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<H2><A NAME="SECTION008130000000000000000"></A><A NAME="os-fd-ops"></A>
<BR>
6.1.3 File Descriptor Operations
</H2>

<P>
These functions operate on I/O streams referred to
using file descriptors.

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1558' xml:id='l2h-1558' class="function">close</tt></b>(</nobr></td>
  <td><var>fd</var>)</td></tr></table></dt>
<dd>
Close file descriptor <var>fd</var>.
Availability: Macintosh, <span class="Unix">Unix</span>, Windows.

<P>
<div class="note"><b class="label">Note:</b>
This function is intended for low-level I/O and must be applied
to a file descriptor as returned by <tt class="function">open()</tt> or
<tt class="function">pipe()</tt>.  To close a ``file object'' returned by the
built-in function <tt class="function">open()</tt> or by <tt class="function">popen()</tt> or
<tt class="function">fdopen()</tt>, use its <tt class="method">close()</tt> method.
</div>
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1559' xml:id='l2h-1559' class="function">dup</tt></b>(</nobr></td>
  <td><var>fd</var>)</td></tr></table></dt>
<dd>
Return a duplicate of file descriptor <var>fd</var>.
Availability: Macintosh, <span class="Unix">Unix</span>, Windows.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1560' xml:id='l2h-1560' class="function">dup2</tt></b>(</nobr></td>
  <td><var>fd, fd2</var>)</td></tr></table></dt>
<dd>
Duplicate file descriptor <var>fd</var> to <var>fd2</var>, closing the latter
first if necessary.
Availability: Macintosh, <span class="Unix">Unix</span>, Windows.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1561' xml:id='l2h-1561' class="function">fdatasync</tt></b>(</nobr></td>
  <td><var>fd</var>)</td></tr></table></dt>
<dd>
Force write of file with filedescriptor <var>fd</var> to disk.
Does not force update of metadata.
Availability: <span class="Unix">Unix</span>.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1562' xml:id='l2h-1562' class="function">fpathconf</tt></b>(</nobr></td>
  <td><var>fd, name</var>)</td></tr></table></dt>
<dd>
Return system configuration information relevant to an open file.
<var>name</var> specifies the configuration value to retrieve; it may be a
string which is the name of a defined system value; these names are
specified in a number of standards (POSIX.1, <span class="Unix">Unix</span> 95, <span class="Unix">Unix</span> 98, and
others).  Some platforms define additional names as well.  The names
known to the host operating system are given in the
<code>pathconf_names</code> dictionary.  For configuration variables not
included in that mapping, passing an integer for <var>name</var> is also
accepted.
Availability: Macintosh, <span class="Unix">Unix</span>.

<P>
If <var>name</var> is a string and is not known, <tt class="exception">ValueError</tt> is
raised.  If a specific value for <var>name</var> is not supported by the
host system, even if it is included in <code>pathconf_names</code>, an
<tt class="exception">OSError</tt> is raised with <tt class="constant">errno.EINVAL</tt> for the
error number.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1563' xml:id='l2h-1563' class="function">fstat</tt></b>(</nobr></td>
  <td><var>fd</var>)</td></tr></table></dt>
<dd>
Return status for file descriptor <var>fd</var>, like <tt class="function">stat()</tt>.
Availability: Macintosh, <span class="Unix">Unix</span>, Windows.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1564' xml:id='l2h-1564' class="function">fstatvfs</tt></b>(</nobr></td>
  <td><var>fd</var>)</td></tr></table></dt>
<dd>
Return information about the filesystem containing the file associated
with file descriptor <var>fd</var>, like <tt class="function">statvfs()</tt>.
Availability: <span class="Unix">Unix</span>.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1565' xml:id='l2h-1565' class="function">fsync</tt></b>(</nobr></td>
  <td><var>fd</var>)</td></tr></table></dt>
<dd>
Force write of file with filedescriptor <var>fd</var> to disk.  On <span class="Unix">Unix</span>,
this calls the native <tt class="cfunction">fsync()</tt> function; on Windows, the
MS <tt class="cfunction">_commit()</tt> function.

