Warning: this is a htmlized version! The original is across this link; it may have some strange characters.
```(find-es "lua5" "lua50ref.e")

(progn
(fooi "e" "é" "c" "ç" "a" "ã" "o" "õ" "o" "ó"
"a" "à" "a" "â" "" "í" "a" "á")
(fooi "Di#erent" "Different" "di#erent" "different" "o#cial" "official"
"e#ects" "effects" "e#ectively" "effectively" "a#ecting" "affecting"
" # " " -> " "a#ect" "affect" "e#cient" "efficient" "o#ers" "offers"
"o#set" "offset" "di#erence" "difference" "turns o# " "turns off "
"su#x" "suffix" "o#er" "offer")
(fooi "+ \n" "+ -\n" "MERCHANTABIL\nITY" "MERCHANTABILITY"
"proce\ndural" "procedural" "auto\nmatically" "automatically"
"con\ncatenation" "concatenation" "discus\nsion" "discussion"
"con\nventional" "conventional" "assign\nments" "assignments"
"state\nment" "statement" "arith\nmetic" "arithmetic"
"appro\npriate" "appropriate" "garbage\ncollection"
"garbage-collection" "pseudo\nindices" "pseudo-indices"
"stan\ndard" "standard" "imple\nmented" "implemented"
"mod\nules" "modules" "ap\nplication" "application"
"char\nacters" "characters" "repre\nsents" "represents"
"cap\ntured" "captured" "nec\nessary" "necessary"
"genera\ntor" "generator" "compil\ners" "compilers"
"de\nscribing" "describing" "comple\ntion" "completion"
"unpro\ntected" "unprotected" "precompile" "pre-compile"
"re\nturning" "returning" "oth\nerwise" "otherwise"
"mes\nsage" "message" "pro\ngrams" "programs" "func\ntional"
"functional" "corou\ntines" "coroutines" "declara\ntions"
"declarations" "oper\nators" "operators" "op\neration"
"operation" "com\npared" "compared" "collec\ntion"
"collection" "trans\nlates" "translates" "zero\nterminated"
"zero-terminated" "distribu\ntion" "distribution"
"environ\nment" "environment" "infor\nmation" "information"
"pseudo\nrandom" "pseudo-random" "de\npends" "depends"
)

(progn
(fooi-re "\n\\([0-9]+\\)\n" "\n\n# «p\\1» (find-lua50page \\1)\n")
(fooi-re "\\([0-9]+\\(\\.[0-9]+\\)*\\)" "(to \"sec\\1\")")
(save-excursion
(ee-goto-position "\nIncompatibilities with")
(fooi-re "\n\\([0-9]+\\(\\.[0-9]+\\)*\\) \\([A-Z][-A-Za-z ]*\\)"
"\n\n# «sec\\1»\n# «\\3»\n\n\\1 \\3\n")
)
(save-excursion
(ee-goto-position "\nStandard Libraries")
(fooi-re "\n\n\\(\\. \\([a-zA-Z_.:]+\\)\\)"
"\n\n# «\\2»\n\\1")
)
(fooi-re "\n\\(``\\([a-z]*\\)'':\\)" "\n# «__\\2»\n\\1")
)

(ee-isearch "\n")
(ee-isearch "")
(fooi "\n" "")
(fooi "\n" "-")

(fooi "" "-")

(fooi-re (concat "\n\\([0-9]+\\(?:\\.[0-9]+\\)*\\) \\([^\n]*\\)"
"\\(?: \\.\\)\\{7\\} \\([0-9]+\\)")
"\n\\1 \\2 \\3 (to \"sec\\1\")")

(progn (jump-to-register ?a) (ee-isearch "\n"))

egrep '^[0-9]+\$' ~/tmp/luaref.txt | l -S

Lua 5.0 Reference Manual
Last revised on April 11, 2003
Lua

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
``Software''), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED ``AS IS'', WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Copies of this manual can be obtained at Lua's official web site,
www.lua.org. The Lua logo was designed by A. Nakonechny. Copyright c#

Lua 5.0 Reference Manual
Roberto Ierusalimschy Luiz Henrique de Figueiredo Waldemar Celes
lua@tecgraf.puc-rio.br
Tecgraf --- Computer Science Department --- PUC-Rio
PUC-RioInf.MCC14/03 Abril 2003

Abstract

Lua is a powerful, light-weight programming language designed for
extending applications. Lua is also frequently used as a
general-purpose, stand-alone language. Lua combines simple procedural
syntax (similar to Pascal) with powerful data description constructs
based on associative arrays and extensible semantics. Lua is
dynamically typed, interpreted from opcodes, and has automatic memory
management with garbage collection, making it ideal for configuration,
scripting, and rapid prototyping.

This document describes version 5.0 of the Lua programming language
and the Application Program Interface (API) that allows interaction
between Lua programs and their host C programs.

Resumo

Lua é uma linguagem de programação poderosa e leve, projetada para
estender aplicações. Lua também é frequentemente usada como uma
linguagem de propósito geral. Lua combina programação procedural (com
sintaxe semelhante à de Pascal) com poderosas construções para
opcodes, e tem gerenciamento automático de memória com coleta de lixo.
Essas características fazem de Lua uma linguagem ideal para
configuração, automação (scripting) e prototipagem rápida.

Este documento descreve a versão 5.0 da linguagem de programação Lua e
a Interface de Programação (API) que permite a interação entre
programas Lua e programas C hospedeiros.

i

ii

Contents
1 Introduction 1
2 The Language 1
2.1 Lexical Conventions . . . . . . . . . . . . . . . . . . 1 (to "sec2.1")
2.2 Values and Types . . . . . . . . . . . . . . . . . . . . 3 (to "sec2.2")
2.2.1 Coercion . . . . . . . . . . . . . . . . . . . . . . . 4 (to "sec2.2.1")
2.3 Variables . . . . . . . . . . . . . . . . . . . . . . . 4 (to "sec2.3")
2.4 Statements . . . . . . . . . . . . . . . . . . . . . . . 4 (to "sec2.4")
2.4.1 Chunks . . . . . . . . . . . . . . . . . . . . . . . . 5 (to "sec2.4.1")
2.4.2 Blocks . . . . . . . . . . . . . . . . . . . . . . . . 5 (to "sec2.4.2")
2.4.3 Assignment . . . . . . . . . . . . . . . . . . . . . . 5 (to "sec2.4.3")
2.4.4 Control Structures . . . . . . . . . . . . . . . . . . 6 (to "sec2.4.4")
2.4.5 For Statement . . . . . . . . . . . . . . . . . . . . 6 (to "sec2.4.5")
2.4.6 Function Calls as Statements . . . . . . . . . . . . 8 (to "sec2.4.6")
2.4.7 Local Declarations . . . . . . . . . . . . . . . . . 8 (to "sec2.4.7")
2.5 Expressions . . . . . . . . . . . . . . . . . . . . . . 8 (to "sec2.5")
2.5.1 Arithmetic Operators . . . . . . . . . . . . . . . . 8 (to "sec2.5.1")
2.5.2 Relational Operators . . . . . . . . . . . . . . . . . 9 (to "sec2.5.2")
2.5.3 Logical Operators . . . . . . . . . . . . . . . . . . 9 (to "sec2.5.3")
2.5.4 Concatenation . . . . . . . . . . . . . . . . . . . . 10 (to "sec2.5.4")
2.5.5 Precedence . . . . . . . . . . . . . . . . . . . . . . 10 (to "sec2.5.5")
2.5.6 Table Constructors . . . . . . . . . . . . . . . . . . 10 (to "sec2.5.6")
2.5.7 Function Calls . . . . . . . . . . . . . . . . . . . . 11 (to "sec2.5.7")
2.5.8 Function Definitions . . . . . . . . . . . . . . . . . 12 (to "sec2.5.8")
2.6 Visibility Rules . . . . . . . . . . . . . . . . . . . . 14 (to "sec2.6")
2.7 Error Handling . . . . . . . . . . . . . . . . . . . . . 14 (to "sec2.7")
2.8 Metatables . . . . . . . . . . . . . . . . . . . . . . . 15 (to "sec2.8")
2.9 Garbage Collection . . . . . . . . . . . . . . . . . . . 20 (to "sec2.9")
2.9.1 Garbage-Collection Metamethods . . . . . . . . . . . . 20 (to "sec2.9.1")
2.9.2 Weak Tables . . . . . . . . . . . . . . . . . . . . . 20 (to "sec2.9.2")
2.10 Coroutines . . . . . . . . . . . . . . . . . . . . . . . 21 (to "sec2.10")
3 The Application Program Interface 22
3.1 States . . . . . . . . . . . . . . . . . . . . . . . . . 22 (to "sec3.1")
3.2 The Stack and Indices . . . . . . . . . . . . . . . . . . 23 (to "sec3.2")
3.3 Stack Manipulation . . . . . . . . . . . . . . . . . . . 23 (to "sec3.3")
3.4 Querying the Stack . . . . . . . . . . . . . . . . . . . 24 (to "sec3.4")
3.5 Getting Values from the Stack . . . . . . . . . . . . . . 25 (to "sec3.5")
3.6 Pushing Values onto the Stack . . . . . . . . . . . . . . 26 (to "sec3.6")
3.7 Controlling Garbage Collection . . . . . . . . . . . . . . 27 (to "sec3.7")
3.8 Userdata . . . . . . . . . . . . . . . . . . . . . . . . 27 (to "sec3.8")
3.9 Metatables . . . . . . . . . . . . . . . . . . . . . . . 28 (to "sec3.9")
3.10 Loading Lua Chunks . . . . . . . . . . . . . . . . . . . 28 (to "sec3.10")
3.11 Manipulating Tables . . . . . . . . . . . . . . . . . . 28 (to "sec3.11")
3.12 Manipulating Environments . . . . . . . . . . . . . . . 30 (to "sec3.12")
3.13 Using Tables as Arrays . . . . . . . . . . . . . . . . . 30 (to "sec3.13")
iii

3.14 Calling Functions . . . . . . . . . . . . . . . . . . . 30 (to "sec3.14")
3.15 Protected Calls . . . . . . . . . . . . . . . . . . . . 31 (to "sec3.15")
3.16 Defining C Functions . . . . . . . . . . . . . . . . . . 32 (to "sec3.16")
3.17 Defining C Closures . . . . . . . . . . . . . . . . . . 33 (to "sec3.17")
3.18 Registry . . . . . . . . . . . . . . . . . . . . . . . . 33 (to "sec3.18")
3.19 Error Handling in C . . . . . . . . . . . . . . . . . . 33 (to "sec3.19")
3.20 Threads . . . . . . . . . . . . . . . . . . . . . . . . 34 (to "sec3.20")
4 The Debug Interface 35
4.1 Stack and Function Information . . . . . . . . . . . . . 35 (to "sec4.1")
4.2 Manipulating Local Variables and Upvalues . . . . . . . . 36 (to "sec4.2")
4.3 Hooks . . . . . . . . . . . . . . . . . . . . . . . . . . 37 (to "sec4.3")
5 Standard Libraries 38
5.1 Basic Functions . . . . . . . . . . . . . . . . . . . . . 39 (to "sec5.1")
5.2 Coroutine Manipulation . . . . . . . . . . . . . . . . . 43 (to "sec5.2")
5.3 String Manipulation . . . . . . . . . . . . . . . . . . . 44 (to "sec5.3")
5.4 Table Manipulation . . . . . . . . . . . . . . . . . . . 49 (to "sec5.4")
5.5 Mathematical Functions . . . . . . . . . . . . . . . . . 50 (to "sec5.5")
5.6 Input and Output Facilities . . . . . . . . . . . . . . . 51 (to "sec5.6")
5.7 Operating System Facilities . . . . . . . . . . . . . . . 53 (to "sec5.7")
5.8 The Reflexive Debug Interface . . . . . . . . . . . . . . 55 (to "sec5.8")
6 Lua Stand-alone 57
Incompatibilities with Previous Versions 58
The Complete Syntax of Lua 60
Index 61
iv

# «sec1»
# «Introduction»

1 Introduction

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, light-weight configuration language for any program
that needs one. Lua is implemented as a library, written in clean C
(that is, in the common subset of ANSI C and C++).

Being an extension language, Lua has no notion of a ``main'' program:
it only works embedded in a host client, called the embedding program
or simply the host. This 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 stand-alone embedding program, lua,
that uses the Lua library to offer a complete Lua interpreter.

Lua is free software, and is provided as usual with no guarantees, as
stated in its copyright notice. The implementation described in this
manual is available at Lua's official web site, www.lua.org. Like any
other reference manual, this document is dry in places. For a
discussion of the decisions behind the design of Lua, see the papers
below, which are available at Lua's web site.

. R. Ierusalimschy, L. H. de Figueiredo, and W. Celes. Lua---an
extensible extension language. Software: Practice & Experience 26 #6
(1996) 635--652.

. L. H. de Figueiredo, R. Ierusalimschy, and W. Celes. The design and
implementation of a language for extending applications. Proceedings
of XXI Brazilian Seminar on Software and Hardware (1994) 273--283.

. L. H. de Figueiredo, R. Ierusalimschy, and W. Celes. Lua: an
extensible embedded language. Dr. Dobb's Journal 21 #12 (Dec 1996)
26--33.

. R. Ierusalimschy, L. H. de Figueiredo, and W. Celes. The evolution
of an extension language: a history of Lua, Proceedings of V Brazilian
Symposium on Programming Languages (2001) B-14--B-28.

Lua means ``moon'' in Portuguese and is pronounced LOO-ah.

# «sec2»
# «The Language»

2 The Language

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. The
language constructs will be explained using the usual extended BNF, in
which { a } means 0 or more a's, and [ a ] means an optional a.
Non-terminals are shown in italics, keywords are shown in bold, and
other terminal symbols are shown in typewriter font, enclosed in
single quotes.

# «sec2.1»
# «Lexical Conventions»

2.1 Lexical Conventions

Identifiers in Lua can be any string of letters, digits, and
underscores, not beginning with a digit. This coincides with the
definition of identifiers in most languages. (The definition of letter
depends on the current locale: any character considered alphabetic by
the current locale can be used in an identifier.)

The following keywords are reserved and cannot be used as identifiers:

# «p1» (find-lua50page 1)

and break do else elseif
end false for function if
in local nil not or
repeat return then true until while

Lua is a case-sensitive language: and is a reserved word, but And and
AND are two different, valid identifiers. As a convention, identifiers
starting with an underscore followed by uppercase letters (such as
_VERSION) are reserved for internal variables used by Lua.

The following strings denote other tokens:

+ - * / ^ =
~= <= >= < > ==
( ) { } [ ]
; : , . .. ...

Literal strings can be delimited by matching single or double quotes,
and can contain the following C-like escape sequences:

\a --- bell
\b --- backspace
\f --- form feed
\n --- newline
\r --- carriage return
\t --- horizontal tab
\v --- vertical tab
\\ --- backslash
\" --- quotation mark
\' --- apostrophe
\[ --- left square bracket
\] --- right square bracket

Moreover, a `\newline' (that is, a backslash followed by a real
newline) results in a newline in the string. A character in a string
may also be specified by its numerical value using the escape sequence
`\ddd ', where ddd is a sequence of up to three decimal digits.
Strings in Lua may contain any 8-bit value, including embedded zeros,
which can be specified as `\0'.

Literal strings can also be delimited by matching double square
brackets [[ . . . ]]. Literals in this bracketed form may run for
several lines, may contain nested [[ . . . ]] pairs, and do not
interpret any escape sequences. For convenience, when the opening `[['
is immediately followed by a newline, the newline is not included in
the string. As an example, in a system using ASCII (in which `a' is
coded as 97, newline is coded as 10, and `1' is coded as 49), the four
literals below denote the same string:

(1) "alo\n123\""
(2) '\97lo\10\04923"'
(3) [[alo
123"]]
(4) [[
alo
123"]]

# «p2» (find-lua50page 2)

Numerical constants may be written with an optional decimal part and
an optional decimal exponent. Examples of valid numerical constants
are

3 3.0 3.1416 314.16e-2 0.31416E1

Comments start anywhere outside a string with a double hyphen (--). If
the text immediately after -- is different from [[, the comment is a
short comment, which runs until the end of the line. Otherwise, it is
a long comment, which runs until the corresponding ]]. Long comments
may run for several lines and may contain nested [[ . . . ]] pairs.

For convenience, the first line of a chunk is skipped if it starts
with #. This facility allows the use of Lua as a script interpreter in
Unix systems (see (to "sec6")).

# «sec2.2»
# «Values and Types»

2.2 Values and Types

Lua is a dynamically typed language. That means that variables do not
have types; only values do. There are no type definitions in the
language. All values carry their own type.

There are eight basic types in Lua: nil , boolean, number , string ,
function, userdata, thread , and table. Nil is the type of the value
nil, whose main property is to be different from any other value;
usually it represents the absence of a useful value. Boolean is the
type of the values false and true. In Lua, both nil and false make a
condition false; any other value makes it true. Number represents real
(double-precision floating-point) numbers. (It is easy to build Lua
interpreters that use other internal representations for numbers, such
as single-precision float or long integers.) String represents arrays
of characters. Lua is 8-bit clean: Strings may contain any 8-bit
character, including embedded zeros ('\0') (see (to "sec2.1")).

Functions are first-class values in Lua. That means that functions can
be stored in variables, passed as arguments to other functions, and
returned as results. Lua can call (and manipulate) functions written
in Lua and functions written in C (see (to "sec2.5.7")).

The type userdata is provided to allow arbitrary C data to be stored
in Lua variables. This type corresponds to a block of raw memory and
has no pre-defined operations in Lua, except assignment and identity
test. However, by using metatables, the programmer can define
operations for userdata values (see (to "sec2.8")). 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.

used to implement coroutines.

The type table implements associative arrays, that is, arrays that can
be indexed not only with numbers, but with any value (except nil).
