"""

Python implementation of the io module.

"""

from __future__ import (print_function, unicode_literals)

import os

import abc

import codecs

import sys

import warnings

import errno

# Import thread instead of threading to reduce startup cost

try:

from thread import allocate_lock as Lock

except ImportError:

from dummy_thread import allocate_lock as Lock

import io

from io import (__all__, SEEK_SET, SEEK_CUR, SEEK_END)

from errno import EINTR

__metaclass__ = type

# open() uses st_blksize whenever we can

DEFAULT_BUFFER_SIZE = 8 * 1024 # bytes

# NOTE: Base classes defined here are registered with the "official" ABCs

# defined in io.py. We don't use real inheritance though, because we don't want

# to inherit the C implementations.

class BlockingIOError(IOError):

"""Exception raised when I/O would block on a non-blocking I/O stream."""

def __init__(self, errno, strerror, characters_written=0):

super(IOError, self).__init__(errno, strerror)

if not isinstance(characters_written, (int, long)):

raise TypeError("characters_written must be a integer")

self.characters_written = characters_written

def open(file, mode="r", buffering=-1,

encoding=None, errors=None,

newline=None, closefd=True):

r"""Open file and return a stream. Raise IOError upon failure.

file is either a text or byte string giving the name (and the path

if the file isn't in the current working directory) of the file to

be opened or an integer file descriptor of the file to be

wrapped. (If a file descriptor is given, it is closed when the

returned I/O object is closed, unless closefd is set to False.)

mode is an optional string that specifies the mode in which the file

is opened. It defaults to 'r' which means open for reading in text

mode. Other common values are 'w' for writing (truncating the file if

it already exists), and 'a' for appending (which on some Unix systems,

means that all writes append to the end of the file regardless of the

current seek position). In text mode, if encoding is not specified the

encoding used is platform dependent. (For reading and writing raw

bytes use binary mode and leave encoding unspecified.) The available

modes are:

========= ===============================================================

Character Meaning

--------- ---------------------------------------------------------------

'r' open for reading (default)

'w' open for writing, truncating the file first

'a' open for writing, appending to the end of the file if it exists

'b' binary mode

't' text mode (default)

'+' open a disk file for updating (reading and writing)

'U' universal newline mode (for backwards compatibility; unneeded

for new code)

========= ===============================================================

The default mode is 'rt' (open for reading text). For binary random

access, the mode 'w+b' opens and truncates the file to 0 bytes, while

'r+b' opens the file without truncation.

Python distinguishes between files opened in binary and text modes,

even when the underlying operating system doesn't. Files opened in

binary mode (appending 'b' to the mode argument) return contents as

bytes objects without any decoding. In text mode (the default, or when

't' is appended to the mode argument), the contents of the file are

returned as strings, the bytes having been first decoded using a

platform-dependent encoding or using the specified encoding if given.

buffering is an optional integer used to set the buffering policy.

Pass 0 to switch buffering off (only allowed in binary mode), 1 to select

line buffering (only usable in text mode), and an integer > 1 to indicate

the size of a fixed-size chunk buffer. When no buffering argument is

given, the default buffering policy works as follows:

* Binary files are buffered in fixed-size chunks; the size of the buffer

is chosen using a heuristic trying to determine the underlying device's

"block size" and falling back on `io.DEFAULT_BUFFER_SIZE`.

On many systems, the buffer will typically be 4096 or 8192 bytes long.

* "Interactive" text files (files for which isatty() returns True)

use line buffering. Other text files use the policy described above

for binary files.

encoding is the name of the encoding used to decode or encode the

file. This should only be used in text mode. The default encoding is

platform dependent, but any encoding supported by Python can be

passed. See the codecs module for the list of supported encodings.

errors is an optional string that specifies how encoding errors are to

be handled---this argument should not be used in binary mode. Pass

'strict' to raise a ValueError exception if there is an encoding error

(the default of None has the same effect), or pass 'ignore' to ignore

errors. (Note that ignoring encoding errors can lead to data loss.)

See the documentation for codecs.register for a list of the permitted

encoding error strings.

newline controls how universal newlines works (it only applies to text

mode). It can be None, '', '\n', '\r', and '\r\n'. It works as

follows:

* On input, if newline is None, universal newlines mode is

enabled. Lines in the input can end in '\n', '\r', or '\r\n', and

these are translated into '\n' before being returned to the

caller. If it is '', universal newline mode is enabled, but line

endings are returned to the caller untranslated. If it has any of

the other legal values, input lines are only terminated by the given

string, and the line ending is returned to the caller untranslated.

