Lua 5.1 Reference Manual

by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, Waldemar Celes

Copyright © 2006-2007 Lua.org, PUC-Rio. Freely available under the
terms of the Lua license.

contents · index

1 - Introduction

Lua is an extension programming language designed to support general
procedural programming with data description facilities. It also offers good
support for object-oriented programming, functional programming, and data-driven
programming. Lua is intended to be used as a powerful, light-weight scripting
language for any program that needs one. Lua is implemented as a library,
written in clean C (that is, in the common subset of ANSI C and
C++).

Being an extension language, Lua has no notion of a "main" program: it only
works embedded in a host client, called the embedding program
or simply the host. This host program can invoke functions to execute a
piece of Lua code, can write and read Lua variables, and can register
C functions to be called by Lua code. Through the use of C functions,
Lua can be augmented to cope with a wide range of different domains, thus
creating customized programming languages sharing a syntactical framework. The
Lua distribution includes a sample host program called lua, which
uses the Lua library to offer a complete, stand-alone Lua interpreter.

Lua is free software, and is provided as usual with no guarantees, as stated
in its license. The implementation described in this manual is available at
Lua's official web site, 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 technical papers
available at Lua's web site. For a detailed introduction to programming in Lua,
see Roberto's book, Programming in Lua (Second Edition).

2 - The Language

This section describes the lexis, the syntax, and the semantics of Lua. In
other words, this section describes which tokens are valid, how they can be
combined, and what their combinations mean.

The language constructs will be explained using the usual extended BNF
notation, in which {a} means 0 or more a's, and
[a] means an optional a. Non-terminals are shown like
non-terminal, keywords are shown like kword, and other terminal symbols
are shown like `=´. The complete syntax of Lua can be found at the end of
this manual.

2.1 - Lexical Conventions

Names (also called identifiers) in Lua can be any string of
letters, digits, and underscores, not beginning with a digit. This coincides
with the definition of names 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.) Identifiers are used to name
variables and table fields.

The following keywords are reserved and cannot be used as names:
and break do else elseif
end false for function if
in local nil not or
repeat return then true until while

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

The following strings denote other tokens:
+ - * / % ^ #
== ~= <= >= < > =
( ) { } [ ]
; : , . .. ...

Literal strings can be delimited by matching single or double
quotes, and can contain the following C-like escape sequences: '\a'
(bell), '\b' (backspace), '\f' (form feed),
'\n' (newline), '\r' (carriage return),
'\t' (horizontal tab), '\v' (vertical tab),
'\\' (backslash), '\"' (quotation mark [double
quote]), and '\'' (apostrophe [single quote]). Moreover, a
backslash followed by a real newline results in a newline in the string. A
character in a string may also be specified by its numerical value using the
escape sequence \ddd, where ddd is a sequence of
up to three decimal digits. (Note that if a numerical escape is to be followed
by a digit, it must be expressed using exactly three digits.) Strings in Lua may
contain any 8-bit value, including embedded zeros, which can be specified as
'\0'.

To put a double (single) quote, a newline, a backslash, or an embedded zero
inside a literal string enclosed by double (single) quotes you must use an
escape sequence. Any other character may be directly inserted into the literal.
(Some control characters may cause problems for the file system, but Lua has no
problem with them.)

Literal strings can also be defined using a long format enclosed by long
brackets. We define an opening long bracket of level n as
an opening square bracket followed by n equal signs followed by another
opening square bracket. So, an opening long bracket of level 0 is written
as [[, an opening long bracket of level 1 is written as
[=[, and so on. A closing long bracket is defined
similarly; for instance, a closing long bracket of level 4 is written as
]====]. A long string starts with an opening long bracket of any
level and ends at the first closing long bracket of the same level. Literals in
this bracketed form may run for several lines, do not interpret any escape
sequences, and ignore long brackets of any other level. They may contain
anything except a closing bracket of the proper level.

For convenience, when the opening long bracket is immediately followed by a
newline, the newline is not included in the string. As an example, in a system
using ASCII (in which 'a' is coded as 97, newline is coded
as 10, and '1' is coded as 49), the five literals below
denote the same string:
a = 'alo\n123"'
a = "alo\n123\""
a = '\97lo\10\04923"'
a = [[alo
123"]]
a = [==[
alo
123"]==]

A numerical constant may be written with an optional decimal part
and an optional decimal exponent. Lua also accepts integer hexadecimal
constants, by prefixing them with 0x. Examples of valid numerical
constants are
3 3.0 3.1416 314.16e-2 0.31416E1 0xff 0x56

A comment starts with a double hyphen (--) anywhere
outside a string. If the text immediately after -- is not an
opening long bracket, the comment is a short comment, which runs until
the end of the line. Otherwise, it is a long comment, which runs until
the corresponding closing long bracket. Long comments are frequently used to
disable code temporarily.

2.2 - Values and Types

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

All values in Lua are first-class values. This means that all values
can be stored in variables, passed as arguments to other functions, and returned
as results.

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; it
usually represents the absence of a useful value. Boolean is the type
of the values false and true. Both nil and false
make a condition false; any other value makes it true. Number
represents real (double-precision floating-point) numbers. (It is easy to build
Lua interpreters that use other internal representations for numbers, such as
single-precision float or long integers; see file luaconf.h.)
String represents arrays of characters. Lua is 8-bit clean: strings may
contain any 8-bit character, including embedded zeros ('\0') (see
§2.1).

Lua can call (and manipulate) functions written in Lua and functions written
in C (see §2.5.8).

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 §2.8).
Userdata values cannot be created or modified in Lua, only through the
C API. This guarantees the integrity of data owned by the host program.

The type thread represents independent threads of execution and it
is used to implement coroutines (see §2.11). Do not confuse
Lua threads with operating-system threads. Lua supports coroutines on all
systems, even those that do not support threads.

The type table implements associative arrays, that is, arrays that
can be indexed not only with numbers, but with any value (except nil).
Tables can be heterogeneous; that is, they can contain values of all
types (except nil). Tables are the sole data structuring mechanism in
Lua; they may be used to represent ordinary arrays, symbol tables, sets,
records, graphs, trees, etc. To represent records, Lua uses the field name as an
index. The language supports this representation by providing
a.name as syntactic sugar for a["name"]. There are
several convenient ways to create tables in Lua (see §2.5.7).

Like indices, the value of a table field can be of any type (except
nil). In particular, because functions are first-class values, table
fields may contain functions. Thus tables may also carry methods (see
§2.5.9).

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

The library function type
returns a string describing the type of a given value.

2.2.1 - Coercion

Lua provides automatic conversion between string and number values at run
time. Any arithmetic operation applied to a string tries to convert this string
to a number, following the usual conversion rules. Conversely, whenever a number
is used where a string is expected, the number is converted to a string, in a
reasonable format. For complete control over how numbers are converted to
strings, use the format function from the string library (see string.format).

2.3 - Variables

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

A single name can denote a global variable or a local variable (or a
function's formal parameter, which is a particular kind of local variable):
var ::= Name

Name denotes identifiers, as defined in §2.1.

Any variable is assumed to be global unless explicitly declared as a local
(see §2.4.7).
Local variables are lexically scoped: local variables can be freely
accessed by functions defined inside their scope (see §2.6).

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

Square brackets are used to index a table:
var ::= prefixexp `[´ exp `]´

The meaning of accesses to global variables and table fields can be changed
via metatables. An access to an indexed variable t[i] is equivalent
to a call gettable_event(t,i). (See §2.8 for a complete
description of the gettable_event function. This function is not
defined or callable in Lua. We use it here only for explanatory purposes.)

The syntax var.Name is just syntactic sugar for
var["Name"]:
var ::= prefixexp `.´ Name

All global variables live as fields in ordinary Lua tables, called
environment tables or simply environments (see §2.9). Each function
has its own reference to an environment, so that all global variables in this
function will refer to this environment table. When a function is created, it
inherits the environment from the function that created it. To get the
environment table of a Lua function, you call getfenv.
To replace it, you call setfenv.
(You can only manipulate the environment of C functions through the debug
library; (see §5.9).)

An access to a global variable x is equivalent to
_env.x, which in turn is equivalent to
gettable_event(_env, "x")

where _env is the environment of the running function. (See §2.8 for a complete
description of the gettable_event function. This function is not
defined or callable in Lua. Similarly, the _env variable is not
defined in Lua. We use them here only for explanatory purposes.)

2.4 - Statements

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

2.4.1 - Chunks

The unit of execution of Lua is called a chunk. A chunk is simply a
sequence of statements, which are executed sequentially. Each statement can be
optionally followed by a semicolon:
chunk ::= {stat [`;´]}

There are no empty statements and thus ';;' is not legal.

Lua handles a chunk as the body of an anonymous function with a variable
number of arguments (see §2.5.9). As such,
chunks can define local variables, receive arguments, and return values.

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

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

2.4.2 - Blocks

A block is a list of statements; syntactically, a block is the same as 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 §2.4.4).

2.4.3 - Assignment

Lua allows multiple assignment. Therefore, the syntax for assignment defines
a list of variables on the left side and a list of expressions on the right
side. The elements in both lists are separated by commas:
stat ::= varlist1 `=´ explist1
varlist1 ::= var {`,´ var}
explist1 ::= exp {`,´ exp}

Expressions are discussed in §2.5.

Before the assignment, the list of values is adjusted to the length
of the list of variables. If there are more values than needed, the excess
values are thrown away. If there are fewer values than needed, the list is
extended with as many nil's as needed. If the list of expressions ends
with a function call, then all values returned by this call enter in the list of
values, before the adjustment (except when the call is enclosed in parentheses;
see §2.5).

The assignment statement first evaluates all its expressions and only then
are the assignments performed. Thus the code
i = 3
i, a[i] = i+1, 20

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

exchanges the values of x and y.

The meaning of assignments to global variables and table fields can be
changed via metatables. An assignment to an indexed variable t[i] =
val is equivalent to settable_event(t,i,val). (See §2.8 for a complete
description of the settable_event function. This function is not
defined or callable in Lua. We use it here only for explanatory purposes.)

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

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

2.4.4 - Control Structures

The control structures if, while, and repeat have the
usual meaning and familiar syntax:
stat ::= while exp do block end
stat ::= repeat block until exp
stat ::= if exp then block {elseif exp then block} [else block] end

Lua also has a for statement, in two flavors (see §2.4.5).

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

In the repeat–until loop, the inner block does not end at the
until keyword, but only after the condition. So, the condition can refer
to local variables declared inside the loop block.

The return statement is used to return values from a function or a
chunk (which is just a function). Functions and chunks may return more than one
value, so the syntax for the return statement is
stat ::= return [explist1]

The break statement is 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.

The return and break statements can only be written as the
last statement of a block. If it is really necessary to return
or break in the middle of a block, then an explicit inner block can be
used, as in the idioms do return end and do break end,
because now return and break are the last statements in their
(inner) blocks.

2.4.5 - For Statement

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

The numeric for loop repeats a block of code while a control variable
runs through an arithmetic progression. It has the following syntax:
stat ::= for Name `=´ exp `,´ exp [`,´ exp] do block end

The block is repeated for name starting at the value of the
first exp, until it passes the second exp by steps of the
third exp. More precisely, a for statement like
for v = e1, e2, e3 do block end

is equivalent to the code:
do
local var, limit, step = tonumber(e1), tonumber(e2), tonumber(e3)
if not (var and limit and step) then error() end
while (step > 0 and var <= limit) or (step <= 0 and var >= limit) do
local v = var
block
var = var + step
end
end

Note the following:

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

var, limit, and
step are invisible variables. The names are here for
explanatory purposes only.

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 v is local to the loop; you cannot use its
value after the for ends or is broken. If you need this value, assign
it to another variable before breaking or exiting the loop.

The generic for statement works over functions, called
iterators. On each iteration, the iterator function is called to
produce a new value, stopping when this new value is nil. The generic
for loop has the following syntax:
stat ::= for namelist in explist1 do block end
namelist ::= Name {`,´ Name}

A for statement like
for var_1, ···, var_n in explist do block end

is equivalent to the code:
do
local f, s, var = explist
while true do
local var_1, ···, var_n = f(s, var)
var = var_1
if var == nil then break end
block
end
end

Note the following:

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

f, s, and
var are invisible variables. The names are here for
explanatory purposes only.

You can use break to exit a for loop.

The loop variables var_i are local to the loop; 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.

2.4.6 - Function Calls as Statements

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

stat ::= functioncall

In this case, all returned values are thrown away. Function calls are
explained in §2.5.8.

2.4.7 - Local Declarations

Local variables may be declared anywhere inside a block. The declaration may
include an initial assignment:
stat ::= local namelist [`=´ explist1]

If present, an initial assignment has the same semantics of a multiple
assignment (see §2.4.3). Otherwise,
all variables are initialized with nil.

A chunk is also a block (see §2.4.1), and so local
variables can be declared in a chunk outside any explicit block. The scope of
such local variables extends until the end of the chunk.

The visibility rules for local variables are explained in §2.6.

2.5 - Expressions

The basic expressions in Lua are the following:
exp ::= prefixexp
exp ::= nil | false | true
exp ::= Number
exp ::= String
exp ::= function
exp ::= tableconstructor
exp ::= `...´
exp ::= exp binop exp
exp ::= unop exp
prefixexp ::= var | functioncall | `(´ exp `)´

Numbers and literal strings are explained in §2.1; variables are
explained in §2.3;
function definitions are explained in §2.5.9; function
calls are explained in §2.5.8; table
constructors are explained in §2.5.7. Vararg
expressions, denoted by three dots ('...'), can only be used when
directly inside a vararg function; they are explained in §2.5.9.

