + + +<p> +This function pops the value from the stack. +The assignment is raw; +that is, it does not invoke metamethods. + + + + + +<hr><h3><a name="lua_Reader"><code>lua_Reader</code></a></h3> +<pre>typedef const char * (*lua_Reader) (lua_State *L, + void *data, + size_t *size);</pre> + +<p> +The reader function used by <a href="#lua_load"><code>lua_load</code></a>. +Every time it needs another piece of the chunk, +<a href="#lua_load"><code>lua_load</code></a> calls the reader, +passing along its <code>data</code> parameter. +The reader must return a pointer to a block of memory +with a new piece of the chunk +and set <code>size</code> 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 <code>NULL</code> or set <code>size</code> to zero. +The reader function may return pieces of any size greater than zero. + + + + + +<hr><h3><a name="lua_register"><code>lua_register</code></a></h3><p> +<span class="apii">[-0, +0, <em>e</em>]</span> +<pre>void lua_register (lua_State *L, const char *name, lua_CFunction f);</pre> + +<p> +Sets the C function <code>f</code> as the new value of global <code>name</code>. +It is defined as a macro: + +<pre> + #define lua_register(L,n,f) \ + (lua_pushcfunction(L, f), lua_setglobal(L, n)) +</pre> + + + + +<hr><h3><a name="lua_remove"><code>lua_remove</code></a></h3><p> +<span class="apii">[-1, +0, –]</span> +<pre>void lua_remove (lua_State *L, int index);</pre> + +<p> +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. + + + + + +<hr><h3><a name="lua_replace"><code>lua_replace</code></a></h3><p> +<span class="apii">[-1, +0, –]</span> +<pre>void lua_replace (lua_State *L, int index);</pre> + +<p> +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. + + + + + +<hr><h3><a name="lua_resume"><code>lua_resume</code></a></h3><p> +<span class="apii">[-?, +?, –]</span> +<pre>int lua_resume (lua_State *L, lua_State *from, int nargs);</pre> + +<p> +Starts and resumes a coroutine in a given thread. + + +<p> +To start a coroutine, +you push onto the thread stack the main function plus any arguments; +then you call <a href="#lua_resume"><code>lua_resume</code></a>, +with <code>nargs</code> 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 <a href="#lua_yield"><code>lua_yield</code></a>, +or all values returned by the body function. +<a href="#lua_resume"><code>lua_resume</code></a> returns +<a href="#pdf-LUA_YIELD"><code>LUA_YIELD</code></a> if the coroutine yields, +<a href="#pdf-LUA_OK"><code>LUA_OK</code></a> if the coroutine finishes its execution +without errors, +or an error code in case of errors (see <a href="#lua_pcall"><code>lua_pcall</code></a>). + + +<p> +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. + + +<p> +To resume a coroutine, +you remove any results from the last <a href="#lua_yield"><code>lua_yield</code></a>, +put on its stack only the values to +be passed as results from <code>yield</code>, +and then call <a href="#lua_resume"><code>lua_resume</code></a>. + + +<p> +The parameter <code>from</code> represents the coroutine that is resuming <code>L</code>. +If there is no such coroutine, +this parameter can be <code>NULL</code>. + + + + + +<hr><h3><a name="lua_setallocf"><code>lua_setallocf</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>void lua_setallocf (lua_State *L, lua_Alloc f, void *ud);</pre> + +<p> +Changes the allocator function of a given state to <code>f</code> +with user data <code>ud</code>. + + + + + +<hr><h3><a name="lua_setfield"><code>lua_setfield</code></a></h3><p> +<span class="apii">[-1, +0, <em>e</em>]</span> +<pre>void lua_setfield (lua_State *L, int index, const char *k);</pre> + +<p> +Does the equivalent to <code>t[k] = v</code>, +where <code>t</code> is the value at the given index +and <code>v</code> is the value at the top of the stack. + + +<p> +This function pops the value from the stack. +As in Lua, this function may trigger a metamethod +for the "newindex" event (see <a href="#2.4">§2.4</a>). + + + + + +<hr><h3><a name="lua_setglobal"><code>lua_setglobal</code></a></h3><p> +<span class="apii">[-1, +0, <em>e</em>]</span> +<pre>void lua_setglobal (lua_State *L, const char *name);</pre> + +<p> +Pops a value from the stack and +sets it as the new value of global <code>name</code>. + + + + + +<hr><h3><a name="lua_setmetatable"><code>lua_setmetatable</code></a></h3><p> +<span class="apii">[-1, +0, –]</span> +<pre>void lua_setmetatable (lua_State *L, int index);</pre> + +<p> +Pops a table from the stack and +sets it as the new metatable for the value at the given index. + + + + + +<hr><h3><a name="lua_settable"><code>lua_settable</code></a></h3><p> +<span class="apii">[-2, +0, <em>e</em>]</span> +<pre>void lua_settable (lua_State *L, int index);</pre> + +<p> +Does the equivalent to <code>t[k] = v</code>, +where <code>t</code> is the value at the given index, +<code>v</code> is the value at the top of the stack, +and <code>k</code> is the value just below the top. + + +<p> +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 <a href="#2.4">§2.4</a>). + + + + + +<hr><h3><a name="lua_settop"><code>lua_settop</code></a></h3><p> +<span class="apii">[-?, +?, –]</span> +<pre>void lua_settop (lua_State *L, int index);</pre> + +<p> +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 <b>nil</b>. +If <code>index</code> is 0, then all stack elements are removed. + + + + + +<hr><h3><a name="lua_setuservalue"><code>lua_setuservalue</code></a></h3><p> +<span class="apii">[-1, +0, –]</span> +<pre>void lua_setuservalue (lua_State *L, int index);</pre> + +<p> +Pops a table or <b>nil</b> from the stack and sets it as +the new value associated to the userdata at the given index. + + + + + +<hr><h3><a name="lua_State"><code>lua_State</code></a></h3> +<pre>typedef struct lua_State lua_State;</pre> + +<p> +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. + + +<p> +A pointer to this structure must be passed as the first argument to +every function in the library, except to <a href="#lua_newstate"><code>lua_newstate</code></a>, +which creates a Lua state from scratch. + + + + + +<hr><h3><a name="lua_status"><code>lua_status</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_status (lua_State *L);</pre> + +<p> +Returns the status of the thread <code>L</code>. + + +<p> +The status can be 0 (<a href="#pdf-LUA_OK"><code>LUA_OK</code></a>) for a normal thread, +an error code if the thread finished the execution +of a <a href="#lua_resume"><code>lua_resume</code></a> with an error, +or <a name="pdf-LUA_YIELD"><code>LUA_YIELD</code></a> if the thread is suspended. + + +<p> +You can only call functions in threads with status <a href="#pdf-LUA_OK"><code>LUA_OK</code></a>. +You can resume threads with status <a href="#pdf-LUA_OK"><code>LUA_OK</code></a> +(to start a new coroutine) or <a href="#pdf-LUA_YIELD"><code>LUA_YIELD</code></a> +(to resume a coroutine). + + + + + +<hr><h3><a name="lua_toboolean"><code>lua_toboolean</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_toboolean (lua_State *L, int index);</pre> + +<p> +Converts the Lua value at the given index to a C boolean +value (0 or 1). +Like all tests in Lua, +<a href="#lua_toboolean"><code>lua_toboolean</code></a> returns true for any Lua value +different from <b>false</b> and <b>nil</b>; +otherwise it returns false. +(If you want to accept only actual boolean values, +use <a href="#lua_isboolean"><code>lua_isboolean</code></a> to test the value's type.) + + + + + +<hr><h3><a name="lua_tocfunction"><code>lua_tocfunction</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>lua_CFunction lua_tocfunction (lua_State *L, int index);</pre> + +<p> +Converts a value at the given index to a C function. +That value must be a C function; +otherwise, returns <code>NULL</code>. + + + + + +<hr><h3><a name="lua_tointeger"><code>lua_tointeger</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>lua_Integer lua_tointeger (lua_State *L, int index);</pre> + +<p> +Equivalent to <a href="#lua_tointegerx"><code>lua_tointegerx</code></a> with <code>isnum</code> equal to <code>NULL</code>. + + + + + +<hr><h3><a name="lua_tointegerx"><code>lua_tointegerx</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>lua_Integer lua_tointegerx (lua_State *L, int index, int *isnum);</pre> + +<p> +Converts the Lua value at the given index +to the signed integral type <a href="#lua_Integer"><code>lua_Integer</code></a>. +The Lua value must be a number or a string convertible to a number +(see <a href="#3.4.2">§3.4.2</a>); +otherwise, <code>lua_tointegerx</code> returns 0. + + +<p> +If the number is not an integer, +it is truncated in some non-specified way. + + +<p> +If <code>isnum</code> is not <code>NULL</code>, +its referent is assigned a boolean value that +indicates whether the operation succeeded. + + + + + +<hr><h3><a name="lua_tolstring"><code>lua_tolstring</code></a></h3><p> +<span class="apii">[-0, +0, <em>e</em>]</span> +<pre>const char *lua_tolstring (lua_State *L, int index, size_t *len);</pre> + +<p> +Converts the Lua value at the given index to a C string. +If <code>len</code> is not <code>NULL</code>, +it also sets <code>*len</code> with the string length. +The Lua value must be a string or a number; +otherwise, the function returns <code>NULL</code>. +If the value is a number, +then <code>lua_tolstring</code> also +<em>changes the actual value in the stack to a string</em>. +(This change confuses <a href="#lua_next"><code>lua_next</code></a> +when <code>lua_tolstring</code> is applied to keys during a table traversal.) + + +<p> +<code>lua_tolstring</code> returns a fully aligned pointer +to a string inside the Lua state. +This string always has a zero ('<code>\0</code>') +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 <code>lua_tolstring</code> +will be valid after the corresponding value is removed from the stack. + + + + + +<hr><h3><a name="lua_tonumber"><code>lua_tonumber</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>lua_Number lua_tonumber (lua_State *L, int index);</pre> + +<p> +Equivalent to <a href="#lua_tonumberx"><code>lua_tonumberx</code></a> with <code>isnum</code> equal to <code>NULL</code>. + + + + + +<hr><h3><a name="lua_tonumberx"><code>lua_tonumberx</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>lua_Number lua_tonumberx (lua_State *L, int index, int *isnum);</pre> + +<p> +Converts the Lua value at the given index +to the C type <a href="#lua_Number"><code>lua_Number</code></a> (see <a href="#lua_Number"><code>lua_Number</code></a>). +The Lua value must be a number or a string convertible to a number +(see <a href="#3.4.2">§3.4.2</a>); +otherwise, <a href="#lua_tonumberx"><code>lua_tonumberx</code></a> returns 0. + + +<p> +If <code>isnum</code> is not <code>NULL</code>, +its referent is assigned a boolean value that +indicates whether the operation succeeded. + + + + + +<hr><h3><a name="lua_topointer"><code>lua_topointer</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>const void *lua_topointer (lua_State *L, int index);</pre> + +<p> +Converts the value at the given index to a generic +C pointer (<code>void*</code>). +The value can be a userdata, a table, a thread, or a function; +otherwise, <code>lua_topointer</code> returns <code>NULL</code>. +Different objects will give different pointers. +There is no way to convert the pointer back to its original value. + + +<p> +Typically this function is used only for debug information. + + + + + +<hr><h3><a name="lua_tostring"><code>lua_tostring</code></a></h3><p> +<span class="apii">[-0, +0, <em>e</em>]</span> +<pre>const char *lua_tostring (lua_State *L, int index);</pre> + +<p> +Equivalent to <a href="#lua_tolstring"><code>lua_tolstring</code></a> with <code>len</code> equal to <code>NULL</code>. + + + + + +<hr><h3><a name="lua_tothread"><code>lua_tothread</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>lua_State *lua_tothread (lua_State *L, int index);</pre> + +<p> +Converts the value at the given index to a Lua thread +(represented as <code>lua_State*</code>). +This value must be a thread; +otherwise, the function returns <code>NULL</code>. + + + + + +<hr><h3><a name="lua_tounsigned"><code>lua_tounsigned</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>lua_Unsigned lua_tounsigned (lua_State *L, int index);</pre> + +<p> +Equivalent to <a href="#lua_tounsignedx"><code>lua_tounsignedx</code></a> with <code>isnum</code> equal to <code>NULL</code>. + + + + + +<hr><h3><a name="lua_tounsignedx"><code>lua_tounsignedx</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>lua_Unsigned lua_tounsignedx (lua_State *L, int index, int *isnum);</pre> + +<p> +Converts the Lua value at the given index +to the unsigned integral type <a href="#lua_Unsigned"><code>lua_Unsigned</code></a>. +The Lua value must be a number or a string convertible to a number +(see <a href="#3.4.2">§3.4.2</a>); +otherwise, <code>lua_tounsignedx</code> returns 0. + + +<p> +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. + + +<p> +If <code>isnum</code> is not <code>NULL</code>, +its referent is assigned a boolean value that +indicates whether the operation succeeded. + + + + + +<hr><h3><a name="lua_touserdata"><code>lua_touserdata</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>void *lua_touserdata (lua_State *L, int index);</pre> + +<p> +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 <code>NULL</code>. + + + + + +<hr><h3><a name="lua_type"><code>lua_type</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_type (lua_State *L, int index);</pre> + +<p> +Returns the type of the value in the given valid index, +or <code>LUA_TNONE</code> for a non-valid (but acceptable) index. +The types returned by <a href="#lua_type"><code>lua_type</code></a> are coded by the following constants +defined in <code>lua.h</code>: +<a name="pdf-LUA_TNIL"><code>LUA_TNIL</code></a>, +<a name="pdf-LUA_TNUMBER"><code>LUA_TNUMBER</code></a>, +<a name="pdf-LUA_TBOOLEAN"><code>LUA_TBOOLEAN</code></a>, +<a name="pdf-LUA_TSTRING"><code>LUA_TSTRING</code></a>, +<a name="pdf-LUA_TTABLE"><code>LUA_TTABLE</code></a>, +<a name="pdf-LUA_TFUNCTION"><code>LUA_TFUNCTION</code></a>, +<a name="pdf-LUA_TUSERDATA"><code>LUA_TUSERDATA</code></a>, +<a name="pdf-LUA_TTHREAD"><code>LUA_TTHREAD</code></a>, +and +<a name="pdf-LUA_TLIGHTUSERDATA"><code>LUA_TLIGHTUSERDATA</code></a>. + + + + + +<hr><h3><a name="lua_typename"><code>lua_typename</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>const char *lua_typename (lua_State *L, int tp);</pre> + +<p> +Returns the name of the type encoded by the value <code>tp</code>, +which must be one the values returned by <a href="#lua_type"><code>lua_type</code></a>. + + + + + +<hr><h3><a name="lua_Unsigned"><code>lua_Unsigned</code></a></h3> +<pre>typedef unsigned long lua_Unsigned;</pre> + +<p> +The type used by the Lua API to represent unsigned integral values. +It must have at least 32 bits. + + +<p> +By default it is an <code>unsigned int</code> or an <code>unsigned long</code>, +whichever can hold 32-bit values. + + + + + +<hr><h3><a name="lua_upvalueindex"><code>lua_upvalueindex</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_upvalueindex (int i);</pre> + +<p> +Returns the pseudo-index that represents the <code>i</code>-th upvalue of +the running function (see <a href="#4.4">§4.4</a>). + + + + + +<hr><h3><a name="lua_version"><code>lua_version</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>const lua_Number *lua_version (lua_State *L);</pre> + +<p> +Returns the address of the version number stored in the Lua core. +When called with a valid <a href="#lua_State"><code>lua_State</code></a>, +returns the address of the version used to create that state. +When called with <code>NULL</code>, +returns the address of the version running the call. + + + + + +<hr><h3><a name="lua_Writer"><code>lua_Writer</code></a></h3> +<pre>typedef int (*lua_Writer) (lua_State *L, + const void* p, + size_t sz, + void* ud);</pre> + +<p> +The type of the writer function used by <a href="#lua_dump"><code>lua_dump</code></a>. +Every time it produces another piece of chunk, +<a href="#lua_dump"><code>lua_dump</code></a> calls the writer, +passing along the buffer to be written (<code>p</code>), +its size (<code>sz</code>), +and the <code>data</code> parameter supplied to <a href="#lua_dump"><code>lua_dump</code></a>. + + +<p> +The writer returns an error code: +0 means no errors; +any other value means an error and stops <a href="#lua_dump"><code>lua_dump</code></a> from +calling the writer again. + + + + + +<hr><h3><a name="lua_xmove"><code>lua_xmove</code></a></h3><p> +<span class="apii">[-?, +?, –]</span> +<pre>void lua_xmove (lua_State *from, lua_State *to, int n);</pre> + +<p> +Exchange values between different threads of the same state. + + +<p> +This function pops <code>n</code> values from the stack <code>from</code>, +and pushes them onto the stack <code>to</code>. + + + + + +<hr><h3><a name="lua_yield"><code>lua_yield</code></a></h3><p> +<span class="apii">[-?, +?, –]</span> +<pre>int lua_yield (lua_State *L, int nresults);</pre> + +<p> +This function is equivalent to <a href="#lua_yieldk"><code>lua_yieldk</code></a>, +but it has no continuation (see <a href="#4.7">§4.7</a>). +Therefore, when the thread resumes, +it returns to the function that called +the function calling <code>lua_yield</code>. + + + + + +<hr><h3><a name="lua_yieldk"><code>lua_yieldk</code></a></h3><p> +<span class="apii">[-?, +?, –]</span> +<pre>int lua_yieldk (lua_State *L, int nresults, int ctx, lua_CFunction k);</pre> + +<p> +Yields a coroutine. + + +<p> +This function should only be called as the +return expression of a C function, as follows: + +<pre> + return lua_yieldk (L, n, i, k); +</pre><p> +When a C function calls <a href="#lua_yieldk"><code>lua_yieldk</code></a> in that way, +the running coroutine suspends its execution, +and the call to <a href="#lua_resume"><code>lua_resume</code></a> that started this coroutine returns. +The parameter <code>nresults</code> is the number of values from the stack +that are passed as results to <a href="#lua_resume"><code>lua_resume</code></a>. + + +<p> +When the coroutine is resumed again, +Lua calls the given continuation function <code>k</code> to continue +the execution of the C function that yielded (see <a href="#4.7">§4.7</a>). +This continuation function receives the same stack +from the previous function, +with the results removed and +replaced by the arguments passed to <a href="#lua_resume"><code>lua_resume</code></a>. +Moreover, +the continuation function may access the value <code>ctx</code> +by calling <a href="#lua_getctx"><code>lua_getctx</code></a>. + + + + + + + +<h2>4.9 – <a name="4.9">The Debug Interface</a></h2> + +<p> +Lua has no built-in debugging facilities. +Instead, it offers a special interface +by means of functions and <em>hooks</em>. +This interface allows the construction of different +kinds of debuggers, profilers, and other tools +that need "inside information" from the interpreter. + + + +<hr><h3><a name="lua_Debug"><code>lua_Debug</code></a></h3> +<pre>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 */ + <em>other fields</em> +} lua_Debug;</pre> + +<p> +A structure used to carry different pieces of +information about a function or an activation record. +<a href="#lua_getstack"><code>lua_getstack</code></a> fills only the private part +of this structure, for later use. +To fill the other fields of <a href="#lua_Debug"><code>lua_Debug</code></a> with useful information, +call <a href="#lua_getinfo"><code>lua_getinfo</code></a>. + + +<p> +The fields of <a href="#lua_Debug"><code>lua_Debug</code></a> have the following meaning: + +<ul> + +<li><b><code>source</code>: </b> +the source of the chunk that created the function. +If <code>source</code> starts with a '<code>@</code>', +it means that the function was defined in a file where +the file name follows the '<code>@</code>'. +If <code>source</code> starts with a '<code>=</code>', +the remainder of its contents describe the source in a user-dependent manner. +Otherwise, +the function was defined in a