"fcntl" — "fcntl" および "ioctl" システムコール
***********************************************

このモジュールでは、ファイル記述子 (file descriptor) に基づいたファイ
ル制御および I/O 制御を実現します。 このモジュールは、 Unix のルーチン
である "fcntl()" および "ioctl()" へのインタフェースです。 これらのシ
ステムコールの完全な説明は、 *fcntl(2)* と *ioctl(2)* のUnix マニュア
ルページを参照してください。

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.

このモジュールには、以下の関数が定義されています:

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.

   最後の型のオブジェクトを除き、動作は "fcntl()" 関数と同じです。

   可変なバッファが渡された場合、動作は *mutate_flag* 引数の値で決定さ
   れます。

   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.

   以下に例を示します:

      >>> 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" – アンロック

   * "LOCK_SH" – 共有ロックを取得

   * "LOCK_EX" – 排他的ロックを取得

   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" – ファイル先頭からの相対位置 ("os.SEEK_SET")

   * "1" – 現在のバッファ位置からの相対位置 ("os.SEEK_CUR")

   * "2" – ファイルの末尾からの相対位置 ("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.

以下に (全ての SVR4 互換システムでの) 例を示します:

   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.

参考:

  "os" モジュール
     もし "os" モジュールに "os.O_SHLOCK" と "os.O_EXLOCK" が 存在する
     場合 (BSD のみ)、 "os.open()" 関数は "lockf()" や "flock()" 関数
     を代替できます。
