Lua 5.1 Reference Manual
Lua 5.1 Reference Manual
Copyright © 2006-2008 Lua.org, PUC-Rio. Freely available under the terms of the Lua license.
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Lua is an extension programming language designed to support general procedural programming with data description facilities. It also offers good support for object-oriented programming, functional programming, and data-driven programming. Lua is intended to be used as a powerful, light-weight scripting language for any program that needs one. Lua is implemented as a library, written in clean C (that is, in the common subset of ANSI C and C++).
Being an extension language, Lua has no notion of a "main" program:
it only works embedded in a host client,
called the embedding program or simply the host.
This host program can invoke functions to execute a piece of Lua code,
can write and read Lua variables,
and can register C functions to be called by Lua code.
Through the use of C functions, Lua can be augmented to cope with
a wide range of different domains,
thus creating customized programming languages sharing a syntactical framework.
The Lua distribution includes a sample host program called lua,
which uses the Lua library to offer a complete, stand-alone Lua interpreter.
Lua is free software,
and is provided as usual with no guarantees,
as stated in its license.
The implementation described in this manual is available
at Lua's official web site, www.lua.org.
Like any other reference manual, this document is dry in places. For a discussion of the decisions behind the design of Lua, see the technical papers available at Lua's web site. For a detailed introduction to programming in Lua, see Roberto's book, Programming in Lua (Second Edition).
This section describes the lexis, the syntax, and the semantics of Lua. In other words, this section describes which tokens are valid, how they can be combined, and what their combinations mean.
The language constructs will be explained using the usual extended BNF notation, in which {a} means 0 or more a's, and [a] means an optional a. Non-terminals are shown like non-terminal, keywords are shown like kword, and other terminal symbols are shown like `=´. The complete syntax of Lua can be found at the end of this manual.
Names (also called identifiers) in Lua can be any string of letters, digits, and underscores, not beginning with a digit. This coincides with the definition of names in most languages. (The definition of letter depends on the current locale: any character considered alphabetic by the current locale can be used in an identifier.) Identifiers are used to name variables and table fields.
The following keywords are reserved and cannot be used as names:
and break do else elseif
end false for function if
in local nil not or
repeat return then true until while
Lua is a case-sensitive language:
and is a reserved word, but And and AND
are two different, valid names.
As a convention, names starting with an underscore followed by
uppercase letters (such as _VERSION)
are reserved for internal global variables used by Lua.
The following strings denote other tokens:
+ - * / % ^ #
== ~= <= >= < > =
( ) { } [ ]
; : , . .. ...
Literal strings
can be delimited by matching single or double quotes,
and can contain the following C-like escape sequences:
'\a' (bell),
'\b' (backspace),
'\f' (form feed),
'\n' (newline),
'\r' (carriage return),
'\t' (horizontal tab),
'\v' (vertical tab),
'\\' (backslash),
'\"' (quotation mark [double quote]),
and '\'' (apostrophe [single quote]).
Moreover, a backslash followed by a real newline
results in a newline in the string.
A character in a string may also be specified by its numerical value
using the escape sequence \ddd,
where ddd is a sequence of up to three decimal digits.
(Note that if a numerical escape is to be followed by a digit,
it must be expressed using exactly three digits.)
Strings in Lua may contain any 8-bit value, including embedded zeros,
which can be specified as '\0'.
To put a double (single) quote, a newline, a backslash, a carriage return, or an embedded zero inside a literal string enclosed by double (single) quotes you must use an escape sequence. Any other character may be directly inserted into the literal. (Some control characters may cause problems for the file system, but Lua has no problem with them.)
Literal strings can also be defined using a long format
enclosed by long brackets.
We define an opening long bracket of level n as an opening
square bracket followed by n equal signs followed by another
opening square bracket.
So, an opening long bracket of level 0 is written as [[,
an opening long bracket of level 1 is written as [=[,
and so on.
A closing long bracket is defined similarly;
for instance, a closing long bracket of level 4 is written as ]====].
A long string starts with an opening long bracket of any level and
ends at the first closing long bracket of the same level.
Literals in this bracketed form may run for several lines,
do not interpret any escape sequences,
and ignore long brackets of any other level.
They may contain anything except a closing bracket of the proper level.
For convenience,
when the opening long bracket is immediately followed by a newline,
the newline is not included in the string.
As an example, in a system using ASCII
(in which 'a' is coded as 97,
newline is coded as 10, and '1' is coded as 49),
the five literals below denote the same string:
a = 'alo\n123"'
a = "alo\n123\""
a = '\97lo\10\04923"'
a = [[alo
123"]]
a = [==[
alo
123"]==]
A numerical constant may be written with an optional decimal part
and an optional decimal exponent.
Lua also accepts integer hexadecimal constants,
by prefixing them with 0x.
Examples of valid numerical constants are
3 3.0 3.1416 314.16e-2 0.31416E1 0xff 0x56
A comment starts with a double hyphen (--)
anywhere outside a string.
If the text immediately after -- is not an opening long bracket,
the comment is a short comment,
which runs until the end of the line.
Otherwise, it is a long comment,
which runs until the corresponding closing long bracket.
Long comments are frequently used to disable code temporarily.
Lua is a dynamically typed language. This means that variables do not have types; only values do. There are no type definitions in the language. All values carry their own type.
All values in Lua are first-class values. This means that all values can be stored in variables, passed as arguments to other functions, and returned as results.
There are eight basic types in Lua:
nil, boolean, number,
string, function, userdata,
thread, and table.
Nil is the type of the value nil,
whose main property is to be different from any other value;
it usually represents the absence of a useful value.
Boolean is the type of the values false and true.
Both nil and false make a condition false;
any other value makes it true.
Number represents real (double-precision floating-point) numbers.
(It is easy to build Lua interpreters that use other
internal representations for numbers,
such as single-precision float or long integers;
see file luaconf.h.)
String represents arrays of characters.
Lua is 8-bit clean:
strings may contain any 8-bit character,
including embedded zeros ('\0') (see §2.1).
Lua can call (and manipulate) functions written in Lua and functions written in C (see §2.5.8).
The type userdata is provided to allow arbitrary C data to be stored in Lua variables. This type corresponds to a block of raw memory and has no pre-defined operations in Lua, except assignment and identity test. However, by using metatables, the programmer can define operations for userdata values (see §2.8). Userdata values cannot be created or modified in Lua, only through the C API. This guarantees the integrity of data owned by the host program.
The type thread represents independent threads of execution and it is used to implement coroutines (see §2.11). Do not confuse Lua threads with operating-system threads. Lua supports coroutines on all systems, even those that do not support threads.
The type table implements associative arrays,
that is, arrays that can be indexed not only with numbers,
but with any value (except nil).
Tables can be heterogeneous;
that is, they can contain values of all types (except nil).
Tables are the sole data structuring mechanism in Lua;
they may be used to represent ordinary arrays,
symbol tables, sets, records, graphs, trees, etc.
To represent records, Lua uses the field name as an index.
The language supports this representation by
providing a.name as syntactic sugar for a["name"].
There are several convenient ways to create tables in Lua
(see §2.5.7).
Like indices, the value of a table field can be of any type (except nil). In particular, because functions are first-class values, table fields may contain functions. Thus tables may also carry methods (see §2.5.9).
Tables, functions, threads, and (full) userdata values are objects: variables do not actually contain these values, only references to them. Assignment, parameter passing, and function returns always manipulate references to such values; these operations do not imply any kind of copy.
The library function type returns a string describing the type
of a given value.
Lua provides automatic conversion between
string and number values at run time.
Any arithmetic operation applied to a string tries to convert
this string to a number, following the usual conversion rules.
Conversely, whenever a number is used where a string is expected,
the number is converted to a string, in a reasonable format.
For complete control over how numbers are converted to strings,
use the format function from the string library
(see string.format).
Variables are places that store values. There are three kinds of variables in Lua: global variables, local variables, and table fields.
A single name can denote a global variable or a local variable (or a function's formal parameter, which is a particular kind of local variable):
var ::= Name
Name denotes identifiers, as defined in §2.1.
Any variable is assumed to be global unless explicitly declared as a local (see §2.4.7). Local variables are lexically scoped: local variables can be freely accessed by functions defined inside their scope (see §2.6).
Before the first assignment to a variable, its value is nil.
Square brackets are used to index a table:
var ::= prefixexp `[´ exp `]´
The meaning of accesses to global variables
and table fields can be changed via metatables.
An access to an indexed variable t[i] is equivalent to
a call gettable_event(t,i).
(See §2.8 for a complete description of the
gettable_event function.
This function is not defined or callable in Lua.
We use it here only for explanatory purposes.)
The syntax var.Name is just syntactic sugar for
var["Name"]:
var ::= prefixexp `.´ Name
All global variables live as fields in ordinary Lua tables,
called environment tables or simply
environments (see §2.9).
Each function has its own reference to an environment,
so that all global variables in this function
will refer to this environment table.
When a function is created,
it inherits the environment from the function that created it.
To get the environment table of a Lua function,
you call getfenv.
To replace it,
you call setfenv.
(You can only manipulate the environment of C functions
through the debug library; (see §5.9).)
An access to a global variable x
is equivalent to _env.x,
which in turn is equivalent to
gettable_event(_env, "x")
where _env is the environment of the running function.
(See §2.8 for a complete description of the
gettable_event function.
This function is not defined or callable in Lua.
Similarly, the _env variable is not defined in Lua.
We use them here only for explanatory purposes.)
Lua supports an almost conventional set of statements, similar to those in Pascal or C. This set includes assignment, control structures, function calls, and variable declarations.
The unit of execution of Lua is called a chunk. A chunk is simply a sequence of statements, which are executed sequentially. Each statement can be optionally followed by a semicolon:
chunk ::= {stat [`;´]}
There are no empty statements and thus ';;' is not legal.
Lua handles a chunk as the body of an anonymous function with a variable number of arguments (see §2.5.9). As such, chunks can define local variables, receive arguments, and return values.
A chunk may be stored in a file or in a string inside the host program. When a chunk is executed, first it is pre-compiled into instructions for a virtual machine, and then the compiled code is executed by an interpreter for the virtual machine.
