Module io
Input and Output Facilities.
The I/O library provides function for file manipulation. There are two different styles for file manipulation. The first one uses implicit file descriptors; that is, there are operations to set a default input file and a default output file, and all input/output operations are over these default files. The second style uses explicit file descriptors.
When using implicit file descriptors, all operations are supplied by table io.
When using explicit file descriptors, the operation io.open returns a file descriptor and then all operations are supplied as methods of the file descriptor.
The table io also provides three predefined file descriptors with their usual meanings from C: io.stdin
, io.stdout
, and io.stderr
.
The I/O library never closes these files.
Unless otherwise stated, all I/O functions return nil on failure (plus an error
message as a second result and a system-dependent error code as a third result)
and some value different from nil on success. On non-Posix systems, the computation
of the error message and error code in case of errors may be not thread safe,
because they rely on the global C variable errno
.
Type io
io.close(file) |
Equivalent to |
io.flush() |
Equivalent to |
io.input(file) |
When called with a file name, it opens the named file (in text mode), and sets its handle as the default input file. |
io.lines(filename) |
Opens the given file name in read mode and returns an iterator function that works like file:lines(...) over the opened file. |
io.open(filename, mode) |
This function opens a file, in the mode specified in the string |
io.output(file) |
Similar to |
io.popen(prog, mode) |
Starts program |
io.read(format) |
Equivalent to |
io.stderr |
io.stderr: Standard error. |
io.stdin |
io.stdin: Standard in. |
io.stdout |
io.stdout: Standard out. |
io.tmpfile() |
Returns a handle for a temporary file. |
io.write(...) |
Equivalent to |
Type file
file:close() |
Closes |
file:flush() |
Saves any written data to |
file:lines(...) |
Returns an iterator function that, each time it is called, reads the file according to the given formats. |
file:read(format) |
Reads the file |
file:seek(whence, offset) |
Sets and gets the file position. |
file:setvbuf(mode, size) |
Sets the buffering mode for an output file. |
file:write(...) |
Writes the value of each of its arguments to the |
Type io
Field(s)
- io.close(file)
-
Equivalent to
file:close
.Without a
file
, closes the default output file.Parameter
-
#file file
: file to close (optional).
Return value
A status code which is system-dependent (optional).
-
- io.flush()
-
Equivalent to
io.output():flush()
.
- io.input(file)
-
When called with a file name, it opens the named file (in text mode), and sets its handle as the default input file.
When called with a file handle, it simply sets this file handle as the default input file. When called without parameters, it returns the current default input file.
In case of errors this function raises the error, instead of returning an error code.
Parameter
-
file
: a filename or a file handle which will used as default input. (optional)
Return value
#file: the default input file handle.
-
- io.lines(filename)
-
Opens the given file name in read mode and returns an iterator function that works like file:lines(...) over the opened file.
When the iterator function detects the end of file, it returns nil (to finish the loop) and automatically closes the file.
The call
io.lines()
(with no file name) is equivalent toio.input():lines()
; that is, it iterates over the lines of the default input file. In this case it does not close the file when the loop ends.In case of errors this function raises the error, instead of returning an error code.
Parameter
-
filename
: a filename or a file handle. (default value isio.input()
)
Return value
iterator function on lines
-
- io.open(filename, mode)
-
This function opens a file, in the mode specified in the string
mode
.It returns a new file handle, or, in case of errors, nil plus an error message. The
mode
string can be any of the following:- "r": read mode (the default);
- "w": write mode;
- "a": append mode;
- "r+": update mode, all previous data is preserved;
- "w+": update mode, all previous data is erased;
- "a+": append update mode, previous data is preserved, writing is only allowed at the end of file.
The
mode
string can also have a 'b
' at the end, which is needed in some systems to open the file in binary mode.Parameters
-
#string filename
: path to the file. -
#string mode
: used to specify the open mode (optional).
Return value
#file: a file handle.
- io.output(file)
-
Similar to
io.input
, but operates over the default output file.Parameter
-
file
: a filename or a file handle which will used as default output. (optional)
Return value
#file: the default ouput file handle.
-
- io.popen(prog, mode)
-
Starts program
prog
in a separated process.Returns a file handle that you can use to read data from this program (if
mode
is"r"
, the default) or to write data to this program (ifmode
is"w"
).This function is system dependent and is not available on all platforms.