Moreover, tables can be heterogeneous, that is, they can contain
values of all types (except nil). Tables are the sole data structuring
mechanism in Lua; they may be used to represent ordinary arrays,
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 a.name as syntactic sugar for
a["name"]. There are several convenient ways to create tables in Lua
(see (to "sec2.5.6")).

Like indices, the value of a table field can be of any type (except
nil). In particular, because functions are first class values, table
fields may contain functions. Thus tables may also carry methods (see
(to "sec2.5.8")).

Tables, functions, and userdata values are objects: variables do not
actually contain these values, only references to them. Assignment,
parameter passing, and function returns always manipulate references
to such values; these operations do not imply any kind of copy.

The library function type returns a string describing the type of a
given value (see (to "sec5.1")).

# «p3» (find-lua50page 3)

# «sec2.2.1»
# «Coercion»

2.2.1 Coercion

Lua provides automatic conversion between string and number values at
run time. Any arithmetic operation applied to a string tries to
convert that string to a number, following the usual rules.
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 of how numbers are converted to strings, use the format
function from the string library (see (to "sec5.3")).

# «sec2.3»
# «Variables»

2.3 Variables

Variables are places that store values. There are three kinds of
variables in Lua: global variables, local variables, and table fields.

A single name can denote a global variable or a local variable (or a
formal parameter of a function, which is a particular form of local
variable):

var -> Name

Variables are assumed to be global unless explicitly declared local
(see (to "sec2.4.7")). Local variables are lexically scoped : Local
variables can be freely accessed by functions defined inside their
scope (see (to "sec2.6")).

Before the first assignment to a variable, its value is nil.

Square brackets are used to index a table:

var -> prefixexp `[' exp `]'

The first expression (prefixexp)should result in a table value; the
second expression (exp) identifies a specific entry inside that table.
The expression denoting the table to be indexed has a restricted
syntax; see (to "sec2.5") for details.

The syntax var.NAME is just syntactic sugar for var["NAME"]:

var -> prefixexp `.' Name

The meaning of accesses to global variables and table fields can be
equivalent to a call gettable_event(t,i). (See (to "sec2.8") for a
complete description of the gettable_event function. This function is
not defined or callable in Lua. We use it here only for explanatory
purposes.)

All global variables live as fields in ordinary Lua tables, called
environment tables or simply environments. Functions written in C and
exported to Lua (C functions) all share a common global environment.
Each function written in Lua (a Lua function) has its own reference to
an environment, so that all global variables in that function will
refer to that environment table. When a function is created, it
inherits the environment from the function that created it. To change
or get the environment table of a Lua function, you call setfenv or
getfenv (see (to "sec5.1")).

An access to a global variable x is equivalent to _env.x, which in
turn is equivalent to

gettable_event(_env, "x")

where _env is the environment of the running function. (The _env
variable is not defined in Lua. We use it here only for explanatory
purposes.)

# «sec2.4»
# «Statements»

2.4 Statements

Lua supports an almost conventional set of statements, similar to
those in Pascal or C. This set includes assignment, control
structures, procedure calls, table constructors, and variable
declarations.

# «p4» (find-lua50page 4)

# «sec2.4.1»
# «Chunks»

2.4.1 Chunks

The unit of execution of Lua is called a chunk . A chunk is simply a
sequence of statements, which are executed sequentially. Each
statement can be optionally followed by a semicolon:

chunk -> { stat [ `;' ] }

Lua handles a chunk as the body of an anonymous function (see (to
"sec2.5.8")). As such, chunks can define local variables and return
values.

A chunk may be stored in a file or in a string inside the host
program. When a chunk is executed, first it is pre-compiled into
opcodes for a virtual machine, and then the compiled code is executed
by an interpreter for the virtual machine.

Chunks may also be pre-compiled into binary form; see program luac for
details. Programs in source and compiled forms are interchangeable;
Lua automatically detects the file type and acts accordingly.

# «sec2.4.2»
# «Blocks»

2.4.2 Blocks

A block is a list of statements; syntactically, a block is equal to a
chunk:

block -> chunk

A block may be explicitly delimited to produce a single statement:

stat -> do block end

Explicit blocks are useful to control the scope of variable
declarations. Explicit blocks are also sometimes used to add a return
or break statement in the middle of another block (see (to
"sec2.4.4")).

# «sec2.4.3»
# «Assignment»

2.4.3 Assignment

Lua allows multiple assignment. 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:

stat -> varlist1 `=' explist1
varlist1 -> var { `,' var }
explist1 -> exp { `,' exp }

Expressions are discussed in (to "sec2.5").

Before the assignment, the list of values is adjusted 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 nil's as needed. If the list of
expressions ends with a function call, then all values returned by
that function call enter in the list of values, before the adjustment
(except when the call is enclosed in parentheses; see (to "sec2.5")).

The assignment statement first evaluates all its expressions and only
then are the assignments performed. Thus the code

i = 3
i, a[i] = i+1, 20

sets a[3] to 20, without affecting a[4] because the i in a[i] is
evaluated (to 3) before it is assigned 4. Similarly, the line

x, y = y, x

# «p5» (find-lua50page 5)

exchanges the values of x and y.

The meaning of assignments to global variables and table fields can be
changed via metatables.

An assignment to an indexed variable t[i] = val is equivalent to
settable_event(t,i,val). (See (to "sec2.8") for a complete description
of the settable_event function. This function is not defined or
callable in Lua. We use it here only for explanatory purposes.)

An assignment to a global variable x = val is equivalent to the
assignment _env.x = val, which in turn is equivalent to

settable_event(_env, "x", val)

where _env is the environment of the running function. (The _env
variable is not defined in Lua. We use it here only for explanatory
purposes.)

# «sec2.4.4»
# «Control Structures»

2.4.4 Control Structures

The control structures if, while, and repeat have the usual meaning
and familiar syntax:

stat -> while exp do block end
stat -> repeat block until exp
stat -> if exp then block { elseif exp then block } [ else block ] end

Lua also has a for statement, in two flavors (see (to "sec2.4.5")).

The condition expression exp of a control structure may return any
value. Both false and nil are considered false. All values different
from nil and false are considered true (in particular, the number 0
and the empty string are also true).

The return statement is used to return values from a function or from
a chunk. Functions and chunks may return more than one value, so the
syntax for the return statement is

stat -> return [ explist1 ]

The break statement can be used to terminate the execution of a while,
repeat, or for loop, skipping to the next statement after the loop:

stat -> break

A break ends the innermost enclosing loop.

For syntactic reasons, return and break statements can only be written
as the last statement of a block. If it is really necessary to return
or break in the middle of a block, then an explicit inner block can be
used, as in the idioms `do return end' and `do break end', because now
return and break are the last statements in their (inner) blocks. In
practice, those idioms are only used during debugging.

# «sec2.4.5»
# «For Statement»

2.4.5 For Statement

The for statement has two forms: one numeric and one generic.

The numeric for loop repeats a block of code while a control variable
runs through an arithmetic progression. It has the following syntax:

stat -> for Name `=' exp `,' exp [ `,' exp ] do block end

The block is repeated for name starting at the value of the first exp,
until it passes the second exp by steps of the third exp. More
precisely, a for statement like

for var = e1, e2, e3 do block end

# «p6» (find-lua50page 6)

is equivalent to the code:

do
local var, _limit, _step = tonumber(e1), tonumber(e2), tonumber(e3)
if not (var and _limit and _step) then error() end
while (_step>0 and var<=_limit) or (_step<=0 and var>=_limit) do
block
var = var + _step
end
end

Note the following:

. All three control expressions are evaluated only once, before the
loop starts. They must all result in numbers.

. _limit and _step are invisible variables. The names are here for
explanatory purposes only.

. The behavior is undefined if you assign to var inside the block.

. If the third expression (the step) is absent, then a step of 1 is used.

. You can use break to exit a for loop.

. The loop variable var is local to the statement; you cannot use its
value after the for ends or is broken. If you need the value of the
loop variable var, then assign it to another variable before breaking
or exiting the loop.

The generic for statement works over functions, called iterators. For
each iteration, it calls its iterator function to produce a new value,
stopping when the new value is nil. The generic for loop has the
following syntax:

stat -> for Name { `,' Name } in explist1 do block end

A for statement like

for var_1, ..., var_n in explist do block end

is equivalent to the code:

do
local _f, _s, var_1 = explist
local var_2, ... , var_n
while true do
var_1, ..., var_n = _f(_s, var_1)
if var_1 == nil then break end
block
end
end

Note the following:

. explist is evaluated only once. Its results are an iterator
function, a state, and an initial value for the first iterator
variable.

. _f and _s are invisible variables. The names are here for
explanatory purposes only.

# «p7» (find-lua50page 7)

. The behavior is undefined if you assign to var_1 inside the block.

. You can use break to exit a for loop.

. The loop variables var_i are local to the statement; you cannot use
their values after the for ends. If you need these values, then assign
them to other variables before breaking or exiting the loop.

# «sec2.4.6»
# «Function Calls as Statements»

2.4.6 Function Calls as Statements

To allow possible side-effects, function calls can be executed as
statements:

stat -> functioncall

In this case, all returned values are thrown away. Function calls are
explained in (to "sec2.5.7").

# «sec2.4.7»
# «Local Declarations»

2.4.7 Local Declarations

Local variables may be declared anywhere inside a block. The
declaration may include an initial assignment:

stat -> local namelist [ `=' explist1 ]
namelist -> Name { `,' Name }

If present, an initial assignment has the same semantics of a multiple
assignment (see (to "sec2.4.3")). Otherwise, all variables are
initialized with nil.

A chunk is also a block (see (to "sec2.4.1")), so local variables can
be declared in a chunk outside any explicit block. Such local
variables die when the chunk ends.

The visibility rules for local variables are explained in (to
"sec2.6").

# «sec2.5»
# «Expressions»

2.5 Expressions

The basic expressions in Lua are the following:

exp -> prefixexp
exp -> nil | false | true
exp -> Number
exp -> Literal
exp -> function
exp -> tableconstructor
prefixexp -> var | functioncall | `(' exp `)'

Numbers and literal strings are explained in (to "sec2.1"); variables
are explained in (to "sec2.3"); function definitions are explained in
(to "sec2.5.8"); function calls are explained in (to "sec2.5.7");
table constructors are explained in (to "sec2.5.6").

An expression enclosed in parentheses always results in only one
value. Thus, (f(x,y,z)) is always a single value, even if f returns
several values. (The value of (f(x,y,z)) is the first value returned
by f or nil if f does not return any values.)

Expressions can also be built with arithmetic operators, relational
operators, and logical operators, all of which are explained below.

# «sec2.5.1»
# «Arithmetic Operators»

2.5.1 Arithmetic Operators

Lua supports the usual arithmetic operators: the binary + (addition),
- (subtraction), * (multiplication), / (division), and ^
(exponentiation); and unary - (negation). If the operands are numbers,

# «p8» (find-lua50page 8)

or strings that can be converted to numbers (see (to "sec2.2.1")),
then all operations except exponentiation have the usual meaning.
Exponentiation calls a global function __pow; otherwise, an
appropriate metamethod is called (see (to "sec2.8")). The standard
mathematical library defines function __pow, giving the expected
meaning to exponentiation (see (to "sec5.5")).

# «sec2.5.2»
# «Relational Operators»

2.5.2 Relational Operators

The relational operators in Lua are

== ~= < > <= >=

These operators always result in false or true.

Equality (==) first compares the type of its operands. If the types
are different, then the result is false. Otherwise, the values of the
operands are compared. Numbers and strings are compared in the usual
way. Objects (tables, userdata, threads, and functions) 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
function), this new object is different from any previously existing
object.

You can change the way that Lua compares tables and userdata using the
``eq'' metamethod (see (to "sec2.8")).

The conversion rules of (to "sec2.2.1") do not apply to equality
comparisons. Thus, "0"==0 evaluates to false, and t[0] and t["0"]
denote different entries in a table.

The operator ~= is exactly the negation of equality (==).

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 (to "sec2.8")).

# «sec2.5.3»
# «Logical Operators»

2.5.3 Logical Operators

The logical operators in Lua are

and or not

Like the control structures (see (to "sec2.4.4")), all logical
operators consider both false and nil as false and anything else as
true.

The operator not always return false or true.

The conjunction operator and returns its first argument if this value
is false or nil; otherwise, and returns its second argument. The
disjunction operator or returns its first argument if this value is
different from nil and false; otherwise, or returns its second
argument. Both and and or use short-cut evaluation, that is, the
second operand is evaluated only if necessary. For example,

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

# «p9» (find-lua50page 9)

# «sec2.5.4»
# «Concatenation»

2.5.4 Concatenation

The string concatenation operator in Lua is denoted by two dots
(`..'). If both operands are strings or numbers, then they are
converted to strings according to the rules mentioned in (to
"sec2.2.1"). Otherwise, the ``concat'' metamethod is called (see (to
"sec2.8")).

# «sec2.5.5»
# «Precedence»

2.5.5 Precedence

Operator precedence in Lua follows the table below, from lower to
higher priority:

or
and
< > <= >= ~= ==
..
+ -
* /
not - (unary)
^

You can use parentheses to change the precedences in an expression.
The concatenation (`..') and exponentiation (`^') operators are right
associative. All other binary operators are left associative.

# «sec2.5.6»
# «Table Constructors»

2.5.6 Table Constructors

Table constructors are expressions that create tables. Every time a
constructor is evaluated, a new table is created. Constructors can be
used to create empty tables, or to create a table and initialize some
of its fields. The general syntax for constructors is

tableconstructor -> `{' [ fieldlist ] `}'
fieldlist -> field { fieldsep field } [ fieldsep ]
field -> `[' exp `]' `=' exp | Name `=' exp | exp
fieldsep -> `,' | `;'

Each field of the form [exp1] = exp2 adds to the new table an entry
with key exp1 and value exp2. A field of the form name = exp is
equivalent to ["name"] = exp. Finally, fields of the form exp are
equivalent to [i] = exp, where i are consecutive numerical integers,
starting with 1. Fields in the other formats do not affect this
counting. For example,

a = {[f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45}

is equivalent to

do
local temp = {}
temp[f(1)] = g
temp[1] = "x" -- 1st exp
temp[2] = "y" -- 2nd exp
temp.x = 1 -- temp["x"] = 1
temp[3] = f(x) -- 3rd exp
temp[30] = 23
temp[4] = 45 -- 4th exp
a = temp
end

# «p10» (find-lua50page 10)

If the last field in the list has the form exp and the expression is a
function call, then all values returned by the call enter the list
consecutively (see (to "sec2.5.7")). To avoid this, enclose the
function call in parentheses (see (to "sec2.5")).

The field list may have an optional trailing separator, as a
convenience for machine-generated code.

# «sec2.5.7»
# «Function Calls»

2.5.7 Function Calls

A function call in Lua has the following syntax:

functioncall -> prefixexp args

In a function call, first prefixexp and args are evaluated. If the
value of prefixexp has type function, then that function is called
with the given arguments. Otherwise, its ``call'' metamethod is
called, having as first parameter the value of prefixexp, followed by
the original call arguments (see (to "sec2.8")).

The form

functioncall -> prefixexp `:' Name args

can be used to call ``methods''. A call v:name(...) is syntactic sugar
for v.name(v,...), except that v is evaluated only once.

Arguments have the following syntax:

args -> `(' [ explist1 ] `)'
args -> tableconstructor
args -> Literal

All argument expressions are evaluated before the call. A call of the
form f{...} is syntactic sugar for f({...}), that is, the argument
list is a single new table. A call of the form f'...' (or f"..." or
f[[...]]) is syntactic sugar for f('...'), that is, the argument list
is a single literal string.

Because a function can return any number of results (see (to
"sec2.4.4")), the number of results must be adjusted before they are
used. If the function is called as a statement (see (to "sec2.4.6")),
then its return list is adjusted to zero elements, thus discarding all
returned values. If the function is called inside another expression
or in the middle of a list of expressions, then its return list is
the first one. If the function is called as the last element of a list
enclosed in parentheses).

Here are some examples:

f() -- adjusted to 0 results
g(f(), x) -- f() is adjusted to 1 result
g(x, f()) -- g gets x plus all values returned by f()
a,b,c = f(), x -- f() is adjusted to 1 result (and c gets nil)
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 values returned by f()
return x,y,f() -- returns x, y, and all values returned by f()
{f()} -- creates a list with all values returned by f()
{f(), nil} -- f() is adjusted to 1 result

If you enclose a function call in parentheses, then it is adjusted to
return exactly one value:

return x,y,(f()) -- returns x, y, and the first value from f()
{(f())} -- creates a table with exactly one element

# «p11» (find-lua50page 11)

As an exception to the free-format syntax of Lua, you cannot put a
line break before the `(' in a function call. That restriction avoids
some ambiguities in the language. If you write

a = f
(g).x(a)

Lua would read that as a = f(g).x(a). So, if you want two statements,
you must add a semi-colon between them. If you actually want to call
f, you must remove the line break before (g).

A call of the form return functioncall is called a tail call . Lua
implements proper tail calls (or proper tail recursion): 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 return has one single
function call as argument; this syntax makes the calling function
returns exactly the returns of the called function. So, all the
following examples are not tails calls:

return (f(x)) -- results adjusted to 1
return 2 * f(x)
return x, f(x) -- additional results
return x or f(x) -- results adjusted to 1

# «sec2.5.8»
# «Function Definitions»

2.5.8 Function Definitions

The syntax for function definition is

function -> function funcbody
funcbody -> `(' [ parlist1 ] `)' block end

The following syntactic sugar simplifies function definitions:

stat -> function funcname funcbody
stat -> local function Name funcbody
funcname -> Name { `.' Name } [ `:' Name ]

The statement

function f () ... end

translates to

f = function () ... end

The statement

function t.a.b.c.f () ... end

translates to

t.a.b.c.f = function () ... end

The statement

local function f () ... end

translates to

# «p12» (find-lua50page 12)

local f; f = function () ... end

A function definition is an executable expression, whose value has
type function. When Lua pre-compiles a chunk, all its function bodies
are pre-compiled too. Then, whenever Lua executes the function
definition, the function is instantiated (or closed). This function
instance (or closure) is the final value of the expression. Different
instances of the same function may refer to different external local
variables and may have different environment tables.

