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\Class \StringIO supports accessing a string as a stream, similar in some ways to class IO.

You can create a \StringIO instance using:

StringIO.new: returns a new \StringIO object containing the given string.
StringIO.open: passes a new \StringIO object to the given block.

Like an \IO stream, a \StringIO stream has certain properties:

Read/write mode: whether the stream may be read, written, appended to, etc.; see Read/Write Mode.
Data mode: text-only or binary; see Data Mode.
Encodings: internal and external encodings; see Encodings.
Position: where in the stream the next read or write is to occur; see Position.
Line number: a special, line-oriented, "position" (different from the position mentioned above); see Line Number.
Open/closed: whether the stream is open or closed, for reading or writing. see Open/Closed Streams.
BOM: byte mark order; see Byte Order Mark.

About the Examples

Examples on this page assume that \StringIO has been required:

require 'stringio'

And that this constant has been defined:

TEXT = <<EOT
First line
Second line

Fourth line
Fifth line
EOT

Stream Properties

Read/Write Mode

Summary

Each section below describes a read/write mode.

Any of the modes may be given as a string or as file constants; example:

strio = StringIO.new('foo', 'a')
strio = StringIO.new('foo', File::WRONLY | File::APPEND)

`'r'`: Read-Only

Mode specified as one of:

String: 'r'.
Constant: File::RDONLY.

Initial state:

strio = StringIO.new('foobarbaz', 'r')
strio.pos    # => 0            # Beginning-of-stream.
strio.string # => "foobarbaz"  # Not cleared.

May be read anywhere:

strio.gets(3) # => "foo"
strio.gets(3) # => "bar"
strio.pos = 9
strio.gets(3) # => nil

May not be written:

strio.write('foo')  # Raises IOError: not opened for writing

`'w'`: Write-Only

Mode specified as one of:

String: 'w'.
Constant: File::WRONLY.

Initial state:

strio = StringIO.new('foo', 'w')
strio.pos    # => 0   # Beginning of stream.
strio.string # => ""  # Initially cleared.

May be written anywhere (even past end-of-stream):

strio.write('foobar')
strio.string # => "foobar"
strio.rewind
strio.write('FOO')
strio.string # => "FOObar"
strio.pos = 3
strio.write('BAR')
strio.string # => "FOOBAR"
strio.pos = 9
strio.write('baz')
strio.string # => "FOOBAR\u0000\u0000\u0000baz"  # Null-padded.

May not be read:

strio.read  # Raises IOError: not opened for reading

`'a'`: Append-Only

Mode specified as one of:

String: 'a'.
Constant: File::WRONLY | File::APPEND.

Initial state:

strio = StringIO.new('foo', 'a')
strio.pos    # => 0      # Beginning-of-stream.
strio.string # => "foo"  # Not cleared.

May be written only at the end; position does not affect writing:

strio.write('bar')
strio.string # => "foobar"
strio.write('baz')
strio.string # => "foobarbaz"
strio.pos = 400
strio.write('bat')
strio.string # => "foobarbazbat"

May not be read:

strio.gets  # Raises IOError: not opened for reading

`'r+'`: Read/Write

Mode specified as one of:

String: 'r+'.
Constant: File::RDRW.

Initial state:

strio = StringIO.new('foobar', 'r+')
strio.pos    # => 0         # Beginning-of-stream.
strio.string # => "foobar"  # Not cleared.

May be written anywhere (even past end-of-stream):

strio.write('FOO')
strio.string # => "FOObar"
strio.write('BAR')
strio.string # => "FOOBAR"
strio.write('BAZ')
strio.string # => "FOOBARBAZ"
strio.pos = 12
strio.write('BAT')
strio.string # => "FOOBARBAZ\u0000\u0000\u0000BAT"  # Null padded.

May be read anywhere:

strio.pos = 0
strio.gets(3) # => "FOO"
strio.pos = 6
strio.gets(3) # => "BAZ"
strio.pos = 400
strio.gets(3) # => nil

`'w+'`: Read/Write (Initially Clear)

Mode specified as one of:

String: 'w+'.
Constant: File::RDWR | File::TRUNC.

Initial state:

strio = StringIO.new('foo', 'w+')
strio.pos    # => 0   # Beginning-of-stream.
strio.string # => ""  # Truncated.

May be written anywhere (even past end-of-stream):

strio.write('foobar')
strio.string # => "foobar"
strio.rewind
strio.write('FOO')
strio.string # => "FOObar"
strio.write('BAR')
strio.string # => "FOOBAR"
strio.write('BAZ')
strio.string # => "FOOBARBAZ"
strio.pos = 12
strio.write('BAT')
strio.string # => "FOOBARBAZ\u0000\u0000\u0000BAT"  # Null-padded.

