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<h1>
<a href="http://www.lua.org/"><img src="logo.gif" alt="" border="0"></a>
Lua 5.1 Reference Manual
</h1>
by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, Waldemar Celes
<p>
<small>
Copyright &copy; 2006-2008 Lua.org, PUC-Rio.
Freely available under the terms of the
<a href="http://www.lua.org/license.html#5">Lua license</a>.
</small>
<hr>
<p>
<a href="contents.html#contents">contents</A>
&middot;
<a href="contents.html#index">index</A>
<!-- ====================================================================== -->
<p>
<!-- $Id: manual.of,v 1.48 2008/08/18 15:24:20 roberto Exp $ -->
<h1>1 - <a name="1">Introduction</a></h1>
<p>
Lua is an extension programming language designed to support
general procedural programming with data description
facilities.
It also offers good support for object-oriented programming,
functional programming, and data-driven programming.
Lua is intended to be used as a powerful, light-weight
scripting language for any program that needs one.
Lua is implemented as a library, written in <em>clean</em> C
(that is, in the common subset of ANSI&nbsp;C and C++).
<p>
Being an extension language, Lua has no notion of a "main" program:
it only works <em>embedded</em> in a host client,
called the <em>embedding program</em> or simply the <em>host</em>.
This host program can invoke functions to execute a piece of Lua code,
can write and read Lua variables,
and can register C&nbsp;functions to be called by Lua code.
Through the use of C&nbsp;functions, Lua can be augmented to cope with
a wide range of different domains,
thus creating customized programming languages sharing a syntactical framework.
The Lua distribution includes a sample host program called <code>lua</code>,
which uses the Lua library to offer a complete, stand-alone Lua interpreter.
<p>
Lua is free software,
and is provided as usual with no guarantees,
as stated in its license.
The implementation described in this manual is available
at Lua's official web site, <code>www.lua.org</code>.
<p>
Like any other reference manual,
this document is dry in places.
For a discussion of the decisions behind the design of Lua,
see the technical papers available at Lua's web site.
For a detailed introduction to programming in Lua,
see Roberto's book, <em>Programming in Lua (Second Edition)</em>.
<h1>2 - <a name="2">The Language</a></h1>
<p>
This section describes the lexis, the syntax, and the semantics of Lua.
In other words,
this section describes
which tokens are valid,
how they can be combined,
and what their combinations mean.
<p>
The language constructs will be explained using the usual extended BNF notation,
in which
{<em>a</em>}&nbsp;means&nbsp;0 or more <em>a</em>'s, and
[<em>a</em>]&nbsp;means an optional <em>a</em>.
Non-terminals are shown like non-terminal,
keywords are shown like <b>kword</b>,
and other terminal symbols are shown like `<b>=</b>&acute;.
The complete syntax of Lua can be found in <a href="#8">&sect;8</a>
at the end of this manual.
<h2>2.1 - <a name="2.1">Lexical Conventions</a></h2>
<p>
<em>Names</em>
(also called <em>identifiers</em>)
in Lua can be any string of letters,
digits, and underscores,
not beginning with a digit.
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.
<p>
The following <em>keywords</em> are reserved
and cannot be used as names:
<pre>
and break do else elseif
end false for function if
in local nil not or
repeat return then true until while
</pre>
<p>
Lua is a case-sensitive language:
<code>and</code> is a reserved word, but <code>And</code> and <code>AND</code>
are two different, valid names.
As a convention, names starting with an underscore followed by
uppercase letters (such as <a href="#pdf-_VERSION"><code>_VERSION</code></a>)
are reserved for internal global variables used by Lua.
<p>
The following strings denote other tokens:
<pre>
+ - * / % ^ #
== ~= &lt;= &gt;= &lt; &gt; =
( ) { } [ ]
; : , . .. ...
</pre>
<p>
<em>Literal strings</em>
can be delimited by matching single or double quotes,
and can contain the following C-like escape sequences:
'<code>\a</code>' (bell),
'<code>\b</code>' (backspace),
'<code>\f</code>' (form feed),
'<code>\n</code>' (newline),
'<code>\r</code>' (carriage return),
'<code>\t</code>' (horizontal tab),
'<code>\v</code>' (vertical tab),
'<code>\\</code>' (backslash),
'<code>\"</code>' (quotation mark [double quote]),
and '<code>\'</code>' (apostrophe [single quote]).
Moreover, a backslash followed by a real newline
results in a newline in the string.
A character in a string can also be specified by its numerical value
using the escape sequence <code>\<em>ddd</em></code>,
where <em>ddd</em> is a sequence of up to three decimal digits.
(Note that if a numerical escape is to be followed by a digit,
it must be expressed using exactly three digits.)
Strings in Lua can contain any 8-bit value, including embedded zeros,
which can be specified as '<code>\0</code>'.
<p>
Literal strings can also be defined using a long format
enclosed by <em>long brackets</em>.
We define an <em>opening long bracket of level <em>n</em></em> as an opening
square bracket followed by <em>n</em> equal signs followed by another
opening square bracket.
So, an opening long bracket of level&nbsp;0 is written as <code>[[</code>,
an opening long bracket of level&nbsp;1 is written as <code>[=[</code>,
and so on.
A <em>closing long bracket</em> is defined similarly;
for instance, a closing long bracket of level&nbsp;4 is written as <code>]====]</code>.
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 can run for several lines,
do not interpret any escape sequences,
and ignore long brackets of any other level.
They can contain anything except a closing bracket of the proper level.
<p>
For convenience,
when the opening long bracket is immediately followed by a newline,
the newline is not included in the string.
As an example, in a system using ASCII
(in which '<code>a</code>' is coded as&nbsp;97,
newline is coded as&nbsp;10, and '<code>1</code>' is coded as&nbsp;49),
the five literal strings below denote the same string:
<pre>
a = 'alo\n123"'
a = "alo\n123\""
a = '\97lo\10\04923"'
a = [[alo
123"]]
a = [==[
alo
123"]==]
</pre>
<p>
A <em>numerical constant</em> can be written with an optional decimal part
and an optional decimal exponent.
Lua also accepts integer hexadecimal constants,
by prefixing them with <code>0x</code>.
Examples of valid numerical constants are
<pre>
3 3.0 3.1416 314.16e-2 0.31416E1 0xff 0x56
</pre>
<p>
A <em>comment</em> starts with a double hyphen (<code>--</code>)
anywhere outside a string.
If the text immediately after <code>--</code> is not an opening long bracket,
the comment is a <em>short comment</em>,
which runs until the end of the line.
Otherwise, it is a <em>long comment</em>,
which runs until the corresponding closing long bracket.
Long comments are frequently used to disable code temporarily.
<h2>2.2 - <a name="2.2">Values and Types</a></h2>
<p>
Lua is a <em>dynamically typed language</em>.
This means that
variables do not have types; only values do.
There are no type definitions in the language.
All values carry their own type.
<p>
All values in Lua are <em>first-class values</em>.
This means that all values can be stored in variables,
passed as arguments to other functions, and returned as results.
<p>
There are eight basic types in Lua:
<em>nil</em>, <em>boolean</em>, <em>number</em>,
<em>string</em>, <em>function</em>, <em>userdata</em>,
<em>thread</em>, and <em>table</em>.
<em>Nil</em> is the type of the value <b>nil</b>,
whose main property is to be different from any other value;
it usually represents the absence of a useful value.
<em>Boolean</em> is the type of the values <b>false</b> and <b>true</b>.
Both <b>nil</b> and <b>false</b> make a condition false;
any other value makes it true.
<em>Number</em> represents real (double-precision floating-point) numbers.
(It is easy to build Lua interpreters that use other
internal representations for numbers,
such as single-precision float or long integers;
see file <code>luaconf.h</code>.)
<em>String</em> represents arrays of characters.
Lua is 8-bit clean:
strings can contain any 8-bit character,
including embedded zeros ('<code>\0</code>') (see <a href="#2.1">&sect;2.1</a>).
<p>
Lua can call (and manipulate) functions written in Lua and
functions written in C
(see <a href="#2.5.8">&sect;2.5.8</a>).
<p>
The type <em>userdata</em> is provided to allow arbitrary C&nbsp;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 <em>metatables</em>,
the programmer can define operations for userdata values
(see <a href="#2.8">&sect;2.8</a>).
Userdata values cannot be created or modified in Lua,
only through the C&nbsp;API.
This guarantees the integrity of data owned by the host program.
<p>
The type <em>thread</em> represents independent threads of execution
and it is used to implement coroutines (see <a href="#2.11">&sect;2.11</a>).
Do not confuse Lua threads with operating-system threads.
Lua supports coroutines on all systems,
even those that do not support threads.
<p>
The type <em>table</em> implements associative arrays,
that is, arrays that can be indexed not only with numbers,
but with any value (except <b>nil</b>).
Tables can be <em>heterogeneous</em>;
that is, they can contain values of all types (except <b>nil</b>).
Tables are the sole data structuring mechanism in Lua;
they can 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 <code>a.name</code> as syntactic sugar for <code>a["name"]</code>.
There are several convenient ways to create tables in Lua
(see <a href="#2.5.7">&sect;2.5.7</a>).
<p>
Like indices,
the value of a table field can be of any type (except <b>nil</b>).
In particular,
because functions are first-class values,
table fields can contain functions.
Thus tables can also carry <em>methods</em> (see <a href="#2.5.9">&sect;2.5.9</a>).
<p>
Tables, functions, threads, and (full) userdata values are <em>objects</em>:
variables do not actually <em>contain</em> these values,
only <em>references</em> to them.
Assignment, parameter passing, and function returns
always manipulate references to such values;
these operations do not imply any kind of copy.
<p>
The library function <a href="#pdf-type"><code>type</code></a> returns a string describing the type
of a given value.
<h3>2.2.1 - <a name="2.2.1">Coercion</a></h3>
<p>
Lua provides automatic conversion between
string and number values at run time.
Any arithmetic operation applied to a string tries to convert
this string to a number, following the 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 <code>format</code> function from the string library
(see <a href="#pdf-string.format"><code>string.format</code></a>).
<h2>2.3 - <a name="2.3">Variables</a></h2>
<p>
Variables are places that store values.
There are three kinds of variables in Lua:
global variables, local variables, and table fields.
<p>
A single name can denote a global variable or a local variable
(or a function's formal parameter,
which is a particular kind of local variable):
<pre>
var ::= Name
</pre><p>
Name denotes identifiers, as defined in <a href="#2.1">&sect;2.1</a>.
<p>
Any variable is assumed to be global unless explicitly declared
as a local (see <a href="#2.4.7">&sect;2.4.7</a>).
Local variables are <em>lexically scoped</em>:
local variables can be freely accessed by functions
defined inside their scope (see <a href="#2.6">&sect;2.6</a>).
<p>
Before the first assignment to a variable, its value is <b>nil</b>.
<p>
Square brackets are used to index a table:
<pre>
var ::= prefixexp `<b>[</b>&acute; exp `<b>]</b>&acute;
</pre><p>
The meaning of accesses to global variables
and table fields can be changed via metatables.
An access to an indexed variable <code>t[i]</code> is equivalent to
a call <code>gettable_event(t,i)</code>.
(See <a href="#2.8">&sect;2.8</a> for a complete description of the
<code>gettable_event</code> function.
This function is not defined or callable in Lua.
We use it here only for explanatory purposes.)
<p>
The syntax <code>var.Name</code> is just syntactic sugar for
<code>var["Name"]</code>:
<pre>
var ::= prefixexp `<b>.</b>&acute; Name
</pre>
<p>
All global variables live as fields in ordinary Lua tables,
called <em>environment tables</em> or simply
<em>environments</em> (see <a href="#2.9">&sect;2.9</a>).
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 <a href="#pdf-getfenv"><code>getfenv</code></a>.
To replace it,
you call <a href="#pdf-setfenv"><code>setfenv</code></a>.
(You can only manipulate the environment of C&nbsp;functions
through the debug library; (see <a href="#5.9">&sect;5.9</a>).)
<p>
An access to a global variable <code>x</code>
is equivalent to <code>_env.x</code>,
which in turn is equivalent to
<pre>
gettable_event(_env, "x")
</pre><p>
where <code>_env</code> is the environment of the running function.
(See <a href="#2.8">&sect;2.8</a> for a complete description of the
<code>gettable_event</code> function.
This function is not defined or callable in Lua.
Similarly, the <code>_env</code> variable is not defined in Lua.
We use them here only for explanatory purposes.)
<h2>2.4 - <a name="2.4">Statements</a></h2>
<p>
Lua supports an almost conventional set of statements,
similar to those in Pascal or C.
