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

This module performs file control and I/O control on file descriptors. It is an interface to the `fcntl()` and `ioctl()` Unix routines. For a complete description of these calls, see fcntl(2) and 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 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`.

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, or a `bytes` object. With an integer value, the return value of this function is the integer return value of the C `fcntl()` call. When the argument is bytes it represents a binary structure, e.g. created by `struct.pack()`. 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. 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 `fcntl()` fails, an `OSError` is raised.

`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. 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 one of an integer, an object supporting the read-only buffer interface (like `bytes`) or an object supporting the read-write buffer interface (like `bytearray`).

  In all but the last case, behaviour is as for the `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 at least as long as what the operating system wants to put there, things should work.

  If 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()` fails, an `OSError` 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])

`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()` fails, an `OSError` exception is raised.

`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 of the file to lock or unlock, and cmd is one of the following values:

  - `LOCK_UN` – unlock

  - `LOCK_SH` – acquire a shared lock

  - `LOCK_EX` – acquire an exclusive lock

  When cmd is `LOCK_SH` or `LOCK_EX`, it can also be bitwise ORed with `LOCK_NB` to avoid blocking on lock acquisition. 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.

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.