/Doc/library/fcntl.rst
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- :mod:`fcntl` --- The :func:`fcntl` and :func:`ioctl` system calls
- =================================================================
- .. module:: fcntl
- :platform: Unix
- :synopsis: The fcntl() and ioctl() system calls.
- .. sectionauthor:: Jaap Vermeulen
- .. index::
- pair: UNIX@Unix; file control
- pair: UNIX@Unix; I/O control
- This module performs file control and I/O control on file descriptors. It is an
- interface to the :cfunc:`fcntl` and :cfunc:`ioctl` Unix routines.
- 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:`fileno` which returns a genuine file descriptor.
- The module defines the following functions:
- .. function:: fcntl(fd, op[, arg])
- Perform the requested operation on file descriptor *fd* (file objects providing
- a :meth:`fileno` method are accepted as well). The operation is defined by *op*
- and is operating system dependent. These codes are also found in the
- :mod:`fcntl` module. 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 :cfunc:`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
- :cfunc:`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 :cfunc:`fcntl` fails, an :exc:`IOError` is raised.
- .. function:: ioctl(fd, op[, arg[, mutate_flag]])
- This function is identical to the :func:`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.
- 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` 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.
- 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:`fileno` method are accepted as well). See the Unix manual
- :manpage:`flock(3)` for details. (On some systems, this function is emulated
- using :cfunc:`fcntl`.)
- .. function:: lockf(fd, operation, [length, [start, [whence]]])
- This is essentially a wrapper around the :func:`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:`fileobj.seek`,
- specifically:
- * :const:`0` -- relative to the start of the file (:const:`SEEK_SET`)
- * :const:`1` -- relative to the current buffer position (:const:`SEEK_CUR`)
- * :const:`2` -- relative to the end of the file (:const:`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 :const:`O_SHLOCK` and :const:`O_EXLOCK` are present
- in the :mod:`os` module, the :func:`os.open` function provides a more
- platform-independent alternative to the :func:`lockf` and :func:`flock`
- functions.