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<li class="nav-item nav-item-this"><a href=""><code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> — The <code class="docutils literal notranslate"><span class="pre">fcntl</span></code> and <code class="docutils literal notranslate"><span class="pre">ioctl</span></code> system calls</a></li>
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<section id="module-fcntl">
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<span id="fcntl-the-fcntl-and-ioctl-system-calls"></span><h1><code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> — The <code class="docutils literal notranslate"><span class="pre">fcntl</span></code> and <code class="docutils literal notranslate"><span class="pre">ioctl</span></code> system calls<a class="headerlink" href="#module-fcntl" title="Link to this heading">¶</a></h1>
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<hr class="docutils" id="index-0" />
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<p>This module performs file and I/O control on file descriptors. It is an
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interface to the <code class="xref c c-func docutils literal notranslate"><span class="pre">fcntl()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">ioctl()</span></code> Unix routines.
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See the <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/fcntl(2)">fcntl(2)</a></em> and <em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/ioctl(2)">ioctl(2)</a></em> Unix manual pages
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for full details.</p>
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<div class="availability docutils container">
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<p><a class="reference internal" href="intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix, not WASI.</p>
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</div>
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<p>All functions in this module take a file descriptor <em>fd</em> as their first
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argument. This can be an integer file descriptor, such as returned by
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<code class="docutils literal notranslate"><span class="pre">sys.stdin.fileno()</span></code>, or an <a class="reference internal" href="io.html#io.IOBase" title="io.IOBase"><code class="xref py py-class docutils literal notranslate"><span class="pre">io.IOBase</span></code></a> object, such as <code class="docutils literal notranslate"><span class="pre">sys.stdin</span></code>
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||
itself, which provides a <a class="reference internal" href="io.html#io.IOBase.fileno" title="io.IOBase.fileno"><code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code></a> that returns a genuine file
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descriptor.</p>
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||
<div class="versionchanged">
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||
<p><span class="versionmodified changed">Changed in version 3.3: </span>Operations in this module used to raise an <a class="reference internal" href="exceptions.html#IOError" title="IOError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">IOError</span></code></a> where they now
|
||
raise an <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a>.</p>
|
||
</div>
|
||
<div class="versionchanged">
|
||
<p><span class="versionmodified changed">Changed in version 3.8: </span>The <code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> module now contains <code class="docutils literal notranslate"><span class="pre">F_ADD_SEALS</span></code>, <code class="docutils literal notranslate"><span class="pre">F_GET_SEALS</span></code>, and
|
||
<code class="docutils literal notranslate"><span class="pre">F_SEAL_*</span></code> constants for sealing of <a class="reference internal" href="os.html#os.memfd_create" title="os.memfd_create"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.memfd_create()</span></code></a> file
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descriptors.</p>
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||
</div>
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||
<div class="versionchanged">
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||
<p><span class="versionmodified changed">Changed in version 3.9: </span>On macOS, the <code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> module exposes the <code class="docutils literal notranslate"><span class="pre">F_GETPATH</span></code> constant,
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which obtains the path of a file from a file descriptor.
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On Linux(>=3.15), the <code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> module exposes the <code class="docutils literal notranslate"><span class="pre">F_OFD_GETLK</span></code>,
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<code class="docutils literal notranslate"><span class="pre">F_OFD_SETLK</span></code> and <code class="docutils literal notranslate"><span class="pre">F_OFD_SETLKW</span></code> constants, which are used when working
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||
with open file description locks.</p>
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||
</div>
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||
<div class="versionchanged">
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||
<p><span class="versionmodified changed">Changed in version 3.10: </span>On Linux >= 2.6.11, the <code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> module exposes the <code class="docutils literal notranslate"><span class="pre">F_GETPIPE_SZ</span></code> and
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<code class="docutils literal notranslate"><span class="pre">F_SETPIPE_SZ</span></code> constants, which allow to check and modify a pipe’s size
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||
respectively.</p>
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||
</div>
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||
<div class="versionchanged">
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||
<p><span class="versionmodified changed">Changed in version 3.11: </span>On FreeBSD, the <code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> module exposes the <code class="docutils literal notranslate"><span class="pre">F_DUP2FD</span></code> and
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<code class="docutils literal notranslate"><span class="pre">F_DUP2FD_CLOEXEC</span></code> constants, which allow to duplicate a file descriptor,
