Lua 2.5 Reference Manual

1 - Introduction

Lua is an extension programming language designed to support general procedural programming features with data description facilities. It is intended to be used as a light-weight, but powerful, configuration language for any program that needs one. Lua has been designed and implemented by W. Celes, R. Ierusalimschy and L. H. de Figueiredo.

Lua is implemented as a library, written in C. Being an extension language, Lua has no notion of a ``main'' program: it only works embedded in a host client, called the embedding program. This host program can invoke functions to execute a piece of code in Lua, 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 many, completely different domains, thus creating customized programming languages sharing a syntactical framework.

Lua is free-distribution software, and provided as usual with no guarantees. The implementation described in this manual is available at the following URL's:

2 - Environment and Chunks

All statements in Lua are executed in a global environment. This environment, which keeps all global variables and functions, is initialized at the beginning of the embedding program and persists until its end.

The global environment can be manipulated by Lua code or by the embedding program, which can read and write global variables using functions in the library that implements Lua.

Global variables do not need declaration. Any variable is assumed to be global unless explicitly declared local (see Section 4.4.5). Before the first assignment, the value of a global variable is nil.

The unit of execution of Lua is called a chunk. The syntax for chunks is:

chunk := {stat | function} [ret] A chunk may contain statements and function definitions, and may be in a file or in a string inside the host program. A chunk may optionally end with a return statement (see Section 4.4.3). When a chunk is executed, first all its functions and statements are compiled, then the statements are executed in sequential order. All modifications a chunk effects on the global environment persist after its end. Those include modifications to global variables and definitions of new functions

Chunks may be pre-compiled; see program luac for details. Text files with chunks and their binary pre-compiled forms are interchangeable. Lua automatically detects the file type and acts accordingly.

3 - Types

Lua is a dynamically typed language. Variables do not have types; only values do. All values carry their own type. Therefore, there are no type definitions in the language.

There are seven basic types in Lua: nil, number, string, function, CFunction, userdata, and table. Nil is the type of the value nil, whose main property is to be different from any other value. Number represents real (floating point) numbers, while string has the usual meaning.

Functions are considered first-class values in Lua. This means that functions can be stored in variables, passed as arguments to other functions and returned as results. When a function is defined in Lua, its body is compiled and stored in a given variable. Lua can call (and manipulate) functions written in Lua and functions written in C; the latter have type CFunction.

The type userdata is provided to allow arbitrary C pointers to be stored in Lua variables. It corresponds to void* and has no pre-defined operations in Lua, besides assignment and equality test. However, by using fallbacks, the programmer may define operations for userdata values; (see Section 4.7).

The type table implements associative arrays, that is, arrays that can be indexed not only with numbers, but with any value (except nil). Therefore, this type may be used not only to represent ordinary arrays, but also symbol tables, sets, records, etc. To represent records, Lua uses the field name as an index. The language supports this representation by providing as syntactic sugar for a["name"]. Tables may also carry methods. Because functions are first class values, table fields may contain functions. The form t:f(x) is syntactic sugar for t.f(t,x), which calls the method f from the table t passing itself as the first parameter.

It is important to notice that tables are objects, and not values. Variables cannot contain tables, only references to them. Assignment, parameter passing and returns always manipulate references to tables, and do not imply any kind of copy. Moreover, tables must be explicitly created before used (see Section 4.5.7).

4 - The Language

This section describes the lexis, the syntax and the semantics of Lua.

4.1 - Lexical Conventions

Lua is a case sensitive language. Identifiers can be any string of letters, digits, and underscores, not beginning with a digit. The following words are reserved, and cannot be used as identifiers:

and do else elseif end function if local nil not or repeat return then until while

The following strings denote other tokens:

~= <= >= < > == = .. + - * / % ( ) { } [ ] ; , .

Literal strings can be delimited by matching single or double quotes, and can contain the C-like escape sequences '\n', '\t' and '\r'. Literal strings can also be delimited by matching [[ ... ]]. Literals in this bracketed form may run for several lines, may contain nested [[ ... ]] pairs, and do not interpret escape sequences. This form is specially convenient for handling text that has quoted strings in it.

Comments start anywhere outside a string with a double hyphen (--) and run until the end of the line. Moreover, if the first line of a chunk file starts with #, this line is skipped

Numerical constants may be written with an optional decimal part, and an optional decimal exponent. Examples of valid numerical constants are:

4 4.0 0.4 4.57e-3 0.3e12

4.2 - Coercion

Lua provides some automatic conversions between values. Any arithmetic operation applied to a string tries to convert that string to a number, following the usual rules. Conversely, whenever a number is used when a string is expected, that number is converted to a string, according to the following rule: if the number is an integer, it is written without exponent or decimal point; otherwise, it is formatted following the %g conversion specification of the printf function in the standard C library. For complete control on how numbers are converted to strings, use the format function (see Section 6.2).

4.3 - Adjustment

Functions in Lua can return many values. Because there are no type declarations, the system does not know how many values a function will return, or how many parameters it needs. Therefore, sometimes, a list of values must be adjusted, at run time, to a given length. If there are more values than are needed, then the last values are thrown away. If there are more needs than values, then the list is extended with as many nil's as needed. Adjustment occurs in multiple assignment and function calls.

4.4 - Statements

Lua supports an almost conventional set of statements, similar to those in Pascal or C. The conventional commands include assignment, control structures and procedure calls. Non-conventional commands include table constructors (see Section 4.5.7), and local variable declarations (see Section 4.4.5).

4.4.1 - Blocks

A block is a list of statements, which are executed sequentially. Any statement can be optionally followed by a semicolon: block := {stat sc} [ret] sc := [';'] For syntactic reasons, a return statement can only be written as the last statement of a block. This restriction also avoids some ``statement not reached'' errors.

