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Unix Specific Services</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="module-fcntl"> <h1>35.10. <tt class="xref docutils literal"><span class="pre">fcntl</span></tt> — The <tt class="xref docutils literal"><span class="pre">fcntl()</span></tt> and <tt class="xref docutils literal"><span class="pre">ioctl()</span></tt> system calls<a class="headerlink" href="#module-fcntl" title="Permalink to this headline">¶</a></h1> <p><em>Platforms: </em>Unix</p> <p id="index-333">This module performs file control and I/O control on file descriptors. It is an interface to the <tt class="xref docutils literal"><span class="pre">fcntl()</span></tt> and <tt class="xref docutils literal"><span class="pre">ioctl()</span></tt> Unix routines.</p> <p>All functions in this module take a file descriptor <em>fd</em> as their first argument. This can be an integer file descriptor, such as returned by <tt class="docutils literal"><span class="pre">sys.stdin.fileno()</span></tt>, or a file object, such as <tt class="docutils literal"><span class="pre">sys.stdin</span></tt> itself, which provides a <tt class="xref docutils literal"><span class="pre">fileno()</span></tt> which returns a genuine file descriptor.</p> <p>The module defines the following functions:</p> <dl class="function"> <dt id="fcntl.fcntl"> <tt class="descclassname">fcntl.</tt><tt class="descname">fcntl</tt><big>(</big><em>fd</em>, <em>op</em><span class="optional">[</span>, <em>arg</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#fcntl.fcntl" title="Permalink to this definition">¶</a></dt> <dd><p>Perform the requested operation on file descriptor <em>fd</em> (file objects providing a <tt class="xref docutils literal"><span class="pre">fileno()</span></tt> method are accepted as well). The operation is defined by <em>op</em> and is operating system dependent. These codes are also found in the <tt class="xref docutils literal"><span class="pre">fcntl</span></tt> module. The argument <em>arg</em> is optional, and defaults to the integer value <tt class="docutils literal"><span class="pre">0</span></tt>. 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="xref docutils literal"><span class="pre">fcntl()</span></tt> call. When the argument is a string it represents a binary structure, e.g. created by <a title="struct.pack" class="reference external" href="struct.html#struct.pack"><tt class="xref docutils literal"><span class="pre">struct.pack()</span></tt></a>. The binary data is copied to a buffer whose address is passed to the C <tt class="xref docutils literal"><span class="pre">fcntl()</span></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 <em>arg</em> 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> <p>If the <tt class="xref docutils literal"><span class="pre">fcntl()</span></tt> fails, an <a title="exceptions.IOError" class="reference external" href="exceptions.html#exceptions.IOError"><tt class="xref docutils literal"><span class="pre">IOError</span></tt></a> is raised.</p> </dd></dl> <dl class="function"> <dt id="fcntl.ioctl"> <tt class="descclassname">fcntl.</tt><tt class="descname">ioctl</tt><big>(</big><em>fd</em>, <em>op</em><span class="optional">[</span>, <em>arg</em><span class="optional">[</span>, <em>mutate_flag</em><span class="optional">]</span><span class="optional">]</span><big>)</big><a class="headerlink" href="#fcntl.ioctl" title="Permalink to this definition">¶</a></dt> <dd><p>This function is identical to the <a title="fcntl.fcntl" class="reference internal" href="#fcntl.fcntl"><tt class="xref docutils literal"><span class="pre">fcntl()</span></tt></a> function, except that the operations are typically defined in the library module <a title="(Unix) POSIX style tty control." class="reference external" href="termios.html#module-termios"><tt class="xref docutils literal"><span class="pre">termios</span></tt></a> and the argument handling is even more complicated.</p> <p>The op parameter is limited to values that can fit in 32-bits.</p> <p>The parameter <em>arg</em> can be one of an integer, absent (treated identically to the integer <tt class="docutils literal"><span class="pre">0</span></tt>), an object supporting the read-only buffer interface (most likely a plain Python string) or an object supporting the read-write buffer interface.</p> <p>In all but the last case, behaviour is as for the <a title="fcntl.fcntl" class="reference internal" href="#fcntl.fcntl"><tt class="xref docutils literal"><span class="pre">fcntl()</span></tt></a> function.</p> <p>If a mutable buffer is passed, then the behaviour is determined by the value of the <em>mutate_flag</em> parameter.</p> <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 as least as long as what the operating system wants to put there, things should work.</p> <p>If <em>mutate_flag</em> is true, then the buffer is (in effect) passed to the underlying <a title="fcntl.ioctl" class="reference internal" href="#fcntl.ioctl"><tt class="xref docutils literal"><span class="pre">ioctl()</span></tt></a> 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 <a title="fcntl.ioctl" class="reference internal" href="#fcntl.ioctl"><tt class="xref docutils literal"><span class="pre">ioctl()</span></tt></a>. 