: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 and I/O control on file descriptors. It is an interface to the :c:func:`fcntl` and :c:func:`ioctl` Unix routines. See the :manpage:`fcntl(2)` and :manpage:`ioctl(2)` Unix manual pages for full details. .. availability:: Unix, not WASI. 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 :mod:`!fcntl` module now contains ``F_ADD_SEALS``, ``F_GET_SEALS``, and ``F_SEAL_*`` constants for sealing of :func:`os.memfd_create` file descriptors. .. versionchanged:: 3.9 On macOS, the :mod:`!fcntl` module exposes the ``F_GETPATH`` constant, which obtains the path of a file from a file descriptor. On Linux(>=3.15), the :mod:`!fcntl` module exposes the ``F_OFD_GETLK``, ``F_OFD_SETLK`` and ``F_OFD_SETLKW`` constants, which are used when working with open file description locks. .. versionchanged:: 3.10 On Linux >= 2.6.11, the :mod:`!fcntl` module exposes the ``F_GETPIPE_SZ`` and ``F_SETPIPE_SZ`` constants, which allow to check and modify a pipe's size respectively. .. versionchanged:: 3.11 On FreeBSD, the :mod:`!fcntl` module exposes the ``F_DUP2FD`` and ``F_DUP2FD_CLOEXEC`` constants, which allow to duplicate a file descriptor, the latter setting ``FD_CLOEXEC`` flag in addition. .. versionchanged:: 3.12 On Linux >= 4.5, the :mod:`fcntl` module exposes the ``FICLONE`` and ``FICLONERANGE`` 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". .. versionchanged:: 3.13 On Linux >= 2.6.32, the :mod:`!fcntl` module exposes the ``F_GETOWN_EX``, ``F_SETOWN_EX``, ``F_OWNER_TID``, ``F_OWNER_PID``, ``F_OWNER_PGRP`` constants, which allow to direct I/O availability signals to a specific thread, process, or process group. On Linux >= 4.13, the :mod:`!fcntl` module exposes the ``F_GET_RW_HINT``, ``F_SET_RW_HINT``, ``F_GET_FILE_RW_HINT``, ``F_SET_FILE_RW_HINT``, and ``RWH_WRITE_LIFE_*`` constants, which allow to inform the kernel about the relative expected lifetime of writes on a given inode or via a particular open file description. On Linux >= 5.1 and NetBSD, the :mod:`!fcntl` module exposes the ``F_SEAL_FUTURE_WRITE`` constant for use with ``F_ADD_SEALS`` and ``F_GET_SEALS`` operations. On FreeBSD, the :mod:`!fcntl` module exposes the ``F_READAHEAD``, ``F_ISUNIONSTACK``, and ``F_KINFO`` constants. On macOS and FreeBSD, the :mod:`!fcntl` module exposes the ``F_RDAHEAD`` constant. On NetBSD and AIX, the :mod:`!fcntl` module exposes the ``F_CLOSEM`` constant. On NetBSD, the :mod:`!fcntl` module exposes the ``F_MAXFD`` constant. On macOS and NetBSD, the :mod:`!fcntl` module exposes the ``F_GETNOSIGPIPE`` and ``F_SETNOSIGPIPE`` constant. .. versionchanged:: 3.14 On Linux >= 6.1, the :mod:`!fcntl` module exposes the ``F_DUPFD_QUERY`` to query a file descriptor pointing to the same file. 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, a :term:`bytes-like object`, or a string. The type and size of *arg* must match the type and size of the argument of the operation as specified in the relevant C documentation. When *arg* is an integer, the function returns the integer return value of the C :c:func:`fcntl` call. When the argument is bytes-like object, it represents a binary structure, for example, created by :func:`struct.pack`. A string value is encoded to binary using the UTF-8 encoding. 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. If the :c:func:`fcntl` call fails, an :exc:`OSError` is raised. .. note:: If the type or size of *arg* does not match the type or size of the operation's argument (for example, if an integer is passed when a pointer is expected, or the information returned in the buffer by the operating system is larger than the size of *arg*), this is most likely to result in a segmentation violation or a more subtle data corruption. .. audit-event:: fcntl.fcntl fd,cmd,arg fcntl.fcntl .. versionchanged:: 3.14 Add support of arbitrary :term:`bytes-like objects `, not only :class:`bytes`. .. versionchanged:: next The size of bytes-like objects is no longer limited to 1024 bytes. .. 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 or 64-bits, depending on the platform. 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 an integer, a :term:`bytes-like object`, or a string. The type and size of *arg* must match the type and size of the argument of the operation as specified in the relevant C documentation. If *arg* does not support the read-write buffer interface or the *mutate_flag* is false, behavior is as for the :func:`~fcntl.fcntl` function. If *arg* supports the read-write buffer interface (like :class:`bytearray`) and *mutate_flag* is true (the default), then the buffer is (in effect) passed to the underlying :c: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 :c: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` call fails, an :exc:`OSError` exception is raised. .. note:: If the type or size of *arg* does not match the type or size of the operation's argument (for example, if an integer is passed when a pointer is expected, or the information returned in the buffer by the operating system is larger than the size of *arg*), this is most likely to result in a segmentation violation or a more subtle data corruption. 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]) .. audit-event:: fcntl.ioctl fd,request,arg fcntl.ioctl .. versionchanged:: 3.14 The GIL is always released during a system call. System calls failing with EINTR are automatically retried. .. versionchanged:: next The size of not mutated bytes-like objects is no longer limited to 1024 bytes. .. 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` call fails, an :exc:`OSError` exception is raised. .. audit-event:: fcntl.flock fd,operation fcntl.flock .. 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: .. data:: LOCK_UN Release an existing lock. .. data:: LOCK_SH Acquire a shared lock. .. data:: LOCK_EX Acquire an exclusive lock. .. data:: LOCK_NB Bitwise OR with any of the other three ``LOCK_*`` constants to make the request non-blocking. 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:`~errno.EACCES` or :const:`~errno.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: * ``0`` -- relative to the start of the file (:const:`os.SEEK_SET`) * ``1`` -- relative to the current buffer position (:const:`os.SEEK_CUR`) * ``2`` -- relative to the end of the file (:const:`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. .. audit-event:: fcntl.lockf fd,cmd,len,start,whence fcntl.lockf 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 :const:`~os.O_SHLOCK` and :const:`~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.