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\begin{document}
%{===============================================================
\thispagestyle{empty}
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{
\parindent=0pt
\vglue1.5in
{\LARGE\bf
The Programming Language Lua}
\hfill
\vskip4pt \hrule height 4pt width \hsize \vskip4pt
\hfill
Reference Manual for Lua version \Version
\\
\null
\hfill
Last revised on \today
\\
\vfill
\centering
\includegraphics[width=0.7\textwidth]{nolabel.ps}
\vfill
\vskip4pt \hrule height 2pt width \hsize
}
\newpage
\begin{quotation}
\parskip=10pt
\footnotesize
\null\vfill
\noindent
Copyright \copyright\ 1994--2001 TeCGraf, PUC-Rio. All rights reserved.
\noindent
Permission is hereby granted, without written agreement and without license
or royalty fees, to use, copy, modify, translate, and distribute
this software and its documentation (hereby called the "package")
for any purpose, including commercial applications, subject to
the following conditions:
\begin{itemize}
\item The above copyright notice and this permission notice shall appear in all
copies or substantial portions of this package.
\item The origin of this package must not be misrepresented; you must not
claim that you wrote the original package. If you use this package in a
product, an acknowledgment in the product documentation would be greatly
appreciated (but it is not required).
\item Altered source versions must be plainly marked as such, and must not be
misrepresented as being the original package.
\end{itemize}
The authors specifically disclaim any warranties, including, but not limited
to, the implied warranties of merchantability and fitness for a particular
purpose. The package provided hereunder is on an ``as is'' basis, and the
authors have no obligation to provide maintenance, support, updates,
enhancements, or modifications. In no event shall TeCGraf, PUC-Rio, or the
authors be held liable to any party for direct, indirect, special,
incidental, or consequential damages arising out of the use of this package
and its documentation.
\noindent
The Lua language and this implementation have been entirely designed and
written by Waldemar Celes, Roberto Ierusalimschy, and Luiz Henrique de
Figueiredo at TeCGraf, PUC-Rio in Brazil.
\noindent
This implementation contains no third-party code.
\noindent
Copies of this manual can be obtained at
\verb|http://www.lua.org|.
\bigskip
\noindent
The Lua logo was designed by A. Nakonechny.
Copyright \copyright\ 1998. All rights reserved.
\end{quotation}
%}===============================================================
\newpage
\title{\Large\bf Reference Manual of the Programming Language Lua \Version}
\author{%
Roberto Ierusalimschy\quad
Luiz Henrique de Figueiredo\quad
Waldemar Celes
\vspace{1.0ex}\\
\smallskip
\small\tt lua@tecgraf.puc-rio.br
\vspace{2.0ex}\\
%MCC 08/95 ---
\tecgraf\ --- Computer Science Department --- PUC-Rio
}
\date{{\small \tt\$Date: 2001/07/19 13:36:18 $ $}}
\maketitle
\pagestyle{plain}
\pagenumbering{roman}
\begin{abstract}
\noindent
Lua is a powerful, light-weight programming language
designed for extending applications.
Lua is also frequently used as a general-purpose, stand-alone language.
Lua combines simple procedural syntax
(similar to Pascal)
with
powerful data description constructs
based on associative arrays and extensible semantics.
Lua is
dynamically typed,
interpreted from bytecodes,
and has automatic memory management with garbage collection,
making it ideal for
configuration,
scripting,
and
rapid prototyping.
This document describes version \Version\ of the Lua programming language
and the Application Program Interface (API)
that allows interaction between Lua programs and their host C~programs.
\end{abstract}
\def\abstractname{Resumo}
\begin{abstract}
\noindent
Lua \'e uma linguagem de programa\c{c}\~ao
poderosa e leve,
projetada para estender aplica\c{c}\~oes.
Lua tamb\'em \'e frequentemente usada como uma linguagem de prop\'osito geral.
Lua combina programa\c{c}\~ao procedural
(com sintaxe semelhante \`a de Pascal)
com
poderosas constru\c{c}\~oes para descri\c{c}\~ao de dados,
baseadas em tabelas associativas e sem\^antica extens\'\i vel.
Lua \'e
tipada dinamicamente,
interpretada a partir de \emph{bytecodes},
e tem gerenciamento autom\'atico de mem\'oria com coleta de lixo.
Essas caracter\'{\i}sticas fazem de Lua uma linguagem ideal para
configura\c{c}\~ao,
automa\c{c}\~ao (\emph{scripting})
e prototipagem r\'apida.
Este documento descreve a vers\~ao \Version\ da linguagem de
programa\c{c}\~ao Lua e a Interface de Programa\c{c}\~ao (API) que permite
a intera\c{c}\~ao entre programas Lua e programas C~hospedeiros.
\end{abstract}
\newpage
\null
\newpage
\tableofcontents
\newpage
\setcounter{page}{1}
\pagestyle{plain}
\pagenumbering{arabic}
\section{Introduction}
Lua is an extension programming language designed to support
general procedural programming with data description
facilities.
Lua is intended to be used as a powerful, light-weight
configuration language for any program that needs one.
Lua is implemented as a library, written in C.
Being an extension language, Lua has no notion of a ``main'' program:
it only works \emph{embedded} in a host client,
called the \emph{embedding} program.
This host program can invoke functions to execute a piece of
code in Lua, can write and read Lua variables,
and can register C~functions to be called by Lua code.
Through the use of C~functions, Lua can be augmented to cope with
a wide range of different domains,
thus creating customized programming languages sharing a syntactical framework.
Lua is free-distribution software,
and is provided as usual with no guarantees,
as stated in its copyright notice.
The implementation described in this manual is available
at the following URL's:
\begin{verbatim}
http://www.lua.org
ftp://ftp.lua.org
\end{verbatim}
Like any other reference manual,
this document is dry in places.
For a discussion of the decisions behind the design of Lua,
see the papers below,
which are available at the web site above.
\begin{itemize}
\item
R.~Ierusalimschy, L.~H.~de Figueiredo, and W.~Celes.
Lua---an extensible extension language.
\emph{Software: Practice \& Experience} {\bf 26} \#6 (1996) 635--652.
\item
L.~H.~de Figueiredo, R.~Ierusalimschy, and W.~Celes.
The design and implementation of a language for extending applications.
\emph{Proceedings of XXI Brazilian Seminar on Software and Hardware} (1994) 273--283.
\item
L.~H.~de Figueiredo, R.~Ierusalimschy, and W.~Celes.
Lua: an extensible embedded language.
\emph{Dr. Dobb's Journal} {\bf 21} \#12 (Dec 1996) 26--33.
\end{itemize}
\section{Environment and Chunks}
All statements in Lua are executed in a \Def{global environment}.
This environment is initialized with a call from the embedding program to
\verb|lua_open| and
persists until a call to \verb|lua_close|,
or the end of the embedding program.
If necessary,
the host programmer can create multiple independent global
environments, and freely switch between them \see{mangstate}.
The global environment can be manipulated by Lua code or
by the embedding program,
which can read and write global variables
using API functions from the library that implements Lua.
\Index{Global variables} in Lua do not need to be declared.
Any variable is assumed to be global unless explicitly declared local
\see{localvar}.
Before the first assignment, the value of a global variable is \nil\ %
(this default can be changed; see \See{tag-method}).
A table is used to keep all global names and values
(tables are explained in \See{TypesSec}).
The unit of execution of Lua is called a \Def{chunk}.
A chunk is simply a sequence of statements,
which are executed sequentially.
Each statement can be optionally followed by a semicolon:
\begin{Produc}
\produc{chunk}{\rep{stat \opt{\ter{;}}}}
\end{Produc}%
Statements are described in \See{stats}.
(The notation above is the usual extended BNF,
in which
\rep{\emph{a}} means 0 or more \emph{a}'s,
\opt{\emph{a}} means an optional \emph{a}, and
\oneormore{\emph{a}} means one or more \emph{a}'s.
The complete syntax of Lua is given on page~\pageref{BNF}.)
A chunk may be stored in a file or in a string inside the host program.
When a chunk is executed, first it is pre-compiled into bytecodes for
a virtual machine, and then the statements are executed in sequential order,
by simulating the virtual machine.
All modifications a chunk effects on the global environment persist
after the chunk ends.
Chunks may also be pre-compiled into binary form and stored in files;
see program \IndexVerb{luac} for details.
Text files with chunks and their binary pre-compiled forms
are interchangeable.
Lua automatically detects the file type and acts accordingly.
\index{pre-compilation}
\section{\Index{Types and Tags}} \label{TypesSec}
Lua is a \emph{dynamically typed language}.
This means that
variables do not have types; only values do.
Therefore, there are no type definitions in the language.
All values carry their own type.
Besides a type, all values also have a \IndexEmph{tag}.
There are six \Index{basic types} in Lua: \Def{nil}, \Def{number},
\Def{string}, \Def{function}, \Def{userdata}, and \Def{table}.
\emph{Nil} is the type of the value \nil,
whose main property is to be different from any other value.
\emph{Number} represents real (double-precision floating-point) numbers,
while \emph{string} has the usual meaning.
\index{eight-bit clean}
Lua is 8-bit clean,
and so strings may contain any 8-bit character,
including embedded zeros (\verb|'\0'|) \see{lexical}.
The \verb|type| function returns a string describing the type
of a given value \see{pdf-type}.
Functions are considered \emph{first-class values} in Lua.
This means that functions can be stored in variables,
passed as arguments to other functions, and returned as results.
Lua can call (and manipulate) functions written in Lua and
functions written in C.
The type \emph{userdata} is provided to allow
arbitrary \Index{C~pointers} to be stored in Lua variables.
This type corresponds to a \verb|void*|
and has no pre-defined operations in Lua,
except assignment and equality test.
However, by using \emph{tag methods},
the programmer can define operations for \emph{userdata} values
\see{tag-method}.
The type \emph{table} implements \Index{associative arrays},
that is, \Index{arrays} that can be indexed not only with numbers,
but with any value (except \nil).
Therefore, this type may be used not only to represent ordinary arrays,
but also symbol tables, sets, records, graphs, trees, etc.
Tables are the main data structuring mechanism in Lua.
To represent \Index{records}, Lua uses the field name as an index.
The language supports this representation by
providing \verb|a.name| as syntactic sugar for \verb|a["name"]|.
Tables may also carry \emph{methods}:
Because functions are first class values,
table fields may contain functions.
The form \verb|t:f(x)| is syntactic sugar for \verb|t.f(t,x)|,
which calls the method \verb|f| from the table \verb|t| passing
the table itself as the first parameter \see{func-def}.
Note that tables are \emph{objects}, and not values.
Variables do not contain tables, only \emph{references} to them.
Assignment, parameter passing, and returns always manipulate references
to tables, and do not imply any kind of copy.
Moreover, tables must be explicitly created before used
\see{tableconstructor}.
\subsection{Tags}
Each type has a \emph{name},
and a numerical identifier,
called a \Index{tag}.
Tags are mainly used by C code,
to avoid the manipulation of strings.
Most operations over types, in the C API,
require a tag to identify the type.
In Lua, all operations over types work
both with type names or tags.
\subsection{User-defined Types}
Lua programs can create new types,
called \Index{User-defined Types}.
A user-defined type is always based on a base type,
either a table or a userdata.
Objects of an extended type have an internal structure
identical to the corresponding base type,
but may have diferent semantics for each operation.
The \verb|newtype| function creates a new type \see{pdf-newtype}.
Types created by Lua programs are always based upon tables;
types created by C can be based upon tables or upon userdata.
The \verb|settagmethod| function defines new semantics for
the operations of this new type \see{tag-method}.
The \verb|settype| function changes the type of a given object
\see{pdf-settype}.
\section{Garbage Collection}\label{GC}
Lua does automatic memory management.
To do that,
Lua runs a \Index{garbage collector} from time to time.
All objects in Lua are subjected to automatic management:
tables, userdata, functions, and strings.
Lua uses two numbers to control its garbage-collection cycles.
One number counts how many bytes of dynamic memory Lua is using,
and the other is a threshold.
When the number of bytes crosses the threshold,
Lua runs the garbage collector,
which reclaims the memory of all ``dead'' objects
(that is, objects no longer accessible from Lua).
The byte counter is corrected,
and then the threshold is reset to twice the value of the byte counter.
Through the C API, you can consult those numbers,
and change the threshold \see{GC-API}.
Setting the threshold to zero actually forces an immediate
garbage-collection cycle,
while setting it to a huge number stops the garbage collector.