Binary operators comprise arithmetic operators (see §2.5.1), relational
operators (see §2.5.2), logical
operators (see §2.5.3), and the
concatenation operator (see §2.5.4). Unary
operators comprise the unary minus (see §2.5.1), the unary
not (see §2.5.3), and the
unary length operator (see §2.5.5).

Both function calls and vararg expressions may result in multiple values. If
the expression is used as a statement (see §2.4.6) (only
possible for function calls), then its return list is adjusted to zero elements,
thus discarding all returned values. If the expression is used as the last (or
the only) element of a list of expressions, then no adjustment is made (unless
the call is enclosed in parentheses). In all other contexts, Lua adjusts the
result list to one element, discarding all values except the first one.

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 results from f()
a,b,c = f(), x -- f() is adjusted to 1 result (c gets nil)
a,b = ... -- a gets the first vararg parameter, b gets
-- the second (both a and b may get nil if there
-- is no corresponding vararg parameter)

a,b,c = x, f() -- f() is adjusted to 2 results
a,b,c = f() -- f() is adjusted to 3 results
return f() -- returns all results from f()
return ... -- returns all received vararg parameters
return x,y,f() -- returns x, y, and all results from f()
{f()} -- creates a list with all results from f()
{...} -- creates a list with all vararg parameters
{f(), nil} -- f() is adjusted to 1 result

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.)

2.5.1 - Arithmetic Operators

Lua supports the usual arithmetic operators: the binary +
(addition), - (subtraction), * (multiplication),
/ (division), % (modulo), and ^
(exponentiation); and unary - (negation). If the operands are
numbers, or strings that can be converted to numbers (see §2.2.1), then all
operations have the usual meaning. Exponentiation works for any exponent. For
instance, x^(-0.5) computes the inverse of the square root of
x. Modulo is defined as
a % b == a - math.floor(a/b)*b

That is, it is the remainder of a division that rounds the quotient towards
minus infinity.

2.5.2 - Relational Operators

The relational operators in Lua are
== ~= < > <= >=

These operators always result in false or true.

Equality (==) first compares the type of its operands. If the
types are different, then the result is false. Otherwise, the values of
the operands are compared. Numbers and strings are compared in the usual way.
Objects (tables, userdata, threads, and functions) are compared by
reference: two objects are considered equal only if they are the
same object. Every time you create a new object (a table, userdata,
thread, or function), this new object is different from any previously existing
object.

You can change the way that Lua compares tables and userdata by using the
"eq" metamethod (see §2.8).

The conversion rules of §2.2.1 do
not apply to equality comparisons. Thus, "0"==0 evaluates to
false, and t[0] and t["0"] denote different
entries in a table.

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

The order operators work as follows. If both arguments are numbers, then they
are compared as such. Otherwise, if both arguments are strings, then their
values are compared according to the current locale. Otherwise, Lua tries to
call the "lt" or the "le" metamethod (see §2.8).

2.5.3 - Logical Operators

The logical operators in Lua are and, or, and not. Like
the control structures (see §2.4.4), all logical
operators consider both false and nil as false and anything else
as true.

The negation operator not always returns false or true.
The conjunction operator and returns its first argument if this value is
false or nil; otherwise, and returns its second argument.
The disjunction operator or returns its first argument if this value is
different from nil and false; otherwise, or returns its
second argument. Both and and or use short-cut evaluation; that
is, the second operand is evaluated only if necessary. Here are some examples:
10 or 20 --> 10
10 or error() --> 10
nil or "a" --> "a"
nil and 10 --> nil
false and error() --> false
false and nil --> false
false or nil --> nil
10 and 20 --> 20

(In this manual, --> indicates the result of the preceding expression.)

2.5.4 - Concatenation

The string concatenation operator in Lua is denoted by two dots
('..'). If both operands are strings or numbers, then they are
converted to strings according to the rules mentioned in §2.2.1. Otherwise,
the "concat" metamethod is called (see §2.8).

2.5.5 - The Length Operator

The length operator is denoted by the unary operator #. The
length of a string is its number of bytes (that is, the usual meaning of string
length when each character is one byte).

The length of a table t is defined to be any integer index
n such that t[n] is not nil and
t[n+1] is nil; moreover, if t[1] is nil,
n may be zero. For a regular array, with non-nil values from 1 to a
given n, its length is exactly that n, the index of
its last value. If the array has "holes" (that is, nil values between
other non-nil values), then #t may be any of the indices that
directly precedes a nil value (that is, it may consider any such
nil value as the end of the array).

2.5.6 - Precedence

Operator precedence in Lua follows the table below, from lower to higher
priority:
or
and
< > <= >= ~= ==
..
+ -
* / %
not # - (unary)
^

As usual, you can use parentheses to change the precedences of an expression.
The concatenation ('..') and exponentiation ('^')
operators are right associative. All other binary operators are left
associative.

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

If the last field in the list has the form exp and the
expression is a function call or a vararg expression, then all values returned
by this expression enter the list consecutively (see §2.5.8). To avoid
this, enclose the function call (or the vararg expression) in parentheses (see
§2.5).

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

2.5.8 - 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 this function is called with the
given arguments. Otherwise, the prefixexp "call" metamethod is called, having as
first parameter the value of prefixexp, followed by the original call arguments
(see §2.8).

The form
functioncall ::= prefixexp `:´ Name args

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

Arguments have the following syntax:
args ::= `(´ [explist1] `)´
args ::= tableconstructor
args ::= String

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

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

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

A call of the form return functioncall is called a
tail call. Lua implements proper tail calls (or proper
tail recursion): in a tail call, the called function reuses the stack entry
of the calling function. Therefore, there is no limit on the number of nested
tail calls that a program can execute. However, a tail call erases any debug
information about the calling function. Note that a tail call only happens with
a particular syntax, where the return has one single function call as
argument; this syntax makes the calling function return exactly the returns of
the called function. So, none of the following examples are tail calls:
return (f(x)) -- results adjusted to 1
return 2 * f(x)
return x, f(x) -- additional results
f(x); return -- results discarded
return x or f(x) -- results adjusted to 1

2.5.9 - 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 () body end

translates to
f = function () body end

The statement
function t.a.b.c.f () body end

translates to
t.a.b.c.f = function () body end

The statement
local function f () body end

translates to
local f; f = function () body end

not to
local f = function () body end

(This only makes a difference when the body of the function contains
references to f.)

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

Parameters act as local variables that are initialized with the argument
values:
parlist1 ::= namelist [`,´ `...´] | `...´

When a function is called, the list of arguments is adjusted to the length of
the list of parameters, unless the function is a variadic or vararg
function, which is indicated by three dots ('...') at the end
of its parameter list. A vararg function does not adjust its argument list;
instead, it collects all extra arguments and supplies them to the function
through a vararg expression, which is also written as three dots. The
value of this expression is a list of all actual extra arguments, similar to a
function with multiple results. If a vararg expression is used inside another
expression or in the middle of a list of expressions, then its return list is
adjusted to one element. If the expression is used as the last element of a list
of expressions, then no adjustment is made (unless the call is enclosed in
parentheses).

As an example, consider the following definitions:
function f(a, b) end
function g(a, b, ...) end
function r() return 1,2,3 end

Then, we have the following mapping from arguments to parameters and to the
vararg expression:
CALL PARAMETERS

f(3) a=3, b=nil
f(3, 4) a=3, b=4
f(3, 4, 5) a=3, b=4
f(r(), 10) a=1, b=10
f(r()) a=1, b=2

g(3) a=3, b=nil, ... --> (nothing)
g(3, 4) a=3, b=4, ... --> (nothing)
g(3, 4, 5, 8) a=3, b=4, ... --> 5 8
g(5, r()) a=5, b=1, ... --> 2 3

Results are returned using the return statement (see §2.4.4). 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 (params) body end

is syntactic sugar for
t.a.b.c.f = function (self, params) body end

2.6 - Visibility Rules

Lua is a lexically scoped language. The scope of variables begins at the
first statement after their declaration and lasts until the end of the
innermost block that includes the declaration. Consider the following example:
x = 10 -- global variable
do -- new block
local x = x -- new 'x', with value 10
print(x) --> 10
x = x+1
do -- another block
local x = x+1 -- another 'x'
print(x) --> 12
end
print(x) --> 11
end
print(x) --> 10 (the global one)

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

Because of the lexical scoping rules, local variables can be freely accessed
by functions defined inside their scope. A local variable used by an inner
function is called an upvalue, or external local variable,
inside the inner function.

Notice that each execution of a local statement defines new local
variables. Consider the following example:
a = {}
local x = 20
for i=1,10 do
local y = 0
a[i] = function () y=y+1; return x+y end
end

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

2.7 - Error Handling

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

Lua code can explicitly generate an error by calling the error
function. If you need to catch errors in Lua, you can use the pcall
function.

2.8 - Metatables

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

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

You can query the metatable of any value through the getmetatable
function.

You can replace the metatable of tables through the setmetatable
function. You cannot change the metatable of other types from Lua (except using
the debug library); you must use the C API for that.

Tables and userdata have individual metatables (although multiple tables and
userdata can share their metatables); values of all other types share one single
metatable per type. So, there is one single metatable for all numbers, and for
all strings, etc.

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

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 the operation.

The code shown here in Lua is only illustrative; the real behavior is hard
coded in the interpreter and it is much more efficient than this simulation. All
functions used in these descriptions (rawget,
tonumber,
etc.) are described in §5.1. In particular, to
retrieve the metamethod of a given object, we use the expression
metatable(obj)[event]

This should be read as
rawget(getmetatable(obj) or {}, event)

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

"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

By using this function, the behavior of the op1 + op2 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(···)
end
end
end

"sub": the - operation. Behavior similar to the "add"
operation.

"mul": the * operation. Behavior similar to the "add"
operation.

"div": the / operation. Behavior similar to the "add"
operation.

"mod": the % operation. Behavior similar to the "add"
operation, with the operation o1 - floor(o1/o2)*o2 as the
primitive operation.

"pow": the ^ (exponentiation) operation. Behavior
similar to the "add" operation, with the function pow (from the
C math library) as the primitive operation.

"unm": the unary - operation.
function unm_event (op)
local o = tonumber(op)
if o then -- operand is numeric?
return -o -- '-' here is the primitive 'unm'
else -- the operand is not numeric.
-- Try to get a handler from the operand
local h = metatable(op).__unm
if h then
-- call the handler with the operand
return h(op)
else -- no handler available: default behavior
error(···)
end
end
end

"concat": the .. (concatenation) operation.
function concat_event (op1, op2)
if (type(op1) == "string" or type(op1) == "number") and
(type(op2) == "string" or type(op2) == "number") then
return op1 .. op2 -- primitive string concatenation
else
local h = getbinhandler(op1, op2, "__concat")
if h then
return h(op1, op2)
else
error(···)
end
end
end

"len": the # operation.
function len_event (op)
if type(op) == "string" then
return strlen(op) -- primitive string length
elseif type(op) == "table" then
return #op -- primitive table length
else
local h = metatable(op).__len
if h then
-- call the handler with the operand
return h(op)
else -- no handler available: default behavior
error(···)
end
end
end

See §2.5.5
for a description of the length of a table.

"eq": the == operation. The function
getcomphandler defines how Lua chooses a metamethod for
comparison operators. A metamethod only is selected when both objects being
compared have the same type and the same metamethod for the selected
operation.
function getcomphandler (op1, op2, event)
if type(op1) ~= type(op2) then return nil end
local mm1 = metatable(op1)[event]
local mm2 = metatable(op2)[event]
if mm1 == mm2 then return mm1 else return nil end
end

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

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

"lt": the < operation.
function lt_event (op1, op2)
if type(op1) == "number" and type(op2) == "number" then
return op1 < op2 -- numeric comparison
elseif type(op1) == "string" and type(op2) == "string" then
return op1 < op2 -- lexicographic comparison
else
local h = getcomphandler(op1, op2, "__lt")
if h then
return h(op1, op2)
else
error(···);
end
end
end

a > b is equivalent to b < a.

"le": the <= operation.
function le_event (op1, op2)
if type(op1) == "number" and type(op2) == "number" then
return op1 <= op2 -- numeric comparison
elseif type(op1) == "string" and type(op2) == "string" then
return op1 <= op2 -- lexicographic comparison
else
local h = getcomphandler(op1, op2, "__le")
if h then
return h(op1, op2)
else
h = getcomphandler(op1, op2, "__lt")
if h then
return not h(op2, op1)
else
error(···);
end
end
end
end

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

"index": The indexing access table[key].
function gettable_event (table, key)
local h
if type(table) == "table" then
local v = rawget(table, key)
if v ~= nil then return v end
h = metatable(table).__index
if h == nil then return nil end
else
h = metatable(table).__index
if h == nil then
error(···);
end
end
if type(h) == "function" then
return h(table, key) -- call the handler
else return h[key] -- or repeat operation on it
end
end

"newindex": The indexing assignment table[key] =
value.
function settable_event (table, key, value)
local h
if type(table) == "table" then
local v = rawget(table, key)
if v ~= nil then rawset(table, key, value); return end
h = metatable(table).__newindex
if h == nil then rawset(table, key, value); return end
else
h = metatable(table).__newindex
if h == nil then
error(···);
end
end
if type(h) == "function" then
return h(table, key,value) -- call the handler
else h[key] = value -- or repeat operation on it
end
end

"call": called when Lua calls a value.
function function_event (func, ...)
if type(func) == "function" then
return func(...) -- primitive call
else
local h = metatable(func).__call
if h then
return h(func, ...)
else
error(···)
end
end
end

2.9 - Environments

Besides metatables, objects of types thread, function, and userdata have
another table associated with them, called their environment. Like
metatables, environments are regular tables and multiple objects can share the
same environment.

Environments associated with userdata have no meaning for Lua. It is only a
convenience feature for programmers to associate a table to a userdata.

