Copyright © 2003 TeCGraf, PUC-Rio. All rights reserved.
Lua is an extension programming language designed to support general procedural programming with data description facilities. It also offers good suport for object-oriented programming, functional programming, and data-driven programming. Lua is intended to be used as a powerful, light-weight configuration language for any program that needs one. Lua is implemented as a library, written in clean C (that is, in the common subset of ANSI C and C++).
Being an extension language, Lua has no notion of a "main" program: it only works embedded in a host client, called the embedding program or simply the host. This host program can invoke functions to execute a piece of Lua code, can write and read Lua variables, and can register C functions to be called by Lua code. Through the use of C functions, Lua can be augmented to cope with a wide range of different domains, thus creating customized programming languages sharing a syntactical framework.
The Lua distribuition includes a stand-alone embedding program,
lua
, that uses the Lua library to offer a complete Lua interpreter.
Lua is free software,
and is provided as usual with no guarantees,
as stated in its copyright notice.
The implementation described in this manual is available
at Lua's official web site, www.lua.org
.
Like any other reference manual, this document is dry in places. For a discussion of the decisions behind the design of Lua, see the papers below, which are available at Lua's web site.
Lua means "moon" in Portuguese and is pronounced LOO-ah.
This section describes the lexis, the syntax, and the semantics of Lua. In other words, this section describes which tokens are valid, how they can be combined, and what their combinations mean.
The language constructs will be explained using the usual extended BNF,
in which
{a} means 0 or more a's, and
[a] means an optional a.
Non-terminals are shown in italics,
keywords are shown in bold,
and other terminal symbols are shown in typewriter
font,
enclosed in single quotes.
Identifiers in Lua can be any string of letters, digits, and underscores, not beginning with a digit. This coincides with the definition of identifiers in most languages. (The definition of letter depends on the current locale: any character considered alphabetic by the current locale can be used in an identifier.)
The following keywords are reserved and cannot be used as identifiers:
and break do else elseif end false for function if in local nil not or repeat return then true until while
Lua is a case-sensitive language:
and
is a reserved word, but And
and AND
are two different, valid identifiers.
As a convention, identifiers starting with an underscore followed by
uppercase letters (such as _VERSION
)
are reserved for internal variables used by Lua.
The following strings denote other tokens:
+ - * / ^ = ~= <= >= < > == ( ) { } [ ] ; : , . .. ...
Literal strings can be delimited by matching single or double quotes, and can contain the following C-like escape sequences:
\a
--- bell
\b
--- backspace
\f
--- form feed
\n
--- newline
\r
--- carriage return
\t
--- horizontal tab
\v
--- vertical tab
\\
--- backslash
\"
--- quotation mark
\'
--- apostrophe
\[
--- left square bracket
\]
--- right square bracket
\
newline´
(that is, a backslash followed by a real newline)
results in a newline in the string.
A character in a string may also be specified by its numerical value
using the escape sequence `\
ddd´,
where ddd is a sequence of up to three decimal digits.
Strings in Lua may contain any 8-bit value, including embedded zeros,
which can be specified as `\0
´.
Literal strings can also be delimited by matching double square brackets
[[
· · · ]]
.
Literals in this bracketed form may run for several lines,
may contain nested [[
· · · ]]
pairs,
and do not interpret any escape sequences.
For convenience,
when the opening `[[
´ is immediately followed by a newline,
the newline is not included in the string.
As an example, in a system using ASCII
(in which `a
´ is coded as 97,
newline is coded as 10, and `1
´ is coded as 49),
the four literals below denote the same string:
(1) "alo\n123\"" (2) '\97lo\10\04923"' (3) [[alo 123"]] (4) [[ alo 123"]]
Numerical constants may be written with an optional decimal part and an optional decimal exponent. Examples of valid numerical constants are
3 3.0 3.1416 314.16e-2 0.31416E1
Comments start anywhere outside a string with a
double hyphen (--
).
If the text immediately after --
is different from [[
,
the comment is a short comment,
which runs until the end of the line.
Otherwise, it is a long comment,
which runs until the corresponding ]]
.
Long comments may run for several lines
and may contain nested [[
· · · ]]
pairs.
For convenience,
the first line of a chunk is skipped if it starts with #
.
This facility allows the use of Lua as a script interpreter
in Unix systems (see 6).
Lua is a dynamically typed language. That means that variables do not have types; only values do. There are no type definitions in the language. All values carry their own type.
There are eight basic types in Lua:
nil, boolean, number,
string, function, userdata, thread, and table.
Nil is the type of the value nil,
whose main property is to be different from any other value;
usually it represents the absence of a useful value.
Boolean is the type of the values false and true.
In Lua, both nil and false make a condition false;
any other value makes it true.
Number represents real (double-precision floating-point) numbers.
(It is easy to build Lua interpreters that use other
internal representations for numbers,
such as single-precision float or long integers.)
String represents arrays of characters.
Lua is 8-bit clean:
Strings may contain any 8-bit character,
including embedded zeros ('\0'
) (see 2.1).
Functions are first-class values in Lua. That means that functions can be stored in variables, passed as arguments to other functions, and returned as results. Lua can call (and manipulate) functions written in Lua and functions written in C (see 2.5.7).
The type userdata is provided to allow arbitrary C data to be stored in Lua variables. This type corresponds to a block of raw memory and has no pre-defined operations in Lua, except assignment and identity test. However, by using metatables, the programmer can define operations for userdata values (see 2.8). Userdata values cannot be created or modified in Lua, only through the C API. This guarantees the integrity of data owned by the host program.
The type thread represents independent threads of execution and it is used to implement coroutines.
The type table implements associative arrays,
that is, arrays that can be indexed not only with numbers,
but with any value (except nil).
Moreover,
tables can be heterogeneous,
that is, they can contain values of all types (except nil).
Tables are the sole data structuring mechanism in Lua;
they may be used to represent ordinary arrays,
symbol tables, sets, records, graphs, trees, etc.
To represent records, Lua uses the field name as an index.
The language supports this representation by
providing a.name
as syntactic sugar for a["name"]
.
There are several convenient ways to create tables in Lua
(see 2.5.6).
Like indices, the value of a table field can be of any type (except nil). In particular, because functions are first class values, table fields may contain functions. Thus tables may also carry methods (see 2.5.8).
Tables, functions, and userdata values are objects: variables do not actually contain these values, only references to them. Assignment, parameter passing, and function returns always manipulate references to such values; these operations do not imply any kind of copy.
The library function type
returns a string describing the type
of a given value (see 5.1).
Lua provides automatic conversion between
string and number values at run time.
Any arithmetic operation applied to a string tries to convert
that string to a number, following the usual rules.
Conversely, whenever a number is used where a string is expected,
the number is converted to a string, in a reasonable format.
For complete control of how numbers are converted to strings,
use the format
function from the string library (see 5.3).
Variables are places that store values. There are three kinds of variables in Lua: global variables, local variables, and table fields.
A single name can denote a global variable or a local variable (or a formal parameter of a function, which is a particular form of local variable):
var ::= NameVariables are assumed to be global unless explicitly declared local (see 2.4.7). Local variables are lexically scoped: Local variables can be freely accessed by functions defined inside their scope (see 2.6).
Before the first assignment to a variable, its value is nil.
Square brackets are used to index a table:
var ::= prefixexp `[´ exp `]´The first expression (prefixexp)should result in a table value; the second expression (exp) identifies a specific entry inside that table. The expression denoting the table to be indexed has a restricted syntax; see 2.5 for details.
The syntax var.NAME
is just syntactic sugar for
var["NAME"]
:
var ::= prefixexp `.´ Name
The meaning of accesses to global variables
and table fields can be changed via metatables.
An access to an indexed variable t[i]
is equivalent to
a call gettable_event(t,i)
.
(See 2.8 for a complete description of the
gettable_event
function.
This function is not defined or callable in Lua.
We use it here only for explanatory purposes.)
All global variables live as fields in ordinary Lua tables,
called environment tables or simply environments.
Functions written in C and exported to Lua (C functions)
all share a common global environment.
Each function written in Lua (a Lua function)
has its own reference to an environment,
so that all global variables in that function
will refer to that environment table.
When a function is created,
it inherits the environment from the function that created it.
To change or get the environment table of a Lua function,
you call setfenv
or getfenv
(see 5.1).
An access to a global variable x
is equivalent to _env.x
,
which in turn is equivalent to
gettable_event(_env, "x")where
_env
is the environment of the running function.
(The _env
variable is not defined in Lua.
We use it here only for explanatory purposes.)
Lua supports an almost conventional set of statements, similar to those in Pascal or C. This set includes assignment, control structures, procedure calls, table constructors, and variable declarations.
The unit of execution of Lua is called a chunk. A chunk is simply a sequence of statements, which are executed sequentially. Each statement can be optionally followed by a semicolon:
chunk ::= {stat [`;´]}
Lua handles a chunk as the body of an anonymous function (see 2.5.8). As such, chunks can define local variables and return values.
A chunk may be stored in a file or in a string inside the host program. When a chunk is executed, first it is pre-compiled into opcodes for a virtual machine, and then the compiled code is executed by an interpreter for the virtual machine.
Chunks may also be pre-compiled into binary form;
see program luac
for details.
Programs in source and compiled forms are interchangeable;
Lua automatically detects the file type and acts accordingly.
block ::= chunk
A block may be explicitly delimited to produce a single statement:
stat ::= do block endExplicit blocks are useful to control the scope of variable declarations. Explicit blocks are also sometimes used to add a return or break statement in the middle of another block (see 2.4.4).
Lua allows multiple assignment. Therefore, the syntax for assignment defines a list of variables on the left side and a list of expressions on the right side. The elements in both lists are separated by commas:
stat ::= varlist1 `=´ explist1 varlist1 ::= var {`,´ var} explist1 ::= exp {`,´ exp}Expressions are discussed in 2.5.
Before the assignment, the list of values is adjusted to the length of the list of variables. If there are more values than needed, the excess values are thrown away. If there are fewer values than needed, the list is extended with as many nil's as needed. If the list of expressions ends with a function call, then all values returned by that function call enter in the list of values, before the adjustment (except when the call is enclosed in parentheses; see 2.5).
The assignment statement first evaluates all its expressions and only then are the assignments performed. Thus the code
i = 3 i, a[i] = i+1, 20sets
a[3]
to 20, without affecting a[4]
because the i
in a[i]
is evaluated (to 3)
before it is assigned 4.
Similarly, the line
x, y = y, xexchanges the values of
x
and y
.
The meaning of assignments to global variables
and table fields can be changed via metatables.
An assignment to an indexed variable t[i] = val
is equivalent to
settable_event(t,i,val)
.
(See 2.8 for a complete description of the
settable_event
function.
This function is not defined or callable in Lua.
We use it here only for explanatory purposes.)
An assignment to a global variable x = val
is equivalent to the assignment
_env.x = val
,
which in turn is equivalent to
settable_event(_env, "x", val)where
_env
is the environment of the running function.
(The _env
variable is not defined in Lua.
We use it here only for explanatory purposes.)
2.4.4 - Control Structures
The control structures
if, while, and repeat have the usual meaning and
familiar syntax:
stat ::= while exp do block end stat ::= repeat block until exp stat ::= if exp then block {elseif exp then block} [else block] endLua also has a for statement, in two flavors (see 2.4.5).
The condition expression exp of a control structure may return any value. Both false and nil are considered false. All values different from nil and false are considered true (in particular, the number 0 and the empty string are also true).
The return statement is used to return values from a function or from a chunk. Functions and chunks may return more than one value, so the syntax for the return statement is
stat ::= return [explist1]
The break statement can be used to terminate the execution of a while, repeat, or for loop, skiping to the next statement after the loop:
stat ::= breakA break ends the innermost enclosing loop.
For syntactic reasons, return and break
statements can only be written as the last statement of a block.
If it is really necessary to return or break in the
middle of a block,
then an explicit inner block can be used,
as in the idioms
`do return end
´ and
`do break end
´,
because now return and break are the last statements in
their (inner) blocks.
In practice,
those idioms are only used during debugging.
The for statement has two forms: one numeric and one generic.
The numeric for loop repeats a block of code while a control variable runs through an arithmetic progression. It has the following syntax:
stat ::= for Name `=´ exp `,´ exp [`,´ exp] do block endThe block is repeated for name starting at the value of the first exp, until it passes the second exp by steps of the third exp. More precisely, a for statement like
for var = e1, e2, e3 do block endis equivalent to the code:
do local var, _limit, _step = tonumber(e1), tonumber(e2), tonumber(e3) if not (var and _limit and _step) then error() end while (_step>0 and var<=_limit) or (_step<=0 and var>=_limit) do block var = var + _step end endNote the following:
_limit
and _step
are invisible variables.
The names are here for explanatory purposes only.
var
inside
the block.
var
is local to the statement;
you cannot use its value after the for ends or is broken.
If you need the value of the loop variable var
,
then assign it to another variable before breaking or exiting the loop.
The generic for statement works over functions, called iterators. For each iteration, it calls its iterator function to produce a new value, stopping when the new value is nil. The generic for loop has the following syntax:
stat ::= for Name {`,´ Name} in explist1 do block endA for statement like
for var_1, ..., var_n in explist do block endis equivalent to the code:
do local _f, _s, var_1 = explist local var_2, ... , var_n while true do var_1, ..., var_n = _f(_s, var_1) if var_1 == nil then break end block end endNote the following:
explist
is evaluated only once.
Its results are an iterator function,
a state, and an initial value for the first iterator variable.
_f
and _s
are invisible variables.
The names are here for explanatory purposes only.
var_1
inside the block.
var_i
are local to the statement;
you cannot use their values after the for ends.
