:mod:`mmap` --- Memory-mapped file support

==========================================

.. module:: mmap

   :synopsis: Interface to memory-mapped files for Unix and Windows.

Memory-mapped file objects behave like both strings and like file objects.

Unlike normal string objects, however, these are mutable.  You can use mmap

objects in most places where strings are expected; for example, you can use

the :mod:`re` module to search through a memory-mapped file.  Since they're

mutable, you can change a single character by doing ``obj[index] = 'a'``, or

change a substring by assigning to a slice: ``obj[i1:i2] = '...'``.  You can

also read and write data starting at the current file position, and

:meth:`seek` through the file to different positions.

A memory-mapped file is created by the :class:`mmap` constructor, which is

different on Unix and on Windows.  In either case you must provide a file

descriptor for a file opened for update. If you wish to map an existing Python

file object, use its :meth:`fileno` method to obtain the correct value for the

*fileno* parameter.  Otherwise, you can open the file using the

:func:`os.open` function, which returns a file descriptor directly (the file

still needs to be closed when done).

For both the Unix and Windows versions of the constructor, *access* may be

specified as an optional keyword parameter. *access* accepts one of three

values: :const:`ACCESS_READ`, :const:`ACCESS_WRITE`, or :const:`ACCESS_COPY`

to specify read-only, write-through or copy-on-write memory respectively.

*access* can be used on both Unix and Windows.  If *access* is not specified,

Windows mmap returns a write-through mapping.  The initial memory values for

all three access types are taken from the specified file.  Assignment to an

:const:`ACCESS_READ` memory map raises a :exc:`TypeError` exception.

Assignment to an :const:`ACCESS_WRITE` memory map affects both memory and the

underlying file.  Assignment to an :const:`ACCESS_COPY` memory map affects

memory but does not update the underlying file.

.. versionchanged:: 2.5

   To map anonymous memory, -1 should be passed as the fileno along with the

   length.

.. versionchanged:: 2.6

   mmap.mmap has formerly been a factory function creating mmap objects. Now

   mmap.mmap is the class itself.

.. class:: mmap(fileno, length[, tagname[, access[, offset]]])

   **(Windows version)** Maps *length* bytes from the file specified by the

   file handle *fileno*, and creates a mmap object.  If *length* is larger

   than the current size of the file, the file is extended to contain *length*

   bytes.  If *length* is ``0``, the maximum length of the map is the current

   size of the file, except that if the file is empty Windows raises an

   exception (you cannot create an empty mapping on Windows).

   *tagname*, if specified and not ``None``, is a string giving a tag name for

   the mapping.  Windows allows you to have many different mappings against

   the same file.  If you specify the name of an existing tag, that tag is

   opened, otherwise a new tag of this name is created.  If this parameter is

   omitted or ``None``, the mapping is created without a name.  Avoiding the

   use of the tag parameter will assist in keeping your code portable between

   Unix and Windows.

   *offset* may be specified as a non-negative integer offset. mmap references

   will be relative to the offset from the beginning of the file. *offset*

   defaults to 0.  *offset* must be a multiple of the ALLOCATIONGRANULARITY.

.. class:: mmap(fileno, length[, flags[, prot[, access[, offset]]]])

   :noindex:

   **(Unix version)** Maps *length* bytes from the file specified by the file

   descriptor *fileno*, and returns a mmap object.  If *length* is ``0``, the

   maximum length of the map will be the current size of the file when

   :class:`mmap` is called.

   *flags* specifies the nature of the mapping. :const:`MAP_PRIVATE` creates a

   private copy-on-write mapping, so changes to the contents of the mmap

   object will be private to this process, and :const:`MAP_SHARED` creates a

   mapping that's shared with all other processes mapping the same areas of

   the file.  The default value is :const:`MAP_SHARED`.

   *prot*, if specified, gives the desired memory protection; the two most

   useful values are :const:`PROT_READ` and :const:`PROT_WRITE`, to specify

   that the pages may be read or written.  *prot* defaults to

   :const:`PROT_READ \| PROT_WRITE`.

   *access* may be specified in lieu of *flags* and *prot* as an optional

   keyword parameter.  It is an error to specify both *flags*, *prot* and

   *access*.  See the description of *access* above for information on how to

   use this parameter.

