1:mod:`fcntl` --- The ``fcntl`` and ``ioctl`` system calls
2=========================================================
3
4.. module:: fcntl
5   :platform: Unix
6   :synopsis: The fcntl() and ioctl() system calls.
7
8.. sectionauthor:: Jaap Vermeulen
9
10.. index::
11   pair: UNIX; file control
12   pair: UNIX; I/O control
13
14----------------
15
16This module performs file control and I/O control on file descriptors. It is an
17interface to the :c:func:`fcntl` and :c:func:`ioctl` Unix routines.  For a
18complete description of these calls, see :manpage:`fcntl(2)` and
19:manpage:`ioctl(2)` Unix manual pages.
20
21All functions in this module take a file descriptor *fd* as their first
22argument.  This can be an integer file descriptor, such as returned by
23``sys.stdin.fileno()``, or an :class:`io.IOBase` object, such as ``sys.stdin``
24itself, which provides a :meth:`~io.IOBase.fileno` that returns a genuine file
25descriptor.
26
27.. versionchanged:: 3.3
28   Operations in this module used to raise an :exc:`IOError` where they now
29   raise an :exc:`OSError`.
30
31
32The module defines the following functions:
33
34
35.. function:: fcntl(fd, cmd, arg=0)
36
37   Perform the operation *cmd* on file descriptor *fd* (file objects providing
38   a :meth:`~io.IOBase.fileno` method are accepted as well).  The values used
39   for *cmd* are operating system dependent, and are available as constants
40   in the :mod:`fcntl` module, using the same names as used in the relevant C
41   header files. The argument *arg* can either be an integer value, or a
42   :class:`bytes` object. With an integer value, the return value of this
43   function is the integer return value of the C :c:func:`fcntl` call.  When
44   the argument is bytes it represents a binary structure, e.g. created by
45   :func:`struct.pack`. The binary data is copied to a buffer whose address is
46   passed to the C :c:func:`fcntl` call.  The return value after a successful
47   call is the contents of the buffer, converted to a :class:`bytes` object.
48   The length of the returned object will be the same as the length of the
49   *arg* argument. This is limited to 1024 bytes. If the information returned
50   in the buffer by the operating system is larger than 1024 bytes, this is
51   most likely to result in a segmentation violation or a more subtle data
52   corruption.
53
54   If the :c:func:`fcntl` fails, an :exc:`OSError` is raised.
55
56
57.. function:: ioctl(fd, request, arg=0, mutate_flag=True)
58
59   This function is identical to the :func:`~fcntl.fcntl` function, except
60   that the argument handling is even more complicated.
61
62   The *request* parameter is limited to values that can fit in 32-bits.
63   Additional constants of interest for use as the *request* argument can be
64   found in the :mod:`termios` module, under the same names as used in
65   the relevant C header files.
66
67   The parameter *arg* can be one of an integer, an object supporting the
68   read-only buffer interface (like :class:`bytes`) or an object supporting
69   the read-write buffer interface (like :class:`bytearray`).
70
71   In all but the last case, behaviour is as for the :func:`~fcntl.fcntl`
72   function.
73
74   If a mutable buffer is passed, then the behaviour is determined by the value of
75   the *mutate_flag* parameter.
76
77   If it is false, the buffer's mutability is ignored and behaviour is as for a
78   read-only buffer, except that the 1024 byte limit mentioned above is avoided --
79   so long as the buffer you pass is at least as long as what the operating system
80   wants to put there, things should work.
81
82   If *mutate_flag* is true (the default), then the buffer is (in effect) passed
83   to the underlying :func:`ioctl` system call, the latter's return code is
84   passed back to the calling Python, and the buffer's new contents reflect the
85   action of the :func:`ioctl`.  This is a slight simplification, because if the
86   supplied buffer is less than 1024 bytes long it is first copied into a static
87   buffer 1024 bytes long which is then passed to :func:`ioctl` and copied back
88   into the supplied buffer.
89
90   If the :c:func:`ioctl` fails, an :exc:`OSError` exception is raised.
91
92   An example::
93
94      >>> import array, fcntl, struct, termios, os
95      >>> os.getpgrp()
96      13341
97      >>> struct.unpack('h', fcntl.ioctl(0, termios.TIOCGPGRP, "  "))[0]
98      13341
99      >>> buf = array.array('h', [0])
100      >>> fcntl.ioctl(0, termios.TIOCGPGRP, buf, 1)
101      0
102      >>> buf
103      array('h', [13341])
104
105
106.. function:: flock(fd, operation)
107
108   Perform the lock operation *operation* on file descriptor *fd* (file objects providing
109   a :meth:`~io.IOBase.fileno` method are accepted as well). See the Unix manual
110   :manpage:`flock(2)` for details.  (On some systems, this function is emulated
111   using :c:func:`fcntl`.)
112
113   If the :c:func:`flock` fails, an :exc:`OSError` exception is raised.
114
115
116.. function:: lockf(fd, cmd, len=0, start=0, whence=0)
117
118   This is essentially a wrapper around the :func:`~fcntl.fcntl` locking calls.
119   *fd* is the file descriptor of the file to lock or unlock, and *cmd*
120   is one of the following values:
121
122   * :const:`LOCK_UN` -- unlock
123   * :const:`LOCK_SH` -- acquire a shared lock
124   * :const:`LOCK_EX` -- acquire an exclusive lock
125
126   When *cmd* is :const:`LOCK_SH` or :const:`LOCK_EX`, it can also be
127   bitwise ORed with :const:`LOCK_NB` to avoid blocking on lock acquisition.
128   If :const:`LOCK_NB` is used and the lock cannot be acquired, an
129   :exc:`OSError` will be raised and the exception will have an *errno*
130   attribute set to :const:`EACCES` or :const:`EAGAIN` (depending on the
131   operating system; for portability, check for both values).  On at least some
132   systems, :const:`LOCK_EX` can only be used if the file descriptor refers to a
133   file opened for writing.
134
135   *len* is the number of bytes to lock, *start* is the byte offset at
136   which the lock starts, relative to *whence*, and *whence* is as with
137   :func:`io.IOBase.seek`, specifically:
138
139   * :const:`0` -- relative to the start of the file (:data:`os.SEEK_SET`)
140   * :const:`1` -- relative to the current buffer position (:data:`os.SEEK_CUR`)
141   * :const:`2` -- relative to the end of the file (:data:`os.SEEK_END`)
142
143   The default for *start* is 0, which means to start at the beginning of the file.
144   The default for *len* is 0 which means to lock to the end of the file.  The
145   default for *whence* is also 0.
146
147Examples (all on a SVR4 compliant system)::
148
149   import struct, fcntl, os
150
151   f = open(...)
152   rv = fcntl.fcntl(f, fcntl.F_SETFL, os.O_NDELAY)
153
154   lockdata = struct.pack('hhllhh', fcntl.F_WRLCK, 0, 0, 0, 0, 0)
155   rv = fcntl.fcntl(f, fcntl.F_SETLKW, lockdata)
156
157Note that in the first example the return value variable *rv* will hold an
158integer value; in the second example it will hold a :class:`bytes` object.  The
159structure lay-out for the *lockdata* variable is system dependent --- therefore
160using the :func:`flock` call may be better.
161
162
163.. seealso::
164
165   Module :mod:`os`
166      If the locking flags :data:`~os.O_SHLOCK` and :data:`~os.O_EXLOCK` are
167      present in the :mod:`os` module (on BSD only), the :func:`os.open`
168      function provides an alternative to the :func:`lockf` and :func:`flock`
169      functions.
170
171