Parameters act as local variables that are initialized with the
argument values:

parlist1 -> namelist [ `,' `...' ]
parlist1 -> `...'

When a function is called, the list of arguments is adjusted to the
length of the list of parameters, unless the function is a variadic or
vararg function, which is indicated by three dots (`...') at the end
of its parameter list. A vararg function does not adjust its argument
list; instead, it collects all extra arguments into an implicit
parameter, called arg. The value of arg is a table, with a field n
that holds the number of extra arguments and with the extra arguments
at positions 1, 2, . . . , n.

As an example, consider the following definitions:

function f(a, b) end
function g(a, b, ...) end
function r() return 1,2,3 end

Then, we have the following mapping from arguments to parameters:

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, arg={n=0}
g(3, 4) a=3, b=4, arg={n=0}
g(3, 4, 5, 8) a=3, b=4, arg={5, 8; n=2}
g(5, r()) a=5, b=1, arg={2, 3; n=2}

Results are returned using the return statement (see (to "sec2.4.4")).
If control reaches the end of a function without encountering a return

The colon syntax is used for defining methods, that is, functions that
have an implicit extra parameter self. Thus, the statement

function t.a.b.c:f (...) ... end

is syntactic sugar for

t.a.b.c.f = function (self, ...) ... end

# «p13» (find-lua50page 13)

# «sec2.6»
# «Visibility Rules»

2.6 Visibility Rules

Lua is a lexically scoped language. The scope of variables begins at
the first statement after their declaration and lasts until the end of
the innermost block that includes the declaration. For instance:

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)

Notice that, in a declaration like local x = x, the new x being
declared is not in scope yet, and so the second x refers to the
outside variable.

Because of the lexical scoping rules, local variables can be freely
accessed by functions defined inside their scope. For instance:

local counter = 0
function inc (x)
counter = counter + x
return counter
end

A local variable used by an inner function is called an upvalue, or
external local variable, inside the inner function.

Notice that each execution of a local statement defines new local
variables. Consider the following example:

a = {}
local x = 20
for i=1,10 do
local y = 0
a[i] = function () y=y+1; return x+y end
end

The loop creates ten closures (that is, ten instances of the anonymous
function). Each of these closures uses a different y variable, while
all of them share the same x.

# «sec2.7»
# «Error Handling»

2.7 Error Handling

Because Lua is an extension language, all Lua actions start from C
code in the host program calling a function from the Lua library (see
(to "sec3.15")). Whenever an error occurs during Lua compilation or
execution, control returns to C, which can take appropriate measures
(such as print an error message).

# «p14» (find-lua50page 14)

Lua code can explicitly generate an error by calling the error
function (see (to "sec5.1")). If you need to catch errors in Lua, you
can use the pcall function (see (to "sec5.1")).

# «sec2.8»
# «Metatables»

2.8 Metatables

Every table and userdata object in Lua may have a metatable. This
metatable is an ordinary Lua table that defines the behavior of the
original table and userdata under certain special operations. You can
change several aspects of the behavior of an object by setting
specific fields in its metatable. For instance, when an object is the
operand of an addition, Lua checks for a function in the field "__add"
in its metatable. If it finds one, Lua calls that function to perform

We call the keys in a metatable events and the values metamethods. In
the previous example, the event is "add" and the metamethod is the

You can query and change the metatable of an object through the
set/getmetatable functions (see (to "sec5.1")).

A metatable may control how an object behaves in arithmetic
operations, order comparisons, concatenation, and indexing. A
metatable can also define a function to be called when a userdata is
garbage collected. For each of those operations Lua associates a
specific key called an event. When Lua performs one of those
operations over a table or a userdata, it checks whether that object
has a metatable with the corresponding event. If so, the value
associated with that key (the metamethod) controls how Lua will
perform the operation.

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; for instance, the
these operations is better explained by a Lua function describing how
the interpreter executes that 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 (rawget,
tonumber, etc.) are described in (to "sec5.1"). In particular, to
retrieve the metamethod of a given object, we use the expression

metatable(obj)[event]

rawget(metatable(obj) or {}, event)

That is, the access to a metamethod does not invoke other metamethods,
and the access to objects with no metatables does not fail (it simply
results in nil).

The function getbinhandler 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.

function getbinhandler (op1, op2, event)
return metatable(op1)[event] or metatable(op2)[event]
end

Using that function, the behavior of the op1 + op2 is

# «p15» (find-lua50page 15)

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

``sub'': the - operation. Behavior similar to the ``add'' operation.

``mul'': the * operation. Behavior similar to the ``add'' operation.

``div'': the / operation. Behavior similar to the ``add'' operation.

``pow'': the ^ (exponentiation) operation.

function pow_event (op1, op2)
local o1, o2 = tonumber(op1), tonumber(op2)
if o1 and o2 then -- both operands are numeric?
return __pow(o1, o2) -- call global `__pow'
else -- at least one of the operands is not numeric
local h = getbinhandler(op1, op2, "__pow")
if h then
-- call the handler with both operands
return h(op1, op2)
else -- no handler available: default behavior
error("...")
end
end
end

``unm'': the unary - operation.

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 and nil
return h(op, nil)

# «p16» (find-lua50page 16)

else -- no handler available: default behavior
error("...")
end
end
end

``concat'': the .. (concatenation) operation.

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

``eq'': the == operation. The function getcomphandler defines how Lua
chooses a metamethod for comparison operators. A metamethod only is
selected when both objects being compared have the same type and the
same metamethod for the selected operation.

function getcomphandler (op1, op2, event)
if type(op1) ~= type(op2) then return nil end
local mm1 = metatable(op1)[event]
local mm2 = metatable(op2)[event]
if mm1 == mm2 then return mm1 else return nil end
end

The ``eq'' event is defined as follows:

function eq_event (op1, op2)
if type(op1) ~= type(op2) then -- different types?
return false -- different objects
end
if op1 == op2 then -- primitive equal?
return true -- objects are equal
end
-- try metamethod
local h = getcomphandler(op1, op2, "__eq")
if h then
return h(op1, op2)
else
return false

# «p17» (find-lua50page 17)

end
end

a ~= b is equivalent to not (a == b).

``lt'': the < operation.

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 = getcomphandler(op1, op2, "__lt")
if h then
return h(op1, op2)
else
error("...");
end
end
end

a > b is equivalent to b < a.

``le'': the <= operation.

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 = getcomphandler(op1, op2, "__le")
if h then
return h(op1, op2)
else
h = getcomphandler(op1, op2, "__lt")
if h then
return not h(op2, op1)
else
error("...");
end
end
end
end

a >= b is equivalent to b <= a. Note that, in the absence of a ``le''
metamethod, Lua tries the ``lt'', assuming that a <= b is equivalent
to not (b < a).

``index'': The indexing access table[key].

# «p18» (find-lua50page 18)

function gettable_event (table, key)
local h
if type(table) == "table" then
local v = rawget(table, key)
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

``newindex'': The indexing assignment table[key] = value.

function settable_event (table, key, value)
local h
if type(table) == "table" then
local v = rawget(table, key)
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
return h(table, key,value) -- call the handler
else h[key] = value -- or repeat operation on it
end

``call'': called when Lua calls a value.

function function_event (func, ...)
if type(func) == "function" then
return func(unpack(arg)) -- primitive call
else
local h = metatable(func).__call
if h then
return h(func, unpack(arg))
else

# «p19» (find-lua50page 19)

error("...")
end
end
end

# «sec2.9»
# «Garbage Collection»

2.9 Garbage Collection

Lua does automatic memory management. That means that you do not have
to worry about allocating memory for new objects and freeing it when
the objects are no longer needed. Lua manages memory automatically by
running a garbage collector from time to time to collect all dead
objects (that is, those objects that are no longer accessible from
Lua). All objects in Lua are subject to automatic management: tables,

Lua uses two numbers to control its garbage-collection cycles. One
number counts how many bytes of dynamic memory Lua is using; the other
is a threshold. When the number of bytes crosses the threshold, Lua
runs the garbage collector, which reclaims the memory of all dead
objects. The byte counter is adjusted, and then the threshold is reset
to twice the new value of the byte counter.

Through the C API, you can query those numbers and change the
threshold (see (to "sec3.7")). Setting the threshold to zero actually
forces an immediate garbage-collection cycle, while setting it to a
huge number effectively stops the garbage collector. Using Lua code
you have a more limited control over garbage-collection cycles,
through the gcinfo and collectgarbage functions (see (to "sec5.1")).

# «sec2.9.1»
# «Garbage-Collection Metamethods»

2.9.1 Garbage-Collection Metamethods

Using the C API, you can set garbage-collector metamethods for
userdata (see (to "sec2.8")). These metamethods are also called
finalizers. 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).

Free userdata with a field __gc in their metatables are not collected
immediately by the garbage collector. Instead, Lua puts them in a
list. After the collection, Lua does the equivalent of the following
function for each userdata in that list:

function gc_event (udata)
local h = metatable(udata).__gc
if h then
h(udata)
end
end

At the end of each garbage-collection cycle, the finalizers for
userdata are called in reverse order of their creation, among those
collected in that cycle. That is, the first finalizer to be called is
the one associated with the userdata created last in the program.

# «sec2.9.2»
# «Weak Tables»

2.9.2 Weak Tables

A weak table is a table whose elements are weak references. 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.

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

# «p20» (find-lua50page 20)

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
value of the __mode field of its metatable. If the __mode field is a
string containing the character `k', the keys in the table are weak.
If __mode contains `v', the values in the table are weak.

After you use a table as a metatable, you should not change the value
of its field __mode. Otherwise, the weak behavior of the tables
controlled by this metatable is undefined.

# «sec2.10»
# «Coroutines»

2.10 Coroutines

Lua supports coroutines, also called semi-coroutines or collaborative
only suspends its execution by explicitly calling a yield function.

You create a coroutine with a call to coroutine.create. Its sole
argument is a function that is the main function of the coroutine. The
create function only creates a new coroutine and returns a handle to
it (an object of type thread); it does not start the coroutine
execution.

When you first call coroutine.resume, passing as its first argument
the thread returned by coroutine.create, the coroutine starts its
execution, at the first line of its main function. Extra arguments
passed to coroutine.resume are given as parameters for the coroutine
main function. After the coroutine starts running, it runs until it
terminates or yields.

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, coroutine.resume returns true, plus any values returned by
the coroutine main function. In case of errors, coroutine.resume
returns false plus an error message.

A coroutine yields by calling coroutine.yield. When a coroutine
yields, the corresponding coroutine.resume 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, coroutine.resume also returns
true, plus any values passed to coroutine.yield. The next time you
resume the same coroutine, it continues its execution from the point
where it yielded, with the call to coroutine.yield returning any extra
arguments passed to coroutine.resume.

The coroutine.wrap function creates a coroutine like coroutine.create,
but instead of returning the coroutine itself, it returns a function
that, when called, resumes the coroutine. Any arguments passed to that
function go as extra arguments to resume. The function returns all the
values returned by resume, except the first one (the boolean error
code). Unlike coroutine.resume, this function does not catch errors;
any error is propagated to the caller.

As an example, consider the next code:

function foo1 (a)
print("foo", a)
return coroutine.yield(2*a)
end
co = coroutine.create(function (a,b)
print("co-body", a, b)
local r = foo1(a+1)
print("co-body", r)
local r, s = coroutine.yield(a+b, a-b)
print("co-body", r, s)

# «p21» (find-lua50page 21)

return b, "end"
end)
a, b = coroutine.resume(co, 1, 10)
print("main", a, b)
a, b, c = coroutine.resume(co, "r")
print("main", a, b, c)
a, b, c = coroutine.resume(co, "x", "y")
print("main", a, b, c)
a, b = coroutine.resume(co, "x", "y")
print("main", a, b)

When you run it, it produces the following output:

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

# «sec3»
# «The Application Program Interface»

3 The Application Program Interface

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

Even when we use the term ``function'', any facility in the API may be
provided as a macro instead. All such macros use each of its arguments
exactly once (except for the first argument, which is always a Lua
state), and so do not generate hidden side-effects.

# «sec3.1»
# «States»

3.1 States

The Lua library is fully reentrant: it has no global variables. The
whole state of the Lua interpreter (global variables, stack, etc.) is
stored in a dynamically allocated structure of type lua_State. A
pointer to this state must be passed as the first argument to every
function in the library, except to lua_open, which creates a Lua state
from scratch.

Before calling any API function, you must create a state by calling
lua_open:

lua_State *lua_open (void);

To release a state created with lua_open, call lua_close:

void lua_close (lua_State *L);

This function destroys all objects in the given Lua state (calling the
corresponding garbage-collection metamethods, if any) and frees all
dynamic memory used by that 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, such as a daemon or a web server, might need to release
states as soon as they are not needed, to avoid growing too large.

# «p22» (find-lua50page 22)

# «sec3.2»
# «The Stack and Indices»

3.2 The Stack and Indices

Lua uses a virtual stack to pass values to and from C. Each element in
this stack represents a Lua value (nil, number, string, etc.).

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. That 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 (to "sec3.16")).

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 index : A positive index represents an absolute
stack position (starting at 1); a negative index represents an offset
from the top of the stack. More specifically, if the stack has n
elements, then index 1 represents the first element (that is, the
element that was pushed onto the stack first) and index n represents
the last element; index -1 also represents the last element (that is,
the element at the top) and index -n represents the first element. We
say that an index is valid if it lies between 1 and the stack top
(that is, if 1 <= abs(index) <= top).

At any time, you can get the index of the top element by calling
lua_gettop:

int lua_gettop (lua_State *L);

Because indices start at 1, the result of lua_gettop is equal to the
number of elements in the stack (and so 0 means an empty stack).

When you interact with Lua API, you are responsible for controlling
stack overflow. The function

int lua_checkstack (lua_State *L, int extra);

grows the stack size to top + extra elements; it returns false if it
cannot grow the stack to that size. This function never shrinks the
stack; if the stack is already larger than the new size, it is left
unchanged.

Whenever Lua calls C, it ensures that at least LUA_MINSTACK stack
positions are available. LUA_MINSTACK is defined in lua.h as 20, so
that usually you do not have to worry about stack space unless your
code has loops pushing elements onto the stack.

Most query functions accept as indices any value inside the available
stack space, that is, indices up to the maximum stack size you have
set through lua_checkstack. Such indices are called acceptable
indices. More formally, we define an acceptable index as follows:

(index < 0 && abs(index) <= top) || (index > 0 && index <= stackspace)

Note that 0 is never an acceptable index.

Unless otherwise noted, any function that accepts valid indices can
also be called with pseudo-indices, which represent some Lua values
that are accessible to the C code but are not in the stack.
Pseudo-indices are used to access the global environment, the
registry, and the upvalues of a C function (see (to "sec3.17")).

# «sec3.3»
# «Stack Manipulation»

3.3 Stack Manipulation

The API offers the following functions for basic stack manipulation:

# «p23» (find-lua50page 23)

void lua_settop (lua_State *L, int index);
void lua_pushvalue (lua_State *L, int index);
void lua_remove (lua_State *L, int index);
void lua_insert (lua_State *L, int index);
void lua_replace (lua_State *L, int index);

lua_settop accepts any acceptable index, or 0, and sets the stack top
to that index. If the new top is larger than the old one, then the new
elements are filled with nil. If index is 0, then all stack elements
are removed. A useful macro defined in the lua.h is

#define lua_pop(L,n) lua_settop(L, -(n)-1)

which pops n elements from the stack.

lua_pushvalue pushes onto the stack a copy of the element at the given
index. lua_remove removes the element at the given position, shifting
down the elements above that position to fill the gap. lua_insert
moves the top element into the given position, shifting up the
elements above that position to open space. lua_replace moves the top
element into the given position, without shifting any element
(therefore replacing the value at the given position). All these
functions accept only valid indices. (You cannot call lua_remove or
lua_insert with pseudo-indices, as they do not represent a stack
position.)

As an example, if the stack starts as 10 20 30 40 50* (from bottom to
top; the `*' marks the top), then

lua_pushvalue(L, 3) --> 10 20 30 40 50 30*
lua_pushvalue(L, -1) --> 10 20 30 40 50 30 30*
lua_remove(L, -3) --> 10 20 30 40 30 30*
lua_remove(L, 6) --> 10 20 30 40 30*
lua_insert(L, 1) --> 30 10 20 30 40*
lua_insert(L, -1) --> 30 10 20 30 40* (no effect)
lua_replace(L, 2) --> 30 40 20 30*
lua_settop(L, -3) --> 30 40*
lua_settop(L, 6) --> 30 40 nil nil nil nil*

# «sec3.4»
# «Querying the Stack»

3.4 Querying the Stack

To check the type of a stack element, the following functions are
available:

int lua_type (lua_State *L, int index);
int lua_isnil (lua_State *L, int index);
int lua_isboolean (lua_State *L, int index);
int lua_isnumber (lua_State *L, int index);
int lua_isstring (lua_State *L, int index);
int lua_istable (lua_State *L, int index);
int lua_isfunction (lua_State *L, int index);
int lua_iscfunction (lua_State *L, int index);
int lua_isuserdata (lua_State *L, int index);
int lua_islightuserdata (lua_State *L, int index);

These functions can be called with any acceptable index.

