# Copyright (C) 2001-2010, Parrot Foundation.
# $Id: pdd22_io.pod 49499 2010-10-10 18:07:25Z dukeleto $

=head1 PDD 22: I/O

=head2 Abstract

Parrot's I/O subsystem.

=head2 Version

$Revision: 49499 $

=head2 Definitions

A "stream" allows input or output operations on a source/destination
such as a file, keyboard, or text console. Streams are also called
"filehandles", though only some of them have anything to do with files.

=head2 Description

=over 4

=item - Parrot I/O objects support both streams and network I/O.

=item - Parrot has both synchronous and asynchronous I/O operations.

=item - Asynchronous operations must interact safely with Parrot's other
concurrency models.

=back

=head2 Implementation

=head3 Platform Implementation

Parrot uses a series of macros and conditional compilation to generate
code to handle IO on various platforms. There is a "portable" version of
the IO system that is used for most systems that use a common POSIX API
for IO, and there are platform-specific files that can be used for systems
that do not subscribe, or don't subscribe faithfully, to POSIX.

A series of macros and conditional compilation are used to determine which
files get added to the build, and which functions are called to implement
each operation.

=head3 Concurrency Model for Asynchronous I/O

Currently, Parrot only implements synchronous I/O operations. Initially,
the asynchronous operations will be implemented separately from the
synchronous ones. There may be an implementation that uses one variant
to implement the other someday, but it's not an immediate priority.

Synchronous opcodes are differentiated from asynchronous opcodes by the
presence of a callback argument in the asynchronous calls.  Asynchronous
calls that don't supply callbacks (perhaps if the user wants to manually
check later if the operation succeeded) are enough of a fringe case that
they don't need opcodes. They can access the functionality via methods
on ParrotIO objects.

Asynchronous operations use a lightweight concurrency model. At the user
level, Parrot follows the callback function model of asynchronous I/O.
At the interpreter level, each asynchronous operation registers a task
with the interpreter's concurrency scheduler. The registered task could
represent a simple Parrot asynchronous I/O operation, a platform-native
asynchronous I/O call, or even synchronous code in a full Parrot thread
(rare but possibly useful for prototyping new features, or for mock
objects in testing).

Communication between the calling code and the asynchronous operation
task is handled by a shared status object. The operation task updates
the status object whenever the status changes, and the calling code can
check the status object at any time.  The status object contains a
reference to the returned result of an asynchronous I/O call. In order
to allow sharing of the status object, asynchronous ops both pass the
status object to the callback PMC, and return it to the calling code.

The lightweight tasks typically used by the asynchronous I/O system
capture no state other than the arguments passed to the I/O call, and
share no variables with the calling code other than the status object.

=head3 FileHandle PMC API

Methods

=over 4

=item C<new>

=begin PIR_FRAGMENT

  $P0 = new [ 'FileHandle' ]

=end PIR_FRAGMENT

Creates a new I/O stream object. [Note that this is usually performed
via the C<open> opcode.]

=item C<open>

=begin PIR_FRAGMENT

  $P0 = $P1.'open'()
  $P0 = $P1.'open'($S2)
  $P0 = $P1.'open'($S2, $S3)

=end PIR_FRAGMENT

Opens a stream on an existing I/O stream object, and returns a status
object. With no arguments, it can be used to reopen a previously opened
I/O stream. $S2 is a file path and $S3 is an optional mode for the
stream (read, write, read/write, etc), using the same format as the
C<open> opcode: 'r' for read, 'w' for write, 'a' for append, and 'p' for
pipe. When the optional mode argument is not passed, the default is read mode.
When the mode is set to write or append, a file is created without warning if
none exists. When the mode is read (without write), a nonexistent file is an
error.

The asynchronous version takes a PMC callback as an additional final
argument. When the open operation is complete, it invokes the callback
with a single argument: a status object containing the opened stream
object.

=item C<close>

=begin PIR_FRAGMENT

  $P0 = $P1.'close'()
  $P0 = $P1.'close'($P2)

=end PIR_FRAGMENT

Closes an I/O stream, but leaves destruction of the I/O object to the
GC. The C<close> method returns a PMC status object.

The asynchronous version takes an additional final PMC callback argument
$P1. When the close operation is complete, it invokes the callback,
passing it a status object. [There's not really much advantage in this
over just leaving the object for the GC to clean-up, but it does give
you the option of executing an action when the stream has been closed.]

