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/Doc/library/posixfile.rst

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  1:mod:`posixfile` --- File-like objects with locking support
  2===========================================================
  3
  4.. module:: posixfile
  5   :platform: Unix
  6   :synopsis: A file-like object with support for locking.
  7   :deprecated:
  8.. moduleauthor:: Jaap Vermeulen
  9.. sectionauthor:: Jaap Vermeulen
 10
 11
 12.. index:: pair: POSIX; file object
 13
 14.. deprecated:: 1.5
 15   The locking operation that this module provides is done better and more portably
 16   by the :func:`fcntl.lockf` call.
 17
 18.. index:: single: fcntl() (in module fcntl)
 19
 20This module implements some additional functionality over the built-in file
 21objects.  In particular, it implements file locking, control over the file
 22flags, and an easy interface to duplicate the file object. The module defines a
 23new file object, the posixfile object.  It has all the standard file object
 24methods and adds the methods described below.  This module only works for
 25certain flavors of Unix, since it uses :func:`fcntl.fcntl` for file locking.
 26
 27To instantiate a posixfile object, use the :func:`open` function in the
 28:mod:`posixfile` module.  The resulting object looks and feels roughly the same
 29as a standard file object.
 30
 31The :mod:`posixfile` module defines the following constants:
 32
 33
 34.. data:: SEEK_SET
 35
 36   Offset is calculated from the start of the file.
 37
 38
 39.. data:: SEEK_CUR
 40
 41   Offset is calculated from the current position in the file.
 42
 43
 44.. data:: SEEK_END
 45
 46   Offset is calculated from the end of the file.
 47
 48The :mod:`posixfile` module defines the following functions:
 49
 50
 51.. function:: open(filename[, mode[, bufsize]])
 52
 53   Create a new posixfile object with the given filename and mode.  The *filename*,
 54   *mode* and *bufsize* arguments are interpreted the same way as by the built-in
 55   :func:`open` function.
 56
 57
 58.. function:: fileopen(fileobject)
 59
 60   Create a new posixfile object with the given standard file object. The resulting
 61   object has the same filename and mode as the original file object.
 62
 63The posixfile object defines the following additional methods:
 64
 65
 66.. method:: posixfile.lock(fmt, [len[, start[, whence]]])
 67
 68   Lock the specified section of the file that the file object is referring to.
 69   The format is explained below in a table.  The *len* argument specifies the
 70   length of the section that should be locked. The default is ``0``. *start*
 71   specifies the starting offset of the section, where the default is ``0``.  The
 72   *whence* argument specifies where the offset is relative to. It accepts one of
 73   the constants :const:`SEEK_SET`, :const:`SEEK_CUR` or :const:`SEEK_END`.  The
 74   default is :const:`SEEK_SET`.  For more information about the arguments refer to
 75   the :manpage:`fcntl(2)` manual page on your system.
 76
 77
 78.. method:: posixfile.flags([flags])
 79
 80   Set the specified flags for the file that the file object is referring to.  The
 81   new flags are ORed with the old flags, unless specified otherwise.  The format
 82   is explained below in a table.  Without the *flags* argument a string indicating
 83   the current flags is returned (this is the same as the ``?`` modifier).  For
 84   more information about the flags refer to the :manpage:`fcntl(2)` manual page on
 85   your system.
 86
 87
 88.. method:: posixfile.dup()
 89
 90   Duplicate the file object and the underlying file pointer and file descriptor.
 91   The resulting object behaves as if it were newly opened.
 92
 93
 94.. method:: posixfile.dup2(fd)
 95
 96   Duplicate the file object and the underlying file pointer and file descriptor.
 97   The new object will have the given file descriptor. Otherwise the resulting
 98   object behaves as if it were newly opened.
 99
100
101.. method:: posixfile.file()
102
103   Return the standard file object that the posixfile object is based on.  This is
104   sometimes necessary for functions that insist on a standard file object.
105
106All methods raise :exc:`IOError` when the request fails.
107
108Format characters for the :meth:`lock` method have the following meaning:
109
110+--------+-----------------------------------------------+
111| Format | Meaning                                       |
112+========+===============================================+
113| ``u``  | unlock the specified region                   |
114+--------+-----------------------------------------------+
115| ``r``  | request a read lock for the specified section |
116+--------+-----------------------------------------------+
117| ``w``  | request a write lock for the specified        |
118|        | section                                       |
119+--------+-----------------------------------------------+
120
121In addition the following modifiers can be added to the format:
122
123+----------+--------------------------------+-------+
124| Modifier | Meaning                        | Notes |
125+==========+================================+=======+
126| ``|``    | wait until the lock has been   |       |
127|          | granted                        |       |
128+----------+--------------------------------+-------+
129| ``?``    | return the first lock          | \(1)  |
130|          | conflicting with the requested |       |
131|          | lock, or ``None`` if there is  |       |
132|          | no conflict.                   |       |
133+----------+--------------------------------+-------+
134
135Note:
136
137(1)
138   The lock returned is in the format ``(mode, len, start, whence, pid)`` where
139   *mode* is a character representing the type of lock ('r' or 'w').  This modifier
140   prevents a request from being granted; it is for query purposes only.
141
142Format characters for the :meth:`flags` method have the following meanings:
143
144+--------+-----------------------------------------------+
145| Format | Meaning                                       |
146+========+===============================================+
147| ``a``  | append only flag                              |
148+--------+-----------------------------------------------+
149| ``c``  | close on exec flag                            |
150+--------+-----------------------------------------------+
151| ``n``  | no delay flag (also called non-blocking flag) |
152+--------+-----------------------------------------------+
153| ``s``  | synchronization flag                          |
154+--------+-----------------------------------------------+
155
156In addition the following modifiers can be added to the format:
157
158+----------+---------------------------------+-------+
159| Modifier | Meaning                         | Notes |
160+==========+=================================+=======+
161| ``!``    | turn the specified flags 'off', | \(1)  |
162|          | instead of the default 'on'     |       |
163+----------+---------------------------------+-------+
164| ``=``    | replace the flags, instead of   | \(1)  |
165|          | the default 'OR' operation      |       |
166+----------+---------------------------------+-------+
167| ``?``    | return a string in which the    | \(2)  |
168|          | characters represent the flags  |       |
169|          | that are set.                   |       |
170+----------+---------------------------------+-------+
171
172Notes:
173
174(1)
175   The ``!`` and ``=`` modifiers are mutually exclusive.
176
177(2)
178   This string represents the flags after they may have been altered by the same
179   call.
180
181Examples::
182
183   import posixfile
184
185   file = posixfile.open('/tmp/test', 'w')
186   file.lock('w|')
187   ...
188   file.lock('u')
189   file.close()
190