<P>
If you're starting with a Python file object <var>f</var>, first do
<code><var>f</var>.flush()</code>, and then do <code>os.fsync(<var>f</var>.fileno())</code>,
to ensure that all internal buffers associated with <var>f</var> are written
to disk.
Availability: Macintosh, <span class="Unix">Unix</span>, and Windows starting in 2.2.3.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1566' xml:id='l2h-1566' class="function">ftruncate</tt></b>(</nobr></td>
  <td><var>fd, length</var>)</td></tr></table></dt>
<dd>
Truncate the file corresponding to file descriptor <var>fd</var>,
so that it is at most <var>length</var> bytes in size.
Availability: Macintosh, <span class="Unix">Unix</span>.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1567' xml:id='l2h-1567' class="function">isatty</tt></b>(</nobr></td>
  <td><var>fd</var>)</td></tr></table></dt>
<dd>
Return <code>True</code> if the file descriptor <var>fd</var> is open and
connected to a tty(-like) device, else <code>False</code>.
Availability: Macintosh, <span class="Unix">Unix</span>.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1568' xml:id='l2h-1568' class="function">lseek</tt></b>(</nobr></td>
  <td><var>fd, pos, how</var>)</td></tr></table></dt>
<dd>
Set the current position of file descriptor <var>fd</var> to position
<var>pos</var>, modified by <var>how</var>: <code>0</code> to set the position
relative to the beginning of the file; <code>1</code> to set it relative to
the current position; <code>2</code> to set it relative to the end of the
file.
Availability: Macintosh, <span class="Unix">Unix</span>, Windows.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1569' xml:id='l2h-1569' class="function">open</tt></b>(</nobr></td>
  <td><var>file, flags</var><big>[</big><var>, mode</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
Open the file <var>file</var> and set various flags according to
<var>flags</var> and possibly its mode according to <var>mode</var>.
The default <var>mode</var> is <code>0777</code> (octal), and the current umask
value is first masked out.  Return the file descriptor for the newly
opened file.
Availability: Macintosh, <span class="Unix">Unix</span>, Windows.

<P>
For a description of the flag and mode values, see the C run-time
documentation; flag constants (like <tt class="constant">O_RDONLY</tt> and
<tt class="constant">O_WRONLY</tt>) are defined in this module too (see below).

<P>
<div class="note"><b class="label">Note:</b>
This function is intended for low-level I/O.  For normal usage,
use the built-in function <tt class="function">open()</tt>, which returns a ``file
object'' with <tt class="method">read()</tt> and <tt class="method">write()</tt> methods (and many
more).
</div>
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1570' xml:id='l2h-1570' class="function">openpty</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
Open a new pseudo-terminal pair. Return a pair of file descriptors
<code>(<var>master</var>, <var>slave</var>)</code> for the pty and the tty,
respectively. For a (slightly) more portable approach, use the
<tt class="module"><a href="module-pty.html">pty</a></tt><a id='l2h-1581' xml:id='l2h-1581'></a> module.
Availability: Macintosh, Some flavors of <span class="Unix">Unix</span>.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1571' xml:id='l2h-1571' class="function">pipe</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
Create a pipe.  Return a pair of file descriptors <code>(<var>r</var>,
<var>w</var>)</code> usable for reading and writing, respectively.
Availability: Macintosh, <span class="Unix">Unix</span>, Windows.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1572' xml:id='l2h-1572' class="function">read</tt></b>(</nobr></td>
  <td><var>fd, n</var>)</td></tr></table></dt>
<dd>
Read at most <var>n</var> bytes from file descriptor <var>fd</var>.
Return a string containing the bytes read.  If the end of the file
referred to by <var>fd</var> has been reached, an empty string is
returned.
Availability: Macintosh, <span class="Unix">Unix</span>, Windows.

<P>
<div class="note"><b class="label">Note:</b>
This function is intended for low-level I/O and must be applied
to a file descriptor as returned by <tt class="function">open()</tt> or
<tt class="function">pipe()</tt>.  To read a ``file object'' returned by the
built-in function <tt class="function">open()</tt> or by <tt class="function">popen()</tt> or
<tt class="function">fdopen()</tt>, or <code>sys.stdin</code>, use its
<tt class="method">read()</tt> or <tt class="method">readline()</tt> methods.
</div>
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1573' xml:id='l2h-1573' class="function">tcgetpgrp</tt></b>(</nobr></td>
  <td><var>fd</var>)</td></tr></table></dt>
<dd>
Return the process group associated with the terminal given by
<var>fd</var> (an open file descriptor as returned by <tt class="function">open()</tt>).
Availability: Macintosh, <span class="Unix">Unix</span>.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1574' xml:id='l2h-1574' class="function">tcsetpgrp</tt></b>(</nobr></td>
  <td><var>fd, pg</var>)</td></tr></table></dt>
<dd>
Set the process group associated with the terminal given by
<var>fd</var> (an open file descriptor as returned by <tt class="function">open()</tt>)
to <var>pg</var>.
Availability: Macintosh, <span class="Unix">Unix</span>.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1575' xml:id='l2h-1575' class="function">ttyname</tt></b>(</nobr></td>
  <td><var>fd</var>)</td></tr></table></dt>
<dd>
Return a string which specifies the terminal device associated with
file-descriptor <var>fd</var>.  If <var>fd</var> is not associated with a terminal
device, an exception is raised.
Availability:Macintosh,  <span class="Unix">Unix</span>.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-1576' xml:id='l2h-1576' class="function">write</tt></b>(</nobr></td>
  <td><var>fd, str</var>)</td></tr></table></dt>
<dd>
Write the string <var>str</var> to file descriptor <var>fd</var>.
Return the number of bytes actually written.
Availability: Macintosh, <span class="Unix">Unix</span>, Windows.

