Reference Manual of the Programming Language Lua 5.0 (alpha)
Reference Manual of the Programming Language Lua 5.0 (alpha)
Copyright © 2002 Tecgraf, PUC-Rio. All rights reserved.
Lua is an extension programming language designed to support general procedural programming with data description facilities. Lua is intended to be used as a powerful, light-weight configuration language for any program that needs one. Lua is implemented as a library, written in C.
Being an extension language, Lua has no notion of a ``main'' program: it only works embedded in a host client, called the embedding program or simply the host. This host program can invoke functions to execute a piece of Lua code, can write and read Lua variables, and can register C functions to be called by Lua code. Through the use of C functions, Lua can be augmented to cope with a wide range of different domains, thus creating customized programming languages sharing a syntactical framework.
Lua is free software, and is provided as usual with no guarantees, as stated in its copyright notice. The implementation described in this manual is available at Lua's official web site, www.lua.org.
Like any other reference manual, this document is dry in places. For a discussion of the decisions behind the design of Lua, see the papers below, which are available at Lua's web site.
Lua means ``moon'' in Portuguese.
This section describes the main concepts of Lua as a language. The syntax and semantics of Lua are described in Section 3. The discussion below is not purely conceptual; it includes references to the C API (see Section 4), because Lua is designed to be embedded in host programs. It also includes references to the standard libraries (see Section 6).
All statements in Lua are executed in a global environment.
This environment is initialized with a call from the embedding program to
lua_open and
persists until a call to lua_close
or the end of the embedding program.
If necessary,
the host programmer can create multiple independent global
environments, and freely switch between them (see Section 4.1).
The unit of execution of Lua is called a chunk. A chunk is simply a sequence of statements. Statements are described in Section 3.3.
A chunk may be stored in a file or in a string inside the host program. When a chunk is executed, first it is pre-compiled into opcodes for a virtual machine, and then the compiled statements are executed by an interpreter for the virtual machine. All modifications a chunk effects on the global environment persist after the chunk ends.
Chunks may also be pre-compiled into binary form and stored in files; see program luac for details. Text files with chunks and their binary pre-compiled forms are interchangeable; Lua automatically detects the file type and acts accordingly.
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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 seven basic types in Lua:
nil, boolean, number,
string, function, userdata, 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 fails,
and any other value makes it succeeds.
Number represents real (double-precision floating-point) numbers.
String represents arrays of characters.
Lua is 8-bit clean,
and so strings may contain any 8-bit character,
including embedded zeros ('\0') (see Section 3.1).
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 Section 3.4.7).
The type userdata is provided to allow the store of arbitrary C data 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 Section 3.7). Userdata values cannot be created or modified in Lua, only through the C API. This guarantees the integrity of data owned by the host program.
The type table implements associative arrays,
that is, arrays that can be indexed not only with numbers,
but with any value (except nil).
Moreover,
tables can be heterogeneous,
that is, they can contain values of all types.
Tables are the sole data structuring mechanism in Lua;
they may be used not only to represent ordinary arrays,
but also symbol tables, sets, records, graphs, trees, etc.
To represent records, Lua uses the field name as an index.
The language supports this representation by
providing a.name as syntactic sugar for a["name"].
There are several convenient ways to create tables in Lua
(see Section 3.4.6).
Like indices, the value of a table field can be of any type. In particular, because functions are first class values, table fields may contain functions. So, tables may also carry methods (see Section 3.4.8).
Tables, functions, and userdata values are objects: variables do not actually contain these values, only references to them. Assignment, parameter passing, and returns from functions always manipulate references to these values, and do not imply any kind of copy.
The library function type returns a string describing the type
of a given value (see Section 6.1).
Each table or userdata object in Lua may have a metatable.
You can change several aspects of the behavior
of an object by setting specific fields in its metatable.
For instance, when an object is the operand of an addition,
Lua checks for a function in the field "__add" in its metatable.
If it finds one,
Lua calls that function to perform the addition.
We call the keys in a metatable events,
and the values metamethods.
In the previous example, "add" is the event,
and the metamethod is the function that performs the addition.
A metatable controls how an object behaves in arithmetic operations, order comparisons, concatenation, and indexing. A metatable can also defines a function to be called when a userdata is garbage collected. Section 3.7 gives a detailed description of which events you can control with metatables.
You can query and change the metatable of an object
through the setmetatable and getmetatable
functions (see Section 6.1).
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 when a string is expected,
the number is converted to a string, in a reasonable format.
The format is chosen so that
a conversion from number to string then back to number
reproduces the original number exactly.
For complete control of how numbers are converted to strings,
use the format function (see Section 6.2).
There are two kinds of variables in Lua: global variables and local variables. Variables are assumed to be global unless explicitly declared local (see Section 3.3.7). Before the first assignment, the value of a variable is nil.
All global variables live as fields in ordinary Lua tables. Usually, globals live in a table called table of globals. However, a function can individually change its global table, so that all global variables in that function will refer to that table. This mechanism allows the creation of namespaces and other modularization facilities.
Local variables are lexically scoped. Therefore, local variables can be freely accessed by functions defined inside their scope (see Section 3.5).
Lua does automatic memory management. That means that you do not have to worry about allocating memory for new objects and freeing it when the objects are no longer needed. Lua manages memory automatically by running a garbage collector from time to time and collecting all dead objects (all objects that are no longer accessible from Lua). All objects in Lua are subject to automatic management: tables, userdata, functions, and strings.
Using the C API,
you can set garbage-collector metamethods for userdata (see Section 3.7).
When it is about to free a userdata,
Lua calls the metamethod associated with event gc in the
userdata's metatable.
Using such facility, you can coordinate Lua's garbage collection
with external resource management
(such as closing files, network or database connections,
or freeing your own memory).
Lua uses two numbers to control its garbage-collection cycles. One number counts how many bytes of dynamic memory Lua is using, and the other is a threshold. When the number of bytes crosses the threshold, Lua runs the garbage collector, which reclaims the memory of all dead objects. The byte counter is corrected, and then the threshold is reset to twice the value of the byte counter.
Through the C API, you can query those numbers,
and change the threshold (see Section 4.8).
Setting the threshold to zero actually forces an immediate
garbage-collection cycle,
while setting it to a huge number effectively stops the garbage collector.
Using Lua code you have a more limited control over garbage-collection cycles,
through the functions gcinfo and collectgarbage
(see Section 6.1).
A weak table is a table whose elements are weak references. A weak reference is ignored by the garbage collector. In other words, if the only references to an object are weak references, then the garbage collector will collect that object.
A weak table can have weak keys, weak values, or both.
A table with weak keys allows the collection of its keys,
but prevents the collection of its values.
A table with both weak keys and weak values allows the collection of
both keys and values.
In any case, if either the key or the value is collected,
the whole pair is removed from the table.
The weakness of a table is set with the setmode function.
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.
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 global
if in local nil not
or repeat return then true
until while
Lua is a case-sensitive language:
and is a reserved word, but And and ánd
(if the locale permits) are two different, valid identifiers.
As a convention, identifiers starting with an underscore followed by
uppercase letters (such as _VERSION)
are reserved for internal variables.
The following strings denote other tokens:
+ - * / ^
~= <= >= < > == =
( ) { } [ ]
; : , . .. ...
Literal strings
can be delimited by matching single or double quotes,
and can contain the C-like escape sequences
`\a' (bell),
`\b' (backspace),
`\f' (form feed),
`\n' (newline),
`\r' (carriage return),
`\t' (horizontal tab),
`\v' (vertical tab),
`\\' (backslash),
`\"' (double quote),
`\'' (single quote),
and `\newline' (that is, a backslash followed by a real newline,
which results in a newline in the string).
A character in a string may also be specified by its numerical value,
through the escape sequence `\ddd',
where ddd is a sequence of up to three decimal digits.
Strings in Lua may contain any 8-bit value, including embedded zeros,
which can be specified as `\0'.
Literal strings can also be delimited by matching [[ ... ]].
Literals in this bracketed form may run for several lines,
may contain nested [[ ... ]] pairs,
and do not interpret escape sequences.
For convenience,
when the opening [[ is immediately followed by a newline,
the newline is not included in the string.
That form is specially convenient for
writing strings that contain program pieces or
other quoted strings.
As an example, in a system using ASCII
(in which `a' is coded as 97,
newline is coded as 10, and `1' is coded as 49),
the four literals below denote the same string:
(1) "alo\n123\""
(2) '\97lo\10\04923"'
(3) [[alo
123"]]
(4) [[
alo
123"]]
Numerical constants may be written with an optional decimal part and an optional decimal exponent. Examples of valid numerical constants are
3 3.0 3.1416 314.16e-2 0.31416E1
Comments start anywhere outside a string with a
double hyphen (--);
If the text after -- is different from [[,
the comment is a short comment,
that runs until the end of the line.
Otherwise, it is a long comment,
that runs until the corresponding ]].
Long comments may run for several lines,
and may contain nested [[ ... ]] pairs.
For convenience,
the first line of a chunk is skipped if it starts with #.