=item C<print>

=begin PIR_FRAGMENT

  $P0 = $P1.'print'($I2)
  $P0 = $P1.'print'($N2)
  $P0 = $P1.'print'($S2)
  $P0 = $P1.'print'($P2)
  $P0 = $P1.'print'($I2, $P3)
  $P0 = $P1.'print'($N2, $P3)
  $P0 = $P1.'print'($S2, $P3)
  $P0 = $P1.'print'($P2, $P3)

=end PIR_FRAGMENT

Writes an integer, float, string, or PMC value to an I/O stream object.
Returns a PMC status object.

The asynchronous version takes an additional final PMC callback
argument $P2. When the print operation is complete, it invokes the callback,
passing it a status object.

=item C<read>

=begin PIR_FRAGMENT

  $S0 = $P1.'read'($I2)
  $P0 = $P1.'read'($I2, $P3)

=end PIR_FRAGMENT

Retrieves a specified number of bytes ($I2) from a stream $P1 into a
string $S0. By default it reads in bytes, but the FileHandle object can
be configured to read in code points instead, by setting the character
set and encoding on the filehandle object. If there are fewer bytes
remaining in the stream than specified in the read request, it returns
the remaining bytes (with no error).

The asynchronous version takes an additional final PMC callback argument
$P3, and only returns a status object $P0. When the read operation is
complete, it invokes the callback, passing it a status object. The
status object contains the return value: a string that may be in bytes
or codepoints depending on the read mode of the I/O object. [The
callback doesn't need to know the read mode of the original operation,
as the information about the character encoding of the return value is
contained in the string.]

=item C<readline>

=begin PIR_FRAGMENT

  $S0 = $P1.'readline'()
  $P0 = $P1.'readline'($P2)

=end PIR_FRAGMENT

Retrieves a single line from a stream $P1 into a string $S1. Calling
C<readline> flags the stream as operating in line-buffer mode (see the
C<buffer_type> method below). The C<readline> operation respects the
read mode of the I/O object the same as C<read> does. Newlines are not
removed from the end of the string.

The asynchronous version takes an additional final PMC callback argument
$P2, and only returns a status object $P0. When the readline operation
is complete, it invokes the callback, passing it a status object and a
string of bytes.

=item C<readall>

=begin PIR_FRAGMENT

  $S0 = $P1.'readall'()
  $P0 = $P1.'readall'($P2)

  $S0 = $P1.'readall'($S2)
  $P0 = $P1.'readall'($S2, $P3)

=end PIR_FRAGMENT

Retrieves the entire contents from a stream $P1 into a string $S0. On a
previously opened stream, C<readall> will read the entire contents of
the file, and return them as a string. It will respect the read mode of
the I/O object the same as C<read> does.

On an unopened stream, if a string file path is passed in $S2,
C<readall> will open a stream on the file, read the contents and close
it again. 

The asynchronous version takes an additional final PMC callback argument
($P2 or $P3), and only returns a status object $P0. When the readall operation
is complete, it invokes the callback, passing it a status object and a
string.

=item C<record_separator>

=begin PIR_FRAGMENT

  $S0 = $P1.'record_separator'()
  $P0.'record_separator'($S1)

=end PIR_FRAGMENT

Accessor (get and set) for the I/O stream's record separator attribute.
The default value is a newline (CR, LF, CRLF, etc. depending on the
platform).

=item C<buffer_type>

=begin PIR_FRAGMENT

  $S0 = $P1.'buffer_type'()
  $P0.'buffer_type'($S1)

=end PIR_FRAGMENT

Accessor (get and set) for the I/O stream's buffer type attribute. The
attribute is set or returned as a string value of 'unbuffered' (bytes sent as
soon as possible), 'line-buffered' (bytes sent when record separator is
encountered), or 'full-buffered' (bytes sent when the buffer is full).

=item C<buffer_size>

=begin PIR_FRAGMENT

  $I0 = $P1.'buffer_size'()
  $P0.'buffer_size'($I1)

=end PIR_FRAGMENT

Accessor (get and set) for the I/O stream's buffer size attribute. The
size is specified in bytes (positive integer value), though the buffer
may hold a varying number of characters when dealing with an encoding of
multi-byte codepoints.  The code that implements the handling of a
particular character set must provide the logic that marks the buffer as
"full" when it can't hold the next codepoint even if there are empty
bytes in the buffer.