May be read anywhere:

strio.rewind
strio.gets(3) # => "FOO"
strio.gets(3) # => "BAR"
strio.pos = 12
strio.gets(3) # => "BAT"
strio.pos = 400
strio.gets(3) # => nil

`'a+'`: Read/Append

Mode specified as one of:

String: 'a+'.
Constant: File::RDWR | File::APPEND.

Initial state:

strio = StringIO.new('foo', 'a+')
strio.pos    # => 0      # Beginning-of-stream.
strio.string # => "foo"  # Not cleared.

May be written only at the end; #rewind; position does not affect writing:

strio.write('bar')
strio.string # => "foobar"
strio.write('baz')
strio.string # => "foobarbaz"
strio.pos = 400
strio.write('bat')
strio.string # => "foobarbazbat"

May be read anywhere:

strio.rewind
strio.gets(3) # => "foo"
strio.gets(3) # => "bar"
strio.pos = 9
strio.gets(3) # => "bat"
strio.pos = 400
strio.gets(3) # => nil

Data Mode

To specify whether the stream is to be treated as text or as binary data, either of the following may be suffixed to any of the string read/write modes above:

't': Text; initializes the encoding as Encoding::UTF_8.
'b': Binary; initializes the encoding as Encoding::ASCII_8BIT.

If neither is given, the stream defaults to text data.

Examples:

strio = StringIO.new('foo', 'rt')
strio.external_encoding # => #<Encoding:UTF-8>
data = "\u9990\u9991\u9992\u9993\u9994"
strio = StringIO.new(data, 'rb')
strio.external_encoding # => #<Encoding:BINARY (ASCII-8BIT)>

When the data mode is specified, the read/write mode may not be omitted:

StringIO.new(data, 'b')  # Raises ArgumentError: invalid access mode b

A text stream may be changed to binary by calling instance method #binmode; a binary stream may not be changed to text.

Encodings

A stream has an encoding; see Encodings.

The initial encoding for a new or re-opened stream depends on its data mode:

Text: Encoding::UTF_8.
Binary: Encoding::ASCII_8BIT.

These instance methods are relevant:

#external_encoding: returns the current encoding of the stream as an Encoding object.
#internal_encoding: returns +nil+; a stream does not have an internal encoding.
#set_encoding: sets the encoding for the stream.
#set_encoding_by_bom: sets the encoding for the stream to the stream's BOM (byte order mark).

Examples:

strio = StringIO.new('foo', 'rt')  # Text mode.
strio.external_encoding # => #<Encoding:UTF-8>
data = "\u9990\u9991\u9992\u9993\u9994"
strio = StringIO.new(data, 'rb') # Binary mode.
strio.external_encoding # => #<Encoding:BINARY (ASCII-8BIT)>
strio = StringIO.new('foo')
strio.external_encoding # => #<Encoding:UTF-8>
strio.set_encoding('US-ASCII')
strio.external_encoding # => #<Encoding:US-ASCII>

Position

A stream has a position, and integer offset (in bytes) into the stream. The initial position of a stream is zero.

Getting and Setting the Position

Each of these methods initializes (to zero) the position of a new or re-opened stream:

::new: returns a new stream.
::open: passes a new stream to the block.
#reopen: re-initializes the stream.

Each of these methods queries, gets, or sets the position, without otherwise changing the stream:

#eof?: returns whether the position is at end-of-stream.
#pos: returns the position.
#pos=: sets the position.
#rewind: sets the position to zero.
#seek: sets the position.

Examples:

strio = StringIO.new('foobar')
strio.pos  # => 0
strio.pos = 3
strio.pos  # => 3
strio.eof? # => false
strio.rewind
strio.pos  # => 0
strio.seek(0, IO::SEEK_END)
strio.pos  # => 6
strio.eof? # => true

Position Before and After Reading

Except for #pread, a stream reading method (see Basic Reading) begins reading at the current position.

Except for #pread, a read method advances the position past the read substring.