Parameters
-
#string prog
: the program to start. -
#string mode
: used to specify the open mode.
Return value
#file: a file handle used to read from or write to the program
prog
. -
- io.read(format)
-
Equivalent to
io.input():read
.Parameter
-
format
: "*n", "*a", "*l" or a number.
Return values
-
A string (or a number) with the characters read
-
#nil: if it cannot read data with the specified format.
-
- io.tmpfile()
-
Returns a handle for a temporary file.
This file is opened in update mode and it is automatically removed when the program ends.
Return value
#file: a file handle for a temporary file.
- io.write(...)
-
Equivalent to
io.output():write
.Parameter
-
...
: must be strings or numbers.
Return values
-
#file: the standard output
-
#nil, #string: an error message, if it failed.
-
Type file
Field(s)
- file:close()
-
Closes
file
.Note that files are automatically closed when their handles are garbage collected, but that takes an unpredictable amount of time to happen.
When closing a file handle created with
io.popen
,file:close
returns the same values returned byos.execute
.Return value
A status code which is system-dependent (optional).
- file:flush()
-
Saves any written data to
file
.
- file:lines(...)
-
Returns an iterator function that, each time it is called, reads the file according to the given formats.
When no format is given, uses "
*l
" as a default. As an example, the constructionfor line in file:lines() do body end
will iterate over all lines of the file. Unlike
io.lines
, this function does not close the file when the loop ends. In case of errors this function raises the error, instead of returning an error code.Parameter
-
...
: reading formats (optional, "*l
" by default)
Return value
iterator function over the file
-
- file:read(format)
-
Reads the file
file
, according to the given formats, which specify what to read.For each format, the function returns a string (or a number) with the characters read, or nil if it cannot read data with the specified format. When called without formats, it uses a default format that reads the entire next line (see below). The available formats are
- "*n": reads a number; this is the only format that returns a number instead of a string.
- "*a": reads the whole file, starting at the current position. On end of file, it returns the empty string.
- "*l": reads the next line (skipping the end of line), returning nil on end of file. This is the default format.
- number: reads a string with up to this number of characters, returning nil on end of file. If number is zero, it reads nothing and returns an empty string, or nil on end of file.
Parameter
-
format
: "*n", "*a", "*l" or a number.
Return values
-
A string (or a number) with the characters read
-
#nil: if it cannot read data with the specified format
- file:seek(whence, offset)
-
Sets and gets the file position.
It is measured from the beginning of the file, to the position given by
offset
plus a base specified by the stringwhence
, as follows:- "set": base is position 0 (beginning of the file);
- "cur": base is current position;
- "end": base is end of file;
In case of success, function
seek
returns the final file position, measured in bytes from the beginning of the file. If this function fails, it returns nil, plus a string describing the error. The default value forwhence
is"cur"
, and foroffset
is 0. Therefore, the callfile:seek()
returns the current file position, without changing it; the callfile:seek("set")
sets the position to the beginning of the file (and returns 0); and the callfile:seek("end")
sets the position to the end of the file, and returns its size.Parameters
-
#string whence
:"set"
,"cur"
or"end"
(optional, default value is"cur"
) -
#number offset
: offset of end position. (optional, default value is0
)
Return values
-
#number: the final file position in bytes, if it succeed.
-
#nil, #string: an error message, if it failed.
- file:setvbuf(mode, size)
-
Sets the buffering mode for an output file.
There are three available modes:
"no"
: no buffering; the result of any output operation appears immediately."full"
: full buffering; output operation is performed only when the buffer is full or when you explicitlyflush
the file (seeio.flush
)."line"
: line buffering; output is buffered until a newline is output or there is any input from some special files (such as a terminal device).
For the last two cases,
size
specifies the size of the buffer, in bytes. The default is an appropriate size.Parameters
-
#string mode
: the buffering mode :"no"
,"full"
or"line"
. -
#number size
: the size of the buffer.(default value is an appropriate size)
- file:write(...)
-
Writes the value of each of its arguments to the
file
.The arguments must be strings or numbers. In case of success, this function returns file. Otherwise it returns nil plus a string describing the error.
Parameter
-
...
: must be strings or numbers.
Return values
-
#file: edited file
-
#nil, #string: an error message, if it failed.
-