path: root/doc/io_streams.rdoc

== \IO Streams

This page describes:

- {Stream classes}[rdoc-ref:io_streams.rdoc@Stream+Classes].
- {Pre-existing streams}[rdoc-ref:io_streams.rdoc@Pre-Existing+Streams].
- {User-created streams}[rdoc-ref:io_streams.rdoc@User-Created+Streams].
- {Basic \IO}[rdoc-ref:io_streams.rdoc@Basic+IO], including:

  - {Position}[rdoc-ref:io_streams.rdoc@Position].
  - {Open and closed streams}[rdoc-ref:io_streams.rdoc@Open+and+Closed+Streams].
  - {End-of-stream}[rdoc-ref:io_streams.rdoc@End-of-Stream].

- {Line \IO}[rdoc-ref:io_streams.rdoc@Line+IO], including:

  - {Line separator}[rdoc-ref:io_streams.rdoc@Line+Separator].
  - {Line limit}[rdoc-ref:io_streams.rdoc@Line+Limit].
  - {Line number}[rdoc-ref:io_streams.rdoc@Line+Number].
  - {Line options}[rdoc-ref:io_streams.rdoc@Line+Options].

- {Character \IO}[rdoc-ref:io_streams.rdoc@Character+IO].
- {Byte \IO}[rdoc-ref:io_streams.rdoc@Byte+IO].
- {Codepoint \IO}[rdoc-ref:io_streams.rdoc@Codepoint+IO].

=== Stream Classes

Ruby supports processing data as \IO streams;
that is, as data that may be read, re-read, written, re-written,
and traversed via iteration.

Core classes with such support include:

- IO, and its derived class File.
- {StringIO}[rdoc-ref:StringIO]: for processing a string.
- {ARGF}[rdoc-ref:ARGF]: for processing files cited on the command line.

Except as noted, the instance methods described on this page
are available in classes \ARGF, \File, \IO, and \StringIO.
A few, also noted, are available in class \Kernel.

=== Pre-Existing Streams

Pre-existing streams that are referenced by constants include:

- $stdin: read-only instance of \IO.
- $stdout: write-only instance of \IO.
- $stderr: read-only instance of \IO.
- \ARGF: read-only instance of \ARGF.

=== User-Created Streams

You can create streams:

- \File:

  - File.new: returns a new \File object;
    the file should be closed when no longer needed.
  - File.open: passes a new \File object to given the block;
    the file is automatically closed on block exit.

- \IO:

  - IO.new: returns a new \IO object for the given integer file descriptor;
    the \IO object should be closed when no longer needed.
  - IO.open: passes a new \IO object to the given block;
    the \IO object is automatically closed on block exit.
  - IO.popen: returns a new \IO object that is connected to the $stdin
    and $stdout of a newly-launched subprocess.
  - Kernel#open: returns a new \IO object connected to a given source:
    stream, file, or subprocess;
    the \IO object should be closed when no longer needed.

- \StringIO:

  - StringIO.new: returns a new \StringIO object;
    the \StringIO object should be closed when no longer needed.
  - StringIO.open: passes a new \StringIO object to the given block;
    the \StringIO object is automatically closed on block exit.

(You cannot create an \ARGF object, but one already exists.)

=== About the Examples

Many examples here use these variables:

  :include: doc/examples/files.rdoc

=== Basic \IO

You can perform basic stream \IO with these methods:

- IO#read: Returns all remaining or the next _n_ bytes read from the stream,
  for a given _n_:

    f = File.new('t.txt')
    f.read     # => "First line\nSecond line\n\nFourth line\nFifth line\n"
    f.rewind
    f.read(30) # => "First line\r\nSecond line\r\n\r\nFou"
    f.read(30) # => "rth line\r\nFifth line\r\n"
    f.read(30) # => nil
    f.close

- IO#write: Writes one or more given strings to the stream:

    $stdout.write('Hello', ', ', 'World!', "\n") # => 14
    $stdout.write('foo', :bar, 2, "\n")

  Output:

    Hello, World!
    foobar2

==== Position

An \IO stream has a nonnegative integer _position_,
which is the byte offset at which the next read or write is to occur.
A new stream has position zero (and line number zero);
method +rewind+ resets the position (and line number) to zero.

