Lua 5.2 Reference Manual

Copyright © 2010 Lua.org, PUC-Rio. All rights reserved.

1 - Introduction

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).

2 - The Language

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 in §8 at the end of this manual.

2.1 - Lexical Conventions

Lua is a free-form language. It ignores spaces (including new lines) and comments between lexical elements (tokens), except as delimiters between names and keywords.

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. 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 can 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 can contain any 8-bit value, including embedded zeros, which can be specified as '\0'.

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. It can contain any text except a closing bracket of the proper level. Literals in this bracketed form can run for several lines, do not interpret any escape sequences, and ignore long brackets of any other level. Any kind of end-of-line sequence (carriage return, newline, carriage return followed by newline, or newline followed by carriage return) is converted to a simple newline. You should not use long strings for non-text data; Use instead a regular quoted literal with explicit escape sequences for control characters.

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 literal strings below denote the same string:

     a = 'alo\n123"'
     a = "alo\n123\""
     a = '\97lo\10\04923"'
     a = [[alo
     123"]]
     a = [==[
     alo
     123"]==]

A numerical constant can 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.

2.2 - Values and Types

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 can 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 can be used to represent ordinary arrays, symbol tables, sets, records, graphs, trees, etc. To represent records, Lua uses the field name as an index. The language supports this representation by providing 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 can contain functions. Thus tables can 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.

2.2.1 - Coercion

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).

2.3 - Variables

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). 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 current environment (see §2.9). (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.)

2.4 - Statements

Lua supports an almost conventional set of statements, similar to those in Pascal or C. This set includes assignments, control structures, function calls, and variable declarations.

2.4.1 - Chunks

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 can be stored in a file or in a string inside the host program. To execute a chunk, Lua first pre-compiles the chunk into instructions for a virtual machine, and then it executes the compiled code with an interpreter for the virtual machine.

Chunks can 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.

2.4.2 - Blocks

A block is a list of statements; syntactically, a block is the same as a chunk:

	block ::= chunk

A block can 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).

2.4.3 - Assignment

Lua allows multiple assignments. Therefore, the syntax for assignment defines a list of variables on the left side and a list of expressions on the right side. The elements in both lists are separated by commas:

	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 that call enter 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, and

     x, y, z = y, z, x

cyclically permutes the values of x, y, and z.

The meaning of assignments to global variables and table fields can be changed via metatables. An assignment to an indexed variable t[i] = val is equivalent to settable_event(t,i,val). (See §2.8 for a complete description of the settable_event function. This function is not defined or callable in Lua. We use it here only for explanatory purposes.)

An assignment to a global variable x = val is equivalent to the assignment _env.x = val, which in turn is equivalent to

     settable_event(_env, "x", val)

where _env is the environment of the running function. (The _env variable is not defined in Lua. We use it here only for explanatory purposes.)

2.4.4 - Control Structures

The control structures if, while, and repeat have the usual meaning and familiar syntax:

	stat ::= while exp do block end
	stat ::= repeat block until exp
	stat ::= if exp then block {elseif exp then block} [else block] end

Lua also has a for statement, in two flavors (see §2.4.5).

The condition expression of a control structure can 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 can return more than one value, and 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.

2.4.5 - For Statement

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:

• All three control expressions are evaluated only once, before the loop starts. They must all result in numbers.
• var, limit, and step are invisible variables. The names shown here are for explanatory purposes only.
• If the third expression (the step) is absent, then a step of 1 is used.
• You can use break to exit a for loop.
• The loop variable 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.
• You can use break to exit a for loop.
• The loop variables 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.

2.4.6 - Function Calls as Statements

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.

2.4.7 - Local Declarations

Local variables can be declared anywhere inside a block. The declaration can 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.

2.4.8 - Lexical Environments

A lexical environment defines a new current environment (see §2.9) for the code inside its block:

	stat ::= in exp do block end

That is, a lexical environment changes the table used to resolve all accesses to global (free) variables inside a block.

Inside a lexical environment, the result of exp becomes the current environment. Expression exp is evaluated only once in the beginning of the statement and it is stored in a hidden local variable named (environment). Then, any global variable (that is, a variable not declared as a local) var inside block is accessed as (environment).var. Moreover, functions defined inside the block also use the current environment as their environments (see §2.9).

A lexical environment does not shadow local declarations. That is, any local variable that is visible just before a lexical environment is still visible inside the environment.

2.5 - Expressions

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 can result in multiple values. If an expression is used as a statement (only possible for function calls (see §2.4.6)), then its return list is adjusted to zero elements, thus discarding all returned values. If an expression is used as the last (or the only) element of a list of expressions, then no adjustment is made (unless the call is enclosed in parentheses). In all other contexts, Lua adjusts the result list to one element, discarding all values except the first one.

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 can get nil if there
                        -- is no corresponding vararg parameter)
     
     a,b,c = x, f()     -- f() is adjusted to 2 results
     a,b,c = f()        -- f() is adjusted to 3 results
     return f()         -- returns all results from f()
     return ...         -- returns all received vararg parameters
     return x,y,f()     -- returns x, y, and all results from f()
     {f()}              -- creates a list with all results from f()
     {...}              -- creates a list with all vararg parameters
     {f(), nil}         -- f() is adjusted to 1 result

Any 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.)

2.5.1 - Arithmetic Operators

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.

2.5.2 - Relational Operators

The relational operators in Lua are

     ==    ~=    <     >     <=    >=

These operators always result in false or true.

Equality (==) first compares the type of its operands. If the types are different, then the result is false. Otherwise, the values of the operands are compared. Numbers and strings are compared in the usual way. Objects (tables, userdata, threads, and functions) are compared by reference: two objects are considered equal only if they are the same object. Every time you create a new object (a table, userdata, 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). A comparison a > b is translated to b < a and a >= b is translated to b <= a.

2.5.3 - Logical Operators

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.)

2.5.4 - Concatenation

The string concatenation operator in Lua is denoted by two dots ('..'). If both operands are strings or numbers, then they are converted to strings according to the rules mentioned in §2.2.1. Otherwise, the "concat" metamethod is called (see §2.8).

2.5.5 - The Length Operator

The length operator is denoted by the unary prefix 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 can be zero. For a regular array, where all non-nil values have keys 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 can 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).

A program can modify the behavior of the length operator for any value but strings through metamethods (see §2.8).

2.5.6 - Precedence

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.

2.5.7 - Table Constructors

Table constructors are expressions that create tables. Every time a constructor is evaluated, a new table is created. A constructor can be used to create an empty table or to create a table and initialize some of its fields. The general syntax for constructors is

	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 can have an optional trailing separator, as a convenience for machine-generated code.

2.5.8 - Function Calls

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-form character 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). In this case, 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

2.5.9 - Function Definitions

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 can refer to different external local variables and can 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 that last expression 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

2.6 - Visibility Rules

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.

2.7 - Error Handling

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.

2.8 - Metatables

Every value in Lua can 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 by 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; that is, there is one single metatable for all numbers, one for all strings, etc. By default, a value has no metatable, but the string library sets a metatable for the string type (see §5.4).

A metatable controls how an object behaves in arithmetic operations, order comparisons, concatenation, length operation, and indexing. A metatable also can define a function to be called when a userdata is garbage collected. For each of these operations Lua associates a specific key called an 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).

• "add": the + 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
  

• "sub": the - operation. Behavior similar to the "add" operation.
• "mul": the * operation. Behavior similar to the "add" operation.
• "div": the / operation. Behavior similar to the "add" operation.
• "mod": the % operation. Behavior similar to the "add" operation, with the operation o1 - floor(o1/o2)*o2 as the primitive operation.
• "pow": the ^ (exponentiation) operation. Behavior similar to the "add" operation, with the function pow (from the C math library) as the primitive operation.
• "unm": the unary - 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
  

• "concat": the .. (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
  

• "len": the # operation.

       function len_event (op)
         if type(op) == "string" then
           return strlen(op)         -- primitive string length
         else
           local h = metatable(op).__len
           if h then
             return (h(op))     -- call handler with the operand
           elseif type(op) == "table" then
             return #op                -- primitive table length
           else  -- no handler available: error
             error(···)
           end
         end
       end
  

  See §2.5.5 for a description of the length of a table.

• "eq": the == 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).

• "lt": the < 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.

• "le": the <= 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).

• "index": The indexing access 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
  

• "newindex": The indexing assignment 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
  

• "call": called when Lua calls a value.

       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
  

2.9 - Environments

Each function and userdata in Lua has a table associated with it, called its environment. Like metatables, environments are regular tables and multiple objects can share the same environment. >From Lua, you can manipulate the environment of an object only through the debug library (see §5.10).

Userdata and C functions are created sharing the environment of the creating C function. Non-nested Lua functions (created by loadfile, loadstring or load) are created sharing the global environment. Nested Lua functions are created sharing the current environment of where it is defined.

Environments associated with userdata have no meaning for Lua. It is only a convenience feature for programmers to associate a table to a userdata.

The environment associated with a C function can be directly accessed by C code (see §3.3). It is used as the default environment for other C functions and userdata created by the function.

The environment associated with a Lua function is used as the current environment of all code in the function outside a lexical environment (see §2.4.8). A lexical environment changes the current environment inside its scope. In any case, the current environment is the table Lua uses to resolve global variables and to initialize the environment of nested functions.

2.10 - Garbage Collection

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, objects that are no longer accessible from Lua). All memory used by Lua is subject to automatic management: tables, userdata, functions, threads, strings, etc.

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. Both use percentage points as units (so that a value of 100 means an internal value of 1).

The garbage-collector pause controls how long the collector waits before starting a new cycle. Larger values make the collector less aggressive. Values smaller than 100 mean the collector will not wait to start a new cycle. A value of 200 means that the collector waits for the total memory in use to double before starting a new cycle.

The step multiplier controls the relative speed of the collector relative to memory allocation. Larger values make the collector more aggressive but also increase the size of each incremental step. Values smaller than 100 make the collector too slow and can result in the collector never finishing a cycle. The default, 200, means that the collector runs at "twice" the speed of memory allocation.

If you set the step multiplier to a very large number (such as 2^30), the collector behaves like a stop-the-world collector. If you then set the pause to 200, the collector behaves as in old Lua versions, doing a complete collection every time Lua doubles its memory usage.

You can change these numbers by calling lua_gc in C or collectgarbage in Lua. With these functions you can also control the collector directly (e.g., stop and restart it).

2.10.1 - Garbage-Collection Metamethods

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.

2.10.2 - Weak Tables

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.

Userdata with finalizers has a special behavior in weak tables. When a userdata is a value in a weak table, it is removed from the table the first time it is collected, before running its finalizer. When it is a key, however, it is removed from the table only when it is really freed, after running its finalizer. This behavior allows the finalizer to access values associated with the userdata through weak tables.

A table with weak keys and strong values is also called an ephemeron table. In an ephemeron table, a value is considered reachable only if its key is reachable. In particular, if the only reference to a key comes from its value, the pair is removed.

After you use a table as a metatable, you should not change the value of its __mode field. Otherwise, the weak behavior of the tables controlled by this metatable is undefined.

2.11 - Coroutines

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 a 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

3 - The Application Program Interface

This section describes the C API for Lua, that is, the set of C functions available to the host program to communicate with Lua. All API functions and related types and constants are declared in the header file lua.h.

Even when we use the term "function", any facility in the API may be provided as a macro instead. All such macros use each of 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.

3.1 - The Stack

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).

3.2 - Stack Size

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 ensure that the stack has extra slots when pushing new elements.