* On output, if newline is None, any '\n' characters written are

translated to the system default line separator, os.linesep. If

newline is '', no translation takes place. If newline is any of the

other legal values, any '\n' characters written are translated to

the given string.

If closefd is False, the underlying file descriptor will be kept open

when the file is closed. This does not work when a file name is given

and must be True in that case.

open() returns a file object whose type depends on the mode, and

through which the standard file operations such as reading and writing

are performed. When open() is used to open a file in a text mode ('w',

'r', 'wt', 'rt', etc.), it returns a TextIOWrapper. When used to open

a file in a binary mode, the returned class varies: in read binary

mode, it returns a BufferedReader; in write binary and append binary

modes, it returns a BufferedWriter, and in read/write mode, it returns

a BufferedRandom.

It is also possible to use a string or bytearray as a file for both

reading and writing. For strings StringIO can be used like a file

opened in a text mode, and for bytes a BytesIO can be used like a file

opened in a binary mode.

"""

if not isinstance(file, (basestring, int, long)):

raise TypeError("invalid file: %r" % file)

if not isinstance(mode, basestring):

raise TypeError("invalid mode: %r" % mode)

if not isinstance(buffering, (int, long)):

raise TypeError("invalid buffering: %r" % buffering)

if encoding is not None and not isinstance(encoding, basestring):

raise TypeError("invalid encoding: %r" % encoding)

if errors is not None and not isinstance(errors, basestring):

raise TypeError("invalid errors: %r" % errors)

modes = set(mode)

if modes - set("arwb+tU") or len(mode) > len(modes):

raise ValueError("invalid mode: %r" % mode)

reading = "r" in modes

writing = "w" in modes

appending = "a" in modes

updating = "+" in modes

text = "t" in modes

binary = "b" in modes

if "U" in modes:

if writing or appending:

raise ValueError("can't use U and writing mode at once")

reading = True

if text and binary:

raise ValueError("can't have text and binary mode at once")

if reading + writing + appending > 1:

raise ValueError("can't have read/write/append mode at once")

if not (reading or writing or appending):

raise ValueError("must have exactly one of read/write/append mode")

if binary and encoding is not None:

raise ValueError("binary mode doesn't take an encoding argument")

if binary and errors is not None:

raise ValueError("binary mode doesn't take an errors argument")

if binary and newline is not None:

raise ValueError("binary mode doesn't take a newline argument")

raw = FileIO(file,

(reading and "r" or "") +

(writing and "w" or "") +

(appending and "a" or "") +

(updating and "+" or ""),

closefd)

result = raw

try:

line_buffering = False

if buffering == 1 or buffering < 0 and raw.isatty():

buffering = -1

line_buffering = True

if buffering < 0:

buffering = DEFAULT_BUFFER_SIZE

try:

bs = os.fstat(raw.fileno()).st_blksize

except (os.error, AttributeError):

pass

else:

if bs > 1:

buffering = bs

if buffering < 0:

raise ValueError("invalid buffering size")

if buffering == 0:

if binary:

return result

raise ValueError("can't have unbuffered text I/O")

if updating:

buffer = BufferedRandom(raw, buffering)

elif writing or appending:

buffer = BufferedWriter(raw, buffering)

elif reading:

buffer = BufferedReader(raw, buffering)

else:

raise ValueError("unknown mode: %r" % mode)

result = buffer

if binary:

return result

text = TextIOWrapper(buffer, encoding, errors, newline, line_buffering)

result = text

text.mode = mode

return result

except:

result.close()

raise

class DocDescriptor:

"""Helper for builtins.open.__doc__

"""

def __get__(self, obj, typ):

return (

"open(file, mode='r', buffering=-1, encoding=None, "

"errors=None, newline=None, closefd=True)\n\n" +

open.__doc__)

class OpenWrapper:

"""Wrapper for builtins.open

Trick so that open won't become a bound method when stored

as a class variable (as dbm.dumb does).

See initstdio() in Python/pythonrun.c.