If you need these values,
then assign them to other variables before breaking or exiting the loop.
2.4.6 - Function Calls as Statements
To allow possible side-effects,
function calls can be executed as statements:
stat ::= functioncallIn this case, all returned values are thrown away. Function calls are explained in 2.5.7.
2.4.7 - Local Declarations
Local variables may be declared anywhere inside a block.
The declaration may include an initial assignment:
stat ::= local namelist [`=´ explist1] namelist ::= Name {`,´ Name}If present, an initial assignment has the same semantics of a multiple assignment (see 2.4.3). Otherwise, all variables are initialized with nil.
A chunk is also a block (see 2.4.1), so local variables can be declared in a chunk outside any explicit block. Such local variables die when the chunk ends.
The visibility rules for local variables are explained in 2.6.
The basic expressions in Lua are the following:
exp ::= prefixexp exp ::= nil | false | true exp ::= Number exp ::= Literal exp ::= function exp ::= tableconstructor prefixexp ::= var | functioncall | `(´ exp `)´
Numbers and literal strings are explained in 2.1; variables are explained in 2.3; function definitions are explained in 2.5.8; function calls are explained in 2.5.7; table constructors are explained in 2.5.6.
An expression enclosed in parentheses always results in only one value.
Thus,
(f(x,y,z))
is always a single value,
even if f
returns several values.
(The value of (f(x,y,z))
is the first value returned by f
or nil if f
does not return any values.)
Expressions can also be built with arithmetic operators, relational operators, and logical operators, all of which are explained below.
+
(addition),
-
(subtraction), *
(multiplication),
/
(division), and ^
(exponentiation);
and unary -
(negation).
If the operands are numbers, or strings that can be converted to
numbers (see 2.2.1),
then all operations except exponentiation have the usual meaning.
Exponentiation calls a global function __pow
;
otherwise, an appropriate metamethod is called (see 2.8).
The standard mathematical library defines function __pow
,
giving the expected meaning to exponentiation
(see 5.5).
2.5.2 - Relational Operators
The relational operators in Lua are
== ~= < > <= >=These operators always result in false or true.
Equality (==
) first compares the type of its operands.
If the types are different, then the result is false.
Otherwise, the values of the operands are compared.
Numbers and strings are compared in the usual way.
Objects (tables, userdata, threads, and functions)
are compared by reference:
Two objects are considered equal only if they are the same object.
Every time you create a new object (a table, userdata, or function),
this new object is different from any previously existing object.
You can change the way that Lua compares tables and userdata using the "eq" metamethod (see 2.8).
The conversion rules of 2.2.1
do not apply to equality comparisons.
Thus, "0"==0
evaluates to false,
and t[0]
and t["0"]
denote different
entries in a table.
The operator ~=
is exactly the negation of equality (==
).
The order operators work as follows. If both arguments are numbers, then they are compared as such. Otherwise, if both arguments are strings, then their values are compared according to the current locale. Otherwise, Lua tries to call the "lt" or the "le" metamethod (see 2.8).
and or notLike the control structures (see 2.4.4), all logical operators consider both false and nil as false and anything else as true.
The operator not always return false or true.
The conjunction operator and returns its first argument if this value is false or nil; otherwise, and returns its second argument. The disjunction operator or returns its first argument if this value is different from nil and false; otherwise, or returns its second argument. Both and and or use short-cut evaluation, that is, the second operand is evaluated only if necessary. For example,
10 or error() -> 10 nil or "a" -> "a" nil and 10 -> nil false and error() -> false false and nil -> false false or nil -> nil 10 and 20 -> 20
2.5.4 - Concatenation
The string concatenation operator in Lua is
denoted by two dots (`..
´).
If both operands are strings or numbers, then they are converted to
strings according to the rules mentioned in 2.2.1.
Otherwise, the "concat" metamethod is called (see 2.8).
or and < > <= >= ~= == .. + - * / not - (unary) ^As usual, you can use parentheses to change the precedences of an expression. The concatenation (`
..
´) and exponentiation (`^
´)
operators are right associative.
All other binary operators are left associative.
2.5.6 - Table Constructors
Table constructors are expressions that create tables.
Every time a constructor is evaluated, a new table is created.
Constructors can be used to create empty tables,
or to create a table and initialize some of its fields.
The general syntax for constructors is
tableconstructor ::= `{´ [fieldlist] `}´ fieldlist ::= field {fieldsep field} [fieldsep] field ::= `[´ exp `]´ `=´ exp | Name `=´ exp | exp fieldsep ::= `,´ | `;´
Each field of the form [exp1] = exp2
adds to the new table an entry
with key exp1
and value exp2
.
A field of the form name = exp
is equivalent to
["name"] = exp
.
Finally, fields of the form exp
are equivalent to
[i] = exp
, where i
are consecutive numerical integers,
starting with 1.
Fields in the other formats do not affect this counting.
For example,
a = {[f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45}is equivalent to
do local temp = {} temp[f(1)] = g temp[1] = "x" -- 1st exp temp[2] = "y" -- 2nd exp temp.x = 1 -- temp["x"] = 1 temp[3] = f(x) -- 3rd exp temp[30] = 23 temp[4] = 45 -- 4th exp a = temp end
If the last field in the list has the form exp
and the expression is a function call,
then all values returned by the call enter the list consecutively
(see 2.5.7).
To avoid this,
enclose the function call in parentheses (see 2.5).
The field list may have an optional trailing separator, as a convenience for machine-generated code.
2.5.7 - Function Calls
A function call in Lua has the following syntax:
functioncall ::= prefixexp argsIn a function call, first prefixexp and args are evaluated. If the value of prefixexp has type function, then that function is called with the given arguments. Otherwise, its "call" metamethod is called, having as first parameter the value of prefixexp, followed by the original call arguments (see 2.8).
The form
functioncall ::= prefixexp `:´ Name argscan be used to call "methods". A call
v:name(...)
is syntactic sugar for v.name(v,...)
,
except that v
is evaluated only once.
Arguments have the following syntax:
args ::= `(´ [explist1] `)´ args ::= tableconstructor args ::= LiteralAll argument expressions are evaluated before the call. A call of the form
f{...}
is syntactic sugar for
f({...})
, that is,
the argument list is a single new table.
A call of the form f'...'
(or f"..."
or f[[...]]
) is syntactic sugar for
f('...')
, that is,
the argument list is a single literal string.
Because a function can return any number of results (see 2.4.4), the number of results must be adjusted before they are used. If the function is called as a statement (see 2.4.6), then its return list is adjusted to zero elements, thus discarding all returned values. If the function is called inside another expression or in the middle of a list of expressions, then its return list is adjusted to one element, thus discarding all returned values except the first one. If the function is called as the last element of a list of expressions, then no adjustment is made (unless the call is enclosed in parentheses).
Here are some examples:
f() -- adjusted to 0 results g(f(), x) -- f() is adjusted to 1 result g(x, f()) -- g gets x plus all values returned by f() a,b,c = f(), x -- f() is adjusted to 1 result (and c gets nil) a,b,c = x, f() -- f() is adjusted to 2 results a,b,c = f() -- f() is adjusted to 3 results return f() -- returns all values returned by f() return x,y,f() -- returns x, y, and all values returned by f() {f()} -- creates a list with all values returned by f() {f(), nil} -- f() is adjusted to 1 result
If you enclose a function call in parentheses, then it is adjusted to return exactly one value:
return x,y,(f()) -- returns x, y, and the first value from f() {(f())} -- creates a table with exactly one element
As an exception to the free-format syntax of Lua,
you cannot put a line break before the `(
´ in a function call.
That restriction avoids some ambiguities in the language.
If you write
a = f (g).x(a)Lua would read that as
a = f(g).x(a)
.
So, if you want two statements, you must add a semi-colon between them.
If you actually want to call f
,
you must remove the line break before (g)
.
A call of the form return
functioncall is called
a tail call.
Lua implements proper tail calls
(or proper tail recursion):
In a tail call,
the called function reuses the stack entry of the calling function.
Therefore, there is no limit on the number of nested tail calls that
a program can execute.
However, a tail call erases any debug information about the
calling function.
Note that a tail call only happens with a particular syntax,
where the return has one single function call as argument;
this syntax makes the calling function returns exactly
the returns of the called function.
So, all the following examples are not tails calls:
return (f(x)) -- results adjusted to 1 return 2 * f(x) return x, f(x) -- adicional results f(x); return -- results discarded return x or f(x) -- results adjusted to 1
The syntax for function definition is
function ::= function funcbody funcbody ::= `(´ [parlist1] `)´ block end
The following syntactic sugar simplifies function definitions:
stat ::= function funcname funcbody stat ::= local function Name funcbody funcname ::= Name {`.´ Name} [`:´ Name]The statement
function f () ... endtranslates to
f = function () ... endThe statement
function t.a.b.c.f () ... endtranslates to
t.a.b.c.f = function () ... endThe statement
local function f () ... endtranslates to
local f; f = function () ... end
A function definition is an executable expression, whose value has type function. When Lua pre-compiles a chunk, all its function bodies are pre-compiled too. Then, whenever Lua executes the function definition, the function is instantiated (or closed). This function instance (or closure) is the final value of the expression. Different instances of the same function may refer to different external local variables and may have different environment tables.
Parameters act as local variables that are initialized with the argument values:
parlist1 ::= namelist [`,´ `...´] parlist1 ::= `...´When a function is called, the list of arguments is adjusted to the length of the list of parameters, unless the function is a variadic or vararg function, which is indicated by three dots (`
...
´) at the end of its parameter list.
A vararg function does not adjust its argument list;
instead, it collects all extra arguments into an implicit parameter,
called arg
.
The value of arg
is a table,
with a field n
that holds the number of extra arguments
and with the extra arguments at positions 1, 2, ..., n
.
As an example, consider the following definitions:
function f(a, b) end function g(a, b, ...) end function r() return 1,2,3 endThen, we have the following mapping from arguments to parameters:
CALL PARAMETERS f(3) a=3, b=nil f(3, 4) a=3, b=4 f(3, 4, 5) a=3, b=4 f(r(), 10) a=1, b=10 f(r()) a=1, b=2 g(3) a=3, b=nil, arg={n=0} g(3, 4) a=3, b=4, arg={n=0} g(3, 4, 5, 8) a=3, b=4, arg={5, 8; n=2} g(5, r()) a=5, b=1, arg={2, 3; n=2}
Results are returned using the return statement (see 2.4.4). If control reaches the end of a function without encountering a return statement, then the function returns with no results.
The colon syntax
is used for defining methods,
that is, functions that have an implicit extra parameter self
.
Thus, the statement
function t.a.b.c:f (...) ... endis syntactic sugar for
t.a.b.c.f = function (self, ...) ... end
Lua is a lexically scoped language. The scope of variables begins at the first statement after their declaration and lasts until the end of the innermost block that includes the declaration. For instance:
x = 10 -- global variable do -- new block local x = x -- new `x', with value 10 print(x) --> 10 x = x+1 do -- another block local x = x+1 -- another `x' print(x) --> 12 end print(x) --> 11 end print(x) --> 10 (the global one)Notice that, in a declaration like
local x = x
,
the new x
being declared is not in scope yet,
and so the second x
refers to the outside variable.
Because of the lexical scoping rules, local variables can be freely accessed by functions defined inside their scope. For instance:
local counter = 0 function inc (x) counter = counter + x return counter endA local variable used by an inner function is called an upvalue, or external local variable, inside the inner function.
Notice that each execution of a local statement defines new local variables. Consider the following example:
a = {} local x = 20 for i=1,10 do local y = 0 a[i] = function () y=y+1; return x+y end endThe loop creates ten closures (that is, ten instances of the anonymous function). Each of these closures uses a different
y
variable,
while all of them share the same x
.
Because Lua is an extension language, all Lua actions start from C code in the host program calling a function from the Lua library (see 3.15). Whenever an error occurs during Lua compilation or execution, control returns to C, which can take appropriate measures (such as print an error message).
Lua code can explicitly generate an error by calling the
error
function (see 5.1).
If you need to catch errors in Lua,
you can use the pcall
function (see 5.1).
Every table and userdata object in Lua may have a metatable.
This metatable is an ordinary Lua table
that defines the behavior of the original table and userdata
under certain special operations.
You can change several aspects of the behavior
of an object by setting specific fields in its metatable.
For instance, when an object is the operand of an addition,
Lua checks for a function in the field "__add"
in its metatable.
If it finds one,
Lua calls that function to perform the addition.
We call the keys in a metatable events
and the values metamethods.
In the previous example, the event is "add"
and the metamethod is the function that performs the addition.
You can query and change the metatable of an object
through the set/getmetatable
functions (see 5.1).
A metatable may control how an object behaves in arithmetic operations, order comparisons, concatenation, and indexing. A metatable can also define a function to be called when a userdata is garbage collected. For each of those operations Lua associates a specific key called an event. When Lua performs one of those operations over a table or a userdata, it checks whether that object has a metatable with the corresponding event. If so, the value associated with that key (the metamethod) controls how Lua will perform the operation.
Metatables control the operations listed next.
Each operation is identified by its corresponding name.
The key for each operation is a string with its name prefixed by
two underscores;
for instance, the key for operation "add" is the
string "__add"
.
The semantics of these operations is better explained by a Lua function
describing how the interpreter executes that operation.
The code shown here in Lua is only illustrative;
the real behavior is hard coded in the interpreter
and it is much more efficient than this simulation.
All functions used in these descriptions
(rawget
, tonumber
, etc.)
are described in 5.1.
In particular, to retrieve the metamethod of a given object,
we use the expression
metatable(obj)[event]This should be read as
rawget(metatable(obj) or {}, event)That is, the access to a metamethod does not invoke other metamethods, and the access to objects with no metatables does not fail (it simply results in nil).
