36.12. "posixfile" — File-like objects with locking support
***********************************************************

1.5 版后已移除: The locking operation that this module provides is
done better and more portably by the "fcntl.lockf()" call.

This module implements some additional functionality over the built-in
file objects.  In particular, it implements file locking, control over
the file flags, and an easy interface to duplicate the file object.
The module defines a new file object, the posixfile object.  It has
all the standard file object methods and adds the methods described
below.  This module only works for certain flavors of Unix, since it
uses "fcntl.fcntl()" for file locking.

To instantiate a posixfile object, use the "posixfile.open()"
function.  The resulting object looks and feels roughly the same as a
standard file object.

The "posixfile" module defines the following constants:

posixfile.SEEK_SET

   Offset is calculated from the start of the file.

posixfile.SEEK_CUR

   Offset is calculated from the current position in the file.

posixfile.SEEK_END

   Offset is calculated from the end of the file.

The "posixfile" module defines the following functions:

posixfile.open(filename[, mode[, bufsize]])

   Create a new posixfile object with the given filename and mode.
   The *filename*, *mode* and *bufsize* arguments are interpreted the
   same way as by the built-in "open()" function.

posixfile.fileopen(fileobject)

   Create a new posixfile object with the given standard file object.
   The resulting object has the same filename and mode as the original
   file object.

The posixfile object defines the following additional methods:

posixfile.lock(fmt[, len[, start[, whence]]])

   Lock the specified section of the file that the file object is
   referring to. The format is explained below in a table.  The *len*
   argument specifies the length of the section that should be locked.
   The default is "0". *start* specifies the starting offset of the
   section, where the default is "0".  The *whence* argument specifies
   where the offset is relative to. It accepts one of the constants
   "SEEK_SET", "SEEK_CUR" or "SEEK_END".  The default is "SEEK_SET".
   For more information about the arguments refer to the *fcntl(2)*
   manual page on your system.

posixfile.flags([flags])

   Set the specified flags for the file that the file object is
   referring to.  The new flags are ORed with the old flags, unless
   specified otherwise.  The format is explained below in a table.
   Without the *flags* argument a string indicating the current flags
   is returned (this is the same as the "?" modifier).  For more
   information about the flags refer to the *fcntl(2)* manual page on
   your system.

posixfile.dup()

   Duplicate the file object and the underlying file pointer and file
   descriptor. The resulting object behaves as if it were newly
   opened.

posixfile.dup2(fd)

   Duplicate the file object and the underlying file pointer and file
   descriptor. The new object will have the given file descriptor.
   Otherwise the resulting object behaves as if it were newly opened.

posixfile.file()

   Return the standard file object that the posixfile object is based
   on.  This is sometimes necessary for functions that insist on a
   standard file object.

All methods raise "IOError" when the request fails.

Format characters for the "lock()" method have the following meaning:

+----------+-------------------------------------------------+
| Format   | Meaning                                         |
+==========+=================================================+
| "u"      | unlock the specified region                     |
+----------+-------------------------------------------------+
| "r"      | request a read lock for the specified section   |
+----------+-------------------------------------------------+
| "w"      | request a write lock for the specified section  |
+----------+-------------------------------------------------+

In addition the following modifiers can be added to the format:

+------------+----------------------------------+---------+
| Modifier   | Meaning                          | Notes   |
+============+==================================+=========+
| "|"        | wait until the lock has been     |         |
|            | granted                          |         |
+------------+----------------------------------+---------+
| "?"        | return the first lock            | (1)     |
|            | conflicting with the requested   |         |
|            | lock, or "None" if there is no   |         |
|            | conflict.                        |         |
+------------+----------------------------------+---------+

Note:

1. The lock returned is in the format "(mode, len, start, whence,
   pid)" where *mode* is a character representing the type of lock
   (‘r’ or ‘w’).  This modifier prevents a request from being granted;
   it is for query purposes only.

Format characters for the "flags()" method have the following
meanings:

+----------+-------------------------------------------------+
| Format   | Meaning                                         |
+==========+=================================================+
| "a"      | append only flag                                |
+----------+-------------------------------------------------+
| "c"      | close on exec flag                              |
+----------+-------------------------------------------------+
| "n"      | no delay flag (also called non-blocking flag)   |
+----------+-------------------------------------------------+
| "s"      | synchronization flag                            |
+----------+-------------------------------------------------+

In addition the following modifiers can be added to the format:

+------------+-----------------------------------+---------+
| Modifier   | Meaning                           | Notes   |
+============+===================================+=========+
| "!"        | turn the specified flags ‘off’,   | (1)     |
|            | instead of the default ‘on’       |         |
+------------+-----------------------------------+---------+
| "="        | replace the flags, instead of the | (1)     |
|            | default ‘OR’ operation            |         |
+------------+-----------------------------------+---------+
| "?"        | return a string in which the      | (2)     |
|            | characters represent the flags    |         |
|            | that are set.                     |         |
+------------+-----------------------------------+---------+

Notes:

1. The "!" and "=" modifiers are mutually exclusive.

2. This string represents the flags after they may have been
   altered by the same call.

Examples:

   import posixfile

   file = posixfile.open('testfile', 'w')
   file.lock('w|')
   ...
   file.lock('u')
   file.close()