<P>
<div class="note"><b class="label">Note:</b>
This function is intended for low-level I/O and must be applied
to a file descriptor as returned by <tt class="function">open()</tt> or
<tt class="function">pipe()</tt>.  To write a ``file object'' returned by the
built-in function <tt class="function">open()</tt> or by <tt class="function">popen()</tt> or
<tt class="function">fdopen()</tt>, or <code>sys.stdout</code> or <code>sys.stderr</code>, use
its <tt class="method">write()</tt> method.
</div>
</dl>

<P>
The following data items are available for use in constructing the
<var>flags</var> parameter to the <tt class="function">open()</tt> function.

<P>
<dl><dt><b><tt id='l2h-1577' xml:id='l2h-1577'>O_RDONLY</tt></b></dt>
<dd>
<dt><b><tt id='l2h-1582' xml:id='l2h-1582'>O_WRONLY</tt></b></dt><dd>
<dt><b><tt id='l2h-1583' xml:id='l2h-1583'>O_RDWR</tt></b></dt><dd>
<dt><b><tt id='l2h-1584' xml:id='l2h-1584'>O_APPEND</tt></b></dt><dd>
<dt><b><tt id='l2h-1585' xml:id='l2h-1585'>O_CREAT</tt></b></dt><dd>
<dt><b><tt id='l2h-1586' xml:id='l2h-1586'>O_EXCL</tt></b></dt><dd>
<dt><b><tt id='l2h-1587' xml:id='l2h-1587'>O_TRUNC</tt></b></dt><dd>
Options for the <var>flag</var> argument to the <tt class="function">open()</tt> function.
These can be bit-wise OR'd together.
Availability: Macintosh, <span class="Unix">Unix</span>, Windows.
</dd></dl>

<P>
<dl><dt><b><tt id='l2h-1578' xml:id='l2h-1578'>O_DSYNC</tt></b></dt>
<dd>
<dt><b><tt id='l2h-1588' xml:id='l2h-1588'>O_RSYNC</tt></b></dt><dd>
<dt><b><tt id='l2h-1589' xml:id='l2h-1589'>O_SYNC</tt></b></dt><dd>
<dt><b><tt id='l2h-1590' xml:id='l2h-1590'>O_NDELAY</tt></b></dt><dd>
<dt><b><tt id='l2h-1591' xml:id='l2h-1591'>O_NONBLOCK</tt></b></dt><dd>
<dt><b><tt id='l2h-1592' xml:id='l2h-1592'>O_NOCTTY</tt></b></dt><dd>
More options for the <var>flag</var> argument to the <tt class="function">open()</tt> function.
Availability: Macintosh, <span class="Unix">Unix</span>.
</dd></dl>

<P>
<dl><dt><b><tt id='l2h-1579' xml:id='l2h-1579'>O_BINARY</tt></b></dt>
<dd>
Option for the <var>flag</var> argument to the <tt class="function">open()</tt> function.
This can be bit-wise OR'd together with those listed above.
Availability: Windows.
</dd></dl>

<P>
<dl><dt><b><tt id='l2h-1580' xml:id='l2h-1580'>O_NOINHERIT</tt></b></dt>
<dd>
<dt><b><tt id='l2h-1593' xml:id='l2h-1593'>O_SHORT_LIVED</tt></b></dt><dd>
<dt><b><tt id='l2h-1594' xml:id='l2h-1594'>O_TEMPORARY</tt></b></dt><dd>
<dt><b><tt id='l2h-1595' xml:id='l2h-1595'>O_RANDOM</tt></b></dt><dd>
<dt><b><tt id='l2h-1596' xml:id='l2h-1596'>O_SEQUENTIAL</tt></b></dt><dd>
<dt><b><tt id='l2h-1597' xml:id='l2h-1597'>O_TEXT</tt></b></dt><dd>
Options for the <var>flag</var> argument to the <tt class="function">open()</tt> function.
These can be bit-wise OR'd together.
Availability: Windows.
</dd></dl>

<P>

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