# «p24» (find-lua50page 24)

lua_type returns the type of a value in the stack, or LUA_TNONE for a
non-valid index (that is, if that stack position is ``empty''). The
types returned by lua_type are coded by the following constants
defined in lua.h: LUA_TNIL, LUA_TNUMBER, LUA_TBOOLEAN, LUA_TSTRING,
LUA_TLIGHTUSERDATA. The following function translates these constants
to strings:

const char *lua_typename (lua_State *L, int type);

The lua_is* functions return 1 if the object is compatible with the
given type, and 0 otherwise. lua_isboolean is an exception to this
rule: It succeeds only for boolean values (otherwise it would be
useless, as any value has a boolean value). They always return 0 for a
non-valid index. lua_isnumber accepts numbers and numerical strings;
lua_isstring accepts strings and numbers (see (to "sec2.2.1"));
lua_isfunction accepts both Lua functions and C functions; and
lua_isuserdata accepts both full and light userdata. To distinguish
between Lua functions and C functions, you can use lua_iscfunction. To
distinguish between full and light userdata, you can use
lua_islightuserdata. To distinguish between numbers and numerical
strings, you can use lua_type.

The API also contains functions to compare two values in the stack:

int lua_equal (lua_State *L, int index1, int index2);
int lua_rawequal (lua_State *L, int index1, int index2);
int lua_lessthan (lua_State *L, int index1, int index2);

lua_equal and lua_lessthan are equivalent to their counterparts in Lua
(see (to "sec2.5.2")). lua_rawequal compares the values for primitive
equality, without metamethods. These functions return 0 (false) if any
of the indices are non-valid.

# «sec3.5»
# «Getting Values from the Stack»

3.5 Getting Values from the Stack

To translate a value in the stack to a specific C type, you can use
the following conversion functions:

int lua_toboolean (lua_State *L, int index);
lua_Number lua_tonumber (lua_State *L, int index);
const char *lua_tostring (lua_State *L, int index);
size_t lua_strlen (lua_State *L, int index);
lua_CFunction lua_tocfunction (lua_State *L, int index);
void *lua_touserdata (lua_State *L, int index);
lua_State *lua_tothread (lua_State *L, int index);
void *lua_topointer (lua_State *L, int index);

These functions can be called with any acceptable index. When called
with a non-valid index, they act as if the given value had an
incorrect type.

lua_toboolean converts the Lua value at the given index to a C
``boolean'' value (0 or 1). Like all tests in Lua, lua_toboolean
returns 1 for any Lua value different from false and nil; otherwise it
returns 0. It also returns 0 when called with a non-valid index. (If
you want to accept only real boolean values, use lua_isboolean to test
the type of the value.)

lua_tonumber converts the Lua value at the given index to a number (by
default, lua_Number is double). The Lua value must be a number or a
string convertible to number (see (to "sec2.2.1")); otherwise,
lua_tonumber returns 0.

# «p25» (find-lua50page 25)

lua_tostring converts the Lua value at the given index to a string
(const char*). The Lua value must be a string or a number; otherwise,
the function returns NULL. If the value is a number, then lua_tostring
also changes the actual value in the stack to a string. (This change
confuses lua_next when lua_tostring is applied to keys.) lua_tostring
returns a fully aligned pointer to a string inside the Lua state. This
string always has a zero ('\0') after its last character (as in C),
but may contain other zeros in its body. If you do not know whether a
string may contain zeros, you can use lua_strlen to get its actual
length. Because Lua has garbage collection, there is no guarantee that
the pointer returned by lua_tostring will be valid after the
corresponding value is removed from the stack. If you need the string
after the current function returns, then you should duplicate it or
put it into the registry (see (to "sec3.18")).

lua_tocfunction converts a value in the stack to a C function. This
value must be a C function; otherwise, lua_tocfunction returns NULL.
The type lua_CFunction is explained in (to "sec3.16"). lua_tothread
converts a value in the stack to a Lua thread (represented as
returns NULL.

lua_topointer converts a value in the stack to a generic C pointer
(void *). The value may be a userdata, a table, a thread, or a
function; otherwise, lua_topointer returns NULL. Lua ensures that
different objects of the same type return different pointers. There is
no direct way to convert the pointer back to its original value.
Typically this function is used for debug information. lua_touserdata
is explained in (to "sec3.8").

# «sec3.6»
# «Pushing Values onto the Stack»

3.6 Pushing Values onto the Stack

The API has the following functions to push C values onto the stack:

void lua_pushboolean (lua_State *L, int b);
void lua_pushnumber (lua_State *L, lua_Number n);
void lua_pushlstring (lua_State *L, const char *s, size_t len);
void lua_pushstring (lua_State *L, const char *s);
void lua_pushnil (lua_State *L);
void lua_pushcfunction (lua_State *L, lua_CFunction f);
void lua_pushlightuserdata (lua_State *L, void *p);

These functions receive a C value, convert it to a corresponding Lua
value, and push the result onto the stack. In particular,
lua_pushlstring and lua_pushstring make an internal copy of the given
string. lua_pushstring can only be used to push proper C strings (that
is, strings that end with a zero and do not contain embedded zeros);
otherwise, you should use the more general lua_pushlstring, which
accepts an explicit size.

You can also push ``formatted'' strings:

const char *lua_pushfstring (lua_State *L, const char *fmt, ...);
const char *lua_pushvfstring (lua_State *L, const char *fmt, va_list argp);

These functions push onto the stack a formatted string and return a
pointer to that string. They are similar to sprintf and vsprintf, but
with some important differences:

. You do not have to allocate the space for the result: The result is
a Lua string and Lua takes care of memory allocation (and
deallocation, through garbage collection).

. The conversion specifiers are quite restricted. There are no flags,
widths, or precisions. The conversion specifiers can be simply `%%'
(inserts a `%' in the string), `%s' (inserts a zero-terminated string,
with no size restrictions), `%f' (inserts a lua_Number), `%d' (inserts
an int), and `%c' (inserts an int as a character).

# «p26» (find-lua50page 26)

The function

void lua_concat (lua_State *L, int n);

concatenates the n values at the top of the stack, pops them, and
leaves the result at the top. If n is 1, the result is that single
string (that is, the function does nothing); if n is 0, the result is
the empty string. Concatenation is done following the usual semantics
of Lua (see (to "sec2.5.4")).

# «sec3.7»
# «Controlling Garbage Collection»

3.7 Controlling Garbage Collection

Lua uses two numbers to control its garbage collection: the count and
the threshold (see (to "sec2.9")). The first counts the amount of
memory in use by Lua; when the count reaches the threshold, Lua runs
its garbage collector. After the collection, the count is updated and
the threshold is set to twice the count value.

You can access the current values of these two numbers through the
following functions:

int lua_getgccount (lua_State *L);
int lua_getgcthreshold (lua_State *L);

Both return their respective values in Kbytes. You can change the
threshold value with

void lua_setgcthreshold (lua_State *L, int newthreshold);

Again, the newthreshold value is given in Kbytes. When you call this
function, Lua sets the new threshold and checks it against the byte
counter. If the new threshold is less than the byte counter, then Lua
immediately runs the garbage collector. In particular
lua_setgcthreshold(L,0) forces a garbage collection. After the
collection, a new threshold is set according to the previous rule.

# «sec3.8»
# «Userdata»

3.8 Userdata

Userdata represents C values in Lua. Lua supports two types of
userdata: full userdata and light userdata.

A full userdata represents a block of memory. It is an object (like a
table): You must create it, it can have its own metatable, and you can
detect when it is being collected. A full userdata is only equal to
itself (under raw equality).

A light userdata represents a pointer. It is a value (like a number):
You do not create it, it has no metatables, it is not collected (as it
was never created). A light userdata is equal to ``any'' light
userdata with the same C address.

In Lua code, there is no way to test whether a userdata is full or
light; both have type userdata. In C code, lua_type returns
LUA_TUSERDATA for full userdata, and LUA_TLIGHTUSERDATA for light
userdata.

You can create a new full userdata with the following function:

void *lua_newuserdata (lua_State *L, size_t size);

This function allocates a new block of memory with the given size,
pushes on the stack a new userdata with the block address, and returns

To push a light userdata into the stack you use lua_pushlightuserdata
(see (to "sec3.6")). lua_touserdata (see (to "sec3.5")) retrieves the
value of a userdata. When applied on a full userdata, it returns the
address of its block; when applied on a light userdata, it returns its
pointer; when applied on a non-userdata value, it returns NULL.

When Lua collects a full userdata, it calls the userdata's gc
metamethod, if any, and then it frees the userdata's corresponding
memory.

# «p27» (find-lua50page 27)

# «sec3.9»
# «Metatables»

3.9 Metatables

The following functions allow you to manipulate the metatables of an
object:

int lua_getmetatable (lua_State *L, int index);
int lua_setmetatable (lua_State *L, int index);

lua_getmetatable pushes on the stack the metatable of a given object.
If the index is not valid, or if the object does not have a metatable,
lua_getmetatable returns 0 and pushes nothing on the stack.

lua_setmetatable pops a table from the stack and sets it as the new
metatable for the given object. lua_setmetatable returns 0 when it
cannot set the metatable of the given object (that is, when the object
is neither a userdata nor a table); even then it pops the table from
the stack.

# «sec3.10»

(lua_State *L, void *data, size_t *size);
const char *chunkname);

The return values of lua_load are:

. 0 --- no errors;
. LUA_ERRSYNTAX --- syntax error during pre-compilation.
. LUA_ERRMEM --- memory allocation error.

If there are no errors, lua_load pushes the compiled chunk as a Lua
function on top of the stack. Otherwise, it pushes an error message.

lua_load automatically detects whether the chunk is text or binary,
and loads it accordingly (see program luac).

time it needs another piece of the chunk, lua_load calls the reader,
passing along its data parameter. The reader must return a pointer to
a block of memory with a new piece of the chunk and set size to the
block size. To signal the end of the chunk, the reader returns NULL.
The reader function may return pieces of any size greater than zero.

In the current implementation, the reader function cannot call any Lua
function; to ensure that, it always receives NULL as the Lua state.

The chunkname is used for error messages and debug information (see
(to "sec4")). See the auxiliary library (lauxlib.c) for examples of
from files and strings.

# «sec3.11»
# «Manipulating Tables»

3.11 Manipulating Tables

Tables are created by calling the function

void lua_newtable (lua_State *L);

# «p28» (find-lua50page 28)

This function creates a new, empty table and pushes it onto the stack.

To read a value from a table that resides somewhere in the stack, call

void lua_gettable (lua_State *L, int index);

where index points to the table. lua_gettable pops a key from the
stack and returns (on the stack) the contents of the table at that
key. The table is left where it was in the stack. As in Lua, this
function may trigger a metamethod for the ``index'' event (see (to
"sec2.8")). To get the real value of any table key, without invoking
any metamethod, use the raw version:

void lua_rawget (lua_State *L, int index);

To store a value into a table that resides somewhere in the stack, you
push the key and then the value onto the stack, and call

void lua_settable (lua_State *L, int index);

where index points to the table. lua_settable pops from the stack both
the key and the value. The table is left where it was in the stack. As
in Lua, this operation may trigger a metamethod for the ``settable''
or ``newindex'' events. To set the real value of any table index,
without invoking any metamethod, use the raw version:

void lua_rawset (lua_State *L, int index);

You can traverse a table with the function

int lua_next (lua_State *L, int index);

where index points to the table to be traversed. The function pops a
key from the stack, and pushes a key-value pair from the table (the
``next'' pair after the given key). If there are no more elements,
then lua_next returns 0 (and pushes nothing). Use a nil key to signal
the start of a traversal.

A typical traversal looks like this:

/* table is in the stack at index `t' */
lua_pushnil(L); /* first key */
while (lua_next(L, t) != 0) {
/* `key' is 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)));
lua_pop(L, 1); /* removes `value'; keeps `key' for next iteration */
}

While traversing a table, do not call lua_tostring directly on a key,
unless you know that the key is actually a string. Recall that
lua_tostring changes the value at the given index; this confuses the
next call to lua_next.

# «p29» (find-lua50page 29)

# «sec3.12»
# «Manipulating Environments»

3.12 Manipulating Environments

All global variables are kept in ordinary Lua tables, called
environments. The initial environment is called the global
environment. This table is always at pseudo-index LUA_GLOBALSINDEX.

To access and change the value of global variables, you can use
regular table operations over an environment table. For instance, to
access the value of a global variable, do

lua_pushstring(L, varname);
lua_gettable(L, LUA_GLOBALSINDEX);

You can change the global environment of a Lua thread using
lua_replace.

The following functions get and set the environment of Lua functions:

void lua_getfenv (lua_State *L, int index);
int lua_setfenv (lua_State *L, int index);

lua_getfenv pushes on the stack the environment table of the function
at index index in the stack. If the function is a C function,
lua_getfenv pushes the global environment. lua_setfenv pops a table
from the stack and sets it as the new environment for the function at
index index in the stack. If the object at the given index is not a
Lua function, lua_setfenv returns 0.

# «sec3.13»
# «Using Tables as Arrays»

3.13 Using Tables as Arrays

The API has functions that help to use Lua tables as arrays, that is,
tables indexed by numbers only:

void lua_rawgeti (lua_State *L, int index, int n);
void lua_rawseti (lua_State *L, int index, int n);

lua_rawgeti pushes the value of the n-th element of the table at stack
position index. lua_rawseti sets the value of the n-th element of the
table at stack position index to the value at the top of the stack,
removing this value from the stack.

# «sec3.14»
# «Calling Functions»

3.14 Calling Functions

Functions defined in Lua and C functions registered in Lua can be
called from the host program. This is done using 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, the function is
called using

void lua_call (lua_State *L, int nargs, int nresults);

nargs is the number of arguments that you pushed onto the stack. All
arguments and the function value are popped from the stack, and the
function results are pushed. The number of results are adjusted to
nresults, unless nresults is LUA_MULTRET. In that 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.

The following example shows how the host program may do the equivalent
to this Lua code:

a = f("how", t.x, 14)

# «p30» (find-lua50page 30)

Here it is in C:

lua_pushstring(L, "t");
lua_gettable(L, LUA_GLOBALSINDEX); /* global `t' (for later use) */
lua_pushstring(L, "a"); /* var name */
lua_pushstring(L, "f"); /* function name */
lua_gettable(L, LUA_GLOBALSINDEX); /* function to be called */
lua_pushstring(L, "how"); /* 1st argument */
lua_pushstring(L, "x"); /* push the string "x" */
lua_gettable(L, -5); /* push result of t.x (2nd arg) */
lua_pushnumber(L, 14); /* 3rd argument */
lua_call(L, 3, 1); /* call function with 3 arguments and 1 result */
lua_settable(L, LUA_GLOBALSINDEX); /* set global variable `a' */
lua_pop(L, 1); /* remove `t' from the stack */

Note that the code above is ``balanced'': at its end, the stack is
back to its original configuration. This is considered good
programming practice.

(We did this example using only the raw functions provided by Lua's
API, to show all the details. Usually programmers define and use
several macros and auxiliary functions that provide higher level
access to Lua. See the source code of the standard libraries for
examples.)

# «sec3.15»
# «Protected Calls»

3.15 Protected Calls

When you call a function with lua_call, any error inside the called
function is propagated upwards (with a longjmp). If you need to handle
errors, then you should use lua_pcall:

int lua_pcall (lua_State *L, int nargs, int nresults, int errfunc);

Both nargs and nresults have the same meaning as in lua_call. If there
are no errors during the call, lua_pcall behaves exactly like
lua_call. However, if there is any error, lua_pcall catches it, pushes
a single value at the stack (the error message), and returns an error
code. Like lua_call, lua_pcall always removes the function and its
arguments from the stack.

If errfunc is 0, then the error message returned is exactly the
original error message. Otherwise, errfunc gives the stack index for
an error handler function. (In the current implementation, that index
cannot be a pseudo-index.) In case of runtime errors, that function
will be called with the error message and its return value will be the
message returned by lua_pcall.

Typically, the error handler function is used to add more debug
information to the error message, such as a stack traceback. Such
information cannot be gathered after the return of lua_pcall, since by
then the stack has unwound.

The lua_pcall function returns 0 in case of success or one of the
following error codes (defined in lua.h):

. LUA_ERRRUN --- a runtime error.

. LUA_ERRMEM --- memory allocation error. For such errors, Lua does
not call the error handler function.

. LUA_ERRERR --- error while running the error handler function.

# «p31» (find-lua50page 31)

# «sec3.16»
# «Defining C Functions»

3.16 Defining C Functions

Lua can be extended with functions written in C. These functions must
be of type lua_CFunction, which is defined as

typedef int (*lua_CFunction) (lua_State *L);

A C function receives a Lua state and returns an integer, the number

In order to communicate properly with Lua, a C function must follow
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, its first argument (if any) is at index 1. 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 discharged by Lua. Like a Lua function, a C function called
by Lua can also return many results.

As an example, the following function receives a variable number of
numerical arguments and returns their average and sum:

static int foo (lua_State *L) {
int n = lua_gettop(L); /* number of arguments */
lua_Number sum = 0;
int i;
for (i = 1; i <= n; i++) {
if (!lua_isnumber(L, i)) {
lua_pushstring(L, "incorrect argument to function `average'");
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 */
}

To register a C function to Lua, there is the following convenience
macro:

#define lua_register(L,n,f) \
(lua_pushstring(L, n), \
lua_pushcfunction(L, f), \
lua_settable(L, LUA_GLOBALSINDEX))
/* lua_State *L; */
/* const char *n; */
/* lua_CFunction f; */

which receives the name the function will have in Lua and a pointer to
the function. Thus, the C function foo above may be registered in Lua
as average by calling

lua_register(L, "average", foo);

# «p32» (find-lua50page 32)

# «sec3.17»
# «Defining C Closures»

3.17 Defining C Closures

When a C function is created, it is possible to associate some values
with it, thus creating a C closure; 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 the
function

void lua_pushcclosure (lua_State *L, lua_CFunction fn, int n);

is used to push the C function onto the stack, with the argument n
telling how many values should be associated with the function
(lua_pushcclosure also pops these values from the stack); in fact, the
macro lua_pushcfunction is defined as lua_pushcclosure with n set to
0.