+
operation.
The function getbinhandler
below defines how Lua chooses a handler
for a binary operation.
First, Lua tries the first operand.
If its type does not define a handler for the operation,
then Lua tries the second operand.
function getbinhandler (op1, op2, event) return metatable(op1)[event] or metatable(op2)[event] endUsing that function, the behavior of the
op1 + op2
is
function add_event (op1, op2) local o1, o2 = tonumber(op1), tonumber(op2) if o1 and o2 then -- both operands are numeric? return o1 + o2 -- `+' here is the primitive `add' else -- at least one of the operands is not numeric local h = getbinhandler(op1, op2, "__add") if h then -- call the handler with both operands return h(op1, op2) else -- no handler available: default behavior error("...") end end end
-
operation.
Behavior similar to the "add" operation.
*
operation.
Behavior similar to the "add" operation.
/
operation.
Behavior similar to the "add" operation.
^
(exponentiation) operation.
function pow_event (op1, op2) local o1, o2 = tonumber(op1), tonumber(op2) if o1 and o2 then -- both operands are numeric? return __pow(o1, o2) -- call global `__pow' else -- at least one of the operands is not numeric local h = getbinhandler(op1, op2, "__pow") if h then -- call the handler with both operands return h(op1, op2) else -- no handler available: default behavior error("...") end end end
-
operation.
function unm_event (op) local o = tonumber(op) if o then -- operand is numeric? return -o -- `-' here is the primitive `unm' else -- the operand is not numeric. -- Try to get a handler from the operand local h = metatable(op).__unm if h then -- call the handler with the operand and nil return h(op, nil) else -- no handler available: default behavior error("...") end end end
..
(concatenation) operation.
function concat_event (op1, op2) if (type(op1) == "string" or type(op1) == "number") and (type(op2) == "string" or type(op2) == "number") then return op1 .. op2 -- primitive string concatenation else local h = getbinhandler(op1, op2, "__concat") if h then return h(op1, op2) else error("...") end end end
==
operation.
The function getcomphandler
defines how Lua chooses a metamethod
for comparison operators.
A metamethod only is selected when both objects
being compared have the same type
and the same metamethod for the selected operation.
function getcomphandler (op1, op2, event) if type(op1) ~= type(op2) then return nil end local mm1 = metatable(op1)[event] local mm2 = metatable(op2)[event] if mm1 == mm2 then return mm1 else return nil end endThe "eq" event is defined as follows:
function eq_event (op1, op2) if type(op1) ~= type(op2) then -- diferent types? return false -- different objects end if op1 == op2 then -- primitive equal? return true -- objects are equal end -- try metamethod local h = getcomphandler(op1, op2, "__eq") if h then return h(op1, op2) else return false end end
a ~= b
is equivalent to not (a == b)
.
<
operation.
function lt_event (op1, op2) if type(op1) == "number" and type(op2) == "number" then return op1 < op2 -- numeric comparison elseif type(op1) == "string" and type(op2) == "string" then return op1 < op2 -- lexicographic comparison else local h = getcomphandler(op1, op2, "__lt") if h then return h(op1, op2) else error("..."); end end end
a > b
is equivalent to b < a
.
<=
operation.
function le_event (op1, op2) if type(op1) == "number" and type(op2) == "number" then return op1 <= op2 -- numeric comparison elseif type(op1) == "string" and type(op2) == "string" then return op1 <= op2 -- lexicographic comparison else local h = getcomphandler(op1, op2, "__le") if h then return h(op1, op2) else h = getcomphandler(op1, op2, "__lt") if h then return not h(op2, op1) else error("..."); end end end end
a >= b
is equivalent to b <= a
.
Note that, in the absence of a "le" metamethod,
Lua tries the "lt", assuming that a <= b
is
equivalent to not (b < a)
.
table[key]
.
function gettable_event (table, key) local h if type(table) == "table" then local v = rawget(table, key) if v ~= nil then return v end h = metatable(table).__index if h == nil then return nil end else h = metatable(table).__index if h == nil then error("..."); end end if type(h) == "function" then return h(table, key) -- call the handler else return h[key] -- or repeat operation on it end
table[key] = value
.
function settable_event (table, key, value) local h if type(table) == "table" then local v = rawget(table, key) if v ~= nil then rawset(table, key, value); return end h = metatable(table).__newindex if h == nil then rawset(table, key, value); return end else h = metatable(table).__newindex if h == nil then error("..."); end end if type(h) == "function" then return h(table, key,value) -- call the handler else h[key] = value -- or repeat operation on it end
function function_event (func, ...) if type(func) == "function" then return func(unpack(arg)) -- primitive call else local h = metatable(func).__call if h then return h(func, unpack(arg)) else error("...") end end end
Lua does automatic memory management. That means that you do not have to worry about allocating memory for new objects and freeing it when the objects are no longer needed. Lua manages memory automatically by running a garbage collector from time to time to collect all dead objects (that is, those objects that are no longer accessible from Lua). All objects in Lua are subject to automatic management: tables, userdata, functions, threads, and strings.
Lua uses two numbers to control its garbage-collection cycles. One number counts how many bytes of dynamic memory Lua is using; the other is a threshold. When the number of bytes crosses the threshold, Lua runs the garbage collector, which reclaims the memory of all dead objects. The byte counter is adjusted, and then the threshold is reset to twice the new value of the byte counter.
Through the C API, you can query those numbers
and change the threshold (see 3.7).
Setting the threshold to zero actually forces an immediate
garbage-collection cycle,
while setting it to a huge number effectively stops the garbage collector.
Using Lua code you have a more limited control over garbage-collection cycles,
through the gcinfo
and collectgarbage
functions
(see 5.1).
Using the C API, you can set garbage-collector metamethods for userdata (see 2.8). These metamethods are also called finalizers. Finalizers allow you to coordinate Lua's garbage collection with external resource management (such as closing files, network or database connections, or freeing your own memory).
Free userdata with a field __gc
in their metatables are not
collected immediately by the garbage collector.
Instead, Lua puts them in a list.
After the collection,
Lua does the equivalent of the following function
for each userdata in that list:
function gc_event (udata) local h = metatable(udata).__gc if h then h(udata) end end
At the end of each garbage-collection cycle, the finalizers for userdata are called in reverse order of their creation, among those collected in that cycle. That is, the first finalizer to be called is the one associated with the userdata created last in the program.
A weak table is a table whose elements are weak references.  In other words, if the only references to an object are weak references, then the garbage collector will collect that object.
A weak table can have weak keys, weak values, or both.
A table with weak keys allows the collection of its keys,
but prevents the collection of its values.
A table with both weak keys and weak values allows the collection of
both keys and values.
In any case, if either the key or the value is collected,
the whole pair is removed from the table.
The weakness of a table is controlled by the value of the
__mode
field of its metatable.
If the __mode
field is a string containing the character `k
´,
the keys in the table are weak.
If __mode
contains `v
´,
the values in the table are weak.
After you use a table as a metatable,
you should not change the value of its field __mode
.
Otherwise, the weak behavior of the tables controlled by this
metatable is undefined.
Lua supports coroutines, also called semi-coroutines or collaborative multithreading. A coroutine in Lua represents an independent thread of execution. Unlike threads in multithread systems, however, a coroutine only suspends its execution by explicitly calling a yield function.
You create a coroutine with a call to coroutine.create
.
Its sole argument is a function
that is the main function of the coroutine.
The create
function only creates a new coroutine and
returns a handle to it (an object of type thread);
it does not start the coroutine execution.
When you first call coroutine.resume
,
passing as argument the thread returned by coroutine.create
,
the coroutine starts its execution,
at the first line of its main function.
Extra arguments passed to coroutine.resume
are given as
parameters for the coroutine main function.
After the coroutine starts running,
it runs until it terminates or yields.
A coroutine can terminate its execution in two ways:
Normally, when its main function returns
(explicitly or implicitly, after the last instruction);
and abnormally, if there is an unprotected error.
In the first case, coroutine.resume
returns true,
plus any values returned by the coroutine main function.
In case of errors, coroutine.resume
returns false
plus an error message.
A coroutine yields by calling coroutine.yield
.
When a coroutine yields,
the corresponding coroutine.resume
returns immediately,
even if the yield happens inside nested function calls
(that is, not in the main function,
but in a function directly or indirectly called by the main function).
In the case of a yield, coroutine.resume
also returns true,
plus any values passed to coroutine.yield
.
The next time you resume the same coroutine,
it continues its execution from the point where it yielded,
with the call to coroutine.yield
returning any extra
arguments passed to coroutine.resume
.
The coroutine.wrap
function creates a coroutine
like coroutine.create
,
but instead of returning the coroutine itself,
it returns a function that, when called, resumes the coroutine.
Any arguments passed to that function
go as extra arguments to resume.
The function returns all the values returned by resume,
except the first one (the boolean error code).
Unlike coroutine.resume
,
this function does not catch errors;
any error is propagated to the caller.
As an example, consider the next code:
function foo1 (a) print("foo", a) return coroutine.yield(2*a) end co = coroutine.create(function (a,b) print("co-body", a, b) local r = foo1(a+1) print("co-body", r) local r, s = coroutine.yield(a+b, a-b) print("co-body", r, s) return b, "end" end) a, b = coroutine.resume(co, 1, 10) print("main", a, b) a, b, c = coroutine.resume(co, "r") print("main", a, b, c) a, b, c = coroutine.resume(co, "x", "y") print("main", a, b, c) a, b = coroutine.resume(co, "x", "y") print("main", a, b)When you run it, it produces the following output:
co-body 1 10 foo 2 main true 4 co-body r main true 11 -9 co-body x y main true 10 end main false cannot resume dead coroutine
3 - The Application Program Interface
This section describes the C API for Lua, that is,
the set of C functions available to the host program to communicate
with Lua.
All API functions and related types and constants
are declared in the header file lua.h
.
Even when we use the term "function", any facility in the API may be provided as a macro instead. All such macros use each of its arguments exactly once (except for the first argument, which is always a Lua state), and so do not generate hidden side-effects.
The Lua library is fully reentrant:
it has no global variables.
The whole state of the Lua interpreter
(global variables, stack, etc.)
is stored in a dynamically allocated structure of type lua_State
.
A pointer to this state must be passed as the first argument to
every function in the library, except to lua_open
,
which creates a Lua state from scratch.
Before calling any API function,
you must create a state by calling lua_open
:
lua_State *lua_open (void);
To release a state created with lua_open
, call lua_close
:
void lua_close (lua_State *L);This function destroys all objects in the given Lua state (calling the corresponding garbage-collection metamethods, if any) and frees all dynamic memory used by that state. On several platforms, you may not need to call this function, because all resources are naturally released when the host program ends. On the other hand, long-running programs, such as a daemon or a web server, might need to release states as soon as they are not needed, to avoid growing too large.
Lua uses a virtual stack to pass values to and from C. Each element in this stack represents a Lua value (nil, number, string, etc.).
Whenever Lua calls C, the called function gets a new stack, which is independent of previous stacks and of stacks of C functions that are still active. That stack initially contains any arguments to the C function, and it is where the C function pushes its results (see 3.16) to be returned to the caller.
For convenience,
most query operations in the API do not follow a strict stack discipline.
Instead, they can refer to any element in the stack by using an index:
A positive index represents an absolute stack position
(starting at 1);
a negative index represents an offset from the top of the stack.
More specifically, if the stack has n elements,
then index 1 represents the first element
(that is, the element that was pushed onto the stack first)
and
index n represents the last element;
index -1 also represents the last element
(that is, the element at the top)
and index -n represents the first element.
We say that an index is valid
if it lies between 1 and the stack top
(that is, if 1 <= abs(index) <= top
).
At any time, you can get the index of the top element by calling
lua_gettop
:
int lua_gettop (lua_State *L);Because indices start at 1, the result of
lua_gettop
is equal to the number of elements in the stack
(and so 0 means an empty stack).
When you interact with Lua API, you are responsible for controlling stack overflow. The function
int lua_checkstack (lua_State *L, int extra);grows the stack size to
top + extra
elements;
it returns false if it cannot grow the stack to that size.
This function never shrinks the stack;
if the stack is already larger than the new size,
it is left unchanged.
Whenever Lua calls C,
it ensures that at least LUA_MINSTACK
positions are available.
LUA_MINSTACK
is defined in lua.h
as 20,
so that usually you do not have to worry about stack space
unless your code has loops pushing elements onto the stack.
Most query functions accept as indices any value inside the
available stack space, that is, indices up to the maximum stack size
you have set through lua_checkstack
.
Such indices are called acceptable indices.
More formally, we define an acceptable index
as follows:
(index < 0 && abs(index) <= top) || (index > 0 && index <= stackspace)Note that 0 is never an acceptable index.
Unless otherwise noted, any function that accepts valid indices can also be called with pseudo-indices, which represent some Lua values that are accessible to the C code but are not in the stack. Pseudo-indices are used to access the global environment, the registry, and the upvalues of a C function (see 3.17).
void lua_settop (lua_State *L, int index); void lua_pushvalue (lua_State *L, int index); void lua_remove (lua_State *L, int index); void lua_insert (lua_State *L, int index); void lua_replace (lua_State *L, int index);
lua_settop
accepts any acceptable index,
or 0,
and sets the stack top to that index.
If the new top is larger than the old one,
then the new elements are filled with nil.
If index
is 0, then all stack elements are removed.
