Lua is an extension programming language designed to support general procedural programming with data description facilities. Lua 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 a wide range of different domains, thus creating customized programming languages sharing a syntactical framework.
Lua is free-distribution software, and provided as usual with no guarantees, as stated in the copyright notice. The implementation described in this manual is available at the following URL's:
http://www.tecgraf.puc-rio.br/lua/ ftp://ftp.tecgraf.puc-rio.br/pub/lua/lua.tar.gz
All statements in Lua are executed in a global environment. This environment, which keeps all global variables, is initialized at the beginning of the embedding program and persists until its end. Optionally, a user can create multiple independent global environments (see Section 5.1).
The global environment can be manipulated by Lua code or by the embedding program, which can read and write global variables using functions from the API library that implements Lua.
Global variables do not need declaration. Any variable is assumed to be global unless explicitly declared local (see Section 4.5.5). Before the first assignment, the value of a global variable is nil; this default can be changed (see Section 4.8).
The unit of execution of Lua is called a chunk. A chunk is simply a sequence of statements:
chunk ::= {stat} [ret]
Statements are described in Section 4.5.
(As usual, {a} means 0 or more a's,
[a] means an optional a and ('a)+ means
one or more a's.)
A chunk may be in a file or in a string inside the host program.
A chunk may optionally end with a return statement (see Section 4.5.3).
When a chunk is executed, first all its code is pre-compiled,
then the statements are executed in sequential order.
All modifications a chunk effects on the global environment persist
after its end.
Chunks may also be pre-compiled into binary form; 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.
Lua is a dynamically typed language. Variables do not have types; only values do. Therefore, there are no type definitions in the language. All values carry their own type. Besides a type, all values also have a tag.
There are six basic types in Lua: nil, number,
string, function, userdata, and table.
Nil is the type of the value nil,
whose main property is to be different from any other value.
Number represents real (double precision floating point) numbers,
while string has the usual meaning.
Lua is eight-bit clean,
and so strings may contain any 8-bit character,
including embedded zeros ('\0').
The function type returns a string describing the type
of a given value (see Section 6.1).
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. Lua can call (and manipulate) functions written in Lua and functions written in C. They can be distinguished by their tags: all Lua functions have the same tag, and all C functions have the same tag, which is different from the tag of Lua functions.
The type userdata is provided to allow
arbitrary C pointers to be stored in Lua variables.
It corresponds to a void* and has no pre-defined operations in Lua,
besides assignment and equality test.
However, by using tag methods,
the programmer can define operations for userdata values
(see Section 4.8).
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.
Tables are the main data structuring mechanism in Lua.
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"].
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 (see Section 4.6.9).
Note 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 4.6.7).
Tags are mainly used to select tag methods when
some events occur.
Tag methods are the main mechanism for extending the
semantics of Lua (see Section 4.8).
Each of the types nil, number and string has a different tag.
All values of each of these types have this same pre-defined tag.
Values of type function can have two different tags,
depending on whether they are Lua or C functions.
Finally,
values of type userdata and table can have
as many different tags as needed (see Section 4.8).
Tags are created with the function newtag,
and the function tag returns the tag of a given value.
To change the tag of a given table,
there is the function settag (see Section 6.1).
This section describes the lexis, the syntax and the semantics of Lua.
Identifiers in Lua can be any string of letters, digits, and underscores, not beginning with a digit. The definition of letter depends on the current locale: Any character considered alphabetic by the current locale can be used in an identifier. The following 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
Lua is a case-sensitive language:
and is a reserved word, but And and \'and
(if the locale permits) are two other different identifiers.
As a convention, identifiers starting with underscore followed by
uppercase letters should not be used in regular programs.
The following strings denote other tokens:
~= <= >= < > == = + - * /
( ) { } [ ] ; , . .. ...
Literal strings can be delimited by matching single or double quotes,
and can contain the C-like escape sequences
'\a' (bell),
'\b' (back space),
'\f' (form feed),
'\n' (new line),
'\r' (carriage return),
'\t' (horizontal tab),
'\v' (vertical tab),
'\\', (backslash),
'\"', (double quote),
and '\'' (single quote).
A character in a string may also be specified by its numerical value,
through the escape sequence '\ddd',
where ddd is a sequence of up to three decimal digits.
Strings in Lua may contain any 8-bit value, including embedded 0.
Literal strings can also be delimited by matching [[ ... ]].
Literals in this bracketed form may run for several lines,
may contain nested [[ ... ]] pairs,
and do not interpret escape sequences.
This form is specially convenient for
writing strings that contain program pieces or
other quoted strings.
As an example, in a system using ASCII,
the following three literals are equivalent:
1) "alo\n123\"" 2) '\97lo\10\04923"' 3) [[alo 123"]]
Comments start anywhere outside a string with a
double hyphen (--) and run until the end of the line.
Moreover,
the first line of a chunk is skipped if it starts with #.
This facility allows the use of Lua as a script interpreter
in Unix systems (see Section 8).
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
All lines that start with a $ sign are handled by a pre-processor.
The $ sign must be immediately
followed by one of the following directives:
Directives may be freely nested.
Particularly, a $endinput may occur inside a $if;
in that case, even the matching $end is not parsed.
A cond part may be:
Lua provides some automatic conversions between values at run time.
Any arithmetic operation applied to a string tries to convert
that string to a number, following the usual rules.
Conversely, whenever a number is used when a string is expected,
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).
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 (see Section 4.5.2) and function calls (see Section 4.6.8).
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 4.6.7), and local variable declarations (see Section 4.5.5).
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'' conditions.
A block may be explicitly delimited:
stat ::= do block endThis is useful to control the scope of local variables (see Section 4.5.5).
