Lua 5.3 Reference Manual

by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, Waldemar Celes

Copyright © 2015 Lua.org, PUC-Rio. Freely available under the terms of the Lua license.

contents · index

1 – Introduction

Lua is an extension programming language designed to support general procedural programming with data description facilities. Lua also offers good support for object-oriented programming, functional programming, and data-driven programming. Lua is intended to be used as a powerful, lightweight, embeddable scripting language for any program that needs one. Lua is implemented as a library, written in clean C, the common subset of Standard C and C++.

As 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. The 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, standalone Lua interpreter, for interactive or batch use.

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.

2 – Basic Concepts

This section describes the basic concepts of the language.

2.1 – 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 both integer numbers and real (floating-point) numbers. String represents immutable sequences of bytes. Lua is 8-bit clean: strings can contain any 8-bit value, including embedded zeros ('\0'). Lua is also encoding-agnostic; it makes no assumptions about the contents of a string.

The type number uses two internal representations, one called integer and the other called float. Lua has explicit rules about when each representation is used, but it also converts between them automatically as needed (see §3.4.3). Therefore, the programmer may choose to mostly ignore the difference between integers and floats or to assume complete control over the representation of each number. Standard Lua uses 64-bit integers and double-precision (64-bit) floats, but you can also compile Lua so that it uses 32-bit integers and/or single-precision (32-bit) floats. The option with 32 bits for both integers and floats is particularly attractive for small machines and embedded systems. (See macro LUA_32BITS in file luaconf.h.)

Lua can call (and manipulate) functions written in Lua and functions written in C (see §3.4.10). Both are represented by the type function.

The type userdata is provided to allow arbitrary C data to be stored in Lua variables. A userdata value represents a block of raw memory. There are two kinds of userdata: full userdata, which is an object with a block of memory managed by Lua, and light userdata, which is simply a C pointer value. Userdata has no predefined operations in Lua, except assignment and identity test. By using metatables, the programmer can define operations for full userdata values (see §2.4). 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 §2.6). Lua threads are not related to operating-system threads. Lua supports coroutines on all systems, even those that do not support threads natively.

The type table implements associative arrays, that is, arrays that can be indexed not only with numbers, but with any Lua value except nil and NaN. (Not a Number is a special numeric value used to represent undefined or unrepresentable results, such as 0/0.) Tables can be heterogeneous; that is, they can contain values of all types (except nil). Any key with value nil is not considered part of the table. Conversely, any key that is not part of a table has an associated value nil.

Tables are the sole data-structuring mechanism in Lua; they can be used to represent ordinary arrays, sequences, 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 §3.4.9).

We use the term sequence to denote a table where the set of all positive numeric keys is equal to {1..n} for some non-negative integer n, which is called the length of the sequence (see §3.4.7).

Like indices, the values of table fields can be of any type. In particular, because functions are first-class values, table fields can contain functions. Thus tables can also carry methods (see §3.4.11).

The indexing of tables follows the definition of raw equality in the language. The expressions a[i] and a[j] denote the same table element if and only if i and j are raw equal (that is, equal without metamethods). In particular, floats with integral values are equal to their respective integers (e.g., 1.0 == 1). To avoid ambiguities, any float with integral value used as a key is converted to its respective integer. For instance, if you write a[2.0] = true, the actual key inserted into the table will be the integer 2. (On the other hand, 2 and "2" are different Lua values and therefore denote different table entries.)

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 (see §6.1).

2.2 – Environments and the Global Environment

As will be discussed in §3.2 and §3.3.3, any reference to a free name (that is, a name not bound to any declaration) var is syntactically translated to _ENV.var. Moreover, every chunk is compiled in the scope of an external local variable named _ENV (see §3.3.2), so _ENV itself is never a free name in a chunk.

Despite the existence of this external _ENV variable and the translation of free names, _ENV is a completely regular name. In particular, you can define new variables and parameters with that name. Each reference to a free name uses the _ENV that is visible at that point in the program, following the usual visibility rules of Lua (see §3.5).

Any table used as the value of _ENV is called an environment.

Lua keeps a distinguished environment called the global environment. This value is kept at a special index in the C registry (see §4.5). In Lua, the global variable _G is initialized with this same value. (_G is never used internally.)

When Lua loads a chunk, the default value for its _ENV upvalue is the global environment (see load). Therefore, by default, free names in Lua code refer to entries in the global environment (and, therefore, they are also called global variables). Moreover, all standard libraries are loaded in the global environment and some functions there operate on that environment. You can use load (or loadfile) to load a chunk with a different environment. (In C, you have to load the chunk and then change the value of its first upvalue.)

2.3 – 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. (When you use Lua standalone, the lua application is the host program.) Whenever an error occurs during the compilation or execution of a Lua chunk, control returns to the host, 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 pcall or xpcall to call a given function in protected mode.

Whenever there is an error, an error object (also called an error message) is propagated with information about the error. Lua itself only generates errors whose error object is a string, but programs may generate errors with any value as the error object. It is up to the Lua program or its host to handle such error objects.

When you use xpcall or lua_pcall, you may give a message handler to be called in case of errors. This function is called with the original error message and returns a new error message. It is called before the error unwinds the stack, so that it can gather more information about the error, for instance by inspecting the stack and creating a stack traceback. This message handler is still protected by the protected call; so, an error inside the message handler will call the message handler again. If this loop goes on for too long, Lua breaks it and returns an appropriate message.

2.4 – Metatables and Metamethods

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" of the value's metatable. If it finds one, Lua calls this function to perform the addition.

The keys in a metatable are derived from the event names; the corresponding values are called 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 using the getmetatable function.

You can replace the metatable of tables using the setmetatable function. You cannot change the metatable of other types from Lua (except by using the debug library (§6.10)); 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 §6.4).

A metatable controls how an object behaves in arithmetic operations, bitwise operations, order comparisons, concatenation, length operation, calls, and indexing. A metatable also can define a function to be called when a userdata or a table is garbage collected (§2.5).

A detailed list of events controlled by metatables is given next. Each operation is identified by its corresponding event name. The key for each event is a string with its name prefixed by two underscores, '__'; for instance, the key for operation "add" is the string "__add". Note that queries for metamethods are always raw; the access to a metamethod does not invoke other metamethods. You can emulate how Lua queries a metamethod for an object obj with the following code: 

     rawget(getmetatable(obj) or {}, "__" .. event_name)

For the unary operators (negation, length, and bitwise not), the metamethod is computed and called with a dummy second operand, equal to the first one. This extra operand is only to simplify Lua's internals (by making these operators behave like a binary operation) and may be removed in future versions. (For most uses this extra operand is irrelevant.)

2.5 – Garbage Collection

Lua performs automatic memory management. This means that you do not have to worry about allocating memory for new objects or freeing it when the objects are no longer needed. Lua manages memory automatically by running a garbage collector 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: strings, tables, userdata, functions, threads, internal structures, 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 (e.g., 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 garbage-collector 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. You should not use values smaller than 100, because they make the collector too slow and can result in the collector never finishing a cycle. The default is 200, which means that the collector runs at "twice" the speed of memory allocation.

If you set the step multiplier to a very large number (larger than 10% of the maximum number of bytes that the program may use), 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. You can also use these functions to control the collector directly (e.g., stop and restart it).

2.5.1 – Garbage-Collection Metamethods

You can set garbage-collector metamethods for tables and, using the C API, for full userdata (see §2.4). 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).

For an object (table or userdata) to be finalized when collected, you must mark it for finalization. You mark an object for finalization when you set its metatable and the metatable has a field indexed by the string "__gc". Note that if you set a metatable without a __gc field and later create that field in the metatable, the object will not be marked for finalization. However, after an object has been marked, you can freely change the __gc field of its metatable.

When a marked object becomes garbage, it is not collected immediately by the garbage collector. Instead, Lua puts it in a list. After the collection, Lua goes through that list. For each object in the list, it checks the object's __gc metamethod: If it is a function, Lua calls it with the object as its single argument; if the metamethod is not a function, Lua simply ignores it.

At the end of each garbage-collection cycle, the finalizers for objects are called in the reverse order that the objects were marked for finalization, among those collected in that cycle; that is, the first finalizer to be called is the one associated with the object marked last in the program. The execution of each finalizer may occur at any point during the execution of the regular code.

Because the object being collected must still be used by the finalizer, that object (and other objects accessible only through it) must be resurrected by Lua. Usually, this resurrection is transient, and the object memory is freed in the next garbage-collection cycle. However, if the finalizer stores the object in some global place (e.g., a global variable), then the resurrection is permanent. Moreover, if the finalizer marks a finalizing object for finalization again, its finalizer will be called again in the next cycle where the object is unreachable. In any case, the object memory is freed only in the GC cycle where the object is unreachable and not marked for finalization.

When you close a state (see lua_close), Lua calls the finalizers of all objects marked for finalization, following the reverse order that they were marked. If any finalizer marks objects for collection during that phase, these marks have no effect.

2.5.2 – 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 that object.

A weak table can have weak keys, weak values, or both. A table with weak keys allows the collection of its keys, but prevents the collection of its values. A table with both weak keys and weak values allows the collection of both keys and values. In any case, if either the key or the value is collected, the whole pair is removed from the table. The weakness of a table is controlled by the __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.

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 through its value, the pair is removed.

Any change in the weakness of a table may take effect only at the next collect cycle. In particular, if you change the weakness to a stronger mode, Lua may still collect some items from that table before the change takes effect.

Only objects that have an explicit construction are removed from weak tables. Values, such as numbers and light C functions, are not subject to garbage collection, and therefore are not removed from weak tables (unless their associated values are collected). Although strings are subject to garbage collection, they do not have an explicit construction, and therefore are not removed from weak tables.

Resurrected objects (that is, objects being finalized and objects accessible only through objects being finalized) have a special behavior in weak tables. They are removed from weak values before running their finalizers, but are removed from weak keys only in the next collection after running their finalizers, when such objects are actually freed. This behavior allows the finalizer to access properties associated with the object through weak tables.

If a weak table is among the resurrected objects in a collection cycle, it may not be properly cleared until the next cycle.

2.6 – 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 by calling 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.