4.4.2 - Assignment

The language allows multiple assignment. Therefore, the syntax defines a list of variables on the left side, and a list of expressions on the right side. Both lists have their elements separated by commas: stat := varlist1 '=' explist1 varlist1 := var {',' var} This statement first evaluates all values on the right side and eventual indices on the left side, and then makes the assignments. Therefore, it can be used to exchange two values, as in x, y = y, x Before the assignment, the list of values is adjusted to the length of the list of variables (see Section 4.3).

A single name can denote a global or a local variable, or a formal parameter:

var := name Square brackets are used to index a table: var := var '[' exp1 ']' If var results in a table value, the field indexed by the expression value gets the assigned value. Otherwise, the fallback settable is called, with three parameters: the value of var, the value of expression, and the value being assigned to it; (see Section 4.7).

The syntax var.NAME is just syntactic sugar for var["NAME"]:

var := var '.' name

4.4.3 - Control Structures

The condition expression of a control structure may return any value. All values different from nil are considered true; only nil is considered false. if's, while's and repeat's have the usual meaning.

stat := while exp1 do block end
| repeat block until exp1
| if exp1 then block {elseif} [else block] end elseif := elseif exp1 then block

A return is used to return values from a function or a chunk. Because they may return more than one value, the syntax for a return statement is:

ret := return [explist1] [sc]

4.4.4 - Function Calls as Statements

Because of possible side-effects, function calls can be executed as statements: stat := functioncall In this case, returned values are thrown away. Function calls are explained in Section 4.5.8.

4.4.5 - Local Declarations

Local variables may be declared anywhere inside a block. Their scope begins after the declaration and lasts until the end of the block. The declaration may include an initial assignment: stat := local declist [init] declist := name {',' name} init := '=' explist1 If present, an initial assignment has the same semantics of a multiple assignment. Otherwise, all variables are initialized with nil.

4.5 - Expressions

4.5.1 - Simple Expressions

Simple expressions are: exp := '(' exp ')' exp := nil exp := 'number' exp := 'literal' exp := var Numbers (numerical constants) and string literals are explained in Section 4.1. Variables are explained in Section 4.4.2.

The non-terminal exp1 is used to indicate that the values returned by an expression must be adjusted to one single value:

exp1 := exp

4.5.2 - Arithmetic Operators

Lua supports the usual arithmetic operators. These operators are the binary + (addition), - (subtraction), * (multiplication), / (division) and ^ (exponentiation), and the unary - (negation). If the operands are numbers, or strings that can be converted to numbers, according to the rules given in Section 4.2, then all operations except exponentiation have the usual meaning. Otherwise, the fallback ``arith'' is called (see Section 4.7). An exponentiation always calls this fallback. The standard mathematical library redefines this fallback, giving the expected meaning to exponentiation (see Section 6.3).

4.5.3 - Relational Operators

Lua provides the following relational operators: < > <= >= ~= == All these return nil as false and a value different from nil\ (actually the number 1) as true.

Equality first compares the types of its operands. If they are different, then the result is nil. Otherwise, their values are compared. Numbers and strings are compared in the usual way. Tables, CFunctions, and functions are compared by reference, that is, two tables are considered equal only if they are the same table. The operator ~= is exactly the negation of equality (==). Note that the conversion rules of Section 4.2 do not apply to equality comparisons. Thus, "0"==0 evaluates to false.

The other operators work as follows. If both arguments are numbers, then they are compared as such. Otherwise, if both arguments can be converted to strings, their values are compared using lexicographical order. Otherwise, the ``order'' fallback is called (see Section 4.7).

4.5.4 - Logical Operators

Like control structures, all logical operators consider nil as false and anything else as true. The logical operators are: and or not The operator and returns nil if its first argument is nil; otherwise it returns its second argument. The operator or returns its first argument if it is different from nil; otherwise it returns its second argument. Both and and or use short-cut evaluation, that is, the second operand is evaluated only if necessary.

4.5.5 - Concatenation

Lua offers a string concatenation operator, denoted by ``..''. If operands are strings or numbers, then they are converted to strings according to the rules in Section 4.2. Otherwise, the fallback ``concat'' is called (see Section 4.7).

4.5.6 - Precedence

Operator precedence follows the table below, from the lower to the higher priority: and or < > <= >= ~= == .. + - * / not - (unary) ^ All binary operators are left associative, except for ^ (exponentiation), which is right associative.

4.5.7 - Table Constructors

Table constructors are expressions that create tables; every time a constructor is evaluated, a new table is created. Constructors can be used to create empty tables, or to create a table and initialize some fields.

The general syntax for constructors is:

tableconstructor := '{' fieldlist '}' fieldlist := lfieldlist | ffieldlist | lfieldlist ';' ffieldlist lfieldlist := [lfieldlist1] ffieldlist := [ffieldlist1]

The form lfieldlist1 is used to initialize lists.

lfieldlist1 := exp {',' exp} [','] The expressions in the list are assigned to consecutive numerical indices, starting with 1. For example: a = {"v1", "v2", 34} is essentialy equivalent to: temp = {} temp[1] = "v1" temp[2] = "v2" temp[3] = 34 a = temp

The next form initializes named fields in a table:

ffieldlist1 := ffield {',' ffield} [','] ffield := name '=' exp For example: a = {x = 1, y = 3} is essentialy equivalent to: temp = {} temp.x = 1 -- or temp["x"] = 1 temp.y = 3 -- or temp["y"] = 3 a = temp

4.5.8 - Function Calls

A function call has the following syntax: functioncall := var realParams Here, var can be any variable (global, local, indexed, etc). If its value has type function or CFunction, then this function is called. Otherwise, the ``function'' fallback is called, having as first parameter the value of var, and then the original call parameters.