Using Lua code you have a more limited control of memory management,
through functions \verb|gcinfo| and \verb|collectgarbage|.
You can set garbage-collector tag methods for user-defined
types based on userdata \see{tag-method}.
Lua calls those functions when it is about to free a userdata
of the corresponding type.
Using this facility, you can coordinate Lua's garbage collection
with external resourse management
(such as closing files or freeing your own memory).
\subsection{Weak Tables}\label{weak-table}
A \IndexEmph{weak table} is a table whose elements are
\IndexEmph{weak references}.
A weak reference is ignored by the garbage collector,
so that if the only references to an object are weak references,
the garbage collector will collect that object.
A weak table can have weak keys, weak values, or both.
A table with weak keys allows the collection of its keys,
but avoids the collection of its values.
A table with both weak keys and weak values allow the collection of both.
In any case, if either the key or the value is collected,
the whole pair is removed from the table.
The weakness of a table is controled by the
function \verb|weakmode| \see{weakmode}.
\section{The Language}
This section describes the lexis, the syntax, and the semantics of Lua.
\subsection{Lexical Conventions} \label{lexical}
\IndexEmph{Identifiers} in Lua can be any string of letters,
digits, and underscores,
not beginning with a digit.
This coincides with the definition of identifiers in most languages,
except that
the definition of letter depends on the current locale:
Any character considered alphabetic by the current locale
can be used in an identifier.
The following words are \emph{reserved},
and cannot be used as identifiers:
\index{reserved words}
\begin{verbatim}
and break do else elseif
end for function global if
in local nil not or
repeat return then until while
\end{verbatim}
(\rwd{global} is reserved for future use.)
Lua is a case-sensitive language:
\T{and} is a reserved word, but \T{And} and \T{\'and}
(if the locale permits) are two different, valid identifiers.
As a convention, identifiers starting with underscore followed by
uppercase letters (such as \verb|_INPUT|)
are reserved for internal variables.
The following strings denote other \Index{tokens}:
\begin{verbatim}
+ - * / ^ %
~= <= >= < > == =
( ) { } [ ]
; : , . .. ...
\end{verbatim}
\IndexEmph{Literal strings}
can be delimited by matching single or double quotes,
and can contain the C-like escape sequences
`\verb|\a|' (bell),
`\verb|\b|' (backspace),
`\verb|\f|' (form feed),
`\verb|\n|' (newline),
`\verb|\r|' (carriage return),
`\verb|\t|' (horizontal tab),
`\verb|\v|' (vertical tab),
`\verb|\\|' (backslash),
`\verb|\"|' (double quote),
`\verb|\'|' (single quote),
and `\verb|\|\emph{newline}' (that is, a backslash followed by a real newline,
which results in a newline in the string).
A character in a string may also be specified by its numerical value,
through the escape sequence `\verb|\|\emph{ddd}',
where \emph{ddd} is a sequence of up to three \emph{decimal} digits.
Strings in Lua may contain any 8-bit value, including embedded zeros,
which can be specified as `\verb|\000|'.
Literal strings can also be delimited by matching \verb|[[| \dots\ \verb|]]|.
Literals in this bracketed form may run for several lines,
may contain nested \verb|[[| \dots\ \verb|]]| pairs,
and do not interpret escape sequences.
When the \verb|[[| is immediatly followed by a newline,
this newline is not included in the string.
This form is specially convenient for
writing strings that contain program pieces or
other quoted strings.
As an example, in a system using ASCII,
the following three literals are equivalent:
\begin{verbatim}
1) "alo\n123\""
2) '\97lo\10\04923"'
3) [[alo
123"]]
4) [[
alo
123"]]
\end{verbatim}
\IndexEmph{Comments} start anywhere outside a string with a
double hyphen (\verb|--|) and run until the end of the line.
Moreover,
the first line of a chunk is skipped if it starts with \verb|#|.
This facility allows the use of Lua as a script interpreter
in Unix systems \see{lua-sa}.
\IndexEmph{Numerical constants} may be written with an optional decimal part
and an optional decimal exponent.
Examples of valid numerical constants are
\begin{verbatim}
3 3.0 3.1416 314.16e-2 0.31416E1
\end{verbatim}
\subsection{\Index{Coercion}} \label{coercion}
Lua provides some automatic conversions between values at run time.
Any arithmetic operation applied to a string tries to convert
that string to a number, following the usual rules.
Conversely, whenever a number is used when a string is expected,
that number is converted to a string, in a reasonable format.
The format is chosen so that
a conversion from number to string then back to number
reproduces the original number \emph{exactly}.
Thus,
the conversion does not necessarily produces nice-looking text for some numbers.
For complete control of how numbers are converted to strings,
use the \verb|format| function \see{format}.
\subsection{Statements}\label{stats}
Lua supports an almost conventional set of \Index{statements},
similar to those in Pascal or C.
The conventional commands include
assignment, control structures, and procedure calls.
Non-conventional commands include table constructors
\see{tableconstructor}
and local variable declarations \see{localvar}.
\subsubsection{Blocks}
A \Index{block} is a list of statements;
syntactically, a block is equal to a chunk:
\begin{Produc}
\produc{block}{chunk}
\end{Produc}%
A block may be explicitly delimited:
\begin{Produc}
\produc{stat}{\rwd{do} block \rwd{end}}
\end{Produc}%
Explicit blocks are useful
to control the scope of local variables \see{localvar}.
Explicit blocks are also sometimes used to
add a \rwd{return} or \rwd{break} statement in the middle
of another block \see{control}.
\subsubsection{\Index{Assignment}} \label{assignment}
Lua allows \Index{multiple assignment}.
Therefore, the syntax for assignment
defines a list of variables on the left side
and a list of expressions on the right side.
The elements in both lists are separated by commas:
\begin{Produc}
\produc{stat}{varlist1 \ter{=} explist1}
\produc{varlist1}{var \rep{\ter{,} var}}
\end{Produc}%
This statement first evaluates all values on the right side
and eventual indices on the left side,
and then makes the assignments.
So, the code
\begin{verbatim}
i = 3
i, a[i] = 4, 20
\end{verbatim}
sets \verb|a[3]| to 20, but does not affect \verb|a[4]|
because the \verb|i| in \verb|a[i]| is evaluated
before it is assigned \verb|4|.
Multiple assignment can be used to exchange two values, as in
\begin{verbatim}
x, y = y, x
\end{verbatim}
Before the assignment, the list of values is adjusted to
the length of the list of variables.
If there are more values than are needed,
the excess values are thrown away.
If there are less values than are needed,
the list is extended with as many \nil's as needed.
If the list of expressions (\M{explist1}) ends with a function call,
all values returned by the function call enter in the list of values,
before the adjust.
A single name can denote a global variable, a local variable,
or a formal parameter:
\begin{Produc}
\produc{var}{name}
\end{Produc}%
Square brackets are used to index a table:
\begin{Produc}
\produc{var}{exp \ter{[} exp \ter{]}}
\end{Produc}%
The first expression (\M{exp}) should result in a table value,
from where the field indexed by the expression \M{exp}
value gets the assigned value.
The syntax \verb|var.NAME| is just syntactic sugar for
\verb|var["NAME"]|:
\begin{Produc}
\produc{var}{exp \ter{.} name}
\end{Produc}%
The meaning of assignments and evaluations of global variables and
indexed variables can be changed by tag methods \see{tag-method}.
Actually,
an assignment \verb|x = val|, where \verb|x| is a global variable,
is equivalent to a call \verb|setglobal("x", val)| and
an assignment \verb|t[i] = val| is equivalent to
\verb|settable_event(t,i,val)|.
See \See{tag-method} for a complete description of these functions
(\verb|setglobal| is in the basic library;
\T{settable\_event} is used for explanatory purposes only).
\subsubsection{Control Structures}\label{control}
The control structures
\rwd{if}, \rwd{while}, and \rwd{repeat} have the usual meaning and
familiar syntax
%(there is also a \rwd{for} statement; see \See{for}):
\index{while-do statement}
\index{repeat-until statement}
\index{if-then-else statement}
\begin{Produc}
\produc{stat}{\rwd{while} exp \rwd{do} block \rwd{end}}
\produc{stat}{\rwd{repeat} block \rwd{until} exp}
\produc{stat}{\rwd{if} exp \rwd{then} block
\rep{\rwd{elseif} exp \rwd{then} block}
\opt{\rwd{else} block} \rwd{end}}
\end{Produc}%
The \Index{condition expression} \M{exp} of a
control structure may return any value.
All values different from \nil\ are considered true;
only \nil\ is considered false.
The \rwd{return} statement is used to return values
from a function or from a chunk.
\label{return}%
\index{return statement}%
Because functions or chunks may return more than one value,
the syntax for the \rwd{return} statement is
\begin{Produc}
\produc{stat}{\rwd{return} \opt{explist1}}
\end{Produc}%
The \rwd{break} statement can be used to terminate the execution of a loop,
skipping to the next statement after the loop:
\index{break statement}
\begin{Produc}
\produc{stat}{\rwd{break}}
\end{Produc}%
A \rwd{break} ends the innermost enclosing loop
(\rwd{while}, \rwd{repeat}, or \rwd{for}).
\NOTE
For syntactic reasons, \rwd{return} and \rwd{break}
statements can only be written as the \emph{last} statements of a block.
If it is really necessary to \rwd{return} or \rwd{break} in the
middle of a block,
an explicit inner block can used,
as in the idiom `\verb|do return end|',
because now \rwd{return} is last statement in the inner block.
\subsubsection{For Statement} \label{for}\index{for statement}
The \rwd{for} statement has two forms,
one for numbers and one for tables.
\newpage
The numerical \rwd{for} loop has the following syntax:
\begin{Produc}
\produc{stat}{\rwd{for} name \ter{=} exp \ter{,} exp \opt{\ter{,} exp}
\rwd{do} block \rwd{end}}
\end{Produc}%
A \rwd{for} statement like
\begin{verbatim}
for var = e1, e2, e3 do block end
\end{verbatim}
is equivalent to the code:
\begin{verbatim}
do
local var, _limit, _step = tonumber(e1), tonumber(e2), tonumber(e3)
if not (var and _limit and _step) then error() end
while (_step>0 and var<=_limit) or (_step<=0 and var>=_limit) do
block
var = var+_step
end
end
\end{verbatim}
Note the following:
\begin{itemize}\itemsep=0pt
\item \verb|_limit| and \verb|_step| are invisible variables.
The names are here for explanatory purposes only.
\item The behavior is \emph{undefined} if you assign to \verb|var| inside
the block.
\item If the third expression (the step) is absent, then a step of~1 is used.
\item Both the limit and the step are evaluated only once,
before the loop starts.
\item The variable \verb|var| is local to the statement;
you cannot use its value after the \rwd{for} ends.
\item You can use \rwd{break} to exit a \rwd{for}.
If you need the value of the index,
assign it to another variable before breaking.
\end{itemize}
The table \rwd{for} statement traverses all pairs
(index,value) of a given table.
It has the following syntax:
\begin{Produc}
\produc{stat}{\rwd{for} name \ter{,} name \rwd{in} exp
\rwd{do} block \rwd{end}}
\end{Produc}%
A \rwd{for} statement like
\begin{verbatim}
for index, value in exp do block end
\end{verbatim}
is equivalent to the code:
\begin{verbatim}
do
local _t = exp
local index, value = next(_t, nil)
while index do
block
index, value = next(_t, index)
end
end
\end{verbatim}
Note the following:
\begin{itemize}\itemsep=0pt
\item \verb|_t| is an invisible variable.
The name is here for explanatory purposes only.
\item The behavior is \emph{undefined} if you assign to \verb|index| inside
the block.
\item The behavior is \emph{undefined} if you change
the table \verb|_t| during the traversal.
\item The variables \verb|index| and \verb|value| are local to the statement;
you cannot use their values after the \rwd{for} ends.
\item You can use \rwd{break} to exit a \rwd{for}.
If you need the value of \verb|index| or \verb|value|,
assign them to other variables before breaking.
\item The order that table elements are traversed is undefined,
\emph{even for numerical indices}.
If you want to traverse indices in numerical order,
use a numerical \rwd{for}.
\end{itemize}
\subsubsection{Function Calls as Statements} \label{funcstat}
Because of possible side-effects,
function calls can be executed as statements:
\begin{Produc}
\produc{stat}{functioncall}
\end{Produc}%
In this case, all returned values are thrown away.