string where +<code>source</code> is that string. +</li> + +<li><b><code>short_src</code>: </b> +a "printable" version of <code>source</code>, to be used in error messages. +</li> + +<li><b><code>linedefined</code>: </b> +the line number where the definition of the function starts. +</li> + +<li><b><code>lastlinedefined</code>: </b> +the line number where the definition of the function ends. +</li> + +<li><b><code>what</code>: </b> +the string <code>"Lua"</code> if the function is a Lua function, +<code>"C"</code> if it is a C function, +<code>"main"</code> if it is the main part of a chunk. +</li> + +<li><b><code>currentline</code>: </b> +the current line where the given function is executing. +When no line information is available, +<code>currentline</code> is set to -1. +</li> + +<li><b><code>name</code>: </b> +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. +The <code>lua_getinfo</code> function checks how the function was +called to find a suitable name. +If it cannot find a name, +then <code>name</code> is set to <code>NULL</code>. +</li> + +<li><b><code>namewhat</code>: </b> +explains the <code>name</code> field. +The value of <code>namewhat</code> can be +<code>"global"</code>, <code>"local"</code>, <code>"method"</code>, +<code>"field"</code>, <code>"upvalue"</code>, or <code>""</code> (the empty string), +according to how the function was called. +(Lua uses the empty string when no other option seems to apply.) +</li> + +<li><b><code>istailcall</code>: </b> +true if this function invocation was called by a tail call. +In this case, the caller of this level is not in the stack. +</li> + +<li><b><code>nups</code>: </b> +the number of upvalues of the function. +</li> + +<li><b><code>nparams</code>: </b> +the number of fixed parameters of the function +(always 0 for C functions). +</li> + +<li><b><code>isvararg</code>: </b> +true if the function is a vararg function +(always true for C functions). +</li> + +</ul> + + + + +<hr><h3><a name="lua_gethook"><code>lua_gethook</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>lua_Hook lua_gethook (lua_State *L);</pre> + +<p> +Returns the current hook function. + + + + + +<hr><h3><a name="lua_gethookcount"><code>lua_gethookcount</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_gethookcount (lua_State *L);</pre> + +<p> +Returns the current hook count. + + + + + +<hr><h3><a name="lua_gethookmask"><code>lua_gethookmask</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_gethookmask (lua_State *L);</pre> + +<p> +Returns the current hook mask. + + + + + +<hr><h3><a name="lua_getinfo"><code>lua_getinfo</code></a></h3><p> +<span class="apii">[-(0|1), +(0|1|2), <em>e</em>]</span> +<pre>int lua_getinfo (lua_State *L, const char *what, lua_Debug *ar);</pre> + +<p> +Gets information about a specific function or function invocation. + + +<p> +To get information about a function invocation, +the parameter <code>ar</code> must be a valid activation record that was +filled by a previous call to <a href="#lua_getstack"><code>lua_getstack</code></a> or +given as argument to a hook (see <a href="#lua_Hook"><code>lua_Hook</code></a>). + + +<p> +To get information about a function you push it onto the stack +and start the <code>what</code> string with the character '<code>></code>'. +(In that case, +<code>lua_getinfo</code> pops the function from the top of the stack.) +For instance, to know in which line a function <code>f</code> was defined, +you can write the following code: + +<pre> + lua_Debug ar; + lua_getglobal(L, "f"); /* get global 'f' */ + lua_getinfo(L, ">S", &ar); + printf("%d\n", ar.linedefined); +</pre> + +<p> +Each character in the string <code>what</code> +selects some fields of the structure <code>ar</code> to be filled or +a value to be pushed on the stack: + +<ul> + +<li><b>'<code>n</code>': </b> fills in the field <code>name</code> and <code>namewhat</code>; +</li> + +<li><b>'<code>S</code>': </b> +fills in the fields <code>source</code>, <code>short_src</code>, +<code>linedefined</code>, <code>lastlinedefined</code>, and <code>what</code>; +</li> + +<li><b>'<code>l</code>': </b> fills in the field <code>currentline</code>; +</li> + +<li><b>'<code>t</code>': </b> fills in the field <code>istailcall</code>; +</li> + +<li><b>'<code>u</code>': </b> fills in the fields +<code>nups</code>, <code>nparams</code>, and <code>isvararg</code>; +</li> + +<li><b>'<code>f</code>': </b> +pushes onto the stack the function that is +running at the given level; +</li> + +<li><b>'<code>L</code>': </b> +pushes onto the stack a table whose indices are the +numbers of the lines that are valid on the function. +(A <em>valid line</em> 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.) +</li> + +</ul> + +<p> +This function returns 0 on error +(for instance, an invalid option in <code>what</code>). + + + + + +<hr><h3><a name="lua_getlocal"><code>lua_getlocal</code></a></h3><p> +<span class="apii">[-0, +(0|1), –]</span> +<pre>const char *lua_getlocal (lua_State *L, lua_Debug *ar, int n);</pre> + +<p> +Gets information about a local variable of +a given activation record or a given function. + + +<p> +In the first case, +the parameter <code>ar</code> must be a valid activation record that was +filled by a previous call to <a href="#lua_getstack"><code>lua_getstack</code></a> or +given as argument to a hook (see <a href="#lua_Hook"><code>lua_Hook</code></a>). +The index <code>n</code> selects which local variable to inspect; +see <a href="#pdf-debug.getlocal"><code>debug.getlocal</code></a> for details about variable indices +and names. + + +<p> +<a href="#lua_getlocal"><code>lua_getlocal</code></a> pushes the variable's value onto the stack +and returns its name. + + +<p> +In the second case, <code>ar</code> should be <code>NULL</code> 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. + + +<p> +Returns <code>NULL</code> (and pushes nothing) +when the index is greater than +the number of active local variables. + + + + + +<hr><h3><a name="lua_getstack"><code>lua_getstack</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_getstack (lua_State *L, int level, lua_Debug *ar);</pre> + +<p> +Gets information about the interpreter runtime stack. + + +<p> +This function fills parts of a <a href="#lua_Debug"><code>lua_Debug</code></a> structure with +an identification of the <em>activation record</em> +of the function executing at a given level. +Level 0 is the current running function, +whereas level <em>n+1</em> is the function that has called level <em>n</em> +(except for tail calls, which do not count on the stack). +When there are no errors, <a href="#lua_getstack"><code>lua_getstack</code></a> returns 1; +when called with a level greater than the stack depth, +it returns 0. + + + + + +<hr><h3><a name="lua_getupvalue"><code>lua_getupvalue</code></a></h3><p> +<span class="apii">[-0, +(0|1), –]</span> +<pre>const char *lua_getupvalue (lua_State *L, int funcindex, int n);</pre> + +<p> +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.) +<a href="#lua_getupvalue"><code>lua_getupvalue</code></a> gets the index <code>n</code> of an upvalue, +pushes the upvalue's value onto the stack, +and returns its name. +<code>funcindex</code> 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.) + + +<p> +Returns <code>NULL</code> (and pushes nothing) +when the index is greater than the number of upvalues. +For C functions, this function uses the empty string <code>""</code> +as a name for all upvalues. + + + + + +<hr><h3><a name="lua_Hook"><code>lua_Hook</code></a></h3> +<pre>typedef void (*lua_Hook) (lua_State *L, lua_Debug *ar);</pre> + +<p> +Type for debugging hook functions. + + +<p> +Whenever a hook is called, its <code>ar</code> argument has its field +<code>event</code> set to the specific event that triggered the hook. +Lua identifies these events with the following constants: +<a name="pdf-LUA_HOOKCALL"><code>LUA_HOOKCALL</code></a>, <a name="pdf-LUA_HOOKRET"><code>LUA_HOOKRET</code></a>, +<a name="pdf-LUA_HOOKTAILCALL"><code>LUA_HOOKTAILCALL</code></a>, <a name="pdf-LUA_HOOKLINE"><code>LUA_HOOKLINE</code></a>, +and <a name="pdf-LUA_HOOKCOUNT"><code>LUA_HOOKCOUNT</code></a>. +Moreover, for line events, the field <code>currentline</code> is also set. +To get the value of any other field in <code>ar</code>, +the hook must call <a href="#lua_getinfo"><code>lua_getinfo</code></a>. + + +<p> +For call events, <code>event</code> can be <code>LUA_HOOKCALL</code>, +the normal value, or <code>LUA_HOOKTAILCALL</code>, for a tail call; +in this case, there will be no corresponding return event. + + +<p> +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. + + +<p> +Hook functions cannot have continuations, +that is, they cannot call <a href="#lua_yieldk"><code>lua_yieldk</code></a>, +<a href="#lua_pcallk"><code>lua_pcallk</code></a>, or <a href="#lua_callk"><code>lua_callk</code></a> with a non-null <code>k</code>. + + +<p> +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 +calling <a href="#lua_yield"><code>lua_yield</code></a> with <code>nresults</code> equal to zero. + + + + + +<hr><h3><a name="lua_sethook"><code>lua_sethook</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>int lua_sethook (lua_State *L, lua_Hook f, int mask, int count);</pre> + +<p> +Sets the debugging hook function. + + +<p> +Argument <code>f</code> is the hook function. +<code>mask</code> specifies on which events the hook will be called: +it is formed by a bitwise or of the constants +<a name="pdf-LUA_MASKCALL"><code>LUA_MASKCALL</code></a>, +<a name="pdf-LUA_MASKRET"><code>LUA_MASKRET</code></a>, +<a name="pdf-LUA_MASKLINE"><code>LUA_MASKLINE</code></a>, +and <a name="pdf-LUA_MASKCOUNT"><code>LUA_MASKCOUNT</code></a>. +The <code>count</code> argument is only meaningful when the mask +includes <code>LUA_MASKCOUNT</code>. +For each event, the hook is called as explained below: + +<ul> + +<li><b>The call hook: </b> 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. +</li> + +<li><b>The return hook: </b> 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. +</li> + +<li><b>The line hook: </b> 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.) +</li> + +<li><b>The count hook: </b> is called after the interpreter executes every +<code>count</code> instructions. +(This event only happens while Lua is executing a Lua function.) +</li> + +</ul> + +<p> +A hook is disabled by setting <code>mask</code> to zero. + + + + + +<hr><h3><a name="lua_setlocal"><code>lua_setlocal</code></a></h3><p> +<span class="apii">[-(0|1), +0, –]</span> +<pre>const char *lua_setlocal (lua_State *L, lua_Debug *ar, int n);</pre> + +<p> +Sets the value of a local variable of a given activation record. +Parameters <code>ar</code> and <code>n</code> are as in <a href="#lua_getlocal"><code>lua_getlocal</code></a> +(see <a href="#lua_getlocal"><code>lua_getlocal</code></a>). +<a href="#lua_setlocal"><code>lua_setlocal</code></a> assigns the value at the top of the stack +to the variable and returns its name. +It also pops the value from the stack. + + +<p> +Returns <code>NULL</code> (and pops nothing) +when the index is greater than +the number of active local variables. + + + + + +<hr><h3><a name="lua_setupvalue"><code>lua_setupvalue</code></a></h3><p> +<span class="apii">[-(0|1), +0, –]</span> +<pre>const char *lua_setupvalue (lua_State *L, int funcindex, int n);</pre> + +<p> +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 <code>funcindex</code> and <code>n</code> are as in the <a href="#lua_getupvalue"><code>lua_getupvalue</code></a> +(see <a href="#lua_getupvalue"><code>lua_getupvalue</code></a>). + + +<p> +Returns <code>NULL</code> (and pops nothing) +when the index is greater than the number of upvalues. + + + + + +<hr><h3><a name="lua_upvalueid"><code>lua_upvalueid</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>void *lua_upvalueid (lua_State *L, int funcindex, int n);</pre> + +<p> +Returns an unique identifier for the upvalue numbered <code>n</code> +from the closure at index <code>funcindex</code>. +Parameters <code>funcindex</code> and <code>n</code> are as in the <a href="#lua_getupvalue"><code>lua_getupvalue</code></a> +(see <a href="#lua_getupvalue"><code>lua_getupvalue</code></a>) +(but <code>n</code> cannot be greater than the number of upvalues). + + +<p> +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. + + + + + +<hr><h3><a name="lua_upvaluejoin"><code>lua_upvaluejoin</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>void lua_upvaluejoin (lua_State *L, int funcindex1, int n1, + int funcindex2, int n2);</pre> + +<p> +Make the <code>n1</code>-th upvalue of the Lua closure at index <code>funcindex1</code> +refer to the <code>n2</code>-th upvalue of the Lua closure at index <code>funcindex2</code>. + + + + + + + +<h1>5 – <a name="5">The Auxiliary Library</a></h1> + +<p> + +The <em>auxiliary library</em> 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. + + +<p> +All functions and types from the auxiliary library +are defined in header file <code>lauxlib.h</code> and +have a prefix <code>luaL_</code>. + + +<p> +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. + + +<p> +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. + + +<p> +Several functions in the auxiliary library are used to +check C function arguments. +Because the error message is formatted for arguments +(e.g., "<code>bad argument #1</code>"), +you should not use these functions for other stack values. + + +<p> +Functions called <code>luaL_check*</code> +always throw an error if the check is not satisfied. + + + +<h2>5.1 – <a name="5.1">Functions and Types</a></h2> + +<p> +Here we list all functions and types from the auxiliary library +in alphabetical order. + + + +<hr><h3><a name="luaL_addchar"><code>luaL_addchar</code></a></h3><p> +<span class="apii">[-?, +?, <em>e</em>]</span> +<pre>void luaL_addchar (luaL_Buffer *B, char c);</pre> + +<p> +Adds the byte <code>c</code> to the buffer <code>B</code> +(see <a href="#luaL_Buffer"><code>luaL_Buffer</code></a>). + + + + + +<hr><h3><a name="luaL_addlstring"><code>luaL_addlstring</code></a></h3><p> +<span class="apii">[-?, +?, <em>e</em>]</span> +<pre>void luaL_addlstring (luaL_Buffer *B, const char *s, size_t l);</pre> + +<p> +Adds the string pointed to by <code>s</code> with length <code>l</code> to +the buffer <code>B</code> +(see <a href="#luaL_Buffer"><code>luaL_Buffer</code></a>). +The string can contain embedded zeros. + + + + + +<hr><h3><a name="luaL_addsize"><code>luaL_addsize</code></a></h3><p> +<span class="apii">[-?, +?, <em>e</em>]</span> +<pre>void luaL_addsize (luaL_Buffer *B, size_t n);</pre> + +<p> +Adds to the buffer <code>B</code> (see <a href="#luaL_Buffer"><code>luaL_Buffer</code></a>) +a string of length <code>n</code> previously copied to the +buffer area (see <a href="#luaL_prepbuffer"><code>luaL_prepbuffer</code></a>). + + + + + +<hr><h3><a name="luaL_addstring"><code>luaL_addstring</code></a></h3><p> +<span class="apii">[-?, +?, <em>e</em>]</span> +<pre>void luaL_addstring (luaL_Buffer *B, const char *s);</pre> + +<p> +Adds the zero-terminated string pointed to by <code>s</code> +to the buffer <code>B</code> +(see <a href="#luaL_Buffer"><code>luaL_Buffer</code></a>). +The string cannot contain embedded zeros. + + + + + +<hr><h3><a name="luaL_addvalue"><code>luaL_addvalue</code></a></h3><p> +<span class="apii">[-1, +?, <em>e</em>]</span> +<pre>void luaL_addvalue (luaL_Buffer *B);</pre> + +<p> +Adds the value at the top of the stack +to the buffer <code>B</code> +(see <a href="#luaL_Buffer"><code>luaL_Buffer</code></a>). +Pops the value. + + +<p> +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. + + + + + +<hr><h3><a name="luaL_argcheck"><code>luaL_argcheck</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>void luaL_argcheck (lua_State *L, + int cond, + int arg, + const char *extramsg);</pre> + +<p> +Checks whether <code>cond</code> is true. +If not, raises an error with a standard message. + + + + + +<hr><h3><a name="luaL_argerror"><code>luaL_argerror</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>int luaL_argerror (lua_State *L, int arg, const char *extramsg);</pre> + +<p> +Raises an error with a standard message +that includes <code>extramsg</code> as a comment. + + +<p> +This function never returns, +but it is an idiom to use it in C functions +as <code>return luaL_argerror(<em>args</em>)</code>. + + + + + +<hr><h3><a name="luaL_Buffer"><code>luaL_Buffer</code></a></h3> +<pre>typedef struct luaL_Buffer luaL_Buffer;</pre> + +<p> +Type for a <em>string buffer</em>. + + +<p> +A string buffer allows C code to build Lua strings piecemeal. +Its pattern of use is as follows: + +<ul> + +<li>First declare a variable <code>b</code> of type <a href="#luaL_Buffer"><code>luaL_Buffer</code></a>.</li> + +<li>Then initialize it with a call <code>luaL_buffinit(L, &b)</code>.</li> + +<li> +Then add string pieces to the buffer calling any of +the <code>luaL_add*</code> functions. +</li> + +<li> +Finish by calling <code>luaL_pushresult(&b)</code>. +This call leaves the final string on the top of the stack. +</li> + +</ul> + +<p> +If you know beforehand the total size of the resulting string, +you can use the buffer like this: + +<ul> + +<li>First declare a variable <code>b</code> of type <a href="#luaL_Buffer"><code>luaL_Buffer</code></a>.</li> + +<li>Then initialize it and preallocate a space of +size <code>sz</code> with a call <code>luaL_buffinitsize(L, &b, sz)</code>.</li> + +<li>Then copy the string into that space.