(find-shttpfile "www.lua.org/ftp/")
(find-es "lua" "lua5.0-beta")
(find-es "lua5" "lua50betaref.e")

(code-ps "lua50" "$S/http/www.lua.org/ftp/refman-5.0-beta.ps.gz")
(find-lua50page 1)



The Lua Programming Language
Reference Manual for Lua version 5.0 (beta)
Last revised on December 17, 2002
Lua

Copyright (c) 2002 Tecgraf, PUC-Rio. All rights reserved.

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) 1998. All
rights reserved.


Reference Manual of the Programming Language Lua 5.0 (beta)
Roberto Ierusalimschy Luiz Henrique de Figueiredo Waldemar Celes
lua@tecgraf.puc-rio.br
Tecgraf --- Computer Science Department --- PUC-Rio
December 17, 2002

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 (beta) 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
descrição de dados, baseadas em tabelas associativas e semântica
extensível. Lua é tipada dinamicamente, interpretada a partir de
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 (beta) 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 (to "sec1")
2 Lua Concepts . . . . . . . . . . . . . . . . . . . . . . 1 (to "sec2")
2.1 Environment and Chunks . . . . . . . . . . . . . . . . 1 (to "sec2.1")
2.2 Table of Globals . . . . . . . . . . . . . . . . . . . 2 (to "sec2.2")
2.3 Values and Types . . . . . . . . . . . . . . . . . . . 2 (to "sec2.3")
2.3.1 Metatables . . . . . . . . . . . . . . . . . . . . . 3 (to "sec2.3.1")
2.4 Coercion . . . . . . . . . . . . . . . . . . . . . . . 3 (to "sec2.4")
2.5 Variables . . . . . . . . . . . . . . . . . . . . . . . 3 (to "sec2.5")
2.6 Garbage Collection . . . . . . . . . . . . . . . . . . 3 (to "sec2.6")
2.6.1 Weak Tables . . . . . . . . . . . . . . . . . . . . . 4 (to "sec2.6.1")
3 The Language . . . . . . . . . . . . . . . . . . . . . . 4 (to "sec3")
3.1 Lexical Conventions . . . . . . . . . . . . . . . . . . 4 (to "sec3.1")
3.2 Variables . . . . . . . . . . . . . . . . . . . . . . . 5 (to "sec3.2")
3.3 Statements . . . . . . . . . . . . . . . . . . . . . . 6 (to "sec3.3")
3.3.1 Chunks . . . . . . . . . . . . . . . . . . . . . . . 6 (to "sec3.3.1")
3.3.2 Blocks . . . . . . . . . . . . . . . . . . . . . . . 6 (to "sec3.3.2")
3.3.3 Assignment . . . . . . . . . . . . . . . . . . . . . 6 (to "sec3.3.3")
3.3.4 Control Structures . . . . . . . . . . . . . . . . . 7 (to "sec3.3.4")
3.3.5 For Statement . . . . . . . . . . . . . . . . . . . . 7 (to "sec3.3.5")
3.3.6 Function Calls as Statements . . . . . . . . . . . . 9 (to "sec3.3.6")
3.3.7 Local Declarations . . . . . . . . . . . . . . . . . 9 (to "sec3.3.7")
3.4 Expressions . . . . . . . . . . . . . . . . . . . . . . 9 (to "sec3.4")
3.4.1 Arithmetic Operators . . . . . . . . . . . . . . . . 9 (to "sec3.4.1")
3.4.2 Relational Operators . . . . . . . . . . . . . . . . 10 (to "sec3.4.2")
3.4.3 Logical Operators . . . . . . . . . . . . . . . . . . 10 (to "sec3.4.3")
3.4.4 Concatenation . . . . . . . . . . . . . . . . . . . . 10 (to "sec3.4.4")
3.4.5 Precedence . . . . . . . . . . . . . . . . . . . . . 11 (to "sec3.4.5")
3.4.6 Table Constructors . . . . . . . . . . . . . . . . . 11 (to "sec3.4.6")
3.4.7 Function Calls . . . . . . . . . . . . . . . . . . . 12 (to "sec3.4.7")
3.4.8 Function Definitions . . . . . . . . . . . . . . . . 13 (to "sec3.4.8")
3.5 Visibility Rules . . . . . . . . . . . . . . . . . . . 14 (to "sec3.5")
3.6 Error Handling . . . . . . . . . . . . . . . . . . . . 15 (to "sec3.6")
3.7 Metatables . . . . . . . . . . . . . . . . . . . . . . 15 (to "sec3.7")
3.7.1 Metatables and Garbage collection . . . . . . . . . . 19 (to "sec3.7.1")
3.8 Coroutines . . . . . . . . . . . . . . . . . . . . . . 20 (to "sec3.8")
4 The Application Program Interface . . . . . . . . . . . . 21 (to "sec4")
4.1 States . . . . . . . . . . . . . . . . . . . . . . . . 21 (to "sec4.1")
4.2 Threads . . . . . . . . . . . . . . . . . . . . . . . . 21 (to "sec4.2")
4.3 The Stack and Indices . . . . . . . . . . . . . . . . . 22 (to "sec4.3")
4.4 Stack Manipulation . . . . . . . . . . . . . . . . . . 23 (to "sec4.4")
4.5 Querying the Stack . . . . . . . . . . . . . . . . . . 23 (to "sec4.5")
4.6 Getting Values from the Stack . . . . . . . . . . . . . 24 (to "sec4.6")
4.7 Pushing Values onto the Stack . . . . . . . . . . . . . 25 (to "sec4.7")
iii