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.

When you call a Lua function without a fixed number of results (see lua_call), Lua ensures that the stack has enough size for all results, but it does not ensure any extra space. So, before pushing anything in the stack after such a call you should use lua_checkstack.

Most query functions accept as indices any value inside the available stack space, that is, indices up to the maximum stack size that 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.

3.3 - Pseudo-Indices

Unless otherwise noted, any function that accepts valid indices also accepts 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 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_ENVIRONINDEX, varname);

3.4 - C Closures

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 (but not greater than 256), produces an acceptable (but invalid) index.

3.5 - Registry

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 by some predefined values. Therefore, integer keys should not be used for other purposes.

When you create a new Lua state, its registry comes with some predefined values. These predefined values are indexed with integer keys defined as constants in lua.h. The following constants are defined:

• LUA_RIDX_MAINTHREAD: At this index the registry has the main thread of the state. (The main thread is the one created together with the state.)
• LUA_RIDX_CPCALL: At this index the registry has the cpcall function. This function allows you to call any lua_CFunction without creating a closure for it, therefore preventing an unprotected memory allocation error.

  The address of the function to be called must be stored in a variable, whose address is passed as the first argument to cpcall, as a light userdata. Note that the light userdata in the first argument should not point to the function to be called, but to a variable containing the pointer to the function. ANSI C does not allow the direct assignment of a function address to the void* in a light userdata.

  Other arguments to cpcall are passed to the called function. The cpcall function itself should be called with lua_pcall, like any C function.

• LUA_RIDX_GLOBALS: At this index the registry has the global environment. This is the C equivalent to the _G global variable.

3.6 - Error Handling in C

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 a memory allocation error, 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.

If an error happens outside any protected environment, Lua calls a panic function (see lua_atpanic) and then calls abort, thus exiting the host application. Your panic function can avoid this exit by never returning (e.g., doing a long jump).

Most functions in the API can throw an error, for instance due to a memory allocation error. The documentation for each function indicates whether it can throw errors.

Inside a C function you can throw an error by calling lua_error.

3.7 - Handling Yields in C

Internally, Lua uses the C longjmp facility to yield a coroutine. Therefore, if a function foo calls an API function and this API function yields (directly or indirectly by calling another function that yields), Lua cannot return to foo any more, because the longjmp removes its frame from the C stack.

To avoid this kind of problem, Lua raises an error whenever it tries to yield accross an API call, except for three functions: lua_yieldk, lua_callk, and lua_pcallk. All those functions receive a continuation function (as a parameter called k) to continue execution after an yield.

To explain continuations, let us set some terminology. We have a C function called from Lua which we will call the original function. This original function may call one of those three functions in the C API, which we will call the callee function, that may yield the current thread. (This will happen when the callee function is lua_yieldk, or when the callee function is either lua_callk or lua_pcallk and the function called by them yields.)

Suppose the running thread yields while executing the callee function. After the thread resumes, it eventually will finish running the callee function. However, the callee function cannot return to the original function, because its frame in the C stack was destroyed by the yield. Instead, Lua calls a continuation function, which was given as an argument to the callee function. As the name implies, the continuation function should continue the task of the original function.

Lua treats the continuation function as if it was the original function. The continuation function receives the same Lua stack from the original function, in the same state it would be if the callee function had returned. (For instance, after a lua_callk the function and its arguments are removed from the stack and replaced by the results from the call.) It also has the same upvalues. Whatever it returns is handled by Lua as if it was the return of the original function.

The only difference in the Lua state between the original function and its continuation is the result of a call to lua_getctx.

3.8 - Functions and Types

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 can 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_Alloc

typedef 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 or some code about what is being allocated; nsize, the new size of the block.

When ptr is not NULL, osize is the previous size of the block pointed by ptr, that is, the size given when it was allocated or reallocated.

When ptr is NULL, osize codes the kind of object that Lua is allocating. osize is any of LUA_TSTRING, LUA_TTABLE, LUA_TFUNCTION, LUA_TUSERDATA, or LUA_TTHREAD when (and only when) Lua is creating a new object of that type. When osize is some other value, Lua is allocating memory for something else.

Lua assumes the following behavior from the allocator function:

When nsize is zero: if ptr is not NULL, the allocator should free the block pointed to by ptr. Anyway, the allocator must return NULL.

When nsize is not zero: if ptr is NULL, the allocator should behave like malloc; when ptr is not NULL, the allocator behaves like realloc. The allocator returns NULL if and only if it cannot fill the request. 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. It also assumes that realloc does not fail when shrinking a block. (ANSI C does not ensure this behavior, but it seems a safe assumption.)

lua_atpanic

[-0, +0, -]

lua_CFunction lua_atpanic (lua_State *L, lua_CFunction panicf);

Sets a new panic function and returns the old one (see §3.6).

The panic function should not try to run anything on the failed Lua state. However, it can still use the debug API (see §3.9) to gather information about the state. In particular, the error message is 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 can do the equivalent to this Lua code:

     a = f("how", t.x, 14)

Here it is in C:

     lua_getfield(L, LUA_ENVIRONINDEX, "f"); /* function to be called */
     lua_pushstring(L, "how");                        /* 1st argument */
     lua_getfield(L, LUA_ENVIRONINDEX, "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_ENVIRONINDEX, "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_callk

[-(nargs + 1), +nresults, e]

void lua_callk (lua_State *L, int nargs, int nresults,
                          int ctx, lua_CFunction k);

This function behaves exactly like lua_call, but allows the called function to yield (see §3.7).

lua_CFunction

typedef 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, -]

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 grant the request, because it would cause the stack to be larger than a fixed maximum size (typically at least a few thousand elements) or because it cannot allocate memory for the new stack 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_compare

[-0, +0, e]

int lua_compare (lua_State *L, int index1, int index2, int op);

Returns 1 if the value at acceptable index index1 satisfies op when compared with the value at acceptable index index2, following the semantics of the corresponding Lua operator (that is, may call metamethods). Otherwise returns 0. Also returns 0 if any of the indices is non valid.

The value of op must be one of the following constants:

• @LUA_OPEQ:compares for equality (==)
• @LUA_OPLT:compares for less than (<)
• @LUA_OPLE:compares for less or equal (<=)

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_copy

[-0, +0, -]

void lua_copy (lua_State *L, int fromidx, int toidx);

Moves the element at the valid index fromidx into the valid index toidx without shifting any element (therefore replacing the value at that position).

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_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 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 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_GCISRUNNING: returns a boolean that tells whether the collector is running (i.e., not stopped).

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_getctx

[-0, +0, -]

int lua_getctx  (lua_State *L, int *ctx);

This function is called by a continuation function (see §3.7) to retrieve the status of the thread and a context information.

When called in the original function, lua_getctx always returns LUA_OK and does not change the value of its argument ctx. When called inside a continuation function, lua_getctx returns LUA_YIELD and sets the value of ctx to be the context information (the value passed as the ctx argument to the callee together with the continuation function).

When the callee is lua_pcallk, Lua may also call its continuation function to handle errors during the call. That is, upon an error in the function called by lua_pcallk, Lua may not return lua_pcallk but instead may call the continuation function. In that case, a call to lua_getctx will return the error code (the value that would be returned by lua_pcallk); the value of ctx will be set to the context information, as in the case of an yield.

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_ENVIRONINDEX, 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_Integer

typedef 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_len

[-0, +1, e]

void lua_len (lua_State *L, int index);

Returns the "length" of the value at the given acceptable index; it is equivalent to the '#' operator in Lua (see §2.5.5). The result is pushed on the stack.

lua_load

[-0, +1, -]

int lua_load (lua_State *L,
              lua_Reader reader,
              void *data,
              const char *source);

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_OK (0): no errors;
• LUA_ERRSYNTAX: syntax error during pre-compilation;
• LUA_ERRMEM: memory allocation error.
• LUA_ERRGCMM: error while running a __gc metamethod. (This error has no relation with the chunk being loaded. It is generated by the garbage collector.)

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 source argument gives a name to the chunk, which is used for error messages and in debug information (see §3.9).

lua_newstate

[-0, +0, -]

lua_State *lua_newstate (lua_Alloc f, void *ud);

Creates a new thread running in a new, independent state. Returns NULL if cannot create the thread/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 thread returned by this function shares with the original thread 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_Number

typedef double lua_Number;

The type of numbers in Lua. By default, it is double, but that can be changed in luaconf.h. Through this configuration file you can change Lua to operate with another type for numbers (e.g., float or long).

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 one of the following codes (defined in lua.h):

• LUA_OK (0): success.
• 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_ERRGCMM: error while running a __gc metamethod. (This error typically has no relation with the function being called. It is generated by the garbage collector.)

lua_pcallk

[-(nargs + 1), +(nresults|1), -]

int lua_pcallk (lua_State *L, int nargs, int nresults, int errfunc,
                          int ctx, lua_CFunction k);

This function behaves exactly like lua_pcall, but allows the called function to yield (see §3.7).

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.

The maximum value for n is 255.

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:

• You do not have to allocate space for the result: the result is a Lua string and Lua takes care of memory allocation (and deallocation, through garbage collection).
• The conversion specifiers are quite restricted. There are no flags, widths, or precisions. The conversion specifiers can only be '%%' (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, a void*. 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]

const char *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.

Returns a pointer to the internal copy of the string.

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]

const char *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.

Returns a pointer to the internal copy of the string.

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 table at the given valid index. The access is raw; that is, it does not invoke metamethods.

lua_rawlen

[-0, +0, -]

size_t lua_rawlen (lua_State *L, int index);

Returns the raw "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 ('#') with no metamethods; for userdata, this is the size of the block of memory allocated for the userdata; for other values, it is 0.

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 table 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_Reader

typedef 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 or set size to zero. 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 without shifting any element (therefore replacing the value at the given position), and then pops the top element.

lua_resume

[-?, +?, -]

int lua_resume (lua_State *L, int narg);

Starts and resumes a coroutine in a given thread.

To start a coroutine, you push onto the thread 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, LUA_OK 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 resume 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 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_ENVIRONINDEX, 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_State

typedef 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 (LUA_OK) for a normal thread, an error code if the thread finished the execution of a lua_resume 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 true for any Lua value different from false and nil; otherwise it returns false. It also returns false 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 can 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 can 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_version

[-0, +0, v]

const lua_Number *lua_version (lua_State *L);

Returns the address of the version number stored in the Lua core. When called with a valid lua_State, returns the address of the version used to create that state. When called with NULL, returns the address of the version running the call.

lua_Writer

typedef 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);

This function is equivalent to lua_yieldk, but it has no continuation (see §3.7). Therefore, when the thread resumes, it returns to the function that called the function calling lua_yield.

lua_yieldk

[-?, +?, -]

int lua_yieldk  (lua_State *L, int nresults, int ctx,
                           lua_CFunction k);

Yields a coroutine.

This function should only be called as the return expression of a C function, as follows:

     return lua_yield (L, nresults, i, k);

When a C function calls lua_yieldk 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.

When the coroutine is resumed again, Lua calls the given continuation function k to continue the execution of the C function that yielded (see §3.7). This continuation function receives the same stack from the previous function, with the results removed and replaced by the arguments passed to lua_resume. Moreover, the continuation function may access the value ctx by calling lua_getctx.