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, xThe two lists may have different lengths. Before the assignment, the list of values is adjusted to the length of the list of variables (see Section 4.4).
A single name can denote a global or a local variable, or a formal parameter:
var ::= nameSquare brackets are used to index a table:
var ::= simpleexp '[' exp1 ']'The simpleexp should result in a table value, from where the field indexed by the expression value gets the assigned value.
The syntax var.NAME is just syntactic sugar for
var["NAME"]:
var ::= simpleexp '.' name
The meaning of assignments and evaluations of global variables and
indexed variables can be changed by tag methods (see Section 4.8).
Actually,
an assignment x = val, where x is a global variable,
is equivalent to a call setglobal('x', val);
an assignment t[i] = val is equivalent to
settable_event(t, i, val).
See Section 4.8 for a complete description of these functions.
(Function setglobal is pre-defined in Lua.
Function settable_event is used only for explanatory purposes.)
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 from a chunk. Because they may return more than one value, the syntax for a return statement is:
ret ::= return [explist1] [sc]
stat ::= functioncallIn this case, all returned values are thrown away. Function calls are explained in Section 4.6.8.
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.
exp ::= '(' exp ')'
exp ::= nil
exp ::= 'number'
exp ::= 'literal'
exp ::= function
exp ::= simpleexp
simpleexp ::= var simpleexp ::= upvalue simpleexp ::= functioncall
Numbers (numerical constants) and string literals are explained in Section 4.1; variables are explained in Section 4.5.2; upvalues are explained in Section 4.7; function definitions (function) are explained in Section 4.6.9; function call are explained in Section 4.6.8.
An access to a global variable x is equivalent to a
call getglobal('x');
an access to an indexed variable t[i] is equivalent to
a call gettable_event(t, i).
See Section 4.8 for a description of these functions.
(Function getglobal is pre-defined in Lua.
Function gettable_event is used only for explanatory purposes.)
The non-terminal exp1 is used to indicate that the values returned by an expression must be adjusted to one single value:
exp1 ::= exp
+ (addition),
- (subtraction), * (multiplication),
/ (division) and ^ (exponentiation),
and unary - (negation).
If the operands are numbers, or strings that can be converted to
numbers (according to the rules given in Section 4.3),
then all operations except exponentiation have the usual meaning.
Otherwise, an appropriate tag method is called (see Section 4.8).
An exponentiation always calls a tag method.
The standard mathematical library redefines this method for numbers,
giving the expected meaning to exponentiation
(see Section 6.3).
< > <= >= ~= ==
All these return nil as false and a value different from nil as true.
Equality first compares the tags 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, userdata 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 4.3
do not apply to equality comparisons.
Thus, "0"==0 evaluates to false,
and t[0] and t["0"] denote different
entries in a table.
The other operators work as follows. If both arguments are numbers, then they are compared as such. Otherwise, if both arguments are strings, then their values are compared using lexicographical order. Otherwise, the ``order'' tag method is called (see Section 4.8).
and or not
Like control structures, all logical operators
consider nil as false and anything else as true.
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 when necessary.
A useful Lua idiom is x = x or v,
which is equivalent to
if x == nil then x = v end
i.e., it sets x to a default value v when
x is not set.
and or
< > <= >= ~= ==
..
+ -
* /
not - (unary)
^
All binary operators are left associative,
except for ^ (exponentiation),
which is right associative.
The general syntax for constructors is:
tableconstructor ::= '{' fieldlist '}'
fieldlist ::= lfieldlist | ffieldlist | lfieldlist ';' ffieldlist | ffieldlist ';' lfieldlist
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 equivalent to:
do
local temp = {}
temp[1] = "v1"
temp[2] = "v2"
temp[3] = 34
a = temp
end
The form ffieldlist1 initializes other fields in a table:
ffieldlist1 ::= ffield {',' ffield} [',']
ffield ::= '[' exp ']' '=' exp | name '=' exp
For example:
a = {[f(k)] = g(y), x = 1, y = 3, [0] = b+c}
is equivalent to:
do
local temp = {}
temp[f(k)] = g(y)
temp.x = 1 -- or temp["x"] = 1
temp.y = 3 -- or temp["y"] = 3
temp[0] = b+c
a = temp
end
An expression like {x = 1, y = 4} is
in fact syntactic sugar for {["x"] = 1, ["y"] = 4}.
Both forms may have an optional trailing comma, and can be used in the same constructor separated by a semi-collon. For example, all forms below are correct:
x = {;}
x = {'a', 'b',}
x = {type='list'; 'a', 'b'}
x = {f(0), f(1), f(2),; n=3}
functioncall ::= simpleexp argsFirst, simpleexp is evaluated. If its value has type function, then this function is called, with the given arguments. Otherwise, the ``function'' tag method is called, having as first parameter the value of simpleexp, and then the original call parameters.
The form:
functioncall ::= simpleexp ':' name argscan be used to call ``methods''. A call
simpleexp:name(...)
is syntactic sugar for
simpleexp.name(simpleexp, ...)except that
simpleexp is evaluated only once.
args ::= '(' [explist1] ')'
args ::= tableconstructor
args ::= 'literal'
explist1 ::= exp1 {',' exp1}
All argument expressions are evaluated before the call.
A call of the form f{...} is syntactic sugar for
f({...}), that is,
the parameter list is a single new table.
A call of the form f'...'
(or f"..." or f[[...]]) is syntactic sugar for
f('...'), that is,
the parameter list is a single literal string.