The relevant methods:

- IO#tell (aliased as +#pos+):
  Returns the current position (in bytes) in the stream:

    f = File.new('t.txt')
    f.tell # => 0
    f.gets # => "First line\n"
    f.tell # => 12
    f.close

- IO#pos=: Sets the position of the stream (in bytes):

    f = File.new('t.txt')
    f.tell     # => 0
    f.pos = 20 # => 20
    f.tell     # => 20
    f.close

- IO#seek: Sets the position of the stream to a given integer +offset+
  (in bytes), with respect to a given constant +whence+, which is one of:

  - +:CUR+ or <tt>IO::SEEK_CUR</tt>:
    Repositions the stream to its current position plus the given +offset+:

      f = File.new('t.txt')
      f.tell            # => 0
      f.seek(20, :CUR)  # => 0
      f.tell            # => 20
      f.seek(-10, :CUR) # => 0
      f.tell            # => 10
      f.close

  - +:END+ or <tt>IO::SEEK_END</tt>:
    Repositions the stream to its end plus the given +offset+:

      f = File.new('t.txt')
      f.tell            # => 0
      f.seek(0, :END)   # => 0  # Repositions to stream end.
      f.tell            # => 52
      f.seek(-20, :END) # => 0
      f.tell            # => 32
      f.seek(-40, :END) # => 0
      f.tell            # => 12
      f.close

  - +:SET+ or <tt>IO:SEEK_SET</tt>:
    Repositions the stream to the given +offset+:

      f = File.new('t.txt')
      f.tell            # => 0
      f.seek(20, :SET) # => 0
      f.tell           # => 20
      f.seek(40, :SET) # => 0
      f.tell           # => 40
      f.close

- IO#rewind: Positions the stream to the beginning (also resetting the line number):

    f = File.new('t.txt')
    f.tell     # => 0
    f.gets     # => "First line\n"
    f.tell     # => 12
    f.rewind   # => 0
    f.tell     # => 0
    f.lineno   # => 0
    f.close

==== Open and Closed Streams

A new \IO stream may be open for reading, open for writing, or both.

A stream is automatically closed when claimed by the garbage collector.

Attempted reading or writing on a closed stream raises an exception.

- IO#close: Closes the stream for both reading and writing.
- IO#close_read: Closes the stream for reading; not in ARGF.
- IO#close_write: Closes the stream for writing; not in ARGF.
- IO#closed?: Returns whether the stream is closed.

==== End-of-Stream

You can query whether a stream is positioned at its end using
method IO#eof? (also aliased as +#eof+).

You can reposition to end-of-stream by reading all stream content:

  f = File.new('t.txt')
  f.eof? # => false
  f.read # => "First line\nSecond line\n\nFourth line\nFifth line\n"
  f.eof? # => true

Or by using method IO#seek:

  f = File.new('t.txt')
  f.eof? # => false
  f.seek(0, :END)
  f.eof? # => true

=== Line \IO

You can read an \IO stream line-by-line using these methods:

- IO#each_line: Passes each line to the block:

    f = File.new('t.txt')
    f.each_line {|line| p line }

  Output:

    "First line\n"
    "Second line\n"
    "\n"
    "Fourth line\n"
    "Fifth line\n"

  The reading may begin mid-line:

    f = File.new('t.txt')
    f.pos = 27
    f.each_line {|line| p line }

  Output:

    "rth line\n"
    "Fifth line\n"

- IO#gets (also in Kernel): Returns the next line (which may begin mid-line):

    f = File.new('t.txt')
    f.gets      # => "First line\n"
    f.gets      # => "Second line\n"
    f.pos = 27
    f.gets      # => "rth line\n"
    f.readlines # => ["Fifth line\n"]
    f.gets      # => nil

- IO#readline (also in Kernel; not in StringIO):
  Like #gets, but raises an exception at end-of-stream.