"""

__doc__ = DocDescriptor()

def __new__(cls, *args, **kwargs):

return open(*args, **kwargs)

class UnsupportedOperation(ValueError, IOError):

pass

class IOBase:

__metaclass__ = abc.ABCMeta

"""The abstract base class for all I/O classes, acting on streams of

bytes. There is no public constructor.

This class provides dummy implementations for many methods that

derived classes can override selectively; the default implementations

represent a file that cannot be read, written or seeked.

Even though IOBase does not declare read, readinto, or write because

their signatures will vary, implementations and clients should

consider those methods part of the interface. Also, implementations

may raise an IOError when operations they do not support are called.

The basic type used for binary data read from or written to a file is

the bytes type. Method arguments may also be bytearray or memoryview of

arrays of bytes. In some cases, such as readinto, a writable object such

as bytearray is required. Text I/O classes work with unicode data.

Note that calling any method (even inquiries) on a closed stream is

undefined. Implementations may raise IOError in this case.

IOBase (and its subclasses) support the iterator protocol, meaning

that an IOBase object can be iterated over yielding the lines in a

stream.

IOBase also supports the :keyword:`with` statement. In this example,

fp is closed after the suite of the with statement is complete:

with open('spam.txt', 'r') as fp:

fp.write('Spam and eggs!')

"""

### Internal ###

def _unsupported(self, name):

"""Internal: raise an exception for unsupported operations."""

raise UnsupportedOperation("%s.%s() not supported" %

(self.__class__.__name__, name))

### Positioning ###

def seek(self, pos, whence=0):

"""Change stream position.

Change the stream position to byte offset pos. Argument pos is

interpreted relative to the position indicated by whence. Values

for whence are:

* 0 -- start of stream (the default); offset should be zero or positive

* 1 -- current stream position; offset may be negative

* 2 -- end of stream; offset is usually negative

Return the new absolute position.

"""

self._unsupported("seek")

def tell(self):

"""Return current stream position."""

return self.seek(0, 1)

def truncate(self, pos=None):

"""Truncate file to size bytes.

Size defaults to the current IO position as reported by tell(). Return

the new size.

"""

self._unsupported("truncate")

### Flush and close ###

def flush(self):

"""Flush write buffers, if applicable.

This is not implemented for read-only and non-blocking streams.

"""

self._checkClosed()

# XXX Should this return the number of bytes written???

__closed = False

def close(self):

"""Flush and close the IO object.

This method has no effect if the file is already closed.

"""

if not self.__closed:

try:

self.flush()

finally:

self.__closed = True

def __del__(self):

"""Destructor. Calls close()."""

# The try/except block is in case this is called at program

# exit time, when it's possible that globals have already been

# deleted, and then the close() call might fail. Since

# there's nothing we can do about such failures and they annoy

# the end users, we suppress the traceback.

try:

self.close()

except:

pass

### Inquiries ###

def seekable(self):

"""Return whether object supports random access.

If False, seek(), tell() and truncate() will raise IOError.

This method may need to do a test seek().

"""

return False

def _checkSeekable(self, msg=None):

"""Internal: raise an IOError if file is not seekable

"""

if not self.seekable():

raise IOError("File or stream is not seekable."

if msg is None else msg)

def readable(self):

"""Return whether object was opened for reading.

If False, read() will raise IOError.

"""

return False

def _checkReadable(self, msg=None):

"""Internal: raise an IOError if file is not readable

"""

if not self.readable():

raise IOError("File or stream is not readable."

if msg is None else msg)

def writable(self):

"""Return whether object was opened for writing.

If False, write() and truncate() will raise IOError.

"""

return False

def _checkWritable(self, msg=None):

"""Internal: raise an IOError if file is not writable

"""

if not self.writable():

raise IOError("File or stream is not writable."

if msg is None else msg)

@property

def closed(self):

"""closed: bool. True iff the file has been closed.

For backwards compatibility, this is a property, not a predicate.

"""

return self.__closed

def _checkClosed(self, msg=None):

"""Internal: raise a ValueError if file is closed

"""

if self.closed:

raise ValueError("I/O operation on closed file."

if msg is None else msg)

### Context manager ###

def __enter__(self):

"""Context management protocol. Returns self."""

self._checkClosed()

return self

def __exit__(self, *args):

"""Context management protocol. Calls close()"""

self.close()

### Lower-level APIs ###

# XXX Should these be present even if unimplemented?

def fileno(self):

"""Returns underlying file descriptor if one exists.