Chunks may also be pre-compiled into binary form;
see program luac for details.
Programs in source and compiled forms are interchangeable;
Lua automatically detects the file type and acts accordingly.
A block is a list of statements; syntactically, a block is the same as a chunk:
block ::= chunk
A block may be explicitly delimited to produce a single statement:
stat ::= do block end
Explicit blocks are useful to control the scope of variable declarations. Explicit blocks are also sometimes used to add a return or break statement in the middle of another block (see §2.4.4).
Lua allows multiple assignment. Therefore, the syntax for assignment defines a list of variables on the left side and a list of expressions on the right side. The elements in both lists are separated by commas:
stat ::= varlist `=´ explist
varlist ::= var {`,´ var}
explist ::= exp {`,´ exp}
Expressions are discussed in §2.5.
Before the assignment, the list of values is adjusted to the length of the list of variables. If there are more values than needed, the excess values are thrown away. If there are fewer values than needed, the list is extended with as many nil's as needed. If the list of expressions ends with a function call, then all values returned by this call enter in the list of values, before the adjustment (except when the call is enclosed in parentheses; see §2.5).
The assignment statement first evaluates all its expressions and only then are the assignments performed. Thus the code
i = 3
i, a[i] = i+1, 20
sets a[3] to 20, without affecting a[4]
because the i in a[i] is evaluated (to 3)
before it is assigned 4.
Similarly, the line
x, y = y, x
exchanges the values of x and y.
The meaning of assignments to global variables
and table fields can be changed via metatables.
An assignment to an indexed variable t[i] = val is equivalent to
settable_event(t,i,val).
(See §2.8 for a complete description of the
settable_event function.
This function is not defined or callable in Lua.
We use it here only for explanatory purposes.)
An assignment to a global variable x = val
is equivalent to the assignment
_env.x = val,
which in turn is equivalent to
settable_event(_env, "x", val)
where _env is the environment of the running function.
(The _env variable is not defined in Lua.
We use it here only for explanatory purposes.)
The control structures if, while, and repeat have the usual meaning and familiar syntax:
stat ::= while exp do block end
stat ::= repeat block until exp
stat ::= if exp then block {elseif exp then block} [else block] end
Lua also has a for statement, in two flavors (see §2.4.5).
The condition expression of a control structure may return any value. Both false and nil are considered false. All values different from nil and false are considered true (in particular, the number 0 and the empty string are also true).
In the repeat–until loop, the inner block does not end at the until keyword, but only after the condition. So, the condition can refer to local variables declared inside the loop block.
The return statement is used to return values from a function or a chunk (which is just a function). Functions and chunks may return more than one value, so the syntax for the return statement is
stat ::= return [explist]
The break statement is used to terminate the execution of a while, repeat, or for loop, skipping to the next statement after the loop:
stat ::= break
A break ends the innermost enclosing loop.
The return and break
statements can only be written as the last statement of a block.
If it is really necessary to return or break in the
middle of a block,
then an explicit inner block can be used,
as in the idioms
do return end and do break end,
because now return and break are the last statements in
their (inner) blocks.
The for statement has two forms: one numeric and one generic.
The numeric for loop repeats a block of code while a control variable runs through an arithmetic progression. It has the following syntax:
stat ::= for Name `=´ exp `,´ exp [`,´ exp] do block end
The block is repeated for name starting at the value of the first exp, until it passes the second exp by steps of the third exp. More precisely, a for statement like
for v = e1, e2, e3 do block end
is equivalent to the code:
do
local var, limit, step = tonumber(e1), tonumber(e2), tonumber(e3)
if not (var and limit and step) then error() end
while (step > 0 and var <= limit) or (step <= 0 and var >= limit) do
local v = var
block
var = var + step
end
end
Note the following:
var, limit, and step are invisible variables.
The names are here for explanatory purposes only.
v is local to the loop;
you cannot use its value after the for ends or is broken.
If you need this value,
assign it to another variable before breaking or exiting the loop.
The generic for statement works over functions, called iterators. On each iteration, the iterator function is called to produce a new value, stopping when this new value is nil. The generic for loop has the following syntax:
stat ::= for namelist in explist do block end
namelist ::= Name {`,´ Name}
A for statement like
for var_1, ···, var_n in explist do block end
is equivalent to the code:
do
local f, s, var = explist
while true do
local var_1, ···, var_n = f(s, var)
var = var_1
if var == nil then break end
block
end
end
Note the following:
explist is evaluated only once.
Its results are an iterator function,
a state,
and an initial value for the first iterator variable.
f, s, and var are invisible variables.
The names are here for explanatory purposes only.
var_i are local to the loop;
you cannot use their values after the for ends.
If you need these values,
then assign them to other variables before breaking or exiting the loop.
To allow possible side-effects, function calls can be executed as statements:
stat ::= functioncall
In this case, all returned values are thrown away. Function calls are explained in §2.5.8.
Local variables may be declared anywhere inside a block. The declaration may include an initial assignment:
stat ::= local namelist [`=´ explist]
If present, an initial assignment has the same semantics of a multiple assignment (see §2.4.3). Otherwise, all variables are initialized with nil.
A chunk is also a block (see §2.4.1), and so local variables can be declared in a chunk outside any explicit block. The scope of such local variables extends until the end of the chunk.
The visibility rules for local variables are explained in §2.6.
The basic expressions in Lua are the following:
exp ::= prefixexp exp ::= nil | false | true exp ::= Number exp ::= String exp ::= function exp ::= tableconstructor exp ::= `...´ exp ::= exp binop exp exp ::= unop exp prefixexp ::= var | functioncall | `(´ exp `)´
Numbers and literal strings are explained in §2.1;
variables are explained in §2.3;
function definitions are explained in §2.5.9;
function calls are explained in §2.5.8;
table constructors are explained in §2.5.7.
Vararg expressions,
denoted by three dots ('...'), can only be used when
directly inside a vararg function;
they are explained in §2.5.9.
Binary operators comprise arithmetic operators (see §2.5.1), relational operators (see §2.5.2), logical operators (see §2.5.3), and the concatenation operator (see §2.5.4). Unary operators comprise the unary minus (see §2.5.1), the unary not (see §2.5.3), and the unary length operator (see §2.5.5).
Both function calls and vararg expressions may result in multiple values. If the expression is used as a statement (see §2.4.6) (only possible for function calls), then its return list is adjusted to zero elements, thus discarding all returned values. If the expression is used as the last (or the only) element of a list of expressions, then no adjustment is made (unless the call is enclosed in parentheses). In all other contexts, Lua adjusts the result list to one element, discarding all values except the first one.
Here are some examples:
f() -- adjusted to 0 results
g(f(), x) -- f() is adjusted to 1 result
g(x, f()) -- g gets x plus all results from f()
a,b,c = f(), x -- f() is adjusted to 1 result (c gets nil)
a,b = ... -- a gets the first vararg parameter, b gets
-- the second (both a and b may get nil if there
-- is no corresponding vararg parameter)
a,b,c = x, f() -- f() is adjusted to 2 results
a,b,c = f() -- f() is adjusted to 3 results
return f() -- returns all results from f()
return ... -- returns all received vararg parameters
return x,y,f() -- returns x, y, and all results from f()
{f()} -- creates a list with all results from f()
{...} -- creates a list with all vararg parameters
{f(), nil} -- f() is adjusted to 1 result
An expression enclosed in parentheses always results in only one value.
Thus,
(f(x,y,z)) is always a single value,
even if f returns several values.
(The value of (f(x,y,z)) is the first value returned by f
or nil if f does not return any values.)
Lua supports the usual arithmetic operators:
the binary + (addition),
- (subtraction), * (multiplication),
/ (division), % (modulo), and ^ (exponentiation);
and unary - (negation).
If the operands are numbers, or strings that can be converted to
numbers (see §2.2.1),
then all operations have the usual meaning.
Exponentiation works for any exponent.
For instance, x^(-0.5) computes the inverse of the square root of x.
Modulo is defined as
a % b == a - math.floor(a/b)*b
That is, it is the remainder of a division that rounds the quotient towards minus infinity.
The relational operators in Lua are
== ~= < > <= >=
These operators always result in false or true.
Equality (==) first compares the type of its operands.
If the types are different, then the result is false.
Otherwise, the values of the operands are compared.
Numbers and strings are compared in the usual way.
Objects (tables, userdata, threads, and functions)
are compared by reference:
two objects are considered equal only if they are the same object.
Every time you create a new object
(a table, userdata, thread, or function),
this new object is different from any previously existing object.
You can change the way that Lua compares tables and userdata by using the "eq" metamethod (see §2.8).
The conversion rules of §2.2.1
do not apply to equality comparisons.
Thus, "0"==0 evaluates to false,
and t[0] and t["0"] denote different
entries in a table.
The operator ~= is exactly the negation of equality (==).
The order operators work as follows. If both arguments are numbers, then they are compared as such. Otherwise, if both arguments are strings, then their values are compared according to the current locale. Otherwise, Lua tries to call the "lt" or the "le" metamethod (see §2.8).
The logical operators in Lua are and, or, and not. Like the control structures (see §2.4.4), all logical operators consider both false and nil as false and anything else as true.
The negation operator not always returns false or true. The conjunction operator and returns its first argument if this value is false or nil; otherwise, and returns its second argument. The disjunction operator or returns its first argument if this value is different from nil and false; otherwise, or returns its second argument. Both and and or use short-cut evaluation; that is, the second operand is evaluated only if necessary. Here are some examples:
10 or 20 --> 10
10 or error() --> 10
nil or "a" --> "a"
nil and 10 --> nil
false and error() --> false
false and nil --> false
false or nil --> nil
10 and 20 --> 20
(In this manual, --> indicates the result of the preceding expression.)
The string concatenation operator in Lua is
denoted by two dots ('..').
If both operands are strings or numbers, then they are converted to
strings according to the rules mentioned in §2.2.1.
Otherwise, the "concat" metamethod is called (see §2.8).
The length operator is denoted by the unary operator #.
The length of a string is its number of bytes
(that is, the usual meaning of string length when each
character is one byte).