This set includes
assignments, control structures, function calls,
and variable declarations.
<h3>2.4.1 - <a name="2.4.1">Chunks</a></h3>
<p>
The unit of execution of Lua is called a <em>chunk</em>.
A chunk is simply a sequence of statements,
which are executed sequentially.
Each statement can be optionally followed by a semicolon:
<pre>
chunk ::= {stat [`<b>;</b>&acute;]}
</pre><p>
There are no empty statements and thus '<code>;;</code>' is not legal.
<p>
Lua handles a chunk as the body of an anonymous function
with a variable number of arguments
(see <a href="#2.5.9">&sect;2.5.9</a>).
As such, chunks can define local variables,
receive arguments, and return values.
<p>
A chunk can be stored in a file or in a string inside the host program.
To execute a chunk,
Lua first pre-compiles the chunk into instructions for a virtual machine,
and then it executes the compiled code
with an interpreter for the virtual machine.
<p>
Chunks can also be pre-compiled into binary form;
see program <code>luac</code> for details.
Programs in source and compiled forms are interchangeable;
Lua automatically detects the file type and acts accordingly.
<h3>2.4.2 - <a name="2.4.2">Blocks</a></h3><p>
A block is a list of statements;
syntactically, a block is the same as a chunk:
<pre>
block ::= chunk
</pre>
<p>
A block can be explicitly delimited to produce a single statement:
<pre>
stat ::= <b>do</b> block <b>end</b>
</pre><p>
Explicit blocks are useful
to control the scope of variable declarations.
Explicit blocks are also sometimes used to
add a <b>return</b> or <b>break</b> statement in the middle
of another block (see <a href="#2.4.4">&sect;2.4.4</a>).
<h3>2.4.3 - <a name="2.4.3">Assignment</a></h3>
<p>
Lua allows multiple assignments.
Therefore, the syntax for assignment
defines a list of variables on the left side
and a list of expressions on the right side.
The elements in both lists are separated by commas:
<pre>
stat ::= varlist `<b>=</b>&acute; explist
varlist ::= var {`<b>,</b>&acute; var}
explist ::= exp {`<b>,</b>&acute; exp}
</pre><p>
Expressions are discussed in <a href="#2.5">&sect;2.5</a>.
<p>
Before the assignment,
the list of values is <em>adjusted</em> to the length of
the list of variables.
If there are more values than needed,
the excess values are thrown away.
If there are fewer values than needed,
the list is extended with as many <b>nil</b>'s as needed.
If the list of expressions ends with a function call,
then all values returned by that call enter the list of values,
before the adjustment
(except when the call is enclosed in parentheses; see <a href="#2.5">&sect;2.5</a>).
<p>
The assignment statement first evaluates all its expressions
and only then are the assignments performed.
Thus the code
<pre>
i = 3
i, a[i] = i+1, 20
</pre><p>
sets <code>a[3]</code> to 20, without affecting <code>a[4]</code>
because the <code>i</code> in <code>a[i]</code> is evaluated (to 3)
before it is assigned&nbsp;4.
Similarly, the line
<pre>
x, y = y, x
</pre><p>
exchanges the values of <code>x</code> and <code>y</code>,
and
<pre>
x, y, z = y, z, x
</pre><p>
cyclically permutes the values of <code>x</code>, <code>y</code>, and <code>z</code>.
<p>
The meaning of assignments to global variables
and table fields can be changed via metatables.
An assignment to an indexed variable <code>t[i] = val</code> is equivalent to
<code>settable_event(t,i,val)</code>.
(See <a href="#2.8">&sect;2.8</a> for a complete description of the
<code>settable_event</code> function.
This function is not defined or callable in Lua.
We use it here only for explanatory purposes.)
<p>
An assignment to a global variable <code>x = val</code>
is equivalent to the assignment
<code>_env.x = val</code>,
which in turn is equivalent to
<pre>
settable_event(_env, "x", val)
</pre><p>
where <code>_env</code> is the environment of the running function.
(The <code>_env</code> variable is not defined in Lua.
We use it here only for explanatory purposes.)
<h3>2.4.4 - <a name="2.4.4">Control Structures</a></h3><p>
The control structures
<b>if</b>, <b>while</b>, and <b>repeat</b> have the usual meaning and
familiar syntax:
<pre>
stat ::= <b>while</b> exp <b>do</b> block <b>end</b>
stat ::= <b>repeat</b> block <b>until</b> exp
stat ::= <b>if</b> exp <b>then</b> block {<b>elseif</b> exp <b>then</b> block} [<b>else</b> block] <b>end</b>
</pre><p>
Lua also has a <b>for</b> statement, in two flavors (see <a href="#2.4.5">&sect;2.4.5</a>).
<p>
The condition expression of a
control structure can return any value.
Both <b>false</b> and <b>nil</b> are considered false.
All values different from <b>nil</b> and <b>false</b> are considered true
(in particular, the number 0 and the empty string are also true).
<p>
In the <b>repeat</b>&ndash;<b>until</b> loop,
the inner block does not end at the <b>until</b> keyword,
but only after the condition.
So, the condition can refer to local variables
declared inside the loop block.
<p>
The <b>return</b> statement is used to return values
from a function or a chunk (which is just a function).
Functions and chunks can return more than one value,
and so the syntax for the <b>return</b> statement is
<pre>
stat ::= <b>return</b> [explist]
</pre>
<p>
The <b>break</b> statement is used to terminate the execution of a
<b>while</b>, <b>repeat</b>, or <b>for</b> loop,
skipping to the next statement after the loop:
<pre>
stat ::= <b>break</b>
</pre><p>
A <b>break</b> ends the innermost enclosing loop.
<p>
The <b>return</b> and <b>break</b>
statements can only be written as the <em>last</em> statement of a block.
If it is really necessary to <b>return</b> or <b>break</b> in the
middle of a block,
then an explicit inner block can be used,
as in the idioms
<code>do return end</code> and <code>do break end</code>,
because now <b>return</b> and <b>break</b> are the last statements in
their (inner) blocks.
<h3>2.4.5 - <a name="2.4.5">For Statement</a></h3>
<p>
The <b>for</b> statement has two forms:
one numeric and one generic.
<p>
The numeric <b>for</b> loop repeats a block of code while a
control variable runs through an arithmetic progression.
It has the following syntax:
<pre>
stat ::= <b>for</b> Name `<b>=</b>&acute; exp `<b>,</b>&acute; exp [`<b>,</b>&acute; exp] <b>do</b> block <b>end</b>
</pre><p>
The <em>block</em> is repeated for <em>name</em> starting at the value of
the first <em>exp</em>, until it passes the second <em>exp</em> by steps of the
third <em>exp</em>.
More precisely, a <b>for</b> statement like
<pre>
for v = <em>e1</em>, <em>e2</em>, <em>e3</em> do <em>block</em> end
</pre><p>
is equivalent to the code:
<pre>
do
local <em>var</em>, <em>limit</em>, <em>step</em> = tonumber(<em>e1</em>), tonumber(<em>e2</em>), tonumber(<em>e3</em>)
if not (<em>var</em> and <em>limit</em> and <em>step</em>) then error() end
while (<em>step</em> &gt; 0 and <em>var</em> &lt;= <em>limit</em>) or (<em>step</em> &lt;= 0 and <em>var</em> &gt;= <em>limit</em>) do
local v = <em>var</em>
<em>block</em>
<em>var</em> = <em>var</em> + <em>step</em>
end
end
</pre><p>
Note the following:
<ul>
<li>
All three control expressions are evaluated only once,
before the loop starts.
They must all result in numbers.
</li>
<li>
<code><em>var</em></code>, <code><em>limit</em></code>, and <code><em>step</em></code> are invisible variables.
The names shown here are for explanatory purposes only.
</li>
<li>
If the third expression (the step) is absent,
then a step of&nbsp;1 is used.
</li>
<li>
You can use <b>break</b> to exit a <b>for</b> loop.
</li>
<li>
The loop variable <code>v</code> is local to the loop;
you cannot use its value after the <b>for</b> ends or is broken.
If you need this value,
assign it to another variable before breaking or exiting the loop.
</li>
</ul>
<p>
The generic <b>for</b> statement works over functions,
called <em>iterators</em>.
On each iteration, the iterator function is called to produce a new value,
stopping when this new value is <b>nil</b>.
The generic <b>for</b> loop has the following syntax:
<pre>
stat ::= <b>for</b> namelist <b>in</b> explist <b>do</b> block <b>end</b>
namelist ::= Name {`<b>,</b>&acute; Name}
</pre><p>
A <b>for</b> statement like
<pre>
for <em>var_1</em>, &middot;&middot;&middot;, <em>var_n</em> in <em>explist</em> do <em>block</em> end
</pre><p>
is equivalent to the code:
<pre>
do
local <em>f</em>, <em>s</em>, <em>var</em> = <em>explist</em>
while true do
local <em>var_1</em>, &middot;&middot;&middot;, <em>var_n</em> = <em>f</em>(<em>s</em>, <em>var</em>)
<em>var</em> = <em>var_1</em>
if <em>var</em> == nil then break end
<em>block</em>
end
end
</pre><p>
Note the following:
<ul>
<li>
<code><em>explist</em></code> is evaluated only once.
Its results are an <em>iterator</em> function,
a <em>state</em>,
and an initial value for the first <em>iterator variable</em>.
</li>
<li>
<code><em>f</em></code>, <code><em>s</em></code>, and <code><em>var</em></code> are invisible variables.
The names are here for explanatory purposes only.
</li>
<li>
You can use <b>break</b> to exit a <b>for</b> loop.
</li>
<li>
The loop variables <code><em>var_i</em></code> are local to the loop;
you cannot use their values after the <b>for</b> ends.
If you need these values,
then assign them to other variables before breaking or exiting the loop.
</li>
</ul>
<h3>2.4.6 - <a name="2.4.6">Function Calls as Statements</a></h3><p>
To allow possible side-effects,
function calls can be executed as statements:
<pre>
stat ::= functioncall
</pre><p>
In this case, all returned values are thrown away.
Function calls are explained in <a href="#2.5.8">&sect;2.5.8</a>.
<h3>2.4.7 - <a name="2.4.7">Local Declarations</a></h3><p>
Local variables can be declared anywhere inside a block.
The declaration can include an initial assignment:
<pre>
stat ::= <b>local</b> namelist [`<b>=</b>&acute; explist]
</pre><p>
If present, an initial assignment has the same semantics
of a multiple assignment (see <a href="#2.4.3">&sect;2.4.3</a>).
Otherwise, all variables are initialized with <b>nil</b>.
<p>
A chunk is also a block (see <a href="#2.4.1">&sect;2.4.1</a>),
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.
<p>
The visibility rules for local variables are explained in <a href="#2.6">&sect;2.6</a>.
<h2>2.5 - <a name="2.5">Expressions</a></h2>
<p>
The basic expressions in Lua are the following:
<pre>
exp ::= prefixexp
exp ::= <b>nil</b> | <b>false</b> | <b>true</b>
exp ::= Number
exp ::= String
exp ::= function
exp ::= tableconstructor
exp ::= `<b>...</b>&acute;
exp ::= exp binop exp
exp ::= unop exp
prefixexp ::= var | functioncall | `<b>(</b>&acute; exp `<b>)</b>&acute;
</pre>
<p>
Numbers and literal strings are explained in <a href="#2.1">&sect;2.1</a>;
variables are explained in <a href="#2.3">&sect;2.3</a>;
function definitions are explained in <a href="#2.5.9">&sect;2.5.9</a>;
function calls are explained in <a href="#2.5.8">&sect;2.5.8</a>;
table constructors are explained in <a href="#2.5.7">&sect;2.5.7</a>.
Vararg expressions,
denoted by three dots ('<code>...</code>'), can only be used when
directly inside a vararg function;
they are explained in <a href="#2.5.9">&sect;2.5.9</a>.
<p>
Binary operators comprise arithmetic operators (see <a href="#2.5.1">&sect;2.5.1</a>),
relational operators (see <a href="#2.5.2">&sect;2.5.2</a>), logical operators (see <a href="#2.5.3">&sect;2.5.3</a>),
and the concatenation operator (see <a href="#2.5.4">&sect;2.5.4</a>).
Unary operators comprise the unary minus (see <a href="#2.5.1">&sect;2.5.1</a>),
the unary <b>not</b> (see <a href="#2.5.3">&sect;2.5.3</a>),
and the unary <em>length operator</em> (see <a href="#2.5.5">&sect;2.5.5</a>).