4.8 Controlling Garbage Collection . . . . . . . . . . . . 26 (to "sec4.8")
4.9 Userdata . . . . . . . . . . . . . . . . . . . . . . . 26 (to "sec4.9")
4.10 Metatables . . . . . . . . . . . . . . . . . . . . . . 27 (to "sec4.10")
4.11 Loading Lua Chunks . . . . . . . . . . . . . . . . . . 27 (to "sec4.11")
4.12 Manipulating Tables . . . . . . . . . . . . . . . . . 27 (to "sec4.12")
4.13 Manipulating Global Variables . . . . . . . . . . . . 29 (to "sec4.13")
4.14 Using Tables as Arrays . . . . . . . . . . . . . . . . 29 (to "sec4.14")
4.15 Calling Functions . . . . . . . . . . . . . . . . . . 29 (to "sec4.15")
4.16 Protected Calls . . . . . . . . . . . . . . . . . . . 30 (to "sec4.16")
4.17 Defining C Functions . . . . . . . . . . . . . . . . . 31 (to "sec4.17")
4.18 Defining C Closures . . . . . . . . . . . . . . . . . 32 (to "sec4.18")
5 The Debug Interface . . . . . . . . . . . . . . . . . . . 32 (to "sec5")
5.1 Stack and Function Information . . . . . . . . . . . . 32 (to "sec5.1")
5.2 Manipulating Local Variables . . . . . . . . . . . . . 34 (to "sec5.2")
5.3 Hooks . . . . . . . . . . . . . . . . . . . . . . . . . 34 (to "sec5.3")
6 Standard Libraries . . . . . . . . . . . . . . . . . . . 35 (to "sec6")
6.1 Basic Functions . . . . . . . . . . . . . . . . . . . . 36 (to "sec6.1")
6.2 String Manipulation . . . . . . . . . . . . . . . . . . 39 (to "sec6.2")
6.3 Table Manipulation . . . . . . . . . . . . . . . . . . 44 (to "sec6.3")
6.4 Mathematical Functions . . . . . . . . . . . . . . . . 45 (to "sec6.4")
6.5 Input and Output Facilities . . . . . . . . . . . . . . 46 (to "sec6.5")
6.6 Operating System Facilities . . . . . . . . . . . . . . 49 (to "sec6.6")
6.7 The Reflexive Debug Interface . . . . . . . . . . . . . 50 (to "sec6.7")
7 Lua Stand-alone . . . . . . . . . . . . . . . . . . . . . 52 (to "sec7")
Incompatibilities with Previous Versions . . . . . . . . . . . . . . . . . 53
The Complete Syntax of Lua . . . . . . . . . . . . . . . . . . . . . . . . 55
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
iv


# «sec1»
# «Introduction»

1 Introduction

Lua is an extension programming language designed to support general
procedural programming with data description facilities. 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 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.

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.

# «sec2»
# «Lua Concepts»

2 Lua Concepts

This section describes the main concepts of Lua as a language. The
syntax and semantics of Lua are described in (to "sec3"). The
discussion below is not purely conceptual; it includes references to
the C API (see (to "sec4")), because Lua is designed to be embedded in
host programs. It also includes references to the standard libraries
(see (to "sec6")).

# «sec2.1»
# «Environment and Chunks»

2.1 Environment and Chunks

All statements in Lua are executed in a global environment . This
environment is initialized with a call from the embedding program to
lua_open and persists until a call to lua_close or the end of the
embedding program. The host program can create multiple independent
global environments, and freely switch between them (see (to
"sec4.1")).

The unit of execution of Lua is called a chunk . A chunk is simply a
sequence of statements. Statements are described in (to "sec3.3").

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 statements are

# «p1» (find-lua50page 1)


executed by an interpreter for the virtual machine. All modifications
a chunk makes to the global environment persist after the chunk ends.

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

# «sec2.2»
# «Table of Globals»

2.2 Table of Globals

????

# «sec2.3»
# «Values and Types»

2.3 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, and any other value makes it true. Number represents
real (double-precision floating-point) numbers. String represents
arrays of characters. Lua is 8-bit clean, and so strings may contain
any 8-bit character, including embedded zeros ('\0') (see (to
"sec3.1")).

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 "sec3.4.7")).

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 "sec3.7")). 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.

The type thread represents independent threads of execution, and it is
used to implement coroutines. (This is an experimental area; it needs
more documentation, and is subject to changes in the future.)

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. Tables are the sole data structuring mechanism in
Lua; they may be used not only to represent ordinary arrays, but also
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 "sec3.4.6")).

Like indices, the value of a table field can be of any type. In
particular, because functions are first class values, table fields may
contain functions. So, tables may also carry methods (see (to
"sec3.4.8")).

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 these values, and do not imply any kind of copy.

The library function type returns a string describing the type of a
given value (see (to "sec6.1")).

# «p2» (find-lua50page 2)


# «sec2.3.1»
# «Metatables»

2.3.1 Metatables

Each table and userdata object in Lua may have a metatable.

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
the addition.

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

A metatable controls 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.
(to "sec3.7") gives a detailed description of which events you can
control with metatables.

You can query and change the metatable of an object through the
setmetatable and getmetatable functions (see (to "sec6.1")).

# «sec2.4»
# «Coercion»

2.4 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 when a string is expected, the
number is converted to a string, in a reasonable format. The format is
chosen so that a conversion from number to string then back to number
reproduces the original number exactly. For complete control of how
numbers are converted to strings, use the format function (see (to
"sec6.2")).

# «sec2.5»
# «Variables»

2.5 Variables

There are two kinds of variables in Lua: global variables and local
variables. Variables are assumed to be global unless explicitly
declared local (see (to "sec3.3.7")). Before the first assignment, the
value of a variable is nil.

All global variables live as fields in ordinary Lua tables. Usually,
globals live in a table called table of globals. However, a function
can individually change its global table, so that all global variables
in that function will refer to that table. This mechanism allows the
creation of namespaces and other modularization facilities.