Function calls are explained in \See{functioncall}.
\subsubsection{Local Declarations} \label{localvar}
\Index{Local variables} may be declared anywhere inside a block.
The declaration may include an initial assignment:
\begin{Produc}
\produc{stat}{\rwd{local} declist \opt{init}}
\produc{declist}{name \rep{\ter{,} name}}
\produc{init}{\ter{=} explist1}
\end{Produc}%
If present, an initial assignment has the same semantics
of a multiple assignment.
Otherwise, all variables are initialized with \nil.
A chunk is also a block,
and so local variables can be declared outside any explicit block.
The scope of local variables begins \emph{after}
the declaration and lasts until the end of the block.
Thus, the code
\verb|local print=print|
creates a local variable called \verb|print| whose
initial value is that of the \emph{global} variable of the same name.
\subsection{\Index{Expressions}}
\subsubsection{\Index{Basic Expressions}}
The basic expressions in Lua are
\begin{Produc}
\produc{exp}{\ter{(} exp \ter{)}}
\produc{exp}{\rwd{nil}}
\produc{exp}{number}
\produc{exp}{literal}
\produc{exp}{var}
\produc{exp}{upvalue}
\produc{exp}{function}
\produc{exp}{functioncall}
\produc{exp}{tableconstructor}
\end{Produc}%
An expression enclosed in parentheses always results
in only one value.
Numbers (numerical constants) and
literal strings are explained in \See{lexical};
variables are explained in \See{assignment};
upvalues are explained in \See{upvalue};
function definitions are explained in \See{func-def};
function calls are explained in \See{functioncall}.
Table constructors are explained in \See{tableconstructor}.
An access to a global variable \verb|x| is equivalent to a
call \verb|getglobal("x")| and
an access to an indexed variable \verb|t[i]| is equivalent to
a call \verb|gettable_event(t,i)|.
See \See{tag-method} for a description of these functions
(\verb|getglobal| is in the basic library;
\T{gettable\_event} is used for explanatory purposes only).
\subsubsection{Arithmetic Operators}
Lua supports the usual \Index{arithmetic operators}:
the binary \verb|+| (addition),
\verb|-| (subtraction), \verb|*| (multiplication),
\verb|/| (division), and \verb|^| (exponentiation);
and unary \verb|-| (negation).
If the operands are numbers, or strings that can be converted to
numbers (according to the rules given in \See{coercion}),
then all operations except exponentiation have the usual meaning.
Otherwise, an appropriate tag method is called \see{tag-method}.
An exponentiation always calls a tag method.
The standard mathematical library redefines this method for numbers,
giving the expected meaning to \Index{exponentiation}
\see{mathlib}.
\subsubsection{Relational Operators}
The \Index{relational operators} in Lua are
\begin{verbatim}
== ~= < > <= >=
\end{verbatim}
These operators return \nil\ as false and a value different from \nil\ as true.
Equality (\verb|==|) first compares the tags of its operands.
If they are different, then the result is \nil.
Otherwise, their values are compared.
Numbers and strings are compared in the usual way.
Tables, userdata, and functions are compared by reference,
that is,
two tables are considered equal only if they are the \emph{same} table.
Every time you create a new table (or userdata, or function) this
new value is different from any previously existing value.
The operator \verb|~=| is exactly the negation of equality (\verb|==|).
\NOTE
The conversion rules of \See{coercion}
\emph{do not} apply to equality comparisons.
Thus, \verb|"0"==0| evaluates to \emph{false},
and \verb|t[0]| and \verb|t["0"]| denote different
entries in a table.
\medskip
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 using lexicographical order.
Otherwise, the ``lt'' tag method is called \see{tag-method}.
\subsubsection{Logical Operators}
The \Index{logical operators} in Lua are
\index{and}\index{or}\index{not}
\begin{verbatim}
and or not
\end{verbatim}
Like the control structures, all logical operators
consider \nil\ as false and anything else as true.
The conjunction operator \verb|and| returns \nil\ if its first argument is \nil;
otherwise, it returns its second argument.
The disjunction operator \verb|or| returns its first argument
if it is different from \nil;
otherwise, it returns its second argument.
Both \verb|and| and \verb|or| use \Index{short-cut evaluation},
that is,
the second operand is evaluated only if necessary.
There are two useful Lua idioms that use logical operators.
The first idiom is
\begin{verbatim}
x = x or v
\end{verbatim}
which is equivalent to
\begin{verbatim}
if x == nil then x = v end
\end{verbatim}
This idiom sets \verb|x| to a default value \verb|v| when \verb|x| is not set.
The second idiom is
\begin{verbatim}
x = a and b or c
\end{verbatim}
which should be read as \verb|x = (a and b) or c|.
This idiom is equivalent to
\begin{verbatim}
if a then x = b else x = c end
\end{verbatim}
provided that \verb|b| is not \nil.
\subsubsection{Concatenation} \label{concat}
The string \Index{concatenation} operator in Lua is
denoted by two dots (`\IndexVerb{..}').
If both operands are strings or numbers, then they are converted to
strings according to the rules in \See{coercion}.
Otherwise, the ``concat'' tag method is called \see{tag-method}.
\subsubsection{Precedence}
\Index{Operator precedence} in Lua follows the table below,
from the lower to the higher priority:
\begin{verbatim}
and or
< > <= >= ~= ==
..
+ -
* /
not - (unary)
^
\end{verbatim}
All binary operators are left associative,
except for \verb|^| (exponentiation),
which is right associative.
\NOTE
The pre-compiler may rearrange the order of evaluation of
associative or commutative operators,
as long as these optimizations do not change normal results.
However, these optimizations may change some results
if you define non-associative (or non-commutative)
tag methods for these operators.
\subsubsection{Table Constructors} \label{tableconstructor}
Table \Index{constructors} are expressions that create tables;
every time a constructor is evaluated, a new table is created.
Constructors can be used to create empty tables,
or to create a table and initialize some of its fields.
The general syntax for constructors is
\begin{Produc}
\produc{tableconstructor}{\ter{\{} fieldlist \ter{\}}}
\produc{fieldlist}{lfieldlist \Or ffieldlist \Or lfieldlist \ter{;} ffieldlist
\Or ffieldlist \ter{;} lfieldlist}
\produc{lfieldlist}{\opt{explist1 \opt{\ter{,}}}}
\produc{ffieldlist}{\opt{ffieldlist1}}
\end{Produc}%
The form \emph{explist1} is used to initialize lists.
The expressions in the list are assigned to consecutive numerical indices,
starting with~1.
For example,
\begin{verbatim}
a = {"v1", "v2", 34}
\end{verbatim}
is equivalent to
\begin{verbatim}
do
local temp = {}
temp[1] = "v1"
temp[2] = "v2"
temp[3] = 34
a = temp
end
\end{verbatim}
If the last expression in the list is a function call,
all values returned by the call enter the list \see{functioncall}.
The form \emph{ffieldlist1} initializes other fields in a table:
\begin{Produc}
\produc{ffieldlist1}{ffield \rep{\ter{,} ffield} \opt{\ter{,}}}
\produc{ffield}{\ter{[} exp \ter{]} \ter{=} exp \Or name \ter{=} exp}
\end{Produc}%
For example,
\begin{verbatim}
a = {[f(k)] = g(y), x = 1, y = 3, [0] = b+c}
\end{verbatim}
is equivalent to
\begin{verbatim}
do
local temp = {}
temp[f(k)] = g(y)
temp.x = 1 -- or temp["x"] = 1
temp.y = 3 -- or temp["y"] = 3
temp[0] = b+c
a = temp
end
\end{verbatim}
An expression like \verb|{x = 1, y = 4}| is
in fact syntactic sugar for \verb|{["x"] = 1, ["y"] = 4}|.
Both forms may have an optional trailing comma,
and can be used in the same constructor separated by
a semi-colon.
For example, all forms below are correct.
\begin{verbatim}
x = {;}
x = {"a", "b",}
x = {type="list"; "a", "b"}
x = {f(0), f(1), f(2),; n=3,}
\end{verbatim}
\subsubsection{Function Calls} \label{functioncall}
A \Index{function call} in Lua has the following syntax:
\begin{Produc}
\produc{functioncall}{exp args}
\end{Produc}%
First, \M{exp} and \M{args} are evaluated.
If the value of \M{exp} has type \emph{function},
then this function is called,
with the given arguments.
Otherwise, the ``function'' tag method is called,
having as first parameter the value of \M{exp},
followed by the original call arguments
\see{tag-method}.
The form
\begin{Produc}
\produc{functioncall}{exp \ter{:} name args}
\end{Produc}%
can be used to call ``methods''.
A call \verb|v:name(...)|
is syntactic sugar for \verb|v.name(v, ...)|,
except that \verb|v| is evaluated only once.
Arguments have the following syntax:
\begin{Produc}
\produc{args}{\ter{(} \opt{explist1} \ter{)}}
\produc{explist1}{\rep{exp \ter{,}} exp}
\produc{args}{tableconstructor}
\produc{args}{literal}
\end{Produc}%
All argument expressions are evaluated before the call.
A call of the form \verb|f{...}| is syntactic sugar for
\verb|f({...})|, that is,
the argument list is a single new table.
A call of the form \verb|f'...'|
(or \verb|f"..."| or \verb|f[[...]]|) is syntactic sugar for
\verb|f('...')|, that is,
the argument list is a single literal string.
Because a function can return any number of results
\see{return},
the number of results must be adjusted before they are used.
If the function is called as a statement \see{funcstat},
then its return list is adjusted to~0,
thus discarding all returned values.
If the function is called inside another expression,
or in the middle of a list of expressions,
then its return list is adjusted to~1,
thus discarding all returned values but the first one.
If the function is called as the last element of a list of expressions,
then no adjustment is made.
Here are some examples:
\begin{verbatim}
f() -- adjusted to 0 results
g(f(), x) -- f() is adjusted to 1 result
g(x, f()) -- g gets x plus all values returned by f()
a,b,c = f(), x -- f() is adjusted to 1 result (and c gets nil)
a,b,c = x, f() -- f() is adjusted to 2
a,b,c = f() -- f() is adjusted to 3
return f() -- returns all values returned by f()
return x,y,f() -- returns x, y, and all values returned by f()
{f()} -- creates a list with all values returned by f()
{f(), nil} -- f() is adjusted to 1 result
\end{verbatim}
If you embrace a function call in parentheses,
then it is adjusted to return exactly one value:
\begin{verbatim}
return x, y, (f()) -- returns x, y, and one value from f()
{(f())} -- create a table with exactly one element
\end{verbatim}
\subsubsection{\Index{Function Definitions}} \label{func-def}
The syntax for function definition is
\begin{Produc}
\produc{function}{\rwd{function} \ter{(} \opt{parlist1} \ter{)}
block \rwd{end}}
\produc{stat}{\rwd{function} funcname \ter{(} \opt{parlist1} \ter{)}
block \rwd{end}}
\produc{funcname}{name \rep{\ter{.} name} \opt{\ter{:} name}}
\end{Produc}%
The statement
\begin{verbatim}
function f () ... end
\end{verbatim}
is just syntactic sugar for
\begin{verbatim}
f = function () ... end
\end{verbatim}
and the statement
\begin{verbatim}
function v.c.f () ... end
\end{verbatim}
is syntactic sugar for
\begin{verbatim}
v.c.f = function () ... end
\end{verbatim}
A function definition is an executable expression,
whose value has type \emph{function}.
When Lua pre-compiles a chunk,
all its function bodies are pre-compiled too.
Then, whenever Lua executes the function definition,
its upvalues are fixed \see{upvalue},
and the function is \emph{instantiated} (or \emph{closed}).
This function instance (or \emph{closure})
is the final value of the expression.
Different instances of the same function
may have different upvalues.
Parameters act as local variables,
initialized with the argument values:
\begin{Produc}
\produc{parlist1}{\ter{\ldots}}
\produc{parlist1}{name \rep{\ter{,} name} \opt{\ter{,} \ter{\ldots}}}
\end{Produc}%
\label{vararg}%
When a function is called,
the list of \Index{arguments} is adjusted to
the length of the list of parameters,
unless the function is a \Def{vararg function},
which is
indicated by three dots (`\verb|...|') at the end of its parameter list.
A vararg function does not adjust its argument list;
instead, it collects all extra arguments into an implicit parameter,
called \IndexLIB{arg}.
The value of \verb|arg| is a table,
with a field~\verb|n| whose value is the number of extra arguments,
and the extra arguments at positions 1,~2,~\ldots,~\verb|n|.