You execute a coroutine by calling coroutine.resume. 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 as arguments to the coroutine's 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 case of normal termination, 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 of how coroutines work, 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

You can also create and manipulate coroutines through the C API: see functions lua_newthread, lua_resume, and lua_yield.

3 – 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.

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 §9 at the end of this manual.

3.1 – 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. Identifiers are used to name variables, table fields, and labels.

The following keywords are reserved and cannot be used as names: 

     and       break     do        else      elseif    end
     false     for       function  goto      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, programs should avoid creating names that start with an underscore followed by one or more uppercase letters (such as _VERSION).

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]). A backslash followed by a real newline results in a newline in the string. The escape sequence '\z' skips the following span of white-space characters, including line breaks; it is particularly useful to break and indent a long literal string into multiple lines without adding the newlines and spaces into the string contents.

Strings in Lua can contain any 8-bit value, including embedded zeros, which can be specified as '\0'. More generally, we can specify any byte in a literal string by its numerical value. This can be done with the escape sequence \xXX, where XX is a sequence of exactly two hexadecimal digits, or with the escape sequence \ddd, where ddd is a sequence of up to three decimal digits. (Note that if a decimal escape sequence is to be followed by a digit, it must be expressed using exactly three digits.)

The UTF-8 encoding of a Unicode character can be inserted in a literal string with the escape sequence \u{XXX} (note the mandatory enclosing brackets), where XXX is a sequence of one or more hexadecimal digits representing the character code point.

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 literal 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 same 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.

Any byte in a literal string not explicitly affected by the previous rules represents itself. However, Lua opens files for parsing in text mode, and the system file functions may have problems with some control characters. So, it is safer to represent non-text data as a quoted literal with explicit escape sequences for non-text 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 (or numeral) can be written with an optional fractional part and an optional decimal exponent, marked by a letter 'e' or 'E'. Lua also accepts hexadecimal constants, which start with 0x or 0X. Hexadecimal constants also accept an optional fractional part plus an optional binary exponent, marked by a letter 'p' or 'P'. A numeric constant with a fractional dot or an exponent denotes a float; otherwise it denotes an integer. Examples of valid integer constants are 

     3   345   0xff   0xBEBADA

Examples of valid float constants are 

     3.0     3.1416     314.16e-2     0.31416E1     34e1
     0x0.1E  0xA23p-4   0X1.921FB54442D18P+1

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.

3.2 – 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 §3.1.

Any variable name is assumed to be global unless explicitly declared as a local (see §3.3.7). Local variables are lexically scoped: local variables can be freely accessed by functions defined inside their scope (see §3.5).

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 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 §2.4 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

An access to a global variable x is equivalent to _ENV.x. Due to the way that chunks are compiled, _ENV is never a global name (see §2.2).

3.3 – 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.

3.3.1 – Blocks

A block is a list of statements, which are executed sequentially: 

	block ::= {stat}

Lua has empty statements that allow you to separate statements with semicolons, start a block with a semicolon or write two semicolons in sequence: 

	stat ::= ‘;’

Function calls and assignments can start with an open parenthesis. This possibility leads to an ambiguity in Lua's grammar. Consider the following fragment: 

     a = b + c
     (print or io.write)('done')

The grammar could see it in two ways: 

     a = b + c(print or io.write)('done')
     
     a = b + c; (print or io.write)('done')

The current parser always sees such constructions in the first way, interpreting the open parenthesis as the start of the arguments to a call. To avoid this ambiguity, it is a good practice to always precede with a semicolon statements that start with a parenthesis: 

     ;(print or io.write)('done')

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 statement in the middle of another block (see §3.3.4).

3.3.2 – Chunks

The unit of compilation of Lua is called a chunk. Syntactically, a chunk is simply a block: 

	chunk ::= block

Lua handles a chunk as the body of an anonymous function with a variable number of arguments (see §3.4.11). As such, chunks can define local variables, receive arguments, and return values. Moreover, such anonymous function is compiled as in the scope of an external local variable called _ENV (see §2.2). The resulting function always has _ENV as its only upvalue, even if it does not use that variable.

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

Chunks can also be precompiled into binary form; see program luac and function string.dump for details. Programs in source and compiled forms are interchangeable; Lua automatically detects the file type and acts accordingly (see load).

3.3.3 – 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 §3.4.

Before the assignment, the list of values is adjusted to the length of the list of variables. If there are more values than needed, the excess values are thrown away. If there are 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 §3.4).

The assignment statement first evaluates all its expressions and only then the assignments are 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 §2.4 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 name x = val is equivalent to the assignment _ENV.x = val (see §2.2).

3.3.4 – 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 §3.3.5).

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 goto statement transfers the program control to a label. For syntactical reasons, labels in Lua are considered statements too: 

	stat ::= goto Name
	stat ::= label
	label ::= ‘::’ Name ‘::’

A label is visible in the entire block where it is defined, except inside nested blocks where a label with the same name is defined and inside nested functions. A goto may jump to any visible label as long as it does not enter into the scope of a local variable.

Labels and empty statements are called void statements, as they perform no actions.

The break statement terminates 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 statement is used to return values from a function or a chunk (which is an anonymous function). Functions can return more than one value, so the syntax for the return statement is 

	stat ::= return [explist] [‘;’]

The return statement can only be written as the last statement of a block. If it is really necessary to return in the middle of a block, then an explicit inner block can be used, as in the idiom do return end, because now return is the last statement in its (inner) block.

3.3.5 – 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
       var = var - step
       while true do
         var = var + step
         if (step >= 0 and var > limit) or (step < 0 and var < limit) then
           break
         end
         local v = var
         block
       end
     end

Note the following:

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)
         if var_1 == nil then break end
         var = var_1
         block
       end
     end

Note the following:

3.3.6 – 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 §3.4.10.

3.3.7 – 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 §3.3.3). Otherwise, all variables are initialized with nil.

A chunk is also a block (see §3.3.2), and so local variables can be declared in a chunk outside any explicit block.

The visibility rules for local variables are explained in §3.5.

3.4 – Expressions

The basic expressions in Lua are the following: 

	exp ::= prefixexp
	exp ::= nil | false | true
	exp ::= Numeral
	exp ::= LiteralString
	exp ::= functiondef
	exp ::= tableconstructor
	exp ::= ‘...’
	exp ::= exp binop exp
	exp ::= unop exp
	prefixexp ::= var | functioncall | ‘(’ exp ‘)’

Numerals and literal strings are explained in §3.1; variables are explained in §3.2; function definitions are explained in §3.4.11; function calls are explained in §3.4.10; table constructors are explained in §3.4.9. Vararg expressions, denoted by three dots ('...'), can only be used when directly inside a vararg function; they are explained in §3.4.11.

Binary operators comprise arithmetic operators (see §3.4.1), bitwise operators (see §3.4.2), relational operators (see §3.4.4), logical operators (see §3.4.5), and the concatenation operator (see §3.4.6). Unary operators comprise the unary minus (see §3.4.1), the unary bitwise not (see §3.4.2), the unary logical not (see §3.4.5), and the unary length operator (see §3.4.7).

Both function calls and vararg expressions can result in multiple values. If a function call is used as a statement (see §3.3.6), 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 expression is enclosed in parentheses). In all other contexts, Lua adjusts the result list to one element, either discarding all values except the first one or adding a single nil if there are no values.

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.)

3.4.1 – Arithmetic Operators

Lua supports the following arithmetic operators:

With the exception of exponentiation and float division, the arithmetic operators work as follows: If both operands are integers, the operation is performed over integers and the result is an integer. Otherwise, if both operands are numbers or strings that can be converted to numbers (see §3.4.3), then they are converted to floats, the operation is performed following the usual rules for floating-point arithmetic (usually the IEEE 754 standard), and the result is a float.

Exponentiation and float division (/) always convert their operands to floats and the result is always a float. Exponentiation uses the ISO C function pow, so that it works for non-integer exponents too.

Floor division (//) is a division that rounds the quotient towards minus infinite, that is, the floor of the division of its operands.

Modulo is defined as the remainder of a division that rounds the quotient towards minus infinite (floor division).

In case of overflows in integer arithmetic, all operations wrap around, according to the usual rules of two-complement arithmetic. (In other words, they return the unique representable integer that is equal modulo 264 to the mathematical result.)

3.4.2 – Bitwise Operators

Lua supports the following bitwise operators:

All bitwise operations convert its operands to integers (see §3.4.3), operate on all bits of those integers, and result in an integer.

Both right and left shifts fill the vacant bits with zeros. Negative displacements shift to the other direction; displacements with absolute values equal to or higher than the number of bits in an integer result in zero (as all bits are shifted out).

3.4.3 – Coercions and Conversions

Lua provides some automatic conversions between some types and representations at run time. Bitwise operators always convert float operands to integers. Exponentiation and float division always convert integer operands to floats. All other arithmetic operations applied to mixed numbers (integers and floats) convert the integer operand to a float; this is called the usual rule. The C API also converts both integers to floats and floats to integers, as needed. Moreover, string concatenation accepts numbers as arguments, besides strings.

Lua also converts strings to numbers, whenever a number is expected.

In a conversion from integer to float, if the integer value has an exact representation as a float, that is the result. Otherwise, the conversion gets the nearest higher or the nearest lower representable value. This kind of conversion never fails.

The conversion from float to integer checks whether the float has an exact representation as an integer (that is, the float has an integral value and it is in the range of integer representation). If it does, that representation is the result. Otherwise, the conversion fails.

The conversion from strings to numbers goes as follows: First, the string is converted to an integer or a float, following its syntax and the rules of the Lua lexer. (The string may have also leading and trailing spaces and a sign.) Then, the resulting number is converted to the required type (float or integer) according to the previous rules.

The conversion from numbers to strings uses a non-specified human-readable format. For complete control over how numbers are converted to strings, use the format function from the string library (see string.format).

3.4.4 – Relational Operators

Lua supports the following relational operators:

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. Strings are compared in the obvious way. Numbers follow the usual rule for binary operations: if both operands are integers, they are compared as integers; otherwise, they are converted to floats and compared as such.

Tables, userdata, and threads are compared by reference: two objects are considered equal only if they are the same object. Every time you create a new object (a table, userdata, or thread), this new object is different from any previously existing object. Closures with the same reference are always equal. Closures with any detectable difference (different behavior, different definition) are always different.