The form:

functioncall := var ':' name realParams can be used to call ``methods''. A call var:name(...) is syntactic sugar for, ...) except that var is evaluated only once.

realParams := '(' [explist1] ')' realParams := tableconstructor explist1 := exp1 {',' exp1} All argument expressions are evaluated before the call; then the list of arguments is adjusted to the length of the list of parameters (see Section 4.3); finally, this list is assigned to the formal parameters. A call of the form f{...} is syntactic sugar for f({...}), that is, the parameter list is a single new table.

Because a function can return any number of results (see Section 4.4.3), the number of results must be adjusted before used. If the function is called as a statement (see Section 4.4.4), its return list is adjusted to 0, thus discarding all returned values. If the function is called in a place that needs a single value (syntactically denoted by the non-terminal exp1), then its return list is adjusted to 1, thus discarding all returned values, except the first one. If the function is called in a place that can hold many values (syntactically denoted by the non-terminal exp), then no adjustment is made.

4.6 - Function Definitions

Functions in Lua can be defined anywhere in the global level of a chunk. The syntax for function definition is:

function := function var '(' [parlist1] ')' block end

When Lua pre-compiles a chunk, all its function bodies are pre-compiled, too. Then, when Lua ``executes'' the function definition, its body is stored, with type function, into the variable var. It is in this sense that a function definition is an assignment to a global variable.

Parameters act as local variables, initialized with the argument values.

parlist1 := name {',' name}

Results are returned using the return statement (see Section 4.4.3). If control reaches the end of a function without a return instruction, then the function returns with no results.

There is a special syntax for defining methods, that is, functions that have an extra parameter self.

function := function var ':' name '(' [parlist1] ')' block end Thus, a declaration like function v:f (...) ... end is equivalent to function v.f (self, ...) ... end that is, the function gets an extra formal parameter called self. Notice that the variable v must have been previously initialized with a table value.

4.7 - Fallbacks

Lua provides a powerful mechanism to extend its semantics, called fallbacks. A fallback is a programmer defined function that is called whenever Lua does not know how to proceed.

Lua supports the following fallbacks, identified by the given strings:

called when an arithmetic operation is applied to non numerical operands, or when the binary ^ operation (exponentiation) is called. It receives three arguments: the two operands (the second one is nil when the operation is unary minus) and one of the following strings describing the offended operator: add sub mul div pow unm Its return value is the final result of the arithmetic operation. The default handler issues an error.
called when an order comparison is applied to non numerical or non string operands. It receives three arguments: the two operands and one of the following strings describing the offended operator: lt gt le ge Its return value is the final result of the comparison operation. The default handler issues an error.
called when a concatenation is applied to non string operands. It receives the two operands as arguments. Its return value is the final result of the concatenation operation. The default handler issues an error.
called when Lua tries to retrieve the value of an index not present in a table. It receives as arguments the table and the index. Its return value is the final result of the indexing operation. The default handler returns nil.
called when Lua tries to retrieve the value of a global variable which has a nil value (or which has not been initialized). It receives as argument the name of the variable. Its return value is the final result of the expression. The default handler returns nil.
called when Lua tries to index a non table value. It receives as arguments the non table value and the index. Its return value is the final result of the indexing operation. The default handler issues an error.
called when Lua tries to assign to an index in a non table value. It receives as arguments the non table value, the index, and the assigned value. The default handler issues an error.
called when Lua tries to call a non function value. It receives as arguments the non function value and the arguments given in the original call. Its return values are the final results of the call operation. The default handler issues an error.
called during garbage collection. It receives as argument the table being collected. After each run of the collector this function is called with argument nil, to signal the completion of the garbage collection. Because this function operates during garbage collection, it must be used with great care, and programmers should avoid the creation of new objects (tables or strings) in this function. The default handler does nothing.
called when an error occurs. It receives as argument a string describing the error. The default handler prints the message on the standard error output (stderr).

The function setfallback is used to change a fallback handler. Its first argument is the name of a fallback condition, and the second argument is the new function to be called. It returns the old handler function for the given fallback.

4.8 - Error Handling

Because Lua is an extension language, all Lua actions start from C code calling a function from the Lua library. Whenever an error occurs during Lua compilation or execution, the ``error'' fallback function is called, and then the corresponding function from the library (lua_dofile, lua_dostring, lua_call, or lua_callfunction) is terminated returning an error condition.

The only argument to the ``error'' fallback function is a string describing the error. The standard I/O library redefines this fallback, using the debug facilities (see Section 7), in order to print some extra information, like the call stack. To provide more information about errors, Lua programs can include the compilation pragma $debug. This pragma must be written in a line by itself. When an error occurs in a program compiled with this option, the error routine is able to print the number of the lines where the calls (and the error) were made. If needed, it is possible to change the ``error'' fallback handler (see Section 4.7).

Lua code can explicitly generate an error by calling the built-in function error (see Section 6.1).

5 - The Application Program Interface

This section describes the API for Lua, that is, the set of C functions available to the host program to communicate with the library. The API functions can be classified in the following categories:

  1. exchanging values between C and Lua;
  2. executing Lua code;
  3. manipulating (reading and writing) Lua objects;
  4. calling Lua functions;
  5. C functions to be called by Lua;
  6. manipulating references to Lua Objects.
All API functions and related types and constants are declared in the header file lua.h.

5.1 - Exchanging Values between C and Lua

Because Lua has no static type system, all values passed between Lua and C have type lua_Object, which works like an abstract type in C that can hold any Lua value. Values of type lua_Object have no meaning outside Lua; for instance, the comparisson of two lua_Object's is undefined.