3.9 - The Debug Interface

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_Debug

typedef 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 linedefined;            /* (S) */
  int lastlinedefined;        /* (S) */
  unsigned char nups;         /* (u) number of upvalues */
  unsigned char nparams;      /* (u) number of parameters */
  char isvararg;              /* (u) */
  char istailcall;            /* (t) */
  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: the source of the chunk that created the function. If source starts with a '@', it means that the function was defined in a file where the file name follows the '@'. If source starts with a '=', the rest of it should describe the source in a user-dependent manner. Otherwise, the function was defined in a string where source is that string.
• 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.
• 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 can be the value of multiple global variables, while others can 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.)
• istailcall: true if this function invocation was called by a tail call. In this case, the caller of this level is not in the stack.
• nups: the number of upvalues of the function.
• nparams: the number of fixed parameters of the function (always 0 for C functions).
• isvararg: whether the function is a vararg function (always true for C functions).

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

[-(0|1), +(0|1|2), m]

int lua_getinfo (lua_State *L, const char *what, lua_Debug *ar);

Returns information about a specific function or function invocation.

To get information about a function invocation, the parameter ar must be a valid activation record that was filled by a previous call to lua_getstack or given as argument to a hook (see lua_Hook).

To get information about a function you push it onto the stack and start the what string with the character '>'. (In that case, lua_getinfo pops the function from the top of the stack.) For instance, to know in which line a function f was defined, you can write the following code:

     lua_Debug ar;
     lua_getfield(L, LUA_ENVIRONINDEX, "f");  /* get global 'f' */
     lua_getinfo(L, ">S", &ar);
     printf("%d\n", ar.linedefined);

Each character in the string what selects some fields of the structure ar to be filled or a value to be pushed on the stack:

• 'n': fills in the field name and namewhat;
• 'S': fills in the fields source, short_src, linedefined, lastlinedefined, and what;
• 'l': fills in the field currentline;
• 't': fills in the field istailcall;
• 'u': fills in the field nups;
• 'f': pushes onto the stack the function that is running at the given level;
• 'L': pushes onto the stack a table whose indices are the numbers of the lines that are valid on the function. (A valid line is a line with some associated code, that is, a line where you can put a break point. Non-valid lines include empty lines and comments.)

This function returns 0 on error (for instance, an invalid option in what).

lua_getlocal

[-0, +(0|1), -]

const char *lua_getlocal (lua_State *L, lua_Debug *ar, int n);

Gets information about a local variable of a given activation record. The parameter ar must be a valid activation record that was filled by a previous call to lua_getstack or given as argument to a hook (see lua_Hook). The index n selects which local variable to inspect (1 is the first parameter or active local variable, and so on, until the last active local variable). lua_getlocal pushes the variable's value onto the stack and returns its name.

Variable names starting with '(' (open parentheses) represent internal variables (loop control variables, temporaries, and C function locals).

Returns NULL (and pushes nothing) when the index is greater than the number of active local variables.

lua_getstack

[-0, +0, -]

int lua_getstack (lua_State *L, int level, lua_Debug *ar);

Get information about the interpreter runtime stack.

This function fills parts of a lua_Debug structure with an identification of the activation record of the function executing at a given level. Level 0 is the current running function, whereas level n+1 is the function that has called level n. When there are no errors, lua_getstack returns 1; when called with a level greater than the stack depth, it returns 0.

lua_getupvalue

[-0, +(0|1), -]

const char *lua_getupvalue (lua_State *L, int funcindex, int n);

Gets information about a closure's upvalue. (For Lua functions, upvalues are the external local variables that the function uses, and that are consequently included in its closure.) lua_getupvalue gets the index n of an upvalue, pushes the upvalue's value onto the stack, and returns its name. funcindex points to the closure in the stack. (Upvalues have no particular order, as they are active through the whole function. So, they are numbered in an arbitrary order.)

Returns NULL (and pushes nothing) when the index is greater than the number of upvalues. For C functions, this function uses the empty string "" as a name for all upvalues.

lua_Hook

typedef void (*lua_Hook) (lua_State *L, lua_Debug *ar);

Type for debugging hook functions.

Whenever a hook is called, its ar argument has its field event set to the specific event that triggered the hook. Lua identifies these events with the following constants: LUA_HOOKCALL, LUA_HOOKRET, LUA_HOOKTAILCALL, LUA_HOOKLINE, and LUA_HOOKCOUNT. Moreover, for line events, the field currentline is also set. To get the value of any other field in ar, the hook must call lua_getinfo.

For call events, event can be LUA_HOOKCALL, the normal value, or LUA_HOOKTAILCALL, for a tail call; in this case, there will be no corresponding return event.

While Lua is running a hook, it disables other calls to hooks. Therefore, if a hook calls back Lua to execute a function or a chunk, this execution occurs without any calls to hooks.

lua_sethook

[-0, +0, -]

int lua_sethook (lua_State *L, lua_Hook f, int mask, int count);

Sets the debugging hook function.

Argument f is the hook function. mask specifies on which events the hook will be called: it is formed by a bitwise or of the constants LUA_MASKCALL, LUA_MASKRET, LUA_MASKLINE, and LUA_MASKCOUNT. The count argument is only meaningful when the mask includes LUA_MASKCOUNT. For each event, the hook is called as explained below:

• The call hook: is called when the interpreter calls a function. The hook is called just after Lua enters the new function, before the function gets its arguments.
• The return hook: is called when the interpreter returns from a function. The hook is called just before Lua leaves the function. You have no access to the values to be returned by the function.
• The line hook: is called when the interpreter is about to start the execution of a new line of code, or when it jumps back in the code (even to the same line). (This event only happens while Lua is executing a Lua function.)
• The count hook: is called after the interpreter executes every count instructions. (This event only happens while Lua is executing a Lua function.)

A hook is disabled by setting mask to zero.

lua_setlocal

[-(0|1), +0, -]

const char *lua_setlocal (lua_State *L, lua_Debug *ar, int n);

Sets the value of a local variable of a given activation record. Parameters ar and n are as in lua_getlocal (see lua_getlocal). lua_setlocal assigns the value at the top of the stack to the variable and returns its name. It also pops the value from the stack.

Returns NULL (and pops nothing) when the index is greater than the number of active local variables.

lua_setupvalue

[-(0|1), +0, -]

const char *lua_setupvalue (lua_State *L, int funcindex, int n);

Sets the value of a closure's upvalue. It assigns the value at the top of the stack to the upvalue and returns its name. It also pops the value from the stack. Parameters funcindex and n are as in the lua_getupvalue (see lua_getupvalue).

Returns NULL (and pops nothing) when the index is greater than the number of upvalues.

lua_upvalueid

[-0, +0, -]

void *(lua_upvalueid) (lua_State *L, int funcindex, int n);

Returns an unique identifier for the upvalue numbered n from the closure at index fidx. Parameters funcindex and n are as in the lua_getupvalue (see lua_getupvalue) (but n cannot be greater than the number of upvalues).

These unique identifiers allow a program to check whether different closures share upvalues. Lua closures that share an upvalue (that is, that access a same external local variable) will return identical ids for those upvalue indices.

4 - The Auxiliary Library

The auxiliary library provides several convenient functions to interface C with Lua. While the basic API provides the primitive functions for all interactions between C and Lua, the auxiliary library provides higher-level functions for some common tasks.

All functions from the auxiliary library are defined in header file lauxlib.h and have a prefix luaL_.

All functions in the auxiliary library are built on top of the basic API, and so they provide nothing that cannot be done with that API.

Several functions in the auxiliary library are used to check C function arguments. Because the error message is formatted for arguments (e.g., "bad argument #1"), you should not use these functions for other stack values.

Functions called luaL_check* always throw an error if the check is not satisfied.

4.1 - Functions and Types

Here we list all functions and types from the auxiliary library in alphabetical order.

luaL_addchar

[-0, +0, m]

void luaL_addchar (luaL_Buffer *B, char c);

Adds the character c to the buffer B (see luaL_Buffer).

luaL_addlstring

[-0, +0, m]

void luaL_addlstring (luaL_Buffer *B, const char *s, size_t l);

Adds the string pointed to by s with length l to the buffer B (see luaL_Buffer). The string may contain embedded zeros.

luaL_addsize

[-0, +0, m]

void luaL_addsize (luaL_Buffer *B, size_t n);

Adds to the buffer B (see luaL_Buffer) a string of length n previously copied to the buffer area (see luaL_prepbuffer).

luaL_addstring

[-0, +0, m]

void luaL_addstring (luaL_Buffer *B, const char *s);

Adds the zero-terminated string pointed to by s to the buffer B (see luaL_Buffer). The string may not contain embedded zeros.

luaL_addvalue

[-1, +0, m]

void luaL_addvalue (luaL_Buffer *B);

Adds the value at the top of the stack to the buffer B (see luaL_Buffer). Pops the value.

This is the only function on string buffers that can (and must) be called with an extra element on the stack, which is the value to be added to the buffer.

luaL_argcheck

[-0, +0, v]

void luaL_argcheck (lua_State *L,
                    int cond,
                    int narg,
                    const char *extramsg);

Checks whether cond is true. If not, raises an error with the following message, where func is retrieved from the call stack:

     bad argument #<narg> to <func> (<extramsg>)

luaL_argerror

[-0, +0, v]

int luaL_argerror (lua_State *L, int narg, const char *extramsg);

Raises an error with the following message, where func is retrieved from the call stack:

     bad argument #<narg> to <func> (<extramsg>)

This function never returns, but it is an idiom to use it in C functions as return luaL_argerror(args).

luaL_Buffer

typedef struct luaL_Buffer luaL_Buffer;

Type for a string buffer.

A string buffer allows C code to build Lua strings piecemeal. Its pattern of use is as follows:

• First you declare a variable b of type luaL_Buffer.
• Then you initialize it with a call luaL_buffinit(L, &b).
• Then you add string pieces to the buffer calling any of the luaL_add* functions.
• You finish by calling luaL_pushresult(&b). This call leaves the final string on the top of the stack.

During its normal operation, a string buffer uses a variable number of stack slots. So, while using a buffer, you cannot assume that you know where the top of the stack is. You can use the stack between successive calls to buffer operations as long as that use is balanced; that is, when you call a buffer operation, the stack is at the same level it was immediately after the previous buffer operation. (The only exception to this rule is luaL_addvalue.) After calling luaL_pushresult the stack is back to its level when the buffer was initialized, plus the final string on its top.

luaL_buffinit

[-0, +0, -]

void luaL_buffinit (lua_State *L, luaL_Buffer *B);

Initializes a buffer B. This function does not allocate any space; the buffer must be declared as a variable (see luaL_Buffer).

luaL_callmeta

[-0, +(0|1), e]

int luaL_callmeta (lua_State *L, int obj, const char *e);

Calls a metamethod.

If the object at index obj has a metatable and this metatable has a field e, this function calls this field and passes the object as its only argument. In this case this function returns true and pushes onto the stack the value returned by the call. If there is no metatable or no metamethod, this function returns false (without pushing any value on the stack).

luaL_checkany

[-0, +0, v]

void luaL_checkany (lua_State *L, int narg);

Checks whether the function has an argument of any type (including nil) at position narg.

luaL_checkint

[-0, +0, v]

int luaL_checkint (lua_State *L, int narg);

Checks whether the function argument narg is a number and returns this number cast to an int.

luaL_checkinteger

[-0, +0, v]

lua_Integer luaL_checkinteger (lua_State *L, int narg);

Checks whether the function argument narg is a number and returns this number cast to a lua_Integer.

luaL_checklong

[-0, +0, v]

long luaL_checklong (lua_State *L, int narg);

Checks whether the function argument narg is a number and returns this number cast to a long.

luaL_checklstring

[-0, +0, v]

const char *luaL_checklstring (lua_State *L, int narg, size_t *l);

Checks whether the function argument narg is a string and returns this string; if l is not NULL fills *l with the string's length.