You can change the way that Lua compares tables and userdata by using the "eq" metamethod (see §2.4).

Equality comparisons do not convert strings to numbers or vice versa. 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 following the usual rule for binary operations. 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 §2.4). A comparison a > b is translated to b < a and a >= b is translated to b <= a.

3.4.5 – Logical Operators

The logical operators in Lua are and, or, and not. Like the control structures (see §3.3.4), 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-circuit 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.)

3.4.6 – 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 described in §3.4.3. Otherwise, the __concat metamethod is called (see §2.4).

3.4.7 – 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).

A program can modify the behavior of the length operator for any value but strings through the __len metamethod (see §2.4).

Unless a __len metamethod is given, the length of a table t is only defined if the table is a sequence, that is, the set of its positive numeric keys is equal to {1..n} for some non-negative integer n. In that case, n is its length. Note that a table like 

     {10, 20, nil, 40}

is not a sequence, because it has the key 4 but does not have the key 3. (So, there is no n such that the set {1..n} is equal to the set of positive numeric keys of that table.) Note, however, that non-numeric keys do not interfere with whether a table is a sequence.

3.4.8 – Precedence

Operator precedence in Lua follows the table below, from lower to higher priority: 

     or
     and
     <     >     <=    >=    ~=    ==
     |
     ~
     &
     <<    >>
     ..
     +     -
     *     /     //    %
     unary operators (not   #     -     ~)
     ^

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.

3.4.9 – 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 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

The order of the assignments in a constructor is undefined. (This order would be relevant only when there are repeated keys.)

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 §3.4.10).

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

3.4.10 – 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 §2.4).

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 ::= LiteralString

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.

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

3.4.11 – Function Definitions

The syntax for function definition is 

	functiondef ::= 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 precompiles a chunk, all its function bodies are precompiled 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.

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 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 §3.3.4). If control reaches the end of a function without encountering a return statement, then the function returns with no results.

There is a system-dependent limit on the number of values that a function may return. This limit is guaranteed to be larger than 1000.

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

3.5 – Visibility Rules

Lua is a lexically scoped language. The scope of a local variable begins at the first statement after its declaration and lasts until the last non-void statement 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.

4 – 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. Except where stated otherwise, 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 the macro LUA_USE_APICHECK defined.

4.1 – 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.

4.2 – Stack Size

When you interact with the 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 enough space for pushing new elements.

Whenever Lua calls C, it ensures that the stack has space for at least LUA_MINSTACK extra slots. 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 space 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.

4.3 – Valid and Acceptable Indices

Any function in the API that receives stack indices works only with valid indices or acceptable indices.

A valid index is an index that refers to a real position within the stack, that is, its position lies between 1 and the stack top (1 ≤ abs(index) ≤ top). Usually, functions that can modify the value at an index require valid 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 registry and the upvalues of a C function (see §4.4).

Functions that do not need a specific stack position, but only a value in the stack (e.g., query functions), can be called with acceptable indices. An acceptable index can be any valid index, including the pseudo-indices, but it also can be any positive index after the stack top within the space allocated for the stack, that is, indices up to the stack size. (Note that 0 is never an acceptable index.) Except when noted otherwise, functions in the API work with acceptable indices.

Acceptable indices serve to avoid extra tests against the stack top when querying the stack. For instance, a C function can query its third argument without the need to first check whether there is a third argument, that is, without the need to check whether 3 is a valid index.

For functions that can be called with acceptable indices, any non-valid index is treated as if it contains a value of a virtual type LUA_TNONE, which behaves like a nil value.

4.4 – C Closures

When a C function is created, it is possible to associate some values with it, thus creating a C closure (see lua_pushcclosure); these values are called upvalues and are accessible to the function whenever it is called.

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.

4.5 – Registry

Lua provides a registry, a predefined table that can be used by any C code to store whatever Lua values it needs to store. The registry table is always located at pseudo-index LUA_REGISTRYINDEX, which is a valid index. Any C library can store data into this table, but it must take care to choose keys that are 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, or any Lua object created by your code. As with variable names, string keys starting with an underscore followed by uppercase letters are reserved for Lua.

The integer keys in the registry are used by the reference mechanism (see luaL_ref) and by some predefined values. Therefore, integer keys must 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:

4.6 – Error Handling in C

Internally, Lua uses the C longjmp facility to handle errors. (Lua will use exceptions if you compile it as C++; search for LUAI_THROW in the source code for details.) 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 recovery point; any error jumps to the most recent active recovery 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 to your own recovery point outside Lua).

The panic function runs as if it were a message handler (see §2.3); in particular, the error message is at the top of the stack. However, there is no guarantee about stack space. To push anything on the stack, the panic function must first check the available space (see §4.2).

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

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

4.7 – Handling Yields in C

Internally, Lua uses the C longjmp facility to yield a coroutine. Therefore, if a C 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 across an API call, except for three functions: lua_yieldk, lua_callk, and lua_pcallk. All those functions receive a continuation function (as a parameter named k) to continue execution after a yield.

We need to set some terminology to explain continuations. We have a C function called from Lua which we will call the original function. This original function then calls one of those three functions in the C API, which we will call the callee function, that then yields the current thread. (This can 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.

As an illustration, consider the following function: 

     int original_function (lua_State *L) {
       ...     /* code 1 */
       status = lua_pcall(L, n, m, h);  /* calls Lua */
       ...     /* code 2 */
     }

Now we want to allow the Lua code being run by lua_pcall to yield. First, we can rewrite our function like here: 

     int k (lua_State *L, int status, lua_KContext ctx) {
       ...  /* code 2 */
     }
     
     int original_function (lua_State *L) {
       ...     /* code 1 */
       return k(L, lua_pcall(L, n, m, h), ctx);
     }

In the above code, the new function k is a continuation function (with type lua_KFunction), which should do all the work that the original function was doing after calling lua_pcall. Now, we must inform Lua that it must call k if the Lua code being executed by lua_pcall gets interrupted in some way (errors or yielding), so we rewrite the code as here, replacing lua_pcall by lua_pcallk: 

     int original_function (lua_State *L) {
       ...     /* code 1 */
       return k(L, lua_pcallk(L, n, m, h, ctx2, k), ctx1);
     }

Note the external, explicit call to the continuation: Lua will call the continuation only if needed, that is, in case of errors or resuming after a yield. If the called function returns normally without ever yielding, lua_pcallk (and lua_callk) will also return normally. (Of course, instead of calling the continuation in that case, you can do the equivalent work directly inside the original function.)

Besides the Lua state, the continuation function has two other parameters: the final status of the call plus the context value (ctx) that was passed originally to lua_pcallk. (Lua does not use this context value; it only passes this value from the original function to the continuation function.) For lua_pcallk, the status is the same value that would be returned by lua_pcallk, except that it is LUA_YIELD when being executed after a yield (instead of LUA_OK). For lua_yieldk and lua_callk, the status is always LUA_YIELD when Lua calls the continuation. (For these two functions, Lua will not call the continuation in case of errors, because they do not handle errors.) Similarly, when using lua_callk, you should call the continuation function with LUA_OK as the status. (For lua_yieldk, there is not much point in calling directly the continuation function, because lua_yieldk usually does not return.)

Lua treats the continuation function as if it were 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 were the return of the original function.

4.8 – 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 raise errors: '-' means the function never raises any error; 'e' means the function may raise errors; 'v' means the function may raise an error on purpose.

lua_absindex

[-0, +0, –] 

int lua_absindex (lua_State *L, int idx);

Converts the acceptable index idx into an absolute index (that is, one that does not depend on the stack top).

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; and nsize, the new size of the block.

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

When ptr is NULL, osize encodes 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, the allocator must behave like free and return NULL.

When nsize is not zero, the allocator must behave like realloc. The allocator returns NULL if and only if it cannot fulfill 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);
     }

Note that Standard C ensures that free(NULL) has no effect and that realloc(NULL,size) is equivalent to malloc(size). This code assumes that realloc does not fail when shrinking a block. (Although Standard C does not ensure this behavior, it seems to be a safe assumption.)

lua_arith

[-(2|1), +1, e] 

void lua_arith (lua_State *L, int op);

Performs an arithmetic or bitwise operation over the two values (or one, in the case of negations) at the top of the stack, with the value at the top being the second operand, pops these values, and pushes the result of the operation. The function follows the semantics of the corresponding Lua operator (that is, it may call metamethods).

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

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 §4.6).

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_getglobal(L, "f");                  /* function to be called */
     lua_pushliteral(L, "how");                       /* 1st argument */
     lua_getglobal(L, "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_setglobal(L, "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,
                lua_KContext ctx,
                lua_KFunction k);

This function behaves exactly like lua_call, but allows the called function to yield (see §4.7).

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 their sum: 

     static int foo (lua_State *L) {
       int n = lua_gettop(L);    /* number of arguments */
       lua_Number sum = 0.0;
       int i;
       for (i = 1; i <= n; i++) {
         if (!lua_isnumber(L, i)) {
           lua_pushliteral(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 n);

Ensures that the stack has space for at least n extra slots. It returns false if it cannot fulfill the request, either because it would cause the stack to be larger than a fixed maximum size (typically at least several thousand elements) or because it cannot allocate memory for the extra space. 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 that create multiple states, such as daemons or web servers, will probably need to close states as soon as they are not needed.

lua_compare

[-0, +0, e] 

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

Compares two Lua values. Returns 1 if the value at index index1 satisfies op when compared with the value at index index2, following the semantics of the corresponding Lua operator (that is, it may call metamethods). Otherwise returns 0. Also returns 0 if any of the indices is not valid.

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

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 §3.4.6).

lua_copy

[-0, +0, –] 

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

Copies the element at index fromidx into the valid index toidx, replacing the value at that position. Values at other positions are not affected.

lua_createtable

[-0, +1, e] 

void lua_createtable (lua_State *L, int narr, int nrec);

Creates a new empty table and pushes it onto the stack. Parameter narr is a hint for how many elements the table will have as a sequence; parameter nrec is a hint for how many other elements the table will have. Lua may use these hints to preallocate memory for the new table. This pre-allocation is useful for performance when you know in advance how many elements the table will have. Otherwise you can use the function lua_newtable.

lua_dump

[-0, +0, e] 

int lua_dump (lua_State *L,
                        lua_Writer writer,
                        void *data,
                        int strip);

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.