<p>
Both function calls and vararg expressions can result in multiple values.
If an expression is used as a statement
(only possible for function calls (see <a href="#2.4.6">&sect;2.4.6</a>)),
then its return list is adjusted to zero elements,
thus discarding all returned values.
If an expression is used as the last (or the only) element
of a list of expressions,
then no adjustment is made
(unless the call is enclosed in parentheses).
In all other contexts,
Lua adjusts the result list to one element,
discarding all values except the first one.
<p>
Here are some examples:
<pre>
f() -- adjusted to 0 results
g(f(), x) -- f() is adjusted to 1 result
g(x, f()) -- g gets x plus all results from f()
a,b,c = f(), x -- f() is adjusted to 1 result (c gets nil)
a,b = ... -- a gets the first vararg parameter, b gets
-- the second (both a and b can get nil if there
-- is no corresponding vararg parameter)
a,b,c = x, f() -- f() is adjusted to 2 results
a,b,c = f() -- f() is adjusted to 3 results
return f() -- returns all results from f()
return ... -- returns all received vararg parameters
return x,y,f() -- returns x, y, and all results from f()
{f()} -- creates a list with all results from f()
{...} -- creates a list with all vararg parameters
{f(), nil} -- f() is adjusted to 1 result
</pre>
<p>
Any expression enclosed in parentheses always results in only one value.
Thus,
<code>(f(x,y,z))</code> is always a single value,
even if <code>f</code> returns several values.
(The value of <code>(f(x,y,z))</code> is the first value returned by <code>f</code>
or <b>nil</b> if <code>f</code> does not return any values.)
<h3>2.5.1 - <a name="2.5.1">Arithmetic Operators</a></h3><p>
Lua supports the usual arithmetic operators:
the binary <code>+</code> (addition),
<code>-</code> (subtraction), <code>*</code> (multiplication),
<code>/</code> (division), <code>%</code> (modulo), and <code>^</code> (exponentiation);
and unary <code>-</code> (negation).
If the operands are numbers, or strings that can be converted to
numbers (see <a href="#2.2.1">&sect;2.2.1</a>),
then all operations have the usual meaning.
Exponentiation works for any exponent.
For instance, <code>x^(-0.5)</code> computes the inverse of the square root of <code>x</code>.
Modulo is defined as
<pre>
a % b == a - math.floor(a/b)*b
</pre><p>
That is, it is the remainder of a division that rounds
the quotient towards minus infinity.
<h3>2.5.2 - <a name="2.5.2">Relational Operators</a></h3><p>
The relational operators in Lua are
<pre>
== ~= &lt; &gt; &lt;= &gt;=
</pre><p>
These operators always result in <b>false</b> or <b>true</b>.
<p>
Equality (<code>==</code>) first compares the type of its operands.
If the types are different, then the result is <b>false</b>.
Otherwise, the values of the operands are compared.
Numbers and strings are compared in the usual way.
Objects (tables, userdata, threads, and functions)
are compared by <em>reference</em>:
two objects are considered equal only if they are the <em>same</em> object.
Every time you create a new object
(a table, userdata, thread, or function),
this new object is different from any previously existing object.
<p>
You can change the way that Lua compares tables and userdata
by using the "eq" metamethod (see <a href="#2.8">&sect;2.8</a>).
<p>
The conversion rules of <a href="#2.2.1">&sect;2.2.1</a>
<em>do not</em> apply to equality comparisons.
Thus, <code>"0"==0</code> evaluates to <b>false</b>,
and <code>t[0]</code> and <code>t["0"]</code> denote different
entries in a table.
<p>
The operator <code>~=</code> is exactly the negation of equality (<code>==</code>).
<p>
The order operators work as follows.
If both arguments are numbers, then they are compared as such.
Otherwise, if both arguments are strings,
then their values are compared according to the current locale.
Otherwise, Lua tries to call the "lt" or the "le"
metamethod (see <a href="#2.8">&sect;2.8</a>).
A comparison <code>a &gt; b</code> is translated to <code>b &lt; a</code>
and <code>a &gt;= b</code> is translated to <code>b &lt;= a</code>.
<h3>2.5.3 - <a name="2.5.3">Logical Operators</a></h3><p>
The logical operators in Lua are
<b>and</b>, <b>or</b>, and <b>not</b>.
Like the control structures (see <a href="#2.4.4">&sect;2.4.4</a>),
all logical operators consider both <b>false</b> and <b>nil</b> as false
and anything else as true.
<p>
The negation operator <b>not</b> always returns <b>false</b> or <b>true</b>.
The conjunction operator <b>and</b> returns its first argument
if this value is <b>false</b> or <b>nil</b>;
otherwise, <b>and</b> returns its second argument.
The disjunction operator <b>or</b> returns its first argument
if this value is different from <b>nil</b> and <b>false</b>;
otherwise, <b>or</b> returns its second argument.
Both <b>and</b> and <b>or</b> use short-cut evaluation;
that is,
the second operand is evaluated only if necessary.
Here are some examples:
<pre>
10 or 20 --&gt; 10
10 or error() --&gt; 10
nil or "a" --&gt; "a"
nil and 10 --&gt; nil
false and error() --&gt; false
false and nil --&gt; false
false or nil --&gt; nil
10 and 20 --&gt; 20
</pre><p>
(In this manual,
<code>--&gt;</code> indicates the result of the preceding expression.)
<h3>2.5.4 - <a name="2.5.4">Concatenation</a></h3><p>
The string concatenation operator in Lua is
denoted by two dots ('<code>..</code>').
If both operands are strings or numbers, then they are converted to
strings according to the rules mentioned in <a href="#2.2.1">&sect;2.2.1</a>.
Otherwise, the "concat" metamethod is called (see <a href="#2.8">&sect;2.8</a>).
<h3>2.5.5 - <a name="2.5.5">The Length Operator</a></h3>
<p>
The length operator is denoted by the unary operator <code>#</code>.
The length of a string is its number of bytes
(that is, the usual meaning of string length when each
character is one byte).
<p>
The length of a table <code>t</code> is defined to be any
integer index <code>n</code>
such that <code>t[n]</code> is not <b>nil</b> and <code>t[n+1]</code> is <b>nil</b>;
moreover, if <code>t[1]</code> is <b>nil</b>, <code>n</code> can be zero.
For a regular array, with non-nil values from 1 to a given <code>n</code>,
its length is exactly that <code>n</code>,
the index of its last value.
If the array has "holes"
(that is, <b>nil</b> values between other non-nil values),
then <code>#t</code> can be any of the indices that
directly precedes a <b>nil</b> value
(that is, it may consider any such <b>nil</b> value as the end of
the array).
<h3>2.5.6 - <a name="2.5.6">Precedence</a></h3><p>
Operator precedence in Lua follows the table below,
from lower to higher priority:
<pre>
or
and
&lt; &gt; &lt;= &gt;= ~= ==
..
+ -
* / %
not # - (unary)
^
</pre><p>
As usual,
you can use parentheses to change the precedences of an expression.
The concatenation ('<code>..</code>') and exponentiation ('<code>^</code>')
operators are right associative.
All other binary operators are left associative.
<h3>2.5.7 - <a name="2.5.7">Table Constructors</a></h3><p>
Table constructors are expressions that create tables.
Every time a constructor is evaluated, a new table is created.
A constructor can be used to create an empty table
or to create a table and initialize some of its fields.
The general syntax for constructors is
<pre>
tableconstructor ::= `<b>{</b>&acute; [fieldlist] `<b>}</b>&acute;
fieldlist ::= field {fieldsep field} [fieldsep]
field ::= `<b>[</b>&acute; exp `<b>]</b>&acute; `<b>=</b>&acute; exp | Name `<b>=</b>&acute; exp | exp
fieldsep ::= `<b>,</b>&acute; | `<b>;</b>&acute;
</pre>
<p>
Each field of the form <code>[exp1] = exp2</code> adds to the new table an entry
with key <code>exp1</code> and value <code>exp2</code>.
A field of the form <code>name = exp</code> is equivalent to
<code>["name"] = exp</code>.
Finally, fields of the form <code>exp</code> are equivalent to
<code>[i] = exp</code>, where <code>i</code> are consecutive numerical integers,
starting with 1.
Fields in the other formats do not affect this counting.
For example,
<pre>
a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 }
</pre><p>
is equivalent to
<pre>
do
local t = {}
t[f(1)] = g
t[1] = "x" -- 1st exp
t[2] = "y" -- 2nd exp
t.x = 1 -- t["x"] = 1
t[3] = f(x) -- 3rd exp
t[30] = 23
t[4] = 45 -- 4th exp
a = t
end
</pre>
<p>
If the last field in the list has the form <code>exp</code>
and the expression is a function call or a vararg expression,
then all values returned by this expression enter the list consecutively
(see <a href="#2.5.8">&sect;2.5.8</a>).
To avoid this,
enclose the function call or the vararg expression
in parentheses (see <a href="#2.5">&sect;2.5</a>).
<p>
The field list can have an optional trailing separator,
as a convenience for machine-generated code.
<h3>2.5.8 - <a name="2.5.8">Function Calls</a></h3><p>
A function call in Lua has the following syntax:
<pre>
functioncall ::= prefixexp args
</pre><p>
In a function call,
first prefixexp and args are evaluated.
If the value of prefixexp has type <em>function</em>,
then this function is called
with the given arguments.
Otherwise, the prefixexp "call" metamethod is called,
having as first parameter the value of prefixexp,
followed by the original call arguments
(see <a href="#2.8">&sect;2.8</a>).
<p>
The form
<pre>
functioncall ::= prefixexp `<b>:</b>&acute; Name args
</pre><p>
can be used to call "methods".
A call <code>v:name(<em>args</em>)</code>
is syntactic sugar for <code>v.name(v,<em>args</em>)</code>,
except that <code>v</code> is evaluated only once.
<p>
Arguments have the following syntax:
<pre>
args ::= `<b>(</b>&acute; [explist] `<b>)</b>&acute;
args ::= tableconstructor
args ::= String
</pre><p>
All argument expressions are evaluated before the call.
A call of the form <code>f{<em>fields</em>}</code> is
syntactic sugar for <code>f({<em>fields</em>})</code>;
that is, the argument list is a single new table.
A call of the form <code>f'<em>string</em>'</code>
(or <code>f"<em>string</em>"</code> or <code>f[[<em>string</em>]]</code>)
is syntactic sugar for <code>f('<em>string</em>')</code>;
that is, the argument list is a single literal string.
<p>
As an exception to the free-format syntax of Lua,
you cannot put a line break before the '<code>(</code>' in a function call.
This restriction avoids some ambiguities in the language.
If you write
<pre>
a = f
(g).x(a)
</pre><p>
Lua would see that as a single statement, <code>a = f(g).x(a)</code>.
So, if you want two statements, you must add a semi-colon between them.
If you actually want to call <code>f</code>,
you must remove the line break before <code>(g)</code>.
<p>
A call of the form <code>return</code> <em>functioncall</em> is called
a <em>tail call</em>.
Lua implements <em>proper tail calls</em>
(or <em>proper tail recursion</em>):
in a tail call,
the called function reuses the stack entry of the calling function.
Therefore, there is no limit on the number of nested tail calls that
a program can execute.
However, a tail call erases any debug information about the
calling function.
Note that a tail call only happens with a particular syntax,
where the <b>return</b> has one single function call as argument;
this syntax makes the calling function return exactly
the returns of the called function.
So, none of the following examples are tail calls:
<pre>
return (f(x)) -- results adjusted to 1
return 2 * f(x)
return x, f(x) -- additional results
f(x); return -- results discarded
return x or f(x) -- results adjusted to 1
</pre>
<h3>2.5.9 - <a name="2.5.9">Function Definitions</a></h3>
<p>
The syntax for function definition is
<pre>
function ::= <b>function</b> funcbody
funcbody ::= `<b>(</b>&acute; [parlist] `<b>)</b>&acute; block <b>end</b>
</pre>
<p>
The following syntactic sugar simplifies function definitions:
<pre>
stat ::= <b>function</b> funcname funcbody
stat ::= <b>local</b> <b>function</b> Name funcbody
funcname ::= Name {`<b>.</b>&acute; Name} [`<b>:</b>&acute; Name]
</pre><p>
The statement
<pre>
function f () <em>body</em> end
</pre><p>
translates to
<pre>
f = function () <em>body</em> end
</pre><p>
The statement
<pre>
function t.a.b.c.f () <em>body</em> end
</pre><p>
translates to
<pre>
t.a.b.c.f = function () <em>body</em> end
</pre><p>
The statement
<pre>
local function f () <em>body</em> end
</pre><p>
translates to
<pre>
local f; f = function () <em>body</em> end
</pre><p>
<em>not</em> to
<pre>
local f = function () <em>body</em> end
</pre><p>
(This only makes a difference when the body of the function
contains references to <code>f</code>.)