Examples:

strio = StringIO.new(TEXT)
strio.string # => "First line\nSecond line\n\nFourth line\nFifth line\n"
strio.pos    # => 0
strio.getc   # => "F"
strio.pos    # => 1
strio.gets   # => "irst line\n"
strio.pos    # => 11
strio.pos = 24
strio.gets   # => "Fourth line\n"
strio.pos    # => 36

strio = StringIO.new('こんにちは')  # Five 3-byte characters.
strio.pos = 0  # At first byte of first character.
strio.read     # => "こんにちは"
strio.pos = 1  # At second byte of first character.
strio.read     # => "\x81\x93んにちは"
strio.pos = 2  # At third byte of first character.
strio.read     # => "\x93んにちは"
strio.pos = 3  # At first byte of second character.
strio.read     # => "んにちは"

strio = StringIO.new(TEXT)
strio.pos = 15
a = []
strio.each_line {|line| a.push(line) }
a         # => ["nd line\n", "\n", "Fourth line\n", "Fifth line\n"]
strio.pos # => 47  ## End-of-stream.

Position Before and After Writing

Each of these methods begins writing at the current position, and advances the position to the end of the written substring:

#putc: writes the given character.
#write: writes the given objects as strings.
Kernel#puts: writes given objects as strings, each followed by newline.

Examples:

strio = StringIO.new('foo')
strio.pos    # => 0
strio.putc('b')
strio.string # => "boo"
strio.pos    # => 1
strio.write('r')
strio.string # => "bro"
strio.pos    # => 2
strio.puts('ew')
strio.string # => "brew\n"
strio.pos    # => 5
strio.pos = 8
strio.write('foo')
strio.string # => "brew\n\u0000\u0000\u0000foo"
strio.pos    # => 11

Each of these methods writes before the current position, and decrements the position so that the written data is next to be read:

#ungetbyte: unshifts the given byte.
#ungetc: unshifts the given character.

Examples:

strio = StringIO.new('foo')
strio.pos = 2
strio.ungetc('x')
strio.pos    # => 1
strio.string # => "fxo"
strio.ungetc('x')
strio.pos    # => 0
strio.string # => "xxo"

This method does not affect the position:

#truncate: truncates the stream's string to the given size.

Examples:

strio = StringIO.new('foobar')
strio.pos    # => 0
strio.truncate(3)
strio.string # => "foo"
strio.pos    # => 0
strio.pos = 500
strio.truncate(0)
strio.string # => ""
strio.pos    # => 500

Line Number

A stream has a line number, which initially is zero:

Method #lineno returns the line number.
Method #lineno= sets the line number.

The line number can be affected by reading (but never by writing); in general, the line number is incremented each time the record separator (default: "\n") is read.

Examples:

strio = StringIO.new(TEXT)
strio.string # => "First line\nSecond line\n\nFourth line\nFifth line\n"
strio.lineno # => 0
strio.gets   # => "First line\n"
strio.lineno # => 1
strio.getc   # => "S"
strio.lineno # => 1
strio.gets   # => "econd line\n"
strio.lineno # => 2
strio.gets   # => "\n"
strio.lineno # => 3
strio.gets   # => "Fourth line\n"
strio.lineno # => 4

Setting the position does not affect the line number:

strio.pos = 0
strio.lineno # => 4
strio.gets   # => "First line\n"
strio.pos    # => 11
strio.lineno # => 5

And setting the line number does not affect the position:

strio.lineno = 10
strio.pos    # => 11
strio.gets   # => "Second line\n"
strio.lineno # => 11
strio.pos    # => 23

Open/Closed Streams

A new stream is open for either reading or writing, and may be open for both; see Read/Write Mode.

Each of these methods initializes the read/write mode for a new or re-opened stream:

::new: returns a new stream.
::open: passes a new stream to the block.
#reopen: re-initializes the stream.

Other relevant methods:

#close: closes the stream for both reading and writing.
#close_read: closes the stream for reading.
#close_write: closes the stream for writing.
#closed?: returns whether the stream is closed for both reading and writing.
#closed_read?: returns whether the stream is closed for reading.
#closed_write?: returns whether the stream is closed for writing.

BOM (Byte Order Mark)

The string provided for ::new, ::open, or #reopen may contain an optional BOM (byte order mark) at the beginning of the string; the BOM can affect the stream's encoding.

The BOM (if provided):

Is stored as part of the stream's string.
Does not immediately affect the encoding.
Is initially considered part of the stream.

utf8_bom = "\xEF\xBB\xBF"
string = utf8_bom + 'foo'
string.bytes               # => [239, 187, 191, 102, 111, 111]
strio.string.bytes.take(3) # => [239, 187, 191]                  # The BOM.
strio = StringIO.new(string, 'rb')
strio.string.bytes         # => [239, 187, 191, 102, 111, 111]   # BOM is part of the stored string.
strio.external_encoding    # => #<Encoding:BINARY (ASCII-8BIT)>  # Default for a binary stream.
strio.gets                 # => "\xEF\xBB\xBFfoo"                # BOM is part of the stream.