- IO#readlines (also in Kernel): Returns all remaining lines in an array;
  may begin mid-line:

    f = File.new('t.txt')
    f.pos = 19
    f.readlines # => ["ine\n", "\n", "Fourth line\n", "Fifth line\n"]
    f.readlines # => []

Each of these reader methods may be called with:

- An optional line separator, +sep+.
- An optional line-size limit, +limit+.
- Both +sep+ and +limit+.

You can write to an \IO stream line-by-line using this method:

- IO#puts (also in Kernel; not in \StringIO): Writes objects to the stream:

    f = File.new('t.tmp', 'w')
    f.puts('foo', :bar, 1, 2.0, Complex(3, 0))
    f.flush
    File.read('t.tmp') # => "foo\nbar\n1\n2.0\n3+0i\n"

==== Line Separator

The default line separator is the given by the global variable <tt>$/</tt>,
whose value is by default <tt>"\n"</tt>.
The line to be read next is all data from the current position
to the next line separator:

  f = File.new('t.txt')
  f.gets # => "First line\n"
  f.gets # => "Second line\n"
  f.gets # => "\n"
  f.gets # => "Fourth line\n"
  f.gets # => "Fifth line\n"
  f.close

You can specify a different line separator:

  f = File.new('t.txt')
  f.gets('l')   # => "First l"
  f.gets('li')  # => "ine\nSecond li"
  f.gets('lin') # => "ne\n\nFourth lin"
  f.gets        # => "e\n"
  f.close

There are two special line separators:

- +nil+: The entire stream is read into a single string:

    f = File.new('t.txt')
    f.gets(nil) # => "First line\nSecond line\n\nFourth line\nFifth line\n"
    f.close

- <tt>''</tt> (the empty string): The next "paragraph" is read
  (paragraphs being separated by two consecutive line separators):

    f = File.new('t.txt')
    f.gets('') # => "First line\nSecond line\n\n"
    f.gets('') # => "Fourth line\nFifth line\n"
    f.close

==== Line Limit

The line to be read may be further defined by an optional integer argument +limit+,
which specifies that the number of bytes returned may not be (much) longer
than the given +limit+;
a multi-byte character will not be split, and so a line may be slightly longer
than the given limit.

If +limit+ is not given, the line is determined only by +sep+.

  # Text with 1-byte characters.
  File.new('t.txt') {|f| f.gets(1) }  # => "F"
  File.new('t.txt') {|f| f.gets(2) }  # => "Fi"
  File.new('t.txt') {|f| f.gets(3) }  # => "Fir"
  File.new('t.txt') {|f| f.gets(4) }  # => "Firs"
  # No more than one line.
  File.new('t.txt') {|f| f.gets(10) } # => "First line"
  File.new('t.txt') {|f| f.gets(11) } # => "First line\n"
  File.new('t.txt') {|f| f.gets(12) } # => "First line\n"

  # Text with 2-byte characters, which will not be split.
  File.new('r.rus') {|f| f.gets(1).size } # => 1
  File.new('r.rus') {|f| f.gets(2).size } # => 1
  File.new('r.rus') {|f| f.gets(3).size } # => 2
  File.new('r.rus') {|f| f.gets(4).size } # => 2

==== Line Separator and Line Limit

With arguments +sep+ and +limit+ given,
combines the two behaviors:

- Returns the next line as determined by line separator +sep+.
- But returns no more bytes than are allowed by the limit.

Example:

  File.new('t.txt') {|f| f.gets('li', 20) } # => "First li"
  File.new('t.txt') {|f| f.gets('li', 2) }  # => "Fi"

==== Line Number

A readable \IO stream has a _line_ _number_,
which is the non-negative integer line number
in the stream where the next read will occur.

