MMAP MODULE:

    This implementation of mmap contains all the functions that are present in
the Cpython implementation. However there are some notable differences :

(I) CONSTRUCTOR
    1.Both the Windows and Unix versions of the constructor take a file object
      as the first argument instead of the fileno.
    2.The Windows version of the mmap constructor does not support the tagname
      parameter.
    3.The Unix version of the constructor does not support the following:
       (a)flags = 'MAP_DENYWRITE', 'MAP_EXECUTABLE'
       (b)prot = 'PROT_EXEC'

(II) FLUSH function 
    This implementation does not support flushing a part of the mapped buffer. 
Even if a chunk of the buffer is requested to be flushed ,the entire buffer 
will be flushed.     

(III) CLOSE function 
    This function is supported , however the implementation makes use of a Sun
specific package called sun.misc.Cleaner.

RATIONALE FOR THE ABOVE DISCREPANCIES:

(I)
    1. In java to memory map a file the map function in the FileChannel class
is used.In Cpython once a file has been memory mapped even if the file is 
closed the mmap can still be accessed and all the operations can be performed
on it.To do this the mmap implementation in CPython creates a duplicate of the 
file descriptor so that even if the file is closed we can still query the file
size,truncate the file etc.(in java ,can only be done with a FileChannel 
instance) using the duplicate file descriptor. So in jython if we use fileno 
as first parameter we effectively get a instance of RawIoObject from which we
can derive the channel,however the channel cannot be duplicated since there 
seems to be no way of duplicating a file channel in java.In such a case it is
impossible to perform the aforementioned file operations when the file is 
closed.Another way would be to open the file again to get another channel to
it.But there is no way to derive the name of the file from RawIoObject instance.
Hence a file object is used instead from which we derive filename and open a 
separate channel to the file.

    2. In java ,the FileChannel map() function does not support specifying the
tagname for Windows environment

    3. Same reason as 2.

(II)
    Same reason as (I)(2,3)

(III)
    While java provides support for mapping a file there is no support for 
unmapping it. This is a RFE that has been reported for java. So I have used a 
widely circulated workaround. However there are other workarounds also like 
using Runtime.exec() to invoke System.gc(). However while testing the patch
on Windows Vista this was the only workaround that seemed to work.

Comments:
   1.The test file has been taken from CPython. It has been modified in 
two ways:
        (a)The mmap constructor takes file object as parameter not fileno
        (b)The os.platform function has been replaced with 
           javashell._getOsType() in order to determine the underlying os.
           This is because the former returns the platform as "java"
 
   2.To make a mmap behave like a string this implementation of PyMmap extends
the PyString class.A separate string is maintained along with the mapped 
buffer and the former is modified whenever the latter is changed.(Not sure 
whether this is the right way to do it....but it serves the purpose).Not 
extending the PyString class leads to errors in the test file where a mmap 
object is passed to the re.search() function since re.search() checks whether 
the argument passed to it is an instance of PyString.

   3.This is actually a doubt.There seems to be a lack of clarity regarding 
how objects in jython can be made to expose the buffer interface . This 
implementation of mmap does not explicitly provide the buffer interface.