Because a function can return any number of results (see Section 4.5.3), the number of results must be adjusted before used. If the function is called as a statement (see Section 4.5.4), then 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 but 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. Note that the only place that can hold many values is the last expression (or the only one) in an assignment or in a return statement; see examples below.
f(); -- adjusted to 0
g(x, f()); -- f() is adjusted to 1
a,b,c = f(), x; -- f() is adjusted to 1 result (and c gets nil)
a,b,c = x, f(); -- f() is adjusted to 2
a,b,c = f(); -- f() is adjusted to 3
return f(); -- returns all values returned by f()
The syntax for function definition is:
function ::= function '(' [parlist1] ')' block end
stat ::= function funcname '(' [parlist1] ')' block end
funcname ::= name | name '.' name
The statement:
function f (...)
...
end
is just syntactic sugar for:
f = function (...)
...
end
A function definition is an executable expression, whose value has type function. When Lua pre-compiles a chunk, all its function bodies are pre-compiled, too. Then, whenever Lua executes the function definition, its upvalues are fixed (see Section 4.7), and the function is instantiated (or ``closed''). This function instance (or ``closure'') is the final value of the expression. Different instances of a same function may have different upvalues.
Parameters act as local variables, initialized with the argument values:
parlist1 ::= '...'
parlist1 ::= name {',' name} [',' '...']
When a function is called,
the list of arguments is adjusted to
the length of the list of parameters (see Section 4.4),
unless the function is a vararg function,
indicated by the dots (...) at the end of its parameter list.
A vararg function does not adjust its argument list;
instead, it collects any extra arguments into an implicit parameter,
called arg.
This parameter is always initialized as a table,
with a field n with the number of extra arguments,
and the extra arguments at positions 1, 2, ...
As an example, suppose definitions like:
function f(a, b) end
function g(a, b, ...) end
Then, we have the following mapping from arguments to parameters:
CALL PARAMETERS
f(3) a=3, b=nil
f(3, 4) a=3, b=4
f(3, 4, 5) a=3, b=4
g(3) a=3, b=nil, arg={n=0}
g(3, 4) a=3, b=4, arg={n=0}
g(3, 4, 5, 8) a=3, b=4, arg={5, 8; n=2}
Results are returned using the return statement (see Section 4.5.3).
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 implicit extra parameter self.
function ::= function name ':' name '(' [parlist1] ')' block end
Thus, a declaration like
function v:f (...) ... endis equivalent to
v.f = function (self, ...) ... endthat is, the function gets an extra formal parameter called
self.
Note that the variable v must have been
previously initialized with a table value.
A function body may refer to its own local variables (which includes its parameters) and to global variables, as long as they are not shadowed by local variables from enclosing functions. A function cannot access a local variable from an enclosing function, since such variables may no longer exist when the function is called. However, a function may access the value of a local variable from an enclosing function, using upvalues.
upvalue ::= '%' nameAn upvalue is somewhat similar to a variable expression, but whose value is frozen when the function wherein it appears is instantiated. The name used in an upvalue may be the name of any variable visible at the point where the function is defined.
Here are some examples:
a,b,c = 1,2,3 -- global variables
function f (x)
local b -- x and b are local to f
local g = function (a)
local y -- a and y are local to g
p = a -- OK, access local 'a'
p = c -- OK, access global 'c'
p = b -- ERROR: cannot access a variable in outer scope
p = %b -- OK, access frozen value of 'b' (local to 'f')
p = %c -- OK, access frozen value of global 'c'
p = %y -- ERROR: 'y' is not visible where 'g' is defined
end -- g
end -- f
Lua provides a powerful mechanism to extend its semantics, called Tag Methods. A tag method is a programmer-defined function that is called at specific key points during the evaluation of a program, allowing the programmer to change the standard Lua behavior at these points. Each of these points is called an event.
The tag method called for any specific event is selected according to the tag of the values involved in the event (see Section 3). The function settagmethod changes the tag method associated with a given pair (tag, event). Its first parameter is the tag, the second is the event name (a string, see below), and the third parameter is the new method (a function), or nil to restore the default behavior. The function returns the previous tag method for that pair. Another function, gettagmethod, receives a tag and an event name and returns the current method associated with the pair.
Tag methods are called in the following events,
identified by the given names.
The semantics of tag methods is better explained by a Lua function
describing the behavior of the interpreter at each event.
The function not only shows when a tag method is called,
but also its arguments, its results and the default behavior.
Please notice that the code shown here is only illustrative;
the real behavior is hard coded in the interpreter,
and it is much more efficient than this simulation.
All functions used in these descriptions
(rawgetglobal, tonumber, call, etc)
are described in Section 6.1.
+ operation is applied to non numerical operands.
The function getbinmethod defines how Lua chooses a tag method
for a binary operation.
First, Lua tries the first operand.
If its tag does not define a tag method for the operation,
then Lua tries the second operand.
If it also fails, then it gets a tag method from tag 0:
function getbinmethod (op1, op2, event)
return gettagmethod(tag(op1), event) or
gettagmethod(tag(op2), event) or
gettagmethod(0, event)
end
function add_event (op1, op2)
local o1, o2 = tonumber(op1), tonumber(op2)
if o1 and o2 then -- both operands are numeric
return o1+o2 -- '+' here is the primitive 'add'
else -- at least one of the operands is not numeric
local tm = getbinmethod(op1, op2, "add")
if tm then
-- call the method with both operands and an extra
-- argument with the event name
return tm(op1, op2, "add")
else -- no tag method available: default behavior
error("unexpected type at arithmetic operation")
end
end
end
- operation is applied to non numerical operands.
Behavior similar to the "add" event.
* operation is applied to non numerical operands.
Behavior similar to the "add" event.
/ operation is applied to non numerical operands.
Behavior similar to the "add" event.