This function uses lua_tolstring to get its result, so all conversions and caveats of that function apply here.

luaL_checknumber

[-0, +0, v]

lua_Number luaL_checknumber (lua_State *L, int narg);

Checks whether the function argument narg is a number and returns this number.

luaL_checkoption

[-0, +0, v]

int luaL_checkoption (lua_State *L,
                      int narg,
                      const char *def,
                      const char *const lst[]);

Checks whether the function argument narg is a string and searches for this string in the array lst (which must be NULL-terminated). Returns the index in the array where the string was found. Raises an error if the argument is not a string or if the string cannot be found.

If def is not NULL, the function uses def as a default value when there is no argument narg or if this argument is nil.

This is a useful function for mapping strings to C enums. (The usual convention in Lua libraries is to use strings instead of numbers to select options.)

luaL_checkstack

[-0, +0, v]

void luaL_checkstack (lua_State *L, int sz, const char *msg);

Grows the stack size to top + sz elements, raising an error if the stack cannot grow to that size. msg is an additional text to go into the error message (or NULL for no additional text).

luaL_checkstring

[-0, +0, v]

const char *luaL_checkstring (lua_State *L, int narg);

Checks whether the function argument narg is a string and returns this string.

This function uses lua_tolstring to get its result, so all conversions and caveats of that function apply here.

luaL_checktype

[-0, +0, v]

void luaL_checktype (lua_State *L, int narg, int t);

Checks whether the function argument narg has type t. See lua_type for the encoding of types for t.

luaL_checkudata

[-0, +0, v]

void *luaL_checkudata (lua_State *L, int narg, const char *tname);

Checks whether the function argument narg is a userdata of the type tname (see luaL_newmetatable) and returns the userdata address (see lua_touserdata).

luaL_checkversion

[-0, +0, -]

void *luaL_checkversion (lua_State *L);

Checks whether the core running the call, the core that created the Lua state, and the code making the call are all using the same version of Lua. Also checks whether the core running the call and the core that created the Lua state are using the same address space.

luaL_dofile

[-0, +?, m]

int luaL_dofile (lua_State *L, const char *filename);

Loads and runs the given file. It is defined as the following macro:

     (luaL_loadfile(L, filename) || lua_pcall(L, 0, LUA_MULTRET, 0))

It returns false if there are no errors or true in case of errors.

luaL_dostring

[-0, +?, -]

int luaL_dostring (lua_State *L, const char *str);

Loads and runs the given string. It is defined as the following macro:

     (luaL_loadstring(L, str) || lua_pcall(L, 0, LUA_MULTRET, 0))

It returns false if there are no errors or true in case of errors.

luaL_error

[-0, +0, v]

int luaL_error (lua_State *L, const char *fmt, ...);

Raises an error. The error message format is given by fmt plus any extra arguments, following the same rules of lua_pushfstring. It also adds at the beginning of the message the file name and the line number where the error occurred, if this information is available.

This function never returns, but it is an idiom to use it in C functions as return luaL_error(args).

luaL_getmetafield

[-0, +(0|1), m]

int luaL_getmetafield (lua_State *L, int obj, const char *e);

Pushes onto the stack the field e from the metatable of the object at index obj. If the object does not have a metatable, or if the metatable does not have this field, returns false and pushes nothing.

luaL_getmetatable

[-0, +1, -]

void luaL_getmetatable (lua_State *L, const char *tname);

Pushes onto the stack the metatable associated with name tname in the registry (see luaL_newmetatable).

luaL_gsub

[-0, +1, m]

const char *luaL_gsub (lua_State *L,
                       const char *s,
                       const char *p,
                       const char *r);

Creates a copy of string s by replacing any occurrence of the string p with the string r. Pushes the resulting string on the stack and returns it.

luaL_len

[-0, +1, e]

int luaL_len (lua_State *L, int index);

Returns the "length" of the value at the given acceptable index as a number; it is equivalent to the '#' operator in Lua (see §2.5.5). Raises an error if the result of the operation is not a number. (This only may happen through metamethods.)

luaL_loadbuffer

[-0, +1, -]

int luaL_loadbuffer (lua_State *L,
                     const char *buff,
                     size_t sz,
                     const char *name);

Loads a buffer as a Lua chunk. This function uses lua_load to load the chunk in the buffer pointed to by buff with size sz.

This function returns the same results as lua_load. name is the chunk name, used for debug information and error messages.

luaL_loadfile

[-0, +1, m]

int luaL_loadfile (lua_State *L, const char *filename);

Loads a file as a Lua chunk. This function uses lua_load to load the chunk in the file named filename. If filename is NULL, then it loads from the standard input. The first line in the file is ignored if it starts with a #.

This function returns the same results as lua_load, but it has an extra error code LUA_ERRFILE if it cannot open/read the file.

As lua_load, this function only loads the chunk; it does not run it.

luaL_loadstring

[-0, +1, -]

int luaL_loadstring (lua_State *L, const char *s);

Loads a string as a Lua chunk. This function uses lua_load to load the chunk in the zero-terminated string s.

This function returns the same results as lua_load.

Also as lua_load, this function only loads the chunk; it does not run it.

luaL_newmetatable

[-0, +1, m]

int luaL_newmetatable (lua_State *L, const char *tname);

If the registry already has the key tname, returns 0. Otherwise, creates a new table to be used as a metatable for userdata, adds it to the registry with key tname, and returns 1.

In both cases pushes onto the stack the final value associated with tname in the registry.

luaL_newstate

[-0, +0, -]

lua_State *luaL_newstate (void);

Creates a new Lua state. It calls lua_newstate with an allocator based on the standard C realloc function and then sets a panic function (see §3.6) that prints an error message to the standard error output in case of fatal errors.

Returns the new state, or NULL if there is a memory allocation error.

luaL_openlibs

[-0, +0, m]

void luaL_openlibs (lua_State *L);

Opens all standard Lua libraries into the given state.

luaL_optint

[-0, +0, v]

int luaL_optint (lua_State *L, int narg, int d);

If the function argument narg is a number, returns this number cast to an int. If this argument is absent or is nil, returns d. Otherwise, raises an error.

luaL_optinteger

[-0, +0, v]

lua_Integer luaL_optinteger (lua_State *L,
                             int narg,
                             lua_Integer d);

If the function argument narg is a number, returns this number cast to a lua_Integer. If this argument is absent or is nil, returns d. Otherwise, raises an error.

luaL_optlong

[-0, +0, v]

long luaL_optlong (lua_State *L, int narg, long d);

If the function argument narg is a number, returns this number cast to a long. If this argument is absent or is nil, returns d. Otherwise, raises an error.

luaL_optlstring

[-0, +0, v]

const char *luaL_optlstring (lua_State *L,
                             int narg,
                             const char *d,
                             size_t *l);

If the function argument narg is a string, returns this string. If this argument is absent or is nil, returns d. Otherwise, raises an error.

If l is not NULL, fills the position *l with the results's length.

luaL_optnumber

[-0, +0, v]

lua_Number luaL_optnumber (lua_State *L, int narg, lua_Number d);

If the function argument narg is a number, returns this number. If this argument is absent or is nil, returns d. Otherwise, raises an error.

luaL_optstring

[-0, +0, v]

const char *luaL_optstring (lua_State *L,
                            int narg,
                            const char *d);

If the function argument narg is a string, returns this string. If this argument is absent or is nil, returns d. Otherwise, raises an error.

luaL_prepbuffer

[-0, +0, -]

char *luaL_prepbuffer (luaL_Buffer *B);

Returns an address to a space of size LUAL_BUFFERSIZE where you can copy a string to be added to buffer B (see luaL_Buffer). After copying the string into this space you must call luaL_addsize with the size of the string to actually add it to the buffer.

luaL_pushresult

[-?, +1, m]

void luaL_pushresult (luaL_Buffer *B);

Finishes the use of buffer B leaving the final string on the top of the stack.

luaL_ref

[-1, +0, m]

int luaL_ref (lua_State *L, int t);

Creates and returns a reference, in the table at index t, for the object at the top of the stack (and pops the object).

A reference is a unique integer key. As long as you do not manually add integer keys into table t, luaL_ref ensures the uniqueness of the key it returns. You can retrieve an object referred by reference r by calling lua_rawgeti(L, t, r). Function luaL_unref frees a reference and its associated object.

If the object at the top of the stack is nil, luaL_ref returns the constant LUA_REFNIL. The constant LUA_NOREF is guaranteed to be different from any reference returned by luaL_ref.

luaL_Reg

typedef struct luaL_Reg {
  const char *name;
  lua_CFunction func;
} luaL_Reg;

Type for arrays of functions to be registered by luaL_register. name is the function name and func is a pointer to the function. Any array of luaL_Reg must end with an sentinel entry in which both name and func are NULL.

luaL_register

[-(0|1), +1, m]

void luaL_register (lua_State *L,
                    const char *libname,
                    const luaL_Reg *l);

Opens a library.

When called with libname equal to NULL, it simply registers all functions in the array l (see luaL_Reg) into the table on the top of the stack. The pointer l may be NULL, representing an empty list.

When called with a non-null libname, luaL_register creates a new table t, sets it as the value of the global variable libname, sets it as the value of package.loaded[libname], and registers on it all functions in the list l. If there is a table in package.loaded[libname] or in variable libname, reuses this table instead of creating a new one.

In any case the function leaves the table on the top of the stack.

luaL_testudata

[-0, +0, m]

void *luaL_testudata (lua_State *L, int narg, const char *tname);

This function works like luaL_checkudata, except that, when the test fails, it returns NULL instead of throwing an error.

luaL_traceback

[-0, +1, m]

luaL_traceback (lua_State *L, lua_State *L1,
                          const char *msg, int level);

Creates and pushes a traceback of the stack L1. If msg is not NULL it is appended at the beginning of the traceback. The level parameter tells at which level to start the traceback.

luaL_typename

[-0, +0, -]

const char *luaL_typename (lua_State *L, int index);

Returns the name of the type of the value at the given index.

luaL_typeerror

[-0, +0, v]

int luaL_typeerror (lua_State *L, int narg, const char *tname);

Generates an error with a message like the following:

     location: bad argument narg to 'func' (tname expected, got rt)

where location is produced by luaL_where, func is the name of the current function, and rt is the type name of the actual argument.

luaL_unref

[-0, +0, -]

void luaL_unref (lua_State *L, int t, int ref);

Releases reference ref from the table at index t (see luaL_ref). The entry is removed from the table, so that the referred object can be collected. The reference ref is also freed to be used again.

If ref is LUA_NOREF or LUA_REFNIL, luaL_unref does nothing.

luaL_where

[-0, +1, m]

void luaL_where (lua_State *L, int lvl);

Pushes onto the stack a string identifying the current position of the control at level lvl in the call stack. Typically this string has the following format:

     chunkname:currentline:

Level 0 is the running function, level 1 is the function that called the running function, etc.

This function is used to build a prefix for error messages.

5 - Standard Libraries

The standard Lua libraries provide useful functions that are implemented directly through the C API. Some of these functions provide essential services to the language (e.g., type and getmetatable); others provide access to "outside" services (e.g., I/O); and others could be implemented in Lua itself, but are quite useful or have critical performance requirements that deserve an implementation in C (e.g., table.sort).