<p>
A function definition is an executable expression,
whose value has type <em>function</em>.
When Lua pre-compiles a chunk,
all its function bodies are pre-compiled too.
Then, whenever Lua executes the function definition,
the function is <em>instantiated</em> (or <em>closed</em>).
This function instance (or <em>closure</em>)
is the final value of the expression.
Different instances of the same function
can refer to different external local variables
and can have different environment tables.
<p>
Parameters act as local variables that are
initialized with the argument values:
<pre>
parlist ::= namelist [`<b>,</b>&acute; `<b>...</b>&acute;] | `<b>...</b>&acute;
</pre><p>
When a function is called,
the list of arguments is adjusted to
the length of the list of parameters,
unless the function is a variadic or <em>vararg function</em>,
which is
indicated by three dots ('<code>...</code>') at the end of its parameter list.
A vararg function does not adjust its argument list;
instead, it collects all extra arguments and supplies them
to the function through a <em>vararg expression</em>,
which is also written as three dots.
The value of this expression is a list of all actual extra arguments,
similar to a function with multiple results.
If a vararg expression is used inside another expression
or in the middle of a list of expressions,
then its return list is adjusted to one element.
If the expression is used as the last element of a list of expressions,
then no adjustment is made
(unless that last expression is enclosed in parentheses).
<p>
As an example, consider the following definitions:
<pre>
function f(a, b) end
function g(a, b, ...) end
function r() return 1,2,3 end
</pre><p>
Then, we have the following mapping from arguments to parameters and
to the vararg expression:
<pre>
CALL PARAMETERS
f(3) a=3, b=nil
f(3, 4) a=3, b=4
f(3, 4, 5) a=3, b=4
f(r(), 10) a=1, b=10
f(r()) a=1, b=2
g(3) a=3, b=nil, ... --&gt; (nothing)
g(3, 4) a=3, b=4, ... --&gt; (nothing)
g(3, 4, 5, 8) a=3, b=4, ... --&gt; 5 8
g(5, r()) a=5, b=1, ... --&gt; 2 3
</pre>
<p>
Results are returned using the <b>return</b> statement (see <a href="#2.4.4">&sect;2.4.4</a>).
If control reaches the end of a function
without encountering a <b>return</b> statement,
then the function returns with no results.
<p>
The <em>colon</em> syntax
is used for defining <em>methods</em>,
that is, functions that have an implicit extra parameter <code>self</code>.
Thus, the statement
<pre>
function t.a.b.c:f (<em>params</em>) <em>body</em> end
</pre><p>
is syntactic sugar for
<pre>
t.a.b.c.f = function (self, <em>params</em>) <em>body</em> end
</pre>
<h2>2.6 - <a name="2.6">Visibility Rules</a></h2>
<p>
Lua is a lexically scoped language.
The scope of variables begins at the first statement <em>after</em>
their declaration and lasts until the end of the innermost block that
includes the declaration.
Consider the following example:
<pre>
x = 10 -- global variable
do -- new block
local x = x -- new 'x', with value 10
print(x) --&gt; 10
x = x+1
do -- another block
local x = x+1 -- another 'x'
print(x) --&gt; 12
end
print(x) --&gt; 11
end
print(x) --&gt; 10 (the global one)
</pre>
<p>
Notice that, in a declaration like <code>local x = x</code>,
the new <code>x</code> being declared is not in scope yet,
and so the second <code>x</code> refers to the outside variable.
<p>
Because of the lexical scoping rules,
local variables can be freely accessed by functions
defined inside their scope.
A local variable used by an inner function is called
an <em>upvalue</em>, or <em>external local variable</em>,
inside the inner function.
<p>
Notice that each execution of a <b>local</b> statement
defines new local variables.
Consider the following example:
<pre>
a = {}
local x = 20
for i=1,10 do
local y = 0
a[i] = function () y=y+1; return x+y end
end
</pre><p>
The loop creates ten closures
(that is, ten instances of the anonymous function).
Each of these closures uses a different <code>y</code> variable,
while all of them share the same <code>x</code>.
<h2>2.7 - <a name="2.7">Error Handling</a></h2>
<p>
Because Lua is an embedded extension language,
all Lua actions start from C&nbsp;code in the host program
calling a function from the Lua library (see <a href="#lua_pcall"><code>lua_pcall</code></a>).
Whenever an error occurs during Lua compilation or execution,
control returns to C,
which can take appropriate measures
(such as printing an error message).
<p>
Lua code can explicitly generate an error by calling the
<a href="#pdf-error"><code>error</code></a> function.
If you need to catch errors in Lua,
you can use the <a href="#pdf-pcall"><code>pcall</code></a> function.
<h2>2.8 - <a name="2.8">Metatables</a></h2>
<p>
Every value in Lua can have a <em>metatable</em>.
This <em>metatable</em> is an ordinary Lua table
that defines the behavior of the original value
under certain special operations.
You can change several aspects of the behavior
of operations over a value by setting specific fields in its metatable.
For instance, when a non-numeric value is the operand of an addition,
Lua checks for a function in the field <code>"__add"</code> in its metatable.
If it finds one,
Lua calls this function to perform the addition.
<p>
We call the keys in a metatable <em>events</em>
and the values <em>metamethods</em>.
In the previous example, the event is <code>"add"</code>
and the metamethod is the function that performs the addition.
<p>
You can query the metatable of any value
through the <a href="#pdf-getmetatable"><code>getmetatable</code></a> function.
<p>
You can replace the metatable of tables
through the <a href="#pdf-setmetatable"><code>setmetatable</code></a>
function.
You cannot change the metatable of other types from Lua
(except by using the debug library);
you must use the C&nbsp;API for that.
<p>
Tables and full userdata have individual metatables
(although multiple tables and userdata can share their metatables).
Values of all other types share one single metatable per type;
that is, there is one single metatable for all numbers,
one for all strings, etc.
<p>
A metatable controls how an object behaves in arithmetic operations,
order comparisons, concatenation, length operation, and indexing.
A metatable also can define a function to be called when a userdata
is garbage collected.
For each of these operations Lua associates a specific key
called an <em>event</em>.
When Lua performs one of these operations over a value,
it checks whether this value has a metatable with the corresponding event.
If so, the value associated with that key (the metamethod)
controls how Lua will perform the operation.
<p>
Metatables control the operations listed next.
Each operation is identified by its corresponding name.
The key for each operation is a string with its name prefixed by
two underscores, '<code>__</code>';
for instance, the key for operation "add" is the
string <code>"__add"</code>.
The semantics of these operations is better explained by a Lua function
describing how the interpreter executes the operation.
<p>
The code shown here in Lua is only illustrative;
the real behavior is hard coded in the interpreter
and it is much more efficient than this simulation.
All functions used in these descriptions
(<a href="#pdf-rawget"><code>rawget</code></a>, <a href="#pdf-tonumber"><code>tonumber</code></a>, etc.)
are described in <a href="#5.1">&sect;5.1</a>.
In particular, to retrieve the metamethod of a given object,
we use the expression
<pre>
metatable(obj)[event]
</pre><p>
This should be read as
<pre>
rawget(getmetatable(obj) or {}, event)
</pre><p>
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 <b>nil</b>).
<ul>
<li><b>"add":</b>
the <code>+</code> operation.
<p>
The function <code>getbinhandler</code> below defines how Lua chooses a handler
for a binary operation.
First, Lua tries the first operand.
If its type does not define a handler for the operation,
then Lua tries the second operand.
<pre>
function getbinhandler (op1, op2, event)
return metatable(op1)[event] or metatable(op2)[event]
end
</pre><p>
By using this function,
the behavior of the <code>op1 + op2</code> is
<pre>
function add_event (op1, op2)
local o1, o2 = tonumber(op1), tonumber(op2)
if o1 and o2 then -- both operands are numeric?
return o1 + o2 -- '+' here is the primitive 'add'
else -- at least one of the operands is not numeric
local h = getbinhandler(op1, op2, "__add")
if h then
-- call the handler with both operands
return (h(op1, op2))
else -- no handler available: default behavior
error(&middot;&middot;&middot;)
end
end
end
</pre><p>
</li>
<li><b>"sub":</b>
the <code>-</code> operation.
Behavior similar to the "add" operation.
</li>
<li><b>"mul":</b>
the <code>*</code> operation.
Behavior similar to the "add" operation.
</li>
<li><b>"div":</b>
the <code>/</code> operation.
Behavior similar to the "add" operation.
</li>
<li><b>"mod":</b>
the <code>%</code> operation.
Behavior similar to the "add" operation,
with the operation
<code>o1 - floor(o1/o2)*o2</code> as the primitive operation.
</li>
<li><b>"pow":</b>
the <code>^</code> (exponentiation) operation.
Behavior similar to the "add" operation,
with the function <code>pow</code> (from the C&nbsp;math library)
as the primitive operation.
</li>
<li><b>"unm":</b>
the unary <code>-</code> operation.
<pre>
function unm_event (op)
local o = tonumber(op)
if o then -- operand is numeric?
return -o -- '-' here is the primitive 'unm'
else -- the operand is not numeric.
-- Try to get a handler from the operand
local h = metatable(op).__unm
if h then
-- call the handler with the operand
return (h(op))
else -- no handler available: default behavior
error(&middot;&middot;&middot;)
end
end
end
</pre><p>
</li>
<li><b>"concat":</b>
the <code>..</code> (concatenation) operation.
<pre>
function concat_event (op1, op2)
if (type(op1) == "string" or type(op1) == "number") and
(type(op2) == "string" or type(op2) == "number") then
return op1 .. op2 -- primitive string concatenation
else
local h = getbinhandler(op1, op2, "__concat")
if h then
return (h(op1, op2))
else
error(&middot;&middot;&middot;)
end
end
end
</pre><p>
</li>
<li><b>"len":</b>
the <code>#</code> operation.
<pre>
function len_event (op)
if type(op) == "string" then
return strlen(op) -- primitive string length
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(&middot;&middot;&middot;)
end
end
end
</pre><p>
See <a href="#2.5.5">&sect;2.5.5</a> for a description of the length of a table.
</li>
<li><b>"eq":</b>
the <code>==</code> operation.
The function <code>getcomphandler</code> 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.
<pre>
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
</pre><p>
The "eq" event is defined as follows:
<pre>
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
</pre><p>
<code>a ~= b</code> is equivalent to <code>not (a == b)</code>.
</li>
<li><b>"lt":</b>
the <code>&lt;</code> operation.
<pre>
function lt_event (op1, op2)
if type(op1) == "number" and type(op2) == "number" then
return op1 &lt; op2 -- numeric comparison
elseif type(op1) == "string" and type(op2) == "string" then
return op1 &lt; op2 -- lexicographic comparison
else
local h = getcomphandler(op1, op2, "__lt")
if h then
return (h(op1, op2))
else
error(&middot;&middot;&middot;)
end
end
end
</pre><p>
<code>a &gt; b</code> is equivalent to <code>b &lt; a</code>.
</li>
<li><b>"le":</b>
the <code>&lt;=</code> operation.
<pre>
function le_event (op1, op2)
if type(op1) == "number" and type(op2) == "number" then
return op1 &lt;= op2 -- numeric comparison
elseif type(op1) == "string" and type(op2) == "string" then
return op1 &lt;= 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(&middot;&middot;&middot;)
end
end
end
end
</pre><p>
<code>a &gt;= b</code> is equivalent to <code>b &lt;= a</code>.
Note that, in the absence of a "le" metamethod,
Lua tries the "lt", assuming that <code>a &lt;= b</code> is
equivalent to <code>not (b &lt; a)</code>.
</li>
<li><b>"index":</b>
The indexing access <code>table[key]</code>.
<pre>
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(&middot;&middot;&middot;)
end
end
if type(h) == "function" then
return (h(table, key)) -- call the handler
else return h[key] -- or repeat operation on it
end
end
</pre><p>
</li>
<li><b>"newindex":</b>
The indexing assignment <code>table[key] = value</code>.
<pre>
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(&middot;&middot;&middot;)
end
end
if type(h) == "function" then
h(table, key,value) -- call the handler
else h[key] = value -- or repeat operation on it
end
end
</pre><p>
</li>
<li><b>"call":</b>
called when Lua calls a value.