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 <a title="fcntl.ioctl" class="reference internal" href="#fcntl.ioctl"><tt class="xref docutils literal"><span class="pre">ioctl()</span></tt></a> and copied back into the supplied buffer.</p> <p>If <em>mutate_flag</em> is not supplied, then from Python 2.5 it defaults to true, which is a change from versions 2.3 and 2.4. Supply the argument explicitly if version portability is a priority.</p> <p>An example:</p> <div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">array</span><span class="o">,</span> <span class="nn">fcntl</span><span class="o">,</span> <span class="nn">struct</span><span class="o">,</span> <span class="nn">termios</span><span class="o">,</span> <span class="nn">os</span> <span class="gp">>>> </span><span class="n">os</span><span class="o">.</span><span class="n">getpgrp</span><span class="p">()</span> <span class="go">13341</span> <span class="gp">>>> </span><span class="n">struct</span><span class="o">.</span><span class="n">unpack</span><span class="p">(</span><span class="s">'h'</span><span class="p">,</span> <span class="n">fcntl</span><span class="o">.</span><span class="n">ioctl</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">termios</span><span class="o">.</span><span class="n">TIOCGPGRP</span><span class="p">,</span> <span class="s">" "</span><span class="p">))[</span><span class="mi">0</span><span class="p">]</span> <span class="go">13341</span> <span class="gp">>>> </span><span class="n">buf</span> <span class="o">=</span> <span class="n">array</span><span class="o">.</span><span class="n">array</span><span class="p">(</span><span class="s">'h'</span><span class="p">,</span> <span class="p">[</span><span class="mi">0</span><span class="p">])</span> <span class="gp">>>> </span><span class="n">fcntl</span><span class="o">.</span><span class="n">ioctl</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">termios</span><span class="o">.</span><span class="n">TIOCGPGRP</span><span class="p">,</span> <span class="n">buf</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span> <span class="go">0</span> <span class="gp">>>> </span><span class="n">buf</span> <span class="go">array('h', [13341])</span> </pre></div> </div> </dd></dl> <dl class="function"> <dt id="fcntl.flock"> <tt class="descclassname">fcntl.</tt><tt class="descname">flock</tt><big>(</big><em>fd</em>, <em>op</em><big>)</big><a class="headerlink" href="#fcntl.flock" title="Permalink to this definition">¶</a></dt> <dd>Perform the lock operation <em>op</em> on file descriptor <em>fd</em> (file objects providing a <tt class="xref docutils literal"><span class="pre">fileno()</span></tt> method are accepted as well). See the Unix manual <em>flock(2)</em> for details. (On some systems, this function is emulated using <tt class="xref docutils literal"><span class="pre">fcntl()</span></tt>.)</dd></dl> <dl class="function"> <dt id="fcntl.lockf"> <tt class="descclassname">fcntl.</tt><tt class="descname">lockf</tt><big>(</big><em>fd</em>, <em>operation</em><span class="optional">[</span>, <em>length</em><span class="optional">[</span>, <em>start</em><span class="optional">[</span>, <em>whence</em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><big>)</big><a class="headerlink" href="#fcntl.lockf" title="Permalink to this definition">¶</a></dt> <dd><p>This is essentially a wrapper around the <a title="fcntl.fcntl" class="reference internal" href="#fcntl.fcntl"><tt class="xref docutils literal"><span class="pre">fcntl()</span></tt></a> locking calls. <em>fd</em> is the file descriptor of the file to lock or unlock, and <em>operation</em> is one of the following values:</p> <ul class="simple"> <li><tt class="xref docutils literal"><span class="pre">LOCK_UN</span></tt> – unlock</li> <li><tt class="xref docutils literal"><span class="pre">LOCK_SH</span></tt> – acquire a shared lock</li> <li><tt class="xref docutils literal"><span class="pre">LOCK_EX</span></tt> – acquire an exclusive lock</li> </ul> <p>When <em>operation</em> is <tt class="xref docutils literal"><span class="pre">LOCK_SH</span></tt> or <tt class="xref docutils literal"><span class="pre">LOCK_EX</span></tt>, it can also be bitwise ORed with <tt class="xref docutils literal"><span class="pre">LOCK_NB</span></tt> to avoid blocking on lock acquisition. If <tt class="xref docutils literal"><span class="pre">LOCK_NB</span></tt> is used and the lock cannot be acquired, an <a title="exceptions.IOError" class="reference external" href="exceptions.html#exceptions.IOError"><tt class="xref docutils literal"><span class="pre">IOError</span></tt></a> will be raised and the exception will have an <em>errno</em> attribute set to <tt class="xref docutils literal"><span class="pre">EACCES</span></tt> or <tt class="xref docutils literal"><span class="pre">EAGAIN</span></tt> (depending on the operating system; for portability, check for both values). On at least some systems, <tt class="xref docutils literal"><span class="pre">LOCK_EX</span></tt> can only be used if the file descriptor refers to a file opened for writing.