This facility allows the use of Lua as a script interpreter
in Unix systems (see Section 7).
Variables are places that store values.
A single name can denote a global variable, a local variable, or a formal parameter in a function (formal parameters are just local variables):
var ::= `Name'
Square brackets are used to index a table:
var ::= prefixexp `[' exp `]'
The first expression should result in a table value,
and the second expression identifies a specific entry inside that table.
The syntax var.NAME is just syntactic sugar for
var["NAME"]:
var ::= prefixexp `.' `Name'
The expression denoting the table to be indexed has a restricted syntax; Section 3.4 for details.
The meaning of assignments and evaluations of global and
indexed variables can be changed via metatables.
An assignment to a global variable x = val
is equivalent to the assignment
_glob.x = val,
where _glob is the table of globals of the running function
((see Section 2.2) for a discussion about the table of globals).
An assignment to an indexed variable t[i] = val is equivalent to
settable_event(t,i,val).
An access to a global variable x
is equivalent to _glob.x
(again, (see Section 2.2) for a discussion about _glob).
An access to an indexed variable t[i] is equivalent to
a call gettable_event(t,i).
See Section 3.7 for a complete description of the
settable_event and gettable_event functions.
(These functions are not defined in Lua.
We use them here only for explanatory purposes.)
Lua supports an almost conventional set of statements, similar to those in Pascal or C. The conventional commands include assignment, control structures, and procedure calls. Non-conventional commands include table constructors and variable declarations.
chunk ::= {stat [`;']}
block ::= chunk
A block may be explicitly delimited to produce a single statement:
stat ::= do block end
Explicit blocks are useful
to control the scope of variable declarations.
Explicit blocks are also sometimes used to
add a return or break statement in the middle
of another block (see Section 3.3.4).
stat ::= varlist1 `=' explist1
varlist1 ::= var {`,' var}
explist1 ::= exp {`,' exp}
Expressions are discussed in Section 3.4.
Before the assignment, the list of values is adjusted to the length of the list of variables. If there are more values than needed, the excess values are thrown away. If there are less values than needed, the list is extended with as many nil's as needed. If the list of expressions ends with a function call, then all values returned by that function call enter in the list of values, before the adjust (except when the call is enclosed in parentheses; see Section 3.4).
The assignment statement first evaluates all its expressions, and only then makes the assignments. So, the code
i = 3
i, a[i] = i+1, 20
sets a[3] to 20, without affecting a[4]
because the i in a[i] is evaluated
before it is assigned 4.
Similarly, the line
x, y = y, x
exchanges the values of x and y.
stat ::= while exp do block end
stat ::= repeat block until exp
stat ::= if exp then block {elseif exp then block} [else block] end
Lua also has a for statement, in two flavors (see Section 3.3.5).
The condition expression exp of a control structure may return any value. All values different from nil and false are considered true (in particular, the number 0 and the empty string are also true); both false and nil are considered false.
The return statement is used to return values from a function or from a chunk. Functions and chunks may return more than one value, and so the syntax for the return statement is
stat ::= return [explist1]
The break statement can be used to terminate the execution of a while, repeat, or for loop, skipping to the next statement after the loop:
stat ::= break
A break ends the innermost enclosing loop.
For syntactic reasons, return and break
statements can only be written as the last statement of a block.
If it is really necessary to return or break in the
middle of a block,
then an explicit inner block can used,
as in the idioms
`do return end' and
`do break end',
because now return and break are the last statements in
their (inner) blocks.
In practice,
those idioms are only used during debugging.
(For instance, a line `do return end' can be added at the
beginning of a chunk for syntax checking only.)
The for statement has two forms, one for numbers and one generic.
The numerical for loop repeats a block of code while a control variable runs through an arithmetic progression. It has the following syntax:
stat ::= for `Name' `=' exp `,' exp [`,' exp] do block end
The block is repeated for name starting at the value of
the first exp, until it reaches the second exp by steps of the
third exp.
More precisely, a for statement like
for var = e1, e2, e3 do block end
is equivalent to the code:
do
local var, _limit, _step = tonumber(e1), tonumber(e2), tonumber(e3)
if not (var and _limit and _step) then error() end
while (_step>0 and var<=_limit) or (_step<=0 and var>=_limit) do
block
var = var+_step
end
end
Note the following:
_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 generators. It calls its generator to produce a new value for each iteration, stopping when the new value is nil. It has the following syntax:
stat ::= for `Name' {`,' `Name'} in explist1 do block end
A for statement like
for var_1, ..., var_n in explist do block end
is equivalent to the code:
do
local _f, _s, var_1 = explist
while 1 do
local var_2, ..., var_n
var_1, ..., var_n = _f(_s, var_1)
if var_1 == nil then break end
block
end
end
Note the following:
explist is evaluated only once.
Its results are a ``generator'' function,
a ``state'', and an initial value for the ``iterator variable''.
_f and _s are invisible variables.
The names are here for explanatory purposes only.
var_i 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.
stat ::= functioncall
In this case, all returned values are thrown away.
Function calls are explained in Section 3.4.7.
stat ::= local namelist [`=' explist1]
namelist ::= `Name' {`,' `Name'}
If present, an initial assignment has the same semantics
of a multiple assignment (see Section 3.3.3).
Otherwise, all variables are initialized with nil.
A chunk is also a block (see Section 3.3.1), and so local variables can be declared outside any explicit block. Such local variables die when the chunk ends.
Visibility rules for local variables are explained in Section 3.5.
The basic expressions in Lua are the following:
exp ::= prefixexp
exp ::= nil | false | true
exp ::= Number
exp ::= Literal
exp ::= function
exp ::= tableconstructor
prefixexp ::= var | functioncall | `(' exp `)'
An expression enclosed in parentheses always results in only one value.
Thus,
(f(x,y,z)) is always a single value,
even if f returns several values.
(The value of (f(x,y,z)) is the first value returned by f
or nil if f does not return any values.)
Numbers and literal strings are explained in Section 3.1; variables are explained in Section 3.2; function definitions are explained in Section 3.4.8; function calls are explained in Section 3.4.7; table constructors are explained in Section 3.4.6.
Expressions can also be built with arithmetic operators, relational operators, and logical operadors, 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 Section 2.4),
then all operations except exponentiation have the usual meaning,
while exponentiation calls a global function pow; ??
otherwise, an appropriate metamethod is called (see Section 3.7).
The standard mathematical library defines function pow,
giving the expected meaning to exponentiation
(see Section 6.4).
== ~= < > <= >=
These operators always result in false or true.
Equality (==) first compares the type of its operands.
If the types are different, then the result is false.
Otherwise, the values of the operands are compared.
Numbers and strings are compared in the usual way.
Tables, userdata, and functions are compared by reference,
that is,
two tables are considered equal only if they are the same table.
Every time you create a new table (or userdata, or function), this new value is different from any previously existing value.
The conversion rules of Section 2.4
do not apply to equality comparisons.
Thus, "0"==0 evaluates to false,
and t[0] and t["0"] denote different
entries in a table.
The operator ~= is exactly the negation of equality (==).
The order operators work as follows. If both arguments are numbers, then they are compared as such. Otherwise, if both arguments are strings, then their values are compared according to the current locale. Otherwise, the ``lt'' or the ``le'' metamethod is called (see Section 3.7).
and or not
Like the control structures (see Section 3.3.4),
all logical operators consider both false and nil as false
and anything else as true.
The operator not always return false or true.
The conjunction operator and returns its first argument if its value is false or nil; otherwise, and returns its second argument. The disjunction operator or returns its first argument if it is different from 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
..').
If both operands are strings or numbers, then they are converted to
strings according to the rules mentioned in Section 2.4.
Otherwise, the ``concat'' metamethod is called (see Section 3.7).
or
and
< > <= >= ~= ==
..
+ -
* /
not - (unary)
^
All binary operators are left associative,
except for ^ (exponentiation),
which is right associative.
The pre-compiler may rearrange the order of evaluation of
associative operators,
and may exchange the operands of commutative operators,
as long as these optimizations do not change normal results.
However, these optimizations may change some results
if you define non-associative (or non-commutative)
metamethods for those operators.
tableconstructor ::= `{' [fieldlist] `}'
fieldlist ::= field {fieldsep field} [fieldsep]
field ::= `[' exp `]' `=' exp | `Name' `=' exp | exp
fieldsep ::= `,' | `;'
Each field of the form [exp1] = exp2 adds to the new table an entry
with key exp1 and value exp2.
A field of the form name = exp is equivalent to
["name"] = exp.
Finally, fields of the form exp are equivalent to
[i] = exp, where i are consecutive numerical integers,
starting with 1.
Fields in the other formats do not affect this counting.
For example,
a = {[f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45}
is equivalent to
do
local temp = {}
temp[f(1)] = g
temp[1] = "x" -- 1st exp
temp[2] = "y" -- 2nd exp
temp.x = 1 -- temp["x"] = 1
temp[3] = f(x) -- 3rd exp
temp[30] = 23
temp[4] = 45 -- 4th exp
a = temp
end
If the last expression in the list is a function call, then all values returned by the call enter the list consecutively (see Section 3.4.7). If you want to avoid this, enclose the function call in parentheses.