<pre>
function function_event (func, ...)
if type(func) == "function" then
return func(...) -- primitive call
else
local h = metatable(func).__call
if h then
return h(func, ...)
else
error(&middot;&middot;&middot;)
end
end
end
</pre><p>
</li>
</ul>
<h2>2.9 - <a name="2.9">Environments</a></h2>
<p>
Besides metatables,
objects of types thread, function, and userdata
have another table associated with them,
called their <em>environment</em>.
Like metatables, environments are regular tables and
multiple objects can share the same environment.
<p>
Threads are created sharing the environment of the creating thread.
Userdata and C&nbsp;functions are created sharing the environment
of the creating C&nbsp;function.
Non-nested Lua functions
(created by <a href="#pdf-loadfile"><code>loadfile</code></a>, <a href="#pdf-loadstring"><code>loadstring</code></a> or <a href="#pdf-load"><code>load</code></a>)
are created sharing the environment of the creating thread.
Nested Lua functions are created sharing the environment of
the creating Lua function.
<p>
Environments associated with userdata have no meaning for Lua.
It is only a convenience feature for programmers to associate a table to
a userdata.
<p>
Environments associated with threads are called
<em>global environments</em>.
They are used as the default environment for threads and
non-nested Lua functions created by the thread
and can be directly accessed by C&nbsp;code (see <a href="#3.3">&sect;3.3</a>).
<p>
The environment associated with a C&nbsp;function can be directly
accessed by C&nbsp;code (see <a href="#3.3">&sect;3.3</a>).
It is used as the default environment for other C&nbsp;functions
and userdata created by the function.
<p>
Environments associated with Lua functions are used to resolve
all accesses to global variables within the function (see <a href="#2.3">&sect;2.3</a>).
They are used as the default environment for nested Lua functions
created by the function.
<p>
You can change the environment of a Lua function or the
running thread by calling <a href="#pdf-setfenv"><code>setfenv</code></a>.
You can get the environment of a Lua function or the running thread
by calling <a href="#pdf-getfenv"><code>getfenv</code></a>.
To manipulate the environment of other objects
(userdata, C&nbsp;functions, other threads) you must
use the C&nbsp;API.
<h2>2.10 - <a name="2.10">Garbage Collection</a></h2>
<p>
Lua performs automatic memory management.
This means that
you have to worry neither about allocating memory for new objects
nor about freeing it when the objects are no longer needed.
Lua manages memory automatically by running
a <em>garbage collector</em> from time to time
to collect all <em>dead objects</em>
(that is, objects that are no longer accessible from Lua).
All memory used by Lua is subject to automatic management:
tables, userdata, functions, threads, strings, etc.
<p>
Lua implements an incremental mark-and-sweep collector.
It uses two numbers to control its garbage-collection cycles:
the <em>garbage-collector pause</em> and
the <em>garbage-collector step multiplier</em>.
Both use percentage points as units
(so that a value of 100 means an internal value of 1).
<p>
The garbage-collector pause
controls how long the collector waits before starting a new cycle.
Larger values make the collector less aggressive.
Values smaller than 100 mean the collector will not wait to
start a new cycle.
A value of 200 means that the collector waits for the total memory in use
to double before starting a new cycle.
<p>
The step multiplier
controls the relative speed of the collector relative to
memory allocation.
Larger values make the collector more aggressive but also increase
the size of each incremental step.
Values smaller than 100 make the collector too slow and
can result in the collector never finishing a cycle.
The default, 200, means that the collector runs at "twice"
the speed of memory allocation.
<p>
You can change these numbers by calling <a href="#lua_gc"><code>lua_gc</code></a> in C
or <a href="#pdf-collectgarbage"><code>collectgarbage</code></a> in Lua.
With these functions you can also control
the collector directly (e.g., stop and restart it).
<h3>2.10.1 - <a name="2.10.1">Garbage-Collection Metamethods</a></h3>
<p>
Using the C&nbsp;API,
you can set garbage-collector metamethods for userdata (see <a href="#2.8">&sect;2.8</a>).
These metamethods are also called <em>finalizers</em>.
Finalizers allow you to coordinate Lua's garbage collection
with external resource management
(such as closing files, network or database connections,
or freeing your own memory).
<p>
Garbage userdata with a field <code>__gc</code> 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:
<pre>
function gc_event (udata)
local h = metatable(udata).__gc
if h then
h(udata)
end
end
</pre>
<p>
At the end of each garbage-collection cycle,
the finalizers for userdata are called in <em>reverse</em>
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.
The userdata itself is freed only in the next garbage-collection cycle.
<h3>2.10.2 - <a name="2.10.2">Weak Tables</a></h3>
<p>
A <em>weak table</em> is a table whose elements are
<em>weak references</em>.
A weak reference is ignored by the garbage collector.
In other words,
if the only references to an object are weak references,
then the garbage collector will collect this object.
<p>
A weak table can have weak keys, weak values, or both.
A table with weak keys allows the collection of its keys,
but prevents the collection of its values.
A table with both weak keys and weak values allows the collection of
both keys and values.
In any case, if either the key or the value is collected,
the whole pair is removed from the table.
The weakness of a table is controlled by the
<code>__mode</code> field of its metatable.
If the <code>__mode</code> field is a string containing the character&nbsp;'<code>k</code>',
the keys in the table are weak.
If <code>__mode</code> contains '<code>v</code>',
the values in the table are weak.
<p>
After you use a table as a metatable,
you should not change the value of its <code>__mode</code> field.
Otherwise, the weak behavior of the tables controlled by this
metatable is undefined.
<h2>2.11 - <a name="2.11">Coroutines</a></h2>
<p>
Lua supports coroutines,
also called <em>collaborative multithreading</em>.
A coroutine in Lua represents an independent thread of execution.
Unlike threads in multithread systems, however,
a coroutine only suspends its execution by explicitly calling
a yield function.
<p>
You create a coroutine with a call to <a href="#pdf-coroutine.create"><code>coroutine.create</code></a>.
Its sole argument is a function
that is the main function of the coroutine.
The <code>create</code> function only creates a new coroutine and
returns a handle to it (an object of type <em>thread</em>);
it does not start the coroutine execution.
<p>
When you first call <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>,
passing as its first argument
a thread returned by <a href="#pdf-coroutine.create"><code>coroutine.create</code></a>,
the coroutine starts its execution,
at the first line of its main function.
Extra arguments passed to <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> are passed on
to the coroutine main function.
After the coroutine starts running,
it runs until it terminates or <em>yields</em>.
<p>
A coroutine can terminate its execution in two ways:
normally, when its main function returns
(explicitly or implicitly, after the last instruction);
and abnormally, if there is an unprotected error.
In the first case, <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> returns <b>true</b>,
plus any values returned by the coroutine main function.
In case of errors, <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> returns <b>false</b>
plus an error message.
<p>
A coroutine yields by calling <a href="#pdf-coroutine.yield"><code>coroutine.yield</code></a>.
When a coroutine yields,
the corresponding <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> returns immediately,
even if the yield happens inside nested function calls
(that is, not in the main function,
but in a function directly or indirectly called by the main function).
In the case of a yield, <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a> also returns <b>true</b>,
plus any values passed to <a href="#pdf-coroutine.yield"><code>coroutine.yield</code></a>.
The next time you resume the same coroutine,
it continues its execution from the point where it yielded,
with the call to <a href="#pdf-coroutine.yield"><code>coroutine.yield</code></a> returning any extra
arguments passed to <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>.
<p>
Like <a href="#pdf-coroutine.create"><code>coroutine.create</code></a>,
the <a href="#pdf-coroutine.wrap"><code>coroutine.wrap</code></a> function also creates a coroutine,
but instead of returning the coroutine itself,
it returns a function that, when called, resumes the coroutine.
Any arguments passed to this function
go as extra arguments to <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>.
<a href="#pdf-coroutine.wrap"><code>coroutine.wrap</code></a> returns all the values returned by <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>,
except the first one (the boolean error code).
Unlike <a href="#pdf-coroutine.resume"><code>coroutine.resume</code></a>,
<a href="#pdf-coroutine.wrap"><code>coroutine.wrap</code></a> does not catch errors;
any error is propagated to the caller.
<p>
As an example,
consider the following code:
<pre>
function foo (a)
print("foo", a)
return coroutine.yield(2*a)
end
co = coroutine.create(function (a,b)
print("co-body", a, b)
local r = foo(a+1)
print("co-body", r)
local r, s = coroutine.yield(a+b, a-b)
print("co-body", r, s)
return b, "end"
end)
print("main", coroutine.resume(co, 1, 10))
print("main", coroutine.resume(co, "r"))
print("main", coroutine.resume(co, "x", "y"))
print("main", coroutine.resume(co, "x", "y"))
</pre><p>
When you run it, it produces the following output:
<pre>
co-body 1 10
foo 2
main true 4
co-body r
main true 11 -9
co-body x y
main true 10 end
main false cannot resume dead coroutine
</pre>
<h1>3 - <a name="3">The Application Program Interface</a></h1>
<p>
This section describes the C&nbsp;API for Lua, that is,
the set of C&nbsp;functions available to the host program to communicate
with Lua.
All API functions and related types and constants
are declared in the header file <a name="pdf-lua.h"><code>lua.h</code></a>.
<p>
Even when we use the term "function",
any facility in the API may be provided as a macro instead.
All such macros use each of their arguments exactly once
(except for the first argument, which is always a Lua state),
and so do not generate any hidden side-effects.
<p>
As in most C&nbsp;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 <a name="pdf-luai_apicheck"><code>luai_apicheck</code></a>,
in file <code>luaconf.h</code>.
<h2>3.1 - <a name="3.1">The Stack</a></h2>
<p>
Lua uses a <em>virtual stack</em> to pass values to and from C.
Each element in this stack represents a Lua value
(<b>nil</b>, number, string, etc.).
<p>
Whenever Lua calls C, the called function gets a new stack,
which is independent of previous stacks and of stacks of
C&nbsp;functions that are still active.
This stack initially contains any arguments to the C&nbsp;function
and it is where the C&nbsp;function pushes its results
to be returned to the caller (see <a href="#lua_CFunction"><code>lua_CFunction</code></a>).
<p>
For convenience,
most query operations in the API do not follow a strict stack discipline.
Instead, they can refer to any element in the stack
by using an <em>index</em>:
A positive index represents an <em>absolute</em> stack position
(starting at&nbsp;1);
a negative index represents an <em>offset</em> relative to the top of the stack.
More specifically, if the stack has <em>n</em> elements,
then index&nbsp;1 represents the first element
(that is, the element that was pushed onto the stack first)
and
index&nbsp;<em>n</em> represents the last element;
index&nbsp;-1 also represents the last element
(that is, the element at the&nbsp;top)
and index <em>-n</em> represents the first element.
We say that an index is <em>valid</em>
if it lies between&nbsp;1 and the stack top
(that is, if <code>1 &le; abs(index) &le; top</code>).
<h2>3.2 - <a name="3.2">Stack Size</a></h2>
<p>
When you interact with Lua API,
you are responsible for ensuring consistency.
In particular,
<em>you are responsible for controlling stack overflow</em>.
You can use the function <a href="#lua_checkstack"><code>lua_checkstack</code></a>
to grow the stack size.
<p>
Whenever Lua calls C,
it ensures that at least <a name="pdf-LUA_MINSTACK"><code>LUA_MINSTACK</code></a> stack positions are available.
<code>LUA_MINSTACK</code> is defined as 20,
so that usually you do not have to worry about stack space
unless your code has loops pushing elements onto the stack.
<p>
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 <a href="#lua_checkstack"><code>lua_checkstack</code></a>.
Such indices are called <em>acceptable indices</em>.
More formally, we define an <em>acceptable index</em>
as follows:
<pre>
(index &lt; 0 &amp;&amp; abs(index) &lt;= top) ||
(index &gt; 0 &amp;&amp; index &lt;= stackspace)
</pre><p>
Note that 0 is never an acceptable index.
<h2>3.3 - <a name="3.3">Pseudo-Indices</a></h2>
<p>
Unless otherwise noted,
any function that accepts valid indices can also be called with
<em>pseudo-indices</em>,
which represent some Lua values that are accessible to C&nbsp;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&nbsp;function (see <a href="#3.4">&sect;3.4</a>).
<p>
The thread environment (where global variables live) is
always at pseudo-index <a name="pdf-LUA_GLOBALSINDEX"><code>LUA_GLOBALSINDEX</code></a>.
The environment of the running C&nbsp;function is always
at pseudo-index <a name="pdf-LUA_ENVIRONINDEX"><code>LUA_ENVIRONINDEX</code></a>.