If strip is true, the binary representation is created without debug information about the function.

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, using the value at the top of the stack as the error object. 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:

For more details about these options, see collectgarbage.

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 given when the memory-allocator function was set.

lua_getfield

[-0, +1, e] 

int 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 index. As in Lua, this function may trigger a metamethod for the "index" event (see §2.4).

Returns the type of the pushed value.

lua_getextraspace

[-0, +0, –] 

void *lua_getextraspace (lua_State *L);

Returns a pointer to a raw memory area associated with the given Lua state. The application can use this area for any purpose; Lua does not use it for anything.

Each new thread has this area initialized with a copy of the area of the main thread.

By default, this area has the size of a pointer to void, but you can recompile Lua with a different size for this area. (See LUA_EXTRASPACE in luaconf.h.)

lua_getglobal

[-0, +1, e] 

int lua_getglobal (lua_State *L, const char *name);

Pushes onto the stack the value of the global name. Returns the type of that value.

lua_geti

[-0, +1, e] 

int lua_geti (lua_State *L, int index, lua_Integer i);

Pushes onto the stack the value t[i], where t is the value at the given index. As in Lua, this function may trigger a metamethod for the "index" event (see §2.4).

Returns the type of the pushed value.

lua_getmetatable

[-0, +(0|1), –] 

int lua_getmetatable (lua_State *L, int index);

If the value at the given index has a metatable, the function pushes that metatable onto the stack and returns 1. Otherwise, the function returns 0 and pushes nothing on the stack.

lua_gettable

[-1, +1, e] 

int lua_gettable (lua_State *L, int index);

Pushes onto the stack the value t[k], where t is the value at the given index and k is the value at the top of the stack.

This function pops the key from the stack, pushing the resulting value in its place. As in Lua, this function may trigger a metamethod for the "index" event (see §2.4).

Returns the type of the pushed value.

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; in particular, 0 means an empty stack.

lua_getuservalue

[-0, +1, –] 

int lua_getuservalue (lua_State *L, int index);

Pushes onto the stack the Lua value associated with the userdata at the given index.

Returns the type of the pushed value.

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. This function cannot be called with a pseudo-index, because a pseudo-index is not an actual stack position.

lua_Integer

typedef ... lua_Integer;

The type of integers in Lua.

By default this type is long long, (usually a 64-bit two-complement integer), but that can be changed to long or int (usually a 32-bit two-complement integer). (See LUA_INT in luaconf.h.)

Lua also defines the constants LUA_MININTEGER and LUA_MAXINTEGER, with the minimum and the maximum values that fit in this type.

lua_isboolean

[-0, +0, –] 

int lua_isboolean (lua_State *L, int index);

Returns 1 if the value at the given index is a boolean, and 0 otherwise.

lua_iscfunction

[-0, +0, –] 

int lua_iscfunction (lua_State *L, int index);

Returns 1 if the value at the given 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 index is a function (either C or Lua), and 0 otherwise.

lua_isinteger

[-0, +0, –] 

int lua_isinteger (lua_State *L, int index);

Returns 1 if the value at the given index is an integer (that is, the value is a number and is represented as an integer), and 0 otherwise.

lua_islightuserdata

[-0, +0, –] 

int lua_islightuserdata (lua_State *L, int index);

Returns 1 if the value at the given 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 index is nil, and 0 otherwise.

lua_isnone

[-0, +0, –] 

int lua_isnone (lua_State *L, int index);

Returns 1 if the given index is not valid, and 0 otherwise.

lua_isnoneornil

[-0, +0, –] 

int lua_isnoneornil (lua_State *L, int index);

Returns 1 if the given index is not valid 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 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 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 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 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 index is a userdata (either full or light), and 0 otherwise.

lua_isyieldable

[-0, +0, –] 

int lua_isyieldable (lua_State *L);

Returns 1 if the given coroutine can yield, and 0 otherwise.

lua_KContext

typedef ... lua_KContext;

The type for continuation-function contexts. It must be a numerical type. This type is defined as intptr_t when intptr_t is available, so that it can store pointers too. Otherwise, it is defined as ptrdiff_t.

lua_KFunction

typedef int (*lua_KFunction) (lua_State *L, int status, lua_KContext ctx);

Type for continuation functions (see §4.7).

lua_len

[-0, +1, e] 

void lua_len (lua_State *L, int index);

Returns the length of the value at the given index. It is equivalent to the '#' operator in Lua (see §3.4.7) and may trigger a metamethod for the "length" event (see §2.4). The result is pushed on the stack.

lua_load

[-0, +1, –] 

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

Loads a Lua chunk without running it. 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:

The lua_load function uses a user-supplied reader function to read the chunk (see lua_Reader). The data argument is an opaque value passed to the reader function.

The chunkname argument gives a name to the chunk, which is used for error messages and in debug information (see §4.9).

lua_load automatically detects whether the chunk is text or binary and loads it accordingly (see program luac). The string mode works as in function load, with the addition that a NULL value is equivalent to the string "bt".

lua_load uses the stack internally, so the reader function must always leave the stack unmodified when returning.

If the resulting function has upvalues, its first upvalue is set to the value of the global environment stored at index LUA_RIDX_GLOBALS in the registry (see §4.5). When loading main chunks, this upvalue will be the _ENV variable (see §2.2). Other upvalues are initialized with nil.

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 it cannot create the thread or the state (due to lack of memory). The argument f is the allocator function; Lua does all memory allocation for this state through this function. The second argument, ud, is an opaque pointer that Lua passes to the allocator in every call.

lua_newtable

[-0, +1, e] 

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, e] 

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 its global environment, 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, e] 

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. The host program can freely use this 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 may change the value at the given index; this confuses the next call to lua_next.

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

lua_Number

typedef double lua_Number;

The type of floats in Lua.

By default this type is double, but that can be changed to a single float. (See LUA_REAL in luaconf.h.)

lua_numbertointeger

int lua_numbertointeger (lua_Number n, lua_Integer *p);

Converts a Lua float to a Lua integer. This macro assumes that n has an integral value. If that value is within the range of Lua integers, it is converted to an integer and assigned to *p. The macro results in a boolean indicating whether the conversion was successful. (Note that this range test can be tricky to do correctly without this macro, due to roundings.)

This macro may evaluate its arguments more than once.

lua_pcall

[-(nargs + 1), +(nresults|1), –] 

int lua_pcall (lua_State *L, int nargs, int nresults, int msgh);

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 msgh is 0, then the error message returned on the stack is exactly the original error message. Otherwise, msgh is the stack index of a message handler. (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 message handler 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 constants (defined in lua.h):

lua_pcallk

[-(nargs + 1), +(nresults|1), –] 

int lua_pcallk (lua_State *L,
                int nargs,
                int nresults,
                int msgh,
                lua_KContext ctx,
                lua_KFunction k);

This function behaves exactly like lua_pcall, but allows the called function to yield (see §4.7).

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, e] 

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 §4.4); these values are then accessible to the function whenever it is called. To associate values with a C function, first these values must 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 will be associated with the function. lua_pushcclosure also pops these values from the stack.

The maximum value for n is 255.

When n is zero, this function creates a light C function, which is just a pointer to the C function. In that case, it never raises a memory error.

lua_pushcfunction

[-0, +1, –] 

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)

Note that f is used twice.

lua_pushfstring

[-0, +1, e] 

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 ISO C function sprintf, but has some important differences:

lua_pushglobaltable

[-0, +1, –] 

void lua_pushglobaltable (lua_State *L);

Pushes the global environment onto the stack.

lua_pushinteger

[-0, +1, –] 

void lua_pushinteger (lua_State *L, lua_Integer n);

Pushes an integer 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, e] 

const char *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. It automatically provides the string length.

lua_pushlstring

[-0, +1, e] 

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 any binary data, including 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 float with value n onto the stack.

lua_pushstring

[-0, +1, e] 

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.

Returns a pointer to the internal copy of the string.

If s is NULL, pushes nil and returns NULL.

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 index onto the stack.

lua_pushvfstring

[-0, +1, e] 

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 indices index1 and index2 are primitively equal (that is, without calling metamethods). Otherwise returns 0. Also returns 0 if any of the indices are not valid.

lua_rawget

[-1, +1, –] 

int lua_rawget (lua_State *L, int index);

Similar to lua_gettable, but does a raw access (i.e., without metamethods).

lua_rawgeti

[-0, +1, –] 

int lua_rawgeti (lua_State *L, int index, lua_Integer n);

Pushes onto the stack the value t[n], where t is the table at the given index. The access is raw; that is, it does not invoke metamethods.

Returns the type of the pushed value.

lua_rawgetp

[-0, +1, –] 

int lua_rawgetp (lua_State *L, int index, const void *p);

Pushes onto the stack the value t[k], where t is the table at the given index and k is the pointer p represented as a light userdata. The access is raw; that is, it does not invoke metamethods.

Returns the type of the pushed value.

lua_rawlen

[-0, +0, –] 

size_t lua_rawlen (lua_State *L, int index);

Returns the raw "length" of the value at the given 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, e] 

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, e] 

void lua_rawseti (lua_State *L, int index, lua_Integer i);

Does the equivalent of t[i] = v, where t is the table at the given 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_rawsetp

[-1, +0, e] 

void lua_rawsetp (lua_State *L, int index, const void *p);

Does the equivalent of t[k] = v, where t is the table at the given index, k is the pointer p represented as a light userdata, 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. This function 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 valid index without shifting any element (therefore replacing the value at the given index), and then pops the top element.

lua_resume

[-?, +?, –] 

int lua_resume (lua_State *L, lua_State *from, int nargs);

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 nargs 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 remove any results from the last lua_yield, put on its stack only the values to be passed as results from yield, and then call lua_resume.

The parameter from represents the coroutine that is resuming L. If there is no such coroutine, this parameter can be NULL.

lua_rotate

[-0, +0, –] 

void lua_rotate (lua_State *L, int idx, int n);

Rotates the stack elements from idx to the top n positions in the direction of the top, for a positive n, or -n positions in the direction of the bottom, for a negative n. The absolute value of n must not be greater than the size of the slice being rotated.

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_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 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 §2.4).