^ operation is applied.
function pow_event (op1, op2)
local tm = getbinmethod(op1, op2, "pow")
if tm then
-- call the method with both operands and an extra
-- argument with the event name
return tm(op1, op2, "pow")
else -- no tag method available: default behavior
error("unexpected type at arithmetic operation")
end
end
- operation is applied to a non numerical operand.
function unm_event (op)
local o = tonumber(op)
if o then -- operand is numeric
return -o -- '-' here is the primitive 'unm'
else -- the operand is not numeric.
-- Try to get a tag method from the operand;
-- if it does not have one, try a "global" one (tag 0)
local tm = gettagmethod(tag(op), "unm") or
gettagmethod(0, "unm")
if tm then
-- call the method with the operand, nil, and an extra
-- argument with the event name
return tm(op, nil, "unm")
else -- no tag method available: default behavior
error("unexpected type at arithmetic operation")
end
end
end
< operation is applied to non numerical
or non string operands.
function lt_event (op1, op2)
if type(op1) == "number" and type(op2) == "number" then
return op1 < op2 -- numeric comparison
elseif type(op1) == "string" and type(op2) == "string" then
return op1 < op2 -- lexicographic comparison
else
local tm = getbinmethod(op1, op2, "lt")
if tm then
return tm(op1, op2, "lt")
else
error("unexpected type at comparison");
end
end
end
> operation is applied to non numerical
or non string operands.
Behavior similar to the "lt" event.
<= operation is applied to non numerical
or non string operands.
Behavior similar to the "lt" event.
>= operation is applied to non numerical
or non string operands.
Behavior similar to the "lt" event.
function concat_event (op1, op2)
if (type(op1) == "string" or type(op1) == "number") and
(type(op2) == "string" or type(op2) == "number") then
return op1..op2 -- primitive string concatenation
else
local tm = getbinmethod(op1, op2, "concat")
if tm then
return tm(op1, op2, "concat")
else
error("unexpected type for concatenation")
end
end
end
"gettable" for its semantics.
newtag.
function getglobal (varname)
local value = rawgetglobal(varname)
local tm = gettagmethod(tag(value), "getglobal")
if not tm then
return value
else
return tm(varname, value)
end
end
The function getglobal is pre-defined in Lua (see Section 6.1).
function setglobal (varname, newvalue)
local oldvalue = rawgetglobal(varname)
local tm = gettagmethod(tag(oldvalue), "setglobal")
if not tm then
return rawsetglobal(varname, newvalue)
else
return tm(varname, oldvalue, newvalue)
end
end
Notice: the function setglobal is pre-defined in Lua (see Section 6.1).
function gettable_event (table, index)
local tm = gettagmethod(tag(table), "gettable")
if tm then
return tm(table, index)
elseif type(table) ~= "table" then
error("indexed expression not a table");
else
local v = rawgettable(table, index)
tm = gettagmethod(tag(table), "index")
if v == nil and tm then
return tm(table, index)
else
return v
end
end
end
function settable_event (table, index, value)
local tm = gettagmethod(tag(table), "settable")
if tm then
tm(table, index, value)
elseif type(table) ~= "table" then
error("indexed expression not a table")
else
rawsettable(table, index, value)
end
end
function function_event (func, ...)
if type(func) == "function" then
return call(func, arg)
else
local tm = gettagmethod(tag(func), "function")
if tm then
local i = arg.n
while i > 0 do
arg[i+1] = arg[i]
i = i-1
end
arg.n = arg.n+1
arg[1] = func
return call(tm, arg)
else
error("call expression not a function")
end
end
end
function gc_event (obj)
local tm = gettagmethod(tag(obj), "gc")
if tm then
tm(obj)
end
end
Moreover, at the end of a garbage collection cycle,
Lua does the equivalent of the call gc_event(nil).
Because Lua is an extension language,
all Lua actions start from C code in the host program
calling a function from the Lua library.
Whenever an error occurs during Lua compilation or execution,
the error method is called,
and then the corresponding function from the library
(lua_dofile, lua_dostring,
lua_dobuffer, or lua_callfunction)
is terminated, returning an error condition.
The only argument to the error method is a string
describing the error.
The default method prints this message to stderr.
If needed, it is possible to change the error method with the
function seterrormethod,
which gets the new error handler as its only parameter
(see Section 6.1).
The standard I/O library uses this facility to redefine the error method,
using the debug facilities (see Section 7),
in order to print some extra information,
such as the call stack.
To provide more information about errors,
Lua programs should include the compilation pragma $debug.
When an error occurs in a program compiled with this option,
the I/O error routine is able to print the number of the
lines where the calls (and the error) were made.
Lua code can explicitly generate an error by calling the built-in
function error (see Section 6.1).
Lua code can ``catch'' an error using the built-in function
call (see Section 6.1).
This section describes the API for Lua, that is, the set of C functions available to the host program to communicate with the Lua library. The API functions can be classified in the following categories:
lua.h.
typedef struct lua_State lua_State; extern lua_State *lua_state;
Before calling any API function, this state must be initialized. This is done by calling
void lua_open (void);This function allocates and initializes some internal structures, and defines all pre-defined functions of Lua. If
lua_state is already different from NULL,
lua_open has no effect;
therefore, it is safe to call this function multiple times.
All standard libraries call lua_open when they are opened.
Function lua_setstate is used to change the current state
of Lua:
lua_State *lua_setstate (lua_State *st);It sets
lua_state to st and returns the old state.
Multiple, independent states may be created.
For that, you must set lua_state back to NULL before
calling lua_open.
An easy way to do that is defining an auxiliary function:
lua_State *lua_newstate (void) {
lua_State *old = lua_setstate(NULL);
lua_open();
return lua_setstate(old);
}
This function creates a new state without changing the current state
of the interpreter.