You can call instance method #set_encoding_by_bom to "activate" the stored BOM; after doing so the BOM:

Is still stored as part of the stream's string.
Determines (and may have changed) the stream's encoding.
Is no longer considered part of the stream.

strio.set_encoding_by_bom
strio.string.bytes      # => [239, 187, 191, 102, 111, 111]  # BOM is still part of the stored string.
strio.external_encoding # => #<Encoding:UTF-8>               # The new encoding.
strio.rewind            # => 0
strio.gets              # => "foo"                           # BOM is not part of the stream.

Basic Stream \IO

Basic Reading

You can read from the stream using these instance methods:

#getbyte: reads and returns the next byte.
#getc: reads and returns the next character.
#gets: reads and returns all or part of the next line.
#read: reads and returns all or part of the remaining data in the stream.
#readlines: reads the remaining data the stream and returns an array of its lines.
Kernel#readline: like #gets, but raises an exception if at end-of-stream.

You can iterate over the stream using these instance methods:

#each_byte: reads each remaining byte, passing it to the block.
#each_char: reads each remaining character, passing it to the block.
#each_codepoint: reads each remaining codepoint, passing it to the block.
#each_line: reads all or part of each remaining line, passing the read string to the block

This instance method is useful in a multi-threaded application:

#pread: reads and returns all or part of the stream.

Basic Writing

You can write to the stream, advancing the position, using these instance methods:

#putc: writes a given character.
#write: writes the given objects as strings.
Kernel#puts writes given objects as strings, each followed by newline.

You can "unshift" to the stream using these instance methods; each writes before the current position, and decrements the position so that the written data is next to be read.

#ungetbyte: unshifts the given byte.
#ungetc: unshifts the given character.

One more writing method:

#truncate: truncates the stream's string to the given size.

Line \IO

Reading:

#gets: reads and returns the next line.
Kernel#readline: like #gets, but raises an exception if at end-of-stream.
#readlines: reads the remaining data the stream and returns an array of its lines.
#each_line: reads each remaining line, passing it to the block

Writing:

Kernel#puts: writes given objects, each followed by newline.

Character \IO

Reading:

#each_char: reads each remaining character, passing it to the block.
#getc: reads and returns the next character.

Writing:

#putc: writes the given character.
#ungetc.: unshifts the given character.

Byte \IO

Reading:

#each_byte: reads each remaining byte, passing it to the block.
#getbyte: reads and returns the next byte.

Writing:

#ungetbyte: unshifts the given byte.

Codepoint \IO

Reading:

#each_codepoint: reads each remaining codepoint, passing it to the block.

Mode	Initial Clear?	Read	Write
`'r'`: read-only	No	Anywhere	Error
`'w'`: write-only	Yes	Error	Anywhere
`'a'`: append-only	No	Error	End only
`'r+'`: read/write	No	Anywhere	Anywhere
`'w+'`: read-write	Yes	Anywhere	Anywhere
`'a+'`: read/append	No	Anywhere	End only

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About the Examples

Stream Properties

Read/Write Mode

Summary

`'r'`: Read-Only

`'w'`: Write-Only

`'a'`: Append-Only

`'r+'`: Read/Write

`'w+'`: Read/Write (Initially Clear)

`'a+'`: Read/Append

Data Mode

Encodings

Position

Getting and Setting the Position

Position Before and After Reading

Position Before and After Writing

Line Number

Open/Closed Streams

BOM (Byte Order Mark)

Basic Stream \IO

Basic Reading

Basic Writing

Line \IO

Character \IO

Byte \IO

Codepoint \IO

Sunbelt Computer Software

PL/B Language Development and Support

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About the Examples

Stream Properties

Read/Write Mode

Summary

'r': Read-Only

'w': Write-Only

'a': Append-Only

'r+': Read/Write

'w+': Read/Write (Initially Clear)

'a+': Read/Append

Data Mode

Encodings

Position

Getting and Setting the Position

Position Before and After Reading

Position Before and After Writing

Line Number

Open/Closed Streams

BOM (Byte Order Mark)

Basic Stream \IO

Basic Reading

Basic Writing

Line \IO

Character \IO

Byte \IO

Codepoint \IO

`'r'`: Read-Only

`'w'`: Write-Only

`'a'`: Append-Only

`'r+'`: Read/Write

`'w+'`: Read/Write (Initially Clear)

`'a+'`: Read/Append