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.

lua_seti

[-1, +0, e] 

void lua_seti (lua_State *L, int index, lua_Integer n);

Does the equivalent to t[n] = v, where t is the value at the given 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 §2.4).

lua_setmetatable

[-1, +0, –] 

void 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 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 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 §2.4).

lua_settop

[-?, +?, –] 

void lua_settop (lua_State *L, int index);

Accepts any 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_setuservalue

[-1, +0, –] 

void lua_setuservalue (lua_State *L, int index);

Pops a value from the stack and sets it as the new value associated to the userdata at the given index.

lua_State

typedef struct lua_State lua_State;

An opaque structure that points to a thread and indirectly (through the thread) to the whole state of a Lua interpreter. The Lua library is fully reentrant: it has no global variables. All information about a state is accessible through this structure.

A pointer to this structure 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.

You can only call functions in threads with status LUA_OK. You can resume threads with status LUA_OK (to start a new coroutine) or LUA_YIELD (to resume a coroutine).

lua_stringtonumber

[-0, +1, –] 

size_t lua_stringtonumber (lua_State *L, const char *s);

Converts the zero-terminated string s to a number, pushes that number into the stack, and returns the total size of the string, that is, its length plus one. The conversion can result in an integer or a float, according to the lexical conventions of Lua (see §3.1). The string may have leading and trailing spaces and a sign. If the string is not a valid numeral, returns 0 and pushes nothing. (Note that the result can be used as a boolean, true if the conversion succeeds.)

lua_toboolean

[-0, +0, –] 

int lua_toboolean (lua_State *L, int index);

Converts the Lua value at the given 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. (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 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);

Equivalent to lua_tointegerx with isnum equal to NULL.

lua_tointegerx

[-0, +0, –] 

lua_Integer lua_tointegerx (lua_State *L, int index, int *isnum);

Converts the Lua value at the given index to the signed integral type lua_Integer. The Lua value must be an integer, or a number or string convertible to an integer (see §3.4.3); otherwise, lua_tointegerx returns 0.

If isnum is not NULL, its referent is assigned a boolean value that indicates whether the operation succeeded.

lua_tolstring

[-0, +0, e] 

const char *lua_tolstring (lua_State *L, int index, size_t *len);

Converts the Lua value at the given 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 Lua value is removed from the stack.

lua_tonumber

[-0, +0, –] 

lua_Number lua_tonumber (lua_State *L, int index);

Equivalent to lua_tonumberx with isnum equal to NULL.

lua_tonumberx

[-0, +0, –] 

lua_Number lua_tonumberx (lua_State *L, int index, int *isnum);

Converts the Lua value at the given 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 §3.4.3); otherwise, lua_tonumberx returns 0.

If isnum is not NULL, its referent is assigned a boolean value that indicates whether the operation succeeded.

lua_topointer

[-0, +0, –] 

const void *lua_topointer (lua_State *L, int index);

Converts the value at the given 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, e] 

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 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 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 valid index, or LUA_TNONE for a non-valid (but acceptable) index. 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_Unsigned

typedef ... lua_Unsigned;

The unsigned version of lua_Integer.

lua_upvalueindex

[-0, +0, –] 

int lua_upvalueindex (int i);

Returns the pseudo-index that represents the i-th upvalue of the running function (see §4.4).

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 state.

This function pops n values from the stack from, and pushes them onto the stack to.

lua_yield

[-?, +?, e] 

int lua_yield (lua_State *L, int nresults);

This function is equivalent to lua_yieldk, but it has no continuation (see §4.7). Therefore, when the thread resumes, it continues the function that called the function calling lua_yield.

lua_yieldk

[-?, +?, e] 

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

Yields a coroutine (thread).

When a C function calls lua_yieldk, 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 will be 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 §4.7). This continuation function receives the same stack from the previous function, with the n results removed and replaced by the arguments passed to lua_resume. Moreover, the continuation function receives the value ctx that was passed to lua_yieldk.

Usually, this function does not return; when the coroutine eventually resumes, it continues executing the continuation function. However, there is one special case, which is when this function is called from inside a line hook (see §4.9). In that case, lua_yieldk should be called with no continuation (probably in the form of lua_yield), and the hook should return immediately after the call. Lua will yield and, when the coroutine resumes again, it will continue the normal execution of the (Lua) function that triggered the hook.

This function can raise an error if it is called from a thread with a pending C call with no continuation function, or it is called from a thread that is not running inside a resume (e.g., the main thread).

4.9 – 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 a function or an activation record. 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:

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), e] 

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

Gets 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_getglobal(L, "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:

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, const lua_Debug *ar, int n);

Gets information about a local variable of a given activation record or a given function.

In the first case, 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; see debug.getlocal for details about variable indices and names.

lua_getlocal pushes the variable's value onto the stack and returns its name.

In the second case, ar must be NULL and the function to be inspected must be at the top of the stack. In this case, only parameters of Lua functions are visible (as there is no information about what variables are active) and no values are pushed onto the stack.

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);

Gets 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 (except for tail calls, which do not count on the stack). 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.

Hook functions cannot have continuations, that is, they cannot call lua_yieldk, lua_pcallk, or lua_callk with a non-null k.

Hook functions can yield under the following conditions: Only count and line events can yield and they cannot yield any value; to yield a hook function must finish its execution calling lua_yield with nresults equal to zero.

lua_sethook

[-0, +0, –] 

void 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:

A hook is disabled by setting mask to zero.

lua_setlocal

[-(0|1), +0, –] 

const char *lua_setlocal (lua_State *L, const 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 a unique identifier for the upvalue numbered n from the closure at index funcindex. 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.

lua_upvaluejoin

[-0, +0, –] 

void lua_upvaluejoin (lua_State *L, int funcindex1, int n1,
                                    int funcindex2, int n2);

Make the n1-th upvalue of the Lua closure at index funcindex1 refer to the n2-th upvalue of the Lua closure at index funcindex2.

5 – 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 and types 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. Nevertheless, the use of the auxiliary library ensures more consistency to your code.

Several functions in the auxiliary library use internally some extra stack slots. When a function in the auxiliary library uses less than five slots, it does not check the stack size; it simply assumes that there are enough slots.

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 raise an error if the check is not satisfied.

5.1 – Functions and Types

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

luaL_addchar

[-?, +?, e] 

void luaL_addchar (luaL_Buffer *B, char c);

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

luaL_addlstring

[-?, +?, e] 

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 can contain embedded zeros.

luaL_addsize

[-?, +?, e] 

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

[-?, +?, e] 

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).

luaL_addvalue

[-1, +?, e] 

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 arg,
                    const char *extramsg);

Checks whether cond is true. If it is not, raises an error with a standard message (see luaL_argerror).

luaL_argerror

[-0, +0, v] 

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

Raises an error reporting a problem with argument arg of the C function that called it, using a standard message that includes extramsg as a comment: 

     bad argument #arg to 'funcname' (extramsg)

This function never returns.

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:

If you know beforehand the total size of the resulting string, you can use the buffer like this:

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_buffinitsize

[-?, +?, e] 

char *luaL_buffinitsize (lua_State *L, luaL_Buffer *B, size_t sz);

Equivalent to the sequence luaL_buffinit, luaL_prepbuffsize.

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 passing 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 arg);

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

luaL_checkinteger

[-0, +0, v] 

lua_Integer luaL_checkinteger (lua_State *L, int arg);

Checks whether the function argument arg is an integer (or can be converted to an integer) and returns this integer cast to a lua_Integer.

luaL_checklstring

[-0, +0, v] 

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

Checks whether the function argument arg 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 arg);

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

luaL_checkoption

[-0, +0, v] 

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

Checks whether the function argument arg 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 arg or when 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 arg);

Checks whether the function argument arg 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 arg, int t);

Checks whether the function argument arg 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 arg, const char *tname);

Checks whether the function argument arg 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, +?, e] 

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_execresult

[-0, +3, e] 

int luaL_execresult (lua_State *L, int stat);

This function produces the return values for process-related functions in the standard library (os.execute and io.close).

luaL_fileresult

[-0, +(1|3), e] 

int luaL_fileresult (lua_State *L, int stat, const char *fname);

This function produces the return values for file-related functions in the standard library (io.open, os.rename, file:seek, etc.).

luaL_getmetafield

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

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 and returns the type of pushed value. If the object does not have a metatable, or if the metatable does not have this field, pushes nothing and returns LUA_TNIL.

luaL_getmetatable

[-0, +1, –] 

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

Pushes onto the stack the metatable associated with name tname in the registry (see luaL_newmetatable). If there is no metatable associated with tname, returns false and pushes nil.

luaL_getsubtable

[-0, +1, e] 

int luaL_getsubtable (lua_State *L, int idx, const char *fname);

Ensures that the value t[fname], where t is the value at index idx, is a table, and pushes that table onto the stack. Returns true if it finds a previous table there and false if it creates a new table.

luaL_gsub

[-0, +1, e] 

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, +0, e] 

lua_Integer luaL_len (lua_State *L, int index);

Returns the "length" of the value at the given index as a number; it is equivalent to the '#' operator in Lua (see §3.4.7). Raises an error if the result of the operation is not an integer. (This case only can happen through metamethods.)

luaL_loadbuffer

[-0, +1, –] 

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

Equivalent to luaL_loadbufferx with mode equal to NULL.

luaL_loadbufferx

[-0, +1, –] 

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

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. The string mode works as in function lua_load.

luaL_loadfile

[-0, +1, e] 

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

Equivalent to luaL_loadfilex with mode equal to NULL.

luaL_loadfilex

[-0, +1, e] 

int luaL_loadfilex (lua_State *L, const char *filename,
                                            const char *mode);

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 #.

The string mode works as in function lua_load.

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 or the file has a wrong mode.

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_newlib

[-0, +1, e] 

void luaL_newlib (lua_State *L, const luaL_Reg l[]);

Creates a new table and registers there the functions in list l.

It is implemented as the following macro: 

     (luaL_newlibtable(L,l), luaL_setfuncs(L,l,0))

The array l must be the actual array, not a pointer to it.

luaL_newlibtable

[-0, +1, e] 

void luaL_newlibtable (lua_State *L, const luaL_Reg l[]);

Creates a new table with a size optimized to store all entries in the array l (but does not actually store them). It is intended to be used in conjunction with luaL_setfuncs (see luaL_newlib).

