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 <li class="nav-item nav-item-this"><a href=""><code class="xref py py-mod docutils literal notranslate">fcntl</code> — The <code class="docutils literal notranslate">fcntl</code> and <code class="docutils literal notranslate">ioctl</code> system calls</a></li>
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 <section id="module-fcntl">
<h1><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">fcntl</code></a> — The <code class="docutils literal notranslate">fcntl</code> and <code class="docutils literal notranslate">ioctl</code> system calls<a class="headerlink" href="#module-fcntl" title="Link to this heading">¶</a></h1>
<hr class="docutils" id="index-0" />
This module performs file and I/O control on file descriptors. It is an
interface to the <code class="xref c c-func docutils literal notranslate">fcntl()</code> and <code class="xref c c-func docutils literal notranslate">ioctl()</code> Unix routines.
See the <a class="manpage reference external" href="https://manpages.debian.org/fcntl(2)">fcntl(2)</a> and <a class="manpage reference external" href="https://manpages.debian.org/ioctl(2)">ioctl(2)</a> Unix manual pages
for full details.
<div class="availability docutils container">
<a class="reference internal" href="intro.html#availability">Availability</a>: Unix, not Emscripten, not WASI.
</div>
All functions in this module take a file descriptor fd as their first
argument. This can be an integer file descriptor, such as returned by
<code class="docutils literal notranslate">sys.stdin.fileno()</code>, or an <a class="reference internal" href="io.html#io.IOBase" title="io.IOBase"><code class="xref py py-class docutils literal notranslate">io.IOBase</code></a> object, such as <code class="docutils literal notranslate">sys.stdin</code>
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">fileno()</code></a> that returns a genuine file
descriptor.
<div class="versionchanged">
Changed in version 3.3: 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">IOError</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">OSError</code></a>.
</div>
<div class="versionchanged">
Changed in version 3.8: The fcntl module now contains <code class="docutils literal notranslate">F_ADD_SEALS</code>, <code class="docutils literal notranslate">F_GET_SEALS</code>, and
<code class="docutils literal notranslate">F_SEAL_*</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">os.memfd_create()</code></a> file
descriptors.
</div>
<div class="versionchanged">
Changed in version 3.9: On macOS, the fcntl module exposes the <code class="docutils literal notranslate">F_GETPATH</code> constant, which obtains
the path of a file from a file descriptor.
On Linux(&gt;=3.15), the fcntl module exposes the <code class="docutils literal notranslate">F_OFD_GETLK</code>, <code class="docutils literal notranslate">F_OFD_SETLK</code>
and <code class="docutils literal notranslate">F_OFD_SETLKW</code> constants, which are used when working with open file
description locks.
</div>
<div class="versionchanged">
Changed in version 3.10: On Linux &gt;= 2.6.11, the fcntl module exposes the <code class="docutils literal notranslate">F_GETPIPE_SZ</code> and
<code class="docutils literal notranslate">F_SETPIPE_SZ</code> constants, which allow to check and modify a pipe’s size
respectively.
</div>
<div class="versionchanged">
Changed in version 3.11: On FreeBSD, the fcntl module exposes the <code class="docutils literal notranslate">F_DUP2FD</code> and <code class="docutils literal notranslate">F_DUP2FD_CLOEXEC</code>
constants, which allow to duplicate a file descriptor, the latter setting
<code class="docutils literal notranslate">FD_CLOEXEC</code> flag in addition.
</div>
<div class="versionchanged">
Changed in version 3.12: On Linux &gt;= 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">fcntl</code></a> module exposes the <code class="docutils literal notranslate">FICLONE</code> and
<code class="docutils literal notranslate">FICLONERANGE</code> constants, which allow to share some data of one file with
another file by reflinking on some filesystems (e.g., btrfs, OCFS2, and
XFS). This behavior is commonly referred to as “copy-on-write”.
</div>
The module defines the following functions:
<dl class="py function">
<dt class="sig sig-object py" id="fcntl.fcntl">
fcntl.fcntl(fd, cmd, arg=0)<a class="headerlink" href="#fcntl.fcntl" title="Link to this definition">¶</a></dt>
<dd>Perform the operation cmd on file descriptor fd (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">fileno()</code></a> method are accepted as well). The values used
for cmd 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">fcntl</code></a> module, using the same names as used in the relevant C