To check the type of a lua_Object, the following function is available:

int lua_type (lua_Object object); plus the following functions: int lua_isnil (lua_Object object); int lua_isnumber (lua_Object object); int lua_isstring (lua_Object object); int lua_istable (lua_Object object); int lua_isfunction (lua_Object object); int lua_iscfunction (lua_Object object); int lua_isuserdata (lua_Object object); All macros return 1 if the object is compatible with the given type, and 0 otherwise. The function lua_isnumber accepts numbers and numerical strings, whereas lua_isstring accepts strings and numbers (see Section 4.2), and lua_isfunction accepts Lua and C functions. The function lua_type can be used to distinguish between different kinds of user data.

To translate a value from type lua_Object to a specific C type, the programmer can use:

double lua_getnumber (lua_Object object); char *lua_getstring (lua_Object object); lua_CFunction lua_getcfunction (lua_Object object); void *lua_getuserdata (lua_Object object); lua_getnumber converts a lua_Object to a floating-point number. This lua_Object must be a number or a string convertible to number (see Section 4.2); otherwise, the function returns 0.

lua_getstring converts a lua_Object to a string (char *). This lua_Object must be a string or a number; otherwise, the function returns 0 (the NULL pointer). This function does not create a new string, but returns a pointer to a string inside the Lua environment. Because Lua has garbage collection, there is no guarantee that such pointer will be valid after the block ends.

lua_getcfunction converts a lua_Object to a C function. This lua_Object must have type CFunction; otherwise, the function returns 0 (the NULL pointer). The type lua_CFunction is explained in Section 5.5.

lua_getuserdata converts a lua_Object to void*. This lua_Object must have type userdata; otherwise, the function returns 0 (the NULL pointer).

Because Lua has automatic memory management and garbage collection, a lua_Object has a limited scope, and is only valid inside the block where it was created. A C function called from Lua is a block, and its parameters are valid only until its end. It is good programming practice to convert Lua objects to C values as soon as they are available, and never to store lua_Objects in C global variables.

All comunication between Lua and C is done through two abstract data types, called lua2C and C2lua. The first one, as the name implies, is used to pass values from Lua to C: parameters when Lua calls C and results when C calls Lua. The structure C2lua is used in the reverse direction: parameters when C calls Lua and results when Lua calls C. Notice that the structure lua2C cannot be directly modified by C code, while the structure C2lua cannot be ``read'' by C code.

The structure lua2C is an abstract array, which can be indexed with the function:

lua_Object lua_lua2C (int number); where number starts with 1. When called with a number larger than the array size, this function returns LUA_NOOBJECT. In this way, it is possible to write C functions that receive a variable number of parameters, and to call Lua functions that return a variable number of results.

The second structure, C2lua, is a stack. Pushing elements into this stack is done by using the following functions:

void lua_pushnumber (double n); void lua_pushstring (char *s); void lua_pushcfunction (lua_CFunction f); void lua_pushusertag (void *u, int tag); void lua_pushnil (void); void lua_pushobject (lua_Object object); plus the macro: void lua_pushuserdata (void *u); All of them receive a C value, convert it to a corresponding lua_Object, and leave the result on the top of C2lua.

User data can have different tags, whose semantics are only known to the host program. Any positive integer can be used to tag a user datum. When a user datum is retrieved, the function lua_type can be used to get its tag.

Please note: most functions in the Lua API use the structures lua2C and C2lua, and therefore change their contents. Great care must be taken, specially when pushing a sequence of objects into C2lua, to avoid using those functions. The family of functions lua_get*, lua_is*, plus the function lua_lua2C, are safe to be called without modifying these structures; the family lua_push* does not modify lua2C. All other functions may change lua2C and C2lua, unless noticed otherwise.

When C code calls Lua repeatedly, as in a loop, objects returned by these calls accumulate, and may create a memory problem. To avoid this, nested blocks can be defined with the functions:

void lua_beginblock (void); void lua_endblock (void); After the end of the block, all lua_Object's created inside it are released. The use of explicit nested blocks is strongly encouraged.

5.2 - Executing Lua Code

A host program can execute Lua chunks written in a file or in a string using the following functions: int lua_dofile (char *filename); int lua_dostring (char *string); Both functions return an error code: 0, in case of success; non zero, in case of errors. More specifically, lua_dofile returns 2 if for any reason it could not open the file. The function lua_dofile, if called with argument NULL, executes the stdin stream. Function lua_dofile is also able to execute pre-compiled chunks. It automatically detects whether the file is text or binary, and loads it accordingly (see program luac). These functions also return, in structure lua2C, any values eventually returned by the chunks.

5.3 - Manipulating Lua Objects

To read the value of any global Lua variable, one uses the function: lua_Object lua_getglobal (char *varname); As in Lua, if the value of the global is nil, then the ``getglobal'' fallback is called.

To store a value previously pushed onto C2lua in a global variable, there is the function:

void lua_storeglobal (char *varname);

Tables can also be manipulated via the API. The function

lua_Object lua_getsubscript (void); expects on the stack C2lua a table and an index, and returns the contents of the table at that index. As in Lua, if the first object is not a table, or the index is not present in the table, the corresponding fallback is called.

To store a value in an index, the program must push the table, the index, and the value onto C2lua, and then call the function:

void lua_storesubscript (void); Again, the ``settable'' fallback is called if a non-table value is used.

Finally, the function

lua_Object lua_createtable (void); creates and returns a new, empty table.