The line number is the number of lines read by certain line-oriented methods
(IO.foreach, IO#each_line, IO#gets, IO#readline, and IO#readlines)
according to the given (or default) line separator +sep+.

A new stream is initially has line number zero (and position zero);
method +rewind+ resets the line number (and position) to zero.

\Method IO#lineno returns the line number.

Reading lines from a stream usually changes its line number:

  f = File.new('t.txt', 'r')
  f.lineno   # => 0
  f.readline # => "This is line one.\n"
  f.lineno   # => 1
  f.readline # => "This is the second line.\n"
  f.lineno   # => 2
  f.readline # => "Here's the third line.\n"
  f.lineno   # => 3
  f.eof?     # => true
  f.close

Iterating over lines in a stream usually changes its line number:

  File.open('t.txt') do |f|
    f.each_line do |line|
      p "position=#{f.pos} eof?=#{f.eof?} lineno=#{f.lineno}"
    end
  end

Output:

  "position=11 eof?=false lineno=1"
  "position=23 eof?=false lineno=2"
  "position=24 eof?=false lineno=3"
  "position=36 eof?=false lineno=4"
  "position=47 eof?=true lineno=5"

==== Line Options

A number of \IO methods accept optional keyword arguments
that determine how lines in a stream are to be treated:

- +:chomp+: If +true+, line separators are omitted; default is +false+.

=== Character \IO

You can process an \IO stream character-by-character using these methods:

- IO#getc: Reads and returns the next character from the stream:

    f = File.new('t.rus')
    f.getc # => "т"
    f.getc # => "е"
    f.getc # => "с"
    f.getc # => "т"
    f.getc # => nil

- IO#readchar (not in \StringIO):
  Like #getc, but raises an exception at end-of-stream:

    f.readchar # Raises EOFError.

- IO#ungetc (not in \ARGF):
  Pushes back ("unshifts") a character or integer onto the stream:

    path = 't.tmp'
    File.write(path, 'foo')
    File.open(path) do |f|
      f.ungetc('т')
      f.read # => "тfoo"
    end

- IO#putc (also in Kernel): Writes a character to the stream:

    File.open('t.tmp', 'w') do |f|
      f.putc('т')
      f.putc('е')
      f.putc('с')
      f.putc('т')
    end
    File.read('t.tmp') # => "тест"

- IO#each_char: Reads each remaining character in the stream,
  passing the character to the given block:

    File.open('t.rus') do |f|
      f.pos = 4
      f.each_char {|c| p c }
    end

  Output:

    "с"
    "т"

=== Byte \IO

You can process an \IO stream byte-by-byte using these methods:

- IO#getbyte: Returns the next 8-bit byte as an integer in range 0..255:

    File.read('t.dat')
    # => "\xFE\xFF\x99\x90\x99\x91\x99\x92\x99\x93\x99\x94"
    File.read('t.dat')
    # => "\xFE\xFF\x99\x90\x99\x91\x99\x92\x99\x93\x99\x94"
    f = File.new('t.dat')
    f.getbyte # => 254
    f.getbyte # => 255
    f.seek(-2, :END)
    f.getbyte # => 153
    f.getbyte # => 148
    f.getbyte # => nil

- IO#readbyte (not in \StringIO):
  Like #getbyte, but raises an exception if at end-of-stream:

    f.readbyte # Raises EOFError.

- IO#ungetbyte (not in \ARGF):
  Pushes back ("unshifts") a byte back onto the stream:

    f.ungetbyte(0)
    f.ungetbyte(01)
    f.read # => "\u0001\u0000"

- IO#each_byte: Reads each remaining byte in the stream,
  passing the byte to the given block:

    f.seek(-4, :END)
    f.each_byte {|b| p b }

  Output:

    153
    147
    153
    148

=== Codepoint \IO

You can process an \IO stream codepoint-by-codepoint using method
+#each_codepoint+:

  a = []
  File.open('t.rus') do |f|
    f.each_codepoint {|c| a << c }
  end
  a # => [1090, 1077, 1089, 1090]