Environments associated with threads are called global environments.
They are used as the default environment for their threads and non-nested
functions created by the thread (through loadfile,
loadstring
or load)
and can be directly accessed by C code (see §3.3).

Environments associated with C functions can be directly accessed by
C code (see §3.3). They are used as
the default environment for other C functions created by the function.

Environments associated with Lua functions are used to resolve all accesses
to global variables within the function (see §2.3). They are used as
the default environment for other Lua functions created by the function.

You can change the environment of a Lua function or the running thread by
calling setfenv.
You can get the environment of a Lua function or the running thread by calling
getfenv.
To manipulate the environment of other objects (userdata, C functions,
other threads) you must use the C API.

2.10 - Garbage Collection

Lua performs automatic memory management. This means that you have to worry
neither about allocating memory for new objects nor about freeing it when the
objects are no longer needed. Lua manages memory automatically by running a
garbage collector from time to time to collect all dead
objects (that is, these objects that are no longer accessible from Lua).
All objects in Lua are subject to automatic management: tables, userdata,
functions, threads, and strings.

Lua implements an incremental mark-and-sweep collector. It uses two numbers
to control its garbage-collection cycles: the garbage-collector pause
and the garbage-collector step multiplier.

The garbage-collector pause controls how long the collector waits before
starting a new cycle. Larger values make the collector less aggressive. Values
smaller than 1 mean the collector will not wait to start a new cycle. A value of
2 means that the collector waits for the total memory in use to double before
starting a new cycle.

The step multiplier controls the relative speed of the collector relative to
memory allocation. Larger values make the collector more aggressive but also
increase the size of each incremental step. Values smaller than 1 make the
collector too slow and may result in the collector never finishing a cycle. The
default, 2, means that the collector runs at "twice" the speed of memory
allocation.

You can change these numbers by calling lua_gc
in C or collectgarbage
in Lua. Both get percentage points as arguments (so an argument of 100 means a
real value of 1). With these functions you can also control the collector
directly (e.g., stop and restart it).

2.10.1 - Garbage-Collection Metamethods

Using the C API, you can set garbage-collector metamethods for userdata
(see §2.8). These
metamethods are also called finalizers. Finalizers allow you to
coordinate Lua's garbage collection with external resource management (such as
closing files, network or database connections, or freeing your own memory).

Garbage userdata with a field __gc in their metatables are not
collected immediately by the garbage collector. Instead, Lua puts them in a
list. After the collection, Lua does the equivalent of the following function
for each userdata in that list:
function gc_event (udata)
local h = metatable(udata).__gc
if h then
h(udata)
end
end

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

2.10.2 - Weak Tables

A weak table is a table whose elements are weak references.
A weak reference is ignored by the garbage collector. In other words, if the
only references to an object are weak references, then the garbage collector
will collect this 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 controlled by the
__mode field of its metatable. If the __mode field is
a string containing the character 'k', the keys in the table
are weak. If __mode contains 'v', the values in the
table are weak.

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

2.11 - Coroutines

Lua supports coroutines, also called collaborative multithreading. A
coroutine in Lua represents an independent thread of execution. Unlike threads
in multithread systems, however, a coroutine only suspends its execution by
explicitly calling a yield function.

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

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

A coroutine can terminate its execution in two ways: normally, when its main
function returns (explicitly or implicitly, after the last instruction); and
abnormally, if there is an unprotected error. In the first case, coroutine.resume
returns true, plus any values returned by the coroutine main function. In
case of errors, coroutine.resume
returns false plus an error message.

A coroutine yields by calling coroutine.yield.
When a coroutine yields, the corresponding coroutine.resume
returns immediately, even if the yield happens inside nested function calls
(that is, not in the main function, but in a function directly or indirectly
called by the main function). In the case of a yield, coroutine.resume
also returns true, plus any values passed to coroutine.yield.
The next time you resume the same coroutine, it continues its execution from the
point where it yielded, with the call to coroutine.yield
returning any extra arguments passed to coroutine.resume.

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

As an example, consider the following code:
function foo (a)
print("foo", a)
return coroutine.yield(2*a)
end

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

print("main", coroutine.resume(co, 1, 10))
print("main", coroutine.resume(co, "r"))
print("main", coroutine.resume(co, "x", "y"))
print("main", coroutine.resume(co, "x", "y"))

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

3 - The Application Program Interface

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

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 their arguments exactly once
(except for the first argument, which is always a Lua state), and so do not
generate any hidden side-effects.

As in most C libraries, the Lua API functions do not check their
arguments for validity or consistency. However, you can change this behavior by
compiling Lua with a proper definition for the macro luai_apicheck, in file
luaconf.h.

3.1 - The Stack

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

Whenever Lua calls C, the called function gets a new stack, which is
independent of previous stacks and of stacks of C functions that are still
active. This stack initially contains any arguments to the C function and
it is where the C function pushes its results to be returned to the caller
(see lua_CFunction).

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
relative to 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).

3.2 - Stack Size

When you interact with Lua API, you are responsible for ensuring consistency.
In particular, you are responsible for controlling stack overflow. You
can use the function lua_checkstack
to grow the stack size.

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

Most query functions accept as indices any value inside the available stack
space, that is, indices up to the maximum stack size you have set through lua_checkstack.
Such indices are called acceptable indices. More formally, we define an
acceptable index as follows:
(index < 0 && abs(index) <= top) ||
(index > 0 && index <= stackspace)

Note that 0 is never an acceptable index.

3.3 - Pseudo-Indices

Unless otherwise noted, any function that accepts valid indices can also be
called with pseudo-indices, which represent some Lua values that are
accessible to C code but which are not in the stack. Pseudo-indices are
used to access the thread environment, the function environment, the registry,
and the upvalues of a C function (see §3.4).

The thread environment (where global variables live) is always at
pseudo-index LUA_GLOBALSINDEX. The
environment of the running C function is always at pseudo-index LUA_ENVIRONINDEX.

To access and change the value of global variables, you can use regular table
operations over an environment table. For instance, to access the value of a
global variable, do
lua_getfield(L, LUA_GLOBALSINDEX, varname);

3.4 - C Closures

When a C function is created, it is possible to associate some values
with it, thus creating a C closure; these values are called
upvalues and are accessible to the function whenever it is called (see
lua_pushcclosure).

Whenever a C function is called, its upvalues are located at specific
pseudo-indices. These pseudo-indices are produced by the macro lua_upvalueindex. The first value
associated with a function is at position lua_upvalueindex(1), and
so on. Any access to lua_upvalueindex(n), where n
is greater than the number of upvalues of the current function, produces an
acceptable (but invalid) index.

3.5 - Registry

Lua provides a registry, a pre-defined table that can be used by any
C code to store whatever Lua value it needs to store. This table is always
located at pseudo-index LUA_REGISTRYINDEX. Any
C library can store data into this table, but it should take care to choose
keys different from those used by other libraries, to avoid collisions.
Typically, you should use as key a string containing your library name or a
light userdata with the address of a C object in your code.

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

3.6 - Error Handling in C

Internally, Lua uses the C longjmp facility to handle errors.
(You can also choose to use exceptions if you use C++; see file
luaconf.h.) When Lua faces any error (such as memory allocation
errors, type errors, syntax errors, and runtime errors) it raises an
error; that is, it does a long jump. A protected environment uses
setjmp to set a recover point; any error jumps to the most recent
active recover point.

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

Inside a C function you can raise an error by calling lua_error.

3.7 - Functions and Types

Here we list all functions and types from the C API in alphabetical
order.

lua_Alloc

typedef void * (*lua_Alloc) (void *ud,
void *ptr,
size_t osize,
size_t nsize);

The type of the memory-allocation function used by Lua states. The allocator
function must provide a functionality similar to realloc, but not
exactly the same. Its arguments are ud, an opaque pointer passed to
lua_newstate;
ptr, a pointer to the block being allocated/reallocated/freed;
osize, the original size of the block; nsize, the new
size of the block. ptr is NULL if and only if
osize is zero. When nsize is zero, the allocator must
return NULL; if osize is not zero, it should free the
block pointed to by ptr. When nsize is not zero, the
allocator returns NULL if and only if it cannot fill the request.
When nsize is not zero and osize is zero, the
allocator should behave like malloc. When nsize and
osize are not zero, the allocator behaves like
realloc. Lua assumes that the allocator never fails when
osize >= nsize.

Here is a simple implementation for the allocator function. It is used in the
auxiliary library by luaL_newstate.

static void *l_alloc (void *ud, void *ptr, size_t osize,
size_t nsize) {
(void)ud; (void)osize; /* not used */
if (nsize == 0) {
free(ptr);
return NULL;
}
else
return realloc(ptr, nsize);
}

This code assumes that free(NULL) has no effect and that
realloc(NULL, size) is equivalent to malloc(size).
ANSI C ensures both behaviors.

lua_atpanic

lua_CFunction lua_atpanic (lua_State *L, lua_CFunction panicf);

Sets a new panic function and returns the old one.

If an error happens outside any protected environment, Lua calls a panic
function and then calls exit(EXIT_FAILURE), thus exiting the
host application. Your panic function may avoid this exit by never returning
(e.g., doing a long jump).

The panic function can access the error message at the top of the stack.

lua_call

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

Calls a function.

To call a function you must use the following protocol: first, the function
to be called is pushed onto the stack; then, the arguments to the function are
pushed in direct order; that is, the first argument is pushed first. Finally you
call lua_call;
nargs is the number of arguments that you pushed onto the stack.
All arguments and the function value are popped from the stack when the function
is called. The function results are pushed onto the stack when the function
returns. The number of results is adjusted to nresults, unless
nresults is LUA_MULTRET.
In this case, all results from the function are pushed. Lua takes care
that the returned values fit into the stack space. The function results are
pushed onto the stack in direct order (the first result is pushed first), so
that after the call the last result is on the top of the stack.

Any error inside the called function is propagated upwards (with a
longjmp).

The following example shows how the host program may do the equivalent to
this Lua code:
a = f("how", t.x, 14)

Here it is in C:
lua_getfield(L, LUA_GLOBALSINDEX, "f"); /* function to be called */
lua_pushstring(L, "how"); /* 1st argument */
lua_getfield(L, LUA_GLOBALSINDEX, "t"); /* table to be indexed */
lua_getfield(L, -1, "x"); /* push result of t.x (2nd arg) */
lua_remove(L, -2); /* remove 't' from the stack */
lua_pushinteger(L, 14); /* 3rd argument */
lua_call(L, 3, 1); /* call 'f' with 3 arguments and 1 result */
lua_setfield(L, LUA_GLOBALSINDEX, "a"); /* set global 'a' */

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

lua_CFunction

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

Type for C functions.

In order to communicate properly with Lua, a C function must use the
following protocol, which defines the way parameters and results are passed: a
C function receives its arguments from Lua in its stack in direct order
(the first argument is pushed first). So, when the function starts,
lua_gettop(L) returns the number of arguments received by the
function. The first argument (if any) is at index 1 and its last argument is at
index lua_gettop(L). To return values to Lua, a C function
just pushes them onto the stack, in direct order (the first result is pushed
first), and returns the number of results. Any other value in the stack below
the results will be properly discarded by Lua. Like a Lua function, a
C function called by Lua can also return many results.

As an example, the following function receives a variable number of numerical
arguments and returns their average and sum:
static int foo (lua_State *L) {
int n = lua_gettop(L); /* number of arguments */
lua_Number sum = 0;
int i;
for (i = 1; i <= n; i++) {
if (!lua_isnumber(L, i)) {
lua_pushstring(L, "incorrect argument");
lua_error(L);
}
sum += lua_tonumber(L, i);
}
lua_pushnumber(L, sum/n); /* first result */
lua_pushnumber(L, sum); /* second result */
return 2; /* number of results */
}

lua_checkstack

int lua_checkstack (lua_State *L, int extra);

Ensures that there are at least extra free stack slots in the
stack. It returns false if it cannot grow the stack to that size. This function
never shrinks the stack; if the stack is already larger than the new size, it is
left unchanged.

lua_close

void lua_close (lua_State *L);

Destroys all objects in the given Lua state (calling the corresponding
garbage-collection metamethods, if any) and frees all dynamic memory used by
this state. On several platforms, you may not need to call this function,
because all resources are naturally released when the host program ends. On the
other hand, long-running programs, such as a daemon or a web server, might need
to release states as soon as they are not needed, to avoid growing too large.

lua_concat

void lua_concat (lua_State *L, int n);

Concatenates the n values at the top of the stack, pops them,
and leaves the result at the top. If n is 1, the result
is the single string on the stack (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 §2.5.4).

lua_cpcall

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

Calls the C function func in protected mode.
func starts with only one element in its stack, a light userdata
containing ud. In case of errors, lua_cpcall
returns the same error codes as lua_pcall,
plus the error object on the top of the stack; otherwise, it returns zero, and
does not change the stack. All values returned by func are
discarded.

lua_createtable

void lua_createtable (lua_State *L, int narr, int nrec);

Creates a new empty table and pushes it onto the stack. The new table has
space pre-allocated for narr array elements and nrec
non-array elements. This pre-allocation is useful when you know exactly how many
elements the table will have. Otherwise you can use the function lua_newtable.

lua_dump

int lua_dump (lua_State *L, lua_Writer writer, void *data);

Dumps a function as a binary chunk. Receives a Lua function on the top of the
stack and produces a binary chunk that, if loaded again, results in a function
equivalent to the one dumped. As it produces parts of the chunk, lua_dump
calls function writer (see lua_Writer)
with the given data to write them.

The value returned is the error code returned by the last call to the writer;
0 means no errors.