The length of a table t is defined to be any
integer index n
such that t[n] is not nil and t[n+1] is nil;
moreover, if t[1] is nil, n may be zero.
For a regular array, with non-nil values from 1 to a given n,
its length is exactly that n,
the index of its last value.
If the array has "holes"
(that is, nil values between other non-nil values),
then #t may be any of the indices that
directly precedes a nil value
(that is, it may consider any such nil value as the end of
the array).
Operator precedence in Lua follows the table below, from lower to higher priority:
or
and
< > <= >= ~= ==
..
+ -
* / %
not # - (unary)
^
As usual,
you can use parentheses to change the precedences of an expression.
The concatenation ('..') and exponentiation ('^')
operators are right associative.
All other binary operators are left associative.
Table constructors are expressions that create tables. Every time a constructor is evaluated, a new table is created. Constructors can be used to create empty tables, or to create a table and initialize some of its fields. The general syntax for constructors is
tableconstructor ::= `{´ [fieldlist] `}´
fieldlist ::= field {fieldsep field} [fieldsep]
field ::= `[´ exp `]´ `=´ exp | Name `=´ exp | exp
fieldsep ::= `,´ | `;´
Each field of the form [exp1] = exp2 adds to the new table an entry
with key exp1 and value exp2.
A field of the form name = exp is equivalent to
["name"] = exp.
Finally, fields of the form exp are equivalent to
[i] = exp, where i are consecutive numerical integers,
starting with 1.
Fields in the other formats do not affect this counting.
For example,
a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 }
is equivalent to
do
local t = {}
t[f(1)] = g
t[1] = "x" -- 1st exp
t[2] = "y" -- 2nd exp
t.x = 1 -- t["x"] = 1
t[3] = f(x) -- 3rd exp
t[30] = 23
t[4] = 45 -- 4th exp
a = t
end
If the last field in the list has the form exp
and the expression is a function call or a vararg expression,
then all values returned by this expression enter the list consecutively
(see §2.5.8).
To avoid this,
enclose the function call (or the vararg expression)
in parentheses (see §2.5).
The field list may have an optional trailing separator, as a convenience for machine-generated code.
A function call in Lua has the following syntax:
functioncall ::= prefixexp args
In a function call, first prefixexp and args are evaluated. If the value of prefixexp has type function, then this function is called with the given arguments. Otherwise, the prefixexp "call" metamethod is called, having as first parameter the value of prefixexp, followed by the original call arguments (see §2.8).
The form
functioncall ::= prefixexp `:´ Name args
can be used to call "methods".
A call v:name(args)
is syntactic sugar for v.name(v,args),
except that v is evaluated only once.
Arguments have the following syntax:
args ::= `(´ [explist] `)´ args ::= tableconstructor args ::= String
All argument expressions are evaluated before the call.
A call of the form f{fields} is
syntactic sugar for f({fields});
that is, the argument list is a single new table.
A call of the form f'string'
(or f"string" or f[[string]])
is syntactic sugar for f('string');
that is, the argument list is a single literal string.
As an exception to the free-format syntax of Lua,
you cannot put a line break before the '(' in a function call.
This restriction avoids some ambiguities in the language.
If you write
a = f
(g).x(a)
Lua would see that as a single statement, a = f(g).x(a).
So, if you want two statements, you must add a semi-colon between them.
If you actually want to call f,
you must remove the line break before (g).
A call of the form return functioncall is called
a tail call.
Lua implements proper tail calls
(or proper tail recursion):
in a tail call,
the called function reuses the stack entry of the calling function.
Therefore, there is no limit on the number of nested tail calls that
a program can execute.
However, a tail call erases any debug information about the
calling function.
Note that a tail call only happens with a particular syntax,
where the return has one single function call as argument;
this syntax makes the calling function return exactly
the returns of the called function.
So, none of the following examples are tail calls:
return (f(x)) -- results adjusted to 1
return 2 * f(x)
return x, f(x) -- additional results
f(x); return -- results discarded
return x or f(x) -- results adjusted to 1
The syntax for function definition is
function ::= function funcbody funcbody ::= `(´ [parlist] `)´ block end
The following syntactic sugar simplifies function definitions:
stat ::= function funcname funcbody
stat ::= local function Name funcbody
funcname ::= Name {`.´ Name} [`:´ Name]
The statement
function f () body end
translates to
f = function () body end
The statement
function t.a.b.c.f () body end
translates to
t.a.b.c.f = function () body end
The statement
local function f () body end
translates to
local f; f = function () body end
not to
local f = function () body end
(This only makes a difference when the body of the function
contains references to f.)
A function definition is an executable expression, whose value has type function. When Lua pre-compiles a chunk, all its function bodies are pre-compiled too. Then, whenever Lua executes the function definition, the function is instantiated (or closed). This function instance (or closure) is the final value of the expression. Different instances of the same function may refer to different external local variables and may have different environment tables.
Parameters act as local variables that are initialized with the argument values:
parlist ::= namelist [`,´ `...´] | `...´
When a function is called,
the list of arguments is adjusted to
the length of the list of parameters,
unless the function is a variadic or vararg function,
which is
indicated by three dots ('...') at the end of its parameter list.
A vararg function does not adjust its argument list;
instead, it collects all extra arguments and supplies them
to the function through a vararg expression,
which is also written as three dots.
The value of this expression is a list of all actual extra arguments,
similar to a function with multiple results.
If a vararg expression is used inside another expression
or in the middle of a list of expressions,
then its return list is adjusted to one element.
If the expression is used as the last element of a list of expressions,
then no adjustment is made
(unless the call is enclosed in parentheses).
As an example, consider the following definitions:
function f(a, b) end
function g(a, b, ...) end
function r() return 1,2,3 end
Then, we have the following mapping from arguments to parameters and to the vararg expression:
CALL PARAMETERS
f(3) a=3, b=nil
f(3, 4) a=3, b=4
f(3, 4, 5) a=3, b=4
f(r(), 10) a=1, b=10
f(r()) a=1, b=2
g(3) a=3, b=nil, ... --> (nothing)
g(3, 4) a=3, b=4, ... --> (nothing)
g(3, 4, 5, 8) a=3, b=4, ... --> 5 8
g(5, r()) a=5, b=1, ... --> 2 3
Results are returned using the return statement (see §2.4.4). If control reaches the end of a function without encountering a return statement, then the function returns with no results.
The colon syntax
is used for defining methods,
that is, functions that have an implicit extra parameter self.
Thus, the statement
function t.a.b.c:f (params) body end
is syntactic sugar for
t.a.b.c.f = function (self, params) body end
Lua is a lexically scoped language. The scope of variables begins at the first statement after their declaration and lasts until the end of the innermost block that includes the declaration. Consider the following example:
x = 10 -- global variable
do -- new block
local x = x -- new 'x', with value 10
print(x) --> 10
x = x+1
do -- another block
local x = x+1 -- another 'x'
print(x) --> 12
end
print(x) --> 11
end
print(x) --> 10 (the global one)
Notice that, in a declaration like local x = x,
the new x being declared is not in scope yet,
and so the second x refers to the outside variable.
Because of the lexical scoping rules, local variables can be freely accessed by functions defined inside their scope. A local variable used by an inner function is called an upvalue, or external local variable, inside the inner function.
Notice that each execution of a local statement defines new local variables. Consider the following example:
a = {}
local x = 20
for i=1,10 do
local y = 0
a[i] = function () y=y+1; return x+y end
end
The loop creates ten closures
(that is, ten instances of the anonymous function).
Each of these closures uses a different y variable,
while all of them share the same x.
Because Lua is an embedded extension language,
all Lua actions start from C code in the host program
calling a function from the Lua library (see lua_pcall).
Whenever an error occurs during Lua compilation or execution,
control returns to C,
which can take appropriate measures
(such as printing an error message).
Lua code can explicitly generate an error by calling the
error function.
If you need to catch errors in Lua,
you can use the pcall function.
Every value in Lua may have a metatable.
This metatable is an ordinary Lua table
that defines the behavior of the original value
under certain special operations.
You can change several aspects of the behavior
of operations over a value by setting specific fields in its metatable.
For instance, when a non-numeric value is the operand of an addition,
Lua checks for a function in the field "__add" in its metatable.
If it finds one,
Lua calls this function to perform the addition.
We call the keys in a metatable events
and the values metamethods.
In the previous example, the event is "add"
and the metamethod is the function that performs the addition.
You can query the metatable of any value
through the getmetatable function.
You can replace the metatable of tables
through the setmetatable
function.
You cannot change the metatable of other types from Lua
(except using the debug library);
you must use the C API for that.
Tables and full userdata have individual metatables (although multiple tables and userdata can share their metatables); values of all other types share one single metatable per type. So, there is one single metatable for all numbers, one for all strings, etc.
A metatable may control how an object behaves in arithmetic operations, order comparisons, concatenation, length operation, and indexing. A metatable can also define a function to be called when a userdata is garbage collected. For each of these operations Lua associates a specific key called an event. When Lua performs one of these operations over a value, it checks whether this value has a metatable with the corresponding event. If so, the value associated with that key (the metamethod) controls how Lua will perform the operation.
Metatables control the operations listed next.
Each operation is identified by its corresponding name.
The key for each operation is a string with its name prefixed by
two underscores, '__';
for instance, the key for operation "add" is the
string "__add".
The semantics of these operations is better explained by a Lua function
describing how the interpreter executes the operation.
The code shown here in Lua is only illustrative;
the real behavior is hard coded in the interpreter
and it is much more efficient than this simulation.
All functions used in these descriptions
(rawget, tonumber, etc.)
are described in §5.1.
In particular, to retrieve the metamethod of a given object,
we use the expression
metatable(obj)[event]
This should be read as
rawget(getmetatable(obj) or {}, event)
That is, the access to a metamethod does not invoke other metamethods, and the access to objects with no metatables does not fail (it simply results in nil).
+ operation.
The function getbinhandler below defines how Lua chooses a handler
for a binary operation.
First, Lua tries the first operand.
