You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
168 lines
7.2 KiB
168 lines
7.2 KiB
: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.
|
|
|