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:
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:
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.
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 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.
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).
This section describes the lexis, the syntax and the semantics of Lua.
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:
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:
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).
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.
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).
A single name can denote a global or a local variable, or a formal parameter:
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"]:
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:
The non-terminal exp1 is used to indicate that the values
returned by an expression must be adjusted to one single value:
+ (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).
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).
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.
^ (exponentiation),
which is right associative.
The general syntax for constructors is:
The form lfieldlist1 is used to initialize lists.
The next form initializes named fields in a table:
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:
var:name(...)
is syntactic sugar for
var is evaluated only once.
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.
Functions in Lua can be defined anywhere in the global level of a chunk. The syntax for function definition is:
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.
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.
self.
Notice that
the variable v must have been previously initialized with a table value.
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:
^ 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:
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.
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).
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:
lua.h.
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:
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:
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:
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:
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:
lua_Object's created inside it are released.
The use of explicit nested blocks is strongly encouraged.
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.
To store a value previously pushed onto C2lua in a global variable, there is the function:
Tables can also be manipulated via the API. The function
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:
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:
lua_getglobal("i") modifies the stack,
and invalidates the previous pushed value.
A correct solution could be:
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
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:
Two special Lua functions have exclusive interfaces:
error and setfallback.
A C function can generate a Lua error calling the function
exit(1).
Fallbacks can be changed with:
lua_Object,
which is the old fallback value,
or nil on failure (invalid fallback name).
This old value can be used for chaining fallbacks.
lua_CFunction,
which is defined as
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:
For more examples, see files strlib.c,
iolib.c and mathlib.c in Lua distribution.
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:
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.
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:
strlib_open, mathlib_open, and iolib_open,
declared in lualib.h.
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 is simply an interface to lua_dostring.
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.
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.
"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.
lua_dofile, lua_dostring, ...).
It never returns.
error is simply an interface to lua_error.
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.
name does not need to be a syntactically valid variable name.
setfallback is simply an interface to lua_setfallback.
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.
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.
n copies of
the string s.
s[i].
If i is absent, then it is assumed to be 1.
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
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".
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:
()%.[*?)
- 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 -;
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.
*,
which matches 0 or more repetitions of characters in the class.
These repetition itens will always match the longest possible sequence.
-,
which also matches 0 or more repetitions of characters in the class.
Unlike *,
these repetition itens will always match the shortest possible sequence.
?,
which matches 0 or 1 occurrence of a character in the class;
%() 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 fallback for the binary operator ^ that,
returns x^y when applied to numbers x^y.
The library provides the following functions:
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).
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.
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.
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.
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.
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.
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:
"." 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),
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
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.
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.
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 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.
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) 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:
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.
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_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.
The Lua interpreter offers two hooks for debugging purposes:
typedef 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 4.8).
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:
EOF.
stat as a Lua chunk.
var=exp as a Lua chunk.
filename as a Lua chunk.
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
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
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.
write has been supersed by
function format;
therefore this facility has been dropped.
read now uses read patterns to specify
what to read;
this is incompatible with the old format options.
strfind now accepts patterns,
so it may have a different behavior when the pattern includes
special characters.
date and time (from iolib)
have been superseded by the new, more powerful version of function date.
append (from iolib) now returns 1 whenever it succeeds,
whether the file is new or not.
int2str (from strlib) has been superseded by new
function format, with parameter "%c".
lua.h provides compatibility macros,
so there is no need to change programs.
lua_pushliteral now is just a macro to
lua_pushstring.
type now returns the string "function"
both for C and Lua functions.
Because Lua functions and C functions are compatible,
this behavior is usually more useful.
When needed, the second result of function type may be used
to distinguish between Lua and C functions.
==,
instead of =.
@(size) has been substituted by {}.
The list constructor (formerly @[...]) and the record
constructor (formerly @{...}) now are both coded like
{...}.
When the construction involves a function call,
like in @func{...},
the new syntax does not use the @.
More important, {\em a construction function must now
explicitly return the constructed table}.
lua_call no longer has the parameter nparam.
lua_pop is no longer available,
since it could lead to strange behavior.
In particular,
to access results returned from a Lua function,
the new macro lua_getresult should be used.
lua_storefield and lua_storeindexed
have been replaced by
lua_errorfunction has been
replaced by the fallback mechanism (see Section 4.8).
lua_getindexed and lua_getfield.