io - input and output facilities (standard library)¶
The I/O library provides 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
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.
io also provides three predefined file descriptors with
their usual meanings from C:
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
file, closes the default output file.
file:flush() over the default output 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. o In case of errors this function raises the error, instead of returning an error code.
Opens the given file name in read mode and returns an iterator function that, each time it is called, returns a new line from the file. Therefore, the construction
for line in io.lines(filename) do body end
will iterate over all lines of the file. When the iterator function
detects the end of file, it returns
nil (to finish the loop) and
automatically closes the file.
io.lines() (with no file name) is equivalent to
io.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
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,
plus an error message.
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.
mode string can also have a ‘
b’ at the end, which is needed
in some systems to open the file in binary mode. This string is
exactly what is used in the standard C function
io.input, but operates over the default output file.
io.popen (prog [, mode])
prog in a separated process and returns a file
handle that you can use to read data from this program (if
"r", the default) or to write data to this program (if
This function is system dependent and is not available on all platforms.
Returns a handle for a temporary file. This file is opened in update mode and it is automatically removed when the program ends.
obj is a valid file handle. Returns the string
obj is an open file handle,
"closed file" if
is a closed file handle, or
obj is not a file handle.
file. Note that files are automatically closed when their
handles are garbage collected, but that takes an unpredictable amount
of time to happen.
Saves any written data to
Returns an iterator function that, each time it is called, returns a new line from the file. Therefore, the construction
for line in file:lines() do body end
will iterate over all lines of the file. (Unlike
function does not close the file when the loop ends.)
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
nilon 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.
file:seek ([whence] [, offset])
Sets and gets the file position, measured from the beginning of the
file, to the position given by
offset plus a base specified by the
whence, 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 for
"cur", and for
offset is 0.
Therefore, the call
file:seek() returns the current file position,
without changing it; the call
file:seek("set") sets the position to
the beginning of the file (and returns 0); and the call
file:seek("end") sets the position to the end of the file, and
returns its size.
file:setvbuf (mode [, size])
Sets the buffering mode for an output file. There are three available modes:
"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 explicitly
flushthe file (see
"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.
Writes the value of each of its arguments to the
arguments must be strings or numbers. To write other values, use
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