</li> + +<li> +Finish by calling <code>luaL_pushresultsize(&b, sz)</code>, +where <code>sz</code> is the total size of the resulting string +copied into that space. +</li> + +</ul> + +<p> +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 <a href="#luaL_addvalue"><code>luaL_addvalue</code></a>.) +After calling <a href="#luaL_pushresult"><code>luaL_pushresult</code></a> the stack is back to its +level when the buffer was initialized, +plus the final string on its top. + + + + + +<hr><h3><a name="luaL_buffinit"><code>luaL_buffinit</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>void luaL_buffinit (lua_State *L, luaL_Buffer *B);</pre> + +<p> +Initializes a buffer <code>B</code>. +This function does not allocate any space; +the buffer must be declared as a variable +(see <a href="#luaL_Buffer"><code>luaL_Buffer</code></a>). + + + + + +<hr><h3><a name="luaL_buffinitsize"><code>luaL_buffinitsize</code></a></h3><p> +<span class="apii">[-?, +?, <em>e</em>]</span> +<pre>char *luaL_buffinitsize (lua_State *L, luaL_Buffer *B, size_t sz);</pre> + +<p> +Equivalent to the sequence +<a href="#luaL_buffinit"><code>luaL_buffinit</code></a>, <a href="#luaL_prepbuffsize"><code>luaL_prepbuffsize</code></a>. + + + + + +<hr><h3><a name="luaL_callmeta"><code>luaL_callmeta</code></a></h3><p> +<span class="apii">[-0, +(0|1), <em>e</em>]</span> +<pre>int luaL_callmeta (lua_State *L, int obj, const char *e);</pre> + +<p> +Calls a metamethod. + + +<p> +If the object at index <code>obj</code> has a metatable and this +metatable has a field <code>e</code>, +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). + + + + + +<hr><h3><a name="luaL_checkany"><code>luaL_checkany</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>void luaL_checkany (lua_State *L, int arg);</pre> + +<p> +Checks whether the function has an argument +of any type (including <b>nil</b>) at position <code>arg</code>. + + + + + +<hr><h3><a name="luaL_checkint"><code>luaL_checkint</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>int luaL_checkint (lua_State *L, int arg);</pre> + +<p> +Checks whether the function argument <code>arg</code> is a number +and returns this number cast to an <code>int</code>. + + + + + +<hr><h3><a name="luaL_checkinteger"><code>luaL_checkinteger</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>lua_Integer luaL_checkinteger (lua_State *L, int arg);</pre> + +<p> +Checks whether the function argument <code>arg</code> is a number +and returns this number cast to a <a href="#lua_Integer"><code>lua_Integer</code></a>. + + + + + +<hr><h3><a name="luaL_checklong"><code>luaL_checklong</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>long luaL_checklong (lua_State *L, int arg);</pre> + +<p> +Checks whether the function argument <code>arg</code> is a number +and returns this number cast to a <code>long</code>. + + + + + +<hr><h3><a name="luaL_checklstring"><code>luaL_checklstring</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>const char *luaL_checklstring (lua_State *L, int arg, size_t *l);</pre> + +<p> +Checks whether the function argument <code>arg</code> is a string +and returns this string; +if <code>l</code> is not <code>NULL</code> fills <code>*l</code> +with the string's length. + + +<p> +This function uses <a href="#lua_tolstring"><code>lua_tolstring</code></a> to get its result, +so all conversions and caveats of that function apply here. + + + + + +<hr><h3><a name="luaL_checknumber"><code>luaL_checknumber</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>lua_Number luaL_checknumber (lua_State *L, int arg);</pre> + +<p> +Checks whether the function argument <code>arg</code> is a number +and returns this number. + + + + + +<hr><h3><a name="luaL_checkoption"><code>luaL_checkoption</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>int luaL_checkoption (lua_State *L, + int arg, + const char *def, + const char *const lst[]);</pre> + +<p> +Checks whether the function argument <code>arg</code> is a string and +searches for this string in the array <code>lst</code> +(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. + + +<p> +If <code>def</code> is not <code>NULL</code>, +the function uses <code>def</code> as a default value when +there is no argument <code>arg</code> or when this argument is <b>nil</b>. + + +<p> +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.) + + + + + +<hr><h3><a name="luaL_checkstack"><code>luaL_checkstack</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>void luaL_checkstack (lua_State *L, int sz, const char *msg);</pre> + +<p> +Grows the stack size to <code>top + sz</code> elements, +raising an error if the stack cannot grow to that size. +<code>msg</code> is an additional text to go into the error message +(or <code>NULL</code> for no additional text). + + + + + +<hr><h3><a name="luaL_checkstring"><code>luaL_checkstring</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>const char *luaL_checkstring (lua_State *L, int arg);</pre> + +<p> +Checks whether the function argument <code>arg</code> is a string +and returns this string. + + +<p> +This function uses <a href="#lua_tolstring"><code>lua_tolstring</code></a> to get its result, +so all conversions and caveats of that function apply here. + + + + + +<hr><h3><a name="luaL_checktype"><code>luaL_checktype</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>void luaL_checktype (lua_State *L, int arg, int t);</pre> + +<p> +Checks whether the function argument <code>arg</code> has type <code>t</code>. +See <a href="#lua_type"><code>lua_type</code></a> for the encoding of types for <code>t</code>. + + + + + +<hr><h3><a name="luaL_checkudata"><code>luaL_checkudata</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>void *luaL_checkudata (lua_State *L, int arg, const char *tname);</pre> + +<p> +Checks whether the function argument <code>arg</code> is a userdata +of the type <code>tname</code> (see <a href="#luaL_newmetatable"><code>luaL_newmetatable</code></a>) and +returns the userdata address (see <a href="#lua_touserdata"><code>lua_touserdata</code></a>). + + + + + +<hr><h3><a name="luaL_checkunsigned"><code>luaL_checkunsigned</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>lua_Unsigned luaL_checkunsigned (lua_State *L, int arg);</pre> + +<p> +Checks whether the function argument <code>arg</code> is a number +and returns this number cast to a <a href="#lua_Unsigned"><code>lua_Unsigned</code></a>. + + + + + +<hr><h3><a name="luaL_checkversion"><code>luaL_checkversion</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>void luaL_checkversion (lua_State *L);</pre> + +<p> +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. + + + + + +<hr><h3><a name="luaL_dofile"><code>luaL_dofile</code></a></h3><p> +<span class="apii">[-0, +?, <em>e</em>]</span> +<pre>int luaL_dofile (lua_State *L, const char *filename);</pre> + +<p> +Loads and runs the given file. +It is defined as the following macro: + +<pre> + (luaL_loadfile(L, filename) || lua_pcall(L, 0, LUA_MULTRET, 0)) +</pre><p> +It returns false if there are no errors +or true in case of errors. + + + + + +<hr><h3><a name="luaL_dostring"><code>luaL_dostring</code></a></h3><p> +<span class="apii">[-0, +?, –]</span> +<pre>int luaL_dostring (lua_State *L, const char *str);</pre> + +<p> +Loads and runs the given string. +It is defined as the following macro: + +<pre> + (luaL_loadstring(L, str) || lua_pcall(L, 0, LUA_MULTRET, 0)) +</pre><p> +It returns false if there are no errors +or true in case of errors. + + + + + +<hr><h3><a name="luaL_error"><code>luaL_error</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>int luaL_error (lua_State *L, const char *fmt, ...);</pre> + +<p> +Raises an error. +The error message format is given by <code>fmt</code> +plus any extra arguments, +following the same rules of <a href="#lua_pushfstring"><code>lua_pushfstring</code></a>. +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. + + +<p> +This function never returns, +but it is an idiom to use it in C functions +as <code>return luaL_error(<em>args</em>)</code>. + + + + + +<hr><h3><a name="luaL_execresult"><code>luaL_execresult</code></a></h3><p> +<span class="apii">[-0, +3, <em>e</em>]</span> +<pre>int luaL_execresult (lua_State *L, int stat);</pre> + +<p> +This function produces the return values for +process-related functions in the standard library +(<a href="#pdf-os.execute"><code>os.execute</code></a> and <a href="#pdf-io.close"><code>io.close</code></a>). + + + + + +<hr><h3><a name="luaL_fileresult"><code>luaL_fileresult</code></a></h3><p> +<span class="apii">[-0, +(1|3), <em>e</em>]</span> +<pre>int luaL_fileresult (lua_State *L, int stat, const char *fname);</pre> + +<p> +This function produces the return values for +file-related functions in the standard library +(<a href="#pdf-io.open"><code>io.open</code></a>, <a href="#pdf-os.rename"><code>os.rename</code></a>, <a href="#pdf-file:seek"><code>file:seek</code></a>, etc.). + + + + + +<hr><h3><a name="luaL_getmetafield"><code>luaL_getmetafield</code></a></h3><p> +<span class="apii">[-0, +(0|1), <em>e</em>]</span> +<pre>int luaL_getmetafield (lua_State *L, int obj, const char *e);</pre> + +<p> +Pushes onto the stack the field <code>e</code> from the metatable +of the object at index <code>obj</code>. +If the object does not have a metatable, +or if the metatable does not have this field, +returns false and pushes nothing. + + + + + +<hr><h3><a name="luaL_getmetatable"><code>luaL_getmetatable</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>void luaL_getmetatable (lua_State *L, const char *tname);</pre> + +<p> +Pushes onto the stack the metatable associated with name <code>tname</code> +in the registry (see <a href="#luaL_newmetatable"><code>luaL_newmetatable</code></a>). + + + + + +<hr><h3><a name="luaL_getsubtable"><code>luaL_getsubtable</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>int luaL_getsubtable (lua_State *L, int idx, const char *fname);</pre> + +<p> +Ensures that the value <code>t[fname]</code>, +where <code>t</code> is the value at index <code>idx</code>, +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. + + + + + +<hr><h3><a name="luaL_gsub"><code>luaL_gsub</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>const char *luaL_gsub (lua_State *L, + const char *s, + const char *p, + const char *r);</pre> + +<p> +Creates a copy of string <code>s</code> by replacing +any occurrence of the string <code>p</code> +with the string <code>r</code>. +Pushes the resulting string on the stack and returns it. + + + + + +<hr><h3><a name="luaL_len"><code>luaL_len</code></a></h3><p> +<span class="apii">[-0, +0, <em>e</em>]</span> +<pre>int luaL_len (lua_State *L, int index);</pre> + +<p> +Returns the "length" of the value at the given index +as a number; +it is equivalent to the '<code>#</code>' operator in Lua (see <a href="#3.4.6">§3.4.6</a>). +Raises an error if the result of the operation is not a number. +(This case only can happen through metamethods.) + + + + + +<hr><h3><a name="luaL_loadbuffer"><code>luaL_loadbuffer</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>int luaL_loadbuffer (lua_State *L, + const char *buff, + size_t sz, + const char *name);</pre> + +<p> +Equivalent to <a href="#luaL_loadbufferx"><code>luaL_loadbufferx</code></a> with <code>mode</code> equal to <code>NULL</code>. + + + + + +<hr><h3><a name="luaL_loadbufferx"><code>luaL_loadbufferx</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>int luaL_loadbufferx (lua_State *L, + const char *buff, + size_t sz, + const char *name, + const char *mode);</pre> + +<p> +Loads a buffer as a Lua chunk. +This function uses <a href="#lua_load"><code>lua_load</code></a> to load the chunk in the +buffer pointed to by <code>buff</code> with size <code>sz</code>. + + +<p> +This function returns the same results as <a href="#lua_load"><code>lua_load</code></a>. +<code>name</code> is the chunk name, +used for debug information and error messages. +The string <code>mode</code> works as in function <a href="#lua_load"><code>lua_load</code></a>. + + + + + +<hr><h3><a name="luaL_loadfile"><code>luaL_loadfile</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>int luaL_loadfile (lua_State *L, const char *filename);</pre> + +<p> +Equivalent to <a href="#luaL_loadfilex"><code>luaL_loadfilex</code></a> with <code>mode</code> equal to <code>NULL</code>. + + + + + +<hr><h3><a name="luaL_loadfilex"><code>luaL_loadfilex</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>int luaL_loadfilex (lua_State *L, const char *filename, + const char *mode);</pre> + +<p> +Loads a file as a Lua chunk. +This function uses <a href="#lua_load"><code>lua_load</code></a> to load the chunk in the file +named <code>filename</code>. +If <code>filename</code> is <code>NULL</code>, +then it loads from the standard input. +The first line in the file is ignored if it starts with a <code>#</code>. + + +<p> +The string <code>mode</code> works as in function <a href="#lua_load"><code>lua_load</code></a>. + + +<p> +This function returns the same results as <a href="#lua_load"><code>lua_load</code></a>, +but it has an extra error code <a name="pdf-LUA_ERRFILE"><code>LUA_ERRFILE</code></a> +if it cannot open/read the file or the file has a wrong mode. + + +<p> +As <a href="#lua_load"><code>lua_load</code></a>, this function only loads the chunk; +it does not run it. + + + + + +<hr><h3><a name="luaL_loadstring"><code>luaL_loadstring</code></a></h3><p> +<span class="apii">[-0, +1, –]</span> +<pre>int luaL_loadstring (lua_State *L, const char *s);</pre> + +<p> +Loads a string as a Lua chunk. +This function uses <a href="#lua_load"><code>lua_load</code></a> to load the chunk in +the zero-terminated string <code>s</code>. + + +<p> +This function returns the same results as <a href="#lua_load"><code>lua_load</code></a>. + + +<p> +Also as <a href="#lua_load"><code>lua_load</code></a>, this function only loads the chunk; +it does not run it. + + + + + +<hr><h3><a name="luaL_newlib"><code>luaL_newlib</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>void luaL_newlib (lua_State *L, const luaL_Reg *l);</pre> + +<p> +Creates a new table and registers there +the functions in list <code>l</code>. +It is implemented as the following macro: + +<pre> + (luaL_newlibtable(L,l), luaL_setfuncs(L,l,0)) +</pre> + + + + +<hr><h3><a name="luaL_newlibtable"><code>luaL_newlibtable</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>void luaL_newlibtable (lua_State *L, const luaL_Reg l[]);</pre> + +<p> +Creates a new table with a size optimized +to store all entries in the array <code>l</code> +(but does not actually store them). +It is intended to be used in conjunction with <a href="#luaL_setfuncs"><code>luaL_setfuncs</code></a> +(see <a href="#luaL_newlib"><code>luaL_newlib</code></a>). + + +<p> +It is implemented as a macro. +The array <code>l</code> must be the actual array, +not a pointer to it. + + + + + +<hr><h3><a name="luaL_newmetatable"><code>luaL_newmetatable</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>int luaL_newmetatable (lua_State *L, const char *tname);</pre> + +<p> +If the registry already has the key <code>tname</code>, +returns 0. +Otherwise, +creates a new table to be used as a metatable for userdata, +adds it to the registry with key <code>tname</code>, +and returns 1. + + +<p> +In both cases pushes onto the stack the final value associated +with <code>tname</code> in the registry. + + + + + +<hr><h3><a name="luaL_newstate"><code>luaL_newstate</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>lua_State *luaL_newstate (void);</pre> + +<p> +Creates a new Lua state. +It calls <a href="#lua_newstate"><code>lua_newstate</code></a> with an +allocator based on the standard C <code>realloc</code> function +and then sets a panic function (see <a href="#4.6">§4.6</a>) that prints +an error message to the standard error output in case of fatal +errors. + + +<p> +Returns the new state, +or <code>NULL</code> if there is a memory allocation error. + + + + + +<hr><h3><a name="luaL_openlibs"><code>luaL_openlibs</code></a></h3><p> +<span class="apii">[-0, +0, <em>e</em>]</span> +<pre>void luaL_openlibs (lua_State *L);</pre> + +<p> +Opens all standard Lua libraries into the given state. + + + + + +<hr><h3><a name="luaL_optint"><code>luaL_optint</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>int luaL_optint (lua_State *L, int arg, int d);</pre> + +<p> +If the function argument <code>arg</code> is a number, +returns this number cast to an <code>int</code>. +If this argument is absent or is <b>nil</b>, +returns <code>d</code>. +Otherwise, raises an error. + + + + + +<hr><h3><a name="luaL_optinteger"><code>luaL_optinteger</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>lua_Integer luaL_optinteger (lua_State *L, + int arg, + lua_Integer d);</pre> + +<p> +If the function argument <code>arg</code> is a number, +returns this number cast to a <a href="#lua_Integer"><code>lua_Integer</code></a>. +If this argument is absent or is <b>nil</b>, +returns <code>d</code>. +Otherwise, raises an error. + + + + + +<hr><h3><a name="luaL_optlong"><code>luaL_optlong</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>long luaL_optlong (lua_State *L, int arg, long d);</pre> + +<p> +If the function argument <code>arg</code> is a number, +returns this number cast to a <code>long</code>. +If this argument is absent or is <b>nil</b>, +returns <code>d</code>. +Otherwise, raises an error. + + + + + +<hr><h3><a name="luaL_optlstring"><code>luaL_optlstring</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>const char *luaL_optlstring (lua_State *L, + int arg, + const char *d, + size_t *l);</pre> + +<p> +If the function argument <code>arg</code> is a string, +returns this string. +If this argument is absent or is <b>nil</b>, +returns <code>d</code>. +Otherwise, raises an error. + + +<p> +If <code>l</code> is not <code>NULL</code>, +fills the position <code>*l</code> with the result's length. + + + + + +<hr><h3><a name="luaL_optnumber"><code>luaL_optnumber</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>lua_Number luaL_optnumber (lua_State *L, int arg, lua_Number d);</pre> + +<p> +If the function argument <code>arg</code> is a number, +returns this number. +If this argument is absent or is <b>nil</b>, +returns <code>d</code>. +Otherwise, raises an error. + + + + + +<hr><h3><a name="luaL_optstring"><code>luaL_optstring</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>const char *luaL_optstring (lua_State *L, + int arg, + const char *d);</pre> + +<p> +If the function argument <code>arg</code> is a string, +returns this string. +If this argument is absent or is <b>nil</b>, +returns <code>d</code>. +Otherwise, raises an error. + + + + + +<hr><h3><a name="luaL_optunsigned"><code>luaL_optunsigned</code></a></h3><p> +<span class="apii">[-0, +0, <em>v</em>]</span> +<pre>lua_Unsigned luaL_optunsigned (lua_State *L, + int arg, + lua_Unsigned u);</pre> + +<p> +If the function argument <code>arg</code> is a number, +returns this number cast to a <a href="#lua_Unsigned"><code>lua_Unsigned</code></a>. +If this argument is absent or is <b>nil</b>, +returns <code>u</code>. +Otherwise, raises an error. + + + + + +<hr><h3><a name="luaL_prepbuffer"><code>luaL_prepbuffer</code></a></h3><p> +<span class="apii">[-?, +?, <em>e</em>]</span> +<pre>char *luaL_prepbuffer (luaL_Buffer *B);</pre> + +<p> +Equivalent to <a href="#luaL_prepbuffsize"><code>luaL_prepbuffsize</code></a> +with the predefined size <a name="pdf-LUAL_BUFFERSIZE"><code>LUAL_BUFFERSIZE</code></a>. + + + + + +<hr><h3><a name="luaL_prepbuffsize"><code>luaL_prepbuffsize</code></a></h3><p> +<span class="apii">[-?, +?, <em>e</em>]</span> +<pre>char *luaL_prepbuffsize (luaL_Buffer *B, size_t sz);</pre> + +<p> +Returns an address to a space of size <code>sz</code> +where you can copy a string to be added to buffer <code>B</code> +(see <a href="#luaL_Buffer"><code>luaL_Buffer</code></a>). +After copying the string into this space you must call +<a href="#luaL_addsize"><code>luaL_addsize</code></a> with the size of the string to actually add +it to the buffer. + + + + + +<hr><h3><a name="luaL_pushresult"><code>luaL_pushresult</code></a></h3><p> +<span class="apii">[-?, +1, <em>e</em>]</span> +<pre>void luaL_pushresult (luaL_Buffer *B);</pre> + +<p> +Finishes the use of buffer <code>B</code> leaving the final string on +the top of the stack. + + + + + +<hr><h3><a name="luaL_pushresultsize"><code>luaL_pushresultsize</code></a></h3><p> +<span class="apii">[-?, +1, <em>e</em>]</span> +<pre>void luaL_pushresultsize (luaL_Buffer *B, size_t sz);</pre> + +<p> +Equivalent to the sequence <a href="#luaL_addsize"><code>luaL_addsize</code></a>, <a href="#luaL_pushresult"><code>luaL_pushresult</code></a>. + + + + + +<hr><h3><a name="luaL_ref"><code>luaL_ref</code></a></h3><p> +<span class="apii">[-1, +0, <em>e</em>]</span> +<pre>int luaL_ref (lua_State *L, int t);</pre> + +<p> +Creates and returns a <em>reference</em>, +in the table at index <code>t</code>, +for the object at the top of the stack (and pops the object). + + +<p> +A reference is a unique integer key. +As long as you do not manually add integer keys into table <code>t</code>, +<a href="#luaL_ref"><code>luaL_ref</code></a> ensures the uniqueness of the key it returns. +You can retrieve an object referred by reference <code>r</code> +by calling <code>lua_rawgeti(L, t, r)</code>. +Function <a href="#luaL_unref"><code>luaL_unref</code></a> frees a reference and its associated object. + + +<p> +If the object at the top of the stack is <b>nil</b>, +<a href="#luaL_ref"><code>luaL_ref</code></a> returns the constant <a name="pdf-LUA_REFNIL"><code>LUA_REFNIL</code></a>. +The constant <a name="pdf-LUA_NOREF"><code>LUA_NOREF</code></a> is guaranteed to be different +from any reference returned by <a href="#luaL_ref"><code>luaL_ref</code></a>. + + + + + +<hr><h3><a name="luaL_Reg"><code>luaL_Reg</code></a></h3> +<pre>typedef struct luaL_Reg { + const char *name; + lua_CFunction func; +} luaL_Reg;</pre> + +<p> +Type for arrays of functions to be registered by +<a href="#luaL_setfuncs"><code>luaL_setfuncs</code></a>. +<code>name</code> is the function name and <code>func</code> is a pointer to +the function. +Any array of <a href="#luaL_Reg"><code>luaL_Reg</code></a> must end with an sentinel entry +in which both <code>name</code> and <code>func</code> are <code>NULL</code>. + + + + + +<hr><h3><a name="luaL_requiref"><code>luaL_requiref</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>void luaL_requiref (lua_State *L, const char *modname, + lua_CFunction