header files. The argument arg 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">bytes</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">fcntl()</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">struct.pack()</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">fcntl()</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">bytes</code></a> object.
The length of the returned object will be the same as the length of the
arg 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.
If the <code class="xref c c-func docutils literal notranslate">fcntl()</code> call fails, an <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate">OSError</code></a> is raised.
Raises an <a class="reference internal" href="sys.html#auditing">auditing event</a> <code class="docutils literal notranslate">fcntl.fcntl</code> with arguments <code class="docutils literal notranslate">fd</code>, <code class="docutils literal notranslate">cmd</code>, <code class="docutils literal notranslate">arg</code>.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="fcntl.ioctl">
fcntl.ioctl(fd, request, arg=0, mutate_flag=True)<a class="headerlink" href="#fcntl.ioctl" title="Link to this definition">¶</a></dt>
<dd>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">fcntl()</code></a> function, except
that the argument handling is even more complicated.
The request parameter is limited to values that can fit in 32-bits.
Additional constants of interest for use as the request 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">termios</code></a> module, under the same names as used in
the relevant C header files.
The parameter arg 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">bytes</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">bytearray</code></a>).
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">fcntl()</code></a>
function.
If a mutable buffer is passed, then the behaviour is determined by the value of
the mutate_flag parameter.
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.
If mutate_flag 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">ioctl()</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">ioctl()</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">ioctl()</code></a> and copied back
into the supplied buffer.
If the <code class="xref c c-func docutils literal notranslate">ioctl()</code> call fails, an <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate">OSError</code></a> exception is raised.
An example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import array, fcntl, struct, termios, os
&gt;&gt;&gt; os.getpgrp()
13341
&gt;&gt;&gt; struct.unpack(&#39;h&#39;, fcntl.ioctl(0, termios.TIOCGPGRP, &quot; &quot;))[0]
13341
&gt;&gt;&gt; buf = array.array(&#39;h&#39;, [0])
&gt;&gt;&gt; fcntl.ioctl(0, termios.TIOCGPGRP, buf, 1)
0
&gt;&gt;&gt; buf
array(&#39;h&#39;, [13341])
</pre></div>
</div>
Raises an <a class="reference internal" href="sys.html#auditing">auditing event</a> <code class="docutils literal notranslate">fcntl.ioctl</code> with arguments <code class="docutils literal notranslate">fd</code>, <code class="docutils literal notranslate">request</code>, <code class="docutils literal notranslate">arg</code>.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="fcntl.flock">
fcntl.flock(fd, operation)<a class="headerlink" href="#fcntl.flock" title="Link to this definition">¶</a></dt>
<dd>Perform the lock operation operation on file descriptor fd (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">fileno()</code></a> method are accepted as well). See the Unix manual
<a class="manpage reference external" href="https://manpages.debian.org/flock(2)">flock(2)</a> for details. (On some systems, this function is emulated
using <code class="xref c c-func docutils literal notranslate">fcntl()</code>.)
If the <code class="xref c c-func docutils literal notranslate">flock()</code> call fails, an <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate">OSError</code></a> exception is raised.
Raises an <a class="reference internal" href="sys.html#auditing">auditing event</a> <code class="docutils literal notranslate">fcntl.flock</code> with arguments <code class="docutils literal notranslate">fd</code>, <code class="docutils literal notranslate">operation</code>.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="fcntl.lockf">
fcntl.lockf(fd, cmd, len=0, start=0, whence=0)<a class="headerlink" href="#fcntl.lockf" title="Link to this definition">¶</a></dt>
<dd>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">fcntl()</code></a> locking calls.
fd 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">fileno()</code></a>
method are accepted as well) of the file to lock or unlock, and cmd
is one of the following values:
<dl class="py data">
<dt class="sig sig-object py" id="fcntl.LOCK_UN">
fcntl.LOCK_UN<a class="headerlink" href="#fcntl.LOCK_UN" title="Link to this definition">¶</a></dt>
<dd>Release an existing lock.
</dd></dl>