It is implemented as a macro. The array l must be the actual array, not a pointer to it.

luaL_newmetatable

[-0, +1, e] 

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 to this new table the pair __name = tname, adds to the registry the pair [tname] = new table, and returns 1. (The entry __name is used by some error-reporting functions.)

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 §4.6) 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, e] 

void luaL_openlibs (lua_State *L);

Opens all standard Lua libraries into the given state.

luaL_optinteger

[-0, +0, v] 

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

If the function argument arg is an integer (or convertible to an integer), returns this integer. 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 arg,
                             const char *d,
                             size_t *l);

If the function argument arg 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 result's length.

luaL_optnumber

[-0, +0, v] 

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

If the function argument arg 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 arg,
                            const char *d);

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

luaL_prepbuffer

[-?, +?, e] 

char *luaL_prepbuffer (luaL_Buffer *B);

Equivalent to luaL_prepbuffsize with the predefined size LUAL_BUFFERSIZE.

luaL_prepbuffsize

[-?, +?, e] 

char *luaL_prepbuffsize (luaL_Buffer *B, size_t sz);

Returns an address to a space of size sz 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, e] 

void luaL_pushresult (luaL_Buffer *B);

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

luaL_pushresultsize

[-?, +1, e] 

void luaL_pushresultsize (luaL_Buffer *B, size_t sz);

Equivalent to the sequence luaL_addsize, luaL_pushresult.

luaL_ref

[-1, +0, e] 

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_setfuncs. name is the function name and func is a pointer to the function. Any array of luaL_Reg must end with a sentinel entry in which both name and func are NULL.

luaL_requiref

[-0, +1, e] 

void luaL_requiref (lua_State *L, const char *modname,
                    lua_CFunction openf, int glb);

If modname is not already present in package.loaded, calls function openf with string modname as an argument and sets the call result in package.loaded[modname], as if that function has been called through require.

If glb is true, also stores the module into global modname.

Leaves a copy of the module on the stack.

luaL_setfuncs

[-nup, +0, e] 

void luaL_setfuncs (lua_State *L, const luaL_Reg *l, int nup);

Registers all functions in the array l (see luaL_Reg) into the table on the top of the stack (below optional upvalues, see next).

When nup is not zero, all functions are created sharing nup upvalues, which must be previously pushed on the stack on top of the library table. These values are popped from the stack after the registration.

luaL_setmetatable

[-0, +0, –] 

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

Sets the metatable of the object at the top of the stack as the metatable associated with name tname in the registry (see luaL_newmetatable).

luaL_Stream

typedef struct luaL_Stream {
  FILE *f;
  lua_CFunction closef;
} luaL_Stream;

The standard representation for file handles, which is used by the standard I/O library.

A file handle is implemented as a full userdata, with a metatable called LUA_FILEHANDLE (where LUA_FILEHANDLE is a macro with the actual metatable's name). The metatable is created by the I/O library (see luaL_newmetatable).

This userdata must start with the structure luaL_Stream; it can contain other data after this initial structure. Field f points to the corresponding C stream (or it can be NULL to indicate an incompletely created handle). Field closef points to a Lua function that will be called to close the stream when the handle is closed or collected; this function receives the file handle as its sole argument and must return either true (in case of success) or nil plus an error message (in case of error). Once Lua calls this field, the field value is changed to NULL to signal that the handle is closed.

luaL_testudata

[-0, +0, e] 

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

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

luaL_tolstring

[-0, +1, e] 

const char *luaL_tolstring (lua_State *L, int idx, size_t *len);

Converts any Lua value at the given index to a C string in a reasonable format. The resulting string is pushed onto the stack and also returned by the function. If len is not NULL, the function also sets *len with the string length.

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

luaL_traceback

[-0, +1, e] 

void 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_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, e] 

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.

6 – 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:

Except for the basic and the 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, the host program can open them individually by using luaL_requiref to call luaopen_base (for the basic library), luaopen_package (for the package library), luaopen_coroutine (for the coroutine library), luaopen_string (for the string library), luaopen_utf8 (for the UTF8 library), luaopen_table (for the table library), luaopen_math (for the mathematical 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.

6.1 – Basic Functions

The basic library provides 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])

Calls error if the value of its argument v is false (i.e., nil or false); otherwise, returns all its arguments. In case of error, message is the error object; when absent, it defaults to "assertion failed!"

collectgarbage ([opt [, arg]])

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

dofile ([filename])

Opens the named file and executes its contents as a Lua chunk. When called without arguments, dofile executes the contents of the standard input (stdin). Returns all values returned by the chunk. In case of errors, dofile propagates the error to its caller (that is, dofile does not run in protected mode).

error (message [, level])

Terminates the last protected function called and returns message as the error object. Function error never returns.

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

A global variable (not a function) that holds the global environment (see §2.2). Lua itself does not use this variable; changing its value does not affect any environment, nor vice versa.

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)

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 key–value pairs (1,t[1]), (2,t[2]), ..., up to the first nil value.

load (chunk [, chunkname [, mode [, env]]])

Loads a chunk.

If chunk is a string, the chunk is this string. If chunk is a function, load calls it repeatedly to get the chunk pieces. Each call to chunk 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 there are no syntactic errors, returns the compiled chunk as a function; otherwise, returns nil plus the error message.

If the resulting function has upvalues, the first upvalue is set to the value of env, if that parameter is given, or to the value of the global environment. Other upvalues are initialized with nil. (When you load a main chunk, the resulting function will always have exactly one upvalue, the _ENV variable (see §2.2). However, when you load a binary chunk created from a function (see string.dump), the resulting function can have an arbitrary number of upvalues.) All upvalues are fresh, that is, they are not shared with any other function.

chunkname is used as the name of the chunk for error messages and debug information (see §4.9). When absent, it defaults to chunk, if chunk is a string, or to "=(load)" otherwise.

The string mode controls whether the chunk can be text or binary (that is, a precompiled chunk). It may be the string "b" (only binary chunks), "t" (only text chunks), or "bt" (both binary and text). The default is "bt".

Lua does not check the consistency of binary chunks. Maliciously crafted binary chunks can crash the interpreter.

loadfile ([filename [, mode [, env]]])

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

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.)

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 (···)

Receives any number of arguments and prints their values to stdout, using the tostring function to convert each argument to a string. print is not intended for formatted output, but only as a quick way to show a value, for instance for debugging. For complete control over the output, use string.format and io.write.

rawequal (v1, v2)

Checks whether v1 is equal to v2, without invoking any metamethod. Returns a boolean.

rawget (table, index)

Gets the real value of table[index], without invoking any metamethod. table must be a table; index may be any value.

rawlen (v)

Returns the length of the object v, which must be a table or a string, without invoking any metamethod. Returns an integer.

rawset (table, index, value)

Sets the real value of table[index] to value, without invoking any metamethod. table must be a table, index any value different from nil and NaN, and value any Lua 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])

When called with no base, tonumber tries to convert its argument to a number. If the argument is already a number or a string convertible to a number, then tonumber returns this number; otherwise, it returns nil.

The conversion of strings can result in integers or floats, according to the lexical conventions of Lua (see §3.1). (The string may have leading and trailing spaces and a sign.)

When called with base, then e must be a string to be interpreted as an integer numeral in that base. 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. If the string e is not a valid numeral in the given base, the function returns nil.

tostring (v)

Receives a value of any type and converts it to a string in a human-readable format. Floats always produce strings with some floating-point indication (either a decimal dot or an exponent). (For complete control of how numbers are converted, use string.format.)

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

type (v)

Returns the type of its only argument, coded as a string. The possible results of this function are "nil" (a string, not the value nil), "number", "string", "boolean", "table", "function", "thread", and "userdata".

_VERSION

A global variable (not a function) that holds a string containing the current interpreter version. The current value of this variable is "Lua 5.3".

xpcall (f, msgh [, arg1, ···])

This function is similar to pcall, except that it sets a new message handler msgh.

6.2 – Coroutine Manipulation

The operations related to coroutines comprise a sub-library of the basic library and come inside the table coroutine. See §2.6 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.isyieldable ()

Returns true when the running coroutine can yield.

A running coroutine is yieldable if it is not the main thread and it is not inside a non-yieldable C function.

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 (when the coroutine yields) or any values returned by the body function (when 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. Any arguments to yield are passed as extra results to resume.

6.3 – Modules

The package library provides basic facilities for loading modules in Lua. It exports one function directly in the global environment: require. Everything else is exported in a table package.

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.searchers sequence. By changing this sequence, we can change how require looks for a module. The following explanation is based on the default configuration for package.searchers.

First require queries package.preload[modname]. If it has a value, this value (which must 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.searchers).

Once a loader is found, require calls the loader with two arguments: modname and an extra value dependent on how it got the loader. (If the loader came from a file, this extra value is the file name.) If the loader returns any non-nil value, require assigns the returned value to package.loaded[modname]. If the loader does not return a non-nil 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 raises an error.

package.config

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

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_5_3 or 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.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 lua_CFunction prototype (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 Standard 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_5_3 or the environment variable LUA_PATH or with a default path defined in luaconf.h, if those environment variables are 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).

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

package.searchers

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) plus an extra value that will be passed to that 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 searcher 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 suffix after (and including) the first hyphen is removed. For instance, if the module name is a.b.c-v2.1, the function name will be luaopen_a_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.

All searchers except the first one (preload) return as the extra value the file name where the module was found, as returned by package.searchpath. The first searcher returns no extra value.

package.searchpath (name, path [, sep [, rep]])

Searches for the given name in the given path.

A path is a string containing a sequence of templates separated by semicolons. For each template, the function replaces each interrogation mark (if any) in the template with a copy of name wherein all occurrences of sep (a dot, by default) were replaced by rep (the system's directory separator, by default), 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 the name foo.a will try to open the files ./foo/a.lua, ./foo/a.lc, and /usr/local/foo/a/init.lua, in that order.

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

6.4 – 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]])

Returns the internal numerical codes of the characters s[i], s[i+1], ..., s[j]. The default value for i is 1; the default value for j is i. These indices are corrected following the same rules of function string.sub.

Numerical codes are not necessarily portable across platforms.

string.char (···)

Receives zero or more integers. Returns a string with length equal to the number of arguments, in which each character has the internal numerical code equal to its corresponding argument.