Note that any new state is built with all predefined functions,
but any additional library (such as the standard libraries) must be
explicitly open in the new state, if needed.
If necessary, a state may be released:
void lua_close (void);This function destroys all objects in the current Lua environment (calling the correspondent garbage collector tag methods), frees all dynamic memory used by the state, and then sets
lua_state to NULL.
Usually, there is no need to call this function,
since these resources are naturally released when the program ends.
If lua_state is already NULL,
lua_close has no effect.
If you are using multiple states, you may find useful the following function, which releases a given state:
void lua_freestate (lua_State *st) {
lua_State *old = lua_setstate(st);
lua_close();
if (old != st) lua_setstate(old);
}
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 comparison of two lua_Object's is undefined.
To check the type of a lua_Object,
the following functions are available:
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 4.3),
and lua_isfunction accepts Lua functions and C functions.
To get the tag of a lua_Object,
the following function is available:
int lua_tag (lua_Object object);
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); long lua_strlen (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 4.3); otherwise, lua_getnumber 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.
Those strings always have a 0 after their last character (like in C),
but may contain other zeros in their body.
If you do not know whether a string may contain zeros,
you can use lua_strlen to get the actual length.
Because Lua has garbage collection,
there is no guarantee that such pointer will be valid after the block ends
(see Section 5.3).
lua_getcfunction converts a lua_Object to a C function.
This lua_Object must have type CFunction;
otherwise, lua_getcfunction returns 0 (the NULL pointer).
The type lua_CFunction is explained in Section 5.7.
lua_getuserdata converts a lua_Object to void*.
This lua_Object must have type userdata;
otherwise, lua_getuserdata returns 0 (the NULL pointer).
lua_Object has a limited scope,
and is only valid inside the block where it has been 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.
A garbage collection cycle can be forced by:
long lua_collectgarbage (long limit);This function returns the number of objects collected. The argument
limit makes the next cycle occur only
after that number of new objects have been created.
If limit=0, then Lua uses an adaptive heuristics to set this limit.
All communication 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.
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.
Note that the structure lua2C cannot be directly modified by C code.
The second structure, C2lua, is an abstract stack. Pushing elements into this stack is done with the following functions and macros:
void lua_pushnumber (double n); void lua_pushlstring (char *s, long len); void lua_pushstring (char *s); void lua_pushusertag (void *u, int tag); void lua_pushnil (void); void lua_pushobject (lua_Object object); void lua_pushcfunction (lua_CFunction f); /* macro */All of them receive a C value, convert it to a corresponding
lua_Object,
and leave the result on the top of C2lua.
Particularly, functions lua_pushlstring and lua_pushstring
make an internal copy of the given string.
Function lua_pushstring can only be used to push proper C strings
(that is, strings that do not contain zeros and end with a zero);
otherwise you should use the more generic lua_pushlstring.
The function
lua_Object lua_pop (void);returns a reference to the object at the top of the C2lua stack, and pops it.
As a general rule, all API functions pop from the stack all elements they use.
Because userdata are objects,
the function lua_pushusertag may create a new userdata.
If Lua has a userdata with the given value (void*) and tag,
that userdata is pushed.
Otherwise, a new userdata is created, with the given value and tag.
If this function is called with
tag equal to LUA_ANYTAG,
then Lua will try to find any userdata with the given value,
regardless of its tag.
If there is no userdata with that value, then a new one is created,
with tag equal to 0.
Userdata can have different tags, whose semantics are only known to the host program. Tags are created with the function:
int lua_newtag (void);The function
lua_settag changes the tag of
the object on the top of C2lua (and pops it);
the object must be a userdata or a table.
void lua_settag (int tag);
tag must be a value created with lua_newtag.
When C code calls Lua repeatedly, as in a loop, objects returned by these calls can accumulate, and may cause a stack overflow. 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 good programming practice
and is strongly encouraged.
int lua_dofile (char *filename); int lua_dostring (char *string); int lua_dobuffer (char *buff, int size, char *name);All these 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.
When called with argument NULL,
lua_dofile executes the stdin stream.
Functions lua_dofile and lua_dobuffer
are both able to execute pre-compiled chunks.
They automatically detect whether the chunk is text or binary,
and load it accordingly (see program luac).
Function lua_dostring executes only source code.
The third parameter to lua_dobuffer (name)
is the ``name of the chunk'',
used in error messages and debug information.
If name is NULL,
Lua gives a default name to the chunk.
In files this name is the file name,
and lua_dostring uses a small prefix
of the string as the chunk name.
These functions return, in structure lua2C, any values eventually returned by the chunks. They also empty the stack C2lua.
lua_Object lua_getglobal (char *varname);As in Lua, this function may trigger a tag method. To read the real value of any global variable, without invoking any tag method, use the raw version:
lua_Object lua_rawgetglobal (char *varname);
To store a value previously pushed onto C2lua in a global variable, there is the function:
void lua_setglobal (char *varname);As in Lua, this function may trigger a tag method. To set the real value of any global variable, without invoking any tag method, use the raw version:
void lua_rawsetglobal (char *varname);
Tables can also be manipulated via the API. The function
lua_Object lua_gettable (void);pops from the stack C2lua a table and an index, and returns the contents of the table at that index. As in Lua, this operation may trigger a tag method. To get the real value of any table index, without invoking any tag method, use the raw version:
lua_Object lua_rawgettable (void);
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_settable (void);Again, the tag method for ``settable'' may be called. To set the real value of any table index, without invoking any tag method, use the raw version:
void lua_rawsettable (void);
lua_Object lua_createtable (void);creates and returns a new, empty table.
int lua_callfunction (lua_Object function);This function returns 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 function lua_lua2C.