An IOError is raised if the IO object does not use a file descriptor.

"""

self._unsupported("fileno")

def isatty(self):

"""Return whether this is an 'interactive' stream.

Return False if it can't be determined.

"""

self._checkClosed()

return False

### Readline[s] and writelines ###

def readline(self, limit=-1):

r"""Read and return a line from the stream.

If limit is specified, at most limit bytes will be read.

The line terminator is always b'\n' for binary files; for text

files, the newlines argument to open can be used to select the line

terminator(s) recognized.

"""

# For backwards compatibility, a (slowish) readline().

if hasattr(self, "peek"):

def nreadahead():

readahead = self.peek(1)

if not readahead:

return 1

n = (readahead.find(b"\n") + 1) or len(readahead)

if limit >= 0:

n = min(n, limit)

return n

else:

def nreadahead():

return 1

if limit is None:

limit = -1

elif not isinstance(limit, (int, long)):

raise TypeError("limit must be an integer")

res = bytearray()

while limit < 0 or len(res) < limit:

b = self.read(nreadahead())

if not b:

break

res += b

if res.endswith(b"\n"):

break

return bytes(res)

def __iter__(self):

self._checkClosed()

return self

def next(self):

line = self.readline()

if not line:

raise StopIteration

return line

def readlines(self, hint=None):

"""Return a list of lines from the stream.

hint can be specified to control the number of lines read: no more

lines will be read if the total size (in bytes/characters) of all

lines so far exceeds hint.

"""

if hint is not None and not isinstance(hint, (int, long)):

raise TypeError("integer or None expected")

if hint is None or hint <= 0:

return list(self)

n = 0

lines = []

for line in self:

lines.append(line)

n += len(line)

if n >= hint:

break

return lines

def writelines(self, lines):

self._checkClosed()

for line in lines:

self.write(line)

io.IOBase.register(IOBase)

class RawIOBase(IOBase):

"""Base class for raw binary I/O."""

# The read() method is implemented by calling readinto(); derived

# classes that want to support read() only need to implement

# readinto() as a primitive operation. In general, readinto() can be

# more efficient than read().

# (It would be tempting to also provide an implementation of

# readinto() in terms of read(), in case the latter is a more suitable

# primitive operation, but that would lead to nasty recursion in case

# a subclass doesn't implement either.)

def read(self, n=-1):

"""Read and return up to n bytes.

Returns an empty bytes object on EOF, or None if the object is

set not to block and has no data to read.

"""

if n is None:

n = -1

if n < 0:

return self.readall()

b = bytearray(n.__index__())

n = self.readinto(b)

if n is None:

return None

del b[n:]

return bytes(b)

def readall(self):

"""Read until EOF, using multiple read() call."""

res = bytearray()

while True:

data = self.read(DEFAULT_BUFFER_SIZE)

if not data:

break

res += data

if res:

return bytes(res)

else:

# b'' or None

return data

def readinto(self, b):

"""Read up to len(b) bytes into b.

Returns number of bytes read (0 for EOF), or None if the object

is set not to block and has no data to read.

"""

self._unsupported("readinto")

def write(self, b):

"""Write the given buffer to the IO stream.

Returns the number of bytes written, which may be less than len(b).

"""

self._unsupported("write")

io.RawIOBase.register(RawIOBase)

from _io import FileIO

RawIOBase.register(FileIO)

class BufferedIOBase(IOBase):

"""Base class for buffered IO objects.

The main difference with RawIOBase is that the read() method

supports omitting the size argument, and does not have a default

implementation that defers to readinto().

In addition, read(), readinto() and write() may raise

BlockingIOError if the underlying raw stream is in non-blocking

mode and not ready; unlike their raw counterparts, they will never

return None.

A typical implementation should not inherit from a RawIOBase

implementation, but wrap one.

"""

def read(self, n=None):

"""Read and return up to n bytes.

If the argument is omitted, None, or negative, reads and

returns all data until EOF.

If the argument is positive, and the underlying raw stream is

not 'interactive', multiple raw reads may be issued to satisfy

the byte count (unless EOF is reached first). But for

interactive raw streams (XXX and for pipes?), at most one raw

read will be issued, and a short result does not imply that

EOF is imminent.