If its type does not define a handler for the operation,
then Lua tries the second operand.
function getbinhandler (op1, op2, event)
return metatable(op1)[event] or metatable(op2)[event]
end
By using this function,
the behavior of the op1 + op2 is
function add_event (op1, op2)
local o1, o2 = tonumber(op1), tonumber(op2)
if o1 and o2 then -- both operands are numeric?
return o1 + o2 -- '+' here is the primitive 'add'
else -- at least one of the operands is not numeric
local h = getbinhandler(op1, op2, "__add")
if h then
-- call the handler with both operands
return (h(op1, op2))
else -- no handler available: default behavior
error(···)
end
end
end
- operation.
Behavior similar to the "add" operation.
* operation.
Behavior similar to the "add" operation.
/ operation.
Behavior similar to the "add" operation.
% operation.
Behavior similar to the "add" operation,
with the operation
o1 - floor(o1/o2)*o2 as the primitive operation.
^ (exponentiation) operation.
Behavior similar to the "add" operation,
with the function pow (from the C math library)
as the primitive operation.
- operation.
function unm_event (op)
local o = tonumber(op)
if o then -- operand is numeric?
return -o -- '-' here is the primitive 'unm'
else -- the operand is not numeric.
-- Try to get a handler from the operand
local h = metatable(op).__unm
if h then
-- call the handler with the operand
return (h(op))
else -- no handler available: default behavior
error(···)
end
end
end
.. (concatenation) operation.
function concat_event (op1, op2)
if (type(op1) == "string" or type(op1) == "number") and
(type(op2) == "string" or type(op2) == "number") then
return op1 .. op2 -- primitive string concatenation
else
local h = getbinhandler(op1, op2, "__concat")
if h then
return (h(op1, op2))
else
error(···)
end
end
end
# operation.
function len_event (op)
if type(op) == "string" then
return strlen(op) -- primitive string length
elseif type(op) == "table" then
return #op -- primitive table length
else
local h = metatable(op).__len
if h then
-- call the handler with the operand
return (h(op))
else -- no handler available: default behavior
error(···)
end
end
end
See §2.5.5 for a description of the length of a table.
== operation.
The function getcomphandler defines how Lua chooses a metamethod
for comparison operators.
A metamethod only is selected when both objects
being compared have the same type
and the same metamethod for the selected operation.
function getcomphandler (op1, op2, event)
if type(op1) ~= type(op2) then return nil end
local mm1 = metatable(op1)[event]
local mm2 = metatable(op2)[event]
if mm1 == mm2 then return mm1 else return nil end
end
The "eq" event is defined as follows:
function eq_event (op1, op2)
if type(op1) ~= type(op2) then -- different types?
return false -- different objects
end
if op1 == op2 then -- primitive equal?
return true -- objects are equal
end
-- try metamethod
local h = getcomphandler(op1, op2, "__eq")
if h then
return (h(op1, op2))
else
return false
end
end
a ~= b is equivalent to not (a == b).
< operation.
function lt_event (op1, op2)
if type(op1) == "number" and type(op2) == "number" then
return op1 < op2 -- numeric comparison
elseif type(op1) == "string" and type(op2) == "string" then
return op1 < op2 -- lexicographic comparison
else
local h = getcomphandler(op1, op2, "__lt")
if h then
return (h(op1, op2))
else
error(···);
end
end
end
a > b is equivalent to b < a.
<= operation.
function le_event (op1, op2)
if type(op1) == "number" and type(op2) == "number" then
return op1 <= op2 -- numeric comparison
elseif type(op1) == "string" and type(op2) == "string" then
return op1 <= op2 -- lexicographic comparison
else
local h = getcomphandler(op1, op2, "__le")
if h then
return (h(op1, op2))
else
h = getcomphandler(op1, op2, "__lt")
if h then
return not h(op2, op1)
else
error(···);
end
end
end
end
a >= b is equivalent to b <= a.
Note that, in the absence of a "le" metamethod,
Lua tries the "lt", assuming that a <= b is
equivalent to not (b < a).
table[key].
function gettable_event (table, key)
local h
if type(table) == "table" then
local v = rawget(table, key)
if v ~= nil then return v end
h = metatable(table).__index
if h == nil then return nil end
else
h = metatable(table).__index
if h == nil then
error(···);
end
end
if type(h) == "function" then
return (h(table, key)) -- call the handler
else return h[key] -- or repeat operation on it
end
end
table[key] = value.
function settable_event (table, key, value)
local h
if type(table) == "table" then
local v = rawget(table, key)
if v ~= nil then rawset(table, key, value); return end
h = metatable(table).__newindex
if h == nil then rawset(table, key, value); return end
else
h = metatable(table).__newindex
if h == nil then
error(···);
end
end
if type(h) == "function" then
h(table, key,value) -- call the handler
else h[key] = value -- or repeat operation on it
end
end
function function_event (func, ...)
if type(func) == "function" then
return func(...) -- primitive call
else
local h = metatable(func).__call
if h then
return h(func, ...)
else
error(···)
end
end
end
Besides metatables, objects of types thread, function, and userdata have another table associated with them, called their environment. Like metatables, environments are regular tables and multiple objects can share the same environment.
Environments associated with userdata have no meaning for Lua. It is only a convenience feature for programmers to associate a table to a userdata.
Environments associated with threads are called
global environments.
They are used as the default environment for their threads and
non-nested functions created by the thread
(through loadfile, loadstring or load)
and can be directly accessed by C code (see §3.3).
Environments associated with C functions can be directly accessed by C code (see §3.3). They are used as the default environment for other C functions created by the function.
Environments associated with Lua functions are used to resolve all accesses to global variables within the function (see §2.3). They are used as the default environment for other Lua functions created by the function.
You can change the environment of a Lua function or the
running thread by calling setfenv.
You can get the environment of a Lua function or the running thread
by calling getfenv.
To manipulate the environment of other objects
(userdata, C functions, other threads) you must
use the C API.
Lua performs automatic memory management. This means that you have to worry neither about allocating memory for new objects nor about freeing it when the objects are no longer needed. Lua manages memory automatically by running a garbage collector from time to time to collect all dead objects (that is, these objects that are no longer accessible from Lua). All objects in Lua are subject to automatic management: tables, userdata, functions, threads, and strings.
Lua implements an incremental mark-and-sweep collector. It uses two numbers to control its garbage-collection cycles: the garbage-collector pause and the garbage-collector step multiplier.
The garbage-collector pause controls how long the collector waits before starting a new cycle. Larger values make the collector less aggressive. Values smaller than 1 mean the collector will not wait to start a new cycle. A value of 2 means that the collector waits for the total memory in use to double before starting a new cycle.
The step multiplier controls the relative speed of the collector relative to memory allocation. Larger values make the collector more aggressive but also increase the size of each incremental step. Values smaller than 1 make the collector too slow and may result in the collector never finishing a cycle. The default, 2, means that the collector runs at "twice" the speed of memory allocation.
You can change these numbers by calling lua_gc in C
or collectgarbage in Lua.
Both get percentage points as arguments
(so an argument of 100 means a real value of 1).
With these functions you can also control
the collector directly (e.g., stop and restart it).
Using the C API, you can set garbage-collector metamethods for userdata (see §2.8). These metamethods are also called finalizers. Finalizers allow you to coordinate Lua's garbage collection with external resource management (such as closing files, network or database connections, or freeing your own memory).
Garbage userdata with a field __gc in their metatables are not
collected immediately by the garbage collector.
Instead, Lua puts them in a list.
After the collection,
Lua does the equivalent of the following function
for each userdata in that list:
function gc_event (udata)
local h = metatable(udata).__gc
if h then
h(udata)
end
end
At the end of each garbage-collection cycle, the finalizers for userdata are called in reverse order of their creation, among those collected in that cycle. That is, the first finalizer to be called is the one associated with the userdata created last in the program. The userdata itself is freed only in the next garbage-collection cycle.
A weak table is a table whose elements are weak references. A weak reference is ignored by the garbage collector. In other words, if the only references to an object are weak references, then the garbage collector will collect this object.
A weak table can have weak keys, weak values, or both.
A table with weak keys allows the collection of its keys,
but prevents the collection of its values.
A table with both weak keys and weak values allows the collection of
both keys and values.
In any case, if either the key or the value is collected,
the whole pair is removed from the table.
The weakness of a table is controlled by the
__mode field of its metatable.
If the __mode field is a string containing the character 'k',
the keys in the table are weak.
If __mode contains 'v',
the values in the table are weak.
After you use a table as a metatable,
you should not change the value of its field __mode.
Otherwise, the weak behavior of the tables controlled by this
metatable is undefined.
Lua supports coroutines, also called collaborative multithreading. A coroutine in Lua represents an independent thread of execution. Unlike threads in multithread systems, however, a coroutine only suspends its execution by explicitly calling a yield function.
You create a coroutine with a call to coroutine.create.
Its sole argument is a function
that is the main function of the coroutine.
The create function only creates a new coroutine and
returns a handle to it (an object of type thread);
it does not start the coroutine execution.
When you first call coroutine.resume,
passing as its first argument
the thread returned by coroutine.create,
the coroutine starts its execution,
at the first line of its main function.
Extra arguments passed to coroutine.resume are passed on
to the coroutine main function.
After the coroutine starts running,
it runs until it terminates or yields.
A coroutine can terminate its execution in two ways:
normally, when its main function returns
(explicitly or implicitly, after the last instruction);
and abnormally, if there is an unprotected error.
In the first case, coroutine.resume returns true,
plus any values returned by the coroutine main function.
In case of errors, coroutine.resume returns false
plus an error message.
A coroutine yields by calling coroutine.yield.
When a coroutine yields,
the corresponding coroutine.resume returns immediately,
even if the yield happens inside nested function calls
(that is, not in the main function,
but in a function directly or indirectly called by the main function).
In the case of a yield, coroutine.resume also returns true,
plus any values passed to coroutine.yield.
The next time you resume the same coroutine,
it continues its execution from the point where it yielded,
with the call to coroutine.yield returning any extra
arguments passed to coroutine.resume.
Like coroutine.create,
the coroutine.wrap function also creates a coroutine,
but instead of returning the coroutine itself,
it returns a function that, when called, resumes the coroutine.
Any arguments passed to this function
go as extra arguments to coroutine.resume.
coroutine.wrap returns all the values returned by coroutine.resume,
except the first one (the boolean error code).