As an example, consider the following definitions:
\begin{verbatim}
function f(a, b) end
function g(a, b, ...) end
function r() return 1,2,3 end
\end{verbatim}
Then, we have the following mapping from arguments to parameters:
\begin{verbatim}
CALL PARAMETERS
f(3) a=3, b=nil
f(3, 4) a=3, b=4
f(3, 4, 5) a=3, b=4
f(r(), 10) a=1, b=10
f(r()) a=1, b=2
g(3) a=3, b=nil, arg={n=0}
g(3, 4) a=3, b=4, arg={n=0}
g(3, 4, 5, 8) a=3, b=4, arg={5, 8; n=2}
g(5, r()) a=5, b=1, arg={2, 3; n=2}
\end{verbatim}
Results are returned using the \rwd{return} statement \see{return}.
If control reaches the end of a function
without encountering a \rwd{return} statement,
then the function returns with no results.
The \emph{colon} syntax
is used for defining \IndexEmph{methods},
that is, functions that have an implicit extra parameter \IndexVerb{self}.
The statement
\begin{verbatim}
function v.c:f (...) ... end
\end{verbatim}
is just syntactic sugar for
\begin{verbatim}
v.c.f = function (self, ...) ... end
\end{verbatim}
Note that the function gets an extra formal parameter called \verb|self|.
\subsection{Visibility and Upvalues} \label{upvalue}
\index{visibility}\index{upvalues}
A function body may refer to its own local variables
(which include its parameters) and to global variables,
as long as they are not \emph{shadowed} by local
variables with the same name from enclosing functions.
A function \emph{cannot} access a local
variable from an enclosing function,
since such variables may no longer exist when the function is called.
However, a function may access the \emph{value} of a local variable
from an enclosing function, using \emph{upvalues},
whose syntax is
\begin{Produc}
\produc{upvalue}{\ter{\%} name}
\end{Produc}%
An upvalue is somewhat similar to a variable expression,
but whose value is \emph{frozen} when the function wherein it
appears is instantiated.
The name used in an upvalue may be the name of any variable visible
at the point where the function is defined,
that is,
global variables and local variables
from the \emph{immediately enclosing} function.
Note that when the upvalue is a table,
only the \emph{reference} to that table
(which is the value of the upvalue) is frozen;
the table contents can be changed at will.
Using table values as upvalues is a technique for having
writable but private state attached to functions.
Here are some examples:
\begin{verbatim}
a,b,c = 1,2,3 -- global variables
local d
function f (x)
local b = {} -- x and b are local to f; b shadows the global b
local g = function (a)
local y -- a and y are local to g
p = a -- OK, access local `a'
p = c -- OK, access global `c'
p = b -- ERROR: cannot access a variable in outer function
p = %b -- OK, access frozen value of `b' (local to `f')
%b = 3 -- ERROR: cannot change an upvalue
%b.x = 3 -- OK, change the table contents
p = %c -- OK, access frozen value of global `c'
p = %y -- ERROR: `y' is not visible where `g' is defined
p = %d -- ERROR: `d' is not visible where `g' is defined
end -- g
end -- f
\end{verbatim}
\subsection{Error Handling} \label{error}
Because Lua is an extension language,
all Lua actions start from C~code in the host program
calling a function from the Lua library.
Whenever an error occurs during Lua compilation or execution,
the function \verb|_ERRORMESSAGE| is called \DefLIB{_ERRORMESSAGE}
(provided it is different from \nil),
and then the corresponding function from the library
(\verb|lua_dofile|, \verb|lua_dostring|,
\verb|lua_dobuffer|, or \verb|lua_call|)
is terminated, returning an error condition.
Memory allocation errors are an exception to the previous rule.
When memory allocation fails, Lua may not be able to execute the
\verb|_ERRORMESSAGE| function.
So, for this kind of error, Lua does not call
the \verb|_ERRORMESSAGE| function;
instead, the corresponding function from the library
returns immediately with a special error code (\verb|LUA_ERRMEM|).
This and other error codes are defined in \verb|lua.h|;
\See{luado}.
The only argument to \verb|_ERRORMESSAGE| is a string
describing the error.
The default definition for
this function calls \verb|_ALERT|, \DefLIB{_ALERT}
which prints the message to \verb|stderr| \see{alert}.
The standard I/O library redefines \verb|_ERRORMESSAGE|
and uses the debug facilities \see{debugI}
to print some extra information,
such as a call stack traceback.
Lua code can explicitly generate an error by calling the
function \verb|error| \see{pdf-error}.
Lua code can ``catch'' an error using the function
\verb|call| \see{pdf-call}.
\subsection{Tag Methods} \label{tag-method}\index{tag method}
A tag method is a programmer-defined function
that defines how Lua operations act over user-defined types
(and, sometimes, over basic types as well).
An \Def{event} is any operation that may invoke a tag method.
Lua selects the tag method called for any specific event
according to the types of the values involved
in the event \see{TypesSec}.
The function \IndexLIB{settagmethod} changes the tag method
associated with a given pair \M{(type, event)}.
Its first parameter is the type (its name or its tag),
the second parameter is the event name (a string; see below),
and the third parameter is the new method (a function),
or \nil\ to restore the default behavior for the pair.
A companion function \IndexLIB{gettagmethod}
receives a type and an event name and returns the
current method associated with the pair.
Tag methods are called in the following events,
identified by the given names.
The semantics of tag methods is better explained by a Lua function
describing the behavior of the interpreter at each event.
Each event-handler function shows how a tag method is called,
its arguments (that is, its signature),
its results,
and the default behavior in the absence of a tag method.
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
(\verb|rawget|, \verb|tonumber|, \verb|call|, etc.)
are described in \See{predefined}.
\begin{description}
\item[``add'':]\IndexTM{add}
called when a \verb|+| operation is applied to non-numerical operands.
The function \verb|getbinmethod| below defines how Lua chooses a tag method
for a binary operation.
First, Lua tries the first operand.
If its type does not define a tag method for the operation,
then Lua tries the second operand.
If it also fails, then it gets a tag method from tag~0.
\begin{verbatim}
function getbinmethod (op1, op2, event)
return gettagmethod(tag(op1), event) or
gettagmethod(tag(op2), event) or
gettagmethod(0, event)
end
\end{verbatim}
Using this function,
the tag method for the ``add'' event is
\begin{verbatim}
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 tm = getbinmethod(op1, op2, "add")
if tm then
-- call the method with both operands
return tm(op1, op2)
else -- no tag method available: default behavior
error("unexpected type at arithmetic operation")
end
end
end
\end{verbatim}
\item[``sub'':]\IndexTM{sub}
called when a \verb|-| operation is applied to non-numerical operands.
Behavior similar to the ``add'' event.
\item[``mul'':]\IndexTM{mul}
called when a \verb|*| operation is applied to non-numerical operands.
Behavior similar to the ``add'' event.
\item[``div'':]\IndexTM{div}
called when a \verb|/| operation is applied to non-numerical operands.
Behavior similar to the ``add'' event.
\item[``pow'':]\IndexTM{pow}
called when a \verb|^| operation (exponentiation) is applied,
even for numerical operands.
\begin{verbatim}
function pow_event (op1, op2)
local tm = getbinmethod(op1, op2, "pow")
if tm then
-- call the method with both operands
return tm(op1, op2)
else -- no tag method available: default behavior
error("unexpected type at arithmetic operation")
end
end
\end{verbatim}
\item[``unm'':]\IndexTM{unm}
called when a unary \verb|-| operation is applied to a non-numerical operand.
\begin{verbatim}
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 tag method from the operand;
-- if it does not have one, try a "global" one (tag 0)
local tm = gettagmethod(tag(op), "unm") or
gettagmethod(0, "unm")
if tm then
-- call the method with the operand and nil
return tm(op, nil)
else -- no tag method available: default behavior
error("unexpected type at arithmetic operation")
end
end
end
\end{verbatim}
\item[``lt'':]\IndexTM{lt}
called when an order operation is applied to non-numerical
or non-string operands.
It corresponds to the \verb|<| operator.
\begin{verbatim}
function lt_event (op1, op2)
if type(op1) == "number" and type(op2) == "number" then
return op1 < op2 -- numeric comparison
elseif type(op1) == "string" and type(op2) == "string" then
return op1 < op2 -- lexicographic comparison
else
local tm = getbinmethod(op1, op2, "lt")
if tm then
return tm(op1, op2)
else
error("unexpected type at comparison");
end
end
end
\end{verbatim}
The other order operators use the \verb|"lt"| tag method
according to the usual equivalences:
\begin{verbatim}
a>b <=> b<a
a<=b <=> not (b<a)
a>=b <=> not (a<b)
\end{verbatim}
\item[``concat'':]\IndexTM{concatenation}
called when a concatenation is applied to non-string operands.
\begin{verbatim}
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 tm = getbinmethod(op1, op2, "concat")
if tm then
return tm(op1, op2)
else
error("unexpected type for concatenation")
end
end
end
\end{verbatim}
\item[``index'':]\IndexTM{index}
called when Lua tries to retrieve the value of an index
not present in a table.
See the ``gettable'' event for its semantics.
\item[``getglobal'':]\IndexTM{getglobal}
called whenever Lua needs the value of a global variable.
This method can only be set for \nil\ and for user-defined types.
Note that
the tag is that of the \emph{current value} of the global variable.
\begin{verbatim}
function getglobal (varname)
-- access the table of globals
local value = rawget(globals(), varname)
local tm = gettagmethod(tag(value), "getglobal")
if not tm then
return value
else
return tm(varname, value)
end
end
\end{verbatim}
The function \verb|getglobal| is defined in the basic library~\see{predefined}.
\NOTE
\verb|getglobal| is ``overloaded'' here.
It is the name both of the event and
of the function that handles the event
to call an eventual tag method
(called \verb|tm| in the above code).
\item[``setglobal'':]\IndexTM{setglobal}
called whenever Lua assigns to a global variable.
This method cannot be set for numbers, strings, and tables and
userdata with the default tag.
\begin{verbatim}
function setglobal (varname, newvalue)
local oldvalue = rawget(globals(), varname)
local tm = gettagmethod(tag(oldvalue), "setglobal")
if not tm then
rawset(globals(), varname, newvalue)
else
tm(varname, oldvalue, newvalue)
end
end
\end{verbatim}
The function \verb|setglobal| is defined in the basic library~\see{predefined}.
\NOTE
See previous note.
\item[``gettable'':]\IndexTM{gettable}
called whenever Lua accesses an indexed variable.
This method cannot be set for tables with the default tag.
\begin{verbatim}
function gettable_event (table, index)
local tm = gettagmethod(tag(table), "gettable")
if tm then
return tm(table, index)
elseif type(table) ~= "table" then
error("indexed expression not a table");
else
local v = rawget(table, index)
tm = gettagmethod(tag(table), "index")
if v == nil and tm then
return tm(table, index)
else
return v
end
end
end
\end{verbatim}
\item[``settable'':]\IndexTM{settable}
called when Lua assigns to an indexed variable.
This method cannot be set for tables with the default tag.
\begin{verbatim}
function settable_event (table, index, value)
local tm = gettagmethod(tag(table), "settable")
if tm then
tm(table, index, value)
elseif type(table) ~= "table" then
error("indexed expression not a table")
else
rawset(table, index, value)
end
end
\end{verbatim}
\item[``function'':]\IndexTM{function}
called when Lua tries to call a non-function value.
\begin{verbatim}
function function_event (func, ...)
if type(func) == "function" then
return call(func, arg)
else
local tm = gettagmethod(tag(func), "function")
if tm then
for i=arg.n,1,-1 do
arg[i+1] = arg[i]
end
arg.n = arg.n+1
arg[1] = func
return call(tm, arg)
else
error("call expression not a function")
end
end
end
\end{verbatim}
\item[``gc'':]\IndexTM{gc}
called when Lua is ``garbage collecting'' a userdata.
This tag method can be set only from~C,
and cannot be set for a userdata with the default tag.
For each userdata to be collected,
Lua does the equivalent of the following function:
\begin{verbatim}
function gc_event (obj)
local tm = gettagmethod(tag(obj), "gc")
if tm then
tm(obj)
end
end
\end{verbatim}
In a garbage-collection cycle,
the tag methods for userdata are called in \emph{reverse}
order of type creation,
that is, the first tag methods to be called are those associated
with the last type created in the program.