The field list may have an optional trailing separator, as a convenience for machine-generated code.
functioncall ::= prefixexp args
In a function call,
first prefixexp and args are evaluated.
If the value of prefixexp has type function,
then that function is called,
with the given arguments.
Otherwise, its ``call'' metamethod is called,
having as first parameter the value of prefixexp,
followed by the original call arguments
(see Section 3.7).
The form
functioncall ::= prefixexp `:' `name' args
can be used to call ``methods''.
A call v:name(...)
is syntactic sugar for v.name(v, ...),
except that v is evaluated only once.
Arguments have the following syntax:
args ::= `(' [explist1] `)'
args ::= tableconstructor
args ::= Literal
All argument expressions are evaluated before the call.
A call of the form f{...} is syntactic sugar for
f({...}), that is,
the argument list is a single new table.
A call of the form f'...'
(or f"..." or f[[...]]) is syntactic sugar for
f('...'), that is,
the argument list is a single literal string.
Because a function can return any number of results (see Section 3.3.4), the number of results must be adjusted before they are used. If the function is called as a statement (see Section 3.3.6), then its return list is adjusted to 0 elements, thus discarding all returned values. If the function is called inside another expression, or in the middle of a list of expressions, then its return list is adjusted to 1 element, thus discarding all returned values but the first one. If the function is called as the last element of a list of expressions, then no adjustment is made (unless the call is enclosed in parentheses).
Here are some examples:
f() -- adjusted to 0 results
g(f(), x) -- f() is adjusted to 1 result
g(x, f()) -- g gets x plus all values returned by f()
a,b,c = f(), x -- f() is adjusted to 1 result (and c gets nil)
a,b,c = x, f() -- f() is adjusted to 2
a,b,c = f() -- f() is adjusted to 3
return f() -- returns all values returned by f()
return x,y,f() -- returns x, y, and all values returned by f()
{f()} -- creates a list with all values returned by f()
{f(), nil} -- f() is adjusted to 1 result
If you enclose a function call in parentheses, then it is adjusted to return exactly one value:
return x,y,(f()) -- returns x, y, and the first value from f()
{(f())} -- creates a table with exactly one element
As an exception to the format-free syntax of Lua,
you cannot put a line break before the ( in a function call.
That restriction avoids some ambiguities in the language.
If you write
a = f
(g).x(a)
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).
The syntax for function definition is
function ::= function funcbody
funcbody ::= `(' [parlist1] `)' block end
The following syntactic sugar simplifies function definitions:
stat ::= function funcname funcbody
stat ::= local function `name' funcbody
funcname ::= `name' {`.' `name'} [`:' `name']
The statement
function f () ... end
translates to
f = function () ... end
The statement
function t.a.b.c.f () ... end
translates to
t.a.b.c.f = function () ... end
The statement
local function f () ... end
translates to
local f; f = function () ... end
A function definition is an executable expression, whose value has type function. When Lua pre-compiles a chunk, all its function bodies are pre-compiled too. Then, whenever Lua executes the function definition, the function is instantiated (or closed). This function instance (or closure) is the final value of the expression. Different instances of the same function may refer to different non-local variables (see Section 3.5) and may have different tables of globals (see Section 2.2).
Parameters act as local variables, initialized with the argument values:
parlist1 ::= namelist [`,' `...']
parlist1 ::= `...'
When a function is called,
the list of arguments is adjusted to
the length of the list of parameters,
unless the function is a vararg function,
which is
indicated by three dots (`...') at the end of its parameter list.
A vararg function does not adjust its argument list;
instead, it collects all extra arguments into an implicit parameter,
called arg.
The value of arg is a table,
with a field n whose value is the number of extra arguments,
and the extra arguments at positions 1, 2, ..., n.
As an example, consider the following definitions:
function f(a, b) end
function g(a, b, ...) end
function r() return 1,2,3 end
Then, we have the following mapping from arguments to parameters:
CALL PARAMETERS
f(3) a=3, b=nil
f(3, 4) a=3, b=4
f(3, 4, 5) a=3, b=4
f(r(), 10) a=1, b=10
f(r()) a=1, b=2
g(3) a=3, b=nil, arg={n=0}
g(3, 4) a=3, b=4, arg={n=0}
g(3, 4, 5, 8) a=3, b=4, arg={5, 8; n=2}
g(5, r()) a=5, b=1, arg={2, 3; n=2}
Results are returned using the return statement (see Section 3.3.4). If control reaches the end of a function without encountering a return statement, then the function returns with no results.
The colon syntax is used for defining methods, that is, functions that have an implicit extra parameter self. Thus, the statement
function t.a.b.c:f (...) ... end
is syntactic sugar for
t.a.b.c.f = function (self, ...) ... end
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,
so the second x refers to the ``outside'' variable.
Because of those lexical scoping rules, local variables can be freely accessed by functions defined inside their scope. For instance:
local counter = 0
function inc (x)
counter = counter + x
return counter
end
Notice that each execution of a local statement ``creates'' new local variables. Consider the following example:
a = {}
local x = 20
for i=1,10 do
local y = 0
a[i] = function () y=y+1; return x+y end
end
In that code,
each function uses a different y variable,
while all of them share the same x.
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 Section 4.16). Whenever an error occurs during Lua compilation or execution, control returns to C, which can take appropriate measures (such as to print an error message).
Lua code can explicitly generate an error by calling the
function error (see Section 6.1).
If you need to catch errors in Lua,
you can use the pcall function (see Section 6.1).
Every table and userdata value in Lua may have a metatable.
This metatable is a table that defines the behavior of
the original table and userdata under some operations.
You can query and change the metatable of an object with
functions setmetatable and getmetatable (see Section 6.1).
For each of those operations Lua associates a specific key, called an event. When Lua performs one of those operations over a table or a userdata, if checks whether that object has a metatable with the corresponding event. If so, the value associated with that key (the metamethod) controls how Lua will perform the operation.
Metatables control the operations listed next.
Each operation is identified by its corresponding name.
The key for each operation is a string with its name prefixed by
two underscores;
for instance, the key for operation ``add'' is the
string "__add".
The semantics of these operations is better explained by a Lua function
describing how the interpreter executes that operation.
The code shown here in Lua is only illustrative;
the real behavior is hard coded in the interpreter,
and it is much more efficient than this simulation.
All functions used in these descriptions
(rawget, tonumber, etc.)
are described in Section 6.1.
+ operation.
The function getbinhandler below defines how Lua chooses a handler
for a binary operation.
First, Lua tries the first operand.
If its type does not define a handler for the operation,
then Lua tries the second operand.
function getbinhandler (op1, op2, event)
return metatable(op1)[event] or metatable(op2)[event]
end
Using that function,
the behavior of the ``add'' operation is
function add_event (op1, op2)
local o1, o2 = tonumber(op1), tonumber(op2)
if o1 and o2 then -- both operands are numeric
return o1+o2 -- '+' here is the primitive 'add'
else -- at least one of the operands is not numeric
local h = getbinhandler(op1, op2, "__add")
if h then
-- call the handler with both operands
return h(op1, op2)
else -- no handler available: default behavior
error("unexpected type at arithmetic operation")
end
end
end
- operation.
Behavior similar to the ``add'' operation.
* operation.
Behavior similar to the ``add'' operation.
/ operation.
Behavior similar to the ``add'' operation.
^ operation (exponentiation) operation.
??
function pow_event (op1, op2)
local h = getbinhandler(op1, op2, "__pow") ???
if h then
-- call the handler with both operands
return h(op1, op2)
else -- no handler available: default behavior
error("unexpected type at arithmetic operation")
end
end
- 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("unexpected type at arithmetic operation")
end
end
end
< operation.
function lt_event (op1, op2)
if type(op1) == "number" and type(op2) == "number" then
return op1 < op2 -- numeric comparison
elseif type(op1) == "string" and type(op2) == "string" then
return op1 < op2 -- lexicographic comparison
else
local h = getbinhandler(op1, op2, "__lt")
if h then
return h(op1, op2)
else
error("unexpected type at comparison");
end
end
end
a>b is equivalent to b<a.
<= operation.
function lt_event (op1, op2)
if type(op1) == "number" and type(op2) == "number" then
return op1 < op2 -- numeric comparison
elseif type(op1) == "string" and type(op2) == "string" then
return op1 < op2 -- lexicographic comparison
else
local h = getbinhandler(op1, op2, "__le")
if h then
return h(op1, op2)
else
h = getbinhandler(op1, op2, "__lt")
if h then
return not h(op2, op1)
else
error("unexpected type at comparison");
end
end
end
end
a>=b is equivalent to b<=a.
Notice that, in the absence of a ``le'' metamethod,
Lua tries the ``lt'', assuming that a<=b is
equivalent to not (b<a).