Then, whenever the C function is called, those values are located at
specific pseudo-indices. Those pseudo-indices are produced by a macro
lua_upvalueindex. The first value associated with a function is at
lua_upvalueindex(n), where n is greater than the number of upvalues of
the current function, produces an acceptable (but invalid) index.

For examples of C functions and closures, see the standard libraries
in the official Lua distribution (src/lib/*.c).

# «sec3.18»
# «Registry»

3.18 Registry

Lua provides a registry, a pre-defined table that can be used by any C
code to store whatever Lua value it needs to store, specially if the C
code needs to keep that Lua value outside the life span of a C
function. This table is always located at pseudo-index
LUA_REGISTRYINDEX. Any C library can store data into this table, as
long as it chooses keys different from other libraries. Typically, you
should use as key a string containing your library name or a light

The integer keys in the registry are used by the reference mechanism,
implemented by the auxiliary library, and therefore should not be used
by other purposes.

# «sec3.19»
# «Error Handling in C»

3.19 Error Handling in C

Internally, Lua uses the C longjmp facility to handle errors. When Lua
faces any error (such as memory allocation errors, type errors, syntax
errors) it raises an error, that is, it does a long jump. A protected
environment uses setjmp to set a recover point; any error jumps to the
most recent active recover point.

If an error happens outside any protected environment, Lua calls a
panic function and then calls exit(EXIT_FAILURE). You can change the
panic function with

lua_CFunction lua_atpanic (lua_State *L, lua_CFunction panicf);

Your new panic function may avoid the application exit by never
returning (e.g., by doing a long jump). Nevertheless, the
corresponding Lua state will not be consistent; the only safe
operation with it is to close it.

Almost any function in the API may raise an error, for instance due to
a memory allocation error. The following functions run in protected
mode (that is, they create a protected environment to run), so they
never raise an error: lua_open, lua_close, lua_load, and lua_pcall.

There is yet another function that runs a given C function in
protected mode:

# «p33» (find-lua50page 33)

int lua_cpcall (lua_State *L, lua_CFunction func, void *ud);

lua_cpcall calls func in protected mode. func starts with only one
element in its stack, a light userdata containing ud. In case of
errors, lua_cpcall returns the same error codes as lua_pcall (see (to
"sec3.15")), plus the error object on the top of the stack; otherwise,
it returns zero, and does not change the stack. Any value returned by

C code can generate a Lua error calling the function

void lua_error (lua_State *L);

The error message (which actually can be any type of object) must be
on the stack top. This function does a long jump, and therefore never
returns.

# «sec3.20»

Lua offers partial support for multiple threads of execution. If you
have a C library that offers multi-threading, then Lua can cooperate
with it to implement the equivalent facility in Lua. Also, Lua
implements its own coroutine system on top of threads. The following
function creates a new thread in Lua:

This function pushes the thread on the stack and returns a pointer to
a lua_State that represents this new thread. The new state returned by
this function shares with the original state all global objects (such
as tables), but has an independent run-time stack.

Each thread has an independent global environment table. When you
create a thread, this table is the same as that of the given state,
but you can change each one independently.

You cannot close the sole (or last) thread of a state. Instead, you
must close the state itself.

To manipulate threads as coroutines, Lua offers the following
functions:

int lua_resume (lua_State *L, int narg);
int lua_yield (lua_State *L, int nresults);

To start a coroutine, you first create a new thread; then you push on
its stack the body function plus any eventual arguments; then you call
lua_resume, with narg being the number of arguments. This call returns
when the coroutine suspends or finishes its execution. When it
returns, the stack contains all values passed to lua_yield, or all
values returned by the body function. lua_resume returns 0 if there
are no errors running the coroutine, or an error code (see (to
"sec3.15")). In case of errors, the stack contains only the error
message. To restart a coroutine, you put on its stack only the values
to be passed as results from yield, and then call lua_resume.

The lua_yield function can only be called as the return expression of
a C function, as follows:

return lua_yield (L, nresults);

When a C function calls lua_yield in that way, the running coroutine
suspends its execution, and the call to lua_resume that started this
coroutine returns. The parameter nresults is the number of values from
the stack that are passed as results to lua_resume.

To exchange values between different threads, you may use lua_xmove:

void lua_xmove (lua_State *from, lua_State *to, int n);

It pops n values from the stack from, and puhses them into the stack
to.

# «p34» (find-lua50page 34)

# «sec4»
# «The Debug Interface»

4 The Debug Interface

Lua has no built-in debugging facilities. Instead, it offers a special
interface by means of functions and hooks. This interface allows the
construction of different kinds of debuggers, profilers, and other
tools that need ``inside information'' from the interpreter.

# «sec4.1»
# «Stack and Function Information»

4.1 Stack and Function Information

The main function to get information about the interpreter runtime
stack is

int lua_getstack (lua_State *L, int level, lua_Debug *ar);

This function fills parts of a lua_Debug structure with an
identification of the activation record of the function executing at a
given level. Level 0 is the current running function, whereas level n+
1 is the function that has called level n. When there are no errors,
lua_getstack returns 1; when called with a level greater than the
stack depth, it returns 0.

The structure lua_Debug is used to carry different pieces of

typedef struct lua_Debug {
int event;
const char *name; /* (n) */
const char *namewhat; /* (n) `global', `local', `field', `method' */
const char *what; /* (S) `Lua' function, `C' function, Lua `main' */
const char *source; /* (S) */
int currentline; /* (l) */
int nups; /* (u) number of upvalues */
int linedefined; /* (S) */
char short_src[LUA_IDSIZE]; /* (S) */
/* private part */
...
} lua_Debug;

lua_getstack fills only the private part of this structure, for later
use. To fill the other fields of lua_Debug with useful information,
call

int lua_getinfo (lua_State *L, const char *what, lua_Debug *ar);

This function returns 0 on error (for instance, an invalid option in
what). Each character in the string what selects some fields of the
structure ar to be filled, as indicated by the letter in parentheses
in the definition of lua_Debug above: `S' fills in the fields source,
linedefined, and what; `l' fills in the field currentline, etc.
Moreover, `f' pushes onto the stack the function that is running at
the given level.

To get information about a function that is not active (that is, not
in the stack), you push it onto the stack and start the what string
with the character `>'. For instance, to know in which line a function
f was defined, you can write

lua_Debug ar;
lua_pushstring(L, "f");
lua_gettable(L, LUA_GLOBALSINDEX); /* get global `f' */
lua_getinfo(L, ">S", &ar);
printf("%d\n", ar.linedefined);

# «p35» (find-lua50page 35)

The fields of lua_Debug have the following meaning:

source If the function was defined in a string, then source is that
string. If the function was defined in a file, then source starts with
a `@' followed by the file name.

short_src A ``printable'' version of source, to be used in error
messages.

linedefined the line number where the definition of the function
starts.

what the string "Lua" if this is a Lua function, "C" if this is a C
function, "main" if this is the main part of a chunk, and "tail" if
this was a function that did a tail call. In the latter case, Lua has

currentline the current line where the given function is executing.
When no line information is available, currentline is set to -1.

name a reasonable name for the given function. Because functions in
Lua are first class values, they do not have a fixed name: Some
functions may be the value of multiple global variables, while others
may be stored only in a table field. The lua_getinfo function checks
how the function was called or whether it is the value of a global
variable to find a suitable name. If it cannot find a name, then name
is set to NULL.

namewhat Explains the name field. The value of namewhat can be
"global", "local", "method", "field", or "" (the empty string),
according to how the function was called. (Lua uses the empty string
when no other option seems to apply.)

nups The number of upvalues of the function.

# «sec4.2»
# «Manipulating Local Variables and Upvalues»

4.2 Manipulating Local Variables and Upvalues

For the manipulation of local variables and upvalues, the debug
interface uses indices: The first parameter or local variable has
index 1, and so on, until the last active local variable. Upvalues
have no particular order, as they are active through the whole
function.

The following functions allow the manipulation of the local variables
of a given activation record:

const char *lua_getlocal (lua_State *L, const lua_Debug *ar, int n);
const char *lua_setlocal (lua_State *L, const lua_Debug *ar, int n);

The parameter ar must be a valid activation record that was filled by
a previous call to lua_getstack or given as argument to a hook (see
(to "sec4.3")). lua_getlocal gets the index n of a local variable,
pushes the variable's value onto the stack, and returns its name.
lua_setlocal assigns the value at the top of the stack to the variable
and returns its name. Both functions return NULL when the index is
greater than the number of active local variables.

The following functions allow the manipulation of the upvalues of a
given function (unlike local variables, the upvalues of a function are
accessible even when the function is not active):

const char *lua_getupvalue (lua_State *L, int funcindex, int n);
const char *lua_setupvalue (lua_State *L, int funcindex, int n);

These functions operate both on Lua functions and on C functions. (For
Lua functions, upvalues are the external local variables that the
function uses, and that consequently are included in its closure.)
funcindex points to a function in the stack. lua_getpuvalue gets the
index n of an upvalue, pushes the upvalue's value onto the stack, and
returns its name. lua_setupvalue assigns the value at the top of the
stack to the upvalue and returns its name. Both functions return NULL
when the index is greater than the number of upvalues. For C
functions, these functions use the empty string "" as a name for all
upvalues.

# «p36» (find-lua50page 36)

As an example, the following function lists the names of all local
variables and upvalues for a function at a given level of the stack:

int listvars (lua_State *L, int level) {
lua_Debug ar;
int i;
const char *name;
if (lua_getstack(L, level, &ar) == 0)
return 0; /* failure: no such level in the stack */
i = 1;
while ((name = lua_getlocal(L, &ar, i++)) != NULL) {
printf("local %d %s\n", i-1, name);
lua_pop(L, 1); /* remove variable value */
}
lua_getinfo(L, "f", &ar); /* retrieves function */
i = 1;
while ((name = lua_getpuvalue(L, -1, i++)) != NULL) {
printf("upvalue %d %s\n", i-1, name);
lua_pop(L, 1); /* remove upvalue value */
}
return 1;
}

# «sec4.3»
# «Hooks»

4.3 Hooks

Lua offers a mechanism of hooks, which are user-defined C functions
that are called during the program execution. A hook may be called in
four different events: a call event, when Lua calls a function; a
return event, when Lua returns from a function; a line event, when Lua
starts executing a new line of code; and a count event, which happens
every ``count'' instructions. Lua identifies these events with the
following constants: LUA_HOOKCALL, LUA_HOOKRET (or LUA_HOOKTAILRET,
see below), LUA_HOOKLINE, and LUA_HOOKCOUNT.

A hook has type lua_Hook, defined as follows:

typedef void (*lua_Hook) (lua_State *L, lua_Debug *ar);

You can set the hook with the following function:

int lua_sethook (lua_State *L, lua_Hook func, int mask, int count);

func is the hook. mask specifies on which events the hook will be
called: It is formed by a disjunction of the constants LUA_MASKCALL,
the hook is called as explained below:

The call hook is called when the interpreter calls a function. The
hook is called just after Lua enters the new function.

The return hook is called when the interpreter returns from a
function. The hook is called just before Lua leaves the function.

# «p37» (find-lua50page 37)

The line hook is called when the interpreter is about to start the
execution of a new line of code, or when it jumps back in the code
(even to the same line). (This event only happens while Lua is
executing a Lua function.)

The count hook is called after the interpreter executes every count
instructions. (This event only happens while Lua is executing a Lua
function.)

A hook is disabled by setting mask to zero.

You can get the current hook, the current mask, and the current count
with the next functions:

lua_Hook lua_gethook (lua_State *L);
int lua_gethookcount (lua_State *L);

Whenever a hook is called, its ar argument has its field event set to
the specific event that triggered the hook. Moreover, for line events,
the field currentline is also set. To get the value of any other field
in ar, the hook must call lua_getinfo. For return events, event may be
LUA_HOOKRET, the normal value, or LUA_HOOKTAILRET. In the latter case,
Lua is simulating a return from a function that did a tail call; in
this case, it is useless to call lua_getinfo.

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,
that execution occurs without any calls to hooks.

# «sec5»
# «Standard Libraries»

5 Standard Libraries

The standard libraries provide useful functions that are implemented
directly through the C API. Some of these functions provide essential
services to the language (e.g., type and getmetatable); others provide
implemented in Lua itself, but are quite useful or have critical
performance to deserve an implementation in C (e.g., sort).

All libraries are implemented through the official C API and are
provided as separate C modules. Currently, Lua has the following
standard libraries:

. basic library;
. string manipulation;
. table manipulation;
. mathematical functions (sin, log, etc.);
. input and output;
. operating system facilities;
. debug facilities.

Except for the basic library, each library provides all its functions
as fields of a global table or as methods of its objects.

To have access to these libraries, the C host program must first call
the functions luaopen_base (for the basic library), luaopen_string
(for the string library), luaopen_table (for the table library),
luaopen_math (for the mathematical library), luaopen_io (for the I/O
and the Operating System libraries), and luaopen_debug (for the debug
library). These functions are declared in lualib.h.

# «p38» (find-lua50page 38)

# «sec5.1»
# «Basic Functions»

5.1 Basic Functions

The basic library provides some core functions to Lua. If you do not
include this library in your application, you should check carefully
whether you need to provide some alternative implementation for some
of its facilities.

# «assert»
. assert (v [, message])

Issues an error when the value of its argument v is nil or false;
otherwise, returns this value. message is an error message; when
absent, it defaults to ``assertion failed!''

# «collectgarbage»
. collectgarbage ([limit])

Sets the garbage-collection threshold to the given limit (in Kbytes)
and checks it against the byte counter. If the new threshold is
smaller than the byte counter, then Lua immediately runs the garbage
collector (see (to "sec2.9")). If limit is absent, it defaults to zero
(thus forcing a garbage-collection cycle).

# «dofile»
. dofile (filename)

Opens the named file and executes its contents as a Lua chunk. When
called without arguments, dofile executes the contents of the standard
input (stdin). Returns any value returned by the chunk. In case of
errors, dofile propagates the error to its caller (that is, it does
not run in protected mode).

# «error»
. error (message [, level])

Terminates the last protected function called and returns message as
the error message. Function error never returns.

The level argument specifies where the error message points the error.
With level 1 (the default), the error position is where the error
function was called. Level 2 points the error to where the function
that called error was called; and so on.

# «_G»
. _G

A global variable (not a function) that holds the global environment
(that is, _G._G = _G). Lua itself does not use this variable; changing
its value does not affect any environment. (Use setfenv to change
environments.)

# «getfenv»
. getfenv (f)

Returns the current environment in use by the function. f can be a Lua
function or a number, which specifies the function at that stack
level: Level 1 is the function calling getfenv. If the given function
is not a Lua function, or if f is 0, getfenv returns the global
environment. The default for f is 1.

If the environment has a "__fenv" field, returns the associated value,

# «p39» (find-lua50page 39)

# «getmetatable»
. getmetatable (object)

If the object does not have a metatable, returns nil. Otherwise, if
the object's metatable has a "__metatable" field, returns the
associated value. Otherwise, returns the metatable of the given
object.

# «gcinfo»
. gcinfo ()

Returns two results: the number of Kbytes of dynamic memory that Lua
is using and the current garbage collector threshold (also in Kbytes).

# «ipairs»
. ipairs (t)

Returns an iterator function, the table t, and 0, so that the
construction

for i,v in ipairs(t) do ... end

will iterate over the pairs (1,t[1]), (2,t[2]), . . . , up to the
first integer key with a nil value in the table.

Loads a file as a Lua chunk (without running it). If there are no
errors, returns the compiled chunk as a function; otherwise, returns
nil plus the error message. The environment of the returned function
is the global environment.

Links the program with the dynamic C library libname. Inside this
library, looks for a function funcname and returns this function as a
C function.

libname must be the complete file name of the C library, including any
eventual path and extension.

This function is not supported by ANSI C. As such, it is only
available on some platforms (Windows, Linux, Solaris, BSD, plus other
Unix systems that support the dlfcn standard).

Loads a string as a Lua chunk (without running it). If there are no
errors, returns the compiled chunk as a function; otherwise, returns
nil plus the error message. The environment of the returned function
is the global environment.

The optional parameter chunkname is the name to be used in error
messages and debug information. To load and run a given string, use
the idiom

# «p40» (find-lua50page 40)

# «next»
. next (table [, index])

Allows a program to traverse all fields of a table. Its first argument
is a table and its second argument is an index in this table. next
returns the next index of the table and the value associated with the
index. When called with nil as its second argument, next returns the
first index of the table and its associated value. When called with
the last index, or with nil in an empty table, next returns nil. If
the second argument is absent, then it is interpreted as nil.

Lua has no declaration of fields; There is no difference between a
field not present in a table or a field with value nil. Therefore,
next only considers fields with non-nil values. The order in which the
indices are enumerated is not specified, even for numeric indices. (To
traverse a table in numeric order, use a numerical for or the ipairs
function.)

The behavior of next is undefined if, during the traversal, you assign
any value to a non-existent field in the table.

# «pairs»
. pairs (t)

Returns the next function and the table t (plus a nil), so that the
construction

for k,v in pairs(t) do ... end

will iterate over all key--value pairs of table t.