Local variables are lexically scoped. Therefore, local variables can
be freely accessed by functions defined inside their scope (see (to
"sec3.5")).

# «sec2.6»
# «Garbage Collection»

2.6 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 and collecting all dead
objects (all objects that are no longer accessible from Lua). All
objects in Lua are subject to automatic management: tables, userdata,
functions, and strings.

Using the C API, you can set garbage-collector metamethods for
userdata (see (to "sec3.7")). When it is about to free a userdata, Lua
calls the metamethod associated with event gc in the userdata's
metatable. Using such facility, you can coordinate Lua's garbage
collection with external resource management (such as closing files,
network or database connections, or freeing your own memory).

Lua uses two numbers to control its garbage-collection cycles. One
number counts how many bytes of dynamic memory Lua is using, and the
other is a threshold. When the number of bytes

# «p3» (find-lua50page 3)


crosses the threshold, Lua runs the garbage collector, which reclaims
the memory of all dead objects. The byte counter is corrected, and
then the threshold is reset to twice the value of the byte counter.

Through the C API, you can query those numbers, and change the
threshold (see (to "sec4.8")). 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 functions gcinfo and collectgarbage (see (to "sec6.1")).

# «sec2.6.1»
# «Weak Tables»

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

# «sec3»
# «The Language»

3 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.

# «sec3.1»
# «Lexical Conventions»

3.1 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:

  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
ánd (if the locale permits) 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.

The following strings denote other tokens:

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

# «p4» (find-lua50page 4)


Literal strings can be delimited by matching single or double quotes,
and can contain the C-like escape sequences `\a' (bell), `\b'
(backspace), `\f' (form feed), `\n' (newline), `\r' (carriage return),
`\t' (horizontal tab), `\v' (vertical tab), `\\' (backslash), `\"'
(double quote), `\'' (single quote), and `\newline' (that is, a
backslash followed by a real newline, which results in a newline in
the string). A character in a string may also be specified by its
numerical value, through 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 [[ . . . ]].
Literals in this bracketed form may run for several lines, may contain
nested [[ . . . ]] pairs, and do not interpret escape sequences. For
convenience, when the opening [[ is immediately followed by a newline,
the newline is not included in the string. That form is specially
convenient for writing strings that contain program pieces or other
quoted strings. 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"]]

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 after -- is different from [[, the comment is a short
comment, that runs until the end of the line. Otherwise, it is a long
comment, that 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 "sec7")).

# «sec3.2»
# «Variables»

3.2 Variables

Variables are places that store values.

A single name can denote a global variable, a local variable, or a
formal parameter in a function (formal parameters are just local
variables):

  var -> Name

Square brackets are used to index a table:

  var -> prefixexp `[' exp `]'

The first expression should result in a table value, and the second
expression identifies a specific entry inside that table.

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

  var -> prefixexp `.' Name

The expression denoting the table to be indexed has a restricted
syntax; (to "sec3.4") for details.

The meaning of assignments and evaluations of global and indexed
variables can be changed via metatables. An assignment to a global
variable x = val is equivalent to the assignment

# «p5» (find-lua50page 5)


_glob.x = val, where _glob is the table of globals of the running
function ((see (to "sec2.2")) for a discussion about the table of
globals). An assignment to an indexed variable t[i] = val is
equivalent to settable_event(t,i,val). An access to a global variable
x is equivalent to _glob.x (again, (see (to "sec2.2")) for a
discussion about _glob). An access to an indexed variable t[i] is
equivalent to a call gettable_event(t,i). See (to "sec3.7") for a
complete description of the settable_event and gettable_event
functions. (These functions are not defined in Lua. We use them here
only for explanatory purposes.)

# «sec3.3»
# «Statements»

3.3 Statements

Lua supports an almost conventional set of statements, similar to
those in Pascal or C. The conventional commands include assignment,
control structures, and procedure calls. Non-conventional commands
include table constructors and variable declarations.

# «sec3.3.1»
# «Chunks»

3.3.1 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 [ `;' ] }

# «sec3.3.2»
# «Blocks»

3.3.2 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
"sec3.3.4")).

# «sec3.3.3»
# «Assignment»

3.3.3 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 "sec3.4").

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 less 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 adjust
(except when the call is enclosed in parentheses; see (to "sec3.4")).

The assignment statement first evaluates all its expressions, and only
then makes the assignments. So, the code

# «p6» (find-lua50page 6)


  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 before it is assigned 4. Similarly, the line

  x, y = y, x

exchanges the values of x and y.

# «sec3.3.4»
# «Control Structures»

3.3.4 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 "sec3.3.5")).

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

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

NOTE : 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 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. (For
instance, a line `do return end' can be added at the beginning of a
chunk for syntax checking only.)

# «sec3.3.5»
# «For Statement»

3.3.5 For Statement

The for statement has two forms, one for numbers and one generic.

The numerical 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 reaches the second exp by steps of the third exp. More
precisely, a for statement like

  for var = e1, e2, e3 do block end

is equivalent to the code:

# «p7» (find-lua50page 7)


  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:

. Both the limit and the step are evaluated only once, before the loop
starts.

. _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 generators. It
calls its generator to produce a new value for each iteration,
stopping when the new value is nil. It 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, ..., var_n = explist
    while 1 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 a ``generator''
function, a ``state'', and an initial value for the ``iterator
variable''.

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

. The behavior is undefined if you assign to any var_i 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.

# «p8» (find-lua50page 8)


# «sec3.3.6»
# «Function Calls as Statements»

3.3.6 Function Calls as Statements

Because of 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 "sec3.4.7").

# «sec3.3.7»
# «Local Declarations»

3.3.7 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 "sec3.3.3")). Otherwise, all variables are
initialized with nil.

A chunk is also a block (see (to "sec3.3.1")), and so local variables
can be declared outside any explicit block. Such local variables die
when the chunk ends.