.. (concatenation) operation.
function concat_event (op1, op2)
if (type(op1) == "string" or type(op1) == "number") and
(type(op2) == "string" or type(op2) == "number") then
return op1..op2 -- primitive string concatenation
else
local h = getbinhandler(op1, op2, "__concat")
if h then
return h(op1, op2)
else
error("unexpected type for concatenation")
end
end
end
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).__gettable
if h == nil then
error("indexed expression not a table");
end
end
if type(h) == "function" then
return h(table, key) -- call the handler
else return h[key] -- or repeat operation with it
end
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).__settable
if h == nil then
error("indexed expression not a table");
end
end
if type(h) == "function" then
return h(table, key,value) -- call the handler
else h[key] = value -- or repeat operation with it
end
function function_event (func, ...)
if type(func) == "function" then
return func(unpack(arg)) -- regular call
else
local h = metatable(func).__call
if h then
tinsert(arg, 1, func)
return h(unpack(arg))
else
error("call expression not a function")
end
end
end
Metatables may also define finalizer methods for userdata values. For each userdata to be collected, Lua does the equivalent of the following function:
function gc_event (obj)
local h = metatable(obj).__gc
if h then
h(obj)
end
end
In a garbage-collection cycle,
the finalizers for userdata are called in reverse
order of their creation,
that is, the first finalizer to be called is the one associated
with the last userdata created in the program
(among those to be collected in the same cycle).
This section describes the API for Lua, that is,
the set of C functions available to the host program to communicate
with Lua.
All API functions and related types and constants
are declared in the header file lua.h.
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, tag methods, etc.)\
is stored in a dynamically allocated structure of type lua_State;
this state must be passed as the first argument to
every function in the library (except lua_open below).
Before calling any API function, you must create a state by calling
lua_State *lua_open (void);
To release a state created with lua_open, call
void lua_close (lua_State *L);
This function destroys all objects in the given Lua environment
(calling the corresponding garbage-collection metamethods, if any)
and frees all dynamic memory used by that state.
Usually, you do not need to call this function,
because all resources are naturally released when your program ends.
On the other hand,
long-running programs -
like a daemon or a web server -
might need to release states as soon as they are not needed,
to avoid growing too large.
With the exception of lua_open,
all functions in the Lua API need a state as their first argument.
Lua offers a partial support for multiple threads of execution. If you have a C library that offers multi-threading, then Lua can cooperate with it to implement the equivalent facility in Lua. Also, Lua implements its own coroutine system on top of threads. The following function creates a new ``thread'' in Lua:
lua_State *lua_newthread (lua_State *L);
The new state returned by this function shares with the original state
all global environment (such as tables, tag methods, etc.),
but has an independent run-time stack.
(The use of these multiple stacks must be ``syncronized'' with C.
How to explain that? TO BE WRITTEN.)
Each thread has an independent table for global variables. When you create a thread, this table is the same as that of the given state, but you can change each one independently.
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.
Lua uses a virtual stack to pass values to and from C. Each element in this stack represents a Lua value (nil, number, string, etc.).
Each C invocation has its own stack. Whenever Lua calls C, the called function gets a new stack, which is independent of previous stacks or of stacks of still active C functions.
For convenience,
most query operations in the API do not follow a strict stack discipline.
Instead, they can refer to any element in the stack by using an index:
A positive index represents an absolute stack position
(starting at 1);
a negative index represents an offset from the top of the stack.
More specifically, if the stack has n elements,
then index 1 represents the first element
(that is, the element that was pushed onto the stack first),
and
index n represents the last element;
index -1 also represents the last element
(that is, the element at the top),
and index -n represents the first element.
We say that an index is valid
if it lies between 1 and the stack top
(that is, if 1 <= abs(index) <= top).
At any time, you can get the index of the top element by calling
int lua_gettop (lua_State *L);
Because indices start at 1,
the result of lua_gettop is equal to the number of elements in the stack
(and so 0 means an empty stack).
When you interact with Lua API, you are responsible for controlling stack overflow. The function
int lua_checkstack (lua_State *L, int extra);
grows the stack size to top + extra elements;
it returns false if it cannot grow the stack to that size.
This function never shrinks the stack;
if the stack is already bigger than the new size,
it is left unchanged.
Whenever Lua calls C,
it ensures that lua_checkstack(L, LUA_MINSTACK) is true,
that is,
at least LUA_MINSTACK positions are still available.
LUA_MINSTACK is defined in lua.h as 20,
so that usually you do not have to worry about stack space
unless your code has loops pushing elements onto the stack.
Most query functions accept as indices any value inside the
available stack space, that is, indices up to the maximum stack size
you (or Lua) have set through lua_checkstack.
Such indices are called acceptable indices.
More formally, we define an acceptable index
as follows:
(index < 0 && abs(index) <= top) || (index > 0 && index <= top + stackspace)
Note that 0 is never an acceptable index.
Unless otherwise noticed, any function that accepts valid indices can also be called with pseudo-indices, which represent some Lua values that are accessible to the C code but are not in the stack. Pseudo-indices are used to access the table of globals (see Section 4.13), the registry, and the upvalues of a C function (see Section 4.18).
void lua_settop (lua_State *L, int index);
void lua_pushvalue (lua_State *L, int index);
void lua_remove (lua_State *L, int index);
void lua_insert (lua_State *L, int index);
void lua_replace (lua_State *L, int index);
lua_settop accepts any acceptable index,
or 0,
and sets the stack top to that index.
If the new top is larger than the old one,
then the new elements are filled with nil.
If index is 0, then all stack elements are removed.
A useful macro defined in the lua.h is
#define lua_pop(L,n) lua_settop(L, -(n)-1)
which pops n elements from the stack.
lua_pushvalue pushes onto the stack a copy of the element
at the given index.
lua_remove removes the element at the given position,
shifting down the elements above that position to fill the gap.
lua_insert moves the top element into the given position,
shifting up the elements above that position to open space.
lua_replace moves the top element into the given position,
without shifting any element (therefore replacing the value at
the given position).
These functions accept only valid indices.
(Obviously, you cannot call lua_remove or lua_insert with
pseudo-indices, as they do not represent a stack position.)
As an example, if the stack starts as 10 20 30 40 50*
(from bottom to top; the * marks the top),
then
lua_pushvalue(L, 3) --> 10 20 30 40 50 30*
lua_pushvalue(L, -1) --> 10 20 30 40 50 30 30*
lua_remove(L, -3) --> 10 20 30 40 30 30*
lua_remove(L, 6) --> 10 20 30 40 30*
lua_insert(L, 1) --> 30 10 20 30 40*
lua_insert(L, -1) --> 30 10 20 30 40* (no effect)
lua_replace(L, 2) --> 30 40 20 30*
lua_settop(L, -3) --> 30 40 20*
lua_settop(L, 6) --> 30 40 20 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_isdataval (lua_State *L, int index);
These functions can be called with any acceptable index.
lua_type returns the type of a value in the stack,
or LUA_TNONE for a non-valid index
(that is, if that stack position is ``empty'').
The types are coded by the following constants
defined in lua.h:
LUA_TNIL,
LUA_TNUMBER,
LUA_TBOOLEAN,
LUA_TSTRING,
LUA_TTABLE,
LUA_TFUNCTION,
LUA_TUSERDATA,
LUA_TLIGHTUSERDATA.
The following function translates such constants to a type name:
const char *lua_typename (lua_State *L, int type);
The lua_is* functions return 1 if the object is compatible
with the given type, and 0 otherwise.
lua_isboolean is an exception to this rule,
and it succeeds only for boolean values
(otherwise it would be useless,
as any value is compatible with a boolean).
They always return 0 for a non-valid index.
lua_isnumber accepts numbers and numerical strings,
lua_isstring accepts strings and numbers (see Section 2.4),
and lua_isfunction accepts both Lua functions and C functions.
To distinguish between Lua functions and C functions,
you should use lua_iscfunction.
To distinguish between numbers and numerical strings,
you can use lua_type.
The API also has functions to compare two values in the stack:
int lua_equal (lua_State *L, int index1, int index2);
int lua_lessthan (lua_State *L, int index1, int index2);
These functions are equivalent to their counterparts in Lua (see Section 3.4.2).
Both functions return 0 if any of the indices are non-valid.
To translate a value in the stack to a specific C type, you can use the following conversion functions:
int lua_toboolean (lua_State *L, int index);
lua_Number lua_tonumber (lua_State *L, int index);
const char *lua_tostring (lua_State *L, int index);
size_t lua_strlen (lua_State *L, int index);
lua_CFunction lua_tocfunction (lua_State *L, int index);
void *lua_touserdata (lua_State *L, int index);
These functions can be called with any acceptable index.
When called with a non-valid index,
they act as if the given value had an incorrect type.
lua_toboolean converts the Lua value at the given index
to a C ``boolean'' value (that is, 0 or 1).
Like all tests in Lua, it returns 1 for any Lua value different from
false and nil;
otherwise it returns 0.
It also returns 0 when called with a non-valid index.
(If you want to accept only real boolean values,
use lua_isboolean to test the type of the value.)
lua_tonumber converts the Lua value at the given index
to a number (by default, lua_Number is double).
The Lua value must be a number or a string convertible to number
(see Section 2.4); otherwise, lua_tonumber returns 0.
lua_tostring converts the Lua value at the given index to a string
(const char*).