A useful macro defined in the lua.h
is
#define lua_pop(L,n) lua_settop(L, -(n)-1)which pops
n
elements from the stack.
lua_pushvalue
pushes onto the stack a copy of the element
at the given index.
lua_remove
removes the element at the given position,
shifting down the elements above that position to fill the gap.
lua_insert
moves the top element into the given position,
shifting up the elements above that position to open space.
lua_replace
moves the top element into the given position,
without shifting any element (therefore replacing the value at
the given position).
All these functions accept only valid indices.
(You cannot call lua_remove
or lua_insert
with
pseudo-indices, as they do not represent a stack position.)
As an example, if the stack starts as 10 20 30 40 50*
(from bottom to top; the `*
´ marks the top),
then
lua_pushvalue(L, 3) --> 10 20 30 40 50 30* lua_pushvalue(L, -1) --> 10 20 30 40 50 30 30* lua_remove(L, -3) --> 10 20 30 40 30 30* lua_remove(L, 6) --> 10 20 30 40 30* lua_insert(L, 1) --> 30 10 20 30 40* lua_insert(L, -1) --> 30 10 20 30 40* (no effect) lua_replace(L, 2) --> 30 40 20 30* lua_settop(L, -3) --> 30 40* lua_settop(L, 6) --> 30 40 nil nil nil nil*
To check the type of a stack element, the following functions are available:
int lua_type (lua_State *L, int index); int lua_isnil (lua_State *L, int index); int lua_isboolean (lua_State *L, int index); int lua_isnumber (lua_State *L, int index); int lua_isstring (lua_State *L, int index); int lua_istable (lua_State *L, int index); int lua_isfunction (lua_State *L, int index); int lua_iscfunction (lua_State *L, int index); int lua_isuserdata (lua_State *L, int index); int lua_islightuserdata (lua_State *L, int index);These functions can be called with any acceptable index.
lua_type
returns the type of a value in the stack,
or LUA_TNONE
for a non-valid index
(that is, if that stack position is "empty").
The types returned by lua_type
are coded by the following constants
defined in lua.h
:
LUA_TNIL
,
LUA_TNUMBER
,
LUA_TBOOLEAN
,
LUA_TSTRING
,
LUA_TTABLE
,
LUA_TFUNCTION
,
LUA_TUSERDATA
,
LUA_TTHREAD
,
LUA_TLIGHTUSERDATA
.
The following function translates these constants to strings:
const char *lua_typename (lua_State *L, int type);
The lua_is*
functions return 1 if the object is compatible
with the given type, and 0 otherwise.
lua_isboolean
is an exception to this rule:
It succeeds only for boolean values
(otherwise it would be useless,
as any value has a boolean value).
They always return 0 for a non-valid index.
lua_isnumber
accepts numbers and numerical strings;
lua_isstring
accepts strings and numbers (see 2.2.1);
lua_isfunction
accepts both Lua functions and C functions;
and lua_isuserdata
accepts both full and light userdata.
To distinguish between Lua functions and C functions,
you can use lua_iscfunction
.
To distinguish between full and light userdata,
you can use lua_islightuserdata
.
To distinguish between numbers and numerical strings,
you can use lua_type
.
The API also contains functions to compare two values in the stack:
int lua_equal (lua_State *L, int index1, int index2); int lua_rawequal (lua_State *L, int index1, int index2); int lua_lessthan (lua_State *L, int index1, int index2);
lua_equal
and lua_lessthan
are equivalent to their counterparts in Lua (see 2.5.2).
lua_rawequal
compares the values for primitive equality,
without metamethods.
These functions return 0 (false) if any of the indices are non-valid.
3.5 - Getting Values from the Stack
To translate a value in the stack to a specific C type, you can use the following conversion functions:
int lua_toboolean (lua_State *L, int index); lua_Number lua_tonumber (lua_State *L, int index); const char *lua_tostring (lua_State *L, int index); size_t lua_strlen (lua_State *L, int index); lua_CFunction lua_tocfunction (lua_State *L, int index); void *lua_touserdata (lua_State *L, int index); lua_State *lua_tothread (lua_State *L, int index); void *lua_topointer (lua_State *L, int index);These functions can be called with any acceptable index. When called with a non-valid index, they act as if the given value had an incorrect type.
lua_toboolean
converts the Lua value at the given index
to a C "boolean" value (0 or 1).
Like all tests in Lua, lua_toboolean
returns 1 for any Lua value
different from false and nil;
otherwise it returns 0.
It also returns 0 when called with a non-valid index.
(If you want to accept only real boolean values,
use lua_isboolean
to test the type of the value.)
lua_tonumber
converts the Lua value at the given index
to a number (by default, lua_Number
is double
).
The Lua value must be a number or a string convertible to number
(see 2.2.1); otherwise, lua_tonumber
returns 0.
lua_tostring
converts the Lua value at the given index to a string
(const char*
).
The Lua value must be a string or a number;
otherwise, the function returns NULL
.
If the value is a number,
then lua_tostring
also
changes the actual value in the stack to a string.
(This change confuses lua_next
when lua_tostring
is applied to keys.)
lua_tostring
returns a fully aligned pointer
to a string inside the Lua state.
This string always has a zero ('\0'
)
after its last character (as in C),
but may contain other zeros in its body.
If you do not know whether a string may contain zeros,
you can use lua_strlen
to get its actual length.
Because Lua has garbage collection,
there is no guarantee that the pointer returned by lua_tostring
will be valid after the corresponding value is removed from the stack.
If you need the string after the current function returns,
then you should duplicate it or put it into the registry (see 3.18).
lua_tocfunction
converts a value in the stack to a C function.
This value must be a C function;
otherwise, lua_tocfunction
returns NULL
.
The type lua_CFunction
is explained in 3.16.
lua_tothread
converts a value in the stack to a Lua thread
(represented as lua_State *
).
This value must be a thread;
otherwise, lua_tothread
returns NULL
.
lua_topointer
converts a value in the stack to a generic
C pointer (void *
).
The value may be a userdata, a table, a thread, or a function;
otherwise, lua_topointer
returns NULL
.
Lua ensures that different objects of the
same type return different pointers.
There is no direct way to convert the pointer back to its original value.
Typically this function is used for debug information.
lua_touserdata
is explained in 3.8.
3.6 - Pushing Values onto the Stack
The API has the following functions to push C values onto the stack:
void lua_pushboolean (lua_State *L, int b); void lua_pushnumber (lua_State *L, lua_Number n); void lua_pushlstring (lua_State *L, const char *s, size_t len); void lua_pushstring (lua_State *L, const char *s); void lua_pushnil (lua_State *L); void lua_pushcfunction (lua_State *L, lua_CFunction f); void lua_pushlightuserdata (lua_State *L, void *p);
These functions receive a C value,
convert it to a corresponding Lua value,
and push the result onto the stack.
In particular, lua_pushlstring
and lua_pushstring
make an internal copy of the given string.
lua_pushstring
can only be used to push proper C strings
(that is, strings that end with a zero and do not contain embedded zeros);
otherwise, you should use the more general lua_pushlstring
,
which accepts an explicit size.
You can also push "formatted" strings:
const char *lua_pushfstring (lua_State *L, const char *fmt, ...); const char *lua_pushvfstring (lua_State *L, const char *fmt, va_list argp);These functions push onto the stack a formatted string and return a pointer to that string. They are similar to
sprintf
and vsprintf
,
but with some important differences:
%%
´ (inserts a `%
´ in the string),
`%s
´ (inserts a zero-terminated string, with no size restrictions),
`%f
´ (inserts a lua_Number
),
`%d
´ (inserts an int
), and
`%c
´ (inserts an int
as a character).
The function
void lua_concat (lua_State *L, int n);concatenates the
n
values at the top of the stack,
pops them, and leaves the result at the top.
If n
is 1, the result is that single string
(that is, the function does nothing);
if n
is 0, the result is the empty string.
Concatenation is done following the usual semantics of Lua
(see 2.5.4).
3.7 - Controlling Garbage Collection
Lua uses two numbers to control its garbage collection: the count and the threshold (see 2.9). The first counts the amount of memory in use by Lua; when the count reaches the threshold, Lua runs its garbage collector. After the collection, the count is updated and the threshold is set to twice the count value.
You can access the current values of these two numbers through the following functions:
int lua_getgccount (lua_State *L); int lua_getgcthreshold (lua_State *L);Both return their respective values in Kbytes. You can change the threshold value with
void lua_setgcthreshold (lua_State *L, int newthreshold);Again, the
newthreshold
value is given in Kbytes.
When you call this function,
Lua sets the new threshold and checks it against the byte counter.
If the new threshold is less than the byte counter,
then Lua immediately runs the garbage collector.
In particular
lua_setgcthreshold(L,0)
forces a garbage collection.
After the collection,
a new threshold is set according to the previous rule.
Userdata represents C values in Lua. Lua supports two types of userdata: full userdata and light userdata.
A full userdata represents a block of memory. It is an object (like a table): You must create it, it can have its own metatable, and you can detect when it is being collected. A full userdata is only equal to itself (under raw equality).
A light userdata represents a pointer. It is a value (like a number): You do not create it, it has no metatables, it is not collected (as it was never created). A light userdata is equal to "any" light userdata with the same C address.
In Lua code, there is no way to test whether a userdata is full or light;
both have type userdata
.
In C code, lua_type
returns LUA_TUSERDATA
for full userdata,
and LUA_TLIGHTUSERDATA
for light userdata.
You can create a new full userdata with the following function:
void *lua_newuserdata (lua_State *L, size_t size);This function allocates a new block of memory with the given size, pushes on the stack a new userdata with the block address, and returns this address.
To push a light userdata into the stack you use
lua_pushlightuserdata
(see 3.6).
lua_touserdata
(see 3.5) retrieves the value of a userdata.
When applied on a full userdata, it returns the address of its block;
when applied on a light userdata, it returns its pointer;
when applied on a non-userdata value, it returns NULL
.
When Lua collects a full userdata,
it calls the userdata's gc
metamethod, if any,
and then it frees the userdata's corresponding memory.
The following functions allow you to manipulate the metatables of an object:
int lua_getmetatable (lua_State *L, int index); int lua_setmetatable (lua_State *L, int index);
lua_getmetatable
pushes on the stack the metatable of a given object.
If the index is not valid,
or if the object does not have a metatable,
lua_getmetatable
returns 0 and pushes nothing on the stack.
lua_setmetatable
pops a table from the stack and
sets it as the new metatable for the given object.
lua_setmetatable
returns 0 when it cannot
set the metatable of the given object
(that is, when the object is neither a userdata nor a table);
even then it pops the table from the stack.
You can load a Lua chunk with lua_load
:
typedef const char * (*lua_Chunkreader) (lua_State *L, void *data, size_t *size); int lua_load (lua_State *L, lua_Chunkreader reader, void *data, const char *chunkname);The return values of
lua_load
are:
LUA_ERRSYNTAX
---
syntax error during pre-compilation.
LUA_ERRMEM
---
memory allocation error.
lua_load
pushes the compiled chunk as a Lua
function on top of the stack.
Otherwise, it pushes an error message.
lua_load
automatically detects whether the chunk is text or binary,
and loads it accordingly (see program luac
).
lua_load
uses an user-supplied reader function to read the chunk.
Everytime it needs another piece of the chunk,
it calls the reader,
passing along its data
parameter.
The reader must return a pointer to a block of memory
with a new piece of the chunk
and set size
to the block size.
To signal the end of the chunk, the reader returns NULL
.
The reader function may return pieces of any size greater than zero.
In the current implementation,
the reader function cannot call any Lua function;
to ensure that, it always receives NULL
as the Lua state.
The chunkname is used for error messages and debug information (see 4).
See the auxiliary library (lauxlib.c
)
for examples of how to use lua_load
and for some ready-to-use functions to load chunks
from files and strings.
Tables are created by calling the function
void lua_newtable (lua_State *L);This function creates a new, empty table and pushes it onto the stack.
To read a value from a table that resides somewhere in the stack, call
void lua_gettable (lua_State *L, int index);where
index
points to the table.
lua_gettable
pops a key from the stack
and returns (on the stack) the contents of the table at that key.
The table is left where it was in the stack.
As in Lua, this function may trigger a metamethod
for the "index" event (see 2.8).
To get the real value of any table key,
without invoking any metamethod,
use the raw version:
void lua_rawget (lua_State *L, int index);
To store a value into a table that resides somewhere in the stack, you push the key and then the value onto the stack, and call
void lua_settable (lua_State *L, int index);where
index
points to the table.
lua_settable
pops from the stack both the key and the value.
The table is left where it was in the stack.
As in Lua, this operation may trigger a metamethod
for the "settable" or "newindex" events.
To set the real value of any table index,
without invoking any metamethod,
use the raw version:
void lua_rawset (lua_State *L, int index);
You can traverse a table with the function
int lua_next (lua_State *L, int index);where
index
points to the table to be traversed.
The function pops a key from the stack,
and pushes a key-value pair from the table
(the "next" pair after the given key).
If there are no more elements, then lua_next
returns 0
(and pushes nothing).
Use a nil key to signal the start of a traversal.
A typical traversal looks like this:
/* table is in the stack at index `t' */ lua_pushnil(L); /* first key */ while (lua_next(L, t) != 0) { /* `key' is at index -2 and `value' at index -1 */ printf("%s - %s\n", lua_typename(L, lua_type(L, -2)), lua_typename(L, lua_type(L, -1))); lua_pop(L, 1); /* removes `value'; keeps `key' for next iteration */ }
While traversing a table,
do not call lua_tostring
directly on a key,
unless you know that the key is actually a string.
Recall that lua_tostring
changes the value at the given index;
this confuses the next call to lua_next
.
3.12 - Manipulating Environments
All global variables are kept in ordinary Lua tables,
called environments.
The initial environment is called the global environment.
This table is always at pseudo-index LUA_GLOBALSINDEX
.