As already noted, most functions from the Lua library receive parameters through C2lua. Because other functions also use this stack, it is important that these parameters be pushed just before the corresponding call, without intermediate calls to the Lua library. For instance, suppose the user wants the value of a[i], where a and i are global Lua variables. A simplistic solution would be:

/* Warning: WRONG CODE */ lua_Object result; lua_pushobject(lua_getglobal("a")); /* push table */ lua_pushobject(lua_getglobal("i")); /* push index */ result = lua_getsubscript(); This code is incorrect because the call lua_getglobal("i") modifies the stack, and invalidates the previous pushed value. A correct solution could be: lua_Object result; lua_Object index = lua_getglobal("i"); lua_pushobject(lua_getglobal("a")); /* push table */ lua_pushobject(index); /* push index */ result = lua_getsubscript();

5.4 - Calling Lua Functions

Functions defined in Lua by a chunk executed with dofile or dostring can be called from the host program. This is done using the following protocol: first, the arguments to the function are pushed onto C2lua (see Section 5.1), in direct order, i.e., the first argument is pushed first. Again, it is important to emphasize that, during this phase, most other Lua functions cannot be called.

Then, the function is called using

int lua_call (char *functionname); or int lua_callfunction (lua_Object function); Both functions return an error code: 0, in case of success; non zero, in case of errors. Finally, the results (a Lua function may return many values) are returned in structure lua2C, and can be retrieved with the macro lua_getresult, which is just another name to the function lua_lua2C.

The following example shows how a C program may call the strsub function in Lua to extract a piece of a string:

/* assume that 's' and 'r' are strings (char *), 'i' and 'j' integers */ lua_pushstring(s); /* 1st argument */ lua_pushnumber(i); /* 2nd argument */ lua_pushnumber(j); /* 3rd argument */ lua_call("strsub"); /* call Lua function */ r = lua_getstring(lua_getresult(1)); /* r = strsub(s, i, j) */

Two special Lua functions have exclusive interfaces: error and setfallback. A C function can generate a Lua error calling the function

void lua_error (char *message); This function never returns. If the C function has been called from Lua, then the corresponding Lua execution terminates, as if an error had occurred inside Lua code. Otherwise, the whole program terminates with a call to exit(1).

Fallbacks can be changed with:

lua_Object lua_setfallback (char *name, lua_CFunction fallback); The first parameter is the fallback name (see Section 4.7), and the second is a CFunction to be used as the new fallback. This function returns a lua_Object, which is the old fallback value, or nil on failure (invalid fallback name). This old value can be used for chaining fallbacks.

5.5 - C Functions

To register a C function to Lua, there is the following macro: #define lua_register(n,f) (lua_pushcfunction(f), lua_storeglobal(n)) /* char *n; */ /* lua_CFunction f; */ which receives the name the function will have in Lua, and a pointer to the function. This pointer must have type lua_CFunction, which is defined as typedef void (*lua_CFunction) (void); that is, a pointer to a function with no parameters and no results.

In order to communicate properly with Lua, a C function must follow a protocol, which defines the way parameters and results are passed.

A C function receives its arguments in structure lua2C; to access them, it uses the macro lua_getparam, again just another name to lua_lua2C. To return values, a C function just pushes them onto the stack C2lua, in direct order (see Section 5.1). Like a Lua function, a C function called by Lua can also return many results.

As an example, the code below shows a CFunction to compute the maximum of a variable number of arguments:

void math_max (void) { int i=1; /* argument count */ double d, dmax; lua_Object o; /* the function must get at least one argument */ if ((o = lua_getparam(i++)) == LUA_NOOBJECT) lua_error("too few arguments to function `max'"); /* and this argument must be a number */ if (!lua_isnumber(o)) lua_error("incorrect argument to function `max'"); dmax = lua_getnumber(o); /* loops until there is no more arguments */ while ((o = lua_getparam(i++)) != LUA_NOOBJECT) { if (!lua_isnumber(o)) lua_error("incorrect argument to function `max'"); d = lua_getnumber(o); if (d > dmax) dmax = d; } /* push the result to be returned */ lua_pushnumber(dmax); } To be available in Lua, this function must be registered: lua_register ("max", math_max);

For more examples, see files strlib.c, iolib.c and mathlib.c in Lua distribution.

5.6 - References to Lua Objects

As noted in Section 5.5, lua_Objects are volatile. If the C code needs to keep a lua_Object outside block boundaries, it must create a reference to the object. The routines to manipulate references are the following:

int lua_ref (int lock); lua_Object lua_getref (int ref); void lua_pushref (int ref); void lua_unref (int ref); The function lua_ref creates a reference to the object that is on the top of the stack, and returns this reference. If lock is true, the object is locked: this means the object will not be garbage collected. Notice that an unlocked reference may be garbage collected. Whenever the referenced object is needed, a call to lua_getref returns a handle to it, whereas lua_pushref pushes the object on the stack. If the object has been collected, then lua_getref returns LUA_NOOBJECT, and lua_pushobject issues an error.

When a reference is no longer needed, it can be freed with a call to lua_unref.

The function lua_pushref does not corrupt the structures lua2C and C2lua, and therefore is safe to be called when pushing parameters onto C2lua.

6 - Predefined Functions and Libraries

The set of predefined functions in Lua is small but powerful. Most of them provide features that allow some degree of reflexivity in the language. Some of these features cannot be simulated with the rest of the Language nor with the standard Lua API. Others are just convenient interfaces to common API functions.

The libraries, on the other hand, provide useful routines that are implemented directly through the standard API. Therefore, they are not necessary to the language, and are provided as separate C modules. Currently there are three standard libraries:

In order to have access to these libraries, the host program must call the functions strlib_open, mathlib_open, and iolib_open, declared in lualib.h.