The Lua value must be a string or a number;
otherwise, the function returns NULL.
If the value is a number,
then lua_tostring also
changes the actual value in the stack to a string.
(This change confuses lua_next
when lua_tostring is applied to keys.)
lua_tostring returns a fully aligned pointer
to a string inside the Lua environment.
This string always has a zero ('\0')
after its last character (as in C),
but may contain other zeros in its body.
If you do not know whether a string may contain zeros,
you can use lua_strlen to get its actual length.
Because Lua has garbage collection,
there is no guarantee that the pointer returned by lua_tostring
will be valid after the corresponding value is removed from the stack.
So, if you need the string after the current function returns,
then you should duplicate it (or put it into the registry (see Section 4.18)).
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 Section 4.17.
lua_touserdata is explained in Section 4.9.
The API has the following functions to push C values onto the stack:
void lua_pushboolean (lua_State *L, int b);
void lua_pushnumber (lua_State *L, lua_Number n);
void lua_pushlstring (lua_State *L, const char *s, size_t len);
void lua_pushstring (lua_State *L, const char *s);
void lua_pushnil (lua_State *L);
void lua_pushcfunction (lua_State *L, lua_CFunction f);
void lua_pushlightuserdata (lua_State *L, void *p);
These functions receive a C value,
convert it to a corresponding Lua value,
and push the result onto the stack.
In particular, lua_pushlstring and lua_pushstring
make an internal copy of the given string.
lua_pushstring can only be used to push proper C strings
(that is, strings that end with a zero and do not contain embedded zeros);
otherwise, you should use the more general lua_pushlstring,
which accepts an explicit size.
You can also push ``formatted'' strings:
const char *lua_pushfstring (lua_State *L, const char *fmt, ...);
const char *lua_pushvfstring (lua_State *L, const char *fmt,
va_list argp);
Both functions push onto the stack a formatted string,
and return a pointer to that string.
These functions are similar to sprintf and vsprintf,
but with some important differences:
%% (inserts a % in the string),
%s (inserts a zero-terminated string, with no size restrictions),
%f (inserts a lua_Number),
%d (inserts an int),
%c (inserts an int as a character).
Lua uses two numbers to control its garbage collection: the count and the threshold (see Section 2.6). The first counts the ammount of memory in use by Lua; when the count reaches the threshold, Lua runs its garbage collector. After the collection, the count is updated, and the threshold is set to twice the count value.
You can access the current values of these two numbers through the following functions:
int lua_getgccount (lua_State *L);
int lua_getgcthreshold (lua_State *L);
Both return their respective values in Kbytes.
You can change the threshold value with
void lua_setgcthreshold (lua_State *L, int newthreshold);
Again, the newthreshold value is given in Kbytes.
When you call this function,
Lua sets the new threshold and checks it against the byte counter.
If the new threshold is smaller than the byte counter,
then Lua immediately runs the garbage collector.
In particular
lua_setgcthreshold(L,0) forces a garbage collectiion.
After the collection,
a new threshold is set according to the previous rule.
Userdata represents C values in Lua. Lua supports two types of userdata: full userdata and light userdata.
A full userdata represents a block of memory. It is an object (like a table): You must create it, it can have its own metatable, you can detect when it is being collected. A full userdata is only equal to itself.
A light userdata represents a pointer. It is a value (like a number): You do not create it, it has no metatables, it is not collected (as it was never created). A light userdata is equal to ``any'' light userdata with the same address.
In Lua code, there is no way to test whether a userdata is full or light;
both have type userdata.
In C code, lua_type returns LUA_TUSERDATA for full userdata,
and LUA_LIGHTUSERDATA for light userdata.
You can create new full userdata with the following function:
void *lua_newuserdata (lua_State *L, size_t size);
It allocates a new block of memory with the given size,
pushes on the stack a new userdata with the block address,
and returns this address.
To push a light userdata into the stack you use
lua_pushlightuserdata (see Section 4.7).
lua_touserdata (see Section 4.6) retrieves the value of a userdata.
When applied on a full userdata, it returns the address of its block;
when applied on a light userdata, it returns its pointer;
when applied on a non-userdata value, it returns NULL.
When Lua collects a full userdata,
it calls its gc metamethod, if any,
and then it frees its corresponding memory.
The following functions allow you do manipulate the metatables of an object:
int lua_getmetatable (lua_State *L, int objindex);
int lua_setmetatable (lua_State *L, int objindex);
Both get at objindex a valid index for an object.
lua_getmetatable pushes on the stack the metatable of that object;
lua_setmetatable sets the table on the top of the stack as the
new metatable for that object (and pops the table).
If the object does not have a metatable,
lua_getmetatable returns 0, and pushes nothing on the stack.
lua_setmetatable returns 0 when it cannot
set the metatable of the given object
(that is, when the object is not a userdata nor a table);
even then it pops the table from the stack.
You can load a Lua chunk with
typedef const char * (*lua_Chunkreader)
(lua_State *L, void *data, size_t *size);
int lua_load (lua_State *L, lua_Chunkreader reader, void *data,
const char *chunkname);
lua_load uses the reader to read the chunk.
Everytime it needs another piece of the chunk,
it calls the reader,
passing along its data parameter.
The reader must return a pointer to a block of memory
with a new part of the chunk,
and set size to the block size.
To signal the end of the chunk, the reader must return NULL.
The reader function may return pieces of any size greater than zero.
In the current implementation,
the reader function cannot call any Lua function;
to ensure that, it always receives NULL as the Lua state.
lua_load automatically detects whether the chunk is text or binary,
and loads it accordingly (see program luac).
The return values of lua_load are:
The chunkname is used for error messages and debug information (see Section 5).
See the auxiliar library (lauxlib)
for examples of how to use lua_load,
and for some ready-to-use functions to load chunks
from files and from strings.
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;
this is convenient for getting multiple values from a table.
As in Lua, this function may trigger a metamethod for the ``gettable'' or ``index'' events (see Section 3.7). To get the real value of any table key, without invoking any metamethod, use the raw version:
void lua_rawget (lua_State *L, int index);
To store a value into a table that resides somewhere in the stack, you push the key and the value onto the stack (in this order), and then call
void lua_settable (lua_State *L, int index);
where index points to the table.
lua_settable pops from the stack both the key and the value.
The table is left where it was in the stack;
this is convenient for setting multiple values in a table.
As in Lua, this operation may trigger a metamethod for the ``settable'' or ``newindex'' events. To set the real value of any table index, without invoking any metamethod, use the raw version:
void lua_rawset (lua_State *L, int index);
You can traverse a table with the function
int lua_next (lua_State *L, int index);
where index points to the table to be traversed.
The function pops a key from the stack,
and pushes a key-value pair from the table
(the ``next'' pair after the given key).
If there are no more elements, then lua_next returns 0
(and pushes nothing).
Use a nil key to signal the start of a traversal.
A typical traversal looks like this:
/* table is in the stack at index `t' */
lua_pushnil(L); /* first key */
while (lua_next(L, t) != 0) {
/* `key' is at index -2 and `value' at index -1 */
printf("%s - %s\n",
lua_typename(L, lua_type(L, -2)), lua_typename(L, lua_type(L, -1)));
lua_pop(L, 1); /* removes `value'; keeps `key' for next iteration */
}
NOTE:
While traversing a table,
do not call lua_tostring on a key,
unless you know the key is actually a string.
Recall that lua_tostring changes the value at the given index;
this confuses the next call to lua_next.
All global variables are kept in an ordinary Lua table. This table is always at pseudo-index LUA_GLOBALSINDEX.
To access and change the value of global variables, you can use regular table operations over the global table. For instance, to access the value of a global variable, do
lua_pushstring(L, varname);
lua_gettable(L, LUA_GLOBALSINDEX);
You can change the global table of a Lua thread using lua_replace.
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 the Lua code:
a = f("how", t.x, 14)
Here it is in C:
lua_pushstring(L, "t");
lua_gettable(L, LUA_GLOBALSINDEX); /* global `t' (for later use) */
lua_pushstring(L, "a"); /* var name */
lua_pushstring(L, "f"); /* function name */
lua_gettable(L, LUA_GLOBALSINDEX); /* function to be called */
lua_pushstring(L, "how"); /* 1st argument */
lua_pushstring(L, "x"); /* push the string "x" */
lua_gettable(L, -5); /* push result of t.x (2nd arg) */
lua_pushnumber(L, 14); /* 3rd argument */
lua_call(L, 3, 1); /* call function with 3 arguments and 1 result */
lua_settable(L, LUA_GLOBALSINDEX); /* set global variable `a' */
lua_pop(L, 1); /* remove `t' from the stack */
Notice that the code above is ``balanced'':
at its end, the stack is back to its original configuration.
This is considered good programming practice.
(We did this example using only the raw functions provided by Lua's API, to show all the details. Usually programmers use several macros and auxiliar functions that provide higher level access to Lua.)
When you call a function with lua_call,
any error inside the called function is propagated upwards
(with a longjmp).