Setting the buffer size turns on full buffering mode for the I/O stream.
The set buffer size is taken as a minimum, the I/O subsystem may round
it up to a standard block size.

The buffer is automatically flushed when the buffer size is changed. If
the new size was larger than the existing data in the buffer, a size
change would be non-disruptive, but if the new size was smaller,
resizing it without flushing would truncate the buffer.

=item C<mode>

=begin PIR_FRAGMENT

  $S0 = $P1.'mode'()

=end PIR_FRAGMENT

Accessor (get only) for the I/O stream's read mode. This returns the mode
string used to open the I/O stream.

=item C<encoding>

=begin PIR_FRAGMENT

  $S0 = $P1.'encoding'()
  $P0.'encoding'($S1)

=end PIR_FRAGMENT

Accessor (get and set) for the I/O stream's encoding attribute. Currently,
the only valid value to set is 'utf8' which turns on UTF-8 reading/writing
mode for the stream. The default behavior is fixed-width 8-bit characters.

=item C<get_fd>

=begin PIR_FRAGMENT

  $I0 = $P1.'get_fd'()

=end PIR_FRAGMENT

For stream objects that are simple wrappers around a Unix filehandle,
C<get_fd> retrieves the Unix integer file descriptor of the object.
This method will simply return -1 on stream objects that aren't Unix
filehandles.

No asynchronous version.

=back

=head3 Status Object PMC API

=over 4

=item C<get_integer> (vtable)

=begin PIR_FRAGMENT

  $I0 = $P1

=end PIR_FRAGMENT

Returns an integer status for the status object, 1 for successful
completion, -1 for an error, and 0 while still running. [Discuss: This
is largely to preserve current expectations of -1 for an error. If we
move away from that, is there a better representation?]

=item C<get_bool> (vtable)

=begin PIR_FRAGMENT

  if $P0 goto L1
  # ...
  L1:

=end PIR_FRAGMENT

Returns a boolean status for the status object, C<true> for successful
completion or while still running, C<false> for an error.

=item C<return>

=begin PIR_FRAGMENT

  $P0 = $P1.'return'()

=end PIR_FRAGMENT

Retrieves the return value of the asynchronous operation from the status
object. Returns a NULL PMC while still running, or if the operation had
no return value.

=item C<error>

=begin PIR_FRAGMENT

  $P0 = $P1.'error'()

=end PIR_FRAGMENT

Retrieves the error object from the status object, if the execution of
the asynchronous operation terminated with an error. The error object is
derived from Exception, and can be thrown from the callback. If there
was no error, or the asynchronous operation is still running, returns a
null PMC.

=item C<throw>

=begin PIR_FRAGMENT

  $P0.'throw'()

=end PIR_FRAGMENT

Throw an exception from the status object if it contains an error
object, otherwise do nothing.

=back

=head3 I/O Iterator PMC API

[Implementation NOTE: this may either be the default Iterator object
applied to a FileHandle or Socket object, a separate Iterator object for
I/O objects, or an Iterator role applied to I/O objects.]

=over 4

=item C<new>

=begin PIR_FRAGMENT

    new $P0, [ 'Iterator' ], $P1

=end PIR_FRAGMENT

Create a new iterator object $P0 from I/O object $P1.

=item C<shift>

=begin PIR_FRAGMENT

      shift $S0, $P1

=end PIR_FRAGMENT

Retrieve the next line/block $S0 from the I/O iterator $P1. The amount
of data retrieved in each iteration is determined by the I/O object's
C<buffer_type> setting: unbuffered, line-buffered, or fully-buffered.

=item C<get_bool> (vtable)

=begin PIR_FRAGMENT

  unless $P0 goto iter_end

=end PIR_FRAGMENT

Returns a boolean value for the iterator, C<true> if there is more data
to pull from the I/O object, C<false> if the iterator has reached the
end of the data. [NOTE: this means that an iterator always checks for
the next line/block of data when it retrieves the current one.]

=back

=head3 I/O Opcodes

The signatures for the asynchronous operations are nearly identical to
the synchronous operations, but the asynchronous operations take an
additional argument for a callback, and the only return value from the
asynchronous operations is a status object. When the callbacks are invoked,
they are passed the status object as their sole argument. Any return
values from the operation are stored within the status object.