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the latter setting <code class="docutils literal notranslate"><span class="pre">FD_CLOEXEC</span></code> flag in addition.</p>
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</div>
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||
<div class="versionchanged">
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||
<p><span class="versionmodified changed">Changed in version 3.12: </span>On Linux >= 4.5, the <a class="reference internal" href="#module-fcntl" title="fcntl: The fcntl() and ioctl() system calls. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code></a> module exposes the <code class="docutils literal notranslate"><span class="pre">FICLONE</span></code> and
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||
<code class="docutils literal notranslate"><span class="pre">FICLONERANGE</span></code> constants, which allow to share some data of one file with
|
||
another file by reflinking on some filesystems (e.g., btrfs, OCFS2, and
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XFS). This behavior is commonly referred to as “copy-on-write”.</p>
|
||
</div>
|
||
<div class="versionchanged">
|
||
<p><span class="versionmodified changed">Changed in version 3.13: </span>On Linux >= 2.6.32, the <code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> module exposes the
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<code class="docutils literal notranslate"><span class="pre">F_GETOWN_EX</span></code>, <code class="docutils literal notranslate"><span class="pre">F_SETOWN_EX</span></code>, <code class="docutils literal notranslate"><span class="pre">F_OWNER_TID</span></code>, <code class="docutils literal notranslate"><span class="pre">F_OWNER_PID</span></code>, <code class="docutils literal notranslate"><span class="pre">F_OWNER_PGRP</span></code> constants, which allow to direct I/O availability signals
|
||
to a specific thread, process, or process group.
|
||
On Linux >= 4.13, the <code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> module exposes the
|
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<code class="docutils literal notranslate"><span class="pre">F_GET_RW_HINT</span></code>, <code class="docutils literal notranslate"><span class="pre">F_SET_RW_HINT</span></code>, <code class="docutils literal notranslate"><span class="pre">F_GET_FILE_RW_HINT</span></code>,
|
||
<code class="docutils literal notranslate"><span class="pre">F_SET_FILE_RW_HINT</span></code>, and <code class="docutils literal notranslate"><span class="pre">RWH_WRITE_LIFE_*</span></code> constants, which allow
|
||
to inform the kernel about the relative expected lifetime of writes on
|
||
a given inode or via a particular open file description.
|
||
On Linux >= 5.1 and NetBSD, the <code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> module exposes the
|
||
<code class="docutils literal notranslate"><span class="pre">F_SEAL_FUTURE_WRITE</span></code> constant for use with <code class="docutils literal notranslate"><span class="pre">F_ADD_SEALS</span></code> and
|
||
<code class="docutils literal notranslate"><span class="pre">F_GET_SEALS</span></code> operations.
|
||
On FreeBSD, the <code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> module exposes the <code class="docutils literal notranslate"><span class="pre">F_READAHEAD</span></code>, <code class="docutils literal notranslate"><span class="pre">F_ISUNIONSTACK</span></code>, and <code class="docutils literal notranslate"><span class="pre">F_KINFO</span></code> constants.
|
||
On macOS and FreeBSD, the <code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> module exposes the <code class="docutils literal notranslate"><span class="pre">F_RDAHEAD</span></code>
|
||
constant.
|
||
On NetBSD and AIX, the <code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> module exposes the <code class="docutils literal notranslate"><span class="pre">F_CLOSEM</span></code>
|
||
constant.
|
||
On NetBSD, the <code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> module exposes the <code class="docutils literal notranslate"><span class="pre">F_MAXFD</span></code> constant.
|
||
On macOS and NetBSD, the <code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code> module exposes the <code class="docutils literal notranslate"><span class="pre">F_GETNOSIGPIPE</span></code>
|
||
and <code class="docutils literal notranslate"><span class="pre">F_SETNOSIGPIPE</span></code> constant.</p>
|
||
</div>
|
||
<p>The module defines the following functions:</p>
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="fcntl.fcntl">
|
||
<span class="sig-prename descclassname"><span class="pre">fcntl.</span></span><span class="sig-name descname"><span class="pre">fcntl</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">fd</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">cmd</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">arg</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">0</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#fcntl.fcntl" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Perform the operation <em>cmd</em> on file descriptor <em>fd</em> (file objects providing
|
||
a <a class="reference internal" href="io.html#io.IOBase.fileno" title="io.IOBase.fileno"><code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code></a> method are accepted as well). The values used
|
||
for <em>cmd</em> are operating system dependent, and are available as constants
|
||
in the <a class="reference internal" href="#module-fcntl" title="fcntl: The fcntl() and ioctl() system calls. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">fcntl</span></code></a> module, using the same names as used in the relevant C
|
||
header files. The argument <em>arg</em> can either be an integer value, or a
|
||
<a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> object. With an integer value, the return value of this
|
||
function is the integer return value of the C <code class="xref c c-func docutils literal notranslate"><span class="pre">fcntl()</span></code> call. When
|
||
the argument is bytes it represents a binary structure, e.g. created by
|
||
<a class="reference internal" href="struct.html#struct.pack" title="struct.pack"><code class="xref py py-func docutils literal notranslate"><span class="pre">struct.pack()</span></code></a>. The binary data is copied to a buffer whose address is
|
||
passed to the C <code class="xref c c-func docutils literal notranslate"><span class="pre">fcntl()</span></code> call. The return value after a successful
|
||
call is the contents of the buffer, converted to a <a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> object.