To access and change the value of global variables, you can use regular table operations over an environment table. For instance, to access the value of a global variable, do
lua_pushstring(L, varname); lua_gettable(L, LUA_GLOBALSINDEX);
You can change the global environment of a Lua thread using lua_replace
.
The following functions get and set the environment of Lua functions:
void lua_getfenv (lua_State *L, int index); int lua_setfenv (lua_State *L, int index);
lua_getfenv
pushes on the stack the environment table of
the function at index index
in the stack.
If the function is a C function,
lua_getfenv
pushes the global environment.
lua_setfenv
pops a table from the stack and sets it as
the new environment for the function at index index
in the stack.
If the object at the given index is not a Lua function,
lua_setfenv
returns 0.
void lua_rawgeti (lua_State *L, int index, int n); void lua_rawseti (lua_State *L, int index, int n);
lua_rawgeti
pushes the value of the n-th element of the table
at stack position index
.
lua_rawseti
sets the value of the n-th element of the table
at stack position index
to the value at the top of the stack,
removing this value from the stack.
Functions defined in Lua and C functions registered in Lua can be called from the host program. This is done using the following protocol: First, the function to be called is pushed onto the stack; then, the arguments to the function are pushed in direct order, that is, the first argument is pushed first. Finally, the function is called using
void lua_call (lua_State *L, int nargs, int nresults);
nargs
is the number of arguments that you pushed onto the stack.
All arguments and the function value are popped from the stack,
and the function results are pushed.
The number of results are adjusted to nresults
,
unless nresults
is LUA_MULTRET
.
In that case, all results from the function are pushed.
Lua takes care that the returned values fit into the stack space.
The function results are pushed onto the stack in direct order
(the first result is pushed first),
so that after the call the last result is on the top.
The following example shows how the host program may do the equivalent to this Lua code:
a = f("how", t.x, 14)Here it is in C:
lua_pushstring(L, "t"); lua_gettable(L, LUA_GLOBALSINDEX); /* global `t' (for later use) */ lua_pushstring(L, "a"); /* var name */ lua_pushstring(L, "f"); /* function name */ lua_gettable(L, LUA_GLOBALSINDEX); /* function to be called */ lua_pushstring(L, "how"); /* 1st argument */ lua_pushstring(L, "x"); /* push the string "x" */ lua_gettable(L, -5); /* push result of t.x (2nd arg) */ lua_pushnumber(L, 14); /* 3rd argument */ lua_call(L, 3, 1); /* call function with 3 arguments and 1 result */ lua_settable(L, LUA_GLOBALSINDEX); /* set global variable `a' */ lua_pop(L, 1); /* remove `t' from the stack */Note that the code above is "balanced": at its end, the stack is back to its original configuration. This is considered good programming practice.
(We did this example using only the raw functions provided by Lua's API, to show all the details. Usually programmers define and use several macros and auxiliary functions that provide higher level access to Lua. See the source code of the standard libraries for examples.)
When you call a function with lua_call
,
any error inside the called function is propagated upwards
(with a longjmp
).
If you need to handle errors,
then you should use lua_pcall
:
int lua_pcall (lua_State *L, int nargs, int nresults, int errfunc);Both
nargs
and nresults
have the same meaning as
in lua_call
.
If there are no errors during the call,
lua_pcall
behaves exactly like lua_call
.
However, if there is any error,
lua_pcall
catches it,
pushes a single value at the stack (the error message),
and returns an error code.
Like lua_call
,
lua_pcall
always removes the function
and its arguments from the stack.
If errfunc
is 0,
then the error message returned is exactly the original error message.
Otherwise, errfunc
gives the stack index for an
error handler function.
(In the current implementation, that index cannot be a pseudo-index.)
In case of runtime errors,
that function will be called with the error message
and its return value will be the message returned by lua_pcall
.
Typically, the error handler function is used to add more debug
information to the error message, such as a stack traceback.
Such information cannot be gathered after the return of lua_pcall
,
since by then the stack has unwound.
The lua_pcall
function returns 0 in case of success
or one of the following error codes
(defined in lua.h
):
LUA_ERRRUN
--- a runtime error.
LUA_ERRMEM
--- memory allocation error.
For such errors, Lua does not call the error handler function.
LUA_ERRERR
---
error while running the error handler function.
Lua can be extended with functions written in C.
These functions must be of type lua_CFunction
,
which is defined as
typedef int (*lua_CFunction) (lua_State *L);A C function receives a Lua state and returns an integer, the number of values it wants to return to Lua.
In order to communicate properly with Lua, a C function must follow the following protocol, which defines the way parameters and results are passed: A C function receives its arguments from Lua in its stack in direct order (the first argument is pushed first). So, when the function starts, its first argument (if any) is at index 1. To return values to Lua, a C function just pushes them onto the stack, in direct order (the first result is pushed first), and returns the number of results. Any other value in the stack below the results will be properly discharged by Lua. Like a Lua function, a C function called by Lua can also return many results.
As an example, the following function receives a variable number of numerical arguments and returns their average and sum:
static int foo (lua_State *L) { int n = lua_gettop(L); /* number of arguments */ lua_Number sum = 0; int i; for (i = 1; i <= n; i++) { if (!lua_isnumber(L, i)) { lua_pushstring(L, "incorrect argument to function `average'"); lua_error(L); } sum += lua_tonumber(L, i); } lua_pushnumber(L, sum/n); /* first result */ lua_pushnumber(L, sum); /* second result */ return 2; /* number of results */ }
To register a C function to Lua, there is the following convenience macro:
#define lua_register(L,n,f) \ (lua_pushstring(L, n), \ lua_pushcfunction(L, f), \ lua_settable(L, LUA_GLOBALSINDEX)) /* lua_State *L; */ /* const char *n; */ /* lua_CFunction f; */which receives the name the function will have in Lua and a pointer to the function. Thus, the C function
foo
above may be registered in Lua as
average
by calling
lua_register(L, "average", foo);
When a C function is created, it is possible to associate some values with it, thus creating a C closure; these values are then accessible to the function whenever it is called. To associate values with a C function, first these values should be pushed onto the stack (when there are multiple values, the first value is pushed first). Then the function
void lua_pushcclosure (lua_State *L, lua_CFunction fn, int n);is used to push the C function onto the stack, with the argument
n
telling how many values should be
associated with the function
(lua_pushcclosure
also pops these values from the stack);
in fact, the macro lua_pushcfunction
is defined as
lua_pushcclosure
with n
set to 0.
Then, whenever the C function is called,
those values are located at specific pseudo-indices.
Those pseudo-indices are produced by a macro lua_upvalueindex
.
The first value associated with a function is at position
lua_upvalueindex(1)
, and so on.
Any access to lua_upvalueindex(n)
,
where n is greater than the number of upvalues of the
current function,
produces an acceptable (but invalid) index.
For examples of C functions and closures,
see the standard libraries in the official Lua distribution
(src/lib/*.c
).
Lua provides a registry,
a pre-defined table that can be used by any C code to
store whatever Lua value it needs to store,
specially if the C code needs to keep that Lua value
outside the life span of a C function.
This table is always located at pseudo-index
LUA_REGISTRYINDEX
.
Any C library can store data into this table,
as long as it chooses keys different from other libraries.
Typically, you should use as key a string containing your library name
or a light userdata with the address of a C object in your code.
The integer keys in the registry are used by the reference mechanism, implemented by the auxiliary library, and therefore should not be used by other purposes.
Internally, Lua uses the C longjmp
facility to handle errors.
When Lua faces any error
(such as memory allocation errors, type errors, syntax errors)
it raises an error, that is, it does a long jump.
A protected environment uses setjmp
to set a recover point,
and any error jumps to the most recent active recover point.
If an error happens outside any protected environment,
Lua calls a panic function,
and then calls exit(EXIT_FAILURE)
.
You can change the panic function with
lua_CFunction lua_atpanic (lua_State *L, lua_CFunction panicf);Your new panic function may avoid the application exit by never returning (e.g. doing a long jump). Nevertheless, the corresponding Lua will not be consistent; the only safe operation with it is to close it.
Almost any function in the API may raise an error,
for instance due to a memory allocation error.
The following functions run in protected mode
(that is, they create a protected environment to run),
so they never raise an error:
lua_open
, lua_close
, lua_load
,
and lua_pcall
.
There is yet another function that runs a given C function in protected mode:
int lua_cpcall (lua_State *L, lua_CFunction func, void *ud);
lua_cpcall
calls func
in protected mode.
func
starts with only one element in its stack,
a light userdata containing ud
.
In case of errors,
lua_cpcall
returns the same error codes as lua_pcall
(see 3.15),
plus the error object on the top of the stack;
otherwise, it returns zero, and does not change the stack.
Any value returned by func
is dicarded.
C code can generate a Lua error calling the function
void lua_error (lua_State *L);The error message (which actually can be any type of object) must be on the stack top. This function does a long jump, and therefore never returns.
Lua offers partial support for multiple threads of execution. If you have a C library that offers multi-threading, then Lua can cooperate with it to implement the equivalent facility in Lua. Also, Lua implements its own coroutine system on top of threads. The following function creates a new thread in Lua:
lua_State *lua_newthread (lua_State *L);This function pushes the thread on the stack and returns a pointer to a
lua_State
that represents this new thread.
The new state returned by this function shares with the original state
all global objects (such as tables),
but has an independent run-time stack.
Each thread has an independent global environment table. When you create a thread, this table is the same as that of the given state, but you can change each one independently.
You destroy threads with lua_closethread
:
void lua_closethread (lua_State *L, lua_State *thread);You cannot close the sole (or last) thread of a state. Instead, you must close the state itself.
To manipulate threads as coroutines, Lua offers the following functions:
int lua_resume (lua_State *L, int narg); int lua_yield (lua_State *L, int nresults);To start a coroutine, you first create a new thread; then you push on its stack the body function plus any eventual arguments; then you call
lua_resume
,
with narg
being the number of arguments.
This call returns when the coroutine suspends or finishes its execution.
When it returns, the stack contains all values passed to lua_yield
,
or all values returned by the body function.
lua_resume
returns 0 if there is no errors running the coroutine,
or an error code (see 3.15).
In case of errors,
the stack contains only the error message.
To restart a coroutine, you put on its stack only the values to
be passed as results from yield
,
and then call lua_resume
.
The lua_yield
function can only be called as the
return expression of a C function, as follows:
return lua_yield (L, nresults);When a C function calls
lua_yield
in that way,
the running coroutine suspends its execution,
and the call to lua_resume
that started this coroutine returns.
The parameter nresults
is the number of values from the stack
that are passed as results to lua_resume
.
To exchange values between different threads,
you may use lua_xmove
:
void lua_xmove (lua_State *from, lua_State *to, int n);It pops
n
values from the stack from
,
and puhses them into the stack to
.
Lua has no built-in debugging facilities. Instead, it offers a special interface by means of functions and hooks. This interface allows the construction of different kinds of debuggers, profilers, and other tools that need "inside information" from the interpreter.
The main function to get information about the interpreter runtime stack is
int lua_getstack (lua_State *L, int level, lua_Debug *ar);This function fills parts of a
lua_Debug
structure with
an identification of the activation record
of the function executing at a given level.
Level 0 is the current running function,
whereas level n+1 is the function that has called level n.
When there is no errors, lua_getstack
returns 1;
when called with a level greater than the stack depth,
it returns 0.
The structure lua_Debug
is used to carry different pieces of
information about an active function:
typedef struct lua_Debug { int event; const char *name; /* (n) */ const char *namewhat; /* (n) `global', `local', `field', `method' */ const char *what; /* (S) `Lua' function, `C' function, Lua `main' */ const char *source; /* (S) */ int currentline; /* (l) */ int nups; /* (u) number of upvalues */ int linedefined; /* (S) */ char short_src[LUA_IDSIZE]; /* (S) */ /* private part */ ... } lua_Debug;
lua_getstack
fills only the private part
of this structure, for later use.
To fill the other fields of lua_Debug
with useful information,
call
int lua_getinfo (lua_State *L, const char *what, lua_Debug *ar);This function returns 0 on error (for instance, an invalid option in
what
).
Each character in the string what
selects some fields of the structure ar
to be filled,
as indicated by the letter in parentheses in the definition of lua_Debug
above:
`S
´ fills in the fields source
, linedefined
,
and what
;
`l
´ fills in the field currentline
, etc.
Moreover, `f
´ pushes onto the stack the function that is
running at the given level.
To get information about a function that is not active
(that is, not in the stack),
you push it onto the stack
and start the what
string with the character `>
´.
For instance, to know in which line a function f
was defined,
you can write
lua_Debug ar; lua_pushstring(L, "f"); lua_gettable(L, LUA_GLOBALSINDEX); /* get global `f' */ lua_getinfo(L, ">S", &ar); printf("%d\n", ar.linedefined);The fields of
lua_Debug
have the following meaning:
source
If the function was defined in a string,
then source
is that string.
If the function was defined in a file,
then source
starts with a `@
´ followed by the file name.
short_src
A "printable" version of source
, to be used in error messages.
linedefined
the line number where the definition of the function starts.
what
the string "Lua"
if this is a Lua function,
"C"
if this is a C function,
"main"
if this is the main part of a chunk,
and "tail"
if this was a function that did a tail call.
In the latter case,
Lua has no other information about this function.
currentline
the current line where the given function is executing.
When no line information is available,
currentline
is set to -1.
name
a reasonable name for the given function.
Because functions in Lua are first class values,
they do not have a fixed name:
Some functions may be the value of multiple global variables,
while others may be stored only in a table field.
The lua_getinfo
function checks how the function was
called or whether it is the value of a global variable to
find a suitable name.