Moreover, at the end of the cycle,
Lua does the equivalent of the call \verb|gc_event(nil)|.
\end{description}
\section{The Application Program Interface}
\index{C API}
This section describes the API for Lua, that is,
the set of C~functions available to the host program to communicate
with Lua.
All API functions and related types and constants
are declared in the header file \verb|lua.h|.
\NOTE
Even when we use the term ``function'',
any facility in the API may be provided as a \emph{macro} instead.
All such macros use each of its arguments exactly once
(except for the first argument, which is always a state),
and so do not generate hidden side-effects.
\subsection{States} \label{mangstate}
The Lua library is fully reentrant:
it does not have any global variables.
\index{state}
The whole state of the Lua interpreter
(global variables, stack, tag methods, etc.)
is stored in a dynamically allocated structure of type \verb|lua_State|;
\DefAPI{lua_State}
this state must be passed as the first argument to
every function in the library (except \verb|lua_open| below).
Before calling any API function,
you must create a state by calling
\begin{verbatim}
lua_State *lua_open (int stacksize);
\end{verbatim}
\DefAPI{lua_open}
The sole argument to this function is the stack size for the interpreter.
(Each function call needs one stack position for each argument, local variable,
and temporary value, plus one position for book-keeping.
The stack must also have some 20 extra positions available.
For very small implementations, without recursive functions,
a stack size of~100 should be enough.)
If \verb|stacksize| is zero,
then a default size of~1024 is used.
To release a state created with \verb|lua_open|, call
\begin{verbatim}
void lua_close (lua_State *L);
\end{verbatim}
\DefAPI{lua_close}
This function destroys all objects in the given Lua environment
(calling the corresponding garbage-collection tag methods, if any)
and frees all dynamic memory used by that state.
Usually, you do not need to call this function,
because all resources are naturally released when your program ends.
On the other hand,
long-running programs ---
like a daemon or a web server ---
might need to release states as soon as they are not needed,
to avoid growing too big.
With the exception of \verb|lua_open|,
all functions in the Lua API need a state as their first argument.
\subsection{Threads}
Lua offers a partial support for multiple threads.
If you have a C library that offers multi-threading or co-routines,
Lua can cooperate with it to implement the equivalent facility in Lua.
The following function creates a new ``thread'' in Lua:
\begin{verbatim}
lua_State *lua_newthread (lua_State *L, int stacksize);
\end{verbatim}
\DefAPI{lua_newthread}
The new state returned by this function shares with the original state
all global environment (such as tables, tag methods, etc.),
but has an independent stack.
(The use of these multiple stacks must be ``syncronized'' with C.
How to explain that? TO BE WRITTEN.)
Each thread has an independent table for global variables.
When you create a thread this table is the same as of the given state,
but you can change each one independently.
You destroy threads with \verb|lua_close|.
When you destroy the last thread of a global state,
the state itself is also destroyed.
\subsection{The Stack and Indices}
Lua uses a \emph{stack} to pass values to and from C.
Each element in this stack represents a Lua value
(nil, number, string, etc.).
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 \emph{index}:
A positive index represents an \emph{absolute} stack position
(starting at~1, not 0 as in C);
a negative index represents an \emph{offset} from the top of the stack.
More specifically, if the stack has \M{n} elements,
index~1 represents the first element
(that is, the first element pushed onto the stack),
and
index~\M{n} represents the last element;
index~\Math{-1} also represents the last element
(that is, the element at the top),
and index \Math{-n} represents the first element.
We say that an index is \emph{valid}
if it lies between~1 and the stack top
(that is, if \verb|1 <= abs(index) <= top|).
\index{stack index} \index{valid index}
At any time, you can get the index of the top element by calling
\begin{verbatim}
int lua_gettop (lua_State *L);
\end{verbatim}
\DefAPI{lua_gettop}
Because indices start at~1,
the result of \verb|lua_gettop| is equal to the number of elements in the stack
(and so 0~means an empty stack).
When you interact with Lua API,
\emph{you are responsible for controlling stack overflow}.
The function
\begin{verbatim}
int lua_stackspace (lua_State *L);
\end{verbatim}
\DefAPI{lua_stackspace}
returns the number of stack positions still available.
Whenever Lua calls C, \DefAPI{LUA_MINSTACK}
it ensures that
at least \verb|LUA_MINSTACK| positions are still available.
\verb|LUA_MINSTACK| is defined in \verb|lua.h| and is at least~16,
so that usually you have to worry about stack space only
when your code has loops pushing elements onto the stack.
Most query functions accept as indices any value inside the
available stack space.
Such indices are called \emph{acceptable indices}.
More formally, we can define an \IndexEmph{acceptable index}
as
\begin{verbatim}
(index < 0 && abs(index) <= top) || (index > 0 && index <= top + stackspace)
\end{verbatim}
Note that 0 is not an acceptable index.
\subsection{Stack Manipulation}
The API offers the following functions for basic stack manipulation:
\begin{verbatim}
void lua_settop (lua_State *L, int index);
void lua_pushvalue (lua_State *L, int index);
void lua_remove (lua_State *L, int index);
void lua_insert (lua_State *L, int index);
\end{verbatim}
\DefAPI{lua_settop}\DefAPI{lua_pushvalue}
\DefAPI{lua_remove}\DefAPI{lua_insert}
\verb|lua_settop| accepts any acceptable index,
or 0,
and sets the stack top to that index.
If the new top is larger than the old one,
then the new elements are filled with \nil.
If \verb|index| is 0, then all stack elements are removed.
A useful macro defined in the API is
\begin{verbatim}
#define lua_pop(L,n) lua_settop(L, -(n)-1)
\end{verbatim}
\DefAPI{lua_pop}
which pops \verb|n| elements from the stack.
\verb|lua_pushvalue| pushes onto the stack a \emph{copy} of the element
at the given index.
\verb|lua_remove| removes the element at the given position,
shifting down the elements on top of that position to fill in the gap.
\verb|lua_insert| moves the top element into the given position,
shifting up the elements on top of that position to open space.
These functions accept only valid indices.
As an example, if the stack starts as \verb|10 20 30 40 50|
(from bottom to top),
then
\begin{verbatim}
lua_pushvalue(L, 3) --> 10 20 30 40 50 30
lua_pushvalue(L, -1) --> 10 20 30 40 50 30 30
lua_remove(L, -3) --> 10 20 30 40 30 30
lua_remove(L, 6) --> 10 20 30 40 30
lua_insert(L, 1) --> 30 10 20 30 40
lua_insert(L, -1) --> 30 10 20 30 40 (no effect)
lua_settop(L, -3) --> 30 10 20
lua_settop(L, 6) --> 30 10 20 nil nil nil
\end{verbatim}
\subsection{Querying the Stack}
To check the type of a stack element,
the following functions are available:
\begin{verbatim}
int lua_tag (lua_State *L, int index);
int lua_rawtag (lua_State *L, int index);
const char *lua_type (lua_State *L, int index);
int lua_isnil (lua_State *L, int index);
int lua_isnumber (lua_State *L, int index);
int lua_isstring (lua_State *L, int index);
int lua_istable (lua_State *L, int index);
int lua_isfunction (lua_State *L, int index);
int lua_iscfunction (lua_State *L, int index);
int lua_isuserdata (lua_State *L, int index);
\end{verbatim}
\DefAPI{lua_type}\DefAPI{lua_tag}
\DefAPI{lua_isnil}\DefAPI{lua_isnumber}\DefAPI{lua_isstring}
\DefAPI{lua_istable}
\DefAPI{lua_isfunction}\DefAPI{lua_iscfunction}\DefAPI{lua_isuserdata}
These functions can be called with any acceptable index.
\verb|lua_tag| returns the tag of a value in the stack,
or \verb|LUA_TNONE| for a non-valid index
(that is, if that stack position is ``empty'').
The tags for the basic types are the following constants:
\verb|LUA_TNIL|,
\verb|LUA_TNUMBER|,
\verb|LUA_TSTRING|,
\verb|LUA_TTABLE|,
\verb|LUA_TFUNCTION|,
\verb|LUA_TUSERDATA|.
\verb|lua_rawtag| is similar to \verb|lua_tag|,
but it returns the tag of the basic (raw) type of a value.
\verb|lua_type| is similar to \verb|lua_tag|,
but it returns the type name of the given value.
The \verb|lua_is*| functions return~1 if the object is compatible
with the given type, and 0 otherwise.
They always return 0 for a non-valid index.
\verb|lua_isnumber| accepts numbers and numerical strings,
\verb|lua_isstring| accepts strings and numbers \see{coercion},
and \verb|lua_isfunction| accepts both Lua functions and C~functions.
To distinguish between Lua functions and C~functions,
you should use \verb|lua_iscfunction|.
To distinguish between numbers and numerical strings,
you can use \verb|lua_rawtag| (or \verb|lua_tag|).
The API also has functions to compare two values in the stack:
\begin{verbatim}
int lua_equal (lua_State *L, int index1, int index2);
int lua_lessthan (lua_State *L, int index1, int index2);
\end{verbatim}
\DefAPI{lua_equal} \DefAPI{lua_lessthan}
These functions are equivalent to their counterparts in Lua.
Specifically, \verb|lua_lessthan| is equivalent to the \verb|lt_event|
described in \See{tag-method}.
Both functions return 0 if any of the indices are non-valid.
To translate a value in the stack to a specific C~type,
you can use the following conversion functions:
\begin{verbatim}
double lua_tonumber (lua_State *L, int index);
const char *lua_tostring (lua_State *L, int index);
size_t lua_strlen (lua_State *L, int index);
lua_CFunction lua_tocfunction (lua_State *L, int index);
void *lua_touserdata (lua_State *L, int index);
\end{verbatim}
\DefAPI{lua_tonumber}\DefAPI{lua_tostring}\DefAPI{lua_strlen}
\DefAPI{lua_tocfunction}\DefAPI{lua_touserdata}
These functions can be called with any acceptable index.
When called with a non-valid index,
they act as if the given value had an incorrect type.
\verb|lua_tonumber| converts the value at the given index
to a floating-point number.
This value must be a number or a string convertible to number
\see{coercion}; otherwise, \verb|lua_tonumber| returns~0.
\verb|lua_tostring| converts a Lua value to a string
(\verb|const char*|).
This value must be a string or a number;
otherwise, the function returns \verb|NULL|.
If the value is a number,
\verb|lua_tostring| also changes the
actual value in the stack to a string.
This function returns a pointer to a string inside the Lua environment.
This pointer is always fully aligned.
The strings always have a zero (\verb|'\0'|)
after their last character (as in C),
but may contain other zeros in their body.
If you do not know whether a string may contain zeros,
you can use \verb|lua_strlen| to get its actual length.
Because Lua has garbage collection,
there is no guarantee that the pointer returned by \verb|lua_tostring|
will be valid after the respective value is removed from the stack.
\verb|lua_tocfunction| converts a value in the stack to a C~function.
This value must be a C~function;
otherwise, \verb|lua_tocfunction| returns \verb|NULL|.
The type \verb|lua_CFunction| is explained in \See{LuacallC}.
\verb|lua_touserdata| converts a value to \verb|void*|.
This value must have type \emph{userdata};
otherwise, \verb|lua_touserdata| returns \verb|NULL|.
\subsection{Pushing values onto the Stack}
The API has the following functions to
push C~values onto the stack:
\begin{verbatim}
void lua_pushnumber (lua_State *L, double n);
void lua_pushlstring (lua_State *L, const char *s, size_t len);
void lua_pushstring (lua_State *L, const char *s);
void lua_pushnil (lua_State *L);
void lua_pushcfunction (lua_State *L, lua_CFunction f);
\end{verbatim}
\DefAPI{lua_pushnumber}\DefAPI{lua_pushlstring}\DefAPI{lua_pushstring}
\DefAPI{lua_pushcfunction}\DefAPI{lua_pushusertag}
\DefAPI{lua_pushnil}\label{pushing}
These functions receive a C~value,
convert it to a corresponding Lua value,
and push the result onto the stack.
In particular, \verb|lua_pushlstring| and \verb|lua_pushstring|
make an \emph{internal copy} of the given string.
\verb|lua_pushstring| can only be used to push proper C~strings
(that is, strings that end with a zero and do not contain embedded zeros);
otherwise you should use the more general \verb|lua_pushlstring|,
which accepts an explicit size.
\subsection{Garbage Collection API}\label{GC-API}
Lua uses two numbers to control its garbage collection \see{GC}.
