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

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" 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, 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. This is
   limited to 1024 bytes.

   If the "fcntl()" call fails, an "OSError" is raised.

   Note:

     If the type or the size of *arg* does not match the type or size
     of the argument of the operation (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 1024 bytes),
     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".

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 1024 bytes, or the
     size of the mutable bytes-like object is too small), 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".

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.