If you need to handle errors,
then you should use lua_pcall:
int lua_pcall (lua_State *L, int nargs, int nresults, int errfunc);
Both nargs and nresults have the same meaning as
in lua_call.
If there are no errors during the call,
lua_pcall behaves exactly like lua_call.
Like lua_call,
lua_pcall always removes the function
and its arguments from the stack.
However, if there is any error,
lua_pcall catches it,
pushes a single value at the stack (the error message),
and returns an error code.
If errfunc is 0,
then the error message returned is exactly the original error message.
Otherwise, errfunc gives the stack index for an
error handler function.
(In the current implementation, that index cannot be a pseudo-index.)
In case of runtime errors,
that function will be called with the error message,
and its return value will be the message returned by lua_pcall.
Typically, the error handler function is used to add more debug
information to the error message, such as a stack traceback.
Such information cannot be gathered after the return of lua_pcall,
since by then the stack has unwound.
The lua_pcall function returns 0 in case of success,
or one of the following error codes
(defined in lua.h):
Some special Lua functions have their own C interfaces. The host program can generate a Lua error calling the function
void lua_error (lua_State *L, const char *message);
This function never returns.
If lua_error is called from a C function that has been called from Lua,
then the corresponding Lua execution terminates,
as if an error had occurred inside Lua code.
Otherwise, the whole host program terminates with a call to
exit(EXIT_FAILURE).
Before terminating execution,
the message is passed to the error handler function,
_ERRORMESSAGE (see Section 3.6).
If message is NULL,
then _ERRORMESSAGE is not called.
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 Section 3.4.4).
lua_CFunction,
which is defined as
typedef int (*lua_CFunction) (lua_State *L);
A C function receives a Lua environment and returns an integer,
the number of values it has returned to Lua.
In order to communicate properly with Lua, a C function must follow the following protocol, which defines the way parameters and results are passed: A C function receives its arguments from Lua in the stack, in direct order (the first argument is pushed first). To return values to Lua, a C function just pushes them onto the stack, in direct order (the first result is pushed first), and returns the number of results. Like a Lua function, a C function called by Lua can also return many results.
As an example, the following function receives a variable number of numerical arguments and returns their average and sum:
static int foo (lua_State *L) {
int n = lua_gettop(L); /* number of arguments */
lua_Number sum = 0;
int i;
for (i = 1; i <= n; i++) {
if (!lua_isnumber(L, i)) {
lua_pushstring(L, "incorrect argument to function `average'");
lua_error(L);
}
sum += lua_tonumber(L, i);
}
lua_pushnumber(L, sum/n); /* first result */
lua_pushnumber(L, sum); /* second result */
return 2; /* number of results */
}
To register a C function to Lua, there is the following convenience macro:
#define lua_register(L,n,f) \
(lua_pushstring(L, n), \
lua_pushcfunction(L, f), \
lua_settable(L, LUA_GLOBALSINDEX))
/* const char *n; */
/* lua_CFunction f; */
which receives the name the function will have in Lua,
and a pointer to the function.
Thus,
the C function `foo' above may be registered in Lua as `average'
by calling
lua_register(L, "average", foo);
When a C function is created, it is possible to associate some values to it, thus creating a C closure; these values are then accessible to the function whenever it is called. To associate values to a C function, first these values should be pushed onto the stack (when there are multiple values, the first value is pushed first). Then the function
void lua_pushcclosure (lua_State *L, lua_CFunction fn, int n);
is used to push the C function onto the stack,
with the argument n telling how many values should be
associated with the function
(lua_pushcclosure also pops these values from the stack);
in fact, the macro lua_pushcfunction is defined as
lua_pushcclosure with n set to 0.
Then, whenever the C function is called,
those values are located at specific pseudo-indices.
Those pseudo-indices are produced by a macro lua_upvalueindex.
The first value associated with a function is at position
lua_upvalueindex(1), and so on.
For examples of C functions and closures, see files
lbaselib.c, liolib.c, lmathlib.c, and lstrlib.c
in the official Lua distribution.
Lua provides a pre-defined table that can be used by any C code to store whatever Lua value it needs to store, especially if the C code needs to keep that Lua value outside the life span of a C function. This table is always located at pseudo-index LUA_REGISTRYINDEX. Any C library can store data into this table, as long as it chooses keys different from other libraries. Typically, you should use as key a string containing your library name, or a light userdata with the address of a C object in your code.
The integer keys in the registry are used by the reference mechanism, implemented by the auxiliar library, and therefore should not be used by other purposes.
Lua has no built-in debugging facilities. Instead, it offers a special interface, by means of functions and hooks, which allows the construction of different kinds of debuggers, profilers, and other tools that need ``inside information'' from the interpreter.
The main function to get information about the interpreter stack is
int lua_getstack (lua_State *L, int level, lua_Debug *ar);
This function fills parts of a lua_Debug structure with
an identification of the activation record
of the function executing at a given level.
Level 0 is the current running function,
whereas level n+1 is the function that has called level n.
Usually, lua_getstack returns 1;
when called with a level greater than the stack depth,
it returns 0.
The structure lua_Debug is used to carry different pieces of
information about an active function:
typedef struct lua_Debug {
lua_Hookevent 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 future use.
To fill the other fields of lua_Debug with useful information,
call
int lua_getinfo (lua_State *L, const char *what, lua_Debug *ar);
This function returns 0 on error
(for instance, an invalid option in what).
Each character in the string what
selects some fields of ar to be filled,
as indicated by the letter in parentheses in the definition of lua_Debug
above:
`S' fills in the fields source, linedefined,
and what;
`l' fills in the field currentline, etc.
Moreover, `f' pushes onto the stack the function that is
running at the given level.
To get information about a function that is not active (that is,
it is not in the stack),
you push the function onto the stack,
and start the what string with the character `>'.
For instance, to know in which line a function f was defined,
you can write
lua_Debug ar;
lua_pushstring(L, "f");
lua_gettable(L, LUA_GLOBALSINDEX); /* get global `f' */
lua_getinfo(L, ">S", &ar);
printf("%d\n", ar.linedefined);
The fields of lua_Debug have the following meaning:
source is that string;
if the function was defined in a file,
then source starts with a @ followed by the file name.
source, to be used in error messages.
"Lua" if this is a Lua function,
"C" if this is a C function,
or "main" if this is the main part of a chunk.
currentline is set to -1.
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.
"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.)
For the manipulation of local variables,
luadebug.h uses indices:
The first parameter or local variable has index 1, and so on,
until the last active local variable.
The following functions allow the manipulation of the local variables of a given activation record:
const char *lua_getlocal (lua_State *L, const lua_Debug *ar, int n);
const char *lua_setlocal (lua_State *L, const lua_Debug *ar, int n);
The parameter ar must be a valid activation record,
filled by a previous call to lua_getstack or
given as argument to a hook (see Section 5.3).
lua_getlocal gets the index n of a local variable,
pushes its value onto the stack,
and returns its name.
lua_setlocal assigns the value at the top of the stack
to the variable and returns its name.
Both functions return NULL on failure,
that is
when the index is greater than
the number of active local variables.
As an example, the following function lists the names of all local variables for a function at a given level of the stack:
int listvars (lua_State *L, int level) {
lua_Debug ar;
int i = 1;
const char *name;
if (lua_getstack(L, level, &ar) == 0)
return 0; /* failure: no such level in the stack */
while ((name = lua_getlocal(L, &ar, i++)) != NULL) {
printf("%s\n", name);
lua_pop(L, 1); /* remove variable value */
}
return 1;
}
The Lua interpreter offers a mechanism of hooks: user-defined C functions that are called during the program execution. A hook may be called in four different events: a call event, when Lua calls a function; a return event, when Lua returns from a function; a line event, when Lua starts executing a new line of code; and a count event, that happens every ``count'' instructions. Lua identifies them with the following enumeration:
typedef enum lua_Hookevent {
LUA_HOOKCALL, LUA_HOOKRET, LUA_HOOKLINE, LUA_HOOKCOUNT
} lua_Hookevent;
A hook has type lua_Hook, defined as follows:
typedef void (*lua_Hook) (lua_State *L, lua_Debug *ar);
You can set the hook with the following function:
int lua_sethook (lua_State *L, lua_Hook func, unsigned long mask);
func is the hook,
and mask specifies at which events it will be called.
It is formed by a disjunction of the constants
LUA_MASKCALL,
LUA_MASKRET,
LUA_MASKLINE,
plus the macro LUA_MASKCOUNT(count).
A hook is disabled with the mask zero.
You can get the current hook and the current mask with the next functions:
lua_Hook lua_gethook (lua_State *L);
unsigned long lua_gethookmask (lua_State *L);
You can get the count inside a mask with the macro lua_getmaskcount.
Whenever a hook is called, its ar argument has its field
event set to the specific event that triggered the hook.
Moreover, for line events, the field currentline is also set.
For the value of any other field, the hook must call lua_getinfo.
While Lua is running a hook, it disables other calls to hooks. Therefore, if a hook calls Lua to execute a function or a chunk, this execution ocurrs without any calls to hooks.
The standard libraries provide useful functions
that are implemented directly through the standard C API.