If it cannot find a name,
then name
is set to NULL
.
namewhat
Explains the name
field.
The value of namewhat
can be
"global"
, "local"
, "method"
,
"field"
, or ""
(the empty string),
according to how the function was called.
(Lua uses the empty string when no other option seems to apply.)
nups
The number of upvalues of the function.
For the manipulation of local variables and upvalues, the debug interface uses indices: The first parameter or local variable has index 1, and so on, until the last active local variable. Upvalues have no particular order, as they are active through the whole function.
The following functions allow the manipulation of the local variables of a given activation record:
const char *lua_getlocal (lua_State *L, const lua_Debug *ar, int n); const char *lua_setlocal (lua_State *L, const lua_Debug *ar, int n);The parameter
ar
must be a valid activation record that was
filled by a previous call to lua_getstack
or
given as argument to a hook (see 4.3).
lua_getlocal
gets the index n
of a local variable,
pushes the variable's value onto the stack,
and returns its name.
lua_setlocal
assigns the value at the top of the stack
to the variable and returns its name.
Both functions return NULL
when the index is greater than
the number of active local variables.
The following functions allow the manipulation of the upvalues of a given function (unlike local variables, the upvalues of a function are accessible even when the function is not active):
const char *lua_getupvalue (lua_State *L, int funcindex, int n); const char *lua_setupvalue (lua_State *L, int funcindex, int n);These functions operate both on Lua functions and on C functions. (For Lua functions, upvalues are the external local variables that the function uses, and that consequently are included in its closure.)
funcindex
points to a function in the stack.
lua_getpuvalue
gets the index n
of an upvalue,
pushes the upvalue's value onto the stack,
and returns its name.
lua_setupvalue
assigns the value at the top of the stack
to the upvalue and returns its name.
Both functions return NULL
when the index is greater than the number of upvalues.
For C functions, these functions use the empty string ""
as a name for all upvalues.
As an example, the following function lists the names of all local variables and upvalues for a function at a given level of the stack:
int listvars (lua_State *L, int level) { lua_Debug ar; int i; const char *name; if (lua_getstack(L, level, &ar) == 0) return 0; /* failure: no such level in the stack */ i = 1; while ((name = lua_getlocal(L, &ar, i++)) != NULL) { printf("local %d %s\n", i-1, name); lua_pop(L, 1); /* remove variable value */ } lua_getinfo(L, "f", &ar); /* retrieves function */ i = 1; while ((name = lua_getpuvalue(L, -1, i++)) != NULL) { printf("upvalue %d %s\n", i-1, name); lua_pop(L, 1); /* remove upvalue value */ } return 1; }
Lua offers a mechanism of hooks, which are
user-defined C functions that are called during the program execution.
A hook may be called in four different events:
a call event, when Lua calls a function;
a return event, when Lua returns from a function;
a line event, when Lua starts executing a new line of code;
and a count event, which happens every "count" instructions.
Lua identifies these events with the following constants:
LUA_HOOKCALL
,
LUA_HOOKRET
(or LUA_HOOKTAILRET
, see below),
LUA_HOOKLINE
,
and LUA_HOOKCOUNT
.
A hook has type lua_Hook
, defined as follows:
typedef void (*lua_Hook) (lua_State *L, lua_Debug *ar);You can set the hook with the following function:
int lua_sethook (lua_State *L, lua_Hook func, int mask, int count);
func
is the hook.
mask
specifies on which events the hook will be called:
It is formed by a disjunction of the constants
LUA_MASKCALL
,
LUA_MASKRET
,
LUA_MASKLINE
,
and LUA_MASKCOUNT
.
The count
argument is only meaningful when the mask
includes LUA_MASKCOUNT
.
For each event, the hook is called as explained below:
count
instructions.
(This event only happens while Lua is executing a Lua function.)
A hook is disabled by setting mask
to zero.
You can get the current hook, the current mask, and the current count with the next functions:
lua_Hook lua_gethook (lua_State *L); int lua_gethookmask (lua_State *L); int lua_gethookcount (lua_State *L);
Whenever a hook is called, its ar
argument has its field
event
set to the specific event that triggered the hook.
Moreover, for line events, the field currentline
is also set.
To get the value of any other field in ar
,
the hook must call lua_getinfo
.
For return events, event
may be LUA_HOOKRET
,
the normal value, or LUA_HOOKTAILRET
.
In the latter case, Lua is simulating a return from
a function that did a tail call;
in this case, it is useless to call lua_getinfo
.
While Lua is running a hook, it disables other calls to hooks. Therefore, if a hook calls back Lua to execute a function or a chunk, that execution occurs without any calls to hooks.
The standard libraries provide useful functions
that are implemented directly through the C API.
Some of these functions provide essential services to the language
(e.g., type
and getmetatable
);
others provide access to "outside" services (e.g., I/O);
and others could be implemented in Lua itself,
but are quite useful or have critical performance to
deserve an implementation in C (e.g., sort
).
All libraries are implemented through the official C API and are provided as separate C modules. Currently, Lua has the following standard libraries:
To have access to these libraries,
the C host program must first call the functions
luaopen_base
(for the basic library),
luaopen_string
(for the string library),
luaopen_table
(for the table library),
luaopen_math
(for the mathematical library),
luaopen_io
(for the I/O and the Operating System libraries),
and luaopen_debug
(for the debug library).
These functions are declared in lualib.h
.
The basic library provides some core functions to Lua. If you do not include this library in your application, you should check carefully whether you need to provide some alternative implementation for some of its facilities.
assert (v [, message])
v
is nil or false;
otherwise, returns this value.
message
is an error message;
when absent, it defaults to "assertion failed!"
collectgarbage ([limit])
Sets the garbage-collection threshold to the given limit
(in Kbytes) and checks it against the byte counter.
If the new threshold is smaller than the byte counter,
then Lua immediately runs the garbage collector (see 2.9).
If limit
is absent, it defaults to zero
(thus forcing a garbage-collection cycle).
dofile (filename)
dofile
executes the contents of the standard input (stdin
).
Returns any value returned by the chunk.
In case of errors, dofile
propagates the error
to its caller (that is, it does not run in protected mode).
Terminates the last protected function called,
and returns error (message [, level])
message
as the error message.
Function error
never returns.
The level
argument specifies where the error
message points the error.
With level 1 (the default), the error position is where the
error
function was called.
Level 2 points the error to where the function
that called error
was called; and so on.
_G
_G._G = _G
).
Lua itself does not use this variable;
changing its value does not affect any environment.
(Use setfenv
to change environments.)
getfenv (f)
f
can be a Lua function or a number,
which specifies the function at that stack level:
Level 1 is the function calling getfenv
.
If the given function is not a Lua function,
or if f
is 0,
getfenv
returns the global environment.
The default for f
is 1.
if the environment has a "__fenv"
field,
returns the associated value, instead of the environment.
If the object does not have a metatable, returns nil.
Otherwise,
if the object's metatable has a "__metatable"
field,
returns the associated value.
Otherwise, returns the metatable of the given object.
gcinfo ()
Returns two results: the number of Kbytes of dynamic memory that Lua is using and the current garbage collector threshold (also in Kbytes).
ipairs (t)
Returns an iterator function, the table t
, and 0,
so that the construction
for i,v in ipairs(t) do ... endwill iterate over the pairs (
1,t[1]
), (2,t[2]
), ...,
up to the first integer key with a nil value in the table.
loadfile (filename)
Loads a file as a Lua chunk (without running it). If there are no errors, returns the compiled chunk as a function; otherwise, returns nil plus the error message. The environment of the returned function is the global environment.
loadlib (libname, funcname)
Links the program with the dynamic C library libname
.
Inside this library, looks for a function funcname
and returns this function as a C function.
libname
must be the complete file name of the C library,
including any eventual path and extension.
This function is not supported by ANSI C.
As such, it is only available on some platforms
(Windows, Linux, Solaris, BSD, plus other Unix systems that
support the dlfcn
standard).
loadstring (string [, chunkname])
The optional parameter chunkname
is the name to be used in error messages and debug information.
To load and run a given string, use the idiom
assert(loadstring(s))()
next (table [, index])
next
returns the next index of the table and the
value associated with the index.
When called with nil as its second argument,
next
returns the first index
of the table and its associated value.
When called with the last index,
or with nil in an empty table,
next
returns nil.
If the second argument is absent, then it is interpreted as nil.
Lua has no declaration of fields;
There is no difference between a
field not present in a table or a field with value nil.
Therefore, next
only considers fields with non-nil values.
The order in which the indices are enumerated is not specified,
even for numeric indices.
(To traverse a table in numeric order,
use a numerical for or the ipairs
function.)
The behavior of next
is undefined if,
during the traversal,
you assign any value to a non-existent field in the table.
pairs (t)
Returns the next
function and the table t
(plus a nil),
so that the construction
for k,v in pairs(t) do ... endwill iterate over all key-value pairs of table
t
.
Calls function f
with
the given arguments in protected mode.
That means that any error inside f
is not propagated;
instead, pcall
catches the error
and returns a status code.
Its first result is the status code (a boolean),
which is true if the call succeeds without errors.
In such case, pcall
also returns all results from the call,
after this first result.
In case of any error, pcall
returns false plus the error message.
print (e1, e2, ...)
stdout
,
using the tostring
function to convert them to strings.
This function is not intended for formatted output,
but only as a quick way to show a value,
typically for debugging.
For formatted output, use format
(see 5.3).
rawequal (v1, v2)
v1
is equal to v2
,
without invoking any metamethod.
Returns a boolean.
rawget (table, index)
table[index]
,
without invoking any metamethod.
table
must be a table;
index
is any value different from nil.
rawset (table, index, value)
table[index]
to value
,
without invoking any metamethod.
table
must be a table,
index
is any value different from nil,
and value
is any Lua value.
require (packagename)
Loads the given package.
The function starts by looking into the table _LOADED
to determine whether packagename
is already loaded.
If it is, then require
returns the value that the package
returned when it was first loaded.
Otherwise, it searches a path looking for a file to load.
If the global variable LUA_PATH
is a string,
this string is the path.
Otherwise, require
tries the environment variable LUA_PATH
.
As a last resort, it uses the predefined path "?;?.lua"
.
The path is a sequence of templates separated by semicolons.
For each template, require
will change each interrogation
mark in the template to packagename
,
and then will try to load the resulting file name.
So, for instance, if the path is
"./?.lua;./?.lc;/usr/local/?/?.lua;/lasttry"a
require "mod"
will try to load the files
./mod.lua
,
./mod.lc
,
/usr/local/mod/mod.lua
,
and /lasttry
, in that order.
The function stops the search as soon as it can load a file,
and then it runs the file.
After that, it associates, in table _LOADED
,
the package name with the value that the package returned,
and returns that value.
If the package returns nil (or no value),
require
converts this value to true.
If the package returns false,
require
also returns false.
However, as the mark in table _LOADED
is false,
any new attempt to reload the file
will happen as if the package was not loaded
(that is, the package will be loaded again).
If there is any error loading or running the file,
or if it cannot find any file in the path,
then require
signals an error.
While running a file,
require
defines the global variable _REQUIREDNAME
with the package name.
The package being loaded always runs within the global environment.
Sets the current environment to be used by the given function.
f
can be a Lua function or a number,
which specifies the function at that stack level:
Level 1 is the function calling setfenv
.
As a special case, when f
is 0 setfenv
changes
the global environment of the running thread.
If the original environment has a "__fenv"
field,
setfenv
raises an error.
setmetatable (table, metatable)
Sets the metatable for the given table.
(You cannot change the metatable of a userdata from Lua.)
If metatable
is nil, removes the metatable of the given table.
If the original metatable has a "__metatable"
field,
raises an error.
tonumber (e [, base])
tonumber
returns that number;
otherwise, it returns nil.
An optional argument specifies the base to interpret the numeral.
The base may be any integer between 2 and 36, inclusive.
In bases above 10, the letter `A
´ (in either upper or lower case)
represents 10, `B
´ represents 11, and so forth,
with `Z
´ representing 35.
In base 10 (the default), the number may have a decimal part,
as well as an optional exponent part (see 2.2.1).
In other bases, only unsigned integers are accepted.
tostring (e)
format
(see 5.3).
If the metatable of e
has a "__tostring"
field,
tostring
calls the corresponding value
with e
as argument,
and uses the result of the call as its result.
Returns the type of its only argument, coded as a string.
The possible results of this function are
type (v)
"nil"
(a string, not the value nil),
"number"
,
"string"
,
"boolean
,
"table"
,
"function"
,
"thread"
,
and "userdata"
.
unpack (list)
return list[1], list[2], ..., list[n]except that the above code can be written only for a fixed n. The number n is the size of the list, as defined for the
table.getn
function.
_VERSION
"Lua 5.0"
.
xpcall (f, err)
This function is similar to pcall
,
except that you can set a new error handler.
xpcall
calls function f
in protected mode,
using err
as the error handler.
Any error inside f
is not propagated;
instead, xpcall
catches the error,
calls the err
function with the original error object,
and returns a status code.
Its first result is the status code (a boolean),
which is true if the call succeeds without errors.
In such case, xpcall
also returns all results from the call,
after this first result.
In case of any error,
xpcall
returns false plus the result from err
.
The operations related to coroutines comprise a sub-library of the basic library, and come inside the table . See 2.10 for a general description of coroutines.
coroutine.create (f)
Creates a new coroutine, with body f
.
f
must be a Lua function.
Returns this new coroutine,
an object with type "thread"
.
coroutine.resume (co, val1, ...)
Starts or continues the execution of coroutine co
.
The first time you resume a coroutine,
it starts running its body.
The arguments val1
, ... go as the arguments to the body function.