<p>
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
<pre>
lua_getfield(L, LUA_GLOBALSINDEX, varname);
</pre>
<h2>3.4 - <a name="3.4">C Closures</a></h2>
<p>
When a C&nbsp;function is created,
it is possible to associate some values with it,
thus creating a <em>C&nbsp;closure</em>;
these values are called <em>upvalues</em> and are
accessible to the function whenever it is called
(see <a href="#lua_pushcclosure"><code>lua_pushcclosure</code></a>).
<p>
Whenever a C&nbsp;function is called,
its upvalues are located at specific pseudo-indices.
These pseudo-indices are produced by the macro
<a name="lua_upvalueindex"><code>lua_upvalueindex</code></a>.
The first value associated with a function is at position
<code>lua_upvalueindex(1)</code>, and so on.
Any access to <code>lua_upvalueindex(<em>n</em>)</code>,
where <em>n</em> is greater than the number of upvalues of the
current function (but not greater than 256),
produces an acceptable (but invalid) index.
<h2>3.5 - <a name="3.5">Registry</a></h2>
<p>
Lua provides a <em>registry</em>,
a pre-defined table that can be used by any C&nbsp;code to
store whatever Lua value it needs to store.
This table is always located at pseudo-index
<a name="pdf-LUA_REGISTRYINDEX"><code>LUA_REGISTRYINDEX</code></a>.
Any C&nbsp;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&nbsp;object in your code.
<p>
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.
<h2>3.6 - <a name="3.6">Error Handling in C</a></h2>
<p>
Internally, Lua uses the C <code>longjmp</code> facility to handle errors.
(You can also choose to use exceptions if you use C++;
see file <code>luaconf.h</code>.)
When Lua faces any error
(such as memory allocation errors, type errors, syntax errors,
and runtime errors)
it <em>raises</em> an error;
that is, it does a long jump.
A <em>protected environment</em> uses <code>setjmp</code>
to set a recover point;
any error jumps to the most recent active recover point.
<p>
Most functions in the API can throw an error,
for instance due to a memory allocation error.
The documentation for each function indicates whether
it can throw errors.
<p>
Inside a C&nbsp;function you can throw an error by calling <a href="#lua_error"><code>lua_error</code></a>.
<h2>3.7 - <a name="3.7">Functions and Types</a></h2>
<p>
Here we list all functions and types from the C&nbsp;API in
alphabetical order.
Each function has an indicator like this:
<span class="apii">[-o, +p, <em>x</em>]</span>
<p>
The first field, <code>o</code>,
is how many elements the function pops from the stack.
The second field, <code>p</code>,
is how many elements the function pushes onto the stack.
(Any function always pushes its results after popping its arguments.)
A field in the form <code>x|y</code> means the function can push (or pop)
<code>x</code> or <code>y</code> elements,
depending on the situation;
an interrogation mark '<code>?</code>' means that
we cannot know how many elements the function pops/pushes
by looking only at its arguments
(e.g., they may depend on what is on the stack).
The third field, <code>x</code>,
tells whether the function may throw errors:
'<code>-</code>' means the function never throws any error;
'<code>m</code>' means the function may throw an error
only due to not enough memory;
'<code>e</code>' means the function may throw other kinds of errors;
'<code>v</code>' means the function may throw an error on purpose.
<hr><h3><a name="lua_Alloc"><code>lua_Alloc</code></a></h3>
<pre>typedef void * (*lua_Alloc) (void *ud,
void *ptr,
size_t osize,
size_t nsize);</pre>
<p>
The type of the memory-allocation function used by Lua states.
The allocator function must provide a
functionality similar to <code>realloc</code>,
but not exactly the same.
Its arguments are
<code>ud</code>, an opaque pointer passed to <a href="#lua_newstate"><code>lua_newstate</code></a>;
<code>ptr</code>, a pointer to the block being allocated/reallocated/freed;
<code>osize</code>, the original size of the block;
<code>nsize</code>, the new size of the block.
<code>ptr</code> is <code>NULL</code> if and only if <code>osize</code> is zero.
When <code>nsize</code> is zero, the allocator must return <code>NULL</code>;
if <code>osize</code> is not zero,
it should free the block pointed to by <code>ptr</code>.
When <code>nsize</code> is not zero, the allocator returns <code>NULL</code>
if and only if it cannot fill the request.
When <code>nsize</code> is not zero and <code>osize</code> is zero,
the allocator should behave like <code>malloc</code>.
When <code>nsize</code> and <code>osize</code> are not zero,
the allocator behaves like <code>realloc</code>.
Lua assumes that the allocator never fails when
<code>osize &gt;= nsize</code>.
<p>
Here is a simple implementation for the allocator function.
It is used in the auxiliary library by <a href="#luaL_newstate"><code>luaL_newstate</code></a>.
<pre>
static void *l_alloc (void *ud, void *ptr, size_t osize,
size_t nsize) {
(void)ud; (void)osize; /* not used */
if (nsize == 0) {
free(ptr);
return NULL;
}
else
return realloc(ptr, nsize);
}
</pre><p>
This code assumes
that <code>free(NULL)</code> has no effect and that
<code>realloc(NULL, size)</code> is equivalent to <code>malloc(size)</code>.
ANSI&nbsp;C ensures both behaviors.
<hr><h3><a name="lua_atpanic"><code>lua_atpanic</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>lua_CFunction lua_atpanic (lua_State *L, lua_CFunction panicf);</pre>
<p>
Sets a new panic function and returns the old one.
<p>
If an error happens outside any protected environment,
Lua calls a <em>panic function</em>
and then calls <code>exit(EXIT_FAILURE)</code>,
thus exiting the host application.
Your panic function can avoid this exit by
never returning (e.g., doing a long jump).
<p>
The panic function can access the error message at the top of the stack.
<hr><h3><a name="lua_call"><code>lua_call</code></a></h3><p>
<span class="apii">[-(nargs + 1), +nresults, <em>e</em>]</span>
<pre>void lua_call (lua_State *L, int nargs, int nresults);</pre>
<p>
Calls a function.
<p>
To call a function you must use the following protocol:
first, the function to be called is pushed onto the stack;
then, the arguments to the function are pushed
in direct order;
that is, the first argument is pushed first.
Finally you call <a href="#lua_call"><code>lua_call</code></a>;
<code>nargs</code> is the number of arguments that you pushed onto the stack.
All arguments and the function value are popped from the stack
when the function is called.
The function results are pushed onto the stack when the function returns.
The number of results is adjusted to <code>nresults</code>,
unless <code>nresults</code> is <a name="pdf-LUA_MULTRET"><code>LUA_MULTRET</code></a>.
In this case, <em>all</em> results from the function are pushed.
Lua takes care that the returned values fit into the stack space.
The function results are pushed onto the stack in direct order
(the first result is pushed first),
so that after the call the last result is on the top of the stack.
<p>
Any error inside the called function is propagated upwards
(with a <code>longjmp</code>).
<p>
The following example shows how the host program can do the
equivalent to this Lua code:
<pre>
a = f("how", t.x, 14)
</pre><p>
Here it is in&nbsp;C:
<pre>
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' */
</pre><p>
Note that the code above is "balanced":
at its end, the stack is back to its original configuration.
This is considered good programming practice.
<hr><h3><a name="lua_CFunction"><code>lua_CFunction</code></a></h3>
<pre>typedef int (*lua_CFunction) (lua_State *L);</pre>
<p>
Type for C&nbsp;functions.
<p>
In order to communicate properly with Lua,
a C&nbsp;function must use the following protocol,
which defines the way parameters and results are passed:
a C&nbsp;function receives its arguments from Lua in its stack
in direct order (the first argument is pushed first).
So, when the function starts,
<code>lua_gettop(L)</code> returns the number of arguments received by the function.
The first argument (if any) is at index 1
and its last argument is at index <code>lua_gettop(L)</code>.
To return values to Lua, a C&nbsp;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&nbsp;function called by Lua can also return
many results.
<p>
As an example, the following function receives a variable number
of numerical arguments and returns their average and sum:
<pre>
static int foo (lua_State *L) {
int n = lua_gettop(L); /* number of arguments */
lua_Number sum = 0;
int i;
for (i = 1; i &lt;= n; i++) {
if (!lua_isnumber(L, i)) {
lua_pushstring(L, "incorrect argument");
lua_error(L);
}
sum += lua_tonumber(L, i);
}
lua_pushnumber(L, sum/n); /* first result */
lua_pushnumber(L, sum); /* second result */
return 2; /* number of results */
}
</pre>
<hr><h3><a name="lua_checkstack"><code>lua_checkstack</code></a></h3><p>
<span class="apii">[-0, +0, <em>m</em>]</span>
<pre>int lua_checkstack (lua_State *L, int extra);</pre>
<p>
Ensures that there are at least <code>extra</code> free stack slots in the stack.
It returns false if it cannot 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.
<hr><h3><a name="lua_close"><code>lua_close</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>void lua_close (lua_State *L);</pre>
<p>
Destroys all objects in the given Lua state
(calling the corresponding garbage-collection metamethods, if any)
and frees all dynamic memory used by this state.
On several platforms, you may not need to call this function,
because all resources are naturally released when the host program ends.
On the other hand, long-running programs,
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.
<hr><h3><a name="lua_concat"><code>lua_concat</code></a></h3><p>
<span class="apii">[-n, +1, <em>e</em>]</span>
<pre>void lua_concat (lua_State *L, int n);</pre>
<p>
Concatenates the <code>n</code> values at the top of the stack,
pops them, and leaves the result at the top.
If <code>n</code>&nbsp;is&nbsp;1, the result is the single value on the stack
(that is, the function does nothing);
if <code>n</code> is 0, the result is the empty string.
Concatenation is performed following the usual semantics of Lua
(see <a href="#2.5.4">&sect;2.5.4</a>).
<hr><h3><a name="lua_cpcall"><code>lua_cpcall</code></a></h3><p>
<span class="apii">[-0, +(0|1), <em>-</em>]</span>
<pre>int lua_cpcall (lua_State *L, lua_CFunction func, void *ud);</pre>
<p>
Calls the C&nbsp;function <code>func</code> in protected mode.
<code>func</code> starts with only one element in its stack,
a light userdata containing <code>ud</code>.
In case of errors,
<a href="#lua_cpcall"><code>lua_cpcall</code></a> returns the same error codes as <a href="#lua_pcall"><code>lua_pcall</code></a>,
plus the error object on the top of the stack;
otherwise, it returns zero, and does not change the stack.
All values returned by <code>func</code> are discarded.
<hr><h3><a name="lua_createtable"><code>lua_createtable</code></a></h3><p>
<span class="apii">[-0, +1, <em>m</em>]</span>
<pre>void lua_createtable (lua_State *L, int narr, int nrec);</pre>
<p>
Creates a new empty table and pushes it onto the stack.
The new table has space pre-allocated
for <code>narr</code> array elements and <code>nrec</code> 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 <a href="#lua_newtable"><code>lua_newtable</code></a>.
<hr><h3><a name="lua_dump"><code>lua_dump</code></a></h3><p>
<span class="apii">[-0, +0, <em>m</em>]</span>
<pre>int lua_dump (lua_State *L, lua_Writer writer, void *data);</pre>
<p>
Dumps a function as a binary chunk.
Receives a Lua function on the top of the stack
and produces a binary chunk that,
if loaded again,
results in a function equivalent to the one dumped.
As it produces parts of the chunk,
<a href="#lua_dump"><code>lua_dump</code></a> calls function <code>writer</code> (see <a href="#lua_Writer"><code>lua_Writer</code></a>)
with the given <code>data</code>
to write them.
<p>
The value returned is the error code returned by the last
call to the writer;
0&nbsp;means no errors.
<p>
This function does not pop the Lua function from the stack.
<hr><h3><a name="lua_equal"><code>lua_equal</code></a></h3><p>
<span class="apii">[-0, +0, <em>e</em>]</span>
<pre>int lua_equal (lua_State *L, int index1, int index2);</pre>
<p>
Returns 1 if the two values in acceptable indices <code>index1</code> and
<code>index2</code> are equal,
following the semantics of the Lua <code>==</code> operator
(that is, may call metamethods).
Otherwise returns&nbsp;0.
Also returns&nbsp;0 if any of the indices is non valid.
<hr><h3><a name="lua_error"><code>lua_error</code></a></h3><p>
<span class="apii">[-1, +0, <em>v</em>]</span>
<pre>int lua_error (lua_State *L);</pre>
<p>
Generates a Lua error.