Note that function lua_callfunction
pops all elements from the C2lua stack.
The following example shows how a C program may do the equivalent to the Lua code:
a,b = f("how", t.x, 4)
lua_pushstring("how"); /* 1st argument */
lua_pushobject(lua_getglobal("t")); /* push value of global 't' */
lua_pushstring("x"); /* push the string 'x' */
lua_pushobject(lua_gettable()); /* push result of t.x (2nd arg) */
lua_pushnumber(4); /* 3rd argument */
lua_callfunction(lua_getglobal("f")); /* call Lua function */
lua_pushobject(lua_getresult(1)); /* push first result of the call */
lua_setglobal("a"); /* sets global variable 'a' */
lua_pushobject(lua_getresult(2)); /* push second result of the call */
lua_setglobal("b"); /* sets global variable 'b' */
Some special Lua functions have exclusive interfaces. 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).
The message is passed to the error handler method.
If message is NULL,
the error handler method is not called.
The error handler method (see Section 4.9) can be changed with:
lua_Object lua_seterrormethod (void);This function sets the object at the top of C2lua as the new error method, and returns the old error method value.
Tag methods can be changed with:
lua_Object lua_settagmethod (int tag, char *event);The first parameter is the tag, and the second is the event name (see Section 4.8); the new method is pushed from C2lua. This function returns a
lua_Object,
which is the old tag method value.
To get just the current value of a tag method,
use the function
lua_Object lua_gettagmethod (int tag, char *event);
It is also possible to copy all tag methods from one tag to another:
int lua_copytagmethods (int tagto, int tagfrom);This function returns
tagto.
#define lua_register(n,f) (lua_pushcfunction(f), lua_setglobal(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 for lua_lua2C.
To return values, a C function just pushes them onto the stack C2lua,
in direct order (see Section 5.2).
Like a Lua function, a C function called by Lua can also return
many results.
When a C function is created, it is possible to associate some upvalues to it; then these values are passed to the function whenever it is called, as common arguments. To associate upvalues to a function, first these values must be pushed on C2lua. Then the function:
void lua_pushcclosure (lua_CFunction fn, int n);is used to put the C function on C2lua, with the argument
n telling how many upvalues must be
associated with the function;
in fact, the macro lua_pushcfunction is defined as
lua_pushcclosure with n set to 0.
Then, any time the function is called,
these upvalues are inserted as the first arguments to the function,
before the actual arguments provided in the call.
For some examples of C functions, see files lstrlib.c,
liolib.c and lmathlib.c in the official Lua distribution.
As noted in Section 5.3, lua_Objects are volatile.
If the C code needs to keep a lua_Object
outside block boundaries,
then 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_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.
Note that an unlocked reference may be garbage collected.
Whenever the referenced object is needed,
a call to lua_getref
returns a handle to it;
if the object has been collected,
lua_getref returns LUA_NOOBJECT.
When a reference is no longer needed,
it can be released with a call to lua_unref.
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:
lua_strlibopen, lua_mathlibopen,
and lua_iolibopen, declared in lualib.h.
func with
the arguments given by the table arg.
The call is equivalent to
func(arg[1], arg[2], ..., arg[arg.n])
If arg.n is not defined,
then Lua stops getting arguments at the first nil value.
By default,
all results from func are just returned by the call.
If the string mode contains "p",
the results are packed in a single table.
That is, call returns just one table;
at index n, the table has the total number of results
from the call;
the first result is at index 1, etc.
For instance, the following calls produce the following results:
a = call(sin, {5}) --> a = 0.0871557 = sin(5)
a = call(max, {1,4,5; n=2}) --> a = 4 (only 1 and 4 are arguments)
t = {x=1}
a = call(next, {t,nil;n=2}, "p") --> a={"x", 1; n=2}
By default,
if an error occurs during the function call,
the error is propagated.
If the string mode contains "x",
then the call is protected.
In this mode, function call does not generate an error,
whatever happens during the call.
Instead, it returns nil to signal the error
(besides calling the appropriated error method).
If provided, errmethod is temporarily set as the error method,
while func runs.
As a particular case, if errmethod is nil,
no error messages will be issued during the execution of the called function.
limit, is a number that
makes the next cycle occur only after that number of new
objects have been created.
If absent, Lua uses an adaptive algorithm to set
this limit.
collectgarbage is equivalent to
the API function lua_collectgarbage.
dofile executes the contents of the standard input (stdin).
If there is any error executing the file,
then dofile 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 equivalent to the API function lua_dofile.
dostring returns nil.
Otherwise, it returns the values returned by the chunk,
or a non nil value if the chunk returns no values.
An optional second parameter (chunkname)
is the ``name of the chunk'',
used in error messages and debug information.
dostring is equivalent to the API function lua_dostring.
newtag is equivalent to the API function lua_newtag.
Lua has no declaration of fields;
semantically, there is no difference between a
field not present in a table or a field with value nil.
Therefore, the function only considers fields with non nil values.
The order in which the indices are enumerated is not specified,
even for numeric indices
(to traverse a table in numeric order, use a counter).
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.
next,
but iterates instead 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.
function over all elements of table.
For each element, the function is called with the index and
respective value as arguments.
If the function returns any non-nil value,
the loop is broken, and the value is returned
as the final value of foreach.
This function could be defined in Lua:
function foreach (t, f)
local i, v = next(t, nil)
while i do
local res = f(i, v)
if res then return res end
i, v = next(t, i)
end
end
function over all global variables.
For each variable,
the function is called with its name and its value as arguments.