The listing below says little about whether the opcodes return error
information. For now assume that they can either return a status object,
or return nothing. Error handling is discussed more thoroughly below in
L<Error Handling>.

=head3 I/O Stream Opcodes

=head4 Opening and closing streams

=over 4

=item C<open>

=begin PIR_FRAGMENT_INVALID

  .loadlib 'io_ops'
  ...
  $P0 = open $S1
  $P0 = open $S1, $S2
  $P0 = open $P1
  $P0 = open $P1, $S2

=end PIR_FRAGMENT_INVALID

Opens a stream object based on a file path in $S1 and returns it.  The
stream object defaults to read/write mode. The optional string argument
$S2 specifies the mode of the stream (read, write, append, read/write,
etc.).  Currently the mode of the stream is set with a string argument
similar to Perl 5 syntax, but a language-agnostic mode string is
preferable, using 'r' for read, 'w' for write, 'a' for append, and 'p'
for pipe.

The asynchronous version takes a PMC callback as an additional final
argument. When the open operation is complete, it invokes the callback
with a single argument: a status object containing the opened stream
object.

=item C<close>

=begin PIR_FRAGMENT_INVALID

  .loadlib 'io_ops'
  ...
  close $P0
  close $P0, $P1

=end PIR_FRAGMENT_INVALID

Closes a stream object. It takes a single string object argument and
returns a status object.

The asynchronous version takes an additional final PMC callback
argument. When the close operation is complete, it invokes the callback,
passing it a status object.

=back

=head4 Retrieving existing streams

These opcodes do not have asynchronous variants.

=over 4

=item *

C<getstdin>, C<getstdout>, and C<getstderr> return a stream object for
standard input, standard output, and standard error, respectively.

=item *

C<fdopen> converts an existing and already open UNIX integer file
descriptor into a stream object. It also takes a string argument to
specify the mode.

=back

=head4 Writing to streams

=over 4

=item C<print>

=begin PIR_FRAGMENT_INVALID

  print $I0
  print $N0
  print $S0
  print $P0
  print $P0, $I1
  print $P0, $N1
  print $P0, $S1
  print $P0, $P1
  print $P0, $I1, $P2
  print $P0, $N1, $P2
  print $P0, $S1, $P2
  print $P0, $P1, $P2

=end PIR_FRAGMENT_INVALID

Writes an integer, float, string, or PMC value to a stream.  It
writes to standard output by default, but optionally takes a PMC
argument to select another stream to write to.

The asynchronous version takes an additional final PMC callback
argument. When the print operation is complete, it invokes the callback,
passing it a status object.

=item C<printerr>

=begin PIR_FRAGMENT_INVALID

  .loadlib 'io_ops'
  ...
  printerr $I0
  printerr $N0
  printerr $S0
  printerr $P0

=end PIR_FRAGMENT_INVALID

Writes an integer, float, string, or PMC value to standard error.

There is no asynchronous variant of C<printerr>. [It's just a shortcut.
If they want an asynchronous version, they can use C<print>.]

=back

=head4 Reading from streams

=over 4

=item C<read>

=begin PIR_FRAGMENT_INVALID

  .loadlib 'io_ops'
  ...
  $S0 = read $I1
  $S0 = read $P1, $I2
  $P0 = read $P1, $I2, $P3

=end PIR_FRAGMENT_INVALID

Retrieves a specified number of bytes, $I2, from a stream, $P2, into a
string, $S0. [Note this is bytes, not codepoints.] By default it reads
from standard input, but it also takes an alternate stream object source
as an optional argument.

The asynchronous version takes an additional final PMC callback
argument, and only returns a status object. When the read operation is
complete, it invokes the callback, passing it a status object and a
string of bytes.

=item C<readline>

=begin PIR_FRAGMENT_INVALID

  .loadlib 'io_ops'
  ...
  $S0 = readline $P1
  $P0 = readline $P1, $P2

=end PIR_FRAGMENT_INVALID

Retrieves a single line from a stream into a string. Calling
C<readline> flags the stream as operating in line-buffer mode.

The asynchronous version takes an additional final PMC callback
argument, and only returns a status object. When the readline operation
is complete, it invokes the callback, passing it a status object and a
string of bytes.

=item C<peek>

=begin PIR_FRAGMENT_INVALID

  .loadlib 'io_ops'
  ...
  $S0 = peek
  $S0 = peek $P1

=end PIR_FRAGMENT_INVALID

[C<peek>, C<seek>, C<tell>, and C<poll> are all candidates for moving from
opcodes to FileHandle object methods.]