6.1 - Predefined Functions

dofile (filename)

This function receives a file name, opens it, and executes its contents as a Lua chunk, or as pre-compiled chunks. When called without arguments, it executes the contents of the standard input (stdin). If there is any error executing the file, it returns nil. Otherwise, it returns the values returned by the chunk, or a non nil value if the chunk returns no values. It issues an error when called with a non string argument. dofile is simply an interface to lua_dofile.

dostring (string)

This function executes a given string as a Lua chunk. If there is any error executing the string, it returns nil. Otherwise, it returns the values returned by the chunk, or a non nil value if the chunk returns no values. dostring is simply an interface to lua_dostring.

next (table, index)

This function 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. It returns the next index of the table and the value associated with the index. When called with nil as its second argument, the function returns the first index of the table (and its associated value). When called with the last index, or with nil in an empty table, it returns nil.

In Lua there is no declaration of fields; semantically, there is no difference between a field not present in a table or a field with value nil. Therefore, the function only considers fields with non nil values. The order in which the indices are enumerated is not specified, even for numeric indices. If the table is modified in any way during a traversal, the semantics of next is undefined.

This function cannot be written with the standard API.

nextvar (name)

This function is similar to the function next, but iterates over the global variables. Its single argument is the name of a global variable, or nil to get a first name. Similarly to next, it returns the name of another variable and its value, or nil if there are no more variables. There can be no assignments to global variables during the traversal; otherwise the semantics of nextvar is undefined.

This function cannot be written with the standard API.

tostring (e)

This function receives an argument of any type and converts it to a string in a reasonable format.

print (e1, e2, ...)

This function receives any number of arguments, and prints their values in a reasonable format. Each value is printed in a new line. This function is not intended for formatted output, but as a quick way to show a value, for instance for error messages or debugging. See Section 6.4 for functions for formatted output.

tonumber (e)

This function receives one argument, and tries to convert it to a number. If the argument is already a number or a string convertible to a number (see Section 4.2), then it returns that number; otherwise, it returns nil.

type (v)

This function allows Lua to test the type of a value. It receives one argument, and returns its type, coded as a string. The possible results of this function are "nil" (a string, not the value nil), "number", "string", "table", "function" (returned both for C functions and Lua functions), and "userdata".

Besides this string, the function returns a second result, which is the tag of the value. This tag can be used to distinguish between user data with different tags, and between C functions and Lua functions.

type is simply an interface to lua_type.

assert (v)

This function issues an ``assertion failed!'' error when its argument is nil.

error (message)

This function issues an error message and terminates the last called function from the library (lua_dofile, lua_dostring, ...). It never returns. error is simply an interface to lua_error.

setglobal (name, value)

This function assigns the given value to a global variable. The string name does not need to be a syntactically valid variable name. Therefore, this function can set global variables with strange names like `m v 1' or 34. It returns the value of its second argument. setglobal is simply an interface to lua_storeglobal.

getglobal (name)

This function retrieves the value of a global variable. The string name does not need to be a syntactically valid variable name.

setfallback (fallbackname, newfallback)

This function sets a new fallback function to the given fallback. It returns the old fallback function. setfallback is simply an interface to lua_setfallback.

6.2 - String Manipulation

This library provides generic functions for string manipulation, such as finding and extracting substrings and pattern matching. When indexing a string, the first character is at position 1, not 0, as in C.

strfind (str, pattern [, init [, plain]])

This function looks for the first match of pattern in str. If it finds one, then it returns the indices on str where this occurence starts and ends; otherwise, it returns nil. If the pattern specifies captures, the captured strings are returned as extra results. A third optional numerical argument specifies where to start the search; its default value is 1. A value of 1 as a forth optional argument turns off the pattern matching facilities, so the function does a plain ``find substring'' operation.

strlen (s)

Receives a string and returns its length.

strsub (s, i [, j])

Returns another string, which is a substring of s, starting at i and runing until j. If j is absent, it is assumed to be equal to the length of s. In particular, the call strsub(s,1,j) returns a prefix of s with length j, whereas the call strsub(s,i) returns a suffix of s, starting at i.

strlower (s)

Receives a string and returns a copy of that string with all upper case letters changed to lower case. All other characters are left unchanged.

strupper (s)

Receives a string and returns a copy of that string with all lower case letters changed to upper case. All other characters are left unchanged.

strrep (s, n)

Returns a string which is the concatenation of n copies of the string s.

ascii (s [, i])

Returns the ASCII code of the character s[i]. If i is absent, then it is assumed to be 1.

format (formatstring, e1, e2, ...)

This function returns a formated 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 printf family of standard C functions. The only differences are that the options/modifiers *, l, L, n, p, and h are not supported, and there is an extra option, q. This option formats a string in a form suitable to be safely read back by the Lua interpreter; that is, the string is written between double quotes, and all double quotes, returns and backslashes in the string are correctly escaped when written. For instance, the call format('%q', 'a string with "quotes" and \n new line') will produce the string: "a string with \"quotes\" and \ new line"

The options c, d, E, e, f, g i, o, u, X, and x all expect a number as argument, whereas q and s expect a string. Note that the * modifier can be simulated by building the appropriate format string. For example, "%*g" can be simulated with "%"..width.."g".

gsub (s, pat, repl [, n])

Returns a copy of s, where all occurrences of the pattern pat have been replaced by a replacement string specified by repl. This function also returns, as a second value, the total number of substitutions made.

If repl is a string, then its value is used for replacement. Any sequence in repl of the form %n with n between 1 and 9 stands for the value of the n-th captured substring.

If repl is a function, then this function is called every time a match occurs, with all captured substrings as parameters (see below). If the value returned by this function is a string, then it is used as the replacement string; otherwise, the replacement string is the empty string.