All libraries are implemented through the official C API and are provided as separate C modules. Currently, Lua has the following standard libraries:

• basic library, which includes the coroutine sub-library;
• package library;
• string manipulation;
• table manipulation;
• mathematical functions (sin, log, etc.);
• bitwise operations;
• input and output;
• operating system facilities;
• debug facilities.

Except for the basic and package libraries, each library provides all its functions as fields of a global table or as methods of its objects.

To have access to these libraries, the C host program should call the luaL_openlibs function, which opens all standard libraries. Alternatively, it can open them individually by calling luaopen_base (for the basic library), luaopen_package (for the package library), luaopen_string (for the string library), luaopen_table (for the table library), luaopen_math (for the mathematical library), luaopen_bitlib (for the bit library), luaopen_io (for the I/O library), luaopen_os (for the Operating System library), and luaopen_debug (for the debug library). These functions are declared in lualib.h and should not be called directly: you must call them like any other Lua C function, e.g., by using lua_call.

5.1 - Basic Functions

The basic library provides some core functions to Lua. If you do not include this library in your application, you should check carefully whether you need to provide implementations for some of its facilities.

assert (v [, message])

collectgarbage ([opt [, arg]])

This function is a generic interface to the garbage collector. It performs different functions according to its first argument, opt:

• "collect": performs a full garbage-collection cycle. This is the default option.
• "stop": stops the garbage collector.
• "restart": restarts the garbage collector.
• "count": returns the total memory in use by Lua (in Kbytes) and a second value with the total memory in bytes modulo 1024. The first value has a fractional part, so the following equality is always true:

       k, b = collectgarbage("count")
       assert(k*1024 == math.floor(k)*1024 + b)
  

• "step": performs a garbage-collection step. The step "size" is controlled by arg (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 arg. Returns true if the step finished a collection cycle.
• "setpause": sets arg as the new value for the pause of the collector (see §2.10). Returns the previous value for pause.
• "setstepmul": sets arg as the new value for the step multiplier of the collector (see §2.10). Returns the previous value for step.
• "isrunning": returns a boolean that tells whether the collector is running (i.e., not stopped).

dofile ([filename])

error (message [, level])

Usually, error adds some information about the error position at the beginning of the message, if the message is a string. The level argument specifies how to get the error position. With level 1 (the default), the error position is where the error function was called. Level 2 points the error to where the function that called error was called; and so on. Passing a level 0 avoids the addition of error position information to the message.

_G

getmetatable (object)

If object does not have a metatable, returns nil. Otherwise, if the object's metatable has a "__metatable" field, returns the associated value. Otherwise, returns the metatable of the given object.

ipairs (t)

If t has a metamethod __ipairs, calls it with t as argument and returns the first three results from the call.

Otherwise, returns three values: an iterator function, the table t, and 0, so that the construction

     for i,v in ipairs(t) do body end

will iterate over the pairs (1,t[1]), (2,t[2]), ···, up to the length of the table, as defined by the length operator (see §2.5.5).

load (ld [, source [, mode]])

Loads a chunk.

If ld is a function, calls it repeatedly to get the chunk pieces. Each call to ld must return a string that concatenates with previous results. A return of an empty string, nil, or no value signals the end of the chunk.

If ld is a string, the chunk is this string.

If there are no errors, returns the compiled chunk as a function; otherwise, returns nil plus the error message. The environment of the returned function is the global environment.

source is used as the source of the chunk for error messages and debug information (see §3.9). When absent, it defaults to "=(load)".

The string mode controls whether the chunk can be text or binary (that is, a precompiled chunk). A letter 't' in mode allows a text chunk; a letter 'b' allows a binary chunk. The default is "bt".

loadin (env, ...)

This function is similar to load, but sets env as the environment of the created function in case of success. The parameters after env are similar to those of load, too.

loadfile ([filename])

Similar to load, but gets the chunk from file filename or from the standard input, if no file name is given.

loadstring (string [, chunkname])

Similar to load, but gets the chunk from the given string.

To load and run a given string, use the idiom

     assert(loadstring(s))()

When absent, chunkname defaults to the given string.

next (table [, index])

Allows a program to traverse all fields of a table. Its first argument is a table and its second argument is an index in this table. next returns the next index of the table and its associated value. When called with nil as its second argument, next returns an initial index and its associated value. When called with the last index, or with nil in an empty table, next returns nil. If the second argument is absent, then it is interpreted as nil. In particular, you can use next(t) to check whether a table is empty.

The order in which the indices are enumerated is not specified, even for numeric indices. (To traverse a table in numeric order, use a numerical for or the ipairs function.)

The behavior of next is undefined if, during the traversal, you assign any value to a non-existent field in the table. You may however modify existing fields. In particular, you may clear existing fields.

pairs (t)

If t has a metamethod __pairs, calls it with t as argument and returns the first three results from the call.

Otherwise, returns three values: the next function, the table t, and nil, so that the construction

     for k,v in pairs(t) do body end

will iterate over all key–value pairs of table t.

See function next for the caveats of modifying the table during its traversal.

pcall (f [, arg1, ···])

Calls function f with the given arguments in protected mode. This means that any error inside f is not propagated; instead, pcall catches the error and returns a status code. Its first result is the status code (a boolean), which is true if the call succeeds without errors. In such case, pcall also returns all results from the call, after this first result. In case of any error, pcall returns false plus the error message.

print (···)

rawequal (v1, v2)

rawget (table, index)

rawset (table, index, value)

This function returns table.

select (index, ···)

If index is a number, returns all arguments after argument number index; a negative number indexes from the end (-1 is the last argument). Otherwise, index must be the string "#", and select returns the total number of extra arguments it received.

setmetatable (table, metatable)

Sets the metatable for the given table. (You cannot change the metatable of other types from Lua, only from C.) If metatable is nil, removes the metatable of the given table. If the original metatable has a "__metatable" field, raises an error.

This function returns table.

tonumber (e [, base])

An optional argument specifies the base to interpret the numeral. The base may be any integer between 2 and 36, inclusive. In bases above 10, the letter 'A' (in either upper or lower case) represents 10, 'B' represents 11, and so forth, with 'Z' representing 35. In base 10 (the default), the number can have a decimal part, as well as an optional exponent part (see §2.1). In other bases, only unsigned integers are accepted.

tostring (e)

If the metatable of e has a "__tostring" field, then tostring calls the corresponding value with e as argument, and uses the result of the call as its result.

type (v)

_VERSION

xpcall (f, err [, arg1, ···])

This function is similar to pcall, except that it sets a new error handler err.

5.2 - Coroutine Manipulation

The operations related to coroutines comprise a sub-library of the basic library and come inside the table coroutine. See §2.11 for a general description of coroutines.

coroutine.create (f)

Creates a new coroutine, with body f. f must be a Lua function. Returns this new coroutine, an object with type "thread".

coroutine.resume (co [, val1, ···])

Starts or continues the execution of coroutine co. The first time you resume a coroutine, it starts running its body. The values val1, ··· are passed as the arguments to the body function. If the coroutine has yielded, resume restarts it; the values val1, ··· are passed as the results from the yield.

If the coroutine runs without any errors, resume returns true plus any values passed to yield (if the coroutine yields) or any values returned by the body function (if the coroutine terminates). If there is any error, resume returns false plus the error message.

coroutine.running ()

Returns the running coroutine plus a boolean, true when the running coroutine is the main one.

coroutine.status (co)

Returns the status of coroutine co, as a string: "running", if the coroutine is running (that is, it called status); "suspended", if the coroutine is suspended in a call to yield, or if it has not started running yet; "normal" if the coroutine is active but not running (that is, it has resumed another coroutine); and "dead" if the coroutine has finished its body function, or if it has stopped with an error.

coroutine.wrap (f)

Creates a new coroutine, with body f. f must be a Lua function. Returns a function that resumes the coroutine each time it is called. Any arguments passed to the function behave as the extra arguments to resume. Returns the same values returned by resume, except the first boolean. In case of error, propagates the error.

coroutine.yield (···)

Suspends the execution of the calling coroutine. The coroutine cannot be running a C function, a metamethod, or an iterator. Any arguments to yield are passed as extra results to resume.

5.3 - Modules

The package library provides basic facilities for loading and building modules in Lua. It exports two of its functions directly in the global environment: require and module. Everything else is exported in a table package.

module (name [, ···])

Creates and returns a module. If there is a table in package.loaded[name], this table is the module. Otherwise, if there is a global table t with the given name, this table is the module. Otherwise creates a new table t and sets it as the value of the global name and the value of package.loaded[name]. This function also initializes t._NAME with the given name, t._M with the module (t itself), and t._PACKAGE with the package name (the full module name minus last component; see below). Finally, module sets t as the new value of package.loaded[name], so that require returns t.

The module function returns the module table t.

If name is a compound name (that is, one with components separated by dots), module creates (or reuses, if they already exist) tables for each component. For instance, if name is a.b.c, then module stores the module table in field c of field b of global a.

This function can receive optional options after the module name, where each option is a function to be applied over the module.

To keep compatibility with old versions of Lua, module also sets t as the new environment of the current function.

require (modname)

Loads the given module. The function starts by looking into the package.loaded table to determine whether modname is already loaded. If it is, then require returns the value stored at package.loaded[modname]. Otherwise, it tries to find a loader for the module.

To find a loader, require is guided by the package.loaders array. By changing this array, we can change how require looks for a module. The following explanation is based on the default configuration for package.loaders.

First require queries package.preload[modname]. If it has a value, this value (which should be a function) is the loader. Otherwise require searches for a Lua loader using the path stored in package.path. If that also fails, it searches for a C loader using the path stored in package.cpath. If that also fails, it tries an all-in-one loader (see package.loaders).

Once a loader is found, require calls the loader with a single argument, modname. If the loader returns any value, require assigns the returned value to package.loaded[modname]. If the loader returns no value and has not assigned any value to package.loaded[modname], then require assigns true to this entry. In any case, require returns the final value of package.loaded[modname].

If there is any error loading or running the module, or if it cannot find any loader for the module, then require signals an error.

package.config

A string describing some compile-time configurations for packages. This string is a sequence of lines:

• The first line is the directory separator string. Default is '\' for Windows and '/' for all other systems.
• The second line is the character that separates templates in a path. Default is ';'.
• The third line is the string that marks the substitution points in a template. Default is '?'.
• The fourth line is a string that, in a path in Windows, is replaced by the executable's directory. Default is '!'.
• The fifth line is a mark to ignore all before it when bulding the luaopen_ function name. Default is '-'.

package.cpath

The path used by require to search for a C loader.

Lua initializes the C path package.cpath in the same way it initializes the Lua path package.path, using the environment variable LUA_CPATH or a default path defined in luaconf.h.

package.loaded

A table used by require to control which modules are already loaded. When you require a module modname and package.loaded[modname] is not false, require simply returns the value stored there.

This variable is only a reference to the real table; assignments to this variable do not change the table used by require.

package.loaders

A table used by require to control how to load modules.

Each entry in this table is a searcher function. When looking for a module, require calls each of these searchers in ascending order, with the module name (the argument given to require) as its sole parameter. The function can return another function (the module loader) or a string explaining why it did not find that module (or nil if it has nothing to say). Lua initializes this table with four functions.

The first searcher simply looks for a loader in the package.preload table.

The second searcher looks for a loader as a Lua library, using the path stored at package.path. The search is done as described in function package.searchpath.