Unlike coroutine.resume,
coroutine.wrap does not catch errors;
any error is propagated to the caller.
As an example, consider the following code:
function foo (a)
print("foo", a)
return coroutine.yield(2*a)
end
co = coroutine.create(function (a,b)
print("co-body", a, b)
local r = foo(a+1)
print("co-body", r)
local r, s = coroutine.yield(a+b, a-b)
print("co-body", r, s)
return b, "end"
end)
print("main", coroutine.resume(co, 1, 10))
print("main", coroutine.resume(co, "r"))
print("main", coroutine.resume(co, "x", "y"))
print("main", coroutine.resume(co, "x", "y"))
When you run it, it produces the following output:
co-body 1 10
foo 2
main true 4
co-body r
main true 11 -9
co-body x y
main true 10 end
main false cannot resume dead coroutine
This section describes the C API for Lua, that is,
the set of C functions available to the host program to communicate
with Lua.
All API functions and related types and constants
are declared in the header file lua.h.
Even when we use the term "function", any facility in the API may be provided as a macro instead. All such macros use each of their arguments exactly once (except for the first argument, which is always a Lua state), and so do not generate any hidden side-effects.
As in most C libraries,
the Lua API functions do not check their arguments for validity or consistency.
However, you can change this behavior by compiling Lua
with a proper definition for the macro luai_apicheck,
in file luaconf.h.
Lua uses a virtual stack to pass values to and from C. Each element in this stack represents a Lua value (nil, number, string, etc.).
Whenever Lua calls C, the called function gets a new stack,
which is independent of previous stacks and of stacks of
C functions that are still active.
This stack initially contains any arguments to the C function
and it is where the C function pushes its results
to be returned to the caller (see lua_CFunction).
For convenience,
most query operations in the API do not follow a strict stack discipline.
Instead, they can refer to any element in the stack
by using an index:
A positive index represents an absolute stack position
(starting at 1);
a negative index represents an offset relative to the top of the stack.
More specifically, if the stack has n elements,
then index 1 represents the first element
(that is, the element that was pushed onto the stack first)
and
index n represents the last element;
index -1 also represents the last element
(that is, the element at the top)
and index -n represents the first element.
We say that an index is valid
if it lies between 1 and the stack top
(that is, if 1 ≤ abs(index) ≤ top).
When you interact with Lua API,
you are responsible for ensuring consistency.
In particular,
you are responsible for controlling stack overflow.
You can use the function lua_checkstack
to grow the stack size.
Whenever Lua calls C,
it ensures that at least LUA_MINSTACK stack positions are available.
LUA_MINSTACK is defined as 20,
so that usually you do not have to worry about stack space
unless your code has loops pushing elements onto the stack.
Most query functions accept as indices any value inside the
available stack space, that is, indices up to the maximum stack size
you have set through lua_checkstack.
Such indices are called acceptable indices.
More formally, we define an acceptable index
as follows:
(index < 0 && abs(index) <= top) ||
(index > 0 && index <= stackspace)
Note that 0 is never an acceptable index.
Unless otherwise noted, any function that accepts valid indices can also be called with pseudo-indices, which represent some Lua values that are accessible to C code but which are not in the stack. Pseudo-indices are used to access the thread environment, the function environment, the registry, and the upvalues of a C function (see §3.4).
The thread environment (where global variables live) is
always at pseudo-index LUA_GLOBALSINDEX.
The environment of the running C function is always
at pseudo-index LUA_ENVIRONINDEX.
To access and change the value of global variables, you can use regular table operations over an environment table. For instance, to access the value of a global variable, do
lua_getfield(L, LUA_GLOBALSINDEX, varname);
When a C function is created,
it is possible to associate some values with it,
thus creating a C closure;
these values are called upvalues and are
accessible to the function whenever it is called
(see lua_pushcclosure).
Whenever a C function is called,
its upvalues are located at specific pseudo-indices.
These pseudo-indices are produced by the macro
lua_upvalueindex.
The first value associated with a function is at position
lua_upvalueindex(1), and so on.
Any access to lua_upvalueindex(n),
where n is greater than the number of upvalues of the
current function,
produces an acceptable (but invalid) index.
Lua provides a registry,
a pre-defined table that can be used by any C code to
store whatever Lua value it needs to store.
This table is always located at pseudo-index
LUA_REGISTRYINDEX.
Any C library can store data into this table,
but it should take care to choose keys different from those used
by other libraries, to avoid collisions.
Typically, you should use as key a string containing your library name
or a light userdata with the address of a C object in your code.
The integer keys in the registry are used by the reference mechanism, implemented by the auxiliary library, and therefore should not be used for other purposes.
Internally, Lua uses the C longjmp facility to handle errors.
(You can also choose to use exceptions if you use C++;
see file luaconf.h.)
When Lua faces any error
(such as memory allocation errors, type errors, syntax errors,
and runtime errors)
it raises an error;
that is, it does a long jump.
A protected environment uses setjmp
to set a recover point;
any error jumps to the most recent active recover point.
Most functions in the API may throw an error, for instance due to a memory allocation error. The documentation for each function indicates whether it can throw errors.
Inside a C function you can throw an error by calling lua_error.
Here we list all functions and types from the C API in alphabetical order. Each function has an indicator like this: [-o, +p, x]
The first field, o,
is how many elements the function pops from the stack.
The second field, p,
is how many elements the function pushes onto the stack.
(Any function always pushes its results after popping its arguments.)
A field in the form x|y means the function may push (or pop)
x or y elements,
depending on the situation;
an interrogation mark '?' means that
we cannot know how many elements the function pops/pushes
by looking only at its arguments
(e.g., they may depend on what is on the stack).
The third field, x,
tells whether the function may throw errors:
'-' means the function never throws any error;
'm' means the function may throw an error
only due to not enough memory;
'e' means the function may throw other kinds of errors;
'v' means the function may throw an error on purpose.
lua_Alloctypedef void * (*lua_Alloc) (void *ud,
void *ptr,
size_t osize,
size_t nsize);
The type of the memory-allocation function used by Lua states.
The allocator function must provide a
functionality similar to realloc,
but not exactly the same.
Its arguments are
ud, an opaque pointer passed to lua_newstate;
ptr, a pointer to the block being allocated/reallocated/freed;
osize, the original size of the block;
nsize, the new size of the block.
ptr is NULL if and only if osize is zero.
When nsize is zero, the allocator must return NULL;
if osize is not zero,
it should free the block pointed to by ptr.
When nsize is not zero, the allocator returns NULL
if and only if it cannot fill the request.
When nsize is not zero and osize is zero,
the allocator should behave like malloc.
When nsize and osize are not zero,
the allocator behaves like realloc.
Lua assumes that the allocator never fails when
osize >= nsize.
Here is a simple implementation for the allocator function.
It is used in the auxiliary library by luaL_newstate.
static void *l_alloc (void *ud, void *ptr, size_t osize,
size_t nsize) {
(void)ud; (void)osize; /* not used */
if (nsize == 0) {
free(ptr);
return NULL;
}
else
return realloc(ptr, nsize);
}
This code assumes
that free(NULL) has no effect and that
realloc(NULL, size) is equivalent to malloc(size).
ANSI C ensures both behaviors.
lua_atpanic[-0, +0, -]
lua_CFunction lua_atpanic (lua_State *L, lua_CFunction panicf);
Sets a new panic function and returns the old one.
If an error happens outside any protected environment,
Lua calls a panic function
and then calls exit(EXIT_FAILURE),
thus exiting the host application.
Your panic function may avoid this exit by
never returning (e.g., doing a long jump).
The panic function can access the error message at the top of the stack.
lua_call[-(nargs + 1), +nresults, e]
void lua_call (lua_State *L, int nargs, int nresults);
Calls a function.
To call a function you must use the following protocol:
first, the function to be called is pushed onto the stack;
then, the arguments to the function are pushed
in direct order;
that is, the first argument is pushed first.
Finally you call lua_call;
nargs is the number of arguments that you pushed onto the stack.
All arguments and the function value are popped from the stack
when the function is called.
The function results are pushed onto the stack when the function returns.
The number of results is adjusted to nresults,
unless nresults is LUA_MULTRET.
In this case, all results from the function are pushed.
Lua takes care that the returned values fit into the stack space.
The function results are pushed onto the stack in direct order
(the first result is pushed first),
so that after the call the last result is on the top of the stack.
Any error inside the called function is propagated upwards
(with a longjmp).
The following example shows how the host program may do the equivalent to this Lua code:
a = f("how", t.x, 14)
Here it is in C:
lua_getfield(L, LUA_GLOBALSINDEX, "f"); /* function to be called */
lua_pushstring(L, "how"); /* 1st argument */
lua_getfield(L, LUA_GLOBALSINDEX, "t"); /* table to be indexed */
lua_getfield(L, -1, "x"); /* push result of t.x (2nd arg) */
lua_remove(L, -2); /* remove 't' from the stack */
lua_pushinteger(L, 14); /* 3rd argument */
lua_call(L, 3, 1); /* call 'f' with 3 arguments and 1 result */
lua_setfield(L, LUA_GLOBALSINDEX, "a"); /* set global 'a' */
Note that the code above is "balanced": at its end, the stack is back to its original configuration. This is considered good programming practice.
lua_CFunctiontypedef int (*lua_CFunction) (lua_State *L);
Type for C functions.
In order to communicate properly with Lua,
a C function must use the following protocol,
which defines the way parameters and results are passed:
a C function receives its arguments from Lua in its stack
in direct order (the first argument is pushed first).
So, when the function starts,
lua_gettop(L) returns the number of arguments received by the function.
The first argument (if any) is at index 1
and its last argument is at index lua_gettop(L).
To return values to Lua, a C function just pushes them onto the stack,
in direct order (the first result is pushed first),
and returns the number of results.
Any other value in the stack below the results will be properly
discarded by Lua.
Like a Lua function, a C function called by Lua can also return
many results.
