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<H1><A NAME="SECTION00101100000000000000000">
8.11 <tt class="module">fcntl</tt> --
The <tt class="function">fcntl()</tt> and <tt class="function">ioctl()</tt> system calls</A>
</H1>
<P>
<A NAME="module-fcntl"></A>
<p class="availability">Availability: <span
class="platform">Unix</span>.</p>
<P>
<a id='l2h-3024' xml:id='l2h-3024'></a><a id='l2h-3025' xml:id='l2h-3025'></a>
<P>
This module performs file control and I/O control on file descriptors.
It is an interface to the <tt class="cfunction">fcntl()</tt> and <tt class="cfunction">ioctl()</tt>
<span class="Unix">Unix</span> routines.
<P>
All functions in this module take a file descriptor <var>fd</var> as their
first argument. This can be an integer file descriptor, such as
returned by <code>sys.stdin.fileno()</code>, or a file object, such as
<code>sys.stdin</code> itself, which provides a <tt class="method">fileno()</tt> which
returns a genuine file descriptor.
<P>
The module defines the following functions:
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-3026' xml:id='l2h-3026' class="function">fcntl</tt></b>(</nobr></td>
<td><var>fd, op</var><big>[</big><var>, arg</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
Perform the requested operation on file descriptor <var>fd</var> (file
objects providing a <tt class="method">fileno()</tt> method are accepted as well).
The operation is defined by <var>op</var> and is operating system
dependent. These codes are also found in the <tt class="module">fcntl</tt>
module. The argument <var>arg</var> is optional, and defaults to the
integer value <code>0</code>. When present, it can either be an integer
value, or a string. With the argument missing or an integer value,
the return value of this function is the integer return value of the
C <tt class="cfunction">fcntl()</tt> call. When the argument is a string it
represents a binary structure, e.g. created by
<tt class="function"><tt class="module"><a href="module-struct.html">struct</a></tt>.pack()</tt>. The binary data is copied to a buffer
whose address is passed to the C <tt class="cfunction">fcntl()</tt> call. The
return value after a successful call is the contents of the buffer,
converted to a string object. The length of the returned string
will be the same as the length of the <var>arg</var> argument. This is
limited to 1024 bytes. If the information returned in the buffer by
the operating system is larger than 1024 bytes, this is most likely
to result in a segmentation violation or a more subtle data
corruption.
<P>
If the <tt class="cfunction">fcntl()</tt> fails, an <tt class="exception">IOError</tt> is
raised.
</dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-3027' xml:id='l2h-3027' class="function">ioctl</tt></b>(</nobr></td>
<td><var>fd, op</var><big>[</big><var>, arg</var><big>[</big><var>, mutate_flag</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
This function is identical to the <tt class="function">fcntl()</tt> function,
except that the operations are typically defined in the library
module <tt class="module"><a href="module-termios.html">termios</a></tt> and the argument handling is even more
complicated.
<P>
The parameter <var>arg</var> can be one of an integer, absent (treated
identically to the integer <code>0</code>), an object supporting the
read-only buffer interface (most likely a plain Python string) or an
object supporting the read-write buffer interface.
<P>
In all but the last case, behaviour is as for the <tt class="function">fcntl()</tt>
function.
<P>
If a mutable buffer is passed, then the behaviour is determined by
the value of the <var>mutate_flag</var> parameter.
<P>
If it is false, the buffer's mutability is ignored and behaviour is
as for a read-only buffer, except that the 1024 byte limit mentioned
above is avoided - so long as the buffer you pass is longer than
what the operating system wants to put there, things should work.
<P>
If <var>mutate_flag</var> is true, then the buffer is (in effect) passed
to the underlying <tt class="function">ioctl()</tt> system call, the latter's
return code is passed back to the calling Python, and the buffer's
new contents reflect the action of the <tt class="function">ioctl()</tt>. This is a
slight simplification, because if the supplied buffer is less than
1024 bytes long it is first copied into a static buffer 1024 bytes
long which is then passed to <tt class="function">ioctl()</tt> and copied back into
the supplied buffer.
<P>
If <var>mutate_flag</var> is not supplied, then in 2.3 it defaults to
false. This is planned to change over the next few Python versions:
in 2.4 failing to supply <var>mutate_flag</var> will get a warning but
the same behavior and in versions later than 2.5 it will default to
true.