The error message (which can actually be a Lua value of any type)
must be on the stack top.
This function does a long jump,
and therefore never returns.
(see <a href="#luaL_error"><code>luaL_error</code></a>).
<hr><h3><a name="lua_gc"><code>lua_gc</code></a></h3><p>
<span class="apii">[-0, +0, <em>e</em>]</span>
<pre>int lua_gc (lua_State *L, int what, int data);</pre>
<p>
Controls the garbage collector.
<p>
This function performs several tasks,
according to the value of the parameter <code>what</code>:
<ul>
<li><b><code>LUA_GCSTOP</code>:</b>
stops the garbage collector.
</li>
<li><b><code>LUA_GCRESTART</code>:</b>
restarts the garbage collector.
</li>
<li><b><code>LUA_GCCOLLECT</code>:</b>
performs a full garbage-collection cycle.
</li>
<li><b><code>LUA_GCCOUNT</code>:</b>
returns the current amount of memory (in Kbytes) in use by Lua.
</li>
<li><b><code>LUA_GCCOUNTB</code>:</b>
returns the remainder of dividing the current amount of bytes of
memory in use by Lua by 1024.
</li>
<li><b><code>LUA_GCSTEP</code>:</b>
performs an incremental step of garbage collection.
The step "size" is controlled by <code>data</code>
(larger values mean more steps) in a non-specified way.
If you want to control the step size
you must experimentally tune the value of <code>data</code>.
The function returns 1 if the step finished a
garbage-collection cycle.
</li>
<li><b><code>LUA_GCSETPAUSE</code>:</b>
sets <code>data</code> as the new value
for the <em>pause</em> of the collector (see <a href="#2.10">&sect;2.10</a>).
The function returns the previous value of the pause.
</li>
<li><b><code>LUA_GCSETSTEPMUL</code>:</b>
sets <code>data</code> as the new value for the <em>step multiplier</em> of
the collector (see <a href="#2.10">&sect;2.10</a>).
The function returns the previous value of the step multiplier.
</li>
</ul>
<hr><h3><a name="lua_getallocf"><code>lua_getallocf</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>lua_Alloc lua_getallocf (lua_State *L, void **ud);</pre>
<p>
Returns the memory-allocation function of a given state.
If <code>ud</code> is not <code>NULL</code>, Lua stores in <code>*ud</code> the
opaque pointer passed to <a href="#lua_newstate"><code>lua_newstate</code></a>.
<hr><h3><a name="lua_getfenv"><code>lua_getfenv</code></a></h3><p>
<span class="apii">[-0, +1, <em>-</em>]</span>
<pre>void lua_getfenv (lua_State *L, int index);</pre>
<p>
Pushes onto the stack the environment table of
the value at the given index.
<hr><h3><a name="lua_getfield"><code>lua_getfield</code></a></h3><p>
<span class="apii">[-0, +1, <em>e</em>]</span>
<pre>void lua_getfield (lua_State *L, int index, const char *k);</pre>
<p>
Pushes onto the stack the value <code>t[k]</code>,
where <code>t</code> is the value at the given valid index.
As in Lua, this function may trigger a metamethod
for the "index" event (see <a href="#2.8">&sect;2.8</a>).
<hr><h3><a name="lua_getglobal"><code>lua_getglobal</code></a></h3><p>
<span class="apii">[-0, +1, <em>e</em>]</span>
<pre>void lua_getglobal (lua_State *L, const char *name);</pre>
<p>
Pushes onto the stack the value of the global <code>name</code>.
It is defined as a macro:
<pre>
#define lua_getglobal(L,s) lua_getfield(L, LUA_GLOBALSINDEX, s)
</pre>
<hr><h3><a name="lua_getmetatable"><code>lua_getmetatable</code></a></h3><p>
<span class="apii">[-0, +(0|1), <em>-</em>]</span>
<pre>int lua_getmetatable (lua_State *L, int index);</pre>
<p>
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&nbsp;0 and pushes nothing on the stack.
<hr><h3><a name="lua_gettable"><code>lua_gettable</code></a></h3><p>
<span class="apii">[-1, +1, <em>e</em>]</span>
<pre>void lua_gettable (lua_State *L, int index);</pre>
<p>
Pushes onto the stack the value <code>t[k]</code>,
where <code>t</code> is the value at the given valid index
and <code>k</code> is the value at the top of the stack.
<p>
This function pops the key from the stack
(putting the resulting value in its place).
As in Lua, this function may trigger a metamethod
for the "index" event (see <a href="#2.8">&sect;2.8</a>).
<hr><h3><a name="lua_gettop"><code>lua_gettop</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_gettop (lua_State *L);</pre>
<p>
Returns the index of the top element in the stack.
Because indices start at&nbsp;1,
this result is equal to the number of elements in the stack
(and so 0&nbsp;means an empty stack).
<hr><h3><a name="lua_insert"><code>lua_insert</code></a></h3><p>
<span class="apii">[-1, +1, <em>-</em>]</span>
<pre>void lua_insert (lua_State *L, int index);</pre>
<p>
Moves the top element into the given valid index,
shifting up the elements above this index to open space.
Cannot be called with a pseudo-index,
because a pseudo-index is not an actual stack position.
<hr><h3><a name="lua_Integer"><code>lua_Integer</code></a></h3>
<pre>typedef ptrdiff_t lua_Integer;</pre>
<p>
The type used by the Lua API to represent integral values.
<p>
By default it is a <code>ptrdiff_t</code>,
which is usually the largest signed integral type the machine handles
"comfortably".
<hr><h3><a name="lua_isboolean"><code>lua_isboolean</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_isboolean (lua_State *L, int index);</pre>
<p>
Returns 1 if the value at the given acceptable index has type boolean,
and 0&nbsp;otherwise.
<hr><h3><a name="lua_iscfunction"><code>lua_iscfunction</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_iscfunction (lua_State *L, int index);</pre>
<p>
Returns 1 if the value at the given acceptable index is a C&nbsp;function,
and 0&nbsp;otherwise.
<hr><h3><a name="lua_isfunction"><code>lua_isfunction</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_isfunction (lua_State *L, int index);</pre>
<p>
Returns 1 if the value at the given acceptable index is a function
(either C or Lua), and 0&nbsp;otherwise.
<hr><h3><a name="lua_islightuserdata"><code>lua_islightuserdata</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_islightuserdata (lua_State *L, int index);</pre>
<p>
Returns 1 if the value at the given acceptable index is a light userdata,
and 0&nbsp;otherwise.
<hr><h3><a name="lua_isnil"><code>lua_isnil</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_isnil (lua_State *L, int index);</pre>
<p>
Returns 1 if the value at the given acceptable index is <b>nil</b>,
and 0&nbsp;otherwise.
<hr><h3><a name="lua_isnone"><code>lua_isnone</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_isnone (lua_State *L, int index);</pre>
<p>
Returns 1 if the given acceptable index is not valid
(that is, it refers to an element outside the current stack),
and 0&nbsp;otherwise.
<hr><h3><a name="lua_isnoneornil"><code>lua_isnoneornil</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_isnoneornil (lua_State *L, int index);</pre>
<p>
Returns 1 if 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 <b>nil</b>,
and 0&nbsp;otherwise.
<hr><h3><a name="lua_isnumber"><code>lua_isnumber</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_isnumber (lua_State *L, int index);</pre>
<p>
Returns 1 if the value at the given acceptable index is a number
or a string convertible to a number,
and 0&nbsp;otherwise.
<hr><h3><a name="lua_isstring"><code>lua_isstring</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_isstring (lua_State *L, int index);</pre>
<p>
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&nbsp;otherwise.
<hr><h3><a name="lua_istable"><code>lua_istable</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_istable (lua_State *L, int index);</pre>
<p>
Returns 1 if the value at the given acceptable index is a table,
and 0&nbsp;otherwise.
<hr><h3><a name="lua_isthread"><code>lua_isthread</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_isthread (lua_State *L, int index);</pre>
<p>
Returns 1 if the value at the given acceptable index is a thread,
and 0&nbsp;otherwise.
<hr><h3><a name="lua_isuserdata"><code>lua_isuserdata</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_isuserdata (lua_State *L, int index);</pre>
<p>
Returns 1 if the value at the given acceptable index is a userdata
(either full or light), and 0&nbsp;otherwise.
<hr><h3><a name="lua_lessthan"><code>lua_lessthan</code></a></h3><p>
<span class="apii">[-0, +0, <em>e</em>]</span>
<pre>int lua_lessthan (lua_State *L, int index1, int index2);</pre>
<p>
Returns 1 if the value at acceptable index <code>index1</code> is smaller
than the value at acceptable index <code>index2</code>,
following the semantics of the Lua <code>&lt;</code> operator
(that is, may call metamethods).
Otherwise returns&nbsp;0.
Also returns&nbsp;0 if any of the indices is non valid.
<hr><h3><a name="lua_load"><code>lua_load</code></a></h3><p>
<span class="apii">[-0, +1, <em>-</em>]</span>
<pre>int lua_load (lua_State *L,
lua_Reader reader,
void *data,
const char *chunkname);</pre>
<p>
Loads a Lua chunk.
If there are no errors,
<a href="#lua_load"><code>lua_load</code></a> pushes the compiled chunk as a Lua
function on top of the stack.
Otherwise, it pushes an error message.
The return values of <a href="#lua_load"><code>lua_load</code></a> are:
<ul>
<li><b>0:</b> no errors;</li>
<li><b><a name="pdf-LUA_ERRSYNTAX"><code>LUA_ERRSYNTAX</code></a>:</b>
syntax error during pre-compilation;</li>
<li><b><a href="#pdf-LUA_ERRMEM"><code>LUA_ERRMEM</code></a>:</b>
memory allocation error.</li>
</ul>
<p>
This function only loads a chunk;
it does not run it.
<p>
<a href="#lua_load"><code>lua_load</code></a> automatically detects whether the chunk is text or binary,
and loads it accordingly (see program <code>luac</code>).
<p>
The <a href="#lua_load"><code>lua_load</code></a> function uses a user-supplied <code>reader</code> function
to read the chunk (see <a href="#lua_Reader"><code>lua_Reader</code></a>).
The <code>data</code> argument is an opaque value passed to the reader function.
<p>
The <code>chunkname</code> argument gives a name to the chunk,
which is used for error messages and in debug information (see <a href="#3.8">&sect;3.8</a>).
<hr><h3><a name="lua_newstate"><code>lua_newstate</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>lua_State *lua_newstate (lua_Alloc f, void *ud);</pre>
<p>
Creates a new, independent state.
Returns <code>NULL</code> if cannot create the state
(due to lack of memory).
The argument <code>f</code> is the allocator function;
Lua does all memory allocation for this state through this function.
The second argument, <code>ud</code>, is an opaque pointer that Lua
simply passes to the allocator in every call.
<hr><h3><a name="lua_newtable"><code>lua_newtable</code></a></h3><p>
<span class="apii">[-0, +1, <em>m</em>]</span>
<pre>void lua_newtable (lua_State *L);</pre>
<p>
Creates a new empty table and pushes it onto the stack.
It is equivalent to <code>lua_createtable(L, 0, 0)</code>.
<hr><h3><a name="lua_newthread"><code>lua_newthread</code></a></h3><p>
<span class="apii">[-0, +1, <em>m</em>]</span>
<pre>lua_State *lua_newthread (lua_State *L);</pre>
<p>
Creates a new thread, pushes it on the stack,
and returns a pointer to a <a href="#lua_State"><code>lua_State</code></a> that represents this new thread.
The new state returned by this function shares with the original state
all global objects (such as tables),
but has an independent execution stack.
<p>
There is no explicit function to close or to destroy a thread.
Threads are subject to garbage collection,
like any Lua object.
<hr><h3><a name="lua_newuserdata"><code>lua_newuserdata</code></a></h3><p>
<span class="apii">[-0, +1, <em>m</em>]</span>
<pre>void *lua_newuserdata (lua_State *L, size_t size);</pre>
<p>
This function allocates a new block of memory with the given size,
pushes onto the stack a new full userdata with the block address,
and returns this address.
<p>
Userdata represent C&nbsp;values in Lua.
A <em>full userdata</em> 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).
<p>
When Lua collects a full userdata with a <code>gc</code> metamethod,
Lua calls the metamethod and marks the userdata as finalized.
When this userdata is collected again then
Lua frees its corresponding memory.