C<peek> retrieves the next byte from a stream into a string, but doesn't
remove it from the stream. By default it reads from standard input, but
it also takes a stream object argument for an alternate source.

There is no asynchronous version of C<peek>. [Does anyone have a line
of reasoning why one might be needed? The concept of "next byte" seems
to be a synchronous one.]

=back

=head4 Retrieving and setting stream properties

=over 4

=item C<seek>

=begin PIR_FRAGMENT_INVALID

  .loadlib 'io_ops'
  ...
  seek $P0, $I1, $I2
  seek $P0, $I1, $I2, $I3
  seek $P0, $I1, $I2, $P3
  seek $P0, $I1, $I2, $I3, $P4

=end PIR_FRAGMENT_INVALID

Sets the current file position of a stream object, $P0, to an integer
byte offset, $I1, from an integer starting position, $I2, (0 for the
start of the file, 1 for the current position, and 2 for the end of the
file). It also has a 64-bit variant that sets the byte offset by two
integer arguments, $I1 and $I2, (one for the first 32 bits of the 64-bit
offset, and one for the second 32 bits). [The two-register emulation for
64-bit integers may be deprecated in the future.]

The asynchronous version takes an additional final PMC callback
argument. When the seek operation is complete, it invokes the callback,
passing it a status object and the stream object it was called on.

=item C<tell>

=begin PIR_FRAGMENT_INVALID

  .loadlib 'io_ops'
  ...
  $I0 = tell $P1
  ($I0, $I1) = tell $P2

=end PIR_FRAGMENT_INVALID

Retrieves the current file position of a stream object.  It also has a
64-bit variant that returns the byte offset as two integers (one for the
first 32 bits of the 64-bit offset, and one for the second 32 bits).
[The two-register emulation for 64-bit integers may be deprecated in the
future.]

No asynchronous version.

=item C<poll>

=begin PIR_FRAGMENT_INVALID

  $I0 = poll $P1, $I2, $I3, $I4

=end PIR_FRAGMENT_INVALID

Polls a stream or socket object for particular types of events (an
integer flag) at a frequency set by seconds and microseconds (the final
two integer arguments). [At least, that's what the documentation in
src/io/io.c says. In actual fact, the final two arguments seem to be
setting the timeout, exactly the same as the corresponding argument to
the system version of C<poll>.]

See the system documentation for C<poll> to see the constants for event
types and return status.

This opcode is inherently synchronous (poll is "synchronous I/O
multiplexing"), but it can retrieve status information from a stream or
socket object whether the object is being used synchronously or
asynchronously.

=back

=head3 Filesystem Opcodes

[Okay, I'm seriously considering moving most of these to methods on the
ParrotIO object. More than that, moving them into a role that is
composed into the ParrotIO object when needed. For the ones that have
the form C<opcodename io_object, arguments>, I can't see that it's
much less effort than C<io_object.'methodname'(arguments)> for either
manually writing PIR or generating PIR. The slowest thing about I/O is
I/O, so I can't see that we're getting much speed gain out of making
them opcodes. The ones to keep as opcodes are C<unlink>, C<rmdir>, and
C<opendir>.]

=over 4

=item *

C<stat> retrieves information about a file on the filesystem. It takes a
string filename or an integer argument of a UNIX file descriptor [or an
already opened stream object?], and an integer flag for the type of
information requested. It returns an integer containing the requested
information.  The following constants are defined for the type of
information requested (see F<runtime/parrot/include/stat.pasm>):

  0    STAT_EXISTS
           Whether the file exists.
  1    STAT_FILESIZE
           The size of the file.
  2    STAT_ISDIR
           Whether the file is a directory.
  3    STAT_ISDEV
           Whether the file is a device such as a terminal or a disk.
  4    STAT_CREATETIME
           The time the file was created.
           (Currently just returns -1.)
  5    STAT_ACCESSTIME
           The last time the file was accessed.
  6    STAT_MODIFYTIME
           The last time the file data was changed.
  7    STAT_CHANGETIME
           The last time the file metadata was changed.
  8    STAT_BACKUPTIME
           The last time the file was backed up.
           (Currently just returns -1.)
  9    STAT_UID
           The user ID of the file.
  10   STAT_GID
           The group ID of the file.

