summaryrefslogtreecommitdiffstats
path: root/Doc/library/fcntl.rst
blob: d0430c282a4991f95a1938cccd77ca645e254db5 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156

:mod:`fcntl` --- The :func:`fcntl` and :func:`ioctl` system calls
=================================================================

.. module:: fcntl
   :platform: Unix
   :synopsis: The fcntl() and ioctl() system calls.
.. sectionauthor:: Jaap Vermeulen


.. index::
   pair: UNIX; file control
   pair: UNIX; I/O control

This module performs file control and I/O control on file descriptors. It is an
interface to the :cfunc:`fcntl` and :cfunc:`ioctl` Unix routines.

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 :meth:`fileno` which returns a genuine file descriptor.

The module defines the following functions:


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

   Perform the requested operation on file descriptor *fd* (file objects providing
   a :meth:`fileno` method are accepted as well). The operation is defined by *op*
   and is operating system dependent.  These codes are also found in the
   :mod:`fcntl` module. 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 :cfunc:`fcntl` call.  When the argument is
   a string it represents a binary structure, e.g. created by :func:`struct.pack`.
   The binary data is copied to a buffer whose address is passed to the C
   :cfunc:`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 :cfunc:`fcntl` fails, an :exc:`IOError` is raised.


.. function:: ioctl(fd, op[, arg[, mutate_flag]])

   This function is identical to the :func:`fcntl` function, except that the
   operations are typically defined in the library module :mod:`termios` and the
   argument handling is even more complicated.

   The op parameter is limited to values that can fit in 32-bits.

   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 :func:`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 :func:`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
   :func:`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 :func:`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.

   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])


.. function:: flock(fd, op)

   Perform the lock operation *op* on file descriptor *fd* (file objects providing
   a :meth:`fileno` method are accepted as well). See the Unix manual
   :manpage:`flock(2)` for details.  (On some systems, this function is emulated
   using :cfunc:`fcntl`.)


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

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

   * :const:`LOCK_UN` -- unlock
   * :const:`LOCK_SH` -- acquire a shared lock
   * :const:`LOCK_EX` -- acquire an exclusive lock

   When *operation* is :const:`LOCK_SH` or :const:`LOCK_EX`, it can also be
   bitwise ORed with :const:`LOCK_NB` to avoid blocking on lock acquisition.
   If :const:`LOCK_NB` is used and the lock cannot be acquired, an
   :exc:`IOError` will be raised and the exception will have an *errno*
   attribute set to :const:`EACCES` or :const:`EAGAIN` (depending on the
   operating system; for portability, check for both values).  On at least some
   systems, :const:`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 :func:`fileobj.seek`,
   specifically:

   * :const:`0` -- relative to the start of the file (:const:`SEEK_SET`)
   * :const:`1` -- relative to the current buffer position (:const:`SEEK_CUR`)
   * :const:`2` -- relative to the end of the file (:const:`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
:func:`flock` call may be better.


.. seealso::

   Module :mod:`os`
      If the locking flags :const:`O_SHLOCK` and :const:`O_EXLOCK` are present
      in the :mod:`os` module (on BSD only), the :func:`os.open` function
      provides an alternative to the :func:`lockf` and :func:`flock` functions.