6.8 – Input and Output Facilities

The I/O library provides two different styles for file manipulation. The first one uses implicit file handles; 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 handles.

When using implicit file handles, all operations are supplied by table io. When using explicit file handles, the operation io.open returns a file handle and then all operations are supplied as methods of the file handle.

The table io also provides three predefined file handles 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. In 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.

io.close ([file])

Equivalent to file:close(). Without a file, closes the default output file.

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 arguments, it returns the current default input file.

In case of errors this function raises the error, instead of returning an error code.

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 no values (to finish the loop) and automatically closes the file.

The call io.lines() (with no file name) is equivalent to io.input():lines("*l"); that is, it iterates over the lines of the default input file. In this case, the iterator does not close the file when the loop ends.

In case of errors this function raises the error, instead of returning an error code.

io.open (filename [, mode])

This function opens a file, in the mode specified in the string mode. In case of success, it returns a new file handle.

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.

io.output ([file])

Similar to io.input, but operates over the default output file.

io.popen (prog [, mode])

This function is system dependent and is not available on all platforms.

Starts program prog in a separated process and 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 (if mode is "w").

io.read (···)

Equivalent to io.input():read(···).

io.tmpfile ()

In case of success, returns a handle for a temporary file. This file is opened in update mode and it is automatically removed when the program ends.

io.type (obj)

Checks whether obj is a valid file handle. Returns the string "file" if obj is an open file handle, "closed file" if obj is a closed file handle, or nil if obj is not a file handle.

io.write (···)

Equivalent to io.output():write(···).

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 by os.execute.

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 construction

     for c in file:lines(1) do body end

will iterate over all characters of the file, starting at the current position. 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.

file:read (···)

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. (In this latter case, the function does not read subsequent formats.) When called without formats, it uses a default format that reads the next line (see below).

The available formats are

• “n“: reads a numeral and returns it as a float or an integer, following the lexical conventions of Lua. (The numeral may have leading spaces and a sign.) This format always reads the longest input sequence that is a valid prefix for a numeral; if that prefix does not form a valid numeral (e.g., an empty string, “0x“, or “3.4e-“), it is discarded and the function returns nil.
• “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.
• “L“: reads the next line keeping the end-of-line character (if present), returning nil on end of file.
• number: reads a string with up to this number of bytes, returning nil on end of file. If number is zero, it reads nothing and returns an empty string, or nil on end of file.

The formats “l“ and “L“ should be used only for text files.

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 string 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, seek returns the final file position, measured in bytes from the beginning of the file. If seek fails, it returns nil, plus a string describing the error.

The default value for whence is "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 flush the file (see io.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.

file:write (···)

Writes the value of each of its arguments to 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.