You can access the current values of these two numbers through the
following functions:
\begin{verbatim}
int lua_getgccount (lua_State *L);
int lua_getgcthreshold (lua_State *L);
\end{verbatim}
\DefAPI{lua_getgcthreshold} \DefAPI{lua_getgccount}
Both return their respective values in Kbytes.
You can change the threshold value with
\begin{verbatim}
void lua_setgcthreshold (lua_State *L, int newthreshold);
\end{verbatim}
\DefAPI{lua_setgcthreshold}
Again, the \verb|newthreshold| value is given in Kbytes.
When you call this function,
Lua sets the new threshold and checks it against the byte counter.
If the new threshold is smaller than the byte counter,
then Lua immediately runs the garbage collector;
after the collection,
a new threshold is set according to the previous rule.
If you want to change the adaptive behavior of the garbage collector,
you can use the garbage-collection tag method for \nil\ %
to set your own threshold
(the tag method is called after Lua resets the threshold).
\subsection{Userdata}
You can create new userdata with the following functions:
\begin{verbatim}
void *lua_newuserdata (lua_State *L, size_t size);
void lua_newuserdatabox (lua_State *L, void *u);
\end{verbatim}
\DefAPI{lua_newuserdata}\DefAPI{lua_newuserdatabox}
The first function, \verb|lua_newuserdata|,
allocates a new block of memory with the given size,
pushes on the stack a new userdata with the block address,
and returns this address.
The second function, \verb|lua_newuserdatabox|,
gets a pointer and pushes on the stack a new userdata
with that pointer.
In this case, Lua does not care about the pointer's value.
By default, all userdata are created with a standard tag,
\verb|LUA_TUSERDATA|.
When Lua collects a userdata created by \verb|lua_newuserdata|,
it automatically frees its corresponding memory.
On the other hand, Lua never uses pointers in
userdata created with \verb|lua_newuserdatabox|;
it is up to you to free any associated memory,
setting a garbage-collection tag method, for instance.
\subsection{Types and Tags}
User-defined types are created with the function
\begin{verbatim}
int lua_newtype (lua_State *L, const char *name, int basictype);
\end{verbatim}
\DefAPI{lua_newtype}
\verb|name| is the name of the new type,
and \verb|basictype| is the basic type for objects with this new type,
which can be \verb|LUA_TUSERDATA| or \verb|LUA_TTABLE|.
The function \verb|lua_settag| changes the tag (i.e., the type) of
the object on top of the stack (without popping it):
\begin{verbatim}
void lua_settag (lua_State *L, int tag);
\end{verbatim}
\DefAPI{lua_settag}
The given \verb|tag| must be a user-defined tag,
and the basic type of the object must be the basic type for that
tag (userdata or table).
The following functions allow you to translate a tag to a type name
and a type name to a tag:
\begin{verbatim}
int lua_name2tag (lua_State *L, const char *name);
const char *lua_tag2name (lua_State *L, int tag);
\end{verbatim}
\DefAPI{lua_name2tag}\DefAPI{lua_tag2name}
\subsection{Executing Lua Code}\label{luado}
A host program can execute Lua chunks written in a file or in a string
by using the following functions:%
\begin{verbatim}
int lua_dofile (lua_State *L, const char *filename);
int lua_dostring (lua_State *L, const char *string);
int lua_dobuffer (lua_State *L, const char *buff,
size_t size, const char *name);
\end{verbatim}
\DefAPI{lua_dofile}\DefAPI{lua_dostring}\DefAPI{lua_dobuffer}%
These functions return
0 in case of success, or one of the following error codes if they fail:
\begin{itemize}
\item \IndexAPI{LUA_ERRRUN} ---
error while running the chunk.
\item \IndexAPI{LUA_ERRSYNTAX} ---
syntax error during pre-compilation.
\item \IndexAPI{LUA_ERRMEM} ---
memory allocation error.
For such errors, Lua does not call \verb|_ERRORMESSAGE| \see{error}.
\item \IndexAPI{LUA_ERRERR} ---
error while running \verb|_ERRORMESSAGE|.
For such errors, Lua does not call \verb|_ERRORMESSAGE| again, to avoid loops.
\item \IndexAPI{LUA_ERRFILE} ---
error opening the file (only for \verb|lua_dofile|).
In this case,
you may want to
check \verb|errno|,
call \verb|strerror|,
or call \verb|perror| to tell the user what went wrong.
\end{itemize}
These constants are defined in \verb|lua.h|.
When called with argument \verb|NULL|,
\verb|lua_dofile| executes the \verb|stdin| stream.
\verb|lua_dofile| and \verb|lua_dobuffer|
are both able to execute pre-compiled chunks.
They automatically detect whether the chunk is text or binary,
and load it accordingly (see program \IndexVerb{luac}).
\verb|lua_dostring| executes only source code,
given in textual form.
The fourth parameter to \verb|lua_dobuffer|
is the ``name of the chunk'',
which is used in error messages and debug information.
If \verb|name| is \verb|NULL|,
then Lua gives a default name to the chunk.
These functions push onto the stack
any values eventually returned by the chunk.
A chunk may return any number of values;
Lua takes care that these values fit into the stack space,
but after the call the responsibility is back to you.
If you need to push other elements after calling any of these functions,
and you want to ``play safe'',
you must either check the stack space
with \verb|lua_stackspace|
or remove the returned elements
from the stack (if you do not need them).
For instance, the following code
loads a chunk in a file and discards all results returned by this chunk,
leaving the stack as it was before the call:
\begin{verbatim}
{
int oldtop = lua_gettop(L);
lua_dofile(L, filename);
lua_settop(L, oldtop);
}
\end{verbatim}
\subsection{Manipulating Global Variables in Lua}
To read the value of a global Lua variable,
you call
\begin{verbatim}
void lua_getglobal (lua_State *L, const char *varname);
\end{verbatim}
\DefAPI{lua_getglobal}
which pushes onto the stack the value of the given variable.
As in Lua, this function may trigger a tag method
for the ``getglobal'' event \see{tag-method}.
To read the real value of a global variable,
without invoking any tag method,
use \verb|lua_rawget| over the table of globals
(see below).
To store a value in a global variable,
you call
\begin{verbatim}
void lua_setglobal (lua_State *L, const char *varname);
\end{verbatim}
\DefAPI{lua_setglobal}
which pops from the stack the value to be stored in the given variable.
As in Lua, this function may trigger a tag method
for the ``setglobal'' event \see{tag-method}.
To set the real value of a global variable,
without invoking any tag method,
use \verb|lua_rawset| over the table of globals
(see below).
All global variables are kept in an ordinary Lua table.
You can get this table calling
\begin{verbatim}
void lua_getglobals (lua_State *L);
\end{verbatim}
\DefAPI{lua_getglobals}
which pushes the current table of globals onto the stack.
To set another table as the table of globals,
you call
\begin{verbatim}
void lua_setglobals (lua_State *L);
\end{verbatim}
\DefAPI{lua_setglobals}
The table to be used is popped from the stack.
\subsection{Manipulating Tables in Lua}
Lua tables can also be manipulated through the API.
To read a value from a table, call
\begin{verbatim}
void lua_gettable (lua_State *L, int index);
\end{verbatim}
\DefAPI{lua_gettable}
where \verb|index| refers to the table.
\verb|lua_gettable| pops a key from the stack,
and returns (on the stack) the contents of the table at that key.
As in Lua, this operation may trigger a tag method
for the ``gettable'' event.
To get the real value of any table key,
without invoking any tag method,
use the \emph{raw} version:
\begin{verbatim}
void lua_rawget (lua_State *L, int index);
\end{verbatim}
\DefAPI{lua_rawget}
To store a value into a table that resides somewhere in the stack,
you push the key and the value onto the stack
(in this order),
and then call
\begin{verbatim}
void lua_settable (lua_State *L, int index);
\end{verbatim}
\DefAPI{lua_settable}
where \verb|index| refers to the table.
\verb|lua_settable| pops from the stack both the key and the value.
As in Lua, this operation may trigger a tag method
for the ``settable'' event.
To set the real value of any table index,
without invoking any tag method,
use the \emph{raw} version:
\begin{verbatim}
void lua_rawset (lua_State *L, int index);
\end{verbatim}
\DefAPI{lua_rawset}
Finally, the function
\begin{verbatim}
void lua_newtable (lua_State *L);
\end{verbatim}
\DefAPI{lua_newtable}
creates a new, empty table and pushes it onto the stack.
\subsection{Using Tables as Arrays}
The API has functions that help to use Lua tables as arrays,
that is,
tables indexed by numbers only:
\begin{verbatim}
void lua_rawgeti (lua_State *L, int index, int n);
void lua_rawseti (lua_State *L, int index, int n);
int lua_getn (lua_State *L, int index);
\end{verbatim}
\DefAPI{lua_rawgeti}
\DefAPI{lua_rawseti}
\DefAPI{lua_getn}
\verb|lua_rawgeti| pushes the value of the \M{n}-th element of the table
at stack position \verb|index|.
\verb|lua_rawseti| sets the value of the \M{n}-th element of the table
at stack position \verb|index| to the value at the top of the stack,
removing the value from the stack.
\verb|lua_getn| returns the number of elements in the table
at stack position \verb|index|.
This number is the value of the table field \verb|n|,
if it has a numeric value,
or the largest numerical index with a non-nil value in the table.
\subsection{Calling Lua Functions}
Functions defined in Lua
(and C~functions registered in Lua)
can be called from the host program.
This is done using the following protocol:
First, the function to be called is pushed onto the stack;
then, the arguments to the function are pushed
\see{pushing} in \emph{direct order}, that is, the first argument is pushed first.
Finally, the function is called using
\begin{verbatim}
int lua_call (lua_State *L, int nargs, int nresults);
\end{verbatim}
\DefAPI{lua_call}
This function returns the same error codes as \verb|lua_dostring| and
friends \see{luado}.
If you want to propagate the error,
instead of returning an error code,
use
\begin{verbatim}
void lua_rawcall (lua_State *L, int nargs, int nresults);
\end{verbatim}
\DefAPI{lua_rawcall}
In both functions,
\verb|nargs| is the number of arguments that you pushed onto the stack.
All arguments and the function value are popped from the stack,
and the function results are pushed.
The number of results are adjusted to \verb|nresults|,
unless \verb|nresults| is \IndexAPI{LUA_MULTRET}.
In that case, \emph{all} results from the function are pushed.
The function results are pushed in direct order
(the first result is pushed first),
so that after the call the last result is on the top.
The following example shows how the host program may do the
equivalent to the Lua code:
\begin{verbatim}
a,b = f("how", t.x, 4)
\end{verbatim}
Here it is in~C:
\begin{verbatim}
lua_getglobal(L, "t"); /* global `t' (for later use) */
lua_getglobal(L, "f"); /* function to be called */
lua_pushstring(L, "how"); /* 1st argument */
lua_pushstring(L, "x"); /* push the string `x' */
lua_gettable(L, -4); /* push result of t.x (2nd arg) */
lua_pushnumber(L, 4); /* 3rd argument */
lua_call(L, 3, 2); /* call function with 3 arguments and 2 results */
lua_setglobal(L, "b"); /* set global variable `b' */
lua_setglobal(L, "a"); /* set global variable `a' */
lua_pop(L, 1); /* remove `t' from the stack */
\end{verbatim}
Notice that the code above is ``balanced'':
at its end, the stack is back to its original configuration.
This is considered good programming practice.
\medskip
Some special Lua functions have their own C~interfaces.
The host program can generate a Lua error calling the function
\begin{verbatim}
void lua_error (lua_State *L, const char *message);
\end{verbatim}
\DefAPI{lua_error}
This function never returns.
If \verb|lua_error| is called from a C~function that has been called from Lua,
then the corresponding Lua execution terminates,
as if an error had occurred inside Lua code.
Otherwise, the whole host program terminates with a call to
\verb|exit(EXIT_FAILURE)|.
Before terminating execution,
the \verb|message| is passed to the error handler function,
\verb|_ERRORMESSAGE| \see{error}.
If \verb|message| is \verb|NULL|,
then \verb|_ERRORMESSAGE| is not called.
\medskip
Tag methods can be changed with
\begin{verbatim}
void lua_settagmethod (lua_State *L, int tag, const char *event);
\end{verbatim}
\DefAPI{lua_settagmethod}
The second parameter is the tag,
and the third is the event name \see{tag-method};
the new method is popped from the stack.