<P>
An example:
<P>
<div class="verbatim"><pre>
>>> import array, fcntl, struct, termios, os
>>> os.getpgrp()
13341
>>> struct.unpack('h', fcntl.ioctl(0, termios.TIOCGPGRP, " "))[0]
13341
>>> buf = array.array('h', [0])
>>> fcntl.ioctl(0, termios.TIOCGPGRP, buf, 1)
0
>>> buf
array('h', [13341])
</pre></div>
</dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-3028' xml:id='l2h-3028' class="function">flock</tt></b>(</nobr></td>
<td><var>fd, op</var>)</td></tr></table></dt>
<dd>
Perform the lock operation <var>op</var> on file descriptor <var>fd</var> (file
objects providing a <tt class="method">fileno()</tt> method are accepted as well).
See the <span class="Unix">Unix</span> manual <span class="manpage"><i>flock</i>(3)</span> for details. (On some
systems, this function is emulated using <tt class="cfunction">fcntl()</tt>.)
</dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
<td><nobr><b><tt id='l2h-3029' xml:id='l2h-3029' class="function">lockf</tt></b>(</nobr></td>
<td><var>fd, operation,
</var><big>[</big><var>length, </var><big>[</big><var>start, </var><big>[</big><var>whence</var><big>]</big><var></var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
This is essentially a wrapper around the <tt class="function">fcntl()</tt> locking
calls. <var>fd</var> is the file descriptor of the file to lock or unlock,
and <var>operation</var> is one of the following values:
<P>
<UL>
<LI><tt class="constant">LOCK_UN</tt> - unlock
</LI>
<LI><tt class="constant">LOCK_SH</tt> - acquire a shared lock
</LI>
<LI><tt class="constant">LOCK_EX</tt> - acquire an exclusive lock
</LI>
</UL>
<P>
When <var>operation</var> is <tt class="constant">LOCK_SH</tt> or <tt class="constant">LOCK_EX</tt>, it
can also be bit-wise OR'd with <tt class="constant">LOCK_NB</tt> to avoid blocking on
lock acquisition. If <tt class="constant">LOCK_NB</tt> is used and the lock cannot
be acquired, an <tt class="exception">IOError</tt> will be raised and the exception
will have an <var>errno</var> attribute set to <tt class="constant">EACCES</tt> or
<tt class="constant">EAGAIN</tt> (depending on the operating system; for portability,
check for both values). On at least some systems, <tt class="constant">LOCK_EX</tt>
can only be used if the file descriptor refers to a file opened for
writing.
<P>
<var>length</var> is the number of bytes to lock, <var>start</var> is the byte
offset at which the lock starts, relative to <var>whence</var>, and
<var>whence</var> is as with <tt class="function">fileobj.seek()</tt>, specifically:
<P>
<UL>
<LI><tt class="constant">0</tt> - relative to the start of the file
(<tt class="constant">SEEK_SET</tt>)
</LI>
<LI><tt class="constant">1</tt> - relative to the current buffer position
(<tt class="constant">SEEK_CUR</tt>)
</LI>
<LI><tt class="constant">2</tt> - relative to the end of the file
(<tt class="constant">SEEK_END</tt>)
</LI>
</UL>
<P>
The default for <var>start</var> is 0, which means to start at the
beginning of the file. The default for <var>length</var> is 0 which means
to lock to the end of the file. The default for <var>whence</var> is also
0.
</dl>
<P>
Examples (all on a SVR4 compliant system):
<P>
<div class="verbatim"><pre>
import struct, fcntl, os
f = open(...)
rv = fcntl.fcntl(f, fcntl.F_SETFL, os.O_NDELAY)
lockdata = struct.pack('hhllhh', fcntl.F_WRLCK, 0, 0, 0, 0, 0)
rv = fcntl.fcntl(f, fcntl.F_SETLKW, lockdata)
</pre></div>
<P>
Note that in the first example the return value variable <var>rv</var> will
hold an integer value; in the second example it will hold a string
value. The structure lay-out for the <var>lockdata</var> variable is
system dependent -- therefore using the <tt class="function">flock()</tt> call may be
better.
<P>
<div class="seealso">
<p class="heading">See Also:</p>
<dl compact="compact" class="seemodule">
<dt>Module <b><tt class="module"><a href="module-os.html">os</a></tt>:</b>
<dd>The <tt class="function">os.open()</tt> function supports locking flags
and is available on a wider variety of platforms than
the <tt class="function">lockf()</tt> and <tt class="function">flock()</tt>
functions, providing a more platform-independent file
locking facility.
</dl>
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