# «pcall»
. pcall (f, arg1, arg2, ...)

Calls function f with the given arguments in protected mode. That
means that any error inside f is not propagated; instead, pcall
catches the error and returns a status code. Its first result is the
status code (a boolean), which is true if the call succeeds without
errors. In such case, pcall also returns all results from the call,
after this first result. In case of any error, pcall returns false
plus the error message.

# «print»
. print (e1, e2, ...)

Receives any number of arguments, and prints their values in stdout,
using the tostring function to convert them to strings. This function
is not intended for formatted output, but only as a quick way to show
a value, typically for debugging. For formatted output, use format
(see (to "sec5.3")).

# «rawequal»
. rawequal (v1, v2)

Checks whether v1 is equal to v2, without invoking any metamethod.
Returns a boolean.

# «rawget»
. rawget (table, index)

Gets the real value of table[index], without invoking any metamethod.
table must be a table; index is any value different from nil.

# «rawset»
. rawset (table, index, value)

Sets the real value of table[index] to value, without invoking any
metamethod. table must be a table, index is any value different from
nil, and value is any Lua value.

# «p41» (find-lua50page 41)

# «require»
. require (packagename)

Loads the given package. The function starts by looking into the table
then require returns the value that the package returned when it was
first loaded. Otherwise, it searches a path looking for a file to

If the global variable LUA_PATH is a string, this string is the path.
Otherwise, require tries the environment variable LUA_PATH. As a last
resort, it uses the predefined path "?;?.lua".

The path is a sequence of templates separated by semicolons. For each
template, require will change each interrogation mark in the template
to packagename, and then will try to load the resulting file name. So,
for instance, if the path is

"./?.lua;./?.lc;/usr/local/?/?.lua;/lasttry"

a require "mod" will try to load the files ./mod.lua, ./mod.lc,
/usr/local/mod/mod.lua, and /lasttry, in that order.

The function stops the search as soon as it can load a file, and then
it runs the file. After that, it associates, in table _LOADED, the
package name with the value that the package returned, and returns
that value. If the package returns nil (or no value), require converts
this value to true. If the package returns false, require also returns
false. However, as the mark in table _LOADED is false, any new attempt
to reload the file will happen as if the package was not loaded (that
is, the package will be loaded again).

If there is any error loading or running the file, or if it cannot
find any file in the path, then require signals an error.

While running a file, require defines the global variable
_REQUIREDNAME with the package name. The package being loaded always
runs within the global environment.

# «setfenv»
. setfenv (f, table)

Sets the current environment to be used by the given function. f can
be a Lua function or a number, which specifies the function at that
stack level: Level 1 is the function calling setfenv.

As a special case, when f is 0 setfenv changes the global environment

If the original environment has a "__fenv" field, setfenv raises an
error.

# «setmetatable»
. setmetatable (table, metatable)

Sets the metatable for the given table. (You cannot change the
metatable of a userdata from Lua.) If metatable is nil, removes the
metatable of the given table. If the original metatable has a
"__metatable" field, raises an error.

# «tonumber»
. tonumber (e [, base])

Tries to convert its argument to a number. If the argument is already
a number or a string convertible to a number, then tonumber returns
that number; otherwise, it returns nil.

An optional argument specifies the base to interpret the numeral. The
base may be any integer between 2 and 36, inclusive. In bases above
10, the letter `A' (in either upper or lower case) represents 10, `B'
represents 11, and so forth, with `Z' representing 35. In base 10 (the
default), the number may have a decimal part, as well as an optional
exponent part (see (to "sec2.2.1")). In other bases, only unsigned
integers are accepted.

# «p42» (find-lua50page 42)

# «tostring»
. tostring (e)

Receives an argument of any type and converts it to a string in a
reasonable format. For complete control of how numbers are converted,
use format (see (to "sec5.3")).

If the metatable of e has a "__tostring" field, tostring calls the
corresponding value with e as argument, and uses the result of the
call as its result.

# «type»
. type (v)

Returns the type of its only argument, coded as a string. The possible
results of this function are "nil" (a string, not the value nil),
"number", "string", "boolean, "table", "function", "thread", and
"userdata".

# «unpack»
. unpack (list)

Returns all elements from the given list. This function is equivalent
to

return list[1], list[2], ..., list[n]

except that the above code can be written only for a fixed n. The
number n is the size of the list, as defined for the table.getn
function.

# «_VERSION»
. _VERSION

A global variable (not a function) that holds a string containing the
current interpreter version. The current content of this string is
"Lua 5.0".

# «xpcall»
. xpcall (f, err)

This function is similar to pcall, except that you can set a new error
handler.

xpcall calls function f in protected mode, using err as the error
handler. Any error inside f is not propagated; instead, xpcall catches
the error, calls the err function with the original error object, 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, xpcall also returns all results from the call, after this first
result. In case of any error, xpcall returns false plus the result
from err.

# «sec5.2»
# «Coroutine Manipulation»

5.2 Coroutine Manipulation

The operations related to coroutines comprise a sub-library of the
basic library and come inside the table coroutine. See (to "sec2.10")
for a general description of coroutines.

# «coroutine.create»
. coroutine.create (f)

Creates a new coroutine, with body f. f must be a Lua function.
Returns this new coroutine, an object with type "thread".

# «p43» (find-lua50page 43)

# «coroutine.resume»
. coroutine.resume (co, val1, ...)

Starts or continues the execution of coroutine co. The first time you
resume a coroutine, it starts running its body. The arguments val1, .
. . go as the arguments to the body function. If the coroutine has
yielded, resume restarts it; the arguments val1, . . . go as the
results from the yield.

If the coroutine runs without any errors, resume returns true plus any
values passed to yield (if the coroutine yields) or any values
returned by the body function (if the coroutine terminates). If there
is any error, resume returns false plus the error message.

# «coroutine.status»
. coroutine.status (co)

Returns the status of coroutine co, as a string: "running", if the
coroutine is running (that is, it called status); "suspended", if the
coroutine is suspended in a call to yield, or if it has not started
running yet; and "dead" if the coroutine has finished its body
function, or if it has stopped with an error.

# «coroutine.wrap»
. coroutine.wrap (f)

Creates a new coroutine, with body f. f must be a Lua function.
Returns a function that resumes the coroutine each time it is called.
Any arguments passed to the function behave as the extra arguments to
resume. Returns the same values returned by resume, except the first
boolean. In case of error, propagates the error.

# «coroutine.yield»
. coroutine.yield (val1, ...)

Suspends the execution of the calling coroutine. The coroutine cannot
be running neither a C function, nor a metamethod, nor an iterator.
Any arguments to yield go as extra results to resume.

# «sec5.3»
# «String Manipulation»

5.3 String Manipulation

This library provides generic functions for string manipulation, such
as finding and extracting substrings, and pattern matching. When
indexing a string in Lua, the first character is at position 1 (not at
0, as in C). Indices are allowed to be negative and are interpreted as
indexing backwards, from the end of the string. Thus, the last
character is at position -1, and so on.

The string library provides all its functions inside the table string.

# «string.byte»
. string.byte (s [, i])

Returns the internal numerical code of the i-th character of s, or nil
if the index is out of range. If i is absent, then it is assumed to be
1. i may be negative.

Note that numerical codes are not necessarily portable across
platforms.

# «string.char»
. string.char (i1, i2, ...)

Receives 0 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 correspondent argument.

Note that numerical codes are not necessarily portable across
platforms.

# «p44» (find-lua50page 44)

# «string.dump»
. string.dump (function)

Returns a binary representation of the given function, so that a later
loadstring on that string returns a copy of the function. function
must be a Lua function without upvalues.

# «string.find»
. string.find (s, pattern [, init [, plain]])

Looks for the first match of pattern in the string s. If it finds one,
then find returns the indices of s where this occurrence starts and
ends; otherwise, it returns nil. If the pattern specifies captures
(see string.gsub below), the captured strings are returned as extra
results. A third, optional numerical argument init specifies where to
start the search; its default value is 1 and may be negative. A value
of true as a fourth, optional argument plain turns off the pattern
matching facilities, so the function does a plain ``find substring''
operation, with no characters in pattern being considered ``magic''.
Note that if plain is given, then init must be given too.

# «string.len»
. string.len (s)

Receives a string and returns its length. The empty string "" has
length 0. Embedded zeros are counted, so "a\000b\000c" has length 5.

# «string.lower»
. string.lower (s)

Receives a string and returns a copy of that string with all uppercase
letters changed to lowercase. All other characters are left unchanged.
The definition of what is an uppercase letter depends on the current
locale.

# «string.rep»
. string.rep (s, n)

Returns a string that is the concatenation of n copies of the string
s.

# «string.sub»
. string.sub (s, i [, j])

Returns the substring of s that starts at i and continues until j; i
and j may be negative. If j is absent, then it is assumed to be equal
to -1 (which is the same as the string length). In particular, the
call string.sub(s,1,j) returns a prefix of s with length j, and
string.sub(s, -i) returns a suffix of s with length i.

# «string.upper»
. string.upper (s)

Receives a string and returns a copy of that string with all lowercase
letters changed to uppercase. All other characters are left unchanged.
The definition of what is a lowercase letter depends on the current
locale.

# «string.format»
. string.format (formatstring, e1, e2, ...)

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 printf family
of standard C functions. The only differences are that the
options/modifiers *, l, L, n, p, and h are not supported, and there is
an extra option, q. The q option formats a string in a form suitable
to be safely read back by the Lua interpreter: The string is written
between double quotes,

# «p45» (find-lua50page 45)

and all double quotes, newlines, and backslashes in the string are
correctly escaped when written. For instance, the call

string.format('%q', 'a string with "quotes" and \n new line')

will produce the string:

"a string with \"quotes\" and \
new line"

The options c, d, E, e, f, g, G, i, o, u, X, and x all expect a number
as argument, whereas q and s expect a string. The * modifier can be
simulated by building the appropriate format string. For example,
"%*g" can be simulated with "%"..width.."g".

String values to be formatted with %s cannot contain embedded zeros.

# «string.gfind»
. string.gfind (s, pat)

Returns an iterator function that, each time it is called, returns the
next captures from pattern pat over string s.

If pat specifies no captures, then the whole match is produced in each
call.

As an example, the following loop

s = "hello world from Lua"
for w in string.gfind(s, "%a+") do
print(w)
end

will iterate over all the words from string s, printing one per line.
The next example collects all pairs key=value from the given string
into a table:

t = {}
s = "from=world, to=Lua"
for k, v in string.gfind(s, "(%w+)=(%w+)") do
t[k] = v
end

# «string.gsub»
. string.gsub (s, pat, repl [, n])

Returns a copy of s in which all occurrences of the pattern pat have
been replaced by a replacement string specified by repl. gsub also
returns, as a second value, the total number of substitutions made.

If repl is a string, then its value is used for replacement. Any
sequence in repl of the form %n, with n between 1 and 9, stands for
the value of the n-th captured substring (see below).

If repl is a function, then this function is called every time a match
occurs, with all captured substrings passed as arguments, in order; if
the pattern specifies no captures, then the whole match is passed as a
sole argument. If the value returned by this function is a string,
then it is used as the replacement string; otherwise, the replacement
string is the empty string.

The optional last parameter n limits the maximum number of
substitutions to occur. For instance, when n is 1 only the first
occurrence of pat is replaced.

Here are some examples:

# «p46» (find-lua50page 46)

x = string.gsub("hello world", "(%w+)", "%1 %1")
--> x="hello hello world world"
x = string.gsub("hello world", "(%w+)", "%1 %1", 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)
end)
--> x="4+5 = 9"
local t = {name="lua", version="5.0"}
x = string.gsub("\$name_\$version.tar.gz", "%\$(%w+)", function (v)
return t[v]
end)
--> x="lua_5.0.tar.gz"

# «Patterns»

Patterns

# «Character Class»

Character Class: A character class is used to represent a set of
characters. The following combinations are allowed in describing a
character class:

x (where x is not one of the magic characters ^\$()%.[]*+-?) ---
represents the character x itself.
. --- (a dot) represents all characters.
%a --- represents all letters.
%c --- represents all control characters.
%d --- represents all digits.
%l --- represents all lowercase letters.
%p --- represents all punctuation characters.
%s --- represents all space characters.
%u --- represents all uppercase letters.
%w --- represents all alphanumeric characters.
%x --- represents all hexadecimal digits.
%z --- represents the character with representation 0.
%x (where x is any non-alphanumeric character) --- represents the
character x . This is the standard way to escape the magic
characters. Any punctuation character (even the non magic) can be
preceded by a `%' when used to represent itself in a pattern.

# «p47» (find-lua50page 47)

[set] --- represents the class which is the union of all characters in
set . A range of characters may be specified by separating the end
characters of the range with a `-'. All classes %x described above may
also be used as components in set . All other characters in set
represent themselves. For example, [%w_] (or [_%w]) represents all
alphanumeric characters plus the underscore, [0-7] represents the
octal digits, and [0-7%l%-] represents the octal digits plus the
lowercase letters plus the `-' character.

The interaction between ranges and classes is not defined. Therefore,
patterns like [%a-z] or [a-%%] have no meaning.

[^set] --- represents the complement of set , where set is interpreted
as above. For all classes represented by single letters (%a, %c,
etc.), the corresponding uppercase letter represents the complement of
the class. For instance, %S represents all non-space characters.

The definitions of letter, space, and other character groups depend on
the current locale. In particular, the class [a-z] may not be
equivalent to %l. The second form should be preferred for portability.

# «Pattern Item»

Pattern Item: A pattern item may be

. a single character class, which matches any single character in the
class;

. a single character class followed by `*', which matches 0 or more
repetitions of characters in the class. These repetition items will
always match the longest possible sequence;

. a single character class followed by `+', which matches 1 or more
repetitions of characters in the class. These repetition items will
always match the longest possible sequence;

. a single character class followed by `-', which also matches 0 or
more repetitions of characters in the class. Unlike `*', these
repetition items will always match the shortest possible sequence;

. a single character class followed by `?', which matches 0 or 1
occurrence of a character in the class;

. %n, for n between 1 and 9; such item matches a substring equal to
the n-th captured string (see below);

. %bxy , where x and y are two distinct characters; such item matches
strings that start with x , end with y , and where the x and y are
balanced. This means that, if one reads the string from left to right,
counting +1 for an x and -1 for a y , the ending y is the first y
where the count reaches 0. For instance, the item %b() matches
expressions with balanced parentheses.

# «Pattern»

Pattern: A pattern is a sequence of pattern items. A `^' at the
beginning of a pattern anchors the match at the beginning of the
subject string. A `\$' at the end of a pattern anchors the match at the
end of the subject string. At other positions, `^' and `\$' have no
special meaning and represent themselves.

# «Captures»

Captures: A pattern may contain sub-patterns enclosed in parentheses;
they describe captures. When a match succeeds, the substrings of the
subject string that match captures are stored (captured) for future
use. Captures are numbered according to their left parentheses. For
instance, in the pattern "(a*(.)%w(%s*))", the part of the string
matching "a*(.)%w(%s*)" is stored as the first capture (and therefore
has number 1); the character matching . is captured with number 2, and
the part matching %s* has number 3.

# «p48» (find-lua50page 48)

As a special case, the empty capture () captures the current string
position (a number). For instance, if we apply the pattern "()aa()" on
the string "flaaap", there will be two captures: 3 and 5.

A pattern cannot contain embedded zeros. Use %z instead.

# «sec5.4»
# «Table Manipulation»

5.4 Table Manipulation

This library provides generic functions for table manipulation. It
provides all its functions inside the table table.

Most functions in the table library assume that the table represents
an array or a list. For those functions, an important concept is the
size of the array. There are three ways to specify that size:

. the field "n" --- When the table has a field "n" with a numerical
value, that value is assumed as its size.

. setn --- You can call the table.setn function to explicitly set the
size of a table.

. implicit size --- Otherwise, the size of the object is one less the
first integer index with a nil value.

For more details, see the descriptions of the table.getn and
table.setn functions.

# «table.concat»
. table.concat (table [, sep [, i [, j]]])

Returns table[i]..sep..table[i+1] ... sep..table[j]. The default value
for sep is the empty string, the default for i is 1, and the default
for j is the size of the table. If i is greater than j, returns the
empty string.

# «table.foreach»
. table.foreach (table, f)

Executes the given f over all elements of table. For each element, f
is called with the index and respective value as arguments. If f
returns a non-nil value, then the loop is broken, and this value is
returned as the final value of foreach.

See the next function for extra information about table traversals.

# «table.foreachi»
. table.foreachi (table, f)

Executes the given f over the numerical indices of table. For each
index, f is called with the index and respective value as arguments.
Indices are visited in sequential order, from 1 to n, where n is the
size of the table (see (to "sec5.4")). If f returns a non-nil value,
then the loop is broken and this value is returned as the result of
foreachi.

# «table.getn»
. table.getn (table)

Returns the size of a table, when seen as a list. If the table has an
n field with a numeric value, this value is the size of the table.
Otherwise, if there was a previous call to table.setn over this table,
the respective value is returned. Otherwise, the size is one less the
first integer index with a nil value.

# «p49» (find-lua50page 49)

# «table.sort»
. table.sort (table [, comp])

Sorts table elements in a given order, in-place, from table[1] to
table[n], where n is the size of the table (see (to "sec5.4")). If
comp is given, then it must be a function that receives two table
elements, and returns true when the first is less than the second (so
that not comp(a[i+1],a[i]) will be true after the sort). If comp is
not given, then the standard Lua operator < is used instead.

The sort algorithm is not stable, that is, elements considered equal
by the given order may have their relative positions changed by the
sort.