Some of these functions provide essential services to the language
(e.g. type and getmetatable);
others provide access to ``outside'' servides (e.g. I/O);
and others could be implemented in Lua itself,
but are quite useful or have critical performance to
deserve an implementation in C (e.g. sort).
All libraries are implemented through the official C API, and are provided as separate C modules. Currently, Lua has the following standard libraries:
To have access to these libraries,
the C host program must call the functions
lua_baselibopen,
lua_strlibopen,
lua_tablibopen,
lua_mathlibopen,
and lua_iolibopen, which 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 facilities.
The basic library also defines a global variable _VERSION with a string containing the current interpreter version. The current content of this string is "Lua 5.0 (alpha)".
v is nil;
otherwise, returns this argument.
This function is equivalent to the following Lua function:
function assert (v, m)
if not v then
error(m or "assertion failed!")
end
return v
end
Sets the garbage-collection threshold for the given limit
(in Kbytes), and checks it against the byte counter.
If the new threshold is smaller than the byte counter,
then Lua immediately runs the garbage collector (see Section 2.6).
If limit is absent, it defaults to zero
(thus forcing a garbage-collection cycle).
dofile executes the contents of the standard input (stdin).
Returns any value returned by the chunk.
message as the error message.
Function error never returns.
function can be a Lua function or a number,
meaning the function at that stack level:
Level 1 is the function calling getglobals.
If the given function is not a Lua function,
returns the ``global'' table of globals.
The default for function is 1.
Returns the metatable of the given object. If the object does not have a metatable, returns nil.
Returns the weak mode of a table, as a string.
Valid values for this string are "" for regular (non-weak) tables,
"k" for weak keys, "v" for weak values,
and "kv" for both.
Returns the table t and a generator function
so that the construction
for i,v in ipairs(t) do ... end
will iterate over the pairs 1, t[1], 2, t[2], ...,
up to the first nil value of the table.
The optional parameter chunkname
is the ``name of the chunk'',
used in error messages and debug information.
To load and run a given string, use the idiom
assert(loadstring(s))()
next returns the next index of the table and the
value associated with the index.
When called with nil as its second argument,
next returns the first index
of the table and its associated value.
When called with the last index,
or with nil in an empty table,
next returns nil.
If the second argument is absent, then it is interpreted as nil.
Lua has no declaration of fields;
semantically, there is no difference between a
field not present in a table or a field with value nil.
Therefore, next only considers fields with non-nil values.
The order in which the indices are enumerated is not specified,
even for numeric indices
(to traverse a table in numeric order,
use a numerical for or the function ipairs).
The behavior of next is undefined if you change
the table during the traversal.
Returns the table t and the function next,
so that the construction
for k,v in pairs(t) do ... end
will iterate over all pairs of key--value of table t.
func with
the given arguments in protected mode.
Its first result is a boolean, true if the call succeeds without errors.
In such case, pcall also returns all results from the call,
after this first result.
In case of any error, pcall returns false plus the error message.
stdout,
using the strings returned by tostring.
This function is not intended for formatted output,
but only as a quick way to show a value,
typically for debugging.
For formatted output, see format (see Section 6.2).
table[index],
without invoking any tag method.
table must be a table;
index is any value different from nil.
table[index] to value,
without invoking any tag method.
table must be a table;
index is any value different from nil;
and value is any Lua value.
Loads the given package.
The function starts by looking into the table _LOADED
whether packagename is already loaded.
If it is, then require is done.
Otherwise, it searches a path looking for a file to load.
If the global variable LUA_PATH is a string,
this string is the path.
Otherwise, require tries the environment variable LUA_PATH.
In the last resort, it uses a predefined path.
The path is a sequence of templates separated by semicolons.
For each template, require will change an eventual interrogation
mark in the template to packagename,
and then will try to load the resulting file name.
So, for instance, if the path is
"./?.lua;./?.lc;/usr/local/?/init.lua;/lasttry"a
require "mod" will try to load the files
./mod.lua,
./mod.lc,
/usr/local/mod/init.lua,
and /lasttry, in that order.
The function stops the search as soon as it can load a file,
and then it runs the file.
If there is any error loading or running the file,
or if it cannot find any file in the path,
then require signals an error.
Otherwise, it marks in table _LOADED
that the package is loaded, and returns.
While running a packaged file,
require defines the global variable _REQUIREDNAME
with the package name.
function can be a Lua function or a number,
meaning the function at that stack level:
Level 1 is the function calling setglobals.
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.
Set the weak mode of a table.
The new mode is described by the mode string.
Valid values for this string are "" for regular (non-weak) tables,
"k" for weak keys, "v" for weak values,
and "kv" for both.
This function returns its first argument (table).
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 Section 2.4). In other bases, only unsigned integers are accepted.
format (see Section 6.2).
"nil" (a string, not the value nil),
"number",
"string",
"table",
"function",
and "userdata".
return list[1], list[2], ..., list[n]except that the above code can be valid only for a fixed n. The number n of returned values is either the value of
list.n, if it is a number,
or one less the index of the first absent (nil) value.
The string library provides all its functions inside the table string.
s.
If i is absent, then it is assumed to be 1.
i may be negative.
Numerical codes are not necessarily portable across platforms.
Numerical codes are not necessarily portable across platforms.
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.
"" has length 0.
Embedded zeros are counted,
and so "a\000b\000c" has length 5.
n copies of
the string s.
s,
starting at i and running until j;
i and j may be negative.
If j is absent, then it is assumed to be equal to -1
(which is the same as the string length).
In particular,
the call string.sub(s,1,j) returns a prefix of s
with length j,
and the call string.sub(s, -i) returns a suffix of s
with length i.
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, returns, and backslashes in the string
are correctly escaped when written.
For instance, the call
string.format('%q', 'a string with "quotes" and \n new line')
will produce the string:
"a string with \"quotes\" and \ new line"
The options c, d, E, e, f,
g, G, i, o, u, X, and x all
expect a number as argument,
whereas q and s expect a string.
The * modifier can be simulated by building
the appropriate format string.
For example, "%*g" can be simulated with
"%"..width.."g".
String values to be formatted with
%s cannot contain embedded zeros.
s
in which all occurrences of the pattern pat have been
replaced by a replacement string specified by repl.
gsub also returns, as a second value,
the total number of substitutions made.
If repl is a string, then its value is used for replacement.
Any sequence in repl of the form %n,
with n between 1 and 9,
stands for the value of the n-th captured substring.
If repl is a function, then this function is called every time a
match occurs, with all captured substrings passed as arguments,
in order (see below);
if the pattern specifies no captures,
then the whole match is passed as a sole argument.
If the value returned by this function is a string,
then it is used as the replacement string;
otherwise, the replacement string is the empty string.
The last, optional parameter n limits
the maximum number of substitutions to occur.
For instance, when n is 1 only the first occurrence of
pat is replaced.
Here are some examples:
x = gsub("hello world", "(%w+)", "%1 %1")
--> x="hello hello world world"
x = gsub("hello world", "(%w+)", "%1 %1", 1)
--> x="hello hello world"
x = gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1")
--> x="world hello Lua from"
x = gsub("home = $HOME, user = $USER", "%$(%w+)", getenv)
--> x="home = /home/roberto, user = roberto" (for instance)
x = gsub("4+5 = $return 4+5$", "%$(.-)%$", dostring)
--> x="4+5 = 9"
local t = {name="Lua", version="4.1"}
x = gsub("$name - $version", "%$(%w+)", function (v) return t[v] end)
--> x="Lua - 4.1"
^$()%.[]*+-?)
- represents the character x itself.
%
when used to represent itself in a pattern.
-.
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.
%a, %c, ...),
the corresponding uppercase letter represents the complement of the class.
For instance, %S represents all non-space characters.
The definitions of letter, space, etc. depend on the current locale.
In particular, the class [a-z] may not be equivalent to %l.
The second form should be preferred for portability.
*,
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;
%b() matches expressions with
balanced parentheses.
^ 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*(.)%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.
A pattern cannot contain embedded zeros. Use %z instead.
Most functions in the table library library assume that the table represents an array or a list. For those functions, an important concept is the size of the array. There are three ways to specify that size:
"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.
func over all elements of table.
For each element, func is called with the index and
respective value as arguments.
If func returns a non-nil value,
then the loop is broken, and this value is returned
as the final value of foreach.
The behavior of foreach is undefined if you change
the table t during the traversal.
func over the
numerical indices of table.
For each index, func is called with the index and
respective value as arguments.
Indices are visited in sequential order,
from 1 to n,
where n is the size of the table (see Section 6.3).
If func returns a non-nil value,
then the loop is broken, and this value is returned
as the final value of foreachi.
n field with a numeric value,
this value is the ``size'' of the table.
Otherwise, if there was a previous call to
table.getn or to table.setn over this table,
the respective value is returned.
Otherwise, the ``size'' is one less the first integer index with
a nil value.
Notice that the last option happens only once for a table.
If you call table.getn again over the same table,
it will return the same previous result,
even if the table has been modified.