|
||
The length of the returned object 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 <code class="xref c c-func docutils literal notranslate"><span class="pre">fcntl()</span></code> call fails, an <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a> is raised.</p>
|
||
<p class="audit-hook">Raises an <a class="reference internal" href="sys.html#auditing"><span class="std std-ref">auditing event</span></a> <code class="docutils literal notranslate"><span class="pre">fcntl.fcntl</span></code> with arguments <code class="docutils literal notranslate"><span class="pre">fd</span></code>, <code class="docutils literal notranslate"><span class="pre">cmd</span></code>, <code class="docutils literal notranslate"><span class="pre">arg</span></code>.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="fcntl.ioctl">
|
||
<span class="sig-prename descclassname"><span class="pre">fcntl.</span></span><span class="sig-name descname"><span class="pre">ioctl</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">fd</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">request</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">arg</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">0</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">mutate_flag</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">True</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#fcntl.ioctl" title="Link to this definition">¶</a></dt>
|
||
<dd><p>This function is identical to the <a class="reference internal" href="#fcntl.fcntl" title="fcntl.fcntl"><code class="xref py py-func docutils literal notranslate"><span class="pre">fcntl()</span></code></a> function, except
|
||
that the argument handling is even more complicated.</p>
|
||
<p>The <em>request</em> parameter is limited to values that can fit in 32-bits.
|
||
Additional constants of interest for use as the <em>request</em> argument can be
|
||
found in the <a class="reference internal" href="termios.html#module-termios" title="termios: POSIX style tty control. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">termios</span></code></a> module, under the same names as used in
|
||
the relevant C header files.</p>
|
||
<p>The parameter <em>arg</em> can be one of an integer, an object supporting the
|
||
read-only buffer interface (like <a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a>) or an object supporting
|
||
the read-write buffer interface (like <a class="reference internal" href="stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytearray</span></code></a>).</p>
|
||
<p>In all but the last case, behaviour is as for the <a class="reference internal" href="#fcntl.fcntl" title="fcntl.fcntl"><code class="xref py py-func docutils literal notranslate"><span class="pre">fcntl()</span></code></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 at least as long as what the operating system
|
||
wants to put there, things should work.</p>
|
||
<p>If <em>mutate_flag</em> is true (the default), then the buffer is (in effect) passed
|
||
to the underlying <a class="reference internal" href="#fcntl.ioctl" title="fcntl.ioctl"><code class="xref py py-func docutils literal notranslate"><span class="pre">ioctl()</span></code></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 class="reference internal" href="#fcntl.ioctl" title="fcntl.ioctl"><code class="xref py py-func docutils literal notranslate"><span class="pre">ioctl()</span></code></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 class="reference internal" href="#fcntl.ioctl" title="fcntl.ioctl"><code class="xref py py-func docutils literal notranslate"><span class="pre">ioctl()</span></code></a> and copied back
|
||
into the supplied buffer.</p>
|
||
<p>If the <code class="xref c c-func docutils literal notranslate"><span class="pre">ioctl()</span></code> call fails, an <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a> exception is raised.</p>
|
||
<p>An example:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span><span class="w"> </span><span class="nn">array</span><span class="o">,</span><span class="w"> </span><span class="nn">fcntl</span><span class="o">,</span><span class="w"> </span><span class="nn">struct</span><span class="o">,</span><span class="w"> </span><span class="nn">termios</span><span class="o">,</span><span class="w"> </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="s1">'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="s2">" "</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="s1">'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>
|
||
<p class="audit-hook">Raises an <a class="reference internal" href="sys.html#auditing"><span class="std std-ref">auditing event</span></a> <code class="docutils literal notranslate"><span class="pre">fcntl.ioctl</span></code> with arguments <code class="docutils literal notranslate"><span class="pre">fd</span></code>, <code class="docutils literal notranslate"><span class="pre">request</span></code>, <code class="docutils literal notranslate"><span class="pre">arg</span></code>.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="fcntl.flock">
|
||