This function does not pop the Lua function from the stack.

lua_equal

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

Returns 1 if the two values in acceptable indices index1 and
index2 are equal, following the semantics of the Lua
== operator (that is, may call metamethods). Otherwise
returns 0. Also returns 0 if any of the indices is non valid.

lua_error

int lua_error (lua_State *L);

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

lua_gc

int lua_gc (lua_State *L, int what, int data);

Controls the garbage collector.

This function performs several tasks, according to the value of the parameter
what:

LUA_GCSTOP: stops the garbage collector.

LUA_GCRESTART: restarts the garbage collector.

LUA_GCCOLLECT: performs a full garbage-collection
cycle.

LUA_GCCOUNT: returns the current amount of memory (in
Kbytes) in use by Lua.

LUA_GCCOUNTB: returns the remainder of dividing the
current amount of bytes of memory in use by Lua by 1024.

LUA_GCSTEP: performs an incremental step of garbage
collection. The step "size" is controlled by data (larger values
mean more steps) in a non-specified way. If you want to control the step size
you must experimentally tune the value of data. The function
returns 1 if the step finished a garbage-collection cycle.

LUA_GCSETPAUSE: sets data/100 as the new
value for the pause of the collector (see §2.10). The function
returns the previous value of the pause.

LUA_GCSETSTEPMUL: sets data/100 as the
new value for the step multiplier of the collector (see §2.10). The function
returns the previous value of the step multiplier.

lua_getallocf

lua_Alloc lua_getallocf (lua_State *L, void **ud);

Returns the memory-allocation function of a given state. If ud
is not NULL, Lua stores in *ud the opaque pointer
passed to lua_newstate.

lua_getfenv

void lua_getfenv (lua_State *L, int index);

Pushes onto the stack the environment table of the value at the given index.

lua_getfield

void lua_getfield (lua_State *L, int index, const char *k);

Pushes onto the stack the value t[k], where t is
the value at the given valid index index. As in Lua, this function
may trigger a metamethod for the "index" event (see §2.8).

lua_getglobal

void lua_getglobal (lua_State *L, const char *name);

Pushes onto the stack the value of the global name. It is
defined as a macro:
#define lua_getglobal(L,s) lua_getfield(L, LUA_GLOBALSINDEX, s)

lua_getmetatable

int lua_getmetatable (lua_State *L, int index);

Pushes onto the stack the metatable of the value at the given acceptable
index. If the index is not valid, or if the value does not have a metatable, the
function returns 0 and pushes nothing on the stack.

lua_gettable

void lua_gettable (lua_State *L, int index);

Pushes onto the stack the value t[k], where t is
the value at the given valid index index and k is the
value at the top of the stack.

This function pops the key from the stack (putting the resulting value in its
place). As in Lua, this function may trigger a metamethod for the "index" event
(see §2.8).

lua_gettop

int lua_gettop (lua_State *L);

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

lua_insert

void lua_insert (lua_State *L, int index);

Moves the top element into the given valid index, shifting up the elements
above this index to open space. Cannot be called with a pseudo-index, because a
pseudo-index is not an actual stack position.

lua_Integer

typedef ptrdiff_t lua_Integer;

The type used by the Lua API to represent integral values.

By default it is a ptrdiff_t, which is usually the largest
signed integral type the machine handles "comfortably".

lua_isboolean

int lua_isboolean (lua_State *L, int index);

Returns 1 if the value at the given acceptable index has type boolean, and
0 otherwise.

lua_iscfunction

int lua_iscfunction (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a C function,
and 0 otherwise.

lua_isfunction

int lua_isfunction (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a function (either C
or Lua), and 0 otherwise.

lua_islightuserdata

int lua_islightuserdata (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a light userdata, and
0 otherwise.

lua_isnil

int lua_isnil (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is nil, and
0 otherwise.

lua_isnone

int lua_isnone (lua_State *L, int index);

Returns 1 if the the given acceptable index is not valid (that is, it refers
to an element outside the current stack), and 0 otherwise.

lua_isnoneornil

int lua_isnoneornil (lua_State *L, int index);

Returns 1 if the the given acceptable index is not valid (that is, it refers
to an element outside the current stack) or if the value at this index is
nil, and 0 otherwise.

lua_isnumber

int lua_isnumber (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a number or a string
convertible to a number, and 0 otherwise.

lua_isstring

int lua_isstring (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a string or a number
(which is always convertible to a string), and 0 otherwise.

lua_istable

int lua_istable (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a table, and
0 otherwise.

lua_isthread

int lua_isthread (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a thread, and
0 otherwise.

lua_isuserdata

int lua_isuserdata (lua_State *L, int index);

Returns 1 if the value at the given acceptable index is a userdata (either
full or light), and 0 otherwise.

lua_lessthan

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

Returns 1 if the value at acceptable index index1 is smaller
than the value at acceptable index index2, following the semantics
of the Lua < operator (that is, may call metamethods). Otherwise
returns 0. Also returns 0 if any of the indices is non valid.

lua_load

int lua_load (lua_State *L,
lua_Reader reader,
void *data,
const char *chunkname);

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

0: no errors;

LUA_ERRSYNTAX: syntax
error during pre-compilation;

LUA_ERRMEM:
memory allocation error.

This function only loads a chunk; it does not run it.

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

The lua_load
function uses a user-supplied reader function to read the chunk
(see lua_Reader).
The data argument is an opaque value passed to the reader function.

The chunkname argument gives a name to the chunk, which is used
for error messages and in debug information (see §3.8).

lua_newstate

lua_State *lua_newstate (lua_Alloc f, void *ud);

Creates a new, independent state. Returns NULL if cannot create
the state (due to lack of memory). The argument f is the allocator
function; Lua does all memory allocation for this state through this function.
The second argument, ud, is an opaque pointer that Lua simply
passes to the allocator in every call.

lua_newtable

void lua_newtable (lua_State *L);

Creates a new empty table and pushes it onto the stack. It is equivalent to
lua_createtable(L, 0, 0).

lua_newthread

lua_State *lua_newthread (lua_State *L);

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

There is no explicit function to close or to destroy a thread. Threads are
subject to garbage collection, like any Lua object.

lua_newuserdata

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

This function allocates a new block of memory with the given size, pushes
onto the stack a new full userdata with the block address, and returns this
address.

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

When Lua collects a full userdata with a gc metamethod, Lua
calls the metamethod and marks the userdata as finalized. When this userdata is
collected again then Lua frees its corresponding memory.

lua_next

int lua_next (lua_State *L, int index);

Pops a key from the stack, and pushes a key-value pair from the table at the
given index (the "next" pair after the given key). If there are no more elements
in the table, then lua_next
returns 0 (and pushes nothing).

A typical traversal looks like this:
/* table is in the stack at index 't' */
lua_pushnil(L); /* first key */
while (lua_next(L, t) != 0) {
/* uses 'key' (at index -2) and 'value' (at index -1) */
printf("%s - %s\n",
lua_typename(L, lua_type(L, -2)),
lua_typename(L, lua_type(L, -1)));
/* removes 'value'; keeps 'key' for next iteration */
lua_pop(L, 1);
}

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

lua_Number

typedef double lua_Number;

The type of numbers in Lua. By default, it is double, but that can be changed
in luaconf.h.

Through the configuration file you can change Lua to operate with another
type for numbers (e.g., float or long).

lua_objlen

size_t lua_objlen (lua_State *L, int index);

Returns the "length" of the value at the given acceptable index: for strings,
this is the string length; for tables, this is the result of the length operator
('#'); for userdata, this is the size of the block of memory
allocated for the userdata; for other values, it is 0.

lua_pcall

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

Calls a function in protected mode.

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

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

Typically, the 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.

lua_pop

void lua_pop (lua_State *L, int n);

Pops n elements from the stack.

lua_pushboolean

void lua_pushboolean (lua_State *L, int b);

Pushes a boolean value with value b onto the stack.

lua_pushcclosure

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

Pushes a new C closure onto the stack.

When a C function is created, it is possible to associate some values
with it, thus creating a C closure (see §3.4); these values are
then accessible to the function whenever it is called. To associate values with
a C function, first these values should be pushed onto the stack (when
there are multiple values, the first value is pushed first). Then lua_pushcclosure
is called to create and push the C function onto the stack, with the
argument n telling how many values should be associated with the
function. lua_pushcclosure
also pops these values from the stack.

lua_pushcfunction

void lua_pushcfunction (lua_State *L, lua_CFunction f);

Pushes a C function onto the stack. This function receives a pointer to
a C function and pushes onto the stack a Lua value of type function
that, when called, invokes the corresponding C function.

Any function to be registered in Lua must follow the correct protocol to
receive its parameters and return its results (see lua_CFunction).

lua_pushcfunction is defined as a macro:
#define lua_pushcfunction(L,f) lua_pushcclosure(L,f,0)

lua_pushfstring

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

Pushes onto the stack a formatted string and returns a pointer to this
string. It is similar to the C function sprintf, but has some
important differences:

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

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

lua_pushinteger

void lua_pushinteger (lua_State *L, lua_Integer n);

Pushes a number with value n onto the stack.

lua_pushlightuserdata

void lua_pushlightuserdata (lua_State *L, void *p);

Pushes a light userdata onto the stack.

Userdata represent C values in Lua. A light userdata represents
a pointer. It is a value (like a number): you do not create it, it has no
individual metatable, and it is not collected (as it was never created). A light
userdata is equal to "any" light userdata with the same C address.

lua_pushlstring

void lua_pushlstring (lua_State *L, const char *s, size_t len);

Pushes the string pointed to by s with size len
onto the stack. Lua makes (or reuses) an internal copy of the given string, so
the memory at s can be freed or reused immediately after the
function returns. The string can contain embedded zeros.

lua_pushnil

void lua_pushnil (lua_State *L);

Pushes a nil value onto the stack.

lua_pushnumber

void lua_pushnumber (lua_State *L, lua_Number n);

Pushes a number with value n onto the stack.

lua_pushstring

void lua_pushstring (lua_State *L, const char *s);

Pushes the zero-terminated string pointed to by s onto the
stack. Lua makes (or reuses) an internal copy of the given string, so the memory
at s can be freed or reused immediately after the function returns.
The string cannot contain embedded zeros; it is assumed to end at the first
zero.

lua_pushthread

int lua_pushthread (lua_State *L);

Pushes the thread represented by L onto the stack. Returns 1 if
this thread is the main thread of its state.

lua_pushvalue

void lua_pushvalue (lua_State *L, int index);

Pushes a copy of the element at the given valid index onto the stack.

lua_pushvfstring

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

Equivalent to lua_pushfstring,
except that it receives a va_list instead of a variable number of
arguments.

lua_rawequal

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

Returns 1 if the two values in acceptable indices index1 and
index2 are primitively equal (that is, without calling
metamethods). Otherwise returns 0. Also returns 0 if any of the
indices are non valid.

lua_rawget

void lua_rawget (lua_State *L, int index);

Similar to lua_gettable,
but does a raw access (i.e., without metamethods).

lua_rawgeti

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

Pushes onto the stack the value t[n], where t is
the value at the given valid index index. The access is raw; that
is, it does not invoke metamethods.

lua_rawset

void lua_rawset (lua_State *L, int index);

Similar to lua_settable,
but does a raw assignment (i.e., without metamethods).

lua_rawseti

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

Does the equivalent of t[n] = v, where t is the
value at the given valid index index and v is the
value at the top of the stack,

This function pops the value from the stack. The assignment is raw; that is,
it does not invoke metamethods.

lua_Reader

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

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

lua_register

void lua_register (lua_State *L,
const char *name,
lua_CFunction f);

Sets the C function f as the new value of global
name. It is defined as a macro:
#define lua_register(L,n,f) \
(lua_pushcfunction(L, f), lua_setglobal(L, n))

lua_remove

void lua_remove (lua_State *L, int index);

Removes the element at the given valid index, shifting down the elements
above this index to fill the gap. Cannot be called with a pseudo-index, because
a pseudo-index is not an actual stack position.

lua_replace

void lua_replace (lua_State *L, int index);

Moves the top element into the given position (and pops it), without shifting
any element (therefore replacing the value at the given position).

lua_resume

int lua_resume (lua_State *L, int narg);

Starts and resumes a coroutine in a given thread.

To start a coroutine, you first create a new thread (see lua_newthread);
then you push onto its stack the main function plus any arguments; then you call
lua_resume,
with narg being the number of arguments. This call returns when the
coroutine suspends or finishes its execution. When it returns, the stack
contains all values passed to lua_yield,
or all values returned by the body function. lua_resume
returns LUA_YIELD
if the coroutine yields, 0 if the coroutine finishes its execution without
errors, or an error code in case of errors (see lua_pcall).
In case of errors, the stack is not unwound, so you can use the debug API over
it. The error message is on the top of the stack. To restart a coroutine, you
put on its stack only the values to be passed as results from
yield, and then call lua_resume.

lua_setallocf

void lua_setallocf (lua_State *L, lua_Alloc f, void *ud);

Changes the allocator function of a given state to f with user
data ud.

lua_setfenv

int lua_setfenv (lua_State *L, int index);

Pops a table from the stack and sets it as the new environment for the value
at the given index. If the value at the given index is neither a function nor a
thread nor a userdata, lua_setfenv
returns 0. Otherwise it returns 1.

lua_setfield

void lua_setfield (lua_State *L, int index, const char *k);

Does the equivalent to t[k] = v, where t is the
value at the given valid index index and v is the
value at the top of the stack,

This function pops the value from the stack. As in Lua, this function may
trigger a metamethod for the "newindex" event (see §2.8).

lua_setglobal

void lua_setglobal (lua_State *L, const char *name);

Pops a value from the stack and sets it as the new value of global
name. It is defined as a macro:
#define lua_setglobal(L,s) lua_setfield(L, LUA_GLOBALSINDEX, s)

lua_setmetatable

int lua_setmetatable (lua_State *L, int index);

Pops a table from the stack and sets it as the new metatable for the value at
the given acceptable index.

lua_settable

void lua_settable (lua_State *L, int index);

Does the equivalent to t[k] = v, where t is the
value at the given valid index index, v is the value
at the top of the stack, and k is the value just below the top.

