: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 a file object, such as ``sys.stdin`` itself, which
provides a :meth:`~io.IOBase.fileno` which returns a genuine file descriptor.
The module defines the following functions:
.. function:: fcntl(fd, op[, arg])
Perform the operation *op* on file descriptor *fd* (file objects providing
a :meth:`~io.IOBase.fileno` method are accepted as well). The values used
for for *op* 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* is optional, and defaults to the integer
value ``0``. 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 :c:func:`fcntl` call. When the argument is
a string 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 string object. The length of the returned string
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:`IOError` is raised.
.. function:: ioctl(fd, op[, arg[, mutate_flag]])
This function is identical to the :func:`~fcntl.fcntl` function, except that the
operations are typically defined in the library module :mod:`termios` and the
argument handling is even more complicated.
The op parameter is limited to values that can fit in 32-bits.
Additional constants of interest for use as the *op* 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, absent (treated identically to the
integer ``0``), an object supporting the read-only buffer interface (most likely
a plain Python string) or an object supporting the read-write buffer interface.
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 as least as long as what the operating system
wants to put there, things should work.
If *mutate_flag* is true, 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 *mutate_flag* 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.
If the :c:func:`ioctl` fails, an :exc:`IOError` 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, op)
Perform the lock operation *op* 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:`IOError` exception is raised.
.. function:: lockf(fd, operation, [length, [start, [whence]]])
This is essentially a wrapper around the :func:`~fcntl.fcntl` locking calls.
*fd* is the file descriptor of the file to lock or unlock, and *operation*
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 *operation* 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:`IOError` 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.
*length* 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 *length* 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 string value. 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.