openf, int glb);</pre> + +<p> +Calls function <code>openf</code> with string <code>modname</code> as an argument +and sets the call result in <code>package.loaded[modname]</code>, +as if that function has been called through <a href="#pdf-require"><code>require</code></a>. + + +<p> +If <code>glb</code> is true, +also stores the result into global <code>modname</code>. + + +<p> +Leaves a copy of that result on the stack. + + + + + +<hr><h3><a name="luaL_setfuncs"><code>luaL_setfuncs</code></a></h3><p> +<span class="apii">[-nup, +0, <em>e</em>]</span> +<pre>void luaL_setfuncs (lua_State *L, const luaL_Reg *l, int nup);</pre> + +<p> +Registers all functions in the array <code>l</code> +(see <a href="#luaL_Reg"><code>luaL_Reg</code></a>) into the table on the top of the stack +(below optional upvalues, see next). + + +<p> +When <code>nup</code> is not zero, +all functions are created sharing <code>nup</code> 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. + + + + + +<hr><h3><a name="luaL_setmetatable"><code>luaL_setmetatable</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>void luaL_setmetatable (lua_State *L, const char *tname);</pre> + +<p> +Sets the metatable of the object at the top of the stack +as the metatable associated with name <code>tname</code> +in the registry (see <a href="#luaL_newmetatable"><code>luaL_newmetatable</code></a>). + + + + + +<hr><h3><a name="luaL_testudata"><code>luaL_testudata</code></a></h3><p> +<span class="apii">[-0, +0, <em>e</em>]</span> +<pre>void *luaL_testudata (lua_State *L, int arg, const char *tname);</pre> + +<p> +This function works like <a href="#luaL_checkudata"><code>luaL_checkudata</code></a>, +except that, when the test fails, +it returns <code>NULL</code> instead of throwing an error. + + + + + +<hr><h3><a name="luaL_tolstring"><code>luaL_tolstring</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>const char *luaL_tolstring (lua_State *L, int idx, size_t *len);</pre> + +<p> +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 <code>len</code> is not <code>NULL</code>, +the function also sets <code>*len</code> with the string length. + + +<p> +If the value has a metatable with a <code>"__tostring"</code> field, +then <code>luaL_tolstring</code> calls the corresponding metamethod +with the value as argument, +and uses the result of the call as its result. + + + + + +<hr><h3><a name="luaL_traceback"><code>luaL_traceback</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>void luaL_traceback (lua_State *L, lua_State *L1, const char *msg, + int level);</pre> + +<p> +Creates and pushes a traceback of the stack <code>L1</code>. +If <code>msg</code> is not <code>NULL</code> it is appended +at the beginning of the traceback. +The <code>level</code> parameter tells at which level +to start the traceback. + + + + + +<hr><h3><a name="luaL_typename"><code>luaL_typename</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>const char *luaL_typename (lua_State *L, int index);</pre> + +<p> +Returns the name of the type of the value at the given index. + + + + + +<hr><h3><a name="luaL_unref"><code>luaL_unref</code></a></h3><p> +<span class="apii">[-0, +0, –]</span> +<pre>void luaL_unref (lua_State *L, int t, int ref);</pre> + +<p> +Releases reference <code>ref</code> from the table at index <code>t</code> +(see <a href="#luaL_ref"><code>luaL_ref</code></a>). +The entry is removed from the table, +so that the referred object can be collected. +The reference <code>ref</code> is also freed to be used again. + + +<p> +If <code>ref</code> is <a href="#pdf-LUA_NOREF"><code>LUA_NOREF</code></a> or <a href="#pdf-LUA_REFNIL"><code>LUA_REFNIL</code></a>, +<a href="#luaL_unref"><code>luaL_unref</code></a> does nothing. + + + + + +<hr><h3><a name="luaL_where"><code>luaL_where</code></a></h3><p> +<span class="apii">[-0, +1, <em>e</em>]</span> +<pre>void luaL_where (lua_State *L, int lvl);</pre> + +<p> +Pushes onto the stack a string identifying the current position +of the control at level <code>lvl</code> in the call stack. +Typically this string has the following format: + +<pre> + <em>chunkname</em>:<em>currentline</em>: +</pre><p> +Level 0 is the running function, +level 1 is the function that called the running function, +etc. + + +<p> +This function is used to build a prefix for error messages. + + + + + + + +<h1>6 – <a name="6">Standard Libraries</a></h1> + +<p> +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., <a href="#pdf-type"><code>type</code></a> and <a href="#pdf-getmetatable"><code>getmetatable</code></a>); +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., <a href="#pdf-table.sort"><code>table.sort</code></a>). + + +<p> +All libraries are implemented through the official C API +and are provided as separate C modules. +Currently, Lua has the following standard libraries: + +<ul> + +<li>basic library (<a href="#6.1">§6.1</a>);</li> + +<li>coroutine library (<a href="#6.2">§6.2</a>);</li> + +<li>package library (<a href="#6.3">§6.3</a>);</li> + +<li>string manipulation (<a href="#6.4">§6.4</a>);</li> + +<li>table manipulation (<a href="#6.5">§6.5</a>);</li> + +<li>mathematical functions (<a href="#6.6">§6.6</a>) (sin, log, etc.);</li> + +<li>bitwise operations (<a href="#6.7">§6.7</a>);</li> + +<li>input and output (<a href="#6.8">§6.8</a>);</li> + +<li>operating system facilities (<a href="#6.9">§6.9</a>);</li> + +<li>debug facilities (<a href="#6.10">§6.10</a>).</li> + +</ul><p> +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. + + +<p> +To have access to these libraries, +the C host program should call the <a href="#luaL_openlibs"><code>luaL_openlibs</code></a> function, +which opens all standard libraries. +Alternatively, +the host program can open them individually by using +<a href="#luaL_requiref"><code>luaL_requiref</code></a> to call +<a name="pdf-luaopen_base"><code>luaopen_base</code></a> (for the basic library), +<a name="pdf-luaopen_package"><code>luaopen_package</code></a> (for the package library), +<a name="pdf-luaopen_coroutine"><code>luaopen_coroutine</code></a> (for the coroutine library), +<a name="pdf-luaopen_string"><code>luaopen_string</code></a> (for the string library), +<a name="pdf-luaopen_table"><code>luaopen_table</code></a> (for the table library), +<a name="pdf-luaopen_math"><code>luaopen_math</code></a> (for the mathematical library), +<a name="pdf-luaopen_bit32"><code>luaopen_bit32</code></a> (for the bit library), +<a name="pdf-luaopen_io"><code>luaopen_io</code></a> (for the I/O library), +<a name="pdf-luaopen_os"><code>luaopen_os</code></a> (for the Operating System library), +and <a name="pdf-luaopen_debug"><code>luaopen_debug</code></a> (for the debug library). +These functions are declared in <a name="pdf-lualib.h"><code>lualib.h</code></a>. + + + +<h2>6.1 – <a name="6.1">Basic Functions</a></h2> + +<p> +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. + + +<p> +<hr><h3><a name="pdf-assert"><code>assert (v [, message])</code></a></h3> +Issues an error when +the value of its argument <code>v</code> is false (i.e., <b>nil</b> or <b>false</b>); +otherwise, returns all its arguments. +<code>message</code> is an error message; +when absent, it defaults to "assertion failed!" + + + + +<p> +<hr><h3><a name="pdf-collectgarbage"><code>collectgarbage ([opt [, arg]])</code></a></h3> + + +<p> +This function is a generic interface to the garbage collector. +It performs different functions according to its first argument, <code>opt</code>: + +<ul> + +<li><b>"<code>collect</code>": </b> +performs a full garbage-collection cycle. +This is the default option. +</li> + +<li><b>"<code>stop</code>": </b> +stops automatic execution of the garbage collector. +The collector will run only when explicitly invoked, +until a call to restart it. +</li> + +<li><b>"<code>restart</code>": </b> +restarts automatic execution of the garbage collector. +</li> + +<li><b>"<code>count</code>": </b> +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: + +<pre> + k, b = collectgarbage("count") + assert(k*1024 == math.floor(k)*1024 + b) +</pre><p> +(The second result is useful when Lua is compiled +with a non floating-point type for numbers.) +</li> + +<li><b>"<code>step</code>": </b> +performs a garbage-collection step. +The step "size" is controlled by <code>arg</code> +(larger values mean more steps) in a non-specified way. +If you want to control the step size +you must experimentally tune the value of <code>arg</code>. +Returns <b>true</b> if the step finished a collection cycle. +</li> + +<li><b>"<code>setpause</code>": </b> +sets <code>arg</code> as the new value for the <em>pause</em> of +the collector (see <a href="#2.5">§2.5</a>). +Returns the previous value for <em>pause</em>. +</li> + +<li><b>"<code>setstepmul</code>": </b> +sets <code>arg</code> as the new value for the <em>step multiplier</em> of +the collector (see <a href="#2.5">§2.5</a>). +Returns the previous value for <em>step</em>. +</li> + +<li><b>"<code>isrunning</code>": </b> +returns a boolean that tells whether the collector is running +(i.e., not stopped). +</li> + +<li><b>"<code>generational</code>": </b> +changes the collector to generational mode. +This is an experimental feature (see <a href="#2.5">§2.5</a>). +</li> + +<li><b>"<code>incremental</code>": </b> +changes the collector to incremental mode. +This is the default mode. +</li> + +</ul> + + + +<p> +<hr><h3><a name="pdf-dofile"><code>dofile ([filename])</code></a></h3> +Opens the named file and executes its contents as a Lua chunk. +When called without arguments, +<code>dofile</code> executes the contents of the standard input (<code>stdin</code>). +Returns all values returned by the chunk. +In case of errors, <code>dofile</code> propagates the error +to its caller (that is, <code>dofile</code> does not run in protected mode). + + + + +<p> +<hr><h3><a name="pdf-error"><code>error (message [, level])</code></a></h3> +Terminates the last protected function called +and returns <code>message</code> as the error message. +Function <code>error</code> never returns. + + +<p> +Usually, <code>error</code> adds some information about the error position +at the beginning of the message, if the message is a string. +The <code>level</code> argument specifies how to get the error position. +With level 1 (the default), the error position is where the +<code>error</code> function was called. +Level 2 points the error to where the function +that called <code>error</code> was called; and so on. +Passing a level 0 avoids the addition of error position information +to the message. + + + + +<p> +<hr><h3><a name="pdf-_G"><code>_G</code></a></h3> +A global variable (not a function) that +holds the global environment (see <a href="#2.2">§2.2</a>). +Lua itself does not use this variable; +changing its value does not affect any environment, +nor vice-versa. + + + + +<p> +<hr><h3><a name="pdf-getmetatable"><code>getmetatable (object)</code></a></h3> + + +<p> +If <code>object</code> does not have a metatable, returns <b>nil</b>. +Otherwise, +if the object's metatable has a <code>"__metatable"</code> field, +returns the associated value. +Otherwise, returns the metatable of the given object. + + + + +<p> +<hr><h3><a name="pdf-ipairs"><code>ipairs (t)</code></a></h3> + + +<p> +If <code>t</code> has a metamethod <code>__ipairs</code>, +calls it with <code>t</code> as argument and returns the first three +results from the call. + + +<p> +Otherwise, +returns three values: an iterator function, the table <code>t</code>, and 0, +so that the construction + +<pre> + for i,v in ipairs(t) do <em>body</em> end +</pre><p> +will iterate over the pairs (<code>1,t[1]</code>), (<code>2,t[2]</code>), ..., +up to the first integer key absent from the table. + + + + +<p> +<hr><h3><a name="pdf-load"><code>load (ld [, source [, mode [, env]]])</code></a></h3> + + +<p> +Loads a chunk. + + +<p> +If <code>ld</code> is a string, the chunk is this string. +If <code>ld</code> is a function, +<code>load</code> calls it repeatedly to get the chunk pieces. +Each call to <code>ld</code> must return a string that concatenates +with previous results. +A return of an empty string, <b>nil</b>, or no value signals the end of the chunk. + + +<p> +If there are no syntactic errors, +returns the compiled chunk as a function; +otherwise, returns <b>nil</b> plus the error message. + + +<p> +If the resulting function has upvalues, +the first upvalue is set to the value of <code>env</code>, +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 <code>_ENV</code> variable (see <a href="#2.2">§2.2</a>). +When you load a binary chunk created from a function (see <a href="#pdf-string.dump"><code>string.dump</code></a>), +the resulting function can have arbitrary upvalues.) + + +<p> +<code>source</code> is used as the source of the chunk for error messages +and debug information (see <a href="#4.9">§4.9</a>). +When absent, +it defaults to <code>ld</code>, if <code>ld</code> is a string, +or to "<code>=(load)</code>" otherwise. + + +<p> +The string <code>mode</code> controls whether the chunk can be text or binary +(that is, a precompiled chunk). +It may be the string "<code>b</code>" (only binary chunks), +"<code>t</code>" (only text chunks), +or "<code>bt</code>" (both binary and text). +The default is "<code>bt</code>". + + + + +<p> +<hr><h3><a name="pdf-loadfile"><code>loadfile ([filename [, mode [, env]]])</code></a></h3> + + +<p> +Similar to <a href="#pdf-load"><code>load</code></a>, +but gets the chunk from file <code>filename</code> +or from the standard input, +if no file name is given. + + + + +<p> +<hr><h3><a name="pdf-next"><code>next (table [, index])</code></a></h3> + + +<p> +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. +<code>next</code> returns the next index of the table +and its associated value. +When called with <b>nil</b> as its second argument, +<code>next</code> returns an initial index +and its associated value. +When called with the last index, +or with <b>nil</b> in an empty table, +<code>next</code> returns <b>nil</b>. +If the second argument is absent, then it is interpreted as <b>nil</b>. +In particular, +you can use <code>next(t)</code> to check whether a table is empty. + + +<p> +The order in which the indices are enumerated is not specified, +<em>even for numeric indices</em>. +(To traverse a table in numeric order, +use a numerical <b>for</b>.) + + +<p> +The behavior of <code>next</code> 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. + + + + +<p> +<hr><h3><a name="pdf-pairs"><code>pairs (t)</code></a></h3> + + +<p> +If <code>t</code> has a metamethod <code>__pairs</code>, +calls it with <code>t</code> as argument and returns the first three +results from the call. + + +<p> +Otherwise, +returns three values: the <a href="#pdf-next"><code>next</code></a> function, the table <code>t</code>, and <b>nil</b>, +so that the construction + +<pre> + for k,v in pairs(t) do <em>body</em> end +</pre><p> +will iterate over all key–value pairs of table <code>t</code>. + + +<p> +See function <a href="#pdf-next"><code>next</code></a> for the caveats of modifying +the table during its traversal. + + + + +<p> +<hr><h3><a name="pdf-pcall"><code>pcall (f [, arg1, ···])</code></a></h3> + + +<p> +Calls function <code>f</code> with +the given arguments in <em>protected mode</em>. +This means that any error inside <code>f</code> is not propagated; +instead, <code>pcall</code> 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, <code>pcall</code> also returns all results from the call, +after this first result. +In case of any error, <code>pcall</code> returns <b>false</b> plus the error message. + + + + +<p> +<hr><h3><a name="pdf-print"><code>print (···)</code></a></h3> +Receives any number of arguments +and prints their values to <code>stdout</code>, +using the <a href="#pdf-tostring"><code>tostring</code></a> function to convert each argument to a string. +<code>print</code> 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 <a href="#pdf-string.format"><code>string.format</code></a> and <a href="#pdf-io.write"><code>io.write</code></a>. + + + + +<p> +<hr><h3><a name="pdf-rawequal"><code>rawequal (v1, v2)</code></a></h3> +Checks whether <code>v1</code> is equal to <code>v2</code>, +without invoking any metamethod. +Returns a boolean. + + + + +<p> +<hr><h3><a name="pdf-rawget"><code>rawget (table, index)</code></a></h3> +Gets the real value of <code>table[index]</code>, +without invoking any metamethod. +<code>table</code> must be a table; +<code>index</code> may be any value. + + + + +<p> +<hr><h3><a name="pdf-rawlen"><code>rawlen (v)</code></a></h3> +Returns the length of the object <code>v</code>, +which must be a table or a string, +without invoking any metamethod. +Returns an integer number. + + + + +<p> +<hr><h3><a name="pdf-rawset"><code>rawset (table, index, value)</code></a></h3> +Sets the real value of <code>table[index]</code> to <code>value</code>, +without invoking any metamethod. +<code>table</code> must be a table, +<code>index</code> any value different from <b>nil</b> and NaN, +and <code>value</code> any Lua value. + + +<p> +This function returns <code>table</code>. + + + + +<p> +<hr><h3><a name="pdf-select"><code>select (index, ···)</code></a></h3> + + +<p> +If <code>index</code> is a number, +returns all arguments after argument number <code>index</code>; +a negative number indexes from the end (-1 is the last argument). +Otherwise, <code>index</code> must be the string <code>"#"</code>, +and <code>select</code> returns the total number of extra arguments it received. + + + + +<p> +<hr><h3><a name="pdf-setmetatable"><code>setmetatable (table, metatable)</code></a></h3> + + +<p> +Sets the metatable for the given table. +(You cannot change the metatable of other types from Lua, only from C.) +If <code>metatable</code> is <b>nil</b>, +removes the metatable of the given table. +If the original metatable has a <code>"__metatable"</code> field, +raises an error. + + +<p> +This function returns <code>table</code>. + + + + +<p> +<hr><h3><a name="pdf-tonumber"><code>tonumber (e [, base])</code></a></h3> + + +<p> +When called with no <code>base</code>, +<code>tonumber</code> tries to convert its argument to a number. +If the argument is already a number or +a string convertible to a number (see <a href="#3.4.2">§3.4.2</a>), +then <code>tonumber</code> returns this number; +otherwise, it returns <b>nil</b>. + + +<p> +When called with <code>base</code>, +then <code>e</code> 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 '<code>A</code>' (in either upper or lower case) +represents 10, '<code>B</code>' represents 11, and so forth, +with '<code>Z</code>' representing 35. +If the string <code>e</code> is not a valid numeral in the given base, +the function returns <b>nil</b>. + + + + +<p> +<hr><h3><a name="pdf-tostring"><code>tostring (v)</code></a></h3> +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 <a href="#pdf-string.format"><code>string.format</code></a>.) + + +<p> +If the metatable of <code>v</code> has a <code>"__tostring"</code> field, +then <code>tostring</code> calls the corresponding value +with <code>v</code> as argument, +and uses the result of the call as its result. + + + + +<p> +<hr><h3><a name="pdf-type"><code>type (v)</code></a></h3> +Returns the type of its only argument, coded as a string. +The possible results of this function are +"<code>nil</code>" (a string, not the value <b>nil</b>), +"<code>number</code>", +"<code>string</code>", +"<code>boolean</code>", +"<code>table</code>", +"<code>function</code>", +"<code>thread</code>", +and "<code>userdata</code>". + + + + +<p> +<hr><h3><a name="pdf-_VERSION"><code>_VERSION</code></a></h3> +A global variable (not a function) that +holds a string containing the current interpreter version. +The current contents of this variable is "<code>Lua 5.2</code>". + + + + +<p> +<hr><h3><a name="pdf-xpcall"><code>xpcall (f, msgh [, arg1, ···])</code></a></h3> + + +<p> +This function is similar to <a href="#pdf-pcall"><code>pcall</code></a>, +except that it