cpython/Doc/library/fcntl.rst

:mod:`fcntl` --- The ``fcntl`` and ``ioctl`` system calls
=========================================================

.. module:: fcntl
   :platform: Unix
   :synopsis: The fcntl() and ioctl() system calls.

.. sectionauthor:: Jaap Vermeulen

.. index::
   pair: UNIX; file control
   pair: UNIX; I/O control

----------------

This module performs file control and I/O control on file descriptors. It is an
interface to the :c:func:`fcntl` and :c:func:`ioctl` Unix routines.  For a
complete description of these calls, see :manpage:`fcntl(2)` and
:manpage:`ioctl(2)` Unix manual pages.

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
``sys.stdin.fileno()``, or an :class:`io.IOBase` object, such as ``sys.stdin``
itself, which provides a :meth:`~io.IOBase.fileno` that returns a genuine file
descriptor.

.. versionchanged:: 3.3
   Operations in this module used to raise an :exc:`IOError` where they now
   raise an :exc:`OSError`.

.. versionchanged:: 3.8
   The fcntl module now contains ``F_ADD_SEALS``, ``F_GET_SEALS``, and
   ``F_SEAL_*`` constants for sealing of :func:`os.memfd_create` file
   descriptors.

The module defines the following functions:


.. function:: fcntl(fd, cmd, arg=0)

   Perform the operation *cmd* on file descriptor *fd* (file objects providing
   a :meth:`~io.IOBase.fileno` method are accepted as well).  The values used
   for *cmd* are operating system dependent, and are available as constants
   in the :mod:`fcntl` module, using the same names as used in the relevant C
   header files. The argument *arg* can either be an integer value, or a
   :class:`bytes` object. With an integer value, the return value of this
   function is the integer return value of the C :c:func:`fcntl` call.  When
   the argument is bytes it represents a binary structure, e.g. created by
   :func:`struct.pack`. The binary data is copied to a buffer whose address is
   passed to the C :c:func:`fcntl` call.  The return value after a successful
   call is the contents of the buffer, converted to a :class:`bytes` 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 :c:func:`fcntl` fails, an :exc:`OSError` is raised.


.. function:: ioctl(fd, request, arg=0, mutate_flag=True)

   This function is identical to the :func:`~fcntl.fcntl` 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 :mod:`termios` 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 :class:`bytes`) or an object supporting
   the read-write buffer interface (like :class:`bytearray`).

   In all but the last case, behaviour is as for the :func:`~fcntl.fcntl`
   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 :func:`ioctl` 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 :func:`ioctl`.  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 :func:`ioctl` and copied back
   into the supplied buffer.

   If the :c:func:`ioctl` fails, an :exc:`OSError` exception is raised.

   An example::

      >>> 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])


.. function:: flock(fd, operation)

   Perform the lock operation *operation* on file descriptor *fd* (file objects providing
   a :meth:`~io.IOBase.fileno` method are accepted as well). See the Unix manual
   :manpage:`flock(2)` for details.  (On some systems, this function is emulated
   using :c:func:`fcntl`.)

   If the :c:func:`flock` fails, an :exc:`OSError` exception is raised.


.. function:: lockf(fd, cmd, len=0, start=0, whence=0)

   This is essentially a wrapper around the :func:`~fcntl.fcntl` locking calls.
   *fd* is the file descriptor (file objects providing a :meth:`~io.IOBase.fileno`
   method are accepted as well) of the file to lock or unlock, and *cmd*
   is one of the following values:

   * :const:`LOCK_UN` -- unlock
   * :const:`LOCK_SH` -- acquire a shared lock
   * :const:`LOCK_EX` -- acquire an exclusive lock

   When *cmd* is :const:`LOCK_SH` or :const:`LOCK_EX`, it can also be
   bitwise ORed with :const:`LOCK_NB` to avoid blocking on lock acquisition.
   If :const:`LOCK_NB` is used and the lock cannot be acquired, an
   :exc:`OSError` will be raised and the exception will have an *errno*
   attribute set to :const:`EACCES` or :const:`EAGAIN` (depending on the
   operating system; for portability, check for both values).  On at least some
   systems, :const:`LOCK_EX` 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
   :func:`io.IOBase.seek`, specifically:

   * :const:`0` -- relative to the start of the file (:data:`os.SEEK_SET`)
   * :const:`1` -- relative to the current buffer position (:data:`os.SEEK_CUR`)
   * :const:`2` -- relative to the end of the file (:data:`os.SEEK_END`)

   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.

Examples (all on a SVR4 compliant system)::

   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)

Note that in the first example the return value variable *rv* will hold an
integer value; in the second example it will hold a :class:`bytes` object.  The
structure lay-out for the *lockdata* variable is system dependent --- therefore
using the :func:`flock` call may be better.


.. seealso::

   Module :mod:`os`
      If the locking flags :data:`~os.O_SHLOCK` and :data:`~os.O_EXLOCK` are
      present in the :mod:`os` module (on BSD only), the :func:`os.open`
      function provides an alternative to the :func:`lockf` and :func:`flock`
      functions.