   *offset* may be specified as a non-negative integer offset. mmap references

   will be relative to the offset from the beginning of the file. *offset*

   defaults to 0.  *offset* must be a multiple of the PAGESIZE or

   ALLOCATIONGRANULARITY.

   This example shows a simple way of using :class:`mmap`::

      import mmap

      # write a simple example file

      with open("hello.txt", "w") as f:

          f.write("Hello Python!\n")

      with open("hello.txt", "r+") as f:

          # memory-map the file, size 0 means whole file

          map = mmap.mmap(f.fileno(), 0)

          # read content via standard file methods

          print map.readline()  # prints "Hello Python!"

          # read content via slice notation

          print map[:5]  # prints "Hello"

          # update content using slice notation;

          # note that new content must have same size

          map[6:] = " world!\n"

          # ... and read again using standard file methods

          map.seek(0)

          print map.readline()  # prints "Hello  world!"

          # close the map

          map.close()

   The next example demonstrates how to create an anonymous map and exchange

   data between the parent and child processes::

      import mmap

      import os

      map = mmap.mmap(-1, 13)

      map.write("Hello world!")

      pid = os.fork()

      if pid == 0: # In a child process

          map.seek(0)

          print map.readline()

          map.close()

   Memory-mapped file objects support the following methods:

   .. method:: close()

      Close the file.  Subsequent calls to other methods of the object will

      result in an exception being raised.

   .. method:: find(string[, start[, end]])

      Returns the lowest index in the object where the substring *string* is

      found, such that *string* is contained in the range [*start*, *end*].

      Optional arguments *start* and *end* are interpreted as in slice notation.

      Returns ``-1`` on failure.

   .. method:: flush([offset, size])

      Flushes changes made to the in-memory copy of a file back to disk. Without

      use of this call there is no guarantee that changes are written back before

      the object is destroyed.  If *offset* and *size* are specified, only

      changes to the given range of bytes will be flushed to disk; otherwise, the

      whole extent of the mapping is flushed.

      **(Windows version)** A nonzero value returned indicates success; zero

      indicates failure.

      **(Unix version)** A zero value is returned to indicate success. An

      exception is raised when the call failed.

   .. method:: move(dest, src, count)

      Copy the *count* bytes starting at offset *src* to the destination index

      *dest*.  If the mmap was created with :const:`ACCESS_READ`, then calls to

      move will throw a :exc:`TypeError` exception.

   .. method:: read(num)

      Return a string containing up to *num* bytes starting from the current

      file position; the file position is updated to point after the bytes that

      were returned.

   .. method:: read_byte()

      Returns a string of length 1 containing the character at the current file

      position, and advances the file position by 1.

   .. method:: readline()

      Returns a single line, starting at the current file position and up to the

      next newline.

   .. method:: resize(newsize)

      Resizes the map and the underlying file, if any. If the mmap was created

      with :const:`ACCESS_READ` or :const:`ACCESS_COPY`, resizing the map will

      throw a :exc:`TypeError` exception.

   .. method:: rfind(string[, start[, end]])

      Returns the highest index in the object where the substring *string* is

      found, such that *string* is contained in the range [*start*, *end*].

      Optional arguments *start* and *end* are interpreted as in slice notation.

      Returns ``-1`` on failure.

   .. method:: seek(pos[, whence])

      Set the file's current position.  *whence* argument is optional and

      defaults to ``os.SEEK_SET`` or ``0`` (absolute file positioning); other

      values are ``os.SEEK_CUR`` or ``1`` (seek relative to the current

      position) and ``os.SEEK_END`` or ``2`` (seek relative to the file's end).

   .. method:: size()

      Return the length of the file, which can be larger than the size of the

      memory-mapped area.

   .. method:: tell()

      Returns the current position of the file pointer.

   .. method:: write(string)

      Write the bytes in *string* into memory at the current position of the

      file pointer; the file position is updated to point after the bytes that

      were written. If the mmap was created with :const:`ACCESS_READ`, then

      writing to it will throw a :exc:`TypeError` exception.

   .. method:: write_byte(byte)

      Write the single-character string *byte* into memory at the current

      position of the file pointer; the file position is advanced by ``1``. If

      the mmap was created with :const:`ACCESS_READ`, then writing to it will

      throw a :exc:`TypeError` exception.