This function pops both the key and the value from the stack. As in Lua, this
function may trigger a metamethod for the "newindex" event (see §2.8).

lua_settop

void lua_settop (lua_State *L, int index);

Accepts any acceptable index, or 0, and sets the stack top to this
index. If the new top is larger than the old one, then the new elements are
filled with nil. If index is 0, then all stack elements
are removed.

lua_State

typedef struct lua_State lua_State;

Opaque structure that keeps the whole state of a Lua interpreter. The Lua
library is fully reentrant: it has no global variables. All information about a
state is kept in this structure.

A pointer to this state must be passed as the first argument to every
function in the library, except to lua_newstate,
which creates a Lua state from scratch.

lua_status

int lua_status (lua_State *L);

Returns the status of the thread L.

The status can be 0 for a normal thread, an error code if the thread finished
its execution with an error, or LUA_YIELD
if the thread is suspended.

lua_toboolean

int lua_toboolean (lua_State *L, int index);

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

lua_tocfunction

lua_CFunction lua_tocfunction (lua_State *L, int index);

Converts a value at the given acceptable index to a C function. That
value must be a C function; otherwise, returns NULL.

lua_tointeger

lua_Integer lua_tointeger (lua_State *L, int idx);

Converts the Lua value at the given acceptable index to the signed integral
type lua_Integer.
The Lua value must be a number or a string convertible to a number (see §2.2.1); otherwise,
lua_tointeger
returns 0.

If the number is not an integer, it is truncated in some non-specified way.

lua_tolstring

const char *lua_tolstring (lua_State *L, int index, size_t *len);

Converts the Lua value at the given acceptable index to a C string. If
len is not NULL, it also sets *len with
the string length. The Lua value must be a string or a number; otherwise, the
function returns NULL. If the value is a number, then lua_tolstring
also changes the actual value in the stack to a string. (This change
confuses lua_next
when lua_tolstring
is applied to keys during a table traversal.)

lua_tolstring
returns a fully aligned pointer to a string inside the Lua state. This string
always has a zero ('\0') after its last character (as in C),
but may contain other zeros in its body. Because Lua has garbage collection,
there is no guarantee that the pointer returned by lua_tolstring
will be valid after the corresponding value is removed from the stack.

lua_tonumber

lua_Number lua_tonumber (lua_State *L, int index);

Converts the Lua value at the given acceptable index to the C type lua_Number
(see lua_Number).
The Lua value must be a number or a string convertible to a number (see §2.2.1); otherwise,
lua_tonumber
returns 0.

lua_topointer

const void *lua_topointer (lua_State *L, int index);

Converts the value at the given acceptable index to a generic C pointer
(void*). The value may be a userdata, a table, a thread, or a
function; otherwise, lua_topointer
returns NULL. Different objects will give different pointers. There
is no way to convert the pointer back to its original value.

Typically this function is used only for debug information.

lua_tostring

const char *lua_tostring (lua_State *L, int index);

Equivalent to lua_tolstring
with len equal to NULL.

lua_tothread

lua_State *lua_tothread (lua_State *L, int index);

Converts the value at the given acceptable index to a Lua thread (represented
as lua_State*). This value must be a thread; otherwise, the
function returns NULL.

lua_touserdata

void *lua_touserdata (lua_State *L, int index);

If the value at the given acceptable index is a full userdata, returns its
block address. If the value is a light userdata, returns its pointer. Otherwise,
returns NULL.

lua_type

int lua_type (lua_State *L, int index);

Returns the type of the value in the given acceptable index, or
LUA_TNONE for a non-valid index (that is, an index to an "empty"
stack position). The types returned by lua_type
are coded by the following constants defined in lua.h:
LUA_TNIL, LUA_TNUMBER, LUA_TBOOLEAN,
LUA_TSTRING, LUA_TTABLE, LUA_TFUNCTION,
LUA_TUSERDATA, LUA_TTHREAD, and
LUA_TLIGHTUSERDATA.

lua_typename

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

Returns the name of the type encoded by the value tp, which must
be one the values returned by lua_type.

lua_Writer

typedef int (*lua_Writer) (lua_State *L,
const void* p,
size_t sz,
void* ud);

The writer function used by lua_dump.
Every time it produces another piece of chunk, lua_dump
calls the writer, passing along the buffer to be written (p), its
size (sz), and the data parameter supplied to lua_dump.

The writer returns an error code: 0 means no errors; any other value
means an error and stops lua_dump
from calling the writer again.

lua_xmove

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

Exchange values between different threads of the same global state.

This function pops n values from the stack from,
and pushes them onto the stack to.

lua_yield

int lua_yield (lua_State *L, int nresults);

Yields a coroutine.

This function should only be called as the return expression of a
C function, as follows:
return lua_yield (L, nresults);

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

3.8 - The Debug Interface

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

lua_Debug

typedef struct lua_Debug {
int event;
const char *name; /* (n) */
const char *namewhat; /* (n) */
const char *what; /* (S) */
const char *source; /* (S) */
int currentline; /* (l) */
int nups; /* (u) number of upvalues */
int linedefined; /* (S) */
int lastlinedefined; /* (S) */
char short_src[LUA_IDSIZE]; /* (S) */
/* private part */
other fields
} lua_Debug;

A structure used to carry different pieces of information about an active
function. lua_getstack
fills only the private part of this structure, for later use. To fill the other
fields of lua_Debug
with useful information, call lua_getinfo.

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.

lastlinedefined: the line number where the definition
of the function ends.

what: the string "Lua" if the function is
a Lua function, "C" if it is a C function,
"main" if it is the main part of a chunk, and "tail"
if it was a function that did a tail call. In the latter case, Lua has no
other information about the function.

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

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

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

nups: the number of upvalues of the function.

lua_gethook

lua_Hook lua_gethook (lua_State *L);

Returns the current hook function.

lua_gethookcount

int lua_gethookcount (lua_State *L);

Returns the current hook count.

lua_gethookmask

int lua_gethookmask (lua_State *L);

Returns the current hook mask.

lua_getinfo

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

Returns information about a specific function or function invocation.

To get information about a function invocation, the parameter ar
must be a valid activation record that was filled by a previous call to lua_getstack
or given as argument to a hook (see lua_Hook).

To get information about a function you push it onto the stack and start the
what string with the character '>'. (In that case,
lua_getinfo pops the function in the top of the stack.) For
instance, to know in which line a function f was defined, you can
write the following code:
lua_Debug ar;
lua_getfield(L, LUA_GLOBALSINDEX, "f"); /* get global 'f' */
lua_getinfo(L, ">S", &ar);
printf("%d\n", ar.linedefined);

Each character in the string what selects some fields of the
structure ar to be filled or a value to be pushed on the stack:

'n': fills in the field name and
namewhat;

'S': fills in the fields source,
short_src, linedefined,
lastlinedefined, and what;

'l': fills in the field currentline;

'u': fills in the field nups;

'f': pushes onto the stack the function that is
running at the given level;

'L': pushes onto the stack a table whose indices are
the numbers of the lines that are valid on the function. (A valid
line is a line with some associated code, that is, a line where you can
put a break point. Non-valid lines include empty lines and comments.)

This function returns 0 on error (for instance, an invalid option in
what).

lua_getlocal

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

Gets information about a local variable of a given activation record. The
parameter ar must be a valid activation record that was filled by a
previous call to lua_getstack
or given as argument to a hook (see lua_Hook).
The index n selects which local variable to inspect (1 is the first
parameter or active local variable, and so on, until the last active local
variable). lua_getlocal
pushes the variable's value onto the stack and returns its name.

Variable names starting with '(' (open parentheses) represent
internal variables (loop control variables, temporaries, and C function
locals).

Returns NULL (and pushes nothing) when the index is greater than
the number of active local variables.

lua_getstack

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

Get information about the interpreter runtime stack.

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

lua_getupvalue

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

Gets information about a closure's upvalue. (For Lua functions, upvalues are
the external local variables that the function uses, and that are consequently
included in its closure.) lua_getupvalue
gets the index n of an upvalue, pushes the upvalue's value onto the
stack, and returns its name. funcindex points to the closure in the
stack. (Upvalues have no particular order, as they are active through the whole
function. So, they are numbered in an arbitrary order.)

Returns NULL (and pushes nothing) when the index is greater than
the number of upvalues. For C functions, this function uses the empty
string "" as a name for all upvalues.

lua_Hook

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

Type for debugging hook functions.

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

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

lua_sethook

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

Sets the debugging hook function.

Argument f is the hook function. mask specifies on
which events the hook will be called: it is formed by a bitwise or of the
constants LUA_MASKCALL, LUA_MASKRET, LUA_MASKLINE, and LUA_MASKCOUNT. The count
argument is only meaningful when the mask includes LUA_MASKCOUNT.
For each event, the hook is called as explained below:

The call hook: is called when the interpreter calls a function. The
hook is called just after Lua enters the new function, before the function
gets its arguments.

The return hook: is called when the interpreter returns from a
function. The hook is called just before Lua leaves the function. You have no
access to the values to be returned by the function.

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

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

A hook is disabled by setting mask to zero.

lua_setlocal

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

Sets the value of a local variable of a given activation record. Parameters
ar and n are as in lua_getlocal
(see lua_getlocal).
lua_setlocal
assigns the value at the top of the stack to the variable and returns its name.
It also pops the value from the stack.

Returns NULL (and pops nothing) when the index is greater than
the number of active local variables.

lua_setupvalue

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

Sets the value of a closure's upvalue. It assigns the value at the top of the
stack to the upvalue and returns its name. It also pops the value from the
stack. Parameters funcindex and n are as in the lua_getupvalue
(see lua_getupvalue).

Returns NULL (and pops nothing) when the index is greater than
the number of upvalues.

4 - The Auxiliary Library

The auxiliary library provides several convenient functions to
interface C with Lua. While the basic API provides the primitive functions for
all interactions between C and Lua, the auxiliary library provides higher-level
functions for some common tasks.

All functions from the auxiliary library are defined in header file
lauxlib.h and have a prefix luaL_.

All functions in the auxiliary library are built on top of the basic API, and
so they provide nothing that cannot be done with this API.

Several functions in the auxiliary library are used to check C function
arguments. Their names are always luaL_check* or
luaL_opt*. All of these functions raise an error if the check is
not satisfied. Because the error message is formatted for arguments (e.g.,
"bad argument #1"), you should not use these functions for other
stack values.

4.1 - Functions and Types

Here we list all functions and types from the auxiliary library in
alphabetical order.

luaL_addchar

void luaL_addchar (luaL_Buffer *B, char c);

Adds the character c to the buffer B (see luaL_Buffer).

luaL_addlstring

void luaL_addlstring (luaL_Buffer *B, const char *s, size_t l);

Adds the string pointed to by s with length l to
the buffer B (see luaL_Buffer).
The string may contain embedded zeros.

luaL_addsize

void luaL_addsize (luaL_Buffer *B, size_t n);

Adds to the buffer B (see luaL_Buffer)
a string of length n previously copied to the buffer area (see luaL_prepbuffer).

luaL_addstring

void luaL_addstring (luaL_Buffer *B, const char *s);

Adds the zero-terminated string pointed to by s to the buffer
B (see luaL_Buffer).
The string may not contain embedded zeros.

luaL_addvalue

void luaL_addvalue (luaL_Buffer *B);

Adds the value at the top of the stack to the buffer B (see luaL_Buffer).
Pops the value.

This is the only function on string buffers that can (and must) be called
with an extra element on the stack, which is the value to be added to the
buffer.

luaL_argcheck

void luaL_argcheck (lua_State *L,
int cond,
int narg,
const char *extramsg);

Checks whether cond is true. If not, raises an error with the
following message, where func is retrieved from the call stack:
bad argument #<narg> to <func> (<extramsg>)

luaL_argerror

int luaL_argerror (lua_State *L, int narg, const char *extramsg);

Raises an error with the following message, where func is
retrieved from the call stack:
bad argument #<narg> to <func> (<extramsg>)

This function never returns, but it is an idiom to use it in C functions
as return luaL_argerror(args).

luaL_Buffer

typedef struct luaL_Buffer luaL_Buffer;

Type for a string buffer.

A string buffer allows C code to build Lua strings piecemeal. Its
pattern of use is as follows:

First you declare a variable b of type luaL_Buffer.

Then you initialize it with a call luaL_buffinit(L, &b).

Then you add string pieces to the buffer calling any of the
luaL_add* functions.

You finish by calling luaL_pushresult(&b). This call
leaves the final string on the top of the stack.

During its normal operation, a string buffer uses a variable number of stack
slots. So, while using a buffer, you cannot assume that you know where the top
of the stack is. You can use the stack between successive calls to buffer
operations as long as that use is balanced; that is, when you call a buffer
operation, the stack is at the same level it was immediately after the previous
buffer operation. (The only exception to this rule is luaL_addvalue.)
After calling luaL_pushresult
the stack is back to its level when the buffer was initialized, plus the final
string on its top.

luaL_buffinit

void luaL_buffinit (lua_State *L, luaL_Buffer *B);

Initializes a buffer B. This function does not allocate any
space; the buffer must be declared as a variable (see luaL_Buffer).

luaL_callmeta

int luaL_callmeta (lua_State *L, int obj, const char *e);

Calls a metamethod.