If the coroutine has yielded,
resume
restarts it;
the arguments val1
, ... go as the results from the yield.
If the coroutine runs without any errors,
resume
returns true plus any values passed to yield
(if the coroutine yields) or any values returned by the body function
(if the coroutine terminates).
If there is any error,
resume
returns false plus the error message.
coroutine.status (co)
Returns the status of coroutine co
, as a string:
"running"
,
if the coroutine is running (that is, it called status
);
"suspended"
, if the coroutine is suspended in a call to yield
,
or if it has not started running yet;
and "dead"
if the coroutine has finished its body function,
or if it has stopped with an error.
coroutine.wrap (f)
Creates a new coroutine, with body f
.
f
must be a Lua function.
Returns a function that resumes the coroutine each time it is called.
Any arguments passed to the function behave as the
extra arguments to resume
.
Returns the same values returned by resume
,
except the first boolean.
In case of error, propagates the error.
coroutine.yield (val1, ...)
Suspends the execution of the calling coroutine.
The coroutine cannot be running neither a C function,
nor a metamethod, nor an iterator.
Any arguments to yield
go as extra results to resume
.
The string library provides all its functions inside the table
string
.
string.byte (s [, i])
i
-th character of s
,
or nil if the index is out of range.
If i
is absent, then it is assumed to be 1.
i
may be negative.
Note that numerical codes are not necessarily portable across platforms.
string.char (i1, i2, ...)
Note that numerical codes are not necessarily portable across platforms.
string.dump (function)
Returns a binary representation of the given function,
so that a later loadstring
on that string returns
a copy of the function.
function
must be a Lua function without upvalues.
string.find (s, pattern [, init [, plain]])
pattern
in the string s
.
If it finds one, then find
returns the indices of s
where this occurrence starts and ends;
otherwise, it returns nil.
If the pattern specifies captures (see string.gsub
below),
the captured strings are returned as extra results.
A third, optional numerical argument init
specifies
where to start the search;
its default value is 1 and may be negative.
A value of true as a fourth, optional argument plain
turns off the pattern matching facilities,
so the function does a plain "find substring" operation,
with no characters in pattern
being considered "magic".
Note that if plain
is given, then init
must be given too.
string.len (s)
""
has length 0.
Embedded zeros are counted,
so "a\000b\000c"
has length 5.
string.lower (s)
string.rep (s, n)
n
copies of
the string s
.
string.sub (s, i [, j])
s
that
starts at i
and continues until j
;
i
and j
may be negative.
If j
is absent, then it is assumed to be equal to -1
(which is the same as the string length).
In particular,
the call string.sub(s,1,j)
returns a prefix of s
with length j
,
and string.sub(s, -i)
returns a suffix of s
with length i
.
string.upper (s)
Returns a formatted version of its variable number of arguments
following the description given in its first argument (which must be a string).
The format string follows the same rules as the string.format (formatstring, e1, e2, ...)
printf
family of
standard C functions.
The only differences are that the options/modifiers
*
, l
, L
, n
, p
,
and h
are not supported,
and there is an extra option, q
.
The q
option formats a string in a form suitable to be safely read
back by the Lua interpreter:
The string is written between double quotes,
and all double quotes, newlines, and backslashes in the string
are correctly escaped when written.
For instance, the call
string.format('%q', 'a string with "quotes" and \n new line')will produce the string:
"a string with \"quotes\" and \ new line"
The options c
, d
, E
, e
, f
,
g
, G
, i
, o
, u
, X
, and x
all
expect a number as argument,
whereas q
and s
expect a string.
The *
modifier can be simulated by building
the appropriate format string.
For example, "%*g"
can be simulated with
"%"..width.."g"
.
String values to be formatted with
%s
cannot contain embedded zeros.
string.gfind (s, pat)
Returns an iterator function that,
each time it is called,
returns the next captures from pattern pat
over string s
.
If pat
specifies no captures,
then the whole match is produced in each call.
As an example, the following loop
s = "hello world from Lua" for w in string.gfind(s, "%a+") do print(w) endwill iterate over all the words from string
s
,
printing one per line.
The next example collects all pairs key=value
from the
given string into a table:
t = {} s = "from=world, to=Lua" for k, v in string.gfind(s, "(%w+)=(%w+)") do t[k] = v end
string.gsub (s, pat, repl [, n])
s
in which all occurrences of the pattern pat
have been
replaced by a replacement string specified by repl
.
gsub
also returns, as a second value,
the total number of substitutions made.
If repl
is a string, then its value is used for replacement.
Any sequence in repl
of the form %
n,
with n between 1 and 9,
stands for the value of the n-th captured substring (see below).
If repl
is a function, then this function is called every time a
match occurs, with all captured substrings passed as arguments,
in order;
if the pattern specifies no captures,
then the whole match is passed as a sole argument.
If the value returned by this function is a string,
then it is used as the replacement string;
otherwise, the replacement string is the empty string.
The optional last parameter n
limits
the maximum number of substitutions to occur.
For instance, when n
is 1 only the first occurrence of
pat
is replaced.
Here are some examples:
x = string.gsub("hello world", "(%w+)", "%1 %1") --> x="hello hello world world" x = string.gsub("hello world", "(%w+)", "%1 %1", 1) --> x="hello hello world" x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1") --> x="world hello Lua from" x = string.gsub("home = $HOME, user = $USER", "%$(%w+)", os.getenv) --> x="home = /home/roberto, user = roberto" x = string.gsub("4+5 = $return 4+5$", "%$(.-)%$", function (s) return loadstring(s)() end) --> x="4+5 = 9" local t = {name="lua", version="5.0"} x = string.gsub("$name_$version.tar.gz", "%$(%w+)", function (v) return t[v] end) --> x="lua_5.0.tar.gz"
a character class is used to represent a set of characters. The following combinations are allowed in describing a character class:
^$()%.[]*+-?
)
--- represents the character x itself.
.
--- (a dot) represents all characters.
%a
--- represents all letters.
%c
--- represents all control characters.
%d
--- represents all digits.
%l
--- represents all lowercase letters.
%p
--- represents all punctuation characters.
%s
--- represents all space characters.
%u
--- represents all uppercase letters.
%w
--- represents all alphanumeric characters.
%x
--- represents all hexadecimal digits.
%z
--- represents the character with representation 0.
%x
(where x is any non-alphanumeric character) ---
represents the character x.
This is the standard way to escape the magic characters.
Any punctuation character (even the non magic)
can be preceded by a `%
´
when used to represent itself in a pattern.
[set]
---
represents the class which is the union of all
characters in set.
A range of characters may be specified by
separating the end characters of the range with a `-
´.
All classes %
x described above may also be used as
components in set.
All other characters in set represent themselves.
For example, [%w_]
(or [_%w]
)
represents all alphanumeric characters plus the underscore,
[0-7]
represents the octal digits,
and [0-7%l%-]
represents the octal digits plus
the lowercase letters plus the `-
´ character.
The interaction between ranges and classes is not defined.
Therefore, patterns like [%a-z]
or [a-%%]
have no meaning.
[^set]
---
represents the complement of set,
where set is interpreted as above.
%a
, %c
, etc.),
the corresponding uppercase letter represents the complement of the class.
For instance, %S
represents all non-space characters.
The definitions of letter, space, and other character groups
depend on the current locale.
In particular, the class [a-z]
may not be equivalent to %l
.
The second form should be preferred for portability.
a pattern item may be
*
´,
which matches 0 or more repetitions of characters in the class.
These repetition items will always match the longest possible sequence;
+
´,
which matches 1 or more repetitions of characters in the class.
These repetition items will always match the longest possible sequence;
-
´,
which also matches 0 or more repetitions of characters in the class.
Unlike `*
´,
these repetition items will always match the shortest possible sequence;
?
´,
which matches 0 or 1 occurrence of a character in the class;
%n
, for n between 1 and 9;
such item matches a substring equal to the n-th captured string
(see below);
%bxy
, where x and y are two distinct characters;
such item matches strings that start with x, end with y,
and where the x and y are balanced.
This means that, if one reads the string from left to right,
counting +1 for an x and -1 for a y,
the ending y is the first y where the count reaches 0.
For instance, the item %b()
matches expressions with
balanced parentheses.
a pattern is a sequence of pattern items.
A `^
´ at the beginning of a pattern anchors the match at the
beginning of the subject string.
A `$
´ at the end of a pattern anchors the match at the
end of the subject string.
At other positions,
`^
´ and `$
´ have no special meaning and represent themselves.
A pattern may contain sub-patterns enclosed in parentheses;
they describe captures.
When a match succeeds, the substrings of the subject string
that match captures are stored (captured) for future use.
Captures are numbered according to their left parentheses.
For instance, in the pattern "(a*(.)%w(%s*))"
,
the part of the string matching "a*(.)%w(%s*)"
is
stored as the first capture (and therefore has number 1);
the character matching .
is captured with number 2,
and the part matching %s*
has number 3.
As a special case, the empty capture ()
captures
the current string position (a number).
For instance, if we apply the pattern "()aa()"
on the
string "flaaap"
, there will be two captures: 3 and 5.
A pattern cannot contain embedded zeros. Use %z
instead.
table
.
Most functions in the table library assume that the table represents an array or a list. For those functions, an important concept is the size of the array. There are three ways to specify that size:
"n"
---
When the table has a field "n"
with a numerical value,
that value is assumed as its size.
setn
---
You can call the table.setn
function to explicitly set
the size of a table.
table.getn
and
table.setn
functions.
table.concat (table [, sep [, i [, j]]])
table[i]..sep..table[i+1] ... sep..table[j]
.
The default value for sep
is the empty string,
the default for i
is 1,
and the default for j
is the size of the table.
If i
is greater than j
, returns the empty string.
table.foreach (table, f)
f
over all elements of table
.
For each element, f
is called with the index and
respective value as arguments.
If f
returns a non-nil value,
then the loop is broken, and this value is returned
as the final value of foreach
.
See the next
function for extra information about table traversals.
table.foreachi (table, f)
f
over the
numerical indices of table
.
For each index, f
is called with the index and
respective value as arguments.
Indices are visited in sequential order,
from 1 to n
,
where n
is the size of the table (see 5.4).
If f
returns a non-nil value,
then the loop is broken and this value is returned
as the result of foreachi
.
Returns the size of a table, when seen as a list.
If the table has an table.getn (table)
n
field with a numeric value,
this value is the size of the table.
Otherwise, if there was a previous call
to table.setn
over this table,
the respective value is returned.
Otherwise, the size is one less the first integer index with
a nil value.
table.sort (table [, comp])
table[1]
to table[n]
,
where n
is the size of the table (see 5.4).
If comp
is given,
then it must be a function that receives two table elements,
and returns true
when the first is less than the second
(so that not comp(a[i+1],a[i])
will be true after the sort).
If comp
is not given,
then the standard Lua operator <
is used instead.
The sort algorithm is not stable, that is, elements considered equal by the given order may have their relative positions changed by the sort.
table.insert (table, [pos,] value)
Inserts element value
at position pos
in table
,
shifting up other elements to open space, if necessary.
The default value for pos
is n+1
,
where n
is the size of the table (see 5.4),
so that a call table.insert(t,x)
inserts x
at the end
of table t
.
This function also updates the size of the table by
calling table.setn(table, n+1)
.
table.remove (table [, pos])
Removes from table
the element at position pos
,
shifting down other elements to close the space, if necessary.
Returns the value of the removed element.
The default value for pos
is n
,
where n
is the size of the table (see 5.4),
so that a call table.remove(t)
removes the last element
of table t
.
This function also updates the size of the table by
calling table.setn(table, n-1)
.
table.setn (table, n)
Updates the size of a table.
If the table has a field "n"
with a numerical value,
that value is changed to the given n
.
Otherwise, it updates an internal state
so that subsequent calls to table.getn(table)
return n
.
This library is an interface to most of the functions of the
standard C math library.
(Some have slightly different names.)
It provides all its functions inside the table math
.
In addition,
it registers the global __pow
for the binary exponentiation operator ^
,
so that x^y
returns xy.
The library provides the following functions:
math.abs math.acos math.asin math.atan math.atan2 math.ceil math.cos math.deg math.exp math.floor math.log math.log10 math.max math.min math.mod math.pow math.rad math.sin math.sqrt math.tan math.frexp math.ldexp math.random math.randomseedplus a variable
math.pi
.
Most of them
are only interfaces to the corresponding functions in the C library.
All trigonometric functions work in radians
(previous versions of Lua used degrees).
The functions math.deg
and math.rad
convert
between radians and degrees.
The function math.max
returns the maximum
value of its numeric arguments.
Similarly, math.min
computes the minimum.
Both can be used with 1, 2, or more arguments.
The functions math.random
and math.randomseed
are interfaces to the simple random generator functions
rand
and srand
that are provided by ANSI C.
(No guarantees can be given for their statistical properties.)
When called without arguments,
math.random
returns a pseudo-random real number
in the range [0,1).
When called with a number n,
math.random
returns a pseudo-random integer in the range [1,n].
When called with two arguments, l and u,
math.random
returns a pseudo-random integer in the range [l,u].
The math.randomseed
function sets a "seed"
for the pseudo-random generator:
Equal seeds produce equal sequences of numbers.
5.6 - Input and Output Facilities
The I/O library provides two different styles for file manipulation. The first one uses implicit file descriptors, that is, there are operations to set a default input file and a default output file, and all input/output operations are over those default files. The second style uses explicit file descriptors.
When using implicit file descriptors,
all operations are supplied by table io
.
When using explicit file descriptors,
the operation io.open
returns a file descriptor
and then all operations are supplied as methods by the file descriptor.
The table io
also provides
three predefined file descriptors with their usual meanings from C:
io.stdin
, io.stdout
, and io.stderr
.
