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
157
158
159
160
161
162
|
:mod:`fcntl` --- The ``fcntl`` and ``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 :c:func:`fcntl` and :c:func:`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 :class:`io.IOBase` object, such as ``sys.stdin``
itself, which provides a :meth:`~io.IOBase.fileno` that returns a genuine file
descriptor.
.. versionchanged:: 3.3
Operations in this module used to raise a :exc:`IOError` where they now
raise a :exc:`OSError`.
The module defines the following functions:
.. function:: fcntl(fd, op[, arg])
Perform the operation *op* on file descriptor *fd* (file objects providing
a :meth:`~io.IOBase.fileno` method are accepted as well). The values used
for *op* are operating system dependent, and are available as constants
in the :mod:`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 :c:func:`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
:c:func:`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 :c:func:`fcntl` fails, an :exc:`OSError` is raised.
.. function:: ioctl(fd, op[, arg[, mutate_flag]])
This function is identical to the :func:`~fcntl.fcntl` function, except
that 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 :mod:`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 :func:`~fcntl.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 (the default), 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.
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:`~io.IOBase.fileno` method are accepted as well). See the Unix manual
:manpage:`flock(2)` for details. (On some systems, this function is emulated
using :c:func:`fcntl`.)
.. function:: lockf(fd, operation, [length, [start, [whence]]])
This is essentially a wrapper around the :func:`~fcntl.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:`OSError` 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:`io.IOBase.seek`, specifically:
* :const:`0` -- relative to the start of the file (:data:`os.SEEK_SET`)
* :const:`1` -- relative to the current buffer position (:data:`os.SEEK_CUR`)
* :const:`2` -- relative to the end of the file (:data:`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
:func:`flock` call may be better.
.. seealso::
Module :mod:`os`
If the locking flags :data:`~os.O_SHLOCK` and :data:`~os.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.
|