If the object at index obj has a metatable and this metatable
has a field e, this function calls this field and passes the object
as its only argument. In this case this function returns 1 and pushes onto the
stack the value returned by the call. If there is no metatable or no metamethod,
this function returns 0 (without pushing any value on the stack).

luaL_checkany

void luaL_checkany (lua_State *L, int narg);

Checks whether the function has an argument of any type (including
nil) at position narg.

luaL_checkint

int luaL_checkint (lua_State *L, int narg);

Checks whether the function argument narg is a number and
returns this number cast to an int.

luaL_checkinteger

lua_Integer luaL_checkinteger (lua_State *L, int narg);

Checks whether the function argument narg is a number and
returns this number cast to a lua_Integer.

luaL_checklong

long luaL_checklong (lua_State *L, int narg);

Checks whether the function argument narg is a number and
returns this number cast to a long.

luaL_checklstring

const char *luaL_checklstring (lua_State *L, int narg, size_t *l);

Checks whether the function argument narg is a string and
returns this string; if l is not NULL fills
*l with the string's length.

luaL_checknumber

lua_Number luaL_checknumber (lua_State *L, int narg);

Checks whether the function argument narg is a number and
returns this number.

luaL_checkoption

int luaL_checkoption (lua_State *L,
int narg,
const char *def,
const char *const lst[]);

Checks whether the function argument narg is a string and
searches for this string in the array lst (which must be
NULL-terminated). Returns the index in the array where the string was found.
Raises an error if the argument is not a string or if the string cannot be
found.

If def is not NULL, the function uses
def as a default value when there is no argument narg
or if this argument is nil.

This is a useful function for mapping strings to C enums. (The usual
convention in Lua libraries is to use strings instead of numbers to select
options.)

luaL_checkstack

void luaL_checkstack (lua_State *L, int sz, const char *msg);

Grows the stack size to top + sz elements, raising an error if
the stack cannot grow to that size. msg is an additional text to go
into the error message.

luaL_checkstring

const char *luaL_checkstring (lua_State *L, int narg);

Checks whether the function argument narg is a string and
returns this string.

luaL_checktype

void luaL_checktype (lua_State *L, int narg, int t);

Checks whether the function argument narg has type
t.

luaL_checkudata

void *luaL_checkudata (lua_State *L, int narg, const char *tname);

Checks whether the function argument narg is a userdata of the
type tname (see luaL_newmetatable).

luaL_dofile

int luaL_dofile (lua_State *L, const char *filename);

Loads and runs the given file. It is defined as the following macro:
(luaL_loadfile(L, filename) || lua_pcall(L, 0, LUA_MULTRET, 0))

It returns 0 if there are no errors or 1 in case of errors.

luaL_dostring

int luaL_dostring (lua_State *L, const char *str);

Loads and runs the given string. It is defined as the following macro:
(luaL_loadstring(L, str) || lua_pcall(L, 0, LUA_MULTRET, 0))

It returns 0 if there are no errors or 1 in case of errors.

luaL_error

int luaL_error (lua_State *L, const char *fmt, ...);

Raises an error. The error message format is given by fmt plus
any extra arguments, following the same rules of lua_pushfstring.
It also adds at the beginning of the message the file name and the line number
where the error occurred, if this information is available.

This function never returns, but it is an idiom to use it in C functions
as return luaL_error(args).

luaL_getmetafield

int luaL_getmetafield (lua_State *L, int obj, const char *e);

Pushes onto the stack the field e from the metatable of the
object at index obj. If the object does not have a metatable, or if
the metatable does not have this field, returns 0 and pushes nothing.

luaL_getmetatable

void luaL_getmetatable (lua_State *L, const char *tname);

Pushes onto the stack the metatable associated with name tname
in the registry (see luaL_newmetatable).

luaL_gsub

const char *luaL_gsub (lua_State *L,
const char *s,
const char *p,
const char *r);

Creates a copy of string s by replacing any occurrence of the
string p with the string r. Pushes the resulting
string on the stack and returns it.

luaL_loadbuffer

int luaL_loadbuffer (lua_State *L,
const char *buff,
size_t sz,
const char *name);

Loads a buffer as a Lua chunk. This function uses lua_load
to load the chunk in the buffer pointed to by buff with size
sz.

This function returns the same results as lua_load.
name is the chunk name, used for debug information and error
messages.

luaL_loadfile

int luaL_loadfile (lua_State *L, const char *filename);

Loads a file as a Lua chunk. This function uses lua_load
to load the chunk in the file named filename. If
filename is NULL, then it loads from the standard
input. The first line in the file is ignored if it starts with a #.

This function returns the same results as lua_load,
but it has an extra error code LUA_ERRFILE if it cannot open/read the
file.

As lua_load,
this function only loads the chunk; it does not run it.

luaL_loadstring

int luaL_loadstring (lua_State *L, const char *s);

Loads a string as a Lua chunk. This function uses lua_load
to load the chunk in the zero-terminated string s.

This function returns the same results as lua_load.

Also as lua_load,
this function only loads the chunk; it does not run it.

luaL_newmetatable

int luaL_newmetatable (lua_State *L, const char *tname);

If the registry already has the key tname, returns 0. Otherwise,
creates a new table to be used as a metatable for userdata, adds it to the
registry with key tname, and returns 1.

In both cases pushes onto the stack the final value associated with
tname in the registry.

luaL_newstate

lua_State *luaL_newstate (void);

Creates a new Lua state. It calls lua_newstate
with an allocator based on the standard C realloc function and
then sets a panic function (see lua_atpanic)
that prints an error message to the standard error output in case of fatal
errors.

Returns the new state, or NULL if there is a memory allocation
error.

luaL_openlibs

void luaL_openlibs (lua_State *L);

Opens all standard Lua libraries into the given state.

luaL_optint

int luaL_optint (lua_State *L, int narg, int d);

If the function argument narg is a number, returns this number
cast to an int. If this argument is absent or is nil,
returns d. Otherwise, raises an error.

luaL_optinteger

lua_Integer luaL_optinteger (lua_State *L,
int narg,
lua_Integer d);

If the function argument narg is a number, returns this number
cast to a lua_Integer.
If this argument is absent or is nil, returns d. Otherwise,
raises an error.

luaL_optlong

long luaL_optlong (lua_State *L, int narg, long d);

If the function argument narg is a number, returns this number
cast to a long. If this argument is absent or is nil,
returns d. Otherwise, raises an error.

luaL_optlstring

const char *luaL_optlstring (lua_State *L,
int narg,
const char *d,
size_t *l);

If the function argument narg is a string, returns this string.
If this argument is absent or is nil, returns d. Otherwise,
raises an error.

If l is not NULL, fills the position
*l with the results's length.

luaL_optnumber

lua_Number luaL_optnumber (lua_State *L, int narg, lua_Number d);

If the function argument narg is a number, returns this number.
If this argument is absent or is nil, returns d. Otherwise,
raises an error.

luaL_optstring

const char *luaL_optstring (lua_State *L,
int narg,
const char *d);

If the function argument narg is a string, returns this string.
If this argument is absent or is nil, returns d. Otherwise,
raises an error.

luaL_prepbuffer

char *luaL_prepbuffer (luaL_Buffer *B);

Returns an address to a space of size LUAL_BUFFERSIZE where you can copy a
string to be added to buffer B (see luaL_Buffer).
After copying the string into this space you must call luaL_addsize
with the size of the string to actually add it to the buffer.

luaL_pushresult

void luaL_pushresult (luaL_Buffer *B);

Finishes the use of buffer B leaving the final string on the top
of the stack.

luaL_ref

int luaL_ref (lua_State *L, int t);

Creates and returns a reference, in the table at index
t, for the object at the top of the stack (and pops the object).

A reference is a unique integer key. As long as you do not manually add
integer keys into table t, luaL_ref
ensures the uniqueness of the key it returns. You can retrieve an object
referred by reference r by calling lua_rawgeti(L, t,
r). Function luaL_unref
frees a reference and its associated object.

If the object at the top of the stack is nil, luaL_ref
returns the constant LUA_REFNIL. The
constant LUA_NOREF is guaranteed to be
different from any reference returned by luaL_ref.

luaL_Reg

typedef struct luaL_Reg {
const char *name;
lua_CFunction func;
} luaL_Reg;

Type for arrays of functions to be registered by luaL_register.
name is the function name and func is a pointer to the
function. Any array of luaL_Reg
must end with an sentinel entry in which both name and
func are NULL.

luaL_register

void luaL_register (lua_State *L,
const char *libname,
const luaL_Reg *l);

Opens a library.

When called with libname equal to NULL, it simply
registers all functions in the list l (see luaL_Reg)
into the table on the top of the stack.

When called with a non-null libname, luaL_register
creates a new table t, sets it as the value of the global variable
libname, sets it as the value of
package.loaded[libname], and registers on it all functions in the
list l. If there is a table in package.loaded[libname]
or in variable libname, reuses this table instead of creating a new
one.

In any case the function leaves the table on the top of the stack.

luaL_typename

const char *luaL_typename (lua_State *L, int idx);

Returns the name of the type of the value at index idx.

luaL_typerror

int luaL_typerror (lua_State *L, int narg, const char *tname);

Generates an error with a message like the following:
location: bad argument narg to 'func' (tname expected, got rt)

where location is produced by luaL_where,
func is the name of the current function, and
rt is the type name of the actual argument.

luaL_unref

void luaL_unref (lua_State *L, int t, int ref);

Releases reference ref from the table at index t
(see luaL_ref).
The entry is removed from the table, so that the referred object can be
collected. The reference ref is also freed to be used again.

If ref is LUA_NOREF
or LUA_REFNIL,
luaL_unref
does nothing.

luaL_where

void luaL_where (lua_State *L, int lvl);

Pushes onto the stack a string identifying the current position of the
control at level lvl in the call stack. Typically this string has
the following format:
chunkname:currentline:

Level 0 is the running function, level 1 is the function that
called the running function, etc.

This function is used to build a prefix for error messages.

5 - Standard Libraries

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

package library;

string manipulation;

table manipulation;

mathematical functions (sin, log, etc.);

input and output;

operating system facilities;

debug facilities.

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

To have access to these libraries, the C host program should call the luaL_openlibs
function, which opens all standard libraries. Alternatively, it can open them
individually by calling luaopen_base
(for the basic library), luaopen_package (for the package
library), luaopen_string (for the
string library), luaopen_table (for
the table library), luaopen_math (for
the mathematical library), luaopen_io
(for the I/O and the Operating System libraries), and luaopen_debug (for the debug library).
These functions are declared in lualib.h
and should not be called directly: you must call them like any other Lua
C function, e.g., by using lua_call.

5.1 - Basic Functions

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

assert (v [, message])
Issues an
error when the value of its argument v is false (i.e., nil
or false); otherwise, returns all its arguments. message is
an error message; when absent, it defaults to "assertion failed!"

collectgarbage (opt [,
arg])

This function is a generic interface to the garbage collector. It performs
different functions according to its first argument, opt:

"stop": stops the garbage collector.

"restart": restarts the garbage collector.

"collect": performs a full garbage-collection cycle.

"count": returns the total memory in use by Lua (in Kbytes).

"step": performs a garbage-collection step. The step "size" is
controlled by arg (larger values mean more steps) in a
non-specified way. If you want to control the step size you must
experimentally tune the value of arg. Returns true if the
step finished a collection cycle.

"setpause": sets arg/100 as the new value for the
pause of the collector (see §2.10).

"setstepmul": sets arg/100 as the new value for the
step multiplier of the collector (see §2.10).

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

error (message [, level])
Terminates
the last protected function called and returns message as the error
message. Function error never returns.

Usually, error adds some information about the error position at
the beginning of the message. The level argument specifies how to
get the error position. With level 1 (the default), the error position is
where the error function was called. Level 2 points the error
to where the function that called error was called; and so on.
Passing a level 0 avoids the addition of error position information to the
message.

_G
A global variable (not a function)
that holds the global environment (that is, _G._G = _G). Lua itself
does not use this variable; changing its value does not affect any environment,
nor vice-versa. (Use setfenv
to change environments.)

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

getmetatable (object)

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

ipairs (t)

Returns three values: an iterator function, the table t, and 0,
so that the construction
for i,v in ipairs(t) do body end

will iterate over the pairs (1,t[1]), (2,t[2]),
···, up to the first integer key absent from the table.

load (func [, chunkname])

Loads a chunk using function func to get its pieces. Each call
to func must return a string that concatenates with previous
results. A return of nil (or no value) signals the end of the chunk.

If there are no errors, returns the compiled chunk as a function; otherwise,
returns nil plus the error message. The environment of the returned
function is the global environment.

chunkname is used as the chunk name for error messages and debug
information.

loadfile ([filename])

Similar to load,
but gets the chunk from file filename or from the standard input,
if no file name is given.

loadstring (string [,
chunkname])

Similar to load,
but gets the chunk from the given string.

To load and run a given string, use the idiom
assert(loadstring(s))()

next (table [, index])

Allows a program to traverse all fields of a table. Its first argument is a
table and its second argument is an index in this table. next
returns the next index of the table and its associated value. When called with
nil as its second argument, next returns an initial index
and its associated value. When called with the last index, or with nil in
an empty table, next returns nil. If the second argument is
absent, then it is interpreted as nil. In particular, you can use
next(t) to check whether a table is empty.

The order in which the indices are enumerated is not specified, even for
numeric indices. (To traverse a table in numeric order, use a numerical
for or the ipairs
function.)

The behavior of next is undefined if, during the
traversal, you assign any value to a non-existent field in the table. You may
however modify existing fields. In particular, you may clear existing fields.

pairs (t)

Returns three values: the next
function, the table t, and nil, so that the construction
for k,v in pairs(t) do body end

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

See function next
for the caveats of modifying the table during its traversal.

pcall (f, arg1, ···)

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

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

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

rawget (table, index)
Gets the real
value of table[index], without invoking any metamethod.
table must be a table; index may be any value.