The third searcher looks for a loader as a C library, using the path given by the variable package.cpath. Again, the search is done as described in function package.searchpath. For instance, if the C path is the string

     "./?.so;./?.dll;/usr/local/?/init.so"

the searcher for module foo will try to open the files ./foo.so, ./foo.dll, and /usr/local/foo/init.so, in that order. Once it finds a C library, this searcher first uses a dynamic link facility to link the application with the library. Then it tries to find a C function inside the library to be used as the loader. The name of this C function is the string "luaopen_" concatenated with a copy of the module name where each dot is replaced by an underscore. Moreover, if the module name has a hyphen, its prefix up to (and including) the first hyphen is removed. For instance, if the module name is a.v1-b.c, the function name will be luaopen_b_c.

The fourth searcher tries an all-in-one loader. It searches the C path for a library for the root name of the given module. For instance, when requiring a.b.c, it will search for a C library for a. If found, it looks into it for an open function for the submodule; in our example, that would be luaopen_a_b_c. With this facility, a package can pack several C submodules into one single library, with each submodule keeping its original open function.

package.loadlib (libname, funcname)

Dynamically links the host program with the C library libname.

If funcname is "*", then it only links with the library, making the symbols exported by the library available to other dynamically linked libraries. Otherwise, it looks for a function funcname inside the library and returns this function as a C function. (So, funcname must follow the protocol (see lua_CFunction)).

This is a low-level function. It completely bypasses the package and module system. Unlike require, it does not perform any path searching and does not automatically adds extensions. libname must be the complete file name of the C library, including if necessary a path and an extension. funcname must be the exact name exported by the C library (which may depend on the C compiler and linker used).

This function is not supported by ANSI C. As such, it is only available on some platforms (Windows, Linux, Mac OS X, Solaris, BSD, plus other Unix systems that support the dlfcn standard).

package.path

The path used by require to search for a Lua loader.

At start-up, Lua initializes this variable with the value of the environment variable LUA_PATH or with a default path defined in luaconf.h, if the environment variable is not defined. Any ";;" in the value of the environment variable is replaced by the default path.

package.preload

A table to store loaders for specific modules (see require).

package.searchpath (name, path)

Searches for the given name in the given path.

A path is string containing a sequence of templates separated by semicolons. For each template, the function changes each interrogation mark in the template by name, and then tries to open the resulting file name. For instance, if the path is the string

     "./?.lua;./?.lc;/usr/local/?/init.lua"

the search for name foo will try to open the files ./foo.lua, ./foo.lc, and /usr/local/foo/init.lua, in that order.

Returns the resulting name of the first file that it can open in read mode (after closing it), or nil plus an error message if none succeeds. (This error message lists all file names it tried to open.)

package.seeall (module)

Sets a metatable for module with its __index field referring to the global environment, so that this module inherits values from the global environment. To be used as an option to function module.

5.4 - String Manipulation

This library provides generic functions for string manipulation, such as finding and extracting substrings, and pattern matching. When indexing a string in Lua, the first character is at position 1 (not at 0, as in C). Indices are allowed to be negative and are interpreted as indexing backwards, from the end of the string. Thus, the last character is at position -1, and so on.

The string library provides all its functions inside the table string. It also sets a metatable for strings where the __index field points to the string table. Therefore, you can use the string functions in object-oriented style. For instance, string.byte(s, i) can be written as s:byte(i).

The string library assumes one-byte character encodings.

string.byte (s [, i [, j]])

Note that numerical codes are not necessarily portable across platforms.

string.char (···)

Note that numerical codes are not necessarily portable across platforms.

string.dump (function)

Returns a string containing a binary representation of the given function, so that a later loadstring on this string returns a copy of the function. function must be a Lua function without upvalues.

string.find (s, pattern [, init [, plain]])

If the pattern has captures, then in a successful match the captured values are also returned, after the two indices.

string.format (formatstring, ···)

     string.format('%q', 'a string with "quotes" and \n new line')

will produce the string:

     "a string with \"quotes\" and \
      new line"

The options c, d, E, e, f, g, G, i, o, u, X, and x all expect a number as argument, whereas q and s expect a string.

This function does not accept string values containing embedded zeros, except as arguments to the q option.

string.gmatch (s, pattern)

As an example, the following loop

     s = "hello world from Lua"
     for w in string.gmatch(s, "%a+") do
       print(w)
     end

will iterate over all the words from string s, printing one per line. The next example collects all pairs key=value from the given string into a table:

     t = {}
     s = "from=world, to=Lua"
     for k, v in string.gmatch(s, "(%w+)=(%w+)") do
       t[k] = v
     end

For this function, a '^' at the start of a pattern does not work as an anchor, as this would prevent the iteration.

string.gsub (s, pattern, repl [, n])

If repl is a string, then its value is used for replacement. The character % works as an escape character: any sequence in repl of the form %n, with n between 1 and 9, stands for the value of the n-th captured substring (see below). The sequence %0 stands for the whole match. The sequence %% stands for a single %.

If repl is a table, then the table is queried for every match, using the first capture as the key; if the pattern specifies no captures, then the whole match is used as the key.

If repl is a function, then this function is called every time a match occurs, with all captured substrings passed as arguments, in order; if the pattern specifies no captures, then the whole match is passed as a sole argument.

If the value returned by the table query or by the function call is a string or a number, then it is used as the replacement string; otherwise, if it is false or nil, then there is no replacement (that is, the original match is kept in the string).

Here are some examples:

     x = string.gsub("hello world", "(%w+)", "%1 %1")
     --> x="hello hello world world"
     
     x = string.gsub("hello world", "%w+", "%0 %0", 1)
     --> x="hello hello world"
     
     x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1")
     --> x="world hello Lua from"
     
     x = string.gsub("home = $HOME, user = $USER", "%$(%w+)", os.getenv)
     --> x="home = /home/roberto, user = roberto"
     
     x = string.gsub("4+5 = $return 4+5$", "%$(.-)%$", function (s)
           return loadstring(s)()
         end)
     --> x="4+5 = 9"
     
     local t = {name="lua", version="5.1"}
     x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t)
     --> x="lua-5.1.tar.gz"

string.len (s)

string.lower (s)

string.match (s, pattern [, init])

string.rep (s, n)

string.reverse (s)

string.sub (s, i [, j])

string.upper (s)

5.4.1 - Patterns

Character Class:

A character class is used to represent a set of characters. The following combinations are allowed in describing a character class:

• x: (where x is not one of the magic characters ^$()%.[]*+-?) represents the character x itself.
• .: (a dot) represents all characters.
• %a: represents all letters.
• %c: represents all control characters.
• %d: represents all digits.
• %l: represents all lowercase letters.
• %p: represents all punctuation characters.
• %s: represents all space characters.
• %u: represents all uppercase letters.
• %w: represents all alphanumeric characters.
• %x: represents all hexadecimal digits.
• %z: represents the character with representation 0.
• %x: (where x is any non-alphanumeric character) represents the character x. This is the standard way to escape the magic characters. Any punctuation character (even the non magic) can be preceded by a '%' when used to represent itself in a pattern.
• [set]: represents the class which is the union of all characters in set. A range of characters can be specified by separating the end characters of the range with a '-'. All classes %x described above can also be used as components in set. All other characters in set represent themselves. For example, [%w_] (or [_%w]) represents all alphanumeric characters plus the underscore, [0-7] represents the octal digits, and [0-7%l%-] represents the octal digits plus the lowercase letters plus the '-' character.

  The interaction between ranges and classes is not defined. Therefore, patterns like [%a-z] or [a-%%] have no meaning.

• [^set]: represents the complement of set, where set is interpreted as above.

For all classes represented by single letters (%a, %c, etc.), the corresponding uppercase letter represents the complement of the class. For instance, %S represents all non-space characters.

The definitions of letter, space, and other character groups depend on the current locale. In particular, the class [a-z] may not be equivalent to %l.

Pattern Item:

A pattern item can be

• a single character class, which matches any single character in the class;
• a single character class followed by '*', which matches 0 or more repetitions of characters in the class. These repetition items will always match the longest possible sequence;
• a single character class followed by '+', which matches 1 or more repetitions of characters in the class. These repetition items will always match the longest possible sequence;
• a single character class followed by '-', which also matches 0 or more repetitions of characters in the class. Unlike '*', these repetition items will always match the shortest possible sequence;
• a single character class followed by '?', which matches 0 or 1 occurrence of a character in the class;
• %n, for n between 1 and 9; such item matches a substring equal to the n-th captured string (see below);
• %bxy, where x and y are two distinct characters; such item matches strings that start with x, end with y, and where the x and y are balanced. This means that, if one reads the string from left to right, counting +1 for an x and -1 for a y, the ending y is the first y where the count reaches 0. For instance, the item %b() matches expressions with balanced parentheses.
• %f[set], a frontier pattern; such item matches an empty string at any position such that the next character belongs to set and the previous character does not belong to set. The set set is interpreted as previously described. The beggining and the end of the subject are handled as if they were the character '\0'.

Pattern:

A pattern is a sequence of pattern items. A '^' at the beginning of a pattern anchors the match at the beginning of the subject string. A '$' at the end of a pattern anchors the match at the end of the subject string. At other positions, '^' and '$' have no special meaning and represent themselves.

Captures:

A pattern can contain sub-patterns enclosed in parentheses; they describe captures. When a match succeeds, the substrings of the subject string that match captures are stored (captured) for future use. Captures are numbered according to their left parentheses. For instance, in the pattern "(a*(.)%w(%s*))", the part of the string matching "a*(.)%w(%s*)" is stored as the first capture (and therefore has number 1); the character matching "." is captured with number 2, and the part matching "%s*" has number 3.

As a special case, the empty capture () captures the current string position (a number). For instance, if we apply the pattern "()aa()" on the string "flaaap", there will be two captures: 3 and 5.

A pattern cannot contain embedded zeros. Use %z instead.

5.5 - Table Manipulation

This library provides generic functions for table manipulation. It provides all its functions inside the table table.

Most functions in the table library assume that the table represents an array or a list. For these functions, when we talk about the "length" of a table we mean the result of the length operator.

table.concat (table [, sep [, i [, j]]])

table.insert (table, [pos,] value)

Inserts element value at position pos in table, shifting up other elements to open space, if necessary. The default value for pos is n+1, where n is the length of the table (see §2.5.5), so that a call table.insert(t,x) inserts x at the end of table t.

table.pack (···)

Returns a new table with all parameters stored into keys 1, 2, etc. and with a field "n" with the total number of parameters.

table.remove (table [, pos])

Removes from table the element at position pos, shifting down other elements to close the space, if necessary. Returns the value of the removed element. The default value for pos is n, where n is the length of the table, so that a call table.remove(t) removes the last element of table t.

table.sort (table [, comp])

The sort algorithm is not stable; that is, elements considered equal by the given order may have their relative positions changed by the sort.

table.unpack (list [, i [, j]])

     return list[i], list[i+1], ···, list[j]

except that the above code can be written only for a fixed number of elements. By default, i is 1 and j is the length of the list, as defined by the length operator (see §2.5.5).

