[DOC] Enhanced RDoc for IO (#6669)

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-== \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]