Visibility rules for local variables are explained in (to "sec3.5").

# «sec3.4»
# «Expressions»

3.4 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 `)'

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.)

Numbers and literal strings are explained in (to "sec3.1"); variables
are explained in (to "sec3.2"); function definitions are explained in
(to "sec3.4.8"); function calls are explained in (to "sec3.4.7");
table constructors are explained in (to "sec3.4.6").

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

# «sec3.4.1»
# «Arithmetic Operators»

3.4.1 Arithmetic Operators

Lua supports the usual arithmetic operators: the binary + (addition),
- (subtraction), * (multiplication), / (division), and ^
(exponentiation); and unary - (negation). If the operands are numbers,
or strings that can be converted to numbers (see (to "sec2.4")), then
all operations except exponentiation have the usual meaning, while
exponentiation calls a global function pow; ?? otherwise, an
appropriate metamethod is called (see (to "sec3.7")). The standard
mathematical library defines function pow, giving the expected meaning
to exponentiation (see (to "sec6.4")).

# «p9» (find-lua50page 9)


# «sec3.4.2»
# «Relational Operators»

3.4.2 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. Tables, userdata, and functions are compared by reference, that
is, two tables are considered equal only if they are the same table.

Every time you create a new table (or userdata, or function), this new
value is different from any previously existing value.

NOTE : The conversion rules of (to "sec2.4") 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, the ``lt'' or the ``le'' metamethod is called (see
(to "sec3.7")).

# «sec3.4.3»
# «Logical Operators»

3.4.3 Logical Operators

The logical operators in Lua are

  and or not

Like the control structures (see (to "sec3.3.4")), 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 its value
is false or nil; otherwise, and returns its second argument. The
disjunction operator or returns its first argument if it is different
from niland 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

# «sec3.4.4»
# «Concatenation»

3.4.4 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
"sec2.4"). Otherwise, the ``concat'' metamethod is called (see (to
"sec3.7")).

# «p10» (find-lua50page 10)


# «sec3.4.5»
# «Precedence»

3.4.5 Precedence

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

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

The .. (concatenation) and ^ (exponentiation) operators are right
associative. All other binary operators are left associative.

# «sec3.4.6»
# «Table Constructors»

3.4.6 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

If the last expression in the list is a function call, then all values
returned by the call enter the list consecutively (see (to
"sec3.4.7")). If you want to avoid this, enclose the function call in
parentheses.

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

# «p11» (find-lua50page 11)


# «sec3.4.7»
# «Function Calls»

3.4.7 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 "sec3.7")).

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
"sec3.3.4")), the number of results must be adjusted before they are
used. If the function is called as a statement (see (to "sec3.3.6")),
then its return list is adjusted to 0 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
adjusted to 1 element, thus discarding all returned values but the
first one. If the function is called as the last element of a list of
expressions, then no adjustment is made (unless the call is 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
  a,b,c = f()    -- f() is adjusted to 3
  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

As an exception to the format-free 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)

# «p12» (find-lua50page 12)


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).

# «sec3.4.8»
# «Function Definitions»

3.4.8 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

  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 non-local
variables (see (to "sec3.5")) and may have different tables of globals
(see (to "sec2.2")).

Parameters act as local variables, 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 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 whose value is
the number of extra arguments, and the extra arguments at positions 1,
2, . . . , n.

As an example, consider the following definitions:

# «p13» (find-lua50page 13)


  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 "sec3.3.4")).
If control reaches the end of a function without encountering a return
statement, then the function returns with no results.

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

# «sec3.5»
# «Visibility Rules»

3.5 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)

# «p14» (find-lua50page 14)


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

Because of those 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

Notice that each execution of a local statement ``creates'' 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

In that code, each function uses a different y variable, while all of
them share the same x.

# «sec3.6»
# «Error Handling»

3.6 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 "sec4.16")). Whenever an error occurs during Lua compilation or
execution, control returns to C, which can take appropriate measures
(such as to print an error message).

Lua code can explicitly generate an error by calling the function
error (see (to "sec6.1")). If you need to catch errors in Lua, you can
use the pcall function (see (to "sec6.1")).

# «sec3.7»
# «Metatables»

3.7 Metatables

Every table and userdata value in Lua may have a metatable. This
metatable is a table that defines the behavior of the original table
and userdata under some operations. You can query and change the
metatable of an object with functions setmetatable and getmetatable
(see (to "sec6.1")).

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, if 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
key for operation ``add'' is the string "__add". The semantics of
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
"sec6.1").

# «p15» (find-lua50page 15)


# «__add»
``add'': the + operation.

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 ``add'' operation is

  function add_event (op1, op2)
    local o1, o2 = tonumber(op1), tonumber(op2)
    if o1 and o2 then -- both operands are numeric
      return o1+o2 -- '+' here is the primitive 'add'
    else -- at least one of the operands is not numeric
      local h = getbinhandler(op1, op2, "__add")
      if h then
	-- call the handler with both operands
	return h(op1, op2)
      else -- no handler available: default behavior
	error("unexpected type at arithmetic operation")
      end
    end
  end

# «__sub»
``sub'': the - operation. Behavior similar to the ``add'' operation.

# «__mul»
``mul'': the * operation. Behavior similar to the ``add'' operation.

# «__div»
``div'': the / operation. Behavior similar to the ``add'' operation.

# «__pow»
``pow'': the ^ operation (exponentiation) operation.

??

  function pow_event (op1, op2)
    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("unexpected type at arithmetic operation")
    end
  end

# «__unm»
``unm'': the unary - operation.

# «p16» (find-lua50page 16)


  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)
      else -- no handler available: default behavior
	error("unexpected type at arithmetic operation")
      end
    end
  end

# «__lt»
``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 = getbinhandler(op1, op2, "__lt")
      if h then
	return h(op1, op2)
      else
	error("unexpected type at comparison");
      end
    end
  end

a>b is equivalent to b<a.