The asynchronous version takes an additional final PMC callback
argument, and only returns a status object. When the stat operation is
complete, it invokes the callback, passing it a status object and an
integer containing the status information.

=item *

C<unlink> deletes a file from the filesystem. It takes a single string
argument of a filename (including the path).

The asynchronous version takes an additional final PMC callback
argument. When the unlink operation is complete, it invokes the
callback, passing it a status object.

=item *

C<rmdir> deletes a directory from the filesystem if that directory is
empty. It takes a single string argument of a directory name (including
the path).

The asynchronous version takes an additional final PMC callback
argument. When the rmdir operation is complete, it invokes the callback,
passing it a status object.

=item *

C<opendir> opens a stream object for a directory. It takes a single
string argument of a directory name (including the path) and returns a
stream object.

The asynchronous version takes an additional final PMC callback
argument, and only returns a status object. When the opendir operation
is complete, it invokes the callback, passing it a status object and a
newly created stream object.

=item *

C<readdir> reads a single item from an open directory stream object. It
takes a single stream object argument and returns a string containing
the path and filename/directory name of the current item. (i.e. the
directory stream object acts as an iterator.)

The asynchronous version takes an additional final PMC callback
argument, and only returns a status object. When the readdir operation
is complete, it invokes the callback, passing it a status object and the
string result.

=item *

C<telldir> returns the current position of C<readdir> operations on a
directory stream object.

No asynchronous version.

=item *

C<seekdir> sets the current position of C<readdir> operations on a
directory stream object. It takes a stream object argument and an
integer for the position. [The system C<seekdir> requires that the
position argument be the result of a previous C<telldir> operation.]

The asynchronous version takes an additional final PMC callback
argument. When the seekdir operation is complete, it invokes the
callback, passing it a status object and the directory stream object it
was called on.

=item *

C<rewinddir> sets the current position of C<readdir> operations on a
directory stream object back to the beginning of the directory. It takes
a stream object argument.

No asynchronous version.

=item *

C<closedir> closes a directory stream object. It takes a single stream
object argument.

The asynchronous version takes an additional final PMC callback
argument. When the closedir operation is complete, it invokes the
callback, passing it a status object.

=back

=head3 Network I/O Methods

Most of these methods take a default set of arguments the same as the standard
UNIX interface. Many also take additional optional arguments. There are no
network-specific opcodes.

=over 4

=item *

C<socket> returns a new socket object from a given address family,
socket type, and protocol number (all integers). The socket object's
boolean value can be tested to see if the socket is open or closed.

The asynchronous version takes an additional final PMC callback
argument, and only returns a status object. When the socket operation is
complete, it invokes the callback, passing it a status object and a new
socket object.

=item *

C<sockaddr> returns a SockAddr object representing a socket address, generated
from a port number (integer) and an address (string).

No asynchronous version.

=item *

C<connect> connects a socket object to an address.

The asynchronous version takes an additional final PMC callback
argument, and only returns a status object. When the socket operation is
complete, it invokes the callback, passing it a status object and the
socket object it was called on. [If you want notification when a connect
operation is completed, you probably want to do something with that
connected socket object.]

=item *

C<recv> receives a message from a connected socket object. It returns
the message in a string.

The asynchronous version takes an additional final PMC callback
argument, and only returns a status object. When the recv operation is
complete, it invokes the callback, passing it a status object and a
string containing the received message.

=item *

C<send> sends a message string to a connected socket object.

The asynchronous version takes an additional final PMC callback
argument, and only returns a status object. When the send operation is
complete, it invokes the callback, passing it a status object.

=item *

C<sendto> sends a message string to an address specified in an address
object (first connecting to the address).

The asynchronous version takes an additional final PMC callback
argument, and only returns a status object. When the sendto operation is
complete, it invokes the callback, passing it a status object.


=item *

C<bind> binds a socket object to the port and address specified by an
address object (the packed result of C<sockaddr>).

The asynchronous version takes an additional final PMC callback
argument, and only returns a status object. When the bind operation is
complete, it invokes the callback, passing it a status object and the
socket object it was called on. [If you want notification when a bind
operation is completed, you probably want to do something with that
bound socket object.]

=item *

C<listen> specifies that a socket object is willing to accept incoming
connections. The integer argument gives the maximum size of the queue
for pending connections.