5.6 - Mathematical Functions

This library is an interface to the standard C math library. It provides all its functions inside the table math.

math.abs (x)

Returns the absolute value of x.

math.acos (x)

Returns the arc cosine of x (in radians).

math.asin (x)

Returns the arc sine of x (in radians).

math.atan (x)

Returns the arc tangent of x (in radians).

math.atan2 (y, x)

Returns the arc tangent of y/x (in radians), but uses the signs of both parameters to find the quadrant of the result. (It also handles correctly the case of x being zero.)

math.ceil (x)

Returns the smallest integer larger than or equal to x.

math.cos (x)

Returns the cosine of x (assumed to be in radians).

math.cosh (x)

Returns the hyperbolic cosine of x.

math.deg (x)

Returns the angle x (given in radians) in degrees.

math.exp (x)

Returns the value e^x.

math.floor (x)

Returns the largest integer smaller than or equal to x.

math.fmod (x, y)

Returns the remainder of the division of x by y that rounds the quotient towards zero.

math.frexp (x)

Returns m and e such that x = m2^e, e is an integer and the absolute value of m is in the range [0.5, 1) (or zero when x is zero).

math.huge

The value HUGE_VAL, a value larger than or equal to any other numerical value.

math.ldexp (m, e)

Returns m2^e (e should be an integer).

math.log (x [, base])

Returns the logarithm of x in the given base. The default for base is $e$ (so that the function returns the natural logarithm of x).

math.max (x, ···)

Returns the maximum value among its arguments.

math.min (x, ···)

Returns the minimum value among its arguments.

math.modf (x)

Returns two numbers, the integral part of x and the fractional part of x.

math.pi

The value of pi.

math.pow (x, y)

Returns x^y. (You can also use the expression x^y to compute this value.)

math.rad (x)

Returns the angle x (given in degrees) in radians.

math.random ([m [, n]])

This function is an interface to the simple pseudo-random generator function rand provided by ANSI C. (No guarantees can be given for its statistical properties.)

When called without arguments, returns a uniform pseudo-random real number in the range [0,1). When called with an integer number m, math.random returns a uniform pseudo-random integer in the range [1, m]. When called with two integer numbers m and n, math.random returns a uniform pseudo-random integer in the range [m, n].

math.randomseed (x)

Sets x as the "seed" for the pseudo-random generator: equal seeds produce equal sequences of numbers.

math.sin (x)

Returns the sine of x (assumed to be in radians).

math.sinh (x)

Returns the hyperbolic sine of x.

math.sqrt (x)

Returns the square root of x. (You can also use the expression x^0.5 to compute this value.)

math.tan (x)

Returns the tangent of x (assumed to be in radians).

math.tanh (x)

Returns the hyperbolic tangent of x.

5.7 - Bitwise operations

This library provides bitwise operations. It provides all its functions inside the table bit. Unless otherwise stated, all functions accept as arguments and return numbers in the range [0,2^32 - 1]. Standard Lua does not use 2-complement representation for numbers, so in standard Lua (that is, with floating-point numbers) you cannot use negative numbers with this library. In particular, -1 is not the same as 0xffffffff.

bit.band (···)

Returns the bitwise and of its operands.

bit.bnot (x)

Returns the bitwise negation of x. For any valid x, the following identity holds:

     assert(bit.bnot(x) == 2^32 - 1 - x)

bit.bor (···)

Returns the bitwise or of its operands.

bit.brotate (x, disp)

Returns the number x rotated disp bits to the left. The number disp may be any representable integer.

For any valid displacement, the following identity holds:

     assert(bit.rotate(x, disp) == bit.rotate(x, disp % 32))

This allows you to consider that negative displacements rotate to the right.

bit.bshift (x, disp)

Returns the number x shifted disp bits to the left. The number disp may be any representable integer. Negative displacements shift to the right. In any direction, vacant bits are filled with zeros. In particular, displacements with absolute values higher than the total number of bits in the representation of x result in zero (all bits are shifted out).

For positive displacements, the following equality holds:

     assert(bit.bshift(b, disp) == (b * 2^disp) % 2^32)

For negative displacements, the following equality holds:

     assert(bit.bshift(b, disp) == math.floor(b * 2^disp))

This shift operation is what is called logical shift. For an arithmetic shift, you should use the arithmetic operators.

bit.btest (···)

Returns a boolean signaling whether the bitwise and of its operands is different from zero.

bit.bxor (···)

Returns the bitwise exclusive or of its operands.

5.8 - Input and Output Facilities

The I/O library provides two different styles for file manipulation. The first one uses implicit file descriptors; that is, there are operations to set a default input file and a default output file, and all input/output operations are over these default files. The second style uses explicit file descriptors.

When using implicit file descriptors, all operations are supplied by table io. When using explicit file descriptors, the operation io.open returns a file descriptor and then all operations are supplied as methods of the file descriptor.

The table io also provides three predefined file descriptors with their usual meanings from C: io.stdin, io.stdout, and io.stderr. The I/O library never closes these files.

Unless otherwise stated, all I/O functions return nil on failure (plus an error message as a second result and a system-dependent error code as a third result) and some value different from nil on success.

io.close ([file])

Equivalent to file:close(). Without a file, closes the default output file.

io.flush ()

Equivalent to file:flush over the default output file.

io.input ([file])

When called with a file name, it opens the named file (in text mode), and sets its handle as the default input file. When called with a file handle, it simply sets this file handle as the default input file. When called without parameters, it returns the current default input file.

In case of errors this function raises the error, instead of returning an error code.

io.lines ([filename])

Opens the given file name in read mode and returns an iterator function that, each time it is called, returns a new line from the file. Therefore, the construction

     for line in io.lines(filename) do body end

will iterate over all lines of the file. When the iterator function detects the end of file, it returns nil (to finish the loop) and automatically closes the file.

The call io.lines() (with no file name) is equivalent to io.input():lines(); that is, it iterates over the lines of the default input file. In this case it does not close the file when the loop ends.

io.open (filename [, mode])

This function opens a file, in the mode specified in the string mode. It returns a new file handle, or, in case of errors, nil plus an error message.

The mode string can be any of the following:

• "r": read mode (the default);
• "w": write mode;
• "a": append mode;
• "r+": update mode, all previous data is preserved;
• "w+": update mode, all previous data is erased;
• "a+": append update mode, previous data is preserved, writing is only allowed at the end of file.

The mode string can also have a 'b' at the end, which is needed in some systems to open the file in binary mode.

io.output ([file])

Similar to io.input, but operates over the default output file.

io.popen (prog [, mode])

Starts program prog in a separated process and returns a file handle that you can use to read data from this program (if mode is "r", the default) or to write data to this program (if mode is "w").

This function is system dependent and is not available on all platforms.

io.read (···)

Equivalent to io.input():read.

io.tmpfile ()

Returns a handle for a temporary file. This file is opened in update mode and it is automatically removed when the program ends.

io.type (obj)

Checks whether obj is a valid file handle. Returns the string "file" if obj is an open file handle, "closed file" if obj is a closed file handle, or nil if obj is not a file handle.

io.write (···)

Equivalent to io.output():write.

file:close ()

Closes file. Note that files are automatically closed when their handles are garbage collected, but that takes an unpredictable amount of time to happen.

If file was created with io.popen, a sucessful close returns the exit status of the process.

file:flush ()

Saves any written data to file.

file:lines ()

Returns an iterator function that, each time it is called, returns a new line from the file. Therefore, the construction

     for line in file:lines() do body end

will iterate over all lines of the file. (Unlike io.lines, this function does not close the file when the loop ends.)

file:read (···)

Reads the file file, according to the given formats, which specify what to read. For each format, the function returns a string (or a number) with the characters read, or nil if it cannot read data with the specified format. When called without formats, it uses a default format that reads the entire next line (see below).

The available formats are

• "*n": reads a number; this is the only format that returns a number instead of a string.
• "*a": reads the whole file, starting at the current position. On end of file, it returns the empty string.
• "*l": reads the next line (skipping the end of line), returning nil on end of file. This is the default format.
• number: reads a string with up to this number of characters, returning nil on end of file. If number is zero, it reads nothing and returns an empty string, or nil on end of file.

file:seek ([whence] [, offset])

Sets and gets the file position, measured from the beginning of the file, to the position given by offset plus a base specified by the string whence, as follows:

• "set": base is position 0 (beginning of the file);
• "cur": base is current position;
• "end": base is end of file;

In case of success, function seek returns the final file position, measured in bytes from the beginning of the file. If this function fails, it returns nil, plus a string describing the error.

The default value for whence is "cur", and for offset is 0. Therefore, the call file:seek() returns the current file position, without changing it; the call file:seek("set") sets the position to the beginning of the file (and returns 0); and the call file:seek("end") sets the position to the end of the file, and returns its size.

file:setvbuf (mode [, size])

Sets the buffering mode for an output file. There are three available modes:

• "no": no buffering; the result of any output operation appears immediately.
• "full": full buffering; output operation is performed only when the buffer is full (or when you explicitly flush the file (see io.flush)).
• "line": line buffering; output is buffered until a newline is output or there is any input from some special files (such as a terminal device).

For the last two cases, size specifies the size of the buffer, in bytes. The default is an appropriate size.

file:write (···)

Writes the value of each of its arguments to the file. The arguments must be strings or numbers. To write other values, use tostring or string.format before write.

In case of success, this function returns file. Otherwise it returns nil plus a string describing the error.

5.9 - Operating System Facilities

This library is implemented through table os.

os.clock ()

Returns an approximation of the amount in seconds of CPU time used by the program.

os.date ([format [, time]])

Returns a string or a table containing date and time, formatted according to the given string format.

If the time argument is present, this is the time to be formatted (see the os.time function for a description of this value). Otherwise, date formats the current time.

If format starts with '!', then the date is formatted in Coordinated Universal Time. After this optional character, if format is the string "*t", then date returns a table with the following fields: year (four digits), month (1--12), day (1--31), hour (0--23), min (0--59), sec (0--61), wday (weekday, Sunday is 1), yday (day of the year), and isdst (daylight saving flag, a boolean).

If format is not "*t", then date returns the date as a string, formatted according to the same rules as the C function strftime.

When called without arguments, date returns a reasonable date and time representation that depends on the host system and on the current locale (that is, os.date() is equivalent to os.date("%c")).

os.difftime (t2, t1)

Returns the number of seconds from time t1 to time t2. In POSIX, Windows, and some other systems, this value is exactly t2-t1.

os.execute ([command])

This function is equivalent to the C function system. It passes command to be executed by an operating system shell. It returns a status code, which is system-dependent. If command is absent, then it returns nonzero if a shell is available and zero otherwise.

os.exit ([code [, close])

Calls the C function exit, with an optional code, to terminate the host program. The default value for code is the success code.

If the optional second argument close is true, closes the Lua state before exiting.

os.getenv (varname)

Returns the value of the process environment variable varname, or nil if the variable is not defined.

os.remove (filename)

Deletes the file or directory with the given name. Directories must be empty to be removed. If this function fails, it returns nil, plus a string describing the error.

os.rename (oldname, newname)

Renames file or directory named oldname to newname. If this function fails, it returns nil, plus a string describing the error.

os.setlocale (locale [, category])

Sets the current locale of the program. locale is a string specifying a locale; category is an optional string describing which category to change: "all", "collate", "ctype", "monetary", "numeric", or "time"; the default category is "all". The function returns the name of the new locale, or nil if the request cannot be honored.

If locale is the empty string, the current locale is set to an implementation-defined native locale. If locale is the string "C", the current locale is set to the standard C locale.

When called with nil as the first argument, this function only returns the name of the current locale for the given category.

os.time ([table])

Returns the current time when called without arguments, or a time representing the date and time specified by the given table. This table must have fields year, month, and day, and may have fields hour, min, sec, and isdst (for a description of these fields, see the os.date function).

The returned value is a number, whose meaning depends on your system. In POSIX, Windows, and some other systems, this number counts the number of seconds since some given start time (the "epoch"). In other systems, the meaning is not specified, and the number returned by time can be used only as an argument to date and difftime.

os.tmpname ()

Returns a string with a file name that can be used for a temporary file. The file must be explicitly opened before its use and explicitly removed when no longer needed.