</p> <p><em>length</em> is the number of bytes to lock, <em>start</em> is the byte offset at which the lock starts, relative to <em>whence</em>, and <em>whence</em> is as with <tt class="xref docutils literal"><span class="pre">fileobj.seek()</span></tt>, specifically:</p> <ul class="simple"> <li><tt class="xref docutils literal"><span class="pre">0</span></tt> – relative to the start of the file (<tt class="xref docutils literal"><span class="pre">SEEK_SET</span></tt>)</li> <li><tt class="xref docutils literal"><span class="pre">1</span></tt> – relative to the current buffer position (<tt class="xref docutils literal"><span class="pre">SEEK_CUR</span></tt>)</li> <li><tt class="xref docutils literal"><span class="pre">2</span></tt> – relative to the end of the file (<tt class="xref docutils literal"><span class="pre">SEEK_END</span></tt>)</li> </ul> <p>The default for <em>start</em> is 0, which means to start at the beginning of the file. The default for <em>length</em> is 0 which means to lock to the end of the file. The default for <em>whence</em> is also 0.</p> </dd></dl> <p>Examples (all on a SVR4 compliant system):</p> <div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">struct</span><span class="o">,</span> <span class="nn">fcntl</span><span class="o">,</span> <span class="nn">os</span> <span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="o">...</span><span class="p">)</span> <span class="n">rv</span> <span class="o">=</span> <span class="n">fcntl</span><span class="o">.</span><span class="n">fcntl</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="n">fcntl</span><span class="o">.</span><span class="n">F_SETFL</span><span class="p">,</span> <span class="n">os</span><span class="o">.</span><span class="n">O_NDELAY</span><span class="p">)</span> <span class="n">lockdata</span> <span class="o">=</span> <span class="n">struct</span><span class="o">.</span><span class="n">pack</span><span class="p">(</span><span class="s">'hhllhh'</span><span class="p">,</span> <span class="n">fcntl</span><span class="o">.</span><span class="n">F_WRLCK</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span> <span class="n">rv</span> <span class="o">=</span> <span class="n">fcntl</span><span class="o">.</span><span class="n">fcntl</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="n">fcntl</span><span class="o">.</span><span class="n">F_SETLKW</span><span class="p">,</span> <span class="n">lockdata</span><span class="p">)</span> </pre></div> </div> <p>Note that in the first example the return value variable <em>rv</em> will hold an integer value; in the second example it will hold a string value. The structure lay-out for the <em>lockdata</em> variable is system dependent — therefore using the <a title="fcntl.flock" class="reference internal" href="#fcntl.flock"><tt class="xref docutils literal"><span class="pre">flock()</span></tt></a> call may be better.</p> <div class="admonition-see-also admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt>Module <a title="Miscellaneous operating system interfaces." class="reference external" href="os.html#module-os"><tt class="xref docutils literal"><span class="pre">os</span></tt></a></dt> <dd>If the locking flags <tt class="xref docutils literal"><span class="pre">O_SHLOCK</span></tt> and <tt class="xref docutils literal"><span class="pre">O_EXLOCK</span></tt> are present in the <a title="Miscellaneous operating system interfaces." class="reference external" href="os.html#module-os"><tt class="xref docutils literal"><span class="pre">os</span></tt></a> module (on BSD only), the <a title="os.open" class="reference external" href="os.html#os.open"><tt class="xref docutils literal"><span class="pre">os.open()</span></tt></a> function provides an alternative to the <a title="fcntl.lockf" class="reference internal" href="#fcntl.lockf"><tt class="xref docutils literal"><span class="pre">lockf()</span></tt></a> and <a title="fcntl.flock" class="reference internal" href="#fcntl.flock"><tt class="xref docutils literal"><span class="pre">flock()</span></tt></a> functions.</dd> </dl> </div> </div> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="pty.html" title="previous chapter">35.9. <tt class="docutils literal"><span class="pre">pty</span></tt> — Pseudo-terminal utilities</a></p> <h4>Next topic</h4> <p class="topless"><a href="pipes.html" title="next chapter">35.11. <tt class="docutils literal"><span class="pre">pipes</span></tt> — Interface to shell pipelines</a></p> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li><a href="../_sources/library/fcntl.txt" rel="nofollow">Show Source</a></li> </ul> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <input type="text" name="q" size="18" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../modindex.html" title="Global Module Index" >modules</a> |</li> <li class="right" > <a href="pipes.html" title="35.11. pipes — Interface to shell pipelines" >next</a> |</li> <li class="right" > <a href="pty.html" title="35.9. pty — Pseudo-terminal utilities" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="../index.html">Python v2.6.6 documentation</a> »</li> <li><a href="index.html" >The Python Standard Library</a> »</li> <li><a href="unix.html" >35. 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