<span class="sig-prename descclassname"><span class="pre">fcntl.</span></span><span class="sig-name descname"><span class="pre">flock</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">fd</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">operation</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#fcntl.flock" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Perform the lock operation <em>operation</em> on file descriptor <em>fd</em> (file objects providing
|
||
a <a class="reference internal" href="io.html#io.IOBase.fileno" title="io.IOBase.fileno"><code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code></a> method are accepted as well). See the Unix manual
|
||
<em class="manpage"><a class="manpage reference external" href="https://manpages.debian.org/flock(2)">flock(2)</a></em> for details. (On some systems, this function is emulated
|
||
using <code class="xref c c-func docutils literal notranslate"><span class="pre">fcntl()</span></code>.)</p>
|
||
<p>If the <code class="xref c c-func docutils literal notranslate"><span class="pre">flock()</span></code> call fails, an <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a> exception is raised.</p>
|
||
<p class="audit-hook">Raises an <a class="reference internal" href="sys.html#auditing"><span class="std std-ref">auditing event</span></a> <code class="docutils literal notranslate"><span class="pre">fcntl.flock</span></code> with arguments <code class="docutils literal notranslate"><span class="pre">fd</span></code>, <code class="docutils literal notranslate"><span class="pre">operation</span></code>.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="fcntl.lockf">
|
||
<span class="sig-prename descclassname"><span class="pre">fcntl.</span></span><span class="sig-name descname"><span class="pre">lockf</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">fd</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">cmd</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">len</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">0</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">start</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">0</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">whence</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">0</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#fcntl.lockf" title="Link to this definition">¶</a></dt>
|
||
<dd><p>This is essentially a wrapper around the <a class="reference internal" href="#fcntl.fcntl" title="fcntl.fcntl"><code class="xref py py-func docutils literal notranslate"><span class="pre">fcntl()</span></code></a> locking calls.
|
||
<em>fd</em> is the file descriptor (file objects providing a <a class="reference internal" href="io.html#io.IOBase.fileno" title="io.IOBase.fileno"><code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code></a>
|
||
method are accepted as well) of the file to lock or unlock, and <em>cmd</em>
|
||
is one of the following values:</p>
|
||
<dl class="py data">
|
||
<dt class="sig sig-object py" id="fcntl.LOCK_UN">
|
||
<span class="sig-prename descclassname"><span class="pre">fcntl.</span></span><span class="sig-name descname"><span class="pre">LOCK_UN</span></span><a class="headerlink" href="#fcntl.LOCK_UN" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Release an existing lock.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py data">
|
||
<dt class="sig sig-object py" id="fcntl.LOCK_SH">
|
||
<span class="sig-prename descclassname"><span class="pre">fcntl.</span></span><span class="sig-name descname"><span class="pre">LOCK_SH</span></span><a class="headerlink" href="#fcntl.LOCK_SH" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Acquire a shared lock.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py data">
|
||
<dt class="sig sig-object py" id="fcntl.LOCK_EX">
|
||
<span class="sig-prename descclassname"><span class="pre">fcntl.</span></span><span class="sig-name descname"><span class="pre">LOCK_EX</span></span><a class="headerlink" href="#fcntl.LOCK_EX" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Acquire an exclusive lock.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py data">
|
||
<dt class="sig sig-object py" id="fcntl.LOCK_NB">
|
||
<span class="sig-prename descclassname"><span class="pre">fcntl.</span></span><span class="sig-name descname"><span class="pre">LOCK_NB</span></span><a class="headerlink" href="#fcntl.LOCK_NB" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Bitwise OR with any of the other three <code class="docutils literal notranslate"><span class="pre">LOCK_*</span></code> constants to make
|
||
the request non-blocking.</p>
|
||
</dd></dl>
|
||
|
||
<p>If <code class="xref py py-const docutils literal notranslate"><span class="pre">LOCK_NB</span></code> is used and the lock cannot be acquired, an
|
||
<a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a> will be raised and the exception will have an <em>errno</em>
|
||
attribute set to <a class="reference internal" href="errno.html#errno.EACCES" title="errno.EACCES"><code class="xref py py-const docutils literal notranslate"><span class="pre">EACCES</span></code></a> or <a class="reference internal" href="errno.html#errno.EAGAIN" title="errno.EAGAIN"><code class="xref py py-const docutils literal notranslate"><span class="pre">EAGAIN</span></code></a> (depending on the