sets a new message handler <code>msgh</code>. + + + + + + + +<h2>6.2 – <a name="6.2">Coroutine Manipulation</a></h2> + +<p> +The operations related to coroutines comprise a sub-library of +the basic library and come inside the table <a name="pdf-coroutine"><code>coroutine</code></a>. +See <a href="#2.6">§2.6</a> for a general description of coroutines. + + +<p> +<hr><h3><a name="pdf-coroutine.create"><code>coroutine.create (f)</code></a></h3> + + +<p> +Creates a new coroutine, with body <code>f</code>. +<code>f</code> must be a Lua function. +Returns this new coroutine, +an object with type <code>"thread"</code>. + + + + +<p> +<hr><h3><a name="pdf-coroutine.resume"><code>coroutine.resume (co [, val1, ···])</code></a></h3> + + +<p> +Starts or continues the execution of coroutine <code>co</code>. +The first time you resume a coroutine, +it starts running its body. +The values <code>val1</code>, ... are passed +as the arguments to the body function. +If the coroutine has yielded, +<code>resume</code> restarts it; +the values <code>val1</code>, ... are passed +as the results from the yield. + + +<p> +If the coroutine runs without any errors, +<code>resume</code> returns <b>true</b> plus any values passed to <code>yield</code> +(if the coroutine yields) or any values returned by the body function +(if the coroutine terminates). +If there is any error, +<code>resume</code> returns <b>false</b> plus the error message. + + + + +<p> +<hr><h3><a name="pdf-coroutine.running"><code>coroutine.running ()</code></a></h3> + + +<p> +Returns the running coroutine plus a boolean, +true when the running coroutine is the main one. + + + + +<p> +<hr><h3><a name="pdf-coroutine.status"><code>coroutine.status (co)</code></a></h3> + + +<p> +Returns the status of coroutine <code>co</code>, as a string: +<code>"running"</code>, +if the coroutine is running (that is, it called <code>status</code>); +<code>"suspended"</code>, if the coroutine is suspended in a call to <code>yield</code>, +or if it has not started running yet; +<code>"normal"</code> if the coroutine is active but not running +(that is, it has resumed another coroutine); +and <code>"dead"</code> if the coroutine has finished its body function, +or if it has stopped with an error. + + + + +<p> +<hr><h3><a name="pdf-coroutine.wrap"><code>coroutine.wrap (f)</code></a></h3> + + +<p> +Creates a new coroutine, with body <code>f</code>. +<code>f</code> 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 <code>resume</code>. +Returns the same values returned by <code>resume</code>, +except the first boolean. +In case of error, propagates the error. + + + + +<p> +<hr><h3><a name="pdf-coroutine.yield"><code>coroutine.yield (···)</code></a></h3> + + +<p> +Suspends the execution of the calling coroutine. +Any arguments to <code>yield</code> are passed as extra results to <code>resume</code>. + + + + + + + +<h2>6.3 – <a name="6.3">Modules</a></h2> + +<p> +The package library provides basic +facilities for loading modules in Lua. +It exports one function directly in the global environment: +<a href="#pdf-require"><code>require</code></a>. +Everything else is exported in a table <a name="pdf-package"><code>package</code></a>. + + +<p> +<hr><h3><a name="pdf-require"><code>require (modname)</code></a></h3> + + +<p> +Loads the given module. +The function starts by looking into the <a href="#pdf-package.loaded"><code>package.loaded</code></a> table +to determine whether <code>modname</code> is already loaded. +If it is, then <code>require</code> returns the value stored +at <code>package.loaded[modname]</code>. +Otherwise, it tries to find a <em>loader</em> for the module. + + +<p> +To find a loader, +<code>require</code> is guided by the <a href="#pdf-package.searchers"><code>package.searchers</code></a> sequence. +By changing this sequence, +we can change how <code>require</code> looks for a module. +The following explanation is based on the default configuration +for <a href="#pdf-package.searchers"><code>package.searchers</code></a>. + + +<p> +First <code>require</code> queries <code>package.preload[modname]</code>. +If it has a value, +this value (which should be a function) is the loader. +Otherwise <code>require</code> searches for a Lua loader using the +path stored in <a href="#pdf-package.path"><code>package.path</code></a>. +If that also fails, it searches for a C loader using the +path stored in <a href="#pdf-package.cpath"><code>package.cpath</code></a>. +If that also fails, +it tries an <em>all-in-one</em> loader (see <a href="#pdf-package.searchers"><code>package.searchers</code></a>). + + +<p> +Once a loader is found, +<code>require</code> calls the loader with two arguments: +<code>modname</code> 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, +<code>require</code> assigns the returned value to <code>package.loaded[modname]</code>. +If the loader does not return a non-nil value and +has not assigned any value to <code>package.loaded[modname]</code>, +then <code>require</code> assigns <b>true</b> to this entry. +In any case, <code>require</code> returns the +final value of <code>package.loaded[modname]</code>. + + +<p> +If there is any error loading or running the module, +or if it cannot find any loader for the module, +then <code>require</code> raises an error. + + + + +<p> +<hr><h3><a name="pdf-package.config"><code>package.config</code></a></h3> + + +<p> +A string describing some compile-time configurations for packages. +This string is a sequence of lines: + +<ul> + +<li>The first line is the directory separator string. +Default is '<code>\</code>' for Windows and '<code>/</code>' for all other systems.</li> + +<li>The second line is the character that separates templates in a path. +Default is '<code>;</code>'.</li> + +<li>The third line is the string that marks the +substitution points in a template. +Default is '<code>?</code>'.</li> + +<li>The fourth line is a string that, in a path in Windows, +is replaced by the executable's directory. +Default is '<code>!</code>'.</li> + +<li>The fifth line is a mark to ignore all text before it +when building the <code>luaopen_</code> function name. +Default is '<code>-</code>'.</li> + +</ul> + + + +<p> +<hr><h3><a name="pdf-package.cpath"><code>package.cpath</code></a></h3> + + +<p> +The path used by <a href="#pdf-require"><code>require</code></a> to search for a C loader. + + +<p> +Lua initializes the C path <a href="#pdf-package.cpath"><code>package.cpath</code></a> in the same way +it initializes the Lua path <a href="#pdf-package.path"><code>package.path</code></a>, +using the environment variable <a name="pdf-LUA_CPATH_5_2"><code>LUA_CPATH_5_2</code></a> +or the environment variable <a name="pdf-LUA_CPATH"><code>LUA_CPATH</code></a> +or a default path defined in <code>luaconf.h</code>. + + + + +<p> +<hr><h3><a name="pdf-package.loaded"><code>package.loaded</code></a></h3> + + +<p> +A table used by <a href="#pdf-require"><code>require</code></a> to control which +modules are already loaded. +When you require a module <code>modname</code> and +<code>package.loaded[modname]</code> is not false, +<a href="#pdf-require"><code>require</code></a> simply returns the value stored there. + + +<p> +This variable is only a reference to the real table; +assignments to this variable do not change the +table used by <a href="#pdf-require"><code>require</code></a>. + + + + +<p> +<hr><h3><a name="pdf-package.loadlib"><code>package.loadlib (libname, funcname)</code></a></h3> + + +<p> +Dynamically links the host program with the C library <code>libname</code>. + + +<p> +If <code>funcname</code> is "<code>*</code>", +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 <code>funcname</code> inside the library +and returns this function as a C function. +So, <code>funcname</code> must follow the <a href="#lua_CFunction"><code>lua_CFunction</code></a> prototype +(see <a href="#lua_CFunction"><code>lua_CFunction</code></a>). + + +<p> +This is a low-level function. +It completely bypasses the package and module system. +Unlike <a href="#pdf-require"><code>require</code></a>, +it does not perform any path searching and +does not automatically adds extensions. +<code>libname</code> must be the complete file name of the C library, +including if necessary a path and an extension. +<code>funcname</code> must be the exact name exported by the C library +(which may depend on the C compiler and linker used). + + +<p> +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 <code>dlfcn</code> standard). + + + + +<p> +<hr><h3><a name="pdf-package.path"><code>package.path</code></a></h3> + + +<p> +The path used by <a href="#pdf-require"><code>require</code></a> to search for a Lua loader. + + +<p> +At start-up, Lua initializes this variable with +the value of the environment variable <a name="pdf-LUA_PATH_5_2"><code>LUA_PATH_5_2</code></a> or +the environment variable <a name="pdf-LUA_PATH"><code>LUA_PATH</code></a> or +with a default path defined in <code>luaconf.h</code>, +if those environment variables are not defined. +Any "<code>;;</code>" in the value of the environment variable +is replaced by the default path. + + + + +<p> +<hr><h3><a name="pdf-package.preload"><code>package.preload</code></a></h3> + + +<p> +A table to store loaders for specific modules +(see <a href="#pdf-require"><code>require</code></a>). + + +<p> +This variable is only a reference to the real table; +assignments to this variable do not change the +table used by <a href="#pdf-require"><code>require</code></a>. + + + + +<p> +<hr><h3><a name="pdf-package.searchers"><code>package.searchers</code></a></h3> + + +<p> +A table used by <a href="#pdf-require"><code>require</code></a> to control how to load modules. + + +<p> +Each entry in this table is a <em>searcher function</em>. +When looking for a module, +<a href="#pdf-require"><code>require</code></a> calls each of these searchers in ascending order, +with the module name (the argument given to <a href="#pdf-require"><code>require</code></a>) as its +sole parameter. +The function can return another function (the module <em>loader</em>) +plus an extra value that will be passed to that loader, +or a string explaining why it did not find that module +(or <b>nil</b> if it has nothing to say). + + +<p> +Lua initializes this table with four searcher functions. + + +<p> +The first searcher simply looks for a loader in the +<a href="#pdf-package.preload"><code>package.preload</code></a> table. + + +<p> +The second searcher looks for a loader as a Lua library, +using the path stored at <a href="#pdf-package.path"><code>package.path</code></a>. +The search is done as described in function <a href="#pdf-package.searchpath"><code>package.searchpath</code></a>. + + +<p> +The third searcher looks for a loader as a C library, +using the path given by the variable <a href="#pdf-package.cpath"><code>package.cpath</code></a>. +Again, +the search is done as described in function <a href="#pdf-package.searchpath"><code>package.searchpath</code></a>. +For instance, +if the C path is the string + +<pre> + "./?.so;./?.dll;/usr/local/?/init.so" +</pre><p> +the searcher for module <code>foo</code> +will try to open the files <code>./foo.so</code>, <code>./foo.dll</code>, +and <code>/usr/local/foo/init.so</code>, 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 "<code>luaopen_</code>" +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 <code>a.v1-b.c</code>, +the function name will be <code>luaopen_b_c</code>. + + +<p> +The fourth searcher tries an <em>all-in-one loader</em>. +It searches the C path for a library for +the root name of the given module. +For instance, when requiring <code>a.b.c</code>, +it will search for a C library for <code>a</code>. +If found, it looks into it for an open function for +the submodule; +in our example, that would be <code>luaopen_a_b_c</code>. +With this facility, a package can pack several C submodules +into one single library, +with each submodule keeping its original open function. + + +<p> +All searchers except the first one (preload) return as the extra value +the file name where the module was found, +as returned by <a href="#pdf-package.searchpath"><code>package.searchpath</code></a>. +The first searcher returns no extra value. + + + + +<p> +<hr><h3><a name="pdf-package.searchpath"><code>package.searchpath (name, path [, sep [, rep]])</code></a></h3> + + +<p> +Searches for the given <code>name</code> in the given <code>path</code>. + + +<p> +A path is a string containing a sequence of +<em>templates</em> separated by semicolons. +For each template, +the function replaces each interrogation mark (if any) +in the template with a copy of <code>name</code> +wherein all occurrences of <code>sep</code> +(a dot, by default) +were replaced by <code>rep</code> +(the system's directory separator, by default), +and then tries to open the resulting file name. + + +<p> +For instance, if the path is the string + +<pre> + "./?.lua;./?.lc;/usr/local/?/init.lua" +</pre><p> +the search for the name <code>foo.a</code> +will try to open the files +<code>./foo/a.lua</code>, <code>./foo/a.lc</code>, and +<code>/usr/local/foo/a/init.lua</code>, in that order. + + +<p> +Returns the resulting name of the first file that it can +open in read mode (after closing the file), +or <b>nil</b> plus an error message if none succeeds. +(This error message lists all file names it tried to open.) + + + + + + + +<h2>6.4 – <a name="6.4">String Manipulation</a></h2> + +<p> +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. + + +<p> +The string library provides all its functions inside the table +<a name="pdf-string"><code>string</code></a>. +It also sets a metatable for strings +where the <code>__index</code> field points to the <code>string</code> table. +Therefore, you can use the string functions in object-oriented style. +For instance, <code>string.byte(s,i)</code> +can be written as <code>s:byte(i)</code>. + + +<p> +The string library assumes one-byte character encodings. + + +<p> +<hr><h3><a name="pdf-string.byte"><code>string.byte (s [, i [, j]])</code></a></h3> +Returns the internal numerical codes of the characters <code>s[i]</code>, +<code>s[i+1]</code>, ..., <code>s[j]</code>. +The default value for <code>i</code> is 1; +the default value for <code>j</code> is <code>i</code>. +These indices are corrected +following the same rules of function <a href="#pdf-string.sub"><code>string.sub</code></a>. + + +<p> +Numerical codes are not necessarily portable across platforms. + + + + +<p> +<hr><h3><a name="pdf-string.char"><code>string.char (···)</code></a></h3> +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. + + +<p> +Numerical codes are not necessarily portable across platforms. + + + + +<p> +<hr><h3><a name="pdf-string.dump"><code>string.dump (function)</code></a></h3> + + +<p> +Returns a string containing a binary representation of the given function, +so that a later <a href="#pdf-load"><code>load</code></a> on this string returns +a copy of the function (but with new upvalues). + + + + +<p> +<hr><h3><a name="pdf-string.find"><code>string.find (s, pattern [, init [, plain]])</code></a></h3> + + +<p> +Looks for the first match of +<code>pattern</code> in the string <code>s</code>. +If it finds a match, then <code>find</code> returns the indices of <code>s</code> +where this occurrence starts and ends; +otherwise, it returns <b>nil</b>. +A third, optional numerical argument <code>init</code> specifies +where to start the search; +its default value is 1 and can be negative. +A value of <b>true</b> as a fourth, optional argument <code>plain</code> +turns off the pattern matching facilities, +so the function does a plain "find substring" operation, +with no characters in <code>pattern</code> being considered magic. +Note that if <code>plain</code> is given, then <code>init</code> must be given as well. + + +<p> +If the pattern has captures, +then in a successful match +the captured values are also returned, +after the two indices. + + + + +<p> +<hr><h3><a name="pdf-string.format"><code>string.format (formatstring, ···)</code></a></h3> + + +<p> +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 <code>sprintf</code>. +The only differences are that the options/modifiers +<code>*</code>, <code>h</code>, <code>L</code>, <code>l</code>, <code>n</code>, +and <code>p</code> are not supported +and that there is an extra option, <code>q</code>. +The <code>q</code> 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 + +<pre> + string.format('%q', 'a string with "quotes" and \n new line') +</pre><p> +may produce the string: + +<pre> + "a string with \"quotes\" and \ + new line" +</pre> + +<p> +Options +<code>A</code> and <code>a</code> (when available), +<code>E</code>, <code>e</code>, <code>f</code>, +<code>G</code>, and <code>g</code> all expect a number as argument. +Options <code>c</code>, <code>d</code>, +<code>i</code>, <code>o</code>, <code>u</code>, <code>X</code>, and <code>x</code> +also expect a number, +but the range of that number may be limited by +the underlying C implementation. +For options <code>o</code>, <code>u</code>, <code>X</code>, and <code>x</code>, +the number cannot be negative. +Option <code>q</code> expects a string; +option <code>s</code> expects a string without embedded zeros. +If the argument to option <code>s</code> is not a string, +it is converted to one following the same rules of <a href="#pdf-tostring"><code>tostring</code></a>. + + + + +<p> +<hr><h3><a name="pdf-string.gmatch"><code>string.gmatch (s, pattern)</code></a></h3> +Returns an iterator function that, +each time it is called, +returns the next captures from <code>pattern</code> over the string <code>s</code>. +If <code>pattern</code> specifies no captures, +then the whole match is produced in each call. + + +<p> +As an example, the following loop +will iterate over all the words from string <code>s</code>, +printing one per line: + +<pre> + s = "hello world from Lua" + for w in string.gmatch(s, "%a+") do + print(w) + end +</pre><p> +The next example collects all pairs <code>key=value</code> from the +given string into a table: + +<pre> + t = {} + s = "from=world, to=Lua" + for k, v in string.gmatch(s, "(%w+)=(%w+)") do + t[k] = v + end +</pre> + +<p> +For this function, a caret '<code>^</code>' at the start of a pattern does not +work as an anchor, as this would prevent the iteration. + + + + +<p> +<hr><h3><a name="pdf-string.gsub"><code>string.gsub (s, pattern, repl [, n])</code></a></h3> +Returns a copy of <code>s</code> +in which all (or the first <code>n</code>, if given) +occurrences of the <code>pattern</code> have been +replaced by a replacement string specified by <code>repl</code>, +which can be a string, a table, or a function. +<code>gsub</code> also returns, as its second value, +the total number of matches that occurred. +The name <code>gsub</code> comes from <em>Global SUBstitution</em>. + + +<p> +If <code>repl</code> is a string, then its value is used for replacement. +The character <code>%</code> works as an escape character: +any sequence in <code>repl</code> of the form <code>%<em>d</em></code>, +with <em>d</em> between 1 and 9, +stands for the value of the <em>d</em>-th captured substring. +The sequence <code>%0</code> stands for the whole match. +The sequence <code>%%</code> stands for a single <code>%</code>. + + +<p> +If <code>repl</code> is a table, then the table is queried for every match, +using the first capture as the key. + + +<p> +If <code>repl</code> is a function, then this function is called every time a +match occurs, with all captured substrings passed as arguments, +in order. + + +<p> +In any case, +if the pattern specifies no captures, +then it behaves as if the whole pattern was inside a capture. + + +<p> +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 <b>false</b> or <b>nil</b>, +then there is no replacement +(that is, the original match is kept in the string). + + +<p> +Here are some examples: + +<pre> + 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" +</pre> + + + +<p> +<hr><h3><a name="pdf-string.len"><code>string.len (s)</code></a></h3> +Receives a string and returns its length. +The empty string <code>""</code> has length 0. +Embedded zeros are counted, +so <code>"a\000bc\000"</code> has length 5. + + + + +<p> +<hr><h3><a name="pdf-string.lower"><code>string.lower (s)</code></a></h3> +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. + + + + +<p> +<hr><h3><a name="pdf-string.match"><code>string.match (s, pattern [, init])</code></a></h3> +Looks for the first <em>match</em> of +<code>pattern</code> in the string <code>s</code>. +If it finds one, then <code>match</code> returns +the captures from the pattern; +otherwise it returns <b>nil</b>. +If <code>pattern</code> specifies no captures, +then the whole match is returned. +A third, optional numerical argument <code>init</code> specifies +where to start the search; +its default value is 1 and can be negative. + + + + +<p> +<hr><h3><a name="pdf-string.rep"><code>string.rep (s, n [, sep])</code></a></h3> +Returns a string that is the concatenation of <code>n</code> copies of +the string <code>s</code> separated by the string <code>sep</code>. +The default value for <code>sep</code> is the empty string +(that is, no separator). + + + + +<p> +<hr><h3><a name="pdf-string.reverse"><code>string.reverse (s)</code></a></h3> +Returns a string that is the string <code>s</code> reversed. + + + + +<p> +<hr><h3><a name="pdf-string.sub"><code>string.sub (s, i [, j])</code></a></h3> +Returns the substring of <code>s</code> that +starts at <code>i</code> and continues until <code>j</code>; +<code>i</code> and <code>j</code> can be negative. +If <code>j</code> is absent, then it is assumed to be equal to -1 +(which is the same as the string length). +In particular, +the call <code>string.sub(s,1,j)</code> returns a prefix of <code>s</code> +with length <code>j</code>, +and <code>string.sub(s, -i)</code> returns a suffix of <code>s</code> +with length <code>i</code>. + + +<p> +If, after the translation of negative indices, +<code>i</code> is less than 1, +it is corrected to 1. +If <code>j</code> is greater than the string length, +it is corrected to that length. +If, after these corrections, +<code>i</code> is greater than <code>j</code>, +the function returns the empty string. + + + + +<p> +<hr><h3><a name="pdf-string.upper"><code>string.upper (s)</code></a></h3> +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. + + + +<h3>6.4.1 – <a name="6.4.1">Patterns</a></h3> + + +<h4>Character Class:</h4><p> +A <em>character class</em> is used to represent a set of characters. +The following combinations are allowed in describing a character class: + +<ul> + +<li><b><em>x</em>: </b> +(where <em>x</em> is not one of the <em>magic characters</em> +<code>^$()%.