Numerical codes are not necessarily portable across platforms.

string.dump (function [, strip])

Returns a string containing a binary representation (a binary chunk) of the given function, so that a later load on this string returns a copy of the function (but with new upvalues). If strip is a true value, the binary representation is created without debug information about the function (local variable names, lines, etc.).

Functions with upvalues have only their number of upvalues saved. When (re)loaded, those upvalues receive fresh instances containing nil. (You can use the debug library to serialize and reload the upvalues of a function in a way adequate to your needs.)

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

Looks for the first match of pattern (see §6.4.1) in the string s. If it finds a match, then find returns the indices of s where this occurrence starts and ends; otherwise, it returns nil. A third, optional numerical argument init specifies where to start the search; its default value is 1 and can be negative. A value of true as a fourth, optional argument plain turns off the pattern matching facilities, so the function does a plain "find substring" operation, with no characters in pattern being considered magic. Note that if plain is given, then init must be given as well.

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

string.format (formatstring, ···)

Returns a formatted version of its variable number of arguments following the description given in its first argument (which must be a string). The format string follows the same rules as the ISO C function sprintf. The only differences are that the options/modifiers *, h, L, l, n, and p are not supported and that there is an extra option, q. The q option formats a string between double quotes, using escape sequences when necessary to ensure that it can safely be read back by the Lua interpreter. For instance, the call 

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

may produce the string: 

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

Options A and a (when available), E, e, f, G, and g all expect a number as argument. Options c, d, i, o, u, X, and x expect an integer. Option q expects a string; option s expects a string without embedded zeros. If the argument to option s is not a string, it is converted to one following the same rules of tostring.

string.gmatch (s, pattern)

Returns an iterator function that, each time it is called, returns the next captures from pattern (see §6.4.1) over the string s. If pattern specifies no captures, then the whole match is produced in each call.

As an example, the following loop will iterate over all the words from string s, printing one per line: 

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

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

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

Returns a copy of s in which all (or the first n, if given) occurrences of the pattern (see §6.4.1) have been replaced by a replacement string specified by repl, which can be a string, a table, or a function. gsub also returns, as its second value, the total number of matches that occurred. The name gsub comes from Global SUBstitution.

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 %d, with d between 1 and 9, stands for the value of the d-th captured substring. 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 repl is a function, then this function is called every time a match occurs, with all captured substrings passed as arguments, in order.

In any case, if the pattern specifies no captures, then it behaves as if the whole pattern was inside a capture.

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 load(s)()
         end)
     --> x="4+5 = 9"
     
     local t = {name="lua", version="5.3"}
     x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t)
     --> x="lua-5.3.tar.gz"

string.len (s)

Receives a string and returns its length. The empty string "" has length 0. Embedded zeros are counted, so "a\000bc\000" has length 5.

string.lower (s)

Receives a string and returns a copy of this string with all uppercase letters changed to lowercase. All other characters are left unchanged. The definition of what an uppercase letter is depends on the current locale.

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

Looks for the first match of pattern (see §6.4.1) in the string s. If it finds one, then match returns the captures from the pattern; otherwise it returns nil. If pattern specifies no captures, then the whole match is returned. A third, optional numerical argument init specifies where to start the search; its default value is 1 and can be negative.

string.pack (fmt, v1, v2, ···)

Returns a binary string containing the values v1, v2, etc. packed (that is, serialized in binary form) according to the format string fmt (see §6.4.2).

string.packsize (fmt)

Returns the size of a string resulting from string.pack with the given format. The format string cannot have the variable-length options 's' or 'z' (see §6.4.2).

string.rep (s, n [, sep])

Returns a string that is the concatenation of n copies of the string s separated by the string sep. The default value for sep is the empty string (that is, no separator). Returns the empty string if n is not positive.

string.reverse (s)

Returns a string that is the string s reversed.

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

Returns the substring of s that starts at i and continues until j; i and j can be negative. If j is absent, then it is assumed to be equal to -1 (which is the same as the string length). In particular, the call string.sub(s,1,j) returns a prefix of s with length j, and string.sub(s, -i) returns a suffix of s with length i.

If, after the translation of negative indices, i is less than 1, it is corrected to 1. If j is greater than the string length, it is corrected to that length. If, after these corrections, i is greater than j, the function returns the empty string.

string.unpack (fmt, s [, pos])

Returns the values packed in string s (see string.pack) according to the format string fmt (see §6.4.2). An optional pos marks where to start reading in s (default is 1). After the read values, this function also returns the index of the first unread byte in s.

string.upper (s)

Receives a string and returns a copy of this string with all lowercase letters changed to uppercase. All other characters are left unchanged. The definition of what a lowercase letter is depends on the current locale.

6.4.1 – Patterns

Patterns in Lua are described by regular strings, which are interpreted as patterns by the pattern-matching functions string.find, string.gmatch, string.gsub, and string.match. This section describes the syntax and the meaning (that is, what they match) of these strings.

Character Class:

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

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

Pattern:

A pattern is a sequence of pattern items. A caret '^' 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.

6.4.2 – Format Strings for Pack and Unpack

The first argument to string.pack, string.packsize, and string.unpack is a format string, which describes the layout of the structure being created or read.

A format string is a sequence of conversion options. The conversion options are as follows:

(A "[n]" means an optional integral numeral.) Except for padding, spaces, and configurations (options "xX <=>!"), each option corresponds to an argument (in string.pack) or a result (in string.unpack).

For options "!n", "sn", "in", and "In", n can be any integer between 1 and 16. All integral options check overflows; string.pack checks whether the given value fits in the given size; string.unpack checks whether the read value fits in a Lua integer.

Any format string starts as if prefixed by "!1=", that is, with maximum alignment of 1 (no alignment) and native endianness.

Alignment works as follows: For each option, the format gets extra padding until the data starts at an offset that is a multiple of the minimum between the option size and the maximum alignment; this minimum must be a power of 2. Options "c" and "z" are not aligned; option "s" follows the alignment of its starting integer.

All padding is filled with zeros by string.pack (and ignored by string.unpack).

6.5 – UTF-8 Support

This library provides basic support for UTF-8 encoding. It provides all its functions inside the table utf8. This library does not provide any support for Unicode other than the handling of the encoding. Any operation that needs the meaning of a character, such as character classification, is outside its scope.

Unless stated otherwise, all functions that expect a byte position as a parameter assume that the given position is either the start of a byte sequence or one plus the length of the subject string. As in the string library, negative indices count from the end of the string.

utf8.char (···)

Receives zero or more integers, converts each one to its corresponding UTF-8 byte sequence and returns a string with the concatenation of all these sequences.

utf8.charpattern

The pattern (a string, not a function) "[\0-\x7F\xC2-\xF4][\x80-\xBF]*" (see §6.4.1), which matches exactly one UTF-8 byte sequence, assuming that the subject is a valid UTF-8 string.

utf8.codes (s)

Returns values so that the construction 

     for p, c in utf8.codes(s) do body end

will iterate over all characters in string s, with p being the position (in bytes) and c the code point of each character. It raises an error if it meets any invalid byte sequence.

utf8.codepoint (s [, i [, j]])

Returns the codepoints (as integers) from all characters in s that start between byte position i and j (both included). The default for i is 1 and for j is i. It raises an error if it meets any invalid byte sequence.

utf8.len (s [, i [, j]])

Returns the number of UTF-8 characters in string s that start between positions i and j (both inclusive). The default for i is 1 and for j is -1. If it finds any invalid byte sequence, returns a false value plus the position of the first invalid byte.

utf8.offset (s, n [, i])

Returns the position (in bytes) where the encoding of the n-th character of s (counting from position i) starts. A negative n gets characters before position i. The default for i is 1 when n is non-negative and #s + 1 otherwise, so that utf8.offset(s, -n) gets the offset of the n-th character from the end of the string. If the specified character is neither in the subject nor right after its end, the function returns nil.

As a special case, when n is 0 the function returns the start of the encoding of the character that contains the i-th byte of s.

This function assumes that s is a valid UTF-8 string.

6.6 – Table Manipulation

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

Remember that, whenever an operation needs the length of a table, the table must be a proper sequence or have a __len metamethod (see §3.4.7). All functions ignore non-numeric keys in the tables given as arguments.

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

Given a list where all elements are strings or numbers, returns the string list[i]..sep..list[i+1] ··· sep..list[j]. The default value for sep is the empty string, the default for i is 1, and the default for j is #list. If i is greater than j, returns the empty string.

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

Inserts element value at position pos in list, shifting up the elements list[pos], list[pos+1], ···, list[#list]. The default value for pos is #list+1, so that a call table.insert(t,x) inserts x at the end of list t.

table.move (a1, f, e, t [,a2])

Moves elements from table a1 to table a2. This function performs the equivalent to the following multiple assignment: a2[t],··· = a1[f],···,a1[e]. The default for a2 is a1. The destination range can overlap with the source range. Index f must be positive.

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. Note that the resulting table may not be a sequence.

table.remove (list [, pos])

Removes from list the element at position pos, returning the value of the removed element. When pos is an integer between 1 and #list, it shifts down the elements list[pos+1], list[pos+2], ···, list[#list] and erases element list[#list]; The index pos can also be 0 when #list is 0, or #list + 1; in those cases, the function erases the element list[pos].

The default value for pos is #list, so that a call table.remove(l) removes the last element of list l.

table.sort (list [, comp])

Sorts list elements in a given order, in-place, from list[1] to list[#list]. If comp is given, then it must be a function that receives two list elements and returns true when the first element must come before the second in the final order (so that not comp(list[i+1],list[i]) will be true after the sort). If comp is not given, then the standard Lua operator < is used instead.

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

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

Returns the elements from the given list. This function is equivalent to 

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

By default, i is 1 and j is #list.

6.7 – Mathematical Functions

This library provides basic mathematical functions. It provides all its functions and constants inside the table math. Functions with the annotation "integer/float" give integer results for integer arguments and float results for float (or mixed) arguments. Rounding functions (math.ceil, math.floor, and math.modf) return an integer when the result fits in the range of an integer, or a float otherwise.

math.abs (x)

Returns the absolute value of x. (integer/float)

math.acos (x)

Returns the arc cosine of x (in radians).

math.asin (x)

Returns the arc sine of x (in radians).

math.atan (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.)

