36.10. "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 a file object, such as
"sys.stdin" itself, which provides a "fileno()" which returns a
genuine file descriptor.

The module defines the following functions:

fcntl.fcntl(fd, op[, arg])

   Perform the operation *op* on file descriptor *fd* (file objects
   providing a "fileno()" method are accepted as well).  The values
   used for for *op* 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* 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 "fcntl()" call.  When the argument is a
   string 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
   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 "fcntl()" fails, an "IOError" is raised.

fcntl.ioctl(fd, op[, arg[, mutate_flag]])

   This function is identical to the "fcntl()" function, except that
   the operations are typically defined in the library module
   "termios" and the argument handling is even more complicated.

   The op parameter is limited to values that can fit in 32-bits.
   Additional constants of interest for use as the *op* 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, 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 "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 "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 *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.

   If the "ioctl()" fails, an "IOError" 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, op)

   Perform the lock operation *op* 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 "IOError" exception is raised.

fcntl.lockf(fd, operation[, length[, start[, whence]]])

   This is essentially a wrapper around the "fcntl()" locking calls.
   *fd* is the file descriptor of the file to lock or unlock, and
   *operation* is one of the following values:

   * "LOCK_UN" – unlock

   * "LOCK_SH" – acquire a shared lock

   * "LOCK_EX" – acquire an exclusive lock

   When *operation* 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 "IOError"
   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.

   *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 "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 *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 "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.