# «__le»
``le'': 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 = getbinhandler(op1, op2, "__le")
      if h then
	return h(op1, op2)
      else
	h = getbinhandler(op1, op2, "__lt")

# «p17» (find-lua50page 17)


	if h then
	  return not h(op2, op1)
	else
	  error("unexpected type at comparison");
	end
      end
    end
  end

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

# «__concat»
``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("unexpected type for concatenation")
      end
    end
  end

# «__index»
``index'': The ``gettable'' operation table[key].

  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("indexed expression not a table");
      end
    end
    if type(h) == "function" then
      return h(table, key) -- call the handler
    else return h[key] -- or repeat operation with it
    end

# «__newindex»
``newindex'': The ``settable'' operation table[key] = value.

# «p18» (find-lua50page 18)


  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("indexed expression not a table");
      end
    end
    if type(h) == "function" then
      return h(table, key,value) -- call the handler
    else h[key] = value -- or repeat operation with it
    end

# «__call»
``call'': called when Lua calls a value.

  function function_event (func, ...)
    if type(func) == "function" then
      return func(unpack(arg)) -- regular call
    else
      local h = metatable(func).__call
      if h then
	tinsert(arg, 1, func)
	return h(unpack(arg))
      else
	error("call expression not a function")
      end
    end
  end

# «sec3.7.1»
# «Metatables and Garbage collection»

3.7.1 Metatables and Garbage collection

Metatables may also define finalizer methods for userdata values. For
each userdata to be collected, Lua does the equivalent of the
following function:

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

In a garbage-collection cycle, the finalizers for userdata are called
in reverse order of their creation, that is, the first finalizer to be
called is the one associated with the last userdata created in the
program (among those to be collected in the same cycle).

# «p19» (find-lua50page 19)


# «sec3.8»
# «Coroutines»

3.8 Coroutines

Lua supports coroutines, also called semi-coroutines, generators, or
colaborative multithreading. A coroutine in Lua represents an
independent thread of execution. Unlike ``real'' threads, however, a
coroutine only suspends its execution by explicitly calling an yield
function.

You create a coroutine with a call to coroutine.create. Its sole
argument is a function, which is the main function of the coroutine.
The coroutine.create 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 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 it 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 falseplus an error message.

A coroutine yields 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, but the first one (the boolean error code).
Unlike coroutine.resume, this function does not catch errors; any
error is propagated to the caller.

As a complete 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)
	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)

# «p20» (find-lua50page 20)


  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

# «sec4»
# «The Application Program Interface»

4 The Application Program Interface

This section describes the API for Lua, that is, the set of C
functions available to the host program to communicate with Lua. All
API functions and related types and constants are declared in the
header file lua.h.

NOTE : 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.

# «sec4.1»
# «States»

4.1 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; this
state must be passed as the first argument to every function in the
library (except lua_open below).

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

  lua_State *lua_open (void);

To release a state created with lua_open, call

  void lua_close (lua_State *L);

This function destroys all objects in the given Lua environment
(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 --- like a daemon or a web server --- might need
to release states as soon as they are not needed, to avoid growing too
large.

With the exception of lua_open, all functions in the Lua API need a
state as their first argument.

# «sec4.2»
# «Threads»

4.2 Threads

Lua offers a 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:

# «p21» (find-lua50page 21)


  lua_State *lua_newthread (lua_State *L);

The new state returned by this function shares with the original state
all global environment (such as tables), but has an independent
run-time stack. (The use of these multiple stacks must be
``syncronized'' with C. How to explain that? TO BE WRITTEN.)

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

You destroy threads with

  void lua_closethread (lua_State *L, lua_State *thread);

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

# «sec4.3»
# «The Stack and Indices»

4.3 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.).

Each C invocation has its own stack. Whenever Lua calls C, the called
function gets a new stack, which is independent of previous stacks or
of stacks of still active C functions. That stack contains initially
any arguments to the C function, and it is where the C function pushes
its results (see (to "sec4.17")).

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

  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 bigger than the new size, it is left
unchanged.

Whenever Lua calls C, it ensures that lua_checkstack(L, LUA_MINSTACK)
is true, that is, at least LUA_MINSTACK positions are still 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 (or
Lua) have set through lua_checkstack. Such indices are called
acceptable indices. More formally, we define an acceptable index as
follows:

# «p22» (find-lua50page 22)


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

Note that 0 is never an acceptable index.

Unless otherwise noticed, 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 table of globals (see (to
"sec4.13")), the registry, and the upvalues of a C function (see (to
"sec4.18")).

# «sec4.4»
# «Stack Manipulation»

4.4 Stack Manipulation

The API offers the following functions for basic stack manipulation:

  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). These functions
accept only valid indices. (Obviously, 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*

# «sec4.5»
# «Querying the Stack»

4.5 Querying the Stack

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

# «p23» (find-lua50page 23)


  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.

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 are coded by the following constants defined in lua.h: LUA_TNIL,
LUA_TNUMBER, LUA_TBOOLEAN, LUA_TSTRING, LUA_TTABLE, LUA_TFUNCTION,
LUA_TUSERDATA, LUA_TLIGHTUSERDATA. The following function translates
such constants to a type name:

  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, and 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 "sec2.4")),
lua_isfunction accepts both Lua functions and C functions, and
lua_isuserdata accepts both full and ligth userdata. To distinguish
between Lua functions and C functions, you should use lua_iscfunction.
To distinguish between full and ligth userdata, you can use
lua_islightuserdata. To distinguish between numbers and numerical
strings, you can use lua_type.