# «table.insert»
. table.insert (table, [pos,] value)

Inserts element value at position pos in table, shifting up other
elements to open space, if necessary. The default value for pos is
n+1, where n is the size of the table (see (to "sec5.4")), so that a
call table.insert(t,x) inserts x at the end of table t. This function
also updates the size of the table by calling table.setn(table, n+1).

# «table.remove»
. table.remove (table [, pos])

Removes from table the element at position pos, shifting down other
elements to close the space, if necessary. Returns the value of the
removed element. The default value for pos is n, where n is the size
of the table (see (to "sec5.4")), so that a call table.remove(t)
removes the last element of table t. This function also updates the
size of the table by calling table.setn(table, n-1).

# «table.setn»
. table.setn (table, n)

Updates the size of a table. If the table has a field "n" with a
numerical value, that value is changed to the given n. Otherwise, it
updates an internal state so that subsequent calls to
table.getn(table) return n.

# «sec5.5»
# «Mathematical Functions»

5.5 Mathematical Functions

This library is an interface to most of the functions of the standard
C math library. (Some have slightly different names.) It provides all
its functions inside the table math. In addition, it registers the
global __pow for the binary exponentiation operator ^, so that x^y
returns x y . The library provides the following functions:

math.abs math.acos math.asin math.atan math.atan2
math.ceil math.cos math.deg math.exp math.floor
math.log math.log10 math.max math.min math.mod
math.frexp math.ldexp math.random math.randomseed

plus a variable math.pi. Most of them are only interfaces to the
corresponding functions in the C library. All trigonometric functions
work in radians (previous versions of Lua used degrees). The functions

The function math.max returns the maximum value of its numeric
arguments. Similarly, math.min computes the minimum. Both can be used
with 1, 2, or more arguments.

The functions math.random and math.randomseed are interfaces to the
simple random generator functions rand and srand that are provided by
ANSI C. (No guarantees can be given for their statistical properties.)
When called without arguments, math.random returns a pseudo-random

# «p50» (find-lua50page 50)

real number in the range [0, 1). When called with a number n,
math.random returns a pseudo-random integer in the range [1, n]. When
called with two arguments, l and u, math.random returns a
pseudo-random integer in the range [l, u]. The math.randomseed
function sets a ``seed'' for the pseudo-random generator: Equal seeds
produce equal sequences of numbers.

# «sec5.6»
# «Input and Output Facilities»

5.6 Input and Output Facilities

The I/O library provides two different styles for file manipulation.
The first one uses implicit file descriptors, that is, there are
operations to set a default input file and a default output file, and
all input/output operations are over those default files. The second
style uses explicit file descriptors.

When using implicit file descriptors, all operations are supplied by
table io. When using explicit file descriptors, the operation io.open
returns a file descriptor and then all operations are supplied as
methods by the file descriptor.

The table io also provides three predefined file descriptors with
their usual meanings from C: io.stdin, io.stdout, and io.stderr.

A file handle is a userdata containing the file stream (FILE*), with a
distinctive metatable created by the I/O library.

Unless otherwise stated, all I/O functions return nil on failure (plus
an error message as a second result) and some value different from nil
on success.

# «io.close»
. io.close ([file])

Equivalent to file:close(). Without a file, closes the default output
file.

# «io.flush»
. io.flush ()

Equivalent to file:flush over the default output file.

# «io.input»
. io.input ([file])

When called with a file name, it opens the named file (in text mode),
and sets its handle as the default input file. When called with a file
handle, it simply sets that file handle as the default input file.
When called without parameters, it returns the current default input
file.

In case of errors this function raises the error, instead of returning
an error code.

# «io.lines»
. io.lines ([filename])

Opens the given file name in read mode and returns an iterator
function that, each time it is called, returns a new line from the
file. Therefore, the construction

for line in io.lines(filename) do ... end

will iterate over all lines of the file. When the iterator function
detects the end of file, it returns nil (to finish the loop) and
automatically closes the file.

The call io.lines() (without a file name) is equivalent to
io.input():lines(), that is, it iterates over the lines of the default
input file.

# «p51» (find-lua50page 51)

# «io.open»
. io.open (filename [, mode])

This function opens a file, in the mode specified in the string mode.
It returns a new file handle, or, in case of errors, nil plus an error
message.

The mode string can be any of the following:

``w'' write mode;
``a'' append mode;
``r+'' update mode, all previous data is preserved;
``w+'' update mode, all previous data is erased;
``a+'' append update mode, previous data is preserved, writing is only
allowed at the end of file.

The mode string may also have a b at the end, which is needed in some
systems to open the file in binary mode. This string is exactly what
is used in the standard C function fopen.

# «io.output»
. io.output ([file])

Similar to io.input, but operates over the default output file.

# «io.tmpfile»
. io.tmpfile ()

Returns a handle for a temporary file. This file is open in update
mode and it is automatically removed when the program ends.

# «io.type»
. io.type (obj)

Checks whether obj is a valid file handle. Returns the string "file"
if obj is an open file handle, "closed file" if obj is a closed file
handle, and nil if obj is not a file handle.

# «io.write»
. io.write (value1, ...)

Equivalent to io.output():write.

# «file:close»
. file:close ()

Closes file.

# «file:flush»
. file:flush ()

Saves any written data to file.

# «p52» (find-lua50page 52)

# «file:lines»
. file:lines ()

Returns an iterator function that, each time it is called, returns a
new line from the file. Therefore, the construction

for line in file:lines() do ... end

will iterate over all lines of the file. (Unlike io.lines, this
function does not close the file when the loop ends.)

Reads the file file, according to the given formats, which specify
what to read. For each format, the function returns a string (or a
number) with the characters read, or nil if it cannot read data with
the specified format. When called without formats, it uses a default
format that reads the entire next line (see below).

The available formats are

``*n'' reads a number; this is the only format that returns a number

``*a'' reads the whole file, starting at the current position. On end
of file, it returns the empty string.

``*l'' reads the next line (skipping the end of line), returning nil
on end of file. This is the default format.

number reads a string with up to that number of characters, returning
nil on end of file. If number is zero, it reads nothing and returns
an empty string, or nil on end of file.

# «file:seek»
. file:seek ([whence] [, offset])

Sets and gets the file position, measured from the beginning of the
file, to the position given by offset plus a base specified by the
string whence, as follows:

``set'' base is position 0 (beginning of the file);
``cur'' base is current position;
``end'' base is end of file;

In case of success, function seek returns the final file position,
measured in bytes from the beginning of the file. If this function
fails, it returns nil, plus a string describing the error.

The default value for whence is "cur", and for offset is 0. Therefore,
the call file:seek() returns the current file position, without
changing it; the call file:seek("set") sets the position to the
beginning of the file (and returns 0); and the call file:seek("end")
sets the position to the end of the file, and returns its size.

# «file:write»
. file:write (value1, ...)

Writes the value of each of its arguments to the filehandle file. The
arguments must be strings or numbers. To write other values, use
tostring or string.format before write.

# «sec5.7»
# «Operating System Facilities»

5.7 Operating System Facilities

This library is implemented through table os.

# «p53» (find-lua50page 53)

. os.clock ()

Returns an approximation of the amount of CPU time used by the
program, in seconds.

# «os.date»
. os.date ([format [, time]])

Returns a string or a table containing date and time, formatted
according to the given string format.

If the time argument is present, this is the time to be formatted (see
the os.time function for a description of this value). Otherwise, date
formats the current time.

If format starts with `!', then the date is formatted in Coordinated
Universal Time. After that optional character, if format is *t, then
date returns a table with the following fields: year (four digits),
month (1--12), day (1--31), hour (0--23), min (0--59), sec (0--61),
wday (weekday, Sunday is 1), yday (day of the year), and isdst
(daylight saving flag, a boolean).

If format is not *t, then date returns the date as a string, formatted
according with the same rules as the C function strftime.

When called without arguments, date returns a reasonable date and time
representation that depends on the host system and on the current
locale (that is, os.date() is equivalent to os.date("%c")).

# «os.difftime»
. os.difftime (t2, t1)

Returns the number of seconds from time t1 to time t2. In Posix,
Windows, and some other systems, this value is exactly t2-t1.

# «os.execute»
. os.execute (command)

This function is equivalent to the C function system. It passes
command to be executed by an operating system shell. It returns a
status code, which is system-dependent.

# «os.exit»
. os.exit ([code])

Calls the C function exit, with an optional code, to terminate the
host program. The default value for code is the success code.

# «os.getenv»
. os.getenv (varname)

Returns the value of the process environment variable varname, or nil
if the variable is not defined.

# «os.remove»
. os.remove (filename)

Deletes the file with the given name. If this function fails, it
returns nil, plus a string describing the error.

# «os.rename»
. os.rename (oldname, newname)

Renames file named oldname to newname. If this function fails, it
returns nil, plus a string describing the error.

# «p54» (find-lua50page 54)

# «os.setlocale»
. os.setlocale (locale [, category])

Sets the current locale of the program. locale is a string specifying
a locale; category is an optional string describing which category to
change: "all", "collate", "ctype", "monetary", "numeric", or "time";
the default category is "all". The function returns the name of the
new locale, or nil if the request cannot be honored.

# «os.time»
. os.time ([table])

Returns the current time when called without arguments, or a time
representing the date and time specified by the given table. This
table must have fields year, month, and day, and may have fields hour,
min, sec, and isdst (for a description of these fields, see the
os.date function).

The returned value is a number, whose meaning depends on your system.
In Posix, Windows, and some other systems, this number counts the
number of seconds since some given start time (the ``epoch''). In
other systems, the meaning is not specified, and the number returned
by time can be used only as an argument to date and difftime.

# «os.tmpname»
. os.tmpname ()

Returns a string with a file name that can be used for a temporary
file. The file must be explicitly opened before its use and removed
when no longer needed.

This function is equivalent to the tmpnam C function, and many people
(and even some compilers!) advise against its use, because between the
time you call this function and the time you open the file, it is
possible for another process to create a file with the same name.

# «sec5.8»
# «The Reflexive Debug Interface»

5.8 The Reflexive Debug Interface

The debug library provides the functionality of the debug interface to
Lua programs. You should exert care when using this library. The
functions provided here should be used exclusively for debugging and
them as a usual programming tool: They can be very slow. Moreover,
setlocal and getlocal violate the privacy of local variables and
therefore can compromise some otherwise secure code.

All functions in this library are provided inside a debug table.

# «debug.debug»
. debug.debug ()

Enters an interactive mode with the user, running each string that the
user enters. Using simple commands and other debug facilities, the
user can inspect global and local variables, change their values,
evaluate expressions, and so on. A line containing only the word cont
finishes this function, so that the caller continues its execution.

Note that commands for debug.debug are not lexically nested with any

# «debug.gethook»
. debug.gethook ()

Returns the current hook settings, as three values: the current hook
function, the current hook mask, and the current hook count (as set by
the debug.sethook function).

# «p55» (find-lua50page 55)

. debug.getinfo (function [, what])

This function returns a table with information about a function. You
can give the function directly, or you can give a number as the value
of function, which means the function running at level function of the
call stack: Level 0 is the current function (getinfo itself); level 1
is the function that called getinfo; and so on. If function is a
number larger than the number of active functions, then getinfo
returns nil.

The returned table contains all the fields returned by lua_getinfo,
with the string what describing which fields to fill in. The default
for what is to get all information available. If present, the option
`f' adds a field named func with the function itself.

For instance, the expression debug.getinfo(1,"n").name returns the
name of the current function, if a reasonable name can be found, and
debug.getinfo(print) returns a table with all available information

# «debug.getlocal»
. debug.getlocal (level, local)

This function returns the name and the value of the local variable
with index local of the function at level level of the stack. (The
first parameter or local variable has index 1, and so on, until the
last active local variable.) The function returns nil if there is no
local variable with the given index, and raises an error when called
with a level out of range. (You can call debug.getinfo to check
whether the level is valid.)

# «debug.getupvalue»
. debug.getupvalue (func, up)

This function returns the name and the value of the upvalue with index
up of the function func. The function returns nil if there is no
upvalue with the given index.

# «debug.setlocal»
. debug.setlocal (level, local, value)

This function assigns the value value to the local variable with index
local of the function at level level of the stack. The function
returns nil if there is no local variable with the given index, and
raises an error when called with a level out of range. (You can call
getinfo to check whether the level is valid.)

# «debug.setupvalue»
. debug.setupvalue (func, up, value)

This function assigns the value value to the upvalue with index up of
the function func. The function returns nil if there is no upvalue
with the given index.

# «debug.sethook»
. debug.sethook (hook, mask [, count])

Sets the given function as a hook. The string mask and the number
count describe when the hook will be called. The string mask may have
the following characters, with the given meaning:

"c" The hook is called every time Lua calls a function;
"r" The hook is called every time Lua returns from a function;
"l" The hook is called every time Lua enters a new line of code.

# «p56» (find-lua50page 56)

With a count different from zero, the hook is called after every count
instructions.

When called without arguments, the debug.sethook function turns off
the hook.

When the hook is called, its first parameter is always a string
describing the event that triggered its call: "call", "return" (or
"tail return"), "line", and "count". Moreover, for line events, it
also gets as its second parameter the new line number. Inside a hook,
running function (level 0 is the getinfo function, and level 1 is the
hook function), unless the event is "tail return". In this case, Lua
is only simulating the return, and a call to getinfo will return
invalid data.

# «debug.traceback»
. debug.traceback ([message])

Returns a string with a traceback of the call stack. An optional
message string is appended at the beginning of the traceback. This
function is typically used with xpcall to produce better error
messages.

# «sec6»
# «Lua Stand-alone»

6 Lua Stand-alone

Although Lua has been designed as an extension language, to be
embedded in a host C program, it is also frequently used as a
stand-alone language. An interpreter for Lua as a stand-alone
language, called simply lua, is provided with the standard
distribution. The stand-alone interpreter includes all standard
libraries plus the reflexive debug interface. Its usage is:

lua [options] [script [args]]

The options are:

- executes stdin as a file;
-e stat executes string stat ;
-l file ``requires'' file;
-i enters interactive mode after running script ;
-v prints version information;
-- stop handling options.

After handling its options, lua runs the given script, passing to it
the given args. When called without arguments, lua behaves as lua -v
-i when stdin is a terminal, and as lua - otherwise. Before running
any argument, the interpreter checks for an environment variable
LUA_INIT. If its format is @filename, then lua executes the file.
Otherwise, lua executes the string itself. All options are handled in
order, except -i. For instance, an invocation like

\$ lua -e'a=1' -e 'print(a)' script.lua

will first set a to 1, then print a, and finally run the file
script.lua. (Here, \$ is the shell prompt. Your prompt may be
different.)

Before starting to run the script, lua collects all arguments in the
command line in a global table called arg. The script name is stored
in index 0, the first argument after the script name goes to index 1,
and so on. The field n gets the number of arguments after the script
name. Any arguments before the script name (that is, the interpreter
name plus the options) go to negative indices. For instance, in the
call

# «p57» (find-lua50page 57)

\$ lua -la.lua b.lua t1 t2

the interpreter first runs the file a.lua, then creates a table

arg = { [-2] = "lua", [-1] = "-la.lua", [0] = "b.lua",
[1] = "t1", [2] = "t2"; n = 2 }

and finally runs the file b.lua.

In interactive mode, if you write an incomplete statement, the
interpreter waits for its completion.

If the global variable _PROMPT is defined as a string, then its value
is used as the prompt. Therefore, the prompt can be changed directly
on the command line:

\$ lua -e"_PROMPT='myprompt> '" -i

(the outer pair of quotes is for the shell, the inner is for Lua), or
in any Lua programs by assigning to _PROMPT. Note the use of -i to
enter interactive mode; otherwise, the program would end just after
the assignment to _PROMPT.

In Unix systems, Lua scripts can be made into executable programs by
using chmod +x and the #! form, as in

#!/usr/local/bin/lua

(Of course, the location of the Lua interpreter may be different in

#!/usr/bin/env lua

is a more portable solution.)

Acknowledgments

The Lua team is grateful to Tecgraf for its continued support to Lua.
We thank everyone at Tecgraf, specially the head of the group, Marcelo
Gattass. At the risk of omitting several names, we also thank the
following individuals for supporting, contributing to, and spreading
the word about Lua: Alan Watson. André Clinio, André Costa, Antonio
Scuri, Asko Kauppi, Bret Mogilefsky, Cameron Laird, Carlos Cassino,
Carlos Henrique Levy, Claudio Terra, David Jeske, Ed Ferguson, Edgar
Toernig, Erik Hougaard, Jim Mathies, John Belmonte, John Passaniti,
John Roll, Jon Erickson, Jon Kleiser, Mark Ian Barlow, Nick Trout,
Noemi Rodriguez, Norman Ramsey, Philippe Lhoste, Renata Ratton, Renato
Borges, Renato Cerqueira, Reuben Thomas, Stephan Herrmann, Steve
Dekorte, Thatcher Ulrich, Tomás Gorham, Vincent Penquerc'h. Thank you!

Incompatibilities with Previous Versions

Lua 5.0 is a major release. There are several incompatibilities with
its previous version, Lua 4.0.

# «p58» (find-lua50page 58)

Incompatibilities with version 4.0

Changes in the Language

. The whole tag-method scheme was replaced by metatables.
. Function calls written between parentheses result in exactly one value.
. A function call as the last expression in a list constructor (like
{a,b,f()}) has all its return values inserted in the list.
. The precedence of or is smaller than the precedence of and.
. in, false, and true are reserved words.
. The old construction for k,v in t, where t is a table, is deprecated
(although it is still supported). Use for k,v in pairs(t) instead.
. When a literal string of the form [[...]] starts with a newline,
this newline is ignored.