[]*+-?</code>) +represents the character <em>x</em> itself. +</li> + +<li><b><code>.</code>: </b> (a dot) represents all characters.</li> + +<li><b><code>%a</code>: </b> represents all letters.</li> + +<li><b><code>%c</code>: </b> represents all control characters.</li> + +<li><b><code>%d</code>: </b> represents all digits.</li> + +<li><b><code>%g</code>: </b> represents all printable characters except space.</li> + +<li><b><code>%l</code>: </b> represents all lowercase letters.</li> + +<li><b><code>%p</code>: </b> represents all punctuation characters.</li> + +<li><b><code>%s</code>: </b> represents all space characters.</li> + +<li><b><code>%u</code>: </b> represents all uppercase letters.</li> + +<li><b><code>%w</code>: </b> represents all alphanumeric characters.</li> + +<li><b><code>%x</code>: </b> represents all hexadecimal digits.</li> + +<li><b><code>%<em>x</em></code>: </b> (where <em>x</em> is any non-alphanumeric character) +represents the character <em>x</em>. +This is the standard way to escape the magic characters. +Any punctuation character (even the non magic) +can be preceded by a '<code>%</code>' +when used to represent itself in a pattern. +</li> + +<li><b><code>[<em>set</em>]</code>: </b> +represents the class which is the union of all +characters in <em>set</em>. +A range of characters can be specified by +separating the end characters of the range, +in ascending order, with a '<code>-</code>', +All classes <code>%</code><em>x</em> described above can also be used as +components in <em>set</em>. +All other characters in <em>set</em> represent themselves. +For example, <code>[%w_]</code> (or <code>[_%w]</code>) +represents all alphanumeric characters plus the underscore, +<code>[0-7]</code> represents the octal digits, +and <code>[0-7%l%-]</code> represents the octal digits plus +the lowercase letters plus the '<code>-</code>' character. + + +<p> +The interaction between ranges and classes is not defined. +Therefore, patterns like <code>[%a-z]</code> or <code>[a-%%]</code> +have no meaning. +</li> + +<li><b><code>[^<em>set</em>]</code>: </b> +represents the complement of <em>set</em>, +where <em>set</em> is interpreted as above. +</li> + +</ul><p> +For all classes represented by single letters (<code>%a</code>, <code>%c</code>, etc.), +the corresponding uppercase letter represents the complement of the class. +For instance, <code>%S</code> represents all non-space characters. + + +<p> +The definitions of letter, space, and other character groups +depend on the current locale. +In particular, the class <code>[a-z]</code> may not be equivalent to <code>%l</code>. + + + + + +<h4>Pattern Item:</h4><p> +A <em>pattern item</em> can be + +<ul> + +<li> +a single character class, +which matches any single character in the class; +</li> + +<li> +a single character class followed by '<code>*</code>', +which matches 0 or more repetitions of characters in the class. +These repetition items will always match the longest possible sequence; +</li> + +<li> +a single character class followed by '<code>+</code>', +which matches 1 or more repetitions of characters in the class. +These repetition items will always match the longest possible sequence; +</li> + +<li> +a single character class followed by '<code>-</code>', +which also matches 0 or more repetitions of characters in the class. +Unlike '<code>*</code>', +these repetition items will always match the shortest possible sequence; +</li> + +<li> +a single character class followed by '<code>?</code>', +which matches 0 or 1 occurrence of a character in the class; +</li> + +<li> +<code>%<em>n</em></code>, for <em>n</em> between 1 and 9; +such item matches a substring equal to the <em>n</em>-th captured string +(see below); +</li> + +<li> +<code>%b<em>xy</em></code>, where <em>x</em> and <em>y</em> are two distinct characters; +such item matches strings that start with <em>x</em>, end with <em>y</em>, +and where the <em>x</em> and <em>y</em> are <em>balanced</em>. +This means that, if one reads the string from left to right, +counting <em>+1</em> for an <em>x</em> and <em>-1</em> for a <em>y</em>, +the ending <em>y</em> is the first <em>y</em> where the count reaches 0. +For instance, the item <code>%b()</code> matches expressions with +balanced parentheses. +</li> + +<li> +<code>%f[<em>set</em>]</code>, a <em>frontier pattern</em>; +such item matches an empty string at any position such that +the next character belongs to <em>set</em> +and the previous character does not belong to <em>set</em>. +The set <em>set</em> is interpreted as previously described. +The beginning and the end of the subject are handled as if +they were the character '<code>\0</code>'. +</li> + +</ul> + + + + +<h4>Pattern:</h4><p> +A <em>pattern</em> is a sequence of pattern items. +A caret '<code>^</code>' at the beginning of a pattern anchors the match at the +beginning of the subject string. +A '<code>$</code>' at the end of a pattern anchors the match at the +end of the subject string. +At other positions, +'<code>^</code>' and '<code>$</code>' have no special meaning and represent themselves. + + + + + +<h4>Captures:</h4><p> +A pattern can contain sub-patterns enclosed in parentheses; +they describe <em>captures</em>. +When a match succeeds, the substrings of the subject string +that match captures are stored (<em>captured</em>) for future use. +Captures are numbered according to their left parentheses. +For instance, in the pattern <code>"(a*(.)%w(%s*))"</code>, +the part of the string matching <code>"a*(.)%w(%s*)"</code> is +stored as the first capture (and therefore has number 1); +the character matching "<code>.</code>" is captured with number 2, +and the part matching "<code>%s*</code>" has number 3. + + +<p> +As a special case, the empty capture <code>()</code> captures +the current string position (a number). +For instance, if we apply the pattern <code>"()aa()"</code> on the +string <code>"flaaap"</code>, there will be two captures: 3 and 5. + + + + + + + + + + + +<h2>6.5 – <a name="6.5">Table Manipulation</a></h2> + +<p> +This library provides generic functions for table manipulation. +It provides all its functions inside the table <a name="pdf-table"><code>table</code></a>. + + +<p> +Remember that, whenever an operation needs the length of a table, +the table should be a proper sequence +or have a <code>__len</code> metamethod (see <a href="#3.4.6">§3.4.6</a>). +All functions ignore non-numeric keys +in tables given as arguments. + + +<p> +For performance reasons, +all table accesses (get/set) performed by these functions are raw. + + +<p> +<hr><h3><a name="pdf-table.concat"><code>table.concat (list [, sep [, i [, j]]])</code></a></h3> + + +<p> +Given a list where all elements are strings or numbers, +returns the string <code>list[i]..sep..list[i+1] ··· sep..list[j]</code>. +The default value for <code>sep</code> is the empty string, +the default for <code>i</code> is 1, +and the default for <code>j</code> is <code>#list</code>. +If <code>i</code> is greater than <code>j</code>, returns the empty string. + + + + +<p> +<hr><h3><a name="pdf-table.insert"><code>table.insert (list, [pos,] value)</code></a></h3> + + +<p> +Inserts element <code>value</code> at position <code>pos</code> in <code>list</code>, +shifting up the elements +<code>list[pos], list[pos+1], ···, list[#list]</code>. +The default value for <code>pos</code> is <code>#list+1</code>, +so that a call <code>table.insert(t,x)</code> inserts <code>x</code> at the end +of list <code>t</code>. + + + + +<p> +<hr><h3><a name="pdf-table.pack"><code>table.pack (···)</code></a></h3> + + +<p> +Returns a new table with all parameters stored into keys 1, 2, etc. +and with a field "<code>n</code>" with the total number of parameters. +Note that the resulting table may not be a sequence. + + + + +<p> +<hr><h3><a name="pdf-table.remove"><code>table.remove (list [, pos])</code></a></h3> + + +<p> +Removes from <code>list</code> the element at position <code>pos</code>, +returning the value of the removed element. +When <code>pos</code> is an integer between 1 and <code>#list</code>, +it shifts down the elements +<code>list[pos+1], list[pos+2], ···, list[#list]</code> +and erases element <code>list[#list]</code>; +The index <code>pos</code> can also be 0 when <code>#list</code> is 0, +or <code>#list + 1</code>; +in those cases, the function erases the element <code>list[pos]</code>. + + +<p> +The default value for <code>pos</code> is <code>#list</code>, +so that a call <code>table.remove(t)</code> removes the last element +of list <code>t</code>. + + + + +<p> +<hr><h3><a name="pdf-table.sort"><code>table.sort (list [, comp])</code></a></h3> + + +<p> +Sorts list elements in a given order, <em>in-place</em>, +from <code>list[1]</code> to <code>list[#list]</code>. +If <code>comp</code> 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 <code>not comp(list[i+1],list[i])</code> will be true after the sort). +If <code>comp</code> is not given, +then the standard Lua operator <code><</code> is used instead. + + +<p> +The sort algorithm is not stable; +that is, elements considered equal by the given order +may have their relative positions changed by the sort. + + + + +<p> +<hr><h3><a name="pdf-table.unpack"><code>table.unpack (list [, i [, j]])</code></a></h3> + + +<p> +Returns the elements from the given table. +This function is equivalent to + +<pre> + return list[i], list[i+1], ···, list[j] +</pre><p> +By default, <code>i</code> is 1 and <code>j</code> is <code>#list</code>. + + + + + + + +<h2>6.6 – <a name="6.6">Mathematical Functions</a></h2> + +<p> +This library is an interface to the standard C math library. +It provides all its functions inside the table <a name="pdf-math"><code>math</code></a>. + + +<p> +<hr><h3><a name="pdf-math.abs"><code>math.abs (x)</code></a></h3> + + +<p> +Returns the absolute value of <code>x</code>. + + + + +<p> +<hr><h3><a name="pdf-math.acos"><code>math.acos (x)</code></a></h3> + + +<p> +Returns the arc cosine of <code>x</code> (in radians). + + + + +<p> +<hr><h3><a name="pdf-math.asin"><code>math.asin (x)</code></a></h3> + + +<p> +Returns the arc sine of <code>x</code> (in radians). + + + + +<p> +<hr><h3><a name="pdf-math.atan"><code>math.atan (x)</code></a></h3> + + +<p> +Returns the arc tangent of <code>x</code> (in radians). + + + + +<p> +<hr><h3><a name="pdf-math.atan2"><code>math.atan2 (y, x)</code></a></h3> + + +<p> +Returns the arc tangent of <code>y/x</code> (in radians), +but uses the signs of both parameters to find the +quadrant of the result. +(It also handles correctly the case of <code>x</code> being zero.) + + + + +<p> +<hr><h3><a name="pdf-math.ceil"><code>math.ceil (x)</code></a></h3> + + +<p> +Returns the smallest integer larger than or equal to <code>x</code>. + + + + +<p> +<hr><h3><a name="pdf-math.cos"><code>math.cos (x)</code></a></h3> + + +<p> +Returns the cosine of <code>x</code> (assumed to be in radians). + + + + +<p> +<hr><h3><a name="pdf-math.cosh"><code>math.cosh (x)</code></a></h3> + + +<p> +Returns the hyperbolic cosine of <code>x</code>. + + + + +<p> +<hr><h3><a name="pdf-math.deg"><code>math.deg (x)</code></a></h3> + + +<p> +Returns the angle <code>x</code> (given in radians) in degrees. + + + + +<p> +<hr><h3><a name="pdf-math.exp"><code>math.exp (x)</code></a></h3> + + +<p> +Returns the value <em>e<sup>x</sup></em>. + + + + +<p> +<hr><h3><a name="pdf-math.floor"><code>math.floor (x)</code></a></h3> + + +<p> +Returns the largest integer smaller than or equal to <code>x</code>. + + + + +<p> +<hr><h3><a name="pdf-math.fmod"><code>math.fmod (x, y)</code></a></h3> + + +<p> +Returns the remainder of the division of <code>x</code> by <code>y</code> +that rounds the quotient towards zero. + + + + +<p> +<hr><h3><a name="pdf-math.frexp"><code>math.frexp (x)</code></a></h3> + + +<p> +Returns <code>m</code> and <code>e</code> such that <em>x = m2<sup>e</sup></em>, +<code>e</code> is an integer and the absolute value of <code>m</code> is +in the range <em>[0.5, 1)</em> +(or zero when <code>x</code> is zero). + + + + +<p> +<hr><h3><a name="pdf-math.huge"><code>math.huge</code></a></h3> + + +<p> +The value <code>HUGE_VAL</code>, +a value larger than or equal to any other numerical value. + + + + +<p> +<hr><h3><a name="pdf-math.ldexp"><code>math.ldexp (m, e)</code></a></h3> + + +<p> +Returns <em>m2<sup>e</sup></em> (<code>e</code> should be an integer). + + + + +<p> +<hr><h3><a name="pdf-math.log"><code>math.log (x [, base])</code></a></h3> + + +<p> +Returns the logarithm of <code>x</code> in the given base. +The default for <code>base</code> is <em>e</em> +(so that the function returns the natural logarithm of <code>x</code>). + + + + +<p> +<hr><h3><a name="pdf-math.max"><code>math.max (x, ···)</code></a></h3> + + +<p> +Returns the maximum value among its arguments. + + + + +<p> +<hr><h3><a name="pdf-math.min"><code>math.min (x, ···)</code></a></h3> + + +<p> +Returns the minimum value among its arguments. + + + + +<p> +<hr><h3><a name="pdf-math.modf"><code>math.modf (x)</code></a></h3> + + +<p> +Returns two numbers, +the integral part of <code>x</code> and the fractional part of <code>x</code>. + + + + +<p> +<hr><h3><a name="pdf-math.pi"><code>math.pi</code></a></h3> + + +<p> +The value of <em>π</em>. + + + + +<p> +<hr><h3><a name="pdf-math.pow"><code>math.pow (x, y)</code></a></h3> + + +<p> +Returns <em>x<sup>y</sup></em>. +(You can also use the expression <code>x^y</code> to compute this value.) + + + + +<p> +<hr><h3><a name="pdf-math.rad"><code>math.rad (x)</code></a></h3> + + +<p> +Returns the angle <code>x</code> (given in degrees) in radians. + + + + +<p> +<hr><h3><a name="pdf-math.random"><code>math.random ([m [, n]])</code></a></h3> + + +<p> +This function is an interface to the simple +pseudo-random generator function <code>rand</code> provided by Standard C. +(No guarantees can be given for its statistical properties.) + + +<p> +When called without arguments, +returns a uniform pseudo-random real number +in the range <em>[0,1)</em>. +When called with an integer number <code>m</code>, +<code>math.random</code> returns +a uniform pseudo-random integer in the range <em>[1, m]</em>. +When called with two integer numbers <code>m</code> and <code>n</code>, +<code>math.random</code> returns a uniform pseudo-random +integer in the range <em>[m, n]</em>. + + + + +<p> +<hr><h3><a name="pdf-math.randomseed"><code>math.randomseed (x)</code></a></h3> + + +<p> +Sets <code>x</code> as the "seed" +for the pseudo-random generator: +equal seeds produce equal sequences of numbers. + + + + +<p> +<hr><h3><a name="pdf-math.sin"><code>math.sin (x)</code></a></h3> + + +<p> +Returns the sine of <code>x</code> (assumed to be in radians). + + + + +<p> +<hr><h3><a name="pdf-math.sinh"><code>math.sinh (x)</code></a></h3> + + +<p> +Returns the hyperbolic sine of <code>x</code>. + + + + +<p> +<hr><h3><a name="pdf-math.sqrt"><code>math.sqrt (x)</code></a></h3> + + +<p> +Returns the square root of <code>x</code>. +(You can also use the expression <code>x^0.5</code> to compute this value.) + + + + +<p> +<hr><h3><a name="pdf-math.tan"><code>math.tan (x)</code></a></h3> + + +<p> +Returns the tangent of <code>x</code> (assumed to be in radians). + + + + +<p> +<hr><h3><a name="pdf-math.tanh"><code>math.tanh (x)</code></a></h3> + + +<p> +Returns the hyperbolic tangent of <code>x</code>. + + + + + + + +<h2>6.7 – <a name="6.7">Bitwise Operations</a></h2> + +<p> +This library provides bitwise operations. +It provides all its functions inside the table <a name="pdf-bit32"><code>bit32</code></a>. + + +<p> +Unless otherwise stated, +all functions accept numeric arguments in the range +<em>(-2<sup>51</sup>,+2<sup>51</sup>)</em>; +each argument is normalized to +the remainder of its division by <em>2<sup>32</sup></em> +and truncated to an integer (in some unspecified way), +so that its final value falls in the range <em>[0,2<sup>32</sup> - 1]</em>. +Similarly, all results are in the range <em>[0,2<sup>32</sup> - 1]</em>. +Note that <code>bit32.bnot(0)</code> is <code>0xFFFFFFFF</code>, +which is different from <code>-1</code>. + + +<p> +<hr><h3><a name="pdf-bit32.arshift"><code>bit32.arshift (x, disp)</code></a></h3> + + +<p> +Returns the number <code>x</code> shifted <code>disp</code> bits to the right. +The number <code>disp</code> may be any representable integer. +Negative displacements shift to the left. + + +<p> +This shift operation is what is called arithmetic shift. +Vacant bits on the left are filled +with copies of the higher bit of <code>x</code>; +vacant bits on the right are filled with zeros. +In particular, +displacements with absolute values higher than 31 +result in zero or <code>0xFFFFFFFF</code> (all original bits are shifted out). + + + + +<p> +<hr><h3><a name="pdf-bit32.band"><code>bit32.band (···)</code></a></h3> + + +<p> +Returns the bitwise <em>and</em> of its operands. + + + + +<p> +<hr><h3><a name="pdf-bit32.bnot"><code>bit32.bnot (x)</code></a></h3> + + +<p> +Returns the bitwise negation of <code>x</code>. +For any integer <code>x</code>, +the following identity holds: + +<pre> + assert(bit32.bnot(x) == (-1 - x) % 2^32) +</pre> + + + +<p> +<hr><h3><a name="pdf-bit32.bor"><code>bit32.bor (···)</code></a></h3> + + +<p> +Returns the bitwise <em>or</em> of its operands. + + + + +<p> +<hr><h3><a name="pdf-bit32.btest"><code>bit32.btest (···)</code></a></h3> + + +<p> +Returns a boolean signaling +whether the bitwise <em>and</em> of its operands is different from zero. + + + + +<p> +<hr><h3><a name="pdf-bit32.bxor"><code>bit32.bxor (···)</code></a></h3> + + +<p> +Returns the bitwise <em>exclusive or</em> of its operands. + + + + +<p> +<hr><h3><a name="pdf-bit32.extract"><code>bit32.extract (n, field [, width])</code></a></h3> + + +<p> +Returns the unsigned number formed by the bits +<code>field</code> to <code>field + width - 1</code> from <code>n</code>. +Bits are numbered from 0 (least significant) to 31 (most significant). +All accessed bits must be in the range <em>[0, 31]</em>. + + +<p> +The default for <code>width</code> is 1. + + + + +<p> +<hr><h3><a name="pdf-bit32.replace"><code>bit32.replace (n, v, field [, width])</code></a></h3> + + +<p> +Returns a copy of <code>n</code> with +the bits <code>field</code> to <code>field + width - 1</code> +replaced by the value <code>v</code>. +See <a href="#pdf-bit32.extract"><code>bit32.extract</code></a> for details about <code>field</code> and <code>width</code>. + + + + +<p> +<hr><h3><a name="pdf-bit32.lrotate"><code>bit32.lrotate (x, disp)</code></a></h3> + + +<p> +Returns the number <code>x</code> rotated <code>disp</code> bits to the left. +The number <code>disp</code> may be any representable integer. + + +<p> +For any valid displacement, +the following identity holds: + +<pre> + assert(bit32.lrotate(x, disp) == bit32.lrotate(x, disp % 32)) +</pre><p> +In particular, +negative displacements rotate to the right. + + + + +<p> +<hr><h3><a name="pdf-bit32.lshift"><code>bit32.lshift (x, disp)</code></a></h3> + + +<p> +Returns the number <code>x</code> shifted <code>disp</code> bits to the left. +The number <code>disp</code> 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). + + +<p> +For positive displacements, +the following equality holds: + +<pre> + assert(bit32.lshift(b, disp) == (b * 2^disp) % 2^32) +</pre> + + + +<p> +<hr><h3><a name="pdf-bit32.rrotate"><code>bit32.rrotate (x, disp)</code></a></h3> + + +<p> +Returns the number <code>x</code> rotated <code>disp</code> bits to the right. +The number <code>disp</code> may be any representable integer. + + +<p> +For any valid displacement, +the following identity holds: + +<pre> + assert(bit32.rrotate(x, disp) == bit32.rrotate(x, disp % 32)) +</pre><p> +In particular, +negative displacements rotate to the left. + + + + +<p> +<hr><h3><a name="pdf-bit32.rshift"><code>bit32.rshift (x, disp)</code></a></h3> + + +<p> +Returns the number <code>x</code> shifted <code>disp</code> bits to the right. +The number <code>disp</code> 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). + + +<p> +For positive displacements, +the following equality holds: + +<pre> + assert(bit32.rshift(b, disp) == math.floor(b % 2^32 / 2^disp)) +</pre> + +<p> +This shift operation is what is called logical shift. + + + + + + + +<h2>6.8 – <a name="6.8">Input and Output Facilities</a></h2> + +<p> +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. + + +<p> +When using implicit file descriptors, +all operations are supplied by table <a name="pdf-io"><code>io</code></a>. +When using explicit file descriptors, +the operation <a href="#pdf-io.open"><code>io.open</code></a> returns a file descriptor +and then all operations are supplied as methods of the file descriptor. + + +<p> +The table <code>io</code> also provides +three predefined file descriptors with their usual meanings from C: +<a name="pdf-io.stdin"><code>io.stdin</code></a>, <a name="pdf-io.stdout"><code>io.stdout</code></a>, and <a name="pdf-io.stderr"><code>io.stderr</code></a>. +The I/O library never closes these files. + + +<p> +Unless otherwise stated, +all I/O functions return <b>nil</b> 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 <b>nil</b> 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 <code>errno</code>. + + +<p> +<hr><h3><a name="pdf-io.close"><code>io.close ([file])</code></a></h3> + + +<p> +Equivalent to <code>file:close()</code>. +Without a <code>file</code>, closes the default output file. + + + + +<p> +<hr><h3><a name="pdf-io.flush"><code>io.flush ()</code></a></h3> + + +<p> +Equivalent to <code>io.output():flush()</code>. + + + + +<p> +<hr><h3><a name="pdf-io.input"><code>io.input ([file])</code></a></h3> + + +<p> +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. + + +<p> +In case of errors this function raises the error, +instead of returning an error code. + + + + +<p> +<hr><h3><a name="pdf-io.lines"><code>io.lines ([filename ···])</code></a></h3> + + +<p> +Opens the given file name in read mode +and returns an iterator function that +works like <code>file:lines(···)</code> over the opened file. +When the iterator function detects the end of file, +it returns <b>nil</b> (to finish the loop) and automatically closes the file. + + +<p> +The call <code>io.lines()</code> (with no file name) is equivalent +to <code>io.input():lines()</code>; +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. + + +<p> +In case of errors this function raises the error, +instead of returning an error code. + + + + +<p> +<hr><h3><a name="pdf-io.open"><code>io.open (filename [, mode])</code></a></h3> + + +<p> +This function opens a file, +in the mode specified in the string <code>mode</code>. +It returns a new file handle, +or, in case of errors, <b>nil</b> plus an error message. + + +<p> +The <code>mode</code> string can be any of the following: + +<ul> +<li><b>"<code>r</code>": </b> read mode (the default);</li> +<li><b>"<code>w</code>": </b> write mode;</li> +<li><b>"<code>a</code>": </b> append mode;</li> +<li><b>"<code>r+</code>": </b> update mode, all previous data is preserved;</li> +<li><b>"<code>w+</code>": </b> update mode, all previous data is erased;</li> +<li><b>"<code>a+</code>": </b> append update mode, previous data is preserved, + writing is only allowed at the end of file.</li> +</ul><p> +The <code>mode</code> string can also have a '<code>b</code>' at the end, +which is needed in some systems to open the file in binary mode. + + + + +<p> +<hr><h3><a name="pdf-io.output"><code>io.output ([file])</code></a></h3> + + +<p> +Similar to <a href="#pdf-io.input"><code>io.input</code></a>, but operates over the default output file. + + + + +<p> +<hr><h3><a name="pdf-io.popen"><code>io.popen (prog [, mode])</code></a></h3> + + +<p> +This function is system dependent and is not available +on all platforms. + + +<p> +Starts program <code>prog</code> in a separated process and returns +a file handle that you can use to read data from this program +(if <code>mode</code> is <code>"r"</code>, the default) +or to write data to this program +(if <code>mode</code> is <code>"w"</code>). + + + + +<p> +<hr><h3><a name="pdf-io.read"><code>io.read (···)</code></a></h3> + + +<p> +Equivalent to <code>io.input():read(···)</code>. + + + + +<p> +<hr><h3><a name="pdf-io.tmpfile"><code>io.tmpfile ()</code></a></h3> + + +<p> +Returns a handle for a temporary file. +This file is opened in update mode +and it is automatically removed when the program ends. + + + + +<p> +<hr><h3><a name="pdf-io.type"><code>io.type (obj)</code></a></h3> + + +<p> +Checks whether <code>obj</code> is a valid file handle. +Returns the string <code>"file"</code> if <code>obj</code> is an open file handle, +<code>"closed file"</code> if <code>obj</code> is a closed file handle, +or <b>nil</b> if <code>obj</code> is not a file handle. + + + + +<p> +<hr><h3><a name="pdf-io.write"><code>io.write (···)</code></a></h3> + + +<p> +Equivalent to <code>io.output():write(···)</code>. + + + + +<p> +<hr><h3><a name="pdf-file:close"><code>file:close ()</code></a></h3> + + +<p> +Closes <code>file</code>. +Note that files are automatically closed when +their handles are garbage collected, +but that takes an unpredictable amount of time to happen. + + +<p> +When closing a file handle created with <a href="#pdf-io.popen"><code>io.popen</code></a>, +<a href="#pdf-file:close"><code>file:close</code></a> returns the same values +returned by <a href="#pdf-os.execute"><code>os.execute</code></a>. + + + + +<p> +<hr><h3><a name="pdf-file:flush"><code>file:flush ()</code></a></h3> + + +<p> +Saves any written data to <code>file</code>. + + + + +<p> +<hr><h3><a name="pdf-file:lines"><code>file:lines (···)</code></a></h3> + + +<p> +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 + +<pre> + for c in file:lines(1) do <em>body</em> end +</pre><p> +will iterate over all characters of the file, +starting at the current position. +Unlike <a href="#pdf-io.lines"><code>io.lines</code></a>, this function does not close the file +when the loop ends. + + +<p> +In case of errors this function raises the error, +instead of returning an error code. + + + + +<p> +<hr><h3><a name="pdf-file:read"><code>file:read (···)</code></a></h3> + + +<p> +Reads the file <code>file</code>, +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 <b>nil</b> 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). + + +<p> +The available formats are + +<ul> + +<li><b>"<code>*n</code>": </b> +reads a number; +this is the only format that returns a number instead of a string. +</li> + +<li><b>"<code>*a</code>": </b> +reads the whole file, starting at the current position. +On end of file, it returns the empty string. +</li> + +<li><b>"<code>*l</code>": </b> +reads the next line skipping the end of line, +returning <b>nil</b> on end of file. +This is the default format. +</li> + +<li><b>"<code>*L</code>": </b> +reads the next line keeping the end of line (if present), +returning <b>nil</b> on end of file. +</li> + +<li><b><em>number</em>: </b> +reads a string with up to this number of bytes, +returning <b>nil</b> on end of file. +If number is zero, +it reads nothing and returns an empty string, +or <b>nil</b> on end of file. +</li> + +</ul> + + + +<p> +<hr><h3><a name="pdf-file:seek"><code>file:seek ([whence [, offset]])</code></a></h3> + + +<p> +Sets and gets the file position, +measured from the beginning of the file, +to the position given by <code>offset</code> plus a base +specified by the string <code>whence</code>, as follows: + +<ul> +<li><b>"<code>set</code>": </b> base is position 0 (beginning of the file);</li> +<li><b>"<code>cur</code>": </b> base is current position;</li> +<li><b>"<code>end</code>": </b> base is end of file;</li> +</ul><p> +In case of success, <code>seek</code> returns the final file position, +measured in bytes from the beginning of the file. +If <code>seek</code> fails, it returns <b>nil</b>, +plus a string describing the error. + + +<p> +The default value for <code>whence</code> is <code>"cur"</code>, +and for <code>offset</code> is 0. +Therefore, the call <code>file:seek()</code> returns the current +file position, without changing it; +the call <code>file:seek("set")</code> sets the position to the +beginning of the file (and returns 0); +and the call <code>file:seek("end")</code> sets the position to the +end of the file, and returns its size. + + + + +<p> +<hr><h3><a name="pdf-file:setvbuf"><code>file:setvbuf (mode [, size])</code></a></h3> + + +<p> +Sets the buffering mode for an output file. +There are three available modes: + +<ul> + +<li><b>"<code>no</code>": </b> +no buffering; the result of any output operation appears immediately. +</li> + +<li><b>"<code>full</code>": </b> +full buffering; output operation is performed only +when the buffer is full or when +you explicitly <code>flush</code> the file (see <a href="#pdf-io.flush"><code>io.flush</code></a>). +</li> + +<li><b>"<code>line</code>": </b> +line buffering; output is buffered until a newline is output +or there is any input from some special files +(such as a terminal device). +</li> + +</ul><p> +For the last two cases, <code>size</code> +specifies the size of the buffer, in bytes. +The default is an appropriate size. + + + + +<p> +<hr><h3><a name="pdf-file:write"><code>file:write (···)</code></a></h3> + + +<p> +Writes the value of each of its arguments to <code>file</code>. +The arguments must be strings or numbers. + + +<p> +In case of success, this function returns <code>file</code>. +Otherwise it returns <b>nil</b> plus a string describing the error. + + + + + + + +<h2>6.9 – <a name="6.9">Operating System Facilities</a></h2> + +<p> +This library is implemented through table <a name="pdf-os"><code>os</code></a>. + + +<p> +<hr><h3><a name="pdf-os.clock"><code>os.clock ()</code></a></h3> + + +<p> +Returns an approximation of the amount in seconds of CPU time +used by the program. + + + + +<p> +<hr><h3><a name="pdf-os.date"><code>os.date ([format [, time]])</code></a></h3> + + +<p> +Returns a string or a table containing date and time, +formatted according to the given string <code>format</code>. + + +<p> +If the <code>time</code> argument is present, +this is the time to be formatted +(see the <a href="#pdf-os.time"><code>os.time</code></a> function for a description of this value). +Otherwise, <code>date</code> formats the current time. + + +<p> +If <code>format</code> starts with '<code>!</code>', +then the date is formatted in Coordinated Universal Time. +After this optional character, +if <code>format</code> is the string "<code>*t</code>", +then <code>date</code> returns a table with the following fields: +<code>year</code> (four digits), <code>month</code> (1–12), <code>day</code> (1–31), +<code>hour</code> (0–23), <code>min</code> (0–59), <code>sec</code> (0–61), +<code>wday</code> (weekday, Sunday is 1), +<code>yday</code> (day of the year), +and <code>isdst</code> (daylight saving flag, a boolean). +This last field may be absent +if the information is not available. + + +<p> +If <code>format</code> is not "<code>*t</code>", +then <code>date</code> returns the date as a string, +formatted according to the same rules as the ANSI C function <code>strftime</code>. + + +<p> +When called without arguments, +<code>date</code> returns a reasonable date and time representation that depends on +the host system and on the current locale +(that is, <code>os.date()</code> is equivalent to <code>os.date("%c")</code>). + + +<p> +On non-Posix systems, +this function may be not thread safe +because of its reliance on C function <code>gmtime</code> and C function <code>localtime</code>. + + + + +<p> +<hr><h3><a name="pdf-os.difftime"><code>os.difftime (t2, t1)</code></a></h3> + + +<p> +Returns the number of seconds from time <code>t1</code> to time <code>t2</code>. +In POSIX, Windows, and some other systems, +this value is exactly <code>t2</code><em>-</em><code>t1</code>. + + + + +<p> +<hr><h3><a name="pdf-os.execute"><code>os.execute ([command])</code></a></h3> + + +<p> +This function is equivalent to the ANSI C function <code>system</code>. +It passes <code>command</code> to be executed by an operating system shell. +Its first result is <b>true</b> +if the command terminated successfully, +or <b>nil</b> otherwise. +After this first result +the function returns a string and a number, +as follows: + +<ul> + +<li><b>"<code>exit</code>": </b> +the command terminated normally; +the following number is the exit status of the command. +</li> + +<li><b>"<code>signal</code>": </b> +the command was terminated by a signal; +the following number is the signal that terminated the command. +</li> + +</ul> + +<p> +When called without a <code>command</code>, +<code>os.execute</code> returns a boolean that is true if a shell is available. + + + + +<p> +<hr><h3><a name="pdf-os.exit"><code>os.exit ([code [, close])</code></a></h3> + + +<p> +Calls the ANSI C function <code>exit</code> to terminate the host program. +If <code>code</code> is <b>true</b>, +the returned status is <code>EXIT_SUCCESS</code>; +if <code>code</code> is <b>false</b>, +the returned status is <code>EXIT_FAILURE</code>; +if <code>code</code> is a number, +the returned status is this number. +The default value for <code>code</code> is <b>true</b>. + + +<p> +If the optional second argument <code>close</code> is true, +closes the Lua state before exiting. + + + + +<p> +<hr><h3><a name="pdf-os.getenv"><code>os.getenv (varname)</code></a></h3> + + +<p> +Returns the value of the process environment variable <code>varname</code>, +or <b>nil</b> if the variable is not defined. + + + + +<p> +<hr><h3><a name="pdf-os.remove"><code>os.remove (filename)</code></a></h3> + + +<p> +Deletes the file (or empty directory, on POSIX systems) +with the given name. +If this function fails, it returns <b>nil</b>, +plus a string describing the error and the error code. + + + + +<p> +<hr><h3><a name="pdf-os.rename"><code>os.rename (oldname, newname)</code></a></h3> + + +<p> +Renames file or directory named <code>oldname</code> to <code>newname</code>. +If this function fails, it returns <b>nil</b>, +plus a string describing the error and the error code. + + + + +<p> +<hr><h3><a name="pdf-os.setlocale"><code>os.setlocale (locale [, category])</code></a></h3> + + +<p> +Sets the current locale of the program. +<code>locale</code> is a system-dependent string specifying a locale; +<code>category</code> is an optional string describing which category to change: +<code>"all"</code>, <code>"collate"</code>, <code>"ctype"</code>, +<code>"monetary"</code>, <code>"numeric"</code>, or <code>"time"</code>; +the default category is <code>"all"</code>. +The function returns the name of the new locale, +or <b>nil</b> if the request cannot be honored. + + +<p> +If <code>locale</code> is the empty string, +the current locale is set to an implementation-defined native locale. +If <code>locale</code> is the string "<code>C</code>", +the current locale is set to the standard C locale. + + +<p> +When called with <b>nil</b> as the first argument, +this function only returns the name of the current locale +for the given category. + + +<p> +This function may be not thread safe +because of its reliance on C function <code>setlocale</code>. + + + + +<p> +<hr><h3><a name="pdf-os.time"><code>os.time ([table])</code></a></h3> + + +<p> +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 <code>year</code>, <code>month</code>, and <code>day</code>, +and may have fields +<code>hour</code> (default is 12), +<code>min</code> (default is 0), +<code>sec</code> (default is 0), +and <code>isdst</code> (default is <b>nil</b>). +For a description of these fields, see the <a href="#pdf-os.date"><code>os.date</code></a> function. + + +<p> +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 <code>time</code> can be used only as an argument to +<a href="#pdf-os.date"><code>os.date</code></a> and <a href="#pdf-os.difftime"><code>os.difftime</code></a>. + + + + +<p> +<hr><h3><a name="pdf-os.tmpname"><code>os.tmpname ()</code></a></h3> + + +<p> +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. + + +<p> +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). + + +<p> +When possible, +you may prefer to use <a href="#pdf-io.tmpfile"><code>io.tmpfile</code></a>, +which automatically removes the file when the program ends. + + + + + + + +<h2>6.10 – <a name="6.10">The Debug Library</a></h2> + +<p> +This library provides +the functionality of the debug interface (<a href="#4.9">§4.9</a>) 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. + + +<p> +All functions in this library are provided +inside the <a name="pdf-debug"><code>debug</code></a> 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. + + +<p> +<hr><h3><a name="pdf-debug.debug"><code>debug.debug ()</code></a></h3> + + +<p> +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 <code>cont</code> finishes this function, +so that the caller continues its execution. + + +<p> +Note that commands for <code>debug.debug</code> are not lexically nested +within any function and so have no direct access to local variables. + + + + +<p> +<hr><h3><a name="pdf-debug.gethook"><code>debug.gethook ([thread])</code></a></h3> + + +<p> +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 <a href="#pdf-debug.sethook"><code>debug.sethook</code></a> function). + + + + +<p> +<hr><h3><a