A file handle is a userdata containing the file stream (FILE*
),
with a distinctive metatable created by the I/O library.
Unless otherwise stated, all I/O functions return nil on failure (plus an error message as a second result) and some value different from nil on success.
io.close ([file])
Equivalent to file:close()
.
Without a file
, closes the default output file.
io.flush ()
Equivalent to file:flush
over the default output file.
io.input ([file])
When called with a file name, it opens the named file (in text mode), and sets its handle as the default input file. When called with a file handle, it simply sets that file handle as the default input file. When called without parameters, it returns the current default input file.
In case of errors this function raises the error, instead of returning an error code.
io.lines ([filename])
Opens the given file name in read mode and returns an iterator function that, each time it is called, returns a new line from the file. Therefore, the construction
for line in io.lines(filename) do ... endwill iterate over all lines of the file. When the iterator function detects the end of file, it returns nil (to finish the loop) and automatically closes the file.
The call io.lines()
(without a file name) is equivalent
to io.input():lines()
, that is, it iterates over the
lines of the default input file.
io.open (filename [, mode])
This function opens a file,
in the mode specified in the string mode
.
It returns a new file handle,
or, in case of errors, nil plus an error message.
The mode
string can be any of the following:
mode
string may also have a b
at the end,
which is needed in some systems to open the file in binary mode.
This string is exactly what is used in the standard C function fopen
.
io.output ([file])
Similar to io.input
, but operates over the default output file.
io.read (format1, ...)
Equivalent to io.input():read
.
io.tmpfile ()
Returns a handle for a temporary file. This file is open in update mode and it is automatically removed when the program ends.
io.type (obj)
Checks whether obj
is a valid file handle.
Returns the string "file"
if obj
is an open file handle,
"closed file"
if obj
is a closed file handle,
and nil if obj
is not a file handle.
io.write (value1, ...)
Equivalent to io.output():write
.
file:close ()
Closes file
.
file:flush ()
Saves any written data to file
.
file:lines ()
Returns an iterator function that, each time it is called, returns a new line from the file. Therefore, the construction
for line in file:lines() do ... endwill iterate over all lines of the file. (Unlike
io.lines
, this function does not close the file
when the loop ends.)
file:read (format1, ...)
Reads the file file
,
according to the given formats, which specify what to read.
For each format,
the function returns a string (or a number) with the characters read,
or nil if it cannot read data with the specified format.
When called without formats,
it uses a default format that reads the entire next line
(see below).
The available formats are
file:seek ([whence] [, offset])
Sets and gets the file position,
measured from the beginning of the file,
to the position given by offset
plus a base
specified by the string whence
, as follows:
seek
returns the final file position,
measured in bytes from the beginning of the file.
If this function fails, it returns nil,
plus a string describing the error.
The default value for whence
is "cur"
,
and for offset
is 0.
Therefore, the call file:seek()
returns the current
file position, without changing it;
the call file:seek("set")
sets the position to the
beginning of the file (and returns 0);
and the call file:seek("end")
sets the position to the
end of the file, and returns its size.
file:write (value1, ...)
Writes the value of each of its arguments to
the filehandle file
.
The arguments must be strings or numbers.
To write other values,
use tostring
or string.format
before write
.
5.7 - Operating System Facilities
This library is implemented through table os
.
os.clock ()
Returns an approximation of the amount of CPU time used by the program, in seconds.
os.date ([format [, time]])
Returns a string or a table containing date and time,
formatted according to the given string format
.
If the time
argument is present,
this is the time to be formatted
(see the os.time
function for a description of this value).
Otherwise, date
formats the current time.
If format
starts with `!
´,
then the date is formatted in Coordinated Universal Time.
After that optional character,
if format
is *t
,
then date
returns a table with the following fields:
year
(four digits), month
(1--12), day
(1--31),
hour
(0--23), min
(0--59), sec
(0--61),
wday
(weekday, Sunday is 1),
yday
(day of the year),
and isdst
(daylight saving flag, a boolean).
If format
is not *t
,
then date
returns the date as a string,
formatted according with the same rules as the C function strftime
.
When called without arguments,
date
returns a reasonable date and time representation that depends on
the host system and on the current locale
(that is, os.date()
is equivalent to os.date("%c")
).
os.difftime (t2, t1)
Returns the number of seconds from time t1
to time t2
.
In Posix, Windows, and some other systems,
this value is exactly t2
-t1
.
os.execute (command)
This function is equivalent to the C function system
.
It passes command
to be executed by an operating system shell.
It returns a status code, which is system-dependent.
os.exit ([code])
Calls the C function exit
,
with an optional code
,
to terminate the host program.
The default value for code
is the success code.
os.getenv (varname)
Returns the value of the process environment variable varname
,
or nil if the variable is not defined.
os.remove (filename)
Deletes the file with the given name. If this function fails, it returns nil, plus a string describing the error.
os.rename (oldname, newname)
Renames file named oldname
to newname
.
If this function fails, it returns nil,
plus a string describing the error.
os.setlocale (locale [, category])
Sets the current locale of the program.
locale
is a string specifying a locale;
category
is an optional string describing which category to change:
"all"
, "collate"
, "ctype"
,
"monetary"
, "numeric"
, or "time"
;
the default category is "all"
.
The function returns the name of the new locale,
or nil if the request cannot be honored.
os.time ([table])
Returns the current time when called without arguments,
or a time representing the date and time specified by the given table.
This table must have fields year
, month
, and day
,
and may have fields hour
, min
, sec
, and isdst
(for a description of these fields, see the os.date
function).
The returned value is a number, whose meaning depends on your system.
In Posix, Windows, and some other systems, this number counts the number
of seconds since some given start time (the "epoch").
In other systems, the meaning is not specified,
and the number returned by time
can be used only as an argument to
date
and difftime
.
os.tmpname ()
Returns a string with a file name that can be used for a temporary file. The file must be explicitly opened before its use and removed when no longer needed.
This function is equivalent to the tmpnam
C function,
and many people (and even some compilers!) advise against its use,
because between the time you call this function
and the time you open the file,
it is possible for another process
to create a file with the same name.
The debug
library provides
the functionality of the debug interface to Lua programs.
You should exert care when using this library.
The functions provided here should be used exclusively for debugging
and similar tasks, such as profiling.
Please resist the temptation to use them as a
usual programming tool:
They can be very slow.
Moreover, setlocal
and getlocal
violate the privacy of local variables
and therefore can compromise some otherwise secure code.
All functions in this library are provided
inside a debug
table.
debug.debug ()
Enters an interactive mode with the user,
running each string that the user enters.
Using simple commands and other debug facilities,
the user can inspect global and local variables,
change their values, evaluate expressions, and so on.
A line containing only the word cont
finishes this function,
so that the caller continues its execution.
Note that commands for debug.debug
are not lexically nested
with any function, so they have no direct access to local variables.
debug.gethook ()
Returns the current hook settings, as three values:
the current hook function, the current hook mask,
and the current hook count (as set by the debug.sethook
function).
debug.getinfo (function [, what])
This function returns a table with information about a function.
You can give the function directly,
or you can give a number as the value of function
,
which means the function running at level function
of the call stack:
Level 0 is the current function (getinfo
itself);
level 1 is the function that called getinfo
;
and so on.
If function
is a number larger than the number of active functions,
then getinfo
returns nil.
The returned table contains all the fields returned by lua_getinfo
,
with the string what
describing which fields to fill in.
The default for what
is to get all information available.
If present,
the option `f
´
adds a field named func
with the function itself.
For instance, the expression debug.getinfo(1,"n").name
returns
the name of the current function, if a reasonable name can be found,
and debug.getinfo(print)
returns a table with all available information
about the print
function.
debug.getlocal (level, local)
This function returns the name and the value of the local variable
with index local
of the function at level level
of the stack.
(The first parameter or local variable has index 1, and so on,
until the last active local variable.)
The function returns nil if there is no local
variable with the given index,
and raises an error when called with a level
out of range.
(You can call debug.getinfo
to check whether the level is valid.)
debug.getupvalue (func, up)
This function returns the name and the value of the upvalue
with index up
of the function func
.
The function returns nil if there is no upvalue with the given index.
debug.setlocal (level, local, value)
This function assigns the value value
to the local variable
with index local
of the function at level level
of the stack.
The function returns nil if there is no local
variable with the given index,
and raises an error when called with a level
out of range.
(You can call getinfo
to check whether the level is valid.)
debug.setupvalue (func, up, value)
This function assigns the value value
to the upvalue
with index up
of the function func
.
The function returns nil if there is no upvalue
with the given index.
debug.sethook (hook, mask [, count])
Sets the given function as a hook.
The string mask
and the number count
describe
when the hook will be called.
The string mask may have the following characters,
with the given meaning:
"c"
The hook is called every time Lua calls a function;
"r"
The hook is called every time Lua returns from a function;
"l"
The hook is called every time Lua enters a new line of code.
count
different from zero,
the hook is called after every count
instructions.
When called without arguments,
the debug.sethook
function turns off the hook.
When the hook is called, its first parameter is always a string
describing the event that triggered its call:
"call"
, "return"
(or "tail return"
),
"line"
, and "count"
.
Moreover, for line events,
it also gets as its second parameter the new line number.
Inside a hook,
you can call getinfo
with level 2 to get more information about
the running function
(level 0 is the getinfo
function,
and level 1 is the hook function),
unless the event is "tail return"
.
In this case, Lua is only simulating the return,
and a call to getinfo
will return invalid data.
debug.traceback ([message])
Returns a string with a traceback of the call stack.
An optional message
string is appended
at the beginning of the traceback.
This function is typically used with xpcall
to produce
better error messages.
Although Lua has been designed as an extension language,
to be embedded in a host C program,
it is also frequently used as a stand-alone language.
An interpreter for Lua as a stand-alone language,
called simply lua
,
is provided with the standard distribution.
The stand-alone interpreter includes
all standard libraries plus the reflexive debug interface.
Its usage is:
lua [options] [script [args]]The options are:
-
executes stdin
as a file;
-e
stat executes string stat;
-l
file "requires" file;
-i
enters interactive mode after running script;
-v
prints version information;
--
stop handling options.
lua
runs the given script,
passing to it the given args.
When called without arguments,
lua
behaves as lua -v -i
when stdin
is a terminal,
and as lua -
otherwise.
Before running any argument,
the interpreter checks for an environment variable LUA_INIT
.
If its format is @filename,
then lua executes the file.
Otherwise, lua executes the string itself.
All options are handled in order, except -i
.
For instance, an invocation like
$ lua -e'a=1' -e 'print(a)' script.luawill first set
a
to 1, then print a
,
and finally run the file script.lua
.
(Here, $
is the shell prompt. Your prompt may be different.)
Before starting to run the script,
lua
collects all arguments in the command line
in a global table called arg
.
The script name is stored in index 0,
the first argument after the script name goes to index 1,
and so on.
The field n
gets the number of arguments after the script name.
Any arguments before the script name
(that is, the interpreter name plus the options)
go to negative indices.
For instance, in the call
$ lua -la.lua b.lua t1 t2the interpreter first runs the file
a.lua
,
then creates a table
arg = { [-2] = "lua", [-1] = "-la.lua", [0] = "b.lua", [1] = "t1", [2] = "t2"; n = 2 }and finally runs the file
b.lua
.
In interactive mode, if you write an incomplete statement, the interpreter waits for its completion.
If the global variable _PROMPT
is defined as a string,
then its value is used as the prompt.
Therefore, the prompt can be changed directly on the command line:
$ lua -e"_PROMPT='myprompt> '" -i(the outer pair of quotes is for the shell, the inner is for Lua), or in any Lua programs by assigning to
_PROMPT
.
Note the use of -i
to enter interactive mode; otherwise,
the program would end just after the assignment to _PROMPT
.
In Unix systems, Lua scripts can be made into executable programs
by using chmod +x
and the #!
form,
as in
#!/usr/local/bin/lua(Of course, the location of the Lua interpreter may be different in your machine. If
lua
is in your PATH
,
then
#!/usr/bin/env luais a more portable solution.)
The Lua team is grateful to Tecgraf for its continued support to Lua. We thank everyone at Tecgraf, specially the head of the group, Marcelo Gattass. At the risk of omitting several names, we also thank the following individuals for supporting, contributing to, and spreading the word about Lua: Alan Watson. André Clinio, André Costa, Antonio Scuri, Asko Kauppi, Bret Mogilefsky, Cameron Laird, Carlos Cassino, Carlos Henrique Levy, Claudio Terra, David Jeske, Ed Ferguson, Edgar Toernig, Erik Hougaard, Jim Mathies, John Belmonte, John Passaniti, John Roll, Jon Erickson, Jon Kleiser, Mark Ian Barlow, Nick Trout, Noemi Rodriguez, Norman Ramsey, Philippe Lhoste, Renata Ratton, Renato Borges, Renato Cerqueira, Reuben Thomas, Stephan Herrmann, Steve Dekorte, Thatcher Ulrich, Tomás Gorham, Vincent Penquerc'h, Thank you!
Lua 5.0 is a major release. There are several incompatibilities with its previous version, Lua 4.0.
{a,b,f()}
) has all its return values inserted in the list.
for k,v in t
, where t
is a table,
is deprecated (although it is still supported).
Use for k,v in pairs(t)
instead.
[[...]]
starts with a newline,
this newline is ignored.
%var
are obsolete;
use external local variables instead.
compat.lua
) that
redefine most of them as global names.
compat.lua
),
functions still work in degrees.
call
function is deprecated.
Use f(unpack(tab))
instead of call(f, tab)
for unprotected calls,
or the new pcall
function for protected calls.
dofile