As an example, the following function receives a variable number of numerical arguments and returns their average and sum:
static int foo (lua_State *L) {
int n = lua_gettop(L); /* number of arguments */
lua_Number sum = 0;
int i;
for (i = 1; i <= n; i++) {
if (!lua_isnumber(L, i)) {
lua_pushstring(L, "incorrect argument");
lua_error(L);
}
sum += lua_tonumber(L, i);
}
lua_pushnumber(L, sum/n); /* first result */
lua_pushnumber(L, sum); /* second result */
return 2; /* number of results */
}
lua_checkstack[-0, +0, m]
int lua_checkstack (lua_State *L, int extra);
Ensures that there are at least extra free stack slots in the stack.
It returns false if it cannot grow the stack to that size.
This function never shrinks the stack;
if the stack is already larger than the new size,
it is left unchanged.
lua_close[-0, +0, -]
void lua_close (lua_State *L);
Destroys all objects in the given Lua state (calling the corresponding garbage-collection metamethods, if any) and frees all dynamic memory used by this state. On several platforms, you may not need to call this function, because all resources are naturally released when the host program ends. On the other hand, long-running programs, such as a daemon or a web server, might need to release states as soon as they are not needed, to avoid growing too large.
lua_concat[-n, +1, e]
void lua_concat (lua_State *L, int n);
Concatenates the n values at the top of the stack,
pops them, and leaves the result at the top.
If n is 1, the result is the single value on the stack
(that is, the function does nothing);
if n is 0, the result is the empty string.
Concatenation is performed following the usual semantics of Lua
(see §2.5.4).
lua_cpcall[-0, +(0|1), -]
int lua_cpcall (lua_State *L, lua_CFunction func, void *ud);
Calls the C function func in protected mode.
func starts with only one element in its stack,
a light userdata containing ud.
In case of errors,
lua_cpcall returns the same error codes as lua_pcall,
plus the error object on the top of the stack;
otherwise, it returns zero, and does not change the stack.
All values returned by func are discarded.
lua_createtable[-0, +1, m]
void lua_createtable (lua_State *L, int narr, int nrec);
Creates a new empty table and pushes it onto the stack.
The new table has space pre-allocated
for narr array elements and nrec non-array elements.
This pre-allocation is useful when you know exactly how many elements
the table will have.
Otherwise you can use the function lua_newtable.
lua_dump[-0, +0, m]
int lua_dump (lua_State *L, lua_Writer writer, void *data);
Dumps a function as a binary chunk.
Receives a Lua function on the top of the stack
and produces a binary chunk that,
if loaded again,
results in a function equivalent to the one dumped.
As it produces parts of the chunk,
lua_dump calls function writer (see lua_Writer)
with the given data
to write them.
The value returned is the error code returned by the last call to the writer; 0 means no errors.
This function does not pop the Lua function from the stack.
lua_equal[-0, +0, e]
int lua_equal (lua_State *L, int index1, int index2);
Returns 1 if the two values in acceptable indices index1 and
index2 are equal,
following the semantics of the Lua == operator
(that is, may call metamethods).
Otherwise returns 0.
Also returns 0 if any of the indices is non valid.
lua_error[-1, +0, v]
int lua_error (lua_State *L);
Generates a Lua error.
The error message (which can actually be a Lua value of any type)
must be on the stack top.
This function does a long jump,
and therefore never returns.
(see luaL_error).
lua_gc[-0, +0, e]
int lua_gc (lua_State *L, int what, int data);
Controls the garbage collector.
This function performs several tasks,
according to the value of the parameter what:
LUA_GCSTOP:
stops the garbage collector.
LUA_GCRESTART:
restarts the garbage collector.
LUA_GCCOLLECT:
performs a full garbage-collection cycle.
LUA_GCCOUNT:
returns the current amount of memory (in Kbytes) in use by Lua.
LUA_GCCOUNTB:
returns the remainder of dividing the current amount of bytes of
memory in use by Lua by 1024.
LUA_GCSTEP:
performs an incremental step of garbage collection.
The step "size" is controlled by data
(larger values mean more steps) in a non-specified way.
If you want to control the step size
you must experimentally tune the value of data.
The function returns 1 if the step finished a
garbage-collection cycle.
LUA_GCSETPAUSE:
sets data/100 as the new value
for the pause of the collector (see §2.10).
The function returns the previous value of the pause.
LUA_GCSETSTEPMUL:
sets data/100 as the new value for the step multiplier of
the collector (see §2.10).
The function returns the previous value of the step multiplier.
lua_getallocf[-0, +0, -]
lua_Alloc lua_getallocf (lua_State *L, void **ud);
Returns the memory-allocation function of a given state.
If ud is not NULL, Lua stores in *ud the
opaque pointer passed to lua_newstate.
lua_getfenv[-0, +1, -]
void lua_getfenv (lua_State *L, int index);
Pushes onto the stack the environment table of the value at the given index.
lua_getfield[-0, +1, e]
void lua_getfield (lua_State *L, int index, const char *k);
Pushes onto the stack the value t[k],
where t is the value at the given valid index.
As in Lua, this function may trigger a metamethod
for the "index" event (see §2.8).
lua_getglobal[-0, +1, e]
void lua_getglobal (lua_State *L, const char *name);
Pushes onto the stack the value of the global name.
It is defined as a macro:
#define lua_getglobal(L,s) lua_getfield(L, LUA_GLOBALSINDEX, s)
lua_getmetatable[-0, +(0|1), -]
int lua_getmetatable (lua_State *L, int index);
Pushes onto the stack the metatable of the value at the given acceptable index. If the index is not valid, or if the value does not have a metatable, the function returns 0 and pushes nothing on the stack.
lua_gettable[-1, +1, e]
void lua_gettable (lua_State *L, int index);
Pushes onto the stack the value t[k],
where t is the value at the given valid index
and k is the value at the top of the stack.
This function pops the key from the stack (putting the resulting value in its place). As in Lua, this function may trigger a metamethod for the "index" event (see §2.8).
lua_gettop[-0, +0, -]
int lua_gettop (lua_State *L);
Returns the index of the top element in the stack. Because indices start at 1, this result is equal to the number of elements in the stack (and so 0 means an empty stack).
lua_insert[-1, +1, -]
void lua_insert (lua_State *L, int index);
Moves the top element into the given valid index, shifting up the elements above this index to open space. Cannot be called with a pseudo-index, because a pseudo-index is not an actual stack position.
lua_Integertypedef ptrdiff_t lua_Integer;
The type used by the Lua API to represent integral values.
By default it is a ptrdiff_t,
which is usually the largest signed integral type the machine handles
"comfortably".
lua_isboolean[-0, +0, -]
int lua_isboolean (lua_State *L, int index);
Returns 1 if the value at the given acceptable index has type boolean, and 0 otherwise.
lua_iscfunction[-0, +0, -]
int lua_iscfunction (lua_State *L, int index);
Returns 1 if the value at the given acceptable index is a C function, and 0 otherwise.
lua_isfunction[-0, +0, -]
int lua_isfunction (lua_State *L, int index);
Returns 1 if the value at the given acceptable index is a function (either C or Lua), and 0 otherwise.
lua_islightuserdata[-0, +0, -]
int lua_islightuserdata (lua_State *L, int index);
Returns 1 if the value at the given acceptable index is a light userdata, and 0 otherwise.
lua_isnil[-0, +0, -]
int lua_isnil (lua_State *L, int index);
Returns 1 if the value at the given acceptable index is nil, and 0 otherwise.
lua_isnone[-0, +0, -]
int lua_isnone (lua_State *L, int index);
Returns 1 if the given acceptable index is not valid (that is, it refers to an element outside the current stack), and 0 otherwise.
lua_isnoneornil[-0, +0, -]
int lua_isnoneornil (lua_State *L, int index);
Returns 1 if the given acceptable index is not valid (that is, it refers to an element outside the current stack) or if the value at this index is nil, and 0 otherwise.
lua_isnumber[-0, +0, -]
int lua_isnumber (lua_State *L, int index);
Returns 1 if the value at the given acceptable index is a number or a string convertible to a number, and 0 otherwise.
lua_isstring[-0, +0, -]
int lua_isstring (lua_State *L, int index);
Returns 1 if the value at the given acceptable index is a string or a number (which is always convertible to a string), and 0 otherwise.
lua_istable[-0, +0, -]
int lua_istable (lua_State *L, int index);
Returns 1 if the value at the given acceptable index is a table, and 0 otherwise.
lua_isthread[-0, +0, -]
int lua_isthread (lua_State *L, int index);
Returns 1 if the value at the given acceptable index is a thread, and 0 otherwise.
lua_isuserdata[-0, +0, -]
int lua_isuserdata (lua_State *L, int index);
Returns 1 if the value at the given acceptable index is a userdata (either full or light), and 0 otherwise.
lua_lessthan[-0, +0, e]
int lua_lessthan (lua_State *L, int index1, int index2);
Returns 1 if the value at acceptable index index1 is smaller
than the value at acceptable index index2,
following the semantics of the Lua < operator
(that is, may call metamethods).
Otherwise returns 0.
Also returns 0 if any of the indices is non valid.
lua_load[-0, +1, -]
int lua_load (lua_State *L,
lua_Reader reader,
void *data,
const char *chunkname);
Loads a Lua chunk.
If there are no errors,
lua_load pushes the compiled chunk as a Lua
function on top of the stack.
Otherwise, it pushes an error message.
The return values of lua_load are:
LUA_ERRSYNTAX:
syntax error during pre-compilation;LUA_ERRMEM:
memory allocation error.This function only loads a chunk; it does not run it.
lua_load automatically detects whether the chunk is text or binary,
and loads it accordingly (see program luac).
The lua_load function uses a user-supplied reader function
to read the chunk (see lua_Reader).
The data argument is an opaque value passed to the reader function.
The chunkname argument gives a name to the chunk,
which is used for error messages and in debug information (see §3.8).
lua_newstate[-0, +0, -]
lua_State *lua_newstate (lua_Alloc f, void *ud);
Creates a new, independent state.
Returns NULL if cannot create the state
(due to lack of memory).
The argument f is the allocator function;
Lua does all memory allocation for this state through this function.
The second argument, ud, is an opaque pointer that Lua
simply passes to the allocator in every call.
lua_newtable[-0, +1, m]
void lua_newtable (lua_State *L);
Creates a new empty table and pushes it onto the stack.