Returns an empty bytes array on EOF.

Raises BlockingIOError if the underlying raw stream has no

data at the moment.

"""

self._unsupported("read")

def read1(self, n=None):

"""Read up to n bytes with at most one read() system call."""

self._unsupported("read1")

def readinto(self, b):

"""Read up to len(b) bytes into b.

Like read(), this may issue multiple reads to the underlying raw

stream, unless the latter is 'interactive'.

Returns the number of bytes read (0 for EOF).

Raises BlockingIOError if the underlying raw stream has no

data at the moment.

"""

data = self.read(len(b))

n = len(data)

try:

b[:n] = data

except TypeError as err:

import array

if not isinstance(b, array.array):

raise err

b[:n] = array.array(b'b', data)

return n

def write(self, b):

"""Write the given buffer to the IO stream.

Return the number of bytes written, which is always len(b).

Raises BlockingIOError if the buffer is full and the

underlying raw stream cannot accept more data at the moment.

"""

self._unsupported("write")

def detach(self):

"""

Separate the underlying raw stream from the buffer and return it.

After the raw stream has been detached, the buffer is in an unusable

state.

"""

self._unsupported("detach")

io.BufferedIOBase.register(BufferedIOBase)

class _BufferedIOMixin(BufferedIOBase):

"""A mixin implementation of BufferedIOBase with an underlying raw stream.

This passes most requests on to the underlying raw stream. It

does *not* provide implementations of read(), readinto() or

write().

"""

def __init__(self, raw):

self._raw = raw

### Positioning ###

def seek(self, pos, whence=0):

new_position = self.raw.seek(pos, whence)

if new_position < 0:

raise IOError("seek() returned an invalid position")

return new_position

def tell(self):

pos = self.raw.tell()

if pos < 0:

raise IOError("tell() returned an invalid position")

return pos

def truncate(self, pos=None):

# Flush the stream. We're mixing buffered I/O with lower-level I/O,

# and a flush may be necessary to synch both views of the current

# file state.

self.flush()

if pos is None:

pos = self.tell()

# XXX: Should seek() be used, instead of passing the position

# XXX directly to truncate?

return self.raw.truncate(pos)

### Flush and close ###

def flush(self):

if self.closed:

raise ValueError("flush of closed file")

self.raw.flush()

def close(self):

if self.raw is not None and not self.closed:

try:

# may raise BlockingIOError or BrokenPipeError etc

self.flush()

finally:

self.raw.close()

def detach(self):

if self.raw is None:

raise ValueError("raw stream already detached")

self.flush()

raw = self._raw

self._raw = None

return raw

### Inquiries ###

def seekable(self):

return self.raw.seekable()

def readable(self):

return self.raw.readable()

def writable(self):

return self.raw.writable()

@property

def raw(self):

return self._raw

@property

def closed(self):

return self.raw.closed

@property

def name(self):

return self.raw.name

@property

def mode(self):

return self.raw.mode

def __repr__(self):

clsname = self.__class__.__name__

try:

name = self.name

except Exception:

return "<_pyio.{0}>".format(clsname)

else:

return "<_pyio.{0} name={1!r}>".format(clsname, name)

### Lower-level APIs ###

def fileno(self):

return self.raw.fileno()

def isatty(self):

return self.raw.isatty()

class BytesIO(BufferedIOBase):

"""Buffered I/O implementation using an in-memory bytes buffer."""

def __init__(self, initial_bytes=None):

buf = bytearray()

if initial_bytes is not None:

buf.extend(initial_bytes)

self._buffer = buf

self._pos = 0

def __getstate__(self):

if self.closed:

raise ValueError("__getstate__ on closed file")

return self.__dict__.copy()

def getvalue(self):

"""Return the bytes value (contents) of the buffer

"""

if self.closed:

raise ValueError("getvalue on closed file")

return bytes(self._buffer)

def read(self, n=None):

if self.closed:

raise ValueError("read from closed file")

if n is None:

n = -1

if not isinstance(n, (int, long)):

raise TypeError("integer argument expected, got {0!r}".format(

type(n)))

if n < 0:

n = len(self._buffer)

if len(self._buffer) <= self._pos:

return b""

newpos = min(len(self._buffer), self._pos + n)

b = self._buffer[self._pos : newpos]

self._pos = newpos

return bytes(b)

def read1(self, n):

"""This is the same as read.