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

This function returns table.

select (index, ···)

If index is a number, returns all arguments after argument
number index. Otherwise, index must be the string
"#", and select returns the total number of extra
arguments it received.

setfenv (f, table)

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

As a special case, when f is 0 setfenv changes the
environment of the running thread. In this case, setfenv returns no
values.

setmetatable (table,
metatable)

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

This function returns table.

tonumber (e [, base])
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 this 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 §2.1). In other bases,
only unsigned integers are accepted.

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 string.format.

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

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

unpack (list [, i [, j]])
Returns
the elements from the given table. This function is equivalent to
return list[i], list[i+1], ···, list[j]

except that the above code can be written only for a fixed number of
elements. By default, i is 1 and j is the length
of the list, as defined by the length operator (see §2.5.5).

_VERSION
A global variable (not a
function) that holds a string containing the current interpreter version. The
current contents of this variable is "Lua 5.1".

xpcall (f, err)

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

xpcall calls function f in protected mode, using
err as the error handler. Any error inside f is not
propagated; instead, xpcall catches the error, calls the
err function with the original error object, and returns a status
code. Its first result is the status code (a boolean), which is true if the call
succeeds without errors. In this case, xpcall also returns all
results from the call, after this first result. In case of any error,
xpcall returns false plus the result from err.

5.2 - Coroutine Manipulation

The operations related to coroutines comprise a sub-library of the basic
library and come inside the table coroutine. See §2.11 for a general
description of coroutines.

coroutine.create (f)

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

coroutine.resume (co [, val1,
···])

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

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

coroutine.running ()

Returns the running coroutine, or nil when called by the main thread.

coroutine.status (co)

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

coroutine.wrap (f)

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

coroutine.yield (···)

Suspends the execution of the calling coroutine. The coroutine cannot be
running a C function, a metamethod, or an iterator. Any arguments to
yield are passed as extra results to resume.

5.3 - Modules

The package library provides basic facilities for loading and building
modules in Lua. It exports two of its functions directly in the global
environment: require
and module.
Everything else is exported in a table package.

module (name [, ···])

Creates a module. If there is a table in package.loaded[name],
this table is the module. Otherwise, if there is a global table t
with the given name, this table is the module. Otherwise creates a new table
t and sets it as the value of the global name and the
value of package.loaded[name]. This function also initializes
t._NAME with the given name, t._M with the module
(t itself), and t._PACKAGE with the package name (the
full module name minus last component; see below). Finally, module
sets t as the new environment of the current function and the new
value of package.loaded[name], so that require
returns t.

If name is a compound name (that is, one with components
separated by dots), module creates (or reuses, if they already
exist) tables for each component. For instance, if name is
a.b.c, then module stores the module table in field
c of field b of global a.

This function may receive optional options after the module name,
where each option is a function to be applied over the module.

require (modname)

Loads the given module. The function starts by looking into the package.loaded
table to determine whether modname is already loaded. If it is,
then require returns the value stored at
package.loaded[modname]. Otherwise, it tries to find a
loader for the module.

To find a loader, first require queries
package.preload[modname]. If it has a value, this value (which
should be a function) is the loader. Otherwise require searches for
a Lua loader using the path stored in package.path.
If that also fails, it searches for a C loader using the path stored in package.cpath.
If that also fails, it tries an all-in-one loader (see below).

When loading a C library, require first uses a dynamic link
facility to link the application with the library. Then it tries to find a
C function inside this library to be used as the loader. The name of this
C function is the string "luaopen_" concatenated with a copy
of the module name where each dot is replaced by an underscore. Moreover, if the
module name has a hyphen, its prefix up to (and including) the first hyphen is
removed. For instance, if the module name is a.v1-b.c, the function
name will be luaopen_b_c.

If require finds neither a Lua library nor a C library for
a module, it calls the all-in-one loader. This loader searches the
C path for a library for the root name of the given module. For instance,
when requiring a.b.c, it will search for a C library for
a. If found, it looks into it for an open function for the
submodule; in our example, that would be luaopen_a_b_c. With this
facility, a package can pack several C submodules into one single library,
with each submodule keeping its original open function.

Once a loader is found, require calls the loader with a single
argument, modname. If the loader returns any value,
require assigns the returned value to
package.loaded[modname]. If the loader returns no value and has not
assigned any value to package.loaded[modname], then
require assigns true to this entry. In any case,
require returns the final value of
package.loaded[modname].

If there is any error loading or running the module, or if it cannot find any
loader for the module, then require signals an error.

package.cpath

The path used by require
to search for a C loader.

Lua initializes the C path package.cpath
in the same way it initializes the Lua path package.path,
using the environment variable LUA_CPATH
(plus another default path defined in luaconf.h).

package.loaded

A table used by require
to control which modules are already loaded. When you require a module
modname and package.loaded[modname] is not false, require
simply returns the value stored there.

package.loadlib (libname,
funcname)

Dynamically links the host program with the C library
libname. Inside this library, looks for a function
funcname and returns this function as a C function. (So,
funcname must follow the protocol (see lua_CFunction)).

This is a low-level function. It completely bypasses the package and module
system. Unlike require,
it does not perform any path searching and does not automatically adds
extensions. libname must be the complete file name of the
C library, including if necessary a path and extension.
funcname must be the exact name exported by the C library
(which may depend on the C compiler and linker used).

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

package.path

The path used by require
to search for a Lua loader.

At start-up, Lua initializes this variable with the value of the environment
variable LUA_PATH or with a default path
defined in luaconf.h, if the environment variable is not defined.
Any ";;" in the value of the environment variable is replaced by
the default path.

A path is a sequence of templates separated by semicolons. For each
template, require
will change each interrogation mark in the template by filename,
which is modname with each dot replaced by a "directory separator"
(such as "/" in Unix); then it will try to load the resulting file
name. So, for instance, if the Lua path is
"./?.lua;./?.lc;/usr/local/?/init.lua"

the search for a Lua loader for module foo will try to load the
files ./foo.lua, ./foo.lc, and
/usr/local/foo/init.lua, in that order.

package.preload

A table to store loaders for specific modules (see require).

package.seeall (module)

Sets a metatable for module with its __index field
referring to the global environment, so that this module inherits values from
the global environment. To be used as an option to function module.

5.4 - String Manipulation

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

The string library provides all its functions inside the table string. It also sets a metatable for strings
where the __index field points to the string table.
Therefore, you can use the string functions in object-oriented style. For
instance, string.byte(s, i) can be written as
s:byte(i).

string.byte (s [, i [,
j]])
Returns the internal numerical codes of the characters
s[i], s[i+1], ···, s[j]. The default
value for i is 1; the default value for j
is i.

Note that numerical codes are not necessarily portable across platforms.

string.char (···)
Receives zero
or more integers. Returns a string with length equal to the number of arguments,
in which each character has the internal numerical code equal to its
corresponding argument.

Note that numerical codes are not necessarily portable across platforms.

string.dump (function)

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

string.find (s, pattern [, init [,
plain]])
Looks for the first match of pattern in the
string s. If it finds a match, then find returns the
indices of s where this occurrence starts and ends; otherwise,
it returns nil. A third, optional numerical argument init
specifies where to start the search; its default value is 1 and 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 as well.

If the pattern has captures, then in a successful match the captured values
are also returned, after the two indices.

string.format (formatstring,
···)
Returns a formatted version of its variable number of
arguments following the description given in its first argument (which must be a
string). The format string follows the same rules as the printf
family of standard C functions. The only differences are that the
options/modifiers *, l, L,
n, p, and h are not supported and that
there is an extra option, q. The q option formats a
string in a form suitable to be safely read back by the Lua interpreter: the
string is written between double quotes, and all double quotes, newlines,
embedded zeros, 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.

This function does not accept string values containing embedded zeros, except
as arguments to the q option.

string.gmatch (s,
pattern)
Returns an iterator function that, each time it is
called, returns the next captures from pattern over string
s. If pattern 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.gmatch(s, "%a+") do
print(w)
end

will iterate over all the words from string s, printing one per
line. The next example collects all pairs key=value from the given
string into a table:
t = {}
s = "from=world, to=Lua"
for k, v in string.gmatch(s, "(%w+)=(%w+)") do
t[k] = v
end

For this function, a '^' at the start of a pattern does not work
as an anchor, as this would prevent the iteration.

string.gsub (s, pattern, repl [,
n])
Returns a copy of s in which all occurrences of
the pattern have been replaced by a replacement string specified by
repl, which may be a string, a table, or a function.
gsub also returns, as its second value, the total number of
substitutions made.

If repl is a string, then its value is used for replacement. The
character % works as an escape character: any sequence in
repl of the form %n, with n between
1 and 9, stands for the value of the n-th captured substring (see
below). The sequence %0 stands for the whole match. The sequence
%% stands for a single %.

If repl is a table, then the table is queried for every match,
using the first capture as the key; if the pattern specifies no captures, then
the whole match is used as the key.

If repl is a function, then this function is called every time a
match occurs, with all captured substrings passed as arguments, in order; if the
pattern specifies no captures, then the whole match is passed as a sole
argument.

If the value returned by the table query or by the function call is a string
or a number, then it is used as the replacement string; otherwise, if it is
false or nil, then there is no replacement (that is, the original
match is kept in the string).

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

Here are some examples:
x = string.gsub("hello world", "(%w+)", "%1 %1")
--> x="hello hello world world"

x = string.gsub("hello world", "%w+", "%0 %0", 1)
--> x="hello hello world"

x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1")
--> x="world hello Lua from"

x = string.gsub("home = $HOME, user = $USER", "%$(%w+)", os.getenv)
--> x="home = /home/roberto, user = roberto"

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

local t = {name="lua", version="5.1"}
x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t)
--> x="lua-5.1.tar.gz"

string.len (s)
Receives a string
and returns its length. The empty string "" has length 0. Embedded
zeros are counted, so "a\000bc\000" has length 5.

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

string.match (s, pattern [,
init])
Looks for the first match of pattern
in the string s. If it finds one, then match returns
the captures from the pattern; otherwise it returns nil. If
pattern specifies no captures, then the whole match is returned. A
third, optional numerical argument init specifies where to start
the search; its default value is 1 and may be negative.

string.rep (s, n)
Returns a
string that is the concatenation of n copies of the string
s.

string.reverse (s)
Returns a
string that is the string s reversed.

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

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

5.4.1 - Patterns

Character Class:

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

x: (where x is not one of the magic
characters ^$()%.[]*+-?) represents the character x
itself.

.: (a dot) represents all characters.

%a: represents all letters.

%c: represents all control characters.

%d: represents all digits.

%l: represents all lowercase letters.

%p: represents all punctuation characters.

%s: represents all space characters.

%u: represents all uppercase letters.

%w: represents all alphanumeric characters.

%x: represents all hexadecimal digits.

%z: represents the character with representation 0.

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

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

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

[^set]: represents the complement of
set, where set is interpreted as above.

For all classes represented by single letters (%a,
%c, etc.), the corresponding uppercase letter represents the
complement of the class. For instance, %S represents all non-space
characters.

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

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:

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

Captures:

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.

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

5.5 - Table Manipulation

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

Most functions in the table library assume that the table represents an array
or a list. For these functions, when we talk about the "length" of a table we
mean the result of the length operator.

table.concat (table [, sep [, i [,
j]]])
Given an array where all elements are strings or numbers,
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 length of the table. If
i is greater than j, returns the empty string.

table.insert (table, [pos,]
value)

Inserts element value at position pos in
table, shifting up other elements to open space, if necessary. The
default value for pos is n+1, where n is
the length of the table (see §2.5.5), so that a
call table.insert(t,x) inserts x at the end of table
t.

table.maxn (table)

Returns the largest positive numerical index of the given table, or zero if
the table has no positive numerical indices. (To do its job this function does a
linear traversal of the whole table.)

table.remove (table [, pos])

Removes from table the element at position pos,
shifting down other elements to close the space, if necessary. Returns the value
of the removed element. The default value for pos is
n, where n is the length of the table, so that a call
table.remove(t) removes the last element of table t.

table.sort (table [,
comp])
Sorts table elements in a given order, in-place,
from table[1] to table[n], where n is the
length of the table. 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.

5.6 - Mathematical Functions

This library is an interface to the standard C math library. It provides
all its functions inside the table math.

math.abs (x)

Returns the absolute value of x.

math.acos (x)

Returns the arc cosine of x (in radians).

math.asin (x)

Returns the arc sine of x (in radians).

math.atan (x)

Returns the arc tangent of x (in radians).

math.atan2 (x, y)

Returns the arc tangent of x/y (in radians), but uses the signs
of both parameters to find the quadrant of the result. (It also handles
correctly the case of y being zero.)

math.ceil (x)

Returns the smallest integer larger than or equal to x.

math.cos (x)

Returns the cosine of x (assumed to be in radians).

math.cosh (x)

Returns the hyperbolic cosine of x.

math.deg (x)

Returns the angle x (given in radians) in degrees.

math.exp (x)

Returns the the value ex.

math.floor (x)

Returns the largest integer smaller than or equal to x.

math.fmod (x, y)

Returns the remainder of the division of x by y.

math.frexp (x)

Returns m and e such that x =
m2e, e is an integer and the absolute value of
m is in the range [0.5, 1) (or zero when x is
zero).

math.huge

The value HUGE_VAL, a value larger than or equal to any other
numerical value.

math.ldexp (m, e)

Returns m2e (e should be an integer).

math.log (x)

Returns the natural logarithm of x.

math.log10 (x)

Returns the base-10 logarithm of x.

math.max (x, ···)

Returns the maximum value among its arguments.

math.min (x, ···)

Returns the minimum value among its arguments.

math.modf (x)

Returns two numbers, the integral part of x and the fractional
part of x.

math.pi

The value of pi.

math.pow (x, y)

Returns xy. (You can also use the expression
x^y to compute this value.)

math.rad (x)

Returns the angle x (given in degrees) in radians.

math.random ([m [, n]])

This function is an interface to the simple pseudo-random generator function
rand provided by ANSI C. (No guarantees can be given for its
statistical properties.)

