# `fcntl` — The `fcntl` and `ioctl` system calls

This module performs file and I/O control on file descriptors. It is an interface to the `fcntl()` and `ioctl()` Unix routines. See the fcntl(2) and 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 `io.IOBase` object, such as `sys.stdin` itself, which provides a `fileno()` that returns a genuine file descriptor.

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

Changed in version 3.8: The `fcntl` module now contains `F_ADD_SEALS`, `F_GET_SEALS`, and `F_SEAL_*` constants for sealing of `os.memfd_create()` file descriptors.

Changed in version 3.9: On macOS, the `fcntl` module exposes the `F_GETPATH` constant, which obtains the path of a file from a file descriptor. On Linux(>=3.15), the `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.

Changed in version 3.10: On Linux >= 2.6.11, the `fcntl` module exposes the `F_GETPIPE_SZ` and `F_SETPIPE_SZ` constants, which allow to check and modify a pipe’s size respectively.

Changed in version 3.11: On FreeBSD, the `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.

Changed in version 3.12: On Linux >= 4.5, the `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”.

Changed in version 3.13: On Linux >= 2.6.32, the `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 `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 `fcntl` module exposes the `F_SEAL_FUTURE_WRITE` constant for use with `F_ADD_SEALS` and `F_GET_SEALS` operations. On FreeBSD, the `fcntl` module exposes the `F_READAHEAD`, `F_ISUNIONSTACK`, and `F_KINFO` constants. On macOS and FreeBSD, the `fcntl` module exposes the `F_RDAHEAD` constant. On NetBSD and AIX, the `fcntl` module exposes the `F_CLOSEM` constant. On NetBSD, the `fcntl` module exposes the `F_MAXFD` constant. On macOS and NetBSD, the `fcntl` module exposes the `F_GETNOSIGPIPE` and `F_SETNOSIGPIPE` constant.

Changed in version 3.14: On Linux >= 6.1, the `fcntl` module exposes the `F_DUPFD_QUERY` to query a file descriptor pointing to the same file.

The module defines the following functions:

fcntl.fcntl(fd, cmd, arg=0, /)

  Perform the operation cmd on file descriptor fd (file objects providing a `fileno()` method are accepted as well). The values used for cmd are operating system dependent, and are available as constants in the `fcntl` module, using the same names as used in the relevant C header files. The argument arg can either be an integer value, a 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 `fcntl()` call.

  When the argument is bytes-like object, it represents a binary structure, for example, created by `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 `fcntl()` call. The return value after a successful call is the contents of the buffer, converted to a `bytes` object. The length of the returned object will be the same as the length of the arg argument.

  If the `fcntl()` call fails, an `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.

  Raises an auditing event `fcntl.fcntl` with arguments `fd`, `cmd`, `arg`.

  Changed in version 3.14: Add support of arbitrary bytes-like objects, not only `bytes`.

  Changed in version 3.15: The size of bytes-like objects is no longer limited to 1024 bytes.

fcntl.ioctl(fd, request, arg=0, mutate_flag=True, /)

  This function is identical to the `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 `termios` module, under the same names as used in the relevant C header files.

  The parameter arg can be an integer, a 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 `fcntl()` function.

  If arg supports the read-write buffer interface (like `bytearray`) and mutate_flag is true (the default), then the buffer is (in effect) passed to the underlying `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 `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 `ioctl()` and copied back into the supplied buffer.

  If the `ioctl()` call fails, an `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])

  Raises an auditing event `fcntl.ioctl` with arguments `fd`, `request`, `arg`.

  Changed in version 3.14: The GIL is always released during a system call. System calls failing with EINTR are automatically retried.

  Changed in version 3.15: The size of not mutated bytes-like objects is no longer limited to 1024 bytes.

fcntl.flock(fd, operation, /)

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

  If the `flock()` call fails, an `OSError` exception is raised.

  Raises an auditing event `fcntl.flock` with arguments `fd`, `operation`.

fcntl.lockf(fd, cmd, len=0, start=0, whence=0, /)

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

  fcntl.LOCK_UN

    Release an existing lock.

  fcntl.LOCK_SH

    Acquire a shared lock.

  fcntl.LOCK_EX

    Acquire an exclusive lock.

  fcntl.LOCK_NB

    Bitwise OR with any of the other three `LOCK_*` constants to make the request non-blocking.

  If `LOCK_NB` is used and the lock cannot be acquired, an `OSError` will be raised and the exception will have an errno attribute set to `EACCES` or `EAGAIN` (depending on the operating system; for portability, check for both values). On at least some systems, `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 `io.IOBase.seek()`, specifically:

  - `0` – relative to the start of the file (`os.SEEK_SET`)

  - `1` – relative to the current buffer position (`os.SEEK_CUR`)

  - `2` – relative to the end of the file (`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.

  Raises an auditing event `fcntl.lockf` with arguments `fd`, `cmd`, `len`, `start`, `whence`.

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 `bytes` object. The structure lay-out for the lockdata variable is system dependent — therefore using the `flock()` call may be better.

See also

Module `os`

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