"""

return self.read(n)

def write(self, b):

if self.closed:

raise ValueError("write to closed file")

if isinstance(b, unicode):

raise TypeError("can't write unicode to binary stream")

n = len(b)

if n == 0:

return 0

pos = self._pos

if pos > len(self._buffer):

# Inserts null bytes between the current end of the file

# and the new write position.

padding = b'\x00' * (pos - len(self._buffer))

self._buffer += padding

self._buffer[pos:pos + n] = b

self._pos += n

return n

def seek(self, pos, whence=0):

if self.closed:

raise ValueError("seek on closed file")

try:

pos.__index__

except AttributeError:

raise TypeError("an integer is required")

if whence == 0:

if pos < 0:

raise ValueError("negative seek position %r" % (pos,))

self._pos = pos

elif whence == 1:

self._pos = max(0, self._pos + pos)

elif whence == 2:

self._pos = max(0, len(self._buffer) + pos)

else:

raise ValueError("invalid whence value")

return self._pos

def tell(self):

if self.closed:

raise ValueError("tell on closed file")

return self._pos

def truncate(self, pos=None):

if self.closed:

raise ValueError("truncate on closed file")

if pos is None:

pos = self._pos

else:

try:

pos.__index__

except AttributeError:

raise TypeError("an integer is required")

if pos < 0:

raise ValueError("negative truncate position %r" % (pos,))

del self._buffer[pos:]

return pos

def readable(self):

if self.closed:

raise ValueError("I/O operation on closed file.")

return True

def writable(self):

if self.closed:

raise ValueError("I/O operation on closed file.")

return True

def seekable(self):

if self.closed:

raise ValueError("I/O operation on closed file.")

return True

class BufferedReader(_BufferedIOMixin):

"""BufferedReader(raw[, buffer_size])

A buffer for a readable, sequential BaseRawIO object.

The constructor creates a BufferedReader for the given readable raw

stream and buffer_size. If buffer_size is omitted, DEFAULT_BUFFER_SIZE

is used.

"""

def __init__(self, raw, buffer_size=DEFAULT_BUFFER_SIZE):

"""Create a new buffered reader using the given readable raw IO object.

"""

if not raw.readable():

raise IOError('"raw" argument must be readable.')

_BufferedIOMixin.__init__(self, raw)

if buffer_size <= 0:

raise ValueError("invalid buffer size")

self.buffer_size = buffer_size

self._reset_read_buf()

self._read_lock = Lock()

def _reset_read_buf(self):

self._read_buf = b""

self._read_pos = 0

def read(self, n=None):

"""Read n bytes.

Returns exactly n bytes of data unless the underlying raw IO

stream reaches EOF or if the call would block in non-blocking

mode. If n is negative, read until EOF or until read() would

block.

"""

if n is not None and n < -1:

raise ValueError("invalid number of bytes to read")

with self._read_lock:

return self._read_unlocked(n)

def _read_unlocked(self, n=None):

nodata_val = b""

empty_values = (b"", None)

buf = self._read_buf

pos = self._read_pos

# Special case for when the number of bytes to read is unspecified.

if n is None or n == -1:

self._reset_read_buf()

chunks = [buf[pos:]] # Strip the consumed bytes.

current_size = 0

while True:

# Read until EOF or until read() would block.

try:

chunk = self.raw.read()

except IOError as e:

if e.errno != EINTR:

raise

continue

if chunk in empty_values:

nodata_val = chunk

break

current_size += len(chunk)

chunks.append(chunk)

return b"".join(chunks) or nodata_val

# The number of bytes to read is specified, return at most n bytes.

avail = len(buf) - pos # Length of the available buffered data.

if n <= avail:

# Fast path: the data to read is fully buffered.

self._read_pos += n

return buf[pos:pos+n]

# Slow path: read from the stream until enough bytes are read,

# or until an EOF occurs or until read() would block.

chunks = [buf[pos:]]

wanted = max(self.buffer_size, n)

while avail < n:

try:

chunk = self.raw.read(wanted)

except IOError as e:

if e.errno != EINTR:

raise

continue

if chunk in empty_values:

nodata_val = chunk

break

avail += len(chunk)

chunks.append(chunk)

# n is more than avail only when an EOF occurred or when

# read() would have blocked.