It is equivalent to lua_createtable(L, 0, 0).
lua_newthread[-0, +1, m]
lua_State *lua_newthread (lua_State *L);
Creates a new thread, pushes it on the stack,
and returns a pointer to a lua_State that represents this new thread.
The new state returned by this function shares with the original state
all global objects (such as tables),
but has an independent execution stack.
There is no explicit function to close or to destroy a thread. Threads are subject to garbage collection, like any Lua object.
lua_newuserdata[-0, +1, m]
void *lua_newuserdata (lua_State *L, size_t size);
This function allocates a new block of memory with the given size, pushes onto the stack a new full userdata with the block address, and returns this address.
Userdata represent C values in Lua. A full userdata represents a block of memory. It is an object (like a table): you must create it, it can have its own metatable, and you can detect when it is being collected. A full userdata is only equal to itself (under raw equality).
When Lua collects a full userdata with a gc metamethod,
Lua calls the metamethod and marks the userdata as finalized.
When this userdata is collected again then
Lua frees its corresponding memory.
lua_next[-1, +(2|0), e]
int lua_next (lua_State *L, int index);
Pops a key from the stack,
and pushes a key-value pair from the table at the given index
(the "next" pair after the given key).
If there are no more elements in the table,
then lua_next returns 0 (and pushes nothing).
A typical traversal looks like this:
/* table is in the stack at index 't' */
lua_pushnil(L); /* first key */
while (lua_next(L, t) != 0) {
/* uses 'key' (at index -2) and 'value' (at index -1) */
printf("%s - %s\n",
lua_typename(L, lua_type(L, -2)),
lua_typename(L, lua_type(L, -1)));
/* removes 'value'; keeps 'key' for next iteration */
lua_pop(L, 1);
}
While traversing a table,
do not call lua_tolstring directly on a key,
unless you know that the key is actually a string.
Recall that lua_tolstring changes
the value at the given index;
this confuses the next call to lua_next.
lua_Numbertypedef double lua_Number;
The type of numbers in Lua.
By default, it is double, but that can be changed in luaconf.h.
Through the configuration file you can change Lua to operate with another type for numbers (e.g., float or long).
lua_objlen[-0, +0, -]
size_t lua_objlen (lua_State *L, int index);
Returns the "length" of the value at the given acceptable index:
for strings, this is the string length;
for tables, this is the result of the length operator ('#');
for userdata, this is the size of the block of memory allocated
for the userdata;
for other values, it is 0.
lua_pcall[-(nargs + 1), +(nresults|1), -]
int lua_pcall (lua_State *L, int nargs, int nresults, int errfunc);
Calls a function in protected mode.
Both nargs and nresults have the same meaning as
in lua_call.
If there are no errors during the call,
lua_pcall behaves exactly like lua_call.
However, if there is any error,
lua_pcall catches it,
pushes a single value on the stack (the error message),
and returns an error code.
Like lua_call,
lua_pcall always removes the function
and its arguments from the stack.
If errfunc is 0,
then the error message returned on the stack
is exactly the original error message.
Otherwise, errfunc is the stack index of an
error handler function.
(In the current implementation, this index cannot be a pseudo-index.)
In case of runtime errors,
this function will be called with the error message
and its return value will be the message returned on the stack by lua_pcall.
Typically, the error handler function is used to add more debug
information to the error message, such as a stack traceback.
Such information cannot be gathered after the return of lua_pcall,
since by then the stack has unwound.
The lua_pcall function returns 0 in case of success
or one of the following error codes
(defined in lua.h):
LUA_ERRRUN:
a runtime error.
LUA_ERRMEM:
memory allocation error.
For such errors, Lua does not call the error handler function.
LUA_ERRERR:
error while running the error handler function.
lua_pop[-n, +0, -]
void lua_pop (lua_State *L, int n);
Pops n elements from the stack.
lua_pushboolean[-0, +1, -]
void lua_pushboolean (lua_State *L, int b);
Pushes a boolean value with value b onto the stack.
lua_pushcclosure[-n, +1, m]
void lua_pushcclosure (lua_State *L, lua_CFunction fn, int n);
Pushes a new C closure onto the stack.
When a C function is created,
it is possible to associate some values with it,
thus creating a C closure (see §3.4);
these values are then accessible to the function whenever it is called.
To associate values with a C function,
first these values should be pushed onto the stack
(when there are multiple values, the first value is pushed first).
Then lua_pushcclosure
is called to create and push the C function onto the stack,
with the argument n telling how many values should be
associated with the function.
lua_pushcclosure also pops these values from the stack.
lua_pushcfunction[-0, +1, m]
void lua_pushcfunction (lua_State *L, lua_CFunction f);
Pushes a C function onto the stack.
This function receives a pointer to a C function
and pushes onto the stack a Lua value of type function that,
when called, invokes the corresponding C function.
Any function to be registered in Lua must
follow the correct protocol to receive its parameters
and return its results (see lua_CFunction).
lua_pushcfunction is defined as a macro:
#define lua_pushcfunction(L,f) lua_pushcclosure(L,f,0)
lua_pushfstring[-0, +1, m]
const char *lua_pushfstring (lua_State *L, const char *fmt, ...);
Pushes onto the stack a formatted string
and returns a pointer to this string.
It is similar to the C function sprintf,
but has some important differences:
%%' (inserts a '%' in the string),
'%s' (inserts a zero-terminated string, with no size restrictions),
'%f' (inserts a lua_Number),
'%p' (inserts a pointer as a hexadecimal numeral),
'%d' (inserts an int), and
'%c' (inserts an int as a character).
lua_pushinteger[-0, +1, -]
void lua_pushinteger (lua_State *L, lua_Integer n);
Pushes a number with value n onto the stack.
lua_pushlightuserdata[-0, +1, -]
void lua_pushlightuserdata (lua_State *L, void *p);
Pushes a light userdata onto the stack.
Userdata represent C values in Lua. A light userdata represents a pointer. It is a value (like a number): you do not create it, it has no individual metatable, and it is not collected (as it was never created). A light userdata is equal to "any" light userdata with the same C address.
lua_pushliteral[-0, +1, m]
void lua_pushliteral (lua_State *L, const char *s);
This macro is equivalent to lua_pushlstring,
but can be used only when s is a literal string.
In these cases, it automatically provides the string length.
lua_pushlstring[-0, +1, m]
void lua_pushlstring (lua_State *L, const char *s, size_t len);
Pushes the string pointed to by s with size len
onto the stack.
Lua makes (or reuses) an internal copy of the given string,
so the memory at s can be freed or reused immediately after
the function returns.
The string can contain embedded zeros.
lua_pushnil[-0, +1, -]
void lua_pushnil (lua_State *L);
Pushes a nil value onto the stack.
lua_pushnumber[-0, +1, -]
void lua_pushnumber (lua_State *L, lua_Number n);
Pushes a number with value n onto the stack.
lua_pushstring[-0, +1, m]
void lua_pushstring (lua_State *L, const char *s);
Pushes the zero-terminated string pointed to by s
onto the stack.
Lua makes (or reuses) an internal copy of the given string,
so the memory at s can be freed or reused immediately after
the function returns.
The string cannot contain embedded zeros;
it is assumed to end at the first zero.
lua_pushthread[-0, +1, -]
int lua_pushthread (lua_State *L);
Pushes the thread represented by L onto the stack.
Returns 1 if this thread is the main thread of its state.
lua_pushvalue[-0, +1, -]
void lua_pushvalue (lua_State *L, int index);
Pushes a copy of the element at the given valid index onto the stack.
lua_pushvfstring[-0, +1, m]
const char *lua_pushvfstring (lua_State *L,
const char *fmt,
va_list argp);
Equivalent to lua_pushfstring, except that it receives a va_list
instead of a variable number of arguments.
lua_rawequal[-0, +0, -]
int lua_rawequal (lua_State *L, int index1, int index2);
Returns 1 if the two values in acceptable indices index1 and
index2 are primitively equal
(that is, without calling metamethods).
Otherwise returns 0.
Also returns 0 if any of the indices are non valid.
lua_rawget[-1, +1, -]
void lua_rawget (lua_State *L, int index);
Similar to lua_gettable, but does a raw access
(i.e., without metamethods).
lua_rawgeti[-0, +1, -]
void lua_rawgeti (lua_State *L, int index, int n);
Pushes onto the stack the value t[n],
where t is the value at the given valid index.
The access is raw;
that is, it does not invoke metamethods.
lua_rawset[-2, +0, m]
void lua_rawset (lua_State *L, int index);
Similar to lua_settable, but does a raw assignment
(i.e., without metamethods).
lua_rawseti[-1, +0, m]
void lua_rawseti (lua_State *L, int index, int n);
Does the equivalent of t[n] = v,
where t is the value at the given valid index
and v is the value at the top of the stack.
This function pops the value from the stack. The assignment is raw; that is, it does not invoke metamethods.
lua_Readertypedef const char * (*lua_Reader) (lua_State *L,
void *data,
size_t *size);
The reader function used by lua_load.
Every time it needs another piece of the chunk,
lua_load calls the reader,
passing along its data parameter.
The reader must return a pointer to a block of memory
with a new piece of the chunk
and set size to the block size.
The block must exist until the reader function is called again.
To signal the end of the chunk, the reader must return NULL.
The reader function may return pieces of any size greater than zero.
lua_register[-0, +0, e]
void lua_register (lua_State *L,
const char *name,
lua_CFunction f);
Sets the C function f as the new value of global name.
It is defined as a macro:
#define lua_register(L,n,f) \
(lua_pushcfunction(L, f), lua_setglobal(L, n))
lua_remove[-1, +0, -]
void lua_remove (lua_State *L, int index);
Removes the element at the given valid index, shifting down the elements above this index to fill the gap. Cannot be called with a pseudo-index, because a pseudo-index is not an actual stack position.
lua_replace[-1, +0, -]
void lua_replace (lua_State *L, int index);
Moves the top element into the given position (and pops it), without shifting any element (therefore replacing the value at the given position).
lua_resume[-?, +?, -]
int lua_resume (lua_State *L, int narg);
Starts and resumes a coroutine in a given thread.