To get the current value of a tag method,
use the function
\begin{verbatim}
void lua_gettagmethod (lua_State *L, int tag, const char *event);
\end{verbatim}
\DefAPI{lua_gettagmethod}
It is also possible to copy all tag methods from one tag
to another:
\begin{verbatim}
int lua_copytagmethods (lua_State *L, int tagto, int tagfrom);
\end{verbatim}
\DefAPI{lua_copytagmethods}
This function returns \verb|tagto|.
\medskip
You can traverse a table with the function
\begin{verbatim}
int lua_next (lua_State *L, int index);
\end{verbatim}
\DefAPI{lua_next}
where \verb|index| refers to the table to be traversed.
The function pops a key from the stack,
and pushes a key-value pair from the table
(the ``next'' pair after the given key).
If there are no more elements, then the function returns 0
(and pushes nothing).
A typical traversal looks like this:
\begin{verbatim}
/* table is in the stack at index `t' */
lua_pushnil(L); /* first key */
while (lua_next(L, t) != 0) {
/* `key' is at index -2 and `value' at index -1 */
printf("%s - %s\n",
lua_typename(L, lua_type(L, -2)), lua_typename(L, lua_type(L, -1)));
lua_pop(L, 1); /* removes `value'; keeps `index' for next iteration */
}
\end{verbatim}
The function
\begin{verbatim}
void lua_concat (lua_State *L, int n);
\end{verbatim}
\DefAPI{lua_concat}
concatenates the \verb|n| values at the top of the stack,
pops them, and leaves the result at the top;
\verb|n|~must be at least 2.
Concatenation is done following the usual semantics of Lua
\see{concat}.
\subsection{Defining C Functions} \label{LuacallC}
To register a C~function to Lua,
there is the following convenience macro:
\begin{verbatim}
#define lua_register(L, n, f) (lua_pushcfunction(L, f), lua_setglobal(L, n))
/* const char *n; */
/* lua_CFunction f; */
\end{verbatim}
\DefAPI{lua_register}
which receives the name the function will have in Lua,
and a pointer to the function.
This pointer must have type \verb|lua_CFunction|,
which is defined as
\begin{verbatim}
typedef int (*lua_CFunction) (lua_State *L);
\end{verbatim}
\DefAPI{lua_CFunction}
that is, a pointer to a function with integer result and a single argument,
a Lua environment.
In order to communicate properly with Lua,
a C~function must follow the following protocol,
which defines the way parameters and results are passed:
A C~function receives its arguments from Lua in the stack,
in direct order (the first argument is pushed first).
To return values to Lua, a C~function just pushes them onto the stack,
in direct order (the first result is pushed first),
and returns the number of results.
Like a Lua function, a C~function called by Lua can also return
many results.
As an example, the following function receives a variable number
of numerical arguments and returns their average and sum:
\begin{verbatim}
static int foo (lua_State *L) {
int n = lua_gettop(L); /* number of arguments */
double sum = 0;
int i;
for (i = 1; i <= n; i++) {
if (!lua_isnumber(L, i))
lua_error(L, "incorrect argument to function `average'");
sum += lua_tonumber(L, i);
}
lua_pushnumber(L, sum/n); /* first result */
lua_pushnumber(L, sum); /* second result */
return 2; /* number of results */
}
\end{verbatim}
This function may be registered in Lua as `\verb|average|' by calling
\begin{verbatim}
lua_register(L, "average", foo);
\end{verbatim}
When a C~function is created,
it is possible to associate some \emph{upvalues} to it
\see{upvalue},
thus creating a \IndexEmph{C~closure};
these values are passed to the function whenever it is called,
as ordinary arguments.
To associate upvalues to a C~function,
first these values should be pushed onto the stack
(when there are multiple upvalues,
the first upvalue is pushed first).
Then the function
\begin{verbatim}
void lua_pushcclosure (lua_State *L, lua_CFunction fn, int n);
\end{verbatim}
\DefAPI{lua_pushcclosure}
is used to push the C~function onto the stack,
with the argument \verb|n| telling how many upvalues should be
associated with the function
(these upvalues are popped from the stack);
in fact, the macro \verb|lua_pushcfunction| is defined as
\verb|lua_pushcclosure| with \verb|n| set to 0.
Then, whenever the C~function is called,
these upvalues are inserted as the \emph{last} arguments to the function,
after the actual arguments provided in the call.
This makes it easy to get the upvalues without knowing how many arguments
the function received (recall that functions in Lua can receive any number of
arguments): The \M{i}-th upvalue is in the stack at index \Math{i-(n+1)},
where \M{n} is the number of upvalues.
For more examples of C~functions and closures, see files
\verb|lbaselib.c|, \verb|liolib.c|, \verb|lmathlib.c|, and \verb|lstrlib.c|
in the official Lua distribution.
\subsection{References to Lua Objects}
If the C~code needs to keep a Lua value
outside the life span of a C~function,
then it must create a \Def{reference} to the value.
The functions to manipulate references are the following:
\begin{verbatim}
int lua_ref (lua_State *L, int lock);
int lua_getref (lua_State *L, int ref);
void lua_unref (lua_State *L, int ref);
\end{verbatim}
\DefAPI{lua_ref}\DefAPI{lua_getref}\DefAPI{lua_unref}
\verb|lua_ref| pops a value from
the stack, creates a reference to it,
and returns this reference.
For a \nil\ value,
the reference is always \verb|LUA_REFNIL|.\DefAPI{LUA_REFNIL}
(\verb|lua.h| also defines a constant \verb|LUA_NOREF| \DefAPI{LUA_NOREF}
that
is different from any valid reference.)
If \verb|lock| is not zero, then the object is \emph{locked}:
this means the object will not be garbage collected.
\emph{Unlocked references may be garbage collected}.
Whenever the referenced object is needed in~C,
a call to \verb|lua_getref|
pushes that object onto the stack;
if the object has been collected,
\verb|lua_getref| returns 0 (and does not push anything).
When a reference is no longer needed,
it should be released with a call to \verb|lua_unref|.
\subsubsection*{Registry}
When Lua starts, it registers a table at position
\IndexAPI{LUA_REFREGISTRY}.
It can be accessed through the macro
\begin{verbatim}
#define lua_getregistry(L) lua_getref(L, LUA_REFREGISTRY)
\end{verbatim}
\DefAPI{lua_getregistry}
This table can be used by C~libraries as a general registry mechanism.
Any C~library can store data into this table,
as long as it chooses a key different from other libraries.
\subsection{Weak Tables}
The following constants and functions control the weak mode of a table:
\begin{verbatim}
#define LUA_WEAK_KEY ...
#define LUA_WEAK_VALUE ...
\end{verbatim}
\begin{verbatim}
void lua_setweakmode (lua_State *L, int mode);
int lua_getweakmode (lua_State *L, int index);
\end{verbatim}
\DefAPI{lua_setweakmode}\DefAPI{lua_getweakmode}
Both functions operate over the table at the top of the stack.
Modes are described as bit sets, so that
\verb|LUA_WEAK_KEY| means weak keys,
\verb|LUA_WEAK_VALUE| means weak values,
\verb|LUA_WEAK_KEY | LUA_WEAK_VALUE| means both,
and zero means none.
\section{Standard Libraries}
The standard libraries provide useful functions
that are implemented directly through the standard API.
Therefore, they are not necessary to the language,
and are provided as separate C~modules.
Currently, Lua has the following standard libraries:
\begin{itemize}
\item basic library;
\item string manipulation;
\item mathematical functions (sin, log, etc);
\item input and output (plus some system facilities).
\end{itemize}
To have access to these libraries,
the C~host program must call the functions
\verb|lua_baselibopen|,
\verb|lua_strlibopen|, \verb|lua_mathlibopen|,
and \verb|lua_iolibopen|, which are declared in \verb|lualib.h|.
\DefAPI{lua_baselibopen}
\DefAPI{lua_strlibopen}
\DefAPI{lua_mathlibopen}
\DefAPI{lua_iolibopen}
\subsection{Basic Functions} \label{predefined}
The basic library provides some core functions to Lua.
Therefore, if you do not include this library in your application,
you should check carefully whether you need to provide some alternative
implementation for some facilities.
(For instance,
without function \verb|_ERRORMESSAGE|,
Lua is unable to show error messages.)
\subsubsection*{\ff \T{_ALERT (message)}}\DefLIB{alert}\label{alert}
Prints its only string argument to \IndexVerb{stderr}.
All error messages in Lua are printed through the function stored
in the \verb|_ALERT| global variable
\see{error}.
Therefore, a program may assign another function to this variable
to change the way such messages are shown
(for instance, for systems without \verb|stderr|).
\subsubsection*{\ff \T{assert (v [, message])}}\DefLIB{assert}
Issues an \emph{``assertion failed!''} error
when its argument \verb|v| is \nil;
otherwise, returns this argument.
This function is equivalent to the following Lua function:
\begin{verbatim}
function assert (v, m)
if not v then
m = m or ""
error("assertion failed! " .. m)
end
return v
end
\end{verbatim}
\subsubsection*{\ff \T{call (func, arg [, mode [, errhandler]])}}\DefLIB{call}
\label{pdf-call}
Calls function \verb|func| with
the arguments given by the table \verb|arg|.
The call is equivalent to
\begin{verbatim}
func(arg[1], arg[2], ..., arg[n])
\end{verbatim}
where \verb|n| is the result of \verb|getn(arg)| \see{getn}.
All results from \verb|func| are simply returned by \verb|call|.
By default,
if an error occurs during the call to \verb|func|,
the error is propagated.
If the string \verb|mode| contains \verb|"x"|,
then the call is \emph{protected}.\index{protected calls}
In this mode, function \verb|call| does not propagate an error,
regardless of what happens during the call.
Instead, it returns \nil\ to signal the error
(besides calling the appropriated error handler).
If \verb|errhandler| is provided,
the error function \verb|_ERRORMESSAGE| is temporarily set to \verb|errhandler|,
while \verb|func| runs.
In particular, if \verb|errhandler| is \nil,
no error messages will be issued during the execution of the called function.
\subsubsection*{\ff \T{collectgarbage ([limit])}}\DefLIB{collectgarbage}
Sets the garbage-collection threshold for the given limit
(in Kbytes), and checks it against the byte counter.
If the new threshold is smaller than the byte counter,
then Lua immediately runs the garbage collector \see{GC}.
If \verb|limit| is absent, it defaults to zero
(thus forcing a garbage-collection cycle).
%\verb|collectgarbage| is equivalent to
%the API function \verb|lua_setgcthreshold|.
\subsubsection*{\ff \T{copytagmethods (tagto, tagfrom)}}
\DefLIB{copytagmethods}
Copies all tag methods from one tag to another;
returns \verb|tagto|.
\subsubsection*{\ff \T{dofile (filename)}}\DefLIB{dofile}
Receives a file name,
opens the named file, and executes its contents as a Lua chunk,
or as pre-compiled chunks.
When called without arguments,
\verb|dofile| executes the contents of the standard input (\verb|stdin|).
If there is any error executing the file,
then \verb|dofile| returns \nil{} plus one of the following strings
describing the error:
\verb|"file error"|, \verb|"run-time error"|,
\verb|"syntax error"|, \verb|"memory error"|, or
\verb|"error in error handling"|.
Otherwise, it returns the values returned by the chunk,
or a non-\nil\ value if the chunk returns no values.
It issues an error when called with a non-string argument.
\subsubsection*{\ff \T{dostring (string [, chunkname])}}\DefLIB{dostring}
Executes a given string as a Lua chunk.
If there is any error executing the string,
then \verb|dostring| returns \nil plus a string describing
the error (see \verb|dofile|).
Otherwise, it returns the values returned by the chunk,
or a non-\nil\ value if the chunk returns no values.
The optional parameter \verb|chunkname|
is the ``name of the chunk'',
used in error messages and debug information.
\subsubsection*{\ff \T{error (message)}}\DefLIB{error}\label{pdf-error}
Calls the error handler \see{error} and then terminates
the last protected function called
(in~C: \verb|lua_dofile|, \verb|lua_dostring|,
\verb|lua_dobuffer|, or \verb|lua_callfunction|;
in Lua: \verb|dofile|, \verb|dostring|, or \verb|call| in protected mode).
If \verb|message| is \nil, then the error handler is not called.
Function \verb|error| never returns.
%\verb|error| is equivalent to the API function \verb|lua_error|.