An optional parameter n limits the maximum number of substitutions to occur. For instance, when n is 1 only the first occurrence of pat is replaced.

As an example, in the following expression each occurrence of the form $name calls the function getenv, passing name as argument (because only this part of the pattern is captured). The value returned by getenv will replace the pattern. Therefore, the whole expression:

gsub("home = $HOME, user = $USER", "$(%w%w*)", getenv) returns a string like: home = /home/roberto, user = roberto


Character Class:

a character class is used to represent a set of characters. The following combinations are allowed in describing a character class:
(where x is any character not in the list ()%.[*?) - represents the character x itself.
- represents all characters.
- represents all letters.
- represents all non letter characters.
- represents all digits.
- represents all non digits.
- represents all lower case letters.
- represents all non lower case letter characters.
- represents all space characters.
- represents all non space characters.
- represents all upper case letters.
- represents all non upper case letter characters.
- represents all alphanumeric characters.
- represents all non alphanumeric characters.
(where x is any non alphanumeric character) - represents the character x. This is the standard way to escape the magic characters ()%.[*?.
- Represents the class which is the union of all characters in char-set. To include a ] in char-set, it must be the first character. A range of characters may be specified by separating the end characters of the range with a -; e.g., A-Z specifies the upper case characters. If - appears as the first or last character of char-set, then it represents itself. All classes %x described above can also be used as components in a char-set. All other characters in char-set represent themselves.
- represents the complement of char-set, where char-set is interpreted as above.

Pattern Item:

a pattern item may be:


a pattern is a sequence of pattern items. A ^ at the beginning of a pattern anchors the match at the beginning of the subject string. A $ at the end of a pattern anchors the match at the end of the subject string.


a pattern may contain sub-patterns enclosed in parentheses, that describe captures. When a match succeeds, the sub-strings 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.

6.3 - Mathematical Functions

This library is an interface to some functions of the standard C math library. In addition, it registers a fallback for the binary operator ^ that, returns x^y when applied to numbers x^y.

The library provides the following functions:

abs acos asin atan atan2 ceil cos floor log log10 max min mod sin sqrt tan random randomseed Most of them are only interfaces to the homonymous functions in the C library, except that, for the trigonometric functions, all angles are expressed in degrees, not radians.

The function max returns the maximum value of its numeric arguments. Similarly, min computes the minimum. Both can be used with an unlimited number of arguments.

The functions random and randomseed are interfaces to the simple random generator functions rand and srand, provided by ANSI C. The function random returns pseudo-random numbers in the range [0,1).

6.4 - I/O Facilities

All input and outpu operations in Lua are done over two current files: one for reading and one for writing. Initially, the current input file is stdin, and the current output file is stdout.

Unless otherwise stated, all I/O functions return nil on failure and some value different from nil on success.

readfrom (filename)

This function may be called in three ways. When called with a file name, it opens the named file, sets it as the current input file, and returns a handle to the file (this handle is a user data containing the file stream FILE*). It does not close the current input file. When called with a file handle, returned by a previous call, it restores the file as the current input. When called without parameters, it closes the current input file, and restores stdin as the current input file.

If this function fails, it returns nil, plus a string describing the error.

System dependent: if filename starts with a |, then a piped input is open, via function popen. Not all systems implement pipes. Moreover, the number of files that can be open at the same time is usually limited and depends on the system.

writeto (filename)

This function may be called in three ways. When called with a file name, it opens the named file, sets it as the current output file, and returns a handle to the file (this handle is a user data containing the file stream FILE*). It does not close the current output file. Notice that, if the file already exists, it will be completely erased with this operation. When called with a file handle, returned by a previous call, it restores the file as the current output. When called without parameters, this function closes the current output file, and restores stdout as the current output file.

If this function fails, it returns nil, plus a string describing the error.

System dependent: if filename starts with a |, then a piped output is open, via function popen. Not all systems implement pipes. Moreover, the number of files that can be open at the same time is usually limited and depends on the system.

appendto (filename)

This function opens a file named filename and sets it as the current output file. It returns the file handle, or nil in case of error. Unlike the writeto operation, this function does not erase any previous content of the file. If this function fails, it returns nil, plus a string describing the error.

Notice that function writeto is available to close an output file.

remove (filename)

This function deletes the file with the given name. If this function fails, it returns nil, plus a string describing the error.

rename (name1, name2)

This function renames file named name1 to name2. If this function fails, it returns nil, plus a string describing the error.

tmpname ()

This function returns a string with a file name that can safely be used for a temporary file.

read ([readpattern])

This function reads the current input according to a read pattern, that specifies how much to read; characters are read from the current input file until the read pattern fails or ends. The function read returns a string with the characters read, even if the pattern succeeds only partially, or nil if the read pattern fails and the result string would be empty. When called without parameters, it uses a default pattern that reads the next line (see below).

A read pattern is a sequence of read pattern items. An item may be a single character class or a character class followed by ? or by *. A single character class reads the next character from the input if it belongs to the class, otherwise it fails. A character class followed by ? reads the next character from the input if it belongs to the class; it never fails. A character class followed by * reads until a character that does not belong to the class, or end of file; since it can match a sequence of zero characteres, it never fails.

A pattern item may contain sub-patterns enclosed in curly brackets, that describe skips. Characters matching a skip are read, but are not included in the resulting string.

Following are some examples of read patterns and their meanings:

write (value1, ...)