To start a coroutine, you first create a new thread
(see lua_newthread);
then you push onto its stack the main function plus any arguments;
then you call lua_resume,
with narg being the number of arguments.
This call returns when the coroutine suspends or finishes its execution.
When it returns, the stack contains all values passed to lua_yield,
or all values returned by the body function.
lua_resume returns
LUA_YIELD if the coroutine yields,
0 if the coroutine finishes its execution
without errors,
or an error code in case of errors (see lua_pcall).
In case of errors,
the stack is not unwound,
so you can use the debug API over it.
The error message is on the top of the stack.
To restart a coroutine, you put on its stack only the values to
be passed as results from yield,
and then call lua_resume.
lua_setallocf[-0, +0, -]
void lua_setallocf (lua_State *L, lua_Alloc f, void *ud);
Changes the allocator function of a given state to f
with user data ud.
lua_setfenv[-1, +0, -]
int lua_setfenv (lua_State *L, int index);
Pops a table from the stack and sets it as
the new environment for the value at the given index.
If the value at the given index is
neither a function nor a thread nor a userdata,
lua_setfenv returns 0.
Otherwise it returns 1.
lua_setfield[-1, +0, e]
void lua_setfield (lua_State *L, int index, const char *k);
Does the equivalent to t[k] = v,
where t is the value at the given valid index
and v is the value at the top of the stack.
This function pops the value from the stack. As in Lua, this function may trigger a metamethod for the "newindex" event (see §2.8).
lua_setglobal[-1, +0, e]
void lua_setglobal (lua_State *L, const char *name);
Pops a value from the stack and
sets it as the new value of global name.
It is defined as a macro:
#define lua_setglobal(L,s) lua_setfield(L, LUA_GLOBALSINDEX, s)
lua_setmetatable[-1, +0, -]
int lua_setmetatable (lua_State *L, int index);
Pops a table from the stack and sets it as the new metatable for the value at the given acceptable index.
lua_settable[-2, +0, e]
void lua_settable (lua_State *L, int index);
Does the equivalent to t[k] = v,
where t is the value at the given valid index,
v is the value at the top of the stack,
and k is the value just below the top.
This function pops both the key and the value from the stack. As in Lua, this function may trigger a metamethod for the "newindex" event (see §2.8).
lua_settop[-?, +?, -]
void lua_settop (lua_State *L, int index);
Accepts any acceptable index, or 0,
and sets the stack top to this index.
If the new top is larger than the old one,
then the new elements are filled with nil.
If index is 0, then all stack elements are removed.
lua_Statetypedef struct lua_State lua_State;
Opaque structure that keeps the whole state of a Lua interpreter. The Lua library is fully reentrant: it has no global variables. All information about a state is kept in this structure.
A pointer to this state must be passed as the first argument to
every function in the library, except to lua_newstate,
which creates a Lua state from scratch.
lua_status[-0, +0, -]
int lua_status (lua_State *L);
Returns the status of the thread L.
The status can be 0 for a normal thread,
an error code if the thread finished its execution with an error,
or LUA_YIELD if the thread is suspended.
lua_toboolean[-0, +0, -]
int lua_toboolean (lua_State *L, int index);
Converts the Lua value at the given acceptable index to a C boolean
value (0 or 1).
Like all tests in Lua,
lua_toboolean returns 1 for any Lua value
different from false and nil;
otherwise it returns 0.
It also returns 0 when called with a non-valid index.
(If you want to accept only actual boolean values,
use lua_isboolean to test the value's type.)
lua_tocfunction[-0, +0, -]
lua_CFunction lua_tocfunction (lua_State *L, int index);
Converts a value at the given acceptable index to a C function.
That value must be a C function;
otherwise, returns NULL.
lua_tointeger[-0, +0, -]
lua_Integer lua_tointeger (lua_State *L, int index);
Converts the Lua value at the given acceptable index
to the signed integral type lua_Integer.
The Lua value must be a number or a string convertible to a number
(see §2.2.1);
otherwise, lua_tointeger returns 0.
If the number is not an integer, it is truncated in some non-specified way.
lua_tolstring[-0, +0, m]
const char *lua_tolstring (lua_State *L, int index, size_t *len);
Converts the Lua value at the given acceptable index to a C string.
If len is not NULL,
it also sets *len with the string length.
The Lua value must be a string or a number;
otherwise, the function returns NULL.
If the value is a number,
then lua_tolstring also
changes the actual value in the stack to a string.
(This change confuses lua_next
when lua_tolstring is applied to keys during a table traversal.)
lua_tolstring returns a fully aligned pointer
to a string inside the Lua state.
This string always has a zero ('\0')
after its last character (as in C),
but may contain other zeros in its body.
Because Lua has garbage collection,
there is no guarantee that the pointer returned by lua_tolstring
will be valid after the corresponding value is removed from the stack.
lua_tonumber[-0, +0, -]
lua_Number lua_tonumber (lua_State *L, int index);
Converts the Lua value at the given acceptable index
to the C type lua_Number (see lua_Number).
The Lua value must be a number or a string convertible to a number
(see §2.2.1);
otherwise, lua_tonumber returns 0.
lua_topointer[-0, +0, -]
const void *lua_topointer (lua_State *L, int index);
Converts the value at the given acceptable index to a generic
C pointer (void*).
The value may be a userdata, a table, a thread, or a function;
otherwise, lua_topointer returns NULL.
Different objects will give different pointers.
There is no way to convert the pointer back to its original value.
Typically this function is used only for debug information.
lua_tostring[-0, +0, m]
const char *lua_tostring (lua_State *L, int index);
Equivalent to lua_tolstring with len equal to NULL.
lua_tothread[-0, +0, -]
lua_State *lua_tothread (lua_State *L, int index);
Converts the value at the given acceptable index to a Lua thread
(represented as lua_State*).
This value must be a thread;
otherwise, the function returns NULL.
lua_touserdata[-0, +0, -]
void *lua_touserdata (lua_State *L, int index);
If the value at the given acceptable index is a full userdata,
returns its block address.
If the value is a light userdata,
returns its pointer.
Otherwise, returns NULL.
lua_type[-0, +0, -]
int lua_type (lua_State *L, int index);
Returns the type of the value in the given acceptable index,
or LUA_TNONE for a non-valid index
(that is, an index to an "empty" stack position).
The types returned by lua_type are coded by the following constants
defined in lua.h:
LUA_TNIL,
LUA_TNUMBER,
LUA_TBOOLEAN,
LUA_TSTRING,
LUA_TTABLE,
LUA_TFUNCTION,
LUA_TUSERDATA,
LUA_TTHREAD,
and
LUA_TLIGHTUSERDATA.
lua_typename[-0, +0, -]
const char *lua_typename (lua_State *L, int tp);
Returns the name of the type encoded by the value tp,
which must be one the values returned by lua_type.
lua_Writertypedef int (*lua_Writer) (lua_State *L,
const void* p,
size_t sz,
void* ud);
The type of the writer function used by lua_dump.
Every time it produces another piece of chunk,
lua_dump calls the writer,
passing along the buffer to be written (p),
its size (sz),
and the data parameter supplied to lua_dump.
The writer returns an error code:
0 means no errors;
any other value means an error and stops lua_dump from
calling the writer again.
lua_xmove[-?, +?, -]
void lua_xmove (lua_State *from, lua_State *to, int n);
Exchange values between different threads of the same global state.
This function pops n values from the stack from,
and pushes them onto the stack to.
lua_yield[-?, +?, -]
int lua_yield (lua_State *L, int nresults);
Yields a coroutine.
This function should only be called as the return expression of a C function, as follows:
return lua_yield (L, nresults);
When a C function calls lua_yield in that way,
the running coroutine suspends its execution,
and the call to lua_resume that started this coroutine returns.
The parameter nresults is the number of values from the stack
that are passed as results to lua_resume.
Lua has no built-in debugging facilities. Instead, it offers a special interface by means of functions and hooks. This interface allows the construction of different kinds of debuggers, profilers, and other tools that need "inside information" from the interpreter.
lua_Debugtypedef struct lua_Debug {
int event;
const char *name; /* (n) */
const char *namewhat; /* (n) */
const char *what; /* (S) */
const char *source; /* (S) */
int currentline; /* (l) */
int nups; /* (u) number of upvalues */
int linedefined; /* (S) */
int lastlinedefined; /* (S) */
char short_src[LUA_IDSIZE]; /* (S) */
/* private part */
other fields
} lua_Debug;
A structure used to carry different pieces of
information about an active function.
lua_getstack fills only the private part
of this structure, for later use.
To fill the other fields of lua_Debug with useful information,
call lua_getinfo.
The fields of lua_Debug have the following meaning:
source:
If the function was defined in a string,
then source is that string.
If the function was defined in a file,
then source starts with a '@' followed by the file name.
short_src:
a "printable" version of source, to be used in error messages.
linedefined:
the line number where the definition of the function starts.
lastlinedefined:
the line number where the definition of the function ends.
what:
the string "Lua" if the function is a Lua function,
"C" if it is a C function,
"main" if it is the main part of a chunk,
and "tail" if it was a function that did a tail call.
In the latter case,
Lua has no other information about the function.
currentline:
the current line where the given function is executing.
When no line information is available,
currentline is set to -1.
name:
a reasonable name for the given function.
Because functions in Lua are first-class values,
they do not have a fixed name:
some functions may be the value of multiple global variables,
while others may be stored only in a table field.
The lua_getinfo function checks how the function was
called to find a suitable name.
If it cannot find a name,
then name is set to NULL.
namewhat:
explains the name field.
The value of namewhat can be
"global", "local", "method",
"field", "upvalue", or "" (the empty string),
according to how the function was called.
(Lua uses the empty string when no other option seems to apply.)
nups:
the number of upvalues of the function.
lua_gethook[-0, +0, -]
lua_Hook lua_gethook (lua_State *L);
Returns the current hook function.
lua_gethookcount[-0, +0, -]
int lua_gethookcount (lua_State *L);
Returns the current hook count.
lua_gethookmask[-0, +0, -]
int lua_gethookmask (lua_State *L);
Returns the current hook mask.
lua_getinfo<