<hr><h3><a name="lua_next"><code>lua_next</code></a></h3><p>
<span class="apii">[-1, +(2|0), <em>e</em>]</span>
<pre>int lua_next (lua_State *L, int index);</pre>
<p>
Pops a key from the stack,
and pushes a key-value pair from the table at the given index
(the "next" pair after the given key).
If there are no more elements in the table,
then <a href="#lua_next"><code>lua_next</code></a> returns 0 (and pushes nothing).
<p>
A typical traversal looks like this:
<pre>
/* table is in the stack at index 't' */
lua_pushnil(L); /* first key */
while (lua_next(L, t) != 0) {
/* uses 'key' (at index -2) and 'value' (at index -1) */
printf("%s - %s\n",
lua_typename(L, lua_type(L, -2)),
lua_typename(L, lua_type(L, -1)));
/* removes 'value'; keeps 'key' for next iteration */
lua_pop(L, 1);
}
</pre>
<p>
While traversing a table,
do not call <a href="#lua_tolstring"><code>lua_tolstring</code></a> directly on a key,
unless you know that the key is actually a string.
Recall that <a href="#lua_tolstring"><code>lua_tolstring</code></a> <em>changes</em>
the value at the given index;
this confuses the next call to <a href="#lua_next"><code>lua_next</code></a>.
<hr><h3><a name="lua_Number"><code>lua_Number</code></a></h3>
<pre>typedef double lua_Number;</pre>
<p>
The type of numbers in Lua.
By default, it is double, but that can be changed in <code>luaconf.h</code>.
<p>
Through the configuration file you can change
Lua to operate with another type for numbers (e.g., float or long).
<hr><h3><a name="lua_objlen"><code>lua_objlen</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>size_t lua_objlen (lua_State *L, int index);</pre>
<p>
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 ('<code>#</code>');
for userdata, this is the size of the block of memory allocated
for the userdata;
for other values, it is&nbsp;0.
<hr><h3><a name="lua_pcall"><code>lua_pcall</code></a></h3><p>
<span class="apii">[-(nargs + 1), +(nresults|1), <em>-</em>]</span>
<pre>int lua_pcall (lua_State *L, int nargs, int nresults, int errfunc);</pre>
<p>
Calls a function in protected mode.
<p>
Both <code>nargs</code> and <code>nresults</code> have the same meaning as
in <a href="#lua_call"><code>lua_call</code></a>.
If there are no errors during the call,
<a href="#lua_pcall"><code>lua_pcall</code></a> behaves exactly like <a href="#lua_call"><code>lua_call</code></a>.
However, if there is any error,
<a href="#lua_pcall"><code>lua_pcall</code></a> catches it,
pushes a single value on the stack (the error message),
and returns an error code.
Like <a href="#lua_call"><code>lua_call</code></a>,
<a href="#lua_pcall"><code>lua_pcall</code></a> always removes the function
and its arguments from the stack.
<p>
If <code>errfunc</code> is 0,
then the error message returned on the stack
is exactly the original error message.
Otherwise, <code>errfunc</code> is the stack index of an
<em>error handler function</em>.
(In the current implementation, this index cannot be a pseudo-index.)
In case of runtime errors,
this function will be called with the error message
and its return value will be the message returned on the stack by <a href="#lua_pcall"><code>lua_pcall</code></a>.
<p>
Typically, the 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 <a href="#lua_pcall"><code>lua_pcall</code></a>,
since by then the stack has unwound.
<p>
The <a href="#lua_pcall"><code>lua_pcall</code></a> function returns 0 in case of success
or one of the following error codes
(defined in <code>lua.h</code>):
<ul>
<li><b><a name="pdf-LUA_ERRRUN"><code>LUA_ERRRUN</code></a>:</b>
a runtime error.
</li>
<li><b><a name="pdf-LUA_ERRMEM"><code>LUA_ERRMEM</code></a>:</b>
memory allocation error.
For such errors, Lua does not call the error handler function.
</li>
<li><b><a name="pdf-LUA_ERRERR"><code>LUA_ERRERR</code></a>:</b>
error while running the error handler function.
</li>
</ul>
<hr><h3><a name="lua_pop"><code>lua_pop</code></a></h3><p>
<span class="apii">[-n, +0, <em>-</em>]</span>
<pre>void lua_pop (lua_State *L, int n);</pre>
<p>
Pops <code>n</code> elements from the stack.
<hr><h3><a name="lua_pushboolean"><code>lua_pushboolean</code></a></h3><p>
<span class="apii">[-0, +1, <em>-</em>]</span>
<pre>void lua_pushboolean (lua_State *L, int b);</pre>
<p>
Pushes a boolean value with value <code>b</code> onto the stack.
<hr><h3><a name="lua_pushcclosure"><code>lua_pushcclosure</code></a></h3><p>
<span class="apii">[-n, +1, <em>m</em>]</span>
<pre>void lua_pushcclosure (lua_State *L, lua_CFunction fn, int n);</pre>
<p>
Pushes a new C&nbsp;closure onto the stack.
<p>
When a C&nbsp;function is created,
it is possible to associate some values with it,
thus creating a C&nbsp;closure (see <a href="#3.4">&sect;3.4</a>);
these values are then accessible to the function whenever it is called.
To associate values with a C&nbsp;function,
first these values should be pushed onto the stack
(when there are multiple values, the first value is pushed first).
Then <a href="#lua_pushcclosure"><code>lua_pushcclosure</code></a>
is called to create and push the C&nbsp;function onto the stack,
with the argument <code>n</code> telling how many values should be
associated with the function.
<a href="#lua_pushcclosure"><code>lua_pushcclosure</code></a> also pops these values from the stack.
<p>
The maximum value for <code>n</code> is 255.
<hr><h3><a name="lua_pushcfunction"><code>lua_pushcfunction</code></a></h3><p>
<span class="apii">[-0, +1, <em>m</em>]</span>
<pre>void lua_pushcfunction (lua_State *L, lua_CFunction f);</pre>
<p>
Pushes a C&nbsp;function onto the stack.
This function receives a pointer to a C function
and pushes onto the stack a Lua value of type <code>function</code> that,
when called, invokes the corresponding C&nbsp;function.
<p>
Any function to be registered in Lua must
follow the correct protocol to receive its parameters
and return its results (see <a href="#lua_CFunction"><code>lua_CFunction</code></a>).
<p>
<code>lua_pushcfunction</code> is defined as a macro:
<pre>
#define lua_pushcfunction(L,f) lua_pushcclosure(L,f,0)
</pre>
<hr><h3><a name="lua_pushfstring"><code>lua_pushfstring</code></a></h3><p>
<span class="apii">[-0, +1, <em>m</em>]</span>
<pre>const char *lua_pushfstring (lua_State *L, const char *fmt, ...);</pre>
<p>
Pushes onto the stack a formatted string
and returns a pointer to this string.
It is similar to the C&nbsp;function <code>sprintf</code>,
but has some important differences:
<ul>
<li>
You do not have to allocate space for the result:
the result is a Lua string and Lua takes care of memory allocation
(and deallocation, through garbage collection).
</li>
<li>
The conversion specifiers are quite restricted.
There are no flags, widths, or precisions.
The conversion specifiers can only be
'<code>%%</code>' (inserts a '<code>%</code>' in the string),
'<code>%s</code>' (inserts a zero-terminated string, with no size restrictions),
'<code>%f</code>' (inserts a <a href="#lua_Number"><code>lua_Number</code></a>),
'<code>%p</code>' (inserts a pointer as a hexadecimal numeral),
'<code>%d</code>' (inserts an <code>int</code>), and
'<code>%c</code>' (inserts an <code>int</code> as a character).
</li>
</ul>
<hr><h3><a name="lua_pushinteger"><code>lua_pushinteger</code></a></h3><p>
<span class="apii">[-0, +1, <em>-</em>]</span>
<pre>void lua_pushinteger (lua_State *L, lua_Integer n);</pre>
<p>
Pushes a number with value <code>n</code> onto the stack.
<hr><h3><a name="lua_pushlightuserdata"><code>lua_pushlightuserdata</code></a></h3><p>
<span class="apii">[-0, +1, <em>-</em>]</span>
<pre>void lua_pushlightuserdata (lua_State *L, void *p);</pre>
<p>
Pushes a light userdata onto the stack.
<p>
Userdata represent C&nbsp;values in Lua.
A <em>light userdata</em> 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&nbsp;address.
<hr><h3><a name="lua_pushliteral"><code>lua_pushliteral</code></a></h3><p>
<span class="apii">[-0, +1, <em>m</em>]</span>
<pre>void lua_pushliteral (lua_State *L, const char *s);</pre>
<p>
This macro is equivalent to <a href="#lua_pushlstring"><code>lua_pushlstring</code></a>,
but can be used only when <code>s</code> is a literal string.
In these cases, it automatically provides the string length.
<hr><h3><a name="lua_pushlstring"><code>lua_pushlstring</code></a></h3><p>
<span class="apii">[-0, +1, <em>m</em>]</span>
<pre>void lua_pushlstring (lua_State *L, const char *s, size_t len);</pre>
<p>
Pushes the string pointed to by <code>s</code> with size <code>len</code>
onto the stack.
Lua makes (or reuses) an internal copy of the given string,
so the memory at <code>s</code> can be freed or reused immediately after
the function returns.
The string can contain embedded zeros.
<hr><h3><a name="lua_pushnil"><code>lua_pushnil</code></a></h3><p>
<span class="apii">[-0, +1, <em>-</em>]</span>
<pre>void lua_pushnil (lua_State *L);</pre>
<p>
Pushes a nil value onto the stack.
<hr><h3><a name="lua_pushnumber"><code>lua_pushnumber</code></a></h3><p>
<span class="apii">[-0, +1, <em>-</em>]</span>
<pre>void lua_pushnumber (lua_State *L, lua_Number n);</pre>
<p>
Pushes a number with value <code>n</code> onto the stack.
<hr><h3><a name="lua_pushstring"><code>lua_pushstring</code></a></h3><p>
<span class="apii">[-0, +1, <em>m</em>]</span>
<pre>void lua_pushstring (lua_State *L, const char *s);</pre>
<p>
Pushes the zero-terminated string pointed to by <code>s</code>
onto the stack.
Lua makes (or reuses) an internal copy of the given string,
so the memory at <code>s</code> can be freed or reused immediately after
the function returns.
The string cannot contain embedded zeros;
it is assumed to end at the first zero.
<hr><h3><a name="lua_pushthread"><code>lua_pushthread</code></a></h3><p>
<span class="apii">[-0, +1, <em>-</em>]</span>
<pre>int lua_pushthread (lua_State *L);</pre>
<p>
Pushes the thread represented by <code>L</code> onto the stack.
Returns 1 if this thread is the main thread of its state.
<hr><h3><a name="lua_pushvalue"><code>lua_pushvalue</code></a></h3><p>
<span class="apii">[-0, +1, <em>-</em>]</span>
<pre>void lua_pushvalue (lua_State *L, int index);</pre>
<p>
Pushes a copy of the element at the given valid index
onto the stack.
<hr><h3><a name="lua_pushvfstring"><code>lua_pushvfstring</code></a></h3><p>
<span class="apii">[-0, +1, <em>m</em>]</span>
<pre>const char *lua_pushvfstring (lua_State *L,
const char *fmt,
va_list argp);</pre>
<p>
Equivalent to <a href="#lua_pushfstring"><code>lua_pushfstring</code></a>, except that it receives a <code>va_list</code>
instead of a variable number of arguments.
<hr><h3><a name="lua_rawequal"><code>lua_rawequal</code></a></h3><p>
<span class="apii">[-0, +0, <em>-</em>]</span>
<pre>int lua_rawequal (lua_State *L, int index1, int index2);</pre>
<p>
Returns 1 if the two values in acceptable indices <code>index1</code> and
<code>index2</code> are primitively equal
(that is, without calling metamethods).
Otherwise returns&nbsp;0.
Also returns&nbsp;0 if any of the indices are non valid.
<hr><h3><a name="lua_rawget"><code>lua_rawget</code></a></h3><p>
<span class="apii">[-1, +1, <em>-</em>]</span>
<pre>void lua_rawget (lua_State *L, int index);</pre>
<p>
Similar to <a href="#lua_gettable"><code>lua_gettable</code></a>, but does a raw access
(i.e., without metamethods).
<hr><h3><a name="lua_rawgeti"><code>lua_rawgeti</code></a></h3><p>
<span class="apii">[-0, +1, <em>-</em>]</span>
<pre>void lua_rawgeti (lua_State *L, int index, int n);</pre>
<p>
Pushes onto the stack the value <code>t[n]</code>,
where <code>t</code> is the value at the given valid index.