|
||
operating system; for portability, check for both values). On at least some
|
||
systems, <code class="xref py py-const docutils literal notranslate"><span class="pre">LOCK_EX</span></code> can only be used if the file descriptor refers to a
|
||
file opened for writing.</p>
|
||
<p><em>len</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
|
||
<a class="reference internal" href="io.html#io.IOBase.seek" title="io.IOBase.seek"><code class="xref py py-func docutils literal notranslate"><span class="pre">io.IOBase.seek()</span></code></a>, specifically:</p>
|
||
<ul class="simple">
|
||
<li><p><code class="docutils literal notranslate"><span class="pre">0</span></code> – relative to the start of the file (<a class="reference internal" href="os.html#os.SEEK_SET" title="os.SEEK_SET"><code class="xref py py-const docutils literal notranslate"><span class="pre">os.SEEK_SET</span></code></a>)</p></li>
|
||
<li><p><code class="docutils literal notranslate"><span class="pre">1</span></code> – relative to the current buffer position (<a class="reference internal" href="os.html#os.SEEK_CUR" title="os.SEEK_CUR"><code class="xref py py-const docutils literal notranslate"><span class="pre">os.SEEK_CUR</span></code></a>)</p></li>
|
||
<li><p><code class="docutils literal notranslate"><span class="pre">2</span></code> – relative to the end of the file (<a class="reference internal" href="os.html#os.SEEK_END" title="os.SEEK_END"><code class="xref py py-const docutils literal notranslate"><span class="pre">os.SEEK_END</span></code></a>)</p></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>len</em> is 0 which means to lock to the end of the file. The
|
||
default for <em>whence</em> is also 0.</p>
|
||
<p class="audit-hook">Raises an <a class="reference internal" href="sys.html#auditing"><span class="std std-ref">auditing event</span></a> <code class="docutils literal notranslate"><span class="pre">fcntl.lockf</span></code> with arguments <code class="docutils literal notranslate"><span class="pre">fd</span></code>, <code class="docutils literal notranslate"><span class="pre">cmd</span></code>, <code class="docutils literal notranslate"><span class="pre">len</span></code>, <code class="docutils literal notranslate"><span class="pre">start</span></code>, <code class="docutils literal notranslate"><span class="pre">whence</span></code>.</p>
|
||
</dd></dl>
|
||
|
||
<p>Examples (all on a SVR4 compliant system):</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">struct</span><span class="o">,</span><span class="w"> </span><span class="nn">fcntl</span><span class="o">,</span><span class="w"> </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="s1">'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 <a class="reference internal" href="stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> object. The
|
||
structure lay-out for the <em>lockdata</em> variable is system dependent — therefore
|
||
using the <a class="reference internal" href="#fcntl.flock" title="fcntl.flock"><code class="xref py py-func docutils literal notranslate"><span class="pre">flock()</span></code></a> call may be better.</p>
|
||
<div class="admonition seealso">
|
||
<p class="admonition-title">See also</p>
|
||
<dl class="simple">
|
||
<dt>Module <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate"><span class="pre">os</span></code></a></dt><dd><p>If the locking flags <a class="reference internal" href="os.html#os.O_SHLOCK" title="os.O_SHLOCK"><code class="xref py py-const docutils literal notranslate"><span class="pre">O_SHLOCK</span></code></a> and <a class="reference internal" href="os.html#os.O_EXLOCK" title="os.O_EXLOCK"><code class="xref py py-const docutils literal notranslate"><span class="pre">O_EXLOCK</span></code></a> are
|
||
present in the <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate"><span class="pre">os</span></code></a> module (on BSD only), the <a class="reference internal" href="os.html#os.open" title="os.open"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.open()</span></code></a>
|
||
function provides an alternative to the <a class="reference internal" href="#fcntl.lockf" title="fcntl.lockf"><code class="xref py py-func docutils literal notranslate"><span class="pre">lockf()</span></code></a> and <a class="reference internal" href="#fcntl.flock" title="fcntl.flock"><code class="xref py py-func docutils literal notranslate"><span class="pre">flock()</span></code></a>
|
||
functions.</p>
|
||
</dd>
|
||
</dl>
|
||
</div>
|
||
</section>
|
||
|
||
|
||
<div class="clearer"></div>
|
||
</div>
|
||
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|
||
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|
||
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|
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||
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|
||
<h4>Next topic</h4>
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<p class="topless"><a href="resource.html"
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