"fcntl" — The "fcntl" and "ioctl" system calls
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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.