When called without arguments, returns a pseudo-random real number in the
range [0,1). When called with a number m,
math.random returns a pseudo-random integer in the range [1,
m]. When called with two numbers m and n,
math.random returns a pseudo-random integer in the range [m,
n].

math.randomseed (x)

Sets x as the "seed" for the pseudo-random generator: equal
seeds produce equal sequences of numbers.

math.sin (x)

Returns the sine of x (assumed to be in radians).

math.sinh (x)

Returns the hyperbolic sine of x.

math.sqrt (x)

Returns the square root of x. (You can also use the expression
x^0.5 to compute this value.)

math.tan (x)

Returns the tangent of x (assumed to be in radians).

math.tanh (x)

Returns the hyperbolic tangent of x.

5.7 - Input and Output Facilities

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

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

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

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

io.close ([file])

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

io.flush ()

Equivalent to file:flush over the default output file.

io.input ([file])

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

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

io.lines ([filename])

Opens the given file name in read mode and returns an iterator function that,
each time it is called, returns a new line from the file. Therefore, the
construction
for line in io.lines(filename) do body end

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

The call io.lines() (with no file name) is equivalent to
io.input():lines(); that is, it iterates over the lines of the
default input file. In this case it does not close the file when the loop ends.

io.open (filename [, mode])

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

The mode string can be any of the following:

"r": read mode (the default);

"w": write mode;

"a": append mode;

"r+": update mode, all previous data is preserved;

"w+": update mode, all previous data is erased;

"a+": append update mode, previous data is preserved, writing is
only allowed at the end of file.

The mode string 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 ([file])

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

io.popen (prog [, mode])

Starts program prog in a separated process and returns a file
handle that you can use to read data from this program (if mode is
"r", the default) or to write data to this program (if
mode is "w").

This function is system dependent and is not available on all platforms.

io.read (···)

Equivalent to io.input():read.

io.tmpfile ()

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

io.type (obj)

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

io.write (···)

Equivalent to io.output():write.

file:close ()

Closes file. Note that files are automatically closed when their
handles are garbage collected, but that takes an unpredictable amount of time to
happen.

file:flush ()

Saves any written data to file.

file:lines ()

Returns an iterator function that, each time it is called, returns a new line
from the file. Therefore, the construction
for line in file:lines() do body 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 (···)

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 this number of
characters, returning nil on end of file. If number is zero, it reads
nothing and returns an empty string, or nil on end of file.

file:seek ([whence] [, offset])

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

"set": base is position 0 (beginning of the file);

"cur": base is current position;

"end": base is end of file;

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

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

file:setvbuf (mode [, size])

Sets the buffering mode for an output file. There are three available modes:

"no": no buffering; the result of any output operation appears
immediately.

"full": full buffering; output operation is performed only when the
buffer is full (or when you explicitly flush the file (see io.flush)).

"line": line buffering; output is buffered until a newline is
output or there is any input from some special files (such as a terminal
device).

For the last two cases, size specifies the size of the buffer,
in bytes. The default is an appropriate size.

file:write (···)

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

5.8 - Operating System Facilities

This library is implemented through table os.

os.clock ()

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

os.date ([format [, time]])

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

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

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

If format is not "*t", then date
returns the date as a string, formatted according to 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 (t2, t1)

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

os.execute ([command])

This function is equivalent to the C function system. It
passes command to be executed by an operating system shell. It
returns a status code, which is system-dependent. If command is
absent, then it returns nonzero if a shell is available and zero otherwise.

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 (varname)

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

os.remove (filename)

Deletes the file or directory with the given name. Directories must be empty
to be removed. If this function fails, it returns nil, plus a string
describing the error.

os.rename (oldname, newname)

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

os.setlocale (locale [,
category])

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

If locale is the empty string, the current locate is set to an
implementation-defined native locale. If locate is the string
"C", the current locate is set to the standard C locale.

When called with nil as the first argument, this function only returns
the name of the current locale for the given category.

os.time ([table])

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

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

os.tmpname ()

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

5.9 - The Debug Library

This library provides the functionality of the debug interface to Lua
programs. You should exert care when using this library. The functions provided
here should be used exclusively for debugging and similar tasks, such as
profiling. Please resist the temptation to use them as a usual programming tool:
they can be very slow. Moreover, several of its functions violate some
assumptions about Lua code (e.g., that variables local to a function cannot be
accessed from outside or that userdata metatables cannot be changed by Lua code)
and therefore can compromise otherwise secure code.

All functions in this library are provided inside the debug table. All functions that operate over a
thread have an optional first argument which is the thread to operate over. The
default is always the current thread.

debug.debug ()

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

Note that commands for debug.debug are not lexically nested
within any function, and so have no direct access to local variables.

debug.getfenv (o)
Returns the
environment of object o.

debug.gethook ([thread])

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

debug.getinfo ([thread,] function [,
what])

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

The returned table may contain all the fields returned by lua_getinfo,
with the string what describing which fields to fill in. The
default for what is to get all information available, except the
table of valid lines. If present, the option 'f' adds a field named
func with the function itself. If present, the option
'L' adds a field named activelines with the table of
valid lines.

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

debug.getlocal ([thread,] level,
local)

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

Variable names starting with '(' (open parentheses) represent
internal variables (loop control variables, temporaries, and C function
locals).

debug.getmetatable
(object)

Returns the metatable of the given object or nil if it
does not have a metatable.

debug.getregistry ()

Returns the registry table (see §3.5).

debug.getupvalue (func,
up)

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

debug.setfenv (object,
table)

Sets the environment of the given object to the given
table. Returns object.

debug.sethook ([thread,] hook, mask [,
count])

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

"c": The hook is called every time Lua calls a
function;

"r": The hook is called every time Lua returns from a
function;

"l": The hook is called every time Lua enters a new
line of code.

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

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

When the hook is called, its first parameter is a string describing the event
that has triggered its call: "call", "return" (or
"tail return"), "line", and "count". For
line events, the hook also gets the new line number as its second parameter.
Inside a hook, you can call getinfo with level 2 to get more
information about the running function (level 0 is the getinfo
function, and level 1 is the hook function), unless the event is
"tail return". In this case, Lua is only simulating the return, and
a call to getinfo will return invalid data.

debug.setlocal ([thread,] level, local,
value)

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

debug.setmetatable (object,
table)

Sets the metatable for the given object to the given
table (which can be nil).

debug.setupvalue (func, up,
value)

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

debug.traceback ([thread,] [message] [,
level])

Returns a string with a traceback of the call stack. An optional
message string is appended at the beginning of the traceback. An
optional level number tells at which level to start the traceback
(default is 1, the function calling traceback).

6 - Lua Stand-alone

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

The options are:

-e stat: executes string stat;

-l mod: "requires" mod;

-i: enters interactive mode after running
script;

-v: prints version information;

--: stops handling options;

-: executes stdin as a file and stops
handling options.

After handling its options, lua runs the given script,
passing to it the given args as string arguments. When called without
arguments, lua behaves as lua -v -i when the standard
input (stdin) is a terminal, and as lua - otherwise.

Before running any argument, the interpreter checks for an environment
variable LUA_INIT. If its format is
@filename, then lua executes the file.
Otherwise, lua executes the string itself.

All options are handled in order, except -i. For instance, an
invocation like
$ lua -e'a=1' -e 'print(a)' script.lua

will first set a to 1, then print the value of a
(which is '1'), and finally run the file script.lua
with no arguments. (Here $ is the shell prompt. Your prompt may be
different.)

Before starting to run the script, lua collects all arguments in
the command line in a global table called arg. The script name is
stored at index 0, the first argument after the script name goes to index 1, and
so on. Any arguments before the script name (that is, the interpreter name plus
the options) go to negative indices. For instance, in the call
$ lua -la b.lua t1 t2

the interpreter first runs the file a.lua, then creates a table
arg = { [-2] = "lua", [-1] = "-la",
[0] = "b.lua",
[1] = "t1", [2] = "t2" }

and finally runs the file b.lua. The script is called with
arg[1], arg[2], ··· as arguments; it can also access
these arguments with the vararg expression '...'.

In interactive mode, if you write an incomplete statement, the interpreter
waits for its completion by issuing a different prompt.

If the global variable _PROMPT contains
a string, then its value is used as the prompt. Similarly, if the global
variable _PROMPT2 contains a string, its
value is used as the secondary prompt (issued during incomplete statements).
Therefore, both prompts can be changed directly on the command line. For
instance,
$ lua -e"_PROMPT='myprompt> '" -i

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

To allow the use of Lua as a script interpreter in Unix systems, the
stand-alone interpreter skips the first line of a chunk if it starts with
#. Therefore, Lua scripts can be made into executable programs by
using chmod +x and the #! form, as in
#!/usr/local/bin/lua

(Of course, the location of the Lua interpreter may be different in your
machine. If lua is in your PATH, then
#!/usr/bin/env lua

is a more portable solution.)

7 - Incompatibilities with the Previous Version

Here we list the incompatibilities that you may found when moving a program
from Lua 5.0 to Lua 5.1. You can avoid most of the incompatibilities
compiling Lua with appropriate options (see file luaconf.h).
However, all these compatibility options will be removed in the next version of
Lua.

7.1 - Changes in the Language

The vararg system changed from the pseudo-argument arg with a
table with the extra arguments to the vararg expression. (See compile-time
option LUA_COMPAT_VARARG in luaconf.h.)

There was a subtle change in the scope of the implicit variables of the
for statement and for the repeat statement.

The long string/long comment syntax ([[string]])
does not allow nesting. You can use the new syntax
([=[string]=]) in these cases. (See compile-time option
LUA_COMPAT_LSTR in luaconf.h.)

7.2 - Changes in the Libraries

Function string.gfind was renamed string.gmatch.
(See compile-time option LUA_COMPAT_GFIND in
luaconf.h.)

When string.gsub
is called with a function as its third argument, whenever this function
returns nil or false the replacement string is the whole match,
instead of the empty string.

Function table.setn was deprecated. Function
table.getn corresponds to the new length operator
(#); use the operator instead of the function. (See compile-time
option LUA_COMPAT_GETN in luaconf.h.)

Function loadlib was renamed package.loadlib.
(See compile-time option LUA_COMPAT_LOADLIB in
luaconf.h.)

Function math.mod was renamed math.fmod.
(See compile-time option LUA_COMPAT_MOD in
luaconf.h.)

Functions table.foreach and table.foreachi are
deprecated. You can use a for loop with pairs or
ipairs instead.

There were substantial changes in function require
due to the new module system. However, the new behavior is mostly compatible
with the old, but require gets the path from package.path
instead of from LUA_PATH.

Function collectgarbage
has different arguments. Function gcinfo is deprecated; use
collectgarbage("count") instead.

7.3 - Changes in the API

The luaopen_* functions (to open libraries) cannot be called
directly, like a regular C function. They must be called through Lua, like a
Lua function.

Function lua_open was replaced by lua_newstate
to allow the user to set a memory-allocation function. You can use luaL_newstate
from the standard library to create a state with a standard allocation
function (based on realloc).

Functions luaL_getn and luaL_setn (from the
auxiliary library) are deprecated. Use lua_objlen
instead of luaL_getn and nothing instead of
luaL_setn.

Function luaL_openlib was replaced by luaL_register.

Function luaL_checkudata now throws an error when the given
value is not a userdata of the expected type. (In Lua 5.0 it returned
NULL.)

8 - The Complete Syntax of Lua

Here is the complete syntax of Lua in extended BNF. (It does not describe
operator precedences.)

chunk ::= {stat [`;´]} [laststat [`;´]]

block ::= chunk

stat ::= varlist1 `=´ explist1 |
functioncall |
do block end |
while exp do block end |
repeat block until exp |
if exp then block {elseif exp then block} [else block] end |
for Name `=´ exp `,´ exp [`,´ exp] do block end |
for namelist in explist1 do block end |
function funcname funcbody |
local function Name funcbody |
local namelist [`=´ explist1]

laststat ::= return [explist1] | break

funcname ::= Name {`.´ Name} [`:´ Name]

varlist1 ::= var {`,´ var}

var ::= Name | prefixexp `[´ exp `]´ | prefixexp `.´ Name

namelist ::= Name {`,´ Name}

explist1 ::= {exp `,´} exp

exp ::= nil | false | true | Number | String | `...´ | function |
prefixexp | tableconstructor | exp binop exp | unop exp

prefixexp ::= var | functioncall | `(´ exp `)´

functioncall ::= prefixexp args | prefixexp `:´ Name args

args ::= `(´ [explist1] `)´ | tableconstructor | String

function ::= function funcbody

funcbody ::= `(´ [parlist1] `)´ block end

parlist1 ::= namelist [`,´ `...´] | `...´

tableconstructor ::= `{´ [fieldlist] `}´

fieldlist ::= field {fieldsep field} [fieldsep]

field ::= `[´ exp `]´ `=´ exp | Name `=´ exp | exp

fieldsep ::= `,´ | `;´

binop ::= `+´ | `-´ | `*´ | `/´ | `^´ | `%´ | `..´ |
`<´ | `<=´ | `>´ | `>=´ | `==´ | `~=´ |
and | or

unop ::= `-´ | not | `#´

Last update: Mon Mar 26 12:59:26 BRT 2007