There is no asynchronous version. C<listen> marks a set of attributes on
the socket object.

=item *

C<accept> accepts a new connection on a given socket object, and returns
a newly created socket object for the connection.

The asynchronous version takes an additional final PMC callback
argument, and only returns a status object. When the accept operation
receives a new connection, it invokes the callback, passing it a status
object and a newly created socket object for the connection. [While the
synchronous C<accept> has to be called repeatedly in a loop (once for
each connection received), the asynchronous version is only called once,
but continues to send new connection events until the socket is closed.]

=item *

C<shutdown> closes a socket object for reading, for writing, or for all
I/O. It takes a socket object argument and an integer argument for the
type of shutdown:

  0    PIOSHUTDOWN_READ
           Close the socket object for reading.
  1    PIOSHUTDOWN_WRITE
           Close the socket object for writing.
  2    PIOSHUTDOWN
           Close the socket object.

=back

=head3 Error Handling

Currently some of the networking opcodes (C<connect>, C<recv>, C<send>,
C<poll>, C<bind>, and C<listen>) return an integer indicating the status
of the call, -1 or a system error code if unsuccessful. Other I/O
opcodes (such as C<accept>) have various different
strategies for error notification, and others have no way of marking
errors at all. We want to unify all I/O opcodes so they use a consistent
strategy for error notification.

=head4 Synchronous operations

Synchronous I/O operations return an integer status code indicating
success or failure in addition to their ordinary return value(s). This
approach has the advantage of being lightweight: returning a single
additional integer is cheap.

[Discuss: should synchronous operations take the same error handling
strategy as asynchronous ones?]


=head4 Asynchronous operations

Asynchronous I/O operations return a status object. The status object
contains an integer status code, string status/error message, and
boolean success value.

An error callback may be set on a status object, though it isn't
required. This callback will be invoked if the asynchronous operation
terminates in an error condition. The error callback takes one argument,
which is the status object containing all information about the failed
call. If no error callback is set, then the standard callback will be
invoked, and the user will need to check for error conditions in the
status object as the first operation of the handler code.

=head4 Exceptions

At some point in the future, I/O objects may also provide a way to throw
exceptions on error conditions. This feature will be enabled by calling
a method on the I/O object to set an internal flag.  The exception
throwing will be implemented as a method call on the status object.

Note that exception handlers for asynchronous I/O operations will likely
have to be set at a global scope because execution will have left the
dynamic scope of the I/O call by the time the error occurs.

=head3 IPv6 Support

The transition from IPv4 to IPv6 is in progress, though not likely to be
complete anytime soon. Most operating systems today offer at least
dual-stack IPv6 implementations, so they can use either IPv4 or IPv6,
depending on what's available. Parrot also needs to support either
protocol. For the most part, the network I/O opcodes should internally
handle either addressing scheme, without requiring the user to specify
which scheme is being used.

IETF recommends defaulting to IPv6 connections and falling back to IPv4
connections when IPv6 fails. This would give us more solid testing of
Parrot's compatibility IPv6, but may be too slow. Either way, it's a
good idea to make setting the default (or selecting one exclusively) an
option when compiling Parrot.

The most important issues for Parrot to consider with IPv6 are:

=over 4

=item *

Support 128 bit addresses. IPv6 addresses are colon-separated
hexadecimal numbers, such as C<20a:95ff:fef5:7e5e>.

=item *

Any address parsing should be able to support the address separated from
a port number or prefix/length by brackets: C<[20a:95ff:fef5:7e5e]:80>
and C<[20a:95ff::]/64>.

=item *

Packed addresses, such as the result of the C<sockaddr> opcode, should
be passed around as an object (or at least a structure) rather than as a
string.

=back

See the relevant IETF RFCs: "Application Aspects of IPv6 Transition"
(http://www.ietf.org/rfc/rfc4038.txt) and "Basic Socket Interface
Extensions for IPv6" (http://www.ietf.org/rfc/rfc3493.txt).

=head2 Links

L<http://en.wikipedia.org/wiki/Asynchronous_I/O> for a relatively
comprehensive list of asynchronous I/O implementation options.

=head2 References

F<src/io/core.c>, F<src/ops/io.ops>, F<include/parrot/io.h>, 
F<runtime/parrot/library/Stream/*>, F<src/io/unix.c>, F<src/io/win32.c>,
Perl 5's IO::AIO, and POE

=cut

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