If the function returns any non-nil value,
the loop is broken, and the value is returned
as the final value of foreachvar.
This function could be defined in Lua:
function foreachvar (f)
local n, v = nextvar(nil)
while n do
local res = f(n, v)
if res then return res end
n, v = nextvar(n)
end
end
format.
tostring.
This function is not intended for formatted output,
but only 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 returns that number;
otherwise, it returns nil.
An optional argument specifies the base to interpret the numeral. The base may be any integer between 2 and 36 inclusive. In bases above 10, the letter `A' (either upper or lower case) represents 10, `B' represents 11, and so forth, with `Z' representing 35.
In base 10 (the default), the number may have a decimal part, as well as an optional exponent part (see Section 4.3). In other bases, only integers are accepted.
"nil" (a string, not the value nil),
"number",
"string",
"table",
"function",
and "userdata".
tag is equivalent to the API function lua_tag.
tag must be a value created with newtag
(see Section 6.1).
It returns the value of its first argument (the table).
For security reasons,
it is impossible to change the tag of a userdata from Lua.
function assert (v, m)
if not v then
m = m or ""
error("assertion failed! " .. m)
end
end
lua_dofile, lua_dostring,
lua_dobuffer, or lua_callfunction;
in Lua: dofile, dostring, or call in protected mode).
If message is nil, the error handler is not called.
Function error never returns.
error is equivalent to the API function lua_error.
table[index],
without invoking any tag method.
table must be a table,
and index is any value different from nil.
table[index] to value,
without invoking any tag method.
table must be a table,
index is any value different from nil,
and value is any Lua value.
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.
Function rawsetglobal returns the value of its second argument.
name does not need to be a
syntactically valid variable name.
Function setglobal returns the value of its second argument.
name does not need to be a
syntactically valid variable name.
name does not need to be a
syntactically valid variable name.
newmethod must be a function or nil,
in which case the error handler does nothing.
Returns the old error handler.
newmethod is nil,
settagmethod restores the default behavior for the given event.
tagto.
pattern in str.
If it finds one, then it returns the indices on str
where this occurrence 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.
If init is negative,
it is replaced by the length of the string minus its
absolute value plus 1.
Therefore, -1 points to the last character of str.
A value of 1 as a fourth optional argument
turns off the pattern matching facilities,
so the function does a plain ``find substring'' operation,
with no characters in pattern being considered ``magic''.
s,
starting at i and running until j.
If i or j are negative,
they are replaced by the length of the string minus their
absolute value plus 1.
Therefore, -1 points to the last character of s
and -2 to the previous one.
If j is absent, it is assumed to be equal to -1
(which is the same as the string length).
In particular,
the call strsub(s,1,j) returns a prefix of s
with length j,
and the call strsub(s, -i) returns a suffix of s
with length i.
n copies of
the string s.
s[i].
If i is absent, then it is assumed to be 1.
If i is negative,
it is replaced by the length of the string minus its
absolute value plus 1.
Therefore, -1 points to the last character of s.
Note that numerical codes are not necessarily portable across platforms.
Note that numerical codes are not necessarily portable across platforms.
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"
Conversions can be applied to the n-th argument in the argument list,
rather than the next unused argument.
In this case, the conversion character % is replaced
by the sequence %d$, where d is a
decimal digit in the range [1,9],
giving the position of the argument in the argument list.
For instance, the call format("%2$d -> %1$03d", 1, 34) will
result in "34 -> 001".
The same argument can be used in more than one conversion.
The options c, d, E, e, f,
g, G, i, o, u, X, and x all
expect a number as argument,
whereas q and s expect a string.
Note that the * modifier can be simulated by building
the appropriate format string.
For example, "%*g" can be simulated with
"%"..width.."g".
Note: function format can only be used with strings that do not contain zeros.
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 passed as arguments,
in order (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.
A last optional parameter n limits
the maximum number of substitutions to occur.
For instance, when n is 1 only the first occurrence of
pat is replaced.
See some examples below:
x = gsub("hello world", "(%w%w*)", "%1 %1")
--> x="hello hello world world"
x = gsub("hello world", "(%w%w*)", "%1 %1", 1)
--> x="hello hello world"
x = gsub("hello world from Lua", "(%w%w*)%s*(%w%w*)", "%2 %1")
--> x="world hello Lua from"
x = gsub("home = $HOME, user = $USER", "$(%w%w*)", getenv)
--> x="home = /home/roberto, user = roberto" (for instance)
x = gsub("4+5 = $return 4+5$", "$(.-)%$", dostring)
--> x="4+5 = 9"
local t = {name="lua", version="3.1"}
x = gsub("$name - $version", "$(%w%w*)", function (v) return %t[v] end)
--> x="lua - 3.1"
t = {n=0}
gsub("first second word", "(%w%w*)",
function (w) %t.n = %t.n+1; %t[%t.n] = w end)
--> t={"first", "second", "word"; n=3}
()%.[*-?)
- represents the character x itself.
()%.[*-?.
] 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 -.
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.
E.g., assuming an ascii character set,
[%dA-Fa-f] specifies the hexa-decimal digits.
The definitions of letter, space, etc. depend on the current locale.
In particular, the class [a-z] may not be equivalent to %l.
The second form should be preferred for more portable programs.
*,
which matches 0 or more repetitions of characters in the class.
These repetition items will always match the longest possible sequence.
-,
which also matches 0 or more repetitions of characters in the class.
Unlike *,
these repetition items will always match the shortest possible sequence.
?,
which matches 0 or 1 occurrence of a character in the class;
%b() matches expressions with
balanced parentheses.
^ at the beginning of a pattern anchors the match at the
beginning of the subject string.
A $ at the end of a pattern anchors the match at the
end of the subject string.