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

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

These functions are equivalent to their counterparts in Lua (see (to
"sec3.4.2")). Both functions return 0 if any of the indices are
non-valid.

# «sec4.6»
# «Getting Values from the Stack»

4.6 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);

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.

# «p24» (find-lua50page 24)


lua_toboolean converts the Lua value at the given index to a C
``boolean'' value (that is, 0 or 1). Like all tests in Lua, it 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 "sec2.4")); otherwise,
lua_tonumber returns 0.

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
environment. 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. So, if you
need the string after the current function returns, then you should
duplicate it (or put it into the registry (see (to "sec4.18"))).

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 "sec4.17").

lua_touserdata is explained in (to "sec4.9").

# «sec4.7»
# «Pushing Values onto the Stack»

4.7 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);

Both functions push onto the stack a formatted string, and return a
pointer to that string. These functions 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, later).

# «p25» (find-lua50page 25)


. 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), %c (inserts an int as a character).

# «sec4.8»
# «Controlling Garbage Collection»

4.8 Controlling Garbage Collection

Lua uses two numbers to control its garbage collection: the count and
the threshold (see (to "sec2.6")). The first counts the ammount 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 smaller than the byte counter, then
Lua immediately runs the garbage collector. In particular
lua_setgcthreshold(L,0) forces a garbage collectiion. After the
collection, a new threshold is set according to the previous rule.

# «sec4.9»
# «Userdata»

4.9 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, you can
detect when it is being collected. A full userdata is only equal to
itself.

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 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_LIGHTUSERDATA for light
userdata.

You can create new full userdata with the following function:

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

It allocates a new block of memory with the given size, pushes on the
stack a new userdata with the block address, and returns this address.

To push a light userdata into the stack you use lua_pushlightuserdata
(see (to "sec4.7")). lua_touserdata (see (to "sec4.6")) 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 its gc metamethod, if any,
and then it frees its corresponding memory.

# «p26» (find-lua50page 26)


# «sec4.10»
# «Metatables»

4.10 Metatables

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

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

Both get at objindex a valid index for an object. lua_getmetatable
pushes on the stack the metatable of that object; lua_setmetatable
sets the table on the top of the stack as the new metatable for that
object (and pops the table).

If the object does not have a metatable, lua_getmetatable returns 0,
and pushes nothing on the stack. lua_setmetatable returns 0 when it
cannot set the metatable of the given object (that is, when the object
is not a userdata nor a table); even then it pops the table from the
stack.

# «sec4.11»
# «Loading Lua Chunks»

4.11 Loading Lua Chunks

You can load a Lua chunk with

  typedef const char * (*lua_Chunkreader)
                          (lua_State *L, void *data, size_t *size);
  int lua_load (lua_State *L, lua_Chunkreader reader, void *data,
                              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).

lua_load uses the reader to read the chunk. Everytime it needs another
piece of the chunk, it calls the reader, passing along its data
parameter. The reader must return a pointer to a block of memory with
a new part of the chunk, and set size to the block size. To signal the
end of the chunk, the reader must return 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 "sec5")). See the auxiliar library (lauxlib) for examples of how
to use lua_load, and for some ready-to-use functions to load chunks
from files and from strings.

# «sec4.12»
# «Manipulating Tables»

4.12 Manipulating Tables

Tables are created by calling the function

  void lua_newtable (lua_State *L);

# «p27» (find-lua50page 27)


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; this is convenient
for getting multiple values from a table.

As in Lua, this function may trigger a metamethod for the ``gettable''
or ``index'' events (see (to "sec3.7")). 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 the value onto the stack (in this order), and then
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;
this is convenient for setting multiple values in a table.

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 */
  }

NOTE: While traversing a table, do not call lua_tostring on a key,
unless you know 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.

# «p28» (find-lua50page 28)


# «sec4.13»
# «Manipulating Global Variables»

4.13 Manipulating Global Variables

All global variables are kept in an ordinary Lua table. 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 the global 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 table of a Lua thread using lua_replace.

# «sec4.14»
# «Using Tables as Arrays»

4.14 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.

# «sec4.15»
# «Calling Functions»

4.15 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 the Lua code:

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

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" */

# «p29» (find-lua50page 29)


  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 */

Notice 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 use several macros
and auxiliar functions that provide higher level access to Lua.)

# «sec4.16»
# «Protected Calls»

4.16 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. Like lua_call, lua_pcall always removes the function and its
arguments from the stack. 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.

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.

¿¿¿¿ Some special Lua functions have their own C interfaces. The host
program 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) is popped
from the stack. This function never returns. If lua_error is called
from a C function that has been called from Lua, then the
corresponding Lua execution terminates, as if an error had occurred
inside Lua code. Otherwise, the whole host program terminates with a
call to exit(EXIT_FAILURE).

The function

# «p30» (find-lua50page 30)


  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 "sec3.4.4")).

# «sec4.17»
# «Defining C Functions»

4.17 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 environment and returns an integer, the
number of values it has returned to Lua.

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. 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))
  /* 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);

# «p31» (find-lua50page 31)


# «sec4.18»
# «Defining C Closures»

4.18 Defining C Closures

When a C function is created, it is possible to associate some values
to it, thus creating a C closure; these values are then accessible to
the function whenever it is called. To associate values to 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
position lua_upvalueindex(1), and so on.

For examples of C functions and closures, see files lbaselib.c,
liolib.c, lmathlib.c, and lstrlib.c in the official Lua distribution.

Registry

Lua provides a pre-defined table that can be used by any C code to
store whatever Lua value it needs to store, especially 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 userdata with the
address of a C object in your code.

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

# «sec5»
# «The Debug Interface»

5 The Debug Interface

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

# «sec5.1»
# «Stack and Function Information»

5.1 Stack and Function Information

The main function to get information about the interpreter 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. Usually, 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
information about an active function:

  typedef struct lua_Debug {
    int event;
    const char *name; /* (n) */

# «p32» (find-lua50page 32)


    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 future
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 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, it is
not in the stack), you push the function 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);

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, or "main" if this is the main part of a chunk.