\subsubsection*{\ff \T{foreach (table, func)}}\DefLIB{foreach}
Executes the given \verb|func| over all elements of \verb|table|.
For each element, the function is called with the index and
respective value as arguments.
If the function returns any non-\nil\ value,
then the loop is broken, and this value is returned
as the final value of \verb|foreach|.
This function could be defined in Lua:
\begin{verbatim}
function foreach (t, f)
for i, v in t do
local res = f(i, v)
if res then return res end
end
end
\end{verbatim}
The behavior of \verb|foreach| is \emph{undefined} if you change
the table \verb|t| during the traversal.
\subsubsection*{\ff \T{foreachi (table, func)}}\DefLIB{foreachi}
Executes the given \verb|func| over the
numerical indices of \verb|table|.
For each index, the function is called with the index and
respective value as arguments.
Indices are visited in sequential order,
from~1 to \verb|n|,
where \verb|n| is the result of \verb|getn(table)| (see below).
If the function returns any non-\nil\ value,
then the loop is broken, and this value is returned
as the final value of \verb|foreachi|.
This function could be defined in Lua:
\begin{verbatim}
function foreachi (t, f)
for i=1,getn(t) do
local res = f(i, t[i])
if res then return res end
end
end
\end{verbatim}
\subsubsection*{\ff \T{gcinfo ()}}\DefLIB{gcinfo}
Returns the number of Kbytes of dynamic memory Lua is using,
and (as a second result) the
current garbage collector threshold (also in Kbytes).
\subsubsection*{\ff \T{getglobal (name)}}\DefLIB{getglobal}
Gets the value of a global variable,
or calls a tag method for ``getglobal''.
Its full semantics is explained in \See{tag-method}.
The string \verb|name| does not need to be a
syntactically valid variable name.
\subsubsection*{\ff \T{getn (table)}}\DefLIB{getn}\label{getn}
Returns the ``size'' of a table, when seen as a list.
If the table has an \verb|n| field with a numeric value,
this value is the ``size'' of the table.
Otherwise, the ``size'' is the largest numerical index with a non-nil
value in the table.
This function could be defined in Lua:
\begin{verbatim}
function getn (t)
if type(t.n) == "number" then return t.n end
local max = 0
for i, _ in t do
if type(i) == "number" and i>max then max=i end
end
return max
end
\end{verbatim}
\subsubsection*{\ff \T{gettagmethod (tag, event)}}
\DefLIB{gettagmethod}
Returns the current tag method
for a given pair \M{(tag, event)}.
This function cannot be used to get a tag method for the ``gc'' event.
(Such tag methods can only be manipulated by C~code.)
\subsubsection*{\ff \T{globals ([table])}}\DefLIB{globals}
Returns the current table of globals.
If the argument \verb|table| is given,
then it also sets this table as the table of globals.
\subsubsection*{\ff \T{newtype (name)}}\DefLIB{newtype}\label{pdf-newtype}
Creates a new type with the given name
(which can be used only for table objects).
Returns the tag of the new type.
\subsubsection*{\ff \T{next (table, [index])}}\DefLIB{next}
Allows a program to traverse all fields of a table.
Its first argument is a table and its second argument
is an index in this table.
\verb|next| returns the next index of the table and the
value associated with the index.
When called with \nil\ as its second argument,
\verb|next| returns the first index
of the table and its associated value.
When called with the last index,
or with \nil\ in an empty table,
\verb|next| returns \nil.
If the second argument is absent, then it is interpreted as \nil.
Lua has no declaration of fields;
semantically, there is no difference between a
field not present in a table or a field with value \nil.
Therefore, \verb|next| only considers fields with non-\nil\ values.
The order in which the indices are enumerated is not specified,
\emph{even for numeric indices}
(to traverse a table in numeric order,
use a numerical \rwd{for} or the function \verb|foreachi|).
The behavior of \verb|next| is \emph{undefined} if you change
the table during the traversal.
\subsubsection*{\ff \T{print (e1, e2, ...)}}\DefLIB{print}
Receives any number of arguments,
and prints their values in \verb|stdout|,
using the strings returned by \verb|tostring|.
This function is not intended for formatted output,
but only as a quick way to show a value,
for instance for debugging.
See \See{libio} for functions for formatted output.
\subsubsection*{\ff \T{rawget (table, index)}}\DefLIB{rawget}
Gets the real value of \verb|table[index]|,
without invoking any tag method.
\verb|table| must be a table,
and \verb|index| is any value different from \nil.
\subsubsection*{\ff \T{rawset (table, index, value)}}\DefLIB{rawset}
Sets the real value of \verb|table[index]| to \verb|value|,
without invoking any tag method.
\verb|table| must be a table,
\verb|index| is any value different from \nil,
and \verb|value| is any Lua value.
\subsubsection*{\ff \T{rawtype (v)}}\DefLIB{rawtype}
Returns the basic (raw) type of its only argument, coded as a string.
The possible results of this function are
\verb|"nil"| (a string, not the value \nil),
\verb|"number"|,
\verb|"string"|,
\verb|"table"|,
\verb|"function"|,
and \verb|"userdata"|.
\subsubsection*{\ff \T{require (module)}}\DefLIB{require}
TO BE WRITTEN.
\subsubsection*{\ff \T{setglobal (name, value)}}\DefLIB{setglobal}
Sets the named global variable to the given value,
or calls a tag method for ``setglobal''.
Its full semantics is explained in \See{tag-method}.
The string \verb|name| does not need to be a
syntactically valid variable name.
\subsubsection*{\ff \T{settype (t, type)}}\DefLIB{settype}\label{pdf-settype}
Sets the type of a given table \see{TypesSec}.
\verb|type| must be the name or the tag of a user-defined type.
\verb|settag| returns the value of its first argument (the table).
For the safety of host programs,
it is impossible to change the tag of a userdata from Lua.
\subsubsection*{\ff \T{settagmethod (tag, event, newmethod)}}
\DefLIB{settagmethod}
Sets a new tag method to the given pair \M{(tag, event)} and
returns the old method.
If \verb|newmethod| is \nil,
then \verb|settagmethod| restores the default behavior for the given event.
This function cannot be used to set a tag method for the ``gc'' event.
(Such tag methods can only be manipulated by C~code.)
\subsubsection*{\ff \T{sort (table [, comp])}}\DefLIB{sort}
Sorts table elements in a given order, \emph{in-place},
from \verb|table[1]| to \verb|table[n]|,
where \verb|n| is the result of \verb|getn(table)| \see{getn}.
If \verb|comp| is given,
then it must be a function that receives two table elements,
and returns true (that is, a value different from \nil)
when the first is less than the second
(so that \verb|not comp(a[i+1], a[i])| will be true after the sort).
If \verb|comp| is not given,
then the standard Lua operator \verb|<| is used instead.
The sort algorithm is \emph{not} stable
(that is, elements considered equal by the given order
may have their relative positions changed by the sort).
\subsubsection*{\ff \T{tag (v)}}\DefLIB{tag}\label{pdf-tag}
Allows Lua programs to test the tag of a value \see{TypesSec}.
It receives one argument, and returns its tag (a number).
%\verb|tag| is equivalent to the API function \verb|lua_tag|.
\subsubsection*{\ff \T{tonumber (e [, base])}}\DefLIB{tonumber}
Tries to convert its argument to a number.
If the argument is already a number or a string convertible
to a number, then \verb|tonumber| returns that number;
otherwise, it returns \nil.
An optional argument specifies the base to interpret the numeral.
The base may be any integer between 2 and 36, inclusive.
In bases above~10, the letter `A' (either upper or lower case)
represents~10, `B' represents~11, and so forth, with `Z' representing 35.
In base 10 (the default), the number may have a decimal part,
as well as an optional exponent part \see{coercion}.
In other bases, only unsigned integers are accepted.
\subsubsection*{\ff \T{tostring (e)}}\DefLIB{tostring}
Receives an argument of any type and
converts it to a string in a reasonable format.
For complete control of how numbers are converted,
use function \verb|format|.
\subsubsection*{\ff \T{tinsert (table, [pos,] value)}}\DefLIB{tinsert}
Inserts element \verb|value| at table position \verb|pos|,
shifting other elements to open space, if necessary.
The default value for \verb|pos| is \verb|n+1|,
where \verb|n| is the result of \verb|getn(table)| \see{getn},
so that a call \verb|tinsert(t,x)| inserts \verb|x| at the end
of table \verb|t|.
This function also sets or increments the field \verb|n| of the table
to \verb|n+1|.
This function is equivalent to the following Lua function,
except that the table accesses are all \emph{raw}
(that is, without tag methods):
\begin{verbatim}
function tinsert (t, ...)
local pos, value
local n = getn(t)
if arg.n == 1 then
pos, value = n+1, arg[1]
else
pos, value = arg[1], arg[2]
end
t.n = n+1;
for i=n,pos,-1 do
t[i+1] = t[i]
end
t[pos] = value
end
\end{verbatim}
\subsubsection*{\ff \T{tremove (table [, pos])}}\DefLIB{tremove}
Removes from \verb|table| the element at position \verb|pos|,
shifting other elements to close the space, if necessary.
Returns the value of the removed element.
The default value for \verb|pos| is \verb|n|,
where \verb|n| is the result of \verb|getn(table)| \see{getn},
so that a call \verb|tremove(t)| removes the last element
of table \verb|t|.
This function also sets or decrements the field \verb|n| of the table
to \verb|n-1|.
This function is equivalent to the following Lua function,
except that the table accesses are all \emph{raw}
(that is, without tag methods):
\begin{verbatim}
function tremove (t, pos)
local n = getn(t)
if n<=0 then return end
pos = pos or n
local value = t[pos]
for i=pos,n-1 do
t[i] = t[i+1]
end
t[n] = nil
t.n = n-1
return value
end
\end{verbatim}
\subsubsection*{\ff \T{type (v)}}\DefLIB{type}\label{pdf-type}
Returns the type name of its only argument.
\subsubsection*{\ff \T{unpack (list)}}\DefLIB{unpack}
Returns all elements from the given list.
This function is equivalent to
\begin{verbatim}
return list[1], list[2], ..., list[n]
\end{verbatim}
except that the above code can be valid only for a fixed \M{n}.
The number of returned values, \M{n},
is the result of \verb|getn(list)| \see{getn},
\subsubsection*{\ff \T{weakmode (table, mode)}}\DefLIB{weakmode}\label{weakmode}
Controls the weakness of a table.
When \verb|mode| is \verb|"?"|,
returns the current mode of the table, as a string;
otherwise, sets the weakmode of the table to the given mode (also a string).
Valid mode strings are \verb|"k"| for weak keys,
\verb|"v"| for weak values,
\verb|"kv"| for both,
and \verb|""| for none (that is, for ``normal'' tables).
\subsection{String Manipulation}
This library provides generic functions for string manipulation,
such as finding and extracting substrings and pattern matching.
When indexing a string in Lua, the first character is at position~1
(not at~0, as in C).
Also,
indices are allowed to be negative and are interpreted as indexing backwards,
from the end of the string. Thus, the last character is at position \Math{-1},
and so on.
\subsubsection*{\ff \T{strbyte (s [, i])}}\DefLIB{strbyte}
Returns the internal numerical code of the \M{i}-th character of \verb|s|.
If \verb|i| is absent, then it is assumed to be~1.
\verb|i| may be negative.
\NOTE
Numerical codes are not necessarily portable across platforms.
\subsubsection*{\ff \T{strchar (i1, i2, \ldots)}}\DefLIB{strchar}
Receives 0 or more integers.
Returns a string with length equal to the number of arguments,
wherein each character has the internal numerical code equal
to its correspondent argument.
\NOTE
Numerical codes are not necessarily portable across platforms.
\subsubsection*{\ff \T{strfind (s, pattern [, init [, plain]])}}
\DefLIB{strfind}
Looks for the first \emph{match} of
\verb|pattern| in \verb|s|.
If it finds one, then \verb|strfind| returns the indices of \verb|s|
where this occurrence starts and ends;
otherwise, it returns \nil.
If the pattern specifies captures (see \verb|gsub| below),
the captured strings are returned as extra results.
A third, optional numerical argument \verb|init| specifies
where to start the search;
its default value is~1, and may be negative.
A value of~1 as a fourth, optional argument \verb|plain|
turns off the pattern matching facilities,
so the function does a plain ``find substring'' operation,
with no characters in \verb|pattern| being considered ``magic''.
Note that if \verb|plain| is given, then \verb|init| must be given too.
\subsubsection*{