The only way to change the value of table.getn is by calling
table.setn or assigning to field "n" in the table.
table[1] to table[n],
where n is the size of the table (see Section 6.3).
If comp is given,
then it must be a function that receives two table elements,
and returns true
when the first is less than the second
(so that not comp(a[i+1],a[i]) will be true after the sort).
If comp is not given,
then the standard Lua operator < is used instead.
The sort algorithm is not stable (that is, elements considered equal by the given order may have their relative positions changed by the sort).
Inserts element value at position pos in table,
shifting other elements up to open space, if necessary.
The default value for pos is n+1,
where n is the size of the table (see Section 6.3),
so that a call table.insert(t,x) inserts x at the end
of table t.
This function also updates the size of the table,
calling table.setn(table, n+1).
Removes from table the element at position pos,
shifting other elements down to close the space, if necessary.
Returns the value of the removed element.
The default value for pos is n,
where n is the size of the table (see Section 6.3),
so that a call tremove(t) removes the last element
of table t.
This function also updates the size of the table,
calling table.setn(table, n-1).
Updates the ``size'' of a table.
If the table has a field "n" with a numerical value,
that value is changed to the given n.
Otherwise, it updates an internal state of the table library
so that subsequent calls to table.getn(table) return n.
This library is an interface to most functions of the standard C math library.
(Some have slightly different names.)
It provides all its functions inside the table .
In addition,
it registers a ??tag method for the binary exponentiation operator ^
that returns xy when applied to numbers x^y.
The library provides the following functions:
math.abs math.acos math.asin math.atan math.atan2
math.ceil math.cos math.deg math.exp math.floor
math.log math.log10 math.max math.min math.mod
math.rad math.sin math.sqrt math.tan math.frexp
math.ldexp math.random math.randomseed
plus a variable math.pi.
Most of them
are only interfaces to the homonymous functions in the C library.
The functions math.deg and math.rad convert
between radians and degrees.
The function math.max returns the maximum
value of its numeric arguments.
Similarly, math.min computes the minimum.
Both can be used with 1, 2, or more arguments.
The functions math.random and math.randomseed
are interfaces to the simple random generator functions
rand and srand, provided by ANSI C.
(No guarantees can be given for their statistical properties.)
When called without arguments,
math.random returns a pseudo-random real number
in the range [0,1).
When called with a number n,
math.random returns a pseudo-random integer in the range [1,n].
When called with two arguments, l and u,
math.random returns a pseudo-random integer in the range [l,u].
The I/O library provides two different styles for file manipulation. The first one uses implicit file descriptors; that is, there are operations to set a default input file and a default output file, and all input/output operations are over those default files. The second style uses explicit file descriptors.
When using implicit file descriptors, all operations are supplied by table io. When using explicit file descriptors, the operation io.open returns a file descriptor, and then all operations are supplied as methods by the file descriptor.
Moreover, the table io also provides
three predefined file descriptors:
io.stdin, io.stdout, and io.stderr,
with their usual meaning from C.
A file handle is a userdata containing the file stream (FILE*),
with a distinctive metatable created by the I/O library.
Unless otherwise stated, all I/O functions return nil on failure (plus an error message as a second result) and some value different from nil on success.
Equivalent to handle:close().
Without a handle, closes the default output file.
Equivalent to fh:flush over the default output file.
When called with a file name, it opens the named file (in text mode), and sets its handle as the default input file (and returns nothing). When called with a file handle, it simply sets that file handle as the default input file. When called without parameters, it returns the current default input file.
In case of errors this function raises the error, instead of returning an error code.
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.
Similar to io.input, but operates over the default output file.
Equivalent to fh:read over the default input file.
Returns a handle for a temporary file. This file is open in read/write mode, and it is automatically removed when the program ends.
Equivalent to fh:write over the default output file.
Closes the file fh.
Saves any written data to the file fh.
Reads the file fh,
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
Sets and gets the file position,
measured in bytes from the beginning of the file,
to the position given by offset plus a base
specified by the string whence, as follows:
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.
Writes the value of each of its arguments to
the filehandle fh.
The arguments must be strings or numbers.
To write other values,
use tostring or format before write.
If this function fails, it returns nil,
plus a string describing the error.
This library is implemented through table os.
Returns an approximation of the amount of CPU time used by the program, in seconds.
Returns a string or a table containing date and time,
formatted according to the given string format.
If the time argument is present,
this is the time to be formatted
(see the time function for a description of this value).
Otherwise, date formats the current time.
If format starts with !,
then the date is formatted in Coordinated Universal Time.
After that optional character,
if format is *t,
then date returns a table with the following fields:
year (four digits), month (1--12), day (1--31),
hour (0--23), min (0--59), sec (0--61),
wday (weekday, Sunday is 1),
yday (day of the year),
and isdst (daylight saving flag, a boolean).
If format is not *t,
then date returns the date as a string,
formatted according with the same rules as the C function strftime.
When called without arguments,
date returns a reasonable date and time representation that depends on
the host system and on the current locale
(that is, os.date() is equivalent to os.date("%c")).
Returns the number of seconds from time t1 to time t2.
In Posix, Windows, and some other systems,
this value is exactly t1-t2.
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.
Calls the C function exit,
with an optional code,
to terminate the host program.
The default value for code is the success code.
Returns the value of the process environment variable varname,
or nil if the variable is not defined.
Deletes the file with the given name. If this function fails, it returns nil, plus a string describing the error.
Renames file named name1 to name2.
If this function fails, it returns nil,
plus a string describing the error.
This function is an interface to the C function setlocale.
locale is a string specifying a locale;
category is an optional string describing which category to change:
"all", "collate", "ctype",
"monetary", "numeric", or "time";
the default category is "all".
The function returns the name of the new locale,
or nil if the request cannot be honored.
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 bt time can be used only as an argument to
date and difftime.
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 the function
and the time you open the file,
it is possible for another process
to create a file with the same name.
The library ldblib provides
the functionality of the debug interface to Lua programs.
You should exert great 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.
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 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 what to get.
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 getinfo(1,"n").name returns
the name of the current function, if a reasonable name can be found,
and getinfo(print) returns a table with all available information
about the print function.
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 getinfo to check whether the level is valid.)
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.)
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 characteres,
with the given meaning:
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", "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).
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).
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] [prog [args]]
The options are:
stdin as a file;
lua runs the given prog,
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 intepreter 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)' prog.lua
will first set a to 1, then print a,
and finally run the file prog.lua.
(Here, $ is the shell prompt. Your prompt may be different.)
Before starting to run the program,
lua collects all arguments in the command line
in a global table called arg.
The program name is stored in index 0,
the first argument after the program goes to index 1,
and so on.
The field n gets the number of arguments after the program name.
Any arguments before the program name
(that is, the interpreter name plus the options)
go to negative indices.
For instance, in the call
$ lua -la.lua b.lua t1 t2
the 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 first pair of quotes is for the shell,
the second 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 a more portable solution is #!/usr/bin/env lua.)
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, Bret Mogilefsky, Cameron Laird, Carlos Cassino, Carlos Henrique Levy, Claudio Terra, David Jeske, 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 Lhost, Renata Ratton, Renato Borges, Renato Cerqueira, Reuben Thomas, Stephan Herrmann, Steve Dekorte, Thatcher Ulrich, Tomás Gorham, Vincent Penquerc'h, Thank you!
{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.
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 do not handle errors, but simply propagate them.
read option *w is obsolete.
format option %n$ is obsolete.
The notation used here is 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.
chunk ::= {stat [`;']}
block ::= chunk
stat ::= varlist1 `=' explist1
| functioncall
| do block end
| while exp do block end
| repeat block until exp
| if exp then block {elseif exp then block} [else block] end
| return [explist1]
| break
| for `Name' `=' exp `,' exp [`,' exp] do block end
| for `Name' {`,' `Name'} in explist1 do block end
| function funcname funcbody
| local function `Name' funcbody
| local namelist [init]funcname ::= `Name' {`.' `Name'} [`:' `Name']
varlist1 ::= var {`,' var}
var ::= `Name' | prefixexp `[' exp `]' | prefixexp `.' `Name'
namelist ::= `Name' {`,' `Name'}
init ::= `=' explist1
explist1 ::= {exp `,'} exp
exp ::= nil false true | `Number'
| `Literal' | function | prefixexp
| tableconstructor | exp binop exp | unop expprefixexp ::= var | functioncall | `(' exp `)'
functioncall ::= prefixexp args | prefixexp `:' `Name' args
args ::= `(' [explist1] `)' | tableconstructor | `Literal'
function ::= function funcbody
funcbody ::= `(' [parlist1] `)' block end
parlist1 ::= `Name' {`,' `Name'} [`,' `...'] | `...'
tableconstructor ::= `{' [fieldlist] `}' fieldlist ::= field {fieldsep field} [fieldsep] field ::= `[' exp `]' `=' exp | name `=' exp | exp fieldsep ::= `,' | `;'
binop ::= `+' | `-' | `*' | `/' | `^' | `..' | `<' | `<=' | `>' | `>=' | `==' | `~='
| and | orunop ::= `-' | not