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 many 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.

# «p33» (find-lua50page 33)


namewhat Explains the previous field. It 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 Number of upvalues of the function.

# «sec5.2»
# «Manipulating Local Variables»

5.2 Manipulating Local Variables

For the manipulation of local variables, luadebug.h uses indices: The
first parameter or local variable has index 1, and so on, until the
last active local variable.

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, filled by a
previous call to lua_getstack or given as argument to a hook (see (to
"sec5.3")). lua_getlocal gets the index n of a local variable, pushes
its 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 on failure, that is when the index is
greater than the number of active local variables.

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

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

# «sec5.3»
# «Hooks»

5.3 Hooks

The Lua interpreter offers a mechanism of hooks: 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, that happens every ``count'' instructions. Lua identifies them
with the following constants: , , , and .

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, unsigned long mask);

# «p34» (find-lua50page 34)


func is the hook, and mask specifies at which events it will be
called. It is formed by a disjunction of the constants LUA_MASKCALL,
LUA_MASKRET, LUA_MASKLINE, plus the macro LUA_MASKCOUNT(count). For
each event, the hook is called as explained below:

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

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

line hook called when the interpreter is about to start the execution
of a new line of code, or when it jumps back (even for the same line).
(For obvious reasons, this event does not happens while Lua is
executing a C function.)

count hook called after the interpreter executes every count
instructions. (For obvious reasons, this event does not happens while
Lua is executing a C function.)

A hook is disabled with the mask zero.

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

  lua_Hook lua_gethook (lua_State *L);
  unsigned long lua_gethookmask (lua_State *L);

You can get the count inside a mask with the macro lua_getmaskcount.

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. For the value of any other field,
the hook must call lua_getinfo.

While Lua is running a hook, it disables other calls to hooks.
Therefore, if a hook calls Lua to execute a function or a chunk, that
execution ocurrs without any calls to hooks.

# «sec6»
# «Standard Libraries»

6 Standard Libraries

The standard libraries provide useful functions that are implemented
directly through the standard C API. Some of these functions provide
essential services to the language (e.g. type and getmetatable);
others provide access to ``outside'' servides (e.g. I/O); and others
could be 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;

# «p35» (find-lua50page 35)


. 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 call the
functions lua_baselibopen (for the basic library), lua_strlibopen (for
the string library), lua_tablibopen (for the table library),
lua_mathlibopen (for the mathematical library), lua_iolibopen (for the
I/O and the Operating System libraries), and lua_dblibopen (for the
debug library), which are declared in lualib.h.

# «sec6.1»
# «Basic Functions»

6.1 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
facilities.

The basic library also defines a global variable _VERSION with a
string containing the current interpreter version. The current content
of this string is "Lua 5.0 (beta)".

# «assert»
. assert (v [, message])

Issues an ``assertion failed!'' error when its argument v is nil;
otherwise, returns this argument. This function is equivalent to the
following Lua function:

  function assert (v, m)
    if not v then
      error(m or "assertion failed!")
    end
    return v
  end

# «collectgarbage»
. collectgarbage ([limit])

Sets the garbage-collection threshold for 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 "sec2.6")). If limit is absent, it defaults to zero
(thus forcing a garbage-collection cycle).

# «dofile»
. dofile (filename)

Receives a file name, 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.

# «error»
. error (message [, level])

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

The level argument affects to 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.

# «p36» (find-lua50page 36)


# «getglobals»
. getglobals (function)

Returns the current table of globals in use by the function. function
can be a Lua function or a number, meaning the function at that stack
level: Level 1 is the function calling getglobals. If the given
function is not a Lua function, returns the ``global'' table of
globals. The default for function is 1.

# «getmetatable»
. getmetatable (object)

Returns the metatable of the given object. If the object does not have
a metatable, returns nil.

# «gcinfo»
. gcinfo ()

Returns the number of Kbytes of dynamic memory Lua is using, and (as a
second result) the current garbage collector threshold (also in
Kbytes).

# «ipairs»
. ipairs (t)

Returns a generator function and the table t, 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
nil value of the table.

# «loadfile»
. loadfile (filename)

Loads a file as a Lua chunk. If there is no errors, returns the
compiled chunk as a function; otherwise, returns nil plus an error
message.

# «loadstring»
. loadstring (string [, chunkname])

Loads a string as a Lua chunk. If there is no errors, returns the
compiled chunk as a function; otherwise, returns nil plus an error
message.

The optional parameter chunkname is the ``name of the chunk'', used in
error messages and debug information.

To load and run a given string, use the idiom

  assert(loadstring(s))()

# «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; semantically, 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 function ipairs).

The behavior of next is undefined if you change the table during the
traversal.

# «p37» (find-lua50page 37)


# «pairs»
. pairs (t)

Returns the function next and the table t, so that the construction

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

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

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

Calls function func with the given arguments in protected mode. That
means that any error inside func is not propagated; instead, pcall
catches the error, returning a status code. Its first result is the
status code (a boolean), 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 strings returned by tostring. This function is not intended
for formatted output, but only as a quick way to show a value,
typically for debugging. For formatted output, see format (see (to
"sec6.2")).

# «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.

# «require»
. require (packagename)

Loads the given package. The function starts by looking into the table
_LOADED whether packagename is already loaded. If it is, then require
is done. Otherwise, it searches a path looking for a file to load.

If the global variable LUA_PATH is a string, this string is the path.
Otherwise, require tries the environment variable LUA_PATH. In the
last resort, it uses a predefined path.