. Upvalues in the form %var are obsolete; use external local variables

Changes in the Libraries

. Most library functions now are defined inside tables. There is a
compatibility script (compat.lua) that redefine most of them as global
names.
. In the math library, angles are expressed in radians. With the
compatibility script (compat.lua), functions still work in degrees.
. The call function is deprecated. Use f(unpack(tab)) instead of
call(f, tab) for unprotected calls, or the new pcall function for
protected calls.
. dofile do not handle errors, but simply propagates them.
. The read option *w is obsolete.
. The format option %n\$ is obsolete.

Changes in the API

. lua_open does not have a stack size as its argument (stacks are
dynamic).
. lua_pushuserdata is deprecated. Use lua_newuserdata or

# «p59» (find-lua50page 59)

The Complete Syntax of Lua

chunk -> { stat [ `;' ] }
block -> chunk
stat -> varlist1 `=' explist1
| functioncall
| do block end
| while exp do block end
| repeat block until exp
| if exp then block { elseif exp then block } [ else block ] end
| return [ explist1 ]
| break
| for Name `=' exp `,' exp [ `,' exp ] do block end
| for Name { `,' Name } in explist1 do block end
| function funcname funcbody
| local function Name funcbody
| local namelist [ init ]
funcname -> Name { `.' Name } [ `:' Name ]
varlist1 -> var { `,' var }
var -> Name | prefixexp `[' exp `]' | prefixexp `.' Name
namelist -> Name { `,' Name }
init -> `=' explist1
explist1 -> { exp `,' } exp
exp -> nil | false | true | Number | Literal
| function | prefixexp | tableconstructor | exp binop exp | unop exp
prefixexp -> var | functioncall | `(' exp `)'
functioncall -> prefixexp args | prefixexp `:' Name args
args -> `(' [ explist1 ] `)' | tableconstructor | Literal
function -> function funcbody
funcbody -> `(' [ parlist1 ] `)' block end
parlist1 -> Name { `,' Name } [ `,' `...' ] | `...'
tableconstructor -> `{' [ fieldlist ] `}'
fieldlist -> field { fieldsep field } [ fieldsep ]
field -> `[' exp `]' `=' exp | name `=' exp | exp
fieldsep -> `,' | `;'
binop -> `+' | `-' | `*' | `/' | `^' | `..'
| `<' | `<=' | `>' | `>=' | `==' | `~='
| and | or
unop -> `-' | not

# «p60» (find-lua50page 60)

Index

_G . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
_LOADED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
_PROMPT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
_REQUIREDNAME . . . . . . . . . . . . . . . . . . . . . . . . . . 42
_VERSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

A

acceptable index . . . . . . . . . . . . . . . . . . . . . . . . 23
``add'' event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
arg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
arithmetic operators . . . . . . . . . . . . . . . . . . . . . . 8
arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
assert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
associative arrays . . . . . . . . . . . . . . . . . . . . . . . . . 3

B

basic types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
boolean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
break statement . . . . . . . . . . . . . . . . . . . . . . . . . . 6

C

C API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Chunkreader . . . . . . . . . . . . . . . . . . . . . . . . 28
lua call . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
lua CFunction . . . . . . . . . . . . . . . . . . . . . . 32
lua checkstack . . . . . . . . . . . . . . . . . . . . 23
lua close . . . . . . . . . . . . . . . . . . . . . . . . . . 22
lua closethread . . . . . . . . . . . . . . . . . . . 34
lua concat . . . . . . . . . . . . . . . . . . . . . . . . . 27
lua cpcall . . . . . . . . . . . . . . . . . . . . . . . . . 34
lua Debug . . . . . . . . . . . . . . . . . . . . . . . . . . 35
lua equal . . . . . . . . . . . . . . . . . . . . . . . . . . 25
LUA ERRERR . . . . . . . . . . . . . . . . . . . . . . . . . 31
LUA ERRMEM . . . . . . . . . . . . . . . . . . . . . 28, 31
lua error . . . . . . . . . . . . . . . . . . . . . . . . . . 34
LUA ERRRUN . . . . . . . . . . . . . . . . . . . . . . . . . 31
LUA ERRSYNTAX . . . . . . . . . . . . . . . . . . . . . . 28
lua getfenv . . . . . . . . . . . . . . . . . . . . . . . . 30
lua getgccount . . . . . . . . . . . . . . . . . . . . 27
lua getgcthreshold . . . . . . . . . . . . . . . . 27
lua gethook . . . . . . . . . . . . . . . . . . . . . . . . 38
lua gethookcount . . . . . . . . . . . . . . . . . . 38
lua gethookmask . . . . . . . . . . . . . . . . . . . 38
lua getinfo . . . . . . . . . . . . . . . . . . . . . . . . 35
lua getlocal . . . . . . . . . . . . . . . . . . . . . . . 36
lua getmetatable . . . . . . . . . . . . . . . . . . 28
lua getstack . . . . . . . . . . . . . . . . . . . . . . . 35
lua gettable . . . . . . . . . . . . . . . . . . . . . . . 29
lua gettop . . . . . . . . . . . . . . . . . . . . . . . . . 23
lua getupvalue . . . . . . . . . . . . . . . . . . . . 36
LUA GLOBALSINDEX . . . . . . . . . . . . . . . . . . 30
lua Hook . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
LUA HOOKCALL . . . . . . . . . . . . . . . . . . . . . . . 37
LUA HOOKCOUNT . . . . . . . . . . . . . . . . . . . . . . 37
LUA HOOKLINE . . . . . . . . . . . . . . . . . . . . . . . 37
LUA HOOKRET . . . . . . . . . . . . . . . . . . . . . . . . 37
LUA HOOKTAILRET . . . . . . . . . . . . . . . . . . . 37
lua insert . . . . . . . . . . . . . . . . . . . . . . . . . 24
lua isboolean . . . . . . . . . . . . . . . . . . . . . . 24
lua iscfunction . . . . . . . . . . . . . . . . . . . 24
lua isfunction . . . . . . . . . . . . . . . . . . . . 24
lua islightuserdata . . . . . . . . . . . . . . 24
lua isnil . . . . . . . . . . . . . . . . . . . . . . . . . . 24
lua isnumber . . . . . . . . . . . . . . . . . . . . . . . 24
lua isstring . . . . . . . . . . . . . . . . . . . . . . . 24
lua istable . . . . . . . . . . . . . . . . . . . . . . . . 24
lua isuserdata . . . . . . . . . . . . . . . . . . . . 24
lua lessthan . . . . . . . . . . . . . . . . . . . . . . . 25
lua load . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
LUA MASKCALL . . . . . . . . . . . . . . . . . . . . . . . 37
LUA MASKCOUNT . . . . . . . . . . . . . . . . . . . . . . 37
LUA MASKLINE . . . . . . . . . . . . . . . . . . . . . . . 37
LUA MASKRET . . . . . . . . . . . . . . . . . . . . . . . . 37
LUA MINSTACK . . . . . . . . . . . . . . . . . . . . . . . 23
LUA MULTRET . . . . . . . . . . . . . . . . . . . . . . . . 30
lua newtable . . . . . . . . . . . . . . . . . . . . . . . 29
lua newthread . . . . . . . . . . . . . . . . . . . . . . 34
lua newuserdata . . . . . . . . . . . . . . . . . . . 27
lua next . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
lua Number . . . . . . . . . . . . . . . . . . . . . . . . . 25
lua open . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
lua pop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
lua pushboolean . . . . . . . . . . . . . . . . . . . 26

# «p61» (find-lua50page 61)

lua pushcclosure . . . . . . . . . . . . . . . . . . 33
lua pushcfunction . . . . . . . . . . . . . . . . . 26
lua pushfstring . . . . . . . . . . . . . . . . . . . 26
lua pushlightuserdata . . . . . . . . . . . . 26
lua pushlstring . . . . . . . . . . . . . . . . . . . 26
lua pushnil . . . . . . . . . . . . . . . . . . . . . . . . 26
lua pushnumber . . . . . . . . . . . . . . . . . . . . 26
lua pushstring . . . . . . . . . . . . . . . . . . . . 26
lua pushvalue . . . . . . . . . . . . . . . . . . . . . . 24
lua pushvfstring . . . . . . . . . . . . . . . . . . 26
lua rawequal . . . . . . . . . . . . . . . . . . . . . . . 25
lua rawget . . . . . . . . . . . . . . . . . . . . . . . . . 29
lua rawgeti . . . . . . . . . . . . . . . . . . . . . . . . 30
lua rawset . . . . . . . . . . . . . . . . . . . . . . . . . 29
lua rawseti . . . . . . . . . . . . . . . . . . . . . . . . 30
lua register . . . . . . . . . . . . . . . . . . . . . . . 32
LUA REGISTRYINDEX . . . . . . . . . . . . . . . . . 33
lua remove . . . . . . . . . . . . . . . . . . . . . . . . . 24
lua replace . . . . . . . . . . . . . . . . . . . . . . . . 24
lua resume . . . . . . . . . . . . . . . . . . . . . . . . . 34
lua setfenv . . . . . . . . . . . . . . . . . . . . . . . . 30
lua setgcthreshold . . . . . . . . . . . . . . . . 27
lua sethook . . . . . . . . . . . . . . . . . . . . . . . . 37
lua setlocal . . . . . . . . . . . . . . . . . . . . . . . 36
lua setmetatable . . . . . . . . . . . . . . . . . . 28
lua settable . . . . . . . . . . . . . . . . . . . . . . . 29
lua settop . . . . . . . . . . . . . . . . . . . . . . . . . 24
lua setupvalue . . . . . . . . . . . . . . . . . . . . 36
lua State . . . . . . . . . . . . . . . . . . . . . . . . . . 22
lua strlen . . . . . . . . . . . . . . . . . . . . . . . . . 25
lua toboolean . . . . . . . . . . . . . . . . . . . . . . 25
lua tocfunction . . . . . . . . . . . . . . . . . . . 25
lua tonumber . . . . . . . . . . . . . . . . . . . . . . . 25
lua topointer . . . . . . . . . . . . . . . . . . . . . . 25
lua tostring . . . . . . . . . . . . . . . . . . . . . . . 25
lua tothread . . . . . . . . . . . . . . . . . . . . . . . 25
lua touserdata . . . . . . . . . . . . . . . . . . . . 25
lua type . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
lua typename . . . . . . . . . . . . . . . . . . . . . . . 25
lua upvalueindex . . . . . . . . . . . . . . . . . . 33
lua xmove . . . . . . . . . . . . . . . . . . . . . . . . . . 34
lua yield . . . . . . . . . . . . . . . . . . . . . . . . . . 34
luaopen base . . . . . . . . . . . . . . . . . . . . . . . 38
luaopen debug . . . . . . . . . . . . . . . . . . . . . . 38
luaopen io . . . . . . . . . . . . . . . . . . . . . . . . . 38
luaopen math . . . . . . . . . . . . . . . . . . . . . . . 38
luaopen string . . . . . . . . . . . . . . . . . . . . 38
luaopen table . . . . . . . . . . . . . . . . . . . . . . 38
C closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
``call'' event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
captures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
character class . . . . . . . . . . . . . . . . . . . . . . . . . . 47
chunk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
collectgarbage . . . . . . . . . . . . . . . . . . . . . . . . 39
comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
concatenation . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
``concatenation'' event . . . . . . . . . . . . . . . . . . . 17
condition expression . . . . . . . . . . . . . . . . . . . . . . 6
constructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
coroutine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
coroutine.create . . . . . . . . . . . . . . . . . . 21, 43
coroutine.resume . . . . . . . . . . . . . . . . . . 21, 44
coroutine.status . . . . . . . . . . . . . . . . . . . . . . 44
coroutine.wrap . . . . . . . . . . . . . . . . . . . . 21, 44
coroutine.yield . . . . . . . . . . . . . . . . . . . 21, 44

D

debug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
debug.debug . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
debug.gethook . . . . . . . . . . . . . . . . . . . . . . . . . 55
debug.getinfo . . . . . . . . . . . . . . . . . . . . . . . . . 56
debug.getlocal . . . . . . . . . . . . . . . . . . . . . . . . 56
debug.getupvalue . . . . . . . . . . . . . . . . . . . . . . 56
debug.sethook . . . . . . . . . . . . . . . . . . . . . . . . . 56
debug.setlocal . . . . . . . . . . . . . . . . . . . . . . . . 56
debug.setupvalue . . . . . . . . . . . . . . . . . . . . . . 56
debug.traceback . . . . . . . . . . . . . . . . . . . . . . . 57
``div'' event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
dofile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

E

eight-bit clean . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
environment tables . . . . . . . . . . . . . . . . . . . . . . . 4
``eq'' event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
exponentiation . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

F

file:close . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
file:flush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
file:lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
file:read . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
file:seek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

# «p62» (find-lua50page 62)

file:write . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
finalizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
for statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
full userdata . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
function call . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
function definitions . . . . . . . . . . . . . . . . . . . . . . 12

G

garbage collector . . . . . . . . . . . . . . . . . . . . . . . . 20
gcinfo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
getfenv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
getmetatable . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
args . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 60
binop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 60
chunk . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 60
exp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8, 60
explist1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 60
field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 60
fieldlist . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 60
fieldsep . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 60
funcbody . . . . . . . . . . . . . . . . . . . . . . . . 12, 60
funcname . . . . . . . . . . . . . . . . . . . . . . . 12, 60
function . . . . . . . . . . . . . . . . . . . . . . . . . 12, 60
functioncall . . . . . . . . . . . . . . . . . . . . . 11, 60
init . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
namelist . . . . . . . . . . . . . . . . . . . . . . . . . . 8, 60
parlist1 . . . . . . . . . . . . . . . . . . . . . . . . . . 13, 60
prefixexp . . . . . . . . . . . . . . . . . . . . . . . . . 8, 60
stat . . . . . . . . . . . . . . . . . . . . . . . . 5--8, 12, 60
tableconstructor . . . . . . . . . . . . . . . . . 10, 60
unop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
var . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4, 60
varlist1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 60

I

identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
if-then-else statement . . . . . . . . . . . . . . . . . . . . . 6
``index'' event . . . . . . . . . . . . . . . . . . . . . . . . 18, 19
io . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
io.close . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
io.flush . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
io.input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
io.lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
io.open . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51, 52
io.output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
io.read . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
io.stderr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
io.stdin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
io.stdout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
io.tmpfile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
io.type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
io.write . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
ipairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
iterators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

K

keywords
and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
break . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
do . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
else . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
elseif . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
false . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
for . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
function . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
if . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
local . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
nil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
not . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
or . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
repeat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
return . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
true . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
until . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
while . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

L

lexical scoping . . . . . . . . . . . . . . . . . . . . . . . . . . 14
light userdata . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
literal strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
loadfile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
loadlib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
loadstring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
local variables . . . . . . . . . . . . . . . . . . . . . . . . . 4, 8
logical operators . . . . . . . . . . . . . . . . . . . . . . . . . . 9
``lt'' event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
lua stand-alone . . . . . . . . . . . . . . . . . . . . . . . . . . 57
LUA INIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
LUA PATH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
luac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 28

# «p63» (find-lua50page 63)

M

math . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.abs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.acos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.asin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.atan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.atan2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.ceil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.cos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.def . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.exp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.frexp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.ldexp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.log10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.max . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.min . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.mod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.pi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.pow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.rad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.random . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.randomseed . . . . . . . . . . . . . . . . . . . . . . . 50
math.sin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.sqrt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
math.tan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
metamethod
add . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
call . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
concatenation . . . . . . . . . . . . . . . . . . . . . . . 17
div . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
eq . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
index . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18, 19
lt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
mul . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
pow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
sub . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
unm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
metamethod . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
metamethods . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
metatable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
``mul'' event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
multiple assignment . . . . . . . . . . . . . . . . . . . . . . 5

N

next . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
nil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
not . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
numerical constants . . . . . . . . . . . . . . . . . . . . . . 3

O

operator precedence . . . . . . . . . . . . . . . . . . . . . 10
or . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
os . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
os.clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
os.date . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
os.difftime . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
os.execute . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
os.exit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
os.getenv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
os.remove . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
os.rename . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
os.setlocale . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
os.time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
os.tmpname . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

P

pairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
panic function . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
pattern item . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
pcall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
``pow'' event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
pre-compilation . . . . . . . . . . . . . . . . . . . . . . . . . . 5
print . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
proper tail calls . . . . . . . . . . . . . . . . . . . . . . . . . 12
pseudo-indices . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

R

rawequal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
rawget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
rawset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
relational operators . . . . . . . . . . . . . . . . . . . . . . . 9
repeat-until statement . . . . . . . . . . . . . . . . . . . . 6
require . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
reserved words . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
return statement . . . . . . . . . . . . . . . . . . . . . . . . . 6

S

self . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

# «p64» (find-lua50page 64)

setfenv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
setmetatable . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
short-cut evaluation . . . . . . . . . . . . . . . . . . . . . . 9
stack index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
string.byte . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
string.char . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
string.dump . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
string.find . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
string.format . . . . . . . . . . . . . . . . . . . . . . . . . 45
string.gsub . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
string.len . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
string.lower . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
string.rep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
string.sub . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
string.upper . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
``sub'' event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

T

table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
table.concat . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
table.foreach . . . . . . . . . . . . . . . . . . . . . . . . . 49
table.foreachi . . . . . . . . . . . . . . . . . . . . . . . . 49
table.getn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
table.insert . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
table.remove . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
table.setn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
table.sort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
tail call . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
thread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
tokens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
tonumber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
tostring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

U

``unm'' event . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
unpack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
upvalue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
userdata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

V

valid index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
values and types . . . . . . . . . . . . . . . . . . . . . . . . . . 3
vararg function . . . . . . . . . . . . . . . . . . . . . . . . . . 13
version 4.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

W

weak references . . . . . . . . . . . . . . . . . . . . . . . . . 20
weak table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
while-do statement . . . . . . . . . . . . . . . . . . . . . . . 6

X

xpcall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

# «p65» (find-lua50page 65)

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