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