The path is a sequence of templates separated by semicolons. For each
template, require will change an eventual 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/?/init.lua;/lasttry"

a require "mod" will try to load the files ./mod.lua, ./mod.lc,
/usr/local/mod/init.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. 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. Otherwise, it marks in table _LOADED that the package is
loaded, and returns.

While running a packaged file, require defines the global variable
_REQUIREDNAME with the package name.

# «p38» (find-lua50page 38)


# «setglobals»
. setglobals (function, table)

Sets the current table of globals to be used by the given function.
function can be a Lua function or a number, meaning the function at
that stack level: Level 1 is the function calling setglobals.

# «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.

# «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 "sec2.4")). In other bases, only unsigned
integers are accepted.

# «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 "sec6.2")).

# «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", "table", "function", 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 valid only for a fixed n. The number
n of returned values is either the value of list.n, if it is a number,
or one less the index of the first absent (nil) value.

# «sec6.2»
# «String Manipulation»

6.2 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. If i
is absent, then it is assumed to be 1. i may be negative.

NOTE : Numerical codes are not necessarily portable across platforms.

# «p39» (find-lua50page 39)


# «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 : Numerical codes are not necessarily portable across platforms.

# «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, and 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 another string, which is a substring of s, starting at i and
running 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 the call 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

# «p40» (find-lua50page 40)


suitable to be safely read back by the Lua interpreter: The string is
written between double quotes, and all double quotes, returns, 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".

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

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

Returns a generator 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 each one in a
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.

If repl is a function, then this function is called every time a match
occurs, with all captured substrings passed as arguments, in order
(see below); 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 last, optional 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:

# «p41» (find-lua50page 41)


  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" (for instance)

  x = string.gsub("4+5 = $return 4+5$", "%$(.-)%$", function (s)
        return loadstring(s)()
      end)
  --> x="4+5 = 9"

  local t = {name="Lua", version="5.0"}
  x = string.gsub("$name - $version", "%$(%w+)", function (v)
        return t[v]
      end)
  --> x="Lua - 5.0"

# «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.

# «p42» (find-lua50page 42)


%x (where x is any non-alphanumeric character) --- represents the
character x . This is the standard way to escape the magic characters.
We recommend that any punctuation character (even the non magic)
should be preceded by a % when used to represent itself in a pattern.

[set] --- represents the class which is the union of all characters in
set . A range of characters 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, . . . ), the
corresponding uppercase letter represents the complement of the class.
For instance, %S represents all non-space characters.

The definitions of letter, space, etc. 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.

# «p43» (find-lua50page 43)


# «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.

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.

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

# «sec6.3»
# «Table Manipulation»

6.3 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 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, func)

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

The behavior of foreach is undefined if you change the table t during
the traversal.

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

Executes the given func over the numerical indices of table. For each
index, func 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 "sec6.3")). If func returns a
non-nil value, then the loop is broken, and this value is returned as
the final value of foreachi.

# «p44» (find-lua50page 44)


# «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.getn or 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.

Notice that the last option happens only once for a table. If you call
table.getn again over the same table, it will return the same previous
result, even if the table has been modified. The only way to change
the value of table.getn is by calling table.setn or assigning to field
"n" in the table.

# «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 "sec6.3")). 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 other
elements up to open space, if necessary. The default value for pos is
n+1, where n is the size of the table (see (to "sec6.3")), 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, calling table.setn(table, n+1).

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

Removes from table the element at position pos, shifting other
elements down 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 "sec6.3")), so that a call tremove(t)
removes the last element of table t. This function also updates the
size of the table, 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 of the table library so that subsequent
calls to table.getn(table) return n.

# «sec6.4»
# «Mathematical Functions»

6.4 Mathematical Functions

This library is an interface to most functions of the standard C math
library. (Some have slightly different names.) It provides all its
functions inside the table . In addition, it registers a ??tag method
for the binary exponentiation operator ^ that returns x y when applied
to numbers 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

# «p45» (find-lua50page 45)


  math.rad   math.sin    math.sqrt       math.tan math.frexp
  math.ldexp math.random math.randomseed

plus a variable math.pi. Most of them are only interfaces to the
homonymous functions in the C library. All trigonometric functions
work in radians. The functions math.deg and math.rad convert between
radians and degrees.

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, provided by ANSI C.
(No guarantees can be given for their statistical properties.) When
called without arguments, math.random returns a pseudo-random 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.

# «sec6.5»
# «Input and Output Facilities»

6.5 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.

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

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 ([handle])

Equivalent to file:close(). Without a handle, 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 (and returns nothing).
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.

# «p46» (find-lua50page 46)


# «io.lines»
. io.lines ([filename])

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

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

will iterate over all lines of the file. When the generator 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.

# «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:

``r'' read mode;
``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.read»
. io.read (format1, ...)

Equivalent to file:read over the default input file.

# «io.tmpfile»
. io.tmpfile ()

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

# «io.write»
. io.write (value1, ...)

Equivalent to file:write over the default output file.

# «file:close»
. file:close ()

Closes the file file.

# «p47» (find-lua50page 47)


# «file:flush»
. file:flush ()

Saves any written data to the file file.

# «file:lines»
. file:lines ()

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

for lines 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.)

# «file:read»
. file:read (format1, ...)

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
       instead of a string.

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

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

number reads a string with up to that number of characters, or 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 in bytes 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.

# «p48» (find-lua50page 48)


# «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 format before write. If this function fails, it returns
nil, plus a string describing the error.

# «sec6.6»
# «Operating System Facilities»

6.6 Operating System Facilities

This library is implemented through table os.

# «os.clock»
. 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 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 (t1, t2)

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

# «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.

# «p49» (find-lua50page 49)


# «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 (name1, name2)

Renames file named name1 to name2. If this function fails, it returns
nil, plus a string describing the error.

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

This function is an interface to the C function setlocale. 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 ti