This function writes the value of each of its arguments to the current output file. The arguments must be strings or numbers. To write other values, use tostring before write. If this function fails, it returns nil, plus a string describing the error.

date ([format])

This function returns a string containing date and time formatted according to the given string format, following the same rules of the ANSI C function strftime. When called without arguments, it returns a reasonable date and time representation that depends on the host system.

exit ([code])

This function calls the C function exit, with an optional code, to terminate the program. The default value for code is 1.

getenv (varname)

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

execute (command)

This function is equivalent to the C function system. It passes command to be executed by an operating system shell. It returns an error code, which is system-dependent.

7 - The Debugger Interface

Lua has no built-in debugging facilities. Instead, it offers a special interface, by means of functions and hooks, which allows the construction of different kinds of debuggers, profilers, and other tools that need ``inside information'' from the interpreter. This interface is declared in the header file luadebug.h.

7.1 - Stack and Function Information

The main function to get information about the interpreter stack is

lua_Function lua_stackedfunction (int level); It returns a handle (lua_Function) to the activation record of the function executing at a given level. Level 0 is the current running function, while level n+1 is the function that has called level n. When called with a level greater than the stack depth, lua_stackedfunction returns LUA_NOOBJECT.

The type lua_Function is just another name to lua_Object. Although, in this library, a lua_Function can be used wherever a lua_Object is required, when a parameter has type lua_Function it accepts only a handle returned by lua_stackedfunction.

Three other functions produce extra information about a function:

void lua_funcinfo (lua_Object func, char **filename, int *linedefined); int lua_currentline (lua_Function func); char *lua_getobjname (lua_Object o, char **name); lua_funcinfo gives the file name and the line where the given function has been defined. If the ``function'' is in fact the main code of a chunk, then linedefined is 0. If the function is a C function, then linedefined is -1, and filename is "(C)".

The function lua_currentline gives the current line where a given function is executing. It only works if the function has been compiled with debug information (see Section 4.8). When no line information is available, it returns -1.

Function lua_getobjname tries to find a reasonable name for a given function. Because functions in Lua are first class values, they do not have a fixed name: Some functions may be the value of many global variables, while others may be stored only in a table field. Function lua_getobjname first checks whether the given function is a fallback. If so, it returns the string "fallback", and name is set to point to the fallback name. Otherwise, if the given function is the value of a global variable, then lua_getobjname returns the string "global", and name points to the variable name. If the given function is neither a fallback nor a global variable, then lua_getobjname returns the empty string, and name is set to NULL.

7.2 - Manipulating Local Variables

The following functions allow the manipulation of the local variables of a given activation record. They only work if the function has been compiled with debug information (see Section 4.8).

lua_Object lua_getlocal (lua_Function func, int local_number, char **name); int lua_setlocal (lua_Function func, int local_number); lua_getlocal returns the value of a local variable, and sets name to point to the variable name. local_number is an index for local variables. The first parameter has index 1, and so on, until the last active local variable. When called with a local_number greater than the number of active local variables, or if the activation record has no debug information, lua_getlocal returns LUA_NOOBJECT. Formal parameters are the first local variables.

The function lua_setlocal sets the local variable local_number to the value previously pushed on the stack (see Section 5.1). If the function succeeds, then it returns 1. If local_number is greater than the number of active local variables, or if the activation record has no debug information, then this function fails and returns 0.

7.3 - Hooks

The Lua interpreter offers two hooks for debugging purposes:

typedef void (*lua_CHFunction) (lua_Function func, char *file, int line); extern lua_CHFunction lua_callhook;

typedef void (*lua_LHFunction) (int line); extern lua_LHFunction lua_linehook;

The first one is called whenever the interpreter enters or leaves a function. When entering a function, its parameters are a handle to the function activation record, plus the file and the line where the function is defined (the same information which is provided by lua_funcinfo); when leaving a function, func is LUA_NOOBJECT, file is "(return)", and line is 0.

The other hook is called every time the interpreter changes the line of code it is executing. Its only parameter is the line number (the same information which is provided by the call lua_currentline(lua_stackedfunction(0))). This second hook is only called if the active function has been compiled with debug information (see Section 4.8).

A hook is disabled when its value is NULL, which is the initial value of both hooks.

8 - Lua Stand-alone

Although Lua has been designed as an extension language, the language can also be used as a stand-alone interpreter. An implementation of such an interpreter, called simply lua, is provided with the standard distribution. This program can be called with any sequence of the following arguments:

prints version information.
runs interactively, accepting commands from standard input until an EOF.
-e stat
executes stat as a Lua chunk.
executes var=exp as a Lua chunk.
executes file filename as a Lua chunk.
All arguments are handled in order. For instance, an invocation like $ lua - a=1 prog.lua will first interact with the user until an EOF, then will set a to 1, and finally will run file prog.lua.

Please notice that the interaction with the shell may lead to unintended results. For instance, a call like

$ lua a="name" prog.lua will not set a to the string "name". Instead, the quotes will be handled by the shell, lua will get only a=name to run, and a will finish with nil, because the global variable name has not been initialized. Instead, one should write $ lua 'a="name"' prog.lua


The authors would like to thank CENPES/PETROBRAS which, jointly with TeCGraf, used extensively early versions of this system and gave valuable comments. The authors would also like to thank Carlos Henrique Levy, who found the name of the game. Lua means moon in Portuguese.

Incompatibilities with Previous Versions

Although great care has been taken to avoid incompatibilities with the previous public versions of Lua, some differences had to be introduced. Here is a list of all these incompatibilities.

Incompatibilities with version 2.4

The whole I/O facilities have been rewritten. We strongly encourage programmers to adapt their code to this new version. However, we are keeping the old version of the libraries in the distribution, to allow a smooth transition. The incompatibilities between the new and the old libraries are:

Incompatibilities with version 2.2

Incompatibilities with version 2.1

Incompatibilities with version 1.1

Last update: Thu Jun 26 08:13:15 EST 1997 by lhf.