"(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.
This library is an interface to some functions of the standard C math library.
In addition, it registers a tag method 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 deg floor log log10 max min mod rad sin sqrt tan frexp ldexp random randomseedplus a global variable PI. 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. Functions deg and rad can be used to convert between radians and degrees.
The function max returns the maximum
value of its numeric arguments.
Similarly, min computes the minimum.
Both can be used with 1, 2 or more arguments.
The functions random and randomseed are interfaces to
the simple random generator functions rand and srand,
provided by ANSI C.
The function random, when called without arguments,
returns a pseudo-random real number in the range [0,1).
When called with a number n,
random returns a pseudo-random integer in the range [1,n].
All input and output operations in Lua are done over two
file handles, one for reading and one for writing.
These handles are stored in two Lua global variables,
called _INPUT and _OUTPUT.
The global variables
_STDIN, _STDOUT and _STDERR
are initialized with file descriptors for
stdin, stdout and stderr.
Initially, _INPUT=_STDIN and _OUTPUT=_STDOUT.
A file handle is a userdata containing the file stream FILE*,
and with a distinctive tag created by the I/O library.
Unless otherwise stated, all I/O functions return nil on failure and some value different from nil on success.
This function may be called in two ways.
When called with a file name, it opens the named file,
sets its handle as the value of _INPUT,
and returns this value.
It does not close the current input file.
When called without parameters,
it closes the _INPUT file,
and restores stdin as the value of _INPUT.
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 opened, 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.
This function may be called in two ways.
When called with a file name,
it opens the named file,
sets its handle as the value of _OUTPUT,
and returns this value.
It does not close the current output file.
Note that, if the file already exists,
then it will be completely erased with this operation.
When called without parameters,
this function closes the _OUTPUT file,
and restores stdout as the value of _OUTPUT.
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 opened, 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.
This function opens a file named filename and sets it as the
value of _OUTPUT.
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.
Note that function writeto is
available to close an output file opened by appendto.
This function deletes the file with the given name. If this function fails, it returns nil, plus a string describing the error.
This function renames file named name1 to name2.
If this function fails, it returns nil,
plus a string describing the error.
This function returns a string with a file name that can safely be used for a temporary file. The file must be explicitly removed when no longer needed.
This function reads the file _INPUT,
or from filehandle if this argument is given,
according to a read pattern, which specifies how much to read;
characters are read from the 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 characters, it never fails.
Note that the behavior of read patterns is slightly different from
the regular pattern matching behavior,
where a * expands to the maximum length such that
the rest of the pattern does not fail.
With the read pattern behavior
there is no need for backtracking the reading.
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:
"." returns the next character, or nil on end of file.
".*" reads the whole file.
"[^\n]*{\n}" returns the next line
(skipping the end of line), or nil on end of file.
This is the default pattern.
"{%s*}%S%S*" returns the next word
(maximal sequence of non white-space characters),
skipping spaces if necessary,
or nil on end of file.
"{%s*}[+-]?%d%d*" returns the next integer
or nil if the next characters do not conform to an integer format.
This function writes the value of each of its arguments to the
file _OUTPUT,
or to filehandle if this argument is given,
The arguments must be strings or numbers.
To write other values,
use tostring or format before write.
If this function fails, it returns nil,
plus a string describing the error.
This 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 and the locale.
This function returns an approximation of the amount of CPU time used by the program, in seconds.
This function calls the C function exit,
with an optional code,
to terminate the program.
The default value for code is 1.
Returns the value of the process environment variable varname,
or nil if the variable is not defined.
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.
This function is an interface to the ANSI C function setlocale.
locale is a string specifying a locale;
category is an optional string describing which category to change:
"all", "collate", "ctype",
"monetary", "numeric", or "time";
the default category is "all".
The function returns the name of the new locale,
or nil if the request cannot be honored.
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.
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 4.9).
When no line information is available,
lua_currentline 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 tag method.
If so, it returns the string "tag-method",
and name is set to point to the event 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 tag method nor a global variable,
then lua_getobjname returns the empty string,
and name is set to NULL.
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 4.9).
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 5.2).
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.
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;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 bytypedef void (*lua_LHFunction) (int line); extern lua_LHFunction lua_linehook;
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 4.9).
A hook is disabled when its value is NULL,
which is the initial value of both hooks.
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:
stat as a Lua chunk.
EOF.
Each line entered is immediately executed.
stdin as a file.
var with string "value".
filename as a Lua chunk.
lua -v -i when stdin is a terminal,
and as lua - otherwise.
All arguments are handled in order. For instance, an invocation like
$ lua -i a=test prog.luawill first interact with the user until an
EOF,
then will set a to "test",
and finally will run the file prog.lua.
When in interactive mode,
a multi-line statement can be written finishing intermediate
lines with a backslash (\).
The prompt presented is the value of the global variable _PROMPT.
Therefore, the prompt can be changed like below:
$ lua _PROMPT='myprompt> ' -i
In Unix systems, Lua scripts can be made into executable programs
by using the #! form,
as in #!/usr/local/bin/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.
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.
lua_open.
However, all standard libraries check whether Lua is already opened,
so any existing program that opens at least one standard
library before calling Lua does not need to be modified.
dostring no longer accepts an optional second argument,
with a temporary error method.
This facility is now provided by function call.
gsub no longer accepts an optional fourth argument
(a callback data, a table).
Closures replace this feature with advantage.
function f[exp] (x) ... end is not
accepted in Lua 3.1.
In these cases,
programs should use an explicit assignment instead, such as
f[exp] = function (x) ... end.
a=b in Lua stand-alone now sets a to the
string b, and not to the value of b.