name="pdf-debug.getinfo"><code>debug.getinfo ([thread,] f [, what])</code></a></h3> + + +<p> +Returns a table with information about a function. +You can give the function directly +or you can give a number as the value of <code>f</code>, +which means the function running at level <code>f</code> of the call stack +of the given thread: +level 0 is the current function (<code>getinfo</code> itself); +level 1 is the function that called <code>getinfo</code> +(except for tail calls, which do not count on the stack); +and so on. +If <code>f</code> is a number larger than the number of active functions, +then <code>getinfo</code> returns <b>nil</b>. + + +<p> +The returned table can contain all the fields returned by <a href="#lua_getinfo"><code>lua_getinfo</code></a>, +with the string <code>what</code> describing which fields to fill in. +The default for <code>what</code> is to get all information available, +except the table of valid lines. +If present, +the option '<code>f</code>' +adds a field named <code>func</code> with the function itself. +If present, +the option '<code>L</code>' +adds a field named <code>activelines</code> with the table of +valid lines. + + +<p> +For instance, the expression <code>debug.getinfo(1,"n").name</code> returns +a table with a name for the current function, +if a reasonable name can be found, +and the expression <code>debug.getinfo(print)</code> +returns a table with all available information +about the <a href="#pdf-print"><code>print</code></a> function. + + + + +<p> +<hr><h3><a name="pdf-debug.getlocal"><code>debug.getlocal ([thread,] f, local)</code></a></h3> + + +<p> +This function returns the name and the value of the local variable +with index <code>local</code> of the function at level <code>f</code> of the stack. +This function accesses not only explicit local variables, +but also parameters, temporaries, etc. + + +<p> +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 <b>nil</b> if there is no variable with the given index, +and raises an error when called with a level out of range. +(You can call <a href="#pdf-debug.getinfo"><code>debug.getinfo</code></a> to check whether the level is valid.) + + +<p> +Variable names starting with '<code>(</code>' (open parenthesis) +represent internal variables +(loop control variables, temporaries, varargs, and C function locals). + + +<p> +The parameter <code>f</code> may also be a function. +In that case, <code>getlocal</code> returns only the name of function parameters. + + + + +<p> +<hr><h3><a name="pdf-debug.getmetatable"><code>debug.getmetatable (value)</code></a></h3> + + +<p> +Returns the metatable of the given <code>value</code> +or <b>nil</b> if it does not have a metatable. + + + + +<p> +<hr><h3><a name="pdf-debug.getregistry"><code>debug.getregistry ()</code></a></h3> + + +<p> +Returns the registry table (see <a href="#4.5">§4.5</a>). + + + + +<p> +<hr><h3><a name="pdf-debug.getupvalue"><code>debug.getupvalue (f, up)</code></a></h3> + + +<p> +This function returns the name and the value of the upvalue +with index <code>up</code> of the function <code>f</code>. +The function returns <b>nil</b> if there is no upvalue with the given index. + + + + +<p> +<hr><h3><a name="pdf-debug.getuservalue"><code>debug.getuservalue (u)</code></a></h3> + + +<p> +Returns the Lua value associated to <code>u</code>. +If <code>u</code> is not a userdata, +returns <b>nil</b>. + + + + +<p> +<hr><h3><a name="pdf-debug.sethook"><code>debug.sethook ([thread,] hook, mask [, count])</code></a></h3> + + +<p> +Sets the given function as a hook. +The string <code>mask</code> and the number <code>count</code> describe +when the hook will be called. +The string mask may have the following characters, +with the given meaning: + +<ul> +<li><b>'<code>c</code>': </b> the hook is called every time Lua calls a function;</li> +<li><b>'<code>r</code>': </b> the hook is called every time Lua returns from a function;</li> +<li><b>'<code>l</code>': </b> the hook is called every time Lua enters a new line of code.</li> +</ul><p> +With a <code>count</code> different from zero, +the hook is called after every <code>count</code> instructions. + + +<p> +When called without arguments, +<a href="#pdf-debug.sethook"><code>debug.sethook</code></a> turns off the hook. + + +<p> +When the hook is called, its first parameter is a string +describing the event that has triggered its call: +<code>"call"</code> (or <code>"tail call"</code>), +<code>"return"</code>, +<code>"line"</code>, and <code>"count"</code>. +For line events, +the hook also gets the new line number as its second parameter. +Inside a hook, +you can call <code>getinfo</code> with level 2 to get more information about +the running function +(level 0 is the <code>getinfo</code> function, +and level 1 is the hook function). + + + + +<p> +<hr><h3><a name="pdf-debug.setlocal"><code>debug.setlocal ([thread,] level, local, value)</code></a></h3> + + +<p> +This function assigns the value <code>value</code> to the local variable +with index <code>local</code> of the function at level <code>level</code> of the stack. +The function returns <b>nil</b> if there is no local +variable with the given index, +and raises an error when called with a <code>level</code> out of range. +(You can call <code>getinfo</code> to check whether the level is valid.) +Otherwise, it returns the name of the local variable. + + +<p> +See <a href="#pdf-debug.getlocal"><code>debug.getlocal</code></a> for more information about +variable indices and names. + + + + +<p> +<hr><h3><a name="pdf-debug.setmetatable"><code>debug.setmetatable (value, table)</code></a></h3> + + +<p> +Sets the metatable for the given <code>value</code> to the given <code>table</code> +(which can be <b>nil</b>). +Returns <code>value</code>. + + + + +<p> +<hr><h3><a name="pdf-debug.setupvalue"><code>debug.setupvalue (f, up, value)</code></a></h3> + + +<p> +This function assigns the value <code>value</code> to the upvalue +with index <code>up</code> of the function <code>f</code>. +The function returns <b>nil</b> if there is no upvalue +with the given index. +Otherwise, it returns the name of the upvalue. + + + + +<p> +<hr><h3><a name="pdf-debug.setuservalue"><code>debug.setuservalue (udata, value)</code></a></h3> + + +<p> +Sets the given <code>value</code> as +the Lua value associated to the given <code>udata</code>. +<code>value</code> must be a table or <b>nil</b>; +<code>udata</code> must be a full userdata. + + +<p> +Returns <code>udata</code>. + + + + +<p> +<hr><h3><a name="pdf-debug.traceback"><code>debug.traceback ([thread,] [message [, level]])</code></a></h3> + + +<p> +If <code>message</code> is present but is neither a string nor <b>nil</b>, +this function returns <code>message</code> without further processing. +Otherwise, +it returns a string with a traceback of the call stack. +An optional <code>message</code> string is appended +at the beginning of the traceback. +An optional <code>level</code> number tells at which level +to start the traceback +(default is 1, the function calling <code>traceback</code>). + + + + +<p> +<hr><h3><a name="pdf-debug.upvalueid"><code>debug.upvalueid (f, n)</code></a></h3> + + +<p> +Returns an unique identifier (as a light userdata) +for the upvalue numbered <code>n</code> +from the given function. + + +<p> +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. + + + + +<p> +<hr><h3><a name="pdf-debug.upvaluejoin"><code>debug.upvaluejoin (f1, n1, f2, n2)</code></a></h3> + + +<p> +Make the <code>n1</code>-th upvalue of the Lua closure <code>f1</code> +refer to the <code>n2</code>-th upvalue of the Lua closure <code>f2</code>. + + + + + + + +<h1>7 – <a name="7">Lua Standalone</a></h1> + +<p> +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 <code>lua</code>, +is provided with the standard distribution. +The standalone interpreter includes +all standard libraries, including the debug library. +Its usage is: + +<pre> + lua [options] [script [args]] +</pre><p> +The options are: + +<ul> +<li><b><code>-e <em>stat</em></code>: </b> executes string <em>stat</em>;</li> +<li><b><code>-l <em>mod</em></code>: </b> "requires" <em>mod</em>;</li> +<li><b><code>-i</code>: </b> enters interactive mode after running <em>script</em>;</li> +<li><b><code>-v</code>: </b> prints version information;</li> +<li><b><code>-E</code>: </b> ignores environment variables;</li> +<li><b><code>--</code>: </b> stops handling options;</li> +<li><b><code>-</code>: </b> executes <code>stdin</code> as a file and stops handling options.</li> +</ul><p> +After handling its options, <code>lua</code> runs the given <em>script</em>, +passing to it the given <em>args</em> as string arguments. +When called without arguments, +<code>lua</code> behaves as <code>lua -v -i</code> +when the standard input (<code>stdin</code>) is a terminal, +and as <code>lua -</code> otherwise. + + +<p> +When called without option <code>-E</code>, +the interpreter checks for an environment variable <a name="pdf-LUA_INIT_5_2"><code>LUA_INIT_5_2</code></a> +(or <a name="pdf-LUA_INIT"><code>LUA_INIT</code></a> if it is not defined) +before running any argument. +If the variable content has the format <code>@<em>filename</em></code>, +then <code>lua</code> executes the file. +Otherwise, <code>lua</code> executes the string itself. + + +<p> +When called with option <code>-E</code>, +besides ignoring <code>LUA_INIT</code>, +Lua also ignores +the values of <code>LUA_PATH</code> and <code>LUA_CPATH</code>, +setting the values of +<a href="#pdf-package.path"><code>package.path</code></a> and <a href="#pdf-package.cpath"><code>package.cpath</code></a> +with the default paths defined in <code>luaconf.h</code>. + + +<p> +All options are handled in order, except <code>-i</code> and <code>-E</code>. +For instance, an invocation like + +<pre> + $ lua -e'a=1' -e 'print(a)' script.lua +</pre><p> +will first set <code>a</code> to 1, then print the value of <code>a</code>, +and finally run the file <code>script.lua</code> with no arguments. +(Here <code>$</code> is the shell prompt. Your prompt may be different.) + + +<p> +Before starting to run the script, +<code>lua</code> collects all arguments in the command line +in a global table called <code>arg</code>. +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 + +<pre> + $ lua -la b.lua t1 t2 +</pre><p> +the interpreter first runs the file <code>a.lua</code>, +then creates a table + +<pre> + arg = { [-2] = "lua", [-1] = "-la", + [0] = "b.lua", + [1] = "t1", [2] = "t2" } +</pre><p> +and finally runs the file <code>b.lua</code>. +The script is called with <code>arg[1]</code>, <code>arg[2]</code>, ... +as arguments; +it can also access these arguments with the vararg expression '<code>...</code>'. + + +<p> +In interactive mode, +if you write an incomplete statement, +the interpreter waits for its completion +by issuing a different prompt. + + +<p> +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 <code>__tostring</code>, +the interpreter calls this metamethod to produce the final message. +Finally, if the error object is <b>nil</b>, +the interpreter does not report the error. + + +<p> +When finishing normally, +the interpreter closes its main Lua state +(see <a href="#lua_close"><code>lua_close</code></a>). +The script can avoid this step by +calling <a href="#pdf-os.exit"><code>os.exit</code></a> to terminate. + + +<p> +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 <code>#</code>. +Therefore, Lua scripts can be made into executable programs +by using <code>chmod +x</code> and the <code>#!</code> form, +as in + +<pre> + #!/usr/local/bin/lua +</pre><p> +(Of course, +the location of the Lua interpreter may be different in your machine. +If <code>lua</code> is in your <code>PATH</code>, +then + +<pre> + #!/usr/bin/env lua +</pre><p> +is a more portable solution.) + + + +<h1>8 – <a name="8">Incompatibilities with the Previous Version</a></h1> + +<p> +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 file <code>luaconf.h</code>). +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. + + + +<h2>8.1 – <a name="8.1">Changes in the Language</a></h2> +<ul> + +<li> +The concept of <em>environment</em> changed. +Only Lua functions have environments. +To set the environment of a Lua function, +use the variable <code>_ENV</code> or the function <a href="#pdf-load"><code>load</code></a>. + + +<p> +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 <a href="#luaL_setfuncs"><code>luaL_setfuncs</code></a> to open a C library +with all functions sharing a common upvalue.) + + +<p> +To manipulate the "environment" of a userdata +(which is now called user value), +use the new functions +<a href="#lua_getuservalue"><code>lua_getuservalue</code></a> and <a href="#lua_setuservalue"><code>lua_setuservalue</code></a>. +</li> + +<li> +Lua identifiers cannot use locale-dependent letters. +</li> + +<li> +Doing a step or a full collection in the garbage collector +does not restart the collector if it has been stopped. +</li> + +<li> +Weak tables with weak keys now perform like <em>ephemeron tables</em>. +</li> + +<li> +The event <em>tail return</em> in debug hooks was removed. +Instead, tail calls generate a special new event, +<em>tail call</em>, so that the debugger can know that +there will not be a corresponding return event. +</li> + +<li> +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. +</li> + +</ul> + + + + +<h2>8.2 – <a name="8.2">Changes in the Libraries</a></h2> +<ul> + +<li> +Function <code>module</code> is deprecated. +It is easy to set up a module with regular Lua code. +Modules are not expected to set global variables. +</li> + +<li> +Functions <code>setfenv</code> and <code>getfenv</code> were removed, +because of the changes in environments. +</li> + +<li> +Function <code>math.log10</code> is deprecated. +Use <a href="#pdf-math.log"><code>math.log</code></a> with 10 as its second argument, instead. +</li> + +<li> +Function <code>loadstring</code> is deprecated. +Use <code>load</code> instead; it now accepts string arguments +and are exactly equivalent to <code>loadstring</code>. +</li> + +<li> +Function <code>table.maxn</code> is deprecated. +Write it in Lua if you really need it. +</li> + +<li> +Function <code>os.execute</code> now returns <b>true</b> when command +terminates successfully and <b>nil</b> plus error information +otherwise. +</li> + +<li> +Function <code>unpack</code> was moved into the table library +and therefore must be called as <a href="#pdf-table.unpack"><code>table.unpack</code></a>. +</li> + +<li> +Character class <code>%z</code> in patterns is deprecated, +as now patterns may contain '<code>\0</code>' as a regular character. +</li> + +<li> +The table <code>package.loaders</code> was renamed <code>package.searchers</code>. +</li> + +<li> +Lua does not have bytecode verification anymore. +So, all functions that load code +(<a href="#pdf-load"><code>load</code></a> and <a href="#pdf-loadfile"><code>loadfile</code></a>) +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 the <code>mode</code> argument of those functions +to restrict them to loading textual chunks. +</li> + +<li> +The standard paths in the official distribution may +change between versions. +</li> + +</ul> + + + + +<h2>8.3 – <a name="8.3">Changes in the API</a></h2> +<ul> + +<li> +Pseudoindex <code>LUA_GLOBALSINDEX</code> was removed. +You must get the global environment from the registry +(see <a href="#4.5">§4.5</a>). +</li> + +<li> +Pseudoindex <code>LUA_ENVIRONINDEX</code> +and functions <code>lua_getfenv</code>/<code>lua_setfenv</code> +were removed, +as C functions no longer have environments. +</li> + +<li> +Function <code>luaL_register</code> is deprecated. +Use <a href="#luaL_setfuncs"><code>luaL_setfuncs</code></a> so that your module does not create globals. +(Modules are not expected to set global variables anymore.) +</li> + +<li> +The <code>osize</code> argument to the allocation function +may not be zero when creating a new block, +that is, when <code>ptr</code> is <code>NULL</code> +(see <a href="#lua_Alloc"><code>lua_Alloc</code></a>). +Use only the test <code>ptr == NULL</code> to check whether +the block is new. +</li> + +<li> +Finalizers (<code>__gc</code> metamethods) for userdata are called in the +reverse order that they were marked for finalization, +not that they were created (see <a href="#2.5.1">§2.5.1</a>). +(Most userdata are marked immediately after they are created.) +Moreover, +if the metatable does not have a <code>__gc</code> field when set, +the finalizer will not be called, +even if it is set later. +</li> + +<li> +<code>luaL_typerror</code> was removed. +Write your own version if you need it. +</li> + +<li> +Function <code>lua_cpcall</code> is deprecated. +You can simply push the function with <a href="#lua_pushcfunction"><code>lua_pushcfunction</code></a> +and call it with <a href="#lua_pcall"><code>lua_pcall</code></a>. +</li> + +<li> +Functions <code>lua_equal</code> and <code>lua_lessthan</code> are deprecated. +Use the new <a href="#lua_compare"><code>lua_compare</code></a> with appropriate options instead. +</li> + +<li> +Function <code>lua_objlen</code> was renamed <a href="#lua_rawlen"><code>lua_rawlen</code></a>. +</li> + +<li> +Function <a href="#lua_load"><code>lua_load</code></a> has an extra parameter, <code>mode</code>. +Pass <code>NULL</code> to simulate the old behavior. +</li> + +<li> +Function <a href="#lua_resume"><code>lua_resume</code></a> has an extra parameter, <code>from</code>. +Pass <code>NULL</code> or the thread doing the call. +</li> + +</ul> + + + + +<h1>9 – <a name="9">The Complete Syntax of Lua</a></h1> + +<p> +Here is the complete syntax of Lua in extended BNF. +(It does not describe operator precedences.) + + + + +<pre> + + chunk ::= block + + block ::= {stat} [retstat] + + stat ::= ‘<b>;</b>’ | + varlist ‘<b>=</b>’ explist | + functioncall | + label | + <b>break</b> | + <b>goto</b> Name | + <b>do</b> block <b>end</b> | + <b>while</b> exp <b>do</b> block <b>end</b> | + <b>repeat</b> block <b>until</b> exp | + <b>if</b> exp <b>then</b> block {<b>elseif</b> exp <b>then</b> block} [<b>else</b> block] <b>end</b> | + <b>for</b> Name ‘<b>=</b>’ exp ‘<b>,</b>’ exp [‘<b>,</b>’ exp] <b>do</b> block <b>end</b> | + <b>for</b> namelist <b>in</b> explist <b>do</b> block <b>end</b> | + <b>function</b> funcname funcbody | + <b>local</b> <b>function</b> Name funcbody | + <b>local</b> namelist [‘<b>=</b>’ explist] + + retstat ::= <b>return</b> [explist] [‘<b>;</b>’] + + label ::= ‘<b>::</b>’ Name ‘<b>::</b>’ + + funcname ::= Name {‘<b>.</b>’ Name} [‘<b>:</b>’ Name] + + varlist ::= var {‘<b>,</b>’ var} + + var ::= Name | prefixexp ‘<b>[</b>’ exp ‘<b>]</b>’ | prefixexp ‘<b>.</b>’ Name + + namelist ::= Name {‘<b>,</b>’ Name} + + explist ::= exp {‘<b>,</b>’ exp} + + exp ::= <b>nil</b> | <b>false</b> | <b>true</b> | Number | String | ‘<b>...</b>’ | functiondef | + prefixexp | tableconstructor | exp binop exp | unop exp + + prefixexp ::= var | functioncall | ‘<b>(</b>’ exp ‘<b>)</b>’ + + functioncall ::= prefixexp args | prefixexp ‘<b>:</b>’ Name args + + args ::= ‘<b>(</b>’ [explist] ‘<b>)</b>’ | tableconstructor | String + + functiondef ::= <b>function</b> funcbody + + funcbody ::= ‘<b>(</b>’ [parlist] ‘<b>)</b>’ block <b>end</b> + + parlist ::= namelist [‘<b>,</b>’ ‘<b>...</b>’] | ‘<b>...</b>’ + + tableconstructor ::= ‘<b>{</b>’ [fieldlist] ‘<b>}</b>’ + + fieldlist ::= field {fieldsep field} [fieldsep] + + field ::= ‘<b>[</b>’ exp ‘<b>]</b>’ ‘<b>=</b>’ exp | Name ‘<b>=</b>’ exp | exp + + fieldsep ::= ‘<b>,</b>’ | ‘<b>;</b>’ + + binop ::= ‘<b>+</b>’ | ‘<b>-</b>’ | ‘<b>*</b>’ | ‘<b>/</b>’ | ‘<b>^</b>’ | ‘<b>%</b>’ | ‘<b>..</b>’ | + ‘<b><</b>’ | ‘<b><=</b>’ | ‘<b>></b>’ | ‘<b>>=</b>’ | ‘<b>==</b>’ | ‘<b>~=</b>’ | + <b>and</b> | <b>or</b> + + unop ::= ‘<b>-</b>’ | <b>not</b> | ‘<b>#</b>’ + +</pre> + +<p> + + + + + + + +<HR> +<SMALL CLASS="footer"> +Last update: +Thu Mar 21 13:01:53 BRT 2013 +</SMALL> +<!-- +Last change: revised for Lua 5.2.2 +--> + +</body></html> +