The default value for x is 1, so that the call math.atan(y) returns the arc tangent of y.

math.ceil (x)

Returns the smallest integral value larger than or equal to x.

math.cos (x)

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

math.deg (x)

Converts the angle x from radians to degrees.

math.exp (x)

Returns the value ex (where e is the base of natural logarithms).

math.floor (x)

Returns the largest integral value 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. (integer/float)

math.huge

The float value HUGE_VAL, a value larger than any other numerical value.

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 argument with the maximum value, according to the Lua operator <. (integer/float)

math.maxinteger

An integer with the maximum value for an integer.

math.min (x, ···)

Returns the argument with the minimum value, according to the Lua operator <. (integer/float)

math.mininteger

An integer with the minimum value for an integer.

math.modf (x)

Returns the integral part of x and the fractional part of x. Its second result is always a float.

math.pi

The value of π.

math.rad (x)

Converts the angle x from degrees to radians.

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

When called without arguments, returns a pseudo-random float with uniform distribution in the range [0,1). When called with two integers m and n, math.random returns a pseudo-random integer with uniform distribution in the range [m, n]. (The value m-n cannot be negative and must fit in a Lua integer.) The call math.random(n) is equivalent to math.random(1,n).

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

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.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.tointeger (x)

If the value x is convertible to an integer, returns that integer. Otherwise, returns nil.

math.type (x)

Returns "integer" if x is an integer, "float" if it is a float, or nil if x is not a number.

math.ult (m, n)

Returns a boolean, true if integer m is below integer n when they are compared as unsigned integers.

6.8 – Input and Output Facilities

The I/O library provides two different styles for file manipulation. The first one uses implicit file handles; 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 handles.

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

The table io also provides three predefined file handles 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. On non-POSIX systems, the computation of the error message and error code in case of errors may be not thread safe, because they rely on the global C variable errno.

io.close ([file])

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

io.flush ()

Equivalent to io.output():flush().

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 works like file:lines(···) over the opened file. When the iterator function detects the end of file, it returns no values (to finish the loop) and automatically closes the file.

The call io.lines() (with no file name) is equivalent to io.input():lines("*l"); 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.

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

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:

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])

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

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").

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.

When closing a file handle created with io.popen, file:close returns the same values returned by os.execute.

file:flush ()

Saves any written data to file.

file:lines (···)

Returns an iterator function that, each time it is called, reads the file according to the given formats. When no format is given, uses "l" as a default. As an example, the construction 

     for c in file:lines(1) do body end

will iterate over all characters of the file, starting at the current position. Unlike io.lines, this function does not close the file when the loop ends.

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

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. (In this latter case, the function does not read subsequent formats.) When called without formats, it uses a default format that reads the next line (see below).

The available formats are

The formats "l" and "L" should be used only for text files.

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:

In case of success, seek returns the final file position, measured in bytes from the beginning of the file. If seek 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:

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 file. The arguments must be strings or numbers.

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

6.9 – 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). This last field may be absent if the information is not available.

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

On non-POSIX systems, this function may be not thread safe because of its reliance on C function gmtime and C function localtime.

os.difftime (t2, t1)

Returns the difference, in seconds, from time t1 to time t2 (where the times are values returned by os.time). In POSIX, Windows, and some other systems, this value is exactly t2-t1.

os.execute ([command])

This function is equivalent to the ISO C function system. It passes command to be executed by an operating system shell. Its first result is true if the command terminated successfully, or nil otherwise. After this first result the function returns a string plus a number, as follows:

When called without a command, os.execute returns a boolean that is true if a shell is available.

os.exit ([code [, close]])

Calls the ISO C function exit to terminate the host program. If code is true, the returned status is EXIT_SUCCESS; if code is false, the returned status is EXIT_FAILURE; if code is a number, the returned status is this number. The default value for code is true.

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 empty directory, on POSIX systems) with the given name. If this function fails, it returns nil, plus a string describing the error and the error code.

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 and the error code.

os.setlocale (locale [, category])

Sets the current locale of the program. locale is a system-dependent 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.

This function may be not thread safe because of its reliance on C function setlocale.

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 (default is 12), min (default is 0), sec (default is 0), and isdst (default is nil). 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 os.date and os.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 POSIX systems, 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.

6.10 – The Debug Library

This library provides the functionality of the debug interface (§4.9) to Lua programs. You should exert care when using this library. Several of its functions violate basic assumptions about Lua code (e.g., that variables local to a function cannot be accessed from outside; that userdata metatables cannot be changed by Lua code; that Lua programs do not crash) and therefore can compromise otherwise secure code. Moreover, some functions in this library may be slow.

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.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,] f [, 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 f, which means the function running at level f 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 f 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,] f, local)

This function returns the name and the value of the local variable with index local of the function at level f of the stack. This function accesses not only explicit local variables, but also parameters, temporaries, etc.

The first parameter or local variable has index 1, and so on, following the order that they are declared in the code, counting only the variables that are active in the current scope of the function. Negative indices refer to vararg parameters; -1 is the first vararg parameter. The function returns nil if there is no 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 parenthesis) represent variables with no known names (internal variables such as loop control variables, and variables from chunks saved without debug information).

The parameter f may also be a function. In that case, getlocal returns only the name of function parameters.

debug.getmetatable (value)

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

debug.getregistry ()

Returns the registry table (see §4.5).

debug.getupvalue (f, up)

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

Variable names starting with '(' (open parenthesis) represent variables with no known names (variables from chunks saved without debug information).

debug.getuservalue (u)

Returns the Lua value associated to u. If u is not a userdata, returns nil.

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 any combination of the following characters, with the given meaning:

Moreover, with a count different from zero, the hook is called also 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.

See debug.getlocal for more information about variable indices and names.

debug.setmetatable (value, table)

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

debug.setupvalue (f, up, value)

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

debug.setuservalue (udata, value)

Sets the given value as the Lua value associated to the given udata. udata must be a full userdata.

Returns udata.

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

If message is present but is neither a string nor nil, this function returns message without further processing. Otherwise, it returns a string with a traceback of the call stack. The 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).

debug.upvalueid (f, n)

Returns a unique identifier (as a light userdata) for the upvalue numbered n from the given function.

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.

debug.upvaluejoin (f1, n1, f2, n2)

Make the n1-th upvalue of the Lua closure f1 refer to the n2-th upvalue of the Lua closure f2.

7 – Lua Standalone

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

     lua [options] [script [args]]

The options are:

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

When called without option -E, the interpreter checks for an environment variable LUA_INIT_5_3 (or LUA_INIT if the versioned name is not defined) before running any argument. If the variable content has the format @filename, then lua executes the file. Otherwise, lua executes the string itself.

When called with option -E, besides ignoring LUA_INIT, Lua also ignores the values of LUA_PATH and LUA_CPATH, setting the values of package.path and package.cpath with the default paths defined in luaconf.h.

All options are handled in order, except -i and -E. 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, and finally run the file script.lua with no arguments. (Here $ is the shell prompt. Your prompt may be different.)

Before running any code, lua collects all command-line arguments in a global table called arg. The script name goes to 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 its options) go to negative indices. For instance, in the call 

     $ lua -la b.lua t1 t2

the table is like this: 

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

If there is no script in the call, the interpreter name goes to index 0, followed by the other arguments. For instance, the call 

     $ lua -e "print(arg[1])"

will print "-e". If there is a script, the script is called with parameters arg[1], ···, arg[#arg]. (Like all chunks in Lua, the script is compiled as a vararg function.)

In interactive mode, Lua repeatedly prompts and waits for a line. After reading a line, Lua first try to interpret the line as an expression. If it succeeds, it prints its value. Otherwise, it interprets the line as a statement. 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 not a string but has a metamethod __tostring, the interpreter calls this metamethod to produce the final message. Otherwise, the interpreter converts the error object to a string and adds a stack traceback to it.

When finishing normally, the interpreter closes its main Lua state (see lua_close). The script can avoid this step by calling os.exit to terminate.

To allow the use of Lua as a script interpreter in Unix systems, the standalone 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.)

8 – Incompatibilities with the Previous Version

Here we list the incompatibilities that you may find when moving a program from Lua 5.2 to Lua 5.3. You can avoid some incompatibilities by compiling Lua with appropriate options (see file luaconf.h). However, all these compatibility options will be removed in the future.

Lua versions can always change the C API in ways that do not imply source-code changes in a program, such as the numeric values for constants or the implementation of functions as macros. Therefore, you should not assume that binaries are compatible between different Lua versions. Always recompile clients of the Lua API when using a new version.

Similarly, Lua versions can always change the internal representation of precompiled chunks; precompiled chunks are not compatible between different Lua versions.

The standard paths in the official distribution may change between versions.

8.1 – Changes in the Language

8.2 – Changes in the Libraries

8.3 – Changes in the API

9 – The Complete Syntax of Lua

Here is the complete syntax of Lua in extended BNF. As usual in extended BNF, {A} means 0 or more As, and [A] means an optional A. (For operator precedences, see §3.4.8; for a description of the terminals Name, Numeral, and LiteralString, see §3.1.) 


	chunk ::= block

	block ::= {stat} [retstat]

	stat ::=  ‘;’ | 
		 varlist ‘=’ explist | 
		 functioncall | 
		 label | 
		 break | 
		 goto Name | 
		 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] 

	retstat ::= return [explist] [‘;’]

	label ::= ‘::’ Name ‘::’

	funcname ::= Name {‘.’ Name} [‘:’ Name]

	varlist ::= var {‘,’ var}

	var ::=  Name | prefixexp ‘[’ exp ‘]’ | prefixexp ‘.’ Name 

	namelist ::= Name {‘,’ Name}

	explist ::= exp {‘,’ exp}

	exp ::=  nil | false | true | Numeral | LiteralString | ‘...’ | functiondef | 
		 prefixexp | tableconstructor | exp binop exp | unop exp 

	prefixexp ::= var | functioncall | ‘(’ exp ‘)’

	functioncall ::=  prefixexp args | prefixexp ‘:’ Name args 

	args ::=  ‘(’ [explist] ‘)’ | tableconstructor | LiteralString 

	functiondef ::= 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 | ‘#’ | ‘~’

Last update: Tue Jan 6 10:10:50 BRST 2015