<dl class="py data">
<dt class="sig sig-object py" id="fcntl.LOCK_SH">
fcntl.LOCK_SH<a class="headerlink" href="#fcntl.LOCK_SH" title="Link to this definition">¶</a></dt>
<dd>Acquire a shared lock.
</dd></dl>

<dl class="py data">
<dt class="sig sig-object py" id="fcntl.LOCK_EX">
fcntl.LOCK_EX<a class="headerlink" href="#fcntl.LOCK_EX" title="Link to this definition">¶</a></dt>
<dd>Acquire an exclusive lock.
</dd></dl>

<dl class="py data">
<dt class="sig sig-object py" id="fcntl.LOCK_NB">
fcntl.LOCK_NB<a class="headerlink" href="#fcntl.LOCK_NB" title="Link to this definition">¶</a></dt>
<dd>Bitwise OR with any of the other three <code class="docutils literal notranslate">LOCK_*</code> constants to make
the request non-blocking.
</dd></dl>

If <code class="xref py py-const docutils literal notranslate">LOCK_NB</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">OSError</code></a> will be raised and the exception will have an errno
attribute set to <a class="reference internal" href="errno.html#errno.EACCES" title="errno.EACCES"><code class="xref py py-const docutils literal notranslate">EACCES</code></a> or <a class="reference internal" href="errno.html#errno.EAGAIN" title="errno.EAGAIN"><code class="xref py py-const docutils literal notranslate">EAGAIN</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">LOCK_EX</code> can only be used if the file descriptor refers to a
file opened for writing.
len is the number of bytes to lock, start is the byte offset at
which the lock starts, relative to whence, and whence 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">io.IOBase.seek()</code></a>, specifically:
<ul class="simple">
<li><code class="docutils literal notranslate">0</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">os.SEEK_SET</code></a>)</li>
<li><code class="docutils literal notranslate">1</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">os.SEEK_CUR</code></a>)</li>
<li><code class="docutils literal notranslate">2</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">os.SEEK_END</code></a>)</li>
</ul>
The default for start is 0, which means to start at the beginning of the file.
The default for len is 0 which means to lock to the end of the file. The
default for whence is also 0.
Raises an <a class="reference internal" href="sys.html#auditing">auditing event</a> <code class="docutils literal notranslate">fcntl.lockf</code> with arguments <code class="docutils literal notranslate">fd</code>, <code class="docutils literal notranslate">cmd</code>, <code class="docutils literal notranslate">len</code>, <code class="docutils literal notranslate">start</code>, <code class="docutils literal notranslate">whence</code>.
</dd></dl>

Examples (all on a SVR4 compliant system):
<div class="highlight-python3 notranslate"><div class="highlight"><pre>import struct, fcntl, os

f = open(...)
rv = fcntl.fcntl(f, fcntl.F_SETFL, os.O_NDELAY)

lockdata = struct.pack(&#39;hhllhh&#39;, fcntl.F_WRLCK, 0, 0, 0, 0, 0)
rv = fcntl.fcntl(f, fcntl.F_SETLKW, lockdata)
</pre></div>
</div>
Note that in the first example the return value variable rv 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">bytes</code></a> object. The
structure lay-out for the lockdata 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">flock()</code></a> call may be better.
<div class="admonition seealso">
See also
<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">os</code></a></dt><dd>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">O_SHLOCK</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">O_EXLOCK</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">os</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">os.open()</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">lockf()</code></a> and <a class="reference internal" href="#fcntl.flock" title="fcntl.flock"><code class="xref py py-func docutils literal notranslate">flock()</code></a>
functions.
</dd>
</dl>
</div>
</section>

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