On some systems (POSIX), this function also creates a file with that name, to avoid security risks. (Someone else might create the file with wrong permissions in the time between getting the name and creating the file.) You still have to open the file to use it and to remove it (even if you do not use it).

When possible, you may prefer to use io.tmpfile, which automatically removes the file when the program ends.

5.10 - The Debug Library

This library provides the functionality of the debug interface to Lua programs. You should exert care when using this library. The functions provided here should be used exclusively for debugging and similar tasks, such as profiling. Please resist the temptation to use them as a usual programming tool: they can be very slow. Moreover, several of these functions violate some assumptions about Lua code (e.g., that variables local to a function cannot be accessed from outside or that userdata metatables cannot be changed by Lua code) and therefore can compromise otherwise secure code.

All functions in this library are provided inside the debug table. All functions that operate over a thread have an optional first argument which is the thread to operate over. The default is always the current thread.

debug.debug ()

Enters an interactive mode with the user, running each string that the user enters. Using simple commands and other debug facilities, the user can inspect global and local variables, change their values, evaluate expressions, and so on. A line containing only the word cont finishes this function, so that the caller continues its execution.

Note that commands for debug.debug are not lexically nested within any function, and so have no direct access to local variables.

debug.getfenv (o)

debug.gethook ([thread])

Returns the current hook settings of the thread, as three values: the current hook function, the current hook mask, and the current hook count (as set by the debug.sethook function).

debug.getinfo ([thread,] function [, what])

Returns a table with information about a function. You can give the function directly, or you can give a number as the value of function, which means the function running at level function of the call stack of the given thread: level 0 is the current function (getinfo itself); level 1 is the function that called getinfo (except for tail calls, which do not count on the stack); and so on. If function is a number larger than the number of active functions, then getinfo returns nil.

The returned table can contain all the fields returned by lua_getinfo, with the string what describing which fields to fill in. The default for what is to get all information available, except the table of valid lines. If present, the option 'f' adds a field named func with the function itself. If present, the option 'L' adds a field named activelines with the table of valid lines.

For instance, the expression debug.getinfo(1,"n").name returns a table with a name for the current function, if a reasonable name can be found, and the expression debug.getinfo(print) returns a table with all available information about the print function.

debug.getlocal ([thread,] level, local)

This function returns the name and the value of the local variable with index local of the function at level level of the stack. (The first parameter or local variable has index 1, and so on, until the last active local variable.) The function returns nil if there is no local variable with the given index, and raises an error when called with a level out of range. (You can call debug.getinfo to check whether the level is valid.)

Variable names starting with '(' (open parentheses) represent internal variables (loop control variables, temporaries, and C function locals).

debug.getmetatable (object)

Returns the metatable of the given object or nil if it does not have a metatable.

debug.getregistry ()

Returns the registry table (see §3.5).

debug.getupvalue (func, up)

This function returns the name and the value of the upvalue with index up of the function func. The function returns nil if there is no upvalue with the given index.

debug.setfenv (object, table)

Sets the environment of the given object to the given table. Returns object.

debug.sethook ([thread,] hook, mask [, count])

Sets the given function as a hook. The string mask and the number count describe when the hook will be called. The string mask may have the following characters, with the given meaning:

• "c": the hook is called every time Lua calls a function;
• "r": the hook is called every time Lua returns from a function;
• "l": the hook is called every time Lua enters a new line of code.

With a count different from zero, the hook is called after every count instructions.

When called without arguments, debug.sethook turns off the hook.

When the hook is called, its first parameter is a string describing the event that has triggered its call: "call" (or "tail call"), "return", "line", and "count". For line events, the hook also gets the new line number as its second parameter. Inside a hook, you can call getinfo with level 2 to get more information about the running function (level 0 is the getinfo function, and level 1 is the hook function).

debug.setlocal ([thread,] level, local, value)

This function assigns the value value to the local variable with index local of the function at level level of the stack. The function returns nil if there is no local variable with the given index, and raises an error when called with a level out of range. (You can call getinfo to check whether the level is valid.) Otherwise, it returns the name of the local variable.

debug.setmetatable (object, table)

Sets the metatable for the given object to the given table (which can be nil).

debug.setupvalue (func, up, value)

This function assigns the value value to the upvalue with index up of the function func. The function returns nil if there is no upvalue with the given index. Otherwise, it returns the name of the upvalue.

debug.traceback ([thread,] [message] [, level])

If message is present but is not a string, this function returns message without further processing. Otherwise, returns a string with a traceback of the call stack. An optional message string is appended at the beginning of the traceback. An optional level number tells at which level to start the traceback (default is 1, the function calling traceback).

6 - Lua Stand-alone

Although Lua has been designed as an extension language, to be embedded in a host C program, it is also frequently used as a stand-alone language. An interpreter for Lua as a stand-alone language, called simply lua, is provided with the standard distribution. The stand-alone interpreter includes all standard libraries, including the debug library. Its usage is:

     lua [options] [script [args]]

The options are:

• -e stat: executes string stat;
• -l mod: "requires" mod;
• -i: enters interactive mode after running script;
• -v: prints version information;
• --: stops handling options;
• -: executes stdin as a file and stops handling options.

After handling its options, lua runs the given script, passing to it the given args as string arguments. When called without arguments, lua behaves as lua -v -i when the standard input (stdin) is a terminal, and as lua - otherwise.

Before running any argument, the interpreter checks for an environment variable LUA_INIT. If its format is @filename, then lua executes the file. Otherwise, lua executes the string itself.

All options are handled in order, except -i. For instance, an invocation like

     $ lua -e'a=1' -e 'print(a)' script.lua

will first set a to 1, then print the value of a (which is '1'), and finally run the file script.lua with no arguments. (Here $ is the shell prompt. Your prompt may be different.)

Before starting to run the script, lua collects all arguments in the command line in a global table called arg. The script name is stored at index 0, the first argument after the script name goes to index 1, and so on. Any arguments before the script name (that is, the interpreter name plus the options) go to negative indices. For instance, in the call

     $ lua -la b.lua t1 t2

the interpreter first runs the file a.lua, then creates a table

     arg = { [-2] = "lua", [-1] = "-la",
             [0] = "b.lua",
             [1] = "t1", [2] = "t2" }

and finally runs the file b.lua. The script is called with arg[1], arg[2], ··· as arguments; it can also access these arguments with the vararg expression '...'.

In interactive mode, if you write an incomplete statement, the interpreter waits for its completion by issuing a different prompt.

In case of unprotected errors in the script, the interpreter reports the error to the standard error stream. If the error object is a string, the interpreter adds a stack traceback to it. Otherwise, if the error object has a metamethod __tostring, the interpreter calls this metamethod to produce the final message. Finally, if the error object is nil, the interpreter does not report the error.

If the global variable _PROMPT contains a string, then its value is used as the prompt. Similarly, if the global variable _PROMPT2 contains a string, its value is used as the secondary prompt (issued during incomplete statements). Therefore, both prompts can be changed directly on the command line or in any Lua programs by assigning to _PROMPT. See the next example:

     $ lua -e"_PROMPT='myprompt> '" -i

(The outer pair of quotes is for the shell, the inner pair is for Lua.) Note the use of -i to enter interactive mode; otherwise, the program would just end silently right after the assignment to _PROMPT.

To allow the use of Lua as a script interpreter in Unix systems, the stand-alone interpreter skips the first line of a chunk if it starts with #. Therefore, Lua scripts can be made into executable programs by using chmod +x and the #! form, as in

     #!/usr/local/bin/lua

(Of course, the location of the Lua interpreter may be different in your machine. If lua is in your PATH, then

     #!/usr/bin/env lua

is a more portable solution.)

7 - Incompatibilities with the Previous Version

Here we list the incompatibilities that you may find when moving a program from Lua 5.1 to Lua 5.2. You can avoid some incompatibilities compiling Lua with appropriate options (see file luaconf.h). However, all these compatibility options will be removed in the next version of Lua.

7.1 - Changes in the Language

• Lua identifiers cannot use locale-dependent letters.
• Doing a step in the garbage collector does not restart the collector if it has been stopped.
• Weak tables with weak keys now perform like ephemeron tables.
• Threads do not have individual environments. All threads share a sinle fixed environment.

7.2 - Changes in the Libraries

• The debug library is not loaded by default. You must explicitly require it.
• Functions setfenv and getfenv are deprecated. To set the environment of a Lua function, use lexical environments or the new function loadin. (If you really need them, you may use their equivalents in the debug library.)
• Function math.log10 is deprecated. Use math.log with 10 as its second argument, instead.
• Function table.maxn is deprecated. Write it in Lua if you really need it.
• Function unpack was moved into the table library and therefore must be called as table.unpack.

7.3 - Changes in the API

• Pseudoindex LUA_GLOBALSINDEX was deprecated. You may use the pseudoindex LUA_ENVIRONINDEX instead, if the C function does not change its standard environment. Otherwise, you should get the global environment from the registry (see §3.5).
• Macros lua_getglobal, lua_setglobal, and lua_register now operate over the function environment instead of the global environment. (This is more consistent with how Lua manipulates global variables.)
• luaL_typerror was renamed luaL_typeerror, to have a correct spelling.
• Function lua_cpcall is deprecated. Use the new cpcall function defined in the registry instead (see §3.5).
• Functions lua_equal and lua_lessthan are deprecated. Use the new lua_compare with appropriate options instead.
• Function lua_objlen was renamed lua_rawlen.

8 - The Complete Syntax of Lua

Here is the complete syntax of Lua in extended BNF. (It does not describe operator precedences.)

	chunk ::= {stat [`;´]} [laststat [`;´]]

	block ::= chunk

	stat ::=  varlist `=´ explist | 
		 functioncall | 
		 do block end | 
		 while exp do block end | 
		 repeat block until exp | 
		 if exp then block {elseif exp then block} [else block] end | 
		 for Name `=´ exp `,´ exp [`,´ exp] do block end | 
		 for namelist in explist do block end | 
		 function funcname funcbody | 
		 local function Name funcbody | 
		 local namelist [`=´ explist] | 
		 in exp do block end 

	laststat ::= return [explist] | break

	funcname ::= Name {`.´ Name} [`:´ Name]

	varlist ::= var {`,´ var}

	var ::=  Name | prefixexp `[´ exp `]´ | prefixexp `.´ Name 

	namelist ::= Name {`,´ Name}

	explist ::= {exp `,´} exp

	exp ::=  nil | false | true | Number | String | `...´ | function | 
		 prefixexp | tableconstructor | exp binop exp | unop exp 

	prefixexp ::= var | functioncall | `(´ exp `)´

	functioncall ::=  prefixexp args | prefixexp `:´ Name args 

	args ::=  `(´ [explist] `)´ | tableconstructor | String 

	function ::= function funcbody

	funcbody ::= `(´ [parlist] `)´ block end

	parlist ::= namelist [`,´ `...´] | `...´

	tableconstructor ::= `{´ [fieldlist] `}´

	fieldlist ::= field {fieldsep field} [fieldsep]

	field ::= `[´ exp `]´ `=´ exp | Name `=´ exp | exp

	fieldsep ::= `,´ | `;´

	binop ::= `+´ | `-´ | `*´ | `/´ | `^´ | `%´ | `..´ | 
		 `<´ | `<=´ | `>´ | `>=´ | `